Key influence of the thermal history on process-induced defects in Czochralski silicon wafers

Using a method to study the grown-in defect density spectra in Czochralski silicon wafers by infrared light scattering tomography, we elucidate the changes in the size distribution of grown-in oxide precipitate nuclei caused by thermal processing at the beginning of a common CMOS device process. The first thermal step, screen oxidation at 900 degrees C, determines which parts of the grown-in defects grow to large stable defect formations and which of them shrink. The cooling rate of the crystal has a considerable influence on the defect evolution during CMOS processing. Low cooling rates result in lower defect densities than high cooling rates, although the total density of as-grown nuclei is the same. However, the maximum of their size distribution is located at a lower stability temperature for the low cooling rate than for the high cooling rate. This is important for their ability to grow during the first thermal step of the CMOS process. It also demonstrates that the choice of the appropriate silicon material is important for defect generation during processing and consequently also for the device yield.