Growth Mechanism of a-C:H Thin Films

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  • What is NFC film and how can it be deposited?

  •        NFC stands for Near-frictionless Carbon. NFC films/coatings were developed at the Argonne National Laboratory (Chicago, USA) by plasma enhanced chemical vapor deposition (PECVD). A source gas of either pure methane or a mixture of methane (CH4) and hydrogen (H2) at 10-40 mtorr pressure at room temperature with an RF bias of 300-400 V was used to prepare super low friction carbon films in the thickness range 1-10 micrometers. It was published by Erdemir, Ali et al in the Journal of Vacuum Science and Technology (J. Vac. Sci. Technol. A 18(4), Jul/Aug 2000 1987-1992) briefly summarized in this presentation.

  • Expand XRR. What can it be used for?

  •        XRR stands for X-ray Reflectivity. It is also called as X-ray Reflectometry or X-ray Specular Reflectivity. In thin films science, this technique is used to characterize density, roughness, and thickness of thin coatings. In general this technique has been used to study a variety of solid and liquid interfaces since its inception in 1954. The central concept behind this technique is the deviation in the intensity of the reflected x-rays, which is then used for characterization.

  • What is the typical hydrogen content in a-C:H films? What does the hydrogen content strongly depend on?

  •        The typical hydrogen content in a-C:H films is from 30% to more than 50%. The hydrogen content strongly depends on the kinetic energy of the impinging ions during plasma-enhanced chemical vapor deposition process.

  • How can one control the hydrogen content in a-C:H films?

  •        The hydrogen content in a-C:H films can be controlled by controlling the impinging ion energy during plasma-enhanced chemical vapor deposition. The hydrogen content increases with decreasing impinging ion energy.

  • How is the hydrogen content related to carbon bonding in a-C:H films?

  •        The more the hydrogen content, the more the sp3 hybridized carbon atoms. Carbon films made at low impinging ion energies with about ~50% hydrogen atom concentration are soft in nature, and are termed as polymer-like amorphous carbon films.

  • What kind of carbon bonding will be observed at high ion energies (> 50 eV)?

  •        At high ion energies (say, > 50 eV), the hydrogen content decreases in the carbon film which increases the amount of sp2 hybridized carbon atoms.

  • What are polymer-like amorphous carbon films?

  •        Polymer-like amorphous carbon films are carbon films made at low energies with almost all the carbon atoms in sp3 hybridized state. Polymer-like amorphous carbon films are very soft in nature.

  • What is the effect of energy of impinging ions on film composition?

  •        The more the energy of impinging ions, the lower the amount of hydrogen atom concentration, the more the sp2 hybridized carbon atoms in the carbon film, and vice versa. (always remember: the more the hydrogen, the more the sp3 carbon atoms)

  • What is surface loss probability, and what are its values for sp1-, sp2-, and sp3- hybridized precursors?

  •        Surface loss probability is the probability that the impinging source ions stick to the surface of the growing film (i.e., 1 minus reflection coefficient) (read more). Its values for sp1-, sp2-, and sp3- hybridized precursors are 0.9, 0.35, and < 0.01, respectively.

  • Explain the growth mechanism of a-C:H films with schematics.

  •        The energy of impinging ions and the surface loss probability have been found to determine the structure of carbon films. However, the growth mechanism of a-C:H films is not fully understood, and is based entirely on plasma experiments. In plasma experiments, the effect of various process parameters (viz. substrate temperature, plasma power, bias voltage, etc.) as a function of growth rate is studied. A simplified pictorial description of the particle beam experiment is shown below (inspired by Hopf, C. et al).

    In this experiment, the very first step is the creation of dangling bonds at the film surface. Dangling bonds are created by abstraction of surface-bonded hydrogen by impinging H and CH3 radicals. Chemisorption of CH3 radicals at dangling bonds at the C:H film surface results in film growth. H and CH3 radicals interact with the C:H film surface simultaneously, which leads to a growth synergy. Schematic of the growth mechanism is shown below.

    (download high resolution image)

    There are two zones during film growth: (1) chemistry dominated zone, and (2) ion dominated zone. The chemistry dominated zone extends approximately 2 nm from the surface into the film. In this zone, impinging H radical activates the surface by abstracting hydrogen, which creates chemisorption sites for incident CH3 radicals. In the ion dominated zone, the distribution of bonded hydrogen in the film is altered in consecutive cascades. Displaced H atoms recombine during such a distribution to form hydrogen molecules (H2), which eventually desorb, and decrease the hydrogen content in the film permanently. However, displaced H atoms may also occupy dangling bonds inside the film in a lattice defect or at the film surface (chemisorption sites).

    >> Keudell, A. von et al