Synthesis And Characterization Of Amorphous Carbon Films For Magnetic Storage Technology

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 (

Heat-assisted magnetic recording (HAMR) is a promising technology for next-generation magnetic storage devices that has the potential to increase magnetic recording density by orders of magnitude (up to 10 Tb/in2). By focusing a laser beam to rapidly heat the magnetic media above the Curie temperature, the coercivity of the magnetic domains over a small area can be sufficiently reduced to allow changes in polarization at a finer scale, thus enabling the reading and writing of data at much greater densities. Several factors, however, have prevented this technology from reaching the market. One major issue is the thermal stability of the amorphous carbon (a-C) overcoat on the magnetic head and its ability to protect critical components, such as read and write poles, near-field transducer, and waveguide, when heated to high temperatures during drive operation. This dissertation focuses on the optimization of a-C overcoats (also referred to as tetrahedral amorphous carbon (ta-C) due to the relatively high content of tetrahedral (sp3) carbon atom hybridization) deposited at very short deposition times (6 sec) and investigates the effects of heating on the nanostructure and intermixing with underlayers. As the overcoat thickness approaches only a few atomic layers, its performance and continuity become of concern, especially when exposed to higher temperatures. Since the tribomechanical and corrosion properties of carbon films have been correlated to the type of carbon atom hybridization, the choice of deposition technique and parameters to control the relative bonding content is crucial. Among the various deposition techniques, filtered cathodic vacuum arc (FCVA) was chosen to develop a-C protective overcoats. The energetic C+ ions film precursors in FCVA are especially beneficial for depositing continuous ultrathin a-C films with low surface roughness. Deposition parameters explored include the ion incidence angle and pulse substrate bias voltage under optimized duty cycle and ion fluence FCVA conditions, with the intent of minimizing a-C film thickness while maintaining adequate mechanical performance. Cross-sectional high-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS) were used to reveal nanostructure variations along the through thickness of a-C films and carbon intermixing with the substrate. The optimized coatings were deposited on an assortment adhesion (NiCr), buffer (SiN, TaOx), and base layers (Au, FeCo) common to HAMR magnetic media and heated for 30 min to simulate accumulation of heating damage. For a-C films 2-3 nm thick, the highest sp3 content was found in the bulk layer and were synthesized under FCVA deposition conditions of 65% duty cycle and -25 to -75 V substrate bias. The HRTEM and EELS analysis revealed no changes in thickness and minor structural changes in the a-C overcoat and generally small amounts of intermixing between the overcoat and the underlayers when operated in an inert hot environment. The findings of this dissertation suggest that proper optimization of such layered coatings can provide a viable solution to thermal damage of HAMR heads.

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

Amorphous, hydrogenated carbon (AHC) films can be deposited on various substrates using several techniques, e.g. plasma deposition and ion beam deposition. The resulting films can be hard, wear resistant and transparent.

There has been tremendous development in the science of carbon in past years. First came the development of the chemical vapor deposition of diamond, followed by the discovery of a new class of molecules - the fullerenes. Carbon nanotubes were discovered and techniques were developed to deposit new phases of amorphous carbon containing mainly sp3 bonding. This book brings together scientists and engineers from all areas of carbon research, both sp2 and sp3 bonded, from the fully amorphous to nanostructured carbon, to the highly ordered nanotubes. It covers a range of subjects including the synthesis and properties of nanotubes, as well as diamond-like carbon deposition and properties. Applications range from nanotubes for hydrogen storage, to electrochemical double-layer capacitors (supercapacitors), field emission displays, hard coatings, and carbon coatings for magnetic storage technology. The book deals with the growth, characterization, properties and applications of nanotubes and field emission from all varieties of carbon, amorphous and diamond-like carbon- growth, properties and applications. It also contains papers on diamond, silicon carbide, carbon nitride and beryllium films.

