Influence of Metal Impurities on Lifetime

Influence of Fe, Ni and Cu contamination on lifetime in Si wafers was studied by microwave photoconductive decay (μ -PCD), surface photovoltage (SPV) and MOS C-t methods. Lifetime degradation was determined by measurement of bulk trap density using deep level transient spectroscopy (DLTS) and observation of surface defects after selective etching by optical microscope. As Fe concentration in the bulk increased, recombination lifetime measured by μ -PCD and SPV method decreased but generation lifetime measured by MOS C-t method depended on the type of contamination. Fe-B pairs in p-type samples and interstitial Fe in n-type samples seem to shorten the recombination lifetime. Surface defects induced by Ni contamination shortened lifetime measured by μ -PCD method, but when measured by SPV method, which measure deeper into the bulk, this lifetime did not change. Also, in the Ni contaminated samples, lifetime measured by MOS C-t method was shortened. MOS C-t method therefore, seems to be sensitive to bulk state near interface of Si/SiO₂. As Cu concentration was increased, both the recombination lifetimes measured in n-type samples shortened and those in p-type samples did not change. It is postulated that Cu-related impurities in n-type samples are electrically active and those in p-type samples are inactive. No surface defects induced by Cu were observed in either case. The defects were, however visible after removal of Al electrode and gate oxide layer of MOS capacitors. The defects caused by Cu contamination as well as those caused by Ni contamination remarkably shorten lifetime measured by MOS C-t.