2. For how long are e-beam resists stable, and what are the optimal storage conditions?
PMMA- and copolymer e-beam resists are not light-sensitive in the visible UV range, they consequently don’t react to light exposure (no safe yellow light required) and are substantially less temperaturesensitive than novolak-based resists. These resists thus age only slowly. Typical age-related changes are a slow thickening of the resist, which however has no negative impact on the quality. In this case, the films just tend do be slightly thicker which can easily be corrected during the coating procedure.
In contrast, novolak-based e-beam resists are as light-sensitive as photoresists. They react to light- and temperature exposure and age much faster during storage. The resists are therefore provided in lightprotected amber glass bottles which should be stored at low temperatures and may only be processed under yellow safe light (λ > 500 nm).
Date of expiry and recommended storage temperature are indicated on the product label of each ebeam resist. Following these recommended storage guidelines, resists are stable until expiry date, at least however for 6 month after date of sale. Repeated opening of resist bottles leads to evaporation of the solvent, causing an increased viscosity of the resist 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 3 – 5 % and consequently also the expose dose.
Examples of our customers and own long-term studies demonstrate that PMMA- and copolymer e-beam resists have already safely been used for up to 2 years without quality loss.
→ 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.