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Electronic Theses and Dissertations
12-2014
Multilayer electret activated by direct contact
silicon electrode.
Mark M. Crain
University of Louisville
Follow this and additional works at: http://ir.library.louisville.edu/etd
Part of the Electrical and Computer Engineering Commons
Recommended Citation
Crain, Mark M., "Multilayer electret activated by direct contact silicon electrode." (2014). Electronic Theses and Dissertations. Paper
1719.
http://dx.doi.org/10.18297/etd/1719
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MULTILAYERELECTRETACTIVATEDBYDIRECTCONTACTSILICONELECTRODE
by
MarkMCrainIII
B.S.M.E.,PurdueUniversity,1991
M.S.E.E.,UniversityofLouisville,1999
ADissertation
SubmittedtotheFacultyofthe
JBSpeedEngineeringSchooloftheUniversityofLouisville
InPartialFulfillmentoftheRequirements
FortheDegreeof
DoctorofPhilosophy
DepartmentofElectricalandComputerEngineering
UniversityofLouisville
Louisville,Kentucky
December2014
Copyright2014byMarkMCrainIII
Allrightsreserved
MULTILAYERELECTRETACTIVATEDBYDIRECTCONTACTSILICONELECTRODE
By
MarkMCrainIII
B.S.M.E.,PurdueUniversity,1991
M.S.E.E.,UniversityofLouisville,1999
ADissertationApprovedon
November11,2014
BythefollowingDissertationCommittee:
DissertationCo‐Director
ShamusMcNamara,Ph.D.
DissertationCo‐Director
RobertKeynton,Ph.D.
RobertCohn,Ph.D.
BruceAlphenaar,Ph.D.
GaminiSumanasekera,Ph.D.
GailDePuy,Ph.D.
ii
DEDICATION
Thisdissertationisdedicatedtomyfamily
Angie,Wyatt,andAndrew.
iii
ACKNOWLEDGMENTS
IwouldliketothankDr.Keyntonforhismentorship.Hehasbeenagreatrole
modelformeandcountlessotherstudents.Iappreciatethetimeandeffortthathe
has put into teaching me how to do research. I would also like to thank Dr.
McNamara for his enthusiasm and helpful discussions. From Dr. Cohn, I learned
perspectiveandIappreciatehishighqualityworkmanship.Heisafantasticwriter.
Dr. Alphenaar is a great teacher, I had been in the microfab business for several
years before taking physical electronics in ECE; I was very happy to learn it from
him.FromDr.Gamini,Ilearnedtotakethingsinstride;worksmart,keepcool;in
otherwords,behavelikeaphysicist.Aspecial“ThankYou”toDr.Depuyforallof
theguidanceandconsultationwhichhasresultedinmylearningagreatdealabout
DOE and statistical interpretation of results. A second special “Thank You” to Dr.
Balooforhistechnicalcommentaryandsupport.
iv
ABSTRACT
MULTILAYERELECTRETACTIVATEDBYDIRECTCONTACTSILICONELECTRODE
MarkMCrainIII
November11,2014
Electrets used in microelectromechanical systems (MEMS) devices are often
formedbycoronacharging,whereionizedgasesaregeneratedinanelectricfieldto
introduce a charge to the electret surface. The purpose of this study was to
investigate a new technique for creating an electret from a plasma enhanced
chemicalvapordeposition(PECVD)multilayerfilmofSiO2/Si3N4/SiO2usingadirect
contactelectrodeofsilicon.Theelectretformationtakesadvantageofdeeptrapsin
siliconnitride,whichareknowntodevelopfromhydrogeninteractionswithsilicon
danglingbondsand,insomestoichiometries,nitrogendanglingbonds.Theelectret
activationprocesshasbeenoptimizedformaximumeffectivesurfacevoltage(ESV).
Thedepositionandactivationprocessfortheelectrethastheadditionalbenefitof
using commercially available equipment present in many microelectronic
fabrication facilities. Standardized processes for depositing the PECVD film stack
andactivatingtheelectretwithawaferlevelbonderhavebeendeveloped.
v
Using this new process, electret films have been produced with positive and
negative effective surface voltages in excess of +/‐194.0 V. Extrapolated lifetimes,
based on thermal decay studies, are calculated to be 57 years and 23 years for
positive and negative electrets respectively if they are maintained in moderate to
lowhumidityenvironmentsbelow125°C.Activationenergylevelsinpositiveand
negativeelectretsare1.4eVand1.2eVrespectively.Thisnewelectretmultilayer
filmstackanddirectchargingmethodproducedthinfilmelectretswithahalf‐life5
times greater than that reported in literature by other groups using PECVD
multilayerelectrets[1,2].
A new application was investigated to see how an electret may benefit
semiconductor‐liquid interactions. The PECVD electret was used to apply a gate
biastothebacksideofadoublesidepolishedsiliconwafertodeterminetheeffect
of gate bias on the etch rates of an anisotropic silicon etch in 25% wt.
tetramethylammonium hydroxide (TMAH). Our results show that the positively
charged electret produced a statistically significant increase in etch rate, when
comparedtoneutralandnegativelychargedelectrets,asthesilicon‐TMAHinterface
approachedthedepletionregionproducedbytheelectret.Themeanvaluesofthe
silicon etch rate were evaluated for the last hour of etching with samples
categorized by electret potentials as positive, negative or neutral. The positive
potentialelectrethadameanetchrateof12.0um/hrforsiliconascomparedto8.8
um/hrand8.6um/hrfornegativelyandneutrallychargeelectretsrespectively.The
onewayAnalysisOfVariance(ANOVA)ofthesiliconetchratesbetweentheneutral
vi
(control)PECVDfilmandthepositiveelectrethadaPvalueof0.009andfallswithin
the1%significancelevel,showingthatitisverylikelythatthepositiveelectretfilm
hasaneffectonthefinaletchrateofthesiliconundernullhypothesistesting.
vii
TABLEOFCONTENTS
DEDICATION...............................................................................................................................................iii ACKNOWLEDGMENTS............................................................................................................................iv ABSTRACT......................................................................................................................................................v LISTOFTABLES.......................................................................................................................................xii LISTOFFIGURES.....................................................................................................................................xiv 1 INTRODUCTION.................................................................................................................................1 2 BACKGROUND....................................................................................................................................5 2.1 IntroductiontoElectrets......................................................................................................5 2.2 Applications................................................................................................................................8 2.3 Insulator,Dielectric,Paraelectric,andFerroelectricProperties.........................9 2.4 Comparisonsofathinfilmelectrettoaparallelplatecapacitor.....................13 2.5 ElectricDisplacementField..............................................................................................17 2.6 EffectiveSurfaceVoltageElectretMeasurement....................................................21 viii
2.7 ElectretFormation...............................................................................................................25 2.7.1 ThermallyAssistedPoling............................................................................................25 2.7.2 CoronaCharging...............................................................................................................26 2.7.3 ElectronBeamImplant..................................................................................................28 2.8 ChargeTrapsinSiO2andSi3N4.......................................................................................28 2.9 StudiesofSiO2/Si3N4MultilayerElectretActivation.............................................30 2.10 ChargeTrappinginSiO2,Si3N4,andMultilayerElectrets....................................32 2.11 AnalysisTechniquesofElectrets...................................................................................37 2.12 ConceptsinModelingTemperatureDependentExponentialDecay..............40 2.13 SiliconEtching........................................................................................................................45 2.14 BandDiagramsofSiliconwithKOHandTMAH......................................................53 2.15 BiasEffectsonAnisotropicEtchingofSilicon..........................................................59 3 FABRICATIONANDMETHODS.................................................................................................65 3.1 PECVDDepositionofSiO2/Si3N4/SiO2.........................................................................65 3.2 WaferLevelThermallyAssistedPolingofElectret................................................67 3.3 ElectretMeasurementandEvaluation........................................................................70 ix
3.4 ProcessOptimization,DesignofExperiments.........................................................71 3.5 ElectretCharacterization...................................................................................................72 3.5.1 IsothermalPotentialDecaySamplePreparation...............................................72 3.5.2 IsothermalPotentialDecayExperiments..............................................................73 3.5.3 EvaluationofIsothermalPotentialDecayTrials................................................74 3.6 4 ApplicationofElectretasanElectrochemicalEtchStop.....................................75 3.6.1 WaferandDiePreparationforElectrochemicalEtchStop............................76 3.6.2 CalculationsfortheJustificationofStrongInversion.......................................79 3.6.3 ElectrochemicalEtchStopEtchCellsandEnvironmentalControl.............83 3.6.4 ElectrochemicalEtchStopTimedStudyOverview...........................................85 3.6.5 ConfirmationofEnvironmentalEtchConditions...............................................88 3.6.6 ElectretStabilityDuringAnisotropicEtching......................................................88 RESULTS.............................................................................................................................................90 4.1 WaferandFilmDimensions.............................................................................................90 4.2 CharacterizationandOptimizationofElectretFormationProcess................91 4.3 ElectretCharacterization.................................................................................................105 x
4.4 ApplicationofElectretasanElectrochemicalEtchStop...................................111 CONCLUSIONS........................................................................................................................................122 REFERENCES..........................................................................................................................................124 CURRICULUMVITAE...........................................................................................................................131 xi
LISTOFTABLES
TABLES
PAGE
Table1.PECVDprocessparameters
66 Table2.Siliconoxidethicknessmeasurements,meanandstandarddeviation.
91 Table3.Siliconnitridethicknessmeasurements,meanandstandarddeviation.
91 Table4.FactorialRegression:Peakversustime,temperature,voltage,CenterPt
93 Table 5. Factorial Regression: Peak positiveESV versus time, temperature, voltage
103 Table 6. Decay rates for positively charged electrets on four wafers at varying
temperaturestopredictthemeanlifetimeoftheelectrets.
107 Table 7. Decay rates for negatively charged electrets on four wafers at varying
temperaturestopredictthemeanlifetimeoftheelectrets.
108 Table8.Trial1etchdepthvstimeforneutral,negative,andpositiveelectrets. 115 Table9.Trial2etchdepthvstimeforneutral,negative,andpositiveelectrets. 116 xii
Table10.Trial3etchdepthvstimeforneutral,negative,andpositiveelectrets. 117 Table11.ESVmeasurementsforetchtestdieintrial3.
118 Table 12. Final silicon etch rate grouped by neutral, negative, and positive ESV
electret.
119 Table13.NullHypothesisPValuesfortheFinalEtchRatewith3separate2sample
T‐testusingMINITAB
119 xiii
LISTOFFIGURES
FIGURES
PAGE
Figure1.Electretmaterialdisplayingfixedsurface,volume,andpolarizedcharges.6 Figure 2. Dielectric materials exhibit a proportional polarizing electric field in
responsetoanactiveexternalelectricfield.
10 Figure 3. Paraelectric properties are exhibited by some dielectric materials as a
nonlinearpolarizationasafunctionofanexternalelectricfield.
10 Figure4.Ferroelectricpropertiesareexhibitedbysomeparaelectricmaterialswith
residualpolarizationwithoutanexternalelectricfield.
11 Figure5.Polarizationvs.electricfieldforaferroelectricmaterial.Spontaneousand
remnantpolarizationlevelsareshown.
12 Figure 6. Venn diagram of insulator classification based on polarization behavior
undertheinfluenceofanexternalelectricfield.
13 Figure7.(a)Parallelplatecapacitorwithfreespacepermittivity.(b)Parallelplate
capacitorwithdielectricmaterial.
15 xiv
Figure 8. A dielectric insulator is sandwiched between two metal layers. (a)
Opposing charges on conductors generate electric fields (arrows: positive to
negative charges) and produce an opposite polarization by charge migration. (b)
The removal of an external electric field on the dielectric reduces the charge
polarizationinthedielectrictozero.
16 Figure 9. A ferroelectric insulator is sandwiched between two metal layers. (a)
Opposing charges on conductors generate electric fields (arrows: positive to
negative charges) and produce a polarization by charge migration; upper detail
leaking charge, lower detail opposing charge. (b) The removal of an external
electricfieldontheinsulatordoesnotnegatethechargepolarization.
17 Figure10.Asimplemodelconsistingofaconductivebackedmultilayerelectretof
silicon oxide and silicon nitride for modeling effective surface voltage and
microphoneoperationalprinciples.
19 Figure 11. Diagram of null seeking feedback vibrating capacitor field meter with
electretandsubstrate.
23 Figure 12. Insulator‐silicon band bending due to electret bias modelled as an
effectivesurfacecharge.
24 Figure13.Coronachargingschematicwithheatassistance[42].
27 Figure14.APCVDSi3N4opencircuitthermallystimulateddecay[50].
30 xv
Figure15.NormalizedeffectivesurfacevoltageTSDofcoronachargedsiliconoxide
electret[52].
32 Figure 16. SONOS band diagram with Poole Frenkel conduction in silicon nitride.
Source:[53]
34 Figure17.Nitrogentosiliconratioeffectonsiliconnitridebandgapisshownwith
silicondanglingbondandnitrogendanglingbondlevels[58].
35 Figure18.Hotcarrierelectroninjectionintosilicondioxideandhydrogenrelease
[63].
37 Figure19.“Ideal”plotofanexponentialdecayplotwithrepresentationsofhalf‐life,
meanlifetimeandrespectiveconcentrations.
43 Figure 20. “Ideal” plot of an Arrhenius function on a semi‐log plot of exponential
decayvsinversetemperature.
44 Figure21.Anisotropicetchingcharacteristicsof(100)siliconwafer.
48 Figure22.Voltammogramprofilefor[100],solidlinep‐typesilicon,dashedlinen‐
typesiliconin40%KOHat60°C[80].
50 Figure 23. Voltammogram profile for [100], solid line p‐type, dashed line n‐type
siliconin25%wt.TMAHat80°C[81].
51 xvi
Figure24.Voltammogramsprovideinsighttoetchingandpassivationpotentialsof
the[100]and[111]siliconfacesinKOHfor(a)p‐typesiliconand(b)n‐typesilicon.
Source:[84].
53 Figure25.Commonsemiconductorbandgapsareshownrelevanttovacuumscale
andSHEelectrodepotential[86].
54 Figure26.ThebanddiagramanddensityofstatediagramforsiliconandKOHalkali
basedetchsolutionrespectively.Source:[66]
55 Figure27BanddiagramoncethesiliconisincontactwithKOHalkalibasedetchant;
(top)p‐typesilicon,(bottom)n‐typesilicon.Source:[66]
55 Figure 28. Band diagram of silicon with 25% TMAH etch solution. (a) n‐type
silicon/alkali based electrolyte (b) p‐type silicon/alkali based electrolyte. Source:
[87].
57 Figure 29. Semiconductor Fermi levels shown at the interface of electret redox
reaction[88].
57 Figure30.P‐typesiliconinanisotropicKOHetchant.Source:[19].
58 Figure31.AnodicbiastoP‐typesiliconinanisotropicKOHetchant.Source:[19] 59 Figure32.npnBJTmodelofpnjunctionofelectrochemicaletchstop[91]
60 Figure33.NPNBJTschematicshownwiththebase‐emitterforwardbiasedandthe
base‐collectorreversebiased[81].
60 xvii
Figure 34. TMAH based three‐electrode electrochemical etch stop using a pn
junction[81].
62 Figure35.BiasedPNjunctionetchstopinKOHusingafourelectrodeconfiguration.
Therelativepotentialsoftheworkelectrode(pnjunction),referenceelectrode,and
counterelectrodeareshownfromlefttoright.Source:[93]
62 Figure 36. A MIS electrochemical etch stop configuration in shown for p‐type
silicon. A region of n‐type silicon is produced for electrical contact to the silicon
substrate. A platinum electrode is submerged in the etchant to bias the silicon‐
etchantinterface[94].
64 Figure37.MISetchstopproducedbyinversionlayer[82].
64 Figure38.100mmdiametersiliconwaferwithPECVDdielectricfilmisloadedwith
2"“cathode”waferinSUSSSB‐6ebondertooling.
68 Figure39.Schematicofthesetupforthethermallyassistedpolingofthemultilayer
filmstackonasiliconwafer.
68 Figure40.Exampletemperatureandvoltagemagnitudeprofileusedfortheelectret
activationprocessintheSUSSbonder.
69 Figure 41. ESV measurements were made over the wafer surface at the
intersectionsofthealphanumericgridatapitchof1.5cmand1.0cm.
70 Figure42.Anisotropicwetetchwithelectretonetchwafer.
76 xviii
Figure43.Eachoftheetchdiecontain9etchcavitiesforevaluationofeffectsofthe
electretontheprogressionofsiliconetchinginthecavities.
77 Figure44.Dielevelelectretactivationwasperformedwithsub‐sizedsilicondieon
thePECVDfilm.Smallpieceofsiliconareplacedtotheedgeofthedietoimprove
groundcontactbetweenthehotplateandelectretdie.
79 Figure45.ElectretOxideSemiconductorbanddiagraminastateofstronginversion.
81 Figure46.Isometricdrawingoftheetchcellassemblywithsampledie.
84 Figure 47. Nine etch cells are shown on the hot plate in the aluminum enclosure
with the lid off to the side. One etch cell is shown with the PTFE lid removed,
showingthesilicondieandit's9respectiveetchcavities.
85 Figure48.Thesurfacetopologyismeasuredforfiveoftheetchcavitiesoneachdie.
Two cross sections of each cavity measured provide a total of four etch depth
measurements.
87 Figure 49. A "Modified" ESV measurement is made after each etching period to
confirmthattheelectretisstillactive.
89 Figure50.NegativepotentialESV“cube”plotofthe23fullfactorialDOEgiventhe
effectsoftemperature(°C),time(hrs),andappliedvoltage(V)ontheESV(V).The
xix
averages of the five peak ESV values for each wafer are shown at the corners and
centeroftheplot.
93 Figure51.The“maineffects”plotofthenegativeESVasafunctionoftemperature
(°C),time(hours),appliedvoltage(V).
94 Figure 52. Negative potential ESV “interaction” plot of temperature (°C), time
(hours),appliedvoltage(V).
94 Figure53:Effectivesurfacevoltageasafunctionofappliedprocesstemperaturefor
samplesactivatedat180Vforonehour.
97 Figure 54: Electret surface voltage as a function of applied process voltage for
samplesactivatedat170°Cforfivehours.
97 Figure 55. ESV contour plot with electret activation at ‐300 V, 5 hrs, 190°C. The
colorcodedlegendontherightprovidesthescaleoftheESVat20Vincrements. 99 Figure 56. ESV contour plot with electret activation at ‐300 V, 1 hrs, 170°C. The
colorcodedlegendontherightprovidesthescaleoftheESVat20Vincrements. 99 Figure 57. ESV contour plot with electret activation at ‐300 V, 5 hrs, 170°C. The
colorcodedlegendontherightprovidesthescaleoftheESVat20Vincrements. 99 Figure 58. ESV contour plot with electret activation at ‐180 V, 1 hrs, 190°C. The
colorcodedlegendontherightprovidesthescaleoftheESVat20Vincrements.100 xx
Figure 59. ESV contour plot with electret activation at ‐180 V, 5 hrs, 170°C. The
colorcodedlegendontherightprovidesthescaleoftheESVat20Vincrements.100 Figure60.ESVcontourplotwithelectretactivationat‐300V,1hr,190°C.Thecolor
codedlegendontherightprovidesthescaleoftheESVat20Vincrements.
100 Figure61.ESVcontourplotwithelectretactivationat‐180V,1hr,170°C.Thecolor
codedlegendontherightprovidesthescaleoftheESVat20Vincrements.
101 Figure 62. ESV contour plot with electret activation at ‐225 V, 3 hrs, 180°C. The
colorcodedlegendontherightprovidesthescaleoftheESVat20Vincrements.101 Figure 63. ESV contour plot with electret activation at ‐180 V, 5 hrs, 190°C. The
colorcodedlegendontherightprovidesthescaleoftheESVat20Vincrements.101 Figure 64.Positive potential ESV“cube” plot ofthe 23full factorial DOE given the
effectsoftemperature(°C),time(hrs),andnegativeappliedvoltage(V)ontheESV
(V).TheaverageofthefivepeakESVvaluesforeachwaferareshownatthecorners
andcenteroftheplot.
103 Figure65.The“maineffects”plotofthepositiveESVasafunctionoftemperature
(°C),time(hrs),appliedvoltage(V).
104 Figure66.PositivepotentialESV“interaction”plotoftemperature(°C),time(hrs),
andappliedvoltage(V).
104 xxi
Figure67.TheITPDplotforasamplefromwafer3showstheESVasafunctionof
time, aged at 300°C. The sample was activated at 300 V for 1 hour at 170°C. The
best fit regression to the exponential decay is provided with the 95% confidence
intervaland95%predictioninterval.
106 Figure68.TheITPDplotforasamplefromwafer3showstheESVasafunctionof
time, aged at 325°C. The sample was activated at 300 V for 1 hour at 170°C. The
best fit regression to the exponential decay is provided with the 95% confidence
intervaland95%predictioninterval.
107 Figure69.Positivelychargedelectret,semi‐logplotoftheexponentialdecayratevs
1000/Temperature. Samples from each of the 4 wafers were subjected to
acceleratedagingat325°C,300°C,275°C,250°C,and200°C.
109 Figure70.Negativelychargedelectret,semi‐logplotoftheexponentialdecayrate
vs 1000/Temperature. Samples from each of the 4 wafers were subjected to
acceleratedagingat325°C,300°C,275°C,250°C,and200°C.
109 Figure71.AbanddiagramofatheMISetchstopwithbiasbetweengateandsilicon
and in open circuit potential between the silicon and electrolyte, alkali based
etchant, showing the drift of electronsinto the electrolyte and continuing theetch
process with hydroxyl ion generation. The figure is modified from Smith and
Soderbarg1993[19].
112 xxii
Figure 72. A linear best fit for etch depth samples as a function of the time
remainingtotheendofthesiliconetcharegroupedbyelectretpotentials;positive,
negative,andneutral.
