Photoresist Processing Trouble-Shooting Etchants and Solvents
Photoresists Ancillaries Storage/Handling/Ageing Substrate Pre-Treatment and Coating Baking Steps Exposure Development Coating and Lift-off Etching and Stripping
Softbake (Prebake) Post Exposure Bake (PEB) Image Reversal Bake Hardbake
After coating, the resist film has a thickness-dependant remaining solvent concentration (e.g. PGMEA). Softbaking minimizes the solvent concentration in order to:
- avoid mask contamination or sticking to the mask,
- prevent bubbling or foaming by N2 during exposure,
- improve the resist adhesion to the substrate,
- minimize the dark erosion during development,
- prevent dissolving one resist layer by a following during multiple coating, and
- prevent bubbling during subsequent thermal processes (coating, dry etching).

After spincoating, the typical average remaining PGMEA concentration in the resist film is between 20% (thin films) and 40% (thick films). Softbaking reduces the remaining solvent concentration in the resist by thermally activated solvent diffusion in the resist bulk, and evaporation as a function of the temperature and the resist surface solvent concentration. A good starting point for a sufficient softbake is 100°C hotplate for 1 minute/μm resist film thickness.

Insufficiently softbaked resist films reveal a high dark erosion during development (resist thinning), with the remaining resist structures too small and less sharp than desired.

Beside solvent reduction, a long/hot softbake thermally decomposes a part of the photo active compound (DNQ-sulfonate) thus reducing the development rate. Therefore, despite a lower dark erosion, the longer necessary development rate may increase the total dark erosion.

Compared to a hotplate, the much more distinct temperature hysteresis of an oven as well as the different heat transfer mechanism (convection in stead of heat conduction) causes - especially for short (few minutes) - baking steps (softbake, reversal bake, hardbake) or substrates with high heat capacity (thick glasses and ceramics) different effective temperatures in the photo resist and time intervals for the required final temperature.
 Softbake and Development Rate Weiterführendes Dokument (pdf) Literature: Baking of Resist Films Weiterführendes Dokument (pdf) Literature: Thick Resist Processing Weiterführendes Dokument (pdf) Literature: Introducing new Resists/Processes
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