INTERFACE DEFECT FORMATION IN MOSFETS BY ATOMIC-HYDROGEN EXPOSURE

The formation of interface defects that occurs when atomic hydrogen, H degrees, is introduced into the gate oxide of MOSFETs at room tempera ture 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 h ydrogen plasma. Baffles prevent UV light from the plasma from reaching the device. The effective channel lengths ranged from 0.9 to 5 mu m. 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 high-low capacitance-voltage measured with a Hg probe. We observe that the induced damage has the same energy dist ribution of interface traps in both types of samples. This distributio n is very similar to that produced by irradiation or hot electron stre ssing. It is characterized by a broad peak at 0.7 eV above the valence band. A charge pumping analysis has been developed to measure interfa ce states as a function of position above the MOSFET source and drain regions. The degradation decreases exponentially from the point of H d egrees entry at the gate edges near the source and drain. This demonst rates that the hydrogen enters the gate oxide of the MOSFET at the cha nnel edges. Furthermore, the decay length is constant within the exper imental uncertainty over three orders of magnitude of H degrees dose a nd two orders of magnitude of interface trap density. The exponential decrease of defect generation with increasing lateral diffusion distan ce is in contrast to the weaker oxide thickness dependence of the defe ct generation rate observed by Cartier et al.[J. Non-Cryst. Sol., in p ress]. These results indicate that defects such as Si-H bonds at the S i/SiO2, interfaces act as catalysts for H-2 formation by reactions suc h as Si- + H degrees --> SiH, SiH + H degrees --> Si- + H-2. Furthermo re, the number of these sites is not changed by the H degrees exposure .