114 xxiii
1 INTRODUCTION
Electretsarematerialswithpermanent“net”and“polarized”chargesthatreside
on the surface or within the material [3]. Electrets are beneficial for providing a
continuous electrostatic potential, over a range of fractional volts to several
thousandvolts,withouttheuseofanexternalpowersupply.Theelectretprovides
an apparent potential at its surface, referred to as the effective surface voltage
(ESV). Thin film electrets have produced some revolutionary devices such as the
electretmicrophoneandnonvolatilememorywhichremainfunctionalfordecades
[4‐6]. Traditionally, field effect transistor (FET) microphones, also known as
electret condenser microphones (ECMs), are made with an electret consisting of a
Mylar or Polytetrafluoroethylene (PTFE) [3, 7, 8]. Integration of surface mount
electretmicrophoneshasbeenlimitedduetotheinabilityofthepolymerelectretto
maintain a charge throughout the surface mount reflow process which can be as
highat260°C.Inorganicelectretmaterials,suchassiliconoxideandsiliconnitride
have been shown to provide stable electret charge at elevated temperatures
exceeding surface mount reflow process [2, 7‐10]. Research has been conducted
withtheseinorganicelectretmaterialstointegratethemintoMEMSmicrophones
providing the benefits of system integration and charge stability [8, 9]. The
mechanicalproperties,togetherwiththeestablishedintegrationofsiliconoxideand
1
silicon nitride films in MEMS platforms, makes these films an excellent choice for
integrating electrets into MEMS devices [11]. The benefits of electrets as a
permanentorsemi‐permanentelectricfieldsourcehasbeenrealizedinmanyother
applications;Xerography[6,12],dosimeters[13,14],pressuresensors[10],energy
harvesters[15],electro‐activepolymer(EAP)actuation[16,17],andsolarcells[18].
Thermally assisted poling, corona charging, and electron beam exposure are
threemethodsfrequentlyusedtoproduceelectretmaterials.Eachmethodhasits
benefitsandlimitations;thebestchoicedependsonthematerialtobecharged,the
application, and cost effectiveness. Corona charging and electron beam exposure
are frequently used in microfabricated devices; corona charging is limited by the
requirementofionexchange,andbothcoronachargingandelectronbeamexposure
arelimitedbysinglepolaritycharging,andcustomequipmenttoformtheelectret.
Thermallyassistedpolingistheoldestmethodofelectretformation.Inthismethod,
amixtureofcarnaubawaxandcolophony(treesap)isheatedtoitsmeltingpoint
while a high electric field is applied between two electrodes in contact with the
mixture; the mixture is cooled to solidification under the electric field to produce
the electret. Direct contact poling is rarely discussed as part of a MEMS based
manufacturingprocessandhasnotbeenaddressedformultilayerSiO2/Si3N4films.
The new process presented does share some similarities to charge trap flash
memory devices, with important differences in increased film thickness and the
openfacedchargedfilmbeinguniquetotheelectretproduced.Thepurposeofthis
study is to develop a PECVD SiO2/Si3N4/SiO2 electret with a direct contact thermally
2
assistedpolingprocessthatrequiresnoionicinterfacewiththePECVDfilmsurfaceas
partofthechargingprocess,canproducebothpositiveandnegativeeffectivesurface
voltageswithasingleprocesspolarization,andcanbeconductedwithcommercially
available semiconductor processing equipment. There are three main aims to this
studywhichwillbeinvestigated.
Theprimaryaimistodevelopahightemperatureelectretthatcanbeprogramed
to provide a positive or negative effective surface voltage. A new method of direct
contactelectretactivatingaPECVDmultilayerofSiO2/Si3N4/SiO2isintroducedwith
a study on optimizing the effective surface voltage of the film generated using a
SUSS SB6+ anodic bonding system. The design of experiment consists of a 23 full
factorialanalysisasafunctionofactivationtemperature,processtime,andapplied
voltage.
Asecondaryaimofthisstudyistodeterminetheactivationenergyoftheelectret
film required to neutralize the effective surface voltage. The activation energy is
determined by measuring the open circuit isothermal decay rate of the electret’s
effect surface voltage over time. The measurement is made at several elevated
temperaturestodeterminetheexponentialdecayrateofthecharge.Thisdataisfit
to the Arrhenius function and the activation energy for positive and negative
electretsisdetermined.
A tertiary aim is to explore the implementation of the electret as part of an
electrochemicaletchstopprocess.Theelectretactsasthe“onwafer”biasrequired
3
forstronginversionandproducesadepletionregionforanetchstop,similartothe
MISetchstoppresentedbyothergroups[19].Theelectrochemicaletchstopmakes
useofareversebiasatthedielectric‐siliconinterfaceasinaMOSFETandotherMIS
devices. Application of the electret in this type of etch stop targets a number of
benefitsincluding;
‐
noexternalpowersupplyforthestronginversionoftheMIS,
‐
noadditionaldopingofthesilicon,asrequiredforap‐njunctionetchstop,
‐
easythermalneutralizationofcharge,
‐
andadjustablethicknesscontrol.
ThePECVDfilmistreatedatthedielevelinadirectcontactactivationprocess
andtestdiearefabricatedtodetermineiftheelectretisapplicabletoanetchstopin
silicon.Theelectretisprotectedfromdirectetchantcontactwiththeuseofspecial
etch cells and the etchant approaches the electret‐silicon interface as the silicon
etches. A number of timed etches are made and the progress of the etch depth is
monitored over time to determine if the charge of the electret produces and
effectivedepletionregiontoterminatetheetchprocess.Theeffectoftheelectret’s
potential on the final etch rate of silicon is determined by the “Null Hypothesis”
method. The “Null Hypothesis” for this set of experiments is that there is no
statistical difference in the final etch rate between samples grouped by electret
potential. An ANOVA is performed to determine if the null hypothesis can be
rejectedbetweenanygroupofsamplesbasedonthechargeoftheelectret.
4
2 BACKGRO
OUND
Theback
kgroundisd
dividedintw
wosectionss.Thefirst halfoftheb
background
dwill
provide a reeview of the physical principles aand formin
ng techniques with hisstoric
co
ontext in th
he developm
ment of electrets. A b
brief review
w of electrett applicatio
ons is
provided to give the reeader an ap
ppreciation for the imp
pact of elecctret techno
ology.
Several prom
minent reseearchers thaat have speent many deecades deveeloping a b
better
understandin
ngofelectretsandhav
vewrittend
detailreview
warticles[6
6,20‐23].M
Much
of the backg
ground presented stem
ms from th
hese articless. The seccond half of the
background coversthe fundamentalsofelectrrochemical etchstops insiliconw
witha
fo
ocus on understandi
u
ing how the
t
effect of the eelectret maay producee an
electrochemicaletchsto
op.
2.1
2 Introd
ductiontoElectrets
An electrret is an inssulating maaterial char acterized b
by the abilitty to maintain a
ch
harge and//or polarizaation whilee free from
m the equiliibrium of aapplied exteernal
electricfield[6].Chargeesarefixed (or“frozen
n”),intothe electret.Thefixedchaarges
are categorized as surfface chargees, space ch
harges, and
d dipole charges, Figu
ure 1.
t
lay at the surfacee of the electret or at the
Surface charrges are neet charges that
5
in
nterface of material su
urfaces. Spaace charges lay within the materiial, although
h the
ch
harges may
y be concentrated in some regioons and may not be homogeneo
ously
dispersed. Dipole chaarges arise when possitive and n
negative ch
harges are held
to
ogetherinssmallgroupingsbuttheesmallgrou
upingsalign
nthemselveesinthegreeater
whole
w
of the material, producingg a “net po larization” in the matterial. Mulltiple
caategories of
o charge may
m be invo
olved in an
n electret aand there ccan be mulltiple
so
ourcesofch
hargetoaccategory.Assanexamp
ple,bothpossitiveandn
negativechaarged
io
onsmayexiistinthebo
odyofanelectretasvoolumecharggesandinducepolarizaation
ofdipolesasswell.Thisswouldbe modelledaastwovolum
mechargessandonedipole
ch
harge. The discharge time
t
duratio
on (or lifetiime) of eacch charging mechanism
m can
vary drasticaally, from minutes
m
to decades,
d
deepending on
n the naturre of the ch
harge,
th
hematerialinwhichitresides,and
dtheenviroonmentalco
onditions.
Figure1.Eleectretmaterrialdisplayiingfixedsurrface,volum
me,andpolaarizedchargges.
The obseervation of electret maaterials wass documentted by Stephen Gray in
n the
early 1700’ss when he observed
o
peermanent surface charrges of dieleectric mateerials.
6
These early electrets where formed as dielectric materials consisting of wax and
rosin developed net charges when cast and cooled in iron molds. Additional
observations had been made by Heaviside in the late 1800’s. Heaviside observed
mobile charges within the insulating material migrating due to external applied
potential and due to fixed polarization within a dielectric material. These mobile
charges effectively mask the fixed charge from external surroundings. It was
around 1920 that Mototaro Eguchi combined these two effects and was able to
create thermoelectrets by applying external electric fields during the cooling and
solidificationprocess[6].Anappreciationforthecomplexnatureofcurrentswithin
the electret during charging and discharging developed in the early 1900’s, as it
became clearer that multiple current sources were acting, some caused by charge
injectionandothersbyionmobilityandbondbreaking[24].Propertiesofelectrets
onionizedgasandonthepolarizationofwaterhavebeenunderstoodfornearlya
century[25].
Inorganic electrets including SiO2, Si3N4, silicon oxynitride, and alumina have
been well studied and they are popular choices in traditional semiconductor and
MEMSapplications[10,26].Theseinorganicelectretsprovideanumberofcharge
trapping mechanisms; making use of dangling bonds, hydrogen defects, and
bandgap interfaces. The wide bandgap of the outer silicon oxide films, in
comparisontotheinteriorsiliconnitridefilm,providesanumberofchargetrapping
opportunities. Composite multilayer inorganic electrets are favored over single
7
laayers due to
o the impro
oved chargee retention over time and temperrature [1, 2
2, 27‐
31].
2.2
2 Appliccations
Electrets are a fund
damental part
p
of the modern fieeld effect ttransistor ((FET)
microphone
m
usedubiqu
uitouslyinm
manyelectroonicdevicestoday,succhasphonees.G.
M.
M Sessler and J. E. Weest are ofteen credited for the first commerccially succeessful
electret micrrophones based
b
on their Mylar eelectroacousstic transdu
ucer inventeed in
1962 [32]. By their own ad
dmission coommercially availablee wax eleectret
microphones
m
s have been available since 193 8 [33]. Electrets havve been useed in
detectingion
nizingradiaation[34]an
ndstillhaveeapplicationindosimeeterapplications
[1
13, 35]. Recent dev
velopmentss include n
nonvolatile memory (NVM)[36, 37],
vibrationaleenergyharvesting[15],andimproovingtheeffficiencyofssolarcells[1
18].
Thechargeofaneleectrethasaneffecton theelectriccfieldofsysstemsaroun
ndit.
As
A an examp
ple, the aco
oustic memb
brane of an
n electret m
microphone vibrates du
ue to
so
ound,acting
gasavibraatingvariabllecapacitorrthatisdep
pendenton thelongitudinal
pressure waaves of the air. The electric field produced b
by the electtret generaates a
ch
hange of potential
p
beetween thee variable ccapacitor p
plates. Forr a microphone
in
ntegratedw
withafieldeeffecttransiistor(FET), thesoundd
dependent gatepotenttialis
trransformed
dbytheFET
Tintoacurrrentoutput whichprovvidesasign
nalforaddittional
amplification
nortransmission.
8
2.3
2 Insulaator,Dielecctric,Paraaelectric,aandFerroeelectricPrroperties
Thecharacteristicsaandproperttieswhichd
defineanelectretoften
nmakeiteaasyto
co
onfuseitwiithothermaaterialcharacteristics.Toclarifyttheworkinggdefinitionofan
electret, it is
i good to review so
ome of thee related ccharacteristiics of insu
ulator
materials.A
m
Anidealinsu
ulatordoesnotconducctcurrent; thereisnochargetransfer
th
hrough the material. The ideal in
nsulator is unaffected by externaal electric fiields.
Dielectricma
D
aterialsareeaffectedby
yelectricfieelds.Theid
dealdielecttricproduceesan
opposing polarizing eleectric field that
t
is prop
portional to
o an active eexternal eleectric
fiield, Figure 2. The polaarization off an ideal d
dielectric is linearly deependent on
n the
magnitudeo
m
ftheextern
nalelectricffieldapplied
d.
Polarization comesinth
heformof electric,ion
nic,ordipolleactionfro
omthematterial.
Theseforms
T
ofpolarizaationaredependenton
nmanyfacto
orsincludin
ng,temperaature,
bindingenerrgywithinm
materialbon
nds,strengtthofelectricfield,phottoninteracttions,
driftanddifffusivity,etc..Paraelectrricmateriallsexhibitan
non‐linearp
polarization
nasa
fu
unctionofth
heexternallyappliedeelectricfield
d,Figure3. Notallpolarizationefffects
dissipatewitththeremo
ovaloftheeexternalelecctricfield.R
Residualpo
olarizationiisthe
reesultofnew
wlytrapped
dcharges,io
onmigration
n,oraresid
dualpolarizzationofdip
poles
in
n the materials. Ferrroelectric materials
m
arre considerred electretts because they
exxhibit polarrization in the
t absencee of an exteernal electriic field [38]]. The resu
ulting
polarizinghy
ysteresisofferroelectriicmaterialccanbeseen
ninFigure4
4.
9
Figure 2. Dielectric
D
materials
m
exxhibit a prooportional polarizing electric field in
reesponsetoaanactiveexxternalelecttricfield.
Figure 3. Paraelectric properties are exhibitted by som
me dielectricc materials as a
nonlinearpo
olarizationaasafunction
nofanexterrnalelectriccfield.
10
Figure4.Ferrroelectricp
propertiesaareexhibiteedbysome paraelectricmaterials with
reesidualpolaarizationwiithoutanexxternalelecttricfield.
Electrets exhibit po
olarization characterisstics similaar to those produced in a
dielectricunderbiasingbyanexterrnalelectriccfield.Unlik
kethedieleectricofatyypical
parallel platte capacitorr, an electreet can main
ntain its po
olarization field witho
out it
beinginduceedbyanextternalfield.Theremn antpolarizaation,Pr,is thepolarizaation
ntheelectreetafterthe externalellectricfield isremoved
d.Thisrem
mnant
reemainingin
polarization is characteeristic of a ferroelectrric materiall due to dip
poles [38]. The
onsofthem
material;surface
electretcan alsomaintaain“net”chargesindiffferentregio
orspacechargetype.R
Remnantpollarizationan
ndspontan
neouspolariizationlevells,Ps,
areshownin
nFigure5. Thepatho
ofhysteresiisshowstheespontaneo
ouspolarizaation
leevel as the polarizatio
on under the
t
stress oof the applied electriic field. In
n the
fo
ormationoffanelectrett,thiswoulldbethepoolarizationvvaluedurin
ngtheactivaation
11
process.Thiishysteresiisprofilem
maybetemp
peraturedep
pendent.A
Athermoeleectret
mayfollowa
m
aparaelectrricpolarizattionprocesssatelevated
dtemperatu
uresresultingin
aremnantpo
olarizationifcooledbeelowtheCurrie‐Weissteemperatureebeforerem
moval
oftheextern
nalelectricffield[38].
Figure5.Polarizationv
vs.electricfiieldforaferrroelectricm
material.SSpontaneoussand
reemnantpolaarizationlevelsaresho
own.
Figure 6,, displays a
a categorizaation of insu
ulators; all ferroelectrric materials are
paraelectric,allparaeleectricmaterrialsaredieelectric,and
dalldielectrricmateriallsare
in
nsulators. An
A electrett can be forrmed in anyy of the reggional defin
nitions with
h the
application surface chaarges, spacee charges, or dipoles.. Some po
opular insu
ulator
materials
m
are listed ineeach region
n of the Ven
nn diagram..Eachof th
he materials are
caategorized in the regiion they are commonlly associateed but this generalizations
sh
hould be approached with caaution. M
Materials o
often carryy some of
f the
12
ch
haracteristiics outside their typiccally defineed region. Silicon nitrride and siilicon
oxide are tw
wo such maaterials. Th
hese materiaals can be modified to
o operate u
under
paraelectric and ferroellectric cond
ditionswith
htheintrod
duction of trraps, impurrities,
and composite construction [39]. All these conditions play an im
mportant paart in
SONOSandM
MNOSdeviccesandthey
yplayaparttinthecom
mpositeelecttretaswell..
m of insulattor classificcation based
d on polariization behaavior
Figure 6. Veenn diagram
d.
undertheinffluenceofanexternaleelectricfield
2.4
2 Comparisonsoffathinfilm
melectrettoaparalllelplateccapacitor
Thereareesimilaritieesandashaaredbackgrooundbetweeenelectrettsanddielecctrics
in
n capacitors that mak
ke it valuab
ble to revieew the principles of a parallel plate
caapacitor beefore discussing the deetails of electret materrials. Figurre 7 (a) sho
ows a
caapacitorcon
nsistingofttwoparallellplatesmad
deofcondu
uctivematerrialinavacuum.
The
T
parallell plate cap
pacitor is “idealized”
“
with the assumption
n that the two
co
onductivep
plateshave alargeoverlappingsu
urfacerelatiivetothediistancebetw
ween
13
them.Thesurfacechargedensity,σp,atthefaceofeachplatevariesproportionally
withthepotential,V,betweentheplates,

,
Equation1
wheretisthedistancebetweenplatesandoisthepermittivityoffreespace.
An ideal capacitor contains identical charge densities on each plate, one net
positiveandtheothernetnegative.Whilethecapacitorcanstorethisenergyina
quasi‐permanent time frame, the charges are mobile within each plate and will
readily migrate to equalize in potential if given the opportunity. As the potential
betweenthetwoconductivelayersisincreased,thecharge(σp)oneachconductive
plate also increases. As the charge density on the conductive plate increases, the
electricfieldbetweenthetwoplatesincreases.AspresentedinthedetailofFigure
7(b), the charge movement within the dielectric is limited and no charge is
transferredfromthedielectrictotheconductiveplates.Mobilechargesanddipoles
in the insulator respond in opposition to this electric field and reduce the net
electric field between the two conductive plates when compared to an identical
parallelplateconfigurationwithoutthedielectric.Theelectricfield,E,betweenthe
parallelconductiveplatesisgivenby,
 
14
,
Equation2
where
w
therelative permittivittyofthedieelectricbetw
weentheco
onductivep
plates
rist
and0isthepermittivity
yoffreespaace.
Figure7.(a)Parallelplaatecapacito
orwithfreeespaceperm
mittivity.(b)Parallel plate
caapacitorwitthdielectriccmaterial.
Figure8((a)providessarepresen
ntationofth
hedielectriicspolarizattioninresp
ponse
to
oanexternaalelectricffieldapplied
dacrosstheeconductiveeplates.On
ncetheexteernal
electric field
d from thee conductiv
ve plates iis removed
d from thee dielectric,, the
polarization of the ideaal dielectricc returns too a state off zero polarrity becausee the
orcesareno
olongeractiingonthessemi‐mobileechargesw
within
exxternalelecctricfieldfo
th
hedielectricc.Figure8(b)provideesarepreseentationoftthedielectrric’spolarizaation
reesponsewh
henthereisnoexternaalelectricfiieldapplied
d;thisisnottthecasefo
oran
electret.
15
Figure 8. A dielectric insulator is
i sandwich
hed betweeen two meetal layers. (a)
Opposing
O
ch
harges on conductorss generate electric fiields (arrow
ws: positivve to
negative chaarges) and produce
p
an
n opposite p
polarization
n by chargee migration.. (b)
The
T
removaal of an exxternal elecctric field oon the diellectric redu
uces the ch
harge
polarizationinthedieleectrictozero
o.
At first glance,
g
the response to
o an applieed external electric fieeld in the lower
detailofFig
d
gure9(a)lo
ookstypical foradielecctricinapaarallelplateecapacitor. The
upper
u
detaiil to Figuree 9(a) show
ws a new cooncept; that surface or space chaarges
developed
d
in
i the insullator are sim
milar in pollarity to thee conductorrs. This typ
pe of
space
s
charg
ge usually develops as
a charges “leak” from
m the cond
ductor into
o the
insulator.T
i
Thechargelleakagemay
yalsooccurrinatypicaaldielectric,butthepollarity
ofthespace
o
echargeis significant whenlookiingatFigurre9(b).In Figure9(b)),the
removaloft
r
theconducttorsisequiv
valenttoth
heremovalo
oftheexterrnalelectricfield
on
o the insu
ulator. In this case, the
t insulatoor is left w
with a resid
dual chargee and
polarization
p
n; this ferro
oelectric ch
haracteristicc has transfformed the insulator tto an
electret.
e
16
Figure 9. A ferroelectrric insulator is sandwiiched betw
ween two m
metal layers.. (a)
Opposing
O
ch
harges on conductorss generate electric fiields (arrow
ws: positivve to
negative chaarges) and produce a polarizatioon by chargge migratio
on; upper d
detail
leeaking charrge, lower detail oppo
osing chargge. (b) Th
he removall of an exteernal
electricfieldontheinsu
ulatordoesn
notnegatetthechargep
polarization
n.
2.5
2 ElectrricDisplacementFieeld
A review
w of electricc displacement, D, is n
needed to understand the relation
nship
between chaarges in a dielectric and the foorces that the chargees can pro
oduce
exxternally. Basic dieelectric maaterials haave a lineaar electricc field ind
duced
polarization,, P, as a fu
unction of the electriic field, E. The polarization caan be
caalculatedassafunction
nofpermitttivityoffreeespace,; relativepermittivityo
ofthe
material,
m
andsuscepttibility,X,assshownin
r;a
1
17
Equation
n3
andresultsinanelectricdisplacement,Ddielectric,forthebasicdielectric
Equation4
.
Theelectricdisplacementprovidesacompletedescriptionoftheresultingforcedue
to the electric field and polarization [40]. It is not required that the relative
permittivityofthematerial,r,beconstant.Therelativepermittivityofparaelectric
materials typically decreases as the electric field increases, as presented earlier in
Figure 3. The displacement field becomes more interesting with ferroelectric
materialswhereresidualpolarizationexistswithoutanyelectricfield.Inthiscase,
thepolarizationofaferroelectricmaterialcanbetreatedasaconstantandforthe
restofthisdiscussionwillbeconsideredasacomponentofelectrets,referredtoas
Pelectret.Electretsmayalsohavespacechargesandsurfacecharges.Indevicedesign
and many practical applications these real charges are lumped together as an
effectivesurfacecharge,σelectret.Thesummationoftheelectricfieldinducedelectric
displacement (Equation 4), the permanent polarization, and the effective surface
chargerepresentthecompleteelectricdisplacementfield
.
