Abstract:
A method and apparatus for creating ablation-free visible markings on a multi-layer hard disk magnetic storage media by laser-induced deformation while maintaining the integrity of the protective carbon layer, and without destroying the multi-layered structure of the media. The apparatus includes a laser generator, a rotatable optical plate and a beamsplitter by which the fluence of the beam can be controlled without altering the power setting to the laser generator, a beam sampler for determining the fluence of the beam, and an optical plate which acts with the beamsplitter to eliminate unwanted reflection of the laser beam. The laser beam is steered by a beamsteerer to a hard disk held in a material handling unit. This technique is highly suitable for marking or labeling finished hard disks for the purposes of identification and traceability, without creating any short-term or long-term contamination problems. The corresponding storage media so marked are also claimed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    Reference is made to and priority claimed from provisional application serial No. 60/089,411 titled “Laser Induced Deformation on Hard Disk Surface,” provisional application serial No. 60/089,465 titled “Laser Marking on Finished Hard Disk Media,” and provisional application serial No. 60/089,429, titled “Laser Marking on Multi-Layered Hard Disk Media,” all filed on Jun. 16, 1998. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to laser marking system, and in particular, to a laser marking apparatus and method for marking the surface of a workpiece with a laser beam.  
           [0004]    2. Description of the Prior Art  
           [0005]    In recent years, the use of lasers in hard disk surface processing is gaining popularity. Laser pulses have already been successfully used to create landing zones with improved tribology performance for the data transducing heads. Lasers have also been identified as a viable tool for the marking or labeling of hard disk surfaces. There are several occasions where disks need to be individually labeled. For example, a hard disk may contain markings indicating the number of reworks it has undergone, to assist the drive manufacturers in determining if a particular disk is suitable for further rework. Marking on an individual disk not only helps to classify the disk but also allows the drive and media manufacturers to identify the product type and trace the origin of the disk should mixing occur. The media manufacturers can more easily and reliably trace faults that result in disk failure if relevant information is tagged to individual disks.  
           [0006]    Currently, there are a few methods of marking the finished disks. Some users mark on the disk surface using a scriber. The scriber actually cuts into the delicate disk surface, abrading and damaging the top layers of the disk. Alternatively, disk marking can be carried out using some forms of ink. Ink marking may use either a jet of the liquid ink or simply a pen with a felt tip to transfer the inscription onto the disk surface. However, ink films can deteriorate with time and give rise to contamination. A disk marking method that is both non-contaminating and non-damaging is needed. The marking method also needs to be fast and efficient in order to be adopted by the manufacturing industry.  
         SUMMARY OF THE INVENTION  
         [0007]    It is therefore an object of the invention to provide an apparatus which uses a laser to produce visible deformations on the surface of a workpiece, especially a hard disk magnetic storage media workpiece.  
           [0008]    It is another object of the invention to provide a method and apparatus for speedily and precisely marking hard disk magnetic storage media with a laser in a way such that surface deformation is visible, yet the protective carbon layer of the disk is intact and free of ablation.  
           [0009]    It is yet another object of the invention to provide a method and laser apparatus for inducing surface deformation for the marking process without introducing contamination to the disk surface.  
           [0010]    According to one aspect of the present invention, there is provided a laser beam generator, a beam conditioning module comprising a rotatable optical plate and a beamsplitter, a beam monitoring module, a beam steering module, and a materials handling unit to handle workpieces being marked. A beam from the laser generator is passed through a rotatable optical plate and a beamsplitter in the beam conditioning module, and a sample of the beam is passed to a beam monitoring module where the fluence of the beam is determined, and if desired, the rotatable optical plate can be rotated to vary the fluence of the conditioned beam leaving the beam conditioning module. The conditioned beam is passed to a beam steering module, which directs the beam to the surface of a workpiece held by the materials handling unit.  
           [0011]    An advantage of the present invention is that a workpiece such as magnetic storage media can be marked using a laser without ablation of the protective carbon layer, and thus no additional cleaning or processing step is required before the storage media is used.  
           [0012]    Another advantage of the present invention is that the fluence of a laser marking beam can be adjusted without changing the power setting of the laser itself.  
           [0013]    A further advantage of the present invention is that flashback of the laser beam from the workpiece or beamsteerer is prevented. 
       
    
    
     IN THE DRAWINGS  
       [0014]    For a more complete understanding of the invention, reference is now made to the detailed description of the embodiments as illustrated in the accompanying drawings, wherein:  
         [0015]    [0015]FIG. 1 is a block diagram of the laser marking apparatus;  
         [0016]    [0016]FIG. 2 a  is a more detailed block diagram of the beam conditioning module of the laser marking apparatus;  
         [0017]    [0017]FIG. 2 b  is a preferred embodiment of the beam conditioning module;  
         [0018]    [0018]FIG. 3 is a more detailed diagram of the laser marking apparatus incorporating a processor;  
         [0019]    [0019]FIG. 4 shows the front and side views of the laser marking apparatus;  
         [0020]    [0020]FIG. 5 is a typical cross section of a finished disk;  
         [0021]    [0021]FIG. 6 is a scanning electron microscopy image of the laser marks formed on the disk surface by a scanning pulsed laser beam;  
         [0022]    [0022]FIG. 7 shows characters formed on the hard disk surface using the laser marking system;  
         [0023]    [0023]FIG. 8 depicts laser marking on a hard disk surface without a lubricating layer;  
         [0024]    [0024]FIGS. 9 a ,  9   b  and  9   c  are data profile plots corresponding to cross-sections of an atomic force microscopy image of the ripple structure shown in FIG. 9 formed during laser marking according to the present invention;  
         [0025]    [0025]FIGS. 10 a ,  10   b , and  10   c  are data profile plots corresponding to cross-sections of an atomic force microscopy image of the ripple structure shown in FIG. 10 formed during laser marking using a laser fluence above that of the present invention;  
         [0026]    [0026]FIG. 11 a  is an auger electron spectroscopy depth profiling performed on the ripple structure of FIG. 9, and FIG. 11 b  is an auger electron spectroscopy depth profiling performed on a non-irradiated region on the same disk specimen;  
         [0027]    [0027]FIG. 12 is an atomic force microscopy image of a mark made on a hard disk using a fluence above the range of the present invention;  
         [0028]    [0028]FIG. 13 are data plots showing the results of auger electron spectroscopy performed on the surface of FIG. 12 at three locations, namely a—the center of the ripple structure, b—the rippling region and c—a non-irradiated region; and  
         [0029]    [0029]FIG. 14 illustrates auger electron spectroscopy data profiles of scans performed at various depths of the central portion of the ripple structure of FIG. 9 showing that the surface carbon layer remains intact.  
         [0030]    [0030]FIG. 15 is an enlarged version of FIG. 9. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    A laser marking process performed on hard disks can produce two kinds of surface deformation. Commonly, the process results in severe melting and inter-diffusion of the upper metallic layers. As the protective carbon layer has also been melted, the interdiffusion between the layers can lead to possible reliability problems due to contamination of the surface layer by the underlying layers. On the other hand, with suitably low laser fluence, the marking process can bring about the necessary visible surface rippling without destroying the carbon layer. The desired fluence can be achieved by attenuating the laser beam using a rotatable optical plate and an optical device. The foregoing and other objects, features and advantages of the invention will be apparent from the following detailed description of the preferred embodiments of invention, which makes reference to the several Figs. of the drawing.  
         [0032]    Description of the Laser-Marking System  
         [0033]    [0033]FIG. 1 is a block diagram of the apparatus of the preferred embodiment used for marking the workpieces. A suitable laser generator  11  is employed to create an output laser beam having a unitary frequency, phase and direction. Output beam  13  from laser generator  11  is passed to a beam conditioning module  15  in which a number of operations including sizing and attenuation are performed to the beam, such that the beam is of the right quality to be delivered to the workpiece  25 . If desired, a sample  16  can be taken of beam  13 , said sample being passed to a beam monitoring module  18 .  
         [0034]    The beam output  19  from the beam conditioning module  15  is passed to a beam steerer  21  in which the position of the beam is manipulated to produce a beam that is directed and focused onto the surface of a workpiece to be marked. In a preferred embodiment, the beam steerer  21  comprises a galvanometer capable of directing and focusing the output beam  23  onto the workpiece  25  which is held at a predetermined position with respect to the beam steerer  21 . Such a position is within the range of focus for the output beam  23 , such that the beam can be scanned across a portion of the surface of the workpiece  25  and the desired mark or pattern of marks created on the surface. Material handling unit  27  comprises an input section  28  by which each caddie or cassette  29  holding one or more workpieces enters and is placed in a suitable position for being marked by the output beam  23 , and after the marking operation is carried out, the caddie or cassette  29  holding one or more marked workpieces  25  is removed to output section  30 , while the next caddie or cassette of one or more unmarked workpieces are positioned at the predetermined position for marking. In an alternative embodiment, the position of output beam  23  can be fixed at a predetermined location within the range of motion of a movable materials handling unit, such as a translating X-Y stage, and the workpiece and materials handling stage can be moved with respect to the fixed position of the output beam  23 , thus creating marks or a pattern of marks on the surface of workpiece  25 .  
         [0035]    [0035]FIG. 2 a  is a more detailed block diagram of the beam conditioning module  15 . In FIG. 2 a , linearly polarized beam  13  passes to beam expander and collimator  31 , in which some of the properties of the beam are altered to produce an expanded beam  33 . The expanded beam passes to a variable beam attenuator  34  by which the fluence of the marking beam striking the workpiece can be altered to a desired level. The variable attenuation of the output beam  33  can be achieved using a rotatable optical plate, preferably half-wave plate, and optical polarizer arrangement. A optical polarizer allows only a beam that is linearly polarized along a certain direction to pass through. By rotating the optical plate, through which the linearly polarized beam passes, about the plate&#39;s symmetry axis in a plane perpendicular to the laser beam, the direction of polarization of the beam is rotated and the component of the beam  45  that can eventually pass through the fixed polarizer is therefore varied, bringing about a variable attenuation to the beam  33 .  
         [0036]    In an alternative embodiment, variable beam attenuation can also be achieved simply by placing a rotatable polarizer in the path of the linearly polarized beam  33 . As the polarizer is rotated about its symmetry axis in a plane perpendicular to the beam  33 , the component of the beam that is allowed to pass through the polarizer is also changed, leading to variable power of the output beam  45 .  
         [0037]    Output beam  45  passes to an optical isolator  47 , such as a quarter wave plate, which serves to prevent reflection of the beam from the workpiece from reaching the laser generator. From optical isolator  47 , the resulting beam  51  passes to an optical sampler  53  that provides a sample beam from which a responsive reading may be obtained. Main beam  19  passes from the optical sampler to the beam steering module  21  (shown in FIG. 1).  
         [0038]    [0038]FIG. 2 b  shows a preferred embodiment of the beam conditioning module. Linearly polarized beam  13  from the laser passes to a beam collimator and expander  31 , in which some of the properties of the beam are altered to produce an expanded beam  33 . The expanded laser beam  33  passes to a rotatable half-wave retardation plate  35 , located in the path of beam  33  and oriented such that the plate can be annularly rotated about its symmetry axis in a plane perpendicular to the laser beam. In passing through the rotatable optical plate  35 , the direction of the plane of polarization of the beam is rotated to an extended depending upon the initial orientation of the incident beam  33  and the degree of rotation of the plate  35  around its axis, thus producing a polarization-shifted output beam  37 . Output beam  37  leaving the rotatable optical plate  35  passes to a beamsplitting cube  41 . The beamsplitting cube  41  splits the incoming beam into a plurality of output beams, commonly two components or beams, a p component beam  43  passes straight through the cube, while the s component beam  45  is redirected such that it exits the beamsplitter cube  41  at a 90 degree angle from the incident beam. Depending on the orientation of rotatable optical plate, the energy of the beam  37  entering the beamsplitter cube  41  can be split from about 97% p and 2% s to about 2% p and 97% s.  
         [0039]    In this embodiment, the intensity or fluence of the output beam  23  striking the workpiece  25  can be adjusted by rotating optical plate  35  on an axis parallel to beam  33  until the desired attenuation of the output beam striking the workpiece  25  is achieved. The rotatable optical plate  35  can be rotated either manually, or by a motor responsive to a signal generated by a processor.  
         [0040]    The beam  45  leaving beamsplitting cube  41  may be directed to a beam monitor, to a beam steering module, or, preferably, through an optical isolator before passing to the beam monitoring and the beam steering modules. The optical isolator serves as a feedback preventer by optically isolating the laser generator  11  from unwanted reflection from further down the path of the beam. In a preferred embodiment, a quarter-wave retardation plate  47   a  is used in conjunction with the beamsplitting cube  41  for the purpose of optical isolation. The quarter-wave plate  47   a  is oriented such that plane-polarized incident beam becomes circularly polarized upon leaving the plate. Optical isolation occurs because a linearly polarized input beam from beamsplitter  41  is transformed by the optical plate into a circularly polarized output beam  51 . Any portion of beam  51  reflected from the beam steerer  21  or the workpiece  25  is changed as it passes back through the quarter wave plate  47   a , to a polarization orthogonal to the polarization of the beam  45  entering the optical plate. When the reflected beam passes back into the beamsplitting cube  41 , the reflected beam will pass straight through the beamsplitting cube  41  and exit the beamsplitting cube from a different face than the one facing the laser generator  11 . Thus, positioning quarter wave plate  47   a  along the path of the beam between the beam steerer  21  and the beamsplitting cube  41  will optically isolate the laser generator and prevent flashback from the workpiece  25  or the beam steerer  21  to the laser generator  11 .  
         [0041]    Beam  51  exiting the quarter wave plate  47   a  passes to a beamsampler  53   a , which deflects sample  16  of beam  51  to a detector  57 . Detector  57  produces a signal  58  responsive to the fluence of beam  51 , and signal  58  is input to a meter  59  having a display responsive to the strength of beam  51 . Upon leaving beamsampler  53   a , the main body of the conditioned beam  19  passes to the beam steering module  21  as shown in FIG. 1.  
         [0042]    As shown in FIG. 3, a further preferred embodiment of the present invention is the use of a processor  101  to control and synchronize various components of the apparatus and facilitate the method for producing marked workpieces using the present apparatus. The processor contains memory, a CPU, a display and an input device such as a keyboard through which the user can interact with the processor, and is capable of receiving one or more signals responsive to the condition of the status of said laser generator, the pattern of marks to be placed on said workpiece, the direction of said selected beam leaving said beam steerer, and the position of the workpiece relative to the beam steerer, as well as being capable of generating one or more signals affecting at least one of the status of said laser generator, the desired pattern of marks to be made on the surface of said workpiece, the direction of the beam leaving the beam steerer, and the position of the workpiece to be marked. The processor  101  communicates with laser  11  through signal  103  by which the processor sets various parameters of the laser, such as frequency of laser pulses and laser power. Processor  101  receives signal  105  responsive to the status of the laser, e.g., Q-switch frequency and power level. The processor can send signal  107  to vary the setting of the beam expander  31 , and thus vary the size of the beam. Processor  101  can also send a signal  111  to a motor  112  capable of rotating rotatable optical plate  35  along its axis and thus alter the intensity or fluence of the output beam  23  reaching the workpiece  25 .  
