US3375342A

US3375342A - Electron beam milling of electrical coatings

Info

Publication number: US3375342A
Application number: US397979A
Authority: US
Inventors: Robinson Preston
Original assignee: Sprague Electric Co
Current assignee: Sprague Electric Co
Priority date: 1963-03-04
Filing date: 1964-09-21
Publication date: 1968-03-26
Anticipated expiration: 1985-03-26

Description

March 26, 1968 P. ROBINSON 3,375,342

ELECTRON BEAM MILLING OF ELECTRICAL COATINGS Original Filed March 4, 1963 Power Source 2O Z Z9T& J

Power 1 Source I Power 19 Source INV ENT OR I Prwionfiobirwon M ATTORNEYS United States Patent 3,375,342 ELECTRON BEAM MILLING OF ELECTRICAL COATINGS Preston Robinson, Williamstown, Mass., assignor to Sprague Electric Company, North Adams, Mass., a corporation of Massachusetts Original application Mar. 4, 1963,-Ser. No. 262,626. Divided and this application Sept. 21, 1964, Ser. No. 397,979

7 Claims. (Cl. 219-69) This application is a division of application Ser. No. 262,626 filed Mar. 4, 1963 (now abandoned), which in turn is a continuation-impart of application Ser. No. 807,- 247 filed Apr. 17, 1959 (now US. Patent 3,080,481 granted Mar. 5, 1963), which in its turn is a continuationin-part of application Ser. No. 553,893 filed Dec. 19,1955 (now US. Patent 2,883,544 granted Apr. 21, 1959). The

present invention relates to a method of making surface film type resistors and more particularly to a process employing an electron beam.

Surface film devices including'resistors and inductances include a type in which a thin film of conductive material coated on a ceramic base is formed into an element by having a portion of the conducting material removed. The conductive material is removed in a path which lies between portions of the conductive area.

'Rapid production of resistors and other semiconductive devices and indu-ctances presents problems in reproducibility, accuracy and flexibility. Resistances in such devices have tended to show variances from specification and rejected units resulting from mechanical damage to the resistance element during production is undesirable. The temperature coelficients of resistance and of expansion in the surface film type of resistor, affect the stability thereof. Therefore, in electrical circuits requiring a high sta= bilityof resistance values over a wide temperature range, a surface film type resistor produced by rapid production may vary from specified limitations because of the me= chanical damage to the conducting film.

One of the problems encountered in producing a spiraled surface film type device is the time and effort involved in creating the insulating groove or out which produces the lengthened conductive element of the device. The conventional means for producing this non-conducting spiral groove is a grinding operation using a sharp Carborundum grinding tool which kni fes into the substrate through the surface film. The speed of forming the non-conducting groove in this manner is limited and consequently the integration of the production of surface film type devices into automated production is hindered.

-Moreover, the grinding technique makes an incision into the substrate to a considerable depth in order to surely and effectively provide the necessary separation between the turns of the conducting material. This deep incision introduces objectionable features to the resultant device which must be overcome. For example, the opening made in the device by the grinding will have a tendency to collect contaminants unless thisis compensated.

An important feature of the surface film type of resistor is stable operation at a high wattage rating. This requires an efiective dissipation of heat equally along the whole length of the resistor. Where the dissipation of the heat is uneven and the temperature gradient along the surface of the resistor is high, there is a detrimental effect on the stability of the resistor. Moreover, it is desirable to provide the highest possible wattage rating for a given hot spot temperature in a resistor.

It is an object of this invention to provide a technique for the preparation of resistor or semiconductor bodiesin a simple manner which accurately reproduces identical completed devices.

3,375,342 Patented Mar. 26, 1968 It is a further object of this invention to provide a method of producing an electrical resistor having a rapid and even dissipation of heat.

Still another object of this invention is to provide a method of producing a resistor having good electrical stability over a wide temperature range.

Another object of this invention is to provide rapid spiralizing of a surface film type device to permit integration of the spiralizing in an automated process.

Still another object of this invention is the removal of a portion of the conductive film in a surface film type device without cutting substantially into the substrate.

These and other objects of this invention will become more apparent upon consideration of the following de scription taken together with the accompanying drawings in which:

FIGURE 1 is an enlarged scale axial sectional view" showing a construction in accordance with the invention;

FIGURE 2 is a fragmentary axial sectional view of a portion of the surface of the construction of FIGURE 1; and

FIGURE 3 is a diagrammatic section of apparatus for making a device of this invention.

