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44934-G5
Fundamental Research in Plasma Enhanced Chemical Vapor Deposition of Amorphous Carbon-Based Films from Hydrocarbon Plasmas
We are pursuing a combined experimental and computational approach to investigate the deposition mechanism of plasma-deposited amorphous carbon (a-C:H) thin films. Specifically, we are exploring the impact of processing conditions such as the power to the plasma source, pressure in the plasma chamber, and choice of precursor(s) on the complex hydrocarbon plasma chemistry and how the gas-phase composition, in turn, affect the film's structural and electrical properties.
Over the past year, this grant has partially supported a PhD student who has been involved in setting up a parallel-plate, capacitively-coupled plasma reactor, which is equipped with in situ surface and gas phase diagnostic tools. The plasma reactor has an in situ attenuated total internal reflection Fourier transform infrared spectroscopy setup to monitor a-C:H growth in real time. The setup provides sub-monolayer detection sensitivity to surface adsorbates and sub-0.1-atomic-% detection sensitivity to IR-active species in the bulk film. This setup is ideal for determining the interaction of various radicals generated in the plasma that impinge onto the substrate surface. To determine the identity of the various stable species present in the plasma, the reactor also has a differentially-pumped quadrupole mass spectrometer and an optical emission spectroscopy (OES) setup. The in situ diagnostic methods are supported with ex situ characterization of the films using Raman spectroscopy and spectroscopic ellipsometry. In combination with experiments, we have also put in place a molecular dynamics code, based on the extended Brenner potential, to simulate film growth from hydrocarbon radicals.
We are currently exploring the issue of H elimination from a-C:H during growth, which is essential for obtaining dense films with a high fraction of sp2-hybridized carbon atoms. Specifically, we are measuring the relative density of the reactive neutral species in the discharge through OES. We are determining if high-density films can be produced at low substrate temperatures, without ion bombardment from hydrocarbon plasmas that contain a high fraction of carbon-rich radicals such as C, C2, C3, and C3H. We are trying to generate these carbon-rich radicals in CH4 and C2H2 plasmas by operating the plasma under conditions that will provide very high dissociation of the parent gas. High power, and dilution with an inert gas such as He, is expected to raise the electron temperature, Te, and promote the dissociation of the parent-gas molecules. Gas-phase polymerization of fragments, such as C, is expected at higher pressures. In the simulations, we will also deposit films using similar radicals and determine if these conditions indeed lead to dense films. Appropriate a-C:H substrates have already been created by melting diamond slabs using MD simulations followed by rapid quenching of the resulting amorphous structure. To tune the density of the films to be similar to that of a-C:H, we first randomly removed an appropriate number of atoms from the diamond slabs prior to melting. After amorphization, the dangling bonds were passivated with H atoms. The resulting films have an sp2/sp3 hybridization ratio and H content that is very similar to experimentally deposited films.
