Protective Overcoat Research Articles

Recording performance enhancement via in-situ annealing of multilayer media structures

A novel technique for incorporating rapid thermal annealing into media sputter fabrication has facilitated the production of flyable media samples. Discs are fabricated with standard processing techniques to control physical grain size and crystallite texture. A CrMn caplayer ranging in thickness between 0.5 and 5 nm is subsequently deposited to provide the Mn-diffusant necessary to achieve post-treatment exchange decoupling. While still in-situ and before application of protective overcoats, the discs are exposed to temperatures between 200/spl deg/C and 350/spl deg/C compatible with most media production processes. A threefold increase in coercive force (peak reaching /spl sim/3800 Oe) and 10 dB improvement in medium signal-to-noise ratio is observed for the optimized process.

Deposition and properties of tetrahedral amorphous carbon films prepared on magnetic hard disks

The areal density of information stored on the hard disk has doubled every two years. This substantial increase in disk storage has resulted from the application of giant magnetoresistance heads, new thin film media, and better electronic recording channels. However, such an increase cannot be easily attained without reducing the separation between the magnetic read-write head and magnetic recording medium surfaces. This can be achieved by using a thinner protective overcoat. In this study, ultrathin tetrahedral amorphous carbon (ta-C) films were deposited on magnetic hard disks (CoCrTa/Cr/NiP/Al–Mg) by a magnetic filtered cathodic arc deposition under different substrate bias voltages. The obtained films exhibited smoother surfaces than the uncoated disks as indicated by the atomic force microscopic measurement. The Raman spectra acquired showed a single asymmetric Lorentzian curve shape. Tetrahedral amorphous carbon coatings were subjected to an accelerated corrosion test in vapors of concentrated hydrochloric acid for 24 h. The corrosion test demonstrated a reduced density of corrosion sites when compared to conventional diamond-like carbon films. Similarly, the scratch resistance of the ta-C coated disks, investigated by a nanoindenter, showed significant improvement in comparison to uncoated disks.

Applied surface analysis in magnetic storage technology

This paper gives a synopsis of today’s challenges and requirements for a surface analysis and materials science laboratory with a special focus on magnetic recording technology. The critical magnetic recording components, i.e. the protective carbon overcoat (COC), the disk layer structure, the read/write head including the giant-magnetoresistive (GMR) sensor, are described and options for their characterization with specific surface and structure analysis techniques are given. For COC investigations, applications of Raman spectroscopy to the structural analysis and determination of thickness, hydrogen and nitrogen content are discussed. Hardness measurements by atomic force microscopy (AFM) scratching techniques are presented. Surface adsorption phenomena on disk substrates or finished disks are characterized by contact angle analysis or so-called piezo-electric mass adsorption systems (PEMAS), also known as quartz crystal microbalance (QCM). A quickly growing field of applications is listed for various X-ray analysis techniques, such as disk magnetic layer texture analysis for X-ray diffraction, compositional characterization via X-ray fluorescence, compositional analysis with high lateral resolution via electron microprobe analysis. X-ray reflectometry (XRR) has become a standard method for the absolute measurement of individual layer thicknesses contained in multi-layer stacks and thus, is the successor of ellipsometry for this application. Due to the ongoing reduction of critical feature sizes, the analytical challenges in terms of lateral resolution, sensitivity limits and dedicated nano-preparation have been consistently growing and can only be met by state-of-the-art Auger electron spectrometers (AES), transmission electron microscopy (TEM) analysis, time-of-flight–secondary ion mass spectroscopy (ToF–SIMS) characterization, focused ion beam (FIB) sectioning and TEM lamella preparation via FIB. The depth profiling of GMR sensor full stacks was significantly improved by the ToF–SIMS Cs method.

Ultrathin TiB2 protective films

TiB2 thin films demonstrate considerable potential for use as protective overcoats in the magnetic recording industry due to their excellent mechanical and tribological properties and good chemical and thermal stability. In the many studies performed on TiB2 films, the relative effectiveness of ultrathin TiB2 films has not been systematically investigated for very thin TiB2 films. In the present investigation, film stress and microstructure in as-sputtered and annealed ultrathin TiB2 films were investigated as a function of thickness. Ultrathin TiB2 films, as thin as 5 nm, were observed to adequately protect an underlying magnetic layer from oxidation up to 400 °C.

