A new type of dielectric bolometer mode of detector pixel using ferroelectric thin film capacitors for infrared image sensor

We have developed a new type of detector pixel circuit operated in an infra red image sensor of dielectric bolometer mode. The detector pixel consists of capacitors of ferroelectric BST (Ba1-xSrxTiO3) thin film, whose dielectr ic constant changes drastically with temperature. Our proposed circuit is a serially connected capacitor-capacitor, where one capacitor is composed of a BST ferroelectric thin film irradiated by infrared light and the other i s nonirradiated one. BST film has been prepared on Si membrane structure by Pulsed Laser Deposition method (PLD). Dielectric constant of the BST film, which is about 450 at 25 degrees C, changes by about 1 to 10%/K at ambient temperature. As a result of on-board evaluation of the assembled circuit w ith a source-follower output, the output level is about 40 mV when a relati ve capacitance change in the capacitor is about 3%. On the other hand, in P SPICE circuit simulations, the output level is about 25 mV when a relative capacitance change in the capacitor of about 100 pF is 1%, The simulated re lationship between the output voltage of the assembled circuit and capacita nce change of the BST film agrees well with that in the experimental result s. It is considered that the circuit has enough output signal level for inp ut of conventional operational amplifier. Voltage responsivity R-v, and spe cific detectivity D* estimated from temperature change of dielectric consta nt are 50 kV/W and 6.5 X 10(9) cm Hz(1/2).W-1, respectively, which means hi gh-sensitivity compared to the other type of IR sensors. The pixel structur e also shows a simple configuration, and is very effective in reducing thei r pixel size and then increasing the pixel density. (C) 1999 Elsevier Scien ce S.A. All rights reserved.