Note: Descriptions are shown in the official language in which they were submitted.
METHOD AND APPARATUS FOR CONTROLLING THE THROAT HEIGHT
OF BATCH FABRICATED THIN FILM MAGNETIC TRANSDUCERS
The present invention relates to 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 ~ips to a final throat height is
provided.
Thin film transducers for reading maynetic disc struc-
tures 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 throa~ height for a given thin
film transducer must be established within a minimum
tolerance in order to provide for transducing e~ficien-
cies capable of handling s~ate of the art data recording
densities.
Lapping of the pole tip ends which define an air bearing
surface is measured by an elec~rical lapping guide (ELG)
structure. A lapping ~uide is located on each end of a
substrate row at each end of the row o transducers. The
E1G is ad]acent to the surface to be lapped and provides
electrical signals identifying the position o 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 main-
tained. In order to achieve the required flatness and
lower the amount of recession of the pole pieces o~ the
transducers as a result o the lapping process, the inal
SA984024X
.... .. . ..
lapping to the final throat hei~ht is accomplished during
a final wash cycleO The wash cycle is fixed to lap for a
calculated wash cycle time which produces a very fine
~lapping of the pole tips. The final lapped dis~ance o~
the priQ~ art represents an estimate based on a predeter-
mined ~ping time versus lapped distance relationship as
curren~ ELG structures do not have sufficient resolution
` to measure the final distance lapped during the fine lap
procédure. The present invention provides an ELG struc-
ture capable of monitoring lapping during a final finelap cycle as well as during the rouyh lapping stages to
achieve final throat height.
Summary of the Invention
It is an object of this invention to provide for an
accurate monitoring of the position of a lapped edge.
It is ano-ther object of this invention to provide for an
accurate monitoring of the throat height of batch fabri-
cated thin film magnetic transducers.
It is a more specific object of the invention to measure
the position of opposite ends of a lapped edge defining
the throat height of batch fabricated thin film magnetic
transducers.
These and other objects are derived rom a method and
apparatus in accordance with the invention. An
electrical lapping guide structure is formed on
preferably two (2) 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 ~i~h at least one
switching junction which changes state during lapping of
the substrate~ The switching junction includes a par-
allel resistance element. The switching junction has a
distinct switching plane which is ~ known distance from
the desired ~inal throat height dimension. When the
lapped edge coincidcs with ~he switching plane, a
SA984024X
distinct stepwise change in resistance is detected across
the switching plane.
In a preferred embodiment of the invention, each lapping
guide in~ludes a plurality of switching junctions and
,, , , ~
shunt ~èslstance elements formed as a sexies circuit.
Eac~ swi~ching junction of a series is located to have a
! switchi`ng plane a known distance from the final throat
height dimension, and different from the remaining
switching junctions. The s ries circuit experiences a
stepwise change in resistance as the lapping progressesu
The lapped substrate edge posi~ion at each of ~he switch-
ing planes is therefore detectable at discrete swi~ching
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 configura-
tion 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 resist-
ance. Thus, as the lapping edge approaches the final
throat height position, the resistance element character-
istic resistance change versus lapping distance charact-
2s eristic is accurately known.
-- Each electrical lapping guide provides for position
measurements of each end of a lapped edgeO 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.
Description of the Fiqures
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
~or carryin~ out an embodiment of the invention.
SA984024X 3
Figure 3 is an isometric view of the electrical lapping
. guide of Figure 2~
Figure 4 is an electrical schematic demonstrating the
operation of the ELG structure of Figures 2 and 3.
, ~ ,
Figure.-:-5 is a plan view of an electrical lapping guide
for carrying out a preferred embodiment of the invention.
Figure 6 is an electrical schematic demonstrating the
operation of the ELG structure of Figure 5.
