A device for lapping a bar of the type which carries a plurality of sliders used in magnetic storage systems includes first, second and third actuators adapted to couple to the bar, and impart a first, second and third controllable force in response to a first, second and third control signal, respectively. An arm couples to the first, second and third actuators and applies a lapping force to the bar which presses the bar against a lapping surface thereby causing material to be removed from the bar. A controller provides the first, second and third control signals to the first, second and third actuators, respectively, to impart a plurality of forces onto the bar. The actuators are controlled to obtain a desired profile of the bar and to obtain a desired distribution of the lapping force across the profile of the bar.
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0. 24. A device for lapping a bar carrying a plurality of sliders, comprising:
an arm; a lapping surface; a carrier comprising: a work surface adapted to couple to the bar and press the bar against the lapping surface; and first, second, third and fourth adjacent control points coupled to the arm and to the work surface; wherein the first control point is coupled to a first actuator, the second control point is coupled to a second actuator, the third control point is coupled to a third actuator and the fourth control point is coupled to a fourth actuator. 14. A device for lapping a bar carrying a plurality of sliders comprising:
an arm; a lapping surface; a first actuator coupled to the arm; a second actuator coupled to the arm; a carrier comprising: a work surface adapted to couple to the bar and press the bar against the lapping surface; a first control point on the work surface coupled to the first actuator and positioned to apply a first force to the bar; and a second control point on the work surface adjacent the first control point coupled to the second actuator and positioned to apply a second force to the bar; wherein the first and second control points are positioned in relatively close proximity such that displacement of the first control point causes significant displacement of the second control point. 1. A device for lapping a bar carrying a plurality of sliders, comprising:
a first actuator adapted to couple to the bar and impart a first controllable force in response to a first control signal; a second actuator adapted to couple to the bar and impart a second controllable force in response to a second control signal; a lapping surface; an arm coupled to the first and second actuators which provides a lapping force to the bar against the lapping surface; and a controller providing the first and second control signals to the first and second actuators, respectively, to impart a plurality of forces on the first and second controllable forces on the bar selected to obtain a desired profile of the bar and to obtain a desired distribution of the lapping force across the profile of the bar, wherein bending caused by the first actuator interferes with bending caused by the second actuator and the controller determines the first and second control signals for the first and second actuators based upon the desired profile of the control points bar and an interrelationship between the bending caused by the first and second control points actuators.
0. 29. A device for lapping a bar carrying a plurality of sliders, comprising:
a first actuator adapted to couple to the bar and impart a first controllable force to the bar proximate to a first control point in response to a first control signal; a second actuator adapted to couple to the bar and impart a second controllable force to the bar proximate to a second control point in response to a second control signal; a lapping surface; an arm adapted to support the bar relative to the lapping surface; and a controller providing the first and second control signals to the first and second actuators, respectively, to impart the first and second controllable forces to bend the bar to obtain a desired profile of the bar and a desired distribution of lapping force across a profile of the bar, wherein the first actuator introduces a first bending profile to the bar and the second actuator introduces a second bending profile to the bar and the first and second control signals are selected as a function of an interrelationship of the first bending profile imparted by the first actuator and the second bending profile imparted by the second actuator at the first and second control points.
0. 37. A device for lapping a bar carrying a plurality of sliders comprising:
an arm; a lapping surface; a carrier coupled to the arm and adapted to support the bar relative to the lapping surface; and a plurality of actuators coupled to the carrier proximate to control points 1 -n spaced along a length of the carrier and the plurality of actuators adapted to supply a plurality of controllable forces to the bar in response to a plurality of control signals supplied to the plurality of actuators to bend the bar, and the plurality of the control signals being determined to provide a desired profile of the bar and a desired distribution of lapping force along a length of the bar based upon
where
is a sensitivity matrix for the controllable force vs. displacement for control points 1 -n where K11 to Knn are not equal to zero;
are the controllable forces proximate to the control points 1 -n; and
is the displacement proximate to the control points 1 -n.
2. The device of
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8. The device of clam 4 wherein the carrier includes a detent and the arm includes a clamp adapted to clamp onto the detent.
