GB2128747A - Gauging the machining of a block using severing of an electrically conducting path - Google Patents

Abstract

A block, e.g. a magnetic head is to be machined from the lower surface 15 to a surface indicated by lines 13a, 13b. This is detected by depositing a conductor 11, an insulator 12 and a further conductor 14b which are so arranged that when machining has been carried out to lines 13a, b a conductive path between conductor 11 and conductor 14b will severed and this is indicated on a continuity meter 22. Additionally, a stepped array of resistive paths which are successively broken as the machining progresses can be provided on the block to monitor the progress, Fig. 6, (not shown). <IMAGE>

Description

SPECIFICATION Machinable prism method of forming the same, machining guide and method of calibrating a machining sensor This invention relates to machinable prisms, methods of forming the same, machining guides and methods of calibrating machining sensors.

In certain manufacturing operations, particularly those for fabricating disc memory thinfilm magnetic heads in situ on an air bearing slider to be carried by a head arm, it is desirable to machine a flying surface until a precisely located line on another surface intersecting the flying surface becomes the line of intersection of the 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 accurately the throat height of the head, 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 fraction of a micron (few tens of microinches) 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 involved. Simply using the top of the slider prism as a reference surface for controlling throat height was satisfactory when grinding ferrite heads, see U.S. Patent Specification 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 deposited conducting materials placed on the surface carrying the head. Two types of sensors are in general use. So-called discrete sensors simply have their electrical continuity broken at some point during machining and hence, provide an indication of machining progress at only a single instant. Analog sensors have an area of resistive material which is slowly removed by machining and hence provide a continuous indication until continuity is broken. With respect to discrete sensors, typically several at different heights are employed. The continuity of each is successively broken by the machining process, thereby providing a series of indications of precisely how much more machining must yet occur to reach the desired final position line.At the limits of or within the desired throat height range, the conductive path of a last discrete sensor will be opened signaling that the machining process should stop.

The use of these machining sensors drastically improves the accuracy with which the edge can be positioned relative to the feature.

However, when dealing with thin-film magnetic heads, one cannot form conventional machining sensors with the same step which defines the throat of the gap. This is because the throat is formed by the deposition of an insulating layer, whereas the machining sensors are conductive patterns and hence are deposited in the steps creating the magnetic legs of the head. It is a known difficulty that successive layers of material deposited by the use of photo-optic masks and forming a composite thin-film structure cannot be registered with respect to each other with perfect accuracy. That is, the masks or patterns which define each of the features of successive layers such as the bottom leg, the throat and the top leg, cannot be placed in precise alignment with the patterns created by previous masking steps during typical manufacturing operations.Therefore, the throat height of a typical thin-film head cannot be controlled to an accuracy greater than the registration between the pattern of the insulating layer of the throat and the conductive pattern of the machining sensors. Experience shows that this inherent inaccuracy results in a substantial percentage of head gaps which have throat heights outside of the required tolerances.

Worse still, even though the throat height is defined at an intermediate stage in the process, one cannot easily tell whether or not the had is good until manufacture is complete, making the relatively high number of reject heads an expensive flaw in these previous systems.

The problem of aligning machining sensors with a feature formed of insulating material such as the throat of a thin-film head is present for both discrete and analog sensors.

In a current manufacturing process, analog sensors are used to indicate the progress of machining of a workpiece carrying several thin-film heads. The machining step sets the throat heights for all the thin film heads simultaneously. An analog sensor is interposed 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 halted 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 throat heights can be reduced to a value within the tolerance range at the same time. Such a process is described in U.S. Patent Application No. 430195, entitled Workpiece Carrier.

IBM TDBVol. 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 layer". How an insulating layer can be registered in the same step with depositing the lapping control layer is not explained.

IBM TDBVol. 23, No. 2, July 1980, p.

776, teaches a method of calibrating an analog lapping guide or machining sensor to compensate for variations in bulk resistivity and film thickness. This method is not involved with determining position of the analog sensor relative to a feature of an insulating layer.

Although the present invention is primarily directed to any novel integer or step, or combination of integers or steps, herein disclosed and/or as shown in the accompanying drawings, nevertheless, according to one particular aspect of the present invention to which, however, the invention is in no way restricted, there is provided a method comprising the steps of: depositing with a mask a feature formed of insulating material on a first surface of a machinable prism; machining a first edge of the first surface from an initial location toward a sensing line; and finally monitoring the output of a continuity tester, said sensing line having a preselected spacing from the feature, said first edge being substantially parallel to the sensing line, and said feature being adjacent a conductive area on the first surface intersected by the sensing line, the method including during the depositing step, depositing a layer of the insulating material on the conductive area in a barrier area thereon lying along the sensing line and extending away from the initial location of the first edge, depositing a layer of conductive material within the boundaries of the barrier area on the insulating material and extending into and contacting the conductive area electrically only between the sensing line and the initial location of the first edge, and attaching the continuity tester between the conductive layer within the barrier area and the conductive area.

