Nanofabrication Using Scanning Near-Field Optical Microscopy

Abstract

The interest in extremely small solid-state devices and high-density data storage has increased rapidly. To realize such applications, new techniques for fabricating nanometer-scale structures are important, because the conventional optical lithography has an insufficient resolution limited by the wavelength of the light and the numerical aperture of the lenses. In addition to electron beam lithography, scanning probe techniques such as scanning tunneling microscopy (STM)’) and atomic force microscopy (AFM)’) have been investigated to perform surface modifications in a simple way. Near-field optical lithography has a potential to fabricate nanometer-scale patterns more rapidly than the techniques based on STM or AFM. Scanning near-field optical microscopy (SNOM) is a useful method to investigate the possibility of near-field optical lithography for nanometer-scale fabrication. The schematic diagram of our SNOM3) system for the fabrication is shown in Fig. 1. An optical fiber probe has an aperture with a subwavelength at the apex. The optical fiber probe is bent and vibrated vertically to control the distance between the sample and the probe tip. The light source is an Ar ion laser (A = 488 nm) or a He-Cd laser (A = 442 nm). Commercial photoresist, which is sensitive to g line (A = 436 nm), is coated on a Si wafer by a spin-coater. The photoresist film is exposed by the light emitted by the aperture of the optical fiber probe. Changing the incident light intensity or the scanning speed controls the exposure conditions. The exposed photoresist film is developed and observed by AFM. Figure 2 shows the AFM image of the positive photoresist film. The groove width in the photoresit film is about 100 nm. It is nearly equal to the aperture size of the optical fiber probe. In Fig. 3, an aluminum line pattern with the width of 100 nm on a Si wafer was fabricated by the lift-off technique. We have demonstrated that subwavelength patterns can be fabricated using SNOM. These results show the possibility of near-field optical lithography for fabricating nanometer-scale structures.