US3142587A - Apparatus for producing electrical conductor films by explosive evaporation - Google Patents

Description

y 1964 E. G. WEBER APPARATUS FOR PRODUCING ELECTRICAL CONDUCTOR FILMS BY EXPLOSIVE EVAPORATION- Filed Jan. 3, 1961 INVENTOR .ZjPW/A/ G. WEBER II I; l l ilmfi WM %;g

United States PatentO APPARATUS FOR PRODUCTN G ELECTRICAL CON- DUCTOR FILMS BY EXPLOSIVE EVAPORATION Erwin G. Weber, Hopkins, Minn, assignor to Sperry Rand Corporation, New York, N.Y., a corporation of Delaware Filed Jan. 3, 1961, Ser. No. 80,301 Claims. (Cl. 118--49.1)

This invention relates to the deposition of thin films.

The invention concerns itself with both the method of depositing films and apparatus for accomplishing the method. The prior art is repleat with ways of effecting thin films, for example as in the Rubens Patent 2,900,282. In that patent reference is made to the vacuum deposition of thin magnetic films with evaporation being effected by inductively or otherwise heating ferromagnetic material and causing its vapor to condense onto a substrate. In the present application magnetic or nonmagnetic films may be deposited in vacuum onto a substrate by explosive- 1y evaporating a conductor which is preferably resistively preheated.

Explosive evaporation offers the advantages of: (1) a short deposition time of less than 1 p.560. which reduces the interaction between the metal vapor and the residual atmosphere in the vacuum to a minimum; (2) absence of contact between the material to be evaporated and other materials at high temperature (for example, crucibles, boats, etc.) prevents changes of the material or of the vaporization process; (3) rapid heat generation prevents fractional distillation; (4) short deposition time minimizes interaction of the condensing Vapor with the substrate (diffusion, chemical reactions, etc.); and (5) exact calibration of the evaporated total mass necessary is easily carried out by a simple length measurement of the conductor exploded.

It is therefore the primary object of this invention to provide a method and apparatus for effecting explosive evaporation of an electrical conductor.

Another object of this invention in conjunction with the foregoing object is the provision of preheating the conductor to supply a part of the energy required to vaporize the conductor completely.

Other objects and advantages of this invention will become apparent to those of ordinary skill in the art after reading the following disclosure and claims in view of the accompanying drawing which is partially schematic.

Novel apparatus for accomplishing the method of this invention is shown in the drawing. Glass bell jar along with its base 12, in which a convenient outlet (not shown) may be disposed, form means for creating a vacuum which is preferably between 1 l0 mm. and -l l0 mm. Hg. The maximum pressure should not exceed 1X 10* mm. Hg and there is no minimum pressure limit. The solid electrical conductor 14 to be explosively evaporated is vertically held at its ends between two high voltage connector balls 16, 18. The upper ball 16 is electrically connected by conductor 20 which feeds through the'top of the bell jar to a high voltage terminal 22. The other high voltage terminal 24 is connected by conductor 26 directly to base plate 12, which forms a common ground for the electrical circuit of the apparatus.

The other internal high voltage ball 18 is electrically connected to a rigid conductor 28 which together normally hang free of electrical or mechanical connection, i.e., with the lower end of conductor 28 spaced from each of the conductive clamp blocks 30 and 32. Block 30 is secured to base plate 12 as by bolt 34, but block 32 is movable from its illustrated position toward block 30, the two blocks being biased toward each other by spring 36. On the right side of block 32, is another spring 38 which 'ice is serially connected with a fuse wire 40 to the terminal post 42 which insulatively passes through base plate 12.

Externally, terminal 42 is serially connected to the base plate by switch 44 and battery 46. Whenever switch 44 is closed, battery 46 provides current via post 42 to fuse 40, thence through spring 38, block 32, and thence to base plate 12 directly if that block is not insulated from the base plate, or through spring 36 and block 30 if it is insulated therefrom. This current causes fuse 40 to blow so that the bias of spring 36 pulls block 32 leftwardly to cause conductor 28 to be securely held between the two blocks. Instead of effecting clamping of the conductor 28 in this manner at the beginning of a deposition cycle, conductor 28 may be permanently secured to block 30 or directly to base plate 12 initially. With the apparatus as shown, the conductor to be exploded need not be drawn tight in connecting it to the balls 16 and 18, but instead the lower holder comprised of ball 18 and conductor 28 is sufficiently weighty to maintain conductor 14 straight and provide to it any desired tension or tautness. On the other hand, conductor 14 may be curved or slightly bent if desired even after the bell jar is pulled down tight against its base.

