基于双表面等离子激元吸收的纳米光刻

光刻技术(lithography)是微纳结构制备的关键技术之一.受限于光的衍射极限,传统光刻方法进一步缩小特征尺寸变得越来越难.表面等离子激元(surface plasmon polariton,SPP)作为光与金属表面自由电子密度振荡相互耦合形成的一种特殊电磁形式,具有波长短、场密度大、异常色散等特点,在突破传统光学衍射极限的研究和应用中具有重要的学术和实用价值.本文针对SPP在光刻胶中的非线性吸收及其在大视场纳米光刻中的应用进行了理论和实验探索.在回顾SPP概念的基础上,阐述了双SPP吸收的概念及其应用于纳米光刻的优势,明确了该效应具有与传统双光子吸收不同的内涵和特性.在800和400nm飞秒激光的作用下,实现了基于双SPP吸收效应的周期干涉条纹,同时验证了双SPP吸收的闽值效应,通过控制曝光计量实现了图形线宽的调控,最小线宽小于真空光波长的1/10.利用SPP波长短、场增强的特点,并结合非线性吸收的闽值效应,单次曝光区域比纳米图形尺度大4 5个数量级,曝光区域的直径可达1.6 mm.同时制备出较为复杂的同心圆环结构.基于双SPP吸收独有的特性以及SPP丰富的模式,有望进一步在大光刻视场、超小尺度图形光刻技术上获得突破.

Nanolithography based on two-surface-plasmon-polariton-absorption

Lithography is one of most important technologies for fabricating micro- and nano-structures. Limited by the light diffraction limit, it becomes more and more difficult to reduce the feature size of lithography. Surface plasmon polariton (SPP) is due to the interaction between electromagnetic wave and oscillation of free-electron on metal surface. For the shorter wavelength, higher field intensity and abnormal dispersion relation, the SPP would play an important role in breaking through the diffraction limit and realizing nanolithography. In this paper, we theoretically and experimentally study the optical nonlinear effect of SPP (two-SPP-absorption) in the photoresist and its application of nanolithography with large field. First, the concept and features of two-SPP-absorption are introduced. Like two-photo-absorption, the two-SPP-absorption based lithography is able to realize nanopatterns beyond the diffraction limit: 1) the absorption rate quadratically depends on the light intensity, which can further squeeze the exposure spot; 2) the pronounced power threshold provides a possibility for precisely controlling the linewidth by manipulating the illumination power. Nevertheless, unlike the two-photo-absorption lithography which focuses light onto a single spot and scans point by point, the two-SPP-absorption method could obtain the subwavelength field pattern by simply illuminating the plasmonic mask. The subwavelength field pattern due to the short wavelength of SPP would further result in the overcoming-diffraction-limit resist pattern. Besides, the highly concentrated SPP field leads to the strong electromagnetic field enhancement at the metal-dielectric interface, which could reduce the input power density of exposure source or enlarge the exposure area. Then the two-SPP absorption is realized under the illuminations of femtosecond lasers with vacuum wavelengths of 800 nm and 400 nm. Meanwhile, the interference periodic patternis realized and it is observed that the linewidth could be adjusted by controlling the exposure dose. The minimum linewidth of resist pattern is only one tenth of the vacuum wavelength. By utilizing the features of two-SPP-absorption, namely shorter wavelength, enhanced field and threshold effect, the lithography field could be of millimeter size, which is about four to five orders of magnitude larger than the characteristic size of nanostructure. Therefore, this two-SPP-absorption scheme could be used for large-area plasmonic lithography beyond the diffraction limit with the help of various plasmonic structures and modes.