Abstract:
A method of manufacturing a disk drive and a disk drive where the width of the read element and the width of the write element are both measured at servo-writing time and the track pitch of the disk drive is set on the basis of those measurements. Disk drives with superior head width combinations are servo-written with a narrower track pitch in order to have a higher storage capacity. Disk drives with inferior head width combinations are detected before servo-writing so that the disk drive may be servo-written with wider track pitch rather than with a nominal track pitch that results in a subsequent drive failure during initial burn-in (IBI). The heads are used more efficiently in that heads that are more capable are used to their ability and less capable heads that would otherwise be disposed of are used at all.

Description:
This application is a Continuation of patent application Ser. No. 09/920,665 filed Jul. 31, 2001, now U.S. Pat. No. 6,885,514. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to magnetic disk drives (disk drives), and more particularly to an efficient method of manufacturing a disk drive by using a servo track writer (STW) for measuring the widths of the read and write elements to set the track pitch. 
   2. Description of the Related Art 
   Referring to  FIG. 1 , a conventional disk drive  10  has a head disk assembly (HDA)  20  including at least one disk  23 , a spindle motor  22  for rapidly rotating the disk  23 , and a head stack assembly (HSA)  40  that includes an actuator assembly  50  and a head gimbal assembly (HGA) (not numbered) with a transducer head  80  for reading and writing data. The HSA  40  is part of a servo control system that positions the transducer head  80  over a particular track on the disk to read or write information from that track. The HSA  40  earns its name from the fact that it generally includes a plurality of HGAs that collectively provide a vertical arrangement of heads called a “head stack.” 
   The transducer heads  80  of several years ago were “merged” devices where reading and writing were accomplished with a single inductive element. The transducer head  80  commonly used today, however, is a magneto-resistive transducer head  80  that has separate read and write elements.  FIG. 2  is a highly simplified representation of a magneto-resistive transducer head  80  having it&#39;s a write element  81  of width W and it&#39;s a read element  82  of width R. The transducer head  80  shown is a “write wide, read narrow” device in that the read element&#39;s width R is typically about 50–65% of the write element&#39;s width W. 
     FIG. 3  is an exploded perspective view of a fully-assembled HDA  20  having servo-writing access ports  25 ,  26  (discussed below) and the controller circuit board  30  that is usually installed after servo-writing. The controller circuit board  30  suitably positions the actuator assembly  50  and then reads or writes user data in accordance with commands from a host system (not shown). 
   Returning to  FIG. 1 , the industry presently prefers a “rotary” or “swing-type” actuator assembly  50  that conventionally comprises an actuator body  51  which rotates on a pivot assembly between limited positions, a coil  52  that extends from one side of the actuator body to interact with a pair of permanent magnets to form a voice coil motor (VCM), and an actuator arm  54  that extends from the opposite side of the actuator body to support the HGA. 
   A disk drive is ultimately used to store user data in one or more “data tracks” that are most commonly arranged as a plurality of concentric data tracks on the surface of its disk or disks. Special servo information is factory-recorded on at least one disk surface so that the disk drive&#39;s servo control system may control the actuator assembly  50 , via the VCM, to accurately position the transducer head to read or write user data to or from the data tracks. In colloquial terms, the servo information provides the servo control system with the “your head is here” data it needs to attain and then maintain a desired head position. In operation, the disk drive&#39;s servo control system intermittently or continuously processes (read only) the pre-recorded servo information just before or while the disk drive processes (reads or writes) user data in the data tracks. 
   Earlier disk early drives used a “dedicated servo” system where one head and one disk surface provide the servo information for all of the other heads and disk surfaces. As shown in  FIG. 4 , however, the industry presently prefers an “embedded servo” system where the servo information is interspersed amongst the data on each surface of each disk. The factory-recorded servo information is contained in servo wedges  300  that are each divided into a plurality of servo sectors  310 . The servo sectors  310  are recorded concentrically in order to provide numerous servo tracks (one entire rotation of servo sectors  310 ). 
   As shown, each servo wedge  300  generally comprises a header region (not separately identified) followed by a plurality of servo bursts. The header region fields include a setup or write splice field WRITE SPLICE, an address mark field AM, an automatic gain control/phase locked oscillator field AGC/PLO, a servo sync mark field SSM, a track identification field TKID, a wedge number field W#. The header region is followed by at least two servo bursts (an A burst and B burst are shown) that are circumferentially sequential and radially offset relative to a burst pair centerline. The servo format used is not critical and is explained here only for background purposes. The purpose of these various fields and available variations are well known to those of ordinary skill in the art. 
