Papers

AVS1997 Session AS+MR+MI-FrM: Hard Disc Processing Issues: II

Friday, October 24, 1997 8:20 AM in Room J2

Friday Morning

Start	Invited?	Item
8:20 AM		AS+MR+MI-FrM-1 Tribology and Lubricant Issues in Thin-Film Hard Disk System Y.W. Chung (Northwestern University) As the hard disk technology moves towards storage densities in the 10 Gbits/in² and higher, one requires lower fly heights, thinner and more wear-resistant overcoats and better lubricants. In the last year or so, two major developments occurred in the hard disk industry. First, amorphous carbon nitride began to be used as a new overcoat material. Second, a new cyclophosphasene lubricant (X-1P) was being incorporated into the lubricant. In this talk, we will explore the relationship between processing and surface properties of amorphous carbon nitride, along with the developments of more wear-resistant overcoats. The unique rheological properties of X-1P giving rise to better durability will be detailed. Storage densities in the 50-100 Gbits/in² are possible by merging the overcoat and magnetic layer into one thinner composite layer. This requires the design and synthesis of materials with the required magnetic and tribological properties. One such design approach, along with some encouraging results, will be presented.
9:00 AM		AS+MR+MI-FrM-3 Flow of Thin Viscous Polymer Films on Rotating Disks: Importance to Disk Drive Lubrication C.M. Mate (IBM Almaden Research Center) For disk drive lubrication, it is desirable for lubricant molecules on disk surfaces to be mobile so that they can quickly replenish lubricant loss during head-disk contacts, but not so mobile that they are spun off the disk. In this study, thin films (< 6 nm thickness) of perfluoropolyether polymer lubricants were deposited on carbon overcoated disks. The disks are spun at high speeds (7000-12,000 rpm) and at elevated temperatures (40-80C) to promote the flow of the lubricant film. On rotating surfaces, the centrifugal acceleration provides a known driving force for flow across surfaces, making it a convenient system for determining the best description of flow of thin liquid films. Even though the film thickness is comparable to the diameter of gyration of the polymer molecules, we find that the flow can still be modeled as that of a continuum, Newtonian liquid. The addition of an alcohol end group to the polymer chain dramatically slows lubricant flow, presumably due to the interaction of the end group with the disk surface. Also, the lubricant films thin an order of magnitude faster when the spinning surfaces are subjected to wind shear forces, indicating that wind shear is a major driving force for lubricant flow on spinning disks.
9:20 AM		AS+MR+MI-FrM-4 Molecular Deformation of Ultrathin Perfluoropolyether Lubricant Films Adsorbed on Amorphous Carbon Substrates H.I. Kim, S.S. Perry (University of Houston) The physical stability and behavior of molecularly thin lubricant films are critical parameters in current disk drive technology. In such applications, the adsorption state of ultrathin films is influenced by both intermolecular and surface interactions. Previous thermal desorption studies of perfluoropolyethers adsorbed on amorphous carbon and amorphous carbon nitride have found relatively strong film-surface interactions. In the present study, atomic force microscopy (AFM) has been used to investigate the local stability of 10 Å Fomblin ZDOL films adsorbed on similar substrates. Friction force microscopy provides evidence for the plastic deformation of these ultrathin films, even in the very low load regime. These regions of molecular deformation are correlated with higher frictional properties and are suggestive of the initial stages of lubricant wear.
