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Photoresists

Lithography is an old art. The dictionary starts by saying that it is the art of drawing pictures on a stone usually covered with a greasy substance. After the picture was drawn in the “grease” exposing the underlying stone, the exposed area could be etched with acid to form a groove. This held ink that could be transferred to paper.

Similar technology was used for etchings based on metal plates. The image was hand drawn before the acid etch was carried out. In the 1930s Kodak introduced a photosensitive material that could be coated on a metal plate to form an image upon exposure to visible light. The exposed regions became insoluble and the unexposed areas could be removed with an organic solvent. The material was called a photoresist. It was based on a soluble polymer (poly vinyl cinnamate) with side groups that became bound to each other (crosslinked) on exposure. This creates giant molecules that are insoluble.

Sometime in the 1940s Kodak introduced Kodak Metal Etch Resist (KMER), a material based on an organic polymer very much like rubber cement. They added a small amount of photosensitive material (a bisazide) that created crosslinked regions that remained after development with xylene. This is a much easier way to make a printing plate.

Photolithographic Process

Fine lithographic patterns can be produced when light interacts with photoresists, which are used in this case to create masks on substrate materials. The mask is therefore used to define the pattern to be produced on the exposed material. Other methods of mask making are:

  • Cut a stencil in the desired pattern and hold over the surface/tape it down
    • Apply paint with a brush or spray can
    • Reusable
  • Cover the surface with protective tape or film (called a frisket)
    • Cut out and remove film sections with a fine knife
      • Paint exposed areas
      • On glass, etch exposed area with fluoride paste; wax was used as a mask
        • Sandblast exposed areas
      • Electroplate a metal surface placing metal selectively where desired
  • Metal etchings
    • Coat metal plate with varnish
    • Remove varnish from selected areas
    • Etch plate in acid
      • Exposed areas become small grooves that retain ink
      • Plate is used to print
  • Silk-screen
    • Cloth of finely woven silk
    • In some areas, paint a layer of wax or other coating that is inert to water-based inks
    • Sandwich the screen (stencil) with paper and apply ink
      • Ink oozes through exposed areas
      • Ink is blocked by the waxy areas
      • Paper picks up a copy of your image
    • Several screens are made, each for a different color ink
      • One stencil can make hundreds or thousands of prints

Photoresist materials are used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface. Negative resists such as those described above are those in which the exposed areas become resistant to chemical etching, so the unexposed areas are removed during the developing process. A negative photoresist becomes tougher when exposed to light.

  • The unexposed area is easily removed with warm water or a solvent
  • The result is a masked surface with exposed and protected areas
  • The pattern is a copy of the negative used when the surface was exposed
  • The underlying surface can be modified as desired
    • Etch with acid; this is how photo-offset printing plates are made
    • Dissolve the entire underlying surface
    • Electroplate to build up a new metal layer
    • Sand blast
    • Paint
    • Chemically alter
  • Surface finish using a stronger solvent and stripping away remaining photoresist

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