<title>Noncontact surface-resistivity measurements on production wafers</title>

Abstract

Existing methods for measuring the resistivity of epitaxial wafers suffer from the following problems: wafer damage, slow turn-around time, poor repeatability and sensitivity to surface conditions. A new technique for in-line non-contact measurement of the resistivity profile in semiconductor wafers, free of the above problems, is described and experimental data presented. In this method, modulated light is directed at the specimen, while the dc surface potential of the specimen is varied between that corresponding to accumulation and that corresponding to deep depletion. The light intensity is sufficiently low and the dc surface potential is varied sufficiently fast to ensure that the surface concentration of optically generated minority carriers is negligible compared to the surface density of the depletion area space charge. The depletion width of the space charge region is calculated from the ac surface photosignal, generated by the modulated light beam, and its dependence on the induced space charge is used to derive various parameters of the specimen including the resistivity, doping concentration profile and epitaxial layer thickness. The high rate of change of the dc surface potential minimizes the effects of surface states recharging, which allows use of this method for characterization of non-passivated wafers. The non-contact nature of the method makes it applicable for monitoring of production wafers.