Carbon Overcoat Research Articles

Broadband infrared absorber based on a sputter deposited hydrogenated carbon multilayer enhancing MEMS-based CMOS thermopile performance.

Based on pulsed DC sputter deposition of hydrogenated carbon, an absorber optical coating with maximized broadband infrared absorptance is reported. Enhanced broadband (2.5-20µm) infrared absorptance (>90%) with reduced infrared reflection is achieved by combining a low-absorptance antireflective (hydrogenated carbon) overcoat with a broadband-absorptance carbon underlayer (nonhydrogenated). The infrared optical absorptance of sputter deposited carbon with incorporated hydrogen is reduced. As such, hydrogen flow optimization to minimize reflection loss, maximize broadband absorptance, and achieve stress balance is described. Application to complementary metal-oxide-semiconductor (CMOS) produced microelectromechanical systems (MEMS) thermopile device wafers is described. A 220% increase in thermopile output voltage is demonstrated, in agreement with modeled prediction.

Structural characterization of ultrathin diamond-like carbon overcoats for high areal density magnetic recording

This paper reports a comprehensive study on chemical structures of ultrathin diamond-like carbon (DLC) overcoats for high areal density magnetic recording. The DLC overcoats were fabricated in a range of thicknesses between 0.5 and 5.0 nm with Si interlayer on NiFe substrates using a pulsed filtered cathodic vacuum arc (PFCVA) technique. The overcoat thicknesses were accurately determined by high–resolution transmission electron microscopy and wavelength dispersive X-ray fluorescence techniques. The surface morphology, chemical composition, microstructure, and tribological properties were systematically investigated by atomic force microscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure, Raman spectroscopy techniques, and a reciprocating wear test. The results showed that the surface morphologies of DLC overcoats were atomically smooth with a root mean square surface roughness below 0.23 nm. The DLC overcoats had high sp3 bonds fraction. However, as the overcoat thickness was thinner than 1 nm, the sp2 bonds fraction suddenly increased resulting in a significant increase in the coefficient of friction. The obtained results demonstrated the limitation of the thickness of the DLC overcoat fabricated by the PFCVA technique as a protective coating for high–density magnetic storage devices.

Graphene overcoats for ultra-high storage density magnetic media

Hard disk drives (HDDs) are used as secondary storage in digital electronic devices owing to low cost and large data storage capacity. Due to the exponentially increasing amount of data, there is a need to increase areal storage densities beyond ~1 Tb/in2. This requires the thickness of carbon overcoats (COCs) to be <2 nm. However, friction, wear, corrosion, and thermal stability are critical concerns below 2 nm, limiting current technology, and restricting COC integration with heat assisted magnetic recording technology (HAMR). Here we show that graphene-based overcoats can overcome all these limitations, and achieve two-fold reduction in friction and provide better corrosion and wear resistance than state-of-the-art COCs, while withstanding HAMR conditions. Thus, we expect that graphene overcoats may enable the development of 4–10 Tb/in2 areal density HDDs when employing suitable recording technologies, such as HAMR and HAMR+bit patterned media

High temperature nanomechanical and nanotribological behavior of sub-5 nm nitrogen-doped carbon overcoat films

Elevated temperature tribological properties of ultra-thin nitrogen-doped carbon overcoat (NCOC) protective film for magnetic disks were investigated. NCOC with three different thicknesses (2.5, 3.5, and 4.5 nm) were examined in terms of both chemical and mechanical properties at temperatures up to 300 °C. The chemical changes were traced by XPS, revealing that the configuration of both carbon and nitrogen elements change partially from sp3- to sp2-hybridizations with exposure of NCOC samples to annealing. The friction coefficient, wear rate, and deformation of the NCOC films were measured using nanoscratch/wear experiments. Three sets of mechanical experiments verified the operating temperature and thickness dependence of the tribological behavior of the NCOC films. The NCOC samples show that the average wear depth is acceptably small after exposure to annealing, thus offering durable films for high temperature applications. Noteworthy, the permanent reduction in mechanical properties associated with annealing reduces in the presence of N dopants.

A Dual Protection System for Heterostructured 3D CNT/CoSe2/C as High Areal Capacity Anode for Sodium Storage.