Equation5
The general model presented in Figure 10 is useful in understanding how the
charge of a thin film electret affects its surroundings. The effects of a moving
conductiveplate,effectivesurfacechargemeasurements,andtheelectret’seffecton
18
seemiconducttor interfaces will all be
b addresseed with thiss fundamen
ntal model. The
basic outlinee follows from
f
Chaptter 2 in Seessler’s “Eleectrets: Topics in Applied
Physics” [6]. The mod
del assumees ideal infiinite planess and the analysis caan be
siimplifiedto
oa1‐Dmod
del.Thelayersconsist ofabilayerrdielectric materialbaacked
with
w
a metaal electrodee. The dieelectric layeer is modeeled as an electret an
nd is
seeparatedfrom the upp
per electrod
de by an airrgap. The voltage,Vo,, across theefree
andbacking electrodep
providefor appliedpottentialorm
measuredpo
otential;currrent
mayalsobe
m
measuredb
betweeneleectrodesasttheconfigurrationchangesstatesb
based
ontheappliccationofboundaryconditions.
A simple mo
odel consisting of a con
nductive baacked multilayerelectrret of
Figure 10.A
deling effecctive surfacce voltage and
siilicon oxide and silicon nitridee for mod
microphone
m
operationalprinciples.
Kirchhofff’ssecondlaaw,withreggardtocharrge,providees
19



Equation6
Thedisplacementfieldsoftheoxidelayerandairlayeraredefinedbythesurface
chargedensityoftheirrespectivemetalelectrodes
Equation7
.
Equation8
Any change in the displacement field intensity is modeled to be due to a fixed
surfacechargedensitybetweenlayers
Equation9
.
Equation10
In the “non‐electret” case, there are no fixed charges in the dielectrics or
interfaces,
0. The displacement field in each region of film is
equal,
, regardless of any applied voltage, Vo. With no
charges located at the oxide‐nitride interface and/or the nitride‐air interface,
0. The charge on metal1 and metal2 are oppositely
chargedandequalinmagnitude,
0.
20
Withdev
vicesthatmakeuseofaachangein airgap,tair,,differentiaationofEquaation
6isuseddetterminehow
wtheeffecttivecharge densityon thesimple metalelecttrode
effectedbyaachangeinttheairgap,tair.
Equatio
on11





reesultsinattransduction
nofairgapthickness,ttair,tovoltagge,V0.The attractivefforce,
F,perunitarrea,A,oreleectrostaticp
pressureP,betweenth
heplatesis

Equatio
on12
.
2.6
2 EffectiveSurfacceVoltageElectretM
Measurem
ment
The effective surface voltage (ESV) of the electreet film is m
measured u
using
noncontactm
methods.Th
hemostcom
mmonlyuseedmeasurem
mentsystem
misnullseeeking
feeedbackvib
bratingcapaacitorfieldm
meter,Figurre11[41][4
42].Them
meterdetecttsthe
effectivesurfacevoltageebyelevatingthepoteentialoftheeexternalcaaseofthep
probe
until it canccels any exxternal field
d detected by the inteernal reson
nating capaacitor
which
w
is exp
posed to an
n external field
f
througgh a small aaperture. E
Effective surface
voltage (ESV
V) measurements can
n be mod elled with a new seet of boun
ndary
co
onditions fo
or the samee model preesented in Figure 10. In the case that theree are
trrapped charges at the oxide‐nitriide interfacce,
21
0
,aand
0.
Heremetal2actsasthefreeelectrode(electrostaticmeterprobe)anditspotentialis
adjusted, using feedback control circuitry, to a point that
whichmeansthat
0and
0 and
0,
.Equation6simplifiesto

Equation13
wherethevoltage,inFigure11,istheeffectivesurfacevoltageoftheelectretfilmas
measuredbytheelectrostaticvoltmeter.Asanexample,withapermittivityof3.45
(10)‐11C/(V*M)andasiliconoxidethicknessof1650nm,aninterfacechargeof4.71
mC/m2 at the blocking oxide‐silicon nitride interface will produce an effective
surfacevoltageof225V.
Isothermalpotentialdecay(ITPD)dataiscollectedbytakingESVmeasurements
as a function of time at a specified temperature [2]. The exponential decay of the
ESVatdifferentelevatedtemperaturesallowsforextrapolationoftheESVatroom
temperature (or the designed operating temperature) so that the system making
useoftheelectretfilmcandetermineitsusefullifetime.Itisimportanttorecognize
thatmultipleslopesmayemergeontheArrheniusplotofthelifetimecoefficientasa
functionoftemperature.Thiscanbeduetoincreasesofthermalenergyrequiredto
initiate charge migration in electron/hole trapping mechanisms, dipole relaxation
times,andionmigration[43].Protectivecoatingspromotinghydrophobicbehavior
oftheelectretmaydeteriorateatelevatedtemperaturesandpresentitinITPDdata
above,thedecompositiontemperatureoftheprotectivecoating.
22
Figure 11. Diagram
D
of
f null seekin
ng feedbackk vibrating capacitor ffield meter with
electretandsubstrate.
The measurement of
o the electrrets effectivve surface vvoltage mayy require sp
pecial
co
onsideration when thee electret is backed b
by a semico
onductor. T
The bulk of the
seemiconducttorisgroun
ndedbuttheereisband bendingin
nthesemico
onductortotake
in
ntoconsideeration.Vieewingtheeeffectivesurrfacevoltaggeasthegatepotentiallofa
MIScapacito
M
or,thetotal voltagemeeasuredasttheeffectiveesurfacevo
oltage
potential acrross the eleectret
,iisthe
, and tthe potentiaal due to b
band bendin
ng in
siiliconis .,showninF
Figure12an
ndgivenby
 .
23
Equation
n14
Figure 12. Insulator‐ssilicon band
d bending due to electret bias modelled as an
effectivesurffacecharge.
24
2.7
2 ElectrretFormattion
Thermalllyassistedp
poling,coronachargingg,andelectrronbeamim
mplantaretthree
frrequently used
u
to produce electrret materialls. Each m
method has its benefitss and
limitations. Thebestch
hoicedepen
ndsonman
nyfactorsin
ncludingthematerialttobe
ch
harged,theapplication
n,andcost.
2.7.1
2
TherrmallyAssistedPolin
ng
Thermalllyassistedp
polingisth
he originalm
method useed in generaating an eleectret
fiilm[6].Theermallyassistedpolinggtypicallyttakesplace throughouttthebulko
ofthe
materialast
m
thesampleiisheatedneearorpastaacriticaltraansitiontem
mperatureo
ofthe
material
m
wh
hile and exxternal potential is applied acro
oss the sample. Oriiginal
exxperimentswerewith blendsofccarnaubawaaxandcolo
ophony(treeesap)that were
brought dow
wn from meelting temp
peratures too solidify under the ap
pplication o
of an
exxternal eleectric field. Likewisse, thermaally assisted activatio
on of PTFE at
teemperatureesnearitsgllasstransitiiontemperaaturewasfo
oundtoimp
provethequ
uality
of electret fo
ormation bo
oth in charge density and increase its lifetim
me temperaature
endurance[6
6].Ferroeleectricdipoleelectretsrrelyonpolaarizationneearorover their
Curietemperature.Electretsform
medfrommu
ultilayerdieelectricssucchasSiO2/Si3N4
reely on therrmal assistaance to incrrease the Poole Frenk
kel conductiion and theermal
ch
hargeinjecttionrequireedtocreatedeeptrapp
pinglayersaandfillthem
mwithchargges.
25
2.7.2 CoronaCharging
Coronachargingisinexpensivetoimplementandiseasytoimplementforlarge
surfaceareas.Withoutheatingthechargematerial,itschargingmethodislimitedto
surface charging and ferroelectric polarization [22]. Heating the electret during
corona charging can assist the electret activation process; thermally assisted
activation with the corona field applying the potential to the electret surface
provides additional opportunities with volume charging and a broader range of
dipolechargingeffects[22].
Corona charging is a frequently used method that makes use of ionized gases;
the process can runat atmospheric pressures. In the case of corona charging, the
ionsproducedfromionizedgas,whichisoftenroomair,arepropelledtothesurface
oftheelectretactivatedmaterial.Thereareanumberofreactionscausedbycorona
chargingthatcancompletetheelectretactivation.Ionscanreactwiththesurfaceof
thefilmandcompleteacovalentbondwhichresultsinanetchargeatthesurface.
The ions can also accumulate at the surface without bonding and act as a flexible
charge distribution that applies a potential through the dielectric film [2]. During
heating this ion induced field applies the potential required for thermally assisted
poling.Asathirdprocess,theionsmaynotbondwiththesurfacebutmaydiffuse
into the surface of the electret film with a resulting net ionization charge
distributionjustbelowthesurface.CoronaactivationofPTFEistransformedfroma
26
su
urface charrge effect to
t space ch
harge activaation with thermally assisted poling
teemperatureesapproachingtheglassstransition
ntemperatu
ureofPTFE[6].
Continuo
ous rolled polymer
p
sheeets and xeerography ssystems typ
pically use wire
dischargesy
ystemstoprromoteevencharging alongthew
widthoftheesheet.Co
orona
ch
hargingfor siliconbassedMEMSd
devicesistrraditionally doneatwaaferlevelw
witha
point dischaarge and griid, as show
wn in Figuree 13, althou
ugh there haave been reecent
developmenttswithinsiitudeviceleevelchargin
ngusingbuiltinmicroeelectrodes[4
44].
Figure13
3.Coronach
hargingsch
hematicwith
hheatassisttance[44].
Electron beam charging of th
he dielectriic typically requires p
processing in a
vacuum. Sources are available
a
wh
hich can prrovide largee surface area coveragge as
well
w as locallized patterrning. Electtron beam ccharging prrovides volu
ume chargin
ng to
th
he electret and can also provide surface an
nd polarizattion chargin
ng with theermal
assistance. Electron beam
b
chargging can prroduce the obvious neegative chaarged
27
reegions in the
t
insulato
or. Electro
on beam ccharging caan also pro
oduce posittively
ch
harged regiions due to
o the second
dary emissiion and scaattering of eelectrons w
within
th
he substrate; secondarry electronss within thee substrate can be kno
ocked out o
of the
su
ubstrate leaaving a nett positive region
r
[45]]. Thermallly assisted
d electron b
beam
ch
harging can
n take advantage of the newly creeated trapss generated by the elecctron
damage[6,2
22].
2.7.3
2
ElecttronBeam
mImplant
Back‐ligh
ht Thyratro
on (BLT) so
ources makke use of tthe Townseend avalancching
io
onizationoffanoblegaas,suchash
hydrogen,u
undervacuu
um.Thefreeelectronsare
propelledby
ythehighellectricfieldtoimplant intotheeleectretsurfacce[46].Elecctron
im
mplanting can
c produce net posittive or negaative electrret films deepending on
n the
kinetic energ
gy of the ellectrons. Low energy electrons w
will embed themselvess just
belowthesu
urfaceoftheefilm.Asttheimplanttenergyisiincreasetheemeandep
pthof
th
heimplantiincreased.W
Withhigheenergy,theeelectronscaanactuallyccausesecon
ndary
electronstobescattered
doutoftheefilm,resulttinginapossitivecharggebecauseo
ofthe
ossofnegattivecharges[45].
lo
2.8
2 Charg
geTrapsin
nSiO2andSi3N4
Siliconoxxideand/orrsiliconnitridehaveseveraladvaantagesaseelectretmatterial
caandidates. Theseinorgganicelectrretsarenottedforwith
hstandinghiightemperaature
environmenttswhilemaaintainingth
heirchargee[7].Inorgganicelectreetscomposedof
28
silicon nitride or silicon oxide can have a thermally stimulated discharge
temperatureashighas500°CwheretraditionalfluorinatedelectretslikePTFEhave
alowerthermallystimulateddischargetemperatureintherangeof200°Cto250°C
[22].Siliconnitrideandsiliconoxidearealsocompatiblewithmicroelectronicand
microfabrication processing. This is particularly advantageous given the
microfabrication capability of getting this fixed charge to interact in a controlled
manneroversuchsmalldimensions.Effectivesurfacevoltagesof‐300Vhavebeen
reportedforSiO2/Si3N4electrets[26].
Charge trapping is an important part of nonvolatile memory (NVM). A
multilayer film of SiO2/Si3N4/SiO2 on a silicon wafer is used to construct the
nonvolatile gate used in silicon‐oxide‐nitride‐oxide‐silicon (SONOS) memory [5,
47].ThemultilayerpropertiesofaSiO2/Si3N4filmonsiliconareconsideredaswide
bandgap materials. Multilayer dielectrics provide additional charge trapping
opportunitieswiththecollectionofinterfacetrappedcharges,andsettingbarriers
for fixed trap charges and mobile ion charges. The charge trapping effects have
beenshowtosurvive15hrsofelevatedtemperatureat150°C[47].Themultilayer
film traps charges in the silicon nitride which hold the state of the gate memory
[48].Thehighhydrogencontentwithinthedepositedsiliconnitridefilmprovides
for Si‐H, dangling bonds and Si‐Si states that act as amphoteric traps for trapping
charges[49].ElevatedtemperaturestudiesonSONOSmemorydeviceshaveshown
that elevated temperatures of 175°C are detrimental to excess electron charge
29
sttatesdueto
othermalellectronemiissionbutth
heelevated
dtemperatu
urehasnoeeffect
onholechargedecayrate[50].
2.9
2 StudieesofSiO2/Si
/ 3N4MulltilayerEleectretActiivation
Research
h has been done on ellectret form
mation of aa PECVD SiO
O2/Si3N4 do
ouble
laayer by corona chargin
ng [2]. Tem
mperature h
has played a role in th
he study bu
ut the
teemperaturee range hass been limiited in ran
nge from ro
oom tempeerature to 8
80°C.
Amajadieta
A
l.[51]noted
dthatsingleelayersofccoronacharrgedoxideo
ornitridedid
dnot
maintain
m
thee electret ch
harge for a period ofm
more thanaa few days without coating
withHMDS.
w
Siliconnitrridecoronaachargedan
ndHMDSco
oatedwasaalsoshownttobe
very suscepttible to higgh humidity
y environments. Amjaadi and Sesssler 1997 [52],
sttudied a single layer of atmosph
heric presssure chemiccal vapor d
deposition Si3N4
ch
hargedina coronafielldatroomttemperatureeandshow
wedthatthepeakcurreentof
th
heopencirccuitthermalllystimulatedcurrentooccurredattabout410°°C,Figure14.
Figure14
4.APCVDSii3N4openciircuittherm
mallystimulaateddecay[52].
30
Leonovprovidesdetailsonaneffectiveprocedureforpreandpostannealingin
conjunction with corona charging of the electret. The results show that
pretreatment of the LPCVD generated silicon nitride with thermal oxide layers
benefit from a dehydration bake when it comes to the decay life of the electret
charge. The results also show that a post processing anneal of around 250°C
initially causes arapidfield loss; it has also be documented by Leonov that a heat
pretreatment at 450°C produced the most durably held field and with only 1‐2%
chargelossascomparedtoatypical20%lossbyprocesswithlowertemperature
pretreatments of the film [28]. The range of temperature ranges used in pre and
post treatments show that multiple charge trapping mechanisms are involved.
Surface treatment modifications to silicon nitrides and silicon oxides have shown
drastic differences in the lifetimes of the electret charge. The lateral surface
conduction on a silicon oxide surface, which normally makes it a poor electret
candidate,iseasilymodifiedwithacoatingofHMDStominimizesurfaceconduction
resultinginanelectretwithalifetimeconstantofover400years[7].Thiscoating
processisperformedtoremovewatervaporandpreventreabsorptionofthewater
intoandonthesurfaceofthefilmsinceithasbeenwelldocumentedthatwaterand
humidity cause neutralization of the effective surface voltage as polarized water
molecules adsorb to the electret and mask the charge [11]. Upon deposition, the
HMDS reacts to form a trimethylsilyl (TMS) monolayer on the multilayer PECVD
electret[7,53].TheTMSmonolayermakesthemultilayersurfacehydrophobicand
prevents the deleterious effects of humidity from neutralizing the electret.
31
Voorthuyzen
V
n et al. ussed a uniq
que form of thermaally stimulaated decayy for
determining the therm
mal energy required tto overcom
me the trap
pped chargees in
co
orona charged silicon oxide elecctret [54]. Here the ssamples weere subjecteed to
elevated tem
mperatures over 20 miinute intervvals, cooled
d, evaluated
d with a Mo
onroe
electrostatic voltmeter,, and then again heateed to a higgher tempeerature then
n the
previous20minuteinteerval,Figuree15.Anottablethermaldecaystaartstoappeearat
teemperatureesover275°°C.
Figure
F
15. Normalizeed effective surface vooltage TSD of corona charged siilicon
oxideelectreet[54].
2.10
2 Charg
geTrappin
nginSiO2,Si3N4,and
dMultilayeerElectretts
The cond
ductivity of silicon nitride play s an important part in the eleectret
properties of
o an oxide‐nitride‐oxid
de (ONO) m
multilayer. Conductivity of the siilicon
nitridecanb
beincreased
dbyformin
ngasilicon richnitridee(SRN)and
dallowsalaarger
portion of the trapped
d charge to migrate too the oxidee/nitride in
nterfaces du
uring
32
poling[55],Figure16.LPCVDhasbeenusedasamethodforproducingSRNfilms.
LPCVDsiliconnitridefilmsaregeneratedinatemperaturerangeof700‐900°Cfrom
dichlorosilane and ammonia gas, although the excess ammonia gas concentration
makesitrelativelyeasytoproducestoichiometricSi3N4.Hydrogenisacomponent
to LPCVD and PECVD nitrides and oxides due to the use of silane and ammonia
gases. LPCVD silicon nitride typically contains a lower hydrogen content verses
PECVDsiliconnitride,8%vs25%respectively.LPCVDsiliconnitridecanbeagood
barrier to hydrogen diffusion and has resistivity on the order of 1016 ohm*cm. In
contrast,thePECVDsiliconnitridehasahighhydrogencontentandtheresistivityis
on the order of 105 to 1021 ohm*cm [56]. The hydrogen content is considered a
liability to producing durable insulators with high breakdown voltages but the
silicon‐hydrogenandnitrogen‐hydrogenbondsproducedefectswhichcanbecome
activechargetrapswhenthehydrogenisreleasedduringheatingandunderlarge
electricfields.Theelectrontrapdensitywithinthebulkofsiliconnitrideincreases
as the temperature decreases from 150°C to room temperature. As this total trap
densityincreaseswithloweringtemperature,theinterfaceelectrontrappingatthe
Si3N4/SiO2interfacedecreases[55].Ultrathinfilmsandfilmsover10nmtakeinto
considerationTrapAssistedTunneling(TAT)[57,58].
Frenkel Poole (FP) conduction is considered the primary source of charge
transfer with fields applied across SiO2 and Si3N4 films over 10 nm [5, 59]. The
Frenkel Poole effect occurs under high electric fields, starting at 2‐5 MV/cm for
siliconnitride,andisduetoelectronsorholesmigratingfromoneshallowtrapsite
33
to
o the next. This “hop
pping” proccess is a fu
unction of ttemperaturee as the ch
harge
teemporarily gainsenoughthermal energytob
breakfreeo
ofthetrap site.Ultraa‐thin
fiilms,lessth
han10nm, andmultilaayerdielecttricstacksw
withultrath
hinfilmssucchas
MNOS
M
and SONOS
S
devices rely on a combinaation of FP conduction with tunneeling,
usually Fow
wler Nordheeim (FN), Fiigure 16. T
The chargees injected through Fo
owler
Nordheim
N
tu
unneling migrate
m
by Frenkel Pooole conducction and b
build up att the
opposingsiliiconnitride‐siliconoxiideinterfacee.
S
band diagram with
w
Poole Frenkel co
onduction in
n silicon nittride.
Figure 16. SONOS
Source:[55]
Thesilico
ondanglinggbondsare amphotericcduetobeiingrelativelywellcenttered
in
nthebandg
gapofthessiliconnitrideregardleessofthesttoichiometrryofnitrogeento
siilicon, Figu
ure 17. Th
he numberr of silicon
n dangling bonds dom
minate nitrrogen
34
dangling bon
nds with an
nd withoutt post depoosition heaat treatmen
nts of 600°C
C but
siilicon danglling bonds are reduced
d by the heeat treatmeent while niitrogen dan
ngling
bonds remaain unchangged [60]. Nitrogen d
dangling bo
onds only become a trap
co
onsiderationastherattioofnitroggentosilicon
nisincreasedintheCV
VDprocess.The
presence of the nitride dangling bonds
b
are seeen near th
he valence b
band, insidee the
siiliconnitrid
debandgap
p,astheban
ndgapwideenswithan increasein
nthenitrogento
siiliconratio,Figure17.
Figure17.N
Nitrogento siliconratio
oeffectonssiliconnitriidebandgaapisshown with
siilicondangllingbondan
ndnitrogendanglingboondlevels[[60].
Hydrogen
n diffusion and the ro
ole of hydroogen in bon
nding playss a factor in
n the
electret charrge created at the SiO2/Si3N4 inteerface [61]. Amphoterric traps, du
ue to
dangling bon
nds, are ressponsible fo
or the chargge trappingg in silicon nitride [50
0, 62‐
64].Hydrog
genhasbeen
nshownto fillthetrap
psofthedaanglingbond
dsofsilicon
nand
nitrogeninthesiliconnitridelayer.Hightemp
peratureineertannealin
ngcanfreetthose
35
hydrogen atoms and increase the number of dangling bonds while annealing in
hydrogencandecreasethedanglingbonds[4,5].
ChargetrapscanbegeneratedinSiO2underthermallyassistedelectricfieldsof
1.5MV/cm[65,66].Thehotcarrierelectronsinjectedintotheoxidearecapableof
releasinghydrogenfromdefectlocationsneartheanodesurfacewhentheelectron
energy is 2eV or greater. These positive charged hydrogen ions can migrate to
cathode region of the oxide creating interface states near the cathode for electron
traps,Figure18.Thisprocessisconsideredindependentofoxidethicknessforfilms
over10nm.Thisprocessagoodcandidateforeachofthedielectriclayersinour
PECVDmultilayerfilm.TheeffectofhydrogenontheperformanceofNVMiswell
documentedandparticularlyaconcerninhydrogenrichPECVDfilms[49,61,67]
Higher energy hot carrier electrons can produce inelastic scattering capable of
producing secondary electron‐hole pairs available to fill deep trap locations [68].
