Laser-texturing data zone on a magnetic disk surface by using degenerative two wave mixing

A laser-optical wave mixing technique is provided for forming a high density textured data zone in an efficient and accurate manner. A single coherent laser beam of wavelength .lambda. is split into two beams with roughly the same intensity. These two beams are then overlapped with a cross angle of 2.theta. and focused at the disk surface, resulting in an intensity grating thereon. The intensity grating creates a wave-form pattern consisting of a series of high-energy laser beam lines that will heat the target surface and make surface topographical changes accordingly.

TECHNICAL FIELD 
 The present invention relates to the recording, storage and reading of 
 magnetic data, particularly rotatable magnetic recording media, such as 
 thin film magnetic disks having textured surfaces for cooperating with 
 magnetic transducer heads. The invention has particular applicability to 
 high density magnetic recording media for exhibiting low noise. 
 BACKGROUND 
 Magnetic disks and disk drives are conventionally employed for storing data
 in magnetizable form. Typically, one or more disks are rotated on a 
 central axis in combination with data transducer heads positioned in close
 proximity to the recording surfaces of the disks and moved generally 
 radially with respect thereto. Magnetic disks are usually housed in a 
 magnetic disk unit in a stationary state with a magnetic head having a 
 specific load elastically in contact with and pressed against the surface 
 of the disk. 
 In operation, the magnetic disk is normally driven by the contact start 
 stop (CSS) method, wherein the head begins to slide against the surface of
 the disk as the disk begins to rotate. Upon reaching a predetermined high 
 rotational speed, the head floats in air at a predetermined distance from 
 the surface of the disk due to dynamic pressure effects caused by the air 
 flow generated between the sliding surface of the head and the disk. 
 During reading and recording operations, the transducer head is maintained
 at a controlled distance from the recording surface, supported on an air 
 bearing as the disk rotates. The magnetic head unit is arranged such that 
 the head can be freely moved in the radial direction of the disk in this 
 floating state allowing data to be recorded on and retrieved from the 
 surface of the disk at a desired position. 
 Upon terminating operation of the disk drive, the rotational speed of the 
 disk decreases and the head begins to slide against the surface of the 
 disk again and eventually stops in contact with and pressing against the 
 disk. Thus, the transducer head contacts the recording surface whenever 
 the disk is stationary, accelerated from a stop and during deceleration 
 just prior to completely stopping. Each time the head and disk assembly is
 driven, the sliding surface of the head repeats the cyclic operation 
 consisting of stopping, sliding against the surface of the disk, floating 
 in the air, sliding against the surface of the disk and stopping. 
 During reading and recording operations, it is desirable to maintain each 
 transducer head as close to its associated recording surface as possible, 
 i.e., to minimize the flying height of the head without contacting or 
 damaging the data storage portion of the disk. This objective becomes 
 particularly significant as the areal data recording density increases. 
 Thus, a smooth recording surface is preferred, as well as a smooth 
 opposing surface of the associated transducer head, for permitting the 
 head and the disk to be positioned in close proximity, with an attendant 
 predictability and consistency of behavior of the air bearing supporting 
 the head. However, if the head surface and recording surface are too flat,
 the precision match of these surfaces gives rise to excessive stiction and
 friction during the start up and stopping phases, thereby causing wear to 
 the head and recording surfaces eventually leading to what is referred to 
 as "head crash." Thus, there are competing goals of reducing head/disk 
 friction and minimizing transducer flying height. 
 In order to satisfy these competing objectives, the recording surfaces of 
 magnetic disks are conventionally provided with a roughened surface to 
 reduce the head/disk friction by techniques referred to as "texturing." 
 Conventional texturing techniques involve polishing the surface of a disk 
 substrate to provide a texture thereon prior to subsequent deposition of 
 layers, such as an underlayer which is typically chromium or a 
 chromium-alloy, a magnetic layer, a protective overcoat which typically 
 comprises carbon, and a lubricant topcoat, wherein the textured surface on
 the substrate is intended to be substantially replicated on the outer 
 surface of the magnetic disk. 
 The escalating requirements for high areal recording density impose 
 increasingly greater requirements on thin film magnetic media in terms of 
 coercivity, squareness, low medium noise and narrow track recording 
 performance. In addition, increasingly high density and larger-capacity 
 magnetic disks require increasingly small flying heights, i.e., the 
 distance by which the head floats above the surface of the disk in the CSS
 drive. The requirement to further reduce the flying height of the head 
 imposed by increasingly higher recording density and capacity render it 
 particularly difficult to satisfy the requirements for controlled 
 texturing to avoid head crash. 
