Abstract:
A method and apparatus for performing interferometric measurement/testing of flying heights of read-write head sliders utilizing an improved rotating disk, the disk having a central opening for use with a spindle for rotation about a central axis, the disk comprising: 
     a substrate comprised of a light transmissive material and including a pair of opposed, smooth, major surfaces; and 
     a wear-resistant, protective overcoat layer on one of the major surfaces for improving the tribological properties thereof; 
     wherein the optical properties of the one surface of the disk are optimized for enhancing the sensitivity of the interferometric measurement/testing by increasing the intensity of reflected light received by a detector of the apparatus.

Description:
CROSS-REFERENCE TO PROVISIONAL APPLICATION 
     This application claims priority from U.S. provisional patent application Ser. No. 60/243,206, filed Oct. 24, 2000, the entire disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to method and apparatus for performing measurement/testing of the flying height of read-write head sliders utilized in disk-type data/information recording, storage, and retrieval systems. More particularly, the present invention relates to method and apparatus for performing flying height measurement/testing with increased sensitivity at very low. flying heights on the order of 5μ inches or less, e.g., 1μ inch or less. 
     BACKGROUND OF THE INVENTION 
     Thin film magnetic and magneto-optical (“MO”) recording media are conventionally employed in disk form for use with disk drives for storing large amounts of data in magnetizable form. Typically, one or more disks are rotated on a central axis in combination with data transducer heads. In operation, a typical contact start/stop (“CSS”) method commences when the head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by the air flow generated between the sliding surface of the head and the disk. During reading and recording operations, the transducer head is maintained at a controlled distance from the recording surface, supported on a bearing of air as the disk rotates, such that the head can be freely moved in both the circumferential and radial directions, allowing data to be recorded on and retrieved from the disk at a desired position. Upon terminating operation of the disk drive, the rotational speed of the disk decreases and the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Thus, the transducer head contacts the recording surface whenever the disk is stationary, accelerated from the static position, and during deceleration just prior to completely stopping. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic sequence consisting of stopping, sliding against the surface of the disk, floating in air, sliding against the surface of the disk, and stopping. 
     It is considered desirable during reading and recording operations, and for obtainment of high areal recording densities, to maintain the transducer head as close to the associated recording surface as is possible, i.e., to minimize the “flying height” of the head slider. Thus, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head, thereby permitting the head and the disk surface to be positioned in close proximity, with an attendant increase in predictability and consistent behavior of the air bearing supporting the head during motion. 
     As should be evident from the above, an experimental method for verification and testing of the fly height of the head slider during both the design and production phases of read-write heads for rotating disk magnetic and MO storage media is necessary. At present, three (3) wavelength interferometry, as for example, disclosed in U.S. Pat. No. 5,280,340 to C. Lacey, the entire disclosure of which is incorporated herein by reference, is the most commonly employed technique for direct measurement of fly heights. According to this technique, a test apparatus (e.g., such as manufactured by Phase Metrics, Inc., San Diego, Calif.) is utilized which comprises an optically flat, very smooth, light transparent (e.g., glass), rotating disk, typically coated on a first (e.g., lower or back side) with a very thin (e.g., 0.5-1.0 nm) layer of a perfluoropolyether lubricant, and a means for controllably positioning a head slider at a very small spacing (i.e., flying height or air gap) from the first surface of the disk. As shown in FIG. 1, white light emanating from a suitable source impinges the second (i.e., upper or front side) surface of the disk (illustratively at substantially normal incidence) and is transmitted through the disk. A first portion of the transmitted incident light travels through the air gap d between the first surface of the disk and the head slider, and reflected thereat back through the disk for ultimate receipt by a suitable detector positioned above the second (front side) surface of the disk; whereas a second portion of the transmitted incident light is reflected at the first (back side) surface of the disk back through the disk for ultimate receipt by the detector. The first and second portions of the transmitted light reflected from the head slider surface and from the first (back side) surface of the disk, respectively, are both constructively and destructively combined by interference in the space before the detector to yield a detector output which produces an intensity vs. wavelength pattern, depending upon the spacing (flying height) between the glass disk and the head slider (the preceding assumes that any portion of the incident light reflected from the second or front side surface of the glass disk is small and that any interference effect resulting therefrom is very small due to the thickness of the disk being much greater than the flying height d). 
