POROUS SILICON
PSST-2002 Short Course
Sunday 10th March 3:00-6:00 pm
FABRICATION, PROCESSING,
MECHANICAL AND THERMAL PROPERTIES
by: Androula G. Nassiopoulou
POROUS SILICON FORMATION BY
ELECTROCHEMICAL DISSOLUTION OF SILICON (II)
Cross sectional view of a
conventional double-tank cell
IMEL,/NCSR Demokritos, PSST 2002, Tenerife
(2)
EFFECT OF ILLUMINATION IN POROUS SILICON
FORMATION IN HF-WATER OR ETHANOLIC SOLUTIONS
For a review, see : Α. Halimaoui in: Properties of porous silicon, edited by: L.T.Canhan EMIS
DATAREVIEWS series No 18 IEE 1997
 Anodization of p-type silicon:  in HF-water or ethanolic solutions
 Anodization of n-type silicon:  in above solutions: need for illumination
Effect of illumination:  electron/hole pair generation
 holes are involved in the chemical reactions for silicon
dissolution
For a doping level < ~1018cm-3 : Silicon dissolution occurs in the dark only at high voltage (>5V)
 Under illumination: porous silicon formation occurs at lower
potentials (<1V) (surface layer: nanoporous, underlying
layer: macroporous)
For a doping level >1018cm-3 :  porous silicon formation mesoporous even in the dark (holes
generated by electric field induced avalanch breakthrough
Using specially “designed” electrolytes: macroporous silicon formation on
n-type silicon without any illumination is possible (current bursts model)
Ref: H.Föll et al., Physica Status Solidi (1) 182,7 (2000)
IMEL,/NCSR Demokritos, PSST 2002, Tenerife
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STAIN ETCH POROUS SILICON FILM GROWTH
References: • G.Di Francia et al, J.Appl. Phys. Vol 77 (1995) p 3549
• Di Francia, Solid State Communications, vol 96 (1995) 79
• Noguchi et al., Appl. Phys. Lett. 62(12) 1993, 1429
Si dissolution without electric
field
 Chemical solution: HF/nitric acid/
water
Key component : hole (h+) generation
cathode: HNO3+3H+NO+2H20+3h+
anode : nh++Si+2H2OSiO2+4H++(4-n)eSiO2+6HFH2SiF6+2H2O
Above reaction: catalysed by HNO2 
“incubation” period
Obtained films :
In general non-uniform (due to random
anodic and cathodic sites)
With Al-150 to 200 nm thick on Si 
instantaneous reaction of silicon with
HF/HNO2/H2O etchant, due to the reaction
of Al and HNO3 to provide holes (selective
formation) of PS)
Use of sonication during stain etching:
 thicker PS films
 more rough PS surface
Simple illumination of Si in 50% HF
with HeNe laser: porous silicon formation
Influence of substrate doping
It influences the incubation time
· p-type silicon: incubation time increases with substrate resistivity
 n-type silicon:
“
“ decreased with increasing resistivity
IMEL,/NCSR Demokritos, PSST 2002, Tenerife
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MULTILAYER STRUCTURES OF POROUS SILICON (I)
Fabrication:
Based on the following properties:
Porosity depends on anodization current density
Porosity depends on illumination parameters in n-type
silicon
Porosity depends strongly on doping concentration
The silicon skeleton in the already etched structures is
not affected during further processing (hole depleted)
Type I multilayers:
The porosity in the layers is
monitored by changing:
 The anodization current density
 The illumination parameters in
n-doped substrates
Ref: M. Thönissen and M.G.Burger in: Properties of porous silicon,
edited by L.T.Canham, EMIS Datareviews Series No 18 IEE, 1997
IMEL/ NCSR Demokritos, PSST 2002, Tenerife
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PRINCIPLE OF OPERATION OF THE
GAS FLOW SENSOR
Gas flow
T1
T1 = T2
T2
T1
IMEL,/NCSR Demokritos, PSST 2002, Tenerife
T1 < T2
T2
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POROUS SILICON FORMATION BY
ELECTROCHEMICAL DISSOLUTION OF SILICON (I)
Electrochemical solution: HF-based
DIFFERENT ANODISATION CELLS
Cross sectional view of a
lateral anodization cell
Cross sectional view of a
conventional single-tank cell
IMEL/NCSR Demokritos, PSST 2002, Tenerife
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MULTILAYER STRUCTURES OF POROUS SILICON (II)
Type II multilayers:
The layered structure is defined before anodization
(alternate layers with different doping concentration)
Interface sharpness:
In type-I multilayers  given by the transition of the
anodization current and its effect
on etching.Transition zone below
15 nm is achieved.
In type-II multilayers  given by the epitaxy
IMEL/NCSR Demokritos, PSST 2002, Tenerife
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APPLICATIONS OF MULTILAYER PS STRUCTURES
 Interference Filters
 Waveguides
 Porous silicon mirrors for biological
applications
IMEL/NCSR Demokritos, PSST 2002, Tenerife
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MULTILAYER STRUCTURES OF POROUS SILICON (III)
(a)
a) Type-I multilayers by varying
anodisation current


