[jeyurkon]

I did a quick and dirty test of the slurry screening technique. I used Nichia's NP-1154 red phosphor. The phosphor was suspended in a dichromated polyvinyl alcohol solution. I used potassium dichromate instead of ammonium dichromate which the recipe called for. Sodium dichromate should also work.

I spread the slurry onto a glass slide and let it dry. I then placed a metal mask over it and exposed it with a UV lamp. It was then developed by letting hot water run over it.

The results can be seen in the attached image. In the center there is an array of dots partially surrounded by a circle. The dots are spaced with a 600 micron pitch and are about 150 microns wide.

You can see problems with it. I was surprised it worked this well. The main reason for the damaged areas is due to the thickness of the coating. The optimal coating thickness is 1.4 phosphor grains. I didn't want to set up a spin coater for this test because of the work involved in cleaning it up afterwards. A spin coater will give a very uniform easily controlled thickness.

When the coating is too thick, the UV light is completely absorbed by the phosphor before reaching the surface of the glass. The result is that there is a PVA layer that doesn't get hardened. This layer softens and releases the hardened layer above it when developed. With the correct thickness, the PVA would have been completely hardened down to the surface and the phosphor would have remained where the PVA was exposed. In this case, some of the areas floated away.

The difference in the horizontal and vertical resolution of the dots also has to do with the thickness. The water was flowing horizontally across the slide and was able to flow undisturbed between rows. The material between columns was protected by the height of the dot in the preceding column. If the coating was as thin as it should have been, then the features wouldn't disturb the flow nearly as much.

It would also have helped to treat the glass so that it was hydrophilic. The most effective means would have been to evaporate a coating of silicon dioxide on to the surface. Another method would have been to clean the glass in a solution of potassium hydroxide and isopropyl alcohol.

BTW, the plate was illuminated with UV light to cause the phosphor to fluoresce when the photograph was taken. Otherwise, the phosphor would have looked white.

It's a simple matter to improve the process to the point of being able to apply the three phosphors using the shadow mask. Whether it would be profitable for a company is another matter.

I had fun in any event.

John