BACKGROUND 
     The present invention relates to disc drive systems and more particularly to a method and apparatus for burnishing asperities or irregularities from the surface of a disc. 
     In data processing systems, magnetic disc drives are used frequently as data storage devices. Data is written onto a rotating magnetic disc by an adjacent read-write head for later retrieval by the same head. The read-write head is located on a slider body, which is mounted to one end of a translatable arm that moves the head in a generally radial direction across the surface of the disc. As the disc spins, the read-write head flies above or below the surface of the disc, with the distance between the head and the surface of the disc depending on the rotational speed of the disc, the elastic force of the arm&#39;s suspension, and the shape and surface features of the slider body. 
     With the disc spinning at thousands of revolutions per minute (rpm), any unwanted interaction between the head and the disc surface can cause both short-term and long-term operational problems. This interaction can range from a thermal asperity to a full head crash. Consequences of contact or near-contact can include a failed read or write process, a temporary performance loss of the read-write head, a permanent defect on the disc surface, or total failure of the drive. These defects must be reduced or removed to provide sufficient clearance for the read-write head throughout the life of the product. Therefore, steps must be taken during the manufacturing process to flatten the disc surface as completely as possible, thereby improving product life and avoiding catastrophic head crashes. Typically, this is done by a burnish process after the disc media is fabricated. 
     During burnishing, the disc is rotated and the arm with the attached burnishing head is translated across the disc surface between an inner and outer diameter. The burnishing head is designed to fly close to the disc so as to physically contact defects protruding from the disc surface. The head is typically designed with burnishing pads and side rails on a contact surface projecting toward the disc to cut asperities and deflect loose particles as the disc rotates. 
     In combination with burnishing, a glide testing apparatus is also used to verify that the disc has been burnished sufficiently to meet quality and reliability requirements. The flying height of the glide head is typically lower than the operating height of the read-write head in the final product. The purpose of the lower flying height is to ensure removal of defects with the goal of improving quality and extending the useful life of the drive. A piezoelectric or thermal sensor or similar sensing means on the glide head is triggered each time that it encounters a defect on the surface. A control device electrically connected to the sensing means and the translator mechanism records the location of each defect in memory. 
     The distance between the disc and read-write head has necessarily decreased with advances in disc drive technology. The read-write head in modern disc drives flies nearly in contact with the disc at all times during normal operation. Therefore, to burnish each operative surface of the disc well below the design clearance of the read-write head, the burnishing methods and the burnishing head must also be improved to meet the increased demands of discs with higher data density. 
     SUMMARY 
     A burnishing head for burnishing and cleaning the surface of a disc includes a slider body having a top mounting surface, bottom surface, burnishing pads, and first and second side rails, which project from the bottom surface of the slider body. The side rails each have an inner surface and outer surface, with at least the first side rail having a serrated outer surface. 
     A disc burnishing apparatus includes a burnishing head, a rotation mechanism for rotating a disc, and a translation mechanism for sweeping the burnishing head across the surface of a disc as the disc is rotated. The burnishing head includes a slider body, an array of burnishing pads, and first and second side rails, at least one of which has a serrated outer surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a top perspective view of a disc burnishing system with a side rail having a serrated outer surface. 
         FIG. 1B  is a side view of the disc burnishing system shown in  FIG. 1A , including the burnishing head with a side rail having a serrated outer surface. 
         FIG. 1C  is an enlarged view of a portion of the burnish head shown in  FIG. 1B . 
         FIG. 2A  is a perspective view of the bottom surface of the burnish head with two side rails and rectangular serrations on the outer surfaces of both rails. 
         FIG. 2B  is a perspective view of the bottom surface of the burnish head with two side rails and triangular, sawtooth-like serrations on the outer surfaces of both rails. 
         FIG. 3A  is a bottom view of the burnish head with two side rails, each with rectangular serrations on the outside surfaces of both rails. 
