Abstract:
A burnishing head comprises at least two rails, each rail having an inner wall and an outer wall. The outer walls are at an angle relative to one another and relative to a central axis of the burnishing head. This angle permits the burnishing head to exhibit improved recovery time if it contacts a disk being burnished. The rail walls are vertical, and the corner between the rail walls and the top surface of the rails is sharp.

Description:
This application claims priority based on our U.S. Provisional patent applications 60/773,190 (filed Feb. 13, 2006) and 60/773,266 (filed Feb. 14, 2006), incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to burnishing heads for burnishing magnetic disks and methods for burnishing magnetic disks. 
     Magnetic disks are typically manufactured with the following method:
     1. An aluminum alloy substrate is electroless plated with a nickel-phosphorus alloy.   2. The plated substrate is textured.   3. One or more underlayers, one or more magnetic layers, and one or more protective overcoats are deposited on the plated, textured substrate. (It is also known to deposit other layers onto the substrate as well.)   4. A lubricant is applied to the protective overcoat.   5. The resulting disk is then burnished.   

     During burnishing, the disk is rotated, and a burnishing head flies over the disk to remove undesired contaminant particles. Such contaminant particles can comprise Al 2 O 3  generated during a “kiss-buff” process or an edge buff process. Enhancing particle removal efficiency during burnishing is an important process objective.  FIG. 1  illustrates a prior art burnishing head  10  burnishing a magnetic disk  12 . Head  10  is held by a suspension  14  while disk  12  rotates in a direction  16 . Head  10  is held at an angle α of about 15° relative to the motion of travel of disk  12 . During burnishing, head  10  removes contaminant particles from the surface of disk  12 . 
       FIGS. 2A ,  2 B and  2 C illustrate side, rear and bottom views of head  10 . As can be seen, head  10  comprises first and second rails  18   a,    18   b  extending from a bottom surface of head  10 . Rails  18   a,    18   b  are parallel to a central axis C of head  10 , and comprise an inclined portion or ramp  20  that assists head  10  to “fly” above disk  12 . Rails  18   a,    18   b  have a height H 1  of about 100 μm, and are formed by a mechanical machining process. Rails  18   a,    18   b  have side walls  22  that are substantially vertical with respect to the body of head  10 , and have a sharp rail corner. (By rail corner we mean the corner where rail side walls  22  meet rail air bearing surface  24 .) 
     It is also known that burnish heads have been made with etching process. Rails formed by etching have a height of about 5 to 10 μm. (It would take a long time to etch rails of substantially greater height.) Some prior art burnishing heads formed by etching have rounded rail corners and some prior art heads formed by etching have fairly sharp rail corners. Also, some prior art burnishing head rails formed by etching have side walls at an angle, e.g. about 60° with respect to the horizontal, whereas other prior art burnishing head rails formed by etching have side walls close to vertical. However, to the best of our knowledge, the etching process conditions used to form prior art rails that have vertical walls when the rails are only about 10 μm high, would result in sloped walls if used to form rails that were much higher, e.g. 75 μm high. 
     (Although burnishing head  10  comprises a pair of rails, it is also known in the art to provide burnishing heads having burnishing surfaces such as those shown in U.S. Pat. No. 4,845,816, issued to Nanis, U.S. Pat. No. 6,267,645, issued to Burga, and U.S. Patent Application publication US 2002/0029448A1.) 
     Burnishing heads differ in structure and function from read-write heads. An example of a read-write head is discussed in U.S. Pat. No. 5,949,614, issued to Chhabra. A read-write head is incorporated into a disk drive. Such a head flies over a magnetic disk during use. A transducer provided at the trailing end of the read-write head reads data from and writes data to the disk. Burnishing heads typically lack such transducers. 
     Another type of head is used to detect asperities on a magnetic disk surface. Such a head comprises a sensor for sensing mechanical impact of the head against asperities. Burnishing heads typically lack transducers of this type as well. Such heads are discussed in by Burga et al. in U.S. Pat. Nos. 5,963,396 and 6,138,502. 
     Unfortunately, from time to time, burnishing head  10  may contact disk  10  during burnishing and stay in the avalanche mode. It takes time for head  10  to “recover” from such contact, resume flight over the surface of disk  10 , and thereafter resume burnishing disk  10 . It would be desirable to reduce the amount of time required for head  10  to recover. Also, the burnishing head  10  shows unstable flying characteristics near the outer edge of the disk  10  since the slider body is not parallel to the direction of the air flow under ABS. This is undesirable for burnishing operation because unstable flying of the head could result in head-disk interaction causing defect generation on the disk. It would be desirable to improve these aspects of burnishing heads. 