MAX phases are a group of nanolaminated ternary carbides and nitrides, with a composition expressed by the general formula Mn+1AXn (?? = 1 ? 3), where M is a transition metal, A is an A-group element, and X is carbon and/or nitrogen. MAX phases have attracted interest due to their unique combination of metallic and ceramic properties, related to their inherently laminated structure of a transition metal carbide (Mn+1Xn) layer interleaved by an A-group metal layer. This Thesis explores synthesis and characterization of magnetic MAX phases, where the A-group element is gallium (Ga). Due to the low melting point of Ga (T = 30 °C), conventional thin film synthesis methods become challenging, as the material is in liquid form at typical process temperatures. Development of existing methods has therefore been investigated, for reliable/reproducible synthesis routes, including sputtering from a liquid target, and resulting high quality material. Routes for minimizing trial-and-error procedures during optimization of thin film synthesis have also been studied, allowing faster identification of optimal deposition conditions and a simplified transfer of essential deposition parameters between different deposition systems. A large part of this Thesis is devoted towards synthesis of MAX phase thin films in the Cr-Mn-Ga-C system. First, through process development, thin films of Cr2GaC were deposited by magnetron sputtering. The films were epitaxial, however with small amount of impurity phase Cr3Ga, as confirmed by X-ray diffraction (XRD) measurements. The film structure was confirmed by scanning transmission electron microscopy (STEM) and the composition by energy dispersive X-ray spectroscopy (EDX) inside the TEM. Inspired by predictive ab initio calculations, the new MAX phase Mn2GaC was successfully synthesized in thin film form by magnetron sputtering. Structural parameters and magnetic properties were analysed. The material was found to have two magnetic transitions in the temperature range 3 K to 750 K, with a first order transition at around 214 K, going from non-collinear antiferromagnetic state at lower temperature to an antiferromagnetic state at higher temperature. The Neél temperature was determined to be 507 K, changing from an antiferromagnetic to a paramagnetic state. Above 800 K, Mn2GaC decomposes. Furthermore, magnetostrictive, magnetoresistive and magnetocaloric properties of the material were iv determined, among which a drastic change in lattice parameters upon the first magnetic transition was observed. This may be of interest for magnetocaloric applications. Synthesis of both Cr2GaC and Mn2GaC in thin film form opens the possibility to tune the magnetic properties through a solid solution on the transition metal site, by alloying the aforementioned Cr2GaC with Mn, realizing (Cr1-xMnx)2GaC. From a compound target with a Cr:Mn ratio of 1:1, thin films of (Cr0.5Mn0.5)2GaC were synthesized, confirmed by TEM-EDX. Optimized structure was obtained by deposition on MgO substrates at a deposition temperature of 600 ºC. The thin films were phase pure and of high structural quality, allowing magnetic measurements. Using vibrating sample magnetometry (VSM), it was found that (Cr0.5Mn0.5)2GaC has a ferromagnetic component in the temperature range from 30 K to 300 K, with the measured magnetic moment at high field decreasing by increasing temperature. The remanent moment and coercive field is small, 0.036 ?B, and 12 mT at 30 K, respectively. Using ferromagnetic resonance spectroscopy, it was also found that the material has pure spin magnetism, as indicated by the determined spectroscopic splitting factor g = 2.00 and a negligible magnetocrystalline anisotropy energy. Fuelled by the recent discoveries of in-plane chemically ordered quaternary MAX phases, so called i-MAX phases, and guided by ab initio calculations, new members within this family, based on Cr and Mn, were synthesized by pressureless sintering methods, realizing (Cr2/3Sc1/3)2GaC and (Mn2/3Sc1/3)2GaC. Their structural properties were determined. Through these phases, the Mn content is the highest obtained in a bulk MAX phase to date. This work has further developed synthesis processes for sputtering from liquid material, for an optimized route to achieve thin films of controlled composition and a high structural quality. Furthermore, through this work, Mn has been added as a new element in the family of MAX phase elements. It has also been shown, that alloying with different content of Mn gives rise to varying magnetic properties in MAX phases. As a result of this Thesis, it is expected that the MAX phase family can be further expanded, with more members of new compositions and new properties.

This book presents the status quo of the structure, preparation, properties and applications of tetrahedrally bonded amorphous carbon (ta-C) films and compares them with related film systems. Tetrahedrally bonded amorphous carbon films (ta-C) combine some of the outstanding properties of diamond with the versatility of amorphous materials. The book compares experimental results with the predictions of theoretical analyses, condensing them to practicable rules. It is strictly application oriented, emphasizing the exceptional potential of ta-C for tribological coatings of tools and components.

World experts in amorphous carbon have been drawn together to produce this comprehensive commentary on the current state and future prospects of amorphous carbon, a highly functional material. Amorphous carbon has a wide range of properties that are primarily controlled by the different bond hybridisations possible in such materials. This allows for the growth of an extensive range of thin films that can be tailored for specific applications. Films can range from those with high transparency and which are hard and diamond-like, through to those which are opaque, soft and graphitic-like. Application areas including field emission cathodes, MEMs, electronic devices, medical and optical coatings are now close to market.