Plasmonic lithography may become a mainstream nano-fabrication technique in the future. Experimental results show that feature size with 22 nm resolution can be achieved by plasmonic lithography [1]. In Pan's experiment, a plasmonic lens is used to focus the laser energy with resolution much higher than the diffraction limit and thereby create features in the thermally sensitive material layer. The energy transport mechanisms are still not fully understood in the plasmonic lithography process. In order to predict the lithography resolution and explore the energy transport mechanisms involved in the process, customized electromagnetic wave and heat transfer models were developed in COMSOL. Parametric studies on both operating parameters and material properties were performed to optimize the lithography process. The parametric studies showed that the lithography process can be improved by either reducing the thickness of the phase change material layer or using a material with smaller real refractive index for that layer.
Moreover, a phase field model was also developed in COMSOL to investigate the phase separation mechanism involved in creating features in plasmonic lithography. By including the effect of bond energy in this model, phase separation was obtained from the phase field model under isothermal conditions with speed much faster than the classical diffusion theory can predict. Mathematical transformation was applied to the phase field model, which was necessary for solving numerical issues to obtain the result of complete phase separation. Under isothermal conditions, the phase field model confirmed the fact that the speed of phase separation is determined by both particle mobility and thermodynamic driving force. The fast phase separation in the phase change material is mainly due to strong thermodynamic driving force from the bond energy.
The phase field model was coupled with a heat transfer model to simulate phase separation under laser pulse heating. In this coupled model, the effect of latent heat is considered when temperature rises from the room temperature to above the melting point of the material. Generally, bond energy causes release of heat during phase separation. This bond energy heat source was also considered in the coupled model. Results from this coupled model show a phase separation region with clear interface between it and the non-phase separated region. Since the phase separation region is removed in the lithography process, this clear interface is related to the high contrast lithography pattern reported from the experiments.
A parametric study was also performed for the coupled phase field and heat transfer model. The parametric study showed that the phase change material average concentration has the most significant effect on the phase separation speed and the size of phase separation region. The parametric study result can also be explained from the concept of particle mobility and thermodynamic driving force.
