DETECTION OF METALLIC CONTAMINANTS ON SILICON BY SURFACE SENSITIVE MINORITY-CARRIER LIFETIME MEASUREMENTS

We present the use of in situ carrier lifetime measurement by radio-fr equency photoconductance decay (RFPCD) to detect metallic contaminatio n during aqueous HF cleaning of silicon wafers. The effect of metal io n deposition from dilute HF solutions on carrier recombination at the Si/HF junction is examined. Ions of the more electropositive metals, s uch as Fe2+ and V2+, do not undergo spontaneous reduction from dilute HF. When present in HF, they do not measurably increase surface recomb ination and hence cannot be detected by RFPCD. Reduction of Cu2+ ions at the Si/HF interface occurs by a slow (conduction band) process whic h involves nucleation of nanometer sized precipitates, whose areal den sity can be measured by RFPCD. Using RFPCD in conjunction with atomic force microscopy, we have found that the presence of 2 ppm of chloride ions in dilute HF increases considerably both nucleation and growth o f the-copper clusters. Air exposure prior to RFPCD measurement in HF i ncreases the recombination activity of deposited copper on the surface drastically. The reason for this effect remains poorly understood at present. In contrast with copper, gold ions deposit from HF on Si by a much faster valence band process. The initial deposition rate of gold ions is diffusion limited, as evidenced by the t(-1/2) dependence of the surface minority carrier lifetime, tau(surf) (t is immersion time in gold-dosed solution).