9. The device of
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The present invention claims the benefit of earlier filed U.S. Provisional Application No. 60/030,276, entitled MULTI-POINT BENDING TOOL (CARRIER) FOR ULTRA-PRECISION MACHINING filed on Nov. 4, 1996
The present invention relates generally to the fabrication of magnetoresistive (MR) and inductive recording sensors or transducers for data storage application. More specifically, the present invention relates to a method and apparatus for bending a bar which carries a plurality of sliders at multiple points during the fabrication process and specifically during the lapping process.
During the fabrication of magnetic heads for use in magnetic data storage applications, an array of transducers are fabricating on a common substrate (also called a wafer) by depositing a plurality of layers onto a surface of the substrate. The array of transducers are patterned using, for example, a photolithographic process in combination with various etching and liftoff processes. The finished substrate or wafer is then optically and/or electrically inspected and subsequently cut into smaller arrays, typically a plurality of bars, i.e. rows of transducers. Next, the individual rows or bars of transducers are machined or "lapped" to obtain a desired dimension. (Lapping is a material removal process described below in more detail.) For MR transducers, this dimension is sometimes referred to as stripe height (SH) and for inductive transducers this dimension is sometimes referred to as throat height (TH). Often, electrical lap guides (ELGs, described below) are deposited upon the same substrate and are used as sensors during the lapping process. Following the lapping process, the recording heads are diced to produce individual transducers or heads which are used to form sliders. These sliders are used to read back and/or write information onto a surface of a magnetic disc, for example, which moves at a high rate of rotation.
In order to establish adequate performance for high efficiency recording heads, it is necessary to achieve the desired strip height or throat height. There are many factors which affect variations in the ultimate stripe height or throat height. These factors include variations in the position and size of elements induced during wafer processing. The step of slicing the substrate into bars can also introduce variations. Mounting induced thermal stress can also cause variations during the processing of the wafers into sliders. Further, the profile of the lapping surface can lead to variations.
Electrical lapping guides (ELGs) are sensors which are deposited onto the wafer during the fabrication process. The output from the ELGs can be used to determine when to stop the lapping process. Typically, the ELGs are fabricated along with the transducers using the same wafer processing steps. This is described, for example, in U.S. Pat. No. 4,477,968 which issued Oct. 23, 1984 and U.S. Pat. No. 4,559,743 which issued Dec. 24, 1985.
Lapping generally refers to machining processes in which material is very slowly, at a controllable rate, removed from a surface. Typically, the process involved applying a work surface of the work piece to a moving surface which is slightly abrasive. One such device is described in U.S. Pat. No. 4,536,992 which issued Aug. 27, 1985. Thus, by controlling the lapping process in response to the output from the ELGs, a closed loop machining process is set up in which the output from the ELGs are used as feedback to the lapping machine.
During the lapping process, the slider is held on a carrier which attaches to the arm of the lapping apparatus. Such a carrier is described in U.S. Pat. No. 4,457,114 which issued Jul. 3, 1984. The carrier in U.S. Pat. No. 4,457,114 uses two actuators to bend the bar during the lapping process. In U.S. Pat. No. 4,457,114, the carrier provides bending of the bar at both ends around the center of the bar. This bending is used to provide non-uniform removal of material from the bar in order to compensate for variations in the bar and the throat height or stripe height of the sensor. In U.S. Pat. No. 4,457,114, the actuators comprises pins which are heated to thereby expand and apply a force to the bar which bends the bar. A variation on this technique is to use three different actuators to apply force to a bar at three different points.
Generally, the prior art has focused on improved ELGs and lapping mechanisms. However, as the data storage industry is continuously driven to higher and higher densities and in an ongoing effort to reduce costs of fabrication, a number of competing factors are observed. First, the sensor height tolerance requirement is getting smaller. Second, the density of heads carried on each bar is getting larger. Third, the aspect ratio of the length to the thickness of each bar is getting larger. Therefore, existing lapping and bending systems are often inadequate for controlling the lapping process. These factors not only lead to heads which are more sensitive to processing induced disturbances, but also lead to bars which are more easily disturbed because they are thinner and the stiffness of the bar is related to the cube of its thickness.