3D electrode design is normally opted for multiple advantages, however, instability/detachment of active material causes the pulverization and degradation of the structure, and ultimately poor cyclic stability. Here, a dually protected, highly compressible, and freestanding anode is presented for sodium‐ion batteries, where 3D carbon nanotube (CNT) sponge is decorated with homogeneously dispersed CoSe2 nanoparticles (NPs) which are protected under carbon overcoat (CNT/CoSe2/C). The 3D CNT sponge delivers enough space for high mass loading while providing high mechanical strength and faster conduction pathway among the NPs. The outer amorphous carbon overcoat controls the formation of solid electrolyte interphase film by avoiding direct contact of CoSe2 with electrolyte, accommodates large volume changes, and ultimately enhances the overall conductivity of cell and assists in transmitting electron to an external circuit. Moreover, the hybrid can be densified up to 11‐fold without affecting its microstructure that results in ultrahigh areal mass loading of 17.4 mg cm−2 and an areal capacity of 7.03 mAh cm−2 along with a high gravimetric capacity of 531 mAh g−1 at 100 mA g−1. Thus, compact and smart devices can be realized by this new electrode design for heavy‐duty commercial applications.

Study on Failure Process of Silicon-Doped Amorphous Carbon by ReaxFF Molecular Dynamics Simulation for HAMR

The failure process of silicon-doped amorphous carbon (a-C:Si) films for heat-assisted magnetic recording (HAMR) disk is studied by reactive force field molecular dynamics (ReaxFF MD) simulations. Insights on the failure process of a-C:Si film are provided by analyzing system potential energy, amount of consumed oxygen molecules, film structure, number of carbon-oxygen bonds and silicon-oxygen bonds, atomic shear strain, radial distribution function, and density. The a-C:Si film graphitizes initially, and then it begins to combust accompanying by delamination of surface oxide layers with the increase in temperature. The mechanism of atomic oxygen diffuses in the film is proposed. The a-C:Si film shows a good thermal stability below 1500 K. It can act as carbon overcoat of HAMR disk.

Nanostructure, structural stability, and diffusion characteristics of layered coatings for heat-assisted magnetic recording head media

The intense laser heating in heat-assisted magnetic recording (HAMR) has been a major hindrance to HAMR technology from becoming commercially viable. Thermal damage of the near-field transducer (NFT) and write pole (WP) embedded in the trailing edge of the magnetic head due to failure of the protective carbon overcoat after prolonged heating at an elevated temperature are major obstacles. Therefore, the main objective of this study was to develop an effective coating method for HAMR heads. This was accomplished by introducing a new class of layered coatings consisting of ultrathin amorphous carbon (a-C) overcoat, adhesion (SiN) layer, and buffer (NiCr or TaOx) layer sequentially deposited onto Au and FeCo base layers to mimic the layer stacking of NFT and WP elements, respectively. The structural stability of the a-C overcoats and diffusion characteristics of each comprising layer under conditions of heating at 350 °C for 30 min in an Ar atmosphere were investigated by high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). For most stacking configurations the HRTEM/STEM and EELS results generally revealed some layer intermixing and minute carbon atom rehybridization in the heated a-C overcoats. The findings of this investigation suggest that further optimization of the developed layered coatings can provide a viable solution to thermal damage of HAMR heads.

Tip-Enhanced Raman Spectroscopy Studies on Amorphous Carbon Films and Carbon Overcoats in Commercial Hard Disk Drives

Far-field Raman spectroscopy and tip-enhanced Raman spectroscopy were used to investigate 20-nm-thick amorphous carbon films and 3-nm-thick carbon overcoats of commercial hard disk drives. Enhancement of the Raman signal on both samples was observed indicating the activation of surface plasmons. The largest enhancement was found for the 3-nm-thick carbon overcoat of a commercial hard disk suggesting that the chemistry of nanometer-thick carbon films can be studied using tip-enhanced Raman spectroscopy with high sensitivity and resolution.