Annealing samples in the temperature range of 200°C to 300°C releases the
hydrogenfromSiandNbondsinsiliconnitrideleavingthemassusceptibletrapping
locations[5].
Impactionizationissignificantwhenfieldstrengthsaregreaterthan7MV/cmin
SiO2.Atthesehighelectricfields,theelectronenergydistributionbroadensandthe
energetictailofhot‐electronenergyissignificantenoughtocrossthesilicon‐oxide
Schottky barrier height. Using deep‐level transient spectroscopy (DLTS), the trap
levelshavebeenmeasuredforONOlayers[69].ONOhasholetrapsthatare0.5‐
36
0.64eVabov
vethevalan
ncebandofsiliconand haselectro
ontrapsinttworanges 0.25‐
0.45eVand0
0.47–0.63eVbelowth
heconductioonbandofssilicon.
Figure18.H
Hotcarriereelectroninjjectionintoosilicondio
oxideandhydrogenrelease
[6
68].
2.11
2 AnalysisTechniiquesofEllectrets
Theappaarentchargeesofanelectretcanbeeintheform
mofsurfacecharges,sspace
ch
harges,and
dorcharged
dipoles.Mu
ultiplecharggingprincip
plescanbeiinvolvedandthe
lifetimeratessofeachtyp
peofchargiingsourceccanbedrastticallydifferrentinthessame
material.As
m
anexamplee,chargesin
njectedfrom
mdirectcon
ntactsourceesmaylast fora
feew days, wh
hile the polarization and
a other h
heterochargge sources w
will last mo
onths
and years [6].
[
A ran
nge of temp
perature baased lifetim
me measureements, surface
voltages, open or closeed circuit current meaasurements, and frequ
uency respo
onses
provideseveeralopportu
unitiesforevaluationo ftheelectreet’soperatio
on.
37
A number of complementary measurement techniques make it possible to
understand the inner workings of the electret’s charge profile and properties. A
review of additional measurement techniques can be reviewed in [6, 38]. The
electricfieldoriginatingformanelectretismeasuredbycontrollingthepositionofa
groundingplanewithavibratingcapacitorfieldmetertoaneffectivesurfacevoltage
(ESV) to the electret. Measurements at elevated temperatures can be used to
provide isothermal potential decay measurements. These two methods will be
discussedingreaterdetailinthefollowingsection.
Thermallystimulateddischargecurrent(TSDC)orthermallystimulatedcurrent
(TSC)ismeasuredfromtheelectretaschargesand/ordipolesrelaxanddischarged
asafunctionoftemperature.Chargesreleasedasafunctiontemperatureprovide
an estimate for the energy level of the trapped charge. Thermally stimulated
discharge (TSD) enables the identification of the relaxation mechanisms of the
electretasafunctiontemperature[70].Twocommonconfigurationsaretheair‐gap
TSD and the direct contact TSD. In the air‐gap TSD configuration the electret is
betweentwoelectrodesbutoneelectrodeisnotindirectcontact,limitingtransfer
ofchargetoorfromtheelectret[38].ThedirectcontactTSDconfigurationis,justas
describedinitsname,withtheelectretsandwichedbetweenelectrodes.Theuseof
bothmethodsonamaterialhelpstoprovideacompletepicture ofthechargeand
polarization responsible for the electret characteristics. Monitoring the current of
the air‐gap configured TSD, due to the discharge of the electret as it is heating,
provides a temperature depended profile of the dipole relaxation temperatures.
38
MonitoringthecurrentofthedirectcontactTSDconfigurationwhiletheelectretis
heating,allowsforbothnetchargeanddipolerelaxationcurrentstobemonitored
simultaneously.Usingbothpermitsseparationofthecurrentdataduetoeachthe
chargemigrationandthedipolerelaxation.
Deep‐leveltransientspectroscopy(DLTS)ortransientcapacitancespectroscopy
(TCS) monitors the variation in capacitance as a gradual change in temperature is
made [38]. There are many variations to this measurement technique, in
semiconductorjunctionsthespacechargeregionispulsedwithaforwardbiasedso
thattraplocationscanberefilled.Thecapacitanceismonitoredovertimetolookat
theemptyingofthesetrapsatvarioustemperatures.
The pulsed electro‐acoustic (PEA) method uses two direct contact electrodes
andanappliedelectricalpulsetoapplyaforcetothetrappedchargesoftheelectret.
This induces a displacement in the electret which propagates to a piezoelectric
crystal mounted on one side of the electret. The piezoelectric crystal in turn
producesanelectricpotentialwhichcanbemonitored.Thetimedelaybetweenthe
input signal and output of the piezo crystal provides information on the physical
depth of the charges within the electret and the intensity of the output relative to
theinputprovidesinformationontheeffectivechargedensity[38].
Itispossibletotellwhatpartoftheelectretselectricfieldisduetopolarization
by looking at the difference in the films permittivity before and after electret
activation. It depends on the magnitude of the field through the material and the
39
frrequency of the electtric field. Polarization
n of a ferrroelectric m
material caan be
measuredw
m
withaSawyeer‐Towerciircuittoprooducethep
polarizationvselectric field
profile.
A Kelvin probe is used
u
in a manner
m
sim
milar to the electrostattic voltmeter to
measuresur
m
rfacephotov
voltage(SPV
V).Thisnoncontactm
methodlookssattheeffeective
su
urfacevoltaageofasem
miconductorrsuchassiliicontoevalluatethech
hargebuildu
upin
th
he space ch
harge region
n (SCR) witth the samp
ple cycling between ligght and no light
exxposure. From
F
these measuremeents, the miinority carrrier diffusio
on length caan be
determined in a nonco
ontact meth
hod. This m
method is a standard practice in
n the
CMOSindusttryformeassuringdefecctdensitiesinthe109‐1
1011cm‐3raange[39].
2.12
2 ConceeptsinMod
delingTem
mperatureeDependeentExponentialDeccay
Theobjecctiveofthisssectionisttointroduceetheconcep
ptsofmodeelingexponeential
decay and then
t
implem
ment those concepts iin defining the characcteristics of the
effective surrface voltaage decay for the PE
ECVD oxidee/nitride/oxide multilayer
he temperaature depeendent decaay of the electret is used to; first,
electret. Th
r
of thee electret aat room teemperature and
exxtrapolate the expectted decay rate
co
ommonopeeratingtemp
peratures;aandsecond,,investigateetheenergyylevelofch
harge
trrappinginthemateriall.
The effecctive surfacce voltage is a result of trapped
d charges aand polarizaation
within
w
the dielectric
d
fillm stack fillm and siliccon substraate as presented earlier in
40
section2.10‐”ChargeTrappinginSiO2,Si3N4, andMultilayerElectrets”.Diffusion
basedmigrationofchargewoulddominateifnoactivationenergyisrequired.The
activationenergyrequiredtoreleasechargetrapsisprovidedthermally.Thetrap
sites are due to bond defects and band interfaces at junctions between dielectric
layersandthesiliconwafer.Thechargesinthesefilledtraplocationshaveagreater
probability,athighertemperatures,ofhavingsufficientenergytomovetoahigher
energystatewheretheycaneitherdriftordiffuse.Externallyappliedelectricfields
andinternalelectricfieldsprovidetheelectricfieldfordrift.
Firstorderexponentialdecayisacommonoccurrenceinnature.Itstemsfroma
quantity,suchasaconcentration,changingwithtimeinconstantproportiontothat
quantity.Theequation
Equation15
representsthisbehaviorwhereN(t)istheconcentrationatanygivenpointoftime,
t, where λ is the exponential decay constant. Given the initial condition of a
concentrationNoattimet=0,theexponentiallydecayingquantityisdeterminedby
integrationofEquation15,resultingin
.
41
Equation16
where No is the initial concentration. The mean lifetime is the average period of
timefortheinitialconcentrationtoreact.Itcanbecalculatedfromtheexponential
decayformula,Equation16.Themeanlifetime(τ)isdeterminedbyintegrationof
thetimeweighteddecay
∾
⟨ ⟩
.
∾
Equation17
Theresultisthatthemeanlifetimeistheinverseoftheexponentialdecayquantity
τ=1/λ.Attime,t=τ,theconcentrationis
Equation18
.
Mean lifetime is easy to extract from curve fits and but it is usually difficult to
visualize e‐1 of the initial concentration. For this reason, “half‐life” is often
presented and provides a reference for the amount of time required for the
concentrationtodroptohalfoftheinitialvalue.MinormanipulationofEquation16
resultsin
⁄
.
Equation19
Half‐lifeismucheasiertovisualizethanexponentialdecayrateormeanlifetime.
Inananalogy,half‐lifeisthetimerequiredforafullcuptobecomehalf‐full.With
exponential decay, the time required for a full cup to become half full is the same
42
amountoftimeittakessthecupto gofromhaalffulltoon
nequarterffull,onequarter
fu
ull to one eighth,
e
and so on. Figgure 19 proovides a vissual summaary of a gen
neral
exxponentialdecaycurveeandassociiatedterms .
Figure19.“IIdeal”ploto
ofanexponentialdecayyplotwith representattionsofhalf‐life,
meanlifetim
m
meandrespeectiveconceentrations.
Exponenttial decay is often deependent oon temperaature as firrst modeled by
Arrhenius
A
in
n 1889. Reactions
R
arre acceleraated with tthermally p
provided kiinetic
energy and are modelled by the mean lifeetime coeffiicient assocciated with
h the
exxponentialdecay.Thereareanum
mberofreaactionmodeelsthatarevvariationso
ofthe
basicArrhen
niusequatio
on.Thesem
modelstakeeintoconsiiderationad
dditionalfactors
su
uchaspressureorsterricreaction
ns;thesemoodelscanbeedrawnon
nifexperimental
data shows that somee of these additional effects mayy be relevaant. The b
basic
Arrhenius
A
equation
e
provides
p
a
a general temperatu
ure depend
dent modeel of
exxponential decay consstant that may
m be app lied to Equ
uation 15 an
nd Equation
n 16.
43
The
T
exponeential decaay constantt of Equattion 16 ch
hanges as a function of
teemperaturee,T,
Equatio
on20
.
where
w
A is a pre‐expo
onential con
nstant, k is Boltzmann
n’s constant, and Ea iss the
activation energy
e
of the
t
reactio
on taking p
place whicch is respo
onsible forr the
co
oncentratio
on as a funcction of tim
me, N(t). Th
he exponenttial decay ffunction is o
often
plottedonasemi‐logsccaleasafun
nctionofinvversetempeerature.Th
hefunctionttakes
alinearappeearanceontthesemi‐loggscale,Figu
ure20.
Figure 20. “IIdeal” plot of an Arrheenius functtion on a seemi‐log plott of exponeential
decayvsinveersetemperrature.
44
2.13
2 Silicon
nEtching
Micromachining of the silicon
n substrate is a criticcal part of the fabricaation
processforb
bothmicroeelectromech
hanicalsysteems(MEMSS)andthein
ntegratedciircuit
(IIC).InMEM
MS,theproccessofmach
hiningintotthesiliconssubstratehaasbeenuseedfor
caarvingoutm
mechanicaldevicespro
oducingshaarptips,cantilevers,andthindefleecting
membranes
m
for atomic force micrroscopes, p
pressure sen
nsors, accelerometers,, and
more.
m
In th
he IC industtry, silicon etching hass been used
d to define MOS geom
metry,
limit diffusio
ons, and prrovide conttact surfacees to specific silicon crystallograaphic
planes[71,72
2].
Etching processes
p
can
c have a wide
w
rangee of effects, on silicon from produ
ucing
crrystallograp
phic flat su
urfaces seleective to th
he crystal o
orientation of the exp
posed
siilicon, such
h as chemiccal mechaniical polishin
ng, to the o
other end o
of the spectrum
producingpo
oroussiliconwithholeediameters varyingfro
om2nmtosseveralmicrons.
The
T
develop
pment of porous
p
siliccon stems from utiliizing the m
minority caarrier
diffusionlen
ngthinthessiliconand theoxidatioonreductio
onreaction producedaatthe
siiliconetchantinterfacee[73].
Etching of silicon can be accomplish
hed with acid solu
utions such
h as
hydrofluoricc/nitric/acettic acid so
olutions or
r base solu
utions such
h as hydraazine,
etthylenediam
mine pyroccatechol (ED
DP), tetram
methylammo
onium hydroxide (TM
MAH),
potassiumhydroxide(K
KOH),ando
otheralkali basesolutiions.Gasb
basedetchiingis
withsiliconsublimation
nviaxenon
ndifluorideorplasmab
based
alsoapopulaarmethodw
45
etching with SF6 and other associated gases. There is plenty of fantastic in depth
literature.Therearemanygreatpublicationsprovidingoverviews,indepththeory,
andapplication[74‐79].
Thefocusofthefollowingsectionswillbeonanisotropicetching.Theprocessof
chemical etching of silicon can be broken down into “chemical” reactions and
“electrochemical” reactions. “Chemical” reactions between the silicon and
electrolytewillhaveanexchangeofatomsandchargewhichareexchangedatthe
interface.Thenetchargetransfer,andthusthenetcurrent,betweenthemiszero.
The charge transfer between the silicon and electrolyte is measured in “partial
currents”wherethe“anodiccurrent”isthetransferofelectronsfromtheelectrolyte
to the solid; “cathodic current” is the transfer of electrons from the solid to the
electrolyte. Hole transfer is the opposite direction of electron transfer for the
respective partial currents. When the charges transfer between the solid and
electrolyte is non‐zero the reaction is considered to have an “electrochemical”
component the reaction. It is this electrochemical component that is frequently
used to produce etch stops in silicon micromachining processes. This
electrochemical component of the reaction can be produced by many factors;
galvanic interaction, external applied potential, photogenerated electrons, or
photogenerated electron/hole pairs. Without an externally applied potential the
reactionoperatesatan“opencircuitpotential”(OCP).
There are multiple steps to the chemical reactions in bulk micromachining
monocrystalline silicon using KOH or TMAH/IPA solutions to anisotropically etch
46
structures into the substrate. The exposed (111) planes in the silicon crystal
structure,showninFigure21,arelesssusceptibletochemicalreactionswiththese
solutions. KOH solutions typically provide mirror quality machined surfaces.
Etchingina(100)planeisfavoredoveretchingina(111)planebyaratioof100to
1atstandardetchingtemperaturesbetween70‐90°C[80].KOHetchingtechnology
isnotCMOScompatibleduetothemobileK+ion,andisthereforeinappropriatefor
someapplications.TMAH/IPAsolutionscanprovidea20to1etchselectivity,again
favoringetchingin(100)planesover(111)planes;andtheTMAH/IPAtechnologyis
CMOScompatible.TheadditionofIPA(2‐proponal)improvessurfacequalityonthe
etchedplanesandisresponsiblefordoublingtheetchselectivityavailablefroman
equalconcentrationofTMAHalone[80].Etchinginthedirectionofthe(100)plane
is favored over etching in the direction of a (111) plane. Any transient currents
betweenthesiliconwaferandtheetchantsolutionareshortlivedoncethewaferis
placedinthesolution.Asimpleanisotropicetchingsetupwiththewaferinsolution
occursatOCPandisconsideredachemicalreaction[81].Thecompletereactionfor
thechemicaletchingofsiliconinalkalibasedsolutionsis
Si+4H2O→Si(OH)4+2H2.
Equation21
althoughthechemicalprocessismorecomplicatedandtakesplaceinanumberof
sequentialstepsforboththe[100]facewithtwobackbonds
＝SiH2+OH‐+H2O→＝SiHOH+H2+OH‐
47
Equation22
Equation23
＝SiHOH+OH‐+H2O→＝Si(OH)2+H2+OH‐
andthe[111]facewith3backbonds
Equation24
(≡Si)2Si(OH)2+２H2O→２(＝Si‐H)＋Si(OH)4
≡Si‐OH‐→≡Si‐OH+e‐
Equation25
≡Si‐OH‐→≡[Si‐OH]++++3e‐
Equation26
[Si‐OH]++++3OH‐→Si(OH)4
Equation27
Si(OH)4+２OH‐→＝SiO2(OH)22‐+２H2O
Equation28
aspresentedbyWindandHines[82].
Etch stopped
structure
54.74
(1
(100)
11
)
(100)
o
Figure21.Anisotropicetchingcharacteristicsof(100)siliconwafer.
Thereareanumberoffactorsthatplayapartintheanisotropicofsiliconwith
alkali based solutions. Silicon atoms along the [100] face have two single back
48
bonds to silicon atoms within the bulk of the silicon. Each silicon atom along the
[100] face surface has two hydrogen terminated surface bonds available for
interaction with the etch solution. Silicon atoms along the [111] silicon/etchant
interfacearemorestablyboundtothebulkofthesiliconsubstratewiththreesingle
backbondsandonlyonehydrogenterminatedbondtoreactwiththeetchsolution
[71].TheactivationenergyforsiliconinKOHisbetween0.52eVand0.69eVinthe
<100>directionand0.8eVand1.0eVinthe<111>direction.Thereisspeculation
thatthereismoreatworkthanadifferenceinactivationenergywhenitcomesto
the etch selectivity between the <100> and <111> directions [71]. The range of
activationenergyisnotedtobedependentonthemolarityoftheKOHsolutionand
thebestetchinganisotropyoccursinhighmolaritysolutions[83].Achemicaletch
stopmechanismhasbeenproposedthat[111]silicon’sresistancetoetchinginhigh
concentration KOH solutions is due to water molecules forming a layer with the
hydroxylated surface silicon atoms preventing additional hydroxyl ions from
reachingthesurfacetocompleteasolublereactionandcontinuetheetchingprocess
[83].
The background concentration of dopants, along with crystallographic
orientation, in silicon also has an effect on the outcome of chemical and
electrochemical reactions [84]. A voltammogram provides valuable insight to
different modes of interaction between the silicon and electrolyte exhibiting
variationsincurrentdensityasafunctionofpotential,Figure22,forKOHsolutions
and,Figure23,forTMAHsolutions.Variationsofpartialcurrentsandtheirimpact
49
onelectrochemicalreacctionscanb
beunderstoood.Theopeencircuitpo
otential(OC
CP)is
th
he point where
w
there is no net current beetween the silicon and
d KOH. An
nodic
potentials arre positive of the OCP and cathod
dic potentiaals are negaative of the OCP.
Thepassivat
T
tionpotentiial(PP)isth
hepointwh
heremaximumnetcurrrentisobseerved
but at which
h point bey
yond the paassivation p
potential th
he silicon su
urface begin
ns to
in
ncreasetoaapointcalledtheFladepotential(F
FP).Atthe Fladepoten
ntialandhigher,
th
heetchingp
processisreduced,ifn
notstopped
dcompletelyyasoxidationofthesiilicon
su
urfacedomiinates[85].
Figure22.Voltammograamprofileffor[100],s olidlinep‐typesilicon
n,dashedlin
nen‐
ty
ypesiliconiin40%KOH
Hat60°C[85].
50
Figure 23. Voltammogr
V
ram profile for [100], solid line p‐type, dasshed line n‐‐type
siiliconin25%
%wt.TMAH
Hat80°C[8
86].
Although
h an anodicc bias can reduce or even stop etching, an
nodic etchin
ng of
siilicon can be
b accompllished when the silicoon being etched is an
nodically biiased
reelativetoth
heOCP.Theeelectronsthatareusu
uallyatthe surfaceoftthesiliconaatthe
siilicon/etchaantinterfacearedrawn
nintotheb
bulkofthessiliconandffreeholesaatthe
valenceband
dofthesilicconinterfacceeffectivellybreaktheebackbond
dsofthesurface
siiliconatomss.[87].Thereactionforranodicetcchingisgiveenin
Si+
+6OH‐+4h+→＝SiO2(OH)2(O‐)2+
+２H2O
n29
Equation
[88], Brressers, et al
a noted thee occurrencce of three modes in electrochem
mical
ettching.Th
hefirstmod
deischemiccalonlyand
doperates attheOCP..Intheseecond
modethesam
m
mpleisheld
datacathodicpotentiaaltotheOC
CPandthessiliconactsaasan
electrodein thedissociationofwaaterandprooducealarggeamounto
ofhydrogen
n.At
potentials ju
ust anodic to the OCP the
t currentt would incrrease but th
he etch ratee did
51
notchangemuchfromtheOCPetchrate.PotentialsjustanodictotheOCPwould
increasetheoxidationrateofthesiliconandassistinproducingahillockfree[100]
surface.Thethirdmodeisastronganodizingpotential.Atthispotentialthecurrent
starts to drop off and etching is reduced due to the heavy oxidation of the silicon
whichproducesabarriertoadditionaletching.Thedifferencesbetweenthep‐type
and n‐type potentials are important when looking at p‐n junction etch stops. It is
alsoimportanttokeepthepotentialsmeasuredbyvoltammograminconsideration
when looking at the electrochemical etching differences between [100] and [111]
silicon planes [89], Figure 24. Through cyclic voltammetry, etching in KOH of a
rectangular region on a [100] silicon wafer shows the effect of current transfer at
given potentials between the silicon substrate and a saturated calomel electrode
(SCE)constructedofmercurychloride[85,89,90]
52
deinsighttooetchingan
ndpassivatiionpotentiaalsof
Figure24.Voltammograamsprovid
he[100]and[111]silicconfacesin
nKOHfor(aa)p‐typesilliconand(b
b)n‐typesillicon.
th
Source:[89]..
2.14
2 BandDiagramsofSiliconwithKOH
HandTMA
AH
Saturated
dCalomelE
Electrodes(S
SCE)madeffrommercu
urychlorideeandtheno
ormal
hydrogen electrode (NHE) are staandards forr referencee of redox p
potentials. The
sttandard hy
ydrogen electrode (SHE) is an id
dealized staandard and is useful w
when
53
co
omparing electrochem
e
mical redox potentials w
with vacuu
um scale of band diagrrams.
Figure25sh
howsthecon
nductionbaandofsilicoontoresideeataSHEp
potentialof ‐0.55
Vandtheval
V
lancebandofsiliconto
oresideat0
0.50V[91].
Figure25.C
Commonsem
miconducto
orbandgap
psareshow
wnrelevant tovacuum scale
andSHEelecctrodepoten
ntial[91].
