Nondestructive evaluation of semiconductor surfaces using the surface acoustic wave convolver

A surface acoustic wave (SAW) device is being developed for use in the non-destructive determination of the electronic properties of semiconductors. Some of the properties that can be determined by the technique are the bulk and surface conductivity, the location in the energy gap of traps, distribution of fast surface states, and interface states, majority carrier capture cross-section and storage times in the depletion layer. This characterization of the semiconductor could be performed at progressive stages of device fabrication thereby improving yield by identifying faulty processing steps. Some investigations have been conducted on silicon, ion-implanted silicon, gallium arsenide, indium arsenide, gallium phosphide and cadmium sulfide. The technique uses surface acoustic waves on a piezoelectric substrate. The electric field associated with the SAW interacts with free carriers of a semiconductor placed near the piezoelectric surface. The interaction generates detectable currents in the semiconductor and attenuates the SAW. Observing these effects while varying external parameters such as temperature, applied acoustic power, SAW frequency, semiconductor surface irradiation and bias voltage, the desired information is obtained. In this paper, this contactless technique is reviewed and some new results are presented in connection with the photoconductivity response time.