GB2150596A

GB2150596A - Mesh structures especially for use in television camera tubes

Info

Publication number: GB2150596A
Application number: GB08331925A
Authority: GB
Inventors: R M Pettigrew; C G Sawyers
Original assignee: PA Consulting Services Ltd
Current assignee: PA Consulting Services Ltd
Priority date: 1983-11-30
Filing date: 1983-11-30
Publication date: 1985-07-03
Also published as: GB8331925D0

Abstract

A first method of making a metal mesh comprises applying a positive photoresist (14, Fig. 1) to a substrate (10) which has a conductive surface (12), applying mask (16) having a mesh configuration over the photoresist, exposing the photoresist which is not protected by the mask, developing and washing away the exposed photoresist to leave the conductive surface exposed in the configuration of the mesh while unexposed photoresist remains in the areas which are to correspond to apertures in the mesh, and electrodepositing mesh material on the exposed conductive surface between the unexposed resist to build up the mesh. A second method of making a metal mesh comprises applying a positive photoresist 34 to a transparent substrate 30, which has a conductive surface 32 applying a mask which has a mesh configuration over the photoresist, with the mesh apertures exposed and the mesh lines obscured, exposing the photoresist which is not protected by the mask, developing and washing away the exposed photoresist to leave the conductive surface exposed in the configuration of the mesh, etching away the exposed conductive surface and removing the photoresist to leave conductive mesh configuration corresponding to the mesh, thereafter applying a negative photoresist 44 to the substrate surface, exposing the resist from the back of the substrate, developing and washing away the unexposed photoresist and electrodepositing mesh metal 48 on the exposed conductive surfaces between the exposed resist to build up the mesh. Using these methods, a mesh can be built up on a substrate which has a flat surface, thereby avoiding the problems associated with prior art methods involving deposition of metal in grooves. In either method it is referrable to apply a release agent such as egg albumen to the conductive surface on which the deposited metal will be built up. <IMAGE>

Description

SPECIFICATION Mesh structures FIELD OF THE INVENTION This invention relates to a method of making a fine metal mesh, such as is used for example as an electrode in certain types of television camera tube. The invention is concerned with electroforming such a mesh, and in particular with the production of a master on which a mesh can be electrodeposited.

BACKGROUND TO THE INVENTION Conventionally, these fine wire meshes are formed from a grid of fine grooves cut into the surface of a substrate. These grooves are filled with a conductive material, usually palladium, by sputtering palladium over the whole of the substrate and then wiping it off so that the palladium which has filled the grooves remains. The metal of the mesh is then electrodeposited onto the remaining palladium but because there is no lateral restraint to stop the deposited metal spreading sideways, the mesh apertures are gradually reduced in size by deposited metal as the process continues.

This process also requires considerable skill to carry out, to correctly wipe the excess palladium from the surface.

British patent Specification 2 063 269 discloses the use of a substrate where the conductive material is deposited wholly within deep grooves of the substrate, so that when electrodeposition takes place, the deposited metal is bounded by the walls of the grooves and does not spread sideways. This technique still requires the difficult and skilled step of wiping excess palladium off the substrate surface prior to deposition.

SUMMARY OF THE INVENTION The invention includes two linked aspects, in one of which a positive photoresist is used and in the other of which a negative photoresist is used.

According to a first aspect of the invention, there is provided a method of making a metal mesh, comprising the steps of applying a positive photoresist to a substrate which has a conductive surface, applying a mask having a mesh configuration over the photoresist, exposing the photoresist which is not protected by the mask, developing and washing away the exposed photoresist to leave the conductive surface exposed in the configuration of the mesh while unexposed photoresist remains in the areas which are to correspond to apertures in the mesh, and electrodepositing mesh material on the exposed conductive surface between the unexposed resist to build up the mesh.

According to a second aspect of the invention, there is provided a method of making a metal mesh, comprising the steps of applying a positive photoresist to a transparent substrate which has a conductive surface, applying a mask which has a mesh configuration over the photoresist, with the mash apertures exposed and the mesh lines obscured, exposing the photoresist which is not protected by the mask, developing and washing away the exposed photoresist to leave the conductive surface exposed in the configuration of the mesh, etching away the exposed conductive surface and removing the photoresist to leave conductive mesh configuration corresponding to the mesh, thereafter applying a negative photoresist to the substrate surface, exposing the resist from the back of the substrate, developing and washing away the unexposed photoresist and electrodepositing mesh metal on the exposed conductive surfaces between the exposed resist to build up the mesh.

Using these methods, a mesh can be built up on a substrate which has a flat surface.

The resist islands located at the mesh apertures confine the lateral build up of deposited metal, so that the resulting mesh has mesh spaces or apertures or exactly the desired dimensions, without any lateral spreading.

The method also has the advantage that it avoids the need for considerable operator skill and gives a thicker, more robust mesh where the mesh lines and the mesh apertures are clearly defined, and their relationship is in accordance with designed parameters.

A release agent such as egg albumen can be applied to the areas of the conductive surface on which the deposited metal will be built up, to aid separation of the mesh from the substrate.

In the second aspect of the invention, once the positive photoresist has been removed, the substrate with its conductive network can be repeatedly cycled through the steps of applying the negative resist, exposure from behind and electrodeposition without the necessity of repeating each time the initial steps with the positive resist.

