The chemistry of a CCl2F2 radio frequency discharge

Abstract

A systematic study of the chemistry of stable molecules and radicals in a low pressure CCl 2 F 2 radio frequency discharge for dry Si etching has been performed. Various particle densities have been measured and modeled. The electron density, needed as an input parameter to model the CCl 2 F 2 dissociation, is measured by a microwave cavity method. The densities of stable molecules, like CClF 3 , CF 4 , 1,2-C 2 Cl 2 F 4 and the etch product SiF 4 , are measured by Fourier transform absorption spectroscopy. The density of the CF 2 radical is measured by means of absorption spectroscopy with a tunable diode laser. Its density is in the order of 10 19 m Ϫ3 . All density measurements are presented as a function of various plasma parameters. Moreover, optical emission intensities of Cl and F have been recorded as a function of plasma parameters. It appears that the feed gas ͑CCl 2 F 2 ) 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 dissociation rate of CCl 2 F 2 is 8ϫ10 Ϫ15 m 3 s Ϫ1 . For this molecule the dissociation rate is substantially higher than the dissociative attachment rate ͑10 Ϫ15 m 3 s Ϫ1 ). The dissociation rate for CClF 3 is 2ϫ10 Ϫ15 m 3 s Ϫ1and for CF 4 about 3ϫ10 Ϫ16 m 3 s Ϫ1 . The total dissociation rate of C 2 Cl 2 F 4 is higher than 5 ϫ 10 Ϫ15 m 3 s Ϫ1 , of which 2.5Ϯ0.5 ϫ 10 Ϫ15 m 3 s Ϫ1 is due to dissociative attachment. Furthermore it has been found that the presence of a silicon wafer strongly affects the plasma chemistry. 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 correlation with the density of the etch product SiF 4 .

Production and destruction of CFx radicals in radio-frequency fluorocarbon plasmas

“…We use the fact that stable molecules are removed by the gas flow, 10 while the radical densities are flow independent. Thus, by comparing the spectra for different gas flow rates the radical absorption lines can be found.…”

Destruction of Freons by the Use of High-Voltage Glow Plasmas

“…RF discharge plasmas have to be discerned from surface, silent, and glow discharge plasmas. Radio frequency (RF) plasmas favor electron impact dissociation over electron attachment dissociation as Stoffels et al observed for dissociation of CCl 2 F 2 (up to 90%) at low pressure (0−400 mTorr) and high power input. Wang et al showed for the same compound that high decomposition rates (up to 94%) and high selectivities to CH 4 and C 2 H 2 (up to 80%) are achievable in a low-pressure hydrogen−argon RF plasma system.…”

nfluence of temperature on the formation of altered layer during silicon etching in CF2Cl2plasma

“…The main products of dissociation of CF 2 Cl 2 molecules are CF 2 Cl, CF 2 radicals, and Cl atoms [9]. In order to reduce the number of frequency probabilities that are freely chosen, only CF 2 radicals are considered.…”