The method may include during the depositing step, depositing insulating material in a staircase pattern along segments of a plurality of sensing lines all spaced from each other, and depositing a plurality of conductive strips electrically connected at a first end to the continuity tester, and each lying across a single sensing line segment and making contact with the conductive area thereacross.

According to a further non-restrictive aspect of the present invention there is provided a machinable prism carrying, on a conducting surface extending to a first edge of the prism, a feature comprising an edge formed in an insulating material layer, said edge along a segment of a feature line, a machining sensor comprising a 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 a conducting layer on the barrier area and electrically contacting the conducting surface only between the sensing line and the first edge.

According to another non-restrictive aspect of the present invention there is provided 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 the guide comprising: a bottom conductive area on the surface lying between the initial location of the first edge and extending to at least the sensing line segment; an insulating barrier area having one edge precisely coincident with the sensing line segment and lying wholly outside the area on the surface between the sensing line segment and the initial location of the first edge; and 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 conductive area.

Preferably 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; the layer of conductive material comprising a connector pad and 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 conductive area between each sensing line and the first edge.

According to yet another non-restrictive aspect of the present invention there is provided a method of calibrating a machining sensor lying on a surface, a first edge of which is to be machined from its initial location to an ideal final position line having a predetermined spacing from a feature line, said sensor having a discrete sensor comprising: a bottom conductive area on the surface lying between the initial location of the first edge and extending to at least the ideal final position line, an insulating barrier area having a sensing line boundary comprising at least one line segment each having precisely known spacing from the feature line and lying between the ideal final position line and the initial location of the first edge, said insulating barrier area extending away from the initial location of the first edge, a layer containing at least one conductive path lying entirely on the barrier area outside the area between the initial location of the first edge and the sensing line boundary, each said conductive path extending across one of the line segments and making electrical contact with the conductive area, and a continuity tester connected between the end of each conductive path remote from the sensing line boundary and the bottom conductive area, and providing an indication when continuity ceases between a conductive path and the bottom conductive area; and an analog sensor comprising a resistive conducting strip lying along the initial location of the first edge and intersected by extensions of each of the line segments of the sensing line boundary crossed by the conductive paths, and the ideal final position line, and resistance measuring means electrically connected across the resistive conducting strip for providing a signal indicative of the resistance of the conducting strip; the method comprises the steps of: machining the first edge of the surface toward the feature line until the continuity tester indicates that continuity between a conductive path and the conductive area has ceased; then without further machining analyzing the signal from the resistance measuring means to determine the resistance in the conducting strip; and calculating a constant of inverse proportionality K from an equation of the form h = K/R by substituting (i) the known spacing distance of the line segment crossed by the conductive path whose continuity with the conductive area ceased, for the distance h between the feature line and the first edge, and (ii) the resistance indicated by the resistance measuring means for the resistance R of the conductive strip, and solving the equation for K; continuing machining of the first edge toward the ideal final position line; while machining, analyzing periodically the signal from the resistance measuring means to determine the resistance of the conductive strip, and calculating therefrom the value h; and ceasing machining when the value of h is reduced to within a predeter mined range of the spacing between the ideal final position and feature lines.

Preferably the analog sensor further com prises first and second connector paths having a known effective length ratio x, at least the second path having appreciable resistance, and each connected to one end of the resistive conducting strip, and a third connector path connected to the junction between the second connector path and the resistive conducting strip, the edge of the resistive conducting strip remote from the first edge being spaced a distance Yoff from the feature line, and the resistance measuring means including a constant current source connected to pass current through the first connector path, the resistive conducting strip and the second connector path in series, first and second voltmeters measuring voltages V, and V2 respectively between the first and third, and second and third, ends of the connector paths unconnected to the resistive conducting strip, the sensing line boundary of the discrete sensor including at least two line segments offset from each other, across each of which extends a conductive path, the method further comprising the steps of: machining the first edge of the surface toward the ideal final position line until the continuity tester indicates that continuity between at least two conductive paths and the conductive area has ceased; recording the voltages V, and V2 at each instant continuity for a conductive path ceases; inserting the known values of throat height for the spacing between the barrier area sensing line segments and the feature line, and the corresponding values for V, and V2 recorded when the associated conductive paths' continuity with the conductive area ceased, into the equation throat height = V2h2/Q(V1-xV2) - Y0ff to produce two linear equations in the two unknowns h2/Q and Yo5; solving the two linear equations simultaneously for the values of h2/Q and Yoff and inserting these values into the equation specifying throat height; continuing machining of the first edge toward the ideal final position line and while machining, periodically recording the voltages V, and V2 and calculating the equation for throat height using the values for V, and V2 most recently recorded and the values for h2/Q and Yoff resulting from solving the two linear equations; and ceasing machining when throat height falls within a desired range. The first and second conductor paths may have nearly identical dimensions and wherein the equation specifying throat height in terms of V1, V2, h2/Q, and Yoff is: throat height = (V2h2/Q(V1-V2)) - Y0ff.

