Method for adjusting curvature of magnetic read/write head sliders

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FIELD OF THE INVENTION

This invention relates generally to laser scribing tools for scribing semiconductor chips and the like. More particularly, it relates to a method for scribing magnetic sliders so that they have an accurate curvature on the air bearing surface (crown curvature and camber curvature).

BACKGROUND OF THE INVENTION

Hard drives utilizing magnetic data storage disks are used extensively in the computer industry. Each magnetic data storage disk in a hard drive has an associated slider which is used to magnetically read and write on a disk surface. In operation, the magnetic data storage disks are rotated and a slider is held very close to the surface of each disk surface. The motion of the disk past the slider allows data communication between the slider and disk surface.

The distance between the slider and disk must be accurately controlled. Typically, the slider is shaped to fly upon a cushion of moving air formed by the rapidly moving disk surface. The surface of the slider closest to the disk surface is called an air bearing surface. The air bearing surface has a shape which is designed to provide a small but stable flying height between the slider and disk. The slider must not touch the disk surface during operation because damage can result. Also, it is desirable to maintain as small a flying height as possible, because this increases the amount of data which can be stored. As flying height is reduced, it becomes increasingly difficult to maintain the flying height accuracy to the degree required for reliable recording and reading of data.

The shape of the slider has a substantial effect upon fly height. More specifically, the flying height is dependent upon the average curvature of the air bearing surface of the slider. The curvature of the air bearing surface is often affected by the manufacturing processes used to make the slider. Lapping of the slider (either the air bearing surface or a surface opposite to the air bearing surface) often causes stress variations in the slider which distort the shape of the air bearing surface. After lapping, it is almost always necessary (for high storage density applications) to adjust the curvature of the air bearing surface to a desired target curvature.

U.S. Pat. No. 5,266,769 to Deshpande et al. discloses a method of adjusting the curvature of the air bearing surface of a slider by scribing a back surface of the slider. The scribing removes material from the back surface, thereby releasing internal stress in the slider and controllably changing the curvature of the air bearing surface. Scribing may be performed with a laser, sandblasting tool or the like. A curvature measuring tool may monitor the curvature of the air bearing surface as material is removed, thereby providing feedback control if desired. A problem with the method of Deshpande is that sliders are most efficiently made in rows, and each slider in a row may have a different amount of stress. This means that each slider must have a different amount of material removed in order for the sliders to have the same air bearing surface curvature. Deshpande does not disclose a method for individually controlling the curvature of sliders in a row. Deshpande assumes that all sliders in a row require the same curvature adjustment. It would be an advance in the art to provide a row of sliders with individually controlled curvature.

Further, Deshpande does not disclose specific, advantageous methods of implementing curvature control. The curvature of a slider may only be changed `in one direction` by removal of material from the back side of the slider and so the target curvature must not be overstepped. Deshpande does not disclose a method for curvature adjustment which assures that the target curvature is not overstepped. Also, the changes in curvature caused by material removal from the back surface of the slider are not entirely predictable. When large changes in curvature are necessary, the final curvature of the slider may be rather inaccurate. Deshpande does not disclose a method which provides the same accuracy in curvature control for large and small curvature adjustments. Therefore, there are many improvements which can be made to the method of Deshpande.

OBJECTS AND ADVANTAGES OF THE INVENTION

Accordingly, it is a primary object of the present invention to provide a method of adjusting curvature of a slider that:

1) can be used to adjust the curvature of individual sliders still joined in a row;

2) assures that a target curvature is not overstepped;

3) provides the same accuracy in final curvature for a wide range in the amount of curvature adjustment required.

These and other objects and advantages will be apparent upon reading the following description and accompanying drawings.

SUMMARY OF THE INVENTION

These objects and advantages are attained by a method for adjusting the curvature of an air bearing surface (ABS) of a slider to match a final target curvature. The slider has a back surface opposite the ABS. The method includes the steps of measuring the ABS curvature, determining a curvature difference between the measured curvature and final target curvature, and scribing lines to correct for a predetermined percentage of the curvature difference. The steps of measuring, determining, and scribing are repeated in at least two installments. The predetermined percentage may be different or the same in succeeding installments. The final installment corrects for 100% of the remaining curvature difference.

Preferably, the scribe lines are spaced apart by a distance sufficient to ensure that the scribe lines act independently to affect the curvature of the ABS. The scribe lines are spaced apart by a distance in the range of about 5-200 microns, preferably in the range of about 20-80 microns, and most preferably in the range of about 35-55 microns.

