Discharge aspects involved in plasma etching

In this paper, the results of a study of the discharge characteristics of a D.C. plasma etcher are presented for the gases CF4and NF3. These results emphasize the importance of discharge chemistry. To study the role of CF3and F in the etching process, a 0.25 Torr mixture of CF4(NF3) and Helium was excited in a D.C. discharge. At low enough power densities such that saturation processes were negligible, a primarily Helium discharge enhanced the 704 nm free fluorine emission over that of a CF discharge through a resonant transfer process, (He^*+ CF4→ He + CF3+ F^**). Even though this line was enhanced by a factor of ∼ 6, the etching process was not affected implying that the products play a small role, if any, in the surface reaction. When the SiO2(Si3N4) sample was biased to draw net ion current from the plasma, the etch rate was greatly enhanced. Measured ion temperatures were found to be too low (< 60 eV) for this to be the result of ion milling. We attribute this enhanced etching to carrier multiplication in the sheath region between the sample and the plasma. Preliminary results will be reported on the performance of NF3as the etchant. Significant (> 10x) increases in etch rate were found with NF3, apparently because all dissociated components of this medium are gases.