Oxygen and Bulk Microdefects in Silicon, 2nd Edition

This chapter discusses the oxygen and bulk microdefects (BMDs) in silicon in detail. Crystalline silicon grown with the Czochralski method contains high concentrations of oxygen. Oxygen atoms initially occupy interstitial sites in the crystal lattice. The main methods for the quantitative determination of interstitial oxygen are infrared spectroscopy, gas fusion analysis (GFA), and secondary ion mass spectroscopy (SIMS). For GFA, silicon samples are inserted in a graphite crucible together with high-purity nickel as a metal flux, and are melted by heating the sample in an inert gas to about 2000°C. In SIMS, the sample is bombarded with an ion beam in an ultrahigh vacuum chamber, and the secondary ions sputtered from the sample are analyzed with a mass spectrometer. Like GFA, SIMS measures the total oxygen concentration. Oxygen precipitates and related defects in the silicon bulk are often referred to as BMDs. The most common method used today for measuring the BMD density is called preferential etching. The preferential etching of defects is based on the use of a special etching solution, which has a higher etch rate around the microdefects compared with the surrounding defect-free silicon. This method can only be used for measuring the BMD density. In principle, scanning infrared microscopy (SIRM) is capable of measuring the density and size distribution of the microdefects as a function of wafer depth. The scattered light is proportional to the volume of the defect, which makes SIRM very sensitive also to defect size variations. Light scattering tomography measure requires a cleaved cross-section sample and is therefore considered a destructive method. In other methods, oxygen precipitates and related defects act as recombination centers in silicon. The density or the size of the defects cannot be directly measured.