Nano-structuring with femtosecond excimer laser pulses

Abstract

Nano-scale fabrication of materials is one of the biggest challenges for nextgeneration devices in science and technology. There is a general trend to reduce the size of opto-mechanical components, and there is a growing need for components with feature sizes below one micron. Also, the fabrication of 2-and 3-dimensional structures, specifically the creation of photonic crystals has attracted tremendous interest in recent years. Such a technical challenge calls for the development of fast, low-cost, and flexible fabrication technologies. Laser direct processing via ablation offers exactly these properties, thus representing a great potential for numerous new applications. It has been shown in a number of studies that short-pulse lasers, generating pico-or femtosecond pulses, provide a much better precision of the ablation process. The reason for the observed improvement is mainly due to two effects. In case of solids with high heat conductivity, the lateral spreading of the absorbed energy outward from the irradiated zone within the laser pulse duration sets a limit for the achievable spatial resolution. A short pulse duration reduces the so-called heat-affected zone (HAZ) [1], thus resulting in sharp boundaries of the fabricated structures. Besides a dramatic reduction of the HAZ, sub-picosecond pulses offer further potentials. Sub-picosecond lasers can be new tools for the micromachining of transparent materials [2]. Ultrashort pulses possess high peak powers while carrying only moderate energies. At such high power levels, multi-photon absorption becomes dominant and ensures the concentration of the irradiated energy in a thin surface layer necessary for precise machining. In this way, high quality structuring of materials like fused silica, fluoride crystals, diamond, sapphire, or Teflon R have become possible. Following the general worldwide trend of miniaturization, many groups tried to fabricate sub-micron features on a variety of materials. However, using the radiation of a Ti:Sapphire laser system, which is the most commonly used short-pulse laser nowadays (with an emission wavelength centred around 800nm), special methods have to be used to obtain structures in the subwavelength range. The first publication on fabrication of sub-micron features on metals with a Ti:Sapphire laser was that of Pronko et al. [3], unfortunately with features of very poor quality. Later, similar results have also been achieved by Korte et al. [4]. The reason for the poor reproducibility and edge quality of these features is the requirement to keep the size below the wavelength (800nm). In this case, for metals and semiconductors, the only possibility is to adjust the laser power in such a way that only the central part of the focal distribution exceeds the ablation threshold of the sample. This is a very critical adjustment, resulting in very limited ablation rates, and responding very sensibly to intensity fluctuations. It is also evident that any variation of the pulse intensity is converted into a change of the feature size. This problem can be entirely solved by using ultraviolet radiation. Since the optical resolution scales with the wavelength, a system running at 248nm (at the KrF excimer wavelength) provides more than three times better spatial resolution compared with a Ti:Sapphire laser. This means that feature sizes below 500nm can easily be created without special control of the peak fluence of the pulse relative to the threshold fluence of the material. In this way, reproducible, clean and sharp structures can be fabricated on the submicron scale on virtually all materials, even on metals and semiconductor surfaces. © 2005 Springer-Verlag Berlin Heidelberg.