Temperature-Induced Changes in the Optical and Material Characteristics of HAMR Media COC and Its Effect on Recording Performance

The carbon overcoat (COC) on heat-assisted magnetic recording (HAMR) media was deposited at increasing temperature. The optical extinction ( ${k}$ ) of this media was measured with ellipsometry and found to increase with temperature from 0.2 to 0.9 over the temperature range from 220 °C to 360 °C. Raman spectroscopy of these COC surfaces had an increasing G-peak position and decreasing G-peak width along with an increasing I(D)/I(G). An increasing sp2 ring/chain and aromatic cluster size and decreasing optical gap were inferred from these changing parameters. HAMR writing on this media was found to require incrementally less laser power (from 100% to 92%) with increasing optical extinction to attain a fixed track width. This was determined to be due to the increased imaginary component of the dielectric function in this film ( 2nk ). The SNR of these written tracks was observed to improve by approximately 0.65 dB with deposition temperature. A finite-element method model of the head and media showed that the thermal gradient of the hot spot in the media increases with optical extinction and increases the SNR. A small decrease in the COC density with temperature was observed and was calculated to further decrease the COC thermal conductivity and thus enhance the increased thermal gradient.

Nanoscale thermal analysis for heat-assisted magnetic recording

The heat assisted magnetic recording (HAMR) technology is expected to increase the areal density of the hard disk drives up to 10 Tb/in.2 This paper presents a numerical thermal study of near field transducers (NFTs), a key component in HAMR. Both an electromagnetic and a heat transfer model are included. Instead of assuming a heat transfer model across the nanoscale gap, we performed a parameter sweep of various models and investigated all the possible results. We found that if the heat transfer coefficient across the head-disk interface (HDI) is high, the carbon overcoat on the head is critical for protecting the NFT from excess heat. The heat flux across the HDI and the thermal efficiency of the NFT are also investigated to better understand the heat transfer mechanism in the HAMR system.

A finite element correction method for sub-20 nm nanoindentation considering tip bluntness

Due to substrate effects and inherent tip bluntness, using current state-of-the-art nanoindentation instrumentation, it is difficult to directly measure mechanical properties of thin films less than 20 nm thick. With the aid of Finite Element Analysis (FEA), we obtain the mechanical properties by correlating simulated force-displacement responses with those from nanoindentation experiments. For shallow indentation experiments that is required for very thin films, it is found that an indenter model with a blunt tip is more realistic and leads to accurate characterization of mechanical properties. The proposed blunt tip model is validated with experiments on a standard fused quartz sample, and then applied to measure the nanomechanical properties of a 90 nm thick metal alloy layer and a 14 nm thick magnetic layer sandwiched between a carbon overcoat and a metal alloy. The magnetic layer properties are found to be 180 GPa for the reduced modulus and 2.9 GPa for the yield strength.

Correlation of Disk Topography Waves with Nanometer-Scale Lubricant Moguls and Magnetic Head Media Spacing

Magnetic recording disk carbon overcoats are lubricated with nanometer-thick films of perfluoropolyether lubricant. It is well-known that lubricant thickness redistribution takes place due to air shear stress oscillation at air bearing resonant frequencies and also due to shear stress oscillation induced by disk topography waves on test tracks. We extended this work to demonstrate correlation between surface topography and lubricant redistribution on whole disk surfaces. Lubricant moguls are shown to form over regions of the disk surface which have topography waves that are half the slider length, and the lubricant thickness peak is out-of-phase down track from the topography peak height. There is a critical relative humidity above 20% beyond which moguls are readily formed by the slider flying at 10 nm without thermal fly-height control. The significance of the lubricant redistribution for drive magnetic performance has long been the subject of debate. The slider flying height modulation measured by magnetic head media spacing was in good agreement with the dynamic air bearing simulation based on the measured disk surface topography. Measured head media spacing image data on the same disk surface before and after lubricant redistribution had the same length scale as the correlation between topography and lubricant thickness variation. These results demonstrate that lubricant thickness redistribution on the order of atomic diameters can degrade magnetic performance, and that the surface topography waves alone can degrade areal density by as much as 2%.