Figure 26
6 depicts a band diagrram of [100
0] silicon aand a KOH eetching solu
ution
beforemakin
ngcontact.Fromthisb
banddiagraamitisappaarentthatth
heFermien
nergy
mersionofaasiliconwafferin
leeveloftheeelectrolyteisshigherthaanthatofsillicon.Subm
th
he electroly
yte creates band bendiing in a sim
milar manneer to a Schottky diodee. As
sh
howninFig
gure27,ban
ndbendingduetoatraansferofch
hargeattheinterfaceocccurs
fo
or both p‐ttype and n‐type
n
silico
on. The su
urface statees of silico
on at the liiquid
in
nterfacealsoproducestraplocationsdeepin
nthesilicon
nbanddue todanglinggand
backbonds[[71].
54
Figure 26
6. The band
d diagram and
a densityy of state diiagram for ssilicon and KOH
alkalibassedetchsolutionrespeectively.Sou
urce:[71]
Figure 27 Band diaagram once the silicon
n is in conttact with KO
OH alkali b
based
bottom)n‐tyypesilicon.Source:[71
1]
etchant;(top)p‐typesilicon,(b
55
TMAH is another common etchant used with silicon and a band diagram is
constructedinthesamefashionasforKOH,Figure28.TheFermilevelofTMAHis
belowtheFermilevelforn‐typesiliconandmoderatelydopedp‐typesiliconatthe
opencircuitpotential(OCP).AccordingtoNemirovsky,thebanddiagramofsilicon
with the electrolyte etchant correctly shows the Fermi levels of the silicon and
electrolyte are not aligned and therefore that the system is not in thermal
equilibrium[92].TheOCPisgeneratedbythetwostagechemicalreaction;first,the
reductionofwatertohydroxylionsandhydrogengas;thesecond,theoxidationof
silicon. These two reactions make up the silicon etching process in alkali based
solutions.
Reactionscanoccurbetweenasemiconductorandelectrolyteutilizingbothvalence
andconductionbandsbetweentheelectrolyteorasingleband.Inthiscase,the
valencebandisresponsibleforthetransferregardlessofwhetheritisn‐typeorp‐
type[93],
Figure29.Astheoxidizingagentisreducedintheelectrolyteofthisexample,
additionalholesdriftintothesemiconductorandaquasi‐Fermilevelisgeneratedin
then‐typesemiconductor.
56
Figure 28. Band
B
diagrram of siliccon with 25% TMAH etch soluttion. (a) n‐‐type
siilicon/alkalielectrolytee(b)p‐typeesilicon/alkkalielectrolyyte.Source:[92].
Figure 29. Semiconduc
S
ctor Fermi levels show
wn at the iinterface off electret rredox
reeaction[93]].
57
Looking atthe chem
mistry ofan
nisotropic ettching in w
will provide thebackground
reequiredto understand
u
d how anMIS structuree can prom
mote an etch
h stop. Thee first
th
hingtolook
katforaP‐ttypewaferisitsnaturaalunbiased
dpotentialin
nananisotrropic
ettchantsuch
hasKOH,Fiigure32.F
Freeelectroncarriersaaccumulate attheregio
onof
siilicon up to
o the silico
on/etchant interface aas is needed
d to keep the Fermi level
co
onstantbetweenmaterrials.Thish
happenswitthN‐typesiiliconasweellbuttoaleesser
exxtent[19]. Thehydro
oxylionsin thewaferp
playthecriiticalrolein
netchingcrrystal
siiliconinthee(100)face.Asiliconaatomfromtthesolidfaccebondswiithtwohydrroxyl
groups and become
b
soluble to the solution leeaving two electrons. These electtrons
reeturn to th
he base solution prod
ducing addittional hydrroxyl ions tto continuee the
reeaction witth silicon. The supply
y of electroons to the base etchaant is cruciial to
providingaccontinuousetchprocesss.Intheusseofanelecctrochemicaaletchstop
p,this
su
upply of electrons fro
om the siliccon to the eetchant is ttruncated b
by adjustingg the
potential of the silicon
n substrate to an anod
dic bias rellative to th
he etch solu
ution,
Figure31.
0.P‐typesiliiconinanisotropicKOH
Hetchant.SSource:[19].
Figure30
58
Figure31.A
AnodicbiasttoP‐typesilliconinanissotropicKO
OHetchant.Source:[19
9]
2.15
2 BiasE
EffectsonA
AnisotropicEtchinggofSilicon
n
As one of
o the few alternativees to a tim ed etch sto
op, the ano
odic bias of the
su
ubstrate is a valuable tool, aidingg in the high
h dimensional tolerance machinin
ng of
many
m
silicon
n based ME
EMS devicees. In manyy cases, exxternal pow
wer sourcess and
electrodes are
a needed to implem
ment this an
nodic bias; a potentio
ostat is useed in
co
onjunctionwithaplatiinumrefereenceelectrodeandasillver/silvercchloridecou
unter
electrode.O
Oftengreateeffortisputtintoohmiiccontactdo
opingandccomplicated
djigs.
Thesehighq
T
qualityelecttricalcontacctsareneed
dedtomakeeaneffectiveandconsistent
co
ontacttoth
hewafer[94
4].Thejigsarerequireedtoseparateelectricalcontacts from
th
heetchsolu
ution.Thep
processofeelectrochem
micaletchsttopbecomeesexpensiveeand
cu
umbersomee.
Anetchsstopisonly
yvaluableiffitcanbeiimplementeedattheco
orrecttime orin
th
hecorrectlocation.An
nanodicpo
otentialinth
heformofaareversebiasP‐Njunction
caan be tuneed to precisse locationss and biaseed to produ
uce well co
ontrolled siilicon
membranes
m
usingetchsstoptechniq
ques.Adraawingoftheefundamen
ntalsetupfo
oran
59
electrochemicaletchsto
opisshowninFigure3
32.Thesilicconetchingstopswithinthe
p‐type silico
on just beffore the metallurgical
m
l junction depth and models siilicon
ettchant solu
ution as a bipolar
b
juncction transiistor (BJT).. The etch
h solution iss the
emitter,theremainingp
ptypesilico
onnearthe endofetch
histhebasee,andthen‐‐type
siilicon is thee collector. The base‐eemitter is fforward biaas and the b
base collecttor is
reeversebias,,Figure33.Thisputstthesystem inaforwardactivemo
odeandrequires
aconstantsu
upplyofcurrrenttomaintaintheettchstop[95
5‐97].
Figure32.np
pnBJTmodelofpnjunctionofelecctrochemicaaletchstop[96]
Figure33.NPNBJTschematicshow
wnwiththeebase‐emittterforward
dbiasedand
dthe
base‐collecto
orreverseb
biased[86].
60
AcerowasabletoshowthatTMAHcanbeusedinanelectrochemicaletchstop
similartoKOH.ThestudyincludedTMAHsolutionsof25%and2.5%wt,withand
without IPA. An n‐type membrane was fabricated using this etch stop using a PN
junctionasthedepletionregion.TheetchstopsetupshowninFigure34makesuse
ofathreeelectrodeconfigurationwherethen‐typeregionofthesiliconisconnected
to the potentiostat. The reverse bias between the p‐type and n‐type regions is
controlled by maintaining a sufficient positive potential, from n‐type to p‐type
material, on the silicon (work electrode) relative to the TMAH etch solution,
(reference electrode). It was noted by the authors that the electrochemical etch
stopofn‐typesiliconoccursatpotentialsanodictotheOCPinthisconfigurationand
thatthecurrentthroughtheworkelectrodepeaksattheonsetofsiliconpassivation.
AshrufpointouttheadvantageofTMAHoverKOHinananodicetchstopprocessin
that TMAH is very sensitive to silicon oxides as a masking layer, where the
selectivitybetween[100]siliconandsiliconoxideismuchlowerwithKOHetchants
[81].Afourelectrodeconfigurationhasalsobeendeveloped,Figure35.Thefour
electrode configuration provides better etch stop control and allows the reverse
bias of the pn junction to be maintained independently of the silicon‐etchant
potential[98].
61
Figure 34. TMAH
T
baseed three‐ellectrode eleectrochemical etch sttop using a pn
ju
unction[86]].
Figure35.BiasedPNjunctionetchstopinKOH
Husingafo
ourelectrod
deconfiguraation.
Therelative
T
oftheworkelectrode(p
pnjunction
n),referenceeelectrode,and
potentialso
co
ounterelecttrodearesh
hownfromllefttoright..Source:[9
98]
62
The metal insulator silicon (MIS) etch stop configuration in p‐type silicon is a
variationofthereversedbiasp‐njunctionetchstop.Thestronginversionlayerof
the biased MIS produces the depletion region.. In the case of the MIS etch stop
(shown in Figure 36), it is critical that the voltage (V2) is tuned and maintained
between the silicon wafer and the etchant solution for the anodic oxidation of the
substrate to occur just prior to reaching the depletion region. It is in this region
wherethequasi‐Fermileveloffreeelectronsdecreasesandelectronsarenolonger
injectedbackintothesolutiontopropagatetheproductionofhydroxidesneededfor
silicontocontinueetching[99].Thecurrentflowbetweenthesiliconsubstrateand
the etchant is maintained by the external voltage, V2, applied between the silicon
substrateandetchant,drawstheanodiccurrentrequiredtostoptheetchoncethe
etch front reaches the strong inversion region of the MIS maintained by external
biasV1.
63
hown for p‐‐type
Figure 36. A MIS elecctrochemicaal etch sto p configuraation in sh
n is produceed for electtrical contact to the siilicon
siilicon. A reegion of n‐ttype silicon
su
ubstrate. A
A platinum electrode is
i submergged in the eetchant to b
bias the silicon‐
ettchantinterrface[99].
Figure37.MISetchstop
pproducedb
byinversion
nlayer[87]].
64
3 FABRIC
CATIONAN
NDMETHODS
The methods for fabricating and ch
haracterizin
ng the eleectret and the
im
mplementattionoftheeelectretasaanelectroch
hemicaletch
hstoparep
presented. First,
th
he depositiion of the PECVD sillicon nitrid
de and siliccon oxide deposition and
su
ubstrate arre defined. Second, th
he method of direct contact theermally asssisted
poling is cov
vered follow
wed by the evaluation of the film’’s effective surface volltage.
Subsequently
y,theproceessforoptimizingelecctretactivattionforneggativepoten
ntials
and the acccelerated lifetime tessting methoods for bo
oth positivee and neggative
potentials arre describeed. Lastly, the
t details for testing the electreet as part o
of an
electrochemicaletchsto
opwithTMA
AHisoutlin
ned.
3.1
3 PECVD
DDepositiionofSiO2/Si3N4/SiiO2
P‐type, boron
b
doped, (100) silicon waferrs (d = 100
0 mm, t = 525 m) aand a
reesistivity off 1‐10 ohm‐cm was ussed in all o f the experriments to ccharacterizee the
electret activ
vation proccess and iso
othermal pootential deccay.. A claadding of siilicon
nitrideandssiliconoxideefilms,simiilarinconsttructiontotthatusedin
nSONOSdevvices,
was
w
producced. Speciifically, thee dielectric was creatted by coaating a film
m of
SiO2/Si3N4/S
SiO2,atanominalthick
knessof165
50nm/250 nm/28nmrespectivelly,on
65
the silicon substrate via PECVD. The silicon nitride and silicon oxide films were
depositedatthesametemperature,butatdifferentpressures,550mtorrand1000
mtorr, respectively, using the silane/argon, ammonia, nitrous oxide and nitrogen
gas rates shown in Table 1. The silicon nitride was deposited using RF/LF power
pulse parameters of 20W for30sec/50W for 2 seconds to minimize film stress in
thesiliconnitridewhilethesiliconoxidewasprocessedwith20WofRFpoweronly.
Prior to the thermally assisted poling, the SiO2/Si3N4/SiO2 films were placed on a
200°Chotplateinatmosphereforonehourtoremoveexcesswaterwhichhadbeen
absorbedbytheSiO2/Si3N4/SiO2fromtheair.This“bakeout”wasfollowedupwith
ahexamethyldisilizane(HMDS)spincoatat4000RPMfor10secondstoproducea
hydrophobic surface on the SiO2/Si3N4/SiO2 and prevent future water absorption
fromhumidair.
Table1.PECVDprocessparameters
Temp.
Pressure
RF
LF
5%SiH4/Ar
NH3
N2O
N2
(°C)
(mtorr)
Power
(W)
Pulse
(sec)
Power
(W)
Pulse
(sec)
(sccm)
(sccm)
(sccm)
(sccm)
silicon
oxide
350
1000
20
‐
‐
‐
170
‐
710
‐
silicon
nitride
350
550
20
30
50
2
200
50
‐
200
ThefilmthicknessofeachSiO2andSi3N4layerwasdeterminedbyfirstrunning
preliminary wafers with a single film layer of SiO2 or Si3N4 and measuring
selectivelyetchandunetchedregionsbysurfaceprofilometry(Dektak8,VeecoInc.,
66
Plainview, New
N
York). The film
m thicknesss of each SSiO2 and SSi3N4 layer was
determined by first run
nning prelim
minary waffers with a single film layer of SiO
O2 or
Si3N4. PECV
VD silicon oxxide deposiition was p erformed fo
or 25 secon
nds, 25 min
nutes,
and 30 minu
utes using one silicon wafer for each respeective time. PECVD siilicon
nitridedepositionwasp
performedffor5minut esand30m
minutesusin
ngonewafeerfor
each respecctive time. The waafers were subsequently photo
olithographiically
CVD film was
w selectivvely etched
d in BOE u
until it beccame
patterned and the PEC
otoresist was
w stripped
d from the substrate in acetonee and
hydrophobicc. The pho
riinsed in DI water. The step heigght of the P ECVD film was then m
measured in
n five
lo
ocationsby contactpro
ofilometry((Dektak8,V
VeecoInc.,P
Plainview,N
NewYork). The
fiive location
ns consisted
d of the cen
nter and foour quadran
nt locationss approxim
mately
2cmfromthewaferedgge.Afullcaassetteof25
5substrate waferswerrerunasab
batch
oncetherequireddepossitiontimesshadbeend
determined..
3.2
3 WaferrLevelTheermallyAsssistedPo
olingofEleectret
The
T thermallly assisted poling waas performeed in an an
nodic bondeer (SB‐6e, SUSS
MicroTecAG
M
G,Garching,Germany)b
byplacingaa2”100P‐tyypesiliconwaferincontact
with
w
the 10
00 mm siliccon substraate contain
ning the mu
ultilayer PE
ECVD depo
osited
siiliconnitrid
deandsilico
onoxidefilm
ms,creatingga“wafersstack”with thePECVD
Dfilm
betweentheetwopolish
hedfacesof
f thesiliconsubstrates,Figure38..Aschemaaticis
sh
howninFig
gure39.Un
ndera5mto
orrvacuum withnitroggenpurge,eeachwafersstack
in
n the study
y was subjeected to a range
r
of volltages (60 V – 540 V)) to identifyy the
67
optimum maaximum volltage needeed for activaation. The voltage waas applied to
o the
to
op wafer accting as a cathode
c
for applying th
he bias acro
oss the PEC
CVD film du
uring
electret activ
vation. Durring the acttivation proocess, the w
wafer stack was heated
d to a
maximumte
m
emperature overarangge(150°C‐‐210°C)to producean
nelectretw
witha
negativeeffeectivesurfacevoltage. Withtheffullvoltage applied,theewaferdw
welled
att the elevatted temperaature for on
ne, three, orr 5 hours aand then alllowed to co
ool to
40°Cbeforetheapplied
dvoltagewaasremoved..
Figure38.100mmdiam
metersilico
onwaferwitthPECVDd
dielectricfilm
misloadedwith
2"“cathode”waferinSU
USSSB‐6ebondertooling.
Figure39.Scchematicof
f thesetupfforthetherm
mallyassisttedpolingo
ofthemultilayer
nasiliconw
wafer.
fiilmstackon
68
As
A an exam
mple, a typical thermaally assisteed poling p
process flo
ow consisteed of
generating a
a negative surface volltage in thee electret w
when the w
wafer stack
k was
su
ubjected to an applied
d voltage off ‐180 V wh
hile being h
heated to 17
70°C, Figure 40.
Under
U
the ap
pplied voltaage, the wafer dwelled
d at 170°C ffor one hou
ur and was then
co
ooled to 40
0°C before removal
r
fro
om the clam
mping electtrodes of th
he SUSS bon
nder.
Oncethesam
O
mplewasactivated,it wasremovvedfromtheSUSSbon
nderandtheetop
electrode sillicon wafer was pried with wafeer tweezers from the P
PECVD film
m and
su
ubstrate. The
T substraate was once again spiin coated w
with HMDS aat 4000 RP
PM to
trreat the surrface to asssure that it will remai n hydropho
obic. Everyy sample in
n this
sttudy was activated
a
in
n a similar manner; h
however, th
he time, teemperature,, and
appliedvoltaagewerevaariedtodeteerminetheooptimumelectretactivvationtreatm
ment.
mperaturean
ndvoltagem
magnitudep
profileused
dfortheeleectret
Figure40.Exxampletem
der.
activationprrocessintheeSUSSbond
69
3.3
3 ElectrretMeasurrementan
ndEvaluation
An electrrostatic volttmeter (Mo
odel #279, Monroe Eleectronics, In
nc., Lyndon
nville,
New
N
York) was
w used to
o measure the
t effectivee surface vo
oltage (ESV
V) of the eleectret
fiilm after acctivation. The
T probe was
w maintained at a d
distance of ttwo millimeters
frromtheelecctretsurfaceandmeassureda2mm
mdiameterrfieldatanyygivenlocaation.
TheSiO
T
bstratewassmappedw
withtheuse ofamicrosscopestagee.The
2/Si3N4/SiO2sub
th
heory of op
peration forr the electro
ostatic volttmeter was presented in section 2.5 ‐
”E
Electric Dissplacement Field”. A detailed m
map of the electret’s eeffective surface
voltage acro
oss the wafeer was used
d to evaluaate the averrage maxim
mum ESV an
nd to
generateadetailedconttourplot,Fiigure41.Th
heaverage ofthefiveh
highestESV
Vdata
points defined the oveerall maxim
mum ESV acchieved und
der the particular pro
ocess
parameters.Eachdatap
pointrepresentsaread
dingatapittchof1.5x1
1.0cmofthee100
mmdiamete
m
erwafer.
Figure 41. ESV meaasurements were maade over tthe wafer surface att the
in
ntersectionssofthealph
hanumericggridatapittchof1.5cm
mand1.0cm
m.
70
3.4
3 ProcessOptimizzation,DesignofExxperimentss
A23fullffactorialdesignofexpeeriments(D
DOE)wasim
mplemented
dtoevaluattethe
main
m
effects of the theermally assiisted polingg process; ttemperature, process time,
and applied voltage [10
00]. The bo
oundary coonditions on
n the DOE w
were 170°C
C and
190°C;oneh
hourandfivehours;and,‐180Van
nd‐300V.A
Amidpoint valueof180°C,‐
240 V and a
a three hou
ur dwell tim
me, respectivvely, was aalso tested. The midp
point
value provid
des addition
nal data wh
hich can bee used to cconfirm thee variable eeffect,
a
the posssibility of nonlinear effects. Eaach sample was
variable inteeractions, and
mappedand
m
theaverageofthefiveehighestsim
milarESVd
datapointsw
wereusedinthe
DOE.
D
The teerm “similar” signifies that the eff
ffective surfface potentiial measureed on
th
he surface of
o the filmp
possessedtthe same poolarity as th
he potentiaal applied byy the
caathode. Fo
or this DO
OE model, “similar”
“
reeferred to a negativee potential. An
additionalan
nalysiswas performed
dtoevaluateethefiveh
highestcomplementaryyESV
datapointsu
usedintheD
DOE;whereetheterm“ccomplemen
ntary”signiffiedtheeffeective
su
urface poteential oppossite of theccathode pottential, i.e. aa positive p
potential forr this
DOEmodel.
D
Basedonthe
B
eresultsof the23fullffactorialDO
OE,Section3
3.4‐“ProcessOptimizaation,
Design
D
of Experiment
E
ts”, both temperature
t
e and app
plied voltage significantly
co
ontributeto
otheproductionoftheeelectretESSV.AsingleefactorDOE
Ewasperforrmed
fo
orthetemperatureand
dvoltagevaariablestod
determinettheextentth
hattheresu
ulting
ESVisaffecttedbytemp
peratureandvoltage. Theeffecto
oftheproceesstemperaature
71
was
w characterized at 150°C, 170°C
C, 190°C an
nd 210°C. Similarly, b
based on th
he 23
DOE,
D
all of the
t experim
ments in thiss sequence were perfo
ormed with
h a potentiaal of ‐
180Vapplieedtotheeleectretsurfaceforaoneehourdwellperiod.In
nadditiontothe
teemperaturee dependen
nt single faactor DOE,, the effectt of applieed voltage was
ch
haracterizedinanexteendedsingleefactorDOE
Eforprocesssingvoltaggesof‐60V,‐180
V,
V ‐300 V, ‐4
420 V and ‐540 V. All
A of the exxperiments in this volltage depen
ndent
seequenceweereconducteedat170°C
Cwithafive hourdwelllperiod.
3.5
3 ElectrretCharactterization
Temperaature has beeen shown to have on
ne of the ggreatest effeects, other than
humidity, on
n decreasing the usefu
ul lifetime oof an electrret [7]. Thus, experim
ments
were
w
design
ned to deteermine the isothermall potential decay (ITP
PD) of the ESV
produced under
u
diffeerent proceessing con
nditions. An extrap
polation of
f the
teemperaturee dependence for the ITPD data was performed to ob
btain the u
useful
mean
m
lifetim
me of thee ESV pro
ovided by the electrret for typ
pical operaation
teemperaturees.Adescrip
ptionofsam
mplepreparrationandeexperimentssperformed
dhas
beendefined
dbelow.
