Abstract:
A magnetic data storage medium includes a dedicated transducing head contact zone for engaging an air bearing slider, primarily when the disk is stationary. The contact zone is textured with at least one elongate ridge extending in the circumferential direction. When a single ridge is formed, it runs in a spiral path in multiple turns with a predetermined radial pitch at least ten times the nominal ridge width. The ridge protrudes axially outward from a nominal surface plane of the contact zone, and is rounded and free of sharp edges. The ridge, or plurality of ridge sections, can be formed by a texturing process that includes directing a laser beam, focused, onto the contact zone surface. While the disk is rotated to maintain a constant circumferential speed relative to the laser, it also is translated radially to provide the desired radial pitch. The laser is operated in a CW (continuous wave) mode, to create a more uniform ridge.

Description:
This application claims the benefit of Provisional Application No. 60/040,788 entitled “Continuous Spiral Line Laser Texture to Improve Take-off and Landing Dynamics of Laser Texture”, filed Mar. 14, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to texturing of magnetic data storage media, and more particularly to the texturing of dedicated transducing head contact zones of such media to minimize system resonance. 
     Laser texturing of magnetic disks, particularly over areas designed for contact with data transducing heads, is known to reduce friction and improve wear characteristics as compared to mechanically textured disks. Traditional laser texturing involves focusing a laser beam onto a disk substrate surface at multiple locations, forming at each location a depression surrounded by a raised rim as disclosed in U.S. Pat. No. 5,062,021 (Ranjan) and U.S. Pat. No. 5,108,781 (Ranjan). An alternative, as disclosed in International Publications No. WO 97/07931 and No. WO 97/43079, is to use a laser beam to form domes or nodules, rather than rims. In some cases, each of the domes is surrounded by a raised rim. The features can be axisymetric, i.e. with circular profiles, or can have non-axisymetric or elliptical profiles. In the latter case, the long axes of the texturing features preferably extend circumferentially relative to the disk. 
     Collectively, the texturing features are formed in a desired pattern or distribution throughout the head contact zone. A particularly preferred pattern is a spiral, formed by rotating the disk at a desired angular speed while at the same time moving a laser radially with respect to the disk. The laser is pulsed to form the individual texturing features. For example, the disk can be rotated at a speed, variable depending on the radial position of the laser, to provide a linear (arcuate) velocity of about one meter per second. Then, operating the laser at 50,000 pulses per second would provide a 20 micron circumferential pitch, i.e. distance between adjacent texturing features. The radial speed of the laser module controls the radial pitch or spacing between adjacent turns of the spiral, which also can be about 20 microns. 
     Although this approach has been highly successful in terms of reducing dynamic friction and improving the wear characteristics of dedicated transducing head contact zones, the regular, repeating pattern of the laser texture features produces strong input excitations based on the fundamental frequency of the circumferential pitch, including higher order harmonics. When the excitation frequencies coincide with natural frequencies of the slider or its gimbal and support system, resonance occurs which results in a high amplitude acoustic emission signal, which can increase the difficulty of determining the glide avalanche breaking point (a disk/transducing head spacing value), and yield a false indication that the disk has failed a glide test. 
     Apart from their contribution to resonance, the regularly spaced apart texturing features are thought to contribute to transducing head disturbances in two further respects. First, an intermittent contact of the peaks of texturing features with the data transducing head during disk accelerations and decelerations can disturb the head. Second, the texturing features contribute to turbulence in the air bearing that supports the transducing head slider during portions of accelerations and decelerations. 
     Therefore, it is an object of the present invention to provide a texturing feature adapted to impart a desired surface roughness to the dedicated transducing head contact zone of a recording medium while minimizing undesirable resonant frequency effects. 
     Another object is to provide a magnetic data storage medium in which a head contact zone has a topography that is directionally controlled, in that surface height gradients occur primarily in the direction perpendicular to the direction of transducing head travel relative to the disk. 
     A further object is to provide a process for laser texturing a data storage medium to form texturizing features that are elongate and highly uniform in the direction of storage media travel. 
     Yet another object is to provide magnetic data storage media that exhibit the highly favorable dynamic friction and wear characteristics of laser textured media, and further exhibit low resonance interactions with transducing heads during head take-offs and landings. 