         FIG. 3B  is a bottom view of the burnish head with two side rails, each with triangular, sawtooth-like serrations on the outside surfaces of both rails. 
         FIG. 3C  is a bottom view of the burnish head with two side rails, each with irregular serrations on the outside surfaces of both rails. 
     
    
    
     DETAILED DESCRIPTION 
     A more efficient burnish process for data storage media such as magnetic discs can reduce the cost of manufacturing by decreasing the number of burnishing cycles necessary on a single machine to achieve the desired clearance. Alternatively or in tandem with a decreased number of cycles, cost savings may also be seen by reducing the number of machines necessary to maintain an adequate rate of production, thereby decreasing the required capital investment. The gains in burnishing efficiency and resulting reduction in clearance may also be leveraged by increasing the recording density of discs, which have the ultimate effect of increasing data storage capacity of disc drives. One method of improving the burnishing process is through the use of burnishing heads with improved cutting efficiency and loose particle deflection and retention. 
       FIG. 1A  schematically depicts disc burnishing system  10 , which performs a burnishing process on a surface of magnetic disc  12 . Burnishing system  10  includes disc  12  to be burnished by burnishing head  20 . Disc  12  is rotated around center  14  in direction  16  by means of rotation mechanism  18 . Adjacent to disc  12 , burnishing head  20  with serrated side rail  22  is mounted to translator arm  24 . Translator arm  24  comprises swing arm  26  and suspension system  28 . Swing arm  26  is mechanically connected at trailing end  36  to translation mechanism  38 . Swing arm  26  is operably connected at the opposite end to suspension system  28 . It should be noted that in  FIG. 1A , head  20  and arm  24  have been magnified relative to disc  12 . 
     In this example of suspension system  28 , load beam  30  connects elastically to flexure  32 . Dimple  34  on load beam  30  protrudes toward flexure  32 , permitting head  20  to move with the topography of disc  12 . The elastic force of suspension system  28  counteracts the air pressure pushing burnishing head  20  away from disc  12 , resulting in burnishing head  20  flying at a substantially constant height over the surface. Data may be recorded on both the top and bottom surfaces of disc  12 , in which case a similar suspension and burnishing head may be provided below disc  12  and operated in tandem with suspension system  28  and burnishing head  20  to burnish the bottom surfaces. Other suspension systems may be substituted for suspension system  28 . 
     As disc  12  rotates in the direction indicated by arrow  16 , translation mechanism  38  moves translator arm  24  in an arc in the direction shown by arrow  40 . This achieves the desired effect of sweeping burnishing head  20  across the top or (bottom) surface of disc  12  between inner radius  42  and outer radius  44 . Arm  24  may be translated continuously during rotation of disc  12  or in a predetermined distance/time combination such that burnishing is performed in concentric regions of disc  12 . 
       FIG. 1A  also illustrates the types of surface defects that need to be removed by the burnishing process. These defects include asperities  46 , loose particles  48 , and contaminants  50 . 
       FIG. 1B  and  FIG. 1C  depict a side view of disc  12 , burnishing head  20 , and translator arm  24  shown in  FIG. 1A . Burnishing head  20  comprises slider body  52  mounted to bottom mounting surface  53  of flexure  32 , and comprises several features including serrated side rails  22  and tapered leading edge  64 . 
     As seen in  FIGS. 1A and 1B , air dragged by the spinning of disc  12  flows toward and under burnishing head  20 . Air flowing under side rails  22  first encounters tapered leading edges  64 , which are shaped to provide lift. The surfaces of side rails  22  proximate to disc  12  act as air bearing surfaces creating lift from the passing air, causing burnishing head  20  to fly at a substantially constant height over the surface of disc  12 . Burnishing head  20  effectively burnishes and cleans the surface of disc  12  by keeping this height as low as possible without damaging disc  12 . This allows the features projecting from surface  54  of burnishing head  20  to reduce or cut off asperities  46 , collect contaminants  48 , and deflect or collect loose particles  50  located on the surface of disc  12 . 
