Study On Deposition Of Diamond-Like Carbon Films By Dielectric Barrier Discharge

For improving the qualities of diamond-like carbon (DLC) film deposited fromdielectric barrier discharge, the relations between plasma parameters and filmqualities have been studied. Firstly, CH₄ and C₂H₆ gases were used to deposit DLCfilms, and, their plasma processes were analyzed. Secondly, the effects of adding Arand He into CH₄ on plasma characteristics and the properties of DLC films depositedwere investigated by measuring the discharge power and analyzing the plasmaelectron temperature. This work reveals the relations between the plasmacharacteristics and film quality. Finally, the theoretical models of ion energy from thedepositing plasmas were established based on drift and diffuse theory. Calculatedresults show that the ion can obtain kinetic energy when traveling through electricfield to form high-quality DLC films, no bias voltage were needed during filmdeposition, thus the whole set of equipment is simple. The contents and results of thiswork are listed as follows:(1) DLC films have been deposited using CH₄ and C₂H₆ gases, and filmproperties have been compared. The results show, that soft polymer-like hydrocarbonfilms are deposited from C₂H₆ gases, and film hardness is independent on gas pressureP and discharge space d. However, at P=100Pa, d=10mm, the dense DLC films aredeposited by C₂H₆/H₂ gases. Film hardness increased from 4 to 19.5GPa withincreasing H₂ fraction from 0ï¼…to 95ï¼…. This indicates that the strong polymerizationprocess occurred in the pure C₂H₆ DBD plasma, which led to the deposition ofpolymer-like films. In contrast, the weak polymerization process occurred in pure CH₄DBD plasma.(2) The effects of Ar and He on DLC deposition were compared usingCH₄/He(Ar) mixtures at the pressure of 100 Pa, discharge space of 10 mm, and thetotal flow rate of 5 sccm. It was found that the hardness of DLC films increased from12.5GPa to 19.4 GPa with increasing the Ar fraction from 0 to 67ï¼…, and increasedfrom 2.5GPa to 23GPa with increasing He fraction from 0 to 58ï¼…. Clearly theproperties of DLC film were greatly improved by adding Ar or He into CH₄.(3) The discharge power of CH₄/He or CH₄/Ar DBD plasmas was measured, theelectron temperature was calculated from the measurement of emission opticalspectrum. The effect of Ar (He) on CH₄+Ar or CH₄+He DBD plasma wereinvestigated by measurement of discharge power and calculation of electrontemperature from emission optical spectrum. At the gas pressure of 100Pa anddischarge space of 10mm, discharge voltage of 25kV, the discharge power of CH₄/Ar decreased from 24.5 to 8.3 mW with increasing the Ar (He) fraction from 20ï¼…to 83ï¼…,while electron temperature decreased from＞6000K to＜2000K. The discharge powerof CH₄/He mixtures increased from 10mW to 23mW when the He fraction increasesfrom 18ï¼…to 67ï¼…, then decreased to＜5mW at the He fraction of 83ï¼…. The electrontemperature was found to be 7146K at the He fraction of 80ï¼…, and 9237K at Hefraction of 90ï¼….(4) The theoretical model based on the drift and diffusion of ions in the dischargegas was used to analyze the kinetic energy of ions. The variation of ion energy withthe product of gas pressure P and discharge space d in CH₄ DBD, and the variation ofion energy with the Ar or He fraction in CH₄/Ar or CH₄／He mixtures were presented.It was found that the hydrocarbon ion energy increased from 5.4eV to 20.2eV withdecreasing the Pd value from 14 to 4.5×133Pa mm, then increased rapidly to 163eV at the Pd value of 2×133Pa mm. With increasing Ar fraction in CH₄/Ar or Hefraction in CH₄/He mixtures from 18ï¼…to 83ï¼…, the kinetic energy of hydrocarbonions increases from 69.6 (43.9) to 92.1 (128.5) eV. The high kinetic energy ofhydrocarbon ions is responsible for the deposition of dense DLC films by DBDplasmas without substrate bias.The experimental results and theoretical analysis showed that the DBD method issuitable for DLC film deposition. During film deposition, the ions can obtain, fromdischarge space electrical field, kinetic energy enough to deposit the dense DLC filmswithout substrate bias, and the total set of equipment are quite simple. Adding Ar orHe to CH₄ facilities improving the properties of DLC films through increasing thehydroncarbon ion kinetic energy. However, the mechanisms of Ar (He) affecting onDBD plasma characteristics are different. In CH₄/Ar plasma, lower breakdownvoltage leads to low ion density, low discharge power, and low electron temperature.However, due to a relatively high mass of Ar, the density of CH_n and H can beimproved through the bi-body collisions between the CH₄ and Ar or Ar^*, andstrengthened surface reaction increased the DLC film hardness. For CH₄/He mixtures,the ionization potential and sub-stable energy level of He is much higher than that ofCH₄, so the breakdown voltage is high, which leads to an increase in the electrontemperature, ionized rate, ion density, and discharge power. However, the times ofdischarge pulses in one cycle of power voltage decrease at high He fractions, whichexplain why the discharge power decreases with the He fraction.