Mechanisms of Transition-Element-Gettering in Silicon
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MECHANISMS OF TRANSITION-ELEMENT-GETTERING IN SILICON
DIETER GILLES Wacker Chemitronic GmbH, PO Box 1140, D-8263 Burghausen, Germany
ABSTRACT The unique properties of 3d-transition elements in silicon are reviewed that are essential for an understanding of gettering phenomena. Transitionelement-gettering techniques are divided into two groups, depending on whether or not the dominant gettering mechanism operates during annealing or cooling down of silicon wafers. Experiments that identify the gettering mechanism are presented for oxygen precipitation-induced gettering (internal gettering) as well as phosphorus-diffusion gettering. It is concluded that only the latter technique operates during high-temperature treatment. INTRODUCTION The reduction in device dimension and the increase in complexity of VLSI and ULSI technology require strictest control of unintended contamination. It is expected that contamination levels of below 1010 atoms/cm 2 have to be reached for every step within a device process [1]. Particularly dangerous are fast-diffusing transition elements. A tremendous effort has been undertaken to determine thermodynamic, electronic and transport properties of transition elements ( for reviews, see [2-4]). Fast interstitial diffusion of 3d-transition elements along with high supersaturation during cooling favor precipitation, which cause short circuits and gate oxide thinning in devices by metallic precipitates. On the other hand, if quenched to room-temperature, it is known experimentally and from theoretical calculations that those impurities introduce multiple deep levels, which act as recombination centers to reduce minority carrier lifetime. The ubiquitous presence of 3d-transition elements, like Fe, Cu, Ni, requires gettering as a safety insurance for high yields in a device line, because it allows one to increase the level of accidental contamination which can be tolerated. Gettering means the removal of metallic impurities in a wafer to predetermined sites away from the active part of the device. Various techniques have been applied successfully, like oxygen precipitation-induced gettering (internal gettering) [5], backside damage-induced gettering [6], or gettering after ion-implantation[71, phosphorus diffusion [8] or polysilicon deposition [9]. As to the gettering mechanism, Schr6ter et al. [3] proposed a classification into relaxation-, segregation-, and injection-induced gettering that will be discussed in the following. Only by identification of the mechanism it is possible to calculate gettering efficiencies and predict gettering optimization processes. Before a discussion of gettering mechanisms a brief summary on 3dtransition metal properties essential for the understanding of the gettering mechanisms is given. Mat. Res. Soc. Symp. Proc. Vol. 262. ยง1992 Materials Research Society
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SOLUBILITY, DIFFUSION, AND PRECIPITATION OF 3d-TRANSITION ELEMENTS IN SILICON The solubilities of 3d-transition elements investigated so far show an enthalpy of solution between 1.5 and 3.05 eV, i.e., a
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