Scanning acoustic microscopy in porous SiC

We report on ultrasonic investigations of acoustic pulses propagating through both the porous surface regions and overgrown epitaxial films on both as-received and porous SiC wafers. In this study, n-type 6-H SiC(0001)Si 3.5° off-axis substrates were half-masked during anodization so that an epitaxial layer may be grown directly on both conventional and porous substrates for comparison purposes. [1]. The research goal of this work is to investigate the suitability of Scanning Acoustic Microscopy (SAM) for use in non-destructive assessment of porous structure in SiC crystals. Both transmission and reflection measurements were made on two crystals containing a 3 and 12 μm thick porous layer respectively. SAM involves the use of peak amplitude and time-of-flight information acquired while raster-scanning over a sample, to produce horizontal cross section images. One of important SAM features is a possibility to perform Tomographic Acoustic Micro Imaging (TAMI) where the entire image can be split into individual cross-sectional scans, analogous to physical sectioning of the sample. SAM in the TAMI mode was performed at an operating frequency up to 260MHz in the pulse-echo mode. A reduction in the absorption of high-frequency acoustic pulses by 25 to 35% was observed in the porous region compared with the control. This result also persists after epi-layer growth on porous SiC. © 2002 Trans Tech Publications.