Patterned multi-track thin film heads for image area record/reproduce on magnetics-on-film

Abstract: Magnetic recording and reproducing linear thin film head arrays (30) particularly for use for recording and reading information on a magnetics-on-film (MOF) layer (28) in the image area of photographic filmstrips (10) and particularly a configuration and mounting of such heads to achieve high compliance with the relatively stiff filmstrip and MOF layer in cameras or other filmstrip handling apparatus without damaging the filmstrip. When constrained in the film transport path, the filmstrip assumes a convex cross film curvature on the MOF layer side approximated by a sixth order polynomial having even terms only. A base line for the deposition of thin film head layers of each head of the array on a linear substrate is approximated by the sixth order polynomial having even terms only. After deposition of the thin film layers, the linear head array is shaped to the cross film curvature, and the lapped head surfaces are aligned with the base line. Preferably, at least one electrical lapping guide is deposited on the substrate in relation to the linear array of magnetic head elements and the base line, wherein the head element layer height is controlled in the head fabrication processing by monitoring the electrical resistance of the electrical lapping guide during shaping of the convex curvature of the head surface.

Description: FIELD OF THE INVENTION This invention relates to magnetic recording and reproducing linear head arrays particularly for use for recording and reading information on a magnetics-on-film (MOF) layer in the image area of photographic filmstrips and particularly to the configuration and mounting of such heads to achieve high compliance with the relatively stiff filmstrip and MOF layer in cameras or other filmstrip handling apparatus without damaging the filmstrip. BACKGROUND OF THE INVENTION In commonly assigned U.S. Patent Nos. 4,933,780 and 4,977,419, a photographic film is disclosed having a virtually transparent magnetic layer covering the non-emulsion side of the film (referred to as a magnetics-on-film or MOF layer) and dedicated recording areas or tracks on the layer for recording information such as film type, film speed, film exposure information and information relevant to the processing and subsequent use (e.g., printing) of the exposed image frames on dedicated longitudinally extending tracks. The systems disclosed therein suggest recording of information during film manufacture, reading and/or recording of information on certain tracks during camera use, and reading and/or recording of print related information during photofinishing using the MOF layer. The information may include voiced messages or sound associated with the photographed scene and may be recorded in digital or analog format on certain of the tracks. The specific camera recording tracks and recording heads disclosed in the '419 and '780 patents are disposed to record in tracks located along the longitudinal edges of the film and outside the MOF tracks in the image frame area. The possibility of recording audio within certain tracks of the MOF layer within the image frame area is described in commonly assigned U.S. Patent No. 5,276,472. Further commonly assigned U.S. Patent Nos. 5,034,836 and 5,041,933 also disclose magnetic recording head configurations for use in recording in longitudinal tracks alongside the filmstrip edges but not in the image frame area. None of the above referenced patents specifically disclose magnetic linear head arrays (i.e. multi-head arrays) for recording or reproducing in a plurality of tracks in the MOF layer on the side of the film opposite to the image frame area. Reading and recording information on tracks of a magnetic coating or stripes of magnetic coatings located in the image frame area on photographic film requires the development of a unique interface between the magnetic head and the photographic film. That interface is described in detail in the above-referenced '352 application. The magnetic head-to-photographic film interface differs in major ways from that normally associated with a head/disk interface in a computer disk drive, a head/tape interface in a tape drive. In a computer disk drive, the head flies or floats on the air film that is created as the flat surface of the disk rotates at high speed. Contact of the magnetic head with the disk is to be avoided at all costs. In a typical audio or video analog tape drive, the tape is flexible enough under tension to conform to the head contour both in the tape feed direction and in the cross-tape direction. The tape path typically wraps the tape over the magnetic head surface as the tape is advanced in the direction of tape advancement. Thus, the magnetic head surface is typically curved in the direction of tape transport or the direction it sweeps over the tape. For example, linear head arrays for recording in parallel tracks of digital audio tape are described in U.S. Patent No. 4,669,016. In the multi-head magnetic head array of the type shown in the '016 patent, a narrow arcuate head surface contact zone in the tape transport direction is provided in conjunction with a generally flattened arc head surface contact zone in the transverse, cross-tape direction to create grooves between the adjacent magnetic heads. The grooves are asserted to provide high contact compliance between the head surfaces and the tape, increase uniformity of wear, and also provide air ducts during high speed tape transport. In an electronic still