Microhotplates for Metal Oxide Semiconductor Gas Sensor Applications-Towards the CMOS-MEMS Monolithic Approach

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.

Figure 1

Comparison of the size and power consumption of the four types of gas sensors.

Figure 2

(a) The response of a laboratory WO₃ based gas sensor and commercial MOX sensor with respect to NO₂ in dry and humid air (b) response of the laboratory sensor as a function of NO₂ concentration (from [33]).

Figure 3

Schematics of (a) a traditional metal oxide gas sensor and (b) a microhotplate metal oxide gas sensor.

Figure 4

Top view of microhotplates with different configurations: (a) closed membrane; (b) suspended membrane; (c) bridge.

Figure 5

The suspended membrane microhotplate structure formed by (a) front-side etching and (b) back-side etching.

Figure 6

Temperature distribution of a suspended membrane microhotplate without gas sensitive layer (from [54]).

Figure 7

Silicon island and metal heat spreader application on a microhotplate.

Figure 8

Heater geometries (a) meander, (b) double spiral, (c) drive wheel.

Figure 9

Coplanar geometry where heating and sensing electrodes lie on the same plane: (a) Au-loaded In₂O₃ nanofiber gas sensor by Xu et al. (from [89]) and (b) SnO₂ nanowire gas sensor by Hwang et al. (from [90]).

Figure 10

Fabrication processes of a suspended membrane microhotplate with backside etching.

Figure 11

Monolithic gas sensor fabricated with silicon on insulator (SOI) CMOS technology (from [50]).

Figure 12

Monolithic CMOS-micro-electro-mechanical systems (MEMS) gas sensors: (a) microhotplate array for carbon monoxide detection (from [108]) (b) microhotplate gas sensor with proportional-integral-derivative (PID) controllers (from [125]), (c) microhotplate gas sensor with mixed-signal architecture (from [125]), (d) microhotplate gas sensor using biased current measurement (from [126]).