Synthesis and Characterization of Amorphous Carbon Films for Magnetic Storage Technology
- Author(s): Xie, Jun
- Advisor(s): Komvopoulos, Kyriakos
- et al.
Increasing demands for high magnetic storage capacity have led to the increase of the recording area density, mainly by reducing the distance between the magnetic media on the hard disk and the magnetic transducer of the head. A factor that has greatly contributed to the profound decrease of the magnetic spacing is excessive thinning of the protective amorphous carbon (a-C) overcoat. However, the remarkable decrease in overcoat thickness raises a concern about its quality and protective capability. In general, a-C films with higher sp3 carbon atom hybridization demonstrate higher density and better tribomechanical and corrosion properties. The sp2 and sp3 contents strongly depend on the film-growth conditions and deposition method.
One of the most common film deposition methods is radio-frequency (RF) sputtering. This method uses low-energy neutral carbon atoms or clusters of atoms as film precursors and has been the workhorse of storage technology for more than four decades. Typically, Ar+ ion bombardment of the growing film during film growth is used to tailor the overcoat structure and properties without affecting its chemical environment. The substrate bias voltage is a key deposition parameter because it directly affects the ion bombardment energy. In this dissertation, the effect of the substrate bias voltage on the growth and properties of ultrathin a-C films was examined and the identified film structure-property interdependencies were explained in the context of an analytical model, which takes into account the effects of irradiation damage and thermal spikes.
Substrate biasing during film deposition may lead to some undesirable effects, such as the development of a high compressive residual stress, which can cause premature overcoat failure by delamination. Experimental studies of this dissertation show that alternating between biasing and non-biasing deposition conditions, multi-layer a-C films consisting of ultrathin hard (bias on) and soft (bias off) layers characterized by high sp3 fraction and greatly reduced compressive residual stress can be synthesized by RF sputtering. An additional advantage is that these multi-layer a-C films exhibit lower surface roughness and improved tribological properties.
Different from deposition methods using neutral carbon atoms as film-forming precursors, such as RF sputtering and other physical vapor deposition methods, filtered cathodic vacuum arc (FCVA) uses energetic C+ ions as film precursors, which is advantageous for depositing ultrathin and very smooth a-C films with superior nanomechanical/tribological properties. The role of important FCVA process parameters, such as substrate bias voltage, which controls the C+ ion energy, in the film growth process were investigated, while considering various means of reducing the a-C film thickness without jeopardizing its structure and properties. The effect of the duty cycle of substrate pulse biasing (i.e., the ratio of the time of substrate biasing over a pulse to the pulse bias period) was examined in terms of film deposition rate, surface topography, and nanostructure. Cross-sectional high-resolution transmission electron microscopy (HRTEM) combined with the scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) studies revealed variations in through-thickness hybridization and density with duty cycle. a-C films with the highest sp3 content and smallest thickness were synthesized under FCVA deposition conditions of 75% and 65% duty cycle, respectively.
EELS studies show that a-C films generally possess a multi-layered structure consisting of surface and interface layers of relatively low sp3 contents and intermediate bulk layer of much higher sp3 content, a result of the deposition mechanisms encountered during ion bombardment. When the a-C film thickness is reduced to only 2–3 nm, the effects of the ultrathin (1–2 nm) surface and interface layers become increasingly more pronounced, resulting in the decrease of the overall sp3 content and, in turn, depletion of the film’s protective capability. To reduce the thickness of the interface layer, a thin (<1 nm) seed carbon layer was deposited under small incidence angle and zero substrate bias to minimize the effect of C+ ion subplantation, followed by deposition of an a-C film under optimal film-growth conditions resulting in high sp3 content. The absence of substrate biasing during the initial deposition allowed the formation of a barrier between the substrate and the bombarding energetic C+ ions. To reduce the thickness of the surface layer, post-deposition Ar+ ion sputter etching of the synthesized a-C film was performed, which effectively sputtered off carbon material, reducing the thickness of the surface layer without degrading the sp3 content of the bulk layer of the a-C film.
Heat-assisted magnetic recording (HAMR) is a relatively new storage technology showing great potential for further increasing the magnetic recording density. This technology utilizes a tightly focused laser beam to rapidly heat the media above the Curie temperature, temporarily reducing the coercivity of magnetic nanodomains such that the magnetic field of the read/write head can induce polarization changes. Among all other issues brought up by the HAMR technology, the thermal stability of the carbon overcoat is of great concern. In this dissertation, ultrathin hydrogenated amorphous carbon (a-C:H) and a-C films synthesized by plasma-enhanced chemical vapor deposition (PECVD) and FCVA, respectively, which are the prime deposition techniques used in current hard-disk drives, were subjected to rapid thermal annealing (RTA) of maximum temperature equal to 650 °C for 1 s. EELS results show structural changes in the PECVD a-C:H films involving carbon diffusion in the substrate and a decrease in sp3 content; however, the FCVA a-C films show remarkable structural stability under the same RTA conditions, indicating that FCVA is an ideal film deposition process for HAMR technology.