| Quasi-LIGA technology draws a high attention in the MEMS fabrication field due to its simplicity and convenience as well as its low-cost. In this thesis, the Nd:YAG double-frequency with 532nm wavelength and triple-frequency with 355nm wavelength laser pulses were used to expose the SU-8 resist and explore a new Laser-LIGA technology by which the microstructure is of high aspect ratio and the exposure dose is exactly controllable based on the consideration of excellent directivity and controllability of laser pulse. The main contents in this thesis involve the photochemical processes occurred in exposed SU-8 resist by both 532nm and 355nm wavelength lasers, technologic process of the exposure lithography, and technical approaches of enhancing the aspect ratio and controlling the exposure dose exactly.1. The samples of SU-8 resist exposed by 532nm laser pulse were analyzed by X-ray photoelectron spectroscopy (XPS). The results showed that the molecule structure has no any changes and the percentage of the F element in the composition has no any evidence of decrease, which indicates that there is no generation process of the acid-activator as the 532nm photon excites the photo-acid generator. In other words, the 532nm photon couldn't induce the isolated molecules to interlink as meshy structure by the epoxy rings of other molecules. Therefore, the 532nm laser should not be used as the exposure source of SU-8 resist to fabricate microstructure, which is further validated in the experiment.2. In the experiment of 355nm laser exposing SU-8 resist, the XPS analysis showed that, the percentage of the F element in the composition decreases largely which maybe mean that the acid-activator HSbF2 is formed and the center peaks of C and O elements in the composition shift hardly which reveals that the main molecular valence states have no change or other chemical processes don't occur except for the acid-activator reaction, when the 355nm photon excites the photo-acid generator. To validate this conclusion further, the Fourier transform infrared (FT-IR) spectroscopy was used to analyze the same exposed sample. The results showed that the intensities of absorbed peaks located at 914 cm-1 and 862 cm-1 decrease evidently for the epoxy ring in SU-8 molecule, the absorbed intensity peak located at 972 cm-1 attributed to the aromatic C-H in-plane bends is also decline. These imply that the main chemical change is focused on the epoxy ring structures in SU-8 molecule and the epoxy rings in the isolated molecules are disassembled according to the absorbed intensity at its characteristic peak decrease after exposure. The evolvement process in chemical can be distinctly described that the epoxy rings of the isolated molecules are firstly disassembled and then interlinked with the contiguous molecules, and finally, a reticular structure connected by lots of molecules is formed under the action of the acid-activator. Therefore, fabricating microstructure is feasible by using 355nm laser exposure SU-8 resist in mechanism.3. Based on the analysis of mechanism, the technologic process of lithography by 355nm pulse laser exposure SU-8 resist is investigated, which refers to the relations of viscosity coefficient and resist thickness with developing temperature and time, exposure dose with lithography depth, respectively. The result suggested that the lithography depth increases with the single pulse energy fluence non-linearly and with the pulse number linearly. Therefore, the technical approach we proposed is identified in experimental that the exposure dose can be exactly controlled by optimizing the ultrashort pulse number and single pulse energy fluence. Finally, a lithography sample of micro-structure pattern with aspect ratio up to 15 and cliffy profile was obtained by 355nm pulse laser exposing SU-8 resist. |
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