Development of a planar coil radio frequency inductively coupled plasma system for material processing

Ng, Kim Hooi (2008) Development of a planar coil radio frequency inductively coupled plasma system for material processing. PhD thesis, University of Malaya.

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A planar coil radio frequency inductively coupled plasma (PC-RFICP) system is set up for this project. The system is powered by a 13.56MHz, 550W, 50 Ω RF generator. The RF power is transferred to the plasma via a planar induction coil. The impedance matching network is a Π-type automatic matching network or a LC resonant circuit matching network (consisting of a step down transformer and a tunable vacuum capacitor, connected in series). The typical E-H mode transition, which is distinctive to the ICP, has been observed in argon plasma. The pure argon plasma is able to transit from the E to H mode plasma at lower RF power compare to argon-nitrogen plasma. No mode transition is observed for pure nitrogen discharge. In the case of the hydrogen and methane-hydrogen admixture plasmas, no E to H mode transition is observed within the limit of the operating power of the RF generator. An increase in plasma light intensity is observed during E to H mode transition. The hysteresis behavior typically observed in an inductively coupled plasma mode transition is also observed when coil current is measured during E to H mode transition and vice versa. The confinement of the RF plasma into a small volume within a glass funnel is able to reduce the RF power required for E to H mode discharge transition. E to H mode transition for hydrogen plasma and hydrogen-methane admixture plasma is observed to occur after the plasma is restricted to a small volume by using a glass funnel. Besides that, the induction heating effect of silicon substrate by the eddy current that is developed on the substrate has been studied. This effect is able to heat up the substrate to 800 to 900oC. Langmuir probe measurement is conducted on the plasma generated by the PC-RFICP system for the pure argon and argon/acetylene admixture discharges to obtain the ion density (ni) and electron temperature (Te). The values of Te obtained for the pure argon discharge under all conditions are in the range of 0.25 to 8.5 eV, while the values of ni measured is in the order of 1017 m-3. On the other hand, the values of Te obtained from the argon/acetylene admixture discharge under all conditions are in the range of 3 to 5 eV, while ni measured is in the order of 1016 m-3. The PC-RFICP system is then used to demonstrate plasma material processing which include nitriding of metallic surface, plasma sputter deposition of titanium nitride film, plasma enhanced chemical deposition of diamond-like carbon (DLC) film, and hydrogenated amorphous carbon (a-C:H) film. The PC-RFICP system operated at 250 W in hydrogen-nitrogen admixture plasma at 1 mbar is used for nitriding of 316 stainless steel substrate. The substrate was biased at -600 V DC. A nitrided layer of 10 ± 2μm was obtained after 2 hours of treatment. The hardness of the nitrided surface is 1365 ± 300 Hv with dominant ε- (Fe,Cr)3N phase present in the nitrided layer. Nitriding of titanium using the PC-RFICP system produces nitrided layers of δ-TiN, ε-Ti2N and α-Ti(N) phases as indicated by XRD analysis. Effects discharge pressure and substrate bias voltage on the formation of these layers is investigated. Surface hardness of the nitrided sample is found to improve by four times compared to untreated titanium. However, surface morphology revealed by AFM and SEM indicated a rougher surface for the nitrided samples and surface roughness is found to increase with the increasing substrate bias voltage. The deposition of TiN film is using the PC-RFICP system is based on plasma enhanced physical vapor deposition (PEPVD) method. The target is biased with negative voltage of –800 V, while the bias voltage of the substrate is varied from 0 to –160 V in steps of 20 V to study its effect on the deposition process. Deposited titanium nitride thin film is golden yellow in color. At low substrate, bias voltage the quality of the film is found to deteriorate as indicated by the dark brown color of the film and increase in the roughness of the film. The effects of discharge pressure and the presence of hydrogen gas are also investigated. A change in the orientation plane from (222) to (111) and (311) for TiN film deposited is observed as discharge pressure is increased. Hydrogen gas is found to prevent the formation of titanium oxy nitride; however, the substrate bias voltage has a larger effect in affecting the “intrinsic feature”of the TiN film than the presence of hydrogen gas. The PC-RFICP system is also used for plasma enhanced chemical vapor deposition (PECVD) of diamond-like carbon (DLC) films on silicon substrate. The effect of the percentage of methane, the influence of the pretreatment using diamond paste on the substrate and the induction heating effect on the DLC films are studied. The different percentages of methane mixed with hydrogen have significant effect on the texture of the surface morphology of the diamond-like carbon thin film. The pretreatment using the diamond paste on the silicon substrate seems to improve the nucleation of the diamond-like carbon film. Induction heating of the silicon substrate which is able to heat up the sample to temperature of about 800-900oC, is important for the nucleation and growth of diamond-like carbon film. The a-C:H film deposited from admixture of acetylene/argon gas using the PC-RFICP system is polymeric and soft. From SEM micrograph analysis, the film deposited onto glass and silicon substrates appear to be homogenous. AFM imaging suggests that the type of growth of the a-C:H film during film deposition is of the columnar growth type. The thickness of the film ranges from 0.45 μm to 1.2 μm, depending on the conditions that is used for deposition. The rate of the deposition of the a-C:H film is found to increase with the increase in applied RF power and discharge pressure. However, the rate of deposition decreases with increase in the flow rate of acetylene and substrate’s distance from the planar coil. Discharge parameters such as applied RF power, discharge pressure, acetylene flow rate, and distance of the substrate from the planar coil are found to affect optical properties such as optical energy gap (Eg) and Urbach energy (Eu) of the a-C:H film. Microstructure of the a-C:H films are also affected by these discharge parameters. Post deposition annealing of the a-C:H film in air at 250 oC is conducted and appear to be a possible solution to improve the adhesion of the film. The a-C:H film demonstrates ohmic characteristic in their electrical property. The conductivity of the film is in the range of 3.0 X 10-8 to 4.7 X 10-8 Scm-1 based on dark current measurement.

Item Type: Thesis (PhD)
Subjects: Q Science > Q Science (General)
Date Deposited: 31 Jul 2013 03:06
Last Modified: 31 Jul 2013 03:06

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