camera context described in U.S. Patent No. 4,910,629, a thin film magnetic sheet or disk is rotated in a plane with respect to a magnetic recording head that is mounted to protrude into the plane to make adequate contact to locally deflect the disk as it rotates. The magnetic head is also mounted to move inward or outward in a radial direction with respect to the center of rotation of the magnetic disk to define a spiral recording track. The magnetic head surface is therefore contoured in both the spiral path and radial directions. In a further U.S. Patent No. 5,146,378, a multi-head, linear, thin film magnetic head array apparently for recording in closely spaced tracks on a magnetic disk is described wherein the head array is aligned in the radial direction of the disk to record in a plurality of tracks as the disk rotates. The linear head array protrudes into the disk plane and deflects it as in the case of a single or dual head of the type described in the '629 patent. In the '378 patent, the head surface is curved in a single radius arc along the length of the linear head array so that the center-most magnetic head(s) protrude further into the disk plane than the magnetic heads elements to either side. The single radius arc is appropriate to a highly flexible, thin film magnetic media that is moved past the linear magnetic head array at relatively high speed and readily conforms to the magnetic head array. Photographic film is of much greater thickness than the magnetic tape, disk or sheet media employed in the above described systems. Typically, film has a thickness on the order of about 0.003 - 0.006 inches or 0.076 - 0.152 mm and is relatively stiff and inflexible in comparison with magnetic tape or disk media. Film is not elastically deformable in localized areas of the size, shapes and depths contemplated by the above-referenced '016, '378 and '629 patents sufficiently to achieve high enough head-to-film compliance to effect reliable recording. The magnetic tape, disk and sheet media employed with intimate head contact of the type described typically are about 0.0005 - 0.0010 inches or 0.013 - 0.025 mm thick and readily deform elastically to comply with the described head contours and penetration depths. When removed from its cartridge, film shows a very observable cross-film curvature across its width that is convex on the non-emulsion side of the film. In addition, the unwrapped and unrestrained film also shows a convex curvature along its length, again on the non-emulsion side of the film. This latter curvature is attributed primarily to a core-set curl that results from the film having been tightly wound on a film cartridge spool. The convex cross-film curvature across the width of the film is primarily caused by the multilayers of emulsion and MOF (if present). These layers have different stretch properties than that of the base film substrate of acetate, PET, or PEN material. The cross-film curvature is also influenced by the bending phenomena known as anticlastic curvature. The degree of cross-film curvature depends on the time and temperature history of the film, the relative humidity, and the thickness of the film layers. Because of the cross-film curvature, it is difficult to achieve good, uniform contact or compliance across the full width of a multi-track recording head located in the image frame area. When photographic film is wrapped around a cylindrical arc contour, as is typically used in a magnetic tape head, most of the cross-film curvature described above is reduced. However, intimate compliance of the film surface to the cylindrical or arcuate head surface is not achieved due to minute air separations or spacings caused by the stiffness of the film. If a pressure pad is applied to the emulsion side of the film in the area immediately opposite the head position, then considerable improvement can be made in reducing the film-to-head spacing. But, the use of image area pressure pad support is not favored due to potential scratching of the emulsion layer and resulting degraded photographic image quality. The prolate ellipsoidal head contours of the linear head array, and the positioning of the head array with respect to the film transport path to effect self loading of the MOF layer against the head contours, disclosed in the above-referenced '352 application, represent a substantial improvement in stationary linear head array design. However, the contouring of the head surfaces can degrade the performance of the outermost head elements, particularly in the case of thin film record/reproduce heads. SUMMARY OF THE INVENTION It is therefore a primary object of the invention to enhance the compliance of a magnetic record and/or reproduce head with the MOF layer of a filmstrip for recording and/or reproducing information accurately in a plurality of tracks in the image area of the MOF layer without negatively affecting the quality of the photographic image. The invention is realized in a first aspect in a thin film magnetic head array for recording and/or reproducing information in tracks of a magnetic layer on photographic film exhibiting a predetermined cross-film, concave curvature extending across the image frame area width of the filmstrip when the filmstrip is constrained in a film transport path for movement with respect to the magnetic head array comprising: means for defining a filmstrip transport path for constraining a filmstrip to exhibit the predetermined cross-film, concave curvature; an elongated head substrate mounted with respect to the filmstrip transport path defining