Alternatively the analog sensor may further comprise first and second connector paths having a known effective length ratio x, at least the second path having appreciable resistance, and each connected to one end of the resistive conducting strip, and a third connector path connected to the junction between the second connector path and the resistive conducting strip, the edge of the resistive conducting strip remote from the first edge being spaced a distance Yoff from the feature line, and the resistance measuring means including a constant current source connected to pass current through the first connector path, the resistive conducting strip and the second connector path in series; first and second voltmeters measuring voltages V, and V2 respectively between the first and third, and second and third ends of the connector paths unconnected to the resistive conducting strip, the sensing line boundary of the discrete sensor including at least one line segment across which extends a conductive path, the method further comprising the steps of: machining the first edge of the surface toward the ideal final position line until the continuity tester indicates that continuity between at least one conductive path and the conductive area has ceased; recording the voltages V, and V2 at the instant continuity for a conductive path ceases; inserting the known value of the spacing between the barrier area sensing line segment and the feature line for the throat height, an approximation for h2/Q, and the corresponding values for V, and V2 recorded when the associated conductive path's continuity with the conductive area ceased, into the equation throat height = V2h2/Q(V1-xV2) - Y0ff to produce a linear equation in the unknown Yo; solving the linear equation for the value of h2/Q and Yofl and inserting this value into the equation specifying throat height; continuing machining of the first edge toward the ideal final position line and while machining periodically recording the voltages V, and V2 and calculating the equation for throat height using the values for V, and V2 most recently recorded and the value for Yoff resulting from solving the linear equation; and ceasing machining when throat height falls within a desired range.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 is a perspective view of a machinable prism according to the present invention showing an intermediate step in a method according to the present invention; Figure 2 is a cross section taken on the line 2-2 of Fig. 1; Figure 3 is a section taken on the line 3-3 of Fig.1; Figure 4 is a view corresponding to Fig. 2 after a machining step; Figure 5 is a view corresponding to Fig. 4 after a machining step; Figure 6 discloses a composite sensor on the machinable prism of Fig. 1 for use with a method according to the present invention; Figure 7a is a magnified perspective view of an individual thin-film resistor of Fig. 6; and Figure 7b is a circuit diagram of the composite sensor of Fig. 6.

Since the present invention has been developed specifically for the purpose of controlling throat height of a thin-film head, the following description is based on an application in this area. It has identical applicability In any case where such machining relative to a feature defined by deposited insulating material must be controlled.

Fig. 1 shows a greatly magnified perspective view of a machinable block or prism 9 formed of a ceramic material, and comprising a thin-film head air-bearing slider as it looks just before the final machining of an airbearing face. A liner 15 is the initial position of the edge of an end face 10, defined by the intersection of the initial position of a flying surface 26 (shown on edge in Figs. 2 and 4) with the face 10. The surface 26 is to be machined until its intersection line with the end face 10 reaches its ideal position coinciding with a sensing plane 13 defined by the two lines 1 3 a and 13b.

On the end face 10 there has been placed a plurality of machining guides or sensors 21 each including a conductive layer or area 11 intersected by the line 1 3a or sensing plane 13, and having any convenient shape. Fig. 2 shows one of the sensors 21 in cross section prior to final machining. On top of the conductive area 11 an insulating layer comprising a barrier area 12 is deposited, having one edge lying along the sensing plane 13, extending away from the initial position of the line 15 at the edge of the end face 10 and lying atop the conductive area 11. The sensing plane 13 should be substantially parallel to the initial position of the line 15 at the edge of the end face 10. A preliminary machining step may be necessary to configure the prism 9 so that this relationship exists.

Another deposited conductive layer forming a conductive area 14 is located entirely within the barrier area 12 on the side of the line 1 3a and extends across the line 1 3a, contacting the conductive area 11 between the sensing line 1 3a and the initial position of the edge at the line 15. Thus the conductive area 14 is completely insulated from the conductive area 11 as to a layer area 14b, i.e. the portion above the line 1 3a, and makes electrical contact with the conductive area 11 in area 1 4a, below the line 1 3a.

