CA1250435A - Machining sensor - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method and article of manufacture allows great accuracy in machining the edge of a face on a prism to a predetermined position respect-ive a feature carried on the face and formed by deposition of insulating material. The preferred application for this invention is in accurately machining the transducing surface of a disk memory transducer assembly so that the throat height of the thin film head carried on an end face of the assembly has a precise value.

Description

il ZS~35 This application is a division of our Canadian Patent Application Serial No. 435,916 filed September 1, 1983.
This invention relates to machining the edge of a face on a prism to a predetermined position in respect of a feature carried on the face.
In certain manufacturing operations, particularly those for fabricating disc memory thin-film magnetic heads 1n situ on the air bearing slider to be carried by the head arm, it is desirable to machine the flying surface until a precisely located line on another surface intersecting the flying surface becomes the line of intersection of the two surfaces. In the thin~film head example, the head is carried on an end face of the slider which is approximately perpendicular to the flying surface, and the line is positioned to specify very accura-tely the thin-film head's throat height, that is the dimension of the flux gap normal to the transducing surface. (The transducing surface, of course, is nearly parallel during disc memory operation, to the medium surface.) Accuracy in throat height to within a few tens of micro-inches is desirable to insure optimum electronic and magnetic characteristics. Machining the flying surface until it coincides with the desired line of intersection then automatically sets throat height to the accuracy with which the line of intersection was set.
Controlling this dimension during fabrication has always been a difficult problem because of the extremely small dimensions and tolerances invo]ved. Simply using the top of the slider prism ~;~S~43S

as a reference surface for controlling throat height was satis-factory when grinding ferrite heads, see United States Patent No. 3,982,318. But tolerance and dimensions are much larger in ferrite head technology.
Respecting thin-film heads, recent innovations allowing accurate control of throat height involves the use of so-called lapping guides or machining sensors, e.g., as disclosed in IBM
Technical Disclosure Bulletin (TDB) Vol. 23, No. 6, November 1980, p. 2550. These guides or sensors are - la -~;~S~435 deposited conducting ~aterials placccl on thc surtace carrying -the thin-~ilm he~d. Two t~es of sensors are in general use. So-called discrcte sellsors simply have their electrical contilluity broken at some point during machinillg and hence provide an indication of machining progress at only a single installt. .~nalog sensors have an area of resistive material which is slo~lly removed by machining and hence provide a continuous indication until continuity is broken. I'~ith respect to di`screte sensors typically several a-t different heights are employed The contilluity of each is successive]y broken by the machining process thereby providing a series of indications of precisely ho~l much more machining must yet occur to reach the desired final position line. At the limits of or `lithill the desired throat heigllt range a last sensor's conductive path will be opened signaling that the machinillg process should stop.
The use of these machinillg sensors drastically improves the accuracy ~1/ith ~Ihich the edge can be positioned relative to the feature.
~-lowcver ~hen dealing witll thin-film m.lglletic heads one cannot form conventional machillillg sensors ~ith thc same step ~hich clefines the throat of the gap. Tllis is becausc the throat is formed by the deposition of an insulatillg layer, ~.~hercas the maclli~ lg sensors a~e collductive patterns and hence are deposited ill the steps crcating the m lglletic legs of tlle hoad. It is a knowll difficulty -that succcssive layers of material depositcd by the use of photo-optic masks ~nd formillg a composite thill-film structure callllot be registered witll respect to cach other ~ith per~cct acctlracy. That is the maslis or p.ltterns ~lhicll clefillc cach of thc fcaturcs ol~ successive layers such as thc hottom leg thc thlo.lt ancl thc top leg callllot be placed ill precise aliglllllellt ~ith thc p.lttcrlls crcatcd l)y previous masliillg stcps clur:illg -typical mallllf.lcturing operatiol)s. Ihclcforc thc thI'O.lt heigllt oF a typical ~2Sa)~35 thin-film head cannot be controlled to an accuracy greater than the registration between the throat insulation-forming pattern and the magnetic leg-machining sensor-forming pattern. Experience shows that this inherent inaccuracy results in a substantial percentage of head gaps which have throat heights outside of the required tol~
erances. Worse still r even though the throat height-defining step occurs intermediately in the process, one cannot easily tell whether or not the head is good until the manufac~uring process is-complete, making the relatively high number of reject heads an expensive flaw in these previous systems.
The problem of aligning machinirlg sensors with a fea-ture formed of insulating material such as the throat defining layer of a thin-film head is present for bothdiscrete and analog sensors.
In a current manufacturing process, analog sensors are used to in-dicate the progress of machining of a workpeice carrying several thin-film heads. The machining step sets the throat heights Eor all the thin-film heads simultaneously. An analog sensor is inter-posed between each pair of heads. It is necessary that the position of each analog sensor vis-a-vis its adjacent heads be known very accurately so that machining can be hal-ted when the throat heights of as many heads as possible are within the desired tolerances.
(Due to various inaccuracies in-the process, it is possible that not all throa-t heights can be reduced to a value within the tolerance range at the same time.) Such a process is described in our Canadian Patent Application No. 432,983 filed July 22, 1983.