The resistors produced by the present invention are coated with a layer of resistive or semiconductive material deposited on an elongated non-conducting base. The conducting layer is formed into a conducting track by a helical insulating path formed around the non-conducting base. The helical non-conducting path is produced on the device through the action of a focused beam of electrons on the coated base to cause the formation of the lengthened resistive path of the conducting track.

This invention produces devices which include metal film resistors in which a non-conducting base such as a ceramic rod is coated with a metal film and thereafter a non-conducting strip is formed by taking away part of the conducting coat in a helical path around the rod. This results in lengthening the resistive path of the remaining conducting material. The conductive material has a resistivity and this resistivity provides the resistance value of the ultimate product. Conductivity and resistivity as used in reference to the surface film type device are relative terms. For example, resistivities can range from below 10 ohm-cm. to about 10 ohm-cm. for silicon, illustrating that in principle a wide range of resistances are possible for this chemical element. Moreover, a satisfactory resistor can be produced with 1 ohm-cm. silicon. It is thus seen that semiconductivity and resistivity have similar connotations.

Referring to FIGURE 1, there is shown a resistor unit 10 produced according to this invention having an insulation core 11 which is shown with a peripheral metal film element 12 in spiral convolution on a surface 13 of the core 11. The spiral shape is produced 'by removing the resistive or semiconductive material of film 12 in the manner of thread cutting. A nonconducting path 14 is the result of spiralizing .and separates turns of the film 12 and forms a portion of the film 12 into a conducting track 15. The film 12 is originally deposited on the surface 13 of the ceramic core 11 to which it bonds firmly. End terminations v16 are of noble metal in order to insure low contact resistance with the resistance film 12. Metal end caps 17 and leads 18 are provided at either end. The end caps 17 are in contact with the end terminations 16.

The enlarged view of FIGURE 2 serves to illustrate the insulation of the succesive turns of the conducting track 15 from each other which is provided by the formation of the intervening turns of the non-conducting path.

The steps of providing the spiralized resistor 10, comprise first coating the ceramic core 11 with the resistive film 12 and thereafter spiralizing the coated core 11 by evaporating the non-conducting path .14 under the im pingernent of an electron beam. The resultant evaporation rapidly removes the material of film 12 as the resistor is rotated on its axis while the electron beam is scanned along the length of resistor 10. This results in a lengthening of the resistive path and hence an increase in resistance value. The core is initially coated to give an initial value Ri and thereafter the spiral non-conducting path is formed extending along the length of the resistor 10 until the desired Rf value is attained. The spiralizing process is carried on automatically, the resistor being rotated on its axis while at the same time, the electron beam is advanced at a constant speed along the length of the resistor 10. The increase in resistance is measured at it oc chrs by integral circuitry and means are provided for detecting the attainment of a given value and for instant cessation of the spiralizing.

The apparatus for treatment of the resistor is shown in FIGURE 3. A vacuum electron beam device 19 uses an electron beam generated from a triode gun in any convenient manner from a source as indicated at 20. Such source can for example, be of the type described in British Patent No. 714,613. It provides a beam which is focused so that it converges at the site of a resistor 10 as shown in FIGURE 3. The resistor 10 is held in chucks 21 which are rotated by suitable driving means not shown. As shown, the chucks 21 seize the leads of the resistor 10 and hold the resistor 10 in the electron beam.

An air tight housing 22 forms a chamber 23 which contains the resistor 10, the chucks 21 and mechanisms for moving a succession of resistors into engagement by the chucks 21. The electron beam source 20 mounted in conjunction with the housing 22 is arranged to direct the beam into the chamber 23 and impinge it upon the resistors 10 while they are rotated by the chucks 21. A conventional vacuum pump 24 connected to the chamber 23 through a conduit 25 serves to evacuate the chamber 23 and provide vacuums down to any desired pressure such as 0.1 micron.

. A suitable mechanism for advancing and positioning successive resistors 10 is indicated in the chamber 23. The illustrated mechanism is comprised of a chain belt conveyor 26 moving a plurality of paired yokes 27 for engaging the leads 18 of individual resistors 10. The advancing yokes 27 move resistors into position for elevation to the chucks 21. Forks 28 are suitably actuated by means not shown to lift the resistor leads 18 and elevate the resistors 10 from the yo'kes 27 to the chucks 21 for the electron beam treatment and then subsequently to lower the resistors 10 back to the yokes 27. The chucks 21 are connected by circuitry not shown, so as to be part of the resistance means measuring circuit described below and are also connected to ground through circuitry not shown.

