Packaging and Interconnection of a Back-Lit CCD Sensor Device
by
Joshua V. Bennett
B.S. Materials Science and Engineering, Massachusetts Institute of Technology
1996
SUBMITTED TO THE DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING
IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
MASSACHUSETTS INSTITUTE
OF TECHNOLOGY
June, 1997
JUN 1 7 2003
©Joshua V. Bennett
LIBRARIES
Signature of Author
____
yr
·
Department of Materials Science and Engineering
May 9, 1997
Approved
by
"
William Robbins
Technical Supervisor, Draper Laboratory
Certified by
Yet-Ming Chag, Kyocera Professr of Ceramics
Thesis Supervisor
( / ,
Accepted by
Linn Hobbs, John F. Elliott Professor of Materials
Chairman, Departmental Graduate Committee
SCI'NCE
Packaging and Interconnection of a Back-Lit CCD Sensor Device
by
Joshua V. Bennett
Submitted to the Department of Materials Science and Engineering
on May 9, 1997 in Partial Fulfillment of the Requirements
for the Degree of Master of Science
ABSTRACT:
This paper describes the engineering and development of a process to electrically and
mechanically bond a large area, backlit charge-coupled device (CCD) sensor to a metallized
substrate, using a materials system compatible with a high vacuum tube configuration. The
CCD sensor is used in light sensor and digital camera devices.
Currently, CCD devices are mechanically bonded to substrates using a variety of
polymeric materials. For maximum resolution, however, the device must operate at high
vacuum. Polymers release monomer molecules when exposed to a vacuum, since the
polymerization reaction is far from equilibrium. Polymers also release retained solvent and
binder molecules into a vacuum. This release of gas pollutes the high vacuum environment
and degrades the tube perfomance.
The CCD chip used in this research is extremely large, compared with standard
semiconductor devices. The CCD chip measures 0.4 x 0.5 inches. At this size scale, thermal
expansion mismatch stresses between die and carrier become an important design
consideration, particularly for a die thinned to 10 micrometers.
The proposed process results in a mechanical bond that is compatible with the high
vacuum requirement, and is applicable to substrates of four possible materials: silicon,
aluminum nitride, Pyrex, and LZS (a zirconia/ silicon nitride composite). The process also
allows for two possible electrical connection techniques.
Thesis Supervisor: Yet-Ming Chiang
Title: Kyocera Professor of Ceramics
ACKNOWLEDGEMENT
May 22, 1997
This thesis was prepared at The Charles Stark Draper Laboratory, Inc.
Publication of this thesis does not constitute approval by Draper or the sponsoring agency of
the findings or conclusions contained herein. It is published for the exchange and stimulation
of ideas.
I hereby assign my copyright of this thesis to The Charles Stark Draper Laboratory, Inc.,
Cambridge, Massachusetts.
Permission is hereby granted by The Charles Stark Iaper Laboratory, Inc., to the
Massachusetts Institute of Technology to reproduce any or all of this thesis.
Table of Contents
1 Acknowledgements
Page
4
2 Glossary of Terms
5
3 List of Figures
6
4 Introduction
7
5 Literature Review
9
6 Device and Process Concerns
15
7 Materials Selection
17
8 Electrical Attachment Process Development
24
9 Mechanical Attachment Process Development
27
10 Results: The Packaging and Interconnection Process
38
11 Testing and Reliability of Bonded Devices
39
12 Conclusions
43
13 Future Research
44
14 Appendices:
A: Phase Diagrams
46
B: Mask Design
47
15 References
48
1 Acknowledgements
First and foremost, I would like to express my gratitude to Bill Robbins and Yet-Ming
Chiang for their constant guidance. This research would have been impossible without their
support and encouragement.
My thanks also goes to DARPA and the Charles Stark Draper Laboratory, who
provided the funding to make this thesis possible.
I also would like to thank Dominic Fulginiti for his technical knowledge and all he has
done to aid this research. Ed Ayers, Sol Kafel, Craig Dement-Myers, and Matthew Lozow
were also instrumental in the progress of this thesis.
Words cannot express my gratitude to my friends, who made my time at MIT bearable.
I especially would like to thank Keith Breinlinger, Anthony Rocco Nole, Jang Don Kim, Neil
Jenkins, Howard Fine, and Luis Ortiz.
Most of all, I would like to thank my loving fiancee, Penny Pliskin. It is impossible to
express how much her constant support and understanding have meant to me in my pursuit of
this degree.
2 Glossary of Terms
CCD: Charge Coupled Device. A CCD receives electrical or photonic current over an area,
and converts this into a usable signal for displays.
Cordierite: A ceramic material, 2MgO-A12 0 3-5SiO 2
Device: In this research, device refers to the electrically and mechanically bonded die and
substrate arrangement.
Die: The silicon chip that contains the actual CCD device.
P-Eucryptite: A ceramic material with a slightly negative thermal expansion coefficient,
Li20-A120 3-2SiO 2
LZS: A composite material consisting of roughly 90% Si 3N4 and 10% ZrO2 . LZS is
manufactured by Ceramic Process Systems, Inc, Chartley, MA.
3 List of Figures
1 Schematic of an EBCCD Apparatus
Page
7
2 Optimal Glass Profile at Die Edge
15
3 Aluminum-Silicon Phase Diagram
16
4 Physical Derivation of the Young-LaPlace Equation
19
5 Creep Map of an FCC Metal
26
6 Furnace Cycle 6
30
7a 2760 Glass on a Pyrex Substrate
34
7b 2760 Glass on a Silicon Substrate
34
8 Anneal Cycle for Bonded Devices
37
4 Introduction
Large area CCD-based digital cameras have a huge number of possible applications.
Because the CCD sensor apparatus can be designed to detect a wider range of wavelengths
than conventional light cameras, they can be used in new applications that are not possible with
conventional cameras. Electronic CCD cameras are currently used to create digital
photographs, as well as in night-vision equipment. It is also possible to use infrared CCD
cameras in medical applications to create a detailed picture based on the heat produced in
different organs or tissue in the body. This is extremely useful in oncology, since cancerous
cells commonly produce a different heat signature than healthy cells.
As the CCD cameras become more complex over time, one desirable quality is lowlevel light detection. One way to increase the quality of images detected at low light levels is
to decrease the pixel size and increase the total sensor area. To understand the physical effects
of changing these variables, one must comprehend the basic workings of a Electron
Bombarded CCD device (EBCCD). A schematic of an EBCCD camera device is drawn below
in Figure 1.
Photon
I
A
GaAs
Imaging
Equipment
I
I
I
.
CCDn
Figure 1: Schematic of an EBCCD sensor apparatus
As shown in figure one, photons hit the photocathode material. The photon excites an
electron from its ground state. The space between the CCD die and the photocathode is held
under vacuum, in the range of 10-11 torr in this research. The electrical potential across this
space accelerates the electron towards the die, which acts as the anode. The electron impinges
on the die active area and activates a pixel. The number of electrons impinging on a pixel
determines the relative intensity of light, or brightness of that portion of the image.
The devices in this research are backside illuminated. This means that the electrons
travel through the back of the die, and impinge on the underside of the active area. This allows
all electrical connections to be removed from the space between the die and photocathode. To
accomplish this physical arrangement, the die is thinned to a thickness of about 10
micrometers. This allows electrons to travel through the silicon die without a loss of
resolution.
