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318 MEMS and Microstructures in Aerospace Applications

SRI report 25 contained data from June 1964 to August 1967 and provided a
reference for choosing materials for use in spacecraft with low outgassing proper-
ties. The SRI apparatus was constructed at Goddard Space Flight Center (GSFC) in
26
1971, and the GSFC report came up with two criteria: a maximum total mass loss
(TML) of 1.0% and a maximum collected volatile condensable material (CVCM) of
0.10%. Eventually, an American Society for Testing and Materials (ASTM) Stand-
ard Test Method was developed and is identified as E 595-77/84/90.
The GSFC equipment is the SRI-described micro-CVCM apparatus, which
condenses micro-quantities of volatile liquids to determine the amount of volatile
condensable materials. The testing is done in vacuum, and critical dimensions of the
apparatus are given in ASTM E 595-77/84/90 in order to produce similar results.
The set up allows 12 samples to be tested at once, with each in its own separate
chamber in a solid copper bar covered by a solid copper cover. The bar is heated to
398 K for 24 h, which forces all volatile materials out through a 6.3-mm diameter
escape hole. Materials are collected on their own chromium-plated disks, 12.7 mm
from the hole, which are kept at a constant 298 K. The volatile materials will collect
on the disk if their condensation temperature is 298 K or higher. The TML is
calculated by taking the percent mass loss of the sample after heating at 398 K, and
the CVCM is determined by calculating the mass of condensable material on the
collector as a percentage of the total initial mass of the sample. The amount of water
vapor regained (WVR) is also sometimes calculated, based on the percentage of
mass gained due to water readsorption or reabsorption in 24 h at 258C when the
sample is in 50% relative humidity.
In the reports, materials are grouped in three ways: Section A groups materials
based on 18 of their probable uses (adhesives, paints, etc.); Section B groups all
materials in one alphabetical list; and Section C groups them by use as in Section A,
but only with those materials that pass TML less than 1% and CVCM less than 0.1%.

14.5.8 HUMIDITY
The strongest surface forces are caused by capillary condensation. As the relative
humidity increases stiction failures will rise. This will affect MEMS devices
exposed to the outside environment waiting for launch and devices exposed to the
environment of planetary exploration missions. Designing with stiction mitigation
strategies is recommended in these cases.

14.6 MATERIALS
14.6.1 SINGLE CRYSTAL SILICON
Single crystal silicon (Si) is the most widely used semiconductor material and is the
most common MEMS substrate. It has a diamond (cubic) crystal structure and an
electronic band gap of 1.1 eV. It can also be doped with impurities to alter its
conductivity, at the expense of introducing defects into the crystal lattice. Single
crystal silicon is a brittle material and instead of undergoing plastic deformation
like metals it yields via catastrophic failure. This is an advantage for sensor

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