Thick, well-adhered, highly stressed tetrahedral amorphous carbon

Diamond-like carbon thin films have attractive optical, electronic and mechanical properties, which make them potentially useful for wide-ranging applications. Although a great deal of research has been conducted on the electronic properties of DLC films, their use in semiconducting applications has been limited. However, there is a growing market for the use of DLC films in mechanical applications. Non-hydrogenated tetrahedral amorphous carbon (ta-C) films deposited by cathodic arc are presently widely used for machining non-ferrous materials. Thin (sub 5 nm) ta-C films are presently being considered as protective overcoats for magnetic disks and heads. However, the most prominent commercial use of a-C films has been in shaving blades. In this paper, the challenges and solutions for the commercial development of ta-C films for mechanical applications are described.

Strongly adhering and thick highly tetrahedral amorphous carbon (ta–C) thin films via surface modification by implantation

Highly tetrahedral amorphous carbon thin films have exceptional mechanical properties that make them ideal for many challenging wear applications such as protective overcoats for orthopaedic prostheses and aerospace components. However, the use of ta–C in many wear applications is limited due to the poor adhesion and the inability to grow thick films because of the large compressive stress. Here we report on a simple modification of the substrate growth surface by 1-keV ion bombardment using a cathodic vacuum arc (CVA) plasma prior to deposition of ta–C films at 100 eV. The 1-keV C+ ion bombardment created a thin intermixed layer consisting of substrate and carbon atoms. The generation of the intermixed carbide layer improved the adhesion and allowed the growth of thick (several μm) ta–C layers on metallic substrates.

Hardness measurements at shallow depths on ultra-thin amorphous carbon films deposited onto silicon and Al 2O 3–TiC substrates

We carried out shallow depth indentations on ultra-thin hydrogenated amorphous carbon (a-C:H) films deposited by plasma enhanced chemical vapour deposition onto silicon and Al 2O 3–TiC ceramic substrates. These measurements are relevant to data storage and microelectronic applications, which require sub-50-nm, wear protective overcoats. We used the continuous stiffness measurement technique to monitor the Young Modulus ( E) and the hardness ( H) as a function of depth for films with thickness varying from 10 to 50 nm. Raman spectroscopy and atomic force spectroscopy show that the films have similar roughness and microstructural bonding environment. For all films, the sub-30-nm region shows a continuous increase of E and H with depth, with no discernible tip calibration artifact. Above 30 nm depth, the effect of the film is predominant, essentially hardening noticeably the silicon and softening slightly the ceramic substrate, with E and H values typical of a-C:H materials. Above 50 nm, the E and H curves are dominated by the properties of the substrates. Initial nano-scratch experiments indicate that the films have better wear resistance than the Al 2O 3–TiC substrate. Therefore, the hardness and wear of the samples show no correlation at these shallow depths.

The effects of texture height and thickness of amorphous carbon nitride coating of a hard disk slider on the friction and wear of the slider against a disk

In order to minimize the stiction force caused by contact of the extremely smooth surfaces of head sliders and disks in hard disk drives, texture is usually applied on the disk surface. For future contact/near-contact recording, the stiction-induced high friction between slider and disk will become a problem. Texture on the slider/disk interface will still be an expected method to reduce friction. Recently, it was suggested to texture the slider surface. A protective coating is usually required on the textured slider surface to reduce wear of the texture. The results showed that texture on the slider surface was effective in reducing the friction between head sliders and disks. On the other hand, the texture and coating on the slider surface increase the spacing between the read/write element and the magnetic layer of the disk. The necessary and effective texture height and coating thickness are still not clear. In the present research, island-type textures with different heights (3–18 mn) were formed on slider surfaces by ion-beam etching. Amorphous carbon nitride (a-CN x ) coatings of different thicknesses (0–50 nm) were coated on the textured slider surfaces as a protective overcoat. The friction and wear properties of these sliders were evaluated by constant-speed drag tests against hard disks coated with diamond-like carbon (DLC). The results show that 2 nm texture on a slider surface is sufficient for low (0.3–0.5) and stable friction of the slider against the disk in a drag test, and coatings thicker than 5 nm show similar wear resistances of the texture on slider surfaces.

Surface and subsurface damages and magnetic recording pattern degradation induced by indentation and scratching

Magnetic recording pattern degradation due to head–disk impact and scratching are simulated by static indentation and scratch testing, respectively, on a pre-recorded thin film magnetic recording disk. Different magnitude of controlled stresses were used to induce stress and physical damage to the magnetic recording disk resulting in erasure and distortion of the magnetic recording pattern. Both nanoindentation and scratching resulted in the elastic–plastic deformation of the multilayer coating of the magnetic recording disk but in different relative magnitude and types of in-plane stresses (which are effective in causing magnetization changes). For residual indentation and scratch depths of the order of the magnetic disk coating thickness, magnetization changes in the recording pattern were observed even though the protective carbon overcoat was not damaged. Large magnetic pattern distortion and erasure results where cracks and pileups, and delamination and buckling damages were observed for deeper indentation marks and scratch grooves, respectively.

