Color interference filters for individual pixels in solid-state electronic image and display devices would be made of thin metal and dielectric films, according to a proposal. The proposed filters would overcome the primary disadvantage (high cost) of dye color filters like those used in liquid-crystal display devices, digital cameras, and camcorders. The proposed filters would also offer advantages of cost and functionality over color interference filters made of alternating dielectric layers with different indices of refraction.
The all-dielectric filters are expensive because of the need for large numbers of layers to obtain adequate discrimination among red, green, and blue (RGB). The proposed filters would provide adequate color discrimination with acceptably broad-band response (pass wavelength bands about 100 nm wide). The proposed filters would be relatively inexpensive because they would contain fewer layers -- typically no more than five layers, and only two layers need to have different thickness for RGB colors, which means it only needs to be masked 2 ¥ (3 - 1) = 4, as contrasted with more than 10 layers for an all-dielectric filter, and needs to be masked 10 ¥ (3 - 1) = 20.
Figure 1 shows aspects of a proposed five-layer metal/dielectric filter containing three layers of silver alternating with two layers of magnesium fluoride. The table in the figure shows the film thicknesses needed to make the filter transmit each of the three primary colors. The corresponding silver layers for all three color filters could be of the same thicknesses; only the magnesium fluoride layers would differ in thickness among the three colors. The total number of distinct layer thicknesses is only five, three for silver and two for magnesium fluoride.
Because of the small number of thicknesses, patterning and other aspects of the fabrication of a device with three primary-color filters in each pixel (see Figure 2) would be relatively easy. The metal patterns could be formed in the presence of photoresist masks temporarily substituting for the magnesium fluoride films. The optical thickness of each photoresist mask would be made equal to that of the magnesium fluoride film to be subsequently deposited in its place. Because it is relatively easy to control the thickness of a photoresist mask, fabrication should be relatively simple and inexpensive.
This work was done by Yu Wang of Caltech for NASA's Jet Propulsion Laboratory.
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