Pulse Compression Acoustic Microscopy Using SAW Filters

Abstract

The basic principles of surface-acoustic.,wave pulse compression acoustic microscopy is described and demonstrated by re- sults obtained at 60 and 750 MHz. A theoretical analysis discusses the behavior of such a system in terms of the required signal processing and the attainable imaging resolution. advantageous. At sufficiently high intensities, the imaging of the microscope will be affected by harmonic genera- tion, sometimes beneficially (g) but more usually in the sense of a large increase in effective attenuation loss (lo). It is clear that we can choose to use extended coded pulses to reduce the intensity at a constant illumination energy and hence the incidence of harmonic effects. We will describe the principles of operation of a pulse- compression microscope, based on surface acoustic wave (SAW) generated chirp pulses and illustrate the advan- tages of such a system by means of results from test sam- ples. A range of micrographs obtained at 60 MHz from metal-metal and metal-ceramic bonds with a processing gain of 17 dB is presented. At higher frequencies, the fre- quency-dependent attenuation in the coupling liquid im- poses two fundamental limitations on the performance of the pulse compression microscope: 1) reduction in the processing gain and 2) reduction in the imaging resolu- tion. Analysis shows, however, that even at high frequen- cies there can be a considerable advantage in using pulse compression. Experimental results at 750 MHz with a chirp bandwidth of 150 MHz have been obtained. We will show that the effective attainable resolution with such a coherent broadband system is in accord with a simple in- tuitive concept, based on monochromatic excitation, at a suitable averaged frequency. The reduction in processing gain due to loss dispersion, which we will calculate, is of course related to the partic- ular choice of coded pulse. However, the loss of resolution is a phenomenon that depends primarily on the spectrum of the pulse; for a pulse of a given spectral width one would not expect any significant differences between the effective resolution using a chirp or a rectangular RF pulse.