The standard bond between a die and substrate in this type of thinned assembly
employs polymeric materials. These bonds can be carried out at temperatures at or near room
temperatures, using epoxy resins or thermoplastic materials. These materials can be filled to
match their thermal expansion coefficients to silicon, so any thermal stresses that occur during
the operation of the device can be minimized. In addition, because the moduli of these
materials are often low, they can deform in compliance with the thermal stresses without
failing due to fracture or permanent deformation.
The assembly considered in this research, however, must be compatible with a high
vacuum environment. Polymeric materials are not compatible with high vacuums. Polymers
are the result of monomer combination reactions, and when the final polymer is exposed to a
vacuum the kinetics of the polymerization reaction will favor a breakdown into monomer
8
molecules. In addition, polymers commonly contain residual solvents or other organic artifacts
of polymerization and processing. Therefore polymeric materials can not be used to create the
bond between the CCD die and substrate in this research.
Lead oxide-based electronic sealing glasses are proposed as a substitute material to
achieve the bond in this research. Glasses are compatible with the high vacuum environment
required for this sensor, and should adhere to the materials proposed for use in this packaging
process. However, glasses must be processed at higher temperatures than polymeric bond
materials, and they commonly have thermal expansion coefficients that are much larger than
silicon. Glasses also have much higher moduli than polymers, and are brittle under tensile
stresses.
It is proposed that these concerns can be overcome to produce a die-substrate bond that
is both durable and practical, and is compatible with the high vacuum environment of the
chosen CCD device. It is theorized that stresses due to thermal expansion coefficient
mismatches can be minimized in a number of ways: minimizing processing temperatures,
adding low thermal expansion coefficient filler materials to the electronic glasses, and
constraining a thin bond glass layer between thicker die and substrate layers.
5 Literature Review
The bonding of large area dies with electronic glass has not been discussed in detail in
any known literature. There is much discussion of polymer-based packaging of large area
sensors, however [1].
5.1 Requirements for bond
There are many mechanisms that can result in a bond between two dissimilar materials
[2,3,4]. First, the bond can be purely mechanical, or a physical interaction of surface
roughness that serves to interlock the materials. Because of the high surface finish of the
materials in this research, and the fact that the bond glass is liquid in the bonding cycle,
mechanical bonding can not be relied on to produce a permanent bond in this materials system.
A bond can also be produced by the interdiffusion of two dissimilar materials. Because
this research proposes firing oxide glasses in contact with passivated silicon, it is expected that
some interdiffusion of these similar materials occurs. This likely contributes to the bond
between the die and glass, as well as the glass and possible substrates such as Pyrex and
silicon.
A chemical bond at the interface between the two materials is also possible. There may
be some possible network bridging when the oxide glass contacts silicon atoms in this
research. This would contribute to the interfacial bonds.
A fourth mechanism for a bond between dissimilar materials is an adsorption and
wetting model. This model necessitates adsorption via molecular or van Der Waals bonding at
the interface. This type of bond will occur if the interfacial energy of the two materials is less
than the surface energy of the two separate materials in the bonding atmosphere [4].
5.2 Glass Bond Literature
The bonding of silicon wafers with borosilicate glass has been discussed [5]. Boron was
deposited on a silicon surface using a physical vapor deposition process, and an oxide layer
was thermally grown. Bonding was carried out at 450'C in an 02 atmosphere. To investigate
10
bond strength, the bonded silicon wafers were scored and broken. If fracture occurred in all
layers without glass peeling away from the silicon surfaces, the bond was considered good.
They report no good bonds using a spun-on glass technique, with a wide variety of borosilicate
glass compositions and process variables. This spin-on glass application technique is closer to
the application techniques used in this research than the PVD/ oxide growth method.
Success in die bonding with silver-filled, conductive PbO-ZnO-B 20 3 glass has been
reported [6]. This glass is used to provide electrical interconnects, as well as mechanical
bonding between substrate and die. This type of glass has produced void and crack free bonds
after sintering at a temperature of roughly 4000 C. However, the experiments used a glass paste
with a binder. Because of the high vacuum environment of the sensor in this research, a glass
without binders is desirable. The glass bond in this research must also be non-conductive. The
lowest temperature, non-conductive bond reported occurs at 450 0 C [6], with the optimal firing
temperature for a lead borate alumina glass at 900 0 C. The materials system in this research
requires glass that can be fired without binders at a temperature near 400'C.
The relationship between voids in IC/substrate bond layers and resultant damage of
chips due to stress concentrations has been investigated [7]. This signifies the need for a
continuous bond glass layer that is free of voids, thereby minimizing any possible sources of
damage to the active die surface.
5.3 Stress Calculations
The calculation of stresses due to thermal processing is vital to the choice of materials
for this application. Thermal expansion stresses cause a variety of problems in the sandwich
type processing. The most important stress component in the device is the amount of stress in
11
the glass layer, both before and after die attachment. This is due to the relative thickness of the
bond layer, and the low yield and rupture stress values characteristic of glasses. Bowing of the
die, and residual stresses in the thinned die are also concerns.
The tensile stress in the glass can be approximated before die attachment by the Stoney
formula. This formula is derived in The Materials Science of Thin Films [8]. This formula is
derived from basic thermoelasticity theory in a biaxial frame of reference, and gives the stress
is small compared to the component in the radial direction, ao.
The Stoney formula assumes a
layer thickness that is much smaller than the substrate thickness. The formula is expressed
below:
1
a2
a61
E,E (a, -a,)AT
Eas s +Ea +EEazaa(4aa +6a a +4a
eq. 1
Es and Ef are the Young's moduli of the substrate and film, respectively, a is the
thermal expansion coefficient, and as and af are the thicknesses of the substrate and film. AT is
the difference in temperature from the bond conditions. Using the final glass composition in
this research, one mil of glass fired on an LZS substrate results in a tensile glaze stress of 23
ksi.
Another estimate of glaze stress can be found in Introduction to Ceramics [9]. This
formula takes into account the relative thickness of the substrate and glass, but is more accurate
at very small glass thicknesses, compared to the substrate thickness. The calculated stresses
from the two equations converge when the difference between the Young's moduli of the glass
and the substrate is small, and when the substrate is significantly thicker than the glass layer.
Because of the less rigorous derivation, the Kingery equation is expected to be a rougher
12
approximation of the value of stress than the Stoney formula. The Kingery formula is shown
below:
ogiaz= E,(To -T')(a
-a) 1-3 a+ + 6 f
as
a.
eq. 2
Values are defined identically to those in the Stoney formula. With a structure consisting of
one mil of glass on an LZS substrate, equation 2 gives a value of 108 ksi in tension. This stress
level is much different than the value calculated from equation 1.
While both equations express the value of stress with the same considerations, the
derivation of the Stoney formula is more vigorous. This is due to the fact that the Stoney
derivation begins with the consideration of materials with similar thicknesses, and then
considers the special case where one material is significantly thicker than the other. The
Kingery equation is derived by disregarding any constraint on the thicker material due to the
thinner material. Both equations predict similar stress levels, as long as the Young's moduli
of the substrate and glass are similar. As these properties become dissimilar, the Stoney
formula is expected to give a better approximation of the actual stress levels.
