handbook of modern sensors

436 14 Light Detectors
(A)
(B)
Fig. 14.26. Platinum-film bolometer: (A) glass membrane over the etched cavity; (B) array of
bolometers.
Fig. 14.27. Germanium-film bolometer floating over the silicon
cavity. (Courtesy of Professor J. Shie.)
sensitivity and where the cost of fabrication is a critical factor, a platinum-film bolometer
is an attractive choice. Platinum has a small but predictive temperature coefficient
of resistivity.
The platinum film (having thickness of about 500 Å) is deposited and photolithographically
patterned over the thin glass membrane. The membrane is supported in the
cavity etched in silicon by tiny extended leads. Thus, the membrane plate is virtually
floating over the V-grooved cavity in the Si substrate. This helps to dramatically minimize
its thermal coupling with the substrate. Figure 14.26B shows a microphotograph
of an array of the Pt bolometers used for the thermal imaging.
In addition to platinum, many other materials may be used as temperaturesensitive
resistors, (e.g., polysilicon, germanium, TaNO, and others). An important
issue when selecting a particular material is its compatibility with a standard CMOS
process so that a full monolithic device can be fabricated on a single silicon chip,
including the interface electronic circuit. Thus, polysilicon is an attractive choice,
along with the deposition of germanium films (Fig. 14.27).
As follows from Eq. (14.27), one of the critical issues which always must be resolved
when designing a bolometer (or any other accurate temperature sensor, for that
matter) is to assure good thermal insulation of the sensing element from a supporting
structure, connecting wires, and interface electronics. Otherwise, heat loss from the
element may result in large errors and reduced sensitivity. One method for achieving
this is to completely eliminate any metal conductors and to measure the temperature
of the bolometer by using a fiber-optic technique, as has been implemented in the
E-field probe fabricated by Luxtron (Mountain View, CA; (U.S. patent 4,816,634).
In the design (Fig. 14.25B), a miniature bolometer is suspended in the end of an
optical probe and its temperature is measured by a fluoroptic temperature sensor (see