METHOD AND APPARATUS FOR CONTROLLING THE THROAT HEIGHT OF BATCH FABRICATED THIN FILM MAGNETIC TRANSDUCERS

Abstract: A method for determining the position of a lapped edge (23) of a substrate (11) during lapping of a row of thin film magnetic transducer elements (30). First and second electrical lapping guide structures (21, 22) are formed on each side of the row of transducer elements. The lapping guide structures include a series of switching junctions (32-35), each switch­ ing junction paralleled by a resistor (38-41). A lapping resistor (31) which provides a change in resistance proportional to the lapping of the row is measured and compared with the position of the lapping plane as determined from each switching junction change of state. Calibration of the resistor versus lapping plane position is effected to permit an accurate determination of the lapped surface from subsequent resistance measurements of the lapping resistor.

Description: The present invention relates to a method and apparatus of manufacturing thin film magnetic transducers. Specifically, a method and apparatus for determining the position of a lapped edge of a substrate during lapping of the transducer pole tips to a final throat height is provided. Thin film transducers for reading magnetic disc structures are batch fabricated through thin film deposition techniques. Typically, transducers are formed in rows and columns on a substrate. The substrate is then cut into a plurality of rows of transducers in a side-by-side relationship with the pole tips of the transducers extending to an edge of the substrate row. In order to achieve a maximum transducing efficiency, the pole tip length must be lapped to a final dimension known as throat height. The final throat height for a given thin film transducer must be established within a minimum tolerance in order to provide for transducing efficiencies capable of handling state of the art data recording densities. Lapping of the pole tip ends which define an air bearing surface is measured by an electrical lapping guide (ELG) structure. A lapping guide is located on each end of a substrate row at each end of the row of transducers. The ELG is adjacent to the surface to be lapped and provides electrical signals identifying the position of the lapped surface. The ELG structure will be used to signal the lapping machinery when the lapped surface has progressed to a final throat height position. Current requirements for throat height control require that flatness of the lapped substrate surface be maintained. In order to achieve the required flatness and lower the amount of recession of the pole pieces of the transducers as a result of the lapping process, the final lapping to the final throat height is accomplished during a final wash cycle. The wash cycle is fixed to lap for a calculated wash cycle time which produces a very fine lapping of the pole tips.Such a prior art lapping guide is described in JP-A-58.77015. The final lapped distance of the prior art represents an estimate based on a predetermined lapping time versus lapped distance relationship as current ELG structures do not have sufficient resolution to measure the final distance lapped during the fine lap procedure. The present invention provides an ELG structure capable of monitoring lapping during a final fine lap cycle as well as during the rough lapping stages to achieve final throat height. The present invention provides a process for an accurate monitoring of the position of a lapped edge. The present invention further provides a process for an accurate monitoring of the throat height of batch fabricated thin film magnetic transducers. More specifically the present invention provides a method of measuring the position of opposite ends of a lapped edge defining the throat height of batch fabricated thin film magnetic transducers. An electrical lapping guide structure is formed on preferably two opposite ends of a substrate bearing a row of thin film magnetic transducers. The substrate is lapped to a final throat height dimension along one edge. The position of the lapped edge is monitored at opposite ends with the electrical lapping guide structures. The electrical lapping guide is formed with at least one switching junction which changes state during lapping of the substrate. The switching junction includes a parallel resistance element. The switching junction has a distinct switching plane which is a known distance from the desired final throat height dimension. When the lapped edge coincides with the switching plane, a distinct stepwise change in resistance is detected across the switching plane. In a preferred embodiment of the invention, each lapping guide includes a plurality of switching junctions and shunt resistance elements formed as a series circuit. Each switching junction of a series is located to have a switching plane a known distance from the final throat height dimension, and different from the remaining switching junctions. The series circuit experiences a stepwise change in resistance as the lapping progresses. The lapped substrate edge position at each of the switching planes is therefore detectable at discrete switching planes. The method of the preferred embodiment further includes depositing a resistance element on the substrate for each lapping guide. The resistance element has a configuration which provides a resistance change as lapping progresses. The resistance element resistance change versus lapped distance is determined by comparing each lapping edge position detected by each switching junction state change with the measured resistance element resistance. Thus, as the lapping edge approaches the final throat height position, the resistance element characteristic resistance change versus lapping distance characteristic is accurately known. Each