In addition, This research also investigated the use of substrates with dissimilar
expansion coefficients that the silicon die. A method of calculating the stress in the bond layer
between two planar materials with dissimilar thermal expansion coefficients has been proposed
[10]. Because the bond layer is constrained by the two significantly thicker layers, the
thermoelastic properties of the bond glass are not considered. The shear modulus of the glass
has a significant effect on the stress level in the glass, so this property is included in the
13
derivation. This formula gives the maximum value of shear stress at the surfaces of the glass,
and does not allow for bowing of the sandwich. Because bowing is not allowed, the value of
stress calculated will be somewhat higher than the true value of stress in a similar physical
setup. Some bowing will occur in any real physical arrangement.
The formula is given below:
(a1 - a 2 )A TG sinh(pr)
,8frcosh(fl)
with:
eq. 3
77
Elt i
E2t2
eq. 4
G and q are the shear modulus and thickness of the glass. r is the radial distance from
the center of the bond. E and a have their usual significance, and t is the thickness of the
sandwiched layers. 1is the full radius of the assembly. In stress calculations in the Appendix,
the die is defined as layer 1, and the substrate is layer 2.
Using equation 3, the maximum shear stress occurs at the extreme edge of the
assembly. At this point, r equals 1,and the hyperbolic sine and cosine terms become a single
hyperbolic tangent function. For all but very small values of/ and r, this hyperbolic tangent
function essentially equals one. For a 1 mil layer of glass on a Pyrex substrate, the hyperbolic
tangent function equals 0.99 at 0.4 millimeters. This would imply that the shear stress at the
die comers is independent of geometry, as long as the die face diagonal is less than 0.8
millimeters. Care must be taken when applying this formula to a real situation.
6 Device and Processing Design Concerns
Steps in the device fabrication process dictate certain limits on the materials and
processing of the die attach process. Most importantly, the die thinning process is extremely
sensitive to the die bond condition. The silicon die is thinned using a HF-based acid etch
process. This acid solution will damage the active side of the die if contact is made.
Therefore, the bond glass must provide a seal to the edge of the die. The non-passivated die
edge, however, shows excellent wetting compatibility with all glasses used in this research.
For the acid etch process to produce a uniform thickness across the profile of the die, no glass
can adhere to the die edges. Figure 2 below shows the optimal glass profile at the die edge to
provide a seal without impeding the thinning process.
die
glass
substrate
not enough glass
die
die
glass
glass
substrate
optimal
substrate
too much glass
Figure 2: Optimal glass profile at die edges.
The CCD die used in this process has conductive lines of aluminum. Aluminum and
pure silicon have a eutectic point at 5770 C. This eutectic point is depressed to with the
addition of boron or other doping elements. The die is composed of P-type silicon. The Al-Si
phase diagram is shown below.
WEIGHT PER CENT SILICON
10
05
700
20
30
1 1401
1 601
50
70
1
1
90
1
wt-*/.si
10
15
600
20
1
5770
(Af)
500
1t500
159
~14120
(1.65)
I
I I
1400
400
0.16(0.17)
0
80
300
(AL+ Si)
7Z
7Z -
-
200
a-
1300
7
200
1
000
rL
0.5 1.0
At-%i
1.5
/S
100
-
900
800
700
6600
600
5770-
1
Si500
A ^^
0
Al
10
20
E
30 40 50 60 70 80
ATOMIC PER CENT SILICON
90
100
SI
Figure 3: The aluminum-silicon phase diagram [8]
If this eutectic temperature is exceeded, the liquid will change the charge-carrying
paths and produce shorts in the die. This would destroy the chip's performance. This
temperature limits the choice of materials used in the bond.
Interdiffusion of aluminum and silicon is also a concern in this physical arrangement.
To lessen the effect of possible interdiffusion, the aluminum used in conductive lines on the die
contains silicon at the solubility limit at room temperature. There is also a passivation layer
under the conductive aluminum layer.
Oxidation of materials in the die is also a concern. As charge-carrying materials in the
die oxidize, their electrical resistivity changes. This could degrade the performance of the die.
7 Materials Selection
The substrate holds the die in place and acts as the mechanical support for the die, and
The substrate also provides electrical interconnects between the die and subsequent packaging
and pins. The substrate must have a Young's modulus high enough to provide mechanical
support to the die, and must have a thermal expansion coefficient that is close to silicon to
minimize thermal expansion mismatch stresses.
The substrate must be electrically insulating to isolate contact bondpads. It is also
desirable to have high thermal conductivity to dissipate heat from the sensor operation. Since
phonon conductivity and electrical conductivity are usually proportional properties, choosing a
material with these qualities is somewhat difficult.
Based on the above criteria, research was limited to four possible substrate materials:
passivated silicon, aluminum nitride, Pyrex, and LZS. LZS is a dispersed composite material,
made up of roughly 91% Si 3N4 and 9% ZrO 2 . Table 1 below shows a comparison of some
thermal and mechanical properties of the four material.
Table 1: Material properties of substrate materials
Density (p)
Young's
CTE (a)
Material
Termal
Conductivity
modulus (E)
Si 3N4
2.25 x10 -6
280 GPa
3.44 g/cm'
30 W/mK
ZrO 2
8.0 x 10-'
180 GPa
5.8 g/cm 3
2.09 W/mK
LZS*
3.3 x 10-
286 GPa
3.79 g/cm 3
30 W/mK
Silicon
3.3 x 10-
170 GPa
2.33 g/cm 3
150 W/mK
A1N
4.2 x 10
300 GPa
3.26 g/cm'
180 W/mK
Pyrex
2.9 x 10-'
63 GPa
-2 g/cm 3
1.1 W/mK
Based on the above criteria, LZS is the most likely candidate for the substrate material.
Experiments were carried out on all four candidate materials, however. It is foreseeable that
other material properties may prove to be more desirable. LZS seems to maximize the
desirable properties in this research.
7.1 Bond Glass
The bond glass connects the die to the substrate. The glass layer must be mechanically
strong to hold the die in place. Because the device operates in a high vacuum environment, the
bond glass must not release gas into the sealed device. This undesirable process is commonly
called outgassing. Because of this outgassing concern, no polymeric materials or organic
binders can be used in the packaging process.
Because of the temperature limits of the die, the glass processing temperature must be
low. Based on comparisons between experiments and the manufacturer's recommendations,
the glass viscosity must be in the range of 10-4 Poise for necessary glass flow in the bonding
process. It is also desirable for the glass to be vitreous, or not experience crystallization and
devitrify after one firing cycle. The process used in this research relies on multiple firing steps,
and the glass must be capable of viscous flow during each heating cycle.
To provide a good bond with both the substrate and the die, the bond glass must wet
and adhere to both surfaces. This is both a physical and chemical process, requiring both
intimate mechanical contact and adhesion between the two materials at the interface.
Adherence is a complex phenomenon, and can not be easily measured. One approach to
quantify adherence is to determine the relative interfacial energies of the materials under
18
investigation. The surface energies between the glass, substrate, and atmosphere can be related
using the Young-LaPlace equation with a droplet of glass on a substrate:
Yls
Yxx
Y=
sv - Ylv COS 0
represent the surface energies of the liquid-solid, solid-atmosphere, and liquid-atmosphere
interfaces. 0 is the angle observed at the edge of the liquid droplet. Figure 4 below shows the
geometric derivation of this equation.