.Zs~ 5 f IsM TDs Vol. 18, No. 1, June 1975, p~ 227, recognizes the difficulty in aligning features of different deposition layers and apparently teaches depositing the lapping control layer with the same step which forms the"registration of the insulating layer forming the gap or covering the gap ~ 3a -~,Z5~ ~5 layer." llow an insulatillg layer can be registerccl ill thc S.llllC stcp with depositing the lapping control layer isn't e~plail~ed.
I~l TDB Vol. 23 No. 2 July lCi~O p. 77~ teaches a method of calibrating an analog lapping guide or machining sensor to compellsate for variations in bulk resistivity and film thickness. This method is not involved Witll determining position of the analog sensor relative to a feature of an insulating layer~
The solution we propose to the problem is to create the machinillg sensor or indicator with the same deposition mask that defines the insulating feature -to bo precisely located relative to tlle final position of the lapped edge. The way ~e accomplish this is by providing a first conductive layer on ' the :Eace whicll is to carry the feature which extends from near the original position of the edge to'be machined through the allowable tolerance band wllich the final position of the edge miay occupy. This can in tlle case oE
thin-Eilm magrletic lleads be convellierltly inclucled in tlle deposition step ormillg the bottom leg of the thin-film llead. Durillg the step wllicll creates the feature from ~hich the final position of the machilled edgc is specifiecl, an acldi.tiollal barrier arecl of inslllLItillg material is deposited on the first concluctive layer alld lying along a so-called sensillg line

2~ substantially parallel to tlle edge ~lliCh the macllillillg will crc;lte using the same mask to create both. TllC sensing line definillg the one edge o-f the barr:ier area is precis'ely positionecl rel.ltive to the feature because both are created l~ith the same mas~ :ili the sallle deposition step.
Tllell a seconcl concl-lctivc l.lyer ;.s clepositc(l on the barrier are.L
cont;lctillg tlle L'irst concluct:ivc layel clirectly ollly bet~eell the initial location of thc eclge nllcl the sellsillg Lille. [1~ thc m.lnuCacturc of thill-f:il ~ZSO'~5 .

hc~ads this step l~ill t~pi~all~ oce~lr ill conj~ ctio~ itl~ tlle dc~lositillr of the top leg oE the magnetic flux path. Those s.~;illed i.n the art lulderstalld that each of these three layers are producecd ~y a series of s-teps incl-lding the use of a precision mas~. usually optical~ to ~Eorm tlie desired pattern in the layer ~iith very higll precision.
The edge of the surface is then machillecl from its initial location to~ard the sensil-g barrier line edge. I~hen the machined edge reaches the sensillg line elec-trical contilluity bet~een the Eirst and second conductive layers is broken (assuming a non-collductive machine tool). A contin-lity tester conllected betl~een the second conductive layer lying on the barrier area and the first conductive layer ~iill indicate an~open circuit indicating position of the machined edge. If the sensing line is intellcled to define the ideal final position oE the line of intersection of the t~o surfaces, thell macllillillg is halted.
Lll factj the preferred applicatioll for this discrete machilling sensor is to cali.l)rate a conventiollal allllog senso:r to precisely determille its position rclative to a feature l;ne precisely cle~fillillg the edge of an insulating feat-lre. This is accompl:ished hy using one or more discrete sensors, each havillg a dif:ferent sensillg line inteIsectillg the sensillg area of tlle allcllog sellsor and eacll precisely posit:ioned relative to the feature line. At each point in the maclli~ lg oper.ltioll ~here a disçrete sensor OPCnSJ the resistal~ce of the a~ log sensor is measurecl. These resis-tallce values may be substituted in a gelleral equatioll oE the Eorm h=K/R
relat-.illg all(llo~ sellsor resistallce R ~ith sp.lcil~g h oE the top edge of the allllog sellsor Erolll the macllillecl edge. T}le ecluatioll call thell l~e solvecl to pl'OVidC a V LIUC fOI~ thc col~stallt 1~ allcl l'Lny other constallts t:o yield an equa-tion clircctly rclclt:ill(r scnsor rcsist.lllcc ~..itllIll;lchi.l~c(l cclge spac;~lg l.`rolll the fCatllrC pOs:itiOII I inc.

