A comparison of MOS gate structures formed by depositing polycrystalline silicon on thin oxides using conventional low pressure chemical vapor deposition and rapid thermal chemical vapor deposition

Abstract

MOS structures with 80A-200A thick gate oxides were fabricated using polycrystalline silicon gate electrodes deposited by rapid thermal chemical vapor deposition (RTCVD) and by conventional chemical vapor deposition (LPCVD). Polycrystalline silicon doping was achieved by BF2 or As implantation followed by rapid thermal annealing (RTA). The Q-C method was employed to study the electrical properties of the capacitors through high and low frequencyC- V profiles. The electrical properties of the MOS structures show that the devices fabricated using RTCVD polycrystalline Si are comparable in quality to those with LPCVD polycrystalline silicon gate electrodes. However, the results also indicate that boron diffusion through the thin oxide is a problem regardless of the deposition technique used for polycrystalline silicon. Boron penetration is accompanied by a shift in threshold voltage, inversion layer capacitance and a high density of midgap interface traps. Dopant diffusion in polycrystalline silicon and through the thin gate oxides was also studied by secondary ion mass spectroscopy (SIMS). The findings of the SIMS analysis correlate well with the electrical measurements. The results indicate that significant boron diffusion can occur through an 80A oxide if an RTA temperature higher than 1000° C is used. On the other hand, both SIMS and electrical measurements suggest that As penetration into the substrate is negligible even at temperatures as high as 1100° C.