[labnetwork] Light filtering in photolithography bays

[Massey, Travis]

Hi all,

LLNL recently changed out its cleanroom lighting to LEDs from fluorescent tubes.  A few months later, we found we were having lithography issues - our resist is getting exposed under ambient light within the litho bays within 5-15 minutes.  The lights in our litho bays all have ~30 year old plastic diffusers that snap on over the tubes, and these diffusers have a filter film inside.  We speculate that the new LEDs may have photobleached the aging photosensitive pigments.  Measurements of the light intensity in the cleanroom at 365, 405, and 435 nm indicate that our litho bays have just as much light at these wavelengths than our non-litho bays!

-- As an alternative to filtering the light, has anyone found LED cleanroom lighting that does not emit in the >450 nm range?  For example, form-compatible LED 'tubes' that only emit at ~580 nm (yellow)?
-- How do all of you solve this problem?  What product do you use to filter your lights (e.g., tube covers, panel covers, applied films, solid dyed plastics, laser safety products...)
-- Have you found that your filters have a finite lifespan?  For example, I've heard in previous discussions that window films are only good for 5 years or so before the wavelength-absorbent pigment decomposes and is ineffective.
-- To what extent have you found computer monitors' light emissions to be a point of concern?
-- Other thoughts/suggestions that we should explore or consider?

Thanks,
Travis Massey
Center for Micro and Nanotechnology (CMNT)
Lawrence Livermore National Laboratory
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