Novel Miniature and Selective Combustion-Type CMOS Gas Sensor for Gas-Mixture Analysis-Part 1: Emphasis on Chemical Aspects

Abstract

There is an ongoing effort to fabricate miniature, low cost, sensitive, and selective gas sensors for domestic and industrial uses. This paper presents a miniature combustion-type gas sensor (GMOS) based on a thermal sensor, where a micromachined CMOS-SOI transistor integrated with a catalytic reaction plate acts as a sensing element. This study emphasizes GMOS performance modeling, technological aspects, and sensing-selectivity issues. Two deposition techniques of a Pt catalytic layer suitable for wafer-level processing were compared, magnetron sputtering and nanoparticle inkjet printing. Both techniques have been useful for the fabrication of GMOS sensor, with good sensitivity to ethanol and acetone in the air. However, a printed Pt nanoparticle catalyst provides almost twice as much sensitivity as compared to that of the sputtered catalyst. Moreover, sensing selectivity in the ethanol/acetone gas mixture was demonstrated for the GMOS with a Pt nanoparticle catalyst. These advantages of GMOS allow for the fabrication of a low-cost gas sensor that requires a low power, and make it a promising technology for future smartphones, wearables, and Internet of Things (IoT) applications.

Keywords: CMOS–SOI–MEMS; Pt nanoparticle; catalytic gas sensor; inkjet printing; selectivity.

Figure 1

Micromachined cavity of (a) thermal sensor (TMOS) and (b) gas sensor (GMOS) based on TMOS sensor.

Figure 2

Overview of typical GMOS sensor die and pixel construction: (a) GMOS die layout containing three pixels for catalytic layer, one reference (“Blind”) and two auxiliary pixels (“In” and “Ref”) for stabilizing operational point of the electrical readout [22]; (b) optical image of released transistor pixel without catalytic layer; (c) cross section on plane A–B of released pixel.

Figure 3

Experiment setup.

Figure 4

Two deposition systems used in this study: (a) magnetron sputtering system ATC 2200 [27]; (b) inkjet printer sciFLEXARRAYER S3 [28] with off-contact-printing schematic.

Figure 5

Pt nanoparticle catalytic-layer characterization: (a) Optical image of pixel with catalyst printed on top of it; (b) SEM micrograph of catalytic layer.

Figure 6

Calculated chemical-oxidation reaction power release due to 100 ppm combustion of ethanol on Pt catalyst as function of heated-pixel temperature. Average G_TH = 60 µW/K, D/δ = 1 m/s [22], E_A ≈ 55.23 kJ/mole, and Z ≈ 10^7.66 m/s [32] were assumed.

Figure 7

Experiment results for ethanol detection over Pt nanoparticle layer and sputtered Pt thin film layer: (a) Voltage signal as function of ethanol concentration at constant heater voltage of 2 V; (b) voltage output signal as a function of heater voltage applied to heating resistor at constant temperature and at constant concentration of 100 ppm of ethanol. Voltage output signal with 5.2 dB gain.

Figure 8

Ethanol measurements at low concentrations over Pt nanoparticle layer. Gradual addition of 1 ppm of ethanol to the chamber from 1 to 10 ppm. Voltage output signal with 20 dB gain at heater-voltage amplitude applied to heating resistor of 3.0 V.

Figure 9

There are two approaches to detect the gases in binary mixtures: using (a) a catalytic layer for two different gases or (b) a specific catalytic layer for each gas.

Figure 10

Ethanol and acetone output signal as function of heater voltage applied to heating resistor, and temperature at constant acetone concentration of 100 ppm.

Figure 11

Voltage signal versus time at heater voltage of (a) 1.8 V and (b) 2.7 V, which correspond to low and high ignition temperatures, respectively. Concentration of each gas was about 100 ppm.