         [0043]    Shutter  133  can send a bi-directional signal  113  to processor  101  to indicate whether the shutter is in an open or closed status, and processor  101  can in response signal the shutter  133  to maintain or change the status. The processor can receive signal  114  from meter  59  responsive to the power of the sampled beam, and can subsequently issue a command to rotate rotatable optical plate  35  along its axis if the desired fluence or intensity of the output beam  23  reaching the workpiece is not within the desired range or at pre-established setpoint.  
         [0044]    The processor  101  is also capable of receiving input from the user on the pattern of markings to be made on the surface of the workpiece  25 , and converting the pattern to a digital representation, which the processor can then transmits to the beam steerer  21  by means of signal  115 . Using signal  115 , the processor can control the position of output beam  23  striking the surface of the workpiece  25 , so as to create a predetermined marking pattern on the surface of the workpiece  25 .  
         [0045]    Signal  117  is generated by beam steerer  21  and sent to the processor  101  responsive to the position of the output beam  23  on the workpiece  25 . Signal  131  is generated by the materials handling unit  27  responsive to the position of the workpiece  25  and sent to the processor  101 , while signal  121  is generated by the processor  101  and sent to the materials handling mechanism to have it change the position of the workpiece  25 . If the laser  11  is operated using a Q-switch, the operation of the Q-switch may be controlled by signal  103  from processor  101  or directly controlled by a signal  123  from the beam steerer  21 , bypassing any delay by the processor  101 .  
         [0046]    If the laser  11  comes equipped with an integral controller, many of these functions can be placed in the correct settings, independent of the external processor  101 , by using the integral controller. Other input signals to the processor  101 , such as a workpiece proximity detector, will be apparent to those skilled in the art.  
         [0047]    The preferred method of carrying out the present invention can be described with reference to FIG. 3. A Q-switched, diode-pumped laser generator  11  operating in the Gaussian mode and with a wavelength of 1064 nanometers is employed. Use of diode laser is preferred because a diode-pumped laser is more efficient, longer lasting, more durable and has a higher-quality laser output than the use of a flashlamp pumping scheme. Use of a Q-switch scheme is advantageous because it causes pulse repetition emission with a relatively high peak pulse energy. Other types of laser generators may be used, so long as they emit a beam at a wavelength that is capable of generating sufficient heat to melt a portion of the desired layer of the workpiece. To ensure pulse-to-pulse uniformity in the marking process, the laser is made to operate in the pre-lasing mode; that is, the Q-switch is set such that between pulses, the laser generator  11  is running slightly over threshold in continuous-wave mode with a low output power. When the Q-switch is turned off, a laser pulse is built up from the already present reproducible continuous-wave pre-lasing signal instead of from a spontaneous emission in the lasing cavity when no prelasing is present. The prelasing operation ensures that the pulse-to-pulse noise is less than about 5% and preferably less than 2%.  
         [0048]    In a preferred embodiment, the beam  13  emerging from laser generator  11  is linearly polarized, and beam  13  passes through a beam collimator and expander  31 . Beam expander  31  alters the size of the beam to achieve the desired beam spot size after the beam is focused on the surface of the workpiece. This size of the spot on the surface of the workpiece determines the actual beam intensity used to mark the workpiece surface, with small spot sizes resulting in markings of high resolution. The expanded beam  33  leaving the beam expander  31  passes through a rotatable optical plate  35 , which has the effect of rotating the polarizing plane of the beam, and then through shutter  133 , which is employed as a safety device to block the laser beam when the laser marking apparatus is not in use. The laser generator  11 , beam expander  31  and shutter  133  receive signals from and send signals to processor  101  responsive to desired or actual settings or levels of operation.  