The invention extends to a mesh formed by either of the methods set forth above.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram illustrating a first embodiment of the invention; Figure la is a view of part of a mask; Figure 2 is a schematic diagram illustrating a second embodiment of the invention; and Figure 2a is a view of part of a mask DESCRIPTION OF PREFERRED EMBODI MENTS In the embodiment shown in Fig. 1, a glass substrate 10 is provided with a chrome conductive layer 12 on one surface. A photoresist 14 is spun on top of the conductive layer and prebaked. This photoresist can for example be Kodak ISOFINE 800 photoresist with about 40% solids content and about 100 centipoise viscosity.

A mask 16 is contacted with the upper surface of the resist 14. This mask has the configuration shown in Fig. la, with dark areas 18 corresponding to the mesh apertures and unmasked areas 20 corresponding to the mesh lines.

The resist 14 is exposed through the mask 16, and the exposed areas (corresponding to the mesh lines) are developed and washed away to leave a network of the chrome conductive surface exposed. A suitable developer is Shipley Microposit positive resist developer, and a suitable solvent is acetone or any other convenient low molecular weight ketone.

The prepared substrate is then subjected to electrodeposition. Copper will normally be deposited to form a copper mesh 22. As can be seen, the copper builds up in the deep channels formed between the resist islands 14.

and as a result the metal is clearly bounded in a lateral sense and the mesh lines are formed as compact bodies. The resulting mesh can then be separated from the substrate which is cleaned and reused. The mesh thus formed is strong and has a high level of definition between the lines and the apertures.

The embodiment which will now be described with reference to Figs. 2 and 2a is a twostage process, where a positive resist is used in a first stage to enable a conductive coating to be partly removed, and a negative resist is used in a second stage to form walls on either side of the conductive coated areas, to confine metal which is electrodeposited on the coated areas. When a number of identical meshes have to be produced, the first of these stages only has to be done once, and only the second stage is repeated for each mesh.

A glass substrate 30 (which must be transparent to the radiatio which exposes the negative photoresist) is provided with a continuous conductive coating 32. This coating can be chrome. A positive photoresist 34 is then spun over the conductive coating 32. A suitable resist is Shipley AZ 1350 positive resist with a viscosity of about 30 centipoise.

This can be applied in a thin layer of about 0.5 micron for a very high fidelity contact print.

A mask 36 is then placed over the resist, and the resist is exposed through the mask.

The mask this time has opaque areas 38 corresponding to the mesh lines and transparent areas 40 corresponding to the mesh apertures. The resist is then developed and the developed areas washed off. Shipley positive resist developer followed by acetone can be used, as before. The areas of chrome now exposed correspond to the mesh apertures.

These exposed chrome areas 42 are then etched away, leaving the glass surface exposed. Next the remaining positive resist is removed with acetone.

The upper surface of the substrate is then coated with a negative photoresist 44 such as Kodak ISOPOLY 752 at about 45 centipoise viscosity. This resist is again spun on to the surface.

The substrate is then exposed from below, for example using ultraviolet radiation, which is possible since the substrate is transparent.

The remaining conductive coating 32 acts as a mask and as a result, only those parts of the resist 44 between the coated areas are exposed and become crosslinked. The resist is then developed using Kodak ISOPOLY negative resist developer (standard grade), and the unexposed areas are removed by washing with a solvent such as Kodak ISOPOLY resist stripper.

Following this, a substrate is produced which has exposed conductive areas in the configuration of the mesh lines, separated by columns of resist. A metal 48 is then electrodeposited onto the conductive areas and builds up between the resist columns which thus ensure that the mesh lines are formed compactly.

After removal of the finished mesh, the substrate is cleaned of resist to reach the condition shown at the bottom of Fig. 2.

To make a second mesh having the same configuration, only the second section of these steps need be repeated, as indicated by the arrow 46. Thus the masking step only has to be done once, instead of in each cycle as in the first embodiment.

In either or both of the embodiments described, a release agent may be applied to the conductive surface before electrodeposition begins. This release agent can be egg albumen or a suitable dilute wax solution.

Copper and nickel are preferred metals for electrodeposition, but any other electrodepositable metal can be used.

Claims

1. A method of making a metal mesh, comprising the steps of applying a positive photoresist to a substrate which has a conductive surface, applying a mask having a mesh configuration over the photoresist, exposing the photoresist which is not protected by the mask, developing and washing away the exposed photoresist to leave the conductive surface exposed in the configuration of the mesh while unexposed photoresist remains in the areas which are to correspond to apertures in the mesh, and electrodepositing mesh material on the exposed conductive surface between the unexposed resist to build up the mesh.

2. A method as claimed in claim 1 wherein a release agent such as egg albumen is applied to the areas of the conductive surface on which the deposited metal will be built up, to aid separation of the mesh from the substrate.

3. A mesh when formed by the method of claim 1 or 2.

4. A method of making a metal mesh, comprising the steps of applying a positive photoresist to a transparent substrate which has a conductive surface, applying a mask which has a mesh configuration over the photoresist, with the mash apertures exposed and the mesh lines obscured, exposing the photoresist which is not protected by the mask, developing and washing away the exposed photoresist to leave the conductive surface exposed in the configuration of the mesh, etching away the exposed conductive surface and removing the photoresist to leave conductive mesh configuration corresponding to the mesh, thereafter applying a negative photoresist to the substrate surface, exposing the resist from the back of the substrate, developing and washing away the unexposed photoresist and electrodepositing mesh metal on the exposed conductive surfaces between the exposed resist to build up the mesh.

5. A method as claimed in claim 4 wherein a release agent such as egg albumen is applied to the areas of the conductive surface on which the deposited metal will be built up, to aid separation of the mesh from the substrate.

6. A mesh when formed by the method of claim 4 or 5.