Scanning Acoustic Microscope Utilizing SAW-BAW Conversion

A 1WMHz acoustic microscope using an integrated grat- ing acoustic scanner is studied. The resolution of the system is approx- imately 68 pm. The real-time acoustic images obtained from this mi- croscope demonstrate the feasibility of the approach. The dependency of the energy velocity of acoustic waves on the propagation direction in the saggital plane of a Y-cut Z-propagating LiNbO, crystal was studied, and piezoelectric effects were included. The energy of velocity disper- sion was calculated and the results were employed for designing the tap positions of a large time-bandwidth product, nonlinear chirp fflter to obtain a large aperture and spherical aberration-free operation. Theoretical analysis reveals that for a given chirped waveform, the sur- face acoustic wave (SAW) propagating in the positive 2 direction gives better resolution than that in the negative 2 direction. Focusing the bulk acoustic wave in the transverse direction is achieved with the curved grooves or metallic strips, and the focus phase error using the curved structure are studied. to reduce the attenuation loss, and the crystals used are normally anisotropic. We had also previously reported a high-frequency GAS device (8) and demonstrated its fo- cusing and scanning capabilities. Electronic focusing in the axial direction was augmented by transverse focusing using an external lens that was massive and bulky. This device was operated at a rather high frequency, approximately 100 MHz, and the anisotropic effect of the substrate was included in the design. In Section I1 the de- sign of the GAS using an anisotropic substrate will be dis- cussed. In Section I11 we will discuss methods for focus- ing the bulk acoustic wave in the transverse direction without the external lens. We also analyze the focus phase error, which is important for obtaining high resolution. To obtain large aperture operation and spherical aber- ration-free focusing, energy velocity dispersion in LiNb03 was calculated (given in the Appendix) and employed for designing the tap positions of a large time-bandwidth product, nonlinear chirp generator. The GAS was used in conjunction with a computer-controlled mechanical scan- ner, which scanned the sample in the transverse direction to the GAS to obtain two-dimensional images. This had not been achieved previously due to the lack of a stable mechanical scanner.