Patent Publication Number: US-6663477-B1

Title: Complaint membrane for restraining a workpiece and applying uniform pressure during lapping to improve flatness control

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates in general to an improved fixture for restraining workpieces, and in particular to improving the flatness control of a workpiece during a lapping process. Still more particularly, the present invention relates to a compliant membrane for restraining a workpiece and applying uniform pressure thereto during a lapping process to improve flatness control thereof. 
     2. Description of the Prior Art 
     Data access and storage devices (DASDs) such as disk drives use magnetic recording heads to read data from or write data to the disks as they spin inside the drive. Each head has a polished air bearing surface (ABS) with flatness parameters, such as crown, camber, and twist. The ABS allows the head to “fly” above the surface of its respective spinning disk. In order to achieve the desired fly height, fly height variance, take-off speed, and other aerodynamic characteristics, the flatness parameters of the ABS need to be tightly controlled. 
     Although a number of processing steps are required to manufacture heads, the ABS flatness parameters are primarily determined during the final lapping process. The final lapping process may be performed on the heads after they have been separated or segmented into individual pieces, or on rows of heads prior to the segmentation step. This process requires the head or row to be restrained while an abrasive plate of specified curvature is rubbed against it. As the plate abrades the surface of the head, the abrasion process causes material removal on the head ABS and, in the optimum case, will cause the ABS to conform to the contour or curvature of the plate. The final lapping process also creates and defines the proper magnetic read sensor and write element material heights needed for magnetic recording. 
     There are a number of factors that affect the accuracy of ABS curvature during the final lapping process. These include diamond size/morphology, lubricant chemistry, lapping tangential surface velocity, plate material, lapping motion/path on the plate, and other lapping parameters. In addition to these parameters, three critical conditions must be satisfied. First, it is essential that the contour of the abrasive plate be tightly controlled since, in the best case, the ABS will conform to the curvature of the plate. In addition, all components of the process, including the head/row, must be restrained without distortion during lapping. Any variance in the restraining forces will cause the parts to distort and/or elastically deform upon removal of the forces. For example, if a head or row is lapped on an absolutely flat surface while it is clamped in a fixture, the part will elastically deform to a non-flat condition when it is released. The amount of deformation is proportional to the amount of elastic distortion created when the part was initially clamped. 
     A third condition affecting the accuracy of the ABS is the lapping force, which is the amount of force exerted by the abrasive plate on the part being lapped. Ideally, the lapping force is minimized to reduce distortion during the lapping process. The holding fixture exerts forces which are normal to the plate for pushing the part against the plate, and tangential to the plate for causing the part to slide over the plate for material removal. Unfortunately, this combination of forces elastically distorts the part (e.g., the head). 
     For example, to lap a flat surface on an initially curved ABS, the normal-directed force of the flat (and assumably non-deformable) plate against the curved ABS causes the ABS to temporarily flatten. The amount of deflection or flattening of the part will depend on the magnitude, direction, and distribution of the force on the part. Under sufficiently high normal-directed force, the entire surface area of the ABS is in contact with the plate. Introducing tangential movement of the part against an abrasive flat plate causes the entire surface area of the ABS to be abraded, not just the non-flat portions of the ABS. Upon removal of the normal-directed force, the ABS will elastically return to a non-flat condition. To minimize the amount of elastic return, it is desirable to provide a low but evenly distributed, normal-directed force on the part. The desired optimum low normal force will depend on a number of factors, such as diamond size/morphology, lubricant chemistry, lapping tangential velocity, and other lapping parameters. Thus, an improved apparatus and method for accurately defining the curvature of an ABS during the final lapping process is needed. 
     SUMMARY OF THE INVENTION 
     A lapping process fixture has a base with a mounting surface and a membrane on the mounting surface for supporting a workpiece. The membrane is bonded to the mounting surface and has adhesive on its outer surface such that the workpiece adheres to it. The membrane extends across and seals an opening to a cavity inside the base. The cavity is filled with fluid that may be sealed or externally pressurized through ports in the base. The fixture restrains the workpiece to minimize distortion of its surface during processing. The workpiece is restrained from normal-directed movement by fluidic pressure such that the normal force is uniformly distributed across the surface area of the workpiece. The external adhesive on the membrane restrains the workpiece from tangential movement. 