       FIG. 1C  depicts an exploded view of some of the features of burnishing head  20  depicted in  FIG. 1B . These magnified features include side rail  22  with serrated outer surface  58 . Serrated outer surface  58  with teeth  60  and notches  62  improve both the overall cutting and cleaning performance of burnishing head  20  compared to smooth or non-serrated side wall outer surfaces. Serrated outer surface  58  provides for increased surface area for reducing and cutting asperities  46  and more area to deflect and collect loose contaminants  48  and particles  50  from the surface of disc  12 . Serrated outer surface  58  can take a variety of forms, with teeth and notches of different shapes. Some examples are shown in  FIGS. 2A-3C . 
       FIG. 2A  depicts a bottom perspective view of burnishing head  20  with rectangular serrations on outer surfaces  58  of both side rails  22 . Side rails  22  and a plurality of burnishing pads  66  project from surface  54  of burnishing head  20 .  FIG. 2A  depicts a bottom perspective view of burnishing head  20  where serrated outer surfaces  58  define teeth  60  and rectangular notches  62 .  FIG. 2B  depicts a bottom perspective view of burnishing head  20  where serrated outer surfaces define teeth  60  and triangular notches  62 . Teeth  60  and notches  62  may also have shapes such as trapezoidal (as shown in  FIG. 16 ) other polyhedral and irregular shapes.  FIGS. 2A and 2B  also show burnishing pads  66  with rectangular and triangular cross sections projecting from surface  54  although other regular or irregular burnishing pad cross sections can also be used. In  FIGS. 2A and 2B , side rails  22  and burnishing pads  66  have been magnified relative to the remainder of burnishing head  20  in order to illustrate some of the features and benefits of serrated outer surface  58 . 
     Serrated outer surfaces  58  shown in  FIGS. 2A and 2B  provide improved burnishing performance as a result of the additional cutting surface area provided by teeth  60  and notches  62 . Various surface defects on disc  12  will come into contact with one or more of these features during the burnishing process. The additional cutting surface areas of teeth  60  as currently disclosed result in more asperities  46  being removed per rotation of disc  12  compared to a burnishing head with straight sided (non-serrated) siderails. With serrated outer surface  58 , many asperities  46  can be reduced or cut off by teeth  60  before reaching burnishing pads  66 , thereby producing a flatter disc surface with fewer protrusions, thus reducing the potention number of cycles required to complete the burnishing process. 
     Performance of burnishing head  20  may be further enhanced by changing the angles at which teeth  60  and notches  62  project from side rails  22  and surface  54 . In the embodiments shown in  FIGS. 2A and 2B , the walls  72  of notches  62  are perpendicular to surface  54  and angle θ equals 90° However, teeth  60  and notches  62  need not be symmetric, nor is it required that θ be 90° as shown. Teeth  60  and notches  62  may project obliquely from bottom surface  54 . For example, if θ is less than 90°, teeth  60  act like a scraper against the surface of disc  12 , cutting more asperities  46 . However, if θ is greater than 90°, teeth  60  act like a broom against the surface of disc  12 , dragging more particles along as disc  12  spins. Angles can be adjusted as necessary to meet manufacturing requirements. 
     Cutting performance can be adjusted by varying the shapes of teeth  60  and notches  62 . Changing these shapes alters the angle of attack of each wall of teeth  60  and notches  62 , which impacts the cutting performance. 
     The relative angle of attack of individual teeth  60  and notches  62  can also be manipulated by increasing or decreasing the overall angle α at which burnishing head  20  is mounted relative to translation arm  24 . Angle α is the angle formed between longitudinal axis  68  of burnishing head  20  and longitudinal axis  70  of translator arm  24 .  FIGS. 2A and 2B  depict angle α as being aligned, e.g. α=0°, however this angle can be adjusted by up to 45° in either direction to optimize performance of burnishing head  20 . Cutting surface area can be enhanced for a particular application by changing angle α to increase or decrease the approach angle of head  20 . In certain embodiments where α≠0°, burnishing head  20  is configured such that more defects on disc  12  strike serrated outer surface  58  first instead of leading edge or pads  66 . In some embodiments, such as the one illustrated in  FIG. 2B , increased cutting surface area is achieved even when α=0° because of triangular teeth  60  and notches  62  and the circular motion of disc  12 . 
