Neutron Activation Study of a Gettering Treatment for Czochralski Silicon Substrates

Abstract

A pre‐process gettering treatment is described for Czochralski silicon substrates which removes essentially any transition group metal contaminants present in the wafer and continues to getter contaminants during subsequent device processing. Laser induced damage (LID) on the back surface of the wafer, a modified high temperature treatment, and formation of oxygen precipitates in the bulk of the wafer only, are used as a three‐step gettering process. Neutron activation analysis (NAA) was used to monitor the gettering behavior of each individual contaminant species and shows that the combined treatment is indeed nearly 100% effective in removing impurities, both from starting substrates and from processed wafers. Removal of impurities from the starting substrates is important in order to prevent the formation of oxidation induced stacking faults (OSF), which are nucleated by impurities and then become decorated with impurities making them much harder to getter (1). Holdtime performance of 4K and 16K dynamic RAM's fabricated on substrates treated in this manner served to evaluate the effectiveness of the treatment. Significant improvement can be achieved for bulk substrates. However, the performance of epitaxial silicon on low resistivity substrates cannot be matched since the low resistivity substrate in the EPI case additionally suppresses the minority carrier diffusion current from the bulk of the wafer. The individual steps of the treatment are: (i) produce laser induced damage on the back surface of the wafers by means of powerful, partially overlapping laser pulses; (ii) ramp from 950° to 1250°C in 0.5% , 0.5% , balance argon; (iii) oxidize at 1250°C for 8 hr in dry , then ramp down to 950°C. Keeping the oxygen concentration to 0.5% during step (ii) is important to prevent formation of a thick oxide film during this step. The long oxidation at 1250°C both depletes interstitial oxygen at the Si surface, thus preventing formation of OSF, and causes formation of oxygen precipitates in the bulk if the initial concentration is high enough. Oxygen precipitates in the bulk are a valuable means of gettering.