YI
Ysv
7 1s
Figure 4: Physical derivation of the Young-LaPlace equation. This equation is derived from
investigations of liquid droplets on solid surfaces
If the angle is less than 900, the liquid wets the solid surface. An angle less than 900
indicates that the system can reduce its energy state by covering the solid with the liquid, since
the interfacial energy between the liquid and solid is lower than the solid-atmosphere energy.
This arrangement will allow the liquid to flow and coat the solid surface.
It is desirable for the bond glass to have a thermal expansion coefficient that is as close
to silicon as possible. This is extremely significant, as the bond glass is in direct contact with
the active side of the die. Even if the liquid glass wets the substrate and die surfaces,
differences in thermal expansion coefficient can cause spalling of the bond glass layer. In
addition, it is desirable to have a glass with a fracture toughness and shear modulus high
enough to resist deformation under any thermal stresses.
19
7.2 Bond Glass Choices
Based on the thermal processing limits of the die, lead oxide-based electronic sealing
glasses were chosen for experimentation. Because initial glass layers are fired on the substrate,
it is permissible to approach the Al-Si eutectic temperature. However, because it is usually
desirable to have some deformation throughout the bond glass layer during the die bond
process, all glasses used in the mechanical bond must have processing temperatures near or
below this eutectic temperature.
Table 2 below shows the glasses used in these experiments, with the manufacturer's
recommended firing temperatures for each.
Table 2: Glass Characteristics
Firing Temperature
Glass
Thermal Expansion
Coefficient [ x 10-6]
EG 2760 (Ferro)
380 0 C
11.35
EG 2008 (Ferro)
410 0 C
7.0
EG 7585 (Ferro)
415 0 C
6.8
EG 2805 (Ferro)
580 0 C
3.37
LS 0111M (Nippon El.)
4600 C
5.0
In all cases except 2805 glass, it is necessary to exceed the recommended firing
temperature of the glass to produce a fully dense layer. Processing at the recommended firing
temperature in all other cases produces a glass layer that is porous.
Table 3 below shows the compositions of the glasses used in this research. All values
shown are the compositions reported by the manufacturers. 2760 is a single-phase lead oxide20
based glass. 2008, 7585, and 2805 are mixtures of 2760 and fillers. The inert fillers serve to
lower the thermal expansion coefficients of the glasses. 0111M is also a filled glass. One
concern with the 2805 glass, as described by the manufacturer, is that the P-eucryptite fillers
can cause the glass to crystallize after multiple firings.
Table 3: Glass compositions
Composition
Glass
EG-2760
<90% PbO, <15% B20 3 , <10% ZnO, <5% SiO 2, <5% A12 0 3
EG-2008
2760 composition + 2MgO-A120 3-5SiO 2 (cordierite)
EG-7585
<70% PbO, <15% SiO 2 , <15% B20 3, <10% A120 3, <5% ZnO, <5% MgO,
<1%BaO, <1%SnO 2
EG-2805
<60% PbO, <20% SiO 2, <15% A120 3, <10% B2 0 3 , <5% ZnO, <5% Li 20, <5%
MgO <1% BaO
or, 2760 composition + cordierite and Li20-A12 0 3 -2SiO2 (P-eucryptite)
Ferro glasses are available in a variety of powder sizes. Table 4 below shows the
power sizes used in this research:
Table 4: Ferro glass mesh sizes
Average diameter (microns)
Powder Type
Maximum Diameter (microns)
VEG
6.0-10.0
44
MVG
4.5
25
SRRG
1.5
10
The VEG grade of all glasses were used, as well as SRRG grade 2760. Visible observations
have shown that the glass powder is composed of flakes. The filled glasses are physical
mixtures of glass flake powder and separate crystalline filler particles. The process proposed
in this paper is designed to be compatible with the VEG grade of all glasses.
21
Adhesion between the glass and the substrates was investigated in relative terms only.
Two types of tests were performed; a visual wetting angle observation, and a scratch test. To
visually inspect the wetting angle, a small amount of glass powder was placed on each
substrate, and the substrate was placed on a hotplate. The assembly was heated until the glass
consolidated, and exhibited flow. The samples were then cooled, and visually inspected under
a microscope. Measurements were made by observing droplets 1-2 mm in diameter. Because
the angles were only visually estimated, the values below only show relative wetting among
the candidate materials. The s in many of the cells signifies that the glass layer spalled from
the substrate. In these cases, the glass did not adhere to the substrate, and no actual wetting
angle could be observed
Table 5: Wetting angles for glasses and substrate materials
Substrate
w
Glass
Pyrex
Silicon
A1N
LZS
Passivated
silicon (die)
2760
s
2008
450
7585
0111M
s
s
s
s
500
550
500
450
450
450
400
300
300
330
Adhesion of the candidate glasses was also investigated relatively using a scratch test.
Glass layers were applied to all candidate substrates by depositing an alcohol/glass slurry using
an airbrush. The samples were then fired at temperatures sufficient to form fully dense layers.
The surfaces were then scratched with a blade, with enough force to remove the glass under
22
the point of the knife. The scratched surface was then observed. Relative adhesion strengths
were qualitatively compared by observing the relative force required to draw the blade across
the surface. This could also only be estimated in relative terms, since this experiment was
performed manually. This scratch test revealed that the differences in thermal expansion
coefficients affected the strength of the bond.
As a result of these experiments, 2008 was chosen as the bond glass for LZS and Pyrex
substrates, and 0111M was chosen for use with AIN and silicon. The manufacturer
discontinued the 0111M glass during this research, so 7585 was chosen as a substitute for A1N
and silicon. The filler particles in the filled glasses remain solid through the firing process.
Therefore, they do not contribute to the glass-die bond if they are in direct contact with the die
face during the bonding process. For this reason, all bonding procedures use a layer of 2760
directly under the die.
7.3 Metallization
In addition to the substrate and bond materials, the mechanical characteristics of the
proposed electrical interconnects must also be investigated. The substrates must be metallized
to provide interconnects between the die and the subsequent packaging. This metallization
must be compatible with both possible die attach mechanisms.
For most interconnects in microelectronics, gold is the desired material. Because of its
low chemical activity, gold provides a surface free of oxides. Surface oxides increase the
resistivity of the contacts. For this reason, the top layer metallization on the substrates is gold.
There are additional layers to increase the metallization adherance on the substrates.
Because most of the substrates have an oxide on the surface, the metallization must be
mechanically and chemically compatible with this oxide layer. The first metallization layer on
the substrate is titanium, because titanium dioxide has roughly the same density as titanium.
This means that as the metal in contact with the substrate oxidizes, it does not expand. This
allows a stress free oxide-oxide bond. A layer of platinum is deposited between the gold and
titanium to act as a diffusion barrier between the two layers.
8 Electrical Attachment Process Development
The die has metallized bond pads of deposited aluminum. The electrical attachment
process must provide an electrical connection between these aluminum bondpads and the gold
metallization on the substrate. There are two proposed methods for connecting the die to the
substrate, a wirebonding procedure and a collapsed ballbond procedure.
