Silicon RFIC UWB Bandpass Filter Using Bulk-Micromachined Trