2. For how long are photoresists stable, and what are the optimal storage conditions?
Photoresists are light-sensitive, they are affected by light exposure and high temperatures, and agerelated changes occur during storage. Resists are therefore filled in light-protected amber glass bottles, stored in a cool place, and may only be processed under yellow safe light (λ>500nm). Date of expiry and recommended storage temperature are indicated on the product label of each bottle. Following these recommended storage guidelines, resists are stable until expiry date, at least however for 6 month after date of sale.
◆ Some resists with recommended storage temperatures between 10~18 °C have an optimum storage temperature of 5~8 °C. In this case, resists may be used even after their expiry date. Examples of our customer demonstrate that resists can be safely used for up to 2 years in this case.
A frequent opening of resist bottles leads to evaporation of the solvent and thus an increased resist viscosity which results in thicker films upon usage. For a resist with a film thickness of 1.4 μm, evaporation of only 1 % of the solvent increases the film thickness by 4 % and consequently also the expose dose required.
◆ Bottles which were stored in the refrigerator should never be opened immediately, since in this case air moisture may precipitate on the cold resist. Resists should be adapted to room temperature before opening.