The next step in the process is to preheat conductor 14 resistively. This is accomplished in the illustrated apparatus by closing switch 48 so that current from battery 50 will proceed through terminal post 52 which is insulated from base plate 12, thence through the upstanding relatively rigid conductor 54, conductor 56 to the fork-shaped connector 58. The V between the tongs 60 and 62 electrically connects to conductor 20, and consequently to ball 16, conductor 14, and the lower conductor holder 18, 28, completing the circuit to the base plate 12. As illustrated, one tong 62 of connector 58 is serially connected by spring 64 and a second fuse wire 66 to a hook 68 which is electrically connected to a second upstanding relatively rigid conductor 70. In this manner, connector 58 is disconnectably held in electrical contact with conductor 20, i.e., to one end of conductor 14. With switch 48 closed, resistive heating of conductor 14occurs. Of course, if switch 48 had been closed before fuse 40 blew, no heating of conductor would have occurred until conductor 28 Was electrically connected, indicating the non-necessity of switch 48 to start the heating period runmng.

As soon as the conductor 14 is heated to a desired temperature, which may be sensed for example by a pyrometer (not shown), switch 48 may be opened it desired, but preferably the resistive heating is stopped by closing switch 72. This, as will be seen, effects a disconnecting of the disconnectable fork 58 from conductor 20, by causing current from battery 74 to flow via conductors 54, 56, connector 58, spring 64, fuse wire 66, and conductor 70 which is insulatively returned through base plate '12 to'the terminal post 76, completing the connection back to switch 72. This current is in excess of that which the fuse 66 can carry, so its blows. At this instant, since the conductor 56 is biased downwardly by spring 78, the forkshaped connector 58 is pulled away from conductor 20 so that even if switch 48 is still closed, the current from battery 50 cannot be returned through conductor 14. As soon as it is noted that the connector 58 is moved out of contact with conductor 20, the double-pole double-throw switch is thrown from its leftward (illustrated) position to the right so as to provide power from the volt terminals 81 to energize relay solenoid 82 causing its switch contact 84 to close. (Preferably, this relay is in an oil bath as indicated.) This immediately discharges condenser 86 and provides across conductor 14 a voltage which is sufiiciently strong to explosively evaporate conductor 14.

The high voltage obtainable from the discharge of condenser 86, which may be a bank of condensers totaling 6 ,uf. capacity for example, may be effected by previously charging that condenser in any desired manner, for exam ple as by the 110 volt source 81 while switch 80 is in its illustrated position. This energizes the charging source 88, and condenser 86 is charged via a high resistance (9M, for example) resistor 90.

When conductor 14 explodes upon the application of the high voltage to terminals 22, 24, the vapor deposits itself on a masked substrate 92. As illustrated the masking effects a plurality of circular apertures 94, for example 8 mm. diameter, so the resulting films deposited through these apertures are circular, but the masking apertures may be of any configuration desired. Alternatively, the substrate need not be masked unless desired. The number of apertures in the mask may of course be any number. Without the additional masking of the cold ends of conductor 14 as effected by masks 96, the films are likely to be heavily perforated due to splattering caused by the ends of conductor 14 being cold relative to the center of the conductor. Masks 96 inhibit this splattering. These masks may take any desired configuration, and may be glass sleeves disposed about the cold ends.

Conductor 14 may be any desired type of electrical conductor; it may be either magnetic or non-magnetic, to cause either magnetic or non-magnetic films respectively. For example, platinum wire (non-magnetic) may be used to effect platinum films. In the magnetic category, conductor 14 may be any magnetic type of wire, for example, one of the so-called Permalloys, which as is well known, include nickel and iron. Perminvar in any of its forms, as well as all other types of binary, ternary, etc., alloys, or non-alloyed magnetic materials may be employed to obtain the type of films desired. One highly important use of this invention is the deposition of one or more metal films of the magnetic type and employment thereof in any device requiring information storage capabilities such as in logic devices and computers generally. For this purpose, conductor 14 is preferably a Permalloy wire, for example of the 80% nickel, balance iron type. More preferably, in order to obtain films which are substantially non-magnetostrictive, the wire has a composition of 81% nickel, remainder iron.

For certain materials, it is desirable that the preheating temperature of conductor 14 cannot be so high as to melt the conductor, though preferably the preheating temperature is adjacent the melting point of the conductor. The purpose of preheating the conductor is to supply thereto some of the energy which is required to allow the high voltage to completely explosively evaporate the conductor. For an 80-20 permalloy wire 4 inches long, 20 mils diameter, the optimal preheating temperature of the wire is around 1000 C. At a much lower temperature the amount of heat generated in the wire by the discharge current of condenser 86 is too small to completely evaporate the wire, and at a higher temperature the evaporation is non-uniform because of apparent weak spots in surfacetension effects, and apparent reduction in conductivity reducing the current caused heat. In both cases splattering of the wire may occur in characteristic ways, depending on factors such as local cross sectional variations or local inhomogeneities. The too cold wire disintegrates into a few big droplets. The too hot wire may disintegrate into many small droplets. The unavoidable cooler ends of the wire always exhibit the too cool splattering and are therefore shielded by masks 96 as above indicated. With a temperature of around 1000 C., the above-mentioned four-inch wire caused films 300-500 A. thick on a substrate one-inch from the wire.

