Experimental Study On Ultrasonic Stress Relief Used In SU-8 Photoresist

With the rapid development of MEMS, UV-LIGA technology based on SU-8 photoresist has been applied widely. SU-8 photoresist has become the initial choice in manufacture of high-aspect-ratio microstructure. However, the aspect ratio and dimension precision of microstructure is largely limited by the well-know internal stress in SU-8 film. The ultrasonic stress relief technology is an important technique of stress relief. By this technique, the residual stress of metal could be relieved, and the viscosity of polymer could be decreased by affecting the movement of molecule chain. The purpose of this paper is to reduce the internal stress by ultrasonic stress relief technology, and to research on the method of stress relief using ultrasonic stress relief technology.In this paper, the principle of vibration stress relief (VSR) used in residual stress relief and ultrasonic vibration used in polymer development is introduced. According to the polymer structure and its mechanical characterization, the principle of stress relief in SU-8 using ultrasonic stress relief is discussed.The type of ultrasonic vibration equipment is chosen, and the work table is designed, which could be fixed on with different kinds of clamps. Using ANSYS, the natural frequencies of the sample are calculated. According to this, the output frequency of the equipment is chosen.Based on the wafer curvature measurement method, the profile measurement model is adopted. Points pick-up method is advanced to decrease the error by uneven resist in measurement. Using Matlab, a profile of substrate is obtained. The experiment results show that the points pick-up method is an effective way to decrease the error in measurement.Two different kinds of clamps are used in ultrasonic stress relief experiment. The results show that the internal stress in SU-8 film could be relived by ultrasonic stress relief by using the ring clamp, and the internal stress could be increased by using the table clamp. The reason for this phenomenon is discussed. The time of ultrasonic stress relief is optimized by using the ring clamp.