means having an array length extending across the image frame area width and a head element surface adapted to contact the magnetic layer approximated by a sixth order polynomial having even terms only and complying with the convex curvature of the filmstrip constrained in the film transport path; a plurality of magnetic head elements deposited in a linear array as thin film layers on the elongated head substrate extending between a head element layer deposition base line and the head surface and having predetermined head element heights, wherein the head element layer deposition base line is approximated by a sixth order polynomial having even terms only and complying with the convex curvature of the filmstrip constrained in the film transport path. The plurality of record/reproduce heads are fabricated in the shape of a plurality of frustums including inner frustums supporting a plurality of active head elements in an interior region in the cross film direction and at least one further outer frustum on each side of the inner frustums in the cross film direction. Preferably, at least one electrical lapping guide is deposited on the substrate in alignment with one of the outer frustums and in relation to the linear array of magnetic head elements and in conformance with the convex curvature of the head surface, wherein the head element layer height is controlled in the head fabrication processing by monitoring the electrical resistance of the electrical lapping guide during shaping, by lapping, of the convex curvature of the head surface. In a preferred embodiment, the magnetic head elements are preferably flux-guided paired MR or PMR head elements with flux elements at the head surfaces that may be contoured during the lapping process thereby loosening lapping tolerances. The invention is preferably implemented within a camera where the filmstrip is transported through a camera exposure gate for exposing image frames in the photosensitive layers of the filmstrip on the opposite side of the filmstrip from the MOF layer. The invention may also be practiced in other filmstrip transport apparatus associated with film manufacture in order to pre-record information in certain tracks or in processing of the exposed filmstrips to develop the image frames or in making prints from the image frames in order to record further information in other tracks or for any other reason that it may prove useful to read information from or write information into the tracks and read or record information into selected tracks of the MOF layer. ADVANTAGES OF THE INVENTION In accordance with the present invention, and due to the various features thereof, an array of thin film head devices that are deposited in a pattern conforming to the curvature of the head bearing surface as defined by the cross-film curvature shape of the photographic film constrained in a film gate is achieved. The pattern enables the close gap depth (for inductive heads) or element height (for MR heads) tolerance required for thin film head structures. In addition, the uniquely shaped in process, monitoring devices (ELGs), deposited in at least one location along the array pattern base line, aid in the fabrication of the head bearing surface contour. In use, the resulting head-to-film interface provided by the head bearing surface contour does not require emulsion side support, nor does it generate any significant surface damage to the MOF surface in the image frame area. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects, advantages and features of the invention will be become apparent from the detailed description given hereinafter in relation to the accompanying drawings, in which: Figure 1 is a perspective view of a camera or other photographic equipment film rail and gate frame for constraining a filmstrip in a film transport path; Figure 2 is an illustration of a linear magnetic head array of thin film magnetic head elements having head surfaces machined into frustums; Figure 3 is a computer drawn perspective view of the large radius, prolate ellipsoidal shapes on the tops of the frustums of the magnetic head arrays of the first and second embodiments after lapping; Figure 4 is a simplified cross-section view of a first embodiment of the thin film layers of the linear head array of Figure 2 prior to lapping; Figure 5 is a simplified cross-section view of the first embodiment of the thin film layers of the linear head array of Figure 2 after lapping; Figures 6 - 8 are simplified cross-section views of the steps of lapping of the outermost electronic lapping guides and dummy head elements of the first embodiment of the head array; Figure 9 is a simplified cross-section view of a second embodiment of the thin film layers of the linear head array of Figure 2 prior to lapping; Figure 10 is a simplified cross-section view of the second embodiment of the thin film layers of the linear head array of Figure 2 after lapping; Figures 11 - 13 are simplified cross-section views of the steps of lapping of the outermost electronic lapping guides and dummy head elements of the second embodiment of the head array; Figure 14 is a top view of a section of a linear array of flux-guided PMR head layers also arranged in an exaggerated and staggered set back manner illustrating a further implementation of the present invention; and Figure 15 is a top view of a further section of a linear array of flux-guided PMR head layers also arranged in an exaggerated and fanned set back manner illustrating a still further implementation of the present invention. The drawings are not necessarily to scale. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The cross-film curvature of a film or filmstrip manifests itself when the filmstrip is attempted to be flattened for transport through a camera exposure gate or a transport path of photofinishing equipment by restraining the film edges. In the transport of filmstrips through a straightened transport path, contact on either side of the film in the image frame area is avoided to minimize the possibility of scratching that area and degrading the photographic filmstrip. Figure 1 depicts in exaggerated scale, the cross-film curvature of a film or filmstrip 10 that occurs when the core set longitudinal curl is straightened out by guidance and constraint of the longitudinal edges of filmstrip 10 in a film rail and gate frame 20 providing a filmstrip transport path. Gate frame 20 has parallel film rails 14 and 16 formed in a elongated channels 24 and 26 closely spaced from a spring biased backing plate 18. As shown in Figure 1, the backing plate 18 is spring biased away from a base plate 36 and is formed with an opening 12 for mounting a linear magnetic head array of the present invention to record or reproduce information in a plurality of linear tracks extending lengthwise in the filmstrip image frame area. In the context of a camera, the filmstrip 10 is intended to extend between the film rails 14, 16 and gate frame 18 from a supply side, across the camera exposure gate and to a take-up side. In photographic equipment, e.g. a photographic printer or a preparation station, the filmstrip 10 is intended to extend between the film rails 14, 16 and gate frame 18 from a supply reel, across an image frame scanning station and to a take-up side. The design of such compact still camera and photographic equipment film transport paths and mechanisms is well known in the art. In either case, the MOF layer 28 on the concave side of filmstrip 10 also bears against the linear magnetic head array of the present invention in the manner described in further detail in the above-referenced '352 application, for example. The filmstrip is not deformed by the linear head array. Instead, the inherent stiffness of the filmstrip when constrained as depicted provides a loading force against the shaped head elements of the linear head arrays of the preferred embodiments of the invention. As described therein, a high degree of compliance is achieved without resorting to any additional tension on the filmstrip 10 or the use of a backing plate or roller or the like to apply pressure or load to the emulsion layer side of the filmstrip 10. In this regard, the linear magnetic head array penetrates the film path a minute depth to create a uniform wrap angle of between 0.5° to 4.5° of the leading and trailing filmstrip wrapped around the linear head array within the opening 12. This ensures that a narrow region of contact between the filmstrip MOF layer 28 and the linear magnetic head array is directly over each magnetic head element. The resulting spring load against the head bearing surface is sufficient to generate a pressure contact at the interface of less than 30 PSI (21 kPa). Because of this natural spring load, no media (i.e., film) tension is required to establish and maintain a robust interface, and the compliance is furthermore enhanced once a pulse of tension is applied to the filmstrip to provide frame stepping motion, in camera applications. Turning now to Figure 2, it depicts a linear magnetic head array 30 of thin film magnetic head elements and dummy head/electronic lapping guides (ELGs) having surfaces machined into the shape of n = 17 frustums 32 0 - 32 16 . The magnetic head array 30 is preferably formed of either thin film inductive magnetic head elements or thin film magnetoresistive (MR) head elements comprising fifteen active inductive head gaps or MR head defining layers, depending on the head type, and two outermost dummy head/ELG resistors formed in head frustums 32 0 and 32 16 as described in greater detail below. The head array therefore can track fifteen tracks on the MOF layer 28 of the filmstrip 10. In use, the magnetic head array 30, fabricated as described below, can be supported on a head stepping mechanism to step the head array between two positions to track a total of 30 tracks in the MOF layer. The frustums 32 0 - 32 16 have relatively sharp machined edges in both directions and are lapped in accordance with the present invention to provide the preferred magnetic head-to-film interface that enables near-contact compliance around each individual head gap and across the full multi-track head array width as disclosed in the above-referenced '352 application. A resulting prolate ellipsoidal contour (as in an upper surface of a football lying on its side) is effected by in-situ lapping of the frustums 32 0 - 32 16 of each head element/ELG surface to match the convex cross-film curvature of the photographic film at the site where the filmstrip contacts the particular head element surface in the linear head array 30. Figure 3 depicts an example of the resulting prolate ellipsoidal head contour 34 of one of the head element frustums 32 n . The frustums 32 0 - 32 16 are lapped in a fixture mimicking the film transport path of Figure 1 by mounting the linear head array 30 in its intended position of use and advancing a photographic film having an abrasive tape applied to the facing film surface through the transport path. After a length of the abrasive tape/film sandwich is passed over the linear head array, all of the initial high pressure points