• SPM in Data Storage

    SPM in Data Storage

    Atomic force microscopy and its enhancements can be useful in almost all fields where applied surface analysis or surface characterization is needed. Data storage is one of these fields. It is represented mainly by magnetic recording media, in particular by hard disk drives and magneto-optic devices. Also, novel approaches in data storage technologies are being intensively developed in which not only AFM but also Scanning Tunneling Microscopy (STM) is employed.

    The principles of magnetic recording has not changed since the invention of this way of information fixation. Hard disk drives are considered in this text, being the most commonly used magnetic data storage device at present.

    Two main elements constitute every hard disk. These are the read/write head usually called "slider" and magnetic disk resembling multilayer sandwich. The head consists of writing and reading modules positioned close to each other. The writing module is a sophisticated solenoid working on the inductive principle. The reading module comprises a special giant magnetoresistive or magnetoimpedance sensor (GMR-, GMI-sensor) that is a multilayer composition of magnetic and nonmagnetic layers. And the hard disk itself contains a number of seed layers, magnetic layers and protective layers on the appropriate substrate (Fig. 1).

    Fig. 1. Schematic of magnetic recording and reading

    As is well known, information on hard disks is stored in terms of bits - microscopic (300 nm or less in width) areas having or not having a local magnetic moment thus expressing a "high" or a "low" level of a digital signal (Fig. 2). The writing module operates inducing local magnetic moments in bit areas on the hard disk magnetic layer. Conversely, bits having remanent magnetization cause measurable change in the resistance of the GMR-sensor of the reading module enabling to distinguish between two levels of the digital signal.

    Fig. 2.1. Topography AFM image

    Fig. 2.2. MFM image of recorded bits

    The progress in hard disk engineering follows the ongoing reduction in size of all the materials and modules involved in the process of magnetic recording. It is sufficient to say that the thickness of each magnetic sublayer and protective carbon coating amounts to merely several nanometers and the head hovers over the disk surface at heights not exceeded 50 nm. Hovering at such small heights requires extraordinary perfection of the disk surface, absence of any defects and particles on it since the presence even of the smallest particle may result in severe damage of the surface with the moving head. Moreover, the effects of nonuniform thermal expansion negligible in the early stages of hard disk development nowadays interfere significantly in the mentioned processes.

    Thus, monitoring of roughness and defectiveness of the disk surface as well as the magnetic head topography with nanometer accuracy is of vital importance in magnetic storage technology.

    Atomic Force Microscopy easily copes with these tasks along with other structure and surface analysis techniques. It is usefully applied to measure surface topography of the hard disk and the topography of the slider in the region facinging the hard disk surface. Reversible displacements of magnetic head layers due to thermal expansion can be observed via AFM under actual write operation conditions. Several attempts have been reported to measure the hardness of protective carbon coating (COC) in order to estimate its wear resistance. The AFM scratching technique is successfully used to determine the scratch resistance of ultrathin protective coatings [1600-1602]. Using image subtraction, scratches down to a residual depth of 1 Å can be evaluated, enabling the study of the very beginning of plastic deformation [1103].

    Using cantilevers with a magnetic coating a powerful technique known as Magnetic Force Microscopy (MFM) is developed for the characterization of bit structure of both the hard disk and the read/write head [1101, 1102, 1144, 1158, 1159, 1179]. For an overview of Magnetic Force Microscopy applications to exploring and characterization of magnetic and magneto-optic materials see reports [1052, 1103, 1142].

    Magnetic force microscopy has become a powerful tool for mapping stray fields very close to the surfaces of magnetic materials since it features high lateral resolution. MFM is now the standard method to measure bit lengths and widths, and furthermore is accepted as one of the most precise techniques for the characterization of bit structure irregularities, which can be correlated with the overwrite and offtrack performance of the magnetic head as determined with regular performance testers [1101, E005]. MFM is employed successfully for characterization of various magnetic carriers such as tapes [1133, 1189], longitudal [1166, 1179, 1188] and the promising perpendicular recording media [1154, 1155, 1157, 1158, 1160, 1164, 1182] as well as GMR (GMI) and magnetoresistive materials for data readout [1100, 1109, 1127]. (A complete list of references to the articles devoted to MFM-related problems can be found in the Reference Collections section of our Library).