     SUMMARY OF THE INVENTION 
     To achieve these and other objects, there is provided a magnetic data storage disk. The disk includes a substrate body formed of a non-magnetizable material and having a substantially planar surface including an annular contact region. The contact region is adapted for surface engagement with a magnetic data transducing head during accelerations and decelerations of the substrate in a circumferential direction with respect to the transducing head. A plurality of elongate ridge sections extend substantially circumferentially along the contact region. Each of the ridge sections has a circumferential length, a width in the radial direction, and an axial height above a nominal surface plane of the contact region. Each ridge section is substantially uniform in height over substantially its entire length. Each ridge section is convex in the direction away from the nominal surface plane. Adjacent ridge sections are spaced radially apart from one another by a radial pitch at least ten times the radial ridge section widths. The ridge sections have substantially the same height, and thus cooperate to determine a substantially uniform surface roughness throughout the contact zone. 
     There are several suitable arrangements of the ridge sections. In one highly preferred arrangement, the ridge sections are part of and cooperate to form a single, continuous spiral-line textured pattern. In this case, each complete turn or revolution of the spiral is conveniently considered as one of the ridge sections. The spiral pattern is favored, since it involves a single, continuous path that facilitates a complete laser texturing of the contact region in a matter of seconds. Disk rotation and radial travel are coordinated to maintain a substantially uniform radial pitch. 
     Alternatively, the ridge section can be formed as a series of concentric rings. In this case, the radial position of the laser module is kept constant during each revolution, and between revolutions is stepped by an amount that determines the radial pitch. 
     The profile of the ridge sections is preferably uniform, and is primarily a function of the impingement energy of the laser beam at the disk surface. Impingement energy, in turn, depends on a combination of factors including laser power, the degree of attenuation if any, and degree of focus of the laser beam, and the mode in which the laser is operated. For example, in the TEM 00  mode, the power distribution is Gaussian, concentrating more energy toward the center of the beam. 
     Preferably the ridge sections have a height in the range of 5-30 nm, more preferably in the range of 5-20 nm. The widths of the ridges are substantially greater than the height, typically at least one micron and more preferably at least two microns. 
     The substrate body typically is formed of aluminum, with a nickel phosphorous layer plated over the aluminum. Subsequent films or layers applied over the Ni—P layer include a non-magnetizable underlayer, a magnetizable thin film recording layer, and a protective carbon layer. Preferably the ridge sections are formed in the nickel phosphorous layer. Then, the underlayer, recording layer and cover layer, being of uniform thickness, replicate the substrate topography including the texturing throughout the contact zone. Alternatively, the ridge sections can be formed in either the underlayer or the thin film recording layer. 
     Further according to the invention, there is provided a process for surface texturing a magnetic data storage medium, including the following steps: 
     a. directing a coherent energy beam toward a magnetic data storage medium, to cause the coherent energy beam to impinge upon a selected surface of the storage medium at an impingement area thereon, wherein the selected surface has a nominal surface defines a nominal surface plane; and 
     b. translating the data storage medium with respect to the coherent energy beam in a manner to cause the impingement area to move along the selected surface in a predetermined direction at a substantially constant speed relative to the data storage medium along a path, momentarily and locally melting the data storage medium at the selected surface and along the path, to form a continuous, elongate ridge along the path in said predetermined direction, extending outwardly away from the nominal surface plane, and having a substantially uniform height in the range of about five to about 30 nm, wherein the elongate ridge is rounded and substantially free of sharp edges. 
     Typically the data storage medium is disk shaped, and the impingement area is moved at least in part by rotating the disk, thus to define a substantially circumferential path. The coherent energy beam is translated radially with respect to the disk, simultaneously with disk rotation. The result is a spiral path, with the rotational and radial movement coordinated to yield a uniform radial pitch. The laser is operated in the CW (continuous wave) mode. 
     Because they are considerably more elongate than the conventional bumps or domes, the ridges and ridge sections enable operation of the laser in the CW mode. While a single, continuous ridge formed in a spiral path perhaps is the best example for demonstrating this advantage, the concentric ring ridge sections provide the advantage as well. Even a short circular ridge, e.g., with a circumferential length of about 10 mm, has a length greater than the characteristic ridge width (approximately 3 microns) by at least three orders of magnitude. The primary benefit of operating the laser in the CW mode is that the power output remains stable over time, meaning that the level of energy at the impingement area likewise is highly stable. Any adjustments to that energy level, whether through changing the laser power or the optical components between the laser source and the disk, are gradual changes in the nature of fine tuning, rather than the abrupt fluctuations inherent in pulsed energy. This simplifies the texturizing process, and affords a greater degree of control over the size and profile of the ridge. 
     In this regard, a key feature is the ability to maintain a uniform height over substantially the entire length of each ridge section, and more generally throughout all of the ridge sections forming the dedicated contact zone. This provides an excellent degree of control over the contact zone surface roughness, in terms of peak height above the nominal surface plane. 
     With respect to resonance, the orientation of the ridge sections and their elongation and uniformity provide a key advantage. More particularly, surface profiles taken in planes perpendicular to ridge extension are substantially uniform. In other words, there is virtually no topography gradient in the circumferential direction. The maximum topography gradient occurs in the radial direction. 
     When the data storage disk is rotationally accelerated and decelerated, to provide respectively for the take-off and landing of the aerodynamically supported head slider, the slider moves relative to the disk in the circumferential direction. Accordingly, the slider encounters virtually no topography gradient, and there is virtually no input excitation frequency (harmonics) to coincide with and amplify natural resonant frequencies of the slider or its support system. As a result, glide avalanche measurements are improved, particularly measurements of glide avalanche breaking point (GABP), reducing the number of disks with satisfactory textures that nonetheless fail glide tests, solely due to resonance effects. 
    
    
     IN THE DRAWINGS 
     For a further appreciation of the above and other features and advantages, reference is made to the following detailed description and to the drawings, in which: 
     FIG. 1 is a plan view of a magnetic data storage disk and a data transducing head supported for generally radial movement relative to the disk; 
     FIG. 2 is an enlarged partial sectional view of the magnetic disk in FIG. 1; 
     FIG. 3 is a partial top plan view of a magnetic data storage disk with a texture pattern of discrete nodules according to the traditional laser texturing approach; 
     FIG. 4 is a schematic representation of a surface profile of the contact zone in FIG. 3, taken in a radial direction; 
     FIG. 5 is a schematic surface profile of the contact zone in FIG. 3, taken in a circumferential direction; 
     FIG. 6 is a diagrammatic view of a texturing device for forming the elongate texturing features of the disk in FIGS. 1 and 2; 
     FIG. 7 is a chart showing an AFM surface profile of a ridge; 
     FIG. 8 is a plan view similar to that in FIG. 3, showing a transducing head landing zone textured in accordance with the present invention; 
     FIG. 9 is a schematic representation of a surface profile of the contact zone in FIG. 8, taken in a radial direction; 
     FIG. 10 is a surface profile of the landing zone in FIG. 8, taken in a circumferential direction; 
     FIG. 11 is a chart illustrating the coincidence of an input excitation frequency with natural resonant frequencies of a transducing head slider and its support system; 
     FIG. 12 is a chart showing the frequency response of a traditional laser texture during transducer slider take-off and landing; 
     FIG. 13 is a chart showing a glide avalanche curve based on a traditional laser texture pattern; 
     FIG. 14 is a chart showing the frequency response of a laser textured ridge in the form of a continuous spiral; 
     FIG. 15 is a partial top plan view of an alternative data storage disk textured according to the present invention; 
     FIG. 16 is a sectional view of the disk in FIG. 15; 
     FIG. 17 is a schematic top view of a further alternative embodiment data storage disk; and 
     FIG. 18 is a schematic top view of yet another alternative embodiment data storage disk. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings, there is shown in FIGS. 1 and 2 a medium for reading and recording magnetic data, in particular a magnetic disk  16  rotatable about a vertical axis and having a substantially planar horizontal upper surface  18 . A rotary actuator (not shown) carries a transducing head support arm  20  in cantilevered fashion. A magnetic data transducing head  22  (including magnetic transducer and air bearing slider) is mounted to the free end of the support arm, through a suspension  24  which allows gimballing action of the head, i.e., limited vertical travel and rotation about pitch and roll axes. The rotary actuator and the support arm pivot to move head  22  in an arcuate path, generally radially with respect to the disk. 
     At the center of disk  22  is an opening to accommodate a disk drive spindle  26  used to rotate the disk. Between the opening and an outer circumferential edge  28  of the disk, upper surface  18  is divided into three annular regions or zones: a radially inward zone  30  used for clamping the disk to the spindle; a dedicated transducing head contact zone  32 ; and a data storage zone  34  that serves as the area for recording and reading the magnetic data. 
     When the disk is at rest, or rotating at a speed substantially below its normal operating range, head  22  contacts upper surface  18 . When the disk rotates at higher speeds, including normal operating range, an air bearing or cushion is formed by air flowing between the head and upper surface  18  in the direction of disk rotation. The air bearing supports the head above the upper surface. Typically the distance between a planar bottom surface  36  of head  22  and upper surface  18 , known as the head “flying height,” is about 10 microinches (254 nm) or less. Lower flying heights permit a higher density storage of data. 