of contact of the frustums 32 n are abraded down so that the resulting head and ELG surfaces are contoured to bear relatively equivalent pressures or loads. The in-situ lapping process produces two contour features: 1) a large radius, prolate ellipsoidal shape on the bearing surfaces of the individual frustums that minimizes film damage as shown in Figure 3; and 2) the convex cross-curvature across the multi-track head width as shown, for example, in Figures 5 and 10, described below. In this process, the heights D n of the head gaps or thin film layers are made relatively equal. In experiments conducted, a 24 mm wide film of 100 µm thickness, interposed with a 25 µm-thick diamond abrasive tape, produced the multi-track linear head array convex cross-curvature of Figure 3 (and Figures 5 and 10). The regions of highest lapping down are noted to be on the surfaces of outermost ELGs and head elements closest to the gate rails, i.e. those having the starting frustums 32 0 - 32 3 and 32 13 - 32 16 . This result reflects the fact that the initial highest pressure points on the linear head array bearing surfaces are focused on these outermost head frustums which are closest to the concave surface of the abrading tape/film. The multi-track head contour of Figure 3 (and Figures 5 and 10) conflicts with the tight tolerance range (e.g., ± 0.5 microns) that is required on height dimension D n of most thin film inductive gaps and MR layers. Thin film magnetic head elements of linear head arrays are typically deposited on a support substrate in straight row and column arrays using photo-lithographic processes. The substrates are then cut into rows to benefit from the economies of batch manufacturing. The rows are machined flat and planar down their lengths to within, for example, ±0.5 micron of the target gap depth or element height dimension. Having thin film head elements deposited in a straight row, and then attempting to apply the aforementioned in-situ lapping process, would damage those head elements located in the outer ends of the linear head array where the contour cross-curvature is the most extreme. To alleviate this problem, in accordance with the present invention, the thin film head elements are deposited onto the support substrate along a setback line 80 (depicted in the first embodiment of Figures 4 and 5, for example) that is curved corresponding to the convex cross-film curvature exhibited in Figure 1. The ELG resistor layers 70 0 and 70 16 and the adjacent MR head element record/reproduce layers or the inductive gaps, depending on the head type, referred to as head defining layers 62 1 - 62 15 for convenience, located near the two ends of the linear head array 30 are set back by a set back distance y ( x ) varying as a function of the distance x along the length or axis with respect to the center of the linear head array 30. In this manner, when the multi-track linear head array 30 is fabricated, the head surface cross curvature requirement and the head gap depth or MR element height D n can be met simultaneously. Unlike the teaching in the above-referenced '378 patent, the setback line curve of the present invention differs from a simple arc section of a circle characterized therein as an "approximately elliptic spherical shape". The cross-film curvature is a more complex curvature approximated by a sixth order polynomial equation having even terms, i.e.: y(x) = a 0 + a 2 x 2 + a 4 x 4 + a 6 x 6 , wherein: a 0 is a constant distance from a reference point along the center axis of the array a 2 = -3.15 x 10 -4 to -2.11 x 10 -4 mm -1 a 4 = 6.50 x 10 -6 to 9.41 x 10 -6 mm -3 a 6 = -1.59 x 10 -7 to -1.297 x 10 -7 mm -5 The values for a 2 , a 4 , a 6 are derived empirically from 24mm wide photographic film constrained as illustrated in Figure 1. These values may differ for film having a different width, e.g. 35mm film. Figure 4 is a simplified cross-section view along the linear axis of a first embodiment of the linear head array 30 of Figure 2 to show the thin film layers thereof prior to the machining of the ELG and head frustums 32 0 - 32 16 and lapping in accordance with Figure 3. Figure 5 is a similar simplified cross-section view along the linear axis of the linear head array 30 of Figure 4 after lapping. In Figures 4 and 5, the ELG resistor layers 70 0 and 70 16 and the head defining layers 62 1 - 62 15 are evenly spaced in the +x and -x array position directions along planar substrate surface 50 on either side of the center of the array substrate 48. For example, as shown in Figure 5, the set back distance y 1 is derived as a function of the array position distance x 1 , and the set back distance y 2 is derived as a function of the array position distance x 2 . The magnetic head array type depicted in the exemplary embodiment of Figures 4 and 5 may be of any of the known thin film types. The invention is preferably implemented in a Paired MR (PMR) head structure as described below. Three leads 56 n , 58 n , 60 n are depicted extending away from each respective' head defining layer 62 n . The head defining layers 62 n are a simplification of an actual PMR head structure having the MR layers coupled to the leads 56 n and 58 n and both MR layers coupled in common to the lead 60 n as described below with respect to Figure 14. A simple MR head would have only one MR layer and a pair of deposited leads. A thin film inductive head (wherein the head defining layer is the head gap) would also have a pair of leads to a field creating structure. An integrated inductive record head and MR reproduce head would