Metal-based wafer level packaging
bond at 380˚C in which the cool-down
rates must be extremely slow in order to
control the additional stress from the
temperature difference between the two
metallurgies and expansion differences
between the substrate materials. In Figure
10, one can truly see the difference that
temperature and heating rate plays in ther-
mal expansion induced stress. The CTE
(coefficient of thermal expansion) for Si is
3.2 ppm, compared to 5.32 ppm for GaAs
and 8.4 ppm for sapphire. Thus the benefit
of the lower temperature process for the
AuSn eutectic compared to the Au-Au
diffusion process allows the Si to sapphire
to be successfully bonded even with the
extreme (3.2 vs. 8.4ppm) CTE mismatch.
conclusions
Several technologies based on metal bond-
ing methods are available as replacements
for glass frit or anodic bonding techniques.
The metal bonds are categorized according
to the type of metallurgical reactions that
are used. These are the diffusion-based
processes and the eutectic reactions. The
former does not involve reflow and gener-
ally requires a higher processing tempera-
!
Figure 10. Gold diffusion bonding of Si to GaAs at 300°C for 1 hour. Cooling rate must be extremely slow to
ture and potentially longer cycle times. The
prevent the subsurface cracking shown in these acoustic images of the bulk GaAs substrate
11,12
.
eutectic process enables reflow similar to
the glass frit processes and is more forgiv- increases in yield which most companies International, July 2001)p. 154.
ing to surface roughness. The choices for are beginning to report. 7. Paul G. Shewmon, Transformations in
eutectic alloys are varied, and processing Metal bonding is a valuable asset to
Metals, (J. Williams Book Company,
temperatures range from slightly above MEMS wafer level packing. The economic
Jenks, OK, 1983)p.58.
150˚C to over 400˚C. Thus it is possible and technological advantages are clear, and
8. Shari Farrens and James Hermanowski,
to find a metallurgy that is compatible with
“Lights-out MEMS Manufacturing”,
these methods will continue to increase
Avd. Packaging, V. Aug/Sept.2006, p.16-
materials requirements as well as post bond in use as markets become more consumer
18.
processing needs like solder packaging. oriented and integration with other com-
9. Shari Farrens, “Stop the Squish Now!
The key motivation for device manu- ponents increase.
How to Control Eutectic Flow in Her-
facturers to transition to metal based wafer
metically Sealed Devices”, Chip Scale
level bonding is the increased hermeticity references Review, V. June 2007, p44-47.
which improves device functionality but 1. Traeger, R.K., “Hermeticity of polymeric 10. William Wong, MSE Ph.D. The-
more importantly enables the continued
lid sealant”, Proc. 25th Electronics sis, “Excimer laser liftoff and processing
scaling of the device to smaller sizes. In
Components Conference, 1976, April, of GaN thin films and light-emitting
addition to device scaling the geometry of
pp 361-367. heterostructures.”, Prof. Nathan Ch-
the sealing rings surrounding the device
2. Chapter 9 “Metal-Metal Interdiffusion” eung, EECS, UC Berkeley) 1999.
J.E.E. Baglin and J.M. Poate Thin Films 11. S. Farrens and S. Sood, “Low
can be diminished further increasing the
Interdiffusion and Reactions (edited by Temperature Au-Au Thermal Compres-
number of die per wafer. In closing it is
J. M. Poate, K. N. Tu and I. W. Mayer). sion Boding of Thermally mismatched
worth noting that while the market growth
Wiley, New York (1978) p.311. Substrates”, to be published 214th Elec.
of products such as accelerometers, gyros
3. Thaddeus B. Massalski, et. al., Binary Chem. Soc. Symposium on Semi-
and RF MEMS is growing at more than Alloy Phase Diagrams, 2nd ed., (ASM conductor Wafer Bonding- Science,
25% per year through 2011, the ASP (aver- International, July 2001)p. 186. Technology and Applications.
age selling price) is expected to drop by
4. D. Starodub, T. Gustafsson, and E. 12. Sumant Sood, Tom Adams, and
8-13%
13
. Thus the analysis in Table 1 is very
Garfunkel, “The reaction of O2 with Ray Thomas, “Acoustic Characterization
poignant. This table compared, through
Al(110): a medium energy ion scattering of Bonded Wafers”, to be published
simple analysis, the number of additional
stuffy of nano-scale oxidation”, Surf. 214th Elec. Chem. Soc. Symposium on
Science , v.552, 2004, pp.199-214. Semiconductor Wafer Bonding- Sci-
die that can be placed on a wafer by switch-
5. S. Farrens, “Vertical Integration: A ence, Technology and Applications.
ing from a large seal area typical of glass
confederacy of Alignment, Bonding, 13. Yole 2007 MEMS Industry Report.
frit to a smaller metal seal. For example,
and Materials Technologies”, Mater. http://www.yole.fr/pagesAn/products/
a 3 mm x 3 mm die size switching from
Res. Soc. Symp. Proc., V. 970, fall 2006, report_MEMS.asp
a 100 µm seal to a 10 µm seal enables an p.215-223.
additional 363 more die. This does not 6. Thaddeus B. Massalski, et. al., Binary
include any assumptions of die scaling or
Alloy Phase Diagrams, 2nd ed., (ASM
16 – Global SMT & Packaging – June 2009 www.globalsmt.net
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