Mechanics and Tribology of MEMS Materials - prod.sandia.gov ...

3 The Role Of Microstructure In SUMMiT TM Polysilicon Failure
3.1 Characterization of Polysilicon Microstructure
This sub-section describes the detailed microstructure characterization of SUMMiT TM
polysilicon using Electron Backscatter Diffraction imaging (EBSD). EBSD is a scanning
electron microscope (SEM) based microstructure mapping technique accomplished by stepping
the incident electron beam across a prepared sample. At each step, the backscattered electron
pattern is captured and used to spatially resolve unique microstructure information, particularily
crystallographic orientation. [3.1] For this study, a cross-section was extracted from a surface
micromachined SUMMiT TM structure containing each of the 5 polysilicon deposition layers.
Cross-sectioning was performed using focused ion-beam milling techniques recently developed
for extracting TEM samples. [3.2] High energy FIB required to section polysilicon also damages
the surface of the cut section. Thus, the cross-section is milled with a low energy beam to clean
and prepare it for EBSD.
Information from an EBSD experiment performed on the prepared cross-section was
assembled to create the microstructure map illustrated in Fig. 3.1a. This map uses an inverse
pole figure colorizing scheme, defined in the legend given in the lower right-hand corner of the
figure. The scheme assigns colors to different grains based on their orientation relative to the
deposition direction. [3.3] The resulting colorized image clearly reveals the microstructure
within each polysilicon layer. The grains have a mixed equi-axed and columnar morphology
with grain sizes ranging approximately between 0.5-3 µm. The figure shows that the grains do
not predominantly exhibit any color, and therefore no preferred crystallographic orientation.
Other features are evident in this figure, including the poly0-poly1 and poly1-poly 2 interfaces,
and the effects of a two-step deposition process in the poly 3 and poly 4 layers. A two-step
deposition process in the poly3 and poly4 layers result in smaller grains near the initially
deposited side.
Fig. 3.1b uses the data from the same EBSD experiment to plot a 〈001〉 micro-pole figure,
representing crystallographic 〈001〉 directions relative to a global coordinate configuration. The
center of the pole figure corresponds to the deposition direction. Small clusters of data in this
figure indicate measurements within single grains (examples and a more detailed description of
〈001〉 micro-pole figures is given in [3.4]). The absence of large scale clustering of data in this
figure also indicates no crystallographic texture. Fig. 3.1c represents a contoured pole figure
generated using the same EBSD data, contours are based on a "times random" crystallographic
orientation density. Times random may be defined as the density of grains with crystallographic
orientations relative to the expected grain-orientation density for a polycrystalline material with
no preferred orientation distribution. [3.5] The contoured pole figure does show slight
crystallographic texture, indicating a less than expected number of grains with their 〈100〉 axis
located near the center of the pole figure, or co-aligned with the deposition direction.
33