   The servo wedges  300  precede a corresponding number of data wedges  400  that are ultimately used to contain data tracks (not shown) that are divided into a plurality of data sectors (not shown) Each data wedge  400  may contain a whole or fractional part of one or more data sectors (not shown). Because the servo information is distributed around the disk within servo sectors  310 , an embedded servo system is sometimes called a “sector servo” system. 
   The servo information is factory recorded at the time of manufacture using a relatively expensive and low-throughput manufacturing fixture called a servo track writer (STW).  FIG. 5  is an exploded perspective view of a simplified servo-track writer (STW)  100  that is figuratively receiving an HDA  20  for servo-writing. The STW  100  records the servo information in special “servo tracks” on each surface of each disk for later use by the servo control system when the drive is in the hands of the user. The servo tracks are generally used throughout the life of the disk drive without modification. 
   In recording the embedded servo information, the STW  100  take temporary control of the drive&#39;s write operation via a suitable electrical connector  102 , repeatedly locates the write element  81  to a desired radial position, and then writes, erases, or does nothing (remains idle) at specific angular positions between the head and a reference position of the disk as the disk rotates beneath the write head. In order to precisely locate the write element  81  where needed, as shown in  FIG. 3 , a conventional HDA  20  has first and second access ports  25 ,  26  (later covered with adhesive labels) for allowing the STW to “reach in” and temporarily control the radial position of the actuator assembly  50  and measure the angular position of the disk while recording the servo information. As to the radial position of the actuator assembly  50 , the conventional STW inserts a moveable “push pin”  101  into the first port  25 , commands the HDA&#39;s VCM to bias the actuator assembly  50  against the push pin, moves the push pin  101  against the bias to move the actuator assembly  50  and the attached head  80 , and measures the position of the push pin  101  with a laser interferometer to control the radial position of the head&#39;s write element  81  carried by the pin-guided actuator assembly  50 . As to the angular position of the write element  81  relative to an index position of the disk, the conventional STW inserts a stationary “clock head” (not shown) into the second port  26 , records a “clock track” containing thousands of “clock marks” and one “index mark” (e.g. an extra clock mark or a gap) on a top-most or bottom-most disk surface, and measures the angular position of the write element  81  relative to the index mark by detecting the index mark and thereafter tracking (i.e. counting) the intermediate clock marks. 
   The conventional STW embeds a servo pattern onto a disk by recording concentric servo tracks in a plurality of discrete “passes.” Each pass consists of moving the push-pin to “step” the transducer head to a desired radial position, allowing the head to “settle,” and during one ensuing revolution of the disk, writing new servo information, erasing overlapping portions of previously written servo information, or remaining idle (neither writing nor erasing). On the first pass, the STW moves the write head to an outer diameter of the disk, and then records magnetic transitions at discrete angular intervals to record the servo information including track identification (track ID) data and servo bursts. During the second and each of the thousands of subsequent passes, the STW steps the write head inward by a fraction of the intended data track pitch (e.g. ½ and ⅓ data track increments), waits for the write head to settle (as much as one full revolution), and then records the servo information during another full revolution, writing more magnetic transitions, trimming overlapping portions of previously recorded transitions, or holding idle, as appropriate for the desired servo pattern. In order to record each concentric servo track, therefore, the STW must repeatedly step, wait, and record. 
   The servo-writing process is a manufacturing bottleneck because each HDA must remain in the STW for an extensive amount of time in order to step, wait, and record each pass that collectively make up the required servo information. 
   Magneto-resistive transducer heads  80  are very small devices that are manufactured in large batches using photolithographic wafer process techniques. As a result, operating characteristics such as the widths of the read and write elements  81 ,  82  tend to vary over a normal distribution curve for a given number of heads, wafers or manufacturers. The presence of separate read and write elements coupled with the wide variability of read width R and write width W is particularly troublesome as it relates to the servo-writing process and narrow range of widths that may presently be used. 
