Abstract
Polyoxide gate dielectric degradation problems were encountered during the process development of a three-dimensional CMOS structure. In prior studies, the gate dielectric degradation was found to occur when the polyoxide was exposed to the oxygen deficient, low pressure silicon rich epitaxial lateral overgrowth (ELO) ambient . The durability of thin polyoxide dielectrics is essential to three-dimensional process, allowing bottom gate control of the vertically stacked PMOS load device. The previous process had approximately a 1000 A minimum thickness limit on the bottom gate dielectric, unacceptable when compared to modern day CMOS technology. This research was directed at developing a durable high quality 100-300 A nitrided polyoxide (NPOX) gate dielectric process. The incorporation and distribution of nitrogen in both ammonia nitrided polyoxide (NPOX) and nitrided silicon dioxide (NOX) dielectric films were studied. The effects of the nitrogen concentration and distribution on the resistance of the NPOX and NOX films to EL0 ambient degradation were tletermined. It was observed that the surface nitrogen concentration had no effect on the durability of the dielectric. However, a bulk nitrogen concentration as low as 8 at% significantly reduced the formation of EL0 ambient induced pinholes in 250A dielectric films. After 40 min. of EL0 stress the electrical yield was raised from 0%, for polyoxide and silicon dioxide dielectric capacitors, to over 80% for NPOX and NOX dielectric capacitors. Analyses of the failed devices suggest that active pinhole generation still existed, however, the bulk nitrogen concentration dramatically reduced the frequency and speed at which these defects were produced. Fixed oxide charges and interface state densities on the order of 1 .2*101 1 were observed after 1100 "C, 10 min. nitridation with NPOX capacitor yields of' 84% after 40 min. of EL0 growth ambient stressing. After 60 minutes of nitridation, the surface became resistant to the EL0 growth ambient induced surface pitting and roughening.
Date of this Version
January 1995