X-ray topography used as a routine tool in the evaluation of the crystal quality of GaAs substrate wafers

Asymmetric crystal x-ray topography (ACT) which can image entire 3" wafers on radiography film using a Cu x-ray source in less than 30 min. is a fast, convenient tool for routine wafer prescreening. An ACT study of over 70 SI LEC GaAs wafers revealed typical defects: low angle grain boundaries, dislocation cell structure, inclusions, precipitates, and included mosaic structure. A comparison of wafers from ten different vendors showed that the concentration of defects can vary significantly. Two correlations were found between crystal quality and wafer manufacturability. First, wafers from boules which had a high density of clustered low angle grain boundaries were found to be too mechanically fragile to survive all the processing steps required in device fabrication. Second, wafers from boules which showed the presence of large included mosaic crystals all showed poor or erratic cleaving characteristics causing the loss of many devices.