Descr~e~ on of the Preferred Embodiment
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 lapplng
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. Elec-
trical lapping guides 21 and 22 provide signals indica-
tive 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
ohmeter 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 suxface 23 with respect to the right and
left ends of substrate ll by directing the output of the
digital ohmmeter 25 to a controller 27~ The controller
27 in turn activates thc actuators 15 and lG to level the
SA984024 X 4
substrate ro~J 11 relative to the lapping plate 2G thereby
insuring that the throa~ heights of all of the trans-
ducers 30 are at the same lengthv
Referring now to Figure 2, there is shown more partic-
`ularly ~ plan view of the electrical lapping guide
structur`é 21 with respect ~o the lapped surface 23, and
the final dimension, identified as the final throat
height whera lapping is to cease. It is, of course,
understood that t~e 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:
P ,~
R = t x w (1)
where R = resistance of lapping guide resistor 31
p - resi.stivity of the material comprislng
resistor 31
Q = the resistor 31 length
-- w = the resistor 31 width
t = the thickness o~ resistor 31.
Lapping of surface 23 will reduce the width w of resistor
31, increasing the resistance R of resistor 31. Measur-
ing this resistance makes it possible to monitor the
positional change of lapping surface 23 as it approaches
the ~inal throat height.
The electrical lapping guides 21 and 22 axe deposited by
thin film deposition techniques during the manufacture of
each of the thin film transducers 30. It is assumed that
the length R, resistivity and thickness t of the
resistor 31 have been controlled during manufactuxe.
Thickness is controlled during the deposition o~ the
SA984024X 5
transducer elernents 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 widt~ of the resistor 31 is determined in the mask
used t~--form the resistor 31. The length of the resistor
is preferably approximately 500 micrometers. The resis-
tivity, thickness and beginning width for the electrical
lapping guide resistor are typically of the following
dimensions and values:
p = .55 micrometers
t - .25 micrometers
Q = 500.
For convenience, t x Q of equation (1) may be repre-
sented 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 (4)
are shown in Figure 2, in the preferred embodiment of the
invention, a total of six t6) 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 ll 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.
SA984024X 6
Another embodiment of the electrical lapping guides 21
and 22 is shown in Figure 5, and the principle of
operation of this apparatus is the same as that for the
embodiment shown in Figure 2. In this embodiment the
switches-32 through 35 are deposited in a series
electr~-c~l circuit between terminals 21a' and 21c'close
to the~`-l~ast thin film magnetic transducer 30 in the row.
The electrical lapping guide resistor 31 is deposited
toward the end of the row in an electrical circuit
between terminals 21c' and 21b'. Since t:his embodiment
is less sensitive to variations in the manufacturing
process, the embodiment shown in Figure 5 is the
preferred embodiment.
When the lapped surface 23 is coincident with the switch-
ing plane 32a of switching junction 32, the switching
junction will change state. Preferably, switching
junction 32 will change from a conducting to a norl-
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 ohmeter 25, connected
to controller 27 of Figure l, 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 measure-
ments appearing between terminals 21a and 21c provide ~or
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
approachin~ the final throat helght, will be accurately
calibrated, permitting the lapped surface 23 to be
SA984024X 7
positioned within a nominal throat height of approxi-
mately 1 to 2 microns to the desired final throat height.
The chan~e in electrical resistance appearing between
tèrmi~als: 21a and 21c can readily be .recognized wi.th
referenc~e to Figure 4. Figure 4 electrically represents
the pos;ition of each switching junctions 32 through 35
with réspect to the final throat height~ It is clear
that as the-lapping surface 23 is lapped away, switches
32 through 35 will, in se~uence, be rendered in an open
circuit ccndition. Each open circuit condition which
results from lapped surface 23 proyressing toward final
throat height, is seen as the addition of resistances 38
through 41 in the series circuit represent~d between
terminals 21a and 21c. Of course, during the switching
of junctions 32 through 3S, the analog resistance
measured between terminals Zla and 21b will be changing
in accordance with the above equation.
The change in electrical resistance appearing between
terminals 21a', 21b' and 21c' of Figure 5 can be
determined by reference to Figure 6. This change in
resistance is the same as that described above with
reference to Figures 2 and 4 for the corresponding
elements.
With the foregoing calculation, it is possible to deter-
mine 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 ~or the transducer array. Addition-
ally, 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 tech-
nique of fine lapping during a wash cycle by setting a
pre-fixed wash cycle time may ~e 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 fi.ne lapping point. Alterna-
tively, lappin~ may be terminated when the last of the
switches, switch 35, changes state.
