Statistical Analysis of Micropipe Defect Distributions in Silicon Carbide Crystals
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Statistical Analysis of Micropipe Defect Distributions in Silicon Carbide Crystals
Authors: Troy Elkington, II-VI Inc., Saxonburg PA; Ejiro Emorhokpor, II-VI Inc., Pine Brook NJ; Tom Kerr, II-VI Inc., Pine Brook NJ; John Chen, II-VI Inc., Saxonburg PA; Kevin Essary, II-VI Inc., Saxonburg PA; Mike Golab, II-VI Inc., Saxonburg PA; Richard H. Hopkins, II-VI Inc., Saxonburg PA; ABSTRACT The density and distribution of micropipes (MPD’s), a screw dislocation with a hollow core, are significant criteria for SiC wafer selection in device fabrication. For this reason the measurement of micropipes in SiC is an important problem. The micropipe distributions of 2’’diameter 6H SiC wafers from three different PVT furnace designs in three different laboratories were statistically characterized. The micropipe distributions were measured in two ways: by KOH etching the wafer and then counting the hexagonal pits formed on the wafer surface using a microscope and imaging software; and by manually counting with an optical transmission microscope the micropipes on a un-etched wafer within a grid of 0.5mm squares superimposed over the wafer. None of the wafers had a random distribution of micropipes i.e. they did not follow a Poisson distribution. The micropipes were found to associate with each other i.e. there is a higher probability of there being other micropipes near a location where a micropipe has been found. In the eight samples characterized, 1.9% of the wafer area contained 32% of the total micropipes. We have analyzed the repeatability of the optical transmission microscopy method of micropipe characterization and found that it is dependent on the micropipe density; the standard deviation of the measurement increases with increasing MPD. The number of micropipes counted increases by about 10% with increasing magnification and 15% when the wafer is double side polished. INTRODUCTION Micropipes are known device killers [1] and so their measurement is an important step in quality control for a manufacturing process. Various methods of identifying micropipes have been reported in the literature including: SWBXT; SEM; AFM; X-ray Topography; Counting pits formed by KOH etching; and various forms of Optical Microscopy [1-6]. There does not appear to be a generally accepted method for counting micropipes particularly in a manufacturing environment. There are two techniques being studied at II-VI. The first is a KOH etch of the wafer followed by inspection for hexagonal pits of a certain size range [1,3]. The second is to inspect the wafers with an optical microscope in transmission mode at a magnification of 100x. The accuracy and repeatability of a measurement method used to accept or reject material in a manufacturing environment must be well understood in order to lower manufacturing costs. In this work the repeatability of the Optical Transmission Microscopy method of micropipe characterization is studied and reported.
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