electrical lapping guide provides for position measurements of each end of a lapped edge. Thus, the level of the lapped edge may be determined and corrective forces applied to the substrate to maintain the lapped edge level with respect to the final throat height dimension of the transducer row. Various embodiments of the invention are given in the independent claims. Figure 1 is a block diagram of apparatus for lapping a substrate edge to a final dimension. Figure 2 is a plan view of an electrical lapping guide for carrying out an embodiment of the invention. Figure 3 is an isometric view of the electrical lapping guide of Figure 2. Figure 4 is an electrical schematic circuit demonstrating the operation of the ELG structure of Figures 2 and 3. Referring now to Figure 1, there is shown an overall block diagram of apparatus for lapping a surface 23 of a substrate row 11 to establish precision throat heights for a plurality of thin film magnetic transducers 30. The apparatus of Figure 1 includes a lapping fixture 12 for holding the substrate 11 in position over a lapping plate 20. Lapping plate 20 is an abrasive surface for accurately lapping surface 23 to a final dimension. The force applied to lapping fixture 12 is derived from first and second pressure actuators 15 and 16. Varying the force applied by the actuators 15 and 16 against substrate 11 controls leveling of the lapped surface 23. Measurements of the deviations from a level surface are provided by electrical lapping guides 21 and 22. Electrical lapping guides 21 and 22 provide signals indicative of the distance lapped, identifying the position of the plane of the lapped surface 23 with respect to a desired final transducer dimension. These electrical measurements are applied to a multiplexer 26. Digital ohmmeter 25 measures the resistance of each electrical lapping guide 21 and 22 through the multiplexer 26. The resulting resistance measurements determine the position of the lapped surface 23 with respect to the right and left ends of substrate 11 by directing the output of the digital ohmmeter 25 to a controller 27. The controller 27 in turn activates the actuators 15 and 16 to level the substrate row 11 relative to the lapping plate 20 thereby insuring that the throat heights of all of the transducers 30 are at the same length. Referring now to Figure 2, there is shown more particularly a plan view of the electrical lapping guide structure 21 with respect to the lapped surface 23, and the final dimension, identified as the final throat height where lapping is to cease. It is, of course, understood that the left hand electrical lapping guide 22 is the mirror image of the right hand electrical lapping guide 21 and will not therefore be further described. Adjacent to the thin film transducer 30 is the electrical lapping guide structure 21 which includes an electrical lapping guide resistor 31. As surface 23 is lapped by the lapping plate 20, the resistance value measured between terminals 21a and 21b will vary according to the following relationship: where R = resistance of lapping guide resistor 31 ρ = resistivity of the material comprising resistor 31 ℓ = the resistor 31 length w = the resistor 31 width t = the thickness of resistor 31. Lapping of surface 23 will reduce the width w of resistor 31, increasing the resistance R of resistor 31. Measuring this resistance makes it possible to monitor the positional change of lapping surface 23 as it approaches the final throat height. The electrical lapping guides 21 and 22 are deposited by thin film deposition techniques during the manufacture of each of the thin film transducers 30. It is assumed that the length ℓ, resistivity ρ and thickness t of the resistor 31 have been controlled during manufacture. Thickness is controlled during the deposition of the transducer elements on the substrate 11. The resistivity is determined by the metallic layer used as the resistor 31. Chromium is preferred, but any other metal such as nickel-iron of the pole pieces could be used. The length and width of the resistor 31 is determined in the mask used to form the resistor 31. The length of the resistor is preferably approximately 500 micrometers. The resistivity and thickness for the electrical lapping guide resistor are typically of the following dimensions and values: ρ = .55 micrometers t = .25 micrometers ℓ = 500 micrometers For convenience,ρ/t × ℓ of equation (1) may be represented as K, and for the preferred embodiment is equal to 1100. Adjacent to the electrical lapping guide resistor 31 and part of the electrical lapping guide structure 21 are a plurality of switches 32 through 35. Although four are shown in Figure 2, in the preferred embodiment of the invention, a total of six switches are incorporated on the substrate adjacent an end of each row of thin film transducers 30. Each of the switches 32 through 35 have a switching junction 32a through 35a, located at a specific distance from the final throat height. The switches are deposited on substrate 11 during the deposition of the transducers 30 onto the substrate 11. By depositing switching elements 32 through 35 during the deposition of the thin film transducer 30, the planes at which the switching junctions change state are accurately located with respect to final throat height. Connected in parallel with each switching junction is a single shunt resistance 38 through 41. The shunt resistance 38-41 can be formed at the same time and of the same material as the resistor 31. The switching junctions are serially connected, forming a series circuit between terminals 21a and 21c. When the lapped surface 23 is coincident with the switching plane 32a of switching junction 32, the switching junction will change state. Preferably, switching junction 32 will change from a conducting to a non- conducting