The Effect of CMOS Processing on Oxygen Precipitation, Wafer Warpage, and Flatness

The interstitial oxygen concentration, wafer warpage, and flatness were measured after each step of a typical n‐well CMOS process. It is found that a critical oxygen concentration of 30.0 ppm exists below which no appreciable precipitation takes place during the process. Precipitation densities at the end of the process range from approximately for wafers having initial oxygen concentrations of 30.0–38.0 ppm, respectively. A well‐defined denuded zone is absent from wafers at the upper end of this concentration range. Changes in wafer flatness were less than the 1.0 μm measurement repeatability. Severe warpage was induced by thermal stress during an ion implant anneal step for wafers having an initial oxygen concentration greater than 32.0 ppm. Reduction of the push/pull temperature for this process and a subsequent glass anneal step from 900° to 650°C made the entire process immune to warpage irrespective of initial oxygen concentration.