Simulation And Experimental Study On Dimensional Errors Of Microelectroformed Structures

SU-8 resist has become a prevalent mould material to electroform MEMS devices because of its mechanical and chemical stability and its ability to produce high-aspect-ratio moulds by UV-LIGA technology. However, electroforming processes based on SU-8 mould still face serious challenges. Thermal expansion and hygroscopic swelling of SU-8 moulds during electroforming process bring serious dimensional errors to final electroformed structures. The low and difficultly controlled accuracy of electroformed device restrict its performance. For improving the controllability and accuracy of electroformed structures, how to quantitatively forecast and reduce the distortions of SU-8 mould is one of the difficult problems urgently to be solved in MEMS. The paper is an exploratory study on the serious and uncontrollable dimensional errors induced by SU-8 swelling.In the paper, the thermal expansion of the SU-8 mould is calculated by ANSYS firstly, and the lower thermal expansion value indicates that hygroscopic swelling plays a leading role in SU-8 mould distortions. Based on SU-8 hygroscopic swelling mechanism, the kinetics model for photoresist swelling, the swelling model for the freestanding planate film and the finite element model for the coupled field between material diffusing and structural deformation are obtained. And then, the SU-8 swelling parameters in the kinetics are measured by experiment. Furthermore, the hygroscopic swelling is calculated by the similar heat conduction module of ANSYS. By this method, the SU-8 moulds swelling in immersing and in electroforming conditions are analyzed respectively.The paper presents an original technique to reduce SU-8 hygroscopic swelling by ultrasonic treatment. Electroforming is performed based on the ultrasonic treated SU-8 mould to produce micro metal structure. The effect of ultrasonic on electroformed structure dimension is studied, and the results indicate that the dimensional errors of the electroformed structure fabricated on the ultrasonic treated mould are 50% lower than the one without ultrasonic treatment. So, the feasibility of reducing SU-8 swelling and improving electroformed structure accuracy by ultrasonic is confirmed. The water contact angle of cured SU-8 is also measured, and the results show that ultrasonic treatment increased the water contact angle from 70.8°to 74.9°,which indicates that the hydrophilicity of SU-8 decreases since exposed to ultrasonic. Based on these results, the mechanism of ultrasonic effect on hygroscopic swelling is proposed from the view of ultrasonic vibration decreasing the number of hydroxyl groups in SU-8. The research presents a novel method to improve the precisions of electroformed structures. It has no influence on the internal stresses of final structures and does not increase the complexities of mask layouts.