Swirl Defects in As-Grown Silicon Crystals
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SWIRL DEFECTS IN AS-GROWN SILICON CRYSTALS
A.J.R.
DE KOCK
Philips Research Laboratories,
Eindhoven, The Netherlands
ABSTRACT During melt-growth of macroscopically dislocationfree bulk silicon crystals (floating-zone and Czochralski technique) microdefects can form due to the condensation of thermal point defects (self-interstitials, vacancies). The formation of these imperfections, generally referred to as "swirl defects", is strongly affected by the growth conditions (e.g. the crystal pulling rate) and crystal purity. The various reported defect formation models will be discussed. Special attention will be paid to the effect of doping on swirl defect formation. INTRODUCTION During growth from the melt of macroscopically dislocation-free silicon crystals microdefects can form, which have a detrimental influence on device performance. Due to their typical spiral-type distribution in cross-sectional crystal slices, these microdefects are generally referred to as "swirl defects". They have been extensively studied in floating-zone (FZ) and Czochralski-grown (CZ) silicon crystals. The results of these investigations have been reviewed in a number of recent articles (1-h). Several models for the formation of swirl defects have been developed (2,4-11). In the majority of these models the defect formation is attributed to the condensation of thermal point defects (self-interstitials and vacancies). In the present paper the most significant properties of swirl defects are
summarized with some emphasis on recently obtained data related to doping.
The various nucleation models are.then briefly described. It is shown that none of the existing models allows a complete interpretation of all experimental data. In principle a more consistent model would be obtained by combining several different elements taken from existing models. DETECTION OF SWIRL DEFECTS Swirl defects in silicon crystals have been studied with the aid of a large number of simple as well as sophisticated diagnostic techniques such as preferential etching (12-17), X-ray transmission topography or infrared transmission microscopy combined with copper or lithium decoration (9,13,14,15,17, 18), double-crystal topograhic techniques (19,20), section topography (21), electron beam induced current mode scanning electron microscopy (22) and high voltage electron microscopy (HVEM) (5,7,23-26). While all these methods give information about the concentration and distribution of swirl defects, only HVEM enables the structural characterization of these microdefects. The presence of swirl defects can also be established more indirectly by means of device processing steps. They can give rise to the formation of extrinsic stacking faults during oxidation (13,14,27,28) and sometimes during epitaxial layer growth (29,30). Because swirl defects often adversely affect device performance, they can be located via e.g. the analysis of leakage current patterns in diode arrays (13,31,32).
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