STUDIES OF WAFER SURFACE CHARGING USING THE THOR MONITOR DEVICE

The THOR monitor is a Texas Instruments in-house gate oxide integrity test device used to qualify the charging performance of production imp lant processes. It has five charge collection antenna on capacitor str uctures with antenna to gate ratios ranging from 100:1 to 7500:1. The gate oxide thickness of 200 Angstrom could be considered to be large, but the addition of photoresist makes this device extremely sensitive to charging damage. A sensitive THOR photoresist configuration has bee n studied in conjunction with advanced beam diagnostics in a situation in which intermittent yield was achieved. A P/150 keV/20 mA implant w as found to give full yield on the test structure for only approximate ly half of the implants before enhancements to the flood system overca me this. Traditional beam diagnostics, such as wheel current, capaciti ve charge sensor voltage and beam profile width could not predict when yield would be good or bad. However, measurement of slow ion spectra before an implant using a cylindrical mirror analyser ion spectrometer gave good correlation between the slow ion spectrum and THOR yield. T he variation in slow ion spectrum could be related to ion source tunin g and demonstrates that although present charge neutralization devices are sufficient for todays requirements, future charging performance i mprovements may well come about from understanding the generation of h igh quality beams and the use of advanced beam diagnostics to aid in a utomatic implanter tuning.