Interface Defect Formation in MOSFETs by Atomic Hydrogen Exposure

Abstract

The formation of interface defects that occurs when atomic hydrogen, H°, is introduced into the gate oxide of MOSFETs at room temperature has been studied, MOSFETs with the edges of the gate oxide at the source and drain exposed to the ambient were placed downstream of a hydrogen plasma. Baffles prevent UV light from the plasma from reaching the device. The effective channel lengths ranged from 0.9 to 5 μm. For comparison, bare thermal oxides were exposed at the same time. The interface state density in the MOSFETs was measured by charge pumping and in the bare oxide by highLow capacitance-voltage measured with a Hg probe. We observe that the induced damage has the same energy distribution of interface traps in both types of samples. This distribution is very similar to that produced by irradiation or hot electron stressing. It is characterized by a broad peak at 0.7 eV above the valence band. A charge pumping analysis has been developed to measure interface states as a function of position above the MOSFET source and drain regions. The degradation decreases exponentially from the point of H° entry at the gate edges near the source and drain. This demonstrates that the hydrogen enters the gate oxide of the MOSFET at the channel edges. Furthermore, the decay length is constant within the experimental uncertainty over three orders of magnitude of H° dose and two orders of magnitude of interface trap density. The exponential decrease of defect generation with increasing lateral diffusion distance is in contrast to the weaker oxide thickness dependence of the defect generation rate observed by Cartier et al.[J. Non-Cryst. Sol., in press]. These results indicate that defects such as Si H bonds at the Si/SiO2 interfaces act as catalysts for H2 formation by reactions such as Si-+ H° → SiH, SiH + H° → Si-+ H2. Furthermore, the number of these sites is not changed by the H° exposure. © 1994 IEEE