Study Of Method And Mechanism Of Ultrasonic Embossing For Polymer Microstructures

MEMS devices made of polymer materials are low cost, simple process, and suitable to mass production, thus its fabrication techniquesure a hotspot in MEMS research field currently. Aiming to realize high-accuracy and high-efficiency production of polymer MEMS devices, ultrasonic embossing for replicating polymer micro structures has been systematically studied in this paper, including molding mechanism, developing new embossing methods and avoiding defects.The molding mechanism of ultrasonic embossing process was investigated, from both of the two aspects:heating process and filling behavior. The filling behavior during ultrasonic embossing process was investigated by theoretical analysis and finite element simulation. Polymer filling behavior mainly occurs near the substrate-mold interface, the interface temperature is the most significant factor affecting the filling process. The relationship between the interface temperature and the ultrasonic parameters was examined by temperature test experiments and theoretical analysis, especially when the interface temperature is high than Tg. The results show that, ultrasonic amplitude significantly impacts on the interface temperature, and the heating rate increases with enlarging ultrasonic amplitude.Ultrasonic embossing at room temperature (UER) for PMMA sheets was studied. The effects of the processing parameters on replication quality, including accuracy and uniformity, were investigated and discussed. The results revealed that, increasing ultrasonic amplitude, ultrasonic time or holding time can significantly increase the replication accuracy, enhancing ultrasonic force or holding force can obviously improve the replication uniformity. Moreover, the influences of pattern on replication quality were studied experimentally. The results verified that, proximity effect among microstructures significantly influences replication accuracy. Proximity effect has marked influence on friction heating rate, but rarely affects viscoelastic heating rate. Proximity effect becomes weaker and friction heating rate increases with increasing center distance between two microstructures, so increasing the center distance can promote the replication accuracy. On the other hand, proximity effect has obvious impact on filling process. There is a maximum replication height with duty (?) and the convex edge can improve the filling stress of the pattern area.Based on the viewpoint that, polymer filling behavior mainly occurs above Tg and viscoelastic heat is the main heat resource above Tg. thermal-activated viscoelastic heat ultrasonic embossing (VHUE) was proposed to improve the replication uniformity. Both of the replication accuracy and uniformity of VHUE can achieve 98%. meanwhile the circle time is less than 70s. Hot plate temperature, ultrasonic amplitude, ultrasonic time and holding pressure are the significant factors on replication accuracy. The optimized hot plate temperature is 105℃. and increasing ultrasonic amplitude, ultrasonic time or holding time can significantly improve the replication accuracy. Moreover, enhancing ultrasonic force or holding force can obviously improve the replication uniformity. The acting mechanism of proximity effect also can reasonably explain the influence regulation of micro pattern on VHUE replication accuracy. For VHUE. smaller microstructure size than URE is necessary to make proximity effect evident. In addition, concave/convex type, the center distance between two microstructures, and the microstructure width are all important factors on replication accuracy for molding high-density microstructure arrays.Formation mechanism and inhibition methods of bubbles during ultrasonic embossing were researched systematically. Thermogravimetric analysis and thermal degradation bubble experiment verified that, bubbles during ultrasonic embossing are caused by thermal degradation of PMMA. Based on the ideal spherical bubble motion theory, equation of bubble wall motion and the cavitation threshold condition were deduced, which bubbles are full of MMA. The bubble phenomenon derived by the cavitation mechanism was highly consistent with the experiment, proved the cavitation mechanism is correct. According to the cavitation mechanism, increasing static pressure and/or reducing temperature can inhibit bubbles. By increasing the ultrasonic pressure, bubbles in pattern area were successfully avoided during UER. In addition, VHUE chips indicated that lower temperature also can avoid bubbles. Bubble wall motion equation was used to study the influence of temperature, static pressure and ultrasonic amplitude on cavitation intensity. Then. Cavitation intensity was estimated. Cavitation intensity is so low and the influence on polymer filling behavior can be ignored.The formation mechanism of rough non-molding surface during UER was studied, and the corresponding suppression method was proposed. The root cause of rough non-molding surface is that, polymer near non-molding surface was softened and flowed. Based on the heating mechanism of ultrasonic embossing, "Two interfaces of different friction coefficient" was put forward to avoid the roughness by inhibiting generating heat. The method was achieved through attaching commercial surface protection films on non-molding surface, and verification experiments were carried out. The results shown that. Non-molding surface roughness was improved remarkably by attaching surface protection films. Moreover, the process repeatability was advanced by selecting optimized film kind.