3.5.1
3
Isoth
hermalPottentialDecaySampllePreparaation
TheDOEusingITPD
Ddatarequiredthedeccayratetob
beevaluated
datanumb
berof
elevatedtem
mperatures.Tothisend
d,eightwafeerswereprreparedatth
hewaferlevvelin
accordancew
withthepro
ocessstandaardspresen
ntedinsectiion0‐”
72
PECVDDepositionofSiO2/Si3N4/SiO2“andsection3.2‐”WaferLevelThermally
AssistedPolingofElectret”.Fouroftheeightwaferswereactivatedattheapplied
potentialofpositive300Vandthefourremainingwaferswereactivatedatnegative
300Vtodeterminetheeffectsofpolarityontheperformanceoftheelectret.Five
sampleswerecleavedfromeachofthesewaferstoperformtheITPDexperiments
for each wafer at five different accelerated aging temperatures (325°C, 300°C,
275°C, 250°C, and 200°C) with the goal of extrapolating the room temperature
lifetime coefficients and activation energy from the Arrhenius plots. In order to
performananalysisofvariance(ANOVA),aminimumoffourwaferswereneededto
demonstratestatisticalsignificanceforthetemperaturestested.
3.5.2 IsothermalPotentialDecayExperiments
ToperformtheITPD,asampleelectretfromeachrespectivewaferwasplaced
on the hot plate (Dataplate PMC 720, Barnstead/Thermolyne, Dubuque, Iowa), at
oneofthespecifiedacceleratedagingtemperatureslistedabove,andtherespective
ESVwasimmediatelymeasuredbytheelectrostaticvoltmeterwithadistanceof2
mm between the probe and the center of the sample. Effective surface voltage
measurements were continually acquired as the sample “aged” at the elevated
temperature to produce decay plots. After all samples were tested, a nonlinear
regressionwasperformedforeachsampletreatedtodeterminethe“bestfit”forthe
datatoanexponentialdecayequationintheformof
73
Equation30
This relationship yielded the lifetime coefficient, τ, a characteristic of each
samplethatwasdeterminedfromtheESVasafunctionoftime.Sincethestarting
ESV value varied between electret samples, the ESV was normalized to an initial
valueofone,theexponentialtermwasunaffectedbythisnormalization.Plotsofthe
exponential decay are presented in Figure 67 and Figure 68 in section 4.2 ‐
“CharacterizationandOptimizationofElectretFormationProcess”.
3.5.3 EvaluationofIsothermalPotentialDecayTrials
The exponential decay data for all samples was utilized to generate a best‐fit
Arrheniusequation
1
Equation31
whereArepresentsthenumberofdecayinteractionspossibleoveragiventime;the
exponential function represents the probability of a decay interaction taking place
as a function of temperature, T, and the activation energy, Ea, of the system and
Boltzmann’sconstant,k=8.617E‐05eV/K.Thisequationwasusedtoextrapolatethe
decay rates of the ESV for temperatures that lied outside the range of accelerated
agingtests.
74
3.6
3 AppliccationofE
ElectretasanElectro
ochemicallEtchStop
p
Thisprojjectinvestiggatedtheusseoftheeleectretasassourceofellectricbias inan
electrochemical etch sttop. As staated in thee Backgroun
nd (or Liteerature Revview)
ch
hapter, bassed on prev
vious work
k by otherss[86, 87, 9
95, 96, 98, 101], electtrical
potentials haave been demonstrate
d
ed to .proviide the elecctric field rrequired forr the
sttrong inverrsion of a semiconduc
s
tor that is required fo
or an electrrochemical etch
sttop.Howev
ver,thesem
methodshav
veimplemeentedtradittionalelectrronicequipm
ment
exxternaltoth
hesubstrateeorgalvaniiceffectsto providetheenecessaryybiastopro
oduce
th
he required
d strong inv
version with
hin the sem
miconductorr. The trap
pped chargges of
th
heelectretp
providethe permanenttbiasneedeedinaMOSSbiasetchstop,makinggthis
th
he first pro
ocess of itss type to operate
o
with
hout an exxternal pow
wer supply. An
electrical biaas generateed electrosttatically prroduces an electrochemical etch stop
without
w
req
quiring add
ditional dop
ping of th
he silicon aand also p
provides fu
uture
possibilities for a patteerned etch stop. For completeneess, experim
ments weree run
withtheelec
w
ctretinboth
hpolarities.Ineachcon
nfiguration,,theelectreetwasdepo
osited
directlyonttheback‐sid
deofthesiliconwafer beingetcheed,Figure4
42.Theeleectret
wasactivate
w
dtoproducceanegativ
veorpositivveESV.Itw
wasexpecteedthatthissetch
sttopwillworkiftheeleectricfieldffromtheeleectretissiggnificanteno
oughtobiaasthe
siiliconintoaadepletion modeatth
hesilicon‐eleectretinterrface.Them
methodsforrthis
ettch stop sttudy is preesented in three parrts. Prelim
minary meth
hods coverr the
preparation of the waffer and die is presenteed. Second
d, the actuaal etch stud
dy on
75
batch treated positive, neutral an
nd negative electrets. Lastly, reaal time effeective
su
urfacevoltaagemeasureementsaremadeastheeetchprogrresses.
Figure42.A
Anisotropicw
wetetchwithelectretoonetchwaffer.
3.6.1
3
WafeerandDiePreparatiionforEleectrochem
micalEtchSStop
Theelecttretissusceeptibletoneeutralization
nandcharggemasking.Becauseoffthis,
th
hesequenceeofactivatiingtheelecctretinadeeviceprocessssequenceeiscritical. Itis
notpossibletoactivatetheelectrettfilmpriorttodicingdu
uetoexposu
uretowaterrand
debris durin
ng the dicingg process. Because off the electreet’s suscepttibility to ch
harge
neutralizatio
on,dicingw
wascompletedpriorto electretacttivationfor allexperim
ments
co
onductedto
odetermineetheeffectiivenessofttheelectret asanelecttrochemicaletch
sttop.
76
All waferrs used forr the etchin
ng study coonsisted of silicon sub
bstrates, double‐
siidedpolisheed,P‐type,b
borondoped
d,(100),wiithresistivittyrangingffrom15ohm
m*cm
to
o25ohm*ccm,andnom
minalthickn
ness380µm
m.Thesubstratesare coatedwith
hthe
PECVD oxidee/nitride/o
oxide layer 1650 nm/2
250 nm/28
8 nm thick respectivelly, in
accordancettothedepossitionparam
meterspreseentedinsecction0–“
D coated w
PECVD Deposition
D
of
o SiO2/Si3N4/SiO2”. E
Each PECVD
wafer was d
diced
withadicing
w
gsaw(DAD321,Disco Inc.,Japan))to20mm by20mm die.Eachw
wafer
yielded9eleectretsampllesforetchstoptestingg.Inorderttoacquiressufficientdaatato
performanaanalysisofsstatisticalsignificance,,atotalof9
9squareop
penings(800µm
to
o a side) were pattern
ned in the PECVD
P
multtilayer film on the seco
ondary poliished
siide of each
h die on thee wafer using conventtional contaact lithograaphy techniq
ques,
providing 9 etch wind
dows for producing
p
ccavities from subsequ
uent anisotrropic
ettching,Figu
ure43.
Figure43.Eaachoftheeetchdieconttain9etch cavitiesforrevaluation
nofeffectso
ofthe
electretonth
heprogresssionofsilico
onetchingin
nthecavitiees.
77
Adielevelvariationoftheelectretactivationwasperformedunderatmospheric
conditionsforonehourat170°Cwithanappliedvoltageofmagnitude300Vona
hotplate(PMC720).Thetopcontactelectrodewasdicedto13mmby13mmfrom
ap‐type(100)siliconwaferwitha1‐20ohm*cmresistivity.BoththePECVDcoated
dieandthetopcontactdiewereplacedonahotplateat200°Cinatmospherefor1
hourforadehydrationbakeandfollowedupwithaHMDSdropandnitrogengun
dry. To activate the electret, the PECVD coated die was placed on an aluminum
faced hot plate (Dataplate PMC 720) set to 40°C and the polished face of the
electrode die was centered on top. A small piece of indium was placed on the
electrodedietoimprovecontactbetweenthedieandtheactivationprobeandsmall
bare silicon pieces are pressed to the edge of the electret etch die to improve
groundingcontact,Figure44.Apowersourcemeter(#2410,KeithleyInstruments,
Inc.,Cleveland,OH)wasusedtoapplya+/‐300Vpotentialbetweenthealuminum
faceofthehotplateandtheelectrodedie.Thehotplatetemperaturewaselevated
to170°Cataramprateof100°C/hrandallowedtodwellfor1hr.Subsequently,the
hot plate was allowed to cool and the 300 V potential was removed when the
temperature fell below 100°C. The electrode die was pried from the electret
substrateinordertomeasuretheelectret’sESVwiththeelectrostaticvoltmeter.
78
Figure44.Dielevelelecctretactivattionwaspeerformedw
withsub‐sizeedsilicondieon
th
hePECVDffilm.Small pieceofsili
p
conareplaacedtothe edgeoftheedietoimp
prove
groundcontaactbetween
nthehotplaateandelecctretdie.
A total of three waffers were processed foor these etcch stop expeeriments. F
From
3diewereffabricatedw
withatargeetedpositiveeESVof+15
50V;3diewere
eachwafer,3
withatargettednegativeeelectretE SVof‐150 V;andthetthreeremaining
faabricatedw
diefromeacchwaferweereunchargged(0V)an
ndusedas controlsam
mplesduringthe
ettchrateexp
periments.
3.6.2
3
Calcu
ulationsfo
ortheJustificationo
ofStrongInversion
Basedon
npriorwork
ksofp‐nju
unctionand MOSstyle etchstops discussedin
nthe
background, it is criticaal that this ESV meetss or exceeds the poten
ntial requireed to
dielectric siilicon
ensure a state of strongg inversion withinthe p‐type siliccon at thed
in
nterfaceforrpositively chargeelecctretsamplees.Anevalu
uationofth
hebanddiaggram
79
for electret‐semiconductor interface provided confirmation that the die for this
anisotropicetchstudywereconsideredviablesampleswithanESVinexcessof150
V, ‐150 V, or within +/‐ 10 V for the 0 V control samples. An electret oxide
semiconductor(EOS)banddiagramlooksmuchlikeaMOSbanddiagram,Figure42.
Theenergybands,conductionandvalance,foragivensemiconductoranddielectric
will behave identically to bias generated by the net charges on a metal of a MOS
structure or fixed “trapped” charges at the surface of the dielectric in an EOS
structure.Themaximumdepletionwidth,Wm,canbeusedasafirstapproximation
for the minimum etch stop, as it has been previously reported that anisotropic
etchingstopsatorbeforereachingthedepletionregionforp‐njunctionsandMOS
style etch stops [19, 87]. For example, 143 nm is calculated as the maximum
depletionwidthforap‐type[100]siliconwaferwithabackgroundimpurity,NA,of
1.5(10)16 atoms/cm3 based on a resistivity, ρ, of 10 ohm‐cm is 0.30 µm with the
potentialbetweenEiandEF,givenasF,is0.3578V.
80
Figure 45. Electret Oxide Sem
miconductorr band diaggram in a state of sttrong
in
nversion.
Themaxiimumdepleetionwidth,Wm,is
2

Equation3
32
where s is the perrmittivity of
o silicon, q
q the chargge of an electron, and
d the
F,
potentialis

ln
n
81
.
Equation33
The minimum surface charge, Qd, required to drive the depletion width to the
maximumvalueis34nC/cm2givenby
.
Equation34
The minimum ESV needed to provide the surface charge required for the
maximumdepletionwidthwas
2 .
Equation35
It was necessary to determine the minimum acceptable ESV to implement the
condition of strong inversion and maximum depletion width; the conservative
estimate was to consider the thickness of the capacitor dielectric, d, to be the
completePECVDfilmstackthickness,whichinthecaseofthisstudy,was1928nm.
Aconservativelylowpermittivityof3.45(10)‐11F/mwasusedasthepermittivityof
the film stack, i, based on the literature of permittivity for PECVD films. The
insulatorcapacitance,Ci,wascalculatedusingtherelationship:

Equation36
Inthiscase,giventhevaluesmentionedabove,thecapacitancewascalculatedtobe
1.79nF/cm2,whichcorrespondstoarequiredminimumESVforastateofmaximum
depletionwidthtobe+19.8V.Therefore,itwasanticipatedthatastronginversion
statewasachievedsincethetargeted150VESVwasoverseventimestheminimum
82
valuenecessarytocreatethechargerequiredforproducingthemaximumdepletion
width.
3.6.3 ElectrochemicalEtchStopEtchCellsandEnvironmentalControl
In order to carefully control the etch stop process, a custom etch cell chamber
wasdesignedandfabricatedtomeetthefollowingcriteria:
‐
Maintain chemical and structural integrity at elevated temperatures of
120°C;
‐
Conductheatfromthehotplateevenlytotheetchsolution;
‐
Limitedchemicalinteractionbetweenetchcellandsolution;
‐
Capableofholding20mlofetchingsolution;
‐
Preventevaporationofthesolutionduringtheetch;and,
‐
Holda20mmby20mmsilicondiewiththicknessrangingfrom350nmto
550nm.
Tomeettherequirements,theetchcellsweremachinedfromvirginPTFEwitha
matching PTFE cap. The O‐rings provide the liquid seal between the etch cell and
silicondie.Thebaseoftheetchcellwasmachinedfrom50mmODand12mmID,
316stainlesssteel.AnisometricdrawingoftheetchcelldesignisshowninFigure
46.
83
die.
Figure46.Isometricdraawingoftheeetchcellasssemblywitthsampled
Temperaatureuniforrmityiscriticaltoasucccessfulevaaluationof theeffecto
ofthe
electretoneetchrates.T
Tothisend
d,theetchccellsareplaacedinaneenclosuredu
uring
ettching to promote
p
a uniform
u
tem
mperature ssurrounding the etch cell by theermal
co
onduction and
a
minimiize convecttive influen
nce that occcurs in a fu
ume hood. The
assemblyof theetchceell,etchcell lid,graphittesheets,aandaluminu
umenclosurreon
th
he hot platte are show
wn in Figurre 47. The enclosure itself is a cast alumiinum
electrical en
nclosure (CN‐5711, BU
UD Industrries, Willou
ughby, OH) with finall box
dimensions of 5½” wid
de by 8½” long and 1½
½” tall. A fflexible 1/8
8” thick grap
phite
sh
heet was used as an interface beetween the ultraflat ho
ot plate and
d the alumiinum
enclosuredu
uetoitsexcellentheatconduction propertiesanditsabillitytodeforrmto
sh
hapetoinsu
ureuniform
mcontactbeetweensurffaces.Simillarly,anadd
ditionalsheeetof
graphitewassplacedbettweenthebaseoftheb
boxandtheeetchcells.
84
N
etch cells are sho
own on the hot plate iin the alum
minum enclo
osure
Figure 47. Nine
with
w
the lid off to the side. One etch cell iss shown w
with the PTF
FE lid remo
oved,
sh
howingthesilicondieaandit's9reespectiveetcchcavities.
3.6.4
3
ElecttrochemicalEtchSto
opTimedStudyOveerview
Cavitiesw
wereetched
din the siliicon dieat regular tim
me intervals of60 min from
th
he front sid
de of the wafer
w
in 25% tetrametthylammon
nium hydroxxide (TMAH
H) at
60°C. The back‐side
b
of
o the wafer consisted
d of a PECV
VD depositeed oxide‐niitride
multilayerfi
m
lminpositive,negativ
ve,andneuttralcharged
dcondition
ns.Astheettched
siilicon mem
mbrane app
proached th
he back‐sid
de electret film, the eetch depth was
measured
m
with
w
an optical proffilometer ((Newview 7300, Zyggo Corporaation,
CT) at eacch time interval. Thee etch rate of samples with possitive,
Middlefield,
M
negative, an
nd neutrally
y charged electret
e
acttivation weere evaluateed at each etch
85
intervaltodetermineifthereisanystatisticaldifferenceinetchrateasthesilicon
etchplaneapproachesthebacksidechargedelectretfilm.
The etching process consisted of loading the die in their respective etch cells,
whichwerethenplacedintheetchingenclosure.Theetchingenclosurewasseton
topofthehotplatethatwasoperatingat70°C.Basedonotheretchstopmethods
that took advantage of MOS inversions or p‐n junction biases, there was no
anticipated effect of the electret on the etch rate when the distance between the
(100) etch face and the electret film was greater than 20 µm. Subsequently, the
silicondiewereeachetchedin20mlof25%TMAHetchtoacavitydepthof300µm
forasingleetchperiodof18hours,leaving60µmto80µmofsiliconremainingto
investigatetheeffectoftheelectretchargeonthesiliconetchrateprocess.Afterthe
18hretchingstep,thedepthoffiveetchcavitiesineachdiewasrecordedwiththe
Zygo optical profilometer. The depth of the center etch cavity and the four edge
cavities in the 3 by 3 array were measured, Figure 43. Each cavity was measured
fromeachofitsfouredges,Figure48.Thus,atotalof20etchdepthmeasurements
wereacquiredforeachdie.
86
Figure48.T
Thesurfacettopologyismeasuredfforfiveofth
heetchcaviitiesoneach
hdie.
Two
T
cross sections
s
of each cavity
y measured
d provide aa total of ffour etch d
depth
measuremen
m
nts.
After thee initial 18 hr etch dep
pth measurrements weere completted, the sam
mples
lo
ocated in th
he etch cells were loaaded with ffresh 25% TMAH and
d the remaining
siilicon was etched usin
ng the sam
me protocol described above for the initial etch,
exxceptthistiimethesam
mpleswereeetchedfor6
60minutein
nsteadof18
8hrs.Attheeend
ofthe60minutes,theT
TMAHwas discardedffromeachccell,andtheecellwasriinsed
withDIwate
w
eranddried
dwithcomp
pressedair. Thesteph
heightsofth
hefivecavitiiesin
eachsamplewereagain
nmeasured
dwiththeoopticalprofiiler.Theettchdepthbeefore
andafterthee60minuteeetchprocesswasused
dtodeterm
mineaneffecctiveetchraateof
th
hesamples foreveryh
hourofetch
hing.Thiseetchingproccesscyclew
wasrepeateed15
tiimesuntiltheetchedssamplereacchedthePE CVDmultilaayerelectreetorthesample
faailed by leaakage. Thee etch deptth data wa s evaluated
d as a funcction of tim
me to
87
determineiftherewasastatisticallysignificantvariationinetchrateastheTMAH
siliconinterfaceapproachesthedepletionregionorenhancedcarrierregionofthe
siliconcausedbytheelectretcharge.
3.6.5 ConfirmationofEnvironmentalEtchConditions
A uniform distribution of heat was confirmed via thermal imaging of the
aluminumlidandPTFElidsremoved.Tocapturetheseimages,thesampleswere
allowed to reach steady state, which took about 15 minutes, as the assembly was
heated to 70°C on the hot plate. A partial immersion thermometer was used to
confirm that the temperature of the solution was uniform amongst the test cells.
While the temperature set point for the hot plate was 70°C, all of the sample
solutionsmeasured60°C+/‐1°C.
3.6.6 ElectretStabilityDuringAnisotropicEtching
Duetothesusceptibilityoftheelectretchargetobeneutralizedwhenexposedto
liquids,itwasimportanttomonitortheESVduringtheanisotropicetchingprocess.
Therefore, the last set of etching experiments, done with the third wafer in the
study, were performed in order to measure the ESV from the electret side before
andafterthesiliconetchstep.Utilizingtheopeninginthestainlesssteelbaseofthe
etchcell,theESVmeasurementswerecollectedoneachdieinthethirdtrialbefore
thesiliconetchproceeded.ThisESVmeasurementwasconsideredamodifiedESV
duetotheinterferenceintheelectricfieldcausedbythesurroundingmetalofthe
88
groundedwaasherthrou
ughwhichth
hemeasureementwasb
beingtaken,Figure49.The
measuremen
m
nt procedurre of the mo
odified ESV
V was follow
wed again aat the end o
of the
ettching proccedure to confirm thatt the electrret charge w
was still in
n place after the
ettchingproccess.Dueto
oresolution
nlimitationsoftheelectrostaticvvoltmeter,ittwas
sttillnotposssibletodeteerminethe durabilityooftheelectrretchargein
nregionsw
where
th
hecompleteebackingoffthesubstratehadbeeenetchedth
hrough.Thiisinformatiionis
taabulatedintheresultstoprovideevidenceth
hattheelecttretcharge wasmaintaained
th
hroughoutttheetchingprocess.
Figure 49. A
A "Modified
d" ESV meaasurement is made affter each ettching perio
od to
co
onfirmthattheelectrettisstillactive.
89
4 RESUL
LTS
Si3N4/SiO2/Si3
/ 4 multtilayer elecctrets weree successfu
ully fabricaated by PE
ECVD
depositionandsubsequ
uentthermaallyassistedpolingusin
ngthetechn
niquesdescrribed
in
nChapterIIII.Thetherrmallyassisstedpoling processwaasoptimizeed,witha23full
faactorial DO
OE and two
o single faactor DOEs,, to achievve the maxximum ESV
V for
negatively charged
c
eleectrets. Th
he ESV of the electrret was ch
haracterized
d by
issothermalp
potentialdeccay(ITPD)todetermin
netheactivaationenerggyofthetrapped
ch
harges in the
t
electrett and extraapolate thee mean lifeetime of th
he ESV at rroom
teemperaturee and operaating tempeeratures. In
n addition, the implem
mentation o
of the
electrets as an anisotro
opic electrochemical ettch stop are presented
d and descrribed
below.
4.1
4 WaferrandFilmDimensio
ons
The
T PECVD SiO2/Si3N4/SiO
/ 2 multiilayer was deposited to a nomin
nal thickness of
1650nm/25
50nm/28nm
mrespectiv
velyasmeassuredbycontactprofilometryofssingle
laayer films on
o silicon su
ubstrates. The substraates were P
P‐type, boro
on doped, ((100)
siilicon waferrs (d = 100
0 mm, t = 525
5 m) w
with a modeerate dopin
ng resultingg in a
reesistivityoff1‐10ohm‐cm.Theressultingsiliconoxidedeepositionfor25second
ds,25
90
minutes,
m
and 30 minu
utes is sho
own in Tab
ble 2. Thee resultingg silicon niitride
depositionfo
or5minutesand30miinutesisshoowninTablle3.
Table2.Silic
T
conoxideth
hicknessmeeasurementts,meanand
dstandardd
deviation.
Deposition D
Time 25 sec 25 min 30 min #1 28 1605 2069 Silicon O
Oxide Thickne ss (nm) Me
easurement LLocation on W
Wafer #2 #3 #
#4
#5 29 29 26 2
27
1711 17
713 172 1 1578
2045 19
926 18977 2066
Average (nm) 27.8 1665.6 2000.6 Std dev (nm) 1.2 61.2 73.8 Table3.Silic
T
connitridetthicknessm
measuremen
nts,meanan
ndstandarddeviation.
