Klaus B. Mogensen Fredrik Eriksson Omar Gustafsson Rikke P. H. Nikolajsen Jörg P. Kutter MIC – Department of Micro and Nanotechnology, Technical University of Denmark, Kgs. Lyngby, Denmark Pure-silica optical waveguides, fiber couplers, and high-aspect ratio submicrometer channels for electrokinetic separation devices A new fabrication procedure for integration of ultraviolet transparent pure-silica planar waveguides, fiber couplers and high-aspect ratio submicrometer channels is pre- sented. Only a single photolithographic mask step is required. The channels are 80– 90 mm deep and the width can be reduced to about 0.5 mm, corresponding to a height- to-width ratio of more than 150. The core of the waveguides consists of pure silicon dioxide, which is favorable over doped silica, due to the absence of absorption centers associated with the dopants. This furthermore improves the long-term stability of the waveguides, because of an increased radiation resistance of the glass. The propaga- tion loss decreases from 1.0 dB/cm at 200 nm to 0.2 dB/cm at 800 nm, which, to our knowledge, is the lowest propagation loss reported for integrated planar waveguides in the ultraviolet wavelength region to date. The effective optical path length is 1.2 mm for an absorbance cell with a nominal length of 1.0 mm, indicating effective suppres- sion of stray light. The limit of detection for paracetamol when present in the entire channel network was determined to 3 mg/mL. Finally, the applicability of the fabricated devices for capillary electrophoresis was evaluated by separation of caffein, para- cetamol and ketoprofone using absorbance detection at 254 nm. Keywords: Absorbance detection / Deep-reactive ion etching / Fiber couplers / High-aspect ratio / Microfluidics / Micrototal analysis system / Miniaturization / Submicrometer channels / UV waveguides DOI 10.1002/elps.200406077 1 Introduction Capillary electrophoresis (CE) and electrochromatogra- phy (CEC) are among the most widely used chemical analysis techniques on microfabricated devices. The main reasons for this are the high separation perfor- mance, particularly for channels with a small cross sec- tional area [1–3], and the ability to use electroosmotic flow for pumping and valving, thereby avoiding bulky external fluidic components [3]. The majority of separation devices so far have been using open channels for chemical anal- ysis by CE, mainly because this was relatively easy to implement. The interest has, however, turned increasingly towards packed channels for chromatographic separa- tions due to the versatility of this separation method [4]. To facilitate this, microfluidic channels have, e.g., been filled with silica particles, similar to those used in HPLC. The main problem with this approach is to make sure that the particles are retained within certain regions of the fluidic network, which can be achieved by integration of frits or weirs in the channels [5, 6], or by letting the parti- cles aggregate at tapered restrictions [7]. In the former approach, packing of channels longer than 1 mm required a combination of electroosmotic and hydrodynamic flow to prevent solvent recirculation and backflow, and in the latter approach packing was sensitive to parameters such as the concentration of particles in the suspension [7]. Another very popular approach is fabrication of con- tinuous porous beds by in situ polymerization of organic monomers [8–10]. In this case, frits are avoided, because the bed is covalently bonded to the channel sidewalls. A further advantage is that much longer columns can be fabricated. A radical different procedure is to etch or mold the sup- port structures within the channels [11–14]. Here, “per- fectly” ordered pillars can be obtained, because they are defined by photolithography. Additionally, this approach allows a bigger freedom in design, and it should thus be possible to test various geometries of structures for opti- Correspondence: Dr. Jörg P. Kutter, MIC – Department of Micro and Nanotechnology, Technical University of Denmark, Bldg. 345 east, DK-2800 Kgs. Lyngby, Denmark E-mail: jku@mic.dtu.dk Fax: +45-45-88-77-62 Abbreviations: DRIE, deep-reactive ion etching; LPCVD, low- pressure chemical vapor deposition; PDMS, poly(dimethylsilox- ane) 3788 Electrophoresis 2004, 25, 3788–3795 * Dedicated to Prof. Thomas Welsch on occasion of his 60 th birthday  2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim