GB2184045A - Method of manufacturing a perforated metal foil - Google Patents

Abstract

The foil for use as, for example, a very fine, high-definition evaporation mask, is made by first applying a film of photo resist (1) to a surface of a stainless steel substrate (2). The resist is then exposed to a collimated beam of ultraviolet light (UV) through a photographic negative (3) and developed to leave portions (5) of resist on the substrate corresponding to the desired apertures in the foil with the surface of the substrate exposed between these portions of resist. Next, the exposed areas of the substrate are electroplated with a layer of nickel (7) having the desired thickness of the finished foil, and finally the resist portions (5) are removed from the substrate with a stripping solution, leaving a nickel foil with the desired apertures lying on the substrate. Due to the lack of adhesion between the nickel plating and the stainless steel of the substrate, the foil can readily be lifted off the substrate. <IMAGE>

Description

SPECIFICATION Method of manufacturing a perforated metal foil The invention relates to a method of manufacturing a perforated metal foil, particularly, though not exclusively, a foil for use as a very fine, high-definition evaporation mask, that is to say, an evaporation mask having a very large number of very small, very accurately defined apertures packed very closely together. Such apertures cannot be formed satisfactorily by punching or machining. The method usually employed is etching, using a photo resist.With etching, however, unless very thin material is used for the foil, which can result in a foil that is not rigid enough to support itself, the unavoidable lateral action of the etchant, i.e., its action in lateral directions parallel to the surfaces of the foil as well as in the desired direction at right angles to these surfaces, tends to produce apertures with undercut walls, which, in section, are not rectilinear and perpendicular to the surfaces of the foil. The apertures thus lack the necessary accuracy of definition. Also, the rate of etching is not readily controllable and consequently may vary across the foil, resulting in further inaccuracies in the definition of the apertures. It is an object of the invention to provide a method which does not suffer from these drawbacks.

According to the invention there is provided a method of manufacturing a perforated metal foil, comprising the steps of applying a film of photo resist to a surface of a stainless steel substrate, exposing the resist through a photographic negative corresponding to the desired pattern of apertures in the foil, developing the resist to leave portions thereof on the substrate corresponding to the desired apertures in the foil with said surface of the substrate exposed between said portions of resist, forming a layer of metal on the exposed areas of said surface of the substrate by electroplating, and subsequently removing said portions of resist from the substrate and separating the layer of plated metal from the substrate.

The photoresist may be a negative or a positive resist. In the case of a negative resist, the portions of the resist which are exposed through the transparent parts of the photographic negative are the portions which are to remain on the substrate after development of the resist and correspond to the desired apertures in the foil; in the case of a positive resist, the portions which are exposed through the photographic negative are removed in the development of the resist to expose the surface of the substrate, the unexposed portions of resist being left on the substrate to form the portions of resist which correspond to the desired apertures in the foil.

In the method according to the invention, the walls of the apertures in the foil are determined by the walls of the portions of resist which remain on the substrate when the resist has been developed. By using a collimated beam of ultraviolet light for exposing the resist, the walls of these portions of resist can be made wholly perpendicular to the surface of the substrate.

Consequently, the walls of the apertures in the foil will be wholly perpendicular to the surfaces of the foil, and the dimensions of each aperture will be constant from one surface of the foil to the other.

The plating on the exposed areas of the surface of the substrate can be built up at a controlled rate to a desired thickness limited only by the thickness of the film of photo resist applied to the surface of the substrate. The resist may be applied in liquid form but the use of a dry-film resist permits thicker plating.

The choice of metal for the plating is determined by the required physical characteristics of the foil, for example, its rigidity. Nickel is preferred because of its strength when plated in a thin layer. For example, a 6 inch square nickel foil 25 Hm thick and having an imperforate border 1 cm. wide is strong enough to support itself.

Stainless steel is chosen for the substrate because no adherent bond will be formed between the plating and the stainless steel and the plated layer can simply be lifted off the substrate.

An example of the method according to the invention using a negative photo resist will now be described with reference to the accompanying drawings, in which: Figures 1 to 4 are sectional views illustrating four successive stages in the method. The drawings are greatly enlarged and are not to scale.