    Moreover, AFM characterization of disk failure regions with respect to topography can be combined with subsequent characterization of magnetic structure of these regions with MFM since the procedure of substitution of nonmagnetic cantilevers with magnetic one takes a few minutes. Such a combination is the best tool to analyze the reasons and details of disk failure.

    As mentioned above Magnetic Force Microscopy can also be successfully used for characterization of the magnetic head. Clear correlations between the geometry of the yoke pole tips and the emerging write field distribution are found. Scanning a magnetized tip over the GMR sensor with varying tip-sensor distance while capturing the sensor's signal provides a method to map the three-dimensional sensitivity of the sensor [1103].

    The application of AFM is not limited by using it only as a surface characterization technique. The principles of scanning probe microscopy themselves are of growing importance in respect to their possible use for information storage. A cursory glance to the design of the read/write system in a magnetic hard disk is enough to notice the substantial resemblance with the system of data acquisition in scanning probe techniques. Very high lateral resolution of about several nanometers reached in the last years due to miniaturization of main components of AFM and improvement of data acquisition techniques, looks now quite preferable if compared with the size of "the smallest" magnetic bit in commercially available storage media taking an area of about 150 x 150 nm (as of middle of 2002). Due to superparamagnetic restrictions [E007] usual magnetic storage media consisting of multigrain bits will soon reach their limit of ~ 50 Gbit/in2. Along with promising patterned media technology [891, 1118, 1137, E006] non-magnetic AFM- and STM-based ROM and read/write techniques are very promising [1464 (see a brief description in SPM-based Nanotechnology section), 1593-1596].

    Bennewitz et al. [1354] discuss the limits of pushing storage density by means of STM to the atomic scale at room temperatures. It is tentatively shown that the smallest possible bit can be coded with a single silicon atom, positioned at lattice sites along self-assembled tracks with a pitch of five atom rows. These tracks are obtained by depositing 0.4 monolayers of gold onto a Si(111) surface at 700° C with a post-anneal at 850° C, thereby forming the well-known Si(111) 5x2 - Au structure. All images are taken by STM with a tunnelling current of 0.2 nA and a sample bias of -2 V. The writing process consists of removing Si atoms from a preformed nearly filled lattice as has been previously performed in well-controllable manner by Dujardin et al. [1203] in the study on removing germanium atoms from Ge(111) surface. As for readout, there is no need to search in two dimensions for the location for a bit. The signal is highly predictable since all atoms have the same shape and occur on well-defined lattice sites. It has been demonstrated that 5 x 4 = 20 atoms cell containing the only bit atom represents the smallest viable one for the underlying 5x2 lattice that keeps bit interactions under control and proves experimentally an early Feynman's prediction that spacing of five atoms between bits is the smallest affordable. The remaining 19 atoms are required to prevent adjacent bits from interacting with each other, which is verified by measuring the autocorrelation. One of the fundamental limitations to devices operating on the atomic scale is speed due to the fact that the signal decreases and becomes noisier especially at room temperatures. Estimated speed by means of one probe would be of 6·106 points/sec, which is respectable but still slower than today's hard disks. The future speed enhancement could be achieved in application of parallelism to such systems. Development of single atom memory is an example of finest nanotechnology.

    Investigations in the field of AFM-based data storage are held intensively by IBM. IBM researchers report on a new technology called "Millipede" [162, 212, 1599] whose prototype was explored in the early 1990s by Mamin and Rugar at the IBM Almaden Research Center. A cantilever equipped with a heater on its tip makes indentation in a plastic substrate that stands for a logical "1". Erasing of data is performed by means of heating either an entire plastic card or its local region. This approach of a single lever allows reaching densities of hundreds Gbit per square inch though at the expense of relatively low data transfer rates up to 10 Mbit/s. The development of a novel silicon cantilever having 6.6 Mhz maximum resonance frequency is reported [163, 1597]. Using this cantilever and a prototype of an AFM recording system with new detection schemes the same speed readout of above 5 Mbit/s was achieved. In principle, using many such cantilevers working in parallel, as is implemented in the "Millipede" project, low data transfer rates can be overcome according to the IBM research group [212, 1598]. In spite of the fact that areal bits density with arrayed cantilevers is about 5 time less than that of the single one, the results are very encouraging to make efforts in this promising direction. Maybe in near future using cheap plastic pieces of postage stamp size containing as much as a library will be normal.

