Abstract:
A glide head calibration technique uses two fly height calibrations on a disk media certifier. The first calibration point uses a spin down on bump technique at a first height, and the second calibration point uses a spin down on disk media roughness at a second lower height. With two height data points, a fly height curve of each glide head is approximated very accurately. Once the fly height curve is derived for each head, any fly height can be dialed-in by the disk media certifier for glide testing. This technique achieves glide fly heights between about 4 nm and 8 nm and does so with improved tolerances.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field  
         [0002]     The present invention relates in general to improved calibration of glide heads and, in particular, to an improved system, method, and apparatus for measuring media roughness and combining that with spin down calibration to form a new and improved glide head calibration for media glide testing.  
         [0003]     2. Description of the Related Art  
         [0004]     Data access and storage systems comprise one or more storage devices that store data on magnetic or optical storage media. For example, a magnetic storage device is known as a direct access storage device (DASD) or a hard disk drive (HDD) and includes one or more disks and a disk controller to manage local operations concerning the disks. The hard disks themselves are usually made of aluminum alloy or a mixture of glass and ceramic, and are covered with a magnetic coating. Typically, two or three disks are stacked vertically on a common spindle that is turned by a disk drive motor at several thousand revolutions per minute (rpm).  
         [0005]     The HDD also has an actuator assembly with an actuator that moves magnetic read/write heads to the desired location on the rotating disk so as to write information to or read data from that location. Within most HDDs, the magnetic read/write head is mounted on a slider. A slider generally serves to mechanically support the head and any electrical connections between the head and the rest of the disk drive system. The slider is aerodynamically shaped to glide over moving air in order to maintain a uniform distance from the surface of the rotating disk, thereby preventing the head from undesirably contacting the disk.  
         [0006]     Typically, a slider is formed with an aerodynamic pattern of protrusions or air bearing design on its air bearing surface (ABS) that enables the slider to fly at a constant height close to the disk during operation of the disk drive. A slider is associated with each side of each platter and flies just over the platter&#39;s surface. Each slider is mounted on a suspension to form a head gimbal assembly (HGA). The HGA is then attached to a semi-rigid actuator arm that supports the entire head flying unit. Several semi-rigid arms may be combined to form a single movable unit having either a linear bearing or a rotary pivotal bearing system.  
         [0007]     The head and arm assembly is linearly or pivotally moved utilizing a magnet/coil structure that is often called a voice coil motor (VCM). The stator of a VCM is mounted to a base plate or casting on which the spindle is also mounted. The base casting with its spindle, actuator VCM, and internal filtration system is then enclosed with a cover and seal assembly to ensure that no contaminants can enter and adversely affect the reliability of the slider flying over the disk. When current is fed to the motor, the VCM develops force or torque that is substantially proportional to the applied current. The arm acceleration is therefore substantially proportional to the magnitude of the current. As the read/write head approaches a desired track, a reverse polarity signal is applied to the actuator, causing the signal to act as a brake, and ideally causing the read/write head to stop directly over the desired track.  
         [0008]     The presence of asperities on the surfaces of the disks can have a deleterious effect on the performance of disk drives. For this reason, a glide test is performed on finished disks to detect asperities that might contact the magnetic head flying at its normal height in a disk drive. In the test, a special glide head containing a piezoelectric transducer (PZT) is flown over a disk at an altitude or height that is below the normal drive fly height. Glide head contact with an asperity creates a PZT voltage response that generally scales with increasing size of the asperity. If the voltage response exceeds a predetermined level, the disk is rejected. As such, quantitative glide testing requires calibration of the voltage response with respect to asperity height.  
         [0009]     In the prior art, there are a number of methods for calibrating glide head fly heights. One method of calibrating glide heads uses a laser-generated, nano-sized protrusion or bump on the surface of a disk that can serve as a calibration asperity. Laser nano-bump generation is a technique that is used throughout the data storage industry. Flying a glide head over a laser nano-bump whose height is known (e.g., by interference or atomic force microscopy) will thus generate a calibrated PZT response. However, when using a single laser calibration bump, statistical variation in the PZT signal from one run to another results in a wide response distribution.  
         [0010]     In another method, the glide head is spun down onto bumps with a known average height arrayed around one or more tracks on a disk media. When the glide head touches the top of the bumps, that linear velocity is used to set the glide test at the average height of the bumps. This “spin down to bump” calibration methodology can only support bump heights down to about 8 nm. The laser melt process used to generate the bump disk media has difficulties maintaining bump height accuracies below 8 nm. However, present and future performance requirements need glide head fly heights down to about 4 nm.  
