FAQs Photoresists

1. What are photoresists composed of, and how do they work?

Photoresists (also photo coatings) are primarily used in micro electronics and micro system technologies for the production of µm- and sub-µm structures. These resists are generally deposited by spin coating in a range between 250
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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 age-related changes occur during storage. Resists are therefore filled in light-protected amber glass bottles,
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3. How may age-related changes influence the quality of a photoresist?

During storage, red azo dyes develop due to a thermal chemical reaction of the light-sensitive component with the novolak, causing a darkening of the resist. Even small amounts of the dye lead to darkening,
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4. What is the optimal pre-treatment of substrates for photoresists?

If new and clean substrates (wafers) are used, a bake at approximately 200 °C for several minutes is sufficient for drying, but the substrates have to be processed quickly afterwards.
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5. What are the adhesion features of photoresists on different wafers?

Adhesion between coating and substrates is a very sensitive feature. Smallest changes of the cleaning procedures or the process parameters can have a fatal impact on the adhesive strength.
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6. What are the optimum coating parameters for photoresists in order to achieve good film images

Prior to the coating procedure, resists have to be adjusted to the temperature of the (preferably air-conditioned) working area. If the resist is too cold, air moisture may precipitate on the resist.
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7. Why may air bubbles develop in photoresist films, and how can they be avoided?

Bubbles after spin deposition are in most cases air bubbles, e.g. if the resist bottle was agitated or moved around, or if the resist was diluted prior to the coating step. Coating procedures performed immediately after bottle opening,
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8. What is the function of the softbake of photoresist films after the coating?

Just recently coated resist films still contain a substantial amount of residual solvent, depending on the respective film thickness. The subsequent bake step at 90 – 100 °C is performed in order to dry the resist films,
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9. How are photo resists exposed, and how can the optimum exposure dose be determined?  How long can coated and exposed substrates be stored prior to exposure?

The exposure is performed using masks in suitable exposure systems such as e.g. steppers (i-, g-line), mask aligners or contact exposure systems in the appropriate spectral working range.
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10. Which developers are optimal for photoresist, and how do factors like developer concentration and temperature influence the result?

During development, a positive tone resist film is structured by a removal of exposed areas, while unexposed areas are removed when negative resists are used. To achieve reproducible results,
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