Interface tribology for 100 Gb/in 2

Critical head/disk interface requirements for 100 Gb/in 2 recording systems are examined with an emphasis on head design, head/disk protective overcoats and disk lubrication. A magnetic spacing of 10 nm that is necessary to achieve the areal density requires implementation of new technologies in each of these areas. To support the 100 Gb/in 2 areal density, novel air bearing surface (ABS) designs conceived to meet the low flying height requirement are presented. Numerical evaluation of various parameters necessary for optimum flying performance is discussed. These simulation studies provide guidance on the tolerance values and other aspects of the fabrication process that are needed to produce heads with low flying standard deviation. In an ultra-high density recording (100 Gb/in 2) system, the media property and the head–media separation distance become extremely important. To minimize the risk of head–disk interactions at low flying heights, super-smooth media with ultra-low glide heights will be needed. Reducing disk flutter at high rotational speeds will be a key issue affecting both the reliability of the interface and the electrical performance. Surface topography of such media will need to be on the order of atomic dimensions. Alternative substrate materials with higher stiffness and damping capacity are discussed as a solution to the above issues. The DLC coating on the air bearing and the transducer is another critical spacing parameter and alternate processing techniques of depositing ultra-thin carbon film are being explored. Lubricant thickness on the order of 1 nm will be applied to accommodate the decrease in the head–media spacing. Tribological measurements will be more crucial in ensuring a reliable head–disk interface for 100 Gb/in 2. These measurements are made even more difficult owing to the shrinking size of the sliders.

A comparative study of the nanoscratching behavior of amorphous carbon films grown under various deposition conditions

Two types of amorphous carbon (a-C) films: one series with a layered structure (sequence of soft/hard carbon layers) resulting in thick, stable and sp3 rich films, and another series of monolithic films rich in sp2, were deposited on Si substrates by magnetron sputtering and were investigated with respect to their nanoscratching behavior. The main purposes of this work are to investigate the scratch performance of a-C films developed in layered structure and to compare this with the films rich in sp2 content. Ultra-low load scratch tests were performed in the load range from 2 to 20mN to explore the deformation mechanisms of such thin films and their adhesion to the substrate. Below 5mN, the scratches on layered structure films exhibit an almost full elastic response while on films rich in sp2 content show massive brittle fragmentation. At 20mN, the scratches on the former films reveal some areas which are completely elastic and some areas which were typical of a buckle failure. These areas may correspond to hard and soft carbon layers, respectively. The coefficient of friction varied from ∼0.2 to ∼0.3, depending on the maximum normal load and being lower for films of the first type. Scratch testing results have shown that layered structured films are resistant to plastic deformation during contact and, together with the stability of the thick layered films, support their potential for protective overcoats.

Nitrogenated amorphous carbon films synthesized by electron cyclotron resonance plasma enhanced chemical vapor deposition

Nitrogenated amorphous carbon (a-CN x :H) films were investigated as protective overcoats for industrial applications. Thin a-CN x :H films have been deposited on silicon by electron cyclotron resonance plasma-enhanced chemical vapor deposition. The substrate bias was found to play an important role in determining the chemical compositions and mechanical properties of the films. The surface roughness and hardness of the films can reach 1.4 Å and 20 GPa, respectively. The influence of mechanical properties by hydrogen was studied. A correlation exists between the background slope of Raman spectra and the hydrogen content as determined by elastic recoil detection analysis.

Impact of a modern firefighting protective uniform on the incidence and severity of burn injuries in New York City firefighters.

The New York City Fire Department (FDNY) is the largest fire department in the United States, with over 11,000 firefighters. In 1994, FDNY changed to a modern firefighting protective uniform. The major difference between traditional and modern uniforms is that modern uniforms include both protective over-coat and over-pant, whereas traditional uniforms include only the over-coat. Furthermore, modern uniforms are manufactured using improved thermal protective textiles that meet or exceed current National Fire Protection Association standards for structural firefighting. The purpose of this study was to determine the impact of the modern uniform on the incidence and severity of FDNY burn injuries. We also evaluated the incidence and severity of other non-burn injuries to determine whether there was serious adverse impact. The number of lower-extremity burns decreased by 85% when 2 years' experience while wearing the modern uniform was compared with 2 years while wearing the traditional uniform. Upper-extremity burns and head burns decreased by 65% and 40%, respectively. Severity indicators (days lost to medical leave, hospital admissions, and skin grafts) for lower- and upper-extremity burn injuries were all substantially reduced. This occurred without significant change in the incidence or severity of trunk burns, heat exhaustion, inhalation injuries (actually decreased), or cardiac events. The reduction in the incidence and severity of burn injuries, the major occupational injury affecting this workforce, has been so dramatic and without untoward effects that the introduction of the modern uniform must be characterized as a sentinel event in the history of firefighter health and safety.

