Progress towards high-performance thermopile imaging arrays

Thermopiles are uncooled, broadband detectors that require no chopper or temperature stabilizer. Their wide operating-temperature range, lack of temperature stabilization, and radiometric accuracy make thermopiles well suited for some space-based scientific imaging applications. These detectors may also offer advantages over bolometers for night vision. Previous work at JPL has produced thermopile linear arrays with D* values over 10(9) cmHz(1/2)/W by combining high-performance thermoelectric materials Bi-Te and Bi-Sb-Te with bulk micromachining processes. To date, however, 2-D thermopile arrays have demonstrated only moderate performance. The purpose of the present work is to improve thermopile 2-D arrays substantially by combining Bi-Te and Bi-Sb-Te thermoelectric materials with a unique pixel structure and low-noise readout circuitry. The initial goal is a 128x128 array with a single multiplexed analog output stream, with system D* values (including readout noise) of 10(9) cmHz(1/2)/W, and with a focal-plane power dissipation of 20 mW. 100 mum square detectors have been demonstrated with D* values of 2x10(8) cmHz(1/2)/W and response times of 4 ms. Models predict D* values well over 10(9) cmHz(1/2)/W for optimized detectors. Modeling of a preliminary readout design shows that for the expected detector resistance of 100 kOhm, the total noise will be 50% higher than the detector Johnson noise. CMOS test chips containing front-end circuits presently display a noise about 2.5 times higher than modeled and a power dissipation of 0.6 muW per pixel.