   In particular, the disk drive market is extremely competitive and drive makers are continually striving for manufacturing efficiencies, increased storage capacities, and higher performance in order to remain profitable. The servo-writing process is of major concern because STWs are so expensive (upward of $100,000) that only limited numbers can be used and it takes a long time to servowrite each disk drive (several minutes per drive). The servo-writing bottleneck is exacerbated by the fact that:
         there is an ever increasing demand for areal density that can only be achieved with ever narrower data tracks (usually specified in tracks per inch or TPI) and ever tighter data densities (usually specified in bits per inch or BPI);   only a subset of the heads come from the manufacturer with read and write widths R, W that are suited for the nominal TPI. Some heads could be used with wider tracks, but they are not used at all. Some heads could be used with narrower tracks, but their enhanced capability is wasted with the nominal TPI, or worse, they are discarded altogether;   it is difficult to accurately identify this small subset of heads with currently available measurement techniques so, even if the heads are “binned” by making such measurements, many of the drives fail at a very late stage of the manufacturing process (after assembly of the HDA, servo-writing, mounting of the controller board and while testing the drive during Initial Burn-In or IBI), because one or more of the heads is too narrow or too wide. These failed drives must generally make a second trip through the servo-writing process, an unfortunate and expensive occurrence.       

   Achieving efficiencies in terms of head use and the overall servo-writing process, therefore, may significantly reduce the overall cost of manufacturing disk drives. Consequently, there remains a need for a method of manufacturing a disk drive that allows more of the heads to be used in the first instance and that reduces the number of drives that must be re-worked and then take a second trip through the entire servo-writing process. 
   SUMMARY OF INVENTION 
   In a first aspect, the invention may be regarded as a method of manufacturing a disk drive formed from a head disk assembly (HDA) containing at least one magnetic disk with a magnetic surface and a head stack assembly (HSA) that includes a transducer head with a write element for writing data to the magnetic disk and a read element for reading data from the magnetic disk, the method comprising the steps of: mounting the HDA in a servo track writer and moving the HSA to desired positions over the magnetic disk; measuring a width of the read element with the servo track writer; measuring a width of the write element with the servo track writer; determining a track pitch based on the measured width of the read element and the measured width of the write element; and writing servo tracks onto the magnetic disk at the determined track pitch. 
   In a second aspect, the invention may be regarded as a disk drive comprising a head disk assembly (HDA) containing at least one magnetic disk that includes a magnetic surface and a head stack assembly (HSA) that includes a transducer head with a write element for writing data to the magnetic disk and a read element for reading data from the magnetic disk, the disk drive produced using the steps of: measuring a width of the read element while the HDA is in a servo track writer; measuring a width of the write element while the HDA is in a servo track writer; determining a track pitch based on the measured width of the read element and the measured width of the write element; and writing servo tracks onto the magnetic disk at the determined track pitch. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The just summarized invention may best be understood with reference to the Figures of which: 
       FIG. 1  is an exploded perspective view of a magnetic disk drive  10  having a head disk assembly  20  (“HDA”) including a head stack assembly  40  (“HSA”) which carries a transducer  80  over concentric data tracks and associated servo bursts on the surface of a disk  23 ; 
       FIG. 2  is a simplified representation of a magneto-resistive transducer head  80  that has two elements that vary in width over a normal distribution curve, namely a write element  81  and a read element  82 . 
       FIG. 3  is an exploded perspective view of a fully-assembled HDA  20  having servo-writing ports  25 ,  26  and the controller circuit board  30  that is usually installed after servo-writing; 
       FIG. 4  is a top plan view of a disk containing embedded servo information that is recorded within concentric servo tracks that are each defined by a corresponding plurality of concentric servo sectors 
       FIG. 5  is an exploded perspective view of a servo-track writer (STW)  100  receiving an HDA  20  for servo-writing; 
       FIG. 6  is a flow-chart of a method of manufacturing according to this invention; 
       FIGS. 7A ,  7 B,  7 C and  7 D illustrate a preferred method of measuring a width of the read and write elements in the method of  FIG. 6 ; 
       FIGS. 8A ,  8 B,  8 C and  8 D illustrate an alternative method of measuring a width of the read and write elements; 
       FIG. 9  illustrates a presently preferred decision table for determining track pitch based on read and write head widths. and 
       FIG. 10  illustrates an alternative approach to determining the track pitch. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   This patent application is directed to a new, innovative method of manufacturing a disk drive and to a disk drive made in accordance with the method. 
     FIG. 6  is a flowchart of a preferred method of manufacturing a disk drive according to this invention. The illustrated method operates on an HDA  20  like that shown in  FIGS. 1 and 3 , i.e. an HDA  20  containing at least one magnetic disk  23  with a magnetic surface and a head stack assembly  40  that includes a two-element transducer head  80 . The HDA  20  is usually placed into the STW  100  in “bare” form (i.e. without the controller circuit board  30 ), but as discussed below, it may be desirable to marry the HDA  20  with the controller circuit board  30  before mounting the disk drive  10  in the STW  100  and recording servo tracks. 