four such deposited leads, and an integrated inductive record head and PMR reproduce head would five such deposited leads. Figures 4 and 5 are intended to be illustrative of any of theses specific head structures. The leads 56 n , 58 n , 60 n and head defining layers 62 n are deposited on the substrate 48 in accordance with conventional MR head deposition technology. Similarly, leads 66 0 , 68 0 extend from ELG resistor layer 70 0 , and leads 66 16 , 68 16 extend from ELG resistor layer 70 16 . The leads 66 0 , 68 0 and the ELG resistor layer 70 0 are also deposited on the substrate 48 in accordance with conventional deposition technology. The deposition of each head defining layer 62 1 - 62 15 and ELG resistor layer 70 0 and 70 17 is therefore in an arc pattern that results in a set back distance x n from planar substrate surface 50 appropriately to maintain the head element height D n for each head defining layer 62 n within a prescribed tolerance range after lapping as illustrated in Figure 5. The pattern of set back line 80 is derived from the sixth order polynomial equation y(x). In this embodiment, the deposited thin film head defining layers 62 n are curved on the interior edge deposition base lines 74 n to follow set back line 80 to correlate with the cross-film curvature approximated by the sixth order polynomial equation. Figures 9 and 10 are plan views similar to Figures 4 and 5 of another embodiment of a thin film MR or inductive head pattern having the deposited head defining layers 62 n stepped in straight line interior edge deposition base lines 76 n to approximately follow the tangents of set back line 80 in a curved pattern to correlate with the cross-film curvature approximated by the sixth order polynomial equation y(x). It will be understood that the individual head element surface contours of Figure 10 follow the prolate ellipsoid contours of Figure 3 in both the longitudinal and transverse directions. The tolerance on the gap depth or element height D n can be maintained for all head defining layers 62 n by incorporating either the arc base line or step base line deposition pattern, or both, on a thin film head substrate that is to be in-situ lapped and contoured. When both are incorporated, for example, the straight line interior edge deposition base lines 76 n may be employed for the innermost head defining layers 62 n , and the curved interior edge deposition base lines 74 n may be used for the outer head defining layers 62 n and the ELG resistor layers 70 0 and 70 16 . In the above-described embodiments, the patterns of layers forming each of the magnetic head elements and the ELGs are laid out in parallel. For example, the leads 56 n , 58 n , 60 n and head defining layers 62 n and the leads 66 0 , 68 0 and 66 16 , 68 16 and ELG resistor layer 70 0 . ELG resistor layer 70 16 are laid out in Figures 4 and 9 oriented perpendicularly to the planar substrate surface 50. It will also be understood that the leads and layers may also be deposited in a fan-like pattern roughly oriented to extend for a short distance perpendicular to the interior edge deposition base lines 74 n or 76 n . In this regard, it will be recalled that the illustrations of the curvature of the set back lines 80 in Figures 4, 5, 9, 10 are highly exaggerated, and that the fan orientation would, in fact, be slight and not necessarily affect the pitch between the head elements and the resulting track pitch. One such fan pattern is depicted in Figure 15 and described below. In a further embodiment, the thin film head elements are individually fabricated and fixed in position to approximately match the eventual convex cross-curvature of the head bearing surfaces. The mounting pattern is in a fan like array approximately following the set back line 80 of the sixth order polynomial y(x) equation. In the preferred embodiments, at least one and preferably two electronic lapping guides (ELGs) 70 0 and 70 16 are included in the head pattern that is photo-lithographically deposited on a substrate 48 wafer surface. The ELGs 70 0 and 70 16 are accurately deposited with respect to the set backs D 0 ' and D 16 ' of the end ELG resistor layers 70 0 and 70 16 , respectively, prior to in-situ lapping. The ELGs resistor layers 70 0 , 70 16 provide closed-loop dimensional feedback on the gap depth or element height D n of the recording head defining layers 62 1 - 62 15 during the in-situ lapping or contouring operation. As the recording head bearing surface is lapped or contoured, the ELG resistor layer heights D 0 and D 16 decrease, resulting in an increase in ELG resistance that is inversely proportional to the height. Given that the ELG resistor layers 70 0 , 70 16 are deposited accurately on substrate 48 with respect to the adjacent head gap layers and the set back line 80, the ELG resistance change can thus be cross-correlated with the actual gap depths or element heights D n of the head layers. To maximize the benefit and accuracy of the ELG as a process monitor for the radially arrayed or stepped thin film head depositions described above, the ELG resistor layers 70 0 and 70 16 are patterned at their deposition base lines 72 0 and 72 16 , respectively, to follow the predetermined arc or stepped set back pattern of the convex cross-curvature of the head surface contour and the set back line 80. Typically, ELG's are deposited in linear rows and columns with the linearly deposited head element thin film layers. With the head surface contour described above, ELG resistor layers deposited with an edge parallel to substrate surface 50 would