Formation of In-Cu Pairs in Silicon During Chemomechanical Polishing
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Formation of In-Cu Pairs in Silicon during Chemomechanical Polishing TH. WICHERT *, R. KELLER, M. DEICHER, W. PFEIFFER, H. SKUDLIK AND D. STEINER Fakultiit ffir Physik, Universitat Konstanz, D 7750 Konstanz, FRG
ABSTRACT
Using the perturbed yy angular correlation technique (PAC) the pairing of Cu with the radioactive acceptor atom 1111n in Si is detected. Because of the identity of the electric field gradients the so-called X defect, observed after chemomechanical polishing of Si wafers and known of neutralizing acceptor atoms in Si, is identified as a Cu atom. It is also shown that as-delivered Si wafers already contain Cu atoms which neutralize acceptor atoms if the wafers are annealed at 1173 K.
INTRODUCTION
During the past years a growing attention has been given to the problem of unintentional deactivation of dopant atoms in semiconductors. 1 Here, hydrogen plays a prominent role since it was discovered that acceptor atoms in Si are passivated by hydrogen atoms. Meanwhile, a fair number of device processing steps has been identified that all lead to the incorporation of H atoms into p-Si followed by the deactivation of acceptor atoms and the formation of acceptor-hydrogen pairs. Among these processing steps also chemomechanical polishing of Si wafers in alkali-based solutions was found to neutralize the acceptor atoms B, Al, Ga, and In and again H was reported to be responsible for this process. 2 This interpretation has meanwhile become part of the current literature though two significant differences are evident if one compares the dynamics and thermal stability of the neutralization of the acceptor atoms effected by either chemomechanical polishing 3 or by the other processing steps1 : i) polishing at ambient temperatures effects a passivation of acceptors across the whole sample of 1 mm thickness within a few hours (for an acceptor concentration of typical 1"1016 cm"3 ) whereas H atoms are known of passivating only the first 10 p•m of the Si wafer at typically 400 K; this implies a substantially higher mobility of the passivating defect X in the polished wafer than of the H atom. ii) The complex formed after polishing dissociates already at room temperature as determined by the decrease of the electrical resistivity of the sample whereas acceptor-H pairs are known to dissociate not below 400 K. A third, direct evidence for the non-identity of the acceptor-X complex induced by polishing and
Mat. Res. Soc. Symp. Proc. Vol. 163. cc1990 Materials Research Society
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the acceptor-H pairs is the difference of the electric field gradients observed by perturbed •• angular correlation experiments (PAC). 4 Therefore, it has to be concluded that the defect X induced by chemomechanical polishing has to be different from H atoms. Using the radioactive probe atom 111In we shall determine the chemical identity of the X defect and demonstrate that chemomechanical polishing of Si leads to the formation of In-Cu pairs. In addition, we shall show that as-delivered Si wafers already contain Cu. The consequences of th
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