Investigation of Femtosecond Laser Irradiation on Fused Silica Etching Selectivity
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Investigation of Femtosecond Laser Irradiation on Fused Silica Etching Selectivity Yves Bellouard1, Ali A. Said2, Mark Dugan2 and Philippe Bado2. 1
CAT/CIE, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180-3590, U.S.A. 2 Translume Inc., 655 Phoenix Drive, Ann Arbor, MI 48108-2222, U.S.A. ABSTRACT Femtosecond laser irradiation has various noticeable effects on fused silica. It can locally increase the index of refraction or modify the material chemical selectivity. Regions that have been exposed to the laser are etched several times faster than unexposed regions. Various observations reported in the literature seem to show that these effects are possibly related to a combination of structural changes and the presence of internal stress. However, a detailed analysis of the contribution of both effects is still lacking. In this paper, we present systematic SEM-based investigations performed on fused silica (aSiO2). Line-patterns were first scanned on the substrate using a femtosecond laser and then etched in a low-concentration HF solution. The effects of various laser parameters like power and scanning speed are analyzed and we show further evidence of an interface between two different etching regimes. INTRODUCTION Kondo et al. [1] reported on a process based on the use of femtosecond lasers and chemical etching to fabricate micro-tunnels in photo-etchable glass. Marcinkevicius et al. [2] applied a similar process to manufacture micro-holes in fused silica. Although, these pioneering works have demonstrated the feasibility of the process, unresolved issues remain. The mechanism responsible for the increased etching rate has yet to be understood. It is not known if the etching mechanism is preferentially driven by the increased level of internal stress resulting from the laser exposure, the presence of voids or micro-cracks, or a laser-induced densification effect. Further the ability to generate arbitrary shapes has – so far – been limited by the laser beam shape. We have used the process demonstrated in [1,2] combined with a volume sampling technique to fabricate fluidic channels and tunnels of virtually any shape [5]. This volume sampling technique consists in stacking and arranging Laser-Affected-Zone (LAZ) units to define the volume to be etched. To be effective this method requires a good understanding of the effect of femtosecond laser on amorphous fused silica and in particular an understanding of how contiguous LAZs interact with each other. Femtosecond-laser irradiation of fused silica produces two previously documented material changes: an increase of the refractive index [3,6] and an increase of the etching rate [2]. The understanding of observed effects is still subject to discussion. The present study is an investigation of the effect of femtosecond-laser irradiation on a-SiO2 that is based on Scanning Electron Microscope observations of chemically etched specimens. In particular, we have investigated the influence of laser power and scanning speed on the surface morphology of e
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