1~5~)435 Accordingly, one purpose of this invention is to increase the accuracy which machining of the edge of a surface can place the edge relative to a feature carried on the surface.
A second purpose is to reduce the scrap rate during such machining operations.
Another purpose is to combine the steps of forming the throat filler material of a thin-film head with the step forming the machining guide when machining a transducer assembly carrying a tnin-film head.
Yet another purpose is to allow more accurate measure-ment of the current status of the machining operation.
Thus, in accordance with one broad aspect of the inven-tion, there is provided a machinable prism having a face carrying a conduc-ting surface exkending to a first edge of the prism and comprising:
a) an insulating material layer overlying said conducting surface and including a feature comprising an edge Eormed in said insulating material layer, said edge lying along a segment of a feature line;
b) a machining sensor comprising an additional barrier area of the insulating layer lying along a segment of a sensing line and on the conducting surface and extending away from the first edge of the prism, said sensing line having a predetermined precise spacing from the feature line; and c) a conducting layer on the barrier area and electrically contacting the conducting surface only between the sensing line and the first edge.

~2~)4~35 66082-1~8D

In accordance with ano-ther broad aspect of the invention there is provided a machining guide lying on a surface of a prism, -6a-~2S~ ~35 a first edge of the surface which is to be machined from its initial location to at least approximately coincide with a sens-ing line segment comprising:
a) a bottom conductive area on the surface lying between the first edge's initial location and extending to at least the sensing line segment;
b) an insulating barrier area having one edge precisely coincident with the sensing line segment and lying wholly outside the area on the surfac~ between sensing line segment and the initial location of the first edge; and c) a layer of conductive material lying entirely on the barrier area outside the area between the initial location of the first edge and the sensing line segment and extending across the line and making electrical contact with the conductor area.

_ ~ _ i~50435 - 6082-15~

Other objects and benefits: of this invention will be evident fro~ the following explanation~
BRIEF DE5CRIPTI~ON OF THE DRAWINGS
Figure 1 is a perspective view of a prism having a surface on which the subject inventive article is located, and showing an intermediate step in the inventive method.

