Scanning Microscopy


A technique for imaging the distribution of ac surface photovoltages induced in a semiconductor wafer by the irradiation with a blue or near-infrared chopped photon beam is applied to non-destructive inspection of faults in Si or GaAs wafers. Faults detected include crystal defects, radiation damage, surface charge-up, striation and junction inhomogeneity. The principles of inspecting for the above mentioned wafer faults are quantitatively presented by numerical analyses of dependence of ac surface photovoltage on wafer electronic characteristics. The minimum detectable changes in minority carrier lifetime, interface trap density, fixed oxide charge density and resistivity for a depleted p-type Si wafer with a resistivity of 1Ωcm are estimated to be approximately 4%, 9%, 0.07% and 0.1%, respectively. For a weakly inverted wafer they are 4%, 250%, 1% and 4%, respectively. Examples of photovoltage images observed in Si wafers with swirl-like defects, grain boundaries, electron beam induced radiation damage. sulfate contamination and plasma induced surface charge up are shown together with images of an ion implanted GaAs wafer.

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