US2279567A - Method of producing thin strips of - Google Patents

Description

April 14, 1942. HQLMAN 2,279,567

METHOD OF PRODUCING THIN STRIPS OF MATERIAL ON A SUPPORTING SURFACE Filed Oct. 26, 1938 INVENTOR #1148597 [Db 19!) f/OL mn/v z 7 MW ATTORNEY Patented Apr. 14, 1942 METHOD OF PRODUCING THIN STRIPS OF IVIATERIAL ON SUPPORTING SURFACES Herbert Edward Holman, Hayes, England, assignor to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Application October 26, 1938, Serial No. 237,068 In Great Britain November 3, 1937 1 Claim.

The present invention relates to methods of coating or plating a supporting body in accordance with a desired pattern, more especially in cases where the desired pattern is a system of fine lines, and was developed in the first place as a means of producing a system of close spaced parallel straight metallic lines on the surface of a glass disc or plate, in order to reduce the length of path of electric currents through semi-conducting material applied to the surface of said plate. However, the invention is in no way limited to this particular application.

One object of the invention is to provide a method of forming a pattern of thin strips or lines of material on the surface of a supporting body which can be carried out readily and economically and which enables the position of the thin strips of material on the supporting surface to be accurately pre-determined.

A further object of the invention is to provide metallic grids or gratings of fine mesh which may be used as transparent electrodes, for example, in television transmitting and receiving tubes, and also to provide series of metallic lines for the purpose of forming, for example, scales for both microscopic and macroscopic optical systems having great durability.

According to the invention a method of plating or coating a surface of a supporting body in accordance with a desired pattern, more especially a pattern of fine lines, is provided, the method comprising the step of coating the surface of the supporting body on which said desired pattern is to be formed with a layer of material which may be engraved, engraving a line in said layer to such a depth as to expose the surface of said supporting body to form an exposed area or areas in conformity with said pattern, plating or coating over the engraved layer with the material of which the coating or plating is to be formed and treating said engraved layer in such a way as to cause it to expand or swell or otherwise disintegrate thereby breaking the plating or coating of coated material along the edges of said exposed area or areas to isolate the strip of material which is in contact with said supporting body.

In the application of the invention, in the particular case above referred to, namely, to the production of a pattern of fine metallic lines on the surface of a glass plate, the surface of the plate was coated with a soft layer of material in the form of a thin film of cellulose acetate, which may be applied in the form of a varnish, the film formed being of the order of .0025 cm. thick. This film is of such texture that it can be cut metal deposit over its entire surface.

through mechanically by a tool which does not necessarily penetrate into the supporting body. The actual thickness of the applied soft layer is not of extreme importance but satisfactory results are more readily obtained when films of the thickness mentioned are employed. After the soft layer of cellulose acetate has been applied it is cut through with a sharp cutting tool, in order to reveal the surface of the glass which lies beneath the soft layer. Preferably to effect the cutting or engraving a mechanically guided cutter is used, for example, the layer may be cut or engraved in a ruling machine having a slide which may be traversed across the surface of the glass plate, this slide having a transverse motion operated by a screw mechanism allowing a definite close spacing of the lines to be made.

The tool used in the engraving process maybe of acute prismatic form, and is preferably held in such a manner as to permit of a degree of freedom in the vertical plane; this obviates any need for great accuracy in levelling the plate, as the tool can rise and fall following the contour of the surface of the plate. The downward force which keeps the tool in contact with the glass and consequently causes it to cut through the cellulose acetate or other soft layer may be supplied either gravitationally or by a spring. If the tool is arranged to be free to move in a vertical direction as described then curved surfaces may be engraved, if desired. In accordance with the next step of the invention the glass plate with the engraved layer is now coated or plated with a be conveniently done by the usual processes of evaporation of metals while heated in vacuo, or

a process of anodic bombardment or sputtering may be used, especially when lines of highly re: fractory metals, such as platinum or tungsten are to be formed; The plating may also be efiected using a method of electro plating in which the metal is caused to be deposited from a suitable electrolytic solution. The thickness of the coated metal is determined by the thickness required in the desired lines. v