In this invention, a surface film type device and more particularly a resistor, may be spiralized by mounting a coated body having a surface film of suitable resistive or semiconductive material on a pair of yokes 27, advancing the yokes by the chain drive 26 into position for engagement by the forks 28 and moving the resistor up to be gripped by the chucks 21. The chucks 21 are then rotated and the electron beam employed to remove a strip of the material to produce the non-conducting path 14. The chamber 23 is evacuated by the suction pump 24 con= nected to the conduit 25 and this evacuation can also serve to remove the vaporized products from the chamber and thus continue to preserve the electron beam efiiciency. When the chamber 23 is evacuated by means of the pump 24. the electron gun of the electron beam source 20 is energized and the accelerated electrons emitted into the chamber 23 are focused by magnetic fields as indicated in FIGURE 3 the electron beam impinges on the rotating surface of the resistor 10 and heats the material of film 12 in a localized area to result in its vaporization.

The apparatus can handle any material that vaporizes and the temperature can be carried to any desired degree.

The width of the beam is readily adjustable by the focusing means of the electron beam source and accordingly the width of the non-conducting path 14 is adjustable. The beam is made to scan along the length of the resistor 10. In fact, the width of the cut can be adjusted during the helixing of a single unit and the pitch of the path 14 can be adjusted by the speed of the scan and the rate of rotation.

The start of the non-conducting path is accurately controllable as is also the ending or the termination of the cut of the non-conducting path. These operations are simply achieved by either deflecting onto or away from the resistor 10 or by focusing or defocusing at the precisely appropriate instant in time.

The electron beam impringing on the film 12 of resistive or semiconductive material causes the material to be evaporated and causes the substrate core in the area of the non-conducting path to be glazed by fusing of the ceramic which takes place under the impingment of the beam. The surface of the non-conducting patch therefore is microscopically smooth.

The electron beam removes the conductive coating completely vaporizing the metal film material in an accurate path and striking the exposed substrate, heats it momen tarily to the fusing point so that just a skin of glazed material is formed. The surface of the non-conducting path, therefore, is free from irregularities and imperfections and microscopically smooth. It is also defined by the edges of the conducting track which are extremely straight and parallel and free from all irregularities. The electron beam in removing the material along the nonconducting path makes a very shallow cut into the sub state. This cut is preferably to the depth of less than 0.0005". The lateral edge formed by the conducting track and the shallow groove is gently rounded to provide a smooth flare with no sharp edges.

The electron beam source is used to remove the resistive or semiconductive material in a path by evaporation. The electron beam source is preferably operated at a high potential of the order or 15 or 20 KV. An electron beam current can be produced which causes the resistive or semiconductive material to evaporate under the impingement of the beam of the semiconductive material. This evaporation can be carried on in such a way as to rapidly remove the resistive or semiconductive material and thus form the non-conducting path. The steps of this procedure can be programmed into an automatic operation.

The electron beam current may be of the order of microamperes and the beam power of the order of 2 watts. The current is subjected to a very sensitive grid bias control. The electron gun having the essential parts of a triode, with a grid bias of 90 v. a representative current out of the gun is microamperes. To cut 0E the electron beam it is possible to increase the grid bias to v. This will serve to cut the current down to below 1 microampere.

The means of controlling the electron beam provides a technique for a quick response to a signal for electron beam termination. The formation of the non-conducting path lengthens the resistive path of the conducting track and produces progressively an increase in resistance value. It is desirable to achieve, within a narrow average deviation, an exact final resistance value for the resistor. This is accomplished by providing a resistance measuring circuit which serves to measure the resistance value of the helix device while the spiralizing is being carried on. When the final resistance value is achieved by the formation of the non-conducting path, the electron beam is cut off. The time between the resistance determination and this cut off has been cut down to as little as 10 milliseconds.

In the operation of the apparatus of FIGURE 3 for the treatment of the resistive or semiconductor material and the formation of the spiral resistor, the resistor 10 is lifted from the yoke 27 on the forks 28 and placed in the chucks 21. The chucks 21 seize the resistor leads l8 and holding the resistor 10 rotate it axially. The elec: tron beam is switched on and focused by adjustment to give the desired impingement on the material. This beam impingement is similar to that described in application Ser.' No. 807,237. The beam modifies the resistive or semiconductive material by evaporating a path. The path is formed by the axial rotation of the resistor and the deflection of the beam along the length of the resistor to trace a spiral'path of evaporated material forming the non-conducting path.