For illustrative purposes here, a simplified diagram of a typical thin-film head 20 is shown adjacent the sensor 21 and in crosssection in Fig. 3 before machining. This comprises a magnetic flux path having legs 17, 18 (see Figs. 3 and 5), a winding 19, and a deposited first insulating layer 24 typically formed o aluminium oxide interposed between the leg 1 7 and the leg 1 8 of the magnetic flux path, thereby creating a flux gap 25. A second insulating layer 16 insulates the winding 19 and defines the interior end of the flux gap 25. This interior end of the flux gap 25 lies along one segment of a feature line 27, shown on end as dots in Figs.

2-5. The spacing between the line 27 and the line 1 3a is formed by the same deposition step and With the same mask, and is therefore known with great precision, since no mask alignment errors are present.

To provide the flux gap 25 of the proper throat height, it is necessary to machine the surface 26 until it coincides with the sensing plane 13 on the end face 10 within a tolerance of 1 .5#m (60 ,~in). The flux gap 25 is physically formed by and essentially com prises deposited non-magnetic insulating material. It will be appreciated that by creating the edge of the barrier area 12 along the line 13a, which defines the point at which machining is to stop, with the same mask and in the same deposition step defining the interior end of the flux gap 25 along the line 27, the throat height of the flux gap will be very accurately defined and much more accurately so defined than if the line 27 and the line 13a were created during separate deposition steps or with different masks.It will also be clear that control of the throat height of the flux gap is only one of many possible applications where this procedure may be used.

The machining 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 the areas 11, 14. Continuously testers 22 are connected to the conductive area 11 and the layer area 14b by connectors 23.

The machining slowly erodes the material between the sensing plane 13 and the initial location of the edge of the end face 10, the line 15. When the material between the sensing plane 13 and the line 15 has been completely eroded, electrical contact between the conductive areas 11, 14a is broken and the continuity testers 22 indicate this condition. The final configuration of the sensor 21 is shown in Fig. 5. The operator monitors the continuity testers 22 and can see the indication by them and stop the machining. Alternatively, the machining device can be connected to the continuity testers 22 to stop automatically its operation once continuity fails.

The reason the initial position of the line 15 must be nearly parallel to the sensing plane 13 is now apparent. When the edge of the end face 10 is machined to coincide with the sensing plane 13, if they are not parallel at that time, some material past the sensing plane 13 will be removed, causing one corner of the sensor 21 to define the end of continuity and the sensors 21 will lose continuity at different times. Thus, at some point in the machining operation, the line 15 should be approximately parallel with the sensing plane 13. The position of the edge of the end face 10 at that point can be considered its initial position. Machining to achieve this relationship may be considered merely a preliminary step. The effect of such non-parallelism can be reduced by making the conductive area 14 more narrow and by placing the 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 inherent width of the head 20 limits the proximity between the sensors 21.

While the sensors 21 and the associated process just described function satisfactorily for certain requirements in small production runs, the commercial requirement for many thousands of heads 20 has led to a preferred use for the sensors 21. To manufacture cheaply and efficiently these heads 20, it is preferred to place several on a single bar, and then machine all of their surfaces 26 simultaneously.

In U.S. Patent Application No. 430,195 entitled "Workpiece Carrier", and mentioned earlier, the preferred use for this invention is described in some detail. 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 throat heights of the heads on the bar within the tolerance range required. To determine current status of the throat height of each head, 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 operation reveal that certain throat heights will be outside the tolerance range when machining has placed all others within the desired tolerance, then the bar is bent to cause additional machining of the surfaces 26 of certain heads to occur relative to the machining of the surfaces 26 of other heads. By properly choosing the amount and location of this bending, a much greater percentage of the throat heights can be 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 suffer from the alignment errors which also plague conventional discrete sensors.

A composite machining sensor which includes an analog sensor 28 continuously providing a signal specifying the position of the line 15 is shown in Fig. 6. A zero throat height or feature line 58 essentially defines the position of the throat relative to which the line 15 is to be positioned by machining. The composite sensor is mounted on the end face 10 of the prism 9 and includes an analog sensing element 31 formed of a resistive conducting strip and three discrete sensors 29 formed from conductor paths 46 to 48, an insulating barrier area 33 beneath them, and a conductive area 49 below the barrier area 33 making electrical contact with ends 50 to 52 respectively of conductor paths 46 to 48.

Sensing line segments 38 to 40 form a staircase pattern along the bottom edge of the barrier area 33 and are offset with respect to each other, are approximately parallel to the line 15 as initially positioned, and have extensions which are a predetermined distance from each other. Each of the sensing line segments 38 to 40 are located at a precisely known spacing from the zero throat height line 58 by virtue of their creation by the same process step and with the same mask as that which produced the interior end of the flux gap 25 of the appurtenant head. The conductor paths 46 to 48 have appreciable electrical resistance and are commonly connected to a connector pad or terminal 43. The conductor paths 46 to 48 cross the sensing line segments 40 to 38 respectively and all make electrical contact with the conductive area 49.