SA984024 X 8
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,`~. An insulative magnetic gap layer 30c is
deposi~e~ between pole pieces 30a and 30b of transducer
30. A~other layer, not shown, such as a conductive coil
layer and other insulation layers 30d are deposited on
thé substrate 11 betwPen ~he pole pieces 30a and 30b and
form a part of the transducer 30. The insulation layer
30d between the pole pieces 30a and 30b determine the
throat height of the transducers. During ~he 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 struc-
tural 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 or the
insulation layers that separates the conductors of a
switchîng element. Thus the registry of the switching
junctions 32-35 with respect to the final th~oat 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 manufac-
ture 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 Ohm
chromium resistor appears across the switching junction
having a closed circuit resistance of appruximately 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 3S throuyh 32 is deposited at the same time the
insulation layer 30d that determines the throat height TH
is deposited or transducer element 30, thus maintaining
SA984024 X 9
~L~ ~7~;g
accurate control over the switching junctions with
respect to final throat height. The geometry of the
conductor elements of each switching junction arz also
maintained congruent with the pole piece 30a, 30b geom-
etxy sin~e these elements are all deposited during thesame p~ocess. The commonly formed insulation layer and
conduc~o`r geometries improve the overall precision
location of switching planes with respect to the flnal
throat height.
The electrical lapping guide structure shown in Figure 5
is also fabricated in a manner similar to ~hat de cribed
above and shown in Figure 3. As described a~ove, the
em~odiment of the ELG shown in Figure 5 is less sensitive
to variations in the manufacturing process. It has been
found that variations in the thickness of the insulation
layers and variations in the thickness of the conductor
elements of each switching junction can cause variations
from the desired value in the final throat height of the
transducer elements 30. It has also been found that
variations in thic~ness of the insulation layers and in
the conductor elements are more likely to occur near the
edge of the substrate 11 than near the center of the
substrate 11. The embodiment of Figure 5 places the last
of the switching junctions, switch 35 near the last
transducer 30 in the row. This is the switching junction
which is the final switching control on throat height,
and the closer proximity reduces the chance of a
substantial difference in thickness of either the
insulation layer or the conductor elements between the
position of the last transducer 30 and the position of
the last switching junction 35. The electrical lapping
guide resistor 31 is near the edge of the substrate where
it has been found that thickness variations are more
likely to occur, but this produces less chance for a
signi~icant impact on final throat height accuracy since
the resistor 31 is calibrated at relative positions
during the lapping operation.
The electrical lapping guide resistor 31 is deposited
between two conductors which form terminals 21a and 21b
in the Figure 2 embodiment or between two conductors
SA98~02~X 10
which form terminals 21bl and 21c' in the Figure 5
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 dPposited on
substrate 11.
: ,,
By mea~u~ing the relative position oE lapped surface 23
with respect to the right and left ends of substrate 11,
it is possible to maintain lapped surfac:e 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 lS and 16
which will correct leveling errors. The force differ-
ential is determined as:
R31L ~ Aw ) / K
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
~w is the difference in OFFSET distance of each
lapping gulde 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 necessaryto level the lapping surface 23 and applying the differ-
ential orce with the actuators 15-16 will achieve a
substantially level condition.
The calculated inal throat height resistance of elec-
trical lapping guide resistor 31 may be represented by
the following:
R3 ~
OFFSET ~ TH
SA939024X ll
.. .. .
where the OFFSET distance of each lapping resistor is
determined by OFFSET = R31 ~ Ddn, Ddn being the switch~
ing position of a given switching junction~
The computer 27 is programmed to measure the resistance
s of eac~ ~eveling resistor of each lapping guide, both
left a~d right, and the serial arrangement of six (6)
i 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
Ddn, and desired throat height, TH, are utilized to
determine the following calculations.
The OFFSET is calculated for each of the leveling resis-
tors by utilizing the following formula:
OFFsET = R 1 - Ddn
The difference in OFFSETs between the right and left
leveling resistors is expressed as:
~w = OFFSETR le~t ~ OFFSETR right
.,
With the difference in OFFSETs, it is possible to deter-
mine a force differential between the actuators 15 and
16, such as to correct for an unlevel lapping surface.