state, presenting a distinct binary change in resistance rather than a gradual change in resistance as lapping occurs. Thus, the current path between terminals 21a and 21c will necessarily be through shunt resistance 38. At this time, when the digital ohmmeter 25, connected to controller 27 of Figure 1, measures a step increase in resistance between terminals 21a and 21c, the position of the lapping surface 23 is accurately known with respect to the final throat height. As each switching plane 32a through 35a is accurately known with respect to final throat height, each discontinuity in resistance measurements appearing between terminals 21a and 21c provide for an accurate check of the lapping surface 23 at various positions with respect to final throat height. Each shunt resistance is located a distance from the final lapping edge to avoid being lapped or severed during lapping. The measurement of a change in state of each switching junction 32 through 35, as seen by the total resistance appearing between terminals 21a and 21c, may be used to calibrate the electrical lapping guide resistor 31. As such, the electrical lapping guide resistor 31, when approaching the final throat height, will be accurately calibrated, permitting the lapped surface 23 to be positioned within a nominal throat height of approximately 1 to 2 microns to the desired final throat height. The change in electrical resistance appearing between terminals 21a and 21c can readily be recognized with reference to Figure 4. Figure 4 electrically represents the position of each switching junctions 32 through 35 with respect to the final throat height. It is clear that as the lapping surface 23 is lapped away, switches 32 through 35 will, in sequence, be rendered in an open circuit condition. Each open circuit condition which results from lapped surface 23 progressing toward final throat height, is seen as the addition of resistances 38 through 41 in the series circuit represented between terminals 21a and 21c. Of course, during the switching of junctions 32 through 35, the analog resistance measured between terminals 21a and 21b will be changing in accordance with the above equation. With the foregoing calculation, it is possible to determine with two ELG structures on opposite sides of the lapped surface, the required force differential to maintain the lapped surface level with respect to the final throat height for the transducer array. Additionally, an end resistance may be determined for each electrical lapping guide resistor 31 identifying a desired throat height. Lapping may then be terminated on this desired throat height. Thus, the prior art technique of fine lapping during a wash cycle by setting a pre-fixed wash cycle time may be avoided as the resistor 31 is sufficiently accurate at this stage in the lapping process and its width close to the end of the lapping process to determine a fine lapping point. Alternatively, lapping may be terminated when the last of the switches, switch 35, changes state. A perspective view of the electrical lapping guide structure shown in Figure 2 appears in Figure 3. Figure 3 demonstrates the relationship between each of the switching junctions 32 through 35, and the final throat height, TH. An insulative magnetic gap layer 30c is deposited between pole pieces 30a and 30b of transducer 30. Another layer, not shown, such as a conductive coil layer and other insulation layers 30d are deposited on the substrate 11 between the pole pieces 30a and 30b and form a part of the transducer 30. The insulation layer 30d between the pole pieces 30a and 30b determines the throat height of the transducers. During the deposition of the pole pieces 30a and 30b, each half of switching junctions 32-35, including conductor 35c and 35b, for example, are deposited on substrate 11. An insulation layer separates the conductors of the switching junction except at switch contact points 32d-35d. Thus, by depositing the conductor elements and insulation layer of switching junctions 32 through 35, corresponding structural elements of transducer 30, i.e., the pole pieces 30a and 30b and the insulation layer 30d, are deposited on substrate 11. The registry of the corresponding switching planes is defined by the leading edge of the insulation layers that separates the conductors of a switching element. Thus the registry of the switching junctions 32-35 with respect to the final throat height is maintained. The electrical lapping guide resistor 31 is also deposited on the substrate 11 at the same time transducer elements 30 are formed. Thus, during manufacture of the transducer array, registry is maintained between the electrical lapping guide components including the contact joints of the switching junctions 32 through 35, and electrical lapping guide resistor 31. The resistors 38 through 41 are deposited as a thin film of chromium with a resistance of 100 Ohms. The 100 Ohms chromium resistor appears across the switching junction having a closed circuit resistance of approximately 1 Ohm. Insulation material is deposited between the conductor elements 35c and 35b to define the switch contact joint 35d. The insulation layer for each of the switches 35 through 32 is deposited at the same time the insulation layer 30d that determines the throat height TH is deposited for transducer element 30, thus maintaining accurate control over the switching junctions with respect to final throat height. The geometry of the conductor elements of each switching junction are also maintained congruent with the pole piece 30a, 30b geometry since these elements are all deposited during the same process. The commonly formed insulation layer and conductor geometries improve the overall precision location of switching planes with respect to the final throat height. The electrical lapping guide resistor 31 