Comparison of CF4/O2 and CF2Cl2/O2 plasmas used for the reactive ion etching of single crystal silicon

Abstract

The etching of single crystal silicon in CF4/O2 and CF2Cl2/O2 gas mixtures in a 13.56 MHz plasma generated in a radial flow reactor at 0.4 W/cm2 and 30 mTorr total pressure has been studied. It was found that for an unloaded reactor, the addition of oxygen to a CF4 plasma increased the lateral and vertical etch rate of single crystal silicon. The maximum lateral etch rate coincided with the maximum vertical etch rate of silicon which occured at 30% of added oxygen (by flow). Simultaneously, the sputtering of gold, used to define the silicon pattern, increased with the proportion of added oxygen. The effect of reactor loading with silicon was also studied, and found to affect the etching anisotropy in a CF4/O2 plasma. Under similar conditions of flow, pressure and power, the CF2Cl2/O2 plasma was found to produce a reduced self bias voltage at the cathode. The etch rate of the gold mask was found to vary with added oxygen in this plasma, as did the silicon etch rate. In general, the anisotropy of etching silicon in the CF2Cl2/O2 plasma was found to be much higher than that in the CF4/O2 plasma at the same oxygen concentration. Possible mechanisms accounting for this difference in anisotropy will be explored.