High-frequency acoustic microscopy studies of buried interfaces in silicon

Acoustic microscopes of the pulse-echo type produce resolution within solid materials that is determined by the acoustic frequency employed as well as the lens design parameters of focal length and beam diameter. At very high frequencies (> ap 200 MHz), however, where the attenuation in the coupling fluid is significant, the prominent frequency of the acoustic pulse is significantly downshifted such that the resolution is reduced accordingly. In addition, spherical aberrations due to velocity mismatch between the different media would be expected to play a significant role in limiting the resolution. While there have been several studies on surface imaging using acoustic microscopy methods, detailed reports on resolution capabilities while imaging buried interfaces are still lacking. In this paper, we report for the first time a systematic study on experimentally determined resolution limits from an assortment of transducers having a nominal frequency of 230 MHz and focal lengths ranging from 0.15 inches to 0.75 inches. A resolution test wafer was designed for these experiments that consisted of a 525 micron thick silicon wafer anodic bonded to a 400 mum glass plate. Another test target was designed that used a 750 micron thick silicon wafer direct bonded to another similar thickness wafer. In each case at the interface between the two materials is a series of varied spaced etched lines and spaces whose visibility to the acoustic beam became the indicator of resolution achieved. The lateral resolution in silicon appears to be nearly the half wavelength limit as predicted by diffraction theory. While imaging through the glass layer, however, it appears that even better resolution is being achieved. In this paper, we also report observations on silicon thickness effects on the observed resolution