Impact of hydrofluorocarbon molecular structure parameters on plasma etching of ultra-low-K dielectric

Abstract

The authors report a systematic study aimed at evaluating the impact of molecular structure parameters of hydrofluorocarbon (HFC) precursors on plasma deposition of fluorocarbon (FC) films and etching performance of a representative ultra-low-k material, along with amorphous carbon. The precursor gases studied included fluorocarbon and hydrofluorocarbon gases whose molecular weights and chemical structures were systematically varied. Gases with three different degrees of unsaturation (DU) were examined. Trifluoromethane (CHF3) is the only fully saturated gas that was tested. The gases with a DU value of one are 3,3,3-trifluoropropene (C3H3F3), hexafluoropropene (C3F6), 1,1,3,3,3-pentafluoro-1-propene (C3HF5), (E)-1,2,3,3,3-pentafluoropropene (C3HF5 isomer), heptafluoropropyl trifluorovinyl ether (C5F10O), octafluorocyclobutane (C4F8), and octafluoro-2-butene (C4F8 isomer). The gases with a DU value of two includes hexafluoro-1,3-butadiene (C4F6), hexafluoro-2-butyne (C4F6 isomer), octafluorocyclopentene (C5F8), and decafluorocyclohexene (C6F10). The work was performed in a dual frequency capacitively coupled plasma reactor. Real-time characterization of deposition and etching was performed using in situ ellipsometry, and optical emission spectroscopy was used for characterization of CF2 radicals in the gas phase. The chemical composition of the deposited FC films was examined by x-ray photoelectron spectroscopy. The authors found that the CF2 fraction, defined as the number of CF2 groups in a precursor molecule divided by the total number of carbon atoms in the molecule, determines the CF2 optical emission intensity of the plasma. CF2 optical emission, however, is not the dominant factor that determines HFC film deposition rates. Rather, HFC film deposition rates are determined by the number of weak bonds in the precursor molecule, which include a ring structure, C=C, C≡C, and C–H bonds. These bonds are broken preferentially in the plasma, and/or at the surface and fragments arriving at the substrate surface presumably provide dangling bonds that efficiently bond to the substrate or other fragments. Upon application of a radio-frequency bias to the substrate, substrate etching is induced. Highly polymerizing gases show decreased substrate etching rates as compared to HFC gases characterized by a lower HFC film deposition rate. This can be explained by a competition between deposition and etching reactions, and an increased energy and etchant dissipation in relatively thicker steady state FC films that form on the substrate surface. Deposited HFC films exhibit typically a high CF2 density at the film surface, which correlates with both the CF2 fractions in the precursor molecular structure and the deposition rate. The FC films deposited using hydrogen-containing precursors show higher degrees of crosslinking and lower F/C ratios than precursors without hydrogen, and exhibit a lower etch rate of substrate material. A small gap structure that blocks direct ion bombardment was used to simulate the sidewall plasma environment of a feature and was employed for in situ ellipsometry measurements. It is shown that highly polymerizing precursors with a DU of two enable protection of low-k sidewalls during plasma exposure from oxygen-related damage by protective film deposition. Dielectric film modifications are seen for precursors with a lower DU.