Preferably, the method includes the step of establishing a set of scribe line locations on the back surface where scribe lines can be located. The scribe line locations are spaced apart by a distance sufficient to ensure that neighboring scribe lines act independently. Also preferably, in each installment, the average curvature change per scribe line is substantially equal to an average curvature contribution for all the scribe line locations.

In an alternative embodiment of the present method, each installment has the same steps of measuring, determining and scribing, but each installment changes the curvature to an intermediate target curvature. The intermediate target curvatures are predetermined. In a process using three installments, for example, there will be two intermediate target curvatures. The target curvature for the third and final installment is the final target curvature. Scribe lines are scribed at the predetermined scribe line locations.

The present invention also includes a method for providing a desired curvature change in an ABS surface of a slider. The method includes the steps of establishing scribe line locations on the back surface of the slider. The scribe line locations are sufficiently spaced apart such that neighboring scribe lines at neighboring scribe line locations act independently. Next, a curvature contribution for each scribe line location is determined. Next, scribe line locations are selected such that a sum of the curvature contributions of the selected locations is equal to the desired curvature change. Finally, scribe lines are scribed at the selected locations.

Preferably, the scribe line locations are spaced apart by a distance sufficient to ensure that neighboring scribe lines at neighboring locations act independently.

Also preferably, scribe line locations are selected such that the average curvature change per scribe line is substantially equal to an average curvature contribution for all scribe line locations.

All methods of the present invention can be applied to both crown curvature and camber curvature.

DESCRIPTION OF THE FIGURES

FIG. 1 (prior art) shows a close-up view of a slider in operation reading/writing data to a magnetic data storage disk.

FIG. 2A shows an example of a slider with negative crown curvature.

FIG. 2B shows an example of a slider with positive crown curvature.

FIG. 3A shows an example of a slider with negative camber curvature.

FIG. 3B shows an example of a slider with positive camber curvature.

FIG. 4 shows how the measurement of curvature is defined in the present invention.

FIG. 5 shows an air bearing surface (ABS) of the slider.

FIG. 6A shows an apparatus according to the present invention which can measure the curvature of the ABS.

FIG. 6B shows four points on the ABS used by the apparatus of FIG. 6A to measure curvature-two points for crown curvature, and two points for camber curvature.

FIG. 6C shows the output of a position sensing detector in the apparatus of FIG. 6A when lasers beams are alternately pulsed.

FIG. 7A shows another apparatus according to the present invention which can measure the curvature of the ABS.

FIG. 7B shows scan lines on the ABS used by the apparatus of FIG. 7A to measure crown curvature and camber curvature.

FIG. 8 shows a table illustrating the signals produced by the apparatus of FIG. 7A for different curvatures of the ABS.

FIG. 9 shows a curvature adjustment tool which can adjust the curvature of sliders still attached in row form.

FIG. 10 shows an exemplary distribution curve of initial curvatures of different sliders in relation to a final target curvature.

FIG. 11 shows a graph of different curvatures of a particular row of sliders.

FIG. 12 shows how the curvature of a particular slider changes while being processed through installments in the method of the invention.

FIG. 13 shows the curvature distribution of a group of sliders before and after being processed.

FIG. 14A shows a back surface of a slider scribed according to the present invention. FIG. 14A is physically aligned with FIG. 14B.

FIG. 14B shows a graph illustrating how the effect on curvature provided by a crown scribe line depends upon the crown scribe line location.

FIG. 15 shows a graph illustrating how two scribe lines located close together affect the net result on slider curvature.

FIG. 16 shows the back surface of a slider with scribe line locations outlined.

FIG. 17 shows a graph illustrating how to select scribe line locations to achieve a desired curvature change.

FIG. 18 illustrates the preferred method of the present invention in which a predetermined percentage of a measured curvature difference is corrected for in each installment.

FIG. 19 is a flow chart illustrating a preferred method of repeatedly adjusting slider curvature by installments based on percentages.

FIG. 20 is a flow chart illustrating a second method of repeatedly adjusting slider curvature by installments based on achieving intermediate target curvatures.

FIG. 21 shows a back surface of a slider having crown scribe lines which provide a desired crown curvature.

FIG. 22 shows a back surface of a slider having camber scribe lines which provide a desired camber curvature.