         [0049]    The beam  37  enters the polarizing beamsplitting cube  41  where the beam is separated into its p- and s-polarization components, which component beams emerge from different faces of the cube. Although either the p- or s-component beams could be used to mark the workpiece, in a preferred embodiment, the s-component is used. While the p-component passes linearly through the cube  41 , the s-component is reflected at an angle, and exits a different face of the cube where it passes to optical plate  47 . The beam  51  exiting optical plate  47  is monitored by a beam sampler  53  in which a sample  16  of the incident beam is removed and directed to detector  57  which generates a responsive signal  58  that is used to create a display on meter  59  or as an input to processor  101 . The intensity of the beam  23  striking the workpiece can be varied in response to signal  58  by rotating rotatable optical plate  35  to attenuate the beam  37  to a greater or lesser degree instead of having to alter the controls of the laser power supply with the resulting undesirable beam power instability.  
         [0050]    Laser beam  19  from the sampler  53  passes to beam steerer  21 , which in a preferred embodiment is a galvanometer. The beam steering module, acting pursuant to a signal  115  from processor  101 , positions and focuses beam  23  on the surface of the workpiece  25 , and on receiving appropriate instructions from the processor  101 , the beam steering module scans the beam across the surface of the workpiece  25  while signals sent to laser  11  initiate laser pulses at appropriate times in the marking cycle to inscribe the desired marks or pattern of marks on the surface of the workpiece  25 . As the laser beam is preferably in the form of pulses, scanning the beam  23  across the surface of the workpiece  25  according to a predetermined pattern input into the processor  101  produces laser-induced dot-like marks along the path of the scan. By scanning the laser beam along closely-spaced multiple lines and controlling the points at which the pulses strike the workpiece, alphanumeric patterns can be formed on the workpiece for identification purposes. The spacing between two adjacent marks is determined by the laser Q-switch frequency and the beam scanning speed, both of which can be controlled by the processor. The spacing of the marks determines the visual contrast of the marked patterns on the surface of the workpiece.  
         [0051]    In the materials handling unit  27 , a workpiece is moved into position for being marked, preferably held in a fixed position while processor  101  signals for beam steerer  21  to vary the position of the beam striking the workpiece, so as to form the desired mark (surface deformation) or pattern of marks. Although workpieces can be handled individually, the materials handling unit typically comprises a magazine or cassette designed to hold multiple workpieces, which workpieces are individually marked. When all of the workpieces in the cassette or magazine have been marked, the magazine or cassette of marked workpieces is replaced with one containing unmarked workpieces.  
         [0052]    [0052]FIG. 4 depicts front and side views of a typical arrangement of the apparatus of the present invention. Cart  301  houses a diode pump  303  for laser  11 . The beam from laser  11  passes through beam conditioning module  15  (not to scale) and a select beam passes to beam steering module  21 , where it emerges as beam  23  directed to the workpiece  25 . Workpiece  25  is positioned for marking by materials handling unit  27 , which is controlled by processor  101 , in this case, a laptop computer. The materials handling unit is capable of handling caddies or cassettes  29  of workpieces as part of a continuous operation.  
         [0053]    Another aspect of the invention relates to marking a workpiece, and typically a multi-layered workpiece such as one which comprises a substrate, a first layer placed over said substrate, said first layer having a first melting point, one or more additional layers placed over said first layer, said additional layers having melting points higher than said first melting point, and a protective layer placed over said additional layer. The invention is especially suited to marking multi-layered workpieces such as magnetic storage media such as a computer hard disk, in which the first layer comprises nickel-phosphorous, the additional layers comprise a chromium layer and a magnetic layer, and the protective layer comprises carbon. The invention is particularly advantageous when applied to marking a finished computer hard disk, which typically comprises multiple layers on an aluminum substrate, as shown in FIG. 5.  
         [0054]    As shown in FIG. 5. the topmost layer  201  of such a disk is commonly an organic lubricant, a few nanometers thick. Below the lubricating layer  201  is a carbon layer  203  about 10 to 30 nanometers thick that serves as a protective coating for the magnetic layer  205  underneath. The magnetic layer  205  comprises mainly cobalt, with some chromium and small traces of platinum and/or tantalum, and is commonly 50 to 70 nanometers thick. Below the magnetic layer  205  is a chromium layer  207  which is typically 100 to 200 nanometers thick, followed by a nickel-phosphorus layer  209  which typically is about 10 micrometers thick, on a substrate  211  such as aluminum or other durable material.  
