[labnetwork] Light filtering in photolithography bays

[Christian Pies]

Hi Travis,

when we planned our new cleanroom facilities, we wanted to switch to LED lighting, but were also quite concerned about accidentally exposing the photoresists. After some discussions with one of our resist suppliers, we learned that wavelengths below 500 nm must be avoided. Unfortunately, most yellow LEDs show a sidepeak around 420 - 450 nm, which is often not visible in the datasheets unless the emission spectrum is shown in a logarithmic representation. The LEDs we originally looked at had a side peak of 0.4% intensity, which was still too much.
Pure yellow light LEDs have a lower efficiency, but covering the lamps with a filter film probably reduces the output in a similar way or even more.

We finally decided for the following LEDs:
https://dammedia.osram.info/media/resource/hires/osram-dam-6665600/LY%20CKBP_EN.pdf
They are in use for almost two years now and we haven't experienced any problems with our resists so far. They are a bit more orange than the typical lighting, which feels a bit unfamiliar at the beginning.

Computer monitors can be an issue. If we leave our coated substrates on the desk in front of the monitor, the resist is typically exposed within a few hours. A short time next to the monitor while setting up the machine is usually uncritical (depending on the sensitivity of the resist). "Light contamination" of the entire room from the monitors has not been a problem in our case. But of course this depends on the local conditions. Turning down the brightness and switching off the monitors when not in use is for sure not a mistake.

Best regards,
Christian


Dr. Christian Pies | Head of the Process & Application Lab
T | +49 6221 728899-0
E | christian.pies at heidelberg-instruments.com

Heidelberg Instruments Mikrotechnik GmbH
Mittelgewannweg 27
69123 Heidelberg

[Heidelberg Instruments]<http://www.heidelberg-instruments.com/>

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Von: labnetwork <labnetwork-bounces at mtl.mit.edu> Im Auftrag von Massey, Travis
Gesendet: Montag, 13. Februar 2023 23:40
An: labnetwork at mtl.mit.edu
Betreff: [labnetwork] Light filtering in photolithography bays

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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