Selective dimensional control of fine wire mesh

Abstract

A method of selectively increasing or building up the thickness of a fine wire or wire mesh by means of an electron beam evaporation and deposition process. The resultant fine wire has a generally elongated rectangular cross-section. The cross-sectional dimension of the resultant fine wire or mesh along the major axis substantially exceeds the dimension along the minor axis.

Description

BACKGROUND OF THE INVENTION

The present invention relates to the manufacture of fine conductive wire and of providing such wire with an elongated generally rectangular cross-section. Fine conductive wire, and more particularly meshes constructed of such fine wire, are used as grids in electronic vidicon camera tubes, electron storage tubes, and other such electron devices. By fine mesh is meant a mesh wherein the wire dimension is between about 0.1 mil. to several mils in diameter. Such fine mesh is typically formed from a thin plate-like member which has a plurality of symmetrical apertures through the member. Such apertures can be provided by forming the member by an electroplating process, or by a etching process. The transmission area of the mesh, that is the percentage of open area, can vary from for example about 20 to 90% transmission, with the number of wires varying for example from about 50 to 2,000 wires per inch. Such mesh is very fragile and requires very careful handling in order to avoid tearing or stretching. Such mesh is also subjected to significant electrical stress due to the high electrical fields in an operating device which can tend to deform the mesh. A present method of increasing the mechanical strength of such fine wire meshes is to increase the cross-sectional area of the wires of the mesh. Such meshes are usually formed by an etching or electro-forming or plating process. All such prior art techniques increase the wire cross-section uniformly so that the resultant wire remains substantially uniform in cross-section with the width equal to the thickness, and this results in a reduction in mesh transmission. This reduction in mesh transmission is undesirable and limits usage of the wire, or limits the increase in cross-sectional area for a given application.

SUMMARY OF THE PRESENT INVENTION

A method of producing fine wire or wire mesh in which the wire has an elongated, generally rectangular cross-section is detailed. An initially uniform cross-section wire is unidirectionally built up by an electron beam evaporation process to significantly strengthen the wire or mesh without reducing the transmission of the formed mesh. The electron beam evaporation chamber is maintained at below about 8 .times. 10.sup.-.sup.6 Torr to insure that a substantially unidirectional build-up occurs.

In this way fine wire or mesh of selected conductive material can be provided which has an elongated generally oval cross-section. The wire typically has an approximate dimension of from 0.1 to 1 mil, in a first direction, and a dimension in a second direction normal to the first direction which exceeds the first direction dimension by greater than about a 2 to 1 ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematical representation of a system for practicing the present invention.

FIG. 2 is an enlarged side elevational view of a fine wire mesh of the present invention.

FIG. 3 is a sectional view taken along lines III--III of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention can be best understood by reference to the exemplary embodiments shown in the drawings. In FIG. 1, an electron beam evaporation chamber 10 is schematically represented. The electron beam evaporation chamber 10 is connected by an inlet manifold 12 via valve 14 to a high vacuum system 16 which permits evacuation of the chamber 10 and maintenance of the chamber 10 at a relatively low pressure of below about 8 .times. 10.sup.-.sup.6 Torr and preferably about 10.sup.-.sup.6 Torr.

A four pocket water cooled copper crucible 18 is disposed within the chamber 10 and is incorporated with the electron beam source 22. The electron beam source 22 is connected by high voltage input line 24 to an externally disposed high voltage power supply 26. The electron beam is focused and deflected through a 270.degree. deflection angle by magnetic focusing and deflection coils, which are not shown. Such electron beam evaporation systems are well known and are commercially available from Airco Temescal Company of Berkeley, California. The electron beam is focused onto the surface of the conductive metal 28 which is disposed within the proper pocket of the multiple crucible 18. The evaporable metal 28 is typically nickel which is supplied as evaporation grade or 99.999% purity nickel shot, which has an average diameter of from about one-eighth to one-quarter inch. The metal pieces melt and flow together to form one large slug in the crucible when heated by the electron beam. A thin shell or skull of metal in contact with the water cooled crucible remains frozen or solid.

The generally planar fine mesh 30, seen in FIG. 2 in greater detail, is disposed within chamber 10 above the crucible 18 and spaced therefrom by a distance of about 6 to 8 inches. The planar mesh is disposed in a plane perpendicular to a line normal to the crucible. The fine mesh is supported above the crucible by means 32. An infrared heater means 34 is provided within the chamber 10 disposed above the mesh 30 and spaced therefrom, by for example about 4 inches. The heater 34 is connected to an appropriate power supply disposed outside of the chamber 10.

The fine mesh 30 is by way of example a generally planar member which is about 0.16 mils thick and has a plurality of apertures therethrough, so that the structural grids are spaced at about 500 grids per inch.

In practicing the present invention the chamber 10 is evacuated to a low pressure of about 10.sup.-.sup.6 Torr and the valve 14 is open during the evaporation process. The infrared heaters are operated at a power input of about 1,500 watts for about 45 minutes prior to the evaporation to heat the mesh 30 to promote adherence of the later deposited material thereon. The electron beam is thereafter initiated, typically at about 5 to 10 kilowatts electron beam power, to heat and evaporate the nickel disposed within the crucible 18. The evaporated nickel will be deposited upon the mesh disposed above the crucible 18 and more particularly upon the mesh surface facing the crucible. The evaporated metal will be unidirectionally deposited upon the exposed surface of the planar mesh, and a significant build up in this direction will occur. The metal deposited in this manner builds up thickness in the direction of the source of the material without a corresponding increase in width of the deposited material or of the mesh. In this way, the thickness of the mesh can be selectively increased without significantly reducing the mesh transmission. The resultant dimension along the direction of build up for the wire described above is to provide a wire which is about 1.6 mils thick. This provides a significant increase in the dimension of the wire in the direction of deposition. There is a slight increase in the width of the wire in the direction normal to the thickness build up, but this is typically of the order of about a 15% increase in the width. This relatively small increase in the width of the wire of the mesh thus minimizes the reduction in transmission of the mesh. The thickness of the resultant wire exceeds the width by about a 10 to 1 ratio.