Deposition D
Time 5 min 30 min #1 45 263 Silicon Niitride Thickneess (nm) Measurement Location
#2 #3 #
#4 #5
41
43 43 4
44 245 25
54 2488 252
Average (nm) 43.2 252.4 Std dev (nm) 1.3 6.2 4.2
4 CharacterizationandOpttimizationofElectreetFormatiionProcesss
The signiificance of temperaturre, process time, and aapplied volttage on thee ESV
produced by
y the thermally assisted poling prrocess was investigated
d using a 23 full
E.EachoftthenineSiO
O2/Si3N4/SiO
O2PECVDco
oatedwaferrswassubjeected
faactorialDOE
to
o thermally
y assisted poling in thee SUSS bon
nder with th
he smaller 2
2” wafer on
n top
actingastheecontactcathode.IntthisDOE,triialswereru
unwithall combinatio
onsof
processtimeesofonean
ndfivehours;processttemperatureesof170°C
Cand190°C;and
appliedactiv
vationpoten
ntialsof150
0Vand300
0V.Anadd
ditionaltriallwasperforrmed
with
w
a midp
point valuee of three hrs. 180°C
C, and 240
0 V for thee process time,
teemperaturee and applieed activatio
on potential, respectivvely. The aaverage of tthe 5
91
peak values for each wafer was used to provide a meaningful value that is
representative of an area significant to provide a yield of useful devices in a
fabrication process. The results for the ANOVA of the 23 full factorial DOE are
presented in Table 4. The five peak negative ESV values measured in the ESV
contourmappingoftheeachsurfacewereaveragedasthedatapointsofthe“cube”
plot,Figure50;“interactioneffects”plot,Figure52;and“maineffects”plot,Figure
51. Evaluation of the P‐values for the ANOVA on the 23 full factorial DOE for
negativeESVsprovidethefollowing:
1) curvature(ornonlinear)effectswerenotsignificant;
2) thetime‐temperatureinteractionswereverysignificant;
3) thetemperature‐appliedvoltageinteractionwasmarginallysignificant;
4) appliedvoltagemaineffectwereverysignificant;
5) timemaineffectwasmarginallysignificant.
Amaximumeffectivesurfacevoltageof‐236.2Vwasachievedatthemaximum
activationtemperatureof190°Cwiththemaximumappliedprocessvoltageof300
VandthemaximumprocesstimeoffivehoursintheDOE.Fromthisresult,itwas
concluded that increasing the ESV by extending these process parameters could
provideinsighttotheprocessinglimitsforelectretactivation.
92
Table4.FactorialRegression:Peakversustime,temperature,voltage,CenterPt
Analysis of Variance
Source
DF
Adj SS
Adj MS F-Value P-Value
Model
8 46839.6
5855.0
6.20
0.000
Linear
3 24533.1
8177.7
8.66
0.000
time
1
7344.1
7344.1
7.78
0.008
temperature
1
624.1
624.1
0.66
0.422
voltage
1 16564.9 16564.9
17.54
0.000
2-Way Interactions
3 20261.1
6753.7
7.15
0.001
time*temp
1 17056.9 17056.9
18.06
0.000
time*voltage
1
348.1
348.1
0.37
0.548
temp*voltage
1
2856.1
2856.1
3.02
0.091
3-Way Interactions 1
828.1
828.1
0.88
0.355
time*temp*voltage
1
828.1
828.1
0.88
0.355
Curvature
1
1217.3
1217.3
1.29
0.264
Error
36 33999.6
944.4
Cube Plot (fitted means) for Peak ESV (V)
Centerpoint
Factorial Point
-110.2
190
-163.6
-152.8
-236.2
-186.2
-167.2
Temperature
(C)
-156.2
-180
Applied Voltage (V)
-176.8
-300
5
-194.2
170
1
Time (hrs)
Figure50.NegativepotentialESV“cube”plotofthe23fullfactorialDOEgiventhe
effectsoftemperature(°C),time(hrs),andappliedvoltage(V)ontheESV(V).The
averages of the five peak ESV values for each wafer are shown at the corners and
centeroftheplot.
93
The“maineeffects”plot ofthenegaativeESVassafunction
noftemperaature
Figure51.T
ours),applieedvoltage((V).
(°C),time(ho
Interaction Plot fo
or Peak ESV (V
V)
Fitted Means
M
170
180
0
190
Temperature * Time (hrs)
Applied Volt * Time
T
(hrs)
Time
(hrs)
1
3
5
-150
M
Mean
off Peak
P k
-175
-200
Time (hrs) * Te
emperature
Applied Volt * Te mperature
-150
-175
Point Type
Corner
Center
Corner
Temperature
170
180
190
Point Type
Corne
er
Cente
er
Corne
er
-200
Time (hrs) * Applied
A
Volt
Temperature * Applied
A
Volt
Applied
Volt
-300
-240
-180
-150
-175
-200
1
3
5
Time (hrs)
-300
Tempera
ature
-240
0
e
Point Type
Corner
Center
Corner
-180
Applied Volt
Figure 52. Negative potential
p
ES
SV “interacction” plot of temperaature (°C), time
(h
hours),appliedvoltagee(V).
Oncethe 23fullfacttorDOEhad
dbeencom
mpleted,itw
wasimportaanttoextendthe
raange of the applied temperaturre and app
plied voltagge process parameterrs to
94
determine the optimal process parameters for the ESV of the electret. A single
factor DOE was performed for the applied temperature and applied voltage
parameters by running additional experiments extending the testing range of the
appliedtemperaturefrom70°Cto230°Candtheappliedvoltagefrom60Vto540V.
Full wafers were processed in the SUSS bonder using the standard PECVD
SiO2/Si3N4/SiO2 (1650 nm/250 nm/28 nm) on a 100 mm p‐type silicon substrate
used in the initial 23 full factorial DOE. The top five negative ESV measurements
from the full contour plots are averaged resulting in a single data point for each
wafer in the extended single factor DOEs The single factor DOE for applied
temperatureshowedthattheappliedtemperaturelinearlyincreasedovertherange
of 70°C to 210°C (Figure 53). However around 190°C, the ESV did not display a
linearrelationshipandtendedtodeteriorateastheprocesstemperatureincreased
further to 230°C. This could be attributed to applied voltages approaching the
breakdown voltage of the dielectric PECVD film with applied voltages at elevated
temperatures.Thiswouldbeanexpectedlimitsincethehighelectricfieldsinthe
PECVD film have been shown to be responsible for the generation of charge
trapping sites in the electret [102]. The additional traps generated at elevated
temperaturesdecreasestheresistivityofthePECVDfilm,resultinginacurrentload
beyondthecapacityoftheSUSSbonderpowersupplyattheappliedvoltage.The
extendedsinglefactorplotofESVduetoappliedvoltageisshowninFigure54.The
singlefactorDOEfortheappliedprocessvoltageillustratedacleartrend,wherean
increaseintheappliedprocessvoltageresultedinanincreaseinESV.Whileithas
95
beendocumentedthatelectricfieldstressingofthedielectricisresponsiblefortrap
generation[66],thesetrapsmaynotbefilledbymobilechargesunlesstheelectret
isprocessedathighertemperatures.Additionally,thereisariskthatahighelectric
field induces a breakdown in the PECVD film. Once a low resistivity path is
produced in the thin film it is not possible to maintain the high potential during
activation and this limits both the yield and ESV produced. This occurred at the
highest applied voltage of ‐540 V. The first trial at this applied potential had an
abruptdropduringtheactivationprocessandthemaximumcurrentsuppliedbythe
SUSSbonderwasinsufficienttomaintainthe‐540V.TheresultingESVofthisfirst
trialwas‐112.2V.Asecondtrialwasrunsuccessfullyproducinganaveragepeak
ESV of ‐226.8 V, providing the highest mean ESV for all samples produced
throughoutthesestudies.Basedontheassumptionsandcalculationspresentedin
section 2.6‐“Effective Surface Voltage Electret Measurement” and Equation 13, an
effectiveplanarchargedensityof4.7mC/m2wasproducedforthisESVof‐226.8V
assumingthatthechargeresidesatthesiliconnitride‐siliconoxide(blockinglayer).
Atrialhadbeenconductedatanappliedpotentialof‐600V,datanotshown,butthe
sample suffered a complete electrical breakdown. The SUSS bonder supplied the
maximum10mAcurrentbuttheresultingappliedpotentialwas0V.Themaximum
effectivesurfacevoltage‐226.8Vwasproducedwithablockingoxideproducedby
PECVD and 1650 nm thick. The maximum ESV was limited by this electrical
breakdownoftheblockingoxide.ThemaximumproducibleESVcanbeincreased
96
byincreasingthethicknessofthePECVDblockingoxideorbyproducingablocking
Effective Surface Voltage (V)
oxidewithahigherbreakdownvoltage.
‐200
‐180
‐160
‐140
‐120
‐100
‐80
70
110
150
190
230
Applied Processing Temperature (oC)
Figure53:Effectivesurfacevoltageasafunctionofappliedprocesstemperaturefor
Effective Surface Voltage (V)
samplesactivatedat170°Cforfivehours.samplesactivatedat180Vforonehour.
‐250
‐200
‐150
‐100
‐50
0
60
240
420
600
Applied Processing Voltage (V)
Figure 54: Electret surface voltage as a function of applied process voltage for
samplesactivatedat170°Cforfivehours.
As previously described in Chapter III, contour maps of the activated electret
wafers were created for each variation in process parameters (Figure 55 ‐Figure
97
61).TheaverageofthefivemaximumESVvaluesforthesamplerunateachDOE
process parameter is represented at the corners and center of the 23 full factorial
DOEcube.Inexaminingthecontourmaps,itcanbeseenthattheeffectivesurface
voltagematchesthepolarityofthecontactwaferwhenthecontactwaferwasused
asthecathode.Theregionsofthecontactwaferandelectretfilmonthesubstrate
acted as a poor conductor, resulting in the ESV having the same polarity as the
appliedpotential.Inaddition,regionsofapositiveESV,oppositethepolarityofthe
cathodepolaritywerealsoseen;however,theseoccurredinareasthatwerenotin
directcontactwiththecathodiccontactwafer.TheregionsofpositiveESVweredue
tothegapbetweentheuppercathodeelectrodeandthesiliconsubstrateactingasa
capacitor.
As a result, two polarities, positive ESV and negative ESV, were created on the
electret surface in a single process of applying a negative applied voltage during
activation. This dual‐polarization was accomplished in regions of contact verses
noncontact applied voltages provides a great opportunity for enabling the
programmingofelectrostaticfieldsonthesurfaceoftheelectret.Forexample,this
processmaybeusedtopattern(orstamp)aspecificelectricfieldonadevicefora
particular application, such as energy harvesters [15] and acoustic sensors [44].
Additional studies are required to determine the limitations of this programming
capabilitytotakeintoaccounttheeffectsoffeaturesize,fieldmagnitude,andcharge
bleedingonthesurfaceoftheelectret,butthesewerenotinvestigatedatthistime.
98
Surface Voltage Plot
300V, 5 hr, 190C
7
ESV (V)
< -180
– -140
– -100
–
-60
–
-20
–
20
–
60
–
100
–
140
–
180
> 180
6
-180
-140
-100
-60
-20
20
60
100
140
Y-Axis (cm)
5
4
3
2
1
0
0
1
2
3
4
X-Axis (cm)
5
6
7
Figure 55. ESV contour plot with electret activation at ‐300 V, 5 hrs, 190°C. The
colorcodedlegendontherightprovidesthescaleoftheESVat20Vincrements. Surface Voltage Plot
300V, 1 hr, 170C
7
ESV (V)
< -180
– -140
– -100
–
-60
–
-20
–
20
–
60
–
100
–
140
–
180
> 180
6
-180
-140
-100
-60
-20
20
60
100
140
Y-Axis (cm)
5
4
3
2
1
0
0
1
2
3
4
X-Axis (cm)
5
6
7
Figure 56. ESV contour plot with electret activation at ‐300 V, 1 hrs, 170°C. The
colorcodedlegendontherightprovidesthescaleoftheESVat20Vincrements.
Surface Voltage Plot
300V, 5 hr, 170C
7
ESV (V)
< -180
– -140
– -100
–
-60
–
-20
–
20
–
60
–
100
–
140
–
180
> 180
6
-180
-140
-100
-60
-20
20
60
100
140
Y-Axis (cm)
5
4
3
2
1
0
0
1
2
3
4
X-Axis (cm)
5
6
7
Figure 57. ESV contour plot with electret activation at ‐300 V, 5 hrs, 170°C. The
colorcodedlegendontherightprovidesthescaleoftheESVat20Vincrements. 99
Surface Voltage Plot
180V, 1 hr, 190C
7
ESV (V)
< -180
– -140
– -100
– -60
– -20
–
20
–
60
– 100
– 140
– 180
> 180
6
-180
-140
-100
-60
-20
20
60
100
140
Y-Axis (cm)
5
4
3
2
1
0
0
1
2
3
4
5
6
7
X-Axis (cm)
Figure 58. ESV contour plot with electret activation at ‐180 V, 1 hrs, 190°C. The
colorcodedlegendontherightprovidesthescaleoftheESVat20Vincrements. Surface Voltage Plot
180V, 5 hr, 170C
8
ESV (V)
< -180
– -140
– -100
– -60
– -20
20
–
60
–
– 100
– 140
– 180
> 180
7
-180
-140
-100
-60
-20
20
60
100
140
Y-Axis (cm)
6
5
4
3
2
1
2
3
4
5
6
7
8
X-Axis (cm)
Figure 59. ESV contour plot with electret activation at ‐180 V, 5 hrs, 170°C. The
colorcodedlegendontherightprovidesthescaleoftheESVat20Vincrements. Surface Voltage Plot
300V, 1 hr, 190C
7
ESV (V)
< -180
– -140
– -100
– -60
– -20
20
–
60
–
– 100
– 140
– 180
> 180
6
-180
-140
-100
-60
-20
20
60
100
140
Y-Axis (cm)
5
4
3
2
1
0
0
1
2
3
4
X-Axis (cm)
5
6
7
Figure60.ESVcontourplotwithelectretactivationat‐300V,1hr,190°C.Thecolor
codedlegendontherightprovidesthescaleoftheESVat20Vincrements. 100
Surface Voltage Plot
180V, 1 hr, 170C
7
ESV (V)
< -180
– -140
– -100
–
-60
–
-20
–
20
–
60
–
100
–
140
–
180
> 180
6
-180
-140
-100
-60
-20
20
60
100
140
Y-Axis (cm)
5
4
3
2
1
0
0
1
2
3
4
X-Axis (cm)
5
6
7
Figure61.ESVcontourplotwithelectretactivationat‐180V,1hr,170°C.Thecolor
codedlegendontherightprovidesthescaleoftheESVat20Vincrements. Surface Voltage Plot
225V, 3hr, 180C
7
ESV (V)
< -180
– -140
– -100
–
-60
–
-20
–
20
–
60
–
100
–
140
–
180
> 180
6
-180
-140
-100
-60
-20
20
60
100
140
Y-Axis (cm)
5
4
3
2
1
0
0
1
2
3
4
X-Axis (cm)
5
6
7
Figure 62. ESV contour plot with electret activation at ‐225 V, 3 hrs, 180°C. The
colorcodedlegendontherightprovidesthescaleoftheESVat20Vincrements. Surface Voltage Plot
180V, 5 hr, 190C
7
ESV (V)
< -180
– -140
– -100
-60
–
–
-20
–
20
–
60
–
100
–
140
–
180
> 180
6
-180
-140
-100
-60
-20
20
60
100
140
Y-Axis (cm)
5
4
3
2
1
0
0
1
2
3
4
X-Axis (cm)
5
6
7
Figure 63. ESV contour plot with electret activation at ‐180 V, 5 hrs, 190°C. The
colorcodedlegendontherightprovidesthescaleoftheESVat20Vincrements. 101
The23fullfactorialDOEwasoriginallydesignedtoinvestigatethedependencies
of process temperature, time, and negative applied voltage on the negative ESV
producedduringelectretactivation.ReviewoftheESVcontourmapsshowedthat
not only were there negative ESVs produced as expected, but there were also
regions of positive ESV produced in regions of the wafer that were not in direct
contact with the cathode used to bias the PECVD film. The data from these
experimentswasrepurposedtoinvestigatehowtheseprocessparametersaffectthe
performance of producing a positive ESV with a negative applied voltage. The
resultsfortheANOVAofthe23fullfactorialDOEarepresentedinTable5.Thefive
peakpositiveESVvaluesmeasuredintheESVcontourmappingoftheeachsurface
wereaveragedasthedatapointsofthe“cube”plot,Figure64;“interactioneffects”
plot,Figure65;and“maineffects”plot,Figure66.EvaluationoftheP‐valuesforthe
ANOVAonthe23fullfactorialDOEforpositiveESVsprovidethefollowing:
1) appliedvoltageprovidedamarginallysignificanteffect;
2) thetime,temperature,andappliedvoltageproduceda3‐wayinteractionthat
wassignificant.
102
Table5.FactorialRegression:PeakpositiveESVversustime,temperature,voltage
Analysis of Variance
Source
DF
Mod
8
Linear
3
time
1
temperature
1
voltage
1
2-Way Interactions
3
time*temp
1
time*voltage
1
temp*voltage
1
3-Way Interactions
1
time*temp*voltage
1
Curvature
1
Error
36
Tota
44
Adj SS
100875
14933
17
5244
9672
8004
6052
314
1638
14364
14364
63574
115086
215961
Adj MS
12609.4
4977.7
16.9
5244.1
9672.1
2667.9
6051.6
313.6
1638.4
14364.1
14364.1
63574.0
3196.8
F-Value
3.94
1.56
0.01
1.64
3.03
0.83
1.89
0.10
0.51
4.49
4.49
19.89
P-Value
0.002
0.217
0.942
0.208
0.091
0.484
0.177
0.756
0.479
0.041
0.041
0.000
Cube Plot (fitted means) for Peak ESV (V)
Centerpoint
Factorial Point
32.8
190
12.6
33.2
100.0
82.0
temperature 9.4
15.8
-180
60.0
170
1
1.8
5
time
voltage
-300
Figure 64.Positive potential ESV“cube” plot ofthe 23full factorial DOE given the
effectsoftemperature(°C),time(hrs),andnegativeappliedvoltage(V)ontheESV
(V).TheaverageofthefivepeakESVvaluesforeachwaferareshownatthecorners
andcenteroftheplot.
103
noftemperaature
Figure65.The“mainefffects”plot oftheposiitiveESVassafunction
(°C),time(hrrs),appliedvoltage(V).
Intera
action Plot fo
or Peak ESV (V)
Fitted Means
M
170
180
190
temperature
e * time
8
80
voltag
ge * time
time
1
3
5
Mean of Peak
4
40
0
time * temperat ure
voltage * temperature
40
time * voltage
e
8
80
0
temperature * voltage
4
40
0
80
1
3
time
5
-300
temperatture
-240
vo
oltage
ype
Point Ty
Corner
Center
Corner
temperature
170
180
190
voltage
-300
-240
-180
Point Type
Corner
Center
Corner
Pointt Type
Corne
er
Cente
er
Corne
er
-180
Figure66.P
PositivepotentialESV““interaction
n”plotofteemperature (°C),time((hrs),
andappliedv
voltage(V)..
104
4.3
4 ElectrretCharactterization
Accelerattedagingteestsoftheeelectretsweereperform
medasdescrribedinChaapter
IIII to determ
mine the mean
m
lifetim
me of the ESSV for both
h positivelyy and negattively
activated wafers.
w
The isothermaal potentiall decay (IT
TPD) of th
he electret was
ev
valuated ov
ver time while being maintained
d at a consttant temperature of eeither
200°C, 250°C
C, 275°C, 300°C, 325°C
C. A samplle was testeed at each o
of the five ITPD
teemperaturees from each of eight wafers.
w
Fou
ur wafers w
were activaated in the SUSS
bonder for one
o hour att 170°C witth an appliied voltage of +300 V, while the four
reemainingw
waferswere activatedfo
oronehourrat170°Cw
withanapp
pliedvoltageeof‐
300 V. The five point averaged maximum
m
eeffective surrface voltagge produced for
positive elecctret samplles was 195.2 V, whille the five point averraged maxim
mum
effective surrface voltage produced
d for the neegative electtret samplees was ‐194
4.2 V.
Based
B
on th
he calculatiions presen
nted in secction 2.6‐ “Effective Surface Vo
oltage
ElectretMeaasurement” andEquatiion13,the effectivesu
urfacecharggedensityiis4.0
2forb
mC/m
m
bothpolaritiies,assumin
ngthecharggeisatthe siliconnitriide‐silicono
oxide
(b
blocking lay
yer). This effective surface charrge densityy is superio
or to the ch
harge
densities of organic ellectrets witth parylenee values th
he leading organic ch
harge
density repo
orted as high as 3.7 mC/m2 [1
103]. The ESV was found to d
decay
exxponentially for both the
t positivee and negattive ESV eleectrets. An
n example o
of the
normalizedIITPDdataaandfitcurveeisshowninFigure67
7andFiguree68forsam
mples
frromwafer3
3.Comparissonofthettwofigures showthattthedecayraate,τ,decreeased
fo
orsamplesaagedathigh
hertemperaatures.Theedecayrateeswerecalcculatedbyfiitting
105
themeasuredESVvaluestothebestfitexponentialdecayequationaspresentedin
ChapterIII.TheτvaluesforeachsampleoftheeightwafersarepresentedinTable
6forpositiveESVsamplesandTable7fornegativeESVsamples.Thedecayratesof
wafer 3 and wafer 4 of the positive ESV ITPD can be seen to have a significantly
lower decay rate than wafer 1 and wafer 2 in the positive ESV study and
significantly lower than all of the wafers in the negative ESV study. It is not clear
whytherageofdecayratesforthepositiveESVwafersissobroad.Figure69and
Figure 70 provide plots of the exponential decay rate, τ, as an inverse function of
temperature on a semi‐log plot for positively charged and negatively charged
electretsrespectively.