     SUMMARY 
     A burnishing head in accordance with our invention comprises rails having outer side walls that are at an angle with respect to a central axis of the head. This is desirable for particle removal. Also, since the central axis of the slider is parallel to that of suspension, it takes less time for the head to recover when head-disk interaction occurs due to better flying characteristics. 
     In one embodiment, the outer walls of the side rails have an angle between 5 and 25° (and typically 15°) with respect to the central axis of the head. It has been demonstrated that this angle prevents contaminant particles from embedding into the disk surface since the particles don&#39;t hit the ramp  20  first, but instead hit the edge of the rail which shoves the particles. If the particles come under the ramp  20  while the disk is spinning, the particles can embed into the disk due to vertical force exerted by the ramp. That is why the rails are at an angle between 5 and 25 degrees. 
     In one embodiment, the burnishing head is held parallel to the direction of the relative motion between the disk and the head. 
     We have discovered that increasing the height of the burnishing rails compared to prior art burnishing heads enhances performance. The rails typically have a height greater than 30 μm, and in one embodiment, between 50 and 100 μm. It is believed that the higher rail walls permit increased free space and air flow for displaced particles to be ejected from the head/disk interface without being reattached. 
     We have also discovered that providing rails with side walls close to vertical also enhances burnishing performance. In one embodiment, the side walls are at an angle greater than 75°, and in one embodiment between 80 and 90°. We believe that having steep rail side walls is superior because if the rail walls are not steep, the vertical component of the force applied by the burnishing head to the contaminant particles tends to drive the particles downward into the disk instead of sweeping the particles off of the disk surface. Also the steep side walls result in a stiffer air bearing due to increased air leakage and results in less compliance to surface abnormalities or particulates. 
     We have also discovered that ensuring that the any radius of curvature between the rail side walls and the rail air bearing surface is minimized. In one embodiment, this radius of curvature is less than 0.5 mils, e.g. between 0.5 and 0.05 mils, and typically between 0.2 and 0.1 mils. We believe that the reason it is desirable to minimize the radius of curvature is that if a rounded corner hits a particle during burnishing the particle does not receive the full desired impact. 
     A burnishing head in accordance with one embodiment of our invention comprises AlTiC. However, other hard materials can also be used, e.g. SiC or carbon. 
     These and other features of a burnishing head in accordance with our invention are described in greater detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates prior art burnishing head burnishing a magnetic disk. 
         FIG. 2A  is a side view of the burnishing head of  FIG. 1 . 
         FIG. 2B  is a rear view of-the burnishing head of  FIG. 2A . 
         FIG. 2C  is a plan view of the bottom of the burnishing head of  FIGS. 2A and 2B . 
         FIG. 3A  is a side view of a burnishing head in accordance with the invention. 
         FIG. 3B  is a rear view of the burnishing head of  FIG. 3A . 
         FIG. 3C  is a plan view of the bottom of the burnishing head of  FIGS. 3A and 3B . 
         FIG. 4  illustrates the burnishing head of  FIGS. 3A to 3C  mounted on a suspension and burnishing a magnetic disk. 
         FIGS. 5A and 5B  illustrate a method for making a burnishing head. 
         FIG. 6A  is a bottom view of a burnishing head constructed in accordance with another embodiment of the invention. 
         FIG. 6B  is a rear view of the burnishing head of  FIG. 6A . 
         FIG. 7  is a table comparing flying performance of burnishing heads in accordance with the invention and burnishing heads in accordance with the prior art. 
         FIG. 8  is a table comparing burnishing performance of burnishing heads having various characteristics. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 3A to 3C  illustrate a burnishing head  100  constructed in accordance with our invention. Burnishing head  100  comprises rails  102   a,    102   b  extending from a generally planar bottom surface  104  of head  100 . Portions  106   a,    106   b  of rails  102   a,    102   b,  adjacent a leading edge  108  of head  100 , are sloped at an angle β for aerodynamic reasons. In one embodiment, angle β is 18 minutes with respect to the rest of the air bearing surfaces  110   a,    110   b  of rails  102   a,    102   b.    
     In one exemplary embodiment, rails  102  extend a height H 2  between 50 and 100 μm from surface  104 . Head  100  has a width W 1  of 60 mils and a length L 1  of 80 mils. Rails  102   a,    102   b  extend a distance greater than half of length L 1 , and typically extend length L 1  or a distance slightly less than length L 1 . Outer walls  112   a,    112   b  of rails  102   a,    102   b  form an angle γ of 15° with respect to a central axis C of head  100 . These dimensions, however, are merely exemplary. 