In the first stage shown in Fig. 1, a layer of dry-film negative photo resist 1 is applied to one side (the upper side in Fig. 1) of a polished stainless steel substrate 2 which has first been thoroughly degreased. Prior to the application of the resist the substrate is warmed to approximately 600C in a warm air oven. On the layer of photo resist is laid a photographic negative 3 having a pattern of transparent parts 4 corresponding to the desired pattern of apertures in the finished foil. Through these parts of the negative the resist 1 is exposed to a collimated beam of ultraviolet light, as indicated by the arrows UV in Fig. 1.

After removal of the photographic negative the resist 1 is developed, for example, in a spray developer unit, to dissolve the unexposed portions of resist and thereby bare the upper surface of the substrate 2 between the hardened, exposed portions 5 of resist which remain on the substrate, as shown in Fig. 2. In their peripheral configuration these portions of resist correspond exactly to the desired apertures in the finished foil. The use of a collimated light beam for exposing the resist produces peripheral walls 6 on the resist portions 5 which are wholly perpendicular to the surface of the substrate.

In the next stage, shown in Fig. 3, a layer of nickel 7 is formed on the exposed areas of the upper surface of the substrate 2 by electroplating in a nickel sulphamate bath, the plating being built up to a thickness no greater and preferably slightly less than that of the resist 1.

In a final stage the plated substrate is immersed in a tank of conventional resist-stripping solution until the resist portions 5 have broken down and can be gently washed away with warm water, leaving the nickel foil 8 with the desired apertures 9 lying on the substrate, as shown in Fig. 4. The nickel plating does not adhere to the stainless steel substrate, so that the foil 8 can be simply lifted off the substrate. The foil is finally washed in de-ionised water, followed by immersion in methanol and drying in a warm air oven.

The exact form of the walls 10 of the apertures 9 in the foil 8 is determined wholly by the walls 6 of the portions 5 of resist which remain on the substrate when the resist has been developed. Since, as explained above, the walls of the resist portions 5 are wholly perpendicular to the surface of the substrate 2, the walls 10 of the apertures in the foil will be wholly perpendicular to the surfaces of the foil and will thus have very accurate definition.

Using the above method a grid-type perforated nickel foil was produced having a thickness of 20 jum and rectangular apertures each with a width of 900 ,um and a length of 1,900m ,um. The apertures were distributed in orthogonal rows with strips of nickel 420 #m wide between the rows in one direction and strips 1,001 Am wide between the rows in the other direction. A dryfilm resist 25 Hm thick was used and the plating, to a thickness of 20,us, was carried out in a nickel sulphamate bath of the following composition:~ Nickel sulphamate 350g/litre Boric acid 3.5g/litre Nickel chloride 3.6g/litre pH. 4.0 C.D. 50 A/ft2: Temperature 50-55 C.

The above method was also used to produce a nickel foil 20 Cim thick and having hexagonal apertures each measuring 140 #m between opposite sides, the strips of nickel between the apertures having a width of 62 ,um.

To give the foil added rigidity when in use as an evaporation mask, for example, the foil may be secured at its edges to a supporting frame.

Claims (5)

1. A method of manufacturing a perforated metal foil, comprising the steps of applying a film of photo resist to a surface of a stainless steel substrate, exposing the resist through a photographic negative corresponding to the desired pattern of apertures in the foil, developing the resist to leave portions thereof on the substrate corresponding to the desired apertures in the foil with said surface of the substrate exposed between said portions of resist, forming a layer of metal on the exposed areas of said surface of the substrate by electroplating, and subsequently removing said portions of resist from the substrate and separating the layer of plated metal from the substrate.

2. A method as claimed in Claim 1, characterised in that the plating metal is nickel.

3. A method as claimed in Claim 1 or 2, characterised in that a collimated beam of ultraviolet light is used for exposing the resist.

4. A method of manufacturing a perforated metal foil, substantially as herein described with reference to the accompanying drawings.

5. A perforated metal foil manufactured by the method claimed in any of Claims 1 to 4.