Photoresists are light-sensitive, their properties change in the presence of light or elevated temperatures. Resists age during storage and are therefore supplied in light-protected amber glass bottles, stored refrigerated and may only be processed under yellow light (λ > 500 nm). Expiration date and recommended storage temperature is each indicated on respective product label. If these temperatures are maintained, unopened resists are stable until the expiration date (usually 2 years after production), at least however 6 month after the date of sale.
A few photoresists can be used far beyond their expiration date if the unopened bottles are stored below optimum temperature (e.g. at 5 – 8 °C as compared to the recommended temperature of 10 – 18 °C).
A frequent opening of resist bottles causes evaporation of the solvent, thickening of the resist and consequently the generation of thicker films. For resists with a film thickness of 1.4 µm, the loss of only 1 % of the solvent already results in 4 % thicker films which thus require higher exposure doses.
Bottles should under no circumstances be opened immediately after removal from the refrigerator, since air humidity may precipitate on the cold resist. Prior to opening, bottles should be adjusted to room temperature.
During storage, thermal chemical reactions of the photosensitive component with the novolac may lead to the formation of red azo dyes which gradually darken the resist. Already small amounts of the dye darken the resist substantially, which however does not seriously affect other resist properties.
Resists stored for several years are outdated and may only be used with considerable restrictions. This also applies to resists stored at too high temperatures or highly diluted resists which age much faster than normal. One consequence is for example a certain particle formation due to precipitation of the photosensitive component. A filtration through 0.2 µm filters may fix this problem only in early stages.
In the long run, photosensitive components are increasingly precipitated from the resist which causes in addition to lower development rates also higher dark erosion and reduced adhesion properties. If resists are stored contrary to the recommendation for prolonged times at high temperatures (e.g. during the summer), nitrogen may be split of from the photosensitive component. Bottles will hiss and foam when opened. In this case, the bottle should be left for 1-2 days with slightly opened lid until the resist calms down. If the incorrect storage did not last too long, the resist may still be used thereafter.