The terminal 43 in turn is connected to the upper selectable terminal of single pole double throw switch 52, and to one terminal each of voltmeters 55, 57.

The sensing element 31 is unitary with the conductive area 49 which forms part of the discrete sensors 29. The ends of sensing element 31 are connected by bridges 35, 36 to resistive conductor paths 34, 32 respectively. The sensing element 31 has an appreciable amount of resistance, initially R1, between the bridges 35, 36. The nominal height h, and length L1 determine its resistance in large part, during machining. As the line 15 of the end face 10 is slowly machined away, the height h, of the sensing element 31 decreases and, naturally, its resistance increases.

The conductor paths 34, 32 connect conductive the bridges 35, 36 respectively to connector pads or terminals 41, 42. The conductor paths 34, 32 themselves have in one preferred embodiment appreciable resistance, again dependent on their lengths L4 and L2 and heights h4 and h2, respectively.

Resistance in the conductor paths 34, 32 is unavoidable because they too are unitary with the sensing element 31, which must have some resistance within it to perform properly its sensing function. The terminal 41 is connected to the terminal of the voltmeter 55 not connected to the terminal 43 such that the voltmeter 55 measures voltage between the terminals 41, 43. The voltmeters 55, 57, the switch 52 and a constant current source 53 are located remote from the end face 10. The terminal 41 is also connected to the lower selectable terminal of the switch 52. The terminal 42 is connected to one terminal of the constant current source 53 and to the terminals of the voltmeter 55 and the voltmeter 57 not connected to the terminal 43.

The terminal of the constant current source 53 not connected to the terminal 42 is connected to the centre or common terminal of the switch 52.

An equation of the form h = K/R can be developed which relates the value of the sensing element 31 height h, = h to the dimensions of the conductor paths 34 and 32 as incorporated in the constant K, and to voltages V1, V2 measured by the voltmeters 55, 57 which provide a current indication of the resistance R of the sensing element 31. As is derived in Appendix, the height h, = V2h2 + /Q (V,-xV2), the voltages V1, V2 measured with the switch 52 in the "down" position shown. It is thus obvious that throat height = V2h2/Q (V,-xV2)~Yo = h,~Yoff/ where Yofl is the spacing between the top of the sensing element 31 and the zero throat height line 58 defining an edge of the throat relative to which the discrete sensor 29 is deposited.In these equations, Q= L2/L, and x = L4/L2. It is relatively easy and preferred to conrol the deposition such that the conductor paths 34, 32 have nearly identical dimensions so that L4 = L2 and x = 1 to within i 2% or less. Even large ( + 4%) errors affect measurements of throat height by only 0.03#m (1,yin) or so.

It is also possible to deposit the conductor path 34 with a very small effective length L4(L4 < < L2) by forming the conductor path 34 with a height and thickness substantially greater than for the conductor path 32. By properly specifying the dimensions of the conductor path 34 formed by the deposition process, x can be set to fall in the range of 0.01 to 0.1 Although the precision with which x is known in this case may be no better than i 10% or even f 20%, since the value of x is quite small, the overall impact on accuracy of measurement of throat height is similar to the case where x = 1 and is known to + 2%.Once the deposition process is stabilized, an average value of x can be determined by either calculation or direct measurement of the resistance of the conductor paths 34, 32 on representative end face 10, allowing x to be treated as a constant thereafter.

There are therefore in either embodiment, two unknowns in the equation for throat height, h2/Q and Yoff With V1, V2, and x known, it is possible to determine the values for h2/Q and Yoff by measuring the values for V1 and V2 at known throat heights. This is accomplished by reference to the discrete sensors 29. As machining of the prism 9 begins, the line 15 moves slowly toward the line segment 38, increasing resistance of and voltage across the sensing element 31. At some point, the line 15 coincides with the line segment 38 causing the conductor path 48 to open. If the switch 52 is in its "up" position near to that time, the voltage V1 measured by the voltmeter 55 will undergo a sudden increase when continuity ends since the resistance between the conductive area 49 and the terminal 43 has increased, while current flow Ic from the constant current source 53 has remain unchanged. Since the voltmeter 55 is assumed to have very large resistance compared to the resistance in the conductor path 34 and the sensing element 31, V1/lc very precisely states the resistance between the conductive area 49 and the terminal 43. At this time, the throat height is known with great precision as the preselected exact spacing between the line segment 38 and the zero throat height line 58.

As soon as the increase in the voltage V, is detected, the switch 52 must be moved to its "down" position, allowing the value of the voltage V1 to be read for use in the equation expressing throat height. The voltage V2 is also read at this time for use in the equation.

