Without suited yellow filters, artificial light as well as daylight will expose substrates coated with photoresist within seconds or minutes with a dose of several mJ/cm2 making reproducible litho-processes impossible. Today, Yellow LED tube with special UV cut material has enough brightness to illuminate the clean room and to enable reproducible litho-processes. Instead of using UV blocking film or yellow sleeve over fluorescent tube, Yellow LED tube is good alternative.
Nocturnal insects are often attracted to light sources that emit large amounts of UV radiation. Yellow LED light with UV block is suitable in open area lighting that may have insect related problems.
How UV block or yellow LED light can help?
Photo-lithography, also termed optical lithography or UV lithography, is a process used in microfabrication to pattern parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical “photoresist“, or simply “resist,” on the substrate. A series of chemical treatments then either engraves the exposure pattern into, or enables deposition of a new material in the desired pattern upon the material underneath the photo resist. For example, in complex integrated circuits, a modern CMOS wafer will go through the photo-lithographic cycle up to 50 times.
Spectral Sensitivity of Photo Resists
The spectral sensitivity of common g-, h-, and i-line broadband photo resists ranges from the near UV to the short wave visible part of the spectrum in the range of 320 to 470 nm. The g-line absorption maximum at 435 nm wavelength is centered in the blue part of the spectrum and drops towards longer wavelengths without sharp absorption edge. Only i-line resists at small resist film thicknesses show a negligible sensitivity beyond approx. 400 nm wavelength.
Maximum Permissible Transmission at Short Wavelengths
The traditional White fluorescent tubes have a strong emission near 405 and 435 nm wavelength, which corresponds to the absorption maxima of common photo resists. Incident sunlight or daylight has also a high intensity in the spectral range absorbed by photo resists, which is not significantly blocked by common windows panes.
Without suited yellow filters, artificial light as well as daylight will expose substrates coated with photo resist within seconds or minutes with a dose of several mJ/cm2 making reproducible litho-processes impossible.
Thus, a yellow filter with a transmission of 0.1 % below 500 nm wavelengths is not suited to allow the storage of coated substrates for several hours at exposed places in the clean room near windows or fluorescent tubes. Under these conditions, positive resists will show an increased dark erosion in the developer, which deteriorates the desired resolution and resist profile. Negative resists may form a cross linked surface which can be not or only time-delayed penetrated by the developer.
Therefore, a yellow filter needs to block wavelengths below 500 nm almost completely in order to allow the storage of photo resist coated substrates over hours and days in the clean room.
Many common yellow foils have a short wavelength transmission (< 500 nm) of approx. 0.1 % or higher, which is not low enough for reproducible litho processes. If unsuited polymers are used for the yellow foil, thermal stress from neighbored light sources can form small cracks in the foil resulting in short wavelength leakage.
The following is the Spectral comparison between the expensive Yellow Sleeve tube vs a Yellow T8 LED tube done by a major Multi-national corporation in Japan.
- 17 W to replace 36 W Yellow sleeve fluorescent.
- Long Lasting thus less interruption due to light and ballast replacement