micromachines Review
Microhotplates for Metal Oxide Semiconductor Gas Sensor Applications—Towards the CMOS-MEMS Monolithic Approach Haotian Liu 1 , Li Zhang 2 , King Ho Holden Li 1,2, * 1 2 3
*
and Ooi Kiang Tan 3
School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore;
[email protected] Temasek Laboratories, Nanyang Technological University, Singapore 67905910, Singapore;
[email protected] School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 67905367, Singapore;
[email protected] Correspondence:
[email protected]; Tel.: +65-6790-6398
Received: 26 September 2018; Accepted: 26 October 2018; Published: 29 October 2018
Abstract: The recent development of the Internet of Things (IoT) in healthcare and indoor air quality monitoring expands the market for miniaturized gas sensors. Metal oxide gas sensors based on microhotplates fabricated with micro-electro-mechanical system (MEMS) technology dominate the market due to their balance in performance and cost. Integrating sensors with signal conditioning circuits on a single chip can significantly reduce the noise and package size. However, the fabrication process of MEMS sensors must be compatible with the complementary metal oxide semiconductor (CMOS) circuits, which imposes restrictions on the materials and design. In this paper, the sensing mechanism, design and operation of these sensors are reviewed, with focuses on the approaches towards performance improvement and CMOS compatibility. Keywords: gas sensor; metal oxide (MOX) sensor; micro-electro-mechanical system (MEMS); microhotplate
1. Introduction Gas sensors have been widely applied in various fields, such as agriculture [1], automotive [2], industrial, indoor air quality monitoring [3] and environmental monitoring [4,5]. Recently, the prevalence of the Internet of Things (IoT) stimulates the development of sensors with small sizes (