, The pressure maintained in the chamber is controllable and may be carried down to an extremely high vacuum such as 0.01 micron. Gases generated within the chamber are removed by evacuation.

The electron beam source is produced according to conditions set forth in application Ser. No. 807,247. This electron beam is introduced into the apparatus 19 through the opening and penetrates into the chamber 23. The electron beam'can be focused and can be aimed. Such focusing and aiming is shown in the above-described British Patent No. 714,613. The resistor body with the resistive or semiconductor material is mounted in the chucks 21 within the chamber 23. The resistor 10 is then subjected to treatment by the electron beam as described. After the steps of treatment, the resistor 10 is removed by release from the chucks 21 into the forks 28 which lower it to a yoke 27. While this treatment is being carried out suction pumps attached to the ducts create a reduced pressure in the chamber and remove gases produced in the chamber.

As mentioned above, the beam scans the length of the resistor. Individual points on the surface of the resistor are subjected to the beam for a time determined by dividing the beam width by the milling rate. The nonconducting path is traced with the combined scanning and rotation ata rate of the order of 150" per minute to provide a resistor cutting rate of the order of I per minute. Such tracing at a rate of at least 100 inches per minute Works very well.

It will be understood that the substrate material may be beryllium oxide. Beryllium oxide is undesirable as a substrate core produced by conventional methods because of the toxicity of the by-products from the worked beryllia material. However with the technique of this invention beryllium oxide may be Worked as the toxicity is not a problem. Beryllium oxide is a highly desirable substrate in this type of resistor, as it assists in providing a high wattage rating for a given hot spot temperature in a resistor. The dissipation of heat generated in this type of resistor is considerably greater at the two ends. The end caps and leads are factors in providing this greater heat dissipation. Moreover, the heat is generated mainly in the spiralized portion rather than in the unspiralized end portions. Accordingly, the temperature distribution along the surface is uneven so that the maximum tem perature or hot spot is in the central area with the temperature falling off at either end. In the beryllium substrate device the heat is most rapidly conducted away by the beryllium substrate material. Accordingly, the resistor stays nearer to a uniform temperature and the rise of temperature in the resistor above the ambient is limited or maintained at a minimum.

Ceramic metallic films may provide desirable surface films for this type of device but present problems in the technique of adjusting the resistance value for an ultimate useful product. The ceramic metallic material after application to the non-conducting base is tenaciously adhesive to the base. The resistance adjusting technique of this invention is particularly applicable to this problem. For example, a cermet of 65% chromium and 35% silicon monoxide as a surface film can be readily adjusted to a desired final resistance by the means and method described herein. Another good resistance material made ,6 useful as an adjustable surface film by this invention is molybdenum disilicide.

The above described invention and its preferred embodiment and the use thereof may be more completely understood by the following example given to illustrate the process and not intended to be limitative.

Example A steatite core coated with a nickel alloy resistive material and having a gold terminal at each end to which a tin solder end cap is attached forming a resistor having an initial resistance of 1300 ohms. The resistor was rotated on its axis at a rotational speed of 175 r.p.m. in a vacuum of 0.1 micron. An electron beam having a beam current of microamperes produced from a beam producirig device under an accelerating voltage of 20 kv. with a grid voltage of -75 v. was impinged on the coat of the resistor and swept along the length of the coat in 8 seconds. The beam vaporized a helical path in the coat and increased the resistance of the resistor to 2 megohms with A wattage.

A flat substrate carrying a conductive film may also be treated with the electron beam apparatus in the evacuated chamber. The resistance of the conductive z'film may be adjusted by vaporizing portions of the film to produce non-conducting portions. The resultant non-conducting portions have a smooth surface even though theyare made up from the substrate after removal of the conductive film.

Great flexibility may be obtained in the electrical values of the product of this process due to the mobility of the electron beam and the speed of its operation on the coated substrate.

Another product of this invention is a surface film type inductor. A surface inductor is made up of a conductive film such as either silver or copper deposited onfan elongated, round or bar-like non-conducting base arid formed into a number of closely spaced turns of a conducting track by a non-conducting path produced in the metallic coat by impingement of an electron beam. As in' the case of the resistor described above, the conductive-material removed to provide the non-conducting path, is volatilized and removed from the vacuum furnace by suction.

