Selective Etching of Thick Si3N4, SiO2 and Si by Using CF4/O2 and C2F6 Gases with or without O2 or Ar Addition

The etching characteristics of thick Si3Ni4, SiO2 and Si in CF4/O-2 (95%/5%) and C2F6 plasmas with or without O-2 or Ar gas as diluents by using reactive ion etching were studied, together with photoresist mask erosion rate during the etching. Etching parameters, such as the CF4/O-2 (or C2F6) flow rate, the O-2 flow rate, the Ar flow rate, the rf power and the process pressure, were changed. The selective etching of Si3N4 over SiO2 and Si was also investigated under various etching conditions. We found that CF4/O-2 gas was more efficient than C2F6 gas owing to the enhanced Si3N4 etching caused by NO for the selective etching of Si3N4 over both SiO2 and Si. In a CF4/O-2 plasma, the etching selectivity was considerably improved with decreasing flow rates of CF4/O-2 and O-2 (or Ar) and with increasing process pressure. Si3N4 etch selectivities over SiO2 and Si of 13.2 and 8.9 were obtained, respectively, with 20 sccm CF4/O-2 at 150 mTorr and 75 W, while maintaining a high Si3N4 etch rare of 306 nm/min. The etching rates were measured by using a surface profiler and a scanning electron microscope (SEM). The etched profiles and the surface morphology were observed by using a SEM. A possible mechanism for these etching phenomena is discussed.