     The cutting performed by serrated outer surface  58  reduces the dependency on burnishing pads  66 . The limited surface area available on bottom surface  54  restricts the available cutting area of burnishing pads  66 . Serrated outer surface  58  leaves behind a lower density of asperities  46  after encountering teeth  60  and notches  62  on outer surface  58 . By the time that asperities  46  reach burnishing pads  66  on subsequent rotations of disc  12 , a higher percentage of remaining asperities  46  are already cut and overall surface smoothness is improved. 
     Not only is cutting performance enhanced by serrated side rails, deflection and accumulation of loose particles  48  and contaminants  50  (shown in  FIG. 1A ) is also improved. Similar to the enhanced cutting performance, improved cleaning is achieved due to the greater contact area on outer surface  58  and plurality of possible contact angles created by teeth  60  and notches  62 . Some loose particles  48  strike side rail  22  in a similar manner to asperities  46 . The plurality of contact angles and increased surface area on outer surface  58  creates more locations to strike particles  48  and impart enough force to deflect them off the surface of disc  12 . Removal of particles  48  is also improved from the presence of notches  62 . 
     Serrated outer surfaces  58 , provide notches  62  of various shapes at several locations to collect particles  48  and contaminants  50 . Notches  62  can be shaped to act like reservoirs collecting particles  48  and other contaminants  50 , preventing buildup in one location. This can allow burnishing head  20  to be used for a longer cycle time between cleaning. In addition, collecting these defects on outer surface  58  may allow better flying stability of burnishing head  20 . 
       FIGS. 3A-3C  depict the interaction of various surface defects of disc  12  with features of the invention.  FIG. 3A  depicts a bottom view of burnishing head  20  with rectangular teeth  60  and resulting notches  62 .  FIG. 3A  is similar to the embodiment depicted in  FIG. 2A .  FIG. 3B  depicts a bottom view of burnishing head  20  with triangular sawtooth-like teeth  60  and resulting notches  62 . The triangular serrations can be any form of triangle, including right, equilateral or isosceles.  FIG. 3B  is similar to the embodiment depicted in  FIG. 2B .  FIG. 3C  depicts a bottom view of burnishing head  20  with a combination of rectangular, triangular, and irregular teeth  60 , along with respective notches  62 . Teeth  60  and notches  62  can be a mix of regular and irregular shapes as depicted, or it can be wholly comprised of irregular shapes. The particular selection of teeth  60  and notches  62  can be targeted to best integrate the use of a burnishing head with serrated outer surfaces into the needs of a particular manufacturing process. 
     As shown in  FIGS. 3A-3C , defects striking outer surface  58  are partially or completely cleared by teeth  60  and notches  62 . The use of serrated outer surfaces  58  on rails  22  increases the opportunities for reduction or removal of asperities  46 . Asperities have a chance of striking various edges of teeth  60  and notches  62  at a plurality of different angles instead of a single wall at a single angle. This cutting effect is depicted in  FIGS. 3A-3C  for three different embodiments of teeth  60  and notches  62 . 
     Similarly, particles  48  are also more likely to be deflected away or captured in notches  62  when compared to a straight outer edge. The greater surface area provided by serrated outer surfaces  58  also acts to deflect some particles  48 , while collecting others in notches  62 . Contaminants  50  also have more potential locations to be collected on outer surface  58  in the form of notches  62 , which provides enhanced cleaning capacity. 
