Impurity Gettering in Silicon by Thin Polycrystalline Films
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IMPURITY GETTERING IN SILICON BY THIN POLYCRYSTALLINE FILMS Y. HAYAMIZU, S. USHIO AND T. TAKENAKA SEH Isobe R&D Center, Shin-Etsu Handotai Co., Isobe, Annaka-shi, Gunma 379-01, Japan
Ltd.,
2-13-1
ABSTRACT Capability of impurity gettering by thin polycrystalline films on the backside of silicon wafer was evaluated by minority-carrier diffusion length. Cu was gettered easily during usual cooling after high temperature annealing. On the other hand, intentional slow cooling or low temperature annealing was necessary for effective Fe gettering. The gettering efficiency for Fe increased with lowering the annealing temperature when Fe was diffused sufficiently. From the quantitative consideration of Fe gettering, we propose the model of impurity gettering based on the chemical equilibrium of impurity reaction in polysilicon films. It was also expected that gettering efficiency increased with the thickness of polysilicon film. INTRODUCTION Impurity gettering in silicon is an indispensable technology to avoid device degradation by heavy metal contamination. Thin polycrystalline films deposited on the backside of silicon wafer, in other words polysilicon back seal (PBS) is one of the frequent external gettering (EG) techniques. Because gettering capability of PBS is usually greater than that of mechanically abraded backside damage [1-3] and PBS has an additional effect of oxygen precipitation enhancement. Recently, it is reported that the efficiency of gettering by PBS depends on the oxygen concentration in the wafers [4] . From the view point of engineering, it is important to understand the interaction between EG and internal gettering (IG) . Before studying the interaction, however, it is necessary to clarify the gettering mechanism of each method. In this paper we report the net gettering capability of PBS for Cu and Fe impurities, since these metals are dominant contaminants occurring during device fabrication process and they show contrastive behavior in silicon [5]. From the quantitative analysis, we discuss the mechanism of PBS gettering based on the chemical equilibrium of Fe reaction in polysilicon films. EXPERIMENT AND RESULTS The samples used in this study were (100) silicon wafers with 100 mm in diameter, 515 gm thick, p-type (boron doped) and n-type (phosphorus doped) with 10 ohm-cm resistivity prepared from float-zone (FZ) single crystals in order to investigate the net gettering capability of PBS. PBS with 1.1 gm thick and backside etched (BE) wafers were arranged for the comparison of their gettering capability. Gettering effects were evaluated using minority-carrier diffusion length by the new constant photon flux surface photovoltage (SPV) method [61 . The SPY method is generally accepted as an useful technique for evaluating the capability of EG [7-9]. Mat. Res. Soc. Symp. Proc. Vol. 262. 01992 Materials Research Society
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400
without
Ionar.0o
=
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FIG. 1. Dependence of effective carrier diffusion length
Cu
200
on the surface metal concen-
Fe .~.
(PBS)
I (PBS) (BE) 200•g (-3OPS)
10
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