<title>Influence of resist-baking on the pattern quality of thick photoresists</title>

Abstract

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 microelectro-mechanical-systems (MEMS). 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, the process of patterning these thick resists, and to deal with the difficulties of the following galvanoplating step. As one of the most critical steps in thick photoresists processing, the baking procedure was investigated. Two positive tone photoresists were processed by means of three different baking methods: air-forced oven, ramped hotplate, and IR radiation. Furthermore, combinations between the methods were tested. It could be shown that IR baking is advantageous compared to the other methods with respect to process duration and energy consumption. Compared to edge steepness, resolution, edge loss, and surface roughness, all methods deliver nearly same results. A minimum width of 2-3 micrometers for the resist bars was found to be necessary to withstand the fabrication process of lines and spaces in about 15 micrometers thick resist. For thicker layers high aspect ratios of more than 10 as well as steep edges of more than 88 degrees could be fabricated. The resist patterns can be molded by using electroplating.