     Burnishing head  100  may be made of any appropriately hard material. For example, in one embodiment, head  100  can comprise AlTiC, SiC or carbon. Alternatively, head  100  can comprise a body of material such as AlTiC and a layer of SiC or carbon deposited thereon, e.g. by sputtering or CVD. (As is known in the art, AlTiC is a two-phase material comprising Al 2 O 3  and TiC.) 
     Referring to  FIG. 3C , walls  112   a,    112   b  of rails  102   a,    102   b  are close to vertical. Also, corners  114   a,    114   b  where walls  112   a,    112   b  meet surfaces  110   a,    110   b  of rails  102   a,    102   b  are typically sharp 90° angles. 
     While walls  112   a,    112   b  are illustrated as vertical (and are preferably vertical), walls  112   a,    112   b  can be slightly off vertical, e.g. at an angle greater than 75°. As explained above, the sharpness of corners  114   a,    114   b  and the vertical nature of walls  112   a,    112   b  improve the performance of head  100 . 
     During use, head  100  is mounted to a suspension  120  as shown in  FIG. 4 . A first motor (not shown) moves suspension  120  (and therefore head  100 ) in a direction  122  while a disk  124  being burnished is rotated by a second motor (also not shown). During burnishing, disk  124  moves at a rate of 600 inches per second (“ips”) relative to head  100 . Head  100  typically flies at about 0.35 microinches above the surface of disk  124 . Typically, during burnishing, one starts at the ID of disk  124  and moves the head outwardly. However, in other embodiments, the head can be moved from the OD toward the ID, although this is less desirable, as it would tend to leave contaminant particles at the ID of the disk, and this could conceivably be part of the data recording zone. 
     In the embodiment of  FIG. 4 , the central axis C of head  100  is parallel to the direction of motion  128  of disk  124 . This is in contrast to the angle α at which head  10  is mounted in  FIG. 1 . 
     In one embodiment, the rails on the burnishing head are formed by etching, e.g. using the following process:
     1. As shown in  FIG. 5A , a copper layer  200  is deposited, e.g. by sputtering, on a body of material  202 . (Body  202  is typically AlTiC.)   2. A photoresist layer  204  is formed on copper layer  200 .   3. Photoresist layer  204  is lithographically patterned. (In lieu of lithographic patterning, in some embodiments e-beam patterning is used.) ( FIGS. 5A and 5B  only show a small portion of body of material  202 . Typically, many burnishing heads are formed in body  202  simultaneously.)   4. The resulting structure is subjected to an etching step using an aqueous ferric chloride (FeCl 3 ) solution to thereby transfer the pattern in photoresist layer  204  to copper layer  200 . The remaining portion of photoresist layer  204  is then removed, e.g. with acetone.   5. Referring to  FIG. 5B , the resulting structure is then subjected to a RIE process using a mixture of fluorine and argon as the process gas. In one embodiment, the source of fluorine is SF 6 , but in other embodiments, other fluorine-containing gasses can be used. Also, in one embodiment, 20 SCCM SF 6  and 15 SCCM of argon flow into the etching apparatus. The etching process continues until etching is performed to a depth from 30 to 100 μm, and in one embodiment, between 65 and 100 μm.   6. Thereafter, the remaining portion of copper layer  202  is removed using an aqueous ferric chloride solution.   7. Body of material  202  is then cut into individual burnishing heads.   8. Portions  106   a  and  106   b  are mechanically formed on the heads.   

     Further details concerning the above-mentioned process are described in U.S. Provisional Patent Application 60/773,225, filed on Feb. 13, 2006 by Simone Guerrier, entitled “Method for Etching a Workpiece”, incorporated herein by reference. This process is merely exemplary. In other embodiments, other process can be used. 
       FIG. 6A  illustrates a burnishing head  250  constructed in accordance with an alternative embodiment of our invention. Burnishing head  250  comprises trapezoidal rails  252   a  and  252   b.  As can be seen, outer walls  254   a,    254   b  of rails  252   a,    252   b  are at an angle θ with respect to the central axis C of head  250 . Angle θ is between 5 and 25°, and typically about 15°. 
       FIG. 6B  is a rear view of head  250 . As in the embodiment of  FIG. 3 , rails  252   a,    252   b  have a height H 3  between 30 and 100 μm, and in one embodiment, 65 μm. The walls of rails  252   a,    252   b  form an angle close to the vertical, e.g. greater than 75° and in one embodiment, between 80 and 90°. 
