Mechanism And Experimental Study On Changing SU-8Photoresist Swelling By Ultrasonic Treatment

In UV-LIGA technology, SU-8photoresist has been widely used in the fabrication process of metal micro devices. SU-8photoresist is a negative tone, epoxy-based, near-UV resist, and has excellent mechanical and chemical corrosion resistance capacity. However, due to the swelling behavior of SU-8resist mould, the structure size and graphics of micro devices are seriously deviated from the expected designed value, the performance of micro devices is restricted, and the fabrication of high-aspect-ratio metal microstructure is also limited. The purpose of this paper is to change the swelling property of cross-linked SU-8photoresist in UV-LIGA technology by ultrasonic treatment, to obtain a novel method of improving the dimensional precision of electroformed micro devices, and further to explore the mechanism of ultrasonic treatment on changing SU-8swelling.The mechanism of SU-8swelling in UV-LIGA process is discussed according to polymer swelling theory and molecular structure of cross-linked SU-8photoresist. Based on volume method used for measuring polymer swelling degree, a calculation model for SU-8swelling ratio is proposed. To quantitative study the effect of ultrasonic treatment on SU-8swelling, a calculation method for SU-8swelling removal ratio is provided.In experiments, the effect of ultrasonic treatment on SU-8swelling during the development and electroforming process are respectively investigated, the surface hydrophilicity of SU-8photoresist in different ultrasonic time is analyzed, and SU-8swelling removal ratio in different ultrasonic time is calculated. In addition, the effect of ultrasonic treatment on dimensional precision of different depth-to-width ratio micro devices is discussed. The experimental results indicate that the SU-8mould swelling in development process is not obvious, and ultrasonic treatment has little effect on SU-8mould swelling during development process. The effect of ultrasonic treatment on SU-8mould swelling mainly occurs in the electroforming process. SU-8swelling and its surface hydrophilicity both first decrease and then increase with increasing the ultrasonic time. By10min ultrasonic treatment, SU-8swelling is greatly reduced, SU-8swelling removal ratio a is up to70%, and the dimensional error of electroformed micro devices has nothing to do with structure depth-to-width ratio.In this paper, based on the ultrasonic mechanical scission of polymer chain and water absorbing mechanism, the mechanism of ultrasonic treatment on changing SU-8swelling is explained from SU-8hydrophilicity and internal stress. The presented method to reduce the swelling of SU-8resist mould and further to improve the dimensional precision of electroformed micro devices, does not depend on the process parameters and not increase the complexities of mask layouts, thus it is a practical new method.