~L254~435 Figure 2 and 4 are cross sectiolls through one oE the machining sensors sho~n in Figure 1 before and after -the machining step respectively~
Figure 3 and 5 are cross sectional viel~s of the feature relat:ive to which the edge positioned by the machining is respectively located before and after machining.
Figure 6 discloses a structure incorporating this discrete sensor in a preferred composite sensor to be employed in mass production of devices such as thin-film heads which have close tolerance dimensions based on the positi.on of an edge oE an insulating area.
Figure 7a is a magnified perspective view of an individual thin-film resistor of Figure 6.
Figure 7b is a circuit schematic of the analog sensor net~ork of Figure 6.
Since this discrete sensor has been cleveloped specifically for the purpose o~ controlling throat heigllt of a thin-filln head the description is based Oll an application in this area. It has identical applicability in any case ~here such machining relative to a feature deEined by deposited insulating material must be controlled.
Figure 1 sho~s a greatly rnagnified perspective vie~ of a machinable prism or block 9 formed of a ceramic material, and comprising a thin-film head air-bearing slider as it looks just before the final macllining of the air-bearing face. Line 15 is the initial position of the edge o-E end face 10, defined by the intersection o the initial pOsitiCIl of Elying surface 2G
(shol~n on edge in Figures 2 and ~1) with face l0. Surface 26 is ~o be machined until its intersection line with encl face 10 reaclles its icleal position coincicling witll a sensing plane 13 clefinecl by the two lines 13a ancl 13b.
B ~te end face 10 there has been pl;lced a m.lcllinirlg sensor or guide 21 including a conductive layer or area 11 intersectecl by sensing line 13a g _ ~2S~ ~35 -or plane 13, and ha~ing any convenient shape. Iigure 2 sho~s this guide 21 in cross section prior to final machining. On top of conductive area 11 an insulating layer comprising barrier area 12 i5 deposited, having one edge lying along the sensing plane 13, extending away from the initial location of line 15 at the edge of face 10 and lying atop concluctive layer 11. Sensing plane 13 should be substantially parallel to the initial location of line 15 at the edge of face 10. A preliminary machining step may be necessary to confi~ure prism 9 so that this relationship e~cists. Another deposited con-ductive layer forming conductive area 14 is located entirely within barrier area 12 on the side of -the sensing line 13a and extends across line 13a, contacting conducting surface 11 between the sensing line 13a and the initial location of the edge at line 15. Thus, conductive layer 14 is completely insulated from conductive layer 11 as to layer area 14b, i.e., the portion above line 13a, and malces electrical contact witll layer 11 in area 14a, below line 13a.
For illustrati.ve purposes here, a si.mp].if:ied diagralll of a typical thin-film heacl 20 is shown acljacent machining guide 21 and in cross section in ~igure 3 before machining. This comprises a pair of magnetic flux paths 17 and lS (see ~igures 3 and 5), a winding 19, and a deposited insulating material 24 typically formed of aluminum oxide interposecl between leg 17 and leg 18 of the magnetic fl~lx path, thereby creating the flux gap 25. A second i.nsulating layer 16 insulates turns 19 and defines the interior end of 1uY gap 25. This interior encl o:f fl~lY gap 25 lies along one segment of a feature line 27, shown on end as clo~s in ~igures 2-5. The spacing between ~eature line 27 and sensing line 13a is formecl by the same cleposition step ancl Wit}l the same maslc, and is therefore kllo\~n with great precision, since no mask alignlllent errors are present.
rO provide a fluA~ gap 25 of the proper thro.lt height, it is necessary to ~achine face 26 until i.t coincicles witll pl.lne 13 on face 10 _9_ ~LZ50~3S

~1ithin a toler~nce of 60 llin. Flux gap 25 ;s physically formed by and essentially comprises deposi.ed non-maglletic insulating material. It ~/ill be clear to one skilled in the art that by creating the edge of barrier area 12 along sensing line 13a, which defines the point at which machlning is to stop"~ith the same mask and in the same deposition step defining the interior end of gap 25 along feature line 27, gap 25 throat height will be very accurately defined and much more accurately so defined than if the feature line 27 and sensing line 13a ~ere created during separate deposition steps or with different masks. It will also be clear that control of throat height of a thin-film head gap is only one of many possible applications wllere this proceclure may be used.
The nlachining is conventional, and can be performed by lapping or other high precision operation, but must be performed by a tool which does not short between layers 11 and 14. Continuity testers 22 are connected to con-ductive surface 11 and conductive layer 14b by connectors 23.
Tlle machining slowly erodes the material be-tween plane 13 and the initial location of the edge of face 10, line 15. Illhen the material between plane 13 and line 15 has been completely eroded, electrical contact between layers 14a and 11 is broken and continuity testers 22 indicate this condition.
2û rhe final configuration of a machining sensor 21 is sho~ln in Figure S. The operator monitors testers 22 and can see the indication by them and stop the machilling. /~lternatively,~the machining devicc can be connected to -testers 22 to automatically stop its operation oncc contin-lity fails.
Tlle reason the initial position of line lS must be nearly parallel to sensing plane 13 is now apparent. I~lllen the edge oE face 10 is macllined tocoincide l~ith plalle 13, iJ: they are not parallel at th;lt tilnc, some m;lterial past plalle 13 ~ill be removed, causillg one corner of the senser 21 to define the end of con~ -lit) ;md the sensors '1 ~ill lose continuity at differellt .

~2S~ 35 times. Thus, at some point in the machining operation, edge 15 should be approximately parallel with sensing plane 13. The position of edge of face 10 at that point can be considered its initial position. Machining to achieve this relationship may be considered merely a preliminary step. The efect of such non-paral]elism can be reduced by making layer 14 more narrow and by placing sensors 21 close together. However, -the likelihood of a defect in the electrical contact between them which totally destroys initial continuity is then greater. The inheren-t width of the feature and itsappurtenant structure (head 20) limits the proximity between sensors 21.
While the sensors 21 and the associated process just described function satisfactorily for cer-tain requirements in small production runs, the commercial requirement for many thousands of magnetic heads 20 has led to a preferred use for these sensors 21. To cheaply and efficien-tly manufacture -these heads 20, we prefer to place several on a sinyle bar, and then machine all of -their flying surfaces 26 simultaneously.
In the Canadian Patent Application NO. 432,983 mentioned earlier, our preferred use for this invention is described in some det-ail. Briefly, this application describes a workpiece support capable of bending the bars on which -the heads are placed so as to place a greater number of -the throa-t heights of the heads on the bar within the -tolerance range required. To determine current status oE each head's throat ~ZS~35 .

