High-aspect-ratio photolithography for MEMS applications

High-aspect-ratio photolithography using a commercially available positive photoresist and a conventional contact mask aligner with standard UV light source is described. A multiple coating process is developed to obtain a photoresist thickness up to 23 μm while maintaining a smooth photoresist surface. Intimate contact between the mask and wafer is found to be most critical for high-resolution photolithography. Vacuum contact is found to be well-suited for this purpose. Additionally, edge bead removal is found to be of significant importance for intimate contact between the mask and the substrate. Prebake, exposure, and development conditions are optimized for resolution and aspect ratio. Maximum prebake temperature still allowing the photoresist to be developed is found to be the optimal temperature for obtaining high resolution. Prebake time distribution is optimized for avoiding residual stress in the photoresist, as well as maintaining high resolution, when multiple coating is applied. Minimum linewidth and spacing of 3.5 μm and 2.5 μm, respectively, and a maximum aspect ratio of 7.7 have been achieved in a photoresist thickness of 23 μm. Postbake improves the chemical resistance to subsequent processes, for example, electroless nickel plating using the photoresist as a mold. However, postbake also causes pattern distortion, which can be severe at times. Therefore, optimal process and design conditions for minimizing the pattern distortion have been studied