is deposited between two conductors which form terminals 21a and 21b in the Figure 2 embodiment. The electrical lapping guide resistor 31 is also preferably of chromium and is deposited at the same time as resistors 38 through 41 are deposited on substrate 11. By measuring the relative position of lapped surface 23 with respect to the right and left ends of substrate 11, it is possible to maintain lapped surface 23 level with respect to final throat height for the entire array of transducers 30 on substrate 11. The controller 27 can be a computer programmed with an algorithm which will determine a force differential for transducers 15 and 16 which will correct leveling errors. The force differential is determined as: where R₃₁L is the resistance of the left electrical lapping guide resistor 31 R₃₁R is the resistance of the right electrical lapping guide resistor 31 Δw is the difference in OFFSET distance of each lapping guide resistor 31. The offset distance is shown in Figures 2 and 3 as the distance from the rearward edge of resistor 31 to the throat height dimension. Continually determining the force differential necessary to level the lapping surface 23 and applying the differential force with the actuators 15-16 will achieve a substantially level condition. The calculated final throat height resistance of electrical lapping guide resistor 31 may be represented by the following: where the OFFSET distance of each lapping resistor is determined by Dd n being the switching position of a given switching junction. The computer 27 is programmed to measure the resistance of each leveling resistor of each lapping guide, both left and right, and the serial arrangement of six switching junctions. The parameters of the resistors and switching junctions which are utilized to determine the level condition of lapping surface 23 include the OFFSET of each leveling resistor, as shown in Figure 3. The OFFSET represents the distance from the desired throat height to the rearward resistor edge, parallel with the lapping surface 23. The distance from the throat to a given detector switching plane, generically described as Dd n , and desired throat height, TH, are utilized to determine the following calculations. The OFFSET is calculated for each of the leveling resistors by utilizing the following formula: The difference in OFFSETs between the right and left leveling resistors is expressed as: With the difference in OFFSETs, it is possible to determine a force differential between the actuators 15 and 16, such as to correct for an unlevel lapping surface. This differential, Fd, previously described is: By calculating the force differential in accordance with the above formulation, the computer 27 will set a force differential for the actuators 15 and 16, according to the following determination: This force determination is a subroutine in programming steps to be described, such that computer 27 will provide the appropriate force differential to the substrate 11 to correct any leveling errors. As a final basic calculation for the system, a final throat height resistance is determined from the electrical lapping guide resistor 31 for each side of the transducer substrate row according to the previously described relationship: In view of the foregoing basic calculations, which provide for leveling information, as well as for a final throat height determination, the following programming steps have been incorporated in the computer 27. These programming steps, described in pseudo-code, are useful for electrical lapping guide structures having the six switching junctions described in the foregoing. Of course, six is not necessary to implement the invention, but is, however, considered preferable. The pseudo-code described is a series of do loops, each beginning from 1 to n where n = number of switching junctions employed in the electrical lapping guide structure. Prior to beginning the lapping process, a number of values are inserted in the computer 27. The first being the value of the constant K, previously described with respect to equation 1. The distance from the final throat height to each switching plane Dd₁ to Dd₆, is inserted in the computer program. The program to be described will, at the user's option, end lapping on a calculated throat height, as inserted in the computer program, or on the last change of state of the switching junction closest to the final throat height. In the event that a calculated throat height is to be used as the endpoint for ceasing lapping operations, this calculated throat height (TH) is loaded into the computer program. A wash cycle distance Wc is also inserted in the program. At the end of the coarse lapping, as is known to those skilled in the art, a wash cycle which affects a finer lapping to the lapped surface 23 permits lapping for a specified distance of Wc. An additional subroutine will be employed in the programming to read the left and right resistance values provided by each electrical lapping guide resistor 31. This subroutine additionally subtracts a lead resistance before making the measured values available for program execution. A lead resistance constant would normally be included in the subroutine, however, this may also be inputted by the operator with the other parameter values. The program is configured in three separate portions which will be described in pseudo-code programming format as DO LOOPS. The first portion comprises a section of the program which will, during lapping to the first of the switching junction, execute the following steps. The result of the first portion of the code is to calculate a force differential and apply the force differential through the actuators 15 and 16 to the substrate 11. This force differential is calculated before the first switching junction, referred to as a detector in