         [0055]    A disk marked pursuant to the present invention will have visible surface deformations created by laser-induced rippling in the nickel phosphorous layer  209 , while the integrity of the protective carbon layer  203  at the point of said deformation is substantially maintained. The laser will have a wavelength from 400 to 10,000 or more nanometers, a duration of  30  to  120  nanoseconds, and a pulse frequency of 1 to 100 kilohertz. In a preferred embodiment, the disk will have been marked by a laser having a wavelength of 1064 nanometers, a pulse length of about 50 nanoseconds, and the fluence of the beam at the surface of the workpiece is within the range of 0.5 to 1.5 joules/square centimeter, preferably 0.8 joule/sq. cm., in a laser spot size having a diameter from 10 to 30 micrometers, and preferably 15 micrometers.  
         [0056]    The laser can be either continuous wave, or pulsed. Preferably, the laser beam is made up of pulses, such that scanning the laser beam across the workpiece surface results in the formation of laser-induced dot-like marks along the line of scan, as shown in FIG. 6. A laser having a spot size of about 30 micrometers was used to form the marks shown in FIG. 6, and within each mark structure there are ripples which are more prominent than others.  
         [0057]    By scanning the laser along closely-spaced multiple lines, patterns of letters and numbers can be formed for labeling or marking purposes, as shown in FIG. 7. The spacing between two adjacent marks is determined by the laser Q-switch frequency and the beam scanning speed. This spacing will subsequently affect the visual contrast of the marked patterns on the workpiece surface. However, the scope of the present invention includes the use of a continuous laser, which will produce a continuous marking line on the surface of the workpiece.  
         [0058]    During a typical laser marking process, the topmost lubricating layer ( 201  on FIG. 5) at the spots where the intense laser pulses strike the workpiece surface would have been evaporated off. However, the visual contrast obtained by use of the present invention is not due to the loss of the lubricating layer. In FIG. 8, the desired visual contrast has been obtained on a workpiece where the lubricating layer is not originally present.  
         [0059]    [0059]FIGS. 9 and 10 show a typical surface morphology, and related data profiles of cross-sections of atomic force microscopy images of the ripple structure formed during laser marking, FIG. 9 being marking formed in accord with the present invention, and FIG. 10 showing the results from use of a higher intensity beam which melts the protective coating. The laser fluence used was about 1 J/sq. cm for FIG. 9 and 2.7 J/sq. cm for FIG. 10. In both cases, a circular ripple structure was obtained. Such a structure is linked to the axial-symmetrical Gaussian-shaped intensity distribution of the laser beam. The ripple periodicity is around 1 to 2 micrometer. No micro-cracks were seen in the vicinity of the structure, and the magnitude of surface deformation around the rim of the ripple structure subsides gradually towards the non-irradiated region. Due to the smaller rippling structure formed, compared to that formed using a spot size of about 30 mcrometer (see e.g., FIG. 6), the outwardly radiating ripples are more uniform than when a larger spot size is used.  
         [0060]    [0060]FIG. 9 b  is a data profile of a cross section taken along line b traversing the central portion of the ripple structure on FIG. 9. The lack of significant peaks or valleys in the profile indicates that the central region of the ripple structure is at about the same level as the surface of the workpiece. FIGS. 9 a  and  9   c  are data profiles of cross sections taken along lines a and c traversing the ripple structure depicted in FIG. 9 on either side of the central portion. The similar heights and depths of the peaks and valleys of the data profile indicates that the rippling occurs quite symmetrically about the surface level of the workpiece, demonstrating that the volume of the workpiece material was more or less conserved before and after laser irradiation.  