Current density: 6 and 104
mA/cm2
Etching time: 4.83 and 1.33 sec
b) Type-I multilayers by varying
the illumination density

(b)
Current density: 6.4 mA/cm2
c) Type-II multilayers on epitaxially
grown silicon layers with varied
doping concentration of 1017
and 1019 cm-3
(c)
Ref: M. Thönissen and M.G.Burger in: Properties of porous silicon,
edited by L.T.Canham, EMIS Datareviews Series No 18 IEE, 1997
IMEL/NCSR Demokritos, PSST 2002, Tenerife
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DERIVATIZED POROUS SILICON
MULTILAYERS AND BIOLOGICAL MIRRORS
Ref: L.T. Canham et al, Phys. Stat. Sol. (a) 182, 521 (2000)
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DRYING OF POROUS SILICON (I)
Crucial in order to avoid cracking
Cracking :
 due to capillary stresses associated with the nanometric
size of the pores
 occurs for PS layers thicker than a critical thickness hc
(hc depends on the porosity and on the surface tension
of the drying liquid)
Example :
Ref. D.Bellet in: Properties of Porous silicon
IMEL/NCSR Demokritos, PSST 2002, Tenerife
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DRYING OF POROUS SILICON (II)
Origin of cracking :  evaporation of the pore liquid gives rise to
capillary tension
Maximum capillary stress :  at the critical point when the
menisci enter the pores
Induced pressure: ΔΡ =2γLV/r, γLV = surface tension, r = pore radius
Example: For water γLV = 72mJ/m2  for r = 5nm  ΔΡ = 30ΜPa (300 bar)
Capillary pressure: not hydrostatic, since normal air drying is out of equilibrium
Measurement of induced tensile stresses :
By measuring wafer curvature
Using X-ray diffraction (measuring lattice parameters)
IMEL/NCSR Demokritos, PSST 2002, Tenerife
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DRYING OF POROUS SILICON (III)
Drying techniques to avoid cracking
a) Water or pentane drying
(pentane:lower surface tension than
water)
b) Supercritical drying
•
•
•
Most efficient drying method (L.T. Canham et al. Nature (UK) Vol 368 (1994) p133)
Used fluid: CO2, drying: above the critical point (40oC, 163 bar)
Result: ultrahigh porosity films
c) Freeze drying
•
The fluid inside the pores is frozen and then sublimed under vacuum
(no interfacial tension)
IMEL/NCSR Demokritos, PSST 2002, Tenerife
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Ref: M. Thönissen and M.G.Burger in: Properties of porous silicon
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AGEING OF POROUS SILICON
It results from the reaction of
the material with its
environment
Intentionally oxidize PS
Isolate the internal
surface by capping
In order to minimize
storage effects:
Modify the internal surface
Impregnate the pores
IMEL/NCSR Demokritos, PSST 2002, Tenerife
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CAPPING OF POROUS SILICON I
Used to avoid ageing
a) Epitaxially deposited capping layers
 CoSi2 and SiGe, deposited at  600oC  stabilization
of strain (Kim et al., J. Appl. Phys, 69 (1991) 2201)
 Si on porous silicon. Fabrication of SOI or bond-andetchback SOI (BESOI)
 GaAs on PS: for monolithic integration of optoelectronics
with Si ICs
 Diamond for high temperature electronics
 PbTe, for mid-infrared (3-5 μm) optoelectronic devices
IMEL/NCSR Demokritos, PSST 2002, Tenerife
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CAPPING OF POROUS SILICON II
b) Organic/polymeric capping layers