     Accordingly, it is an object of the present invention to provide an improved fixture for restraining workpieces. 
     It is an additional object of the present invention to improve the flatness control of a workpiece during a lapping process. 
     Still another object of the present invention is to provide a compliant membrane for restraining a workpiece and applying uniform pressure thereto during a lapping process to improve flatness control thereof. 
    
    
     The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the preferred embodiment of the present invention, taken in conjunction with the appended claims and the accompanying drawings. 
     DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the features, advantages and objects of the invention, as well as others which will become apparent, are attained and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the drawings illustrate only a preferred embodiment of the invention and is therefore not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
     FIG. 1 is an isometric view of a first embodiment of a fixture constructed in accordance with the invention and shown supporting a plurality of workpieces. 
     FIG. 2 is a sectional end view of the fixture of FIG. 1 shown lapping one of the workpieces. 
     FIG. 3 is a top plan view of a second embodiment of the fixture of FIG.  1 . 
     FIG. 4 is a sectional side view of the fixture of FIG. 3 taken along the line  4 — 4  of FIG.  3 . 
     FIG. 5 is a top plan view of a third embodiment of the fixture of FIG.  1 . 
     FIG. 6 is a sectional side view of the fixture of FIG. 5 taken along the line  6 — 6  of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, a first embodiment of the invention is shown as a lapping process fixture  11 . Fixture  11  comprises a generally rectangular base  13  having a mounting surface  15  for supporting a plurality of discrete workpieces  17 . Although fixture  11  is shown supporting three rows  17  of magnetic read/write head stock, each having a plurality of air bearing surfaces (ABS) thereon, more or fewer rows may be supported by fixture  11 , as well as other types and sizes of workpieces. In addition, fixture  11  may be adapted for use with different types of processing techniques other than lapping. 
     Each workpiece  17  is located on a thin flexible sheet or membrane  21  that is slightly larger in surface area than the workpiece  17  it supports. Membranes  21  are substantially planar and parallel to mounting surface  15 . As shown in FIG. 2, each membrane  21  has an outer surface  23  and an inner surface  25 . In the preferred embodiment, outer and inner surfaces  23 ,  25  are each coated with adhesive, such that workpiece  17  adheres to the outer surface  23 , and the inner surface  25  bonds to a portion of the mounting surface  15  of base  13  of fixture  11 . Membranes  21  may utilize other forms of attachment as well, but typically comprise double-sided strips of conventional adhesive dicing tape. Each membrane  21  spans or extends across a generally rectangular, internal cavity  31  inside base  13 . Each cavity  31  has a opening  33  in mounting surface  15  that is sealed by its respective membrane  21 . Workpiece  17  is located within the surface area defined by opening  33 . 
     A fluid  35  such as a gas or liquid is located inside each of the cavities  31 . In the preferred embodiment, fluid  35  comprises pressurized air or water. Fluid  35  provides membrane  21  with a resilient outer surface  23  for supporting workpiece  17 . The cavities  31  may be filled with fluid  35  through one or more ports  37 . In one version, ports  37  are sealed after fluid  35  is pressurized. In another version, fluid  35  is pressurized via an external pressure source, such as a pump  38 , which delivers fluid  35  through at least one port  37 . When liquid is used as fluid  35 , the second port  37  may be used to purge air during the filling process. 
     In operation, fixture  11  is provided as a means of holding the rows  17  such that distortion of their ABS due to restraining or holding forces is minimized. Each membrane  21  supports one row  17  while it is processed with a lapping device  41  having a lapping surface  43 . Since the row  17  is located completely within the area defined by opening  33  in base  15 , the row  17  is fully supported by membrane  21  and is substantially restrained from movement in a direction normal to membrane  21  and mounting surface  15  by the pressure of fluid  35 . The thin membrane  21  bends elastically very easily due to its low bending moment of inertia. Because membrane  21  has very low stiffness to bending, distortion of workpiece  17  in the normal direction is low. Moreover, since the normal-directed support is provided by fluid pressure, the force is uniformly distributed across the attached surface of workpiece  17 . 
