Fabrication of high-aspect-ratio metallic micro-structures by reverse exposure method

Abstract

High-aspect-ratio microstructures (HARMST) are commonly used in the integration ofcomponents to make functional microdevices and HARMST fabrication plays animportant role in the micro-eletromechanical systems (MEMS) industry. Among themethods used to fabricate HARMST, LIGA is one of the best. Although the LIGAprocess is well developed, it is not used widely because it needs expensive X-ray sourcesfor exposing the resist. Another way for fabricating HARMST is to use a thickphotoresist and expose it by UV-lithography. SU-8 resist is the forerunner of commercialhigh-viscous photoresists in high-aspect-ratio applications. This is due to the low opticalabsorption of SU-8 near UV range which results in vertical sidewalls and uniformexposure. UV sensitive characteristics, high viscosity, and high functionality are some ofthe advantages of SU-8. In comparison to LIGA, it has provided the possibility toproduce HARMST at lower cost.This project looks into the use of SU-8 as an electroplating mold to fabricate high-aspectratiometallic microstructures. For the fabrication of SU-8 mold without cracks anddelamination, key parameters affecting SU-8 photolithography process are first studied.These include pre-baking and post-exposure-baking time and temperature, exposure time,and relaxation time after pre-baking and before development. For UV exposure, apreliminary investigation is first conducted on two exposure methods: top-side andreverse-side. The reverse-side method is then chosen as it has several advantages incomparison to the top-side method, such as no UV light reflection as the light passesthrough the substrate, which is a drawback in typical UV lithography. Due to the hardcontact without air gap between photomask and photoresist, the image resolution is highwith no edge bead. Also in the reverse-side method, the UV light exposure window iswider and easier to control than the more commonly used top-side method.After reverse-side exposure, the fabricated SU-8 mold is plated by nickel electroplating.Electroplating parameters have been studied and a pulse-plating method has beenidentified to have a good result for electroplating.Finally, methods of fabrication micro-gear structures have been studied. The aim is tofind a method to separate the metallic micro-gear from the SU-8 mold easily. Sacrificialphotoresist, sacrificial photoresist and transparent resist, PDMS, and sacrificial coppermethods have been investigated for the fabrication of the micro-gear structure. Theconclusion is that using copper as a sacrificial material is more practical than the othermethods. Using this method, a high-aspect-ratio metallic microstructure has beenfabricated in an inexpensive way.