無潤滑ディスク DLC の摩擦特性の温度依存性に関する研究

To further increase recording density of disk storage drives, the heat assisted magnetic recording (HAMR) can solve the coercive force problem of thermal fluctuation and is regarded as the key technology for achieving recording density of more than 1Tb/in2 (1)(2). In this technology, it is suggested that there are serious issues for the diamond like carbon (DLC) overcoat on the disk surface. Because that is heated to high temperature with laser beams(2). In this study, first, we measured temperature dependence of friction properties of non-lubricated DLC surface on magnetic disks. Second, we considered the influence of the accumulated smear on test pin surface during the friction test. As a result, it was estimated that the smear on pin surface accumulated by the laser irradiation was originated from the hydrocarbon contamination adsorbed on DLC surface.

Thin Film Properties of Ammonium Sulfonate Ionic Liquids Having a Long Alkyl Chain

Abstract New well-lubricated ILs including ammonium sulfonate having a long alkyl chain have been developed. The materials based on ammonium sulfonate have a higher ΔpKa between the Brønsted acid and the Brønsted base of the salts and do not decompose even at 300 °C, making them more thermally stable than the perfluoropolyether (PFPE). Therefore, octadecyl ammonium sulfonates have a lower frictional coefficient on the carbon overcoat film even after heat treatment at high temperatures. The ILs are adsorbed on the media surface with high orientation. The alkyl chains of the C16 and C18 sulfonates in the bulk have a trans conformation, nevertheless, the films have a gauche conformation. A sufficient length of the hydrocarbon chain leads to both macro and micro-friction reductions. Dimethyl ammonium sulfonate has sufficient solubility in fluorinated solvent, which may make commercial applications possible.

The Effect of Diamondlike Carbon Overcoat on the Tribological Performance of the Dimple/Gimbal Interface in Hard Disk Drives1

Fretting wear at the dimple/gimbal interface of a hard disk drive suspension was investigated for stainless steel dimples in contact with stainless steel gimbals coated with diamondlike carbon (DLC) of different thicknesses and different elastic moduli. Scanning electron microscopy (SEM) was used to evaluate the size and characteristics of the wear scar of both the dimple and the gimbal. Fretting wear and fatigue-type cracks were found predominantly on the dimple. For different dimple/gimbal combinations tested in this study, the least amount of wear was obtained for the case of a 690 nm thick DLC overcoat. Numerical simulations were performed to calculate the maximum principal stress in the dimple and the gimbal with the goal of correlating wear and the maximum principal stress. The maximum principal stress in both the dimple and the gimbal was found to increase with an increase of the elastic modulus of the DLC overcoat on the gimbal. On comparing the experimental and simulation results, we conclude that wear and fatigue crack formation can be explained by the different level of the maximum principal stress in both the dimple and the gimbal.

Atomic Scale Interface Manipulation, Structural Engineering, and Their Impact on Ultrathin Carbon Films in Controlling Wear, Friction, and Corrosion.

Reducing friction, wear, and corrosion of diverse materials/devices using <2 nm thick protective carbon films remains challenging, which limits the developments of many technologies, such as magnetic data storage systems. Here, we present a novel approach based on atomic scale interface manipulation to engineer and control the friction, wear, corrosion, and structural characteristics of 0.7-1.7 nm carbon-based films on CoCrPt:oxide-based magnetic media. We demonstrate that when an atomically thin (∼0.5 nm) chromium nitride (CrNx) layer is sandwiched between the magnetic media and an ultrathin carbon overlayer (1.2 nm), it modifies the film-substrate interface, creates various types of interfacial bonding, increases the interfacial adhesion, and tunes the structure of carbon in terms of its sp(3) bonding. These contribute to its remarkable functional properties, such as stable and lowest coefficient of friction (∼0.15-0.2), highest wear resistance and better corrosion resistance despite being only ∼1.7 nm thick, surpassing those of ∼2.7 nm thick current commercial carbon overcoat (COC) and other overcoats in this work. While this approach has direct implications for advancing current magnetic storage technology with its ultralow thickness, it can also be applied to advance the protective and barrier capabilities of other ultrathin materials for associated technologies.

