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Microtechnologies for Science Instrumentation Applications 141

7.3.4 MICROMACHINED BOLOMETERS
Bolometers are an important application for MEMS devices in infrared spectrometry.
Most IR detectors require cryogenic cooling, bolometers can be used at ambient
temperatures and are almost wavelength independent. While bolometers are used as
detectors from microwaves to the visible spectrum, but visible MEMS fabrication has
given this technology a new dimension. We now can fabricate bolometers as mech-
anical structures, which are the size of a wavelength, with thermal masses so small
that even the smallest amount of absorbed energy is detectable in arrays with standard
video array sizes. In a commercially available imaging array from Sarnoff Corpor-
ation, bimetallic cantilevers deflect upon absorption and change the capacity of the
respective pixel. 67 The bi-material cantilever deflects upon absorption and changes
the capacity in this pixel. The small dimensions of MEMS technology allow the
bimetallic cantilever to be thermally insolated from the substrate with a very thin
element and to have such a low thermal mass that the absorbed energy creates a
temperature change large enough to measurably deflect the cantilever. Other bolom-
eter designs developed for satellite-based infrared imaging use active and reference
detectors arranged in Wheatstone bridge configurations. 3,68,69 The energy absorbed
in the optical stack formed by the materials changes the temperature and therefore the
resistance of the active pixel.
The same approach can be used not only for infrared radiation, but also for other
radiation such as x-rays. NASA GSFC has been working on a high-resolution x-ray
spectrometer for the Constellation–X mission. 35 The spectrometer is microma-
chined and consists of a Bi or Cu multilayer absorber for stopping and thermalizing
the incident x-rays, an e-beam evaporated Mo or Au proximity bilayer with
sputtered Nb leads for sensing the resultant temperature rise, and a silicon nitride
membrane to provide a weak thermal link to the thermal sink so that the calorimeter
can return to its equilibrium temperature. The x-ray spectrometers have achieved
resolutions of about 28 eV at 3.3 keV x-rays. MEMS are an enabling technology for
these position sensitive spectrometers, which require small sizes for resolution as
well as for small thermal capacities.

7.4 MEMS SENSORS FOR IN SITU ANALYSIS

All of the scientific spacecraft instruments discussed so far are essentially remote
sensing devices, measuring photons, fields, or particles incident upon an orbiting
spacecraft or space telescope. Equally important is the ability to measure the
chemical composition or other properties of a sample encountered on a planet’s
surface or in its atmosphere. Robotic spacecraft carrying mass spectrometers, for
example, have been used in the exploration of Venus, Mars, Jupiter, the Moon, the
comet Halley, and most recently Saturn and its moon Titan. Other devices such as
x-ray spectrometers, x-ray fluorescence and diffraction instruments, nuclear mag-
netic resonance force microscopes, and scanning electron microscopes have been
either flown or proposed for use in a planetary exploration mission to identify the
composition of planetary samples in situ. In all cases, existing spacecraft instru-

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