height, frequent measurements of each of these throat heights occur during the final machining phase. Accurately calibrated analog machining sensors are located adjacent each head on the bar. If indications from these sensors early in the final machining opera-tion reveal that certain throat heights will be out of tolerance when machining has placed all others within the desired tolerance, then the bar is bent to cause additional machining of the flying surfaces of cer-tain heads to occur rel-ative to the machining of other -lla-~S~3435 heads' surfaces. By properly choosing the amount and IOCatiO;I of this bend:i.ng a much greater percentage of the heads' throat he:ights can he caused to fall within the tolerance range at the completion of machining. But of course, the sensors providing this information must accurately measure throat height at frequent intervals. Because such analog sensors have constituent elements formed by conductive deposits they surfer from the alignment errors ~rhich also plague conventional di.screte sensors.
1~ composite machining sensor ~rhich includes an analog sensor 2S
continuously providing a signal specifying the position of the machined edge 15 is shol~n in Figure 6. The zero throat height or feature line 58 essentially defines the position of the feature relative to ~rhich line or edge 15 is to be positioned by machining. The composite sensor is mounted on end face 10 of prism 9 and includes an analog sensing element 31 formed of a resistive conducting strip and three discrete sensors formed from concluctor paths ~16-48, insulating ~arrier area 33 beneath them ancl a concluctive area 49 belo~r the barrier area 33 malcing electrical contact ~ith ends 50-52 respcctively of conductor paths 46-48. Sensing line segments 38-40 torm a staircase pattern along the bottom edge of barrier area 33 and are of:tset ~rith respect to each other are appro.~imately parallel to e~ge 15 as in-itially positione~l and 20 have e~tensions l~rhi.ch areapredetermined distance from each other. Each of the sensillg line segments 38-~10 are located at a precisely kno~rll spacing from the ~ero throat heigllt or feat~re line 58 by virtue of tlleir cre-tion by the same process step aTId ~rith the same masli as that ~Yhi.cll procluced the :interior end of the i`lu~ gap of the appurtenant hca-l or otller device. Conductor patlls ~6-~18 have apprcci.l~lc clectrica]. resistance and arc comlllollly connected to conncctor pad or terlllinal ;13. Patlls ~16-~lS cross line segments ~10-38 respecti.vcly and all mal;e electrical con-tact l~ith conduct.i\e arcL ~9~ lermi.llai ~13 :in turn is conllectcd to Lhe upper selectLble termi.llal oE single pole doublc thro~r (~SI'DT) ~L2S~ ~35 .~

s~itch 52 and to one terminal cach of voltmeters 55 and 57.
.~nalog sen~ing element 31 is unitary ~ith the conductive area 49 ~hicll forms part of tl~e discrete sensors 29. The ends of sensing element 31 are connected by bridges 3' and 36 to resistive conductor paths 34 and 32 respectively Element 31 has an appreciable amount of resistance initially Rl bet~een bridges 35 and 36. Tlle nominal lleigllt hl and length Ll determine its resistance in large part, during machining. As the bottom edge 15 of end surface as face 10 is slo~ily machined a~ay the heig}lt hl o element 31 decreases and, naturally, its resistance increases.
Paths 34 and 32 connect conductive bridges 35 and 36 to connector pads or terminals 41 and 42 respectively. Conductor paths 3~1 ancl 32 them-selves have in one preferred embodiment appreciable resistance again dependent on their lengths L4 and L2 and heights h~ and h2, respectively. Resistance in conductive paths 34 and 32 is unavoidable because they too are unitary ~ith analog sensing element 31 "~hich must have some resistance ~ithin it to properly perform its sensing function. Connector pad 41 is connect~d to the tel~ninal of voltmeter 55 not connected to pad 43 sucll that voltmeter 55 mea.sures voltage bet~een pads 41 and 43. (Voltmeters 55 and 57 s~;.tch 52 and constant current source 53 are located remote from face 10.) Pad 41 is also colmected to the lo\~er selectable terminal of SPDT s~itcll 52. Pad 42 :is connected to one terminal o~ constant current source 53 ancl to the terminals of-voltmeter 55 and voltmeter 57 not cohnected to pad ~13. The terminal of constant c~lrrent source 53 not colmected to pad 42 is connected to the center or comlllon terminal of SPDT s~itch 52~
~!e h.lve deve]opecl an equation of tlle form Il=K/R ~ icll relatcs the value of sensor 31 height hl=ll to the dimensions of concluctors 3~1 and 32 as incorporatecl in the constant K and to voltages measuLecl by voltmetcrs 55 ancl 57 ~hicll provide a current imclicatioll o the analog sensor ~1 resistallce l~.
I ,, ~2S~435 ,, ~