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    ID Reference list (newly come references are marked red)
    162 Ultrahigh density, high-data-rate NEMS-based AFM data storage system
    J. Brugger, P. Vettiger, M. Despont, U. Durig, M. Lutwyche, G. Binnig, U. Drechsler, W. Haberle, H. Rothuizen, R. Stutz, R. Widmer
    Microelectronic Engineering, 46 (1999), 1-4, 11-17
    163 6.6 MHz silicon AFM cantilever for high-speed readout in AFM-based recording
    K. Itoh, H. Koyanagi, K. Etoh, S. Hosaka, A. Kikukawa
    Microelectronic Engineering, 46 (1999), 1-4, 109-112
    212 VLSI-NEMS chip for parallel AFM data storage
    J. Brugger, P. Vettiger, M. Despont, H. Rohrer, U. Durig, M. Lutwyche, G. Binnig, U. Drechsler, W. Haberle, H. Rothuizen, R. Stutz, R. Widmer
    Sensors and Actuators A: Physical, 80 (2000), 2, 100-107
    846 Friction and head and disk interface durability in contact recording
    K. Schouterden, B.M. Lairson, C.S. Gudeman, K. Chun
    Wear, 216 (1998), 1, 70-76
    891 Preparation and characterization of low-dimensional nanostructures
    L.F. Chi, S. Rakers, H. Fuchs, L. Augustin, C. Rothig, F. Starrberg, T. Schwaack, S. Hoppner
    Applied Surface Science, 141 (1999), 3-4, 219-227
    1052 Scanning probe microscopy for nanometer inspections and industrial applications
    W. Gutmannsbauer, H.J. Hug, E. Meyer
    Microelectronic Engineering, 32 (1996), 1-4, 389-409
    1100 A magnetic force microscopy and Kerr effect study of magnetic domains and cross-tie walls in magnetoresistive NiFe shapes
    H. Joisten, S. Lagnier, M.H. Vaudaine, L. Vieux-Rochaz, J.L. Porteseil
    Journal of Magnetism and Magnetic Materials, 233 (2001), 3, 230-235
    1101 A study of recorded bit patterns using TEM and MFM
    B.K. Middleton, J. Rose, J.K. Birtwistle, J.J. Miles, P. Sivasamy, E.W. Hill, J.N. Chapman, S.M. Casey
    Journal of Magnetism and Magnetic Materials, 193 (1999), 1-3, 470-473
    1102 Analysis of two-dimensional medium noise and magnetic cluster with MFM for Co82Cr13Ta5 longitudinal magnetic recording media
    J. Chen, H. Saito, S. Ishio, K. Kobayashi
    Journal of Magnetism and Magnetic Materials, 188 (1998), 1-2, 260-267
    1103 Applied surface analysis in magnetic storage technology
    J. Windeln, C. Bram, H.-L. Eckes, D. Hammel, J. Huth, J. Marien, H. Rohl, C. Schug, M. Wahl, A. Wienss Applied Surface Science, 179 (2001), 1-4, 168-181
    1109 Correlation between GMI effect and domain structure in electrodeposited Co-P tubes. J.M. Garcia, A. Asenjo, J.P. Sinnecker, M. Vazquez
    Journal of Magnetism and Magnetic Materials, 215-216 (2000), 352-354
    1118 Fabrication and magnetic properties of CoPt perpendicular patterned media
    T. Aoyama, S. Okawa, K. Hattori, H. Hatate, Y. Wada, K. Uchiyama, T. Kagotani, H. Nishio, I. Sato
    Journal of Magnetism and Magnetic Materials, 235 (2001), 1-3, 174-178
    1127 Irradiation effects on the surface morphology and on the magnetic microstructure of giant magnetoresistance La0.7Sr0.3MnO3 thin films studied by magnetic force microscopy
    J.F. Hamet, F. Elard, C. Mathieu, J. Wolfman, R. Desfeux, C. Simon, A. Da Costa
    Journal of Magnetism and Magnetic Materials, 196-197 (1999), 123-125
    1133 Magnetic force microscopic study of magnetic tapes recorded at MHz frequencies
    T. Sato, M. Ishibashi, K. Aso
    Journal of Magnetism and Magnetic Materials, 193 (1999), 1-3, 430-433
    1137 Magnetic force microscopy of high-density perpendicular magnetic recording media
