Micromachined Coplanar Waveguide Band-Pass Filter for W-Band Applications Dan Neculoiu 1,2 , Alina-Cristina Bunea 1,2 1 IMT Bucharest, Bucharest, Romania 2 Politehnica University of Bucharest, Bucharest, Romania dan.neculoiu@imt.ro , alina.bunea@imt.ro Abstract—In this paper we present the design and measurements for a novel configuration of a membrane supported coplanar waveguide band-pass filter for W band applications. The topology is based on identical symmetrical elementary cells connected in cascade. The design process is based on the image impedance representation and a full-wave 3D electromagnetic (EM) model is developed. A two-cell test structure was fabricated through silicon micromachining. The measured pass-band is of 28 %, centered around 91.5 GHz. The measured losses are of 3.77 dB at the central frequency and the reflection losses go down to -20 dB, in good agreement with the simulated results. This structure was designed to be easily adapted to the BiCMOS technology with localized back-side etch (LBE) process. Keywords—band-pass filter, electromagnetic modeling, millimeter waves, silicon micromachining. I. INTRODUCTION In the mid-90s, high-resistivity silicon micromachining proved to be a good choice for millimeter-wave filter and antenna fabrication. Membrane supported circuits offer many advantages, the most important consist in the reduction of the losses as well as in the reduction of the dispersion effects. Several realizations of micromachined band-pass filters (BPF) for millimeter-waves were reported in the literature ([1]-[3]). For the W-band (75 – 110 GHz) a 17.7 % bandwidth BPF with 1.4 dB insertion losses was presented in [1] and it uses a shielded membrane microstrip approach that is based on two micromachined silicon wafers. In [2] a narrow band Chebyshev BPF at 60 GHz with 3.4 dB insertion loss is described. It is based on a microstrip structure packaged in a micromachined cavity, using a stack of 3 micromachined high resistivity silicon wafers. A wideband BPF at 45 GHz with mid-band insertion losses of only 0.8 dB and a 3 dB bandwidth of 29% is presented in [3]. Using the coplanar waveguide (CPW) topology, this approach requires only one silicon micromachined substrate. In present days the technological advancement of the SiGe technology at W-band frequencies and above requires high performance low loss BPFs and antennas. A standard BiCMOS process with an additional localized back-side etch (LBE) module was used to fabricate a on-chip dipole antenna at 77 GHz in [4]. This technology opens a window of opportunity for the investigation of membrane-like supported circuits for the 75 – 220 GHz frequency range. This paper presents the design, modeling and the experimental results for a membrane supported BPF operating in the W-band. Because of the high cost of the BiCMOS technology, a test BPF structure was designed and tested in the standard high-resistivity silicon micromachining technology. The EM modeling is briefly described in the next section. The obtained results can be easily implemented in the BiCMOS with LBE module technology. The design, based on a new topology of the elementary cell, is presented section III. It takes advantage of the decreased value of the free space wavelength in the W-band frequency range to obtain a very compact filter. Filter test structures with two cascaded cells were fabricated and measured on wafer. There is a very good agreement between simulated and measured results. II. ELECTROMAGNETIC MODELING The band-pass filter consists of several identical elementary cells connected in cascade. The new elementary cell proposed in this paper is shown in Fig.1. It consists of one coupled line CPW section (Z oe – even mode impedance; Z oo – odd mode impedance; L c – coupling physical length; θ – coupling electrical length) and two symmetrical CPW lines (Z c – characteristic impedance; L p – physical length; θ p – electrical length). In contrast to [3], in this design the coupled line and the simple CPW lines are collinear and can be easily included in the BiCMOS with LBE module technology. Fig. 1. Layout of the elementary cell This work was supported by the European Union 7th Framework Program, under the project NANOTEC and the Romanian Ministry of National Education, under the project no. PN-II-ID-PCE-2011-3-0830.