Gettering of copper and nickel in p/p+ epitaxial wafers

We studied whether the theory and practice of segregation gettering are applicable to copper and nickel in p/p+ epitaxial wafers of different substrate resistivities. The gettering test consisted of a reproducible spin-on contamination of the wafers, followed by a metal drive-in at 800 degrees C for 30 min under argon with a cooling rate of 50 degrees C/min at the end. For evaluating the metal profile in the wafer, we developed a new analytical method: wafers were etched step by step using a mixture of hydrofluoric and nitric acid. Then these "stratigraphical" etching solutions were analyzed by inductively coupled plasma mass spectrometry. This enabled the evaluation of stratigraphical metal concentration profiles in the wafers with detection limits of between 10(12) and 10(12) atoms/cm(3). Copper was found to be gettered in the substrate due to the high boron concentration. No precipitated copper was detected at the stress field of the epi/substrate interface. We explained the behavior of copper in terms of its increased solubility in heavily boron-doped silicon. Calculations of the gettering efficiency were in good agreement with our experimental results. Nickel, by contrast, was not gettered by the heavily boron-doped substrate wafers. it also did not precipitate at the stress field of the epitaxial layer/substrate wafer interface. Instead, it was found only in the near-surface layers. From the gettering behavior of nickel we conclude that interstitial nickel in silicon is uncharged. (C) 2000 The Electrochemical Society.