FIG. 23 shows a graph illustrating how the effect on curvature provided by a camber scribe line depends upon the camber scribe line location.

FIG. 24 shows a slider which has both crown scribe lines and camber scribe lines.

FIGS. 25A and 25B show sliders which have herringbone scribe lines that affect both crown and camber curvature.

FIG. 26 shows a back surface of a slider which has angled scribe lines that affect both crown curvature and camber curvature.

FIG. 27 shows a back surface of a row which has crown scribe lines.

DETAILED DESCRIPTION

The present invention provides a method for adjusting the curvature of magnetic sliders used for transducers in data storage hard drives.

FIG. 1 shows a close-up side view of a slider 20 held above a hard drive disk 22 which is moving in the direction of the arrow 24. The slider 20 has an air bearing surface (ABS) 26 facing the disk 22. The slider also has a back surface 28. The slider 20 is inclined with respect to the disk 22 so that air moving along with the disk provides aerodynamic lift to the slider 20. For reliable operation and maximum data storage in a hard drive, a fly height 30 should be small, stable, and well defined. The shape and curvature of the air bearing surface 26 is very important for determining the fly height 30.

The shape of the air bearing surface 26 is described in terms of two types of curvature, crown and camber. FIG. 2A shows a side view of a slider which has a concave air bearing surface 26 which appears curved when viewed from the side. The slider is said to have negative crown. Negative crown is caused by residual stress in the slider, which is often a result of lapping the ABS. Negative crown is generally undesirable for air bearing surfaces. FIG. 2B shows a slider 20 having an ABS with positive crown. A small value of positive crown is generally desirable for air bearing surfaces 26. An ABS with a small, positive value of crown provides a more stable and predictable fly height 30.

It is noted that it is the average curvature of the ABS 26 which is important. Therefore, the curvature can be expressed as a single number indicating the average curvature. The exact shape of the curvature does not have a significant effect upon fly height 30, provided that the average curvature is a well-defined value. The fly height 30 is approximately proportional to the average curvature. For example, a typical fly height sensitivity is about 0.25 nanometers of fly height 30 change for each nanometer of crown curvature change.

FIG. 3A shows a front view of a slider 20 with a curved ABS which appears curved when viewed from the front (i.e. in the direction of the arrow 24). The disk 22 is moving into the page. The slider is said to have negative camber. Negative camber is generally undesirable for air bearing surfaces because the ABS is more likely to contact the disk. FIG. 3B shows a slider having positive camber. A small value of positive camber is generally desirable for air bearing surfaces 26. An ABS with a small, positive value of camber curvature provides a more stable and predictable fly height 30.

FIG. 4 shows how crown and camber (most generally, curvature) is measured in the present invention. Curvature is measured in units of distance defined by the length 32. Length 32 is the distance between the highest and lowest points on the ABS 26. A negative curvature value indicates that the ABS is concave; a positive curvature value indicates that the ABS is convex (this is true for both crown and camber curvature). Length 32 is either the crown curvature or camber curvature, depending upon the orientation of the slider 20.

FIG. 5 shows a close-up view of the slider ABS. The ABS 26 does not extend over an entire bottom surface 34 of the slider 20. Portions 36 of the bottom surface of the slider are recessed and therefore do not substantially affect the fly height 30. The ABS 26 is polished and raised above the remaining portions 36. Therefore, when measuring curvature, it is only necessary to measure the curvature of the ABS 26 and not the portions 36.

FIG. 6A shows an apparatus according to the present invention for measuring the curvature of the ABS 26. Two lasers 40, 42 supply laser beams 43a, 43b to a first beamsplitter 44. The lasers 40, 42 and beamsplitter are located so that the beams after having passed through the beamsplitter 44 are spaced apart by a distance 46 and parallel. The slider 20 whose curvature is to be measured is placed a distance away from the beamsplitter 44 in the path of the laser beams 43a, 43b. A lens 45 is located in front of the slider 20. The beams 43a, 43b are incident upon the ABS 26 at two spaced apart points 47, 48. The apparatus measures the curvature between the points 47, 48. FIG. 6B shows a top view of the ABS 26 which shows where the points 47, 48 are located on the ABS 26. Of course, the points 47, 48 can be located anywhere on the ABS 26 where a curvature measurement is required. If camber curvature is being measured, then the beams will be incident upon the ABS 26 at locations 63, 65 (i.e. horizontally spaced apart locations). The beams 43a, 43b reflect from the ABS 26 and enter a second beamsplitter 50 which directs the reflected beams to a position sensing detector 52. The beams are incident on the detector 52 at spaced apart points 53, 54. The distance between the points 53, 54 determines the output of the detector 52 and is indicative of the curvature of the ABS 26.