     For data recording and reading operations, rotation of the disk and pivoting of the support arm are controlled in concert to selectively position transducing head  22  near desired locations within data zone  34 . Following a data operation, the disk is decelerated and support arm  20  is moved radially inward toward contact zone  32 . By the time the disk decelerates sufficiently to allow head/disk contact, the head is positioned over the contact zone. Thus, head contact with other regions of the disk surface is avoided. Before the next data operation, the disk is accelerated, initially with head  22  engaged with disk  16  within the contact zone. Support arm  20  is not pivoted until the head is supported by an air bearing, above the contact zone. 
     Magnetic disk  16  is formed by mechanically finishing an aluminum substrate disk  38  to provide a substantially flat upper surface. Typically a nickel-phosphorous alloy has been plated onto the upper surface of the substrate disk, to provide a non-magnetizable layer  40  with a uniform thickness in the range of about 2-12 microns. Following plating, the exposed upper surface  42  of the Ni—P alloy layer is polished to a roughness of about 0.1 micro inch (2.54 nm) or less. 
     After mechanical finishing, substrate surface  42 , at least along contact zone  32 , is laser textured to provide a desired surface roughness. Laser texturing involves melting the substrate disk at and near surface  42 , forming texturing features as will be described in greater detail below. 
     Fabrication of disk  16  involves the application of several layers after texturing. The first of these is a chrome underlayer  44  with a typical thickness of about 10-100 nm. Next is a magnetic thin film recording layer  46 , where the data are stored, typically at a thickness of about 10-50 nm. The final layer is a protective carbon layer  48 , in the range of 5-30 nm in thickness. Layers  44 ,  46  and  48  are substantially uniform in thickness, and thus replicate the texture of substrate surface  42 . 
     As previously mentioned, traditional laser texturing involves forming discrete nodules (also called bumps or domes) in the substrate disk at surface  42 . The size and shape of the nodules depends on the level of laser beam energy impinging upon surface  42 . Typically the nodules are formed in a spiral path, having a circumferential pitch governed by the disk rotational speed and laser pulsing interval during texturing. A radial pitch, i.e., the radial distance between consecutive turns of the spiral path, is determined by disk rotation and the rate of radial shifting of the laser relative to the disk. The surface profile views in FIGS. 4 and 5 illustrate radial pitch and circumferential pitch, respectively. 
     FIG. 6 shows a laser texturing device  50  for forming laser textured features in accordance with the present invention. Device  50  includes a neodynium: yttrium vanadium oxide (Nd:=YVO 4 ) diode laser  52 , and beam expanding and collimating optics to produce a beam  54  in the form of a circular cylinder, the diameter of which varies with the application and optical components involved. The optical components include a variable beam attenuator  56 , a beam expander  58  and a lens  60  for focusing the beam onto surface  42  of the disk. Attenuator  56  can be a neutral density filter, e.g., a glass plate bearing a film applied by sputtering unevenly to provide a transmissivity gradient through the plate. 
     As explained in the aforementioned U.S. Pat. No. 5,062,021 and Publication WO 97/07931, the focusing of laser energy onto the metallic surface of the substrate disk causes highly localized melting at the surface. Although the material resolidifies rapidly, there is sufficient material flow to form a nodule which projects outwardly, or in the case of a horizontal surface projects upwardly, from the nominal surface plane. 
     The desired texturing pattern and features are formed by rotating disk  16  using a spindle  62 , and by radially translating the disk relative to the laser beam, e.g., by a motor  64  rotating a shaft  66  to move a non-rotating portion of spindle  62  upwardly and downwardly as viewed in the figure. 
     To trace the preferred spiral path, disk rotation and radial translation occur simultaneously. A substantial departure from previous systems resides in the fact that laser  52  is operated in the CW (continuous wave) mode during texturing. As a result, a single, continuous ridge is formed on surface  42  along the spiral path. 
     FIG. 7 is a chart showing an AMF (atomic force microscopy) profile of surface  42 , where the ridge, indicated at  68 , is shown in a profile taken on a radial plane perpendicular to the surface. The vertical scale is in nm, and the horizontal scale is in microns. Ridge  68  has a round, upwardly convex profile with a peak height of about 15-20 nm, and a width at its base of about 3 microns. The width can vary within a range of 1-5 microns. The height can vary within a range of about 5-30 nm, and more preferably 5-10 nm. The height is the more critical parameter, since the height throughout the ridge determines the surface roughness of the contact zone. The finer lines on opposite sides of ridge  68  illustrate the more acicular character of the mechanically finished disk surface. 