As is derivod in the ~ppendi~ sensor height hl=V2h2/Q (Vl - xV2) Vl and V~
measured with s~ritch 52 in the down position sllown. lt is thus obvious that g V2~2/Q (Vl-xV~)-YOff=hl - Yoff ~here Y ff is the spacing bet~een the top of analog sensing element ~l and tlle ~ero throat height or feature line 58 clefining an edge of the feature relative to ~hich discrete sensor 29 is deposi.ted. In these equations Q=L2/Ll and ~=L~/L2. It is relatively oasv to control the deposition such that paths 3~1 and 32 11ave nearly identical dimensions so that L~ = L2 and ~=l to withir1 + 2o or less and 1~e prefer in one embodiment to do this. Even larger (~ 4%) errors affect throat height measurements by only a microinch or so.
It is also possible to deposit path 34 ~Yith a very small effective L~! (La ~ ~ ~2) by forl~ing path 34 with heigllt and thickness substantially greater than for path 32. By properly specifying the dimensions of path 34 formed by the deposltion process x can be set to fall in the range of .Ol to .l. Although the precision ~ith which ~ is 1cno~n in this case may be no better than + 10% or even ~ 20% since the value of ~ :is (:luito small the overall :impact on throat hei.ght measurement accuracy is si.mil.1l to tho case ~here x=l and is kno~n to 2%. Once the cLeposition process is stabli~ed, an average value of ~ can be cloter1l1ined by eithor calculations or clirect measuremonts of the resistance of pat11s 3~ ancl ~2 on represe1lt.ltive pris faces lO allo~ing c to be treated as a co1lstallt thereafter.
Tllere are therefore :in either embodimoTlt t~o unknowns in the throat heig11t e~uation, h2 /Q and Yoff. ~ith Vl V2 and .~ kno~1l it is possïble to determine the values for h2/Q and Yoff by n1e;lsuri1lg the v.llues for Vl and V2 at ~no~n throat heig11ts. T11is :is accompli.shod by ro~orence to discreto Se1lsorS 29. ~s mac11:i1ling of prism '~ begins li.ne 15 moves slo~ly to~arcl line 3S increasi1lg resi.sta1lce of alld volt;loe across an1log sensor ole11lent 3l. .~t son1e pOillt, lino 15 coincides ~-ith l:ine 3S causing the sensor comprisi1lg conductor patl1 ~18 to oper.... [f s~itc11 52 :is in 1'~5~3~

its up position near to that time the voltage ~!1 mca~ured by voltmeter 55 ~ill undergo a su~den increase when continuity ends since the resistance bet~een conductive area 43 and pad ~3 ilas increased ~hile current flo~ Ic from constant current source 53 llzs remaine~ unch;lnged. (Since voltmeter 55 is asswlled to have very large resistance compared to the resistance in path

3-~ and element 31 Vl/I very precisely states the resistance bet~een area ~9 and terminal 43.) At this time throat height is kno~n ~itll great precision as the preselected exact spacing bet~een line segment 3S and ~ero throat heigllt line 58.
As soon as the increase in Vl is cletected s~itch 52 must be moved to its do~n position allowing the value of Vl to be read for use in the equation expressing throat height. V2 is also read at this time for use in the equation. Although dimensions of the ~eposited resistors can not be precisely set by the deposition process Ll and L2 as ~ell as h2 and Yo~f are knol~n ~ith reasonable initial accuracy, llaving been formed by the same mask.
~tt the time that line 15 coincides ~ith line 3~, throat heigllt is kno~n wi-th great precision. Substituting the approximatiolls for Q (=L2/Ll) and h2 the measured values for Vl, V2, and the exLct thro.lt heigllt into the equatioll for throat heig]lt above, yields a better approximatioll for Yoff increasing the precision of its value substantially.
l~ith s~itch 52 again in tlle up position, machining continues until line 15 coincides with line 39 causing the discrete switcll comprising path ~l7 to open and another jUlllp in the value of Vl to occur. Again a second precise vialue for throat height is available. At this point ~ith t~o values fo-r thro;lt heigllt ~110~ ith great accuracy and ~ith t~o valucs each for Vl an~ V2 for those throat height valucs also accur.ltely lino~n it is possible to solve t~o throat heigllt equat;olls simultalleously for the value of h2/Q
~nd Yoff. ~fter this point throat height ~ill he linonin ~;ith great accuracy ~ZS~435 ,,.

