Thick-layer resists for surface micromachining

Abstract

Interest in thick-photoresist applications is steadily growing.In addition to bump fabrication and wire interconnect technology (WIT), theprocess of patterning thick-layer photoresists by UV lithography is speciallyqualified for applications in microelectromechanical systems (MEMS).Specialized equipment and new photoresists have been developed or are underdevelopment to cope with the new challenges in the field of preparingextremely thick photoresist layers, the process of patterning these thickresists, and to deal with the difficulties of the following galvanoplatingstep.As one of the most critical steps in thick-photoresist processing, the bakingprocedure was investigated. Positive tone photoresists (AZ 4562, ma-P 100)were processed by means of three different baking methods: air-forced oven,ramped hotplate, and IR radiation.It could be shown that IR baking is advantageous compared to the other methodswith respect to process duration and energy consumption. As for edgesteepness, resolution, edge loss, and surface roughness, all methods delivernearly the same results. A minimum width of 2-3 µm for the resistbars was found to be necessary to withstand the fabrication process of linesand spaces in about 15 µm thick resists. For thicker layers, highaspect ratios of about 10 as well as steep edges of more than 88° couldbe fabricated.The development of SU-8, a chemically amplified negative tone photoresist forthe 300-450 nm region opened totally new dimensions for the UV depthlithography. Even under development, SU-8 delivers results otherwise onlyachievable by x-ray lithography.The deposition of photoresist on highly-structured surfaces demands advancedmethods. Electrodeposition of resist is one solution. PEPR 2400 was used forpatterning by UV light in order to generate resist patterns around a freestanding silicon bar. The achieved resist patterns were moulded by usingelectroplating. For microsystem applications some metals and alloys weredeposited. Three-dimensional micro components were fabricated as demonstratorsfor the new technique. Electrodeposition allows the use of materials withinteresting properties which could not be provided by standard processes inmicroelectronics.