Superpositioning of negative dc bias in dual-frequency capacitively coupled plasmas (dc-superposed (DS)-CCP) was realized for the selective etching of carbon-doped silicon oxide (SiOCH) films over carbon-doped amorphous silicon (SiC) films, while the dc bias exceeded about −800 V. When a dc bias of −1200 V was superposed on 60 MHz VHF power on the top electrode opposed to a wafer on the bottom electrode biased with 13.56 MHz power, a selectivity of above 50 for SiOCH over SiC was obtained. From characterization of the plasma density and various chemical species in the gaseous phase, such as CF2, CF and atomic N, the density of CF2 significantly decreased with the application of dc bias ranging from −800 to −1200 V. This indicated that CF2 radicals were consumed at the surface of the counter electrode which was made of silicon. The bulk densities of the species including CF2 were decreased, especially due to excess surface loss caused by the bombardment of highly energetic ions accelerated by the superposed dc bias, as well as the rf sheath for the superposition of the negative dc bias. The DS-CCP technology is thus concluded to be indispensable for yielding highly selective etching of SiOCH over SiC.

No abstract