 Texture on magnetic recording media surfaces has been required, also, in 
 data storage zones to orient the crystallization of the magnetic layer 
 along circumferential lines to improve the signal-to-noise ratio and other
 magnetic performance. Conventional techniques comprise a mechanical 
 operation, such as polishing. One such technique is to apply slurries with
 coolant for scratching the substrate surface. The slurries are inserted 
 between a tape and the substrate with a certain normal force applied to 
 the tape while the disk is in relative motion to the tape. The substrate 
 surface is scratched by the slurry particles during this process, the 
 resulting scratched lines known as surface texture lines. Because of the 
 random distribution of the slurry particle sizes, these texture lines are 
 randomly spaced with different scratch widths and depths. Also, because of
 the inconsistency of slurry concentration supplied to each disk, the 
 scratch line width and depth vary from disk to disk. With conventional 
 mechanical texturing techniques, it is extremely difficult to provide a 
 clean textured surface due to debris formed by mechanical abrasions. 
 Moreover, the surface inevitably becomes scratched during mechanical 
 operations, which contributes to poor glide characteristics and higher 
 defects. Such relatively crude mechanical polishing, with attendant 
 non-uniformities and debris, does not provide a surface with an adequately
 specular finish or with adequate microtexturing to induce proper 
 crystallographic orientation of a subsequently deposited magnetic layer on
 which to record and read information, i.e., a data zone. 
 FIG. 1 is illustrative of surface profiles obtained from typical mechanical
 texturing techniques. Asperities between scratch lines, which are created 
 by the mechanical texturing method, vary greatly in size of up to the 
 order of 50 .ANG. high on a surface of roughness average Ra of only about 
 5 .ANG.. The surface profile is a relatively random profile, with no 
 specified number of peaks, nor defined heights of the asperities and 
 depths of the valleys. As recording density requirements continue to 
 increase, the size of each magnetic bit becomes smaller. As a result of 
 random spacing of texture lines and random unacceptable scratch asperities
 and depths, more defects are found during magnetic testing. 
 An alternative technique to mechanical texturing comprises the use of a 
 laser light beam focused on an upper surface of a nonmagnetic substrate. 
 See, for example, Ranjan et al., U.S. Pat. No. 5,062,021, in which an NiP 
 plated Al substrate is polished to a specular finish, and then the disk is
 rotated while directing pulsed laser energy over a limited portion of the 
 radius, to provide a textured landing zone leaving the data zone specular.
 The landing zone comprises a plurality of individual laser spots 
 characterized by a central depression surrounded by a substantially 
 circular raised rim. 
 Another laser texturing technique is reported by Baumgart et al. "A New 
 Laser Texturing Technique for High Performance Magnetic Disk Drives," IEEE
 Transactions on Magnetics, Vol. 31, No. 6, pp. 2946-2951, November 1995. 
 See, also, U.S. Pat. Nos. 5,550,696 and 5,595,791. 
 The above-identified copending application Ser. No. 09/125,152, applies 
 laser texturing to obtain an ultra-fine pattern with elongated asperities 
 having low asperity height. While there are no deep valleys on the media 
 surface, the elongated asperities are randomly elongated, created by a 
 laser beam that is randomly modulated and focused on the data storage 
 media surface. Although asperity elongation provides a more limited 
 randomness in the circumferential direction, nonuniformity in height 
 imposes negative effects on signal-to-noise ratio and magnetic performance
 as data density becomes increasingly greater. 
 Accordingly, there exists a need for a magnetic recording medium having 
 data storage surfaces configured to accommodate the decrease in bit size 
 concomitant with higher density storage. Such a configuration should 
 provide an acceptable limit in the number of bits that are disqualified or
 missing in magnetic testing, which in the prior art are due to random 
 spacing of deep scratches or texture lines. A further need exists for a 
 laser micro-machining technique to form such high density storage surfaces
 in a practical manner. 
 Copending U.S. patent application Ser. No. 09/311,358 addresses these needs
 with a method for forming a textured data zone on a magnetic recording 
 disk in which a focused laser beam is continuously applied to a substrate 
 in a path of generally circumferential direction on the disk surface 
 between inner and outer radii of a data storage zone. Application of the 
 laser beam occurs while the substrate is rotated at a relatively constant 
 first speed and the focused beam is moved radially at a relatively 
 constant second speed, significantly slower. The resulting configuration 
 is a continuous grooved structure in which a plurality of generally 
 parallel and circumferential continuous ridges are separated by grooves. 