9:40 AM		AS+MR+MI-FrM-5 Lubricant Degradation Mechanisms in Disk Drives G.H. Vurens (HDI Instrumentation) The decomposition of perfluoropolyether (PFPE) lubricants is studied with Fourier Transform Infrared Spectroscopy (FTIR), Thermal Programmed Desorption (TPD), Electron Stimulated Desorption (ESD) and Optical Surface Analysis (OSA). A comparison is made between the degradation products from thermal decomposition, electron decomposition, catalytic decomposition and the degradation products observed by head-disk interaction. It is shown that the degradation products from the head-disk interaction are closely related to the electron degradation products. OSA is an ellipsometry like technique (1), which is used to perform in-situ lubricant degradation studies. The results show that addition of X-1P (available from Dow Chemical) to PFPE lubricants leads to significantly less lubricant degradation, possibly due to anti-oxidant properties of X-1P. ¹(1) S.W. Meeks, W.E. Weresin and H.J. Rosen, ASME/STLE Tribology Conference, Maui, Hawaii, 1994 as paper 94-Trib-18
10:20 AM		AS+MR+MI-FrM-7 Carbon Overcoat Composition and Structure using Secondary Ion Mass Spectrometry (SIMS) J.W. Erickson, J. Sheng, Y. Gao (Charles Evans and Associates); H. Pham (Stormedia Corporation); I.L. Singer (Naval Research Laboratory) The composition and structure of carbon overcoat layers on hard discs was studied systematically using secondary ion mass spectrometry (SIMS). The amounts of H, C, N, O, and F in the carbon overcoat were measured along with Co and Cr out-diffusion from the underlying magnetic layer. A specialized SIMS technique was used to measure the stoichiometry of diamond-like carbon, hydrogenated carbon, and carbon nitride overcoat layers or sublayers. Very thin overcoat layers and sub-layers can also be studied since the instrumental depth resolution for SIMS, expressed as the full-width-at-half-maximum, can be decreased to <2 nm. Structural information on the physical uniformity of the overcoat was obtained by monitoring dimer and trimer secondary ions such as C2 and C3 from the carbon overcoat, or Co2 and Co3 from the magnetic layer. As a consequence of auto-correlation effects in the secondary ion formation kinetics, atomic diffusion can be readily distinguished from non-uniform layers and some interface defects. The SIMS results are compared to those obtained by other techniques, emphasizing the relative value and cost of information (and potential cost-reductions). Finally, the tribological issues affecting hard disc durability and wear are discussed. Topographical mapping of asperities and tribological measurements of hardness, friction, and stiction provide important insights into overcoat functionality. Partly for this reason the correlation of compositional and structural results to functional properties may be strong, weak, or absent depending on the type of overcoat and the deposition method.
10:40 AM		AS+MR+MI-FrM-8 Calibrating Perfluoropolyether Lubricant Thickness Measurements M.F. Toney (IBM Almaden Research Center); D. Pocker (IBM Storage Systems Division); M. Mate (IBM Almaden Research Center) Tribology performance of the head-disk interface in disk drives is sensitive to the amount of lubricant present on disk surfaces. Too little lubricant results in poor durability; too much results in high stiction forces. Consequently, the lubricant thickness must be controlled to within a few Ångstroms, which requires measurement techniques with accuracies much better than an Ångstrom for lubricant films typically a few tens of Ångstroms thick. In this work, X-ray reflectivity (XRR) has been used as an absolute measurement of the thickness of perfluoropolyether (Zdol and Z type) lubricant layers on silicon wafers to check the validity of ESCA and ellipsometry thickness measurements. Excellent agreement is found between the XRR and ESCA measurements if the sum of organic contamination and lubricant thickness determined by ESCA is used and if a 25Å mean free path is used. ESCA and XRR thickness measurements are about 5Å thicker than those by ellipsometry, which is attributed to the displacement of contamination on the silicon wafer when the lubricant is deposited. Changes in layer thickness measured by ellipsometry are only about 5(+/-6)% larger than changes in thickness measured by XRR and ESCA, indicating that ellipsometry can measure, with good accuracy, changes in lubricant thickness at monolayer coverages. Consequently, ESCA and ellipsometry, if contamination displacement is taken into account, can be used to calibrate other techniques for measuring lubricants on disks, such as Fourier Transform Infrared (FTIR) spectroscopy. The measurements also show that the amount of contamination absorbed on the SiOx surfaces increases over several weeks and that PFPE lubricant layers are not effective at preventing contamination adsorption.
