Highly sensitive lifetime mapping equipment using the UV laser excited differential microwave photoconductivity decay technique was developed to observe the defect and contamination distribution in ultrathin silicon-on-insulator ͑SOI͒ layers. The technique was successively applied to the evaluation of state-of-the-art SOI wafers. Through theoretical analysis, it was quantitatively confirmed that the lifetime is shortened as the SOI film becomes thin, due to an increase in interface recombination. The large improvement in sensitivity attained by the differential detection enabled us to evaluate nonuniformities in the active layer quality by overcoming the difficulty of detection normally experienced by the decrease in the thickness of the SOI layer. Using light illumination with an extremely short penetration depth and the barrier effect of the buried oxide layer, it was found that the generated carriers were confined to the active layer, and lifetime data reflecting the quality of the SOI layer could be obtained. In the lifetime map for an SOI layer with a thickness of 52 nm, numerous islandlike spots with long lifetime, which were believed to be caused by HF defects or thin silicon areas, were detected.

An ultraviolet (UV) irradiation effect on minority-carrier recombination lifetime measured by a noncontact laser/microwave (LM) method is investigated for silicon wafers with native oxide. After UV irradiation, the surface recombination velocity greatly decreased resulting in the increase in the effective recombination lifetime (τeff). The effect disappears rapidly with time after the irradiation, and τeff recovers to the initial value after several minutes at room temperature. Since the UV irradiation process is noncontact and nondestructive, the irradiation is proposed to minimize the surface effect of sample, in turn, to obtain the bulk lifetime (τb) with a noncontact LM lifetime measurement method.

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