The coated and plated glass plate is then transferred to a bath of acetone, which by its solvent action causes the cellulose acetate to expand so as to completely break up the metallic film deposited on it thus isolating the lines of metal supported on the glass surfaces exposed in the bottom of the engraved grooves in the varnish. The unwanted metal plate may be removed by the mere immersion in acetone followed if necessary by a gentle wiping with cotton wool saturated This may the method of carrying the invention into prac- -tlce may be clearly understood, one embodiment of the invention will now be described with reference by way of example, to the accompanying drawing, in which Figure 1 is a perspective view of a piece of glass plate on which it is desired to form a pattern of fine lines of conducting material, the plate having been coated with varnish in accordance with the first step of the invention,

Figure 2 is a view of the plate shown in Figure 1 after the desired pattern which the conductive material to be applied to the glass plate is required to form has been engraved in the soft layer of varnish or the like,

Figure 3 is an elevational view of the plate in Figures 1 and 2 to show more clearly the formation of the plate and the applied varnish coating and plated layer,

Figure 4 is a view similar to that of Figure 3 showing the plate after the varnish has been treated to cause it to swell, and

Figure 5 is a view similar to Figures 1 and 2 showing the glass plate in its final condition.

Referring to Figure 1 of the drawing, 1 is the glass plate on which the lines of conductive material are to be formed and 2 is the layer of cellulose acetate varnish which may be applied in any suitable way, for example by being brushed over the surface'of the plate.

Figure 2 shows the plate after being engraved,-

it being assumed that three lines of conductive material are to be formed on the plate. lines are shown to an enlarged scale for the purpose of clarity. The ruling or engraving process results inthe formation of grooves 3 in the layer of cellulose acetate 2. The engraving process being effective substantially to remove the applied varnish from the surface of the supporting body I, within the limits of each. groove as indicated at 3a in Figure 2, for example.

Figure 3 shows in elevation the plate shown in Figure2 to which has been applied the plating 4 of metal. This figure also shows clearly the profile of the grooves 3. Preferably this profile is such that the sides 3b of the groove are nearly vertical so that the plating 4 of these sides will be very thin so that it permits a very rapid lateral penetration of the acetone or other solvent which is used to causethe varnish layer to expand when the plate is dipped therein. In this way it is ensured that the plating is broken or cracked due to the expansion of the underlying varnish layer accurately along lines corresponding toboth sides of each groove so that the plat ing material at the bottom of each groove is released completely from the rest of the layer 4 which will be removed in due course when the varnish is wiped off or washed away. The action of the solvent in causing the swelling of the layer 2' is more clearly illustrated in Figure 4, the en- These 2,279,667 with acetone, this operation leaving the plating hanced swelling of the varnish which takes place at the sides of the grooves particularly at the initial stages of the solvent being indicated at 2a. After the removal of tl.-e varnish layer the plate I is left with the applied plated lines 4a in the areas defined by the position of the grooves 3.

After the final washing to remove all traces of cellulose acetate, the giass plate now bearing the metallic line pattern may be placed in an oven preferably of the vacuum type and then slowly raised to the softening point of the glass. This firmly attaches the metal lines of the glass and then they can then only be removed by heavy scraping or by dissolving in acids or other very strong action. If desired, either before the glass surface is coated with varnish it may be treated with an etching agent for example, hydrofluoric acid to produce a matt effect on the surface of the glass. Alternatively, the etching may be performed after the coating process and may serve to produce grooves in the glass support itself to receive the plated strips or lines 4. Alternatively, of course, the supporting plate I might be itself engraved by the engraving tool which cuts through the varnish or other soft layer.

