Application of optical lithography for high aspect ratio microstructures

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Interest in thick photoresist applications is steadily growing. Besides the bump fabrication and wire interconnect technology, the process of patterning thick layer photoresists by UV lithography is specially qualified for applications in micro electro mechanical systems. Specialized equipment and new photoresists have been developed or are under development to cope with the new challenges in the field of preparing extremely thick photoresist layers, to plan the process of patterning these thick resists, and to deal with the difficulties of the following galvanoplating step. A technology called three-dimensional (3D) UV-microforming was developed, consisting of a resist preparation process for very thick photoresists (positive or negative tone), UV lithographic steps, resist development, moulding procedures by galvanodeposition, and finally stripping and cleaning for finishing the structures. A minimum width of 3 μm for the resist bars was found to be necessary to withstand the fabrication process of lines and spaces in 15 μm thick resist. For thicker layers high aspect ratios of more than 10 as well as steep edges of more than 88° could be fabricated. The resist patterns were moulded by using electroplating. For microsystem applications metals and alloys can be deposited into the resist patterns. 3D UV-microforming, a combination of UV patterning of thick layer resists and galvanoplating, enables the low-cost fabrication of a wide variety of micro components for many different users.