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| back-end.md [2026/05/06 17:19] – [Processes] yusufabdillah | back-end.md [2026/05/06 17:29] (current) – [Processes] yusufabdillah |
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| #### 2.3. Polyimide Photolithography | #### 2.3. Polyimide Photolithography |
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| When a polyimide is photoactive, it can be patterned directly with photolithography to form vias and openings through the dielectric material. If the polyimide is not photoactive, then a separate photoresist mask will be used to pattern the polyimide after it is coated and pre-baked. The goal is to expose parts of the seed layer or pad underneath for bump formation while leaving the rest of the wafer insulated and providing mechanical support. Proper alignment of the openings is critical because they must match the pad or redistribution features with good overlay, especially for fine-pitch wafers. | The photolithography technique involves defining locations on the polyimide film where openings are going to be etched. The wafer, which by now has been coated with a photosensitive polyimide film, is accurately positioned beneath a photomask bearing the layout using specialized equipment referred to as a stepper. An ultraviolet (UV) beam of light falls on the mask causing chemical modifications in the exposed parts of the polyimide film. Since polyimide often functions as a negative photoresist, the exposed regions become cross-linked and insoluble, while the unexposed areas remain dissolvable. Important factors for this stage include resolution, sensitivity, and alignment; otherwise, one might have lithography "hotspots," which will destroy any chance of establishing connections [([[https://link.springer.com/referencework/10.1007/978-981-99-2836-1|Handbook of Integrated Circuit Industry]], Wang, Yangyuan, et al., eds. Springer Nature, 2023)]. |
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