The Recording And Large Scale Replication Of Crossed Holographic Grating Arrays Using Multiple Beam Interferometry

In the photographic industry, two dimensional periodic arrays, or hole patterns, that have a periodicity on the order of one to several micrometers, can have many significant applications. One use of such an array is the regular positioning of photosensitive elements, including, for example, photographic emulsion grains, into film systems of exceptionally low noise. A method is described for producing the array patterns using the technique of multiple beam interferometry applied to the already well known techniques of holographic grating formation. In the first process a square hole pattern is formed using two sequential two beam interference exposures in a thick layer of positive photoresist. Upon development an array of hills and valleys is formed, with the valleys corresponding to the holes and occupying about half of the available surface area. In the second process a hexagonal close packed hole pattern (honeycomb array) is formed using a single simultaneous three-beam coherent exposure of the photoresist. In the third process a square close packed hole pattern (egg-crate array) is formed using a single simultaneous four-beam exposure. The second and third processes produce surface depressions in the photoresist layer that occupy practically all of the available surface area. Because the periodic arrays are required in large quantities, the patterns formed in photoresist must be replicated. A nickel master is made from the original photoresist by electroplating, and this master is then used to repeatedly emboss the pattern into long plastic sheets. A description is given of the special industrial requirements for making high quality embossed patterns in this way on a large scale.