An Integrated Micromachined Thermopile Sensor With a Chopper Interface Circuit for Contact-Less Temperature Measurements

Abstract

This paper presents a sensor and its readout circuit suitable for contact-less temperature measurements. The sensor, designed employing a MEMS technology provided by STMicroelectronics, consists of a miniaturized micromachined silicon thermopile with 180-V/W responsivity, 540-kΩ output resistance, and 0.64-mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> active absorbing area. The interface circuit, fabricated in a standard 130-nm CMOS process, employs chopper technique in order to provide amplification of the sensor output signal, which behaves substantially as a DC while minimizing offset and noise contributions at low frequency. Given the sensor characteristics, a single-ended architecture was preferred over the most straightforward fully-differential approach. The interface circuit reduces the offset by a factor 255, achieving an input referred offset standard deviation equal to 1.365 μV, measured across 29 samples. The thermopile sensor and the interface circuit, integrated in two separate test-chips, were characterized and extensively measured both as standalone devices and together as a system to perform contact-less temperature measurements. Adding a metal cap to the thermopile sensor in order to reduce the environmental noise, a measurement accuracy of approximately ±0.2 °C was obtained, thus verifying the system suitability for human body temperature detection.