Mechanism analysis of ultrasonic treatment on SU-8 swelling in UV-LIGA technology

In SU-8 UV-LIGA technology, the precision and controllability of electroformed metal structure are not satisfied because of SU-8 photoresist swelling during electroforming process. To reduce SU-8 swelling and further improve the dimensional precision of electroformed microstructure, the authors originally introduce ultrasonic treatment in the fabrication process of SU-8 mould. The experimental results indicate that the swelling property of cross-linked cured SU-8 varies with ultrasonic time. SU-8 swelling is diminished when it is treated less than 15 min by ultrasonic, but the swelling is increased if the ultrasonic time is extended to 20 min. To explore ultrasonic treatment mechanism of SU-8 swelling, the contact angle between SU-8 and electroforming solutions in different ultrasonic time was measured, and the result presents that SU-8 surface hydrophilicity is affected by ultrasonic treatment. Besides, the contact angle trend is consistent with the curve of SU-8 swelling removal ratio in different ultrasonic time. Subsequently, ultrasonic treatment mechanism of SU-8 swelling is proposed based on ultrasonic mechanical scission of polymer chain and moisture diffusion mechanism.