The wire described above as nickel can be other conductive metals such as copper, aluminum, gold, titanium, and similar conductive elements and alloys. The evaporable metal may be the same metal as the wire substrate or can be another compatible metal. Thus the evaporable metal may by way of example be gold, nickel, tantalum, or silver.

It is sometimes desirable in depositing a selected metal upon the wire substrate to first deposit a thin layer of an adherence promoting metal. For a nickel wire substrate, it is desirable to first electron beam evaporate and deposit a tantalum layer of about 1000 Angstroms thickness, again upon the surface of the wire or mesh exposed to or facing the crucible. A plurality of crucibles are provided within the chamber in practicing this embodiment. The crucibles are movable into the position indicated in FIG. 1 where the electron beam is focused onto the metal surface within the crucible. The evaporated tantalum will deposit on the nickel substrate and actually tend to embed itself in the nickel wire surface. The nickel containing crucible is then moved to the beam focus point to permit evaporation and deposition of nickel onto the freshly deposited tantalum. The rate of deposition of the metal upon the wire mesh is a function of the electron beam power input and for example for nickel with an electron beam power input of 5KW approximately 1.5 mils. is deposited in about 10 minutes. The thickness of metal deposited is observed by means of a crystal monitor. The sensor of which is disposed within the chamber so that as the metal is deposited upon the crystal surface the resonant frequency of the crystal changes as a function of the metal thickness. The thickness of the film is shown on a digital display.

It should be apparent that the wire or mesh substrate may be an insulator as well as being a conductive substrate. In the same way, the deposited material may be other than a conductive metal. The basis for producing the unidirectional build-up will work with any material that is evaporable at the low chamber pressure of less than about 8 .times. 10.sup.-.sup.6 Torr.

The structural members provided by the present invention have improved mechanical rigidity which broadens their application in electronic devices.

Claims

We claim:

1. Method of producing fine wire having an elongated generally rectangular cross-section, which wire has a cross-section which in a first direction is of a predetermined dimension, and in a second direction normal to the first direction has a significantly greater dimension, which method comprises:

a. disposing a wire within an evacuated chamber which is maintained at less than about 8 .times. 10.sup.-.sup.6 Torr pressure, with the wire disposed above an evaporable source of material disposed within said chamber;

b. heating said wire to promote adherence of the later deposited material;

c. directing an electron beam onto said source of material to evaporate said material and deposit said material upon the heated wire surface exposed to the source of material so that the dimension in the direction normal to the surface exposed to said material source is increased.

2. The method specified in claim 1, wherein said wire comprises a generally planar fine wire mesh.

3. The method specified in claim 1, wherein the wire is preferably nickel of from 0.1 to 1 mil thick, and a first tantalum layer is electron beam evaporated and deposited thereon, after which nickel is electron beam evaporated and deposited on the tantalum.

4. The method specified in claim 3, wherein the fine wire is initially approximately from 0.1 to 1 mil in thickness and the deposition increase is substantially only in the wire thickness.

5. The method specified in claim 1 wherein successive layers of different material are sequentially evaporated by electron beam heating and deposited upon the heated wire surface.

6. The method specified in claim 5, wherein for a nickel wire starting material, tantalum is first evaporated and deposited upon the nickel wire, and thereafter nickel is evaporated and deposited atop the tantalum.

7. The method specified in claim 1 wherein the electron beam power is up to about 10,000 watts.

8. The method specified in claim 1, wherein the wire is heated at a power input of about 500-1500 watts for about 45 minutes prior to the electron beam evaporation deposition process.

9. The method specified in claim 1 wherein the wire comprises a fine mesh, with the wire thickness being between 0.1 to 1 mil, and the mesh has up to about 2000 wires per inch.

10. Fine wire of selected conductive material having an elongated generally rectangular cross-section produced by the method of:

a. disposing a uniform cross-section wire mesh within an evacuated chamber which is maintained at less than about 8 .times. 10.sup.-.sup.6 Torr pressure, with the wire mesh being disposed above an evaporable source of material disposed within said chamber;

b. heating said wire mesh to promote adherence of the later deposited material;

c. directing an electron beam onto said source of material to evaporate said material and deposit said material upon the heated wire surface exposed to the source of material so that the wire dimension in the direction normal to the surface exposed to said material source is increased.

11. Method of producing a generally planar fine conductive mesh which has a generally rectangular cross-section, with the cross-section dimension in the direction normal to the plane of the mesh being substantially greater than the other cross-section dimension, which method comprises;

a. disposing a uniform cross-section conductive mesh within an evacuated chamber which is maintained at less than about 8.times.10.sup.-.sup.6 Torr pressure, with the mesh disposed in a plane generally normal to the path between the mesh and an evaporable compatible conductive metal;

b. heating the mesh to promote adherence of the later deposited compatible conductive metal;

c. directing an electron beam onto the compatible conductive metal to evaporate same, and thereby deposit same, upon the heated mesh surface exposed to the evaporated compatible conductive metal, so that the mesh cross-section dimension in the direction normal to the exposed surface and the plane of the mesh is increased, while the mesh cross-section dimension in the other direction is substantially unchanged.