Nitrogenated carbon layer on magnetic recording disks

Abstract Hydrogenated carbon films have been used in the magnetic recording industry as a protective overcoat for memory disks for many years. In recent years, with shortening of the gap between the head and the disk (as in proximity heads), the inclusion of nitrogen in the films to form hydro/nitrogenated films has also aroused tremendous interest. In this study, nitrogenated amorphous carbon films (without hydrogen) (a-C:N) of thickness ranging from 5 to 20 nm were deposited on rigid magnetic disks via the magnetron sputtering of a graphite target. The deposition was conducted in an argon/nitrogen atmosphere under RF (radio frequency) substrate bias power. A scanning probe microscope (SPM) with a contact mode of atomic force microscopy (AFM) mode was used in the roughness determination. A Raman spectroscope was employed to study the structural evolution of the coatings with varying nitrogen/argon proportion and flow rate, radio frequency substrate bias and target power density. The surface contact angle between the a-C:N coating and deionized water was measured using a Rame–Hart goinometer. Under the deposition conditions, the carbon thickness decreased with gas flow rate, but increases with nitrogen content. An increase in contact angle with bias power and decrease with flow rate were also observed. The roughness (Ra) of the coatings did not change because the coating was too thin to change the original substrate roughness. Also studied were the Raman peak positions, and the D-band and G-band intensity ratio and their relationship with the processing parameters.

Tribology of proximity magnetoresistive head–disk interface for 5 Gb/in.2 recording

Low fly-height magnetoresistive (MR) sliders and low glide-height laser-texture thin film disks were introduced to meet the tribological challenges of proximity MR recording, with which an areal density of 5 Gb/in.2 has been achieved when using the sliders with dual-stripe MR heads and the disks with low-noise media. The 30% pico sliders employed two air-bearing designs with a fly height around 12.5 nm. The thin film disks used superfinish substrates with a glide-avalanche-height falling well below 10.0 nm. In the contact start/stop (CSS) zones, small crater-shape laser texture bumps were generated to meet both low stiction and low glide-height requirements. An 80 Å thin amorphous nitrogenated carbon was added over the magnetic layers as a protective overcoat. A layer of perfluoropolyether with an additive of phosphazene compounds was applied on the disk surface to improve the reliability of head–disk interfaces. The near contact head–disk interface survived for 20 k-cycle hot/wet and hot/dry CSS tests without wear and high stiction. Twenty-four-hour hot/wet park-stiction values after 20 k CSS cycles were within the acceptable range. Remarkably, thermal-asperity tests showed no hits by using the proximity heads on the ultralow glide-height thin film disks.

Magnetic hard disk overcoats in the 3–5 nm thickness range

Protective properties of 3–5 nm thick carbon overcoat layers deposited on magnetic hard disks by ion beam deposition (IBD) and plasma-enhanced chemical vapor deposition (PECVD) were investigated. It is found that these overcoats are superior to the sputtered carbon films at thicknesses below 5 nm. Low-stiction performance of 3-nm-thick IBD films without any detectable wear was observed during 50 000 contact start-stop cycles at 55 °C and 10% relative humidity. Surface concentration of cobalt ions is reduced by as much as an order of magnitude as compared to the media with sputtered overcoats. Polarization resistivity values for IBD and PECVD overcoats are by an order of magnitude higher. The results show that thickness of protective carbon overcoats on magnetic hard disks can be reduced to 3–5 nm without compromising media reliability requirements. This reduction is critical for the continuing growth of storage density.

