Fast temperature programmed sensing for micro-hotplate gas sensors

Abstract

Abstruct-We describe an operating mode of a gas sensor that greatly enhances the capability of the device to determine the composition of a sensed gas. The device consists of a micromachined hotplate with integrated heater, heat distribution plate, electrical contact pads, and sensing film. The temperature programmed sensing (TPS) technique uses millisecond timescale temperature changes to modify the rates for adsorption, desorption, and reaction of gases on the sensing surface during sensor operation. A repetitive train of temperature pulses produces a patterned conductance response that depends on the gas composition, as well as the temperature pulse width, amplitude, and specific sequence of pulses. Results are shown for the vapors of water, ethanol, methanol, formaldehyde, and acetone.EMICONDUCTOR gas sensors are devices that can pro-S vide sensitive response to a gas of interest at moderate cost. The operating principle is a change in an electrical property of a sensing film, usually conductance, caused by a coverage of adsorbed or chemisorbed gas species. Semiconducting oxides such as SnOz are commonly used as a base sensing material. The devices typically operate at elevated temperatures to activate the reactions that produce a sensor response and to reduce the effects of humidity. One drawback of these devices is that a response may be produced by more than one gas. To enhance the "selectivity" of a device, catalytic metals are incorporated into the sensing material, and operating temperature ,is optimized to reduce the effects of competing reactions. While most devices in use today are discrete sensors, arrays of different sensors have been proposed as an additional means to enhance selectivity by using pattem recognition to analyze the overlapping responses of array elements.An alternative route to selectivity is to take advantage of the temperature-dependence of surface kinetics by operating sensors in a dynamic mode, in which temperature is varied during operation. Cyclic alteration between two temperatures has been considered for the enhancement of sensitivity and selectivity, and minimization of the effects of humidity [1]-[5]. Hiranaka et al. describe the use of a single temperature pulse to create transient response which has a characteristic time that is gas-dependent [6]. Temperature-programmed desorption, in which the conductance is measured during a temperature ramp, has been used to investigate the desorption of 0 2 , . IEEE Log Number 9411758.Fig. 1. Schematic top view (left) and side view (right) of the device. Top: Scanning electron micrograph of a micro-hotplate. Bottom: and Hz from tin oxide [7] and the desorption of 0 2 from ultrathin metal films [8]. While the temperature modulation for the above experiments is done on the time scale of seconds to minutes, with micro-machined structures it is possible to modulate temperature on a millisecond time scale [9]. This works describes a temperature-programmed sensing (TPS) technique that uses short temperature pulses applied to micro-hotplate...

Array Optimization and Preprocessing Techniques for Chemical Sensing Microsystems

“…In a less straightforward but even more effective manner, temperature pulses have been applied to these same types of sensors to produce concatenated transient responses over temperature that effectively enable metal oxide sensors to discriminate between carbon monoxide and nitric dioxide [42] and among water, ethanol, methanol, formaldehyde, and acetone [43].…”

MEMS-based formaldehyde gas sensor integrated with a micro-hotplate

“…Gas-sensing devices typically operate at elevated temperatures in order to activate the reactions which produce a sensor response and to reduce humidity effects (Cavicchi and Suehle 1995). A drawback of these devices is that the observed response may actually be the result of the simultaneous presence of more than one gas.…”

Calibration of microhotplate conductometric gas sensors by non-linear multivariate regression methods

“…This would theoretically enhance selectivity to individual target analytes. Similar thermal modulation has been applied to arrays of semi-selective piezoelectric sensors [36] and arrays of Taguchi gas sensors [27,37]. However, the optimal calibration of the microhotplate sensors is potentially more difficult to achieve and, consequentially, this work focuses on calibration of steady state analysis of MHP sensors.…”