Gettering of Dissolved and Segregated Impurities in Multicrystalline Silicon Wafers
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GETTERING OF DISSOLVED AND SEGREGATED IMPURITIES IN MULTICRYSTALLINE SILICON WAFERS
M. STEMMER, I. PERICHAUD, S. MARTINUZZI Laboratoire de Photodlectricitd des Semi-conducteurs, case 231, Facultd des Sciences et Techniques de Marseille-St.J6r6me, 13397 Marseille Cedex 13, France.
ABSTRACT
Phosphorus gettering by diffusion from a POCl 3 source was applied to matched wafers cut out of the same region of a cast ingot. Light Beam Induced Current mappings with wavelengths in the range between 840 and 980 nm lead to follow the variation of minority carrier diffusion length after gettering at 9000C for 120 and 240 inn, especially near extended crystallographic defects like dislocations and grain boundaries. The mappings show that after the gettering treatments, the local values of L increase due to the reduction of the recombination strength of extended defects and to the improvement of the homogeneous regions of the grains. As SIMS analyses indicate that Fe, Cu and Ni atoms are gettered, it is reasonable to assume that these impurities were initially dissolved in the grains and also segregated at the extended defects. INTRODUCTION
It is well known that metallic impurities, particularly fast diffusers, generate recombination centers in silicon. These impurities could be dissolved, precipitated or segregated by extended crystallographic defects like stacking faults, dislocations and eventually grain boundaries. The recombination strength of these defects is enhanced by impurity atom segregation, and it is necessary to reduce the impurity concentration in the wafers in order to improve their electrical properties. External gettering is a means of reducing or eliminating impurities by removing them from the bulk and localizing them near the surfaces. During the gettering, the impurities which could be removed must be intersitial atoms which can diffuse rapidly to the gettering sites, which are extended crystallographic defects, interfaces, vacancies or high solubility regions.The gettering mechanism could be divided in three steps as shown by figure 1 : 10/ Extraction of impurities from substitutional sites, from precipitates and complexes. The extracted impurities are transfered to interstitial position and are added to interstitial 3d metallic atoms like Fe.
Mat. Res. Soc. Symp. Proc. Vol. 262. C1992 Materials Research Society
976
INTERSTITIAL METALS
S..
(
1
~.• •steps
Fig. 1. The three basic of the gettering
mechanism of metallic impurities
FAST DIFFUSION PRECPITATES SUBSTITUTIONAL METALS
o
EXTRACTION
E5RN SITES
CAPTURE
20/ Fast diffusion through the crystal. 30/
Capture by gettering sites.
Notice that the extraction of impurities from substitutional sites and from precipitates could be favoured by the injection in the bulk of self interstitials in excess [1 ;2]. Phosphorus diffusion within the surface is a well known technique of external gettering which presents the advantage to be one of the preparation steps of most devices. When phosphorus diffusion is carried out from a POCI 3 source, a high co
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