Dielectric constant reduction using porous substrates in finline millimetre and submillimetre detectors Chris E. North a , Michael D. Audley b , Dorota M. Glowacka b , David Goldie b , Paul K. Grimes a , Bradley R. Johnson a , Bruno Maffei c , Simon J. Melhuish c , Lucio Piccirillo c , Giampaolo Pisano c , Vassilka N. Tsaneva b , Stafford Withington b , Ghassan Yassin a a University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, UK b Cavendish Laboratory, University of Cambridge, Cambridge, UK c University of Manchester, Manchester, UK ABSTRACT Finlines are planar structures which allow broadband and low loss transition from waveguide to planar circuits. Their planar structure and large substrate makes them ideal for integration with other planar circuits and components, allowing the development of an on chip polarimeter. We have developed a method of extending the employment of finlines to thick substrates with high dielectric constants by drilling or etching small holes into the substrate, lowering the effective dielectric constant. We present the results of scale model measurements at 15 GHz and cryogenic measurements at 90 GHz which illustrate the excellent performance of finline transitions with porous substrates and the suitability of this technique for extending the bandwidth of finline transitions. Keywords: finline, millimetre-wave, substrate, dielectric, porous, bandwidth 1. INTRODUCTION One of the key components in a millimetre or submillimetre astronomical detector is the coupling between the horn and the detector. Modern detectors employ superconducting planar circuits, normally comprising miniature microstrip lines, so an efficient coupling from waveguide to microstrip is required. The next generation of high sensitivity polarimetric instruments also require excellent performance in terms of cross-polarisation over a very wide frequency band. Finline transitions provide a broadband, low cross-polarisation, low-loss transition from waveguide to a number of planar circuit geometries including microstrips. 1 Their performance has previously been proven in SIS (Superconductor-Insulator-Superconductor) mixers at 200–700 GHz. 2–4 Current and future instruments use finlines to couple horns to bolometers such as TES (Transition Edge Sensors) 5 and CEBs (Cold Electron Bolometers). 6, 7 Prime examples are CMB polarisation instruments which are searching for the very faint signature of gravity waves caused by inflation in the early universe. Ground and balloon-borne experiments have limitations on the available frequency bands imposed by the atmosphere, and need to use all available transmissive bands as efficiently as possible. The metal fins are supported on a dielectric substrate, commonly made of quartz or silicon. The supporting substrate, which can be of high dielectric constant, sits in a groove in the waveguide wall. The presence of this thick dielectric can allow the transmission of higher order modes at the high frequency end of the frequency band, limiting the bandwidth of the finline transition and consequently the performance of the entire detector chip. One solution to this problem is to make small (≪ λ) holes in the substrate. These structures have the effect of lowering the dielectric constant of the substrate and shifting the transmission of higher order modes to higher frequencies. In Section 2 we will describe finlines and the problem of higher order modes. In Section 3 we will discuss the proposed solution, while in Sections 4 and 5 we will detail the results of measurements made at 15 and 90 GHz. Further author information: (Send correspondence to C.E.N.) C.E.N.: E-mail: c.north1@physics.ox.ac.uk, Telephone: 44 1865 283011 Millimeter and Submillimeter Detectors and Instrumentation for Astronomy IV edited by William D. Duncan, Wayne S. Holland, Stafford Withington, Jonas Zmuidzinas Proc. of SPIE Vol. 7020, 70202G, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.788479 Proc. of SPIE Vol. 7020 70202G-1 2008 SPIE Digital Library -- Subscriber Archive Copy