Testing of porous silicon membranes as a novel humidity sensor
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Testing of porous silicon membranes as a novel humidity sensor L. Quercia*, M. Della Noce, V. La Ferrara and G. Di Francia ENEA Centro Ricerche Portici, I-80055 Portici (Italy) *e-mail: [email protected] ABSTRACT A new gas sensor device based on a porous silicon membrane has been fabricated. The sensible membrane has been bonded to a 1 cm2 Al2O3 substrate [1], where electrical contacts have been previously deposited by vacuum evaporation. In this work we present the results of DC measurements showing a good response of this device to humidity gradients. In this frame, the most striking properties of our device are its selectivity and a fast and full recovery after exposure to humidity levels up to 90%. Moreover, the new fabrication process used to realize the sensor merges the advantages typical of porous silicon with thin film ones, by means of a simple process. INTRODUCTION The possible applications of humidity sensors in automated systems for environmental control encompass many industrial and domestic fields. Very different operating temperatures and a variety of humidity ranges are needed for these purposes. The trend towards using automatic control systems, gaining importance because of the falling cost of interface circuits and microprocessors, push towards humidity sensors using changes in electrical parameters. Humidity sensors must satisfy a number of requirements, among which a short response time, a high sensitivity, negligible hysteresis and a possibly large operating range for both humidity and temperature. Many materials have been used, like ceramics [2], polymers, porous Al2O3 and eventually porous silicon [3,4]. We have recently reported [5] on the fabrication of a silicon membrane sensor device exhibiting a fairly high sensitivity to NO2 (down to 0.1 ppm). Device operates in DC and at room temperature, showing a nice selectivity while its long-term stability is currently under investigation. The simple fabrication process and its tunability in porosity and in average silicon skeleton size, have suggested to test similarly produced membranes as humidity sensors. One of the most interesting features this kind of device exhibits is its ability to work at room temperature (RT), an interesting advantage in most cases and a requisite in some ones. In particular food, medical and hazardous environments absolutely require RT operation. In this work we investigate the feasibility of fabricating a relative humidity (R.H.) sensor based on a silicon membrane. At this aim, several series of membranes have been realized and placed on an Al2O3 substrate where gold electrical contacts had been previously deposited. We have found that only luminescent membranes show R.H. sensitivity. Two optimized series of membranes (A and B) have been used to realize devices that have been tested in different environments (N2, synthetic air, CO, CO2, NO2) and in the R.H. range 10% - 90%. EXPERIMENTAL Porous silicon was prepared starting from a 1 Ωcm p-type CZ silicon wafer in electrochemical etching, using a solution HF:IPA
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