Silicon oxide and silicon nitride etch mechanisms in nitrogen trifluoride/ethylene plasma

Low-pressure inductive plasma was used to study SiO 2 and Si3N4 etching with the NF3/C 2H4-based gas chemistry. NF3 and C2H 4 were used so that fluorine and carbon could be supplied from parent gases other than strongly global warming fluorocarbons, which are conventionally used in industry. Experiments were performed at low pressure (<10 mTorr) and low flow rates to minimize production of global-warming CFx species. Selective etching of SiO2 to Si3N4 was not achieved. Etch rates of SiO2 over a wide range of conditions are less than 0.8 times the Si3N4 etch rates. Changing ion energy distributions at the oxide or nitride surface by varying the bias frequency significantly changed the etch rates of both materials but did not strongly affect etch mechanisms or the selectivity. Ex-situ X-ray photoelectron spectroscopy was employed to determine the characteristics of a very thin steady-state film, in order to establish likely etch mechanisms. CHx F, CF2, and CF3 were produced but in small concentrations compared to CHx and CN, as supported by mass spectrometry and XPS results. The C 1s spectra from etched oxide samples show a large percentage of HxC-CHx structures compared to other carbon-containing species. On the other hand, Si3N4 appears to react easily with HxC-CHx structures, yielding CN-bearing etch products and SiC. Although nitride can react quickly with saturated hydrocarbon structures, a too large concentration of HxC-CHx decelerates the removal process of CN-containing etch products from the surface and can lower the nitride etch rate. The lack of selectivity is not limited to C2H 4. Etch rate and selectivity results of NF3-based discharges fed with C2H2, C4H10, and CH 3F are similar to those of the NF3/C2H4 plasma. Comparisons of normalized F 1s spectra of nitride and oxide etched under the same conditions show that relative concentrations of CF2 and CF3 on SiO2 are much lower than the concentrations on Si3N4. It appears that SiO2 preferentially reacts with CF2 and CF3 only but not with HxC-CH x or CHxF. Differences in the abilities of SiO2 and Si3N4 to react with HxC-CHx structures contributed to higher etch rates of Si3N4.