Surface Plasmon Assisted Laser Nanolithography Using Metalic Mask

Traditional photolithography has a resolution at wavelength scale due to optical diffraction. In this paper, a high-density direct photolithography method beyond diffraction limit by utilizing surface plasmons (SPs) was developed on virtually any substrate. Simulation results by Finite Different Time Domain (FDTD) method have shown that surface plasmon excited on both the mask and the substrate helps to confine the light behind the apertures of the mask. Numerical simulations have demonstrated that very high density sub-wavelength patterns can be transferred using this method. In experiments, a polarized laser beam of 355nm wavelength was used as a light source to photo-initiate a 80nm-thick photoresist on a silicon substrate with 50nm Ti coating. 100nm line aperture patterns were made on gold film on quartz substrate as mask. Experimental results showed that illumination intensity control is crucial to the lithography results. The feature size using such method could be further scaled down, limited theoretically by the validity of dielectric function of the material, and practically by the fabrication of mask.