Abstract:
A system and method are disclosed for calibrating a hard disc drive magnetic head flying height tester by a calibration standard, which includes a mock slider and mock disc, by optical interference techniques.

Description:
BACKGROUND INFORMATION  
         [0001]    The present invention relates to optical gap measuring tool calibration. More specifically, the invention relates to a system and method for calibrating a hard disc drive magnetic head flying height tester by optical interference techniques.  
           [0002]    [0002]FIG. 1 provides an illustration of a typical hard disc drive. In the art of hard disc drives, magnetic read/write heads  102  are commonly integrated in a slider  102  designed to respond to a flow of air moving with the rotating disc  104  over which the slider  102  travels. The head/slider  102  ‘flies’ close to the surface of the disc  104 . In manufacturing such heads/sliders  102 , it is often necessary to test hydrodynamic characteristics of the heads  102  to verify their performance. It is important that the head  102  not travel too far from or close to the disc  104  surface. Further, it is important to prevent the head  102  from traveling at an improper angle with respect to the disc surface  104 . A head  102  traveling too high above the disc surface  104  will result in a lower than desired areal density. A head  102  traveling too low can cause an interface failure between the head  102  and disc  104 .  
           [0003]    In order to test the flying height of the head, a flying height tester is commonly used. Optical interference techniques are often employed to determine the distance between head and disc. A monochromatic light source is directed at a transparent surrogate disc, such as a glass disc, rotating at speeds similar to that of a magnetic disc, and the head assembly being tested is secured in a holder in its normal flying orientation in relation to the disc. The monochromatic light is directed at the disc at a predetermined angle to the surface thereof. The light is reflected from the surface of the disc closest to the head, as well as from the surface of the flying head itself, and impinges onto a light sensitive sensor.  
           [0004]    The interference effects created by the combined reflections from the disc and the slider surface provide the flying height information. A computer receives data from the flying height tester and calculates the perceived flying height and angle of the head. As hard drives become smaller and increase in data storage capacity, the desired head flying height continually reduces. Therefore, the accuracy of a flying height tester, and thus its calibration, are of critical concern.  
           [0005]    [0005]FIG. 2 illustrates a typical device used to calibrate a flying height tester. A calibration standard, such as is depicted in U.S. Pat. No. 5,552,884, is often utilized. As can be seen in FIG. 2 a , the calibration standard includes a mock head  48  in contact with a transparent disc  44  via a load spring  52 . The transparent disc  44  has a plurality of grooves  60  formed in a surface facing the mock head  48 . A cover case  56  is attached to the glass disc  44  at one end and provides a sealed environment for the interface between the mock head  48  assembly and the transparent disc  44 . Several problems exist with the utilization of this device. For example, in establishing H 1   204 , which is important in evaluating flying height (explained below), the nature of the design causes problems with using optical interference means. Measurement of H 1   205  must not be taken too close to a ridge&#39;s  64  edge, or else one (or both) of the measurement light beam&#39;s return paths  206 , 208  may travel a portion through air (separated by the walls at  120  and  124 ). The differences in optical properties between air and the transparent disc (glass, etc.) disrupts the travel path and thus causes inaccurate optical interference measurement results (i.e., the resultant beams  206  and  208  are not at the correct positions and/or the correct distance apart for accurate measurement). Therefore, H 1  measurements may only be taken towards the center of the ridges  64  (if at all). This prevents appropriate compensation for surface irregularities  76  in the mock disc  48 . Also, a separate device must be used to determine a minimum and maximum light intensity for the flying height tester, a necessary step in calibration, as explained below. This separate device adds cost and complexity to the calibration process.  
           [0006]    It is therefore desirable to have a system and method for calibrating flying height testers that avoids the above-mentioned problems, as well as having additional benefits.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 provides an illustration of a typical hard disc drive.  
         [0008]    [0008]FIG. 2 illustrates a typical device used to calibrate a flying height tester.  
         [0009]    [0009]FIG. 3 illustrates a flying height tester calibration standard according to an embodiment of the present invention.  
         [0010]    [0010]FIG. 4 illustrates surface irregularity compensation and provides further detailed illustrations of two mock heads according to an embodiment of the present invention.  
         [0011]    [0011]FIG. 5 provides a graphical illustration of the ‘unique fit’ solution utilized for providing a continuous spectrum of uniquely-valued combinations associatable to a range of head/disc gaps under principles of an embodiment of the present invention.  
         [0012]    [0012]FIG. 6 provides an illustration of a mock head design according to an alternative embodiment of the present invention.  
         [0013]    [0013]FIG. 7 provides illustrations of three mock head designs according to alternative embodiments of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0014]    [0014]FIG. 3 illustrates a flying height tester calibration standard according to an embodiment of the present invention. As can be seen in FIG. 3 a , in one embodiment, the calibration standard  100  includes a transparent mock disc  10  and one or more mock heads  20  placed in substantial contact with the mock disc  10  by one or more load springs  40 . In this embodiment, a cover  50  is utilized to protect the standard from contaminants such as dust and debris. In this embodiment, two screws  71 , 72  are used to secure the cover  50  (and thus, the mock heads  20 ) to the mock disc  10 . In this embodiment, the mock disc  10  is made of a substantially smooth, transparent material such as glass. Further, in this embodiment, the mock head  20  is provided an overcoat by thin film chemical deposition to emulate the optical properties of a head/slider.  
         [0015]    In one embodiment, the height standard  100  plays two roles: a light intensity calibration tool and a height calibration tool. As a light intensity calibration tool, an inclined surface  22  on one or more of the mock heads  20  is utilized. As shown in FIG. 3 b , in one embodiment, the light source  80  of the flying height tester is moved (with respect to the standard) along the inclined surface  22 . As the tester is passed over the inclined surface  22 , optical interference techniques (described below) yield an oscillating, continuous spectrum containing segments of high intensity light as well as darker segments. From this continuous spectrum, values for both maximum light intensity and minimum light intensity received at the detector  90  can be established. In this embodiment, the values of light intensity are stored in a computer (not shown) associated to the flying height tester.  
         [0016]    After establishing the range of light intensity for the flying height tester, in an embodiment, the depth (flying height) of at least one surface recess  302  is measured with the flying height tester to determine at least one ‘observed’ distance between the disc  10  and surface  23  of recessed portion  302 . In this embodiment, the physical dimensions of the mock head  20  may be determined by a device, such as an atomic force microscope (AFM), and thus, the ‘actual’ distance between the disc  10  and the surface  23  of the recessed portion  302  can be compared to the ‘observed’ distance for calibration of the flying height tester. The differential between ‘actual’ and ‘observed’ distance is used to adjust the flying height tester for calibration. In one embodiment, multiple recessed portions  302  of differing depths (heights) are provided to improve calibration (calibration for different heights). Also, because the dimensions of the inclined surface  22  are known, it can be used to perform gap calibration as well (i.e., depth being known at any position x).  
         [0017]    As explained, in one embodiment of the present invention, to calibrate a flying height tester, the calibration standard  100  is placed in the flying height tester in place of the original glass disc (not shown) of the tester under the tester&#39;s light source  80 . As shown in FIGS. 3 b  and  3   c , in calibrating the flying height tester, height measurements are taken by the tester, yielding ‘observed’ distances. The ‘observed’ distances are compared with the ‘actual’ distances at those locations. In one embodiment, a linear translator and computer (not shown) are utilized to position the standard  100  appropriately for measurement. In this embodiment, at each measurement point, monochromatic light  88   a  is directed at the (transparent) mock disc  10  by the light source  80 , as shown in FIG. 3 b . The light  88   a  impinges the disc  10  at an angle incident θ to a first mock disc surface  12  and continues through the (glass) mock disc  10  along path  88   b  to a second mock disc surface  11 , where it splits and is partially reflected. The reflected portion follows path  88   c  through the disc  10  to the first surface  12 , and follows path  88   d  to a sensor  90  of the flying height tester (not shown). The remaining light follows path  88   e  to the mock slider (head) surface  22  where it is reflected to the mock disc  10  via path  88   f.  The light impinges the second surface  11  of the mock disc  10 , follows path  88   g  through the disc  10  and follows path  88   h  to the tester sensor  90 . The slight angular deviations between paths at the air/disc interface are due to the Snell effect. Both the height h 2  and the incident angle θ have been exaggerated in FIG. 3 b  for illustrative purposes. Path  88   a  is actually substantially normal to the mock disc surface  12  with typical flying height testers.  
         [0018]    [0018]FIG. 4 illustrates surface irregularity compensation and provides further detailed illustrations of two mock heads according to an embodiment of the present invention. As seen in FIG. 4 a , because of surface irregularities upon the top of each mock head  20 , the distance, He, from disc to mock head surface  21  varies with position. In one embodiment of the present invention, the mock head&#39;s surface profile may be determined by a device such as a profilometer. This surface profile, combined with the knowledge of the ‘actual’ dimensions of the mock head  20  (by AFM, etc.) enable improved calibration. The true depth Ha of the recessed portion of the mock head  20  is slightly different than the apparent depth H 1  (because of high points  402  on the mock head  20  surface). Utilizing Ha as the ‘actual’ distance provides a more accurate value. In an embodiment, the acquired surface irregularity information may be used by the flying height tester computer to provide a correction factor or a series of correction factors for the calibration.  
         [0019]    [0019]FIGS. 4 b  and  4   c  further illustrate a mock head slider  20  with a recessed surface  23  and inclined surface  22  (see FIG. 4 b ) and a mock head slider  20  with a series of recessed surfaces (grooves)  23  at varying depths (see FIG. 4 c ) under an embodiment of the present invention. In one embodiment, recessed surface  23  length L 1  is greater than 50 microns, and the recessed surface  23  depth (flying height) H 1  is greater than 2 nanometers. In one embodiment, inclined surface height (rise) H 2  is between 12 and 13 microinches (0.31-0.33 microns), and inclined surface  22  length (run) L 2  approaches 100 mils (2,540 microns). As stated above, the mock heads  20  can be used together in a calibration standard  100  (see FIG. 3 a ), or they can be used alone in a calibration standard  100 .  
         [0020]    [0020]FIG. 5 provides a graphical illustration of the ‘unique fit’ solution utilized for providing a continuous spectrum of uniquely-valued combinations associatable to a range of head/disc gaps under principles of an embodiment of the present invention. In one embodiment, light of multiple wavelengths (e.g., three wavelengths  501 , 502 , 503 ) is directed at the surface to be measured. In one embodiment, upon varying the distance between the mock head and mock disc to obtain the maximum and minimum light intensity (for light intensity calibration), multiple curves may be developed. After calibrating light intensity at the different wavelengths (equalizing amplitude), the wavelengths displayed superimposed provide multiple curves that may be utilized for a ‘unique fit’ solution spectrum. By optical interference, light intensity  524  received by the detector oscillates repeatedly between the maximum  526  and the minimum  528  as the distance measured increases (or decreases). Although each curve passes through the same light intensity values multiple times as the measured distance increases (or decreases) through the range of possible values, the combination of values  511 , 512 , 513  provided by the multiple-wavelength light source is unique for each distance in the range of possible distances  522 . This ‘unique fit’ solution provides a range of light intensity combinations that is directly and uniquely associatable to the range of possible distances to be measured.  
         [0021]    According to embodiments of the present invention, a calibration device is provided for both light intensity/unique fit theory curves (inclined surface; See, e.g., FIG. 4 b ) and for specific depth (flying height) measurement calibration (recessed surface; See, e.g., FIG. 4 c ). In this embodiment, both mock heads are provided in the same calibration standard (as opposed to requiring a separate standard/device). As stated previously, typical calibration standards in the art provide no more than a series of grooves for gap calibration (on the disc side, not on the head side). For light intensity calibration and the development of theory curves, a separate component (a wedge piece) would need to be added, adding cost to the manufacture and operation. Therefore, in addition to the advantages of having varying-depth grooves on the mock head (as opposed to on the mock disc; as explained above), having all parts integrated in a single calibration standard is advantageous from both a complexity and a cost standpoint. Further, the process of forming grooves (by, e.g., ion milling or chemical etching) in a mock disk of glass, for example, is more difficult because of its hardness than forming similar grooves in a mock head (substrate). Further, etching glass with such methods produces surface roughness (irregularities) as large as 0.4 microinches (˜10 nanometers) or more, exacerbating calibration difficulties.  
         [0022]    Further, employing optical interference techniques with calibration grooves  60  formed in the mock disc  44 , such as in the prior art (see FIG. 2 a ), causes significant inaccuracies. If a measurement location is too close to the edge of a ridge  64 , one or more of the light beam&#39;s return paths may pass through the air  212  (glass-air-glass, rather than just glass), altering the path of the light (see FIG. 2 c ). Because the distance in which one of the light beam travels through air defines the height measurement perceived, the light should travel through consistent paths through the glass (i.e., uniform thickness mock disc, such as the present invention).  
         [0023]    [0023]FIG. 6 provides an illustration of a mock head design according to an alternative embodiment of the present invention. In this embodiment, the mock head  20  has two separate inclined surfaces  22 , 24 . In this embodiment they can be formed with differing slopes (H 2 /L 2  and H 4 /L 4 ). An inclined surface  22 , 24  with a shallow slope could be used for fine adjustment calibration and an inclined surface  22 , 24  with a steeper slope could be used for large range adjustment.  
         [0024]    [0024]FIG. 7 provides illustrations of three mock head designs according to alternative embodiments of the present invention. As shown in FIG. 7 a , in one embodiment, the mock head  20  has a cylindrically convex (curved) portion  702  and a recessed surface portion  704 . In this embodiment, the cylindrical portion  702  is used for light intensity calibration and gap spectrum calibration (via light intensity curves, as explained above). In this embodiment, the dimensions of the cylindrical portion  702  may be determined by AFM and known geometric principles to yield ‘actual’ (flying height) distances H  706  (similar to inclined surface  22 ; see FIG. 3 b ). Similar to above, in this embodiment, the recessed portion  704  is utilized for specific flying height calibration. As illustrated in FIG. 7 b , in another embodiment, a mock head  20  with a cylindrical portion  702  is utilized in the calibration standard. In this embodiment, the cylindrical portion  702  is used for light intensity calibration, gap spectrum calibration (via light intensity curves), and specific flying height calibration. In this embodiment, specific gap measurement calibration (via ‘actual’ vs. ‘measured’ differential) is taken at a desired location. As stated the ‘actual’ distance is known by a device such as an AFM. In another embodiment, the curved surface  702  of the designs shown in FIG. 7 a  and  7   b  is a spherical (convex) surface. In an alternative embodiment, as shown in FIG. 7 c , a curved surface  762  (e.g., spherical, cylindrical, etc.) occupies the top portion of a mock head  20  with an inclined surface portion, providing further flexibility of calibration.  
         [0025]    Although several embodiments are specifically illustrated and described herein, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.