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| back-end.md [2026/05/06 17:07] – [Processes] yusufabdillah | back-end.md [2026/05/06 17:29] (current) – [Processes] yusufabdillah |
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| #### 2.2. Polyimide Spin coating | #### 2.2. Polyimide Spin coating |
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| Polyimide is a common dielectric material used in wafer bumping because it provides electrical insulation, with mechanical stress relief, and protection between the redistribution layer and the bump structure. Spin coating works by dispensing liquid polyimide precursor onto the wafer surface (RDL) followed by centrifugal force spreads the film. This dielectric layer also helps to relieve stress between silicon and the underbump metallization (UBM). A good polyimide coating needs to be uniform across the wafer, because thickness variation can cause problems via depth, stress, warpage, and bump height. | Polyimide spin coating refers to the method by which a homogeneous insulating or passivating layer of polyimide can be applied on the wafer. In this step, a small quantity of the liquid form of polyimide resin is dispensed at the center of a spinning silicon wafer. Under centrifugal action, the resin gets distributed towards the edges, where the speed of rotation, viscosity, and temperature dictate the thickness of the deposited film. At this point, there is usually formation of an edge bead on the wafer edges as a result of solvent evaporation, which then removed using a solvent spray (EBR). Polyimides are preferred in bumping because of their flexibility, heat resistance, and stress-buffer capabilities [([[https://www.sciencedirect.com/book/edited-volume/9780128177860/handbook-of-silicon-based-mems-materials-and-technologies|Handbook of silicon based MEMS materials and technologies]], Tilli, Markku, et al., eds. Elsevier, 2020)]. |
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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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