8.1 Wirebonding
In the proposed wirebond process, the wirebond is connected to the underside of the
aluminum bondpad on the die. The backside of the bondpad is exposed using a selective
etching process. After the standard thinning etch procedure, the back side of the die active area
is masked, and the silicon under the aluminum bondpads at the edges of the die is etched away.
The etching process can be controlled so that only the metal bondpad is left. The glass bond
under the die supports this thin layer of metal. The exposed bondpad can be wirebonded to the
substrate metallization at room temperature, after the mechanical bond is accomplished. This
wirebond can be created using gold wire, using standard wirebonding procedures.
8.2 Ballbonding
The second attachment method is more complex. It consists of a ball bond, with a
metal ball deposited on the aluminum bondpads on the die. The standard ballbond is made
using gold wire, leaving a four mil diameter ball attached to the aluminum bondpads. The die
is then aligned with the substrate, and put under a load, and then heated to the glass firing
temperature. The ball collapses, and provides an electrical interconnect between the die and
the substrate.
The attachment is loaded using arrangements of 2 or 4 lb. spring clips. The yield stress
of gold is 207 MPa in tension. One two pound clip, assuming contact with all 30 ballbonds,
corresponds to a compressive force of 177 MPa. Any additional clips will cause plastic
deformation at room temperature. This will lead to deformation and prevent a permanent
connection. It is desirable for the bonding process to be carried out with a creep mechanism,
allowing for interdiffusion and mechanical connection between the bondpad and the gold balls.
The ballbond attachment is carried out at a homologous temperature of roughly 0.54, based on
the 1337K melting point of gold. With the same arrangement of one two pound clip, this
corresponds to low temperature power-law creep. The mechanism for this deformation is
dislocation motion, assisted by diffusion through the dislocation core. Figure 5 below is a
creep deformation map for a FCC metal, showing deformation regimes in terms of homologous
temperature (T/Tm) and normalized shear (o/p). All pure FCC metals will show the same
mode of deformation at similar homologous temperatures and normalized shear stresses. For
this reason, the deformation map for gold and silver will be essentially the same. A
Compressional stress state can be resolved onto pure shear axes by dividing the stress value by
25
1.73. Figure 5 shows the deformation behavior in a pure shear mode. Because only order-ofmagnitude approximations are necessary in these calculations, deformation in compression can
be roughly equated with deformation under shear.
TEMPERATURE (C)
S4 --
O
C
200
400
00
Oc
b'
E
z
in-
LdJ
Ix
W
_m
LJ0
0I
0
Ln
W
Ix
a:
(I)
0
w
4-J
Cr
W
w
I
O
Z
U,
HOMOLOGOUS TEMPERATURE, T/T.
Figure 5: Creep map of an FCC metal. In this case, the metal is silver.
With the addition of glass, the contact area is raised significantly. Assuming a glass
pad of 0.4 in. x 0.5 in, the overall stress in the bond, using one four pound clip, is now 287KPa.
This implies that diffusional creep controls deformation in the gold balls.
These calculations show the large variation in gold deformation mechanisms based on
the contact area of the glass. To ensure consistent bonding mechanics, the glass must be flat,
and roughly the same height as the glass. This can be accomplished by carefully hand lapping
of the ballbonded die and the glass bond layer. Lapping the die for about two seconds on a
26
3000 mesh diamond lap produces balls that are uniformly 1.2 to 1.7 mils high.
9 Mechanical Attachment Process Development
To mechanically attach the die to the substrate, a bond glass layer is first deposited on
the substrate. This glass layer is built up through a series of depositions and firings. The glass
may undergo a surface preparation between subsequent layers. After the bond glass layer is
complete, a die is aligned with the substrate, and the whole assembly is fired under a load. The
whole assembly will then go through a compaction and stress anneal cycle.
9.1 Glass Deposition
Powdered glass is applied to the substrate using an airbrush. All glasses used in the
final process are manufactured by the Ferro Company, and are available in various mesh sizes.
Experiments were performed using the VEG and SRRG mesh sizes, and the VEG mesh was
selected for processing. This selection was based on ease of handling in the airbrush process.
The powdered glass is mixed with isopropyl alcohol to form a slurry. Based on initial
experiments with spraying procedure, a solution of 7.5 g of glass in 20 mL of alchohol gives an
even coat of glass without clogging the airbrush. Solutions with less alcohol gave a glass layer
that was less uniform after firing, so 20 mL was chosen as the optimal airbrush solution.
Constant agitation is necessary throughout the spraying process. To ensure a consistent
glass mixture, the solution is made in 20 mL batches and mixed fully before each spraying.
Then a small volume of the solution is poured into the airbrush fluid receptacle. This provides
enough solution to coat roughly ten substrates. The substrates are coated using nitrogen as the
propellant, at 20 psi. The spraying distance is roughly 8 inches, and the spraying time is
27
roughly one second per 0.4 x 0.5 square inches of substrate area. After coating, the excess
solution is returned to the 20 mL container. The alcohol evaporates from the substrate in a few
seconds, leaving an even layer of the powdered glass on the substrate. After firing, the above
procedure produces a glass layer that is roughly 0.5 mils thick.
9.2 Glass Firing
All glass layers were fired in belt furnaces. To guard against die degradation due to
oxidation, die bonding cycles were all carried out under a nitrogen atmosphere. There is some
concern that a reducing atmosphere may cause reduction of the lead oxide in the bond glass[7],
but this did not appear to be significant enough to have an effect in this research. This concern
suggests that the total number of firing cycles should be kept to a minimum to lessen the
amount of time that the glass is exposed to a reducing environment.
The table below shows the belt furnace profiles used in this research. Cycle numbers 2
and 3 are brazing profiles for standard production, and all glass firing cycles are derived from
these two basic profiles.
Because all the filled glasses in this research require higher firing temperatures than the
2760 glass, all filled glasses are fired in cycles 1 or 2. The four zones identified in table 6 are
equidistant through the length of the furnace. The percentages in each zone refer to the power
to the zone heaters. The furnaces are controlled through feedback from two thermocouples,
located at two of the four zones. Furnace 1 is controlled by tuning to the temperature of zones
one and four, and furnace 2 by the temperature of zones two and three.
Table 6: Firing Furnace Cycles
Cycle
Furnace
Zone 1
Zone 2
Zone 3
Zone 4
Atm.
Number
1
1
1
500 0C
2
100%
4
2
100%
5
2
90%
6
2
100%
7
2
90%
8
2
90%
9
3000
70%
70%
4700
100%
3
Notes
Speed
100%
2
Belt
2
90%
70%
4200
4300
80%
80%
4000
4100
80%
80%
3850
4050
60%
65%
3850
3650
90%
65%
4050
3650
100%
65%
4200
3850
100%
65%
4200
3850
100%
65%
3 in/min
100%
3000
70%
Air
Filled glass
firing cycle
Air
3 in/min
Default
N2
4 in/min
Default
N2
4 in/min
Non-intermixing
100%
100%
100%
attempt
Air
2 in/min
100%
Die bond
attempt
Air
2 in/min
65%
Die bond
attempt
Air
2 in/min
65%
Die bond
attempt
N2
4 in/min
2760 firing cycle
N2
2 in/min
Die bond cycle
65%
65%
Oxide glasses have a large gradient in its thermal expansion coefficient near their glass
transition temperature. The manufacturer suggests that any residual stresses arising from a
cooling cycle through this temperature can be removed using an anneal cycle at a temperature
of 3150 . The transition temperature, which the manufacturer identifies as the softening
temperature, occurs at 350'. To minimize residual stresses in the glass during firing cycles, a
slow cool through the transition temperature is desired. It is also desirable to fire the glass
layers at as low temperature as possible, to further lessen residual stresses. Cycle 3 also had a
slower ramp profile, with a fast cool. This is undesirable for glass firing. Further cycles
attempted to produce a profile with a slow cool. Figure 6 below shows a profile obtained by
running a thermocouple through firing cycle six. This is believed to be the slowest cooling
profile obtained.