 An underlayer, magnetic layer, protective overcoat and lubricant topcoat 
 are then deposited, the textured surface of the substrate surface being 
 substantially replicated on subsequently deposited layers. One or more 
 data storage tracks are then formed in the magnetic layer of each of the 
 resulting ridges. 
 This method provides improved magnetic layer crystallization orientation 
 with a more regular geometric texture configuration within acceptable 
 ranges. However, use of a single laser beam, accurately focused at a very 
 fine point on the substrate surface, requires continuous application 
 through a number of revolutions equal in number to the number of 
 revolutions of the groove that is formed. As the substrate surface is 
 traversed radially at rate in the range of about 0.001 inches per second 
 (IPS) to 0.010 IPS, the time during which relatively constant power and 
 high accuracy focus must be continuously administered between the inner 
 and outer disk radius is considerable. 
 There continues to be a need for new laser-optical techniques for 
 micro-machining on magnetic recording media substrate surfaces to form 
 regularly-spaced ridges/grooves with enhanced magnetic layer 
 crystallization orientation. 
 SUMMARY OF THE INVENTION 
 The present invention fulfills the aforementioned needs. An advantage of 
 the present invention is that a new laser-optical wave mixing technique is
 provided for forming a high density textured data zone in an efficient and
 accurate manner. A single coherent laser beam of wavelength .lambda. is 
 split into two beams with the same intensity. These two beams are then 
 overlapped with a cross angle of 2.theta. and focused at the disk surface,
 resulting in an intensity grating thereon. The intensity grating creates a
 wave-form pattern consisting of a series of high-energy laser beam lines 
 that will heat the target surface and make surface topographical changes 
 accordingly. 
 This technique is realized, at least in part, by positioning a beam 
 expander in the output beam path of a laser source, splitting the expanded
 beam output into two coherent component beams of the same polarization and
 approximately the same intensity, and closely focusing these beams at the 
 substrate surface. The resulting overlapped laser beam is applied to the 
 disk during rotation of the disk and while the focus point is moved 
 radially with respect to the disk. Focusing is effected by means of a lens
 and two mirrors, each mirror being positioned in a path of a component 
 beam to reflect the beam toward the focus point. The lens may be placed 
 advantageously downstream of the beam expander to focus the expanded beam 
 at the beam splitter. Alternatively, a lens may be positioned 
 advantageously in both reflected paths downstream of the mirrors to focus 
 both beams on the disk surface. 
 Additional aspects and advantages of the present invention will become 
 readily apparent to those skilled in this art from the following detailed 
 description, wherein the embodiments of the invention are described, 
 simply by way of illustration of the best mode contemplated for carrying 
 out the invention. As will be realized, the invention is capable of other 
 and different embodiments, and its several details are capable of 
 modifications in various obvious respects, all without departing from the 
 invention. Accordingly, the drawings and description are to be regarded as
 illustrative in nature, and not as restrictive.

DETAILED DESCRIPTION OF THE INVENTION 
 FIG. 2 illustrates the use of laser micro-machining techniques to form the 
 substrate surface with regularly spaced ridges and grooves to enhance the 
 magnetic layer crystallization orientation. Disk substrate 15, having 
 upper and lower surfaces, is allocated a data zone 17 between inner radius
 19 and outer radius 21, at which one or both substrate surfaces are 
 processed to form data storage areas. Laser head 23 produces a coherent 
 laser beam, which is expanded by beam expander 25. The expanded beam 
 output by the beam expander is focused by lens 27 and applied to beam 
 splitter 29. The applied beam is split substantially equally in the 
 preferred embodiments. Satisfactory operation can be obtained with 
 different beam split ratios. The split component beams are directed to 
 mirrors 31 and 33, which appropriately reflect both of these component 
 beams to a focused point on surface 17 of the substrate surface. 
 Application of the laser beams occurs while the substrate is rotating and 
 in relative radial movement with respect to the focused laser beam. Either
 the laser beam generating apparatus or the disk may be driven for radial 
 movement with respect to the other. 
 It can thus be appreciated that a single coherent laser beam of wavelength 
 .lambda. is split into two beams with the same intensity. Then these two 
 beams with a cross angle of 2.theta. will be overlapped and focused at the
 disk surface, resulting in an intensity grating at the disk surface. The 
 intensity grating creates a wave-form pattern consisting of a series of 
 high-energy laser beam lines that will heat the target surface and make 
 surface topographical changes accordingly. The spacing or period .GAMMA. 
 between grating lines is determined as: 
EQU .GAMMA.=.lambda./(2 sin .theta.). 
 The spacing or period .GAMMA. is also proportional to the resulting surface
 texture line-densities. By controlling the laser power and the relative 
 movement between the final focused laser beam and the disk, the disk 
 surface can be laser-textured to have high-low profiles. 
