Abstract:
A system and method for servo formatting magnetic media using contact magnetic printing is disclosed. The apparatus includes a dipole magnet that supplies a magnetic field to a magnetic media, a holder for holding the magnetic media, a set of stampers with servo patterns and a press for pressing the magnetic media firmly against the stampers while the magnetic media and stampers are being exposed to a magnetic field and a set of elastomer pads to distribute and equalize the force of the press on the magnetic media. Servo formatting, using the contact magnetic printing apparatus, is done by first positioning the magnetic media against the stampers having servo patterns on them. The magnetic media/stampers stack are then loaded and aligned in a system for contact magnetic printing, applying a force on the magnetic media and stamper so that the two are in firm contact with each other. A sequence of magnetic fields is then applied to the stampers and magnetic media which duplicates the servo patterns of the stamper onto the magnetic media.

Description:
[0001]    This application claims priority from U.S. provisional application serial No. 60/350,181, filed on Jan. 15, 2002. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to the field of disc drive storage, and more particularly to contact magnetic printing used to create magnetic patterns on magnetic recording media.  
           [0004]    2. Description of the Related Art  
           [0005]    Conventional disc drives use magnetic properties of materials to store and retrieve data. Typically, disc drives are incorporated into electronic equipment, such as computer systems and home entertainment equipment, to store large amounts of data in a form that can be quickly and reliably retrieved. The major components of a disc drive include magnetic media in the form of a disk, read-write heads, a motor and software. The magnetic media is rotated by a motor at a constant high speed while the read-write head, which rests on a head gimble assembly, glides over the magnetic media reading and writing signals to the media. The surface of each magnetic media is divided into a series of data tracks, which are radially spaced apart and which extend circumferentially around the magnetic media disc. The data tracks store data in the form of magnetic flux transitions within the radial extent of the tracks on the disc surfaces.  
           [0006]    Typically, each data track is divided into a number of data sectors that store fixed sized blocks of user data. Embedded among the sectors on each track are servo fields that enable the disc drive to control the position of heads used to transfer the user data between the discs and a host computer. More particularly, the heads are mounted to a rotary actuator assembly which includes a coil of a voice coil motor, so that the position of the heads relative to the tracks can be maintained by the application of current to the coil by a closed loop digital servo system in response to the servo information read by the servo fields.  
           [0007]    The servo fields are written to the discs during the manufacture of the disc drives using an extremely precise servo track writer as illustrated in FIG. 1A. Conventional servo track writers include an anti-vibration table  110 , a recording media  115 , a spindle motor with a hub  120 , which rotates the media in a direction  125 , a read-write head  130 , an arm  135  and a controller  140 . Anti-vibration table  110  is a conventional table designed to reduce vibration. Spindle motor with hub  120  rotates recording media  115  at a constant rate while read-write head  130  reads and writes servo signals to recording media  115 . Read-write head  130  is attached to an arm  135  which driven by controller  140 . Controller  140  contains a laser based positioning system which moves arm  135  by receiving feedback from a closed loop detection system and engaging an actuator assembly that advances the position of the read-write head to the servo writing position. Additionally, controller  140  includes control circuitry for providing servo information to be stored in the servo fields. Since servo fields are used to define tracks, precise control and positioning of the read-write head is required during the servo field writing to the recording media  115  surface.  
           [0008]    The typical manufacturing process of magnetic disk with a servo pattern in illustrated in FIG. 1B. In a first step  150 , a disk is prepared for sputtering by being textured and cleaned. Next in step  155 , various layers including a magnetic layer and a protective layer are deposited on top of the disk. The magnetic layer usually consists of a cobalt based alloy and is used to record information via magnetic signals whereas the protective layer usually consists of a diamond like carbon layer. Next in step  160 , a lube layer is deposited over the media with the magnetic layer. In step  165 , the lubed disked undergoes a buff process wherein the media is smoothed by rubbing a pad over the top of the surface. After being buffed, the media is glide tested in step  170  for defects that could cause a head to crash thereon. Next in step  175 , the media is certified by writing signals to it and reading signals from it. Typically, after certification is done in step  175  the media is shipped to a hard drive building facility where it is servo formatted and installed into a hard drive. The servo formatting is then done in step  180  with a conventional servo writer as was discussed with reference to FIG. 1A above. Finally, in step  185  the disk is tested again and installed into a finished hard drive.  
           [0009]    In order to facilitate reliable operation of the disc drive, proper radial alignment of the servo fields is essential. If errors are introduced in the placement of the servo fields, position error signals (PES) generated by the servo system during subsequent operation of the drive are detected at corresponding frequencies. The PES is a measure of the relative position of a selected head with respect to an associated track and is used primarily during track following operations to maintain the head over the center of the track. Frequency dependent PES for a given track result in the repeated adjustment of the position of the head by the servo system in an attempt to maintain the head over the center of the track during each revolution of the disc. When such frequencies are sufficiently severe, the correction required to account for these frequencies can require a significant amount of correction limiting the overall track density that can be achieved in a disc drive design. One source of error that occurs during the servo writer process is the spindle motor, which includes bearing assemblies with characteristic frequencies that are generated from the rotation of the balls and ball cages within the inner and outer bearing raceways. These bearing frequencies can result in low frequency errors being laid down in the servo pattern.  
           [0010]    Accordingly, there is a need for an improved servo writer system and method that permits the writing of servo patterns early on in the process while reducing the number of servo data errors written to discs of a disc drive.  
         SUMMARY OF THE INVENTION  
         [0011]    This limitation is overcome by using a contact magnetic printing system and apparatus for servo formatting magnetic media used in hard drives.  
           [0012]    A system for contact magnetic printing servo patterns onto a magnetic media that does not involve the use of conventional servo writers is disclosed. The apparatus includes a dipole magnet that supplies a magnetic field to a magnetic media, a holder for holding the magnetic media, two stampers with servo patterns and a press for pressing the stampers firmly against the magnetic media while the magnetic media and stampers are exposed to a magnetic field. An elastomer pad is also used to deliver and spread evenly the force of the press on the magnetic media/stamper stack. In the case of single sided printing one stamper can be used instead of two.  
           [0013]    Additionally, a method for contact magnetic printing servo patterns onto a magnetic media that does not involve the use of conventional servo writers is disclosed. The magnetic media to be servo patterned is first positioned against two stampers so that each stamper is abutted against opposite sides of the magnetic media. In the case of single sided printing only one stamper is used and that stamper is abutted against the one side of the magnetic media that is to be servo written. The magnetic media/stampers stack is then loaded and aligned in a system for contact magnetic printing, applying a force on the magnetic media/stampers stack so that they are in firm contact with each other at the interface. A sequence of magnetic fields, of sufficient strength, is then applied for a set time to the magnetic media/stampers stack while it is subjected to the force. An example of a typical sequence of magnetic fields includes applying a first magnetic field of approximately 15 KOe in one direction for a few milliseconds and then applying a second field of approximately 3 KOe is the opposite direction for a few milliseconds. Finally, the magnetic field is removed, the magnetic media/stampers stack is unloaded from the contact magnetic printing apparatus and the stampers and magnetic media are separated.  
       
    
    
     BRIEF DESCRIPTION OF THE INVENTION  
       [0014]    [0014]FIG. 1A is a block diagram showing the writing of servo patterns using conventional servo writers.  
         [0015]    [0015]FIG. 1B is a flowchart illustrating the prior art method of printing servo patterns.  
         [0016]    [0016]FIG. 2 is a block diagram showing a contact magnetic printer using actuated magnet poles for contact pressure source in accordance with an embodiment of the invention.  
         [0017]    [0017]FIG. 3 is a block diagram showing the detailed configuration of the sample holder  245 .  
         [0018]    [0018]FIG. 4A is a top view of stamper  330 .  
         [0019]    [0019]FIG. 4B shows details of a portion of a track  450 .  
         [0020]    [0020]FIG. 5 shows the stamper in the presence of a magnetic field.  