Normalized Thermally Stimulated Discharge at 300 C Wafer 3
1.2
Normailized ESV = 1.01876 * exp(-time * 1.20544)
Normalized ESV (V/Vo)
1.0
Regression
95% CI
95% PI
0.8
0.6
0.4
0.2
0.0
0
1
2
3
Time (hrs)
4
5
Figure67.TheITPDplotforasamplefromwafer3showstheESVasafunctionof
time, aged at 300°C. The sample was activated at 300 V for 1 hour at 170°C. The
best fit regression to the exponential decay is provided with the 95% confidence
intervaland95%predictioninterval.
106
Normalized Thermally Stimulated Discharge at 325 C Wafer 3
Normalized ESV (V/Vo)
1.0
Normalized ESV = 0.989112 * exp(-time * 1.92092)
0.8
Regression
95% C I
95% PI
0.6
0.4
0.2
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Time (hrs)
Figure68.TheITPDplotforasamplefromwafer3showstheESVasafunctionof
time, aged at 325°C. The sample was activated at 300 V for 1 hour at 170°C. The
best fit regression to the exponential decay is provided with the 95% confidence
intervaland95%predictioninterval.
Table 6. Decay rates for positively charged electrets on four wafers at varying
temperaturestopredictthemeanlifetimeoftheelectrets.
Temperature Wafer1
(°C)
τ(hrs)
Wafer2
τ(hrs)
Wafer3 Wafer4
τ(hrs) τ(hrs)
Average
τ(hrs)
Stddev
τ(hrs)
200
1036.80
943.40
392.20
518.10
722.63
273.16
250
30.81
47.38
36.06
24.15
34.60
8.50
275
6.27
10.70
7.03
5.11
7.28
2.09
300
1.79
2.67
0.83
3.32
2.15
0.94
325
0.35
0.30
0.52
0.98
0.54
0.27
107
Table 7. Decay rates for negatively charged electrets on four wafers at varying
temperaturestopredictthemeanlifetimeoftheelectrets.
Temperature Wafer1
(°C)
τ(hrs)
Wafer2
τ(hrs)
Wafer3
τ(hrs)
Wafer4
τ(hrs)
Average
τ(hrs)
Stddev
τ(hrs)
200
968.45
369.64
976.86
529.20
711.04
267.65
250
57.77
88.81
75.94
84.45
76.74
11.89
275
30.13
13.28
7.37
11.82
15.65
8.64
300
16.43
5.26
4.66
3.64
7.50
5.19
325
1.14
1.16
1.58
1.03
1.23
0.21
108
Exponential Decay Rate (1/hrs)
10
0
1
0.1
0.01
0.001
1.7
1.8
1.9
2.0
1000/Temperaturre (1000/K)
2.1
Figure69.P
Positivelych
hargedelecttret,semi‐loogplotofth
heexponenttialdecayraatevs
1000/Tempeerature. Samples
S
fro
om each oof the 4 w
wafers werre subjecteed to
acceleratedaagingat325
5°C,300°C,2
275°C,250°°C,and200°°C.
Negativelycchargedelectret,semi‐‐logplotof theexponeentialdecayyrate
Figure70.N
mperature. Samples from each
h of the 4 wafers were subjecteed to
vs 1000/Tem
acceleratedaagingat325
5°C,300°C,2
275°C,250°°C,and200°°C.
Mean lifeetime values for both positive
p
an d negative ESV were eextrapolated for
mperatures tested.Theeseextrapolated
teemperatureesbelowtheeaccelerateedagingtem
109
values arebased on calculations of the activationenergy for positive andnegative
ESV samples within the range of the accelerated aging temperature range. The
calculatedactivationenergyalsoprovidesinsightandconfirmationtothecauseand
locationofthechargetrapsintheelectretinconjunctionwithpublishedvaluesfrom
othergroups[55].Thebestfitoftheexponentialdecayconstantforthepositively
chargedelectretis
.
1.2869 10
.
Equation37
The activation energy of the positive electret is 1.41 eV. The coefficient of
determination(R‐sq)fortheexponentialdecayrateofthepositiveelectretis97.3%.
Thebestfitoftheexponentialdecayconstantforthenegativelychargedelectretis
8.356 10
.
.
Equation38
The activation energy of the negative electret is 1.20 eV. The coefficient of
determinationfortheexponentialdecayrateofthenegativeelectretis94.8%.The
isothermalpotentialdecayoftheelectretisextrapolatedbyevaluatingEquation37
and Equation 38 at a temperature of 125°C. The positive electret has a mean
lifetime value of 57.7 years, while the mean lifetime value of the negative electret
was23.9yearsifmaintainedinanenvironmentat125°C.Therateofdecayforthis
multilayerelectretis5timesslowerthantheelevatedtemperaturerateofdecayfor
PECVDelectretlayersproducedbyothergroups[2,51].
110
Stoichiomeetric Si3N4 produce
p
traaps with acttivation eneergy levels of 1.4 eV, w
while
energy level of traps in
n silicon ricch LPCVD siilicon nitrid
de have beeen measureed by
other group
ps to be in
n the 0.80 eV
e range [5
55]. The eexperimentaally determ
mined
activation en
nergy requirements of the multi layer PECV
VD oxide/niitride film were
1.41 eVand 1.20 eVforr positiveeelectrets and
d negative electrets reespectively. The
valuesappeaarreasonab
blegiventhedocumenttedeffects ofahighhyydrogencon
ntent
fo
or our PECV
VD silicon nitride
n
film
m and, the u
unquantified
d effects that the nitride is
onlypartofaamultilayerrstackwith
hPECVDsili conoxidelaayers.
4.4
4 AppliccationofE
ElectretasanElectro
ochemicallEtchStop
p
After succcessfully fabricating
f
the electrets, it wass desired tto explore new
applications that woulld benefit from the implementaation of th
he electret.. In
particular,th
hisstudyexxploredtheuseofthe electretas partofan electrochem
mical
ettchstopinttheanisotro
opicetchinggofsiliconw
withTMAH..Theelectrretwasused
dasa
reeplacementtforanexteernalbiasty
ypicallyused
dinaMISeetchstoppro
ocess.Thistype
of bias has been supp
plied in MIS and p‐n
n junction electrochem
mical etch stop
m of externaal electrodees, as presented in thee backgroun
nd of
teechniques in the form
seection3.6–
–“Applicatio
onofElectretasanEleectrochemiccalEtchStop
p”.Withouttthis
bias, chemiccal etching in the [100
0] silicon diirection wo
ould continu
ue unabated. A
model
m
of thee etch band
d diagram for
f the impllemented eetch is show
wn in Figure 71.
Withthebia
W
as,adepletionregionisproduced
dandthean
nodicpartiaalcurrentth
hatis
generated during
d
etchiing between the subsstrate and electrolyte are elimin
nated,
111
bringingtheetchingprocesstoasstopinproxximitytoth
hedepletion
nregion[19
9,98,
99].
AbanddiagrramofatheeMISetchsttopwithbiaasbetweengateandsiilicon
Figure71.A
and in open
n circuit po
otential between the silicon and
d electrolyyte, alkali b
based
ettchant, show
wing the drrift of electrrons into th
he electrolyyte and continuing the etch
process with
h hydroxyl ion generation. Thee figure is modified ffrom Smith
h and
Soderbarg19
993[19].
The
T raw datta of the av
verage etch
h depth meaasurementss from the electrochem
mical
ettch study outlined
o
in
n section 3.6.4 ‐ ”Elecctrochemicaal Etch Sto
op Timed SStudy
112
Overview”ispresentedinFigure72withthedataprovidedinTable8,Table9,and
Table10.Themeanfinalsiliconetchrateofthepositiveelectretsampleswas11.9
μm/hr while the mean final silicon etch rate for neutral and negative electret
samples were 8.6 μm/hr and 8.8 μm/hr respectively. The initial ESV of each
respectivesampleispresentatthebottomofthetable.AdditionalqualitativeESV
measurementsweretakenofthesamplesintrial3withthesamplesloadedintothe
etchcells.Duringtheetchstopexperimentsfortrial1and2,onequestionthatwas
raised was the integrity of the electret during the etching process. Specifically,
preliminarystudiesindicatedthatwhentheelectretcameintocontactwithafluid,
theelectretisneutralized.Thus,intrial3,theexperimentalprotocolwaschanged
toincludethemeasurementoftheelectretwhilemountedintheetchcelltoconfirm
that the electret was maintained throughout the etching process. The qualitative
measurements were made before etching and once the etching process was
completedwiththeintentionofconfirmingthattheintegrityoftheelectretcharge
was maintained throughout the etching process. The results of the these
measurements, Table 11, show that 67% of the samples maintained a significant
portionoftheirESVevenoncetheetchanthadetchedthroughthesilicontoPECVD
multilayer film in the cavity regions of the substrate. This is an acceptable yield
consideringthelargeremovalofmaterialandthemaskingofchargepossibledueto
etchsolutionindirectcontactwiththebacksideofelectret.
113
Scatterplot of Etch Depth vs Remaining Etch Time
Etch Depth (micrometers)
375
Charge
Negative
Neutral
Positive
350
325
300
275
250
0
2
4
6
Remaining Etch Time (hrs)
8
10
Figure 72. A linear best fit for etch depth samples as a function of the time
remainingtotheendofthesiliconetcharegroupedbyelectretpotentials;positive,
negative,andneutral.
114
Table8.Trial1etchdepthvstimeforneutral,negative,andpositiveelectrets.
Neutral Trial 1 Die 1 Die 2* Negative Die 3 Time (hr) Die 1 Die 2 Positive Die 3 Die 3 0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 18 300.3 330.7 284.7 239.9 289.3 335.4 293.2 264.8 318.4 19 310.5 292.8 250.3 298.0 350.4 304.5 276.0 331.4 20 318.0 297.9 257.1 303.2 357.0 312.4 282.4 338.2 21 332.7 307.1 267.3 311.0 365.8 324.7 290.9 350.8 22 342.6 314.9 273.2 320.2 368.6 333.5 301.0 359.6 23 358.1 327.2 281.1 330.1 346.0 313.6 372.0 24 368.0 334.7 288.6 338.5 358.2 323.7 25 342.0 295.9 348.3 370.2 337.1 26 352.5 302.8 357.8 349.0 27 360.3 313.7 369.6 365.9 28 366.0 323.1 29 336.2 30 346.2 31 355.1 32 368.7 ESV (V) 0 0 0 ‐150 ‐250 ‐150 150 150 250 115
Die 2 Etch Depth (microns) *Die2oftheneutraldiesetfracturedintheetchcell.
Die 1 Table9.Trial2etchdepthvstimeforneutral,negative,andpositiveelectrets.
Neutral Trial 2 Die 1 Die 2 Negative Die 3* Time (hr) Die 1 Die 2 Positive Die 3 Die 1 Die 2 Die 3 Etch Depth (microns) 0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 18 274.2 284.7 288.9 206.8 208.9 217.0 289.3 264.7 271.6 19 279.1 293.0 299.5 277.8 280.6 20 286.9 300.0 306.0 285.2 288.7 21 296.1 309.6 316.9 298.5 299.7 22 308.5 326.2 323.5 328.3 314.2 309.8 23 317.4 329.6 328.2 338.7 324.4 318.9 24 326.8 338.2 336.0 349.4 337.5 329.8 25 335.3 348.6 344.0 360.5 346.1 339.6 26 345.0 358.6 352.9 359.6 350.3 27 353.6 367.8 361.4 361.5 28 364.5 ESV (V) 0 0 0 ‐150 ‐225 ‐150 175 150 250 *Die3oftheneutraldiesetfracturedintheetchcellandwasreplacedwitha
neutraldiefromanextrawafer.
116
Table10.Trial3etchdepthvstimeforneutral,negative,andpositiveelectrets.
Neutral Trial 3 Die 1* Die 2 Negative Die 3 Time (hr) Die 1 Die 2 Positive Die 3 Die 3 0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 18 223.9 312.5 295.7 282.8 298.9 288.4 288.5 299.5 285.7 19 319.4 303.3 290.2 310.9 297.9 298.4 310.3 294.3 20 327.6 309.6 297.9 318.8 302.7 306.6 319.1 302.8 21 334.7 318.4 305.7 327.4 308.7 316.7 329.9 313.0 22 344.9 326.3 316.9 338.7 320.4 326.7 340.2 321.4 23 351.4 333.9 323.1 345.6 328.5 333.3 348.8 330.0 24 361.4 344.7 329.9 358.7 335.0 345.9 360.9 340.7 25 355.5 337.3 369.6 342.4 354.3 350.5 26 361.8 346.5 351.0 360.7 27 355.4 358.7 28 361.6 366.4 ESV (V) 0 0 0 ‐150 ‐160 ‐150 160 170 150 117
Die 2 Etch Depth (µm) *Die1oftheneutraldiesetfracturedintheetchcell.
Die 1 Table11.ESVmeasurementsforetchtestdieintrial3.
Neutral
Negative
Positive
Die1
Die2
Die3
Die1
Die2
Die3
Die1
Die2
Die3
Die
ESV(V)
0
0
0
‐150
‐160
‐150
160
170
150
DieinEtchCell
InitialESV(V) FinalESV(V)
0
0
0
0
0
0
‐65
‐20
‐110
‐80
‐92
0
133
100
123
98
47
0
A one‐way ANOVA was performed to determine if there a significant difference in
theetchratebetweentheelectret,positive,neutral,andnegativeESVgroupsinthe
last stages of etching before reaching the electret. The final etch rate was
determinedbytakingthedifferencebetweenthefinaletchdepthmeasurementand
the prior etch depth measurement. The mean value of the final etch rate for the
positiveESVelectretis11.9μm/hrversesthemeanvalueofthefinaletchratefor
neutralandnegativeESVelectretsof8.6μm/hrand8.8μm/hrrespectively,Table
12. The “Null Hypothesis” is thatthereis nostatistical differencein the final etch
ratebetweensamplesgroupedbyelectretpotential.Thenullhypothesisistested
byevaluatingtheP‐Valuesofthefinaletchratesgroupedbychargewith3separate
2 sample T‐test using MINITAB. The P‐value, which represents the probability of
obtaining repeated differences between groups of data, is presented for the final
etch rate between positive‐neutral, negative‐neutral, and negative‐positive ESV
groups, Table 13. By “null hypothesis”, no significant difference in etch rate was
118
identifiedbetweenthenegativelychargedelectretandneutralsamples;theP‐value
of the ANOVA between the two sample groups was over 0.90. However, the final
etch rate of silicon for the positive electret was found to be significantly different
thanboththeneutralandnegativeelectrets.TheP‐valuefor theANOVAbetween
the positive electret and neutral samples was less than 0.01 and the P‐value
betweenthepositiveelectretandneutralelectretwas0.07.Asafinalanalysisofthe
nullhypothesis,allthreeofthechargegroupswerecomparedsimultaneouslyusing
anANOVAwitharesultingP‐valueof0.042andaresultingstrongsignificancetothe
rejectionofthenullhypothesis.
Table 12. Final silicon etch rate grouped by neutral, negative, and positive ESV
electret.
Final Etch Rate (μm/hr) Mean Standard Deviation Neutral Negative
Positive 8.6 8.8 11.9 1.9 3.9 2.3 Table13.NullHypothesisPValuesfortheFinalEtchRatewith3separate2sample
T‐testusingMINITAB
P Value Neutral Positive Positive Negative
0.009 0.903 ‐ 0.073 Asindicatedintherawdata,andconfirmedby“nullhypothesis”tests,asurgein
etch rate occurred at the end of the etching process for the positively charged
electrets.Onepossibleexplanationforthisbehavioristhattherewasanincreasein
119
electrons available in the silicon at the silicon‐electret interface due to the strong
inversion of the gate bias. This abundance of electrons in the p‐type silicon
provided a “stock” of electrons available to the electrolyte interface as the silicon
layer thins; thereby, adjusting the Fermi level in accordance to the changing open
circuit potential OCP. The extra supply of electrons diffused to the silicon‐
electrolyte interface as a minority carrier and was injected into the electrolyte
producing a short term increase in OH‐ groups and silicon etching [19]. This is a
behaviorsimilartotheetchrateonewouldseeinacathodicallybiasedsiliconetch
inabasesolution[104].
It is customary to run an electrochemical etch stop with a controlled positive
potential applied between the silicon substrate and the electrolyte. With this
positive bias, electrons are drawn to the silicon‐gate interface and away from the
electrolyte,breakingtheetchcyclewhichrequiresacontinuoussupplyofhydroxyl
ionsforsilicontoetch[19].Withoutthisbiasbetweenthesubstrateandelectrolyte,
Figure71,thefreeelectronsthataregeneratedduringtheetchprocessreturntothe
analyteandreducethewaterproducingadditionalhydroxylionsforsiliconetching
andhydrogengasasabyproduct.Atmost,withnoappliedbiasbetweenthesilicon
and electrolyte, a transient etch rate response can be expected due to the electric
fieldproducedbytheelectret.Itisreasonablethatthepositivelychargedelectret
elicitsa fasteretch rate thantheneutral and negative electret in the final stage of
thesiliconetching.AsolepositivebiasofthegateintheMISetchstopprovidesa
120
rich source of electrons as the silicon at the interface of the gate is in strong
inversionandpastthethresholdvoltage,Vth.[19].
121
5 CONCLUSIONS
TheelectretactivationprocessforathinmultilayerSiO2/Si3N4/SiO2PECVDfilm
has been optimized using a silicon wafer as the contact electrode in a thermally
assisted poling process. Effective surface voltages in excess of ‐225 V were
produced with cathodic contact using processes and equipment standard to
microfabrication facilities. The optimum process for maximizing the negative ESV
wasdeterminedtobefivehoursat170°Cwithanappliedvoltageof‐540Vwiththe
SUSSSB‐6e.ThemaximumfivepointaveragedESVproducedforpositiveelectret
sampleswas195.2V,whilethemaximumfivepointaveragedESVproducedforthe
negativeelectretsampleswas‐194.2V.Theactofsinglepolaritychargingproduced
regions of positively and negatively charged electret where the regions in direct
contact with the silicon contact electrode during the activation step developed an
ESVcomplementarytothepolarizationoftheelectrode.Ontheotherhand,regions
not in direct contact developed a charged ESV opposite to the polarization of the
electrode during activation. Longer activation periods resulted in larger
complementingandopposingESVvalues.
Characterization of the films by isothermal potential decay show that the
inorganic positive electret has an extrapolated mean lifetime value of 57.7 years;
whiletheextrapolatedmeanlifetimevalueofthenegativeelectretwas23.9yearsat
122
acontinuoustemperatureof125°C.Thermalneutralizationoftheelectretthrough
isothermalpotentialdecaystudiesdemonstratedanactivationenergyof1.4eVand
1.2eVforthepositiveelectretsandnegativeelectrets,respectively.
When using the electret as a MOS biased etch stop, the data demonstrated the
positivelychargedelectretproducedastatisticallysignificantincreaseinetchrate,
when compared to the neutral and negatively charged electrets. However, overall
the MOS bias caused by the electret did not produce a definitive etch stop for the
configurationpresentedinthisstudy.Theseresultssuggestthat,toactasanetch
stop, a positive bias must be maintained on the silicon wafer with respect to the
etchant.
123
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CURRICULUMVITAE
MARKCRAIN
PROFESSIONALEXPERIENCE
Research Engineer/Graduate Teaching Assistant‐‐‐University of Louisville,
2011‐Present
 Completing Ph.D. in Electrical Engineering under the support of the
Bioengineering Department “The Development of Multilayer Thin Film
ElectretsforMicrofabricationProcessing”
 MicrofabricationSpecialistfor“OpticalandElectricalPhytoplanktonDetection,
Size,andCountFlowCytometryOnaChip”
CleanroomManager/ResearchEngineer‐‐‐UniversityofLouisville,1999‐2011
 Developed the University of Louisville MicroNano Technology Center to
provide basic microfabrication services to Universities, government and
industry‐‐bothlocalandinternational.
 Managed4technicalstaff,2administrativestaff,and3‐5studentworkstudies.
 Team member for the design and installation of the new cleanroom facility
locatedintheUniversityofLouisvilleBelknapResearchBuilding
 Produced numerous contributions in the development of microfabrication
technologyforgrantfundedresearch.Areasofextensiveresearchexperience
includecapillaryelectrophoresis,capacitivepressuretransducers,thermaland
opticalmicrophones,chemicalpre‐concentrators,andradiationdetectors.
 Managed capital equipment selections, purchases, and installations.
Committee member determining equipment needs. Managed “Requests for
Proposals”fromequipmentvendors.Managedcommissioningandinstallation
termsandacceptancetesting.
 Operated an active outreach program, brought many thousands of young
students(K‐12)throughthecleanroomprovidinginteractivetours.
 Developed and implemented safety protocols with the support of University
HealthandSafety.
 DevelopedsignificantprojectrelationshipswithNSWCCrane,Zyvex,andMPD,
Inc.
131
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
Cleanroom manager on organization committee for the University,
Government, and Industry Micro/nano Symposium held at the University of
Louisville2008andshowcasingournewfacility.
On search committee for the University of Louisville Vice‐President for
ResearchandIndustry2010
GraduateResearchAssistant‐‐‐UniversityofLouisville,1997‐1999
 Designed and implemented of astudent laboratory course for thefabrication
andtestingofamicrofabricatedpiezoresistivepressuretransducer.
 Developed of many microfabrication processes, the production of
corresponding SOPs, training students and research staff, ordering supplies,
andmaintainingresearchequipment.
 InstructoroftheUofLMicrofabricationLaboratoryCourse(1997‐2003).
Project Manager‐‐‐North American Stainless (New Installation Department),
1993‐1996
 Developed written equipment specifications; prepared bid packages and
contracts; supervised the start‐up and installation of new process lines and
supportequipment.
 Responsible for major installation projects including a cold rolling mill,
multiple finishing lines and mill duty overhead cranes (over $50M in
equipment).
Laboratory Manager North American Stainless (Quality‐Control Department),
1991‐1993
 Coordinated the design and construction of the product certification
laboratoryduringplantstart‐up.
 Responsible for purchasing laboratory equipment, hiring and training
laboratory personnel; and creating SOPs required for certification and
evaluation of mechanical and metallurgical properties of processed stainless
steelinaccordancewithASTM,ISO9000,andspecificindustrystandards.
EDUCATION
UniversityofLouisville,Louisville,KY
Completing Electrical Engineering Doctor of Philosophy (Ph.D) degree, Fall
2014
MSinElectricalEngineering(MSEE),August1999
PurdueUniversity,WestLafayette,IN
BSinMechanicalEngineering(BSME),May1991
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