     More specifically, and with reference to FIG. 2, the total reflected light intensity vs. wavelength resulting from the constructive and destructive interference of the first and second portions of the reflected incident light is modulated at a specific air gap or flying height d to produce a generally sinusoidally-shaped intensity vs. wavelength pattern having spaced-apart maxima and minima, and is compared with a calibration curve to determine the actual flying height. If incident light of a particular wavelength is utilized for the measurement, a half-cycle of the reflected intensity modulation corresponds to a change in the air gap or flying height d equal to one quarter (¼) of the particular incident wavelength. For example, for yellow/green incident light of 560 nm wavelength, the wavelength spacing between adjacent peaks of reflected light intensity of the intensity vs. wavelength pattern corresponds to a change in air gap or flying height of about 140 nm. 
     However, unlike typical interferometric measurements, the distances or spacings to be measured in air gap or flying height applications are much less than the wavelengths of the light utilized for the measurement, typically on the order of 25 nm or less; consequently, only a small portion of the peak-to-peak reflected light intensity vs. wavelength modulation pattern can be utilized for flying height measurement. Therefore, in order to maximize the sensitivity of the measurement, it is advantageous for the air gaps or flying heights corresponding to the relatively slowly changing reflected light intensities at the maximum and minimum of the modulation pattern of the reflected light intensity to be far from the air gap or flying height region of interest, where the reflected light intensity is desired to change rapidly with change in air gap or flying height. Further, in order to maximize measurement sensitivity at spacings of 25 nm or less, it is considered essential that overall intensity losses arising from the disk substrate due to, inter alia, internal reflection and absorption within the glass disk and external reflection and scattering therefrom, be minimized. 
     As indicated above, the continuing requirement for decreased flying heights for obtaining increased areal recording densities of magnetic recording media has necessitated continuing improvements in the sensitivity and accuracy of flying height measurements at very low head-to-disk spacings. However, accurate determination of flying heights below about 25 nm utilizing lubricated optical glass disks, as described supra, have become ever more problematic. While the sources or origins of the difficulties are several and varied, they are, in essence, dominated by the fact that the glass material utilized for the disk is a poor tribological surface for interaction with head sliders which are typically provided at their sliding surface(s) with a wear-resistant coating, e.g., of diamond-like carbon (“DLC”). Moreover, in addition to physical damage imparted to the glass disk surface and the head slider due to their intermittent contacting as in the CSS operation described above, the head flying over the lubricant-coated glass disk surface often incurs undesirable lubricant/contaminant buildup, resulting in alteration of the air bearing characteristics and instability of the flying height. 
     While it is well known that addition of a lubricated hard carbon overcoat (e.g., of DLC) to a rotating disk surface can significantly improve the tribological performance thereof by affording protection against friction and wear induced by contact with the DLC-coated head slider, a difficulty arises in that the optical properties of sputtered carbon (utilized for the DLC overcoat) are such that at the minimum thickness (i.e., about 5 nm) necessary to improve the tribology of the head slider-disk interface, the total change in reflected light intensity at the detector during interferometric air gap or flying height measurement as described above, e.g., for green-yellow light of about 562 nm wavelength, over a fly height range of from near 0 to about 25 nm, is reduced to about one-fourth (¼) of the intensity change obtained with bare (i.e., uncoated) glass. The disadvantageous reduction in intensity change attendant upon the use of glass disks with lubricated DLC overcoats, hence measurement sensitivity reduction, is attributed both to increased internal reflection within the glass disk at the disk/DLC interface due to the poor refractive index (n) match of the two materials, and to an increased amount of light absorption and scattering within the DLC layer. 
     In addition to the reduction in measurement sensitivity attributable to the above-described optical effects such as absorption and scattering, an additional reduction in measurement sensitivity results from the fact that the composite optical properties of carbon-coated glass substrates are such that the position of the minimum in the intensity vs. fly height curve is shifted into the fly height region of interest. This phase shift is a general property of multi-layer stacks, and must be taken into consideration as a factor affecting measurement sensitivity, hence precision and accuracy. 
     Accordingly, there exists a need for an improved method, apparatus, and disk means for performing interferometric measurement/testing of the fly height of a read-write head slider over the surface of a light transparent, rotating disk (e.g., of glass) having at least a wear-resistant protective overcoat layer thereon for improving the tribological properties thereof, which method, apparatus, and disk means overcome the drawbacks and disadvantages associated with the prior art means and methodology and affords increased measurement sensitivity at very low flying heights below about 5μ inches, while providing full compatibility with all aspects of interferometric air gap or flying height measurement/testing. 
     The present invention addresses and solves problems attendant upon the use of interferometric techniques for the measurement/testing of very small air gaps or flying heights of read-write head sliders utilized in very high areal recording density rotating disk-based, magnetic data/information recording, storage, and retrieval media and systems, while preserving the essential features of conventional interferometric air gap measurement apparatus and technology. An advantage afforded by the present invention is the ability to fabricate and utilize disks required for fly height measurement which comprise coated glass substrates akin to those utilized for magnetic disks, which coated glass substrates may be prepared by means of techniques and instrumentalities conventionally employed in the manufacture of magnetic recording media. Moreover, the means and methodology of the present invention can be utilized for gap or spacing measurement/testing as may be required for all manner of devices, for example, devices utilizing probe scanning techniques, e.g., Atomic Force Microscopes (“AFM”). 
     DISCLOSURE OF THE INVENTION 
     An advantage of the present invention is an improved method for performing interferometric measurement/testing of flying heights of read-write head sliders utilized in e.g., magnetic data/information recording, storage, and retrieval. 
     Another advantage of the present invention is an improved rotatable disk for use in performing interferometric measurement/testing of flying heights of read-write head sliders. 
     Still another advantage of the present invention is an improved apparatus for performing measurement/testing of flying heights of read-write head sliders of data/information recording, storage, and retrieval systems. 
     Additional advantages and other aspects and features of the present invention will be set forth in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims. 
     According to the present invention, the foregoing and other advantages are obtained in part by a method for performing interferometric measurement/testing of the fly height of a read-write head slider over a surface of a rotating disk  10 , as shown in FIG. 7, the disk surface having at least a wear-resistant, protective overcoat layer  14  thereon for improving tribological properties thereof, the method comprising transmitting at least a portion of an incident light beam through the rotating disk for reflection by an air bearing surface of the head slider facing the disk surface through an air gap, the method further comprising optimizing the optical properties of the disk surface for enhancing the sensitivity of the interferometric measurement/testing by increasing the intensity of reflected light received by a suitable detector. 
     According to embodiments of the present invention, the protective overcoat layer  14  comprises a material having a high refractive index from about 1.8 to about 2.4 (at an interferometric measurement wavelength of about 562 nm), e.g., a diamond-like carbon (DLC) material; and the disk surface further comprises a lubricant topcoat layer  22  over the protective overcoat layer  14 . 
     In accordance with a particular embodiment of the present invention, the disk  10  comprises a light transmissive substrate  12  having a high refractive index from about 1.9 to about 2.5, the high refractive index substrate  12  being index-matched to the high refractive index protective overcoat layer  14 , whereby loss of reflected light intensity due to internal reflection within the disk  10  is substantially eliminated, or at least reduced, and the phase shift characteristic of multi-layer stacks is minimized, or at least reduced; e.g., the protective overcoat layer  14  comprises a high refractive index, diamond-like carbon (DLC) material and the disk  10  comprises a high refractive index glass substrate  12 . 
     According to another embodiment of the present invention, the disk  20  comprises a light transmissive substrate  18  having a low refractive index from about 1.4 to about 1.6, as shown in FIG. 8, with an underlayer  22  of a material having a very high index of refraction n from about 1.9 to about 2.6 and a very low extinction coefficient k interposed between the disk surface and said high refractive index protective overcoat layer  14 , with the primary considerations being both the gain or loss in measurement sensitivity due to the phase shift and intensity loss(es). For example, the very high refractive index, very low extinction coefficient underlayer  22  comprises a material selected from the group consisting of ZnS, SiN, TiO 2 , ZrO 2 , Ta 2 O 5 , HfO 2 , TiN, BN, and multi-layer metal structures, with ZnS and SiN presently preferred; the high refractive index protective overcoat layer  14  comprises a diamond-like carbon (DLC) material; and the thickness of each of the protective overcoat layer  14  and underlayer  22  is selected to provide enhancement in reflected light intensity received by the detector for air gaps, hence fly heights of the head slider, of about 5μ inches and below, e.g., for air gaps not greater than about 1μ inch. 
     Another aspect of the present invention is a disk  10  for use in an apparatus for performing interferometric measurement/testing of flying heights of read-write head sliders, the disk  10  having a central opening for use with a spindle for rotation about a central axis, the disk  10  comprising: 
     a substrate  12  comprised of a light transmissive material and including a pair of opposed, smooth, major surfaces; and 
     a wear-resistant, protective overcoat layer  14  on one of the major surfaces for improving the tribological properties thereof; 
     wherein the optical properties of the one surface of the disk  10  are optimized for enhancing the sensitivity of the interferometric measurement/testing by increasing the intensity of reflected light received by a detector of said apparatus. 
     According to embodiments of the present invention, the protective overcoat layer  14  comprises a diamond-like carbon (DLC) material having a high refractive index from about 1.8 to about 2.4 (at an interferometric measurement wavelength of about 562 nm); and a lubricant topcoat layer  16  is provided over the protective overcoat layer  14 . 
     In accordance with an embodiment of the present invention, the light transmissive substrate  12  comprises a glass material having a high refractive index from about 1.9 to about 2.5, the high refractive index glass material being index-matched to the high refractive index DLC material of the protective overcoat layer  14 , whereby loss of reflected light intensity due to internal reflection within the disk  10  is substantially eliminated, or at least reduced. 
     According to another embodiment of the present invention, the light transmissive substrate  18  comprises a glass material having a low refractive index from about 1.4 to about 1.6; and the disk  20  further comprises an underlayer  22  of a material having a very high index of refraction n from about 1.9 to about 2.6 and a very low extinction coefficient k from 0 to about 0.5 interposed between the one surface of the low refractive index glass substrate  18  and the high refractive index, protective overcoat layer  14  of DLC material; wherein the very high refractive index, very low extinction coefficient underlayer  22  comprises a material selected from the group consisting of ZnS, SiN, TiO 2 , ZrO 2 , Ta 2 O 5 , HfO 2 , TiN, BN, and multi-layer metal structures, with ZnS and SiN presently preferred; and the thickness of each of the protective overcoat layer  14  and the underlayer  22  is selected to provide enhancement in reflected light intensity received by the detector for flying heights of the head slider between about 0 and about 5μ in. 
     Yet another aspect of the present invention is an apparatus for performing interferometric measurement/testing of flying heights of read-write head sliders, comprising: 
     a rotatable disk  10  comprised of a light transmissive substrate material; and 
     means for optimizing the optical properties of one side of the disk  10  for enhancing the sensitivity of the measurement/testing for flying heights between about 0 and about 5μ in. 
     According to embodiments of the present invention, the one side of the disk  10  includes protective overcoat  14  and lubricant topcoat 16 layers thereon. 
    
    
     Additional advantages and aspects of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein embodiments of the present invention are shown and described, simply by way of illustration of the best mode contemplated for practicing the present invention. As will be described, the present invention is capable of other and different embodiments, and its several details are susceptible of modification in various obvious respects. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as limitative. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following detailed description of the embodiments of the present invention can best be understood when read in conjunction with the following drawings, wherein: 
     FIG. 1 schematically illustrates, in simplified cross-sectional form, the critical optical paths involved in the measurement of an air gap or flying height “d” by means of interferometry; 
     FIG. 2 is a sample of a graph of relative reflected white light intensity vs. wavelength obtained in an interferometric measurement of air gap or flying height utilizing an optical path arrangement as in FIG. 1; 
     FIGS.  3 (A)- 3 (B) are graphs of calculated relative reflected light intensity vs. fly height (“FH”) for various glass disk substrates according to embodiments of the present invention; 
     FIGS. 4 and 5 are graphs for showing the variation with wavelength of n and k of SiN and a-C:H, respectively; and 
     FIG. 6 is a graph for showing the correlation between fly heights measured with standard glass disks and the glass/SiN/a-C:H disks of the present invention. 
     FIG. 7 is an illustration of a disk according to an embodiment of the present invention. 
     FIG. 8 is an illustration of a disk according to another embodiment of the present invention. 
     FIG. 9 is an illustration of a disk according to another embodiment of the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     The present invention is based upon the recognition that the sensitivity of interferometric measurement/testing of the air gap or flying height of a read-write head slider operating at very low spacings from a rotating disk  10 ,  20 ,  30  surface, e.g., from about 0 to about 5μ inches, can be significantly enhanced by optimizing the optical properties of the surface of the light transmissive rotating disk  10 ,  20 ,  30  facing the surface of the head slider across the air gap. 
     A key feature according to an embodiment of the present invention is the use of a light transmissive disk  10  comprised of a high index of refraction substrate material  12 , which substrate material  12  is index-matched to a high refractive index, protective overcoat layer  14 , whereby loss of reflected light intensity due to internal reflection within the disk  10  is substantially eliminated, or at least reduced; whereas, a key feature according to another embodiment of the present invention, is the use of a light transmissive disk  20  comprised of a low refractive index material, and an underlayer  22  of a very high refractive index material is provided between the light transmissive disk  20  and the high refractive index protective overcoat layer  14 , whereby the intensity of reflected light received by a detector of an interferometric air gap measurement/testing apparatus is substantially increased, thereby providing a corresponding increase in obtainable measurement sensitivity at very small air gaps or flying heights. 
     More specifically, according to a first embodiment of the present invention, the poor tribological properties of uncoated glass and the poor optical properties of carbon-coated (e.g., DLC-coated) glass are overcome by use of a glass substrate material  12  having a high refractive index (i.e., from about 1.9 to about 2.5) which is better matched to that of the high refractive index (i.e., from about 1.8 to about 2.4) carbon-based protective overcoat  14 , typically a DLC material, such as sputtered a-C:H. According to this embodiment, losses in reflected light intensity due to internal reflection at the glass/DLC interface can be reduced, the amount of reduction depending upon the degree of index matching. For example, perfect index matching can provide an intensity sensitivity corresponding to about 85% of that obtainable with bare (i.e., no protective overcoat), lubricated low refractive index glass. However, in this case, as well as in the case of lubricated bare, low refractive index glass, a significant loss in absolute reflected light intensity occurs due to unwanted reflections at the air/a-C:H and air/glass interfaces. 
     According to a second, more preferable embodiment of the present invention, a greater increase in reflected light intensity, hence an increase in measurement sensitivity of a factor of 2-3 over that obtainable with lubricated, bare low index glass disks, can be obtained by forming a light transmissive disk  20  wherein an appropriately thick (e.g., quarter wavelength) underlayer  22  of a very high refractive index (i.e., having a value of refractive index n from about 1.9 to about 2.6), very low extinction coefficient material (i.e., having a value of extinction coefficient k from about 0 to about 0.5) is sandwiched between a glass substrate  18  of a low refractive index material (i.e., having a value of refractive index n from about 1.4 to about 1.6) and a high refractive index protective overcoat layer  14  (e.g., a DLC material such as sputtered a-C:H having a high refractive index from about 1.8 to about 2.4). According to the invention, suitable high refractive index, low extinction coefficient materials for use according to the second embodiment include, but are not necessarily limited to, materials selected from the group consisting of ZnS, SiN, TiO 2 , ZrO 2 , Ta 2 O 5 , HfO 2 , TiN, BN, and multi-layer metal structures. 
     Simulations have been performed in order to evaluate several different materials combinations and layer thicknesses for each of the first and second embodiments, utilizing standard interferometric formulae and calculations, in which the system included, in the optical path, a fly test disk  30 , as shown in FIG. 9, comprising, in sequence, of a semi-infinite medium  18  (i.e., the glass disk substrate), quarter wavelength thick high refractive index, low extinction coefficient underlayer  22 , protective overcoat  14  (sputtered a-C:H), and lubricant topcoat  16  (a perfluoropolyether); an air gap (i.e., the fly height); and a head slider comprising, in sequence, a DLC-based wear-resistant coating of sputtered a-C:H, a Si-based head underlayer, and a semi-infinite medium (i.e., the head slider body material). The result of the simulation is the calculated real reflectance of the combination as the air gap or fly height is varied over the range from −50 to +150 nm, the calculation being extended into the negative fly height region because the minimum of the reflected intensity vs. fly height curve can occur at theoretically negative fly heights for some combinations of materials. The materials, optical properties, and thicknesses thereof used for the various combinations are given below in Table 1. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Layer 
                 Material 
                 n 
                 k 
                 Thickness 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Substrates 
                 normal glass 
                 1.53 
                 0.0 
                 semi-infinite 
               
               
                   
                 high index glass 
                 1.90 
                 0.0 
                 semi-infinite 
               
               
                   
                 ZnS disk 
                 2.39 
                 0.0 
                 semi-infinite 
               
               
                 Underlayer 
                 ZnS 
                 2.39 
                 0.0 
                  0-100 nm 
               
               
                   
                 SiN 
                 2.00 
                 0.03 
                  0-100 nm 
               
               
                 Overcoat 
                 a-C:H 
                 1.85 
                 1.6 
                   5 nm 
               
               
                 Lubricant 
                 Fluoropolymer 
                 1.30 
                 0.0 
                  ˜5 nm 
               
               
                 Air Gap 
                 air 
                 1.0 
                 0.0 
                 −50-150 nm 
               
               
                 Head Slider 
                 DLC coating 
                 2.2 
                 0.4 
                 3.5 nm 
               
               
                   
                 Si Underlayer 
                 2.53 
                 0.0 
                 1.5 nm 
               
               
                   
                 Head Slider Mat&#39;l 
                 2.25 
                 0.45 
                 semi-infinite 
               
               
                   
               
             
          
         
       
     
     Results of the simulation for the various different combinations of materials and thicknesses thereof are graphically shown in FIG. 3, wherein the abscissa indicates fly heights in the range from −2 to +6μ inches and the ordinate indicates relative reflected light intensities in arbitrary units. Flying height (“FH”) measurement sensitivities for the different combinations of materials and thicknesses were estimated from the calculated reflected light intensity changes, over the very low flying height range of 0-1μ inch (i.e., 0 to 25 nm), relative to the reflected light intensity calculated for lubricated bare glass disks. The results are presented in the following Table 2. 
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Relative 
               
               
                   
                   
                 Reflected Light 
               
               
                   
                   
                 Intensity Change, 
               
               
                 Figure No. 
                 Configuration 
                 0-1 μin. 
               
               
                   
               
             
             
               
                 3(A), 
                 Bare lubricated low index glass (currently 
                 ˜100% 
               
               
                 3(B) 
                 used) 
               
               
                 3(A) 
                 Low index glass with 5 nm a-C:H DLC 
                  ˜25% 
               
               
                   
                 layer 
               
               
                 3(B) 
                 ZnS substrate with 5 nm a-C:H DLC layer 
                 ˜175% 
               
               
                 3(A) 
                 Low index glass with 60 nm SiN 
                 ˜240% 
               
               
                   
                 underlayer and 5 nm a-C:H DLC layer 
               
               
                 3(B) 
                 Low index glass with 56 nm ZnS 
                 ˜270% 
               
               
                   
                 underlayer and 5 nm a-C:H DLC layer 
               
               
                   
               
             
          
         
       
     
     From the above, it is evident that each of the embodiments according to the present invention, i.e., wherein the refractive indices of the disk substrate  12  and protective overcoat layer  14  are each high and matched, as in the first embodiment; or wherein a very high refractive index, very low extinction coefficient, quarter wavelength thick underlayer  22  is provided between a low refractive index glass substrate  18  and a high refractive index protective overcoat layer  14 , as in the second embodiment, can provide a substantial enhancement in total reflected light intensity change which, in turn, yields a significant increase in measurement sensitivity for flying heights in the air gap region below about 1μ inch (i.e., &lt;25 nm). 
     In order to provide further verification of the efficacy of the inventive concept, three (3)-wavelength (i.e., blue, green, and yellow light) flying height measurement/testing was performed with glass disk substrates  18  provided with an about 60 nm thick silicon nitride (SiN) underlayer  22 , a protective overcoat layer  14  comprised of an about 5 nm thick layer of hydrogen-doped carbon (a-C:H), wherein n and k of each of SiN and a-C:H are wavelength dependent and determined from the graphs of FIGS. 4 and 5, respectively, and an about 1 nm thick layer of a PFPE (perfluoropolyethylene) lubricant  16 . Results are given below in Table 3. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 Reflected Light Intensity (arbitrary unit) 
                   
               
             
          
           
               
                   
                   
                   
                 Standard Test 
               
               
                   
                 Light 
                 a-C:H/SiN/Glass Disk 
                 Glass Disk 
               
               
                   
                   
               
               
                   
                 Blue 
                   
                   
               
               
                   
                 Max. 
                 603 
                 594 
               
               
                   
                 Min. 
                 472 
                 494 
               
               
                   
                 Green 
               
               
                   
                 Max 
                 714 
                 657 
               
               
                   
                 Min. 
                 545 
                 556 
               
               
                   
                 Yellow 
               
               
                   
                 Max. 
                 695 
                 581 
               
               
                   
                 Min. 
                 328 
                 357 
               
               
                   
                 Fly Height 
                 0.727 μin. 
                 0.390 μin. 
               
               
                   
                 Std. Dev., σ 
                 0.022 μin. 
                 0.046 μin. 
               
               
                   
                   
               
             
          
         
       
     
     As is apparent from Table 3, for each of the three (3) wavelengths utilized for the interferometric flying height measurement, the glass disk media  30  fabricated according to the invention and including a high refractive index, low extinction coefficient SiN underlayer  22  intermediate the glass substrate  18  and the carbon-based protective overcoat layer  14  agreed with predictions indicated by the above simulations and exhibited a greater change in reflected light intensity between the minimum and maximum of the reflected light intensity vs. wavelength modulation pattern than that provided by the conventional glass disk media  30  not including the underlayer  22  according to the invention. Further, the lower σ value for the fly height measurements with the inventive disk media  30  indicate a better tribological interface, vis-a-vis that of the conventional, i.e., standard, glass disk media. The differences in absolute fly height in the table result from lack of availability of suitable calibration data for the inventive disk media. Finally, FIG. 6 graphically shows the generally good correlation between fly heights measured using a standard glass disk and the a-C:H/SiN/glass disk  30  of the present invention. 
     Thus, the present invention advantageously provides improved disk media  10 ,  20 ,  30  for use in interferometric measurement of very low flying heights of read-write heads such as are utilized in very high areal recording density magnetic media and systems, i.e., below about 1μ in. The inventive media  10 ,  20 ,  30  substantially eliminate, or at least reduce unwanted loss in reflected light intensity and provide a substantial increase in the change in reflected light intensity from minimum to maximum of the reflected light intensity vs. wavelength modulation curve, thereby increasing the sensitivity of fly height measurement at very low fly heights. The disk media  10 ,  20 ,  30  of the present invention are especially useful when employed in conjunction with interferometric flying height apparatus utilizing the three (3) wavelength method and enjoy particular utility in the development of high recording density media for computer-related applications. In addition, the inventive media  10 ,  20 ,  30  can be readily fabricated by means of conventional methodologies, e.g., sputtering techniques. 
     In the previous description, numerous specific details are set forth, such as specific materials, structures, processes, etc., in order to provide a better understanding of the present invention. However, the present invention can be practiced without resorting to the details specifically set forth. In other instances, well-known processing materials and techniques have not been described in detail in order not to unnecessarily obscure the present invention. 
     Only the preferred embodiments of the present invention and but a few examples of its versatility are shown and described in the present disclosure. It is to be understood that the present invention is capable of use in various other combinations and environments and is susceptible of changes and/or modifications within the scope of the inventive concept as expressed herein.