Temperature Programmed Sensing

Because of the rapid thermal time constant (about 1 ms) of microhotplates, it is possible to vary temperature rapidly in real time. This feature can be used to obtain kinetic data about the transducing reaction. Different gases will have different energies for adsorption, desorption, and reaction on the metal oxide surface, and therefore the response of the sensor at different temperatures will depend on the gas being sensed. Because the sensing film is a semiconductor, however, in a simple linear temperature ramp (akin to temperature programmed desorption,) these changes would be masked by the simple thermal excitation of donor electrons. Therefore we use a temperature pulse sequence as a pump-probe measurement, as indicated schematically. By always measuring the conductance at a fixed temperature, only changes in the surface from the temperature pulses cause changes in the conductance.

A simple example is the desorption of water into vacuum. Dosing water on a tin oxide surface in a vacuum system causes the conductance to rise to a higher value. Pumping the water out of the vacuum system leaves a water dosed surface with a higher conductance than the non-dosed base value. By applying a sequence of increasing amplitude temperature pulses and measuring the conductance at room temperature, between each pulse, we begin to observe a drop in the conductance when the pulse amplitude is sufficient to desorb some of the water. Note that for wider pulses, the characteristic conductance drop occurs at a lower temperature. Such data can be used to deduce the activation energy for desorption.

In an atmospheric sensing environment, the reactions are more complex, but the operating principles are similar. In this case we apply a repeated sequence of temperature pulse, for example, the linear ramp shown below.

The pulse sequence need not be a linear ramp. By changing the temperature program, the processes on the sensing surface can be altered, and a different response pattern is obtained. For example, methanol and ethanol produce similar responses for a linear pulse ramp applied to the sensor as shown above. It would be desirable to modify the pulse sequence to produce a greater difference between the response patterns.

The results plotted to the right are the response data to a program consisting of ten 100 ms pulses at each of eight temperatures ranging from 20 °C to 370 °C with a 50 °C increment.


Microsensor arrays can be fabricated with integrated control and analysis electronics, including pattern recognition circuitry. In analyzing a gas component, the temperature program can be altered in real time to bring out response characteristics that distinguish a particular analyte. Wavelet network and other modeling techniques are being used to optimize the temperature program of the sensor and for recognizing and analyzing response patterns.

References

Cavicchi, R.E., Suehle, J.S., Kreider, K.G., Gaitan, M., and Chaparala, P., "Fast Temperature Programmed Sensing for Micro-Hotplate Gas Sensors," IEEE Electron Device Letters, 16, 286-288 (1995).

Cavicchi, R.E., Suehle, J.S., Kreider, K.G., Gaitan, M., and Chaparala, P., "Optimized temperature pulse sequences for the enhancement of chemically-specific response patterns from micro-hotplate gas sensors," Proc. of Transducers Ô95/Eurosensors IX, (Norstedts Tryckeri AB, Stockholm, Sweden), 823-826 (1995).

Cavicchi, R.E., Suehle, J.S., Chaparala, P., Poirier, G.E., Kreider, K.G., Gaitan, M., and Semancik, S., "Microhotplate temperature control for sensor fabrication, study and operation," Proc. of the 5th International Meeting on Chemical Sensors (Rome), 1136-1139 (1994).

Cavicchi, R.E., Suehle, J.S., Chaparala, P., Kreider, K.G., Gaitan, M. and Semancik, S., "Microhotplate Gas Sensor," Proc. of the 1994 Solid State Sensor and Actuator Workshop (Hilton Head, S.C.), 53-56 (1994).

Semancik, S., Cavicchi, R. E., Gaitan, M., and Suehle, J. S.,"Temperature-controlled micromachined arrays for chemical sensor fabrication and operation" U.S. Patent 5,345,213 (1994).

Kunt, T.A., McAvoy, T.J., Cavicchi, R.E., and Semancik, S., "Optimization of temperature programmed sensing for gas identification using micro-hotplate sensors," submitted to Sensors and Actuators.