Investigation On The Improvement In Interface Adhesion Strength Between SU-8Photoresist And Metal Substrate By Ultrasonic

With the development of MEMS technology, the demand for metal micro devices is gradually increasing. UV-LIGA technology based on SU-8photoresist is one of the effective methods to fabricate metal micro devices. During the fabrication process of metal micro devices by SU-8UV-LIGA technology, the direct application of metal substrate is considered for its advantages of fewer procedures, short electro forming time and nonperishable substrate. However, during SU-8lithographic process, poor adhesion strength between SU-8photoresist and metal substrate makes it difficult to bind, which contributes to the debonding of the SU-8photoresist from the substrate or even the failure of lithography. In this paper, the poor adhesion strength problem between SU-8photoresist and metal substrate was investigated. The interface adhesion strength was quantitatively evaluated and the method to improve the interface adhesion strength was explored. This research is of great significance to improve the product yield and the stability for the micro device fabrication.In this paper, the quantitative measurement methods of the interface adhesion strength between SU-8photoresist and metal substrate were established. Several evaluation methods on interface adhesion property were evaluated. The interface adhesion strengths between SU-8photoresist and metal substrate were measured by pull-off test, indentation test and scratch test. The theoretical models of indentation test and scratch test were established. The results showed that the pull-off test and the indentation test are not applicable to measure the practical interface adhesion strength between SU-8photoresist and metal substrate, while the scratch test is adapted to measure the interface adhesion strength. According to the chemical and physical characters of polymer and the property of SU-8photoresist, the causes of the internal stress in the film were analyzed. The measurement method and the corresponding calculation model adapted to the internal stress in SU-8photoresist film were established.Considering the adhesion property between SU-8photoresist and metal substrate is sensitive to the soft bake parameters, molecular dynamics simulation was performed to investigate the effect and mechanism of soft bake temperature and time on the interface adhesion strength between SU-8photoresist and Ni substrate in this paper. The interface between SU-8photoresist and Ni substrate was constructed. The molecular dynamics simulation was performed on it and then the effects of the soft bake parameters on the interface strength were obtained. The results show that the interface adhesion strength firstly increases and then decreases with the increase of the soft bake temperature, and decreases with the increase of the soft bake time. The effect mechanism was analyzed considering the concentration of cyclopentanone near the interface, the interfacial intermolecular interaction energy in C5H8O/Ni system and in SU-8/Ni system, the effect of the temperature on the solubility of SU-8solute in solvent, the interfacial molecules distance in different temperatures and the interfacial molecule configuration of cyclopentanone.The effect and mechanism of the ultrasonic treatment on the internal stress in SU-8photoresist was investigated. In this paper, the mechanical properties of cross-linked element were described by spring mass system. The effects of ultrasonic time and ultrasonic power on the internal stress of SU-8photoresist were investigated by using a self-made ultrasonic device. The experimental results show that when the ultrasonic time is less than10min, the internal stress gradually decreases, while when the ultrasonic time is longer than10min, the internal stress increases. In addition, under the range of0-144W, the internal stress increases with the ultrasonic power. Besides, the effect of the ultrasonic treatment on the SU-8photoresist micro structure was investigated. The results show that the internal stress of the SU-8photoresist micro structure was reduced with the ultrasonic treatment, which improved the adhesion strength between the film and the substrate.Based on the investigation of the internal stress treated by ultrasonic, a novel method to improve the interface adhesion between SU-8photoresist and metal substrate by ultrasonic treatment was proposed and verified by experiments. Due to the input of ultrasonic vibration into the film, the internal stress of SU-8photoresist was reduced and the interface adhesion strength was improved. The interface adhesion strength firstly increases and then decreases with the increase of ultrasonic time. The interface adhesion work is0.427J/m2at an optimized time of10min, which is higher than that of the sample without ultrasonic treatment by39.5%. In addition, under the experiment conditions, the interface adhesion strength firstly increases and then decreases with the ultrasonic power.