This differential, Fd, previously described is:
Fd = ( K - Aw ) - K
By calculating the force differential in accordance with
the above formulation, the computer 27 will set a force
differential for the actuators lS and 16, according to
the following determination:
SA984024X 12
. . .
Fd < 0 Left force < Right force
Fd = 0 Left force = Right force
Fd > 0 Left force ~ Right force
This ~orce determination is a subroutine in programming
steps ~o~be described, such that computer 27 will provide
the ap~ropriate force differentlal to the substrate 11 to
correc~ any leveling errors.
As a final basic calculation for the system, a final
throat height resistance is determined from the elec-
trical lapping guide resistor 31 for each side of the
- transducer substrate row according to the previously
described relationship:
~C
R31~side)
OFFSET (side) TH
In view of the foregoing basic calculations, which
provide ~or 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
(6~ switching junctions described in the foregoing. Of
course, six (6) is not necessary to implement the inven-
tion, 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 irst being
the value of the constant K, previously described with
respect to equation 1. The distance from the final
throat height to each s~itching plane Ddl 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 com~uter
program, or on the last change of state of the switching
junction closest to the ~inal throat height. In the
SA984024X 13
event that a calculated throat height is to be used as
the endpoint for ceasing lapping operations, this calcu-
lated throa~ height (TH) is loaded into the computer
program~ A wash cycle distance Wc is also inserted in
the prog~am. At the end of -the coarse lapping, as is
lcnown 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.
i
An additional subrou~ine will be employed in the program-
ming to read the left and right resistance values pro-
vided by each electrical lapping guide resistor 31. This
subroutine additionally subtracts a leacl 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 l3) separate ~ortions
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.
Do until first switching junctions activate and level
resistor values stored
Call read Left & Right R31 values
Calc force differential
Read level R31 values right and left
Fd=R31Left R31Right Equation (1)
Call set mechanics
If both left and right detectors activate
Then
Store both level R values in
R1ll and Rrn respectively
SA9~40~X 14
s
I one detector activates
Store level R valu~ in Rside
Rside - 1.0 (detected side)
~; Rside - 0.0 (undetected side)
~ , Calc force differential
`~ Call set mechanics
~
- Do until other detector activates
If other detector activates
Then
Call read Left and Right R values
Store other level R value in Rsi.de
The result o~ the first portion of the code is to calcu-
late a force differential and apply the force differen-
tial through the actuators 15 and 16 to the substrate 11.
This force differential is calcu1ated before the first
switching junction, referred to as a detector in the
pseudo-code, o each side of the substrate changes state
by subtracting the resistance levels measured at each
si~e 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 correspo~ding switching junctlons on each electrical
lapping guide structure change state simultaneously, then
the value of each electrical lapping guide resistor, left
and right t are stored.
:
: In the event that on.ty 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 ~orce differential is calcu-
lated and the SET MECE~ANICS apply the calculated force
~or the actuators 15 and ~6.
When the seco~d of the switching junctions of th~ oppo-
site electrical lappin~ guide structure is found to
SA984024X 15
change s~ate, the le~t and right R31 resistor values are
read and stored.
Thus, after a pair of switches on opposite sides of the
substrate ln different ELG structures changes state, the
S first ~o~tion of the programming is completedO
, ;.
The second portion of the programming below consists of
performing a series of calculations when lapping prog-
resses from the first pair of switching junctions which
have been activated to the third pair to change state.
Do N from 1 to 3
Calc offsets right and left
wside = Rsiden Ddn Equation (2)
Store offsets in Wsiden
Calc ~ offset
W W1n Wrn Equation (3)
Store offset in ~Wn
Do until next detector activates
Call read Left & Right R values
Calc new force differential
Fd = ( - W J ~ K . Equation (4)
R3 1L R3 1R
Call set mechanics
End do until next detectors activate
SA984024X 16
If both detectors activate
Then
; Store both level R values in Rside
~, .