     In addition, the adhesive coating on outer surface  23  of membrane  21  substantially restrains workpiece  17  from movement in a direction that is tangential to membrane  21 . The adhesive on membrane  21  provides the tangential force needed to drag the ABS along the lap plate  41 . This allows workpiece  17  to be lapped against lap plate  41  such that its ABS will conform to the shape of lapping surface  43 . Membrane  21  provides excellent transfer of tangential force because the tangential force is in the tension axis of the material of membrane  21 . 
     Referring now to FIGS. 3 and 4, a second embodiment of the invention is shown as lapping process fixture  51 . Fixture  51  is very similar to fixture  11  and has a base  53  with a mounting surface  55 . However, fixture  51  has much larger membranes  61  for supporting larger arrays of workpieces  57  such as slider rows with ABS. As before, each membrane  61  is slightly larger in surface area than the workpiece  57  it supports and has an outer surface  63  and an inner surface  65 . In the preferred embodiment, outer and inner surfaces  63 ,  65  are each coated with adhesive, such that workpiece  57  adheres to the outer surface  63 , and the inner surface  65  bonds to a portion of the mounting surface  55  of base  53 . Each membrane  61  extends across and seals an opening  73  in a cavity  71  inside base  53 . As described for the first embodiment, cavities  71  are filled with a fluid  75  that may or may not be sealed or externally pressurized through ports in base  53 . 
     In operation, fixture  51  restrains rows  57  to minimize distortion of their ABS during lapping. The rows  57  are substantially restrained from movement in the normal direction by the fluidic pressure such that the normal force is uniformly distributed across the attached surface of rows  57 . The adhesive coating on membrane  61  substantially restrains rows  57  from tangential movement in order to drag the ABS along the lap plate. Rows  57  are lapped against the lap plate such that their ABS conform to the shape of lapping surface. 
     Referring now to FIGS. 5 and 6, a third embodiment of the invention is shown as lapping process fixture  81 . Fixture  81  is a simplified version of the previous fixtures including a base  83  with a mounting surface  85 , but fixture  81  only has one membrane  87  for supporting an array of discrete sliders or heads  89  with ABS. Membrane  87  is bonded to mounting surface  85  and has adhesive on its outer surface  91  such that heads  89  adhere to it. Membrane  87  extends across and seals an opening  93  in a cavity  95  inside base  83 . Cavity  95  is filled with fluid that may or may not be sealed or externally pressurized through ports in base  83 . Fixture  81  restrains heads  89  to minimize distortion of their ABS during lapping. The heads  89  are restrained from normal-directed movement by fluidic pressure such that the normal force is uniformly distributed across the surface area of each head  89 . The external adhesive on membrane  87  restrains heads  89  from tangential movement to drag the ABS along the lap plate. Heads  89  are lapped against the lap plate such that their ABS conform to the shape of lapping surface. 
     Fixture  81  is also provided with a plurality of wear pads  97  which assist in providing a fixed spacing between the lapping plate  99  and fixture  81 . During the lapping procedure, fixture  81  rests against plate  99  via wear pads  97 . Thus, both the ABS of heads  89  and wear pads  97  are abraded simultaneously. The fixed spacing provided by wear pads  97  will slowly decrease with wear. 
     The invention has several advantages including the ability to restrain a workpiece in such a manner that minimizes the restraining forces exerted on the workpiece, thereby minimizing distortion of the workpiece during lapping processes. The highly compliant fixture allows the ABS to be more uniformly, quickly, and accurately lapped to conform to the shape of the lapping surface. Assuming negligible force is need to deflect the membrane in the normal direction of the supporting membrane, the fluid will cause the membrane to conform to the curvature of the head/row at the adhesive attachment region and, hence, minimize distortion of the workpiece. This will allow tighter control of curvature in ABS for the lapping process. 
     While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.