Note: Descriptions are shown in the official language in which they were submitted.
HIGH RESOLUTION LEV~LING RESISTOR
The present invention relates to the manufacture of thin film
magnetic -transducersO Specifically, an electrical lapping
guide is described which permits arl accurate measurement of
S the throat height obtained from lapping the transducing gap
to a final dimension.
Thin ~ilm transducers provide for high density data recording
on a magnetic disk carrier. Thin film transducers are batch
fabricated by depositing a plurality of thin film transducing
elements on a substrate, which are commonly aligned with
parallel pole pieces. The entire substrate is lapped along a
plane substantially parallel with -the pole tip ends of the
transducers. The transducing efficiency in large measure is
determined by the length of the transducing gap, referred to
as its final throat height. Accurate control o-~er the length
of this transducing gap provides optimum transducing
efficiency.
The prior art has utilized electrical lapping guide
structures to measure the distance lapped to con-trol the
final throat hei~ht. These structures typically embody first
and second resistance elements called lapping guide resistors
haviny an edge parallel to the lapping surface, disposed on
opposite ends of the substrate. During fine lapping of the
substrate, the resistor edge supporting the pole tips is
lapped, yielding a resistance change which is proportional to
the distance lappedO The position of the lapped edge with
respect to final throat height is monitored by measuring the
resistance of each of the resistance elements.
During the initial stages of lapping, the rough lap, the
electrical lapping guide provides a coarse indication of
lapping by producing a distinct and detectable resistance
step change. The step change is effected by placing in
parallel with the lapping guide resistor an electrical
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SA984-023
element which is broken by the lapping at a known distance
from the final lapping plane. Such lapping guide structures
are exemplified in U.S. Patent No. 3,821,815. These prior
art lapping structures which utilize a resistor for measuring
the final lapping distance to a final throat height are
rectangular in shape, and provide a non-linear resistance
change as a ~unction of the lapped distance~ These
non-linear measurements impose a limitation on obtaining very
accurate throat height tolerances. During coarse lapping,
the resistance vs lapping distance slope is so steep as to
make lappiny distance measurements very uncertain. During
the ~inal lapping stages, the resistance element provides a
very flat resistance versus lapping distance characteristic,
making a final measurement of throat height imprecise. Thus,
these resistance elements find use only over a small lapping
range.
Summary of the Invention
It is an object of this invention to provide a method for
accurately measuring the lapping distance in the manufacture
of batch fabricated thin film magnetic heads.
.
It is a more specific object of this invention to provide an
electrical lapping guide resistor having a resistance which
changes linearly over the lapped distance.
These and other objects are accomplished by a lapping guide
2~ resistor in accordance with the invention. An electrical
lapping guide resistor is provided which produces a
substantially linear change in resistance as a function of
the lapping distance. The resistor is deposi-ted on two
opposite sides of the transducer array. During lapping of
the transducer array substrate, the resistance of the lapping
guide resistor changes indicating the position of the lapped
edge. The monitored resistance changes provide an accurate
measurement of the throat height of the transducer array.
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S~984-023 3
In a preferred embodiment of the invention, resistors of
chromium material are deposited on each end of the trans~ucer
array. The resistors are configured in a shape such that the
effective length of the reslstors increases as the effective
width decreases during lapping. As such, the resistance
change is approximately linear, enhancing the resolution of
controlling lapping distance, and expanding the lapping
distance over which resistance measurements are useable. The
preferred embodiment of the invention includes a pair of
separated conductors which lie along first and second
converging front edges of the deposited chromium resistance
material ~orming along with the conductors a lapping guide
resistor. The front edges of the resistor converge toward
the edge of the substrate surface to be lapped. A -third
rearward edge of the resistor is parallel to the surface to
be lapped. Preferably, the slope of the resistor's first and
second converging edges decreases in the rearward direction,
further linearizing the resistance change versus lapped
distance. As the lapping commences, the resistor is also
lapped, providing a resistance change measured between the
conductors proportional to -the lapped distance. The unique
slope of the resistor edges provide for a linear resistance
change as lapping progresses.
