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ETH Zurich Develops a Single-chip Microsensor System to Analyse Gases

November 14, 2001

There are many target application areas envisaged for the gas sensor chip. Air quality in closed rooms can be monitored. The presence of dangerous - even poisonous - substances in the air can be detected, such as benzene from petrol pumps or perchloroethylene from dry-cleaning installations, and warnings can then be issued.

Low-cost Mass Production Becomes Possible

The newly developed single-chip sensor system, which is presented this week in Nature, is assembled on a silicon chip measuring just 7 by 7 mm, which allows for its integration in to lightweight handheld or even credit-card-size units. Its small dimensions and the broad range of candidate applications make low-cost mass production of the silicon chip possible using existing manufacturing facilities of the microelectronics industry. Apart from the sensors, the entire driving and signal-conditioning electronics - as well as a digital interface, which serves to control and run the system - are already integrated on the chip. The microsensor area of the chip is coated with special polymers that absorb molecules from the air, predominantly volatile organic compounds (VOCs). The volume absorption of the VOCs causes a change in physical properties of the polymer (mass, dielectric coefficient and release of heat), which in turn generates specific electrical signals.

Sensors Generate a Typical Signal Pattern for Each Gas

Three different types of transducers are arranged on each chip. The first type, a micro-machined resonating cantilever, monitors the mass change upon volatile absorption with a precision of less than a picogram (a billionth of a milligram). The second transducer monitors changes in the heat budget upon absorption or desorption (condensation or vaporisation heat) of the organic volatile in the polymer. The third type of transducer records the changes in dielectric properties, which makes it very sensitive to polar molecules, such as alcohol or water. Since the various volatile organics have different chemical and dielectric characteristics, as well as varying masses, the three transducers produce distinct signals that enable volatile identification. The signal intensity depends linearly on the volatile concentration. Consequently, it is possible to quantify the concentration of hazardous compounds in air.

Detection and Monitoring of Different Gases

Different polymers preferentially absorb different groups of volatiles, or to adapt an adage, «like dissolves like». Polar polymers, for example, preferentially absorb polar gases, whereas non-polar polymers tend to absorb non-polar gases. In order to obtain more information, several sensor chips coated with different polymers can be arranged in an array-type configuration. Even such a sensor array, however, cannot be compared with the performance and the broad applicability of a human nose. Gas sensor chips always have to be optimised for defined target applications, and, therefore, only cover a specific spectrum of chemicals.

On-going Development of Sensor Chips

Over the past ten years, a variety of sensor chips such as flow sensors, temperature sensors or magnetic-field sensors have been developed at the Physical Electronics Laboratory at the ETH Zurich. The gas sensor microsystem chip forms part of a collaborative effort of the University of Tübingen, Germany, the University of Bologna, Italy and the ETH Zurich to develop a handheld instrument for gas analysis. The project is financially supported by the Körber Foundation, Germany, within the framework of the European Science Award.

ETH Zuerich