STRIATIONS, SWIRLS, AND STACKING FAULTS IN CZOCHRALSKI-GROWN SILICON

Abstract

In this investigation, controlled thermal annealing and oxidation treatments were carried out on wafers obtained from seed-end and tang-end regions of (100)-oriented, 75 mm-diameter, Czochralski-grown, "typical" silicon single crystals. The radial variation of resistivity was characterized with four-point probe and spreading resistance probe measurements. The defects were studied by preferential etching and optical microscopy, using Wright etch for characterizing the individual etch figures, whereas the overall distribution of defects was obtained by using a modified form of Sirtl etch. The preferential etching was carried out in a Teflon barrel under controlled conditions. Transmission electron microscopy (TEM) was carried out on selected samples to study the defect structure in the as-grown crystal as well as after specific thermal treatments. In the p-type as well as n-type crystals studied in this work, the relative radial gradient as well as the magnitude of resistivity are greater at the seed-end than at the tang-end. An annealing treatment at 650°C for 100 min on seed-end wafers stabilized the resisitivity by destroying oxygen-donor complexes. Such an annealing treatment on tang-end wafers has a minor effect on the resistivity of the sample, which was uniform initially. The "swirl" patterns, as revealed by preferential etching, showed that they are more pronounced in seed-end wafers and are almost absent in tang-end wafers. A pre-annealing treatment at 650°C in argon for 100 min followed by a high-temperature (≥800°C) treatment precipitates the swirl pattern much more intensely, in comparison to just the high temperature treatment without any preanneal at 650°C. For comparable oxide thicknesses (0.5 μm) for thermal oxidation in steam at three different temperatures (900°C, 1050°C, 1200°C), it was found that the swirl pattern was most severe at 900°C and the dissolution of the defect structure progressively increased with increasing temperature. It was found in this investigation that bulk-type stacking faults are generated after argon annealing at 1050°C. This is in contrast to the generally prevailing confusion that thermal oxidation is essential for generation of stacking faults in silicon. It must be distinguished here that the formation of surface-type stacking faults requires thermal oxidation, whereas bulk-type stacking faults nucleate at individual swirl defects due to precipitation of dissolved oxygen. TEM work done in this investigation showed that as-grown CZ silicon defect structure consists of an assortment of precipitates, small dislocation lines, and a helical type of long (∼24 μm) dislocation line, and another long linear defect with periodically spaced nodes. The annealing treatment at 650°C as well as thermal oxidation at 900°C produce a spectrum of precipitates and small dislocations.