Method for making matt diffusion patterns
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the United States counter-part of West German Application Ser. No. P 29 46 235.9, dated Nov. 16, 1979, whose priority is hereby claimed for this application. 
     BACKGROUND OF THE INVENTION 
     This invention relates to an improved method of making matt diffusion patterns, in particular to methods of making matt diffusion patterns along with opaque and transparent patterns. Such diffusion patterns are utilized, for example, in photographic camera view-finders, and on theodolite grid plates. 
     Hitherto, matt diffusion patterns have been made by applying the desired pattern in the form of an opaque layer to a ground glass pattern carrier plate. This process has not produced completely satisfactory results, however, because holes have tended to form in the opaque areas and thus to produce imperfect patterns. Also, this means of producing patterns has the limitation that only matt and opaque patterns can be produced thereby on a single carrier plate. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method of making matt diffusion patterns which avoids these and other problems of the prior art. In particular, the present invention is directed to a photolithographical method of making matt diffusion patterns along with either or both of opaque and transparent patterns on a transparent pattern carrier plate. 
     According to this invention, a photoexposure mask corresponding to the desired matt pattern is made by photolithography, which mask is then used in subsequent photolithographic processes to form the matt diffusion pattern on pattern carrier plates. 
     To generate the photoexposure mask, a photoresist layer is applied to a mask of the desired matt diffusion pattern, exposed through a matt diffusion plate, and developed. An opaque layer is then applied over the mask, and the photoresist layer which remained after developing, along with those portions of the opaque layer lying thereon, is removed. 
     The photoexposure mask thus formed is used to photolitographically create the desired matt diffusion pattern on the pattern carrier plates. In a preferred embodiment of the invention, a transparent pattern carrier plate has applied thereto an opaque layer. The opaque layer is covered with a photoresist layer which is then photographically exposed through the photoexposure mask described above, and the photoresist layer is developed. The layer-bearing face of the pattern carrier plate is then treated with etching substances to etch away the opaque layer and the surface of the pattern carrier plate in those areas which were exposed by developing of the photoresist layer. The surface of the pattern carrier plate is etched deeply so that the remaining opaque layer is underetched slightly. This underetching removes small points of the opaque layer from the matt diffusion pattern areas. 
     Finally, portions of the remaining opaque layer are removed to form the transparent pattern. 
     One important advantage of this method of producing matted patterns is that matt diffusion patterns can be produced along with both transparent and opaque patterns on a single pattern carrier plate. 
     Another advantage of this method of producing patterns is that matt diffusion patterns can be produced along with transparent and opaque patterns by means of using a single exposure mask. 
     Yet another advantage of this method is that improved patterns can be made because holes no longer tend to form in the opaque portions of the pattern. 
     The invention itself, together with further objects and attendant advantages, will be best understood by reference to the following detailed description taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1a through 1e show stages in the production of the exposure mask. 
     FIG. 1a shows the photolithographic formation of the opaque portions of the mask pattern; 
     FIG. 1b shows the exposure mask with the opaque portions of the mask pattern formed; 
     FIG. 1c shows the exposure step of the photolithographic formation of the diffusion portion of the mask pattern; 
     FIG. 1d shows the mask after application of the diffusion pattern forming opaque layer; 
     FIG. 1e shows the final exposure mask. 
     FIGS. 2a through 2d show stages in the production of the matt diffusion pattern on a pattern carrier plate; 
     FIG. 2a shows the carrier plate as it initially appears before processing; 
     FIG. 2b shows the carrier plate after photolithographic processing to imprint thereon the image of the exposure mask of FIG. 1e; 
     FIG. 2c shows the carrier plate after etching of the carrier plate surface; 
     FIG. 2d shows the final pattern carrier plate with the matt diffusion pattern. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Referring now to the drawings, FIG. 1 shows stages in the formation of the matt diffusion exposure mask 1. 
     As FIG. 1a shows, the exposure mask 1 is formed from a polished transparent substrate or layer carrier 2 which carries on one face an opaque layer 3. The opaque layer 3 is preferably a layer of chromium. The opaque layer 3 is covered with a layer of photoresistant photolacquer 4. 
