Introduction

Abstract

Progress in DRAM technology requires improved lithography. It is estimated that technology nodes should reach to 16 nm by the year 2019 [1]. Recent improvements in the immersion lithography using an excimer laser (wavelengths of 193 nm and 157 nm) have resulted in the technology nodes as small as 45 nm. Further decreases in the node size are expected using an extreme ultraviolet (EUV) light source with a wavelength of 13. 5 nm. However, the resolution of the linewidth is limited by the diffraction limit of the light. To overcome this limitation, it is necessary to use the optical near-field to exceed the diffraction limit of light. The concept of the optical near-field was proposed [2, 3, 4] as a localized electric field at a metallic aperture (Fig. 1.1). Based on scalar theory [5], when the size of the aperture is much smaller than the wavelength, the scattered light has a larger wavenumber than the incident light. Additionally, to hold the law of momentum conservation, the wavenumber normal to the aperture has a negative value.