         [0021]    [0021]FIG. 6 shows the stamper  333  along with a conventional magnetic media in the presence of a magnetic field in accordance with one embodiment of the invention.  
         [0022]    [0022]FIG. 7 is a flowchart showing the preferred method of making perpendicular contact print magnetic media in accordance with an embodiment of the invention.  
         [0023]    [0023]FIG. 8 shows further details of the CSPM step  750 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    The invention provides a system and method for creating magnetic patterns on magnetic recording media. In particular, the invention provides a system and method for servo formatting magnetic media using contact magnetic printing.  
         [0025]    [0025]FIG. 2 represents a contact magnetic printer, in accordance with one embodiment of the invention, including a frame  210 , a press  215 , a driving rod  220 , a stage  225 , a first magnetic pole  230 , a second magnetic pole  235 , a yoke  240  and a sample holder  245 .  
         [0026]    Frame  210  is a support structure typically made of sturdy materials such as iron, aluminum or stainless steel. The main function of frame  210  is to hold up the contact magnetic printer and its components. Additionally, frame  210  can include an antivibration mechanism, such as an air bearing surface, for decoupling the contact magnetic printer from the floor.  
         [0027]    Press  215  is a hydraulic press used to apply force to the sample in a uniaxial direction. Press  215  typically consists of a hydraulic press, with a pressure gauge, that is capable of applying forces of up to hundreds of tons and measuring those forces through the pressure gauge. Alternatively, other forms of presses, such as screw presses, can be used to achieve the same goals. It will be recognized by those skilled in the art that a variety of presses are available to supply force to samples which are adaptable to the constraints of the particular environment or the pressures needed for the contact magnetic printing. Driving rod  220  is typically made of a strong material such as stainless steel and is used to transfer the force supplied by press to  210  to stage  225 , which supports the first magnetic pole  230 .  
         [0028]    First magnetic pole  230  and second magnetic pole  235  are typically electromagnets that each include windings of electrical conductors and a core. The electrical conductors can be closely wound copper tubing, which allow for water cooling when operating at high electrical currents, as is well known by those skilled in the art. The core can be constructed from a material having high magnetic permeability, high saturation magnetization, low remanence and low coercivity. Similarly yoke  240  is constructed from of a material having high magnetic permeability, high saturation magnetization, low remanence and low coercivity. Both the yoke  240  and the core of electromagnets  230  and  235  can be made of materials such as permalloy or mu-metal. The faces of both first magnetic pole  230  and second magnetic pole  235  are typically flat within 25 microns over 100 milimeters in diameter area, and are parallel with each other to within 50 microns. These dimensional requirements are useful for providing uniform pressure as well as for achieving magnetic field uniformity.  
         [0029]    In one embodiment, press  215  is used to control the vertical position of the first magnetic pole  230 , which is supported by the driving rod  220  and stage  225 . In this embodiment the second magnetic pole  235  is generally fixed to the yoke  240  of the magnet. By pushing the first magnetic pole  230  in the direction of the fixed second magnetic pole  235  with press  215  a uniaxial force is created on the sample holder  245 . In an alternative embodiment the vertical position of both the first magnetic pole  230  and the second magnetic pole  234  can be adjusted. In this alternative embodiment one pole can be used for applying high forces whereas the second pole can be used for applying low forces. In another embodiment, the first magnetic pole  230  can have a first pole section and a second pole section wherein the first pole section is fixed but the second pole section is adjustable and can be adjusted to apply a force to the sample holder  245 . In another embodiment the press  215  can be positioned between the first magnetic pole  230  and the second magnetic pole  235  such that the press  215  supplies the force directly to the sample holder  245 .  
         [0030]    The combination of the first magnetic pole  230 , second magnetic pole  235  and yoke  240  form the dipole magnet while the combination of press  215 , driving rod  220  and stage  225  form the pressing assembly. The apparatus for contact magnetic printing is the combination of the dipole magnet and the pressing assembly. The dipole magnet is used to apply a uniform magnetic field between the pole faces and the pressing assembly is used to supply a force on the sample holder  245 .  