         [0061]    Since the melting points of carbon—3800 K, cobalt—1768 K, platinum—2041 K, tantalum—3290 K and chromium—2130 K are all higher than that of the nickel-phosphorus—1200 K, the fluence of the laser was such that the laser only melted part of the nickel phosphorus layer while the upper two metallic layers and the carbon layer still remained reasonably solid. The interfacial stress exerted by the upper two metallic layers and the carbon layer acts as a restoring force controlling the movement of melted nickel phosphorus. Within a confined space, the volumetric change during rapid localized melting and subsequent re-solidification therefore brings about the rippling observed.  
         [0062]    [0062]FIG. 10 b  is a data profile of a cross section taken along line b traversing the central portion of the ripple structure on FIG. 10. FIG. 10 b  shows a laser-induced structure with a different surface morphology from FIG. 9 b . The structure of FIG. 10 b  has a raised circular central region surrounded by an annular rippling region. The data profiles of cross sections of the ripples are depicted in FIGS. 10 a  and  10   c , representing cross sections taken along lines a and c traversing either side of the central portion of the ripple structure depicted in FIG. 10.  
         [0063]    [0063]FIG. 11 a  is an auger electron spectroscopy depth profiling performed on the center of the ripple structure of FIG. 9, and FIG. 11 b  is an auger electron spectroscopy depth profiling performed on a non-irradiated region on the same disk specimen. On the left axis of FIG. 11 a , it can be seen that the concentration of carbon nearest the surface (at the least depth) of a ripple approximates    100   %, similar as what is shown in FIG. 11 b  for the concentration of carbon nearest the surface of a non-irradiated region of the same disk. Thus, auger electron spectroscopy depth profiling performed on the center of the structure in FIG. 9, when compared to profiling performed on a non-irradiated region on the same specimen, indicates that the carbon layer has remained very much intact, and that the interfaces between the metallic layers are well preserved.  
         [0064]    Referring now to FIG. 12, there is depicted an atomic force microscopy image of a mark made on a hard disk using a laser fluence above that of the present invention, and three locations are marked on the surface, namely, a the center of the circular region, b the rippling region and c a non-irradiated region. Auger electron spectroscopy was performed on the surface at these locations and the results are shown in FIG. 13. The plot of the results indicates that at locations b and c, the surface layer contains only carbon, whereas at the surface at point a contains a mixture of carbon, chromium, and cobalt. Thus, at the circular central region of a mark formed using a laser fluence higher than that taught in the art, one or more of the upper metallic layers have melted together with the nickel-phosphorus layer, resulting in severe mass diffusion and material mixing. Due to the fact that the different layers have either been melted together or inter-diffused into one another, no surface rippling was obtained. The interfaces between different layers are almost no longer distinguishable. As the carbon layer, serving as a protective layer for the disk, has already been mixed with other materials, such a laser-induced deformation can lead to potential disk failures.  
         [0065]    On the other hand, auger electron spectroscopy data profiles of scans taken at various depths of the central portion of the ripple structure of FIG. 9 are shown in FIG. 14. The auger electron spectroscopy was carried out repeatedly after every 5 minutes of sputter etching, and the spectrum at the respective depth was plotted. The first spectrum at the bottom corresponds to the surface level, while the topmost spectrum corresponds to a depth obtained after  95  minutes of sputter etching. The peak for carbon at the lowermost profiles remains well differentiated from the peaks for cobalt. Similarly, the peaks for cobalt remain well differentiated from the peak for chromium in the profiles at the corresponding depth. Subsequent profiles show that there is some interdiffusion of the chromium and nickel-phosphorous interfaces. Thus, the laser-induced deformation carried out according to the teachings of the present invention have been limited to formation of surface rippling required in the marking process, while leaving the protective upper layer substantially intact and functional.  
         [0066]    While the invention has been particularly shown and described with reference to certain preferred embodiments, it will be understood by those skilled in the art that various alterations and modifications in form and detail may be made therein. Accordingly, it is intended that the following claims cover all such alterations and modifications as they fall within the true spirit and scope of the invention.