Opportunities and Challenges of Atomistic Modeling to Simulate Amorphous Carbon Properties for Computer Hard-Disk Applications

Amorphous carbon is used as coating material for computer hard-disks magnetic media and recording heads. There has been significant improvement in understanding amorphous carbon’s properties based on experimental observations. High data storage density requirement in the coming years necessitates the use of an ultrathin carbon overcoat while maintaining or enhancing its tribological, thermal, optical, and corrosion properties for better recording performance and reliability. Along with experimental techniques, atomistic simulations can be a useful tool to provide fundamental understanding, especially in the cases where experiments are not adequate. This review gives an overview of how atomistic modeling can provide insights into amorphous carbon properties and discuss challenges for such modeling.

The effect of Argon ion irradiation on the thickness and structure of ultrathin amorphous carbon films

Carbon films synthesized by plasma-enhanced chemical vapor deposition (PECVD) and filtered cathodic vacuum arc (FCVA) exhibit a layered structure consisting of a bottom (interface) and a top (surface) layer rich in sp2 atomic carbon bonding and a middle (bulk) layer of much higher sp3 content. Because of significant differences in the composition, structure, and thickness of these layers, decreasing the film thickness may negatively affect its properties. In this study, transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) were used to examine the effect of Ar+ ion irradiation on the structure and thickness of ultrathin films of hydrogenated amorphous carbon (a-C:H) and hydrogen-free amorphous carbon (a-C) deposited by PECVD and FCVA, respectively. The TEM and EELS results show that 2-min ion irradiation decreases the film thickness without markedly changing the film structure and composition, whereas 4-min ion irradiation results in significant film thinning and a moderate decrease of the sp3 content of the bulk layer. This study demonstrates that Ar+ ion irradiation is an effective post-deposition process for reducing the thickness and tuning the structure of ultrathin carbon films. This capability has direct implications in the synthesis of ultrathin protective carbon overcoats for extremely high-density magnetic recording applications.

Laser Heating-Induced Degradation of Ultrathin Media Carbon Overcoat for Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) is a technique for overcoming the superparamagnetic limit and enabling large increases in the storage density of hard disk drives. A tiny area of the magnetic recording media has to be heated up to a high temperature with a laser to lower the coercivity temporarily before information can be written on the area. The possible degradation of the ultrathin media carbon overcoat induced by the laser heating is a concern. In this paper, the laser heating-induced degradation of ultrathin a-C:Nx, a-C:Hx, and a-C overcoats on HAMR media is studied. Surface topography changes induced by the laser heating are evaluated with atomic force microscopy and structure changes with the visible Raman spectrum and X-ray photoelectron spectroscopy (XPS). Laser heating areas are analyzed with time-of-flight secondary ion mass spectrometry and XPS to reveal the underlying mechanism of the degradations. It is found that at a proper HAMR writing temperature and for a total laser heating duration corresponding to a five-year drive life, the surface topography, structure, and composition of the a-C:Nx overcoat are changed by the laser heating. However, the surface topographies of the a-C:Hx and a-C overcoats are not changed. The structure and composition of the a-C:Hx and a-C overcoats are also not changed much. Interlayer diffusion between the carbon overcoat and the underlying magnetic layer is confirmed in the laser heating area for all the three overcoats.

Ultrathin PFPE/DLC Hybrid Overcoat for Magnetic Disks by Photoelectron-assisted Chemical Vapor Deposition

We developed a new method for using photoelectron-assisted chemical vapor deposition to deposit perfluoropolyether (PFPE) lubricant on a diamond-like carbon (DLC) overcoat. PFPE lubricant in the vapor phase was deposited on the DLC surface of a magnetic disk with a bias voltage between the disk surface and a counter electrode above the disk surface during ultraviolet irradiation. The bias voltage accelerated photoelectron emission from the DLC surface, and the photoelectrons partially dissociated the PFPE molecules in the vapor phase. The dissociated molecules then chemically bonded to the DLC surface. According to the experimental results, the bonded ratio of PFPE molecules to the DLC surface was approximately 100%. An ultrathin PFPE/DLC hybrid overcoat was produced as a monolithic film. This hybrid film exhibited a much lower surface energy than the reference Z-tetraol lubricant film and a high water contact angle of over 100°. This hybrid film can increase the head-media clearance and improve some read/write characteristics by using the actual heads.