Metal-based wafer level packaging
!
Figure 4. Image degradation from sample effects.
!
Figure 3. Aluminum–germanium phase diagram showing simple eutectic reaction
at 56.1wt% Ge and at 420°C.
Figure 6. a) Bubbles formed
in the eutectic alloy due to
overheating; b) smearing of the
eutectic alloy in the direction of
non-parallel force application.
After eutectic bonds
are cooled, there is a
possibility that microvoids
form in the eutectic micro-
structure. These voids may
be due to the Kirkendall
effect, which occurs when
one element diffuses more
quickly than the other
and the lattice sites left
behind are empty
7
. Or,
Figure 5. Suggested male and female targets for IR or ISA alignment. Note dual
rather, they are filled with
!
scale verniers provide either 0.25µm or 0.1µm resolution.
vacancies. If a substantial
vacancy concentration ex-
eutectic reaction is at 51.6 wt% Ge and has layer on both sides of
!
ists, then the vacancies can
a solid to liquid transition (eutectic point) the interface, then the wafers are simply cluster and lead to microvoiding. In most
at 420˚C. In most eutectic bonding ap- aligned, brought into a
!
contact and pressed cases this can be overcome by adjusting
plications, the rule of thumb is to remain together. After contact is established, the the cooling rates and the amount of hyper
at 7-15˚C above the eutectic temperature. wafers are heated to the eutectic tempera- eutectic heating.
Going to higher temperatures will increase ture, melted and re-solidified. Alloys can
the fluidity of the alloy and can lead to be deposited by sputtering of alloy targets alignment
excess flow into unwanted regions in the or electroplated in some but not necessar- Alignment accuracy is affected by the qual-
die. However, if the temperature is not ily all cases. The advantage of the direct ity of the alignment keys, the method of
uniform, the viscosity of the melt will melting of alloy layers is speed because the imaging used during wafer-to-wafer align-
vary. Some areas on the wafer may be diffusion step can be avoided. ment and the thermal response of the wa-
solid and others liquid, and the wafers will Alternatively, the aluminum can be de- fers during heating and bonding. There are
crack under the applied force as bending posited on one wafer and the germanium three primary alignment methods used for
moments develop in regions with varying on the other substrate. Then the wafers metal bonding. These are BSA (backside
compliance. are pressed together and heated (below alignment), ISA (inter-substrate face-to-face
The next decision that must be made 420˚C) until the interface mixes. Note alignment), and IR (infrared) alignment.
for a eutectic bond is whether or not the that limited solid solubility means that the In general, metal bonding provides a huge
bond will be done by melting the alloy diffusion is only a few percent, and grain benefit over other types of bonds whenever
or by a combination of diffusing pure boundary reactions will play a major role the fiducials can be defined with the metal
materials together in the solid state to in the success or failure of the bonds when layers. The sharp contrast between the
reach the eutectic composition and then completed with this strategy. After mixing metal and semiconducting layers provides
melting the alloy. If the alloy, for example the material, the wafers are reflowed and excellent human or automated image
51.6 wt% Ge, can be deposited as an alloy cooled. resolution.
Backside polished wafers are recom-
12 – Global SMT & Packaging – June 2009 www.globalsmt.net
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