   The method begins at step  210  by mounting the HDA  20  in the STW  100  for moving the HSA  40  to desired positions over the magnetic disk.  FIG. 5  further illustrates this step. 
   At steps  220 -A and  220 -B, the method proceeds by measuring a width “R” of the read element  82  with the STW  100  and by measuring a width “W” of the write element  81  with the STW  100 . The required measurement steps  220 -A and  220 -B can be accomplished together or separately and in succession depending on the approach taken. In addition, the measurement steps  220 -A and  220 -B may be accomplished with a “smart” STW  100  that includes sufficient data processing capabilities to process a bare HDA  20  or by using a more conventional STW to process a fully-assembled disk drive  10  (HDA  20  and controller board  30 ), the STW operating in cooperation with suitable firmware on the controller circuit board  30 . 
   The presently preferred approach is using an STW  100  that processes a bare HDA  20 . A preferred STW  100  is manufactured by Xyratex. However, other STW&#39;s may be used. 
   At step  230 , the method proceeds by determining a track pitch based on the measured width “R” of the read element  82  and the measured width “W” of the write element  81 . 
   At step  240 , the method proceeds with the STW  100  writing servo tracks onto the magnetic disk at the determined track pitch. 
   Measuring Methods 
   Any suitable measuring algorithm may be used, but  FIGS. 7A to 7D  graphically illustrate a first preferred method of measuring a width of the read and write elements  81 ,  82  in the method steps  220 -A and  220 -B of  FIG. 6 . 
   In  FIG. 7A , corresponding to step  221 , the STW  100  moves the transducer head  80  to a known location and then causes the head&#39;s write element  81  to record a test track  90  of unknown width “W”. 
   In  FIG. 7B  or step  222 , as suggested by the vertical arrow, the STW  100  moves the transducer head  80  radially away from the known position at which the test track  90  was recorded to another second known position and, as suggested by the horizontal arrow, begins to try to read the test track  90  with the read element  82  of unknown width “R”. The radial displacement between the first and second known positions should be sufficient to ensure that the read element  82  does not initially overlap the test track  90 . 
   In  FIG. 7C  or step  223 , as suggested by the upwardly pointing arrow, the STW  100  repeatedly moves the head  80  back toward the first known position a little at a time and, after each such movement, tries to read the test track  90 . The result of many such passes after moving the head  80  sufficiently beyond the first known position is the development of convolution data  91  that ramps up, flattens out, and then ramps down. Each data point in the convolution data  91 , of course, is associated with a position count (e.g. 10,000) that is available to the STW. 
   In  FIG. 7D  or step  224 , as suggested by a more detailed view of the convolution data  91  In conjunction with some hypothetical position counts associated with each point of inflection, the STW  100  computes the read element&#39;s width “R” and the write elements width “W” from the mathematical relationships between W−R, W+R and the four position counts. In the example shown, “W”=900 and “R”=500. By way of example, if each count represents 0.02 micro-inches, this translates to a write element  81  with a width of 18 micro-inches and a read element  82  having a width of 10 micro-inches. 
     FIGS. 8A to 8D  illustrate an alternative measuring method that involves three written tracks W 1 , W 2  and W 3 . The length of the first and subsequent written tracks W 1 , W 2  and W 3  is preferably short, in the neighborhood of 500 bytes, so that external perturbations do not affect the accuracy of the measurement. It is preferable, in other words, to make the measurements over a short distance rather than over full revolutions. 
   This alternative approach may be desirable in that it inherently includes an erase band effect (typically on the order of 2%) in connection with the determining the width of the write element  81 . 
   At step  8 A, the STW  100  causes the write element  81  to record a first written track W 1  at a known position on the disk. The written track W 1  includes an initial data area (hatched) followed by a DC erase area. The data area may be written with any suitable patter such as the 2T pattern common recorded in a PLL/AGC field. 
   At step  8 B, the STW  100  steps the head  80  radially away from W 1  and then causes the write element  81  to record a second written track W 2  using the same pattern of an initial data area (hatched) followed by a DC erase area. The step distance must be such that the distance between the two written tracks W 1  and W 2  is less than the minimum width expected for a write element  81 . 