Although dimensions of the deposited resistors cannot be precisely set by the deposition process, the lengths L1, L2 as well as the height h2 and the spacing Y0ff are known with reasonable initial accuracy, having been formed by the same mask. At the time that the line 15 coincides with the line segment 38, the throat height is known with great precision. Substituting the approximations for = L2/L1) and h2, the measured values for voltages V1, V2, and the exact throat height into the equation for throat height above, yields a better approximation for Y0ff, increasing the precision of its value substantially.

With the switch 52 again in the "up" position, machining continues until the line 15 coincides with the line segment 39, causing the conductor path 47 to open and another jump in the value of the voltage V, to occur. Again, a second precise value for the throat height is available.At this point, with two values for throat height known with great accuracy and with two values each for the voltages V, and V2 for those throat height values also accurately known, it is possible to solve two throat height equations simultaneously for the value of h,/Q and Yoe After this point, the throat height will be known with great accuracy by simply measuring the values of the voltages V1, V2 and calculating it using the just-determined values for h2/Q and Toff4 Thus, the voltmeters 55, 57 function as an ohmeter in conjunction with the foregoing equation for throat height, to determine the resistance R, after calibration.

For the particular application for which this method has been developed, it is necessary that each composite sensor be particularly effective in indicating when throat heights range from 0.5 to 2.0ym (20 to 80yin.). With that tolerance band, we have found it convenient to place the line segment 38 5.1#m (200 yin.) from the zero throat height line 58, the line segment 39 at 2.0#m (80 yin.) from the zero throat height line 58, and the line segment 40 at 0.5#m (20,yin.) from the zero throat height line 58. It will be recalled that these line segments can be placed at accurately known distances from the zero throat height line 58.Thus, during machining when the discrete sensor formed by the line segment 39 and the conductor path 47 is severed, then it is known that the upper limit for throat height has been reached by the adjacent heads. When the discrete sensor comprising the line segment 40 and the conductor path 46 opens, then it is known that the adjacent head has fallen out of tolerance and must be discarded. The ideal final position line to which the line 15 is machined, may be anywhere within the range of throat heights of 0.5 to 2.0#m (20 to 80 yin.).

Because of the relatively good accuracy with which the lengths L1, L2 and the height h2 are initially known being all defined by the same mask, in contrast to the lower initial accuracy with which Yoff is known, the great accuracy with which throat height is known when the discrete sensor comprising the conductor path 48 and the line segment 38 opens, allows one to determine Yoff with substantially increased accuracy. Yoff is initially known to 9 :::: 1 27#m (:::;50 yin.) whereas the value of h2/Q has an inherent inaccuracy of only about 0.25#m ( + 10 yin.). When machining has proceeded such that the line 15 coincides with the line segment 39 and the discrete sensor comprising the conductor path 47 opens, then a better value for h2/Q and Yoff can be calculated by solving for h2/Q and Yoff simultaneously using the two values for throat height previously measured. This yields a somewhat greater accuracy of around ::l::0.1 3#m 0.13,um ( 5 tin.) for the final com- putations of throat height calculated by the equation above as machining of the prism 9 along the line 15 occurs.

Accordingly, if a large number of these composite elements are simultaneously employed during machining on the prism 9 carrying many thin-film heads, it is possible to stop machining at a time which permits the maximum number of heads adjacent to the discrete sensors to have the correct throat height. Alternatively, the prism 9 is to be bent during the machining process, one can sense what direction of bending is necessary to result in the greatest possible yield of heads.

APPENDIX Referring first to Fig. 7a, the conductor path 32 is shown to have length, height, and thickness dimensions respectively of L2, h2, and t2 current flow being parallel to the length dimension.

The schematic diagram of Fig. 7b reflects the electrical circuit of the composite sensor of Fig. 6 on the end face 10 and is amenable to mathematical analysis as follows using the symbols: R = resistance P = resistivity of film t = film thickness h = resistor height L = resistor length A = cross sectional area of resistor The conductor paths of Fig. 6 will hereafter in this analysis be referred to as resistors, but the use of reference numerals will be consistent from Fig. 6 to Figs. 7ato 7b.

The following equations governing the resis tance of each resistor can be written: R4 = PL4/th4 = CL4/h2 R2 = PL2/th2 = CL2/h2 R, = PL,/th, = CL,/hl (Assuming P and t are uniform across the entire surface of the prism 9 and that h2 = h4 allows C to be substituted for P/t. These are reasonable assumptions).

Solving for h, in terms of resistance and resistor size gives: R, + R4 = C((L4/h2) + (L1/h1)) Substituting the value of C = R2h2/L2 into this equation yields R, + R4 = (R2h2/L2)(L4/h2) + (R2h2/L2)(L,/h,) which can be rewritten as h,L2(R, + R4)~h,L4R2 = R2h2L1.

