Metal-based wafer level packaging
Metal Temperature Applied Force Time Atmosphere
Range
†
Al 400-450˚C >70 KN 20-45 min Vac or H2/N2
Au 350-450˚C >40 KN 20-45 min Vac or H2/N2
Cu 380-450˚C >30 KN 20-60 min Vac or H2/N2
†
Applied force depends on wafer diameter and pattern density. Table values are representative of 200 mm wafers.
Table 2. Typical processing parameters for metal diffusion bonding.
!
Figure 2. Images on the right are the wafer as
can be used at lower temperatures than the oxygen from the surface diffuses through received. The wafer was then heated in a bonding
other options. the Al
2
O
3
layer to provide the source of
chamber to 450˚C for 30 minutes. Right upper
Diffusion rates vary dramatically additional oxigen until the surface is fully
images were cleaned with formic acid cleaning prior
depending upon the reaction path. Surface passivated. However, the surface oxide can
to heating and the right lower wafer was not cleaned.
diffusion is diffusion along the terraces be cracked, and bonding can be completed
of a surface. Since there are no atomically by using a high applied force. The composition and temperature
flat surfaces on MEMS wafers, the atoms Gold diffusion is the lowest tempera- define the reaction and are unique to only
will move from surface site to surface site ture process of the three and is successfully a few materials systems. Table 3 shows
to reduce the terracing and free energy. managed at temperatures as low as 380°C. the alloys most often used for wafer level
This is the most rapid diffusion process, Unlike copper and aluminum, gold does bonding. The choices are alloys of gold,
since the atomic motion occurs relatively not readily form an oxide and under aluminum or copper, since these materials
unimpeded. normal processing conditions; it is not are already used in semiconductor fabrica-
Grain boundary diffusion is the next necessary to use surface cleaning prior to tion labs and in most cases have estab-
most rapid reaction pathway. Since most bonding. lished processing and deposition methods.
deposited layers are polycrystalline, there Copper, on the other hand, read- The most common choice is the gold- tin
are numerous boundaries between the ily forms a surface oxide. The oxide can system with interest in other alloys fairly
grains in which 1:1 atomic lattice matching be successfully removed and the surface evenly distributed.
is incomplete. This leaves empty space in passivated by the use of formic acid vapor Choosing the correct eutectic alloy
which the atoms can migrate freely. cleaning. Vapor cleaning with formic acid for an application is most often deter-
Diffusion through the bulk of the crys- is used in 3D vertical integration to ensure mined by the processing temperature and
tal is the slowest of the three mechanisms. high conductivity of interconnects
5
. The compatibility of the materials with the
Exchange of atoms or vacancies within the vapor cleaning is demonstrated in Figure existing manufacturing flow. In addition
lattice enables the mixing to occur. The 2. A patterned copper wafer was exposed to alloy selection, it is equally important
onset of bulk diffusion is typically 1/3 to to elevated temperatures with and without to determine the method of eutectic alloy
1/2 of the melting point of the material the formic acid cleaning. The cleaned formation.
and increases exponentially with tempera- wafers were able to withstand post clean- Eutectic alloys have several advantages
ture in a Arrhenius relation
2
. However, ing exposures to high temperature in the over the diffusion processes, including
oxidation and impurities in the metal films bonder and subsequently showed improved lower processing temperatures and reflow.
also play a significant role in diffusion reac- electrical performance. The reflow process enables the interface to
tions and generally reduce the diffusion self-planarize and minimize the effects of
rates significantly. Thus manufacturing eutectic bonding surface topography or less-than-ideal CMP
techniques need to include clean deposi- The other major category of metal-based (chemical mechanical polishing) steps.
tion practices, and bonding using surface bonding used in MEMS packaging is the However, the reflow must be controlled
oxide removal and re-oxidation prevention eutectic bond. A eutectic alloy is some- through proper thermal and force applica-
are recommended. times called a “solder;” however this is not tion in the bonder.
Table 2 gives the recommended process- necessary the correct metallurgical term. A Figure 3 is the Al-Ge phase diagram
6
.
ing ranges for the three leading metal- eutectic alloy is a two-component alloy that This is a simple eutectic phase diagram
diffusion bonding techniques. Aluminum undergoes a direct solid to liquid phase with no intermetallic phase formation. The
bonding is generally not pure aluminum temperature at a specific composition and aluminum has a melting point of 660˚C,
but rather the metallization alloy used in temperature. and germanium melts at 938˚C. The
the fab, which includes up to 4% Cu or
other binary additions. Both aluminum Eutectic Alloy Eutectic Composition Eutectic Temperature
and copper bonding require temperatures
Al-Ge 49/51 wt% 419˚C
above 400˚C to achieve a good hermetical-
ly sealed interface. Aluminum also requires
Au-Ge 28/72 wt% 361˚C
a large applied force, which is apparently
Au-In 0.6/99.4 wt% 156˚C
needed to crack the surface oxide that
spontaneously forms on any aluminum
Al-Si 97.1/2.9 wt% 363˚C
surface. The surface oxide of aluminum is Au-Sn 80/20 wt% 280˚C
not soluble in the matrix (<2e-8 wt%) and
Cu-Sn 5/95 wt% 231˚C
tracer studies have proven that aluminum
does not penetrate the oxide
3,4
. Rather, Table 3. Eutectic alloy commonly used in MEMS wafer level packaging.
www.globalsmt.net Global SMT & Packaging – June 2009 – 11
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