Indian Journal of Pure & Applied Physics Vol. 45, April 2007, pp. 336-344 Micromachined conductor backed coplanar waveguides for millimeter wave circuit application Shiban K Koul*, Abhishek Kumbhat & Ananjan Basu Center for Applied Research in Electronics, Indian Institute of Technology Delhi, Hauz-Khas, New Delhi 110 016 *E-mail: shiban_koul@hotmail.com Received 7 June 2006; revised 6 November 2006; accepted 14 February 2007 Conductor Backed Coplanar Waveguide (CBCPW) lines on Si micromachined substrate are in use for more than a decade, but their detailed analysis is still not available in the open literature. This paper presents design data and closed form expressions on characteristic parameters of CBCPW lines on Si-micromachined structures that are valid up to 40 GHz. Effect of various physical parameters are studied. Procedure for deriving lumped equivalent circuit models for discontinuities on Si-Micromachined substrate as a function of discontinuity physical parameters is also presented. Scattering parameters obtained from equivalent circuit models agree well with those obtained from full wave FEM for all the discontinuity structures. To show the usefulness of the data generated on discontinuities, design of a Ka-band band pass filter is described. The results presented are useful in designing micromachined components at microwave and millimeter wave frequencies. Keywords: Micromachined CBCPW line, RF MEMS, Discontinuity Models, Millimeter waves IPC Code: B81B7/02, H01P 1 Introduction Micromachined microwave and millimeter wave components have been identified as an area that has the potential to provide a major impact on existing systems in communications. The motivation behind fabrication of high frequency circuits on micromachined silicon substrates is due to precise dimensions, repeatability and reduced weight, cost, size, losses and power dissipation. Designing micromachined components at microwave and millimeter wave frequencies will require careful selection of a planar transmission structure that can be conveniently fabricated using conventional Si- micromachining technique. Micromachined transmission lines are printed on high-resistivity Si with a thin membrane of monolayer (SiO 2 ) or multilayer (SiO 2 –Si 3 N 4 –SiO 2 ) dielectrics and Si removed underneath the line by anisotropic etching (wet etching using KOH). Access to both signal and ground lines on the same plane, less radiation losses, lower dispersion, removal of via holes and high power handling capability makes Conductor Backed Coplanar Waveguide (CBCPW) most versatile and easy to use among all micromachined transmission lines. Various structural configurations for CBCPW line employing micromachining have been proposed by different researchers 1-7 , which show very low loss, less dispersion and nearly TEM propagation. Some of the micromachined line configurations are shown in Fig. 1. Figure 1(a) shows dielectric membrane supported CPW line 1 , in which silicon is etched away from the backside and treating front side dielectric (in general thermally grown SiO 2 ) as etch stop layer. To provide mechanical strength and increase effective dielectric constant, the dielectric membrane is supported by thin silicon membrane left during backside etching as shown in Fig. 1(b). Such a configuration can be used to develop synthesized substrates, enhancing performance of circuits such as stepped impedance low pass filter 2-3 . Figure 1(c) shows micro shield CPW line 4-6 in which the metallization from backside onto cavity sidewalls is done protecting selected areas of membrane from metallization. The metallized lower shielding cavity minimizes signal cross-talk between adjacent lines, decreases parasitic effects associated with discontinuities and supports TEM mode propagation over a large bandwidth. Figure 1(d) shows the multilayer micromachined structure that is generally used to suppress parasitic waveguide mode in CBCPW line. Here the micromachined substrate is