the pseudo-code, of each side of the substrate changes state by subtracting the resistance levels measured at each side of the substrate of each electrical lapping guide resistor 31 and 32. The SET MECHANICS subroutine called to establish this force differential on the actuators 15 and 16. If corresponding switching junctions on each electrical lapping guide structure change state simultaneously, then the value of each electrical lapping guide resistor, left and right, are stored. In the event that only one switching junction on one of the electrical lapping guide structures changes state, indicating a non- level condition, the two ELG resistors 31 of each side of the substrate are measured and set to 1 and 0, respectively. The force differential is calculated and the SET MECHANICS apply the calculated force for the actuators 15 and 16. When the second of the switching junctions of the opposite electrical lapping guide structure is found to change state, the left and right R₃₁ resistor values are read and stored. Thus, after a pair of switches on opposite sides of the substrate in different ELG structures changes state, the first portion of the programming is completed. The second portion of the programming below consists of performing a series of calculations when lapping progresses from the first pair of switching junctions which have been activated to the third pair to change state. The OFFSETs, W side , for the right and left ELG structures are calculated and stored in a matrix. The differential offset w is also stored as lapping progresses between the first pair of switching junctions to change state and the third pair to change state. As lapping proceeds from the first pair of switching junctions to change state a force differential is calculated based upon equation 4. This force differential is applied through the set mechanics to the actuators 15 and 16 until the subsequent pair of switching junctions, one in each ELG structure, change state. In the event that only one switching junction on one side of the substrate 11 changes state, a force differential is calculated as was accomplished earlier until the remaining switching junction of the other electrical lapping guide structure changes state. When both switching junctions have changed state, the two resistors R₃₁ are measured and their resistance values stored. The above routine is repeated until the third pair of the switching junctions changes state. The OFFSETs, W side , and difference in offsets, Δw, are calculated at the completion of the above steps and stored. The following steps executed by the computer program of computer 27 occur during the lapping of substrate 11 between switching junctions 3 to 6. As lapping progresses from the third switching junction pair to change state, the stored differential offsets Ww are examined and the largest differential offset is identified. An average of the offsets determined is computed according to equation (5). When the next pair of switches on opposite sides of substrate 11 change state, each resistor 31 is measured and a force differential calculated. The force differential is applied through the set mechanics to the actuators 15 and 16. If the operator of computer 27 has entered a calculated throat height at which lapping is to cease, then the computer determines the largest of the previous offset values determined, and averages the offset measurements according to equation (6). The final throat height resistance is calculated by equation (7) and compared with the actual measured resistance of each ELG resistor 31 of each side of the substrate 11. When the calculated final throat height resistance is equal to the actual measured resistance of resistor 31, equation (8) will terminate lapping. As was mentioned, typically in lapping substrates 11, a final lapping distance is obtained in a wash cycle. The wash cycle will typically lap from approximately 5 microns within final throat height to a final throat height measurement. The final throat height measurement indicated as the ideal throat height, may, of course, be set to any distance from this height. In the event a throat height is selected other than this final throat height, this nominal throat height is utilized in equation (9) with the wash cycle lapping distance, W c , to calculate a final lapping resistance, RW for each ELG resistor 31. The wash cycle resistance is continuously compared with the measured ELG resistance 31 of each side of substrate 11. When the wash cycle resistance equals the measured resistance, the wash cycle is effected for final lapping of the substrate. In the event that the lapping is not to end on a calculated throat height, but rather is to end on the changing of state of one of the switching junction, the programming steps will calculate a wash cycle resistance RW according to equation (10). The wash cycle resistance is compared with each measured resistance of resistors 31, and the switching junction on which the lapping is to end is monitored. The following sequences of steps are executed depending on whether one of the remaining detector pairs is activated, or both. These calculations are repeated for each of the remaining detector pairs which are to change state during lapping. The values of the OFFSETs and differential offsets, Δw, are continuously determined and the wash cycle resistance computed again from averaging the calculated offsets and calculated differential offsets. Thus, there is described with respect to one program a method for reducing the resistance information derived from the electrical lapping guide structure of the present invention. Those skilled in the art will recognize yet other ways of reducing these measurements to effectively control leveling and lapping of batch fabricated thin film magnetic transducers to a final throat height.

