Using a method to study the grown-in defect density spectra in Czochralski silicon wafers, we elucidate the changes in the size distribution of grown-in oxide precipitate nuclei caused by thermal processing in a common complementary metal-oxide semiconductor device process. The first thermal step determines which parts of the grown-in defects grow to large stable defects and how many harmful defects appear in the defect-denuded zone. The cooling rate of the crystal considerably influences the defect evolution during complementary metal oxide semiconductor processing. The choice of appropriate silicon material for a device process or adjusting processing conditions to suit the material are important for defect generation during processing and, consequently, also for device yield.