11:00 AM		AS+MR+MI-FrM-9 Evaluation of the Lubricant Thickness Measurement by XPS/ESCA on Current Hard Disk Surfaces. P.E. Sobol, D.S. Spaulding, E.L. Principe (Charles Evans and Associates); A. Greene, R.E. Linder, F. Chu (Surface Science Laboratories); C.C. Martner (Hyundai Electronics America) XPS/ESCA is commonly used for the evaluation of thickness of thin overlayers, in particular a large number of analyses are performed for monitoring thickness of lubricants on hard disk surfaces. A simple protocol ¹ is commonly in use, but this was developed for the measurement of fluorocarbon lubricants on composite Fe oxide impregnated epoxy disk surface. Evolution of the hard disk surface through the use of thin film sputter depositions for carbon overcoating, the nitrogenation and hydrogenation of the carbon overcoat, the introduction of new lubricants containing non-fluorocarbon components and the thinner lubricant levels used has created the need for a re-evaluation of the lube thickness measurement. The significance and accuracy of the protocol is evaluated relative to a number of variables including: lube composition, carbon overcoat composition, intensity measurement algorithm, and lube-substrate interaction. In particular measurements are made on a number of overcoated substrates from various sources to determine the effects of manufacturing variability on assumptions made in the thickness calculation. Measurements are also made from the various lubricant/substrate combinations now in use to develop constants and evaluate the systematic errors in the calculation algorithm. Effects of incomplete surface coverage and lube/substrate interaction on accuracy are also evaluated by angle resolved XPS measurements and comparison with other techniques including AFM, ToF-SIMS and lubricant extraction. ¹R. E. Linder and P. B. Mee, `ESCA Determination of Fluorocarbon Lubricant Film Thickness on Magnetic Disk Media' IEEE Transactions of Magnetics, Vol. MAG-18 No. 6, 11/82
11:20 AM		AS+MR+MI-FrM-10 Evidence for Galvanic Corrosion of Aluminum, Iron, and Nickel by a Polyperfluoroether Lubricant J.T. Dickinson, S.C. Langford, W. Faultersack (Washington State University); H. Yoshizaki (National Defense Academy) While investigating electrical signals and charge transfer due to contact on aluminum surfaces in the presence of a polyperfluoroether lubricant (Fomblin Z-DOL), transient electrical signals were discovered that are galvanic in origin. Galvanic current and potential measurements were carried out in three experimental configurations: (1) an Al stylus was translated across a Z-DOL-coated Al substrate in a typical wear geometry, (2) an Al electrode was brought down perpendicularly onto an identical, Z-DOL-coated Al electrode, and (3) in a simple electrolytic cell. All configurations yielded readily measurable currents with corresponding potential differences of about 300 mV. Oxidation of the Al stylus with phosphoric acid changed the sign of the resulting current. Corrosion in galvanic cells was confirmed by the detection of significant amounts of Al in the oil, whereas analysis of oil employed in non-galvanic cells (no external electrical connection) showed negligible Al. Scanning force microscopy of polished Al surfaces exposed to Z-DOL in galvanic cells surface smoothing due to an electropolishing effect. Similar currents are observed using degased Z-DOL, 99.999% pure Al, and single-crystal Al(100) crystal surfaces; i.e., the observed currents are not due to dissolved gases (e.g., O₂, H₂O) in the Z-DOL, impurities in the Al, or grain boundary effects. Due to the high resistivity of Z-DOL, these currents are quite sensitive to the thickness of the oil film. In wear situations (1), asperity contact and near-contact produce current transients that depend on the local surface chemistry. We present model reactions that explain our observations. Currents are also observed with metallic Fe and Ni electrodes, indicating that corrosion of magnetic materials is possible. The consequences of galvanic corrosion including lubricant and substrate degradation in applications of PFPE lubricants, e.g., magnetic storage, is discussed. This work was supported by the National Science Foundation Surface Engineering and Tribology Program under Grant CMS-9414405.