be inaccurate and possibly even partly or wholly severed and opened in the final head in-situ lapping process. In one embodiment, the ELG resistor layers 70 0 and 70 16 are patterned in a trapezoidal shape by angling the deposition base lines 72 0 and 72 16 as shown in the second embodiment of Figures 9 - 13. The trapezoid dimensions and positioning are chosen such that when the surface of the linear head array 30 is final contoured as shown in Figure 13, the deposition base lines 72 0 and 72 16 of the ELG resistor layers 70 0 and 70 16 are parallel to the tangent of the head surface contour and the set back line 80. This latter embodiment enables using the ELGs in an initial lapping operation to provide the transverse contour of the linear head array surface as shown in Figure 12 as well as the final contouring operation resulting in the contour in the direction of the linear head array as shown in Figure 13. Figures 6 - 8 show a further improvement on the generally trapezoidal shaped ELG, by curving the deposition base lines 72 0 and 72 16 of the ELG resistor layers 70 0 and 70 16 to conform to, while being offset from, the set back line 80. The results of the initial transverse contour and the final in-situ lapping contour are shown in Figures 7 and 8. When final contoured, the ELG reference edge will exactly match the desired convex curvature of the head bearing surface as reflected by set back line 80. This enhances the ability to fine tune the depth of the in-situ lapping by comparing the measured resistance with a reference resistance value during the lapping process. As depicted in Figures 4 - 13, the deposition base lines 72 0 and 72 16 of the ELG resistor layers 70 0 and 70 16 are spaced somewhat from the set back line to effect a short height. In theory, they could be coincident with one another, but it is desired to have a shorter ELG resistor layer height to provide a relatively high resistance target within a resistance tolerance range. The resistance is measured during the lapping process and halted when the target resistance is achieved after the excess layer height is lapped away in the lapping process. All of these embodiments are applicable toward either MR or inductive thin film heads. In the case of the MR head element, the heights D n of the head defining layers 62 n shown in Figures 4, 5 and 9, 10 are determined by the pattern of the set back line 80 and must be controlled carefully to remain within the tight tolerance range (e.g., ± 0.5 microns). An additional embodiment of the present invention that relaxes the tight element height tolerance requirement involves the use of a magnetic flux guided MR structure of the type shown in U.S. Patent Nos. 5,258,883 and 5,291,363 or a flux guided PMR structure. The preferred embodiment for reading the small magnetic signals from an MOF layer comprises a PMR head structure, e.g. flux guided PMR head structure layers 64 1 - 64 15 used in conjunction with a magnetic flux guides 78 1 - 78 15 as shown in partial arrays in Figures 14 and 15. Figures 14 and 15 show top views of the end and center flux guided PMR structure layers 64 1 , 64 15 and 64 8 , respectively, laid down on a substrate 48 with respect to substrate surface 50 in a highly exaggerated set back pattern (Figure 14) and in a highly exaggerated fan pattern (Figure 15). The set back pattern and the fan pattern may also be employed in patterning the thin film magnetic head defining layers 62 n of the arrays of Figures 4 - 13 as well. A pair of MR structures are superimposed on one another to form the PMR structure. The PMR output signals provided on leads 56 n , 58 n , 60 n are processed differentially to reduce thermal noise effects common to both MR's, increasing the signal-to-noise ratio in the detection of small signals from the MOF layers over that attained in detection from a single MR head element. The flux guides 78 n "conduct" magnetic flux from the media-bearing contoured head surface to the PMR structure, thereby increasing (improving) the tolerance range allowed during the process of lapping away the planar substrate surface 50. In the flux guided PMR head configuration, the magnetic flux guide material extends to the media-bearing surface, while the PMR layers remain subsurface and unexposed in the lapping process. The electrical and magnetic bias properties of the PMR element are independent of the flux guides 78 n and are therefore independent of the final lapped height. The overall effective magnetic "spacing loss" (a loss of signal detection which is exponential with the spacing between head and media) is reduced with the flux guides 78 n . A PMR head layer structure without flux guides 78 n must be set back from the media-bearing contoured head surface to prevent shorting between the MR pair by lapping of the head bearing surface. The result is that the latter has a tight MR element height lapping tolerance (±0.5 microns typical), chosen to minimize the distance that the PMR layers are set back. A flux guided structure can tolerate a wider range (5.5 microns typical) in element height without significantly sacrificing signal output when used in conjunction with the arc or stepped MR deposition pattern described above with respect to Figures 4, 5 and 9, 10. The significance of using flux guides 78 n in the head structure is that they ease the lapping tolerance enough to allow for variations in the final contour achieved. Flux guides 78 n , therefore, enable the use of the PMR structure, which provides excellent detection of the small signals from the filmstrip MOF layer.

