In Situ Observation of Stress-Induced Au-Si Phase Transformation
D. Huitink1, L. Peng2, R. Ribeiro2, and H. Liang1,2*
Department of Mechanical Engineering1 and Materials Science and Engineering,2 Texas A&M
University, College Station, TX 77843-3123
Materials
Gold and silicon were chosen in this study for their relatively low energy of mixing, as indicated
by the Au-Si low eutectic temperature. To coat the tip, a vacuum enabled thermal evaporator was
employed to deposit an intermediate ~5 nm thick layer of germanium (99.9999%, Alfa Aesar) on
the probe, followed by a 250 nm layer of gold (99.99%, Alfa Aesar). This enhances the adhesion
of the gold to the probe.
According to the equilibrium phase diagram (not shown), the gold (Au) and silicon (Si) system
forms a eutectic with small quantity of about 5%wt Si at 363 0C and exists as a perfectly mixed
liquid over a fairly broad temperature and composition range.1 Previously, equilibrium eutectic
systems have been used to fabricate nanorods and nanowires at a relatively low-temperature
through vapor-liquid-solid synthesis methods.2, 3, 4, 5, 6, 7 Among those, metastable phases of
AuSi and Au7Si appeared with different structural and electronic properties.
Silicon alloyed with gold forms a eutectic alloy which can be used as a low melting point solder
for silicon devices.8, 9 During soldering, it is desirable for silicon to have a high diffusion rate
inside the gold.10, 11
AFM Experiments
An AFM facilitated the control of sliding the probe on the surface of the silicon substrate in a
method analogous to traditional friction tests in the mild wear regime. After initially contacting
the surface of the Si, the AFM controller increased the loading on the gold coated probe
tip/surface contact point by increasing the deflection of the probe cantilever in 5 µm increments.
The deflection of the cantilever is directly proportional to the force at the probe-surface interface,
where the proportionality constant is a function of the cantilever composition and dimensions.
The AFM then controlled the reciprocating linear motion of the probe tip along the surface of the
substrate with a velocity of 480 µm/s. After this procedure, an alternate, uncoated AFM probe
*
Corresponding Author: hliang@tamu.edu
was used in "close-contact" mode (i.e. no physical contact) to observe any morphological
changes to the surface of the silicon wafer. Unlike in traditional wear tests, where a trough or
wear track is observed where sliding occurred, a deposition of new material was observed on the
surface of the Si. A line of 55 nm width and 1.3 nm height formed on the surface, as shown in
Figure 1. Furthermore, it was previously demonstrated that the formation of this structure is not
directionally dependent (or related to the crystal direction of the substrate) as indicated by nonlinear structures created using a patterning software. 1
In Situ TEM Experiments
Inside this system, the nanoindentation tip can be coarsely positioned in three dimensions with
screws, and further finely positioned and actuated with a 3D piezoceramic actuator which can
control the movement and applied force according to the applied voltage. A digital video
recorder equipped with the TEM captured the events of the indentation in real time for brightfield or dark-field imaging conditions.
A worn diamond nanoindenter tip of Berkovich-type geometry was sputtered with ~1 µm gold
(Mohs Hardness of 2.5-3,12 although likely softer due to the porosity resulting from sputtering)
and pressed into Si (Mohs Hardness of ~713) while monitoring by TEM. Prior to indentation, the
Si was ion milled by focused ion beam (FIB) to reduce the thickness of the Si to less than
approximately 250 nm in order to facilitate TEM observation. To achieve this electron
transparent thin feature, two opposing trenches are milled with the ion beam on a Si wafer. This
process was optimized to reduce the amount of damage by the 30 kV Ga+ ion beam in the FIB.
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