During storage, red azo-dyes develop due to a thermal chemical reaction of the light-sensitive component with the novolac, causing a darkening of the resist. Even small amounts of the dye lead to darkening, which however has no significant effect on general resist properties.
Photoresists which were stored for several years are too old and may only be used with considerable limitations. This also applies to resists stored at high temperatures and to highly diluted resists which age faster than normal. Possible effects are the formation of particles due to a precipitation of the light-sensitive component. Filtration with a pore size of 0.2 µm may be helpful at an early stage. After repeated filtrations however, the steadily declining concentration of the light-sensitive component causes lower development rates, an increased dark field loss, as well as reduced adherence of the resist.
If resists are stored for prolonged times at higher temperatures against all recommendations (e.g. during the summer), nitrogen may be cleaved from the light-sensitive component which is indicated by fizzling and foaming after the bottle is opened. In this case, the lid of the bottle should be opened a bit and the bottle left undisturbed for 1 – 2 days until the resist has settled again. If the resist has not been kept for too long under improper storage conditions, it may still be used.