A Low-Temperature, Whole-Wafer-Imaging System for Defect and Impurity Mapping

We have developed a whole-wafer, low-temperature imaging system for making absorption and photoluminescence maps of wafers up to 100 mm (4 in) in diameter. The cryostat requires no cryogens and can maintain a temperature of 15 K in the sample chamber indefinitely. He gas provides the thermal contact with the samples, allowing rapid (15 min) interchange of samples. The transmitted or emitted light from the wafer is imaged on a CCD (charge-coupled device) camera rather than using a raster scanning mechanism. The resultant parallel collection advantage allows images to be acquired in much less time. The system is ideally suited for the routine characterization of large numbers of wafers. This apparatus has been used to map the neutral EL2 concentration and near gap photoluminescence efficiency for a number of semi-insulating GaAs wafers.