Other materials than cellulose acetate may be used for the engraved layer provided they can be engraved satisfactorily and caused to expand in some suitable manner either by the use of a solvent as described or, for example, by the method of heating or warming. Also the method of the invention is not limited in its application to the formation of lines of metal or conducting material as lines of any material which may be plated, for example, by being evaporated, may be formed on the surface of a supporting body. For example, lithium borate, quartz, glass known under the name Pyrex (registered trade-mark) or other glass may be evaporated into the grooves so as to form a non-conductive grid-like system of lines on a metal surface. Also it will be appreciated that the engraving process may be applied equally well to cut any pattern or configuration of lines either by using suitable mechanical arrangement or by engraving the lines manually, these lines being extremely narrow when necessary. As the vaporated or sputtered plate is applied in molecular form, it would appear that the only limit to the width of the grooves will be that applied by the cutting technique, and as the structure of a cellulose film for example, is such as to permit of division without splintering, such a limit is extremely small. In fact, it is found that the close spacing permitted is such that metallic grids or gratings can be produced having a very wide application. For example, screens made in accordance with the invention may be used for stabilising the screens of television receiving tubes. For example in British Patent specificationNo. 381,306 and as shown at 14a in Figure 2 of the drawings thereof, it is proposed to provide a thin conducting layer between the fluorescent screen and the wall of th cathode ray tube. This thin conducting layer constitutes a stabilising screen. However, difficulty is experienced in making a stabilising screen both sufliciently electrically conductive to be effective in stabilising a fluorescent screen and sufficiently transparent not to impair the equality of the image as observed by' a viewer. By forming a stabilising screen according to the invention it is found that substantially the whole screen may be rendered conductive without undue loss of light as the area rendered opaque due to the provision of the grid or grating can be reduced to a small proportion of the whole. In addition, the ruled lines may, by virtue of the process used in forming them, be of such dimensions as to preclude resolution by the scanning beam as the width of these lines may be less than that of a picture point.

The invention is also applicable to the preparation of transparent signal plates for use in television transmittin tubes of the .kind described in British Patent specification 455,123 wherein the tubes ar provided with a mosaic screen comprising a sheet of substantially transparent electrically insulating material having at tachcd to one side thereof a substantially transparent layer of electrically conductive material and to the other side thereof a multiplicity of mutually insulated electrically-conducting elements coated with photo-electrically-active material. In such tubes the transparent electrode may, according to the invention, b produced in the form of grids or gratings as described above or the transparent layer may be formed by forming a grid or grating according to the invention on the mosaic screen and superimposing over the grating a very thin film of metal which would by itself not be sufliciently conductive for the purposes described in the aid patent specification, the necessary conductivity being provided by the metallic lines or bar's of the underlying grating produced according to the invention.

Gratings produced according to the invention may also be used in optical measuring devices for which purpose scales andgraticules of all types both' for microscopic and macroscopic purposes may be provided. Scales according to the invention may be formed with accurate and clean cut lines and will withstand higher temperatures than scales produced by photographic processes in which the scale is formed as an image on the film, or in the case where cross filaments of spiders web or viscose are used. Furthermore, scales prepared according to the invention are extremely durable. Such scales will withstand abrasion and provided suitable non-corrosive metals such as platinum or gold .are used the scales will also resist atmospheric action, sea water, and even immersion in strong acids and alkalis. For this reason scales prepared according to the invention may be used with great advantage in marine equipment, aircraft equipment and military equipment where robustness is a very desirable characteristic. A further advantage of the invention resides in the fact that a metallic grid or grating according to the invention may be formed on any curved surface either cylindrical or spherical. If it is required to form a grid or grating according to the invention on a curved surface then, when a mechenical engraving process is employed, th engraving tool used in engraving the soft layer should be provided with means for revolving about the centre of curvature in order to prevent distortion arising when the cutter is not directed towards the centre of the sphere. For example, the cutting or engraving tool mounting may be suitably pivoted.

According to the invention also, diffraction gratings may be provided in which the ruled lines are of extremely small width, for example, reflection diffraction gratings may be made by first producing a system of silver or other suitable metallic lines upon a glass plate. These may then be chemically blackened in order to render them non-reflecting and the other side of the glass plate may then be silvered. If the plate is then viewed from the ruled sidea system of black or non-reflective lines will be seen with the silvered reflector surface visible between them.