In another embodiment of the invention, the electrical
lapping guide resistor comprises a first coarse lap
resistance section abutting a second fine lap resistance
section. The coarse lap resistance section includes a wider
base edge contlnuous with the forward edge of the fine lap
resistance section, and two additional edyes which converge
towards the lapping surface. The coarse lapping resistor
will provide during early stayes of lapping a resistance
versus lapping distance slope of less than .1 ohm/micron.
~, During a final lapping stage, the fine lap resistance section
will provide a resistance change of approximately 25
ohms/micron.
Description of the Figures
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SA984-023 4
Figure 1 is an illustration of a conventional electrical
lapping guide resistor on one side of an array of thin film
transducers.
Figure 2 illustrates the measured resistance versus lapping
5 distance of a conventional electrical lapping guide resistor
(a) and that of a preferred embodiment of the invention (b).
Figure 3 is an illustration of one embodiment of the
invention having a geometry which effectively linearizes the
change in resistance versus lapping distance.
Figure 4 is an illustration of a preferred embodiment of the
invention whicn further linearizes resistance versus lapping
distance measurements.
Figure 4A is a chart showing horizon~al and vertical lapping
distances according to the embodiment of Figure 4.
.
Figure 5 is an illustration of another embodiment of the
` invention which provides for a rough lapping distance
:~ measurement as well as a fine lapping distance measurement.
Figure 6 is an illustration of the resistance versus lapping
distance characteristic for the embodiment of Fi.gure 5.
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Description of the Preferred Embodiments
Referring now to Figure 1, there is shown an end of a
substrate 11 having a plurality of deposited thin film
transducers 1~. An electrical lapping guide resistor 14 is
:.: shown having a front edge 14a parallel to a lapped surfaca
lla of the substrate, and parallel to a final throat height
~ 15 of the pole pieces 12a of the transducer 12. The
.~ electrical lapping guide resistor 14 comprises a strap 14b of
~: a metallic material, preferably chromium, deposited on the
~ substrate and bounded by two conductors 16 and 17. The ratio
~ 30 of the resistance of the chromium strap to the conductors is
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SA984-023 5
Z:l. A similar lapping guide resistor is deposited on the
opposite end of the substrate 11.
During lapping of the substrate 11 to accomplish a final
throat height of the transducer 12, a rough lapping to
approximately 10 um is accomplished by optically monitoring
the position of the lapped edye. At the conclusion of the
rough lapping s-tage, the ends of the substrate bearing the
lapping resistors are level to within 5 um with respect to
final throat height. From 10 um to final throat height, the
resistance of each strap resistor 14b is used to monltor any
leveling error. The detected leveling error is utilized to
balance the lapping force on each end of the substrate. The
final lapping distance from 10 um to the final throat height
is completed by monitoring the position of the lapped edge
~ 15 using the resistance measurements of the two electrical
;~ lapping guides 14.
.
;~ The prior art electrical lapping guides 14 of Figure 1 have a
non-linear response curve as shown by the solid line curve of
Figure 2. This curve is substantially propor~ional to
~ 20 Rx = Rs . ( ) + Rc , where
`~ X ~ H
Rx is the two point resistance measurement between the
conductors 16 and 17;
~i Rs is the sheet resistance of the chromi~n strap 14b;
Rc is the contact and lead resistance of connections to
conductors 16 and 17;
L is the effective length of;the chromium strap 14b;
X is the lapping distance or width from the front edge 14a of
the strap 14b to the final throat height; and
~ ~ H is the final width of the lapping resistor at final throat
`` ~ 30 height.
