PMMA-, copolymer and styrene acrylate resist are not light-sensitive in the visible UV range; they consequently don’t react to light exposure (no yellow light required) and are substantially less temperature-sensitive than novolac-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, films just tend to be slightly thicker which can easily be corrected during the coating procedure.
In contrast, novolac-based e-beam resists are about as light-sensitive as photoresists. They react to light- and temperature exposure and age much faster during storage. The resists are thus provided in light-protected amber glass bottles which should be stored at low temperatures and only be processed under yellow safe light (λ > 500 nm). Exceptions are experimental samples SX AR-N 7530/1 and SX AR-N 7730/1. Due to their specific acid generators are both resists also not sensitive in the visible UV (> 300 nm) and thus do not require yellow light.
Date of expiry and recommended storage temperature are indicated on the product label of each e-beam 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 an increase of 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.