The team from the Department of Mechanical Engineering has made important progress in the field of micro-nano manufacturing based on colloidal self-assembly
Tsinghua News Network, October 18th: Recently, the Department of Mechanical Engineering of Tsinghua University has made important progress in the field of micro-nano manufacturing based on colloidal self-assembly.
Microsphere lithography (SLPL) technology is based on traditional lithography, using self-assembled microsphere films instead of masks, and utilizing the optical effects of microspheres to achieve sub-wavelength scale features. Compared with the traditional mask lithography method, it has the advantages of high cost-effectiveness and easy implementation, and has gained more and more attention due to its advantages of low cost, high throughput, easy implementation and high precision. However, microsphere lithography is mainly used to produce simple pattern shapes, such as circular arrays. Although there have been many reports in the literature that complex patterns can be formed with a single exposure, there has been a long-term lack of systematic theoretical analysis, especially predictive analysis of the achievability of the various non-circular shapes that can be achieved. At the same time, numerical simulation of microsphere lithography technology suffers from the problems of numerous conditions and difficult analysis, making it difficult to draw instructive conclusions directly from the simulation results.
In response to the above problems, the research team based on the tension gradient-induced rapid large-area self-assembly method of nanoparticles at the liquid-air interface proposed in the previous study, using a densely packed and ordered colloidal particle self-assembly structure as a template to replace the sophisticated and complex traditional nanolithography The method successfully prepared non-circular pattern shapes of rings and holes in the rings, and applied Mie theory to systematically analyze and summarize the various patterns that can be obtained through the microsphere lithography process. The theory is It is beneficial to use relatively intuitive analytical solutions to calculate, predict and optimize the results of the microsphere lithography process, and analyze the physical laws of various influencing factors. The pattern shape obtained by microsphere lithography is mainly controlled by two variables, namely normalized diameter (ratio of microsphere diameter to exposure wavelength ) and normalized refractive index (ratio of microsphere refractive index to medium refractive index ) . Most non-circular patterns obtained in previous literature can be directly predicted by this simplified model.
After the plane light passes through the microsphere, the light intensity distribution on the section tangent to the microsphere can be mainly divided into three areas, including the peak area, the middle valley area and the outer platform area. Among them, the peak region has the ability to form sub-wavelength microstructures and is the most noteworthy part that is most likely to achieve nanoscale processing. To obtain diverse shapes in this region, a lower normalized refractive index is the most critical factor. When the normalized diameter increases, the number of peaks in the central peak area also increases. However, excessive size will lead to large errors between theory and experiment and bring difficulties in the control of exposure parameters. It is usually difficult to obtain multiple concentric structures.
Figure 2. Part of the simulation and experimental results
Based on this theory, the research team obtained single-ring and hole-ring structures through experiments, expanding the application range of microsphere lithography technology. Using a simplified model of Mie scattering theory, various possible patterns were comprehensively sorted out and predicted, further improving the theoretical basis of microsphere lithography technology. These findings could help promote broader applications of microsphere lithography in future scientific research and industry.