Design, fabrication and characterization of a front-etched micromachined thermopile for IR detection

Abstract

This paper presents the design, fabrication and experimental results of a front-etched CMOS compatible micromachined thermopile IR detector. The N-polysilicon/Al thermocouples were embedded in a 2.5 µm thick SiO2–Si3N4–SiO2 sandwich membrane, and XeF2 front-side isotropic post-etching was adopted to release and thermally isolate the thermopile structure. Due to the isotropy of XeF2 etching, a lot of flexibility was allowed in etching window layout and thermopile structure design. Etching windows in the dielectric absorber area were designed to avoid cutting off the heat transfer path from the absorber to hot junctions, and aluminum strips were patterned in the absorber to ensure the temperature was uniform across the absorber area. Two different thermopile structures, circle and rectangle, were designed and fabricated to investigate detector performance with respect to the thermopile structure. The steady-state behavior of fabricated detectors was anticipated by thermal ANSYS simulation. Due to the fact that the circular structure can get a higher temperature difference between hot and cold junctions, the circular thermopile detector has a quicker response, two times higher responsivity and detectivity than the rectangular thermopile detector. The circular thermopile detector exhibits a responsivity of 102.0 V W−1, a detectivity of 9.2 × 107 cm Hz1/2 W−1 and a time constant of 16.8 ms, in air at room temperature.