The present invention includes a lapping apparatus which provides improved control during the lapping process. In one embodiment, a device for lapping a bar which carries a plurality of sliders includes a first actuator adapted to couple to the bar and to impart a first controllable force in response to a first control signal and a second actuator adapted to couple to the bar through the actuators and applies a lapping force to the bar and against the lapping surface. A controller provides first and second control signals to the first and second actuators, respectively, to thereby impart a plurality of forces on the bar. The forces are selected to obtain a desired profile of the bar and to obtain a desired distribution of the lapping surface across the profile of the bar.
In one embodiment, seven different actuators are used to provide seven separate control points for bending of the bar.
The present invention provides a method and apparatus for accurately lapping a bar carrying a plurality of heads of the type used to write and/or read back information from the surface of a magnetic storage disc.
It has been known in the prior art to control the profile of the carrier. This control is used to more accurately control the material removal. Specifically, as illustrated by
One aspect of the present invention includes the recognition that there are two parameters which must be controlled during lapping. These parameters are bending of the bar profile and balancing of the force applied to the bar. Bending of the bar refers to adjusting the profile of the bar such that the bar becomes relatively straight (or is otherwise shaped as desired). Balancing, on the other hand, is the distribution of pressure across the bar. For example, in the prior art design of
Another aspect of the present invention includes the recognition that the use of additional control points on a carrier can be used to provide more accurate control of a bar during a lapping process. The present invention includes determination of the number of control points needed to achieve a desired degree of control for a bar having a desired length. For example, given a bar profile for I bars defined by J data pints, xij, yij where i=1, 2, . . . I, and j=1, 2, . . . J. A polynomial curve fit function is formed using a least squares fit analysis:
where k equals 2, 3, . . . and is the order of the curve and a, a2, . . . are the coefficients of the curve. Next, the root mean square (RMS) of the residuals for each k are calculated according to the following formula:
Using Equation 2, the number of control points (k) required to achieve the desired amount of control can be determined by assuming that a (k-1)th order curve can be bent straight and setting RMSk to the desired minimum variation in the bar profile. One can then calculate the necessary order of the RMS curve fit which also provides the number of control points. For example, if RMSk is less than 1 pinch Equation 2 is solved with k equal to 10. This analysis has been verified experimentally using carriers having five control points over a 2 inch long bar and five control points over a 1 inch long bar. The standard deviation of the 2 inch long bar was 2.3 μinch while the standard deviation of the 1 inch long bar was 0.78 μinches. This leads to the conclusion that nine control points is sufficient to obtain a variation of less than 1 μinch in the finished bar profile for a standard 2 inch long bar. Table 1 shows that between 9 and 10 control points are needed to achieve control to within 1 μinch in a 2 inch bar.
TABLE 1 | ||||||
RMS of residuals between | ||||||
Curve Fit | individual point and | RMS over | ||||
Parameter | curve on each bar | 15 × 24 | STD measure = | |||
# | mean | median | 75% | 90% | points | 1.67 μinch |
2 | 39.32 | 33.44 | 60.28 | 72.85 | 46.67 | 46.64 |
3 | 19.87 | 13.59 | 27.57 | 46.91 | 25.13 | 25.07 |
4 | 8.96 | 6.54 | 10.96 | 18.78 | 11.4 | 11.28 |
5 | 6.06 | 4.48 | 6.54 | 9.63 | 7.5 | 7.31 |
6 | 4.67 | 3.39 | 5.77 | 8.63 | 5.6 | 5.35 |
7 | 3.21 | 2.57 | 3.82 | 7.26 | 3.62 | 3.21 |
8 | 2.56 | 2.23 | 2.97 | 4.66 | 2.82 | 2.27 |
9 | 2.09 | 2.02 | 2.61 | 2.98 | 2.19 | 1.42 |
10 | 1.69 | 1.62 | 2.01 | 2.78 | 1.74 | 0.49 |
In operation, actuators (not shown in
Another aspect of the present invention includes the use non-uniform spacing between control points in order to improve control or more evenly distribute control. For example, referring back to
One aspect of the present invention includes characterizing the carrier for subsequent use during the lapping process.
where:
The sensitivity matrix {overscore (K)} defines the behavior of profile 52 in response to forces applied at each of the actuator coupling points 120-132. The major diagonal components in {overscore (K)} describe the direct effect of forces applied at individual bending points. The off-diagonal components in {overscore (K)} describe the coupling effect between the various points.