     In the above-described embodiments, both the outer rail walls are at an angle θ with respect to the head&#39;s central axis C. It is primarily important for the rail wall closest to the OD (for the case in which the burnishing head is moved from the ID to the OD) to be at angle θ. The opposite wall of the opposite rail is typically at this angle for reasons of symmetry and flying stability. (For the case in which the burnishing head is moved from the OD toward the ID, the angle of the rail wall closest to the ID is of primary importance.) 
     As mentioned above, one of the major advantages of a head in accordance with the invention is an improvement in flyability, e.g. as shown in  FIG. 7 . In  FIG. 7  an experiment was performed in which burnishing heads were moved toward a disk OD during burnishing. The disk radius was 1.87 inches. Heads  351 ,  352  and  353  were prior burnishing heads as shown in  FIGS. 1 and 2 . As can be seen in  FIG. 7 , on the average, heads of this design could only reach about 1.855 inches before the onset of avalanching. (Avalanching occurs when the head stops flying and drags on the disk.) After avalanching, heads  351 ,  352  and  353  were pulled back toward the disk ID. As can be seen, heads  351 ,  352  and  353  did not recover and begin flying again until they were on an average radius of 1.808 inches. 
     In contrast, heads  301 ,  302  and  303  (in accordance with the design of  FIGS. 6A and 6   b ) achieved superior performance. In particular, they did not begin avalanching until they reached a radius (on average) of 1.868 inches, and they recovered at an average radius of 1.859 inches. Thus, heads of this design exhibited superior flying performance. 
     Although heads in accordance with the design of  FIGS. 6A and 6B  exhibited superior flying performance, it is also necessary for burnishing heads to exhibit good particle removal during burnishing. Heads having the  FIGS. 6A and 6B  design are not easily formed by machining. We experimented with etching techniques to determine whether such heads could be formed by etching.  FIG. 8  is a table illustrating the results achieved during experiments with burnishing heads  401  to  406 , each having selected characteristics as discussed below. Burnishing head  401  was a prior art burnishing head as shown in  FIGS. 1 and 2  formed by machining. The rails for head  401  had a height of 100 μm. During the experiments, a disk was examined with optical inspection apparatus to determine the number of contaminant particles thereon, dipped in lubricant which contained additional Al 2 O 3  contaminant particles, and examined again to get a new count of contaminant particles. The disk was then burnished with a burnishing head. During burnishing, the head swept from the ID to the OD and then back to the ID. The disk was then examined again with the above-mentioned apparatus to determine how many contaminant particles were removed. 
     Head  401  eliminated a number of contaminant particles equal to the number of particles added to the disk when it was dipped in lubricant, i.e. the number of contaminant particles removed equals 100% of the number of particles added during dipping. (During this experiment, the particles removed during burnishing were not necessarily all the exact same particles placed on the disk due to dipping. However, the number of particles removed during burnishing was the same as the number of particles placed on the disk due to dipping.) 
     Head  402  was similar to head  401 , except a) head  402  was made by etching, b) the rail height for head  402  was 10 μm, c) the rail walls were at 60°, and d) the radius of curvature at the corner of the rails for head  402  were larger (e.g. a couple of mils) than for head  401  (which had sharp corners). As can be seen, head  402  yielded poor burnishing performance, removing a number of particles equal to only 84.1% of the particles that were added during the lubricant dip. 
     Head  403  was the same as head  402 , except that the rail height was 75 μm instead of 10 μm. As can be seen, this caused the particle removal efficiency to rise to 96.0%. 
     Head  404  was the same as head  403 , except the rail corners were much sharper in head  404 . This design change caused the particle removal efficiency to rise to 98.4%. 
     Head  405  was the same as head  404 , except that the rail walls were vertical. This caused the particle removal efficiency to rise to 102.0%. (This efficiency was possible because this head removed not only a number of particles equal to what was added when the disk was dipped in the contaminant particle-containing lubricant, but also contaminant particles present on the disk before dipping.) 
     Head  406  was of the design in accordance with  FIGS. 6A and 6B . Head  406  had a rail height of 75 μm, vertical walls and sharp corners. As can be seen, head  406  exhibited a particle removal efficiency of 101.0% 
     The above-mentioned experiments show that one can form a burnishing head that achieves both good burnishing performance and good flyability. 
     While the invention has been described with respect to a specific embodiment, those skilled in the art will appreciate that changes can be made in form and detail without departing form the spirit and scope of the invention. For example, the burnishing head can be made using different manufacturing techniques, have different mechanical dimensions, and be made from different materials. A burnishing head in accordance with our invention need not have all the characteristics, and meet all of the objectives set forth above. Also, one can rotate a disk at different velocities during burnishing. Accordingly, all such changes come within the invention.