Preferably, the optical path length from the ABS to the detector (i.e. from point 47 to point 53, and from point 48 to point 54) is in the range of about 25-300 millimeters.

Preferably, the second beamsplitter 50 is a polarizing beamsplitter and a quarter-wave plate 51 is located between the second beamsplitter 50 and ABS 26. Proper alignment between the polarizing beamsplitter 50 and quarter-wave plate assures that all the light reflected from the ABS is directed toward the detector 52.

It is noted that the slider 20 may be made of a composite ceramic material having a predetermined grain size (e.g. TiC and Alumina composite ceramics). The beam spot sizes at locations 47 and 48 must be large compared to the grain size in order to obtain accurate curvature measurements. The beam spot sizes should also be small enough to provide sufficient spatial resolution.

If the ABS 26 is flat, then the distance between points 47 and 48 is the same as the distance between points 53 and 54. If the ABS 26 is concave (but not excessively concave), then the distance 47-48 will be greater than the distance 53-54. If the ABS 26 is convex, then the distance 47-48 will be less than the distance 53-54. Therefore, the apparatus shown can measure positive and negative curvature between any two points on the ABS 26. By appropriately orienting the beams 43a, 43b with respect to the slider 20, both crown and camber of the slider 20 can be measured.

In operation, beams 43a and 43b are alternately pulsed such that only one beam is on at a time. Preferably, the lasers 40, 42 are diode lasers and are alternately switched. This produces an output from the detector 52 shown in FIG. 6C. The output can be described as being a DC-biased square wave signal. The voltage step difference (V.sub.2 -V.sub.1) is proportional to the distance between points 53 and 54, which provides a measure of the curvature of the ABS 26. In a particular embodiment the lasers are pulsed at a rate of about 100 kHz. If each curvature measurement is averaged over 10 cycles, then a curvature measurement is provided every 100 microseconds. Preferably, a lock-in amplifier tuned to the frequency of the pulsed lasers 40, 42 (i.e. 100 kHz in the above example) is used to measure the voltage step difference. Alternatively, an RMS voltage meter is used to measure the voltage step difference.

The accuracy of the measurements depends upon the total distance traversed by the beams after being reflected from the ABS 26. Typically, the optical path length between points 47 and 53 is about 25 to 300 millimeters. An apparatus according to the present invention can measure curvature to within 1 nanometer.

FIG. 7A shows another embodiment of the present invention for measuring the crown and camber curvature of the ABS 26. A laser 54 directs a laser beam 56 towards a scanner 58. The scanner 58 can be a scanning mirror or an acousto-optical scanner, for example. The scanner may scan the laser beam 56 in one or two dimensions. The scanner directs the beam 56 through a beamsplitter 60 and through a scan lens 62. The scan lens 62 is one focal length away from the beam pivot scan point 61. The slider 20 is located behind the lens 62. The beam 56 strikes the ABS 26 substantially perpendicularly.

The beamsplitter 60 directs a reflected beam from the ABS 26 towards the position sensing detector 52. The detector 52 is preferably located as far away as possible from a scan lens focal plane 67 of the lens 62 such that the reflected beam is always incident upon the detector 52. Typically, the distance between focal plane 67 and detector 52 is in the range of about 1-3 millimeters. This assures that there will be an oscillatory signal from the detector for many different values of curvature. In FIG. 7A, the detector is shown located in front of the focal plane 67 (i.e. between the beamsplitter 60 and focal plane 67), however, the detector can also be located behind the focal plane 67.

Alternatively, the detector 52 is located one focal length from the lens 62 so that the surface of the detector is coincident with the focal plane 67. In this case, if the ABS 26 is flat, then the beam will always be incident upon the same point of the detector 52 as the beam is scanned and no oscillatory signal is produced. The amplitude of the signal from the detector indicates the magnitude of curvature, and the phase of the signal indicates whether the curvature is positive or negative (i.e. convex or concave).

FIG. 7B shows a top view of the ABS 26 showing a beam trajectory 66 over