   At step  8 C, the STW  100  steps the head  80  radially back from W 2  and then causes the write element  82  to record a third written track W 3 . In this case, however, the write element  81  is controlled so at to record a DC erase area followed by a data area (hatched). At this point, the data area of the first and second written tracks W 1 , W 2  have been erased back by the DC erase portion of W 3  while the data area of written track W 3  is full width. The beginning of the third written track W 3  may be delayed somewhat, as shown, so that a clean unmodified portion of either written track W 1  or W 2  is available for gain normalization that establishes a fixed gain to be used during the gathering of data. In the context of an STW  100 , however, it may be sufficient to use the same fixed gain for all drives and thereby eliminate the need to perform gain normalization on a drive-by-drive basis. 
   At step  8 D, the STW  100  incrementally steps the head in small radial amounts while measuring the power of the signal detected by the read element  82  in the two successive regions labeled “Adjacent Track Measurement” and “Target Track Measurement” for each pass (or on two successive passes as the same position) corresponding to two collection of data  92 ,  93 . Thus, the STW  100  incrementally gathers a large number of samples  92 ,  93  that, if all were gathered, would appear as shown to the right of  FIG. 8D . 
   The cross-over points between the two sets of data  92 ,  93  correspond to the boundaries between the first two written track W 1 , W 2  and the third written track W 3 , any erase band effect recorded by the write element  81  and detected by the read element  82 . The STW  100 , of course, does not need to gather all of the data and then mathematically process it as the STW  100  can simply hunt for the two positions where the data values  92 ,  93  are equal and perform a simple subtraction to determined the value “W+Erase”. 
   The width “R” of the read element  82  may be determined from the width of the saturated flat spot of data curve  93  which is equal to W−R. As the erase bands will impose some degree of error on this measurement, a realistic approach to determining the width “R” is to simply establish the point of the two boundaries as the position at which the signal is 90% of the maximum power value. 
   There are other possible ways to measure the width “R” of the write element. One alternative approach, for example, involves writing a track, moving the head by a small amount (say 5% of a track pitch), and DC erasing the written track to leave a sliver that is narrower than the write elements width “R”. The read head  82  may then be repeatedly passed over the sliver, with small amounts of radial movement between passes, in order to develop a convolution of data that is representative of the read element&#39;s width “R”. 
   Determining Track Pitch 
   A variety of approaches may be taken in terms of implementing the details of step  230 , i.e. in determining a track pitch based on the measured width “W” of the write element  81  and the measured width “R” of the read element  82 . 
     FIG. 9  illustrates a presently preferred decision table for determining track pitch based on read and write head widths. If the widest write head width is beyond a “wide” threshold, then the TPI is set to “low” regardless of the read head width. If the widest write head width is within a “nominal” range, then the TPI is set to “low”, “nominal” or “high” as a function of the widest read head width. If the widest write head width is below a “narrow” threshold, then the TPI is set to “high” unless the widest read head width is beyond a “wide” threshold in which case the TPI is set to “low”. 
     FIG. 10 , however, illustrates an alternative approach where the HDA  20  includes a plurality of transducer heads  80 , where the measuring steps are performed for each transducer head  80  to establish a collection of width measurements; and where the determining step is accomplished based on the collection of width measurements. 
   As suggested by  FIG. 10 , the collection of width measurements may be algorithmically analyzed in a two-dimensional selection system where the “x”-axis corresponds to write element widths “W” and the “y”-axis corresponds to read element widths “Y”. The origin or (0,0) point corresponds to the pair of widths (W, R) that are designated as nominal. For example, it may be that a particular vendor is requested to deliver heads  80  that nominally have a write width “W” of 20 micro-inches and a read width “R” of 15 micro-inches. 
   In operation, based on the write element width “W” and the read element width “R” that are measured for each head  80  in steps  220 -A and  220 -B, a data point is developed and algorithmically mapped into the coordinate system of  FIG. 10 . If all of the data points are inside of the nominal zone  401 , then the STW  100  servo-writes the HDA  20  with a nominal track pitch and corresponding TPI. If one or more of the head&#39;s data points is wider than usual and located to the right of the nominal zone  401 , then the STW  100  servo-writes the HDA  20  with a wider than nominal track pitch. If one or more of the head&#39;s data points is narrower than usual and located to the left of the nominal zone  401 , then the STW  100  servo-writes the HDA  20  with a narrower than nominal track pitch.