We report on a technology for the fabrication of hollow capillary optical leaky waveguides in fused silica glass. The fabrication process is based on lithography, wet chemical etching and aligned direct bonding. We have developed a single-layer photoresist soft mask which allows for channel etch depths up to 33 µm in fused silica glass. To our knowledge, such etch depths have never been achieved before in fused silica glass with single-layer soft etch masks. Aligned direct glass-glass bonding is used for the first time to obtain channels with almost circular profiles with diameters between 16 and 66 µm. Capillary optical leaky waveguides embedded into microfluidic devices can be used, for example, for capillary electrophoresis and hyper Rayleigh scattering.

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Fabrication processes for wet chemical and dry etching of hollow capillary leaky optical waveguides in high-purity fused silica for extended path cells for improved optical detection in analytical chemistry are described. We focus on microstructures with etch depths on the order of 80 µm. Special attention is paid to the preparation of the etch masks for the two different etch technologies. The fused silica wet chemical etching technique uses buffered hydrofluoric acid with ultrasonic agitation achieving etch rates > 100 nm/min. We succeeded in developing an etch process based on a singlelayer photoresist (AZ 5214E, Clariant Corp.) soft mask, which gives excellent results due to special adhesion promotion and a photoresist hardening cycle after the developing step. This procedure allows for the production of channels of nearly semi-cylindrical profiles with etch depths of up to 87 µm. For the dry etch process a ~10 µm thick Ni layer is used as a hard mask realized with electroplating and a thick photoresist. The etch process is performed in an ECR (Electron Cyclotron Resonance) chamber using CF 4 gas. The resulting etch rate for fused silica is about 138 nm/min. Etch depths of (accidentally also) 87 µm are achieved.

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