A Comparison of Wet and Dry Etching to Fabricate a Micro-Photonic Structure for use in OCT. Paul Jansz*, Graham Wild and Steven Hinckley Optical Research Laboratory, Centre for Communication Engineering Research, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA, Australia 6027 ABSTRACT In conventional time-domain Optical Coherence Tomography (OCT), a moving mirror is used as a reference optical delay line. This motion can result in instrument degradation, and in some situations it is preferable to have no moving parts. Stationary optical delay lines using a variety of methods have been proposed. Of particular interest, due to its low cost, is the use of a micro-photonic stationary optical delay line, made up of an addressable Stepped Mirror Structure (SMS) using a liquid crystal optical switch. Here the individual steps of the SMS can be selected by the liquid crystal array. For use in OCT, the discrete nature of the SMS needs to be overcome by having the step height less than the coherence length of the low coherent light source. Typical coherence lengths in current OCT systems are on the order of 10µm. Hence, micrometer size steps require the use of a relevant fabrication method. In this paper, we compare SMSs fabricated using wet and dry etching methods. Specifically, Reactive Ion Etching (RIE) using CF 4 /O 2 and chemical bath etching, using a solution of HF, HNO 3 and Acetic acid. Three inch diameter silicon wafers, 400µm thick, were etched by both methods. The RIE was used to produce a SMS with five 5µm high steps each step approximately 1 cm wide. The wet etching produced an SMS with three 15µm steps approximately 2 cm wide. The overall structures of the SMSs were compared using optical profilometry. The RIE step quality was far superior to the wet etch method due to the ability to control the anisotropy of the RIE method. Keywords: Optical Coherence Tomography, Reactive Ion Etching, Stationary Optical Delay Line, Wet Etching. 1. INTRODUCTION Optical Coherence Tomography (OCT) is an interferometric technique that uses the auto-modulation effect of broad- band (low coherent) Near Infra-Red (NIR) light to present a reflection intensity map of a tissue’s cross section. OCT can be used to a depth of up to 3 mm, with 5-10 µm axial resolution [1] , depending on the wavelength and bandwidth. OCT has medical applications in ophthalmology [2] , and many other uses [3,4] . Time Domain OCT (TD OCT) and Frequency Domain OCT (FD OCT) represent the two operational modes of OCT. In TD OCT, the axial scan rate and the signal-to-noise ratio (SNR) depend on the frequency bandwidth of the detection system. In state-of-the-art TD OCT, featuring an Optical Delay Line (ODL), the frame rate is limited to 4-8 frames per second. Even though the ODL can operate at higher rates, the SNR is severely compromised at video-frame rates. FD OCT, which operates without an ODL, can operate at video-frame rate because the SNR does not depend on the scan rate. Since FD OCT performance is poor for turbid samples, TD OCT has some niche application, although video-frame rate ODL is not really necessary [5] . However, conventional TD OCT requires the use of a reference ODL which repeatedly scans a depth range equivalent to the penetration depth of the beam into the sample. This movement results in instrument degradation. Also, in some situations, it is necessary to have no moving parts. Hence non-mechanical, low-cost, controllable, long-range and long- life ODLs can present some value for TD OCT techniques. Such Stationary ODLs (SODLs) in OCT will have application to a wide market. It will be more affordable, due to negligible maintenance, and the use of solid state components. With the use of Spatial Light Modulators (SLM), the OCT instrument could be more compact, which translates into portability. Therefore it is intended that this research will improve an already mature medical imaging technology by delivering a more portable and affordable solution [5] . * p.jansz@ecu.edu.au Biomedical Applications of Micro- and Nanoengineering IV and Complex Systems, edited by Dan V. Nicolau, Guy Metcalfe Proc. of SPIE Vol. 7270, 727016 · © 2008 SPIE · CCC code: 1605-7422/08/$18 · doi: 10.1117/12.810973 Proc. of SPIE Vol. 7270 727016-1 2008 SPIE Digital Library -- Subscriber Archive Copy