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
The hardbake sometimes performed after development intends to increase the thermal, chemical, and physical stability of developed resist structures for subsequent processes (e.g. electroplating, wet-chemical and dry-chemical etching). Hereby the following mechanisms have to be considered:

Coated photo resists react with atmospheric oxygen and embrittle above approx. 130°C. The different thermal expansion coefficient of resist/substrate can lead to the cracking of the resist making it useless as mask for wet or drying-chemical etching or electroplating. If the hardbake cannot be waived, nor the hardbake temperature reduced, the cracking can be suppressed by a slow cooling (e.g. -3°C/min ramp, or by keeping the substrate in/on the switched-off oven/hotplate for soft cooling, if feasible).

The resist structures start roundening/flowing above their softening point (110-130°C for most positive tone and image reversal AZ® resists). In this case thermally more stable resists (e.g. AZ® 66xx positive resist, AZ® 5214E, TI 35ES image reversal resists in image reversal mode, or AZ® nLOF 2000 negative resist) may be a good alternative.

During high (> 130°C) temperatures, the thermal cross-linking of the resist matrix (Novolak) increases the alkaline stability of the resist (as desired). However, at hardbake temperatures < 130-140°C the thermally induced decay of the photo active compound (=inhibitor for alkaline solubility) causes a decrease in the alkaline stability.

The intrinsic high alkaline stability of AZ® and TI resists, together with an optimum substrate pretreatment (e.g. adhesion promoter TI PRIME), in many cases make the hardbake redundant, which simplifies the processing and arranges following wet-chemical processes more reproducible. For harsh attack (e.g. mesa etching with HNO3), however, a hardbake at 130-140°C is sometimes inevitable.
e-Mail Anfrage Product request: AZ 66xx series Weiterführendes Dokument (pdf) Literature: Baking of Resist Films e-Mail Anfrage Product request: AZ nLOF 2000 series e-Mail Anfrage Product request: AZ TI PRIME adhesion promoter  Optimum Substrate Pre-Treatment
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