F2-laser microfabrication for photonics and biophotonics

Abstract

The F2 laser defines the short-wavelength forefront of commercial laser systems. As such, the 157-nm lasing wavelength is highly attractive for defining features to ∼100nm in size during laser processing [1], and extensions to sub-100nm processing were under development for the next generation semiconductor lithography exposure tool [2, 3]. The F2 laser further provides a high photon energy of 7.9 eV that drives strong interactions with nearly all materials, opening a broad base of laser applications to wideband-gap materials such as transparent glasses like fused silica or robust polymers like Teflon. The F2-laser light can smoothly microsculpt optical surfaces with precise ∼ 10-nm depth control without generating the microcracks normally produced by longer-wavelength lasers [4, 5, 6, 7]. The strong interactions can be further tailored to more gentle internal modification of glasses to imprint buried refractive index structures like volume gratings or optical waveguides [5, 6, 8]. Such processes merit a broad range of new applications such as custom fabrication of micro-optic components, diffractive optical elements, microfluidic channels for laboratory on a chip (LOAC), planar or bulk 3-D optical circuits, and repair and phase trimming of optical functions. Microfabrication also extends to weakly absorbing inorganics or polymers like polyethylene, PMMA, and PTFE [4, 9, 10, 11, 12, 13, 14, 15, 16] for potential biomedical, electronic, and MEMs applications, and to sapphire [17] and semiconductor materials [18]. Commercial exploitation of F2-laser processing applications has been hampered in the past by the need for robust vacuum-ultraviolet (VUV) optics that can withstand the hard 157-nm photons and by special requirements for optical beam systems that flush out ambient air, a strong absorber of 157-nm light. A large research effort to meet F2-laser lithographic requirements set in the International Technology Roadmap for Semiconductors by Sematech [2] has expedited the development of 157-nm optics and related nanofabrication technology [19]. Although 193-nm immersion lithography has recently superseded F 2-laser lithography, high quality CaF2 and dielectric coatings now rou tinely provide > 98% transmittance per optic at 157nm and new gas flushing systems offer high transmittance over several meters of optical path length. To this end, the University of Toronto group [20, 21] has co-developed with MicroLas Lasersystems and Laser-Laboratorium Göttingen a high-resolution optical processing system for high-fluence machining and refractive index structuring of optical glasses. Such systems are proving to be robust laser technology, providing the convincing metrics to unlock the first generationof microfabrication applications for the F2laser. This chapter is primarily devoted to the research study undertaken with this system in Toronto. The chapter begins by describing the optical tooling system and its micromachining performance. Section 13.3 is devoted to micromachining of optical glasses, and describes the process precision and control of surface morphology. Section 13.4 presents several micromachining examples, including microvias, microchannels, mask fabrication, gratings, and diffractive optical elements. Section 13.5 examines the photosensitivity responses for imprinting refractive index structures inside various glasses. The latter two sections examine prospects for F2-laser microfabrication of photonic components in the telecommunication and general optics manufacturing fields. The final section looks at special opportunities for fabricating laboratory-on-a-chip devices,including new means for integrating photonic functions with microfluidics. The objective of this chapter is to demonstrate that optical tools are well developed for F2-laser material processing, and application opportunities are waiting for exploitation. © 2005 Springer-Verlag Berlin Heidelberg.