Thus: h, = R2h2L,/{L2(R, + R,)- L,R,) (1) Since lc is by definition constant, then R, + R4 = V1/lc and R2 = V2/lc where V, is the voltage drop across both resistors 31 and 34, as measured by the voltmeter 55 and V2 is the voltage across resistor 32 measured by the voltmeter 57. Both measurements occur with the switch 52 in its "down" position.

The voltmeters 55 and 57 both have internal resistances very large compared to that in the series path of the conductive area 49, the conductor paths 46 to 48, and the terminal 43 (Fig. 6). Thus the voltage across this series path is negligible when measuring voltages between the terminal 43 and the terminal 41 or 42. The conductor paths 46 to 48 serve double duty in a sense, functioning as elements of the discrete sensors 29 and also as connector paths between the voltmeters 55, 57 and the junction between the resistors 31, 32. Once machining reaches the line segment 40, the voltages V1, V2 can no longer be measured since the voltage adjacent the bridge 36 is unavailable. Note that the composite sensor will typically be only a few thousandths of a centimetre (or inch) wide.

Substituting these values for R, and R2 into equation (1) yields h, = (V2/lc)(h2L1)/(V1/lc)L2 - (V2/lc)L4j or h, = V2h2L, /(V, L2-V2L4) (2) If we set x = L4/L2 and Q = L2/L, so that L4 = xL2 and L2 = QL1, then L4 = xQL,. Substituting these values of L2 and L4 into equation (2) above yields h, = V2h2/Q(V,-xV2)- (3) In Fig. 6, by definition h, = Yoff + throat height, where h, is the current height of the sensing element 31. Substituting the value of h, from equation (3) into this equation above yields throat height = (V2h2/Q(V1-xV2)) - Y0ff

Claims (14)

1. A method comprising the steps of: depositing with a mask a feature formed of insulating material on a first surface of a machinable prism; machining a first edge of the first surface from an initial location toward a sensing line; and finally monitoring the output of a continuity tester, said sensing line having a preselected spacing from the feature, said first edge being substantially parallel to the sensing line, and said feature being adjacent a conductive area on the first surface intersected by the sensing line, the method including during the depositing step, depositing a layer of the insulating material on the conductive area in a barrier area thereon lying along the sensing line and extending away from the initial location of the first edge; depositing a layer of conductive material within the boundaries of the barrier area on the insulating material and extending into and contacting the conductive area electrically only between the sensing line and the initial location of the first edge, and attaching the continuity tester between the conductive layer within the barrier area and the conductive area.

2. A method as claimed in claim 1 including, during the depositing step, depositing insulating material in a staircase pattern along segments of a plurality of sensing lines all spaced from each other, and depositing a plurality of conductive strips electrically connected at a first end to the continuity tester, and each lying across a single sensing line segment and making contact with the conductive area thereacross.

3 A machinable prism carrying, on a conducting surface extending to a first edge of the prism,a feature comprising an edge formed in an insulating material layer, said edge lying along a segment of a feature line, a machining sensor comprising a 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 a conducting layer on the barrier area and electrically contacting the conducting surface only between the sensing line and the first edge.

4. 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, the guide comprising: a bottom conductive area on the surface lying between the initial location of the first edge, and extending to at least the sensing line segment; an insulating barrier area having one edge precisely coincident with the sensing line segment and lying wholly outside the area on the surface between the sensing line segment and the initial location of the first edge; and 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 conductive area.

5. A machining guide as claimed in claim 4 in which 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; the layer of conductive material comprising a connector pad and 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 conductive area.

between each sensing line and the first edge.

6. A method of calibrating a machining sensor lying on a surface, a first edge of which is to be machined from its initial location to an ideal final position line having a predetermined spacing from a feature line, said sensor having a discrete sensor comprising: a bottom conductive area on the surface lying between the initial location of the first edge, and extending to at least the ideal final position line, an insulating barrier area having a sensing line boundary comprising at least one line segment each having precisely known spacing from the feature line and lying between the ideal final position line and the initial location of the first edge, said insulating barrier area extending away from the initial location of the first edge, a layer containing at least one conductive path lying entirely on the barrier area outside the area between the initial location of the first edge and the sensing line boundary, each said conductive path extending across one of the line segments and making electrical contact with the conductive area, and a continuity tester connected between the end of each conductive path remote from the sensing line boundary and the bottom conductive area, and providing an indication when continuity ceases between a conductive path and the bottom conductive area; and an analog sensor comprising a resistive conducting strip lying along the initial location of the first edge and intersected by extensions of each of the line segments of the sensing line boundary crossed by the conductive paths, and the ideal final position line, and resistance measuring means electrically connected across the resistive conducting strip for providing a signal indicative of the resistance of the conducting strip; the method comprises the steps of machining the first edge of the surface toward the feature line until the continuity tester indicates that continuity between a conductive path and the conductive area has ceased; then without further machining analyzing the signal from the resistance measuring means to determine the resistance in the conducting strip; and calculating a constant of inverse proportionality K from an equation of the form h = k/R by substituting i) the known spacing distance of the line segment crossed by the conductive path whose continuity with the conductive area ceased, for the distance h between the feature line and the first edge, and ii) the resistance indicated by the resistance measuring means for the resistance R of the conductive strip, and solving the equation for K; continuing machining of the first edge toward the ideal final position line; while machining, analyzing periodically the signal from the resistance measuring means to determine the resistance of the conductive strip, and calculating threfrom the value h; and ceasing machining when the value of h is reduced to within a predetermined range of the spacing between the ideal final position and feature lines.

7. A method as claimed in claim 6 in which the analog sensor further comprises first and second connector paths having a known effective length ratio x, at least the second path having appreciable resistance, and each connected to one end of the resistive conducting strip, and a third connector path connected to the junction between the second connector path and the resistive conducting strip, the edge of the resistive conducting strip remote from the first edge being spaced a distance Yoe from the feature line, and the resistance measuring means including a constant current source connected to pass current through the first connector path, the resistive conducting strip and the second connector path in series, first and second voltmeters measuring voltages V, and V2 respectively between the first and third, and second and third ends of the connector paths unconnected to the resistive conducting strip, the sensing line boundary of the discrete sensor including at least two line segments offset from each other, across each of which extends a conductive path, the method further comprising the steps of: machining the first edge of the surface toward the ideal final position line until the continuity tester indicates that continuity between at least two conductive paths and the conductive area has ceased; recording the voltages V, and V2 at each instant continuity for a conductive path ceases; inserting the known values of throat height for the spacing between the barrier area sensing line segments and the feature line, and the corresponding values for V, and V2 recorded when the associated conductive paths' continuity with the conductive area ceased, into the equation throat height = V2h2/Q(V1-xV2) - #otr' to produce two linear equations in the two unknowns h2/Q and Yo; solving the two linear equations simultaneously for the values of h2/Q and Yoff and inserting these values into the equation specifying throat height; continuing machining of the first edge toward the ideal final position line and while machining, periodically recording the voltages V, and V2 and calculating the equation for throat height using the values for V, and V2 most recently recorded and the values for h2/Q and Y,, resulting from solving the two linear equations; and ceasing machining when throat height falls within a desired range.

8. A method as claimed in claim 7 in which the first and second conductor paths have nearly identical dimensions and wherein the equation specifying throat height in terms of V1, V2, h2/Q, and Yofl is: throat height = {V2h2/O(V1-VA} - Y0#.

9. A method as claimed in claim 6 in which the analog sensor further comprises first and second connector paths having a known effective length ratio x, at least the second path having appreciable resistance, and each connected to one end of the resistive conducting strip, and a third connector path connected to the junction between the second connector path and the resistive conducting strip, the edge of the resistive conducting strip remote from the first edge being spaced a distance Yoff from the feature line, and resistance measuring means including a constant current source connected to pass current through the first connector path, the resistive conducting strip and the second connector path in series; first and second voltmeters measuring voltages V, and V2 respectively between the first and third, and second and third ends of the connector paths unconnected to the resistive conducting strip, the sensing line boundary of the discrete sensor including at least one line segment across which extends a conductive path, the method further comprising the steps of: machining the first edge of the surface toward the ideal final position line until the continuity tester indicates that continuity between at least one conductive path and the conductive area has ceased; recording the voltages V, and V2 at the instant continuity for a conductive path ceases; inserting the known value of the spacing between the barrier aea sensing line segment and the feature line for the throat height, an approximation for h2/Q, and the corresponding values for V1 and V2 recorded when the associated conductive path's continuity with the conductive area ceased, into the equation throat height = V2h2/O(V1- xV2) - Y0ff to produce a linear equation in the unknown Yo; solving the linear equation for the value of h,/Q and Yoff and inserting this value into the equation specifying throat height; continuing machining of the first edge toward the ideal final position line and while machining, periodically recording the voltages V1 and V2 and calculating the equation for throat height using the values for V1 and V2 most recently recorded and the value for Yoff resulting from solving the linear equation; and ceasing machining when throat height falls within a desired range.

10. A method as claimed in claim 1 and substantially as herein described with reference to the accompanying drawings.

11. A method as claimed in claim 6 and substantially as hrein described with reference to the accompanying drawings.

12. A machinable prism substantially as herein described with reference to and as shown in the accompanying drawings.

13. A machining guide substantially as herein described with reference to and as shown in the accompanying drawings.

14. Any novel integer or step, or combination of integers or steps, hereinbefore described, irrespective of whether the present claim is within the scope of, or relates to the same or a different invention from that of, the preceding claims.