     The photolacquer layer 4 is photographically exposed through an auxiliary mask 5. The auxiliary mask 5 is opaque but for areas which correspond to the desired final matt diffusion pattern, which are either cut out of the auxiliary mask 5 or left transparent. 
     After exposure, the photolacquer layer 4 is developed to remove those portions of the photolacquer layer that were exposed. Etching substances are then applied to etch away those portions of the opaque layer 3 which have no photolacquer layer 4 covering them. After etching, the remains of the photolacquer layer 4 are removed. Thus, exposure of the photolacquer layer 4 through the auxiliary mask 5, followed by photolithographic processing, removes from the opaque layer 5 the areas of the matt diffusion pattern of the final diffusion plate, as shown in FIG. 1b. 
     The partially formed mask 1 of FIG. 1b is next covered with a new layer of photolacquer 6 and is photographically exposed through a matt diffusion plate 7, as shown in FIG. 1c. The matt diffusion plate 7 is preferably a plate of glass with ground unpolished surfaces. 
     The photolacquer layer 6 is developed to remove the exposed portions of the photolacquer layer 6, and a new opaque layer 8 is applied over the developed surface to produce the structure of FIG. 1d. The opaque layer 8 is preferably a metal or a metal oxide, most preferably again a layer of chromium. 
     The remaining areas of photolacquer layer 6 are then removed, thus removing also those areas of the opaque layer 8 which lie upon the photolacquer layer 6, leaving behind as remains of the opaque layer 8 the opaque particles 9 as shown in FIG. 1e. The exposure mask 1 is now completed. Thus, exposure of the photolacquer layer 6 through the matt diffusion plate 7, followed by photolithographic processing which, instead of the usual etching step, includes the depositing of a new opaque layer 8, forms the exposure mask of FIG. 1e. 
     Chromium particles 9 adhere directly to the layer carrier 2 in those areas of the pattern which are desired to be diffusing. Areas desired to be transparent or opaque on the final pattern carrier plate are opaque on the exposure mask 1. 
     FIG. 2 shows stages in the process of producing the matt diffusion pattern on a pattern carrier plate, which process utilizes the exposure mask of FIG. 1e. 
     FIG. 2a shows the unprocessed pattern carrier plate 14 which comprises of a transparent carrier plate 10, an opaque layer 11 deposited on one face of the carrier plate 10, and a layer of photoresistant photolacquer 12 covering the opaque layer 11. The carrier plate 10 is preferably glass and the opaque layer 11 is preferably chromium. 
     The photolacquer layer 12 is photographically exposed through the exposure mask 1 to imprint therein the image of the exposure mask 1. The photolacquer layer 12 is then developed to remove the exposed areas of the layer 12, and an etching substance is applied to etch away those portions of the opaque layer 11 which are not covered by photolacquer layer 12. This photolithographic process forms in the opaque layer 11 the image of the exposure mask 1, including opaque particles 13 which are an image of the opaque particles 9 of the exposure mask 1, as shown in FIG. 2c. 
     Next, the remains of the photolacquer layer 12 are removed and an etching substance is applied to etch away those portions of the surface of carrier plate 10 which are not covered by opaque layer 11. The etching of the carrier plate 10 is made deep, so that the opaque particles 13 are underetched, as shown in FIG. 2c, and thus removed. 
     Removal of the opaque particles 13 by etching leaves the etched surface areas of the carrier plate 10 jagged, as shown in FIG. 2d. These jagged areas form the matt diffusion pattern on the pattern carrier plate 14. 
     Finally, transparent areas of the final pattern are formed by removing those parts of the opaque layer 11 which correspond to those transparent areas. The formation of the transparent areas is preferably again accomplished photolithographically. 
     The exposure mask 1 can be reused in subsequent photolithographic processes to form additional diffusion plates 14. 
     This completes the production of the final pattern carrier plate in which matt diffusion patterns have been formed along with opaque and transparent patterns. Of course, it should be understood that various changes and modifications to the preferred embodiment described above will be apparent to those skilled in the art, which changes and modifications can be made without departing from the spirit or scope of the present invention.