THE CHEMISTRY OF A CCL2F2 RADIO-FREQUENCY DISCHARGE

A systematic study of the chemistry of stable molecules and radicals i n a low pressure CCl2F2 radio frequency discharge for dry Si etching h as been performed. Various particle densities have been measured and m odeled. The electron density, needed as an input parameter to model th e CCl2F2 dissociation, is measured by a microwave cavity method. The d ensities of stable molecules, like CClF3, CF4, 1,2-C2Cl2F4 and the etc h product SiF4, are measured by Fourier transform absorption spectrosc opy. The density of the CF2 radical is measured by means of absorption spectroscopy with a tunable diode laser. Its density is in the order of 10(19) m(-3). All density measurements are presented as a function of various plasma parameters. Moreover, optical emission intensities o f Cl and F have been recorded as a function of plasma parameters. It a ppears that the feed gas (CCl2F2) is substantially dissociated (about 70%-90%) in the discharge. Based on the obtained data the dissociation rates of several molecules have been estimated. The measured total di ssociation rate of is 8x10(-15) m(3) s(-1). For this molecule the diss ociation rate is substantially higher than the dissociative attachment rate (10(-15) m(3) s(-1)). The dissociation rate for CClF3 is 2X10(-1 5) m(3) s(-1) about 3x10(-16). The total dissociation rate of C2Cl2F4 is higher than 5X10(-15) m(3) s(-1), Of which 2.5+/-0.5X10(-15) m(3) s (-1) is due to dissociative attachment. Furthermore it has been found that the presence of a silicon wafer strongly affects the plasma chemi stry. Optical emission measurements show that the densities of halogen radicals are significantly depleted in presence of Si. Moreover, the densities of several halocarbon molecules display a negative correlati on with the density of the etch product SiF4. (C) 1995 American Vacuum Society.