Claims: A method for measuring the position of a lapped edge of a substrate supporting thin film transducers in a process for batch fabricating such transducers, comprising the steps of: depositing a plurality of switching junctions (32-35) on the substrate, each of the switching junctions having a distinct switching plane parallel to and a known distance away from a desired position for the lapped edge; depositing a shunt resistance (38-41) in parallel with each of the switching junctions and spaced apart from the lapped edge to avoid severing by lapping; depositing a resistor element (31) on the substrate, the resistor element having a resistance which changes with lapping of the substrate; forming a circuit comprising each of the switching junctions (32-35) connected in a series circuit; lapping the substrate from an initial edge of the substrate toward the desired position for the lapped edge wherein each of the switching junctions changes from a closed state to an open state when the lapping plane is coincident with the switching junction (32-35) switching plane whereby the shunt resistance (38-41) in parallel with the switching junction which changes state is connected into the series circuit; detecting stepwise changes in the series circuit resistance during lapping of the substrate, each of the changes indicating a change of state of one of the switching junctions thereby identifying the position of the lapped edge to be coincident with the switching plane of the switching junction which changes state; measuring the resistance of the said resistor element (31) whose resistance changes with the lapping of the substrate each time a switching junction changes state; characterlsed by determining a lapping edge position versus resistance characteristic for the lapping resistor each time a switching junction (32-35) changes state, whereby the lapping resistor is recalibrated each time a switching junction changes state; and determining the lapping edge position from the lapping edge position versus resistance characteristic; and measuring subsequent positions of the lapped edge by measuring the resistance. A method as claimed in claim 1 wherein the resistor element (31)is positioned on the substrate between the series circuit and the supported thin film transducers. A method as claimed in claim 2 wherein the switching junctions (32-35) are spaced apart on the substrate with the switching junction having a switching plane closest to a final throat height being adjacent to the resistor element (31). An apparatus for batch fabricating thin film transducers, comprising an electrical lapping guide for measuring the position of a lapped edge of a substrate supporting the thin film transducers, said electrical lapping guide comprising; a plurality of switching junctions (32-35) formed on the substrate, in known spaced relation to the thin film transducers, each of the switching junctions having a distinct switching plane parallel to and a known distance away from a desired position for the lapped edge; a shunt resistance (38-41) formed in parallel with each of the switching junctions and spaced apart from the lapped edge to avoid severing by lapping; an electrical circuit having first and second terminals (21a and 21c); and characterised by having a resistor element (31) having two terminals (21a and 21b) formed on the substrate adjacent to the plurality of switching junctions, the resistor element (31) having a resistance which changes with lapping of the substrate, whereby, upon connection of means for measuring resistance across the first and second terminals (21a and 21c), stepwise changes in the series circuit resistance can be detected during lapping of the substrate, each of the changes indicating a change of state of one of the switching junctions (32-35) thereby identifying the position of the lapped edge to be coincident with the switching plane of the switching junction which changes state, and whereby, upon connection of means for measuring resistance across the two terminals(21a and 21b), a lapping edge position versus resistance characteristic can be determined for the resistor element each time a switching junction changes state, whereby the resistor element is recalibrated each time a switching junction changes state. An apparatus as claimed in claim 4 wherein the plurality of switching junctions (32-35) are closer to the thin film transducers than the resistor element (31).

Inventors: CHURCH, MARK ANTHONY
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1987
Main classification: G11B5
IPC codes: G11B5