 Laser head 23 may produce a continuous-waveform (CW-laser) laser or a 
 pulsed laser, which may run at high pulse frequency with long pulse width 
 of wavelength in the order of 1064 nm. The substrate material may comprise
 any material conventionally employed for substrates in manufacturing 
 magnetic recording media, for example, nickel-phosphorus coated aluminum 
 or aluminum alloy. From the starting, inner radius 19 of the data zone, 
 while the disk is rotating, the focused laser beam is moved radially 
 towards the outer radius at a proper linear speed until the ending radius 
 21 of the data zone is reached. The focused laser beam provides accurate 
 heat energy to the surface while the disk is rotating and sliding. 
 Mixing the two component beam waveforms produces intensified line sets at 
 the substrate surface. The length path difference between these two 
 waveforms is smaller than the coherent length of the laser beam to thereby
 to produce a high contrast intensity grating pattern. This grating pattern
 is used for surface texturing. FIG. 3 is an illustration of the mixing of 
 the beams to obtain intensity grating at surface 17. The optical path 
 difference (OPD) between the two incident beams at the position x is 
EQU OPD=2.vertline.x.vertline. sin .theta. 
 For two coherent beams with the same polarization, the constructive 
 interference, i.e., maximum intensity, occurs when 
EQU OPD=n.multidot..lambda.(n: integer) 
 where I=I.sub.1 +I.sub.2 +2 E.sub.1 E.sub.2 (I.sub.1, I.sub.2 are 
 intensities, and E.sub.1, E.sub.2 are fields of the two incident beam at 
 the surface, respectively; .lambda. is the wavelength of the single source
 coherent laser beam.) 
 Destructive interference, i.e., minimum intensity, occurs when 
EQU OPD=(n+1/2).multidot..lambda.(n: integer) 
 where I=I.sub.1 +I.sub.2 -2 E.sub.1 E.sub.2 
 Therefore, a spatial intensity pattern is formed at the overlapped spot of 
 the disk surface. This pattern has been termed the "intensity grating." 
 The coherence length is the distance between two positions along the light 
 propagation direction that still maintains a fixed phase relation to 
 effect interference. The coherence length can be determined as 
 ##EQU1## 
 where .DELTA..nu. and .DELTA.(1/.lambda.) is the line width of a Gaussian 
 beam in frequency (Hz) and wave number (cm.sup.-1) units, respectively. 
 The optical path difference should be less than the coherence length of the
 laser line, preferably within 10% of the coherence length. Satisfactory 
 results were obtained with, for example, laser power larger than 0.03 
 .mu.J/(.mu.m)).sup.2 when using a nanosecond pulsed laser. With a 
 CW-laser, laser power larger than 5 mW/(.mu.m).sup.2 produced satisfactory
 results. 
 FIG. 4 is a diagram of a second embodiment of the invention by in which 
 lens 27 has been relocated. In this embodiment, the focusing lens is the 
 last component in the optical train. This diffraction limited focusing 
 lens will focus the two component collimated parallel beams to the same 
 spot. The lens has a shorter focal length than the lens of FIG. 2 as it is
 closer to the surface. This construction permits obtaining the required 
 light intensity with less laser power. 
 Satisfactory results were obtained with, for example, an intensity of 0.03 
 .mu.J (.mu.m).sup.2 using a nanosecond pulse laser. For a typical high 
 repetition laser with 30 .mu.J/pulse, the focused spot size can be 1000 
 (.mu.m).sup.2, which corresponds to a diameter of about 35 .mu.m. Using a 
 lens with a focal length between 100-150 mm, a laser beam has been found 
 to satisfactorily focus down to this required spot size. 
 Satisfactory parameters for the invention were found to be, for coherence 
 length L.sub.c, a range of about 1 cm to 15 cm, for OPD, a range of about 
 0 to 0.2 L.sub.c, for laser power, a range of 1 W to 50 W, and for lens 
 focal length, a range of about 5 cm to 20 cm. Particularly advantageous 
 results were obtained for coherence length L.sub.c, a range of about 5 cm 
 to 10 cm, for OPD, a range of about 0 to 0.1 L.sub.c, for laser power, a 
 range of 10 W to 20 W, and for lens focal length, a range of about 10 cm 
 to 15 cm. 
 Only certain embodiments of the invention and but a few examples of its 
 versatility are shown and described in the present disclosure. It is to be
 understood that the invention is capable of use in various other 
 combinations and environments and is capable of changes or modifications 
 within the scope of the inventive concept as expressed herein.