Study of i-Line Photosensitive Materials with a Wide Depth of Focus for Fine Pitch Redistribution Layers

In this study, we investigated the design of photosensitive materials with a wide depth of focus (DOF) for use in fine pitch redistribution layers. First, we developed a photosensitive material having a DOF exceeding 4.0 µm with 0.8-µm line/space (L/S) patterns and that having a DOF of 4.0 µm with $\mathrm{0}.\mathrm{7}-\mu \mathrm{m}$ L/S patterns, at a numerical aperture (NA) of 0.5. Second, we simulated the DOF to match the experimental DOF using lithography simulation software (PROLITH). Subsequently, we simulated the DOF at an NA of 0.24 and obtained a $\mathrm{10}.\mathrm{8}-\mu \mathrm{m}$ DOF with $\mathrm{0}.\mathrm{8}-\mu \mathrm{m}$ L/S patterns and a $\mathrm{6}.\mathrm{8}-\mu \mathrm{m}$ DOF with $\mathrm{0}.\mathrm{7}-\mu \mathrm{m}$ L/S patterns of the photosensitive material. For a detailed insight of the performance at the NA of 0.24, we conducted an aerial image simulation; we also simulated the impact of three dissolution parameters-$-\mathrm{R}_{\text{max}},\ \mathrm{R}_{\text{min}}$, and development time—on the DOF. For the $\mathrm{0}.\mathrm{7}-\mu \mathrm{m}$ L/S patterns, we found that precise $\mathrm{R}_{\text{min}}$ control is essential for a wide DOF. In fact, for an $\mathrm{R}_{\text{max}}$ ranging from 150 to 1000 nm/s, a precise $\mathrm{R}_{\text{min}}$ control of less than 0.4 nm/s is required for a $\mathrm{7}.\mathrm{0}-\mu \mathrm{m}$ DOF.