Figure 6: Cycle 6.
The cooling profile obtained in the belt furnaces is too fast to provide for any
significant stress relief, so an anneal cycle was added. The development of this anneal cycle
will be discussed in section 9.7, Anneal Cycle.
9.3 Masking
The substrate is metallized prior to the glass deposition steps. The metal runs must be
free of glass to ensure electrical contact between the carrier, substrate, and die. A masking
fixture was designed to shield the metalized area of the substrate during the glass spraying
process steps. The mask consists of an aluminum plate with recessed cavities for the
substrates, and a mylar sheet with an opening that matches the dimensions of the desired glass
bond area. The design of this mask assembly is included as Appendix B.
There are two possible mask sizes, which correspond to the two possible electrical
attachment techniques. The ballbond attachment technique uses a masked area that is smaller
than the die, to allow the ballbonds at the edge of the die to contact the metallized areas of the
substrate. The final bonded die assembly must have glass at the edge of the die to prevent
damage to the active side during thinning. This arrangement relies on surface tension to cause
the glass to wick to the die edge after bonding.
With the second electrical attachment technique, the masked area is slightly larger than
the die. The wirebonds can be attached to the metallized area beyond the glass, and not
necessarily directly under the metallized pad on the die. This technique does not rely on glass
wicking to produce a fully sealed die edge.
After the glass is applied with the airbrush, the substrate is visually inspected with a
microscope. Any overspray is removed with a vacuum pencil. Experiments with the SRRG
grade of 2760 glass show more residual overspray than the VEG grade. This is due to the
difficulty of removing the overspray of the extremely fine SRRG grade particles.
This vacuum process is tedious, and likely to be a bottleneck in a production-scale
process. Therefore, other masking techniques have been proposed. One is to remove glass
using a photoetch process. Using this method, no masking apparatus is necessary, and the
entire substrate can be coated with glass. After the final glass deposition step, the glass above
the metallized area can be etched away. This technique would also eliminate the need for glass
movement in the ballbond-type arrangement.
9.4 Optimal Layer Thicknesses
The total bond glass layer thickness must be thick enough to produce a continuous bond
between the substrate and die. In the ballbond-attached arrangement, there must be enough
glass to allow for flow to the die edges in the time duration of the bonding furnace cycle. The
glass layer also must have a flat surface, with a surface that is uniform enough to provide
contact with the die over the entire die surface. Flow occurs in the vitreous glass phases in the
glass layer, but the filler particles remain solid throughout all furnace cycles.
During initial research, spalling and substrate adhesion concerns led to a necessary 3:1
filled glass: 2760 glass ratio. To obtain the necessary surface to provide for complete bonding
over the die surface, glass layers of 3-4 mils were necessary. This deviated from the original
concepts of using very thin layers, but glass surface irregularities were determined to be the
dominant factor in interfacial bonding. Using the stress models proposed in section 4.3, this
gives a level of stress of 23 ksi, for a 4 mil layer of glass on an LZS substrate.
It was discovered that hand lapping can greatly improve the surface of the glass,
without the need for a 3-4 mil glass layer. Lapping will remove the non-vitreous material
along with the glass, and produce a very flat layer. Substrates are now lapped after deposition
32
of the initial filled glass, before the application of the 2760 glass layers. Because the added
2670 glass is completely vitreous, no lapping is required after 2760 application.
The filled glass, 2008 or 7585, is applied as before, and a layer of 2.5-3 mils is built up.
The substrate is then lapped until the filled glass layer is uniformly 1 mil thick. This ensures a
continuous layer after lapping. A 0.5 mil layer of 2760 glass is then applied on top of the filled
glass. At the firing temperature, the matrix glass in the filled glass and the applied 2760 both
flow. The result is a 1.5 mil layer of glass with a uniform flat surface of vitreous glass. A 1.5
mil layer of glass on a LZS substrate is calculated to have a residual stress level of 23 ksi (164
MPa).
9.5 Crazing
General lead oxide based glass can support a tensile stress of 12 ksi [9]. Because all the
glasses in this research are lead oxide based, it is assumed that they will have an ultimate
tensile stress of about 12 ksi. Based on a Stoney formula approximation, the tensile stress in
glass deposited on an AIN substrate is 23 ksi. Since AIN has the highest thermal expansion
coefficient of all the substrate materials (4.2 x 10-6), this arrangement should have the lowest
possible residual stress. Because all glass/ substrate samples exceed the ultimate tensile stress
of the glass, crazing is evident on the glass surface. Crazing is a network of cracks over the
surface, which relieve the residual stress.
Crazing could be detrimental to die performance in a number of reasons. The small
cracks could retain atmosphere when the device is operated under a vacuum. The gas would
pollute the high vacuum environment and degrade the resolution of the CCD. Also, if the
cracking occurs after the die is bonded to the glass, the sudden strain could damage the active
surface of the die.
Figures 7a and 7b below show crazing of 2760 glass on Pyrex and silicon. Crazing
grain sizes scale roughly with the calculated stress values. The crazing grain size of the glass
on Pyrex, Figure 7a. is slightly larger than the grain size of the glass on silicon, Figure 7b.
top,%Ij
Figure 7a: 2760 glass on Pyrex
Figure 7b: 2760 glass on silicon
The severity of crazing is greatly reduced with the addition of the filled glass layer. and
an anneal treatment. Samples using the final glass composition and process have been etched
to the point where only the passivation layer is left on top of the glass, and cracks are not
observed in the passivation layer. Crazing is still evident in the glass, however.
9.6 Die Attach
The die is attached to the substrate by placing the die against the bond glass on the
substrate, and heating the arrangement until the glass becomes vitreous. A load is applied to
the arrangement, so the bond glass layer can deform and provide intimate contact with the die
surface. The bond glass surface must be extremely flat before bonding can occur.
A template jig was designed to align the die ballbonds and the metallized pads on the
substrate. Because the chip is bonded in a flip-chip arrangement and all substrate materials
(except Pyrex) are opaque, this can not be done using visual alignment. The jig has an opening
to allow for the application of a spring clip. The spring clip holds the arrangement together
during the initial bonding cycle.
The amount of force on the unbonded arrangement is critical for device performance.
The arrangement can be clamped using from one to four clamps, and the clamps are available
in either two or four pound loads. Careful control of glass layer and ballbond height is critical
in the bonding operation is critical, as the amount of stress on the glass and ballbonds is
dependant on the contact area of the arrangement. If the ballbonds are higher than the glass,
then all the stress from the clip is distributed among the ballbonds, and not carried by the glass.
