Influence of resist baking on the pattern quality of thick photoresist
Besides bump fabrication and wire interconnect technology (WIT), the process of patterning thick layer photoresists by UV lithography is specially qualified for applications in microelectromechanical systems (MEMS). Specialised 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 photoresist processing, the baking procedure was investigated. Two positive tone photoresists (AZ 4562, ma-PV100) were processed by means of three different baking methods: air-forced oven, ramped hotplate, and IR radiation baking. Furthermore, combinations of the methods were tested. It was shown that IR baking is advantageous compared to the other methods with respect to process duration and energy consumption. When compared by edge s teepness, resolution, edge loss, and surface roughness, all methods deliver nearly the same results. A minimum width of 2-3 mu m for the resist bars was found to be necessary to withstand the fabrication process of lines and spaces in 15 mu m 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 moulded using electroplating.