Claims: A thin film magnetic head array (30) for recording and/or reproducing information in tracks of a magnetic layer (28) on photographic filmstrip (10) exhibiting a predetermined cross-film, concave curvature extending across the image frame area width of the filmstrip (10) when the filmstrip (10) is constrained in a film transport path for movement with respect to the magnetic head array (30) comprising: means for defining a filmstrip transport path for constraining a filmstrip (10) to exhibit the predetermined cross-film, concave curvature; an elongated head substrate (48) mounted with respect to said filmstrip transport path defining means having an array length extending across the image frame area width and a head element surface adapted to contact the magnetic layer approximated by a sixth order polynomial having even terms only and complying with the convex curvature of the filmstrip constrained in the film transport path; a plurality of magnetic head elements (32n) deposited in a linear array as thin film layers (62n) on said elongated head substrate (48) extending between a head element layer deposition base line (80) and the head surface and having predetermined head element heights, wherein said head element layer deposition base line (80) is approximated by a sixth order polynomial having even terms only and complying with the convex curvature of the filmstrip constrained in the film transport path. The magnetic head array of Claim 1, wherein the sixth order polynomial is represented by y(x) = a o + a 2 x 2 + a 4 x 4 + a 6 x 6 , where y(x) is the convex curvature of the head layer deposition base line as a function of the distance x along the magnetic head array from the centermost of the magnetic head elements that approximates the convex curvature, and the constants are within the ranges: a 2 = -3.15 x 10 -4 to -2.11 x 10 -4 mm -1 ; a 4 = 6.50 x 10 -6 to 9.41 x 10 -6 mm -3 ; and a 6 = -1.59 x 10 -7 to -1.297 x 10 -7 mm -5 . The magnetic head array of Claim 2 wherein the magnetic head elements are deposited on the substrate having stepped height offsets between adjacent head elements arranged in such a manner as to approximate the convex curvature of the head element layer deposition base line. The magnetic head array of Claim 1 wherein the magnetic head elements are deposited on the substrate having stepped height offsets between adjacent head elements arranged in such a manner as to approximate the convex curvature of the head element layer deposition base line. The magnetic head array of Claim 1 further comprising at least one electrical lapping guide deposited on the substrate in relation to the linear array of magnetic head elements and in conformance with the convex curvature of the head surface and wherein the head element layer height is controlled in the head fabrication processing by monitoring the electrical resistance of the electrical lapping guide during shaping of the convex curvature of the head element surface. The magnetic head array of Claim 1 wherein the head element predetermined layer height is controlled in the fabrication of the magnetic head array by mounting the magnetic head array in operating relation to a guiding means and lapping the head surfaces with an abrasive strip exhibiting the filmstrip cross film concave curvature passed through said guiding means in the path of transport of said filmstrip to lap down the head surface into a curvature conforming with said convex curvature. The magnetic head array of Claim 1 wherein the magnetic head elements of the magnetic head array further comprise compound, three-dimensional bearing surfaces further comprising a plurality of shallow frustums mounted in a linear array in the cross-film direction each supporting a head element layer and shaped to create localized pressure regions when penetrated into the concave curvature of the filmstrip. The magnetic head array of Claim 1 wherein the magnetic head array elements comprise thin film, magnetoresistive reproduce heads deposited in thin film layers on said elongated head substrate. The magnetic head array of Claim 1 wherein the magnetic head elements are deposited on the substrate in a fan array such that the adjacent head elements are arranged in such a manner as to approximate the convex curvature of the head element layer deposition base line. The magnetic head array of Claim 1 wherein the magnetic head array elements comprise thin film, paired magnetoresistive reproduce heads deposited in thin film layers on said elongated head substrate with flux guides formed between the paired magnetoresistive reproduce heads and the head surface.

Inventors: CZARNECKI WLODZIMIERZ STANLEY, GANDOLA KENT RAPHAEL, KOEPPE PETER VANDERSALM, CZARNECKI, WLODZIMIERZ STANLEY, GANDOLA, KENT RAPHAEL, KOEPPE, PETER VANDERSALM
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1996
Main classification: G11B5
IPC codes: G11B5, G03B17, G11B15, H01F10