         [0011]     One solution to this problem is to spin down onto the disk media with a substrate roughness that is very near the desired low fly height. Fortunately, most glass substrate roughness is in the range of about 3 to 4 nm. Substrate texturing techniques can generate spin down response up to about 7 nm. However, this solution alone has some deficiencies. A good glide head calibration technique should do more than just (a) set the right fly height for glide testing. The technique should also check each glide head for (b) proper flying characteristics and (c) uniform PZT response. Thus, since parameters (b) and (c) are not provided, the glide head PZT signal generated by spinning down onto the media roughness does not hold enough information to satisfy the requirements of a good calibration technique.  
         [0012]     A second problem with spinning down onto disk media roughness is that the current glide PZT channel has problems handling the PZT response of the glide head flying at the media roughness height. Typical glide channels were designed to analyze glide heads hitting discreet media defects producing PZT signals with very fast rise and decay times. These transitions in the PZT signal trigger software interrupts allowing each discreet hit to be counted. The PZT signal of a glide head on media roughness can resemble a DC phenomena shift from low to high and stay high due to the many contacts around the track. When this happens, one cannot determine if the glide head is hitting one discreet defect or many contacts due to the media roughness.  
         [0013]     The PZT RMS signal could be used to measure the disk media roughness with spin down. However, one significant weakness of RMS is that a single defect with sufficient amplitude can greatly affect the RMS reading of the entire track and thereby render useless any information gleaned therefrom. Thus, an improved system, method, and apparatus for glide head calibration at lower fly heights would be desirable.  
       SUMMARY OF THE INVENTION  
       [0014]     One embodiment of the present invention provides a new glide head calibration as a two-point fly height calibration on a disk media certifier. The first calibration point is a spin down on bump technique at a relatively high height (e.g., 8 to 10 nm). The second calibration point is an improved spin down on disk media roughness at a relatively low height (e.g., 3 to 4 nm). With two height data points, a fly height curve (i.e., linear velocity vs. height) of each head can be approximated very accurately. Depending on the design of the glide head, a linear or power regression curve can be used. Once this fly height curve is derived for each head, any fly height can be dialed-in by the disk media certifier for glide testing.  
         [0015]     This new technique achieves glide fly heights below 8 nm and does so with improved tolerances. A significant advantage of this technique is that a fly height curve of each glide head can be generated directly on a disk media certifier. Another option is to rely on fly height curve data from an optical fly height certifier and transfer the data to the disk media certifier. This option has many correlation issues and can contribute to glide fly height tolerance errors up to about +/−2 nm. Another advantage is that the calibration is common across media products with the same form factor but different fly height requirements. Using the same high and low height calibration disks, products can be calibrated that require any glide head fly height between about 4 and 8 nm. In addition, the first calibration at some higher fly height retains all of the benefits of checking the glide head for proper flying characteristics and uniform PZT response.  
         [0016]     One advantage of using glide PZT peak detection over PZT RMS for the media roughness measurement is improved accuracy. A defect on the track can cause the RMS signal to prematurely trigger media roughness contact. In the case of using the PZT peak detection, one defect on the track gets averaged out if the media roughness contact point is defined as contact with some large percentage of the entire track.  
         [0017]     In one embodiment, the first calibration point at some high fly height is determined by a glide head that is calibrated in situ on a disk glide tester that is subsequently used for disk testing. One embodiment is to use a disk that has fabricated bumps with calibrated heights. A verification is first made to insure that observed contacts are between the glide head and the bumps. Then the flying height of the glide head is then raised above the bumps and then gradually reduced by decreasing the speed between the glide head and the disk until contact occurs. In this manner, the differences in flying height from mounting on different testers is solved by calibrating the glide head on the tester that is to be used in manufacturing for testing disks. In addition, the surface topography is dominated by lubricant and sputtered carbon for both the disk with bumps and the disks to be tested. Accordingly, there is no significant difference in flying heights generated by different surface topographies.  
         [0018]     In another embodiment, the glide head is dynamically scanned over the calibrated bumps. Dynamic scanning is accomplished by moving the glide head radially over the disk at the approximate radial location of the calibrated bump. This insures that the roll of the glide head is taken into account during the fly height calibration and that the lowest portion of the glide head is the portion that is in contact with the bump.  
         [0019]     The second or low fly height calibration point is a new “spin down” technique requiring a new glide PZT channel and calibration algorithm. Like the first calibration point, this technique involves spin down on media roughness peaks with known average height and using peak detection. The touch down point is defined as the first linear velocity where there is contact with some significant percentage of the track sectors. Due to the nature of the PZT response of many contacts with media roughness peaks, the first calibration point technique does not work here without enhancements to the glide PZT channel.  
         [0020]     The channel enhancements needed for this new calibration technique include a new clock circuit and multiplexing capabilities. The new clock circuit allows the glide PZT peak detection to count the many glide head contacts with the media roughness. The clock circuit effectively adds artificial signal transitions when the media roughness multiple contacts cause the PZT signal to go into the DC shift phenomenon. Essentially, this forces the channel to trigger a software interrupt at the start of each sector and count if the PZT signal is still high. The multiplexer allows the PZT signal from the glide head to travel through the glide channel in either path, depending on the desired mode. It can operate in discreet defect detection mode or in media roughness measurement mode. Software can multiplex the channel to the discreet defect detection mode for the high fly height calibration, multiplex to media roughness measurement mode for the low fly height calibration, and then multiplex back to defect detection for glide testing.  
         [0021]     After both calibration points are successful for any glide head, a fly height curve can be generated with either a linear or a power function curve fit. This curve is then used to set up the proper linear velocity for each glide head&#39;s required glide fly height.  
         [0022]     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 present invention, taken in conjunction with the appended claims and the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     So that the manner in which the features and advantages of the invention, as well as others that 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 an embodiment of the invention and therefore are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments.  
         [0024]      FIG. 1  is a schematic isometric drawing of one embodiment of a calibration system constructed in accordance with the present invention.  
         [0025]      FIG. 2  is a partial isometric view of a head gimbal assembly utilized by the system of  FIG. 1 .  
         [0026]      FIG. 3  is circuit block diagram constructed in accordance with the present invention.  
         [0027]      FIG. 4  is a plot of a glide head fly height showing, on the x-axis, linear velocity, and, on the y-axis, fly height from a surface of the disk, and illustrating a glide head calibration methodology of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     Referring to  FIG. 1 , one embodiment of an apparatus  11  for calibrating a glide head utilizes a rotating, magnetic disk  13  having a large plurality of tracks. The disk  13  also has a plurality of precisely formed glide bumps  41  that are used during the calibration process, as will be described below. Apparatus  11  comprises an actuator  15  with a movable arm  17  and a suspension  19  on one end. Arm  17  and disk  13  move in the directions indicated by arrows  21 ,  23 , respectively. Arm  17  provides the seek motion when changing tracks on disk  13 .  
         [0029]     As shown in  FIG. 2 , a slider or flying/glide head  31  is bonded to the end of suspension  19 . In the embodiment shown, glide head  31  is nano size (approximately 2050×1600×450 microns) and formed from ceramic or intermetallic materials. Glide head  31  may also be pico size (approximately 1250×1000×300 microns). Glide head  31  is pre-loaded against the surface of disk  13  (typically in the range two to ten grams) by suspension  19 . It is glide head  31  that is calibrated with respect to the surface of disk  13 .  
         [0030]     Glide head  31  carries a piezoelectric element  33  (shown schematically) on its upper surface adjacent to suspension  19 . The contacting side or air bearing  35  of head  31  which touches disk  13  is located opposite element  33 . Element  33  produces an electrical voltage (PZT) signal when head  31  is mechanically excited, such as when air bearing  35  touches a protruding asperity on the surface of disk  13 . The stronger the contact between head  31  and the asperity, the higher the signal produced. Thus, with proper calibration, head  31  can be used to determine the size of the disk defects.  
         [0031]     Referring now to  FIGS. 3 and 4 , one embodiment of a system  51  for glide head calibration with a two-point fly height calibration on a disk media certifier is shown. The system  51  comprises a first calibration point  53  derived from a spin down on bump technique at a relatively high height based on fabricated bumps  41  with calibrated heights on the disk  13 . A second calibration point  55  derived from a spin down on disk media roughness  57  technique at a relatively low height. A fly height curve  59  of linear velocity vs. fly height of a glide head  35  is approximated with the first and second calibration points  53 ,  55 .  
         [0032]     For example, the relatively high height (of bump  41 ) is approximately 8 to 10 nm, and the relatively low height (of roughness  57 ) is approximately 3 to 4 nm. A number of methods may be used to derive fly height curve  59 , including a linear regression curve and a power regression curve. The best fit will depend on the geometries of the glide head air bearing slider design. The fly height curve  59  is used to set up a linear velocity for a required glide fly height for the glide head  35  such that any fly height can be dialed-in for the glide head  35  by the disk media certifier for glide testing. A less accurate method would be data for the fly height curve  59  as determined by an optical fly height certifier, in which case the data is transferred to the disk media certifier. The new method is an improvement and replacement for the latter.  
         [0033]     In a general sense, the first calibration point  53  may be derived by making contact between the glide head  35  and the bumps  41  on the disk  13 , raising the flying height of the glide head  35  above the bumps  41 , and then gradually reducing the fly height by decreasing the speed between the glide head  35  and the disk  13  until contact occurs again.  
         [0034]     More particularly, the first calibration point  53  may be determined by: flying the glide head  35  over the rotating disk  13 , verifying that observed contacts are between the glide head  35  and the bump  41 , reducing the speed between the glide head  35  and the disk  13  by a discrete interval, dynamically scanning the disk  13  at each discrete interval of speed, and calibrating the flying height of the glide head  35  when a value of the spin down speed has been successfully determined. After verification, the initial flying height of the glide head  35  is higher than the calibrated bump  41 , which may number from two to forty equally-spaced apart, calibrated bumps  41  on the disk  13 . Additional details pertaining to this portion of the present invention are contained in Applicant&#39;s Attorney Docket No. SJO920010183US1, which is incorporated herein by reference.  
         [0035]     The second calibration point  55  is a touch down point on the disk  13  by the glide head  35  and is defined as the first linear velocity where there is contact with a significant percentage of the track sectors. The second calibration point  55  may utilize a calibration algorithm, a glide PZT channel  61 , a clock circuit  63 , and multiplexing capabilities  65 . The glide PZT channel  61  alone cannot handle typical touch down signals (media roughness) because there is only one signal low to high transition. This generates only a single interrupt and only one count for a track of potentially many touch down contacts. By definition, we are looking for some percentage of sectors in a track with counts to define the first linear velocity.  
         [0036]     The clock circuit  63 , which can be controlled by software, is added so that when enabled will generate interrupts at the start of each sector as long as the touch down signal stays higher than the clip level of the single interrupt. The clock circuit  63  also continues to digitize the glide signal as long as the interrupts are generated. This allows the proper counting of sectors with contacts in the determination of the first linear velocity for the second calibration point  55 . The multiplexing capabilities  65  allow the same glide channel to function for both the first calibration point  53  which does not need the clock circuit  63  and the second calibration point  55  which does need the clock circuit  63 .  
         [0037]     The present invention also comprises a method of glide head calibration with a two-point fly height calibration on a disk media certifier. One embodiment of the method comprises deriving a first calibration point from a spin down on bump technique at a relatively high height, such as flying the glide head at 8 to 10 nm; deriving a second calibration point from a spin down on disk media roughness technique at a relatively low height, such as flying the glide head at 3 to 4 nm; and approximating a fly height curve of linear velocity vs. fly height of a glide head with the first and second calibration points. The final step may comprise using one of a linear regression curve and a power regression curve to determine the fly height curve.  
         [0038]     The method may further comprise using the fly height curve to set up a linear velocity for a required glide fly height for the glide head such that any fly height can be dialed-in for the glide head by the disk media certifier for glide testing. In one alternate embodiment, derivation of the second calibration point may comprise using a glide PZT channel, a calibration algorithm, a clock circuit, and multiplexing capabilities, wherein a touch down point is defined as a first linear velocity where there is contact with a significant percentage of the track sectors. In one version, the multiplexer allows the PZT signal from the glide head to function for both the first calibration point, which does not need the clock circuit, and the second calibration point which does need the clock circuit, depending on a desired mode.  
         [0039]     The method may further comprise generating interrupts with the clock circuit at a start of each sector as long as a touch down signal stays higher than a clip level of a single interrupt, and digitizing the glide signal with the clock circuit as long as the interrupts are generated, and counting sectors with contacts in determining the first linear velocity for the second calibration point. In addition, the method may comprise deriving the first calibration point by contact made between the glide head and the bumps on the disk, raising the flying height of the glide head above the bumps, and then gradually reducing the flying height by decreasing the speed between the glide head and the disk until contact occurs again.  
         [0040]     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.