| 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. |
Product request: AZ 66xx series
Literature: Baking of Resist Films
Optimum Substrate Pre-Treatment