The sensitivity matrix {overscore (K)} can be established by quantifying the carrier displacement response to individually applied known bending forces. This may be performed either through actual experimental measurements or using FEM modeling techniques. During the lapping process, the equation is solved in reverse. First, the normalized bar bow profile is formed using ELG feedback information. The profile of the bar is leveled using balancing of the fixed control points. The carrier deflection required to bend the bar straight is calculated in accordance with the equation:
Where U1-U7 represent a flat profile. Next, equation 3 is solved for {overscore (F)}:
Where F1-F7 are the forces which must be applied by each actuator to achieve the desired profile described by {overscore (U)}.
In operation, the lapping process is controlled by control system 262. Controller 278 retrieves instructions and parameters from memory 274. For example, the matrix of Equation 4 may be stored in memory 274. Instructions and information are received from user input 270 and the status of the lapping process may be displayed on display 272. Additionally, lapping system 198 may include a bar code reader (not shown) to read bar code information 140 for use by controller 278. Feedback regarding the progress of the lapping operation is received through ELG input 276 and provided to controller 278. Controller 278 solves Equation 3 for {overscore (F)} and responsively controls actuators 220-232 and 264 using driver 280. Driver 280 may comprise, for example, a transistor circuit providing a power output to actuate the actuators. Actuators 220-232 and 264 may be any appropriate actuator which is capable of providing a controlled movement such as a hydraulic system, a voice coil, a pneumatic actuator, a piezo electric system, thermal, magnetorestrictive, etc. Those skilled in the art will recognize that the present invention is not limited to any particular actuator. Actuator 264 is used to provide a balance control to balance distribution of the force applied to bar 10. Actuators 220-232 are used to apply the individual forces of vector {overscore (F)}. The total amount of force applied on bar 10 may be controlled by a weight or another actuator (not shown). As shown in
It will be understood that any appropriate orientation of cutouts or other mechanisms to allow relative movement of the control points is within the scope of the present invention.
The present invention provides a lapping system having a carrier which allows improved control of the bending of a bar during a lapping process. The present invention includes numerous features including an increased number of actuators and closer spacing between actuators. Further, in contrast to prior art designs, in the present invention actuators are placed adjacent one another without an intermediate fixed region. Further, in the present invention, the actuator mechanism is placed on the arm of the lapping machine such that the mechanism need not be placed on each bar. Attachment of the bar to the carrier may be through any appropriate technique. In one embodiment, the bar may be slid into a slot carried on the profile of the carrier. Further, the actuators of the present invention may be used to either push or pull the bar and thereby allow deformations in either direction. Preferably, bending of the bar is centered around the normal profile of the bar. This reduces any extra bending stress placed on the bar during lapping. In one preferred embodiment, the lapping of a 2 inch long bar is controlled to within a standard deviation of less than 1 μinch using a minimum of nine separate control points, seven for bending and two for balancing the lapping force. This number may be increased or decreased as appropriate based upon the length of the bar and the desired minimum standard deviation. Clamping of the carrier may be through any appropriate technique and is not limited to the specific clamps described herein. Further, the actuators may be coupled too the carrier using other techniques. As used herein, the term "control point" may be either a fixed control point (shown as a fulcrum in the schematic drawings) or an actuated control point.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, any number of control points, formed by any appropriate technique, in any type of carrier, actuated or fixed by any means may be used. Further, any type of lapping may be used, and the arm can be integral with the carrier. The control points could also be formed integral with the bar itself. This might require additional connections to the bar.
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