The minimum preheat temperature is related to the total amount of energy provided by the discharge of condenser 86 and the amount of energy required to successfully cause the explosive evaporation. That is, if the condenser discharge energy was in excess of the amount of energy required to explode a preheated wire, this excess energy could be used to explosively evaporate a cold wire. The minimum preheat temperature then would be that temperature to which the wire must be preheated in order that sufiicient capacity discharge energy is available to successfully explode the wire. Though the wire may be exploded without any preheating, the capacitor discharge energy, provided by the presently used equipment, alone does not provide sufficient energy to completely transform the solid wire to its vapor state. Preheating is required to effect complete transformation.

After the desired preheating temperature is reached, it is held for some desired time, for example ten minutes, to allow outgassing. The limiting factor with respect to the maximum preheat time is the possibility of oxidizing the surface of the wire which would change the chemical characteristics of the wire with resultant changes in conductivity. Therefore, the preheating is concluded prior to the occurrence of significant oxidation. Of course, the higher the vacuum, the less the possibility of oxidation, and the more outgassing that can be effected. The higher the vacuum, the smoother and less structured are the vapor-clouds leaving the wire.

As between a plurality of 8 mm. diameter circular permalloy films deposited onto ordinary glass microscope slides, a remarkable uniformity with regard to the type of hysteresis loop, coercive force, and anisotropy field was noted. For most films the coercive force was in the range of from 2*l0 oe. and the anisotropy field was always greater than 100 oe. Uniformity from evaporating one wire to the next can be expected if the wires have like characteristics if the preheat temperature is the same, and switch connects to coil 82 at the same time relative to the closing of switch 72. As exemplary, this time element may be approximately 0.1 sec. maximum, and the minimum time is that required for spring 78 to remove fork 58 from the high voltage feed-through 20 sufliciently to prevent a spark from jumping between the fork 58 and the feed-through 20 when the high voltage is applied. If wire 14 cools slowly after preheating stops or has been heated far in excess of the temperature required, the maximum time can be extended slightly.

In certain cases it has been found desirable to permit the wire to cool subsequent to the preheating step and prior to the time that the explosion operation is initiated. In these cases, a superior outgassing of the material may be achieved, this being due to the elimination of the high vapor pressure which would normally be expected to exist between the inner phase of the heated wire as it is being heated. For example, the thermal-conduction of the balls 16 and 18, which have a cooling effect upon the end portion of the wire 14, sets up thermal gradients along the length of the wire body. The presence of the gradients along the length of the wire provides a source of integral zones of relatively high vapor pressure along the wire. Thus, the technique of preheating to a relatively high temperature followed by a cooling from this high temperature can be utilized to eliminate the formation of these zones of high vapor pressure.

Throwing switch 80 rightwardly at a predetermined time after closing switch 72 may be effected automatically in any of numerous manners.

Preferably, the voltage to which condenser 86 is charged is around 20 kv., though it may vary considerably from this voltage, if desired or necessary in accordance with the characteristics of the particular type electrical conductor being exploded. 20 kv. is sufficient to cause explosion of 81 Permalloy wire which is six-inches long and 25 mils in diameter, or which is four inches by 30 mils diameter. There is no apparent maximum limit for the high voltage and the minimum is that necessary to cause the desired explosion of a particular type and size of wire used.

While the technique described above has been related to the explosion of materials substantially from the solid phase, it should be pointed out that the technique is adaptable to the treating of materials which are maintained in the liquid or substantially liquid phase. Such a technique is particularly adaptable for use in connection with materials which have a relatively high vapor pressure. In this connection, the liquid phase may be achieved by either reducing the pressure or preheating of the solid to a sufficiently high temperature to transform the solid into the liquid. For certain materials, no specific modification of the equipment embodiment disclosed hereinabove is required inasmuch as the surface tension of these materials may be sufiiciently high to maintain the material in a certain desired physical form. For other materials, however, particularly those having a strong tendency to sublime at the low pressure encountered, it may become necessary to encapsulate or otherwise enclose the substance within a certain enclosure prior to explod ing the same. Any suitable enclosure may be employed, such as, for example, polyethylene or the like. This encapsulation technique is also particularly adaptable to use in connection with materials haw'ng unreasonably high vapor pressures, such as, for example, lead, zinc or the like. In each case, the explosive evaporation is accomplished by following the techniques as set forth hereinabove and there is accordingly no unusual problem encountered in the explosion operation per se.

