A computational model for plasma chemical reactions has been developed. An ab initio molecular orbital method was used to determine dissociation paths and their threshold energies. Plasma characteristics were calculated by a plasma kinetic method. The radical compositions in C 4 F 8 , with additional gases such as Ar, He and CH 2 F 2 , were calculated. Radicals influencing the selective etching of SiO 2 over Si 3 N 4 were analysed. With increased microwave power or decreased flow rate, CF 2 density decreased and CF, C and F densities increased. The increase of radicals with abundant carbon relative to fluorine would result in high etch selectivity of SiO 2 over Si 3 N 4 and a low etch rate. Increases in the concentration of F radicals are correlated with increases in SiO 2 etch rates. Electron temperature was high with He addition, and dropped with C 4 F 8 alone and Ar addition discharges. On the contrary, the electron density was high in the reverse order. The highest etch selectivity was obtained with He addition. A high electron temperature discharge would be one solution to obtain high etch selectivity of SiO 2 over Si 3 N 4 .
A computational model for chemical reactions in plasmas has been developed and applied to the gas-phase chemistry of dry etching of silicon dioxide. An ab-initio molecular orbital method is used to determine the dissociation processes and the threshold energies for gases and neutral radicals. Plasma chemistry is calculated by the plasma chemical kinetic method. The chemical compositions of the dry etching plasmas were investigated for fluorocarbon gases. Calculated ion fluxes, electron temperatures, and densities agreed with experimental results within factors of three. The differences in plasma characteristics between CHF3/CH2F2 and C4F8 were attributed to differences in the products and threshold energies of the dissociation reactions. Correlations could be found between the composition of radicals in a plasma and the etch selectivity in C4F8.
Plasma compositions in BCl3/Cl2 were calculated using a molecular orbital method and a plasma kinetic method. The key factors for plasma compositions and their relations to the etch characteristics of Al-based multilayers were investigated. The etch rate of Al seemed to be correlated with the Cl density rather than the Cl2 density. The etch rates of TiN and the resist were also related to the Cl density and the ion density. Therefore the ion energy distribution (IED) impinging on the wafer was also calculated and the rf frequency dependence of the selectivity of the TiN/resist was evaluated using an etching model. Even if the rf frequency increased to 10 MHz, where the IED had a single peak, the predicted selectivity was slightly improved. A narrower IED was required.
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