Ultrasonic Stress Relief Mechanism And Experimental Study Of Polymer Photoresist

Polymer SU-8 photoresist is an epoxy-based, near-UV, negative tone resist. It has excellent physical, mechanical and chemical corrosion resistance capacity, and thus has become an ideal photoresist material for the fabrication of high aspect ratio structure in MEMS. However, the MEMS fabrication of using SU-8 is largely limited by its well-known high internal stress, which affects the overall pattern quality of the structures. Large internal stress can result in cracks in SU-8 layer and poor adhesion to substrate. These phenomena are aggravated as the film goes thicker. High internal stress has become one of the most primary problems and restricts structure stability and higher aspect ratio of SU-8 resist. The purpose of this paper is to reduce the internal stress in cured SU-8 photoresist layer by ultrasonic stress relief technology, and the process parameters of ultrasonic stress relief are optimized.The vibration stress relief mechanism of metal is presented. According to physical and chemical properties of polymer and characteristic of SU-8 photoresist, the occurrence reason of SU-8 internal stress is obtained. In terms of polymer structure and its mechanical characterization, the ultrasonic stress relief mechanism of SU-8 photoresist layer has been discussed.Based on the improved Stoney's formula, a theoretical calculation model for SU-8 internal stress is proposed. The profile method is corrected by reducing uneven thickness of SU-8 layer. The verification experiments are performed, and the surface profile curve of coated Si wafer is fitted by Matlab. The experiment results show that the corrected profile method is effective.The effect of ultrasonic stress relief on internal stress of SU-8 layer is studied by experiments with self-developed ultrasonic stress relief equipment and clamping apparatuses. The quantitative analysis experiments and SU-8 micro structure fabrication experiments show that the internal stress in SU-8 layer can be reduced effectively by ultrasonic stress relief technology.The process parameters of ultrasonic stress relief technology are optimized in terms of SU-8 stress relief rate. Three factors and three levels orthogonal experiment is designed to measure and calculate internal stress relief rate of SU-8 photoresist with 9 groups of different ultrasonic stress relief process parameters. The fuzzy neural network is trained by orthogonal experiment datum, and the prediction model is built between internal stress relief rate and main process parameters:position, power input and relief time. Meanwhile, the network prediction results are consistent with the experimental results. The internal stress in cured SU-8 photoresist layer can be relieved by ultrasonic stress relief with optimum process parameters.