Thus, it is apparent that there is provided by this invention a device in which the various objects and advantages herein set forth are successfully achieved.

Modifications of this invention not described herein will become apparent to those of ordinary skill in the art upon reading this disclosure. Therefore, it is intended that the material contained in the foregoing description and the accompanying drawing be interpreted as illus trative and not limitative, the scope of the invention being defined in the appended claims.

What is claimed is:

1. Apparatus for producing at least one film from an electrical conductor comprising vacuum means, an integral length of electrical conductor disposed within said vacuum means, means inside said vacuum means for holding said conductor by its respective end portions, a substrate adjacent said conductor when so held, means for resistively heating said conductor, means for applying across the heated conductor a voltage sufficient to explosively evaporate said conductor onto said substrate as said film, and means disposed between said substrate and the conductor end portions, a part of which are exposed between said holding means and cold relative to the central length of said conductor when resistively heated as aforesaid, to prevent splattering of said cold ends under such substrate when said conductor is explosively evaporated.

2. Apparatus for producing at least one film from an electrical conductor comprising vacuum means, means inside said vacuum means for holding said conductor by its respective end portions, a substrate adjacent said conductor when so held, means for resistively heating said conductor, and means for applying across the heated conductor a voltage sufiicient to explosively evaporate said conductor onto said substrate as said film, said holding means arranged to hold said conductor vertically and including a lower-end weighting holder which is normally free of electrical or mechanical connection to cause said conducter to be taut, and means inside said vacuum means for causing the said weighting holder to continue holding said conductor taut and to complete at a predetermined time an electrical connection of the conductor to said heating and voltage applying means, said last mentioned means including two clamping means biased toward each other and a fuse normally holding the clamping means apart with a part of said Weighting holder being disposed between said clamping means, and means for causing said fuse to blow at said predetermined time to allow clamping of said weighting holder by said clamping means.

3. Apparatus for producing at least one film from an electrical conductor comprising vacuum means, means inside said vacuum means for holding said conductor by its respective end portions, a substrate adjacent said conductor when so held, means for resistively heating said conductor, and means for applying across the heated conductor a voltage sufficient to explosively evaporate said conductor onto said substrate as said film, said heating means including disconnectable means inside said vacuum means, electrically connected to one end of said conductor, and said apparatus further including means for disconnecting said disconnectable means.

4. Apparatus as in claim 3 wherein said disconnecting means includes a fuse normally holding said disconnectable means and said one end of the conductor in an electrical connection and further includes means for blowing said fuse before said voltage is applied.

5. Apparatus as in claim 4 wherein said disconnecting means also includes bias means for positively pulling said disconnectable means away from its connection to said conductor after said fuse blows.

References Cited in the file of this patent UNITED STATES PATENTS 2,976,174 Howard Mar. 21, 1961 FOREIGN PATENTS 702,937 Germany Jan. 23, 1941 OTHER REFERENCES Physical Review, vol. 79 (1950), (p. 213 relied on).

Claims (1)

1. APPARATUS FOR PRODUCING AT LEAST ONE FILM FROM AN ELECTRICAL CONDUCTOR COMPRISING VACUUM MEANS, AN INTEGRAL LENGTH OF ELECTRICAL CONDUCTOR DISPOSED WITHIN SAID VACUUM MEANS, MEANS INSIDE SAID VACUUM MEANS FOR HOLDING SAID CONDUCTOR BY ITS RESPECTIVE END PORTIONS, A SUBSTRATE ADJACENT SAID CONDUCTOR WHEN SO HELD, MEANS FOR RESISTIVELY HEATING SAID CONDUCTOR, MEANS FOR APPLYING ACROSS THE HEATED CONDUCTOR A VOLTAGE SUFFICIENT TO EXPLOSIVELY EVAPORATE SAID CONDUCTOR ONTO SAID SUBSTRATE AS SAID FILM, AND MEANS DISPOSED BETWEEN SAID SUBSTRATE AND THE CONDUCTOR END PORTIONS, A PART OF WHICH ARE EXPOSED BETWEEN SAID HOLDING MEANS AND COLD RELATIVE TO THE CENTRAL LENGTH OF SAID CONDUCTOR WHEN RESISTIVELY HEATED AS AFORESAID, TO PREVENT SPLATTERING OF SAID COLD ENDS UNDER SUCH SUBSTRATE WHEN SAID CONDUCTOR IS EXPLOSIVELY EVAPORATED.

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