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

Abstract

We have developed a new type of detector pixel circuit operated in an infrared image sensor of dielectric bolometer mode. The detector pixel consists of capacitors of ferroelectric BST (Ba<SUB>1-x</SUB>Sr<SUB>x</SUB>TiO<SUB>3</SUB>) thin film, whose dielectric 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 is nonirradiated one. BST film has been prepared on Si membrane structure by laser ablation. Dielectric constant of the BST film, which is about 450 at 25 degrees Celsius, changes by about 1 to 10%/K in ambient temperature. The maximum change is as large as about 100/K, and more than about 10%/K relative change in the dielectric constant or the corresponding capacitance has been induced. As a result of on-board evaluation of the assembled circuit with a source-follower output, the output level is about 40 mV when a relative capacitance change in the capacitor is about 3%. On the other hand, in PSPICE 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 relationship between the output voltage and capacitance change of the BST film in the assembled circuit agrees well with that in the experimental results. It is considered that the circuit has enough output signal level for input of conventional operational amplifier. Calculated thermal responsivity R<SUB>v</SUB>, and specific detectivity D* are 50 kV/W and 6.5 X 10<SUP>9</SUP> cm (DOT) Hz<SUP>1/2</SUP>/W, respectively, which means high-sensitivity compared to the other type of IR sensors. The pixel structure also shows a simple configuration, and then is very effective in reducing their pixel size and then increasing the pixel density.