Hydrogen-plasma etching of hydrogenated amorphous silicon: A study by a combination of spectroscopic ellipsometry and trap-limited diffusion model

The kinetics of etching hydrogenated amorphous silicon by a hydrogen plasma has been studied by in-situ spectroscopic ellipsometry measurements. The formation of a hydrogen-rich sublayer is clearly emphasized. Its thickness increases from 7 to 27 nm when the temperature during the hydrogen-plasma treatment is raised from 100 to 250°C. This effect is interpreted by solving the differential equation for trap-limited hydrogen diffusion through a mobile surface. By assigning the thickness of this sublayer to the mean diffusion distance of hydrogen we determined values of the effective diffusion coefficient of hydrogen higher than 10^-14cm²s ^-1 with an activation energy of 0.22 eV. The density of hydrogen traps is found to decrease from 7.3 × 10¹⁸ to 4.5 × 10¹⁷cm^-3 as the temperature of the hydrogen treatment increases from 100 to 250°C with an activation energy of 0.43 eV. This effect is interpreted by a thermal equilibrium involving hydrogen transitions between shallow states and hydrogen-trapping sites.