Linear arrays of thermopile infrared detectors made of high-performance thermoelectric materials have been fabricated on silicon substrates by micromachining processes. Such detector arrays can be useful in dispersive spectrometers for chemical analyses, including exhaust and environmental monitoring, in inexpensive thermal imaging systems for predictive and preventative maintenance, such as looking for hot spots on train wheels or power generating equipment, and in horizon sensors for satellite attitude control.
For some applications, thermopiles offer advantages over other uncooled infrared detectors. Thermopiles can operate over a broad temperature range without temperature stabilization. They are passive devices, generating a voltage proportional to the incident infrared power without electrical bias. They require no chopper. Thus, for some applications, thermopiles can be supported by simpler, lower-power, more-reliable ancillary components than are needed for the operation of such infrared devices as bolometers, pyroelectric or ferroelectric detectors. Another advantage is that if thermopiles are read out with high-input-impedance amplifiers, they exhibit negligible excess low-frequency (1/f) noise. Thermopile response is typically highly linear over many orders of magnitude in incident infrared power.
Prior to the development of the present high-performance devices, arrays of thermopiles had been fabricated by micromachining of silicon, but those arrays contained metal or silicon-based thermoelectric materials, which are characterized by low thermoelectric figures of merit. [A material's thermoelectric figure of merit is defined by Z = a2/rl, where a is the Seebeck coefficient, r is the electrical resistivity, and l is the thermal conductivity.] The signal-to noise ratio of an infrared detector can be described by the specific detectivity, (D*). The D* of a thermopile is approximately proportional to Z1/2.
The present devices are 63-element linear arrays, with each element containing 11 Bi-Te/Bi-Sb-Te thin-film thermocouples, which are supported on a silicon nitride membrane over a hole in the silicon substrate to maximize the thermal isolation of the thermocouple junctions from the substrate (see figure). The thermocouple films were deposited by sputtering from targets of Bi2.0Te3.5 and Bi0.55Sb1.45Te3.0. The Bi-Sb-Te-Se family of compounds has the highest known thermoelectric figure of merit at room temperature. The thin-film thermoelectric wires are electrically connected to each other and gold interconnect wiring with contact pads made of gold film deposited over titanium film.
When exposed to radiation from a 1,000 K black-body source, the detectors exhibited zero frequency responsivity values of 1,100 V/W and specific detectivites of D* = 1.4 × 109 cm·Hz1/2/W, with a response time of 99 ms. The only measurable noise at frequencies above 20 mHz was Johnson noise from the detector resistance. These performance figures are the best reported to date for an array of thermopile detectors.
This work was done by Marc Foote, Eric Jones, and Thierry Caillat of Caltech for NASA's Jet Propulsion Laboratory.
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