     FIG. 8 is a top view showing part of contact zone  32  of disk  16 , showing consecutive turns or ridge sections  68   a ,  68   b  and  68   c  of the ridge. The radial pitch, as seen in FIG. 9, is 50 microns. Ridge Sections  68   a-c  are highly uniform in surface profile due to their formation as parts of a continuous texturing operation during which the parameters that control intensity at the beam impingement area, and the rate of impingement area travel relative to the disk, are controlled to keep the intensity and speed substantially constant. The radial pitch can vary, but in general it is preferred to provide a radial pitch at least ten times the ridge width. Larger radial pitch values that still yield satisfactory performance are preferred, because the spiral ridge can be formed more rapidly. Further, it has been found that unduly high density of nodules or bumps adversely affects stiction performance, and density in terms of close radial spacing between adjacent ridge sections may have the same undesirable result. 
     FIG. 10 schematically represents the profile of ridge  68  along the entire spiral path, which includes multiple turns or ridge sections to provide a contact zone with a radial dimension sufficient to accommodate transducing head  22 . Because laser  52  is operated in the CW mode, and because the linear (circumferential) velocity of disk  16  relative to the laser beam can be precisely controlled, the resulting ridge has a high consistency throughout its entire length, both in height and in transverse (radial) profile. The consistency in height is beneficial from the standpoint that ridge  68  determines a uniform surface roughness throughout the contact zone. More important, however, is the orientation of ridge  68 , which imparts a highly directional quality to the topography of the contact zone. More particularly, maximum gradients in the height of the textured surface occur in the radial direction (e.g., FIG.  9 ), while in the circumferential direction there are virtually no height gradients. 
     The advantage of this result can be understood from FIG. 11, a plot of disk velocity (circumferential velocity) at a given radial location that has nodules formed by conventional laser texturing, i.e., with a uniform circumferential pitch. A line  70  illustrates the increase in input excitation frequency in linear relation to increases in disk speed. Several slider or slider support system resonant frequencies are indicated at  72 ,  74  and  76 . At points where the input excitation frequency and its harmonics intersect the resonant frequencies, a strong resonant response results. FIG. 12 is a chart showing a frequency response of an array of patterned laser-formed nodules during head take-off and landing. 
     FIG. 13 is a chart showing a glide avalanche curve for the disk, with spikes  78  indicating a match of the excitation frequencies (or harmonics) with natural frequencies of the air bearing. Resonance effects can cause an erroneous test result indicating failure of a disk. For example, if the threshold of the glide test were set to 1.5 volts, the disk under test would fail a 0.95 microinch glide test. Without the resonance effect the disk would pass the test. 
     FIG. 14 is a chart similar to that in FIG. 12, but in contrast illustrates the frequency response of a texture pattern like ridge  68 , i.e., a spiral-line laser texture, during head take-off and landing. Transducing head excitation is considerably reduced, particularly at high frequencies. A corresponding glide avalanche curve (not shown) would be smoother, and thus be more useful in assessing disk performance. 
     FIGS. 15 and 16 illustrate an alternative embodiment data storage medium, in particular a glass ceramic substrate  80  provided with a metallic layer  82 , e.g., chromium, sputtered or otherwise deposited onto the glass substrate. The metallic layer is exposed to a CW laser beam while the substrate and metallic layer are rotated and translated radially, to form a ridge  84  along a spiral path substantially as previously described. To ensure that the topography is determined by ridge formation rather than by localized micro fracturing, metallic layer  82  should have a thickness of at least about 100 nm. 
     FIG. 17 schematically illustrates a further alternative embodiment data storage disk  85 , particularly a transducing head contact zone  86  of the disk. The laser texturing consists of a series of concentric rings  88   a ,  88   b , etc. To form rings  88 , device  50  steps disk  85  radially according to a predetermined radial pitch, then maintains a constant radius as the disk is rotated to form one of the rings. 
     FIG. 18 illustrates yet another alternative embodiment data storage disk, having a contact zone  90  in which the texturing features are arcuate ridge segments, spaced radially apart from one another. Formation of these texturing features requires a stepping of the disk in the radial direction. 
     Thus, in accordance with the present invention, the transducing head contact zones of data storage disks are textured to provide an enhanced surface roughness that improves dynamic friction and wear, yet also eliminates the problem of input excitation frequencies that yield on duly high acoustic energy signals during the take-off and landing of the head slider. This result is achieved by providing a directional character to the topography, in particular virtually eliminating gradients in topography in the circumferential direction. The preferred features are circumferentially extending, elongate segments that can be formed separately from one another in circles or arc segments, or be combined in a single, continuous spiral ridge.