If the ballbond height is not uniform, and not all the ballbonds contact the substrate, the stress
levels in the higher ballbonds can be extremely high. Preliminary experiments have
demonstrated that stresses in the ballbonds can exceed the compressional yield stress of silicon,
and cause chips in the die directly under the ballbond. Ballbond lapping produces ballbonds of
uniform height, and alleviates this problem. To ensure electrical connection, it is desirable to
have ballbonds that are slightly higher (0.2 mils) than the glass pad. This ensures that the
ballbond will connect with the substrate metallization.
35
After clamping the die/substrate arrangement in the jig, it is heated in cycle 9. This
seals the die, but does not allow for enough glass deformation to produce a good bond. After
this initial bond, the die can be removed with the application of a small shear stress. However,
this bond cycle provides a seal of the active surface of the die in a nitrogen atmosphere. This is
believed to ensure against oxidation degredation of the die.
9.7 Anneal Cycle
After the initial seal cycle, the arrangement is bonded in a small (440 cm 3 ) box furnace,
in air. Because the ballbonds and glass are now in contact at both interfaces, a larger bonding
force can be applied without damage to the silicon surface. In addition, flat pieces of alumina
are placed on both sides of the bonded arrangement to distribute the stress equally over the
surfaces.
This furnace cycle heats the arrangement to a temperature where the glass is again
viscous, and then cools to anneal the glass at 315 0C. This anneal temperature is below the
softening temperature of 2760 glass. According to the manufacturer, an anneal at this
temperature for ten minutes will remove the residual stress from bonding. This furnace cycle
is diagrammed below:
415C
400
T
time
Figure 8: Anneal cycle for bonded devices
This cycle produces a bond that is uniform, and can withstand manual applied shear stresses.
If the glass surface is sufficiently uniform before bonding, the bond will have no visible
porosity. Some crazing is still evident under the die, although to a much lesser degree than
samples solely bonded in a belt furnace.
If the sample is electrically bonded using the ballbond method, the glass must flow
outwards past the connected ballbonds to seal the entire active area of the die. Many
experiments were performed to determine what process variables would allow consistent flow
of glass to the die edge during the final bond/anneal cycle. However, it seems too difficult to
produce repeatable results on a laboratory scale. Substituting the mask with the larger opening
for wirebonding, with a vacuuming step to remove the glass from the metallization runs before
firing, should produce more repeatible results. The new photoetching process should eliminate
the need for any mask apparatus.
10 Results: The Packaging and Interconnection Process
This development suggests a process that will produce an optimal bond between the die
and substrate, using the proposed materials systems.
10.1 Glass Layer Firing
The first step in the packaging process is the choice of substrate material. This process
is compatible with four substrate materials. To minimize extraneous variables in the process,
aluminum nitride and silicon substrates follow an identical filled glass procedure; and Pyrex
and LZS substrates follow a common procedure.
2008 glass is used as the first layer on Pyrex and LZS substrates, and 7585 glass is used
on silicon and aluminum nitride. This glass is deposited using an airbrush procedure,
described in section 8.1, Glass Deposition. Successive glass layers are deposited and fired
until the total glass thickness reaches roughly 3.5 mils. This ensures a continuous layer after
lapping. The glass is then hand lapped to a thickness of 1 mil. It is necessary at this point to
have a continuous layer of glass.
After the sample has been lapped to a glass layer thickness of 1 mil, 2760 glass layers
are deposited and fired using the firing cycle 8. The glass layer thickness is measured after
each firing, and the glass is built up to a total glass layer thickness of 1.5 mils.
10.2 Bond Cycles
If the die is to be electrically connected to the substrate using the ballbond procedure,
the ballbonded die must be face lapped. A hand lap of 2-3 seconds on a 3000 mesh lap will
ensure uniform ballbond height. After a 1.5 mil glass layer is deposited and fired on the
38
substrate, it is aligned with a die in the alignment jig. One four pound clip is placed on the diesubstrate assembly, and the arrangement is fired in cycle 9, under nitrogen. This will seal the
assembly.
The assembly is then sandwiched between two flat pieces of alumina to distribute the
bonding load. Four four-pound clips are then attached to the sandwich, and the whole
assembly is fired in the anneal/ bond cycle described in section 8.7, Anneal Cycle. The device
is then ready to be thinned and connected to the CCD sensor apparatus.
11 Testing and Reliability of Bonded Devices
The reliability of these devices can not be fully tested until a whole assembly is placed
in operation. However, there are numerous ways to quantify the success of the mechanical and
electrical bonds. While these tests provide some measure of how well a bonded assembly will
perform, only field tests of the whole CCD device under operating conditions will give a true
representation of possible performance.
11.1 Manual Mechanical Bond Test
After a die is bonded to a substrate, adhesion is investigated in a purely qualitative
basis by applying a small amount of manual force to the die. This only ensures that the die will
not separate from the substrate with regular handling during subsequent testing, and that some
contact is made between the die and the glass.
11.2 Thermal Cycling
It is desirable for the mechanical bond to be able to withstand the cyclic stresses
involved in repeated thermal cycling. To test cyclic thermal exposure resistance, bonded
samples using all four possible substrate materials were put through two separate thermal
exposure profiles. The tests were carried out in air, and attempted to emulate the range of
temperatures that a finished device would be exposed to during operation. The first set of
samples were exposed to 21 cycles from temperatures of -250 C to 750 C. The cycles were
somewhat sinusoidal, and had a frequency of 1.3 cycles per hour. The second set of samples
were cycled from -300 C to 750C with a frequency of 0.8 cycles per hour for a total of 12
cycles.
No degradation of the bonds was observed after these thermal cycles. It is assumed that
the thermal cycles used in this experiment are less severe than any possible use conditions, so
degradation of the bond due to temperature fluctuations during use should not be a major
concern.
11.3 Die Connectivity
To test the success of electrical bonding attempts, all bonded devices were tested for
electrical connectivity. This was accomplished by contacting the metallized runs on the
substrate with electrical point probes, and measuring electrical characteristics across circuits on
the die. This gave a discrete method to determine electrical connection status at each ballbond.
This test also attempted to determine allowable thermal exposure of the dies by comparing
impedance changes with duration of thermal exposure. No changes in electrical behavior were
observed, however.
Table 7 shows the results of electrical connectivity tests for devices. The column
labeled 'bonds connected' gives the number of bonds that showed a standard electrical
response for diodes in the known circuits. This column also reports the total number of bonds
that were probed. In most cases, all 31 bonds were not probed.
This table shows the increase in connected bonds chronologically, with sample
numbers. The increase in connected bonds through subsequent experiments should be noted.
It should also be noted that all samples from number 43 onwards show full electrical
connectivity of all probed bonds. This demonstrates that the electrical and mechanical bonding
procedures are compatible.
Sample numbers 28-42 are the results of experiments with the 2805 glass. These
experiments were discontinued, as the 2805 does not exhibit the required viscoelastic
characteristics in the range of temperatures required for bonding.
This table also presents the results of some experiments with 2004 glass. This glass is
another 2760 derivative that has recently become available. 2004 glass has good viscous
properties at the bonding temperature, and shows promise to replace the 2008 glass in the
process described in this research.
