Very High Selective n⁺ poly-Si RIE with Carbon Elimination

The effect of carbon contamination was studied in low-temperature HBr RIE of n + -doped poly-Si etching. With a resist mask, which is a notable carbon source, selectivity (poly-Si/SiO 2 ) was 10 to 20. With a SiO 2 mask and carbon elimination from various sources such as gas, piping system and reactor wall, SiO 2 etching rate was suppressed, and selectivity of more than 300 was achieved with a self-bias voltage of 400 V. Addition of a small amount of oxygen after carbon elimination removed residual carbon and enhanced the selectivity up to 3000. The anisotropic profile was not affected by carbon elimination since the side-wall protection by reaction products (SiBr x ) was the mechanism of anisotropy. The effect of carbon on the selectivity was explained by the thermodynamics on the basis of bond strengths of reaction products.