~;
; ~ ~The resistance vs lapping distance characteristic for the
prior art electrical lapping guide resistor limits the usable
response to a fine lapping distance of 10 um. As is evident
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SA984-023 6
from Figure 2, during rough lapping of the substrate the
response curve A for -the first 6 um of lapping, shown by the
solid line, is too sensitive to yield an ideal lapping
measurement. At the low end of the response curve, the
change in resistance is small as the lapping distance X
decreases, yielding a poor signal to noise ratio. As the
lapping distance approaches zero, the change in resistance
becomes very large with a small removal of material during
lapping. Thus, the dynamic range of the lapping guide
resistor i5 limited to approxirnately the knee of ~he curve A
since over -this area a decrease in lapped distance gives a
reasonable change in resistance RX. The curve B in dashed
lines of Figure 2 shows a resistance RX versus lapped
distance according to the present invention.
To linéarize the resistance of the electrical lapping guide,
the resistor configuration of Figure 3 may be employed. The
resistor comprises a layer of chromium 20 as a strap. The
resistor has a rear edge 20c parallel to the final throat
height 15. The side edges 20a, 20b of the resistor 20
converge towards the lapped surface lla at an angle to the
lapped surface lla. The final length, L finish, of the
~` resistor at the throat height 15 can be approximately 480 um.
~;~ The beginning length, L start, is 20 um. The electrical
lapping guide of Figure 3 provides for an effective increase
in length of the resistor 20 as lapping progresses, thus
llnearizing the resistance versus lapping distance as shown
in curve B of Figure 2. Additional linearization of the
resistance versus lapping distance is obtained by sloping
;~ resistor edges 20a, 20b and conductors 16 and 17, as shown in
Figure 4, such that the slope of the resistor edges 20a, 20b
and conductors 16 and 17 in the earlier stage of lapping is
slightly higher than the slope of the remaining portion. It
was further observed that improved resolution can be obtained
during rough lapping by increasing the spacing between
conductors 16 and 17. The x/v coordinates in microns fox the
edges shown in Figure 4 are shown in the Figure 4a. The X
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SA984-023 7
coordinates are the distance from the central line to the
conductor 17. The Y coordinates are the width of the strap
20.
To achieve an even higher range of lappiny for the resistors
which will cover both rough and final lappiny, the embodiment
of Figure 5 is shown. An extension portion 22 was added to
the resistor 20 of Figure 4. The e~tension portion 22
comprises a coarse lapping resistance section K having a base
portion ln contact wi-th the forward edge of the fine lapping
resistance section L. The extension portion 22 which begins
at the fine lap dimension W, equal to 10 um, extends
approximately the distance W1, of 70 um to a coarse lap
beyinning edge along two converging edges 23, 24. The
~ resistance change during a coarse lap is from approximately 4
; 15 to 15 ohms. The fine lap resistance change is appro~imately
25 to 275 oh~s for a resolution of about 25 ohinstl um. The
performance of the embodiment of Figure 5 compared with the
conventional lapping resistor of ~igure 1 for a 50:1 strap to
conductor sheet resistance ratio is shown in Figure 6. The
resolution during coarse lap, noise immunity and improved
; linearization during fine lap is evident from curve ~c) o~
the Figure. Conventional lapping resistors exhibit the
characteristics la~ and (b) also shown in Figure 6. In curve
(c), the resistance changes slowly but measurably during the
rough lap, i.e., lapping distance W1. Then the resistance
change increases a large amount over the lapping distance W,
which is the width of the strap resistor 20. Better control
is required and obtained for the fine lap portion since the
resistance RX is more directly representative of the lapping
distance W and therefore of the final throat height.
Thus, it is seen tha-t by advantageously contouring the edges
of a lapping resistor, the effective change of lapping
distance versus resistance may be controlled. Tapering of
the respective resistor edges will provide for a resistance
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SA984-023 8
change which is more linear and provides a higher noise
immunity than conventional lapping structures.
While the invention ha's been particularly shown and described
with reference to pre~erred embodimcnts thereof, it will be
understood by those skilled in the art that various changes
may be made without departing from the spirit and scope of
the invention.
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