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

strength. The subsequent chapter discusses the observed strength behavior, using this improved
test methodology.
1.2 Improvement of MEMS Tensile Strength Evaluation Methodology
Several testing techniques have been published with widely varying tensile strengths
appearing in the literature - between 1 to 4 GPa [1.1-1.6]. Much of the variation between authors
has been explained in terms of microstructural differences due to deposition conditions, sample
size effects and release processing. A previous cross comparison exercise involving direct and
indirect testing techniques using the same material, but different releases techniques, reported
significant variations [1.7].
Tensile data was collected from five investigators that employed two essentially different
types of samples, with further variations in size within each group. The larger sized group of
samples were designed to be gripped with an electrostatic force applied to the enlarged end of a
sample, the tensile force application and measurement were performed with a macro scale
system; slightly different versions of this system were used by Tsuchiya [1.8], and Sharpe and
Coles [1.9]. Chasiotis and Knauss also tested this size sample, but used the electrostatic force
only to assist in the adhesive bonding of the sample to the grip [1.10]. Samples of four sizes
were tested by these three labs, with widths of 6 and 20 µm and lengths of 250 and 1000 mm.
The second sample type, tested by Read [1.11], and by LaVan at Sandia [1.12], is 1.8 µm wide
and 15 to 1000 µm long.
All of the samples were produced using Sandia National Labs SUMMiT TM IV polysilicon
process – they were patterned in the poly1-2 composite layer that is 2.5 mm thick. Samples of
all sizes were produced side by side on the same die, five or more die were sent to each
participant. The films were deposited as n-type, fine grained polysilicon from silane in a low
pressure chemical vapor deposition (LPCVD) furnace at ~580°C. The intervening sacrificial
oxide layers were also deposited in an LPCVD furnace from tetraethylorthosilicate (TEOS) at
~720°C. This process usually uses 6-inch, (100) n-type silicon wafers of 2 to 20 ohm/cm
resistivity covered by 6000 Å of thermal oxide followed by 8000 Å of LPCVD silicon nitride for
electrical isolation. Thickness was accurately controlled during the deposition process and was
measured, along with width, in a calibrated SEM after release (accuracy 0.1 µm). The samples
were released, coated with a self-assembling monolayer (SAM) such as octadecyltrichlorosilane
(ODTS) or perfluorodecyltrichloro-silane (FDTS) as an anti-stiction coating and then dried with
super-critical CO2. The microstructure and crystallographic texture of this polysilicon have been
well characterized. The texture is random. The grain morphology is columnar, with a mean
column diameter of 300-400 nm. Most of the grains bridge from the top to bottom surface of the
film. More details of the process may be found in [1.13].
As shown in Fig. 1.2, the strength values measured at Sandia, had both a higher mean
strength and a higher scatter in strength. Since all other participants obtained values within a
reasonable scatter band of each other, the Sandia methodology was suspected of inducing
anomalous strength values.
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