     While burnishing pads  66  perform significant cutting tasks during burnishing, increased cutting efficiency and cleaning efficiency cannot be realistically achieved simply by increasing the number of burnishing pads  66 . Air must be free to pass in the voids between burnishing pads  66  or else the flying stability of burnishing head  20  is sacrificed. In each of the figures, burnishing pads  66  are arranged in a matrix pattern on the bottom of burnishing head  20  to balance cutting efficiency and flying stability of burnishing head  20 . Though pads  66  are depicted as diamond shapes in the drawings, burnishing pads  66  may be any single shape or combination of shapes depending on the application. 
     With serrated outer surfaces  58  of rails  22 , more defect cutting, deflecting of particles, and capturing of particles and contaminants occurs on outer surface  58 . As such, particles  48  and contaminants  50  are trapped or collected before they can become trapped under burnishing head  20 . Limiting the number of trapped particles between burnishing pads  66  and between burnishing pads  66  and side rails  22  maintains consistent air flow over the air bearing surfaces of side rails  22 . This results in a more consistent flying height, and in even more efficient cutting and cleaning. In addition, the enhanced cleaning capacity results in fewer loose particles, which minimizes possible damage to disc  12 . High speed contact with burnishing pads  66  or the air bearing surface of side rails  22  can result in embedding of particles  48  into the surface, causing permanent damage to disc  12 . 
     Table 1 below illustrates the improvement in burnishing efficiency achieved with burnishing heads with serrated side rails versus burnishing heads with non-serrated side rails. 
                                       TABLE 1                   Comparison of burnishing performed by serrated and       non-serrated side rails.                Head Type   Serrated rails   Non-serrated                       Glide Yield   69%   39%           Mean Hard Hit Count/100 surfaces   1.11   4.53           Mean Soft Hit Count/100 surfaces   2.19   23.76           Mean Glide Noise/100 surfaces   0.461   0.764                        
The data show a significant improvement in reducing defects by the burnishing head with serrated side rails, as can be seen by the increase in glide yield and the decrease in the various defects. Glide yield is a measure of discs passing a glide test after a defined burnishing process. The mean hard hit count per 100 surfaces is a measure of the average number of times that a glide head physically contacted an asperity during testing. The mean soft hit count per 100 surfaces is the average number of times that an asperity was high enough to affect the glide head but not high enough to make physical contact. The mean glide noise is the relative amount of background noise that the glide head measures over the entire disc surface. As shown in Table 1, burnishing heads having serrated side rails clearly show improved burnishing efficiency in all typical measurements over burnishing heads with non-serrated side rails.
 
     Certain shapes and arrangements of teeth and notches on the side rails of burnishing head  20  will exhibit better surface cleaning, while other arrangements will exhibit better cutting efficiency. The needs of a particular burnishing application will determine the selection of the shape and arrangement of notches and teeth to balance cutting and cleaning requirements and optimize the overall disc manufacturing process. Several other factors that affect burnishing include variations in initial disc quality, disc rotation speed, disc material, and burnishing material, and angle α formed by burnishing head axis  68  and translation arm axis  70 . These factors may also be taken into account in the design of the serrated side rails for a particular burnishing application. 
     The relative proportion of each defect can also affect the choice of shapes used on outer surface  58 . Discs with more particles  48  and contaminants  50  are burnished better if outer surface  58  has larger notches  62 , which act to collect these defects. In contrast, larger teeth  60  with more cutting area will more effectively burnish discs with more asperities  46 . While complex shapes may increase both cutting and cleaning efficiency, the costs of fabricating such shapes on a micron or submicron scale may also increase. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the scope of the invention as claimed. The implementations described above and other implementations are within the scope of the following claims.
A disc burnishing head includes an array of burnishing pads and a first and second side rail projecting from a bottom surface of a slider body. The side rails each have an inside surface facing the burnishing pads and an outside surface facing outward. The outside surface of at least the first side rail is serrated. The serrations define a plurality of teeth and notches on the outside surface of the first side rail for cutting, cleaning, and conditioning defects from the surface of a disc.