~ , .,
!` ~ Else
~`: If one detector activates
Then
Store level resistor values in Rside
Rside=l.0 (detected side~
Rside~0.0 (undetected side)
.. 10 Calc force differential Equation (l)
Fd = R3lL ~ R3lR
Do until other side detected
Call set mechanics
End do until other side detected
Read and store other level resistor in Rside
End do N from l to 3
Calc & store next offset in Wside E~uation t2)
Calc ~ store next ~W in ~Wside Equation (3)
The OFFSETs, Wside, for ~he right and let ELG structures
Z0 are calcula~ed 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 ~apping proceeds from the first pair of switching
: 25 junctions:to change state a force differential is calcu-
lated 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 ll chan~cs state, a force diferential
is calculated as was accomplished earlier until the
remaining switching junction of the oth~r electrical
SA984024X 17
lapping guide structure changes state. When both switch-
ing junctions have changed state, the two resistors R
are measured and their resistance values stored.
~ he above routine is repeated until the third pair of the
switchi~g~junctions changes state.
.
The OFFSETs, Wside, and difference in offsets, Qw, are
calculated at the completion of the above steps and
stored.
The following steps executed by the computer program of
computer 27 occur during ~he lapping of substrate 11
between switching junctions 3 to 6.
Do N from 3 to 6
Find the largest absolute value of
QWl to ~Wn and store its real value W1
Check error correction value
n ~ Equation (5)
w - - - ~wk - Wl~
k=1 n ~ 1
Do until next detector pairs activate
Call read level R values
Calc force differential
Fd= ( K _ W~ - K
R31L R31R
CaIl set mechanics
SA98402~X 18
If end on calculated throat height
Then
Find largest offset values for left
and rlght and store off
,
~ Average offsets
, ~
oFrsÉTs = L~ Wside ~ - OFF Equation (6)
n-l
. . .
Calc final throat resistance
Reside = K Equation (7)
FFside T
If REside ~ ~side Equation (8
Then
Print data `~
Exit program
If not in wash cycle
: Then
15 If end on calculated throat height
''
Then :
Calc wash cycle resistance
'Rw= K Equati~n (9)
OFFS ide~Th ~wc
:
: : If Rw-Rside < 0
~ .
Thcn
Fix wash cycle in on condition
SA984024X l9
7~i
Else
Calc wash cycle resistance
Rw = - K _ Equation (10)
OFFside+Dd6 C
-- If Rw-Rside < 0
Then
Fix wash cycle in on condition
As lapping progresses from the third switching junction
pair to change state, the stored differential of~sets Ww
are examined and the largest differential o~fset 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 differen-
tial calculated. The force differential is applied
through the set mechanics to the actuators 15 and 16.
If the operator o~ computer 27 has entered a calculated
throat height at which lapping is to cease, then the
computer determines the largest of the previous o~fset
values determined, and averages the offset measurements
according to equation ~6). The final throat hei~ht
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 resis~ance 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 typicall~ lap from approximately 5
- 30 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 selectcd other than this final throat
SA984024X 20
height, this nominal throat height is utilized in equa~
tion (9) with the wash cycle lapping distance, Wc, to
calculate a final lapping resistance, RW for each ELG
~resistor 31. The wash cycle resistance i5 continuously
compared with the measured ELG resistance 31 of each side
o subs~rate 11. When the wash cycle resistance equals
i . ;
the measured resistance, the wash cycle is efected for
final lapping of the su~strate.
In the event that the lapping is not to end on a calcu-
lated throat height, but rather is to end on the changing
of state of one of the switching junction, the programm-
ing 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 acti-
vated, or both.
If one detector activates
Then
Store level R value in Rside
Calc ~ store next offset in
W . Equation (2)
slde
If both offsets stored
Then
Calc and store next ~W in ~Wn
Else
If both detectors have activated
SA98~1024 X 21
Then
S~ore level R values in
Rln an n
~ ~; Calc & store next offsets in
S ~,Wln and Wrn Equation (2)
~, .. ; :
Calc & store next ~W in
~W Equation (3)
End do until next
End do from 3 to 6
Print data
Exit program
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, ~w,
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 lappinct guide structure of the
- present invention. Those skilled in the art will recog-
nize yet other ways of r~ducing these measurements to
effectively control leveling and lapping of batch fabri-
cated thin film magnetic transducers to a final throat
height.
SA98~02~X 22
