Si monolithic microbolometers of ferroelectric BST thin film combined with readout FET for uncooled infrared image sensor

Abstract A silicon monolithic ferroelectric thin-film bolometer coupled with a readout FET has been developed for uncooled infrared imaging applications by means of Si-bulk micromachining and pulsed-laser-deposited (PLD) barium strontium titanate Ba 1− x Sr x TiO 3 (BST) thin films. It is a new type of dielectric bolometer (DB) mode based on the strong temperature dependence of capacitance upon the ferroelectric phase transition. The pixel circuit is a serial pair of capacitors where a sensing capacitor is fabricated on a thermally isolated membrane and a reference capacitor on bulk area. And a new pulse-biased-operation mode has been implemented to sense the voltage change at the interface node between sensitive and the reference capacitors. In order to avoid crack and deformation on the thermally insulated structure, a stress-balanced structure by multi-layered membrane has been adopted, where the ferroelectric capacitor is formed on a triple layer of NSG/SiN/SiO 2 -stacked films. A BST (75/25) film on membrane is found to show positive temperature-coefficient of dielectric constant (TCD) ranging from 1 to 6%/K. Upon infrared irradiation on the membrane part, a capacitance difference arising from a temperature rise is significantly induced within the two capacitors. Thermal signals have been confirmed with one pixel bolometer tested under an infrared light irradiation. A new monolithic process flow is developed to combine an n-MOSFET process and a Si-bulk micromachining process, and ferroelectric capacitors on the stress-balanced membrane are able to be formed monolithically with MOSFETs for source-follower output buffer. The pixel structure also shows a simple configuration, and is very effective in reducing their pixel size and then increasing the pixel density. Finally, it is especially noted that the operation in the detector pixel in the DB mode is confirmed on the monolithically integrated device structure.