Mid- and High-Z Doping of High Purity, Density Tunable Amorphous Carbon Films on Complex Geometries
Steven Falabella | 20-ERD-042
Homogenous, amorphous carbon coatings with tunable density and composition have the potential to improve the performance of inertial confinement fusion (ICF) ablators. This project investigated the use of an LLNL-designed, radio-frequency (RF), hollow-cathode (HC) plasma source with a confining magnetic field to deposit thick (greater than 10 microns), amorphous, diamond-like carbon films from gaseous precursors. The properties of the resultant films were measured by a combination of profilometry, Rutherford Backscatter Spectroscopy (RBS) , Elastic Recoil Detection Analysis (ERDA), x-ray diffraction, Raman Spectroscopy, and nano-indention. We investigated the dependence of the deposition rate, film density, elemental composition, HC self-bias and residual stress as functions of RF power, magnetic field, gas composition, operational pressure, and substrate bias. In addition, we added aluminum to the growing film as a test dopant by magnetron sputtering concurrent with the hollow-cathode DLC deposition. Finally, we designed and constructed a water-cooled, rocking substrate stage to coat small spheres, to investigate the feasibility of coating ICF ablators using the Hollow-Cathode system.
By varying the gas composition and deposition conditions, we were able to produce amorphous, hydrogenated carbon coatings with densities between 1.1 and 1.6 g/cc, and incorporate up to 6% aluminum as a test dopant. We used the rocking substrate stage to demonstrate coating test spheres, although the process was not optimized. In general, the HC-plasma source demonstrated that it could produce thick, amorphous, hydrogenated carbon coatings, with low stress, but more optimizations will be required before its suitability for ablator production can be determined.
Carbon is currently the material of choice for ICF ablator capsules. Both "glow discharge polymer" (hydrocarbon) and "high density carbon" (polycrystalline diamond) have limitations as ablator materials. These include low density, and inhomogeneities from crystallite grain boundaries, respectively. Diamond has the additional challenge of adding dopants to the dense crystalline lattice. To overcome these limitations, we sought to demonstrate the rapid deposition of >50 micron-thick amorphous diamond-like carbon (DLC) coatings on complex geometries, including spherical (for ICF capsules). By varying the deposition parameters of the hollow-cathode plasma discharge, amorphous DLC can be produced with significant sp3 bonding, and with densities between ~1.2 and ~3.1 g/cc. This is a range of interest, spanning the range of densities from hydrocarbon plastics to polycrystalline diamond. While we were only able to produce carbon films with density up to approximately 1.6 g/cc during this project, we demonstrated that amorphous films produced using a hollow-cathode source could be doped and have the low intrinsic stress required for thick coatings. In addition to advancing fusion energy experiments, this project developed science and technology tools and capabilities to help meet future program needs.
Publications, Presentations, and Patents
Kucheyev, S. O. et. al. "Recent Developments in Plasma-Assisted Deposition for LaserTargets at LLNL." 8th Target Fabrication Workshop (TFW-2022), Oxford, UK. September 26-28, 2022.
Miller, J. et al. "Hydrogenated Amorphous Carbon from Magnetized Hollow Cathode Discharges." 47th International Conference of Metallurgical Coatings and Thin Films, Virtual. 26 April 2021.
Miller, J. et al. "Diamond-Like Carbon Coatings by Hollow Cathode Chemical Vapor Deposition." 24th Target Fabrication Specialist Meeting. Virtual. June 6-9 2022.
Miller, J. et al., 2020. "Hollow-Cathode Chemical Vapor Deposition of Thick, Low-Stress Diamond-Like Carbon Films." Thin Solid Films Vol. 714 (2020). https://doi.org/10.1016/j.tsf.2020.138394.