Gettering of Impurities in Silicon

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GETTERING OF IMPURITIES IN SILICON A. OURMAZD AT&T Bell Laboratories Holmdel, NJ 07733

ABSTRACT Despite the apparent dissimilarities between different gettering methods, we show that many can be understood in terms of two basic mechanisms. The first involves the interaction of selfinterstitials (emitted, for example, by P in-diffusion and precipitation, 0 precipitation, or Ar implantation) with the impurities to be gettered. The second relies on a strain-field/point defect interaction (for example around a dislocation), which appears capable of enhancing the diffusivity of the impurities. In either case, gettering can be viewed as a particular and particularly useful instance of the wider class of defect/defect interactions. INTRODUCTION The technological importance of gettering lies in its remarkable ability to remove metallic impurities from certain regions of a wafer by localizing them in other parts. This technological utility has resulted in intensive research, producing many detailed descriptions of conditions under which particular gettering procedures operate efficiently. Early attempts at an understanding of the basic physical mechanisms at work were limited to investigating the equilibrium consequences of the introduction of the gettering species (e.g. phosphorus), with little attention to the kinetics of the gettering process. A remarkable aspect of gettering is the wide variety of means by which it can come about; phosphorus in-diffusion, oxygen precipitation, and argon implantation are but a few examples. The very different natures of the techniques suggest that gettering may come about more as a consequence of the effect of the gettering agent on the host crystal than a direct interaction between the gettering agent and the impurity to be gettered. Phosphorus gettering is accomplished by the in-diffusion of a large concentration of phosphorus into the region where the impurities are to be localized. This effect is traditionally described in terms of the consequent raising of the Fermi level. An impurity with an acceptor level can remove an electron from the conduction band, thereby reducing the energy of the system. Thermodynamically, this leads to a solubility enhancement for acceptor impurities. In this way, the heavily P-doped layer acts as a sink for the impurities, capturing and localizing those that arrive by diffusion. The thermodynamic logic of this argument is, of course, correct. Indeed, in cases where the total number of impurities in the wafer can be accommodated in the P-doped layer by the solubility enhancement, and no kinetic questions are considered, this model is able to explain the overall features of the "gettering" process [1]. However, gettering is not limited to impurities with acceptor levels (see below). Moreover, the raising of the Fermi level prior to the in-diffusion of the metallic impurities is not an effective gettering procedure [2]. For these and other kinetic reasons described below, the solubility enhancement effect at best operates in parallel with a more general pr

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