Spectroscopic studies of fluorescent emission in plasma etching of Si and SiO 2 and the mechanism of gas-surface interactions

Abstract

Results are presented of an extensive study of the spectral emission of a discharge in a CF 4/8% O 2 mixture, a system of major importance for plasma etching of Si and SiO 2 in silicon device technology. A commercially available single wafer etching apparatus (E.T. Associates, Nailsea, UK) was used in this study. Spectra have been recorded in the range 1900–7100 Å at moderate resolution (∼0.3 Å) resulting in the identification of F, F +, O, O +, C, CO, CO +, CO 2 + and CF 2, under typical operating conditions (0.18 torr, 0.3 W cm −2, discharge frequency: 62.5 kHz). Si and Si + were also observed in the presence of Si and thermally grown SiO 2 substrates. In a new technique, using a fine quartz light probe (0.8 mm dia), the intensity of emission of F, F +, CO and CO 2 + has been measured in the presence of Si and SiO 2 substrates as a function of position across the discharge parallel to the anode and cathode surfaces. Major conclusions relate largely to etching of Si. We have established that Si is etched by the action of F atoms with a contribution from CO (a 3Π) and that the rate of attack is considerably faster for Si(100) than for (111). Moreover, our results clearly imply a very small sheath potential around a substrate in the discharge. Energy is imparted to the surface through the action of metastable atoms and molecules such as quartet states of F, singlet states of F + or CO (a 3Π), rather than through the impact of ions of high kinetic energy. On a practical note spatially resolved data indicate how the major industrial problem of etch non-uniformity may be significantly alleviated. Through the observation of the behaviour of a variety of electronic states of F, F +, O, O + and C in the presence of Si we suggest that gas-surface reactions follow the same spin conservation rule as that of gas phase reactions. This principle should be useful in understanding plasma etching in general and in the future choice of suitable gases.