         [0031]    In an alternative embodiment, the first magnetic pole  230  and the second magnetic pole  235  can be permanent magnets. Permanent magnets can include magnetic materials such as Neodymium-Iron-Boron (NdFeB) or Samarium-Cobalt (SmCo). It will be recognized by those skilled in the art that these and other magnet designs have both advantages and disadvantages depending on the application.  
         [0032]    Sample holder  245  is designed to hold conventional magnetic media, which resemble thin circular disks with a circular hole punched out in the center, as discussed in detail with reference to FIG. 3 below. Typical dimensions of magnetic media can be thickness of approximately 3 mm, circular outside diameter of approximately 90 mm diameter and circular inside diameter of approximately 25 mm.  
         [0033]    [0033]FIG. 3 is a block diagram showing the detailed configuration of the sample holder  245  including the first magnetic pole  230 , a second magnetic pole  235 , a first elastomer pad  310 , a second elastomer pad  320 , a first stamper  330 , a second stamper  335  and a magnetic media  340 . First elastomer pad  310  and second elastomer pad  320  are made of an elastic material, such as silicon rubber, which conforms to the geometry of an object which it is pressed against. First stamper  330  and second stamper  335  contain data patterns thereon, as discussed in detail with reference to FIG. 4 below, which are transferred to the magnetic media  340 . Although the data patterns on first stamper  330  and second stamper  335  can be the same, they do not have to contain the same data patterns. In fact, since different sides of the magnetic media  340  typically require different servo patterns, first stamper  330  and second stamper  335  usually have different servo patterns. Additionally, first stamper  330  and second stamper  335  are made out of some magnetic material such as nickel, iron or cobalt and is used to direct the magnetic field as described in detail with reference to FIGS  5  and  6  below.  
         [0034]    When a force is applied to the magnetic media  340 , using the press  215 , the first elastomer pad  310  and second elastomer pad  320  are compressed and deform according to the surface profiles of first stamper  330  and second stamper  335 . As a result, uniform force is achieved at the sample holder  245  allowing for simultaneous uniform force and magnetic field at the contact.  
         [0035]    [0035]FIG. 4A is a top view of stamper  330  including patterns  410 ,  415 ,  420 ,  425 ,  430 ,  435 ,  440 ,  445  and track  450 . Stamper  330  is typically made out of a soft magnetic material such as Nickel. The patterns are typically punched into the material so that a magnetic field exiting the stamper mimics the pattern on the stamper near the surface of the stamper and as is further discussed with reference to FIG. 5 below. Patterns  410 ,  415 ,  420 ,  425 ,  430 ,  435 ,  440  and  445  are typically binary codes which translate into commands that align the head to the drive and give information about track width and location. An example of a typical binary code that is found in patterns  410 ,  415 ,  420 ,  425 ,  430 ,  435 ,  440  and  445  is discussed in detailed with reference to FIG. 4B, below. Track  450  is a collection of data located at a fixed radius from the center of magnetic media  340 , from which the head reads and writes to during one revolution of magnetic media  340 .  
         [0036]    [0036]FIG. 4B shows a portion of a track  450  at a selected radius on the magnetic media  340  between any two adjacent patterns, such as pattern  410  and  415 , illustrating the arrangement of respective servo fields  460  from patterns  410 - 445  and user data fields  465 . Each pattern  410 - 445  contains one servo field  460  for each track  450  wherein each servo field  460  preferably includes an automatic gain control field  470 , an index field  475 , a gray code field  480  and a position field  485 . The automatic gain control field  470  provides an oscillating signal that prepares the servo circuitry within a hard drive for remaining portions of the servo field  154 , the index field  475  provides an angular reference for the servo circuitry within a hard drive, the gray code field  480  provides a unique track address to indicate radial position for the track  450 , and the position field  485  provides an arrangement of servo patterns that allows the servo circuitry within a hard drive to perform intra-track positioning. It will be apparent to those skilled in the art that other servo field configurations can be readily employed, including different arrangements of servo fields as well as a dedicated servo scheme wherein one disc surface is used to store servo data and the remaining discs are used to store user data.  
         [0037]    [0037]FIG. 5 shows the stamper  330  in the presence of a magnetic field including first elastomer pad  310 , second elastomer pad  320 , first stamper  330 , a magnetic north pole  520 , a magnetic south pole  530  and magnetic field lines  540 , in accordance with one embodiment of the invention. Magnetic north pole  520  and magnetic south pole  530  are both typically iron cores surrounded by windings of copper wire. The magnetic field lines  540  point from the magnetic north pole  520  to the magnetic south pole  530  and are perpendicular to both the magnetic north pole  520  surface and the magnetic south pole  530  surface. Since first stamper  330  is made from magnetic materials and are each placed in between the magnetic field lines  540 , the magnetic field lines enter and exit first stamper  330  at perpendicular angles to both the top and bottom surface of first stamper  330 . The characteristics of the magnetic field lines  540 , in accordance with the present invention, are governed by solutions to Maxwell&#39;s equation with appropriate boundary conditions as will be recognized by those skilled in the art. Since the magnetic field lines  540  are constrained to contact any magnetic surface at right angles, the magnetic field lines  540  curve in the space between magnetic surfaces so that they both satisfy Maxwell&#39;s equations at any point in the space and the boundary value condition that the magnetic field lines  540  be perpendicular to a magnetic surface. This curving of the magnetic field lines  540  occurs as soon as the magnetic field lines  540  leave the magnetic surface and therefore the profile of the magnetic field lines  540  is different at the surface of first stamper  330  than at a position far removed from the stamper. Therefore, in order to insure that the magnetic field lines  540  have the same pattern as first stamper  330 , the magnetic media  340  is positioned as close as possible to the first stamper  330 , as further discussed with reference to FIG. 6 below. Additionally, since the first elastomer pad  310  and the second elastomer pad  320  are made of a non-conductive and non-magnetic material the magnetic field lines  540  penetrate through both elastomers as if they were free space.  
         [0038]    [0038]FIG. 6 shows the magnetic media  340  in the contact magnetic printing environment of FIG. 5 including first elastomer pad  310 , second elastomer pad  320 , first stamper  330 , second stamper  334 , a magnetic north pole  520 , a magnetic south pole  530  and magnetic field lines  540  and the magnetic media  340 , in accordance with one embodiment of the invention. Magnetic media  340  is positioned to make direct and firm contact with first stamper  330  and second stamper  335 . Since one application of this embodiment is to duplicate the stamper patterns onto the magnetic media  340  through the use of the magnetic field lines  540  which curve in free space as was discussed with reference to FIG. 5 above, it is desirable to have the magnetic media  340  as close as possible to first stamper  330  and second stamper  335  to ensure that the magnetic field lines  540  have the same pattern as first stamper  330  and second stamper  335  at the respective surfaces of the magnetic media  340 . Since the magnetic media  340  is positioned so that it abuts firmly against first stamper  330  and second stamper  335 , the magnetic field line  540  do not have a chance to disperse before reaching magnetic media  340  and consequently the patterns on first stamper  330  and second stamper  335  are preserved within the magnetic field lines when they reach the magnetic media  340 . Since the magnetic media  340  is magnetized by the magnetic field lines  540  and the magnetic field lines  540  have the same pattern as first stamper  330  and second stamper  335 , the first stamper  330  pattern and second stamper pattern  335  are each transferred to opposite sides of the magnetic media in the form of a magnetic imprint. The strength of the magnetic field is chosen so that it is high enough to magnetize the magnetic media  340  and is typically about 4500 gauss. However, the magnitude of the required magnetic field is dependent on the materials used to construct the layer.  