Table 7: Results of electrical connectivity tests
notes
thermal history
bonds
Test
sample
connected
number
6
13/25
final treatment in
4 mils of glass
cycle 2
rebonded- one side of die connected
6
13/23
rebonded again, same result
7
11/26
one bond only works with pressure on die
9
25/25
bond cycle only
no glass
11
4/16
bond cycle only
no glass,
substrate is broken, some metallization gone
13-27
no glass- ballbond experiments
28-42
experiments with 2805 glass
43
29/29
before final anneal
glass lapped- final thickness 1.2 mils
die with polyimide coating
43
29/29
43
29/29
44
25/25
44
25/25
bonded with 8 lb load
after die thinning
full process
probe tested
experiments with 2004 glass
44-56
57
after final anneal
25/25
SiC coating on die, 2004 glass
12 Conclusions
This research demonstrates that it is possible to package silicon CCD dies with
materials that are compatible with extremely high vacuum environments. Bonds using
materials with dissimilar thermal expansion coefficients can be accomplished, as long as the
bond layer is physically constrained to lessen the effects of this dissimilar thermal expansion
coefficient. This research suggests that crazing can occur in the bond glass layer without
causing extensive damage to the operation of the device. However, the extent of this crazing
should be minimized through further research.
The results of die thinning experiments have shown that the level of physical constraint
drastically changes when the die is thinned to 10 pým. This suggests that physical constraint
alone is not enough to remove the effects of thermal expansion coefficient mismatch.
However, the proposed process demonstrates that the glass layer can be engineered to
minimize these effects on the active area of the die. The manual and thermal cycling test
results demonstrate that the assembled device should be able to withstand any mechanical
stress during final device assembly and use.
The results reported in Table 7 demonstrate that the process can produce a
mechanically bonded assembly that is fully electrically connected. Therefore this process can
be used in this application, with the expectation of a fully connected and working device. It
can be observed that all samples from sample number 43 onwards demonstrate dies that are
fully electrically connected. This reproducibility demonstrates the robustness of this process.
Therefore, slight variations in process variables will not cause failures in bonded devices, and
it is likely that the process can be scaled to an industrial level.
This research suggests that while it may not be possible to fully eliminate the effects of
thermal expansion coefficient mismatch stress, it is possible to design a process to minimize
these effects to a point where they do not effect the performance of the CCD sensor.
13 Future Research
The initial successes of the 2004 glass show promise. Dies have been attached to Pyrex
substrates using this glass, and no crazing has been exhibited after bonding. This is significant,
since the thermal expansion coefficient of this glass is roughly the same as the 2008 and 7585
glass. There are many problems yet to be addressed considering this glass, however. A
continuous 2004 glass layer has not been achieved, so a glass layer free of both porosity and
crazing has not been observed. This is significant, as porosity is theorized to remove some of
the stress that would cause crazing in a continuous layer. There are also concerns that porosity
could cause pollution of the high vaccum environment in service.
To lessen the extent of porosity in glass layers, proposed experiments will investigate
glass porosity in glass layers fired under vacuum. Theoretically, the low pressure environment
will discourage the formation of pores in the viscous molten glass.
Photographs of thinned dies show no surface effects of the glass porosity when bonded
using 2004 glass. This suggests that a continuous glass layer may not be necessary for optimal
die performance. These results also suggest that porosity may be more desirable than crazing.
Experiments are proposed to investigate the effects of coatings on the die surface. It is
proposed that any possible damage to the die surface due to crazing could be lessened with the
application of a barrier coating. Polyimide, spun-on glass, and silicon carbide coatings have
been proposed. Preliminary experiments using the polyimide coatings suggest that damage to
44
the die surface can be prevented. However, this polyimide is an organic material, and not
compatible with the high vacuum environment of the sensor assembly. Experimentation must
be performed using inorganic coatings, such as silicon carbide or the spun-on glass.
Li20-A1203-SiO2
S10,
14 Appendices
14.1 Phase Diagrams
PbO-SiO2
m•
2050*
Rustum Roy and E. F. Osborn, J. Am. Ceram.
Soc., 71 [6] 2086 (1949); slightly modified by
M. Krishna Murthy and F. A. Hummel, ibid..
37 [1117 (1954).
PbO-B 20 3 -SiOz
R. F. Geller, A. S. Creamer, and E. N. Bunting, J. Research Natl. Bur. Standards, 13 [2] 243 (1934); RP 705.
40
Of
i
PbO-AlsOr-SiOj; high-PbO portion.
R. F. Geller and E. N. Bunting, J. Research Natl. Bur.
Standards 31, 264 (1943); RP 1564.
46
R. F. Geller and E. N. Bunting,
J. Research Natl. Bur. Standards, 23
18] 279 (1939); RP 123.
14.2 Mask Design
[-V
-
0.375"
0.5"
SII
'0'I
I
2.750"
pin offset
3.25"
0.75"
0.5"
O
Pins: 0.122" dia.
O
Pin holes in cover:
0.125" dia.
3.25" sq.
Material:
Aluminum, 6061T6
2.750"
Cover - 0.250" thick
O
O
Cavity 1:
0.65" x 0.55" on
center
Depth: 0.032"
12.750"
hole
offset
O
O
2.50" sq.
47
3.25"
Cavity 2:
0.65" x 0.55" on an
(+0.5", +0.75") offset
Depth: 0.040"
Cavity 3:
0.65" x 0.55" on an
(-0.5", -0.75") offset
Depth: 0.015"
Josh Bennett
x2984, rm 4251
15 References
1 Beebe, D. J. and Denton, D. D. A Flexible Polyimide-BasedPackagefor Sensors. Sensors and Actuators. Vol.
A44. 1994. pp 5 7 -6 4 .
2 Loehman, R. E. and Tomsia, A. P. Joining of Ceramics. Ceramic Bulletin. Vol. 67, n. 2. pp 375-380.
3 Pask, J. A. From Technology to the Science of Glass/Metaland Ceramic/MetalSealing. Ceramic Bulletin.
Vol. 66, n. 11. 1987. pp 1587-1592.
4 Shanahan, M. E. R. Adhesion and Wetting: Similarities and Differences. Rubber World. Vol. 205, n 1. pp
28-36.
5 Field, L. A. and Muller, R. S. Fusing Silicon Wafers with Low Melting Temperature Glass. Sensors
and Actuators. Vol. A23. 1990. pp 935-938.
6 Geiger, G. Glass in ElectronicPackagingApplications. Ceramic Bulletin. Vol. 69, n. 7. 1990. pp 1131-1136.
7 Shukla, R. K. and Mencinger, N. P. A CriticalReview of VLSI Die-Attachments in High Reliability
Applications. Solid State Technology, July 1985. pp 67-74.
8 Ohring, M. The Materials Science of Thin Films. Academic Press, Inc. San Diego, 1992.
9 Kingery, W. D. Introduction to Ceramics. Wiley. New York, 1976.
10 Chen, W. T. and Nelson, C. W. Thermal Stresses in Bonded Joints. IBM Journal of Research Development.
Vol. 23, n. 2. March, 1979. pp 179-188.
11 Frost, H. J. and Ashby, M. F. Deformation-Mechanism Maps. Pergamon Press. Oxford, 1982. p 25.