Also grids or gratings prepared according to the invention may be employed in the process of photographic reproduction in which ruled screens are required. In such processes it is common to employ diamond ruled gratings having their ruled surfaces in such a position and the rulings so placed as to cross at 90. The spaces between these lines have to be filled in so as to be opaque. This is difficult when very fine rulings are required. However, according to the invention grids and gratings can be produced hav- A further application of the invention is to i the manufacture of non-inductive high resist.- ances. The groove may be traced in zig-zag form by keeping the cutting tool in contact with the glass while making a lateral movement of the glass or the tool each time the end of the line is reached. If this lateral movement is small the ruled lines will be very closely spaced and the, resistance obtained will thus be substantially non-inductive. If desired each alternate line may be connected to a common bar at the termination of the line, and this arrangement may be used in-photoelectric devices where a light sensitive substance is required to have a low transverse resistance.

In cases where a complicated system of metallised or otherwise coated or plated lines is to be produced on a plate, and the degree of accuracy required in ruling is not too high, the layer of material which is applied over the supporting body and engraved may be engraved by a photographicprocess instead of being engraved mechanically as described above. For example in accordance with this further method of carrying the present invention into practice the surface of the supporting body on which a line or strip of coated material is formed maybe coated with a layer of cellulose-acetate. After the cellulose-acetate layer has been allowed to harden by exposure to air the layer may be covered with a layer of bichromated gum or glue such as is used in the process of photo engraving of printing blocks, care being taken not to expose the bichromated gum to light. The layer of hichromated gum is then exposed to light through a stencilor mask bearing. the pattern which it is desired that the coated or plated material shall follow, the parts of the glue exposed representing the regions from which the layer of coated or plated material which will be subsequently removed. The mask or stencil through which the bichromated layer is exposed to light may be a photographic negative corresponding to the pattern to be traced by the coated or plated material.

The exposed parts of the bichromated gum are rendered insoluble in water so that by washing the coated hard body in water the unexpo sed portion of the gum may be washed away leaving the aforesaid layer of material covered with a tracery in hardened gum corresponding to the pattern formed by the transparent parts of the stencil or mask through which the gum has been exposed. The surface of a supporting body thus treated may be then immersed in a solvent such as acetone which will attack the exposed parts oi-the cellulose acetate where it is not covered by the film of gum so that the exposed parts of the acetate film may be removed. After this has been carried out the remaining gum may be removed by immersion in a bath of weak sodium-hydroxide (NaOH) and washing by a stream of water. Thus the supporting body will now be coated by a thin filmof cellulose-acetate pierced in accordance with the pattern formed by the'opaque parts of the,

original stencil or screen.

The supporting body coated with the photoengraved layer of cellulose-acetate film may be treated precisely in the same manner as the hard body coated with the mechanical engraved layer, as described above, the layer of coated or plated material of which it is desired to form a pattern on the 'hard body being sprayed or plated, for example by electrolytic deposition or by cathode sputtering, over the acetate layer and the acetate being treated with a solvent such as acetone to cause it to expand and to shatter the film of coated or plated material along the edges of the engraved grooves or other region from which the engraved layer has been removed.

Thus in accordance with this further embodiment of the invention, plates or grids may be provided with very complicated patterns without the necessity of ruling the engraved lines individually, though it will be realised that the photo-- the line spacing in the grid formed is required to be of the highest order.

The method of photographic engraving might also be useful in forming a pattern oi'vfine lines on a supporting body formed 01' a material which ing the surface of the supporting body with a comparatively thin film of cellulose acetate, forming the desired pattern on said film by engraving said film to such a depth as to expose the surface of the supporting body, etching the exposed areas of said body, coating over the engraved-layer with the material of which the desired coating is to be formed, immersing the entire supporting body in an acetone bath so as to cause any remaining cellulose acetate to disintegrate thereby breaking the coating of coated material along the edges of the pattern to render thereby the plating outside of the patgraphic method of engraving the soft layer is not applicable in cases where the accuracy of tern readily removable and to isolate the coated material which is in contact with the supporting body, and removing the plating outside the exposed areas.

HERBERT EDWARD HOLMAN.

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