Metal-based wafer level packaging
Metal-based wafer level
packaging
by Shari Farrens, Ph.D., SUSS MicroTec, Waterbury Center, VT, USA
introduction
Metal-based wafer bonding for
The permeability rate of a material is a
WLP has several advantages,
rate at which gas atoms diffuse through a
material. Permeation rates can be com-
including enhanced hermetic-
pared for materials of equal thickness and
ity and the facilitatation of
under standardized atmospheres. Figure 1
vertical integration. These
shows this type of analysis for a generalized
advantages allow for reduction
categories of sealing materials; polymers,
in die size and cost savings glass and metals. By definition, materials
with improved device perfor- with less than one day of sealing capacity
mance. Until recently, first
(1e-14 gm/cm-s-Torr) are considered Non-
level packaging for MEMS
Hermetic, as indicated by the red line
1
. If
was done using glass frit or
a comparison is made between the glass
anodic bond process. The glass-
seal and a metal seal, both 10 µm thick, the
based bonding methods are
lifetime of the seal is a few years versus a
century or more. The metal seals used for
!
used in over 80% of volume
Figure 1. The Permeability of materials as a function
MEMS packaging have seal geometries of of the thickness of the material. Highlighted arrows
MEMS production for high
1-2 µm and could conceivably be reduced
compare metal and glass lifetimes.
volume products such as pres-
to less than 1 µm if mechanical integrity
sure sensors, accelerometers
(strength) could be ensured.
methods have numerous benefits and can
and gyroscopes. All of these The combination of improved herme-
be successful replacements for traditional
products, as well as RF resona- ticity and smaller seal rings enables signifi-
packaging techniques.
tors, require vacuum packag-
cant die size reduction. With reduced die
ing. The physical properties of
size, the number of die per wafer increases
Diffusion bonding
glass and frit sealing materials
as does the yield, all contributing to cost re-
Diffusion bonding is when two metals
translate into seal geometries
ductions. Table 1 compares the area savings
are pressed together under applied force
that are in the range of 100s
when the sealing geometry is reduced. The
and heat, which enables atoms to migrate
table assumes a constant dicing street size
of microns.
from lattice site to lattice site, “stitching”
of 75 microns. Using this type of analysis,
Using advanced bonding
the interface together. Diffusion processes
a 9 mm
2
die that was formerly sealed by
and bond alignment equip-
require intimate contact between the
a glass-based technology, can be replaced
surfaces, since the atoms move by lattice vi-
ment, in combination with
with a 2-3 µm wide metal seal, and several
bration. Generally speaking, softer metals,
metal bonding methods, hundred more die can be placed on the
such as copper or aluminum, are excellent
significant improvements in same sized wafer.
choices for diffusion bonding due to their
COO and device performance
The two types of metal seals available
ductile properties and rapid diffusion rates.
can be realized.
for wafer level bonding are diffusion-based
Gold is another excellent candidate and
and eutectic. Both of these fabrication
Die size (mm x mm) 3 5 7 10
Keywords: Wafer Level Packag-
ing, Metal Bonding, Diffusion
Effective die area w/50 µm seals 10.1 26.8 51.5 103.5
Bonding, Eutectic Bonding Effective die area w/25 µm seals 9.8 26.3 50.8 102.5
Effective die area w/10 µm seals 9.6 26.0 50.3 101.9
Max added die/wafer (100 µm > 50 µm) 187 44 17 6
This article was presented as a paper at
Max added die/wafer (50 µm > 25 µm) 101 23 9 3
the International Wafer-Level Packaging Max added die/wafer (25 µm > 10 µm) 63 14 5 2
Conference, held in San Jose, California,
Table 1. Comparison of effective die size and number of additional die added when seal geometry area is reduced.
October 13-16, 2008.
10 – Global SMT & Packaging – June 2009 www.globalsmt.net
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