    F.B. Dunning, W.H. Liu, L. Mei, K. Ho, B.M. Lairson
    Journal of Magnetism and Magnetic Materials, 187 (1998), 2, 268-272
    1142 Magnetic force microscopy of thin film media for high density magnetic recording
    L. Abelmann, S. Porthun, C. Lodder
    Journal of Magnetism and Magnetic Materials, 182 (1998), 1-2, 238-273
    1144 Magnetic force microscopy studies of bit erasure in particulate magnetic recording media
    H.V. Kuo, C.A. Merton, E. Dan Dahlberg
    Journal of Magnetism and Magnetic Materials, 226 (2001), 2046-2047
    1154 Magnetization reversal processes in perpendicular anisotropy thin films observed with magnetic force microscopy
    J. Schmidt, E. Dan Dahlberg, C. Merton, S. Foss, G. Skidmore
    Journal of Magnetism and Magnetic Materials, 190 (1998), 1-2, 81-88
    1155 Magnetization structures of CoCr-alloy perpendicular magnetic recording media
    Y. Honda, Y. Hirayama, K. Ito, M. Futamoto
    Journal of Magnetism and Magnetic Materials, 176 (1997), 20-24
    1157 Medium noise properties of Co/Pd multilayer films for perpendicular magnetic recording
    K. Ouchi, N. Honda, T. Kiya, L. Wu
    Journal of Magnetism and Magnetic Materials, 193 (1999), 89-92
    1158 MFM analysis of recorded bit patterns of perpendicular media
    M. Kitano, E. Miyashita, K. Kuga, R. Taguchi, T. Tamaki, H. Okuda, H. Uwazumi, Y. Sakai, A. Kumagai, A. Otsuki
    Journal of Magnetism and Magnetic Materials, 235 (2001), 459-464
    1159 MFM analysis of recorded bits written by trimmed and untrimmed MR heads
    M. Takahashi, K. Takano, G.N. Phillips, T. Suzuki
    Journal of Magnetism and Magnetic Materials, 193 (1999), 434-436
    1160 MFM imaging of FePd stripe domains. Evolution with Pt buffer layer thickness
    M. Vazquez, A. Asenjo, A. Hernando, P.A. Caro, A. Cebollada, D. Garca, F. Briones, D. Ravelosona, J.M. Garca
    Journal of Magnetism and Magnetic Materials, 196-197 (1999), 23-25
    1164 MFM study of magnetic interaction between recording and soft magnetic layers
    Y. Honda, K. Tanahashi, Y. Hirayama, A. Kikukawa, M. Futamoto
    Journal of Magnetism and Magnetic Materials, 235 (2001), 1-3, 126-132
    1166 MFM study of the effects of thickness and composition in high recording density CoCrTa/Cr media
    X. Yang, M. Maeda, M. Yasui, Y. Okumura, Y. Okawa
    Journal of Magnetism and Magnetic Materials, 148 (1995), 3, 466-474
    1179 Quantitative analysis of written bit transitions in 5 Gbit/in2 media by magnetic force microscopy
    G.N. Phillips, T. Suzuki
    Journal of Magnetism and Magnetic Materials, 175 (1997), 1-2, 115-124
    1182 Shape instability in out of equilibrium magnetic domains observed in ultrathin magnetic films with perpendicular anisotropy
    J.E. Mazille, Y. Samson, R. Hoffmann, B. Gilles, A. Marty, V. Gehanno
    Journal of Magnetism and Magnetic Materials, 192 (1999), 3, 409-418
    1188 Thermal stability and micromagnetic properties of high-density CoCrPtTa longitudinal media
    E.N. Abarra, P. Glijer, H. Kisker, T. Suzuki, I. Okamoto
    Journal of Magnetism and Magnetic Materials, 175 (1997), 1-2, 148-158
    1189 Track edges in metal-evaporated tape and thin metal-particle tape
    S. Lalbahadoersing, M.H. Siekman, J.P.J. Groenland, S.B. Luitjens, J.C. Lodder
    Journal of Magnetism and Magnetic Materials, 219 (2000), 2, 248-251
    1203 Vertical Manipulation of Individual Atoms by a Direct STM Tip-Surface Contact on Ge(111)
    G. Dujardin, A. Mayne, O. Robert, F. Rose, C. Joachim, and H. Tang
