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114 MEMS and Microstructures in Aerospace Applications

TRL ‘‘push’’ technology to a mission-enabling ‘‘pull’’ technology. Thus, this highly
sensitive detector (noise equivalent power ~ 10 18 W/rt-Hz at 100 mK) is the first
‘‘success story’’ for JPL-developed microelectromechanical system (MEMS) tech-
nologies. The device consists of a high-purity, neutron transmutation doped (NTD),
single crystal Ge thermistor chip mounted on a ‘‘spider web’’ suspension compris-
ing metallized, suspended SiN filaments. The spider web structure has several
advantages: (a) it provides a large area for microwave absorption; (b) it has low
heat capacity; (c) it provides excellent thermal isolation for the NTD chip from the
surrounding environment; and (d) it also has a low cross section for cosmic rays.
The detection mechanism consists of the NTD chips measuring the local tempera-
ture rise due to the absorbed microwave radiation.

6.2.2 MEMS-BASED SUN SENSOR
Sun sensors are used commonly as part of the attitude control systems of spacecraft.
JPL has developed a miniaturized sun sensor with a mass of less than 30 g and with
8
power consumption less than 20 mW. The device in Figure 6.2 consists of a focal-
plane array photodetector above which a microfabricated, silicon chip with several
hundred small apertures is mounted. The focal plane captures the image of the
aperture array upon illumination by the sun. The orientation of the spacecraft with
respect to the sun is then computed to accuracies of better than a few arcminutes by
analysis of the resultant image on the focal plane detector. The simplicity and
robustness of the device have made it a candidate technology for the Mars Surface
Laboratory mission to be launched in 2009.

6.2.3 MEMS VIBRATORY GYROSCOPE
MEMS-based miniature gyroscope development has become a very active area of
research and development for a number of research groups around the world. The
main performance parameter used for classifying gyroscopes is the angular bias
stability or the minimum uncertainty in rotation rate as a function of the time over
which the measurements are averaged or integrated. For inertial grade performance,

FIGURE 6.2 MEMS-based sun sensor device. (a) Fully assembled device consisting of
the (b) microfabricated silicon mask mounted over a focal plane array detector. (Source:
NASA/JPL.)

© 2006 by Taylor & Francis Group, LLC