A 32-element micromachined thermal imager with on-chip multiplexing

Abstract

Abstract This paper describes a multiplexed infrared (IR) thermal line imager realized in silicon using micromachined dielectric windows and thermopile detector elements. The imager consists of 32 detectors, each having 40 series-connected thermocouples, arranged in two staggered 16-element linear arrays. The thermopiles are formed using n+ polysilicon-gold thermocouples supported on 1.2 μm thick diaphragms of LPCVD silicon dioxide and silicon nitride. The detectors provide a responsivity of 64 V W−1, a noise voltage less than ≈ 50 nV Hz− 1 2 at 20 Hz, and a specific detectivity of 7×107 cm Hz 1 2 W−1. The detector signals are multiplexed out using on-chip enhanced depletion (ED)-NMOS circuitry. The imager chip measures 5.5 mm×11 mm and requires seven masks. It has been used for non-contact measurements of wafer temperature during reactive ion etching and rapid thermal processing and for the measurement of dielectric thicknesses using IR absorption techniques.