    Phys. Rev. Lett. 80 (1998) 3085
    1354 Atomic scale memory at a silicon surface
    R. Bennewitz, J. N. Crain, A. Kirakosian, J.-L. Lin, J. L. McChesney, D. Y. Petrovykh and F. J. Himpsel
    Nanotechnology 13 (2002) 499-502
    1385 Read/write mechanisms and data storage system using atomic force microscopy and MEMS technology
    Hyunjung Shin, Seungbum Hong, Jooho Moon and Jong Up Jeon
    Ultramicroscopy, 91 (2002), 1-4, pp. 103-110
    1387 Observation of recording pits on phase-change film using a scanning probe microscope
    Toshiya Nishimura, Masato Iyoki and Shoji Sadayama
    Ultramicroscopy, 91 (2002), 1-4, pp. 119-126
    1464 Terabit-per-square-inch data storage with the atomic force microscope
    E. B. Cooper, S. R. Manalis, H. Fang, H. Dai, K. Matsumoto, S. C. Minne, T. Hunt, and C. F. Quate
    Appl. Phys. Lett. 75 (1999), 22, 3566-3568
    1593 Ultrahigh-density atomic force microscopy data storage with erase capability
    G. Binnig, M. Despont, U. Drechsler, W. Haberle, M. Lutwyche, P. Vettiger, H.J. Mamin, B.W. Chui, T.W. Kenny
    Appl. Phys. Lett. 74 1999 1329-1331
    1594 High-density data storage using proximal probe techniques
    H.J. Mamin, B.D. Terris, L.S. Fan, S. Hoen, R.C. Barrett, D. Rugar
    IBM J. Res. Dev. 39 1995 681-700
    1595 Automated parallel high-speed atomic force microscopy
    S.C. Minne, G. Yaralioglu, S.R. Manalis, J.D. Adams, A. Atalar, C.F. Quate
    Appl. Phys. Lett. 72 1998 2340-2342
    1596 Micromachined heaters with 1-ls thermal time constants for AFM thermomechanical data storage
    B.W. Chui, H.J. Mamin, B.D. Terris, D. Rugar, K.E. Goodson, and T.W. Kenny
    Proc. IEEE Transducers '97, Chicago, USA, June 1997
    1597 Megahertz silicon atomic force microscopy (AFM) cantilever and high-speed readout in AFM-based recording
    S. Hosaka, K. Etoh, K. Kikukawa, H. Koyanagi
    J. Vac. Sci. Technol. B 18 (2000) 94-99
    1598 5x5 2-D AFM cantilever arrays a first step towards terabit storage device
    M. Lutwyche, C. Andreoli, G. Binnig, J. Brugger, U. Drechsler, W. Haerberle, H. Rohrer, H. Rothuizen, P. Vettiger, G. Yaralioglu, C.F. Quate
    Sensors and Actuators A 73 (1999) 89-94
    1599 The "Millipede" - More than one thousand tips for future AFM data storage
    P. Vettiger et al.
    IBM J. Res. Develop. 44, 3, May 2000
    1600 Scratching resistance of diamond-like carbon coatings in the sub-nanometer regime
    A. Wienss, G. Persch-Schuy, M. Vogelgesang, U. Hartmann
    Appl. Phys. Lett. 75 (1999) 1077-1079
    1601 Subnanometer scale tribological properties of nitrogen containing carbon coatings used in magnetic storage devices
    A. Wienss, G. Persch-Schuy, R. Hartmann, P. Joeris, U. Hartmann
    J. Vac. Sci. Technol. A 18 (2000) 2023-2036
    1602 Mechanical properties of d.c. magnetron-sputtered and pulsed vacuum arc deposited ultra-thin nitrogenated carbon coatings
    A. Wienss, M. Neuhauser, H.-H. Schneider, G. Persch-Schuy, J. Windeln, T. Witke, U. Hartmann
    Diamond Related Mater., 10 (2001), 3-7, 1024-1029
    E005 A. Wienss, G. Persch-Schuy
    IBM Technical Report, TR 05.501, 1999.
    E006 Writing and Reading Perpendicular Magnetic recording media patterned by a focus ion beam
    J. Lohan et al.
    Applied Physics Letters, 78, 7, February, 2001
    E007 The Future of Magnetic Data Storage Technology
    D. A. Thompson, J. S. Best
    IBM J. Res. Develop. 44, 3, May 2000
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