Internal Gettering in Bipolar Process: Effect on Circuit Performance and Relationship to Oxygen Precipitation Kinetics

Abstract

The effect of internal gettering on circuit performance has been studied for and linear logic bipolar circuits. The gettering has been introduced by controlled precipitation of oxygen in silicon wafers during processing, and its effect has been measured as reduction of circuit leakage current, which has been reflected in yield improvement. The efficiency of internal gettering has been found to be a strong function of the amount of precipitated oxygen and precipitate's morphology. It has also been found that the same factors affect the mechanical strength (warpage) of silicon wafers. The best gettering and yield improvement was obtained for relatively low amounts of precipitated oxygen (about 10 ppm), which resulted in the desired precipitate's morphology. An increase in the amount of precipitated oxygen decreased the gettering efficiency and also reduced the mechanical strength of the silicon wafers. The present work showed that, in addition to well‐known factors, such as the interstitial oxygen concentration in virgin wafers, oxygen‐precipitation kinetics during bipolar processing is strongly affected by other factors, such as concentration of nucleation centers for oxygen precipitation. They control the rate of oxygen precipitation and precipitate's morphology. Since the concentration of these centers is related to the crystal‐growth conditions, differences in precipitation kinetics are found in the wafers provided by various commercial suppliers. This can lead to unusual results. For instance, lower amounts of precipitated oxygen have been found in wafers with an initial oxygen concentration of 40 ppm than in wafers with 28 or 31 ppm coming from different suppliers. We believe that our results clearly demonstrate that internal gettering after optimization can be used to improve circuit yield in bipolar processing lines.