         [0039]    [0039]FIG. 7 is a flowchart showing the preferred method for servo formatting magnetic media using contact magnetic printing, in accordance with an embodiment of the invention. In a first step  710 , a disk is prepared for sputtering by being textured and cleaned. Next in step  715 , various layers including a magnetic layer and a protective layer are deposited on top of the disk. The magnetic layer usually consists of a cobalt-based alloy and is used to record information via magnetic signals whereas the protective layer usually consists of a diamond like carbon layer. Next in step  720 , a lube layer is deposited over the media with the magnetic layer. In step  725 , the lubed disked undergoes a buff process wherein the media is smoothed by rubbing a pad over the top of the surface. After being buffed, the media is glide tested in step  730  for defects that could cause a head to crash thereon. Next in step  735 , the magnetic media is servo formatted using contact magnetic printing (CMP) as further discussed with reference to FIG. 8 below. In step  740 , the servo-formatted disk undergoes a second buff process wherein the media is smoothed by rubbing a pad over the top of the surface. After being buffed, the media is glide tested again in step  745  for defects that could cause a head to crash thereon. Next, in step  750 , the magnetic media is certified by testing its read-write functionality. Finally in step  755 , the certified magnetic media is installed in a hard drive.  
         [0040]    [0040]FIG. 8 shows further details of the CMP step  735 . First in step  810 , the magnetic media  340  is loaded into the contact magnetic printer by aligning the magnetic media  340  with first stamper  330  and second stamper  335  on the other side of magnetic media  340 . The first stamper  330 , magnetic media  340  and second stamper  335  are positioned in between first elastomer pad  310  and second elastomer pad  320  which is between the magnetic north pole  520  and the magnetic south pole  530 . Next in step  815 , the first stamper  330 , magnetic media  340  and second stamper  335  are aligned within the center of the magnetic poles. Next in step  820 , a force is applied to the first stamper  330 , magnetic media  340  and second stamper  335  forcing all three to make firm contact with each other, such firm contact to minimize magnetic field divergence at the surface of the magnetic layer of the magnetic media when the magnetic field is applied. The force is applied with press  215  indirectly through a first elastomer pad  310  and a second elastomer pad  320 . Next in step  825  a sequence of magnetic fields are applied between magnetic north pole  520  and magnetic south pole  530  by running current through the windings in the poles. An example of a typical sequence of magnetic fields includes applying a first magnetic field of approximately 15 KOe in one direction for a few milliseconds and then applying a second field of approximately 3 KOe is the opposite direction for a few milliseconds. The first magnetic field of 15 KOe is typically called the DC erase field and is used to prepare that magnetic media  340  for servo formatting. The second magnetic field of 3 KOe is applied in the opposite direction to the DC erase field and is used to servo pattern the magnetic media  340 . Although the magnetic field strengths are chosen to be 15 KOe and 3 KOe respectively, the only magnetic field requirements are that the magnetic fields be sufficiently uniform and strong to servo format the magnetic media  340 . Typically the uniformity of the magnetic field is less than 0.5 percent. After the magnetic field is turned ON in either the DC erase or servo writing cases, the magnetic media  340  remains in the field for a waiting time of greater than a few fractions of a second in step  830 . If a pulsed magnet is used the magnetic media  340  may only remain in the field for a few milliseconds. Next in step  835  the magnetic field is removed. Finally in step  840  the first stamper  330 , magnetic media  340  and second stamper  335  is unloaded and the magnetic media  340  is moved onto the next step of the process.  
         [0041]    In an alternative embodiment the DC erase magnetic field and servo writing field can be applied in different magnetic presses. The first magnetic press can be built to go to higher fields whereas the second press can be built to apply a smaller field. One advantage of having a dual set of magnetic presses is that need to have polarity reversal capabilities is eliminated. Additionally, it may be advantages for factory throughput to have a dual set of magnetic presses.  
         [0042]    It will also be recognized by those skilled in the art that, while the invention has been described above in terms of preferred embodiments, it is not limited thereto. Various features and aspects of the above-described invention may be used individually or jointly. Further, although the invention has been described in the context of its implementation in a particular environment and for particular applications, those skilled in the art will recognize that its usefulness is not limited thereto and that the present invention can be utilized in any number of environments and implementations.