Synthesis, Characterization, and Molecular Dynamics Analysis of Ultrathin Amorphous Carbon Films
- Author(s): Wang, Na
- Advisor(s): Komvopoulos, Kyriakos
- et al.
Increasing demands for high magnetic storage capacity have led to the increase of the recording area density by more than 100,000 times over the past 30 years. Among all the approaches considered to increase the area density, reducing the magnetic spacing is an effective solution that directly impacts the thickness and quality of the carbon overcoat. One of the methods of carbon overcoat deposition is chemical vapor deposition, which uses carbon-containing precursor gases as the source of carbon radicals and atoms to form the carbon overcoat. The produced carbon film is characterized by high hydrogen content (20%-50%), depending on the carbon-to-hydrogen ratio of the precursor gas and process parameters. Because of the hydrogen content, CVD-deposited hydrogenated amorphous carbon (a-C:H) deposited by CVD exhibit density of 1.7-2.2 g/cm3, which is much lower than the density (~3 g/cm3) of hydrogen-free amorphous carbon (a-C) films deposited by filtered cathodic vacuum arc (FCVA).
The superior nanomechanical/tribological properties of FCVA-deposited a-C films have been widely-reported; however, most studies have examined relatively thick (tens of nanometers) a-C films, while current demands require much thinner films of thickness in the range of 1-4 nm. FCVA-deposited a-C films overcoats are desirable protective overcoats for HDDs provided they can maintain their demonstrated high quality even for thickness as low as 1 nm. In this dissertation, an in-depth study of the structure of FCVA-deposited a-C films deposited on silicon was carried out using high-resolution transmission electron microscopy (HRTEM) and analytical electron energy loss spectroscopy (EELS). Both low- and high (core)-loss EELS spectra of Si and C were analyzed to determine the elemental content and through-thickness structure of ~20-nm-thick a-C films. Calculations of atomic carbon hybridization based on EELS spectra were used to track the film structure evolution. The average content of carbon hybridization in the top few nanometers of the a-C film, determined from EELS analysis, was found to be ~50%, much less than 73% of the bulk film. This multilayer structure was also validated by X-ray photoelectron spectroscopy (XPS). Results indicate that the minimum thickness of a-C films deposited by the FCVA method under conditions of optimum substrate bias (-100 V) should be equal to 3-3.5 nm, which is the total thickness of the buffer and surface layers.
The effects of other important FCVA process parameters on film growth were also investigated to explore the prospect of further decreasing the a-C film thickness. The incidence angle effect of energetic C+ ions bombarding onto the growing film surface was studied in terms of the deposition rate, topography, and film structure. Cross-section TEM measurements combined with Monte Carlo (T-DYN) simulations revealed that the deposition yield (rate) is independent of the ion fluence but varies with the incidence angle according to a relationship derived from sputtering theory. XPS and atomic force microscopy (AFM) studies were also performed to examine carbon hybridization and film topography. The optimum incidence angle for FCVA deposition was found equal to 45o.
A relatively new technology that shows potential for further breakthroughs in magnetic recording is heat-assisted magnetic recording (HAMR). This technology utilizes a tightly focused laser beam to heat and temporarily reduce the coercivity of magnetic nanodomains below that of the magnetic field applied by the magnetic head. Impulsive laser heating (typically <1 ns) raises the temperature in the magnetic medium above its Curie temperature, i.e., the temperature above which the coercivity of the magnetic medium decreases significantly, thus enabling rapid data encoding by the magnetic field of the head. Among all other issues raised by this new technology, the thermal stability of the carbon overcoat of HAMR disks is of great concern. To address this concern, the stability of CVD- and FCVA-deposited carbon overcoats d by exposing them laser heating of different power. FCVA-deposited a-C films demonstrated higher stability than CVD-deposited a-C:H films, suggesting that hydrogen in the CVD-deposited films diffused at relatively lower temperatures before the commencement of structural changes in the carbon film. The thermal stability of the FCVA-deposited a-C films also showed a decreasing trend with decreasing film thickness.
The thermal stability of a-C:H films synthesized by plasma-enhanced CVD (PECVD) was also studied by rapid thermal annealing (RTA) experiments, to accurately estimate the critical temperature for a-C:H film degradation. X-ray reflectivity (XRR) and XPS measurements did not reveal any discernible changes in the film thickness and the sp3 content due to RTA, indicating that oxidization and graphitization of the a-C:H films was either secondary or negligible during RTA. However, Raman spectra showed significant changes with annealing temperature increasing above 450 oC. Hydrogen depletion, the increase of the sp2 cluster size, and the enhancement of carbon network ordering are the most likely factors affecting the structural stability of the a-C:H films. The obtained results suggest that the structural stability of a-C:H films deposited on current hard disks by PECVD can be preserved, provided they are not heated to temperatures above ~400 °C during the read/write operation process of HAMR HDDs.
In addition to the above experimental studies, molecular dynamics simulations were also performed to study the growth mechanism of a-C film due to an atom-by-atom deposition process, which is physically close to the real FCVA deposition process. Using the second-generation reactive empirical bond order (REBO) potential, a-C films with different structures were simulated by varying the carbon atom incident energy in the range of 1-120 eV. Atomic hybridization and ring connectivity were used to study the film microstructure. A multi-layer film structure consisting of an intermixing layer, bulk film, and surface layer was observed for relatively high carbon atom deposition energy, in agreement with the subplantation model. The highest film density (3.3 g/cm3) and sp3 fraction (~43%) and best intermediate-range order were obtained for ~80 eV incident energy, which is in good agreement with experimental findings.