Structural and tribological characterization of protective amorphous diamond-like carbon and amorphous CNx overcoats for next generation hard disks

Further insight into processing-structure-property relationships have been carried out for existing and candidate carbon-based protective overcoats used in the magnetic recording industry. Specifically, 5 nm thick amorphous diamond-like carbon (a:C) and nitrogenated diamond-like carbon (a:CNx) overcoats were deposited by low deposition rate sputtering onto a thin film disk consisting of either CoCrPt/CrV/NiP/AlMg or CoCrPt/CrV/glass. The wear durability and frictional behavior of these hard disks were ascertained using a recently developed depth sensing reciprocating nanoscratch test. It was determined that the CN0.14/CoCrPt/CrV/glass disk exhibited the most wear resistance, least amount of plastic deformation, and lowest kinetic friction coefficient after the last wear event. Core level x-ray photoelectron spectroscopy (XPS) results of sputter cleaned overcoats indicated that nitrogen up to 14 at. % incorporated into the amorphous network resulted in these improvements near the overcoat/magnetic layer interface, since there was an increase in the number of N-sp3 C bonded sites in a predominantly N-sp2 C bonded matrix. However, nonsputter cleaned overcoats exhibited a more graphitic pyridine-like (nondoping configuration) structure near the surface as evidenced by the increase in C=N versus C–N bonds and the valence band XPS determined appearance of the 2p-π band near the Fermi level (EF). Therefore, XPS sputter cleaning revealed a gradient in the chemical nature of the overcoats through the thickness. In addition, micro-Raman spectroscopy established that a further increase of nitrogen (⩾18 at. %) weakened the overcoat structure due to the formation of terminated sites in the amorphous carbon network, since nitrogen failed to connect the sp2 domains within the network. This, in conjunction with an increase in the intensity of the 2p-π band from the valence band XPS spectra and the increase in the G-band position and ID/IG ratio from the Raman spectra, confirmed the increase in the size and number of sp2 bonds in the CN0.18 overcoat.

Bonding of Ethers and Alcohols to a-CNx Films

The surface chemistry of fluorinated ethers and fluorinated alcohols adsorbed on amorphous nitrogenated carbon (a-CNx) have been studied as models for the interaction of perfluoropolyalkyl ether (PFPE) lubricants with the surfaces of magnetic data storage media. Temperature-programmed desorption experiments conducted in ultrahigh vacuum using small fluorinated ethers and fluorinated alcohols have measured their desorption energies and have provided insight into the nature of bonding between PFPEs and a-CNx films. Preliminary results indicate that ether linkages interact with a-CNx films through electron donation from the oxygen lone pair electrons. In contrast, alcohol end groups show evidence of hydrogen bonding with a-CNx films. The models derived from this study can aid in the future development of both lubricants and protective overcoats to ensure that the magnetic hard disks display optimal wear resistance and performance.

Spreading Profiles of Molecularly Thin Perfluoropolyether Films

Perfluoropolyethers (PFPEs) are widely used as lubricants on magnetic recording media. The mobility of the PFPE on the protective carbon overcoat of the media is widely accepted to be intimately coupled to the resulting tribological performance. The flow properties of molecularly thin films of nonpolar PFPEZ and polar PFPE Zdol fractions on solid surfaces were investigated by measuring the spreading profiles. The spreading of Zdol exhibits terraced profiles with the formation of a molecular foot, a shoulder and a vertical step. To describe these features of Zdol spreading, we measured the Zdol thickness dependence of the surface energy, which is then used to calculate the thickness dependence of the disjoining pressure. The polar component of the Zdol surface energy exhibits oscillations as a function of PFPE thickness. The resulting oscillations in the disjoining pressure can be used to qualitatively describe the origins of terraced spreading. The characteristic Zdol spreading profile and surface energy oscillations of Zdol are attributed to molecular layering induced by polar interactions between the Zdol end-groups and the surface. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Toronto, Ontario, Canada, October 26–28, 1998

The nano-scratch tester (NST) as a new tool for assessing the strength of ultrathin hard coatings and the mar resistance of polymer films

At present, functional coatings are used in increasingly demanding applications that require good adhesion and specific resistance to damage. This has prompted a rising interest in improving the mechanical properties of polymer coatings, especially their scratch and mar resistance, as well as ultra-thin hard coatings as used for protective overcoats in the magnetic storage industry. The nano-scratch tester (NST) is a new instrument overcoming the limitations of both the classical stylus scratch test (normal force range) and the atomic force microscope technique (short sliding distances), allowing scratch lengths of up to 10 mm. Tangential force and penetration depth are simultaneously measured during the scratching process, both in a multipass contact fatigue mode. For high resolution inspection of the deformed or damaged area, a scanning force microscope (SFM) is integrated into the system. Experimental results are presented for two very different types of material; polymeric clear-coat samples which give a range of mar resistance and DLC ultra-thin films used as protective overcoats for hard disks. The results indicate very good reproducibility and confirm the application of this new instrument for the accurate characterization of elasticity, hardness, adhesion and mechanical integrity in coated systems where the film thickness is less than 1 μm.