A new method of manufacturing high aspect ratio structures using SU8 negative photoresist
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1272-KK09-07
A new method of manufacturing high aspect ratio structures using SU8 negative photoresist. S.K.Persheyev, M.J.Rose Amorphous Materials Group, Carnegie Laboratory of Physics, Electronics Engineering and Physics Division, University of Dundee, Scotland, UK
Abstract SU8 Negative photoresist is finding high demand in applications such as MEMS sensors and waveguides. The possibility of photolithographic patterning and high physical and dielectric properties are attracting ever more users among workers in the electronics industry and increasingly in biomedical applications. In our work we employ an original method of exposing of SU8 and create high aspect ratio structures on glass and other substrates. Dry plasma etching results of negative epoxy-based photoresist by Inductively Coupled Plasma system using gases O2 and CF4 are presented.
1.
Introduction
The negative epoxy-based SU8 photoresist, due to its special mechanical properties such as durability, water impermeability and dielectric nature upon polymerisation, is often used as a resin for making high aspect ratio, functional MEMS (Micro-Electro-Mechanical Systems)1-4 device structures and packaging, for cantilevers,5 and “Lab-on-a-chip” 6 systems. SU8 is ideally suited to the fabrication of devices for micro-fluidic devices, 7, 8 and in bio-MEMS,2 due its biocompatibility and chemical resistance. The negative photoresist is structured by UV photolithography3,4,8-10 and structures consisting of multiple layers can be created. The common issues that need to be overcome for conventional through top mask exposure is low adherence for low exposure times and top feature T shape broadening (a phenomenon known as T-topping) for overexposed films. Another limitation is low structure profile flexibility while the same mask is applied.
2. Experimental methods In our experiments a Karl Suss MJB3 mask aligner was used for UV exposure of the SU8 films whilst soft and hard baking of the films was carried out on a standard hot plate at 95oC. Corning 7059 glass was coated with 120nm of chromium by magnetron sputtering and then patterned with a special mask allowing the formation of 3-5 µm round shaped holes in the metal film. After cleaning the samples were then prebaked at 165 oC in order to remove surface water moisture prior to SU8 deposition. The prebaked samples were immediately spin coated with SU8 and soft baked at 95 oC for 2 minutes, then exposed under the mask aligner upside down without any mask (the patterned chromium acting as a mask as shown on fig.1a,b), with just a clear quartz plate to hold samples. UV exposure was followed by hard (95 oC, 5min) baking for cross-linking of the exposed negative photoresist. For the removal of non cross-linked films after exposure Microposit EC Solvent was applied and then isopropanol (IPA) was used for rinsing cleaning and removal of residual photoresist. SEM images were obtained by using a JEOL JSM-7400F Field Emission Scanning Electron Microscope with the samples preliminarily coated with thin layer of Au-Pd met
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