A new type of dielectric bolometer mode of detector pixel using ferroelectric thin film capacitors for 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 1− x Sr x TiO 3) 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 Pulsed Laser Deposition method (PLD). Dielectric constant of the BST film, which is about 450 at 25°C, changes by about 1 to 10%/K at ambient temperature. 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 of the assembled circuit and capacitance change of the BST film 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. Voltage responsivity R v, and specific detectivity D* estimated from temperature change of dielectric constant are 50 kV/W and 6.5×10 9 cm·Hz 1/2·W −1, respectively, which means high-sensitivity compared to the other type of IR sensors. The pixel structure also shows a simple configuration, and is very effective in reducing their pixel size and then increasing the pixel density.