Paraffin on the surface of PS (Tischler et al). Short term stabilization of PL
Capping with conducting polymers, as plyaniline, polypyrrole
Polymer within the pores
c) Metallic capping layers

Ti or Co silicides
d) Al deposition – protection in ambient air

Reduces C and O pick-up, retains F
e) Al capping – Protection during analysis

f)
avoids oxidation and carbonization of samples, and H or F desorption
Dielectric capping



CVD deposited SiO2 on medium porosity Si  minimizes ion-beam induced
ageing
Ion-implanted O or N, or PECVD-deposited SiO2, Si3N4
No result on PL stabilization
IMEL/NCSR Demokritos, PSST 2002, Tenerife
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SURFACE MODIFICATION OF POROUS SILICON
Surface of freshly etched porous silicon:
hydrogen-passivated (SiH, SiH2, SiH3)
Surface modification :
good electronic passivation
limited stability
 Oxidation
Anodic, chemical, thermal
 Nitridation
Rapid thermal annealing in N2 or NH3
 Organic chemical derivatisation
Stabilisation by organic groups, process stopped at a monolayer
• Surface covered with SiH and SiCH2CH3 upon dissociative adsorption
of diethylsilace (Dillon et al. (1992)
• Grafting of trimethylsiloxy groups. Substitution of - H with - OSi (CH3)3
Anderson et al 1993)
• Thermal derivatisation with alcohols (Hory et al. 1995, Kim et al. 1997)
• Grafting of alkoxy groups (Li et al. 1994)
 Electrochemical derivatisation
Ref: J.N. Chazalviel et al in: Properties of Porous silicon
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STABILIZATION AND FUNCTIONALIZATION VIA
HYDROSILYLATION AND ELECTROGRAFTING REACTIONS
Substitution of the silicon hydride bonds with
silicon carbon bonds
LAM (Lewis acid mediated) reaction
(hydrosilylation)
Light-promoted hydrosilylation
Cathodic electrografting
Ref: J.M. Buriak, Adv. Mat. 11, 265 (1999)
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ELASTIC PROPERTIES OF PS
They differ drastically from those of bulk silicon
Young’s modulus of P.S
Measured by
Brillouin scattering
used to investigate
the surface acoustic
waves on a PS-layer
X-ray diffraction
Microechography and measurement of
acoustic signature
(measuring reflection and transmission
parameters versus frequency)
Nanoindentation investigation
(Nanoindenter: it measures force and
displacement as an indentation is performed
on the material using a very low load)
General tendency:
PS is less stiff than bulk silicon (with lower Young’s modulus values)
IMEL/NCSR Demokritos, PSST 2002, Tenerife
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YOUNG’S MODULUS VALUES OF POROUS SILICON
Ref: M. Thönissen and M.G.Burger in: Properties of porous silicon,
edited by L.T.Canham, EMIS Datareviews Series No 18 IEE, 1997
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THERMAL CONDUCTIVITY OF POROUS SILICON
Very different from that of bulk silicon
Bulk silicon :
145 W m-1K-1
Porous silicon : depends on porosity
[1,2]
[1,2]
D [3]
1-10
10
-
none
65%
40
1.2 (3)
(1)
(2)
(3)
A. Drost et al. Sens. Mat. (Japan), vol 7 (1995) p 111
W.Lang et al. Mater. Res. Soc. Symp. Proc. (USA) vol. 358 (1995) p561
A.G.Nassiopoulou et al. Phys. St. Sol. (a) 182, 307 (2000)
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40
38
36
34
32
o
T ( C)
30
28
26
24
Resistor on bulk silicon
22
20
-200
-100
0
100
200
Distance from heater (μm)
Temperature distribution around a
heater on bulk silicon
Ref: A.G. Nassiopoulou and G. Kaltsas, Phys. Stat. Solidi (9) 182, 307 (2000).
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160
140
120
Temperature
distribution around a
heater on porous silicon
o
Resistor
on PS
100
T ( C)
80
60
40
20
-200
-100
0
100
200
Distance from heater (μm)
600
500
Temperature distribution
around a heater on a free
standing silicon
membrane
400
T(C)
300
200
100
0
-200
Resistor on a free
standing silicon
-100 membrane
100
0
Distance from heater (μm)
200
Ref: A.G. Nassiopoulou and G. Kaltsas, Phys. Stat. Solidi (9) 182, 307 (2000).
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LOCAL FORMATION AND PATTERNING OF POROUS SILICON
Necessary in applications using monolithic integration of the
corresponding devices and structures on the silicon substrate
LITHOGRAPHIC PATTERNING
Most commonly used masking materials:

Photo resists:
Common photoresists (AZ5214): withstand etching solutions only for short
anodization time.
Use of SU8 photoresist: long anodization time (V.V. Starkov et al (this Conference))
 Silicon dioxide: For anodization times of a few minutes
 Stoichiometric silicon nitride or silicon carbide:
Resistant to the anodization solution but they show problems related to
stress effects and cracking
 Non-stroichiometric nitride, deposited by LPCVD  good mask
 Double layer of polysilicon/SiO2
Perfect mask for porous silicon micromachining.
IMEL/ NCSR Demokritos, PSST 2002, Tenerife
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Examples of local anodization through a lithographic mask
Silicon nitride mask
SiO2 mask:anodization time: 1 min
Ref: Α.Nassiopoulou et al.
Thin Solid Films 255 (1995) 329
Ref: Α.Nassiopoulou et al
Thin Solid Films 255 (1995) 329
Polysilicon mask
Ref: G.Kaltsas and A.G.Nassiopoulou,
Sensors and Actuators A65(1998) 175
IMEL/ NCSR Demokritos, PSST 2002, Tenerife
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LITHOGRAPHIC PATTERNING USING POLY/SiO2 MASK
APPLICATION IN MICROMACHINING
Reference: G.Kaltsas and A.G.Nassiopoulou, Sensors and Actuators A65 (1998) 175
IMEL/NCSR
IMEL, NCSR Demokritos, PSST 2002, Tenerife
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POROUS SILICON MICROMACHINING
Use of porous silicon as sacrificial layer for
the formation of free standing membranes
on top of a cavity
Examples of free
standing polysilicon
membranes and
cantilevers.
Ref: G. Kaltsas and A.G. Nassiopoulou, Sensors and Actuators, A65 (1998) 175-179.
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DRY ETCHING OF POROUS SILICON
 As prepared PS layers are etched
6-7 times faster compared to Si.
Typical etch rates for SF6 :
RIE : 6.8 μm/min (Si:1.5μm/min)
HDP : 66 μm/min (Si:10 μm/min)
 Etching rate depends on:
 The porosity
 Aging of the layer.
 Thermal treatment.
 The etch rate of thermally treated PS
layers is significantly smaller than that
of freshly etched PS.
HDP : 0.33 μm/min
Ref: A. Tserepi et al, PSST 2002, abstract book page 187.
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500
o
Heater Temperature ( C)
SUSPENDED POROUS SILICON MICROHOTPLATES FOR GAS SENSORS
100μm
400
300
200
100
o
Temperature Increase 15 C/mW
0
0
10
20
Power (mW)
30
40
High temperatures (>400oC) can be obtained with
very low energy consumption (<30mW)
Ref: C. Tsamis and A.G. Nassiopoulou, unpublished results.
IMEL,/NCSR Demokritos, PSST 2002, Tenerife
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Suspended Porous Silicon membranes
with Pt heater (60x60μm2)
P=42mW
P=50mW
P=46.6mW
P=57mW
Ref: C. Tsamis and A.G. Nassiopoulou, unpublished results.
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EXAMPLE: GAS FLOW SENSOR
Direction of gas flow
Cold thermopile contacts
Hot thermopile contacts
Al
Pads
Porous silicon area
Heating resistor
Bulk Silicon
Ref: G. Kaltsas and A.G. Nassiopoulou, Sensors and Actuators 76 (1999) 133-138.
IMEL,/NCSR Demokritos, PSST 2002, Tenerife
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Sensor characteristics
Ref: G. Kaltsas and A.G. Nassiopoulou, unpublished results
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