CW Backside Laser Gettering
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CW BACKSIDE LASER GETTERING
GILBERT HAWKINS AND GEORGE ERIKSON Eastman Kodak Company, Research Laboratories Rochester, New York 14650
ABSTRACT A variety of backside damage techniques are available for These gettering heavy-metal contaminants in silicon wafers. include mechanical damage, ion implantation, thin film deposiIn each case, strain tion, and pulsed-laser surface melting. fields and microscopic defects induced by the processing trap impurities as they diffuse through the wafer during subsequent high-temperature processing steps. We examine the defect structures produced by CW laser gettering, describe the dependence of gettering efficiency on wafer oxygen content and processing conditions, and demonstrate that CW laser processing can be an effective gettering technique even when the number of laser scan lines is reduced to make wafer processing acceptably rapid.
INTRODUCTION Transition-metal impurities are a primary source of generationIn many device applicarecombination centers in high-purity silicon. tions, minority-carrier generation from g-r centers is highly detrimental. Impuriti ]may be intrinsic to the starting material or may be process induced. Both the type of contaminant and its location in the wafer are Midgap impurities important in determining the effects of g-r currents. near the silicon[•foxide interface and in the depletion region are partialthough contaminants dispersed in the bulk or collected cularly harmful, at gettering sites may also influence device performance, especially at high temperatures. The location of contaminants in the final device wafer is process dependent and is correlated with wafer oxygen and carbon content. Since these parameters are difficult to control, it is not surprising that minority-carrier generation rates vary greatly from wafer to wafer and run to run, even under clean processing conditions. by choosing This undesired variation can be reduced in two ways: design and processing conditions to minimize the effects of contaminants or by gettering contaminants away from the active surface region of the wafer, This paper describes CW-laser either to the interior or to the backside. backside gettering. Previous workbrs [4 have used a variety of techniques to effect backside damage gettering, including mechanical damage, ion implantation, thin In each case, strain film deposition, and pulsed-laser surface mel~ting. fields and microscopic defects induced by the processing are believed to trap heavy-metal impurities as they diffuse through the wafer during subThese techniques suffer sequent high-temperature device processing steps. (1) Most involve a considerable number of processfrom two disadvantages: ing steps. For example, phosphorus backside implantation necessitates special frontside protection and, for p-type wafers, requires removal of (ii) The defects responsible for the implant for ohmic backside contact. gettering may themselves be annealed at high temperatures and, depending upon the detailed nature of the defects, may become ineffective at some point
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