Manipulation of etch selectivity of silicon nitride over silicon dioxide to a-carbon by controlling substate temperature with a CF4/H2 plasma

The effects of substrate temperature (Ts) on the etch rate (ER) of the PECVD-prepared SiN, SiO2 and amorphous carbon (a-C) films, and their selectivity were investigated with a CF4/H2 plasma. The ERs for the SiN at all Ts were higher than that for the SiO2 films. As Ts was decreased from 50 to −20 °C, the ER for the SiN decreased. Contrarily, the ER of the SiO2 films increased. The etching selectivity of SiN over SiO2 reached to near unity when the Ts was −20 °C. At the same time, the ER of for the a-C films was found to be around 0.1 nm/s and irrespective of Ts. The fluorocarbon (FC) thickness was greater for the SiO2 films than that of the SiN. The lower ER for the SiO2 was therefore attributed to the thicker FC layer and resultant etching mechanism. As the Ts was decreased, the FC thickness on the SiO2 films decreased, which led to the ER decrease. The decrease of ER for the SiN etching at the low temperature was likely due to the higher stability of the surface N–H modification layer, compared with that processed at 20 °C, which was confirmed by the in situ FTIR.