According to results presented above, we postulate
the phase formation mechanism as follows. The
eutectic 57Sn –43Bi alloy has relatively low melting
point of 139 jC. When the Bi and Sn layers of
eutectic composition are deposited, they form the
typical eutectic lamellar structure due to the long
deposition time and the heat inside the chamber. As
the Au layer is deposited over the Bi –Sn eutectic
alloy, Au atoms diffuse into the Sn-rich phase and Birich
phase to form Au –Sn intermetallic compounds,
resulting in the capping layer. At high temperature, the
Sn-rich phase contains significantly amount of Bi.
When the sample is removed from the chamber and
kept at room temperature for a few weeks, the Bi
atoms in the Sn-rich phase gradually diffuse from the
Sn-rich phase into the capping layer, thus expanding
the thickness of the capping layer. The capping layer
covers most of the surface area, minimizing the
exposure of the inner Sn-rich and Bi-rich phases to
oxygen in air. Since AuSn intermetallics are very
stable and oxidation tendency of Bi is relatively weak
[13], the capping layer can effectively prevent oxidation
on the inner phases, making fluxless bonding
possible.
In conclusion, we have studied the microstructure
and intermetallic crystal properties of Au/Sn/Bi thin
films deposited on a silicon wafer in high vacuum.
The microstructure is composed of a capping layer on
a typical Sn –Bi eutectic alloy of lamellar structure.
The capping layer is formed by Au –Sn intermetallic
compounds embedded in a Bi-rich matrix. The average
sizes of AuSn2 and AuSn4 crystallite are 756 and
667 A˚ , respectively. The lattice constants of AuSn4
are determined to be a = 6.3694, b = 6.4092 and
c = 11.4785 A˚ , closed to bulk values within 2%.