~y simply meLsurin-T the vLlues oE the ~l and ~2 and calcul;ltingT it Usi1lg thc just-.letermilleci values for h2~Q and Y ff. rhuS, voltmeters 55 ancl 57 function as an ohn~neter in conjunction with the foregoing equation for throat llcig]lt to determine resistance Rl after calibration.
For the particular application for which we have devcloped this method it is necessary that eaclL composite sensor be particularly effcctive in indicating w11en throat heights range Erom 20 to 80 ~in. ~i.th that tolerance band "le have found it convenient to place a first sensing line 38 of barrier areL 33 at 200 ~in. from the zero throat height line 58, a second sensing line 39 at S0 ~in. from line 58, and sensing line 40 at 20 ~in. from line 58.
Recall that thcse cliscrcte sensors can be placed at accurately known distances from zero throat height line 58. Thus during machining w11cn the inclividuLl sensor formed by line 39 and conductor ~7 is severed then the operator knows that the upper limit for throat height has been reacheLI by the adjacent heads.
I~hen the sensor coinprising sensing line 40 and path ~IG opens then the operator knows that tlle adjacent head ilas fallen out of tolcrance ancl mus-t be cliscarclecl.
The ideal final position line to ~hich linc 15 is mac11incd may be a1ly~1lere within the throat height range of 20 - S0 ~in.
Because of the relatively good accuracy with whic11 Ll L2 ancl h2 are initially known being all dcfined by the san1e mask in contrast to the lower initial accuracy ~ith which Yoff is linown the great accuracy-ilith w11ic1 t11roat height is known when the sensor comprising concluctor ~18 and barrier line aS opcns allows one to detcrmi1le YoEf ~ith substantially incrc;lsecl accuracy. In our met11od Y ff is initiaLlly know1l to r 5n 1~ in. w11ereas thc value of h~/Q has a1l i~lherent inaccuracy oE only about + lO ~:in. 1~'11cn mac11irlin(1 has proceeded such th;lt li.ne 15 coincicles wi th linc a9 a1ld the cliscrete scnsor com-prising path ~17 loses electrical continuitv then a bcttcr val;.~c -for h,/Q .1nd YoEf ca1l bc calc~ll.ltecl by solvin.1 for hz/Q a1l(1 Y ff si1nl1lt1ncously L1si.ng the two ~2S~ 35 ,~ ~

values for throat height previously measured. This yields a somewhat greater accuracy of arouncl 5 ~in. for the final comput.ltions of throat height calculated by the throat height equation as machining of prism 9 along line 15 occurs.
Accordingly, if a large nwnber of these composite elements are simultaneously employed during machining on a prism 9 carrying many thin-film heads, it is possible to stop machining at a time which permits the maximum nwnber of heads adjacent to the sensors to have the correct throat height.
Alternatively, if one wishes to employ the aforementioned invention permitting prism 9 to be bent during the machining process, one can sense what direction of bending is necessary to result in the greatest possible yield of good heads.
APPENDI~
lleferring first to Figure 7a, the styli~ed thin-film resistor 32 is shown to have length, height, and thickness dirllensions respectively of L~, h2, and t2. Current flow is parallel to the length climension.
Tlle schematic diagram of Figure 7b ref]ects the electrical circuit on surface 10 in Eigure 6 ancl is amenable to mathemat~cal analysis as follows using the symbols:
R = resistance : P = resistivity o-E film t = film thickness h = resistor height L = resistor length A = cross scctional are.L of resistor Ihe concluctor patlls or areas oE l:igure 6 will hercaEter in this analysis be referred to as resistors, but the use of reference nwllcr.lls will be consistent from Figure 6 to r igllres 7a ancl 7b.

~5~35 .

h'e can write the fol1O~ing equations go~;ernillg tlle resistance of each resistor:
R4 = PL~/th~ = CL4/1l2 R2 = PL2/tll2 = CL~ 2 Rl = PLl/thl CLl/ I
(Assuming P and t are uniform across the entire surface of pris~n 9 and that h2=1l~ allo~Ys C to be substituted for P/t. These are reasonable assumptions.) l~e next solve for hl in terms of resistance and resistor si~e:

Rl+R4 = C [ ~L4/h2) ~ (Ll/ ~) ]
Substituting the value of C=R21l2jL2 into this equation yields Rl+R4 = (R2h2/L2) (L~ (R2ll2/L2) ( 1/ 1 WhiC}l can be rewritten as lL2(Rl+R4) - ~1lL~I,R2 = R2~12L
R21~2I~l/ [L2(Rl+R~ L~ 2] (1) Since Ic is by definition constant, thell Rl ~ R~ = Vl/IC ancl R2 =
V2/IC 1~here Vl is the voltage drop across both resistors 31 and S~l, as measured by voltmeter 55 and V2 is the voltage across resisto-r S2 mc?asurecl by voltmeter 57. Both measurements occur ~itll s~itch 52 in its "do~in" position.

yoltmeters 55 and 57 both have interll.ll resist.lnces very large compared to that in tho series path of area 49, patlls 46 - 48, ancl pacl 4~ igure 6).
Thus voltage across this series path is nogligible ~llon measurin~ voltages betl~eell pacl 4~ and pad 41 or ~l2. I~aths 4G- 4S serve clouble duty in a sonse, functiollillg as elelllents of cLiscrete sensors 2~ and .llso ~s conllector patlls betl~een voltmetors 55 ancl 57 ancl tlle junction bet~eell resistorx ~1 alld ~2.Once machinillg reaclles line segmellt 40, volt.lges Vl allcl V~ call no lon~er be me.lsured since the voltage adj.lcellt bridge ~6 is urrLLv.lil;ll)le~ Note that the cntire sensor ~9 ~ill typically be only a fe~ thous.lndtlls of .In inch ~icle.
-lS-Substituting these values for Rl and R2 into equation ~l) yields hl = (V2/IC)~h2Ll)/[(vl/Ic)L2 (V2/ c) 4]
or hl = V2h2Ll/(VlL2 V2 4) (2) If we set x = L4/L2 and Q = L2/Ll so that L4 = xL2 and L2 = QLl, tllen L4 = xQLl. Substituting these values of L2 and L4 into equation (2) above yields hl = V2h2/Q(Vl-xV2).
In ~igure 6, by definition hl = Y ff -~ throat height, where h is the current height of sensing element 31. Substituting the value of h from equation (3) into this equation above yields throat height = [V2h2/Q
tVl-xV2)~ - YOff~

Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A machinable prism having a face carrying a conducting surface extending to a first edge of the prism and comprising:
a) an insulating material layer overlying said conducting surface and including a feature comprising an edge formed in said insulating material layer, said edge lying along a segment of a feature line;
b) a machining sensor comprising an additional barrier area of the insulating layer lying along a segment of a sensing line and on the conducting surface and extending away from the first edge of the prism, said sensing line having a predetermined precise spacing from the feature line; and c) a conducting layer on the barrier area and electri-cally contacting the conducting surface only between the sensing line and the first edge.

2. A machining guide lying on a surface of a prism, a first edge of the surface which is to be machined from its initial location to at least approximately coincide with a sensing line segment comprising:
a) a bottom conductive area on the surface lying between the first edge's initial location and extending to at least the sensing line segment;
b) an insulating barrier area having one edge precisely coincident with the sensing line segment and lying wholly outside the area on the surface between sensing line segment and the initial location of the first edge; and c) a layer of conductive material lying entirely on the barrier area outside the area between the initial location of the first edge and the sensing line segment and extending across the line and making electrical contact with the conductor area.

3. The apparatus of claim 2, wherein:
a) the barrier area lies along a staircase pattern edge including at least first and second sensing line segments offset with respect to each other and parallel to the first edge and whose extensions are a predetermined distance from each other;
b) the conducting layer lying on the barrier area comprises:
i) a connector pad and ii) a plurality of legs, each leg having appreciable resistance along its length and attached at a first end to the pad, at least one of the legs extending across each sensing line segment between its end points and making electrical contact with the conducting area between each sensing line and the first edge.