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120 MEMS and Microstructures in Aerospace Applications

each containing 72 shutters) and is assembled within a 9 10 3 cm enclosure.
The instrument is scheduled to fly on a NASA NMP ST5 technology demonstration
flight in May 2005. The key factors that led to the selection of this technology for
the demonstration flight are the simplicity and robustness of the core technology,
the mission-enabling nature of the technology (can enable 20-kg class satellites),
and the strong technical team.

6.2.9 TUNNELING INFRARED SENSOR ON THE SAPPHIRE SATELLITE
The University CubeSat Project 17 represents a rapid and low-cost approach to
testing new technologies in a LEO space environment. Pioneered by Prof. Bob
Twiggs, head of the Space Systems Development Laboratory at Stanford Univer-
sity, this is an exciting movement that has spread to several universities worldwide.
The CubeSat development is closely related to the PICOSAT satellite development
described below. Prof. Twiggs’s group has had a long history of experience in
launching and operating nano- and picosatellites. One of the SSDL satellites named
SAPPHIRE 18 carried a MEMS-based tunneling infrared sensor (TIS) payload. The
TIS was used as a horizon detector on SAPPHIRE.
Kenny et al. 19 developed the TIS initially at JPL, following up with further
development of it at Stanford University. They modified the pneumatic infrared
detector invented in 1947 by Marcel Golay 20 using MEMS-based silicon microma-
chining techniques and utilized quantum mechanical electron tunneling as the
displacement transducer. As shown in Figure 6.7, the TIS consists of a stack of
three silicon chips. The top two chips enclose a volume of air that expands upon
absorption of infrared energy. The enclosed cavity is bounded by a flexible, metal-
lized, silicon nitride membrane, which forms one of the electrodes for the tunneling
transducer. The second tunneling electrode is a metallized silicon micromachined tip
on the bottom chip. This conductive tip is surrounded at its base by a larger, planar
electrode. The purpose of this planar electrode is to electrostatically attract the
membrane to within electron-tunneling distance (~ 1 nm) of the tip. Once tunneling
is initiated, the distance is maintained constant using feedback electronics. The
infrared signal is subsequently measured by the change in the bias voltage on the
planar electrode when the membrane moves outwards towards the tip. The TIS is
highly sensitive and falls within the general class of uncooled infrared detectors.

6.2.10 FREE MOLECULE MICRO-RESISTOJET
The Free Molecule Micro-Resistojet (FMMR) 21 was developed by the Air Force
Research Laboratory (AFRL) in collaboration with the University of Southern
California and JPL. This novel MEMS-based micropropulsion device is based on
resistively heating molecules within a Knudsen flow regime (Knudsen number ~ 1)
in order to increase their kinetic energy as they exit the thruster, and thereby impart
momentum to the spacecraft. The design is extremely simple: The propellant is
solid ice at an ambient temperature of 245 K with a vapor pressure of 50 Pa.
The water molecules pass through 100-mm-wide silicon-micromachined slots,

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