Nanofabrication by Photons

Abstract

Since the planar processing for integrated circuits (ICs) manufacturing started nearly half century ago, optical lithography, often called photolithography, has become the convenient choice for making planar microstructures. In optical lithography, a mask or photomask, also called reticle, is imaged onto a flat substrate surface coated with a thin layer of polymer material called photo-resist. The photon energy is focused onto the photoresist, causing polymer chain scission or cross-linking. The mask pattern is then delineated into the photoresist after development. There is a whole spectrum of photons, as shown in Fig. 2.1, which can be explored for lithographic patterning. In the early days when the features size of ICs were a few micrometers, optical lithography was done with visible light and was no different from copying photos in a photography workshop. As the circuit feature dimension shrank, illuminating light with shorter wavelength was employed. This was the time when mercury lamps were used as the illumination source, with UV-wavelength emission at G-line (l=436 nm) and I-line (l = 365 nm) being the choices of wavelengths for exposure of photoresists. In the early days of IC manufacturing, 1-mm feature dimension had once been considered as a formidable barrier for optical lithography using the aforemen-tioned illuminating wavelength [1]. Then excimer lasers at deep UV (DUV) wavelength l=248 nm (KrF excimer laser) and l=193 nm (ArF excimer laser) became the new illumination sources for optical lithography. New photoresists, particularly the chemically amplified (CA) resists, were developed to suit the shorter-wavelength irradiation. Next from 193 nm further going down the wavelength were vacuum UV (VUV) at 157 nm, extreme UV (EUV) at 13 nm, and X-ray at 1 nm. The technical difficulty inevitably increases with the shorter-wavelength illumination being used for optical lithography. Some of technical barriers have been insurmountable or the solutions were too expensive, as there is always an economical leverage in determining the champion technology. The time came when a new limit at around 130 nm was predicted for the exit of optical lithography and the entry of next-generation lithography (NGL) techniques. However, optical lithography has marched on.