,An advantageous example of an inductance prepared according to this invention is the provision of a silvercoat on a magnesium oxide core. The silver coat is transformed into a spiral conductor by forming a non-conducting path with the electron beam. The silver conducting layer is particularly adapted to effective cutting by removal with the electron beam.

In a modification of this invention a resistor body is provided having a ceramic substrate carrying tafilm of resistive or semiconductive material and overlain with a layer of ceramic to form a covering. This body can be spiralized according to this invention to provide a spiraled surface film resistor. The non-conducting path can be cut from this body by the electron beam in the same manner as described above for forming the non-conducting path in the above described embodiment. The electron beam vaporizes and removes the overlying layer of ceramic at the non-conducting path. In this way a ceramic coating can be provided before the formation of the helix. Any possibility of contamination during the step of applying the coating is thus avoided. A feature of the product of this method is the small percentage of exposure of the resistor material during and after the path cutting operation. Only the cut edge of the resistive material is exposed and as the exposed edge is almost dimensionless actually it is not sensitive to exposure.

According to an additional optional step in the treatment of the resistor, the resistor may have a protective surface applied by polymerizing a gas in the chamber for deposit on the resistor after the cutting of the non-conducting path 14 is complete. The chamber may hold a gas which when subjected to the electron beam will result in the application of a protective layer to the semiconductive material. In FIGURE 1 the resistor shown in axial cross section is shown to have a resistive or semiconductive material of the film 12 removed according to the process of this invention by the electron beam evaporation and apparatus of FIGURE 3 in the non-conducting path 14. The removed portions of the material 10 are the result of removal of the resistive or semiconductive material by direct evaporation. The electron beam source 20 operates at a sufficiently high potential so that the effect of its impingement on the semiconductive material of the film 12 results in an evaporation of the semiconductive material by directing the beam against the path 14. This path is removed from the film 12 as indicated by FIG- URES 1 and 2. To effect this evaporation the resistor 10 is positioned in the chucks within the chamber under a reduced atmosphere to cause the electron beam to be projected against it without undue loss of efficiency. Suction pumps evacuate the chamber. The electron beam from the beam source strikes the resistor 10 and is focused by previous adjustment to give the desired beam impingement. An electron beam source operates at a sufficiently high potential so that the effect of its impingement on the resistive or semiconductive material of film 12 results in an evaporation of the semiconductive material by directing the beam along the path 14. This evaporation is carried on in such a way as to rapidly remove portions of the resistive or semiconductive material ofv the film 12 and thus change its shape to the conducting track 15.

After the removal of the material 12 from the nonconducting path 14 the resistor 10 is subjected to the next step of resistor manufacture. This may include the application of the protective coating.

An important advantage of the electron beam helixing is the smooth edge of the path when it is cut. This smooth edge results from the melting. In the first place, the profile has a much reduced curvature surface. With the mechanical method a jagged edge is formed. This is undesirable since it produces sharp points which can be areas of electrical stress concentration. These areas of concentration are points at which break-down can start. The shape and stability of the conducting path are important. The electron beam provides a very even or smooth edge conducting path. This is demonstrated in the control of the magnetic field at the edge of a superconductor. In a superconductor if the flow along the edge is not disturbed by the irregularity of the edge, the superconductive phenomenon is preserved. Therefor with the electron beam method of cutting a smooth edge is provided so that the conducting path has the desired straight shape and is stable and preserves the superconductivity.

Among other advantages this invention provides a means of producing precise resistors having a tolerance of as low as A of 1%. The effective narrow cut of the non-conducting path produced by this invention permits a great reduction in size of the metal film resistor for the same value: Correspondingly, there is provided a considerable increase in wattage rating for the same size unit.

Another advantage is the stabilized temperature coefficient of the resistance permitted by the good heat dissipation made possible. Moreover, it is possible to match a core and a coat having compatible temperature coefficients of expansion. A most important advantage is the faster helixing with fewer rejects which makes this invention particularly adaptable to automatic production. This is particularly so since fewer rejects will be expected because of the lower noise in the resultant unit permitted by the cleaner cut of the non-conducting path and the lack of damage to the substrate.

The electron beam apparatus as disclosed herein provides a means for adjusting the electrical values of the surface film device very precisely. The resistance measuring means together with fast response to a determination of the attainment of the desired resistance permits'extremely precise control of the adjustment The groove or cut which makes up the non-conducting turns within the conducting track is smooth and sharp and cooperates with the microscopically smooth, straight edge of the conductive material. These features provide a surface which is not conducive to entrapment of contaminants.

A second advantage of the foregoing grooving technique is found in the modification which employs a ceramic coat over the metal film before the spiralizing takes place. With the ceramic coat in place and the nonconducting path cut through the coat, a device is provided at the end of the spiralizing procedure whichneeds protection only over the extremely thin edge of the metal film. Thus the product is expedited and simplified.

The flexibility of this means and method will also be appreciated. It will operate on any material or combination of materials that vaporize. It is thus effective in making spiralized surface film devices with hard-to-handle materials that have other extremely advantageous properties. The characteristics of the resultant product are correspondingly improved. Another facet of the vaporizing in a vacuum aspect of this invention is found in the advantage gained from being able to process in the vacuum, materials otherwise unavailable for use because of their toxicity. Beryllia, for example, is generally unattractive as a material but can be safely used according to this invention.

The above described embodiments and particularly the embodiment treating a resistive metal alloy film on a steatite Alsimag 513 rod have been found to be effective in practice. However, without departing from the spirit of this invention various modifications may be made as exemplified in the modifications indicated above,' and therefore it is intended that the invention be limited only by the scope of the appended claims.

What is claimed is:

1. The process of forming electrical resistors comprising the steps of applying a tenaciously adhering ceramicmetallic coating on a ceramic base, projecting a focused 15 to 20 kilovolt beam of electrons against the coating and simultaneously scanning the electron beam along the coating in a narrow path to volatilize the coating where the beam impinges on it and form a non-conducting path with microscopically straight edges in said coating at a rate of at least inches per minute while at the same time glazing the surface of the base exposed by the volatilized coating.

2. A process for preparing an electric circuit member, comprising providing a ceramic support carrying a thin electrically conductive coating film, milling out a linear gap in the film at a speed of at least about 100 inches per minute, the milling being effected with a focused 15 to 20 kilovolt electron beam that leaves the milled gap with microscopically straight edges and at the same time glazes the surface of the ceramic exposed at the gap.

3. The combination of claim 2 in which the milling is effected at a speed of the order of inches per minute.

4. A process for preparing electrical resistors, which process comprises providing a resistor blank having a thin electrically resistive film coated on a ceramic base, milling out the film along a path that magnifies its electrical length, the milling being effected at a speed of at least about 100 inches per minute with a focussed 15 to 20 kilovolt electron beam that leaves the milled path with microscopically straight edges and at the same time glazes the surface of the ceramic along the milled path.

5. The combination of claim 4 in which the milling is effected at a speed of the order of 15() inches per minute and the resistor blank is a cylindrical core carrying the resistive film on its outer surface and also having noble metal coatings spaced on either side of the milled out section to define terminal connection sites.

6. In the process of milling out a linear gap in an elec trically conductive resistor coating on a ceramic support with a focussed electron beam to form a member having a predetermined electrical characteristic, the improvement according to which the milling is carried out with a 15 to 20"kilovolt beam at a speed of at least about 100 inches per minute to provide a milled path with microscopically straight edges and glaze the surface or the ceramic exposed by the milling, the electrical characteristic of the milled coating is measured while the milling is taking place, and the milling is terminated within 10 milliseconds of the time when the measurement shows that the desired value is reached.

7, A process for preparing electrical resistors, which process comprises providing a resistor blank having a thin 15 electrically resistive film coated on a support and a ceramic overlayer covering the resistive film, and milling out the film along a path that magnifies its electrical length, the milling being efiected at a speed of at least.

about 100 inches per minute with a focussed 15 to 20 kilovolt electron beam that leaves the milled path with microscopically straight edges.

References Cited.

UNITED STATES PATENTS 1,859,112 5/1932 Silberstein 338l95 3,056,881 10/ 1962 Schwarz. 3,140,379 7/1964 Schleich et al. 219-69 OTHER REFERENCES The Electron Beam as a Production Tool, Moore, Hamilton Standard TP61, December 6-7, 1961, page 8 relied on,

RICHARD M. WOOD, Primary Examiner,

JOSEPH V. TRUHE, Examiner,

R. F, STAUBLY, Assistant Examiner,