Uncooled Infrared Image Sensor of Dielectric Bolometer Mode Using Ferroelectric BST Thin Film Prepared by Metal Organic Decomposition

Abstract

A monolithic dielectric bolometer type uncooled infrared image sensor fabrication technology has been developed by integrating both Si bulk micromachining and ferroelectric thin film preparation technique. The operating principle of these dielectric bolometers has been explained and the fabrication process of the Si membrane structure and ferroelectric thin film have been elaborated. Metal Organic Decomposition (MOD) has been applied to prepare Barium Strontium Titanate (Ba_1−xSr_xTiO₃) ferroelectric thin film on micromachined Si wafer to fabricate the sensor. The detector pixel circuit is a capacitor-capacitor serially connected circuit, with one capacitor of BST film on Si membrane structure and the other on Si bulk structure. When irradiated by IR light, the capacitance of the IR detecting capacitor on membrane structure changed as a result of the change in dielectric constant against temperature of BST ferroelectric film. A stress-balanced structure of multi-layered membrane has been developed in order to avoid crack and deformation in the sensor fabrication process. Temperature Coefficient of Dielectric constant (TCD) of the MOD made BST (Ba:St 75:25) thin film is about 1%/K. Uniform and reproducible capacitance behaviours in the BST ferroelectric thin film capacitor on micromachined Si substrate have been confirmed. Chopperless operation has been attained and IR response evaluation of the fabricated sensor also has been carried out with R_v of 0.4 kV/W and D* of 9.8×10⁷ cmHz^l/2/W respectively.