STUDY ON THE ULTRANANOCRYSTALLINE DIAMOND FILM SYNTHESIS

Abstract

The ultrananocrystalline diamond (UNCD) film synthesis by direct-current plasma assisted chemical vapor deposition (DC-PACVD) and hot filament chemical vapor deposition (HF-CVD) was investigated. In particular, I devoted attention to investigation of 1) the stabilization of DC plasma by optimization of cathode material, precursor gas and interelectrode distance (IED); 2) the refinement of grain size by controlling of the electron temperature and cathode temperature under the DC-PACVD environment and growth temperature under the HF-CVD environment; and 3) the enhancement of nucleation density by ultrasonic seeding treatment, substrate surface modification and improvements of DC-PACVD process. Furthermore, I provide novel insights on the grain-refining mechanism in DC-PACVD as well as HF-CVD using hydrogen-rich precursor gas without positive ion bombardment on the growth front, and the seeding mechanism with additional enhancement in the electrostatic interaction between nano-diamond particles and substrate. The grain refinement and nucleation enhancement of diamond enabled the void-free ultrathin (U)NCD coating as thin as 30 nm, thereby providing its suitability for applications in the mechanical seal, micro-electro-mechanical-systems (MEMS) and thin film waveguide resonance sensor. The UNCD deposition by direct-current plasma assisted chemical vapor deposition on 4 in. Si wafer using CH4-H2 as well as CH4-Ar gas chemistry containing additive nitrogen was investigated. CH4/N2/H2 (5/0.5/94.5%) and CH4/N2/H2/Ar (0.5/5/6/88.5%) gas mixtures were compared as the precursor gas. Molybdenum and tungsten were compared as cathode material. I demonstrated that (1) the elimination of the positive column, by adopting very small interelectrode distance, gave some important and beneficial effects; (2) the plasma stability and impurity incorporation was sensitive to the cathode material and the precursor gas; (3) using the conventional CH4/H2 precursor gas and tungsten cathode, the mirror-smooth 4 in. UNCD film of excellent phase-purity and grain size below 10 nm could be deposited even in the absence of the positive column. The high electric field in the unusually narrow interelectrode space and the consequent high electron kinetic energy, in conjunction with the unusually high electron current thereof, directed to the substrate, was proposed to be the source of the grain refinement to achieve UNCD at such high pressure (110–150 Torr), in the absence of the usual ion bombardment assistance. New insights into the grain-refining mechanism in the UNCD film synthesis under the direct-current diode discharge environment using the hydrogen-rich gas chemistry with the growth front on the anode were provided. The grain size and crystal structure of the diamond films were analyzed as a function of the interelectrode electric field (IEEF) at a given growth temperature. A strong grain refinement from MCD toward UNCD domain was enabled by increasing IEEF from 260 V/cm to 940 V/cm, which was correlated well to the IEEF-dependence of local electron temperature (i.e. kinetic energy) near the growth front. The generation of the bi-radical sites via the electron-stimulated desorption (ESD) of the hydrogen from the hydrogenate diamond growth front, which should increase with electron stimulation strength thereon, was proposed to be responsible for the grain refinement. Such insights reconciled the uniqueness of UNCD synthesis by DC-PACVD in relatively high pressure and under the positive substrate bias in the hydrogen-rich gas chemistry, under which conditions the microwave plasma chemical vapor deposition (MP-CVD) or HF-CVD was unable to synthesize the UNCD to date. Some novel aspects of nanocrystalline diamond (NCD) film nucleation and growth by DC-PACVD were investigated, which focused on the effect of methane injection timing at ramp stage and cathode temperature as well. NCD films were deposited for 4 h on a 4 in. Si wafer which was ultrasonically seeded in methanol slurry of diamond powder with a 5 nm average diameter. The nucleation density was found to be sensitive to methane injection timing in the ramp stage. In addition, the cathode temperature greatly affected the nucleation density, grain size and growth rate. The synthesis of ultrathin, mirror-smooth and void-free UNCD film was investigated using DC-PACVD. The seeding process was investigated in the previously reported “two-step” seeding scheme, where the substrate was pretreated in microwave hydrocarbon plasma prior to the ultrasonic seeding to enhance seed density; in the present study, DC plasma and hot filament process were adopted for the pretreatment, instead of the conventional microwave plasma. Two types of nano-diamond seed powders of similar grain sizes but with different zeta potentials were also compared. Contrary to the previous report, the pretreatments deteriorated the seed density relative to that of the non-treated substrate. By contrast, the seed density was drastically improved by using a proper type of the nano-diamond seed powder. The seed density variation according to the substrate pretreatments and the type of the seed powders was attributed to the relative values of the zeta potentials of the substrates and that of the seed powders, which indicated the electrostatic nature of the seeding process. The variation of the substrate surface zeta potentials was attributed to the variation in the surface terminations induced by the respective pretreatments. The present DC-PACVD environment ensured that the secondary nucleation was also active enough to generate the densely packed UNCD grains in the growth stage. Consequently, the ultrathin, mirror-smooth and void-free UNCD film of 30 nm in thickness was enabled. The effect of surface modification on the dispersion of nano-diamond seeds on the SiO2-coated Si substrate to enable the ultrathin UNCD coating on the substrate was investigated by the DC-PACVD using hydrogen-rich chemistry. The ultrasonically dispersed seed density on the SiO2-coated Si wafer was so much lower than that on the pristine Si wafer that the void-free ultrathin UNCD coating was impossible. For surface modification, I exposed the substrate to (1) the hydrogen/hydrocarbon plasma in the DC-PACVD chamber or (2) the hydrocarbon atmosphere in the HF-CVD chamber, prior to the ultrasonic seeding. The exposure to hydrocarbon or to its plasma greatly reduced the seed density, while exposure to hydrogen plasma drastically enhanced it by a factor of 6, which enabled a void-free ultrathin UNCD coating as thin as 30 nm. The effects of the pretreatments on the seed density were explained by Si−OH or Si−CH3 termination and by the consequent change on (1) the zeta potentials of the substrate and (2) that of the nano-diamond particles in the seeding suspension. The NCD thin film growth was systematically investigated for application in the thin film waveguide resonance sensor. The NCD thin film was grown on the Si wafer or the SiO2-coated sapphire substrate using the HF-CVD. The waveguide modes of the NCD layer were studied by prism coupler technique using laser with varying incident angle. A novel aspect was disclosed in the grain size dependence on the growth temperature at the relatively low methane content in the precursor gas, which was important for optical property: the grain size increased with decreasing growth temperature, which was opposite to the conventional knowledge prevailing in MCD domain. I proposed a mechanism in terms of the classical nucleation theory to reconcile the observation. An optical waveguide mode resonance based on the microstructure-controlled transparent NCD thin film waveguide was demonstrated in the visible region, which provided a strong potential for waveguide mode resonance sensor applications.