A monolithic CMOS microhotplate-based gas sensor system

Abstract

A monolithic CMOS microhotplate-based conductance-type gas sensor system is described. A bulk micromachining technique is used to create suspended microhotplate structures that serve as sensing film platforms. The thermal properties of the microhotplates include a 1-ms thermal time constant and a 10 C mW thermal efficiency. The polysilicon used for the microhotplate heater exhibits a temperature coefficient of resistance of 1.067 10 3 C. Tin(IV) oxide and titanium(IV) oxide (SnO 2 TiO 2) sensing films are grown over postpatterned gold sensing electrodes on the microhotplate using low-pressure chemical vapor deposition (LPCVD). An array of microhotplate gas sensors with different sensing film properties is fabricated by using a different temperature for each microhotplate during the LPCVD film growth process. Interface circuits are designed and implemented monolithically with the array of microhotplate gas sensors. Bipolar transistors are found to be a good choice for the heater drivers, and MOSFET switches are suitable for addressing the sensing films. An on-chip operational amplifier improves the signal-to-noise ratio and produces a robust output signal. Isothermal responses demonstrate the ability of the sensors to detect different gas molecules over a wide range of concentrations including detection below 100 nanomoles/mole. I. INTRODUCTION C HEMICAL microsensors represent one important application for microelectromechanical systems (MEMS) technology. Microhotplate devices belong to the MEMS family and can be fabricated in commercial CMOS technology using micromachining techniques [1]. Thermally isolated microhotplate structures can be utilized for conductance-type gas sensing [2] or as microscopic infrared sources [3]. The CMOS compatible process realizes a class of devices that are based on thermo-electromechanical effects and are compatible with existing very-large-scale-integration (VLSI) circuit design techniques [4]-[6]. In this paper, a monolithic integration of a gas sensor system based on CMOS-compatible microhotplate technology is presented. There are numerous applications avail-Manuscript

Multifunctional Platform with CMOS-Compatible Tungsten Microhotplate for Pirani, Temperature, and Gas Sensor

“…This platform could be used as the heating plate for metal-oxide gas sensors with extra chemically sensitive material on the surface of the microhotplate [6,9,26]. In our experiment, two aluminum electrodes were fabricated on the surface of the microhotplate during the CMOS process.…”

“…With better control and optimization of CMOS technology, the manufacturing yield of the sensors can be dramatically increased [5]. The noise level of the sensor would be decreased by integrating the sensor and the signal processing circuit in one chip [6]. With the fast development of various kinds of sensors, the idea of multifunctional platforms for the different sensors is generated [7].…”

Design and fabrication of a CMOS-compatible MHP gas sensor

“…[5][6][7] By integrating MHP gas sensor array with IC on one chip for signal processing and temperature compensation, intelligent gas sensors can be achieved, thus, CMOS-compatible MHP gas sensors are of great interest. [6][7][8][9][10][11] Heaters, free-standing membrane, gas sensing films and electrodes are essential factors for MHP gas sensors. [6][7][8]11 Many CMOS-compatible MHP gas sensors use poly-Si resisters as heaters.…”

“…[6][7][8][9][10][11] Heaters, free-standing membrane, gas sensing films and electrodes are essential factors for MHP gas sensors. [6][7][8]11 Many CMOS-compatible MHP gas sensors use poly-Si resisters as heaters. [10][11][12] However, during operating at high temperature for a long time, the resistances drift of poly-Si cause long-term stability problems.…”

“…With nonconventional layout design, MHPs with the tungsten resistor as the heater and thermometer have been developed, 15,16 which is a better choice for CMOScompatible micro gas sensors. For the free-standing membrane, the dielectric multilayer depends on the CMOS processes, 6,7,11,12 and the release of the membrane is carried out in the post-CMOS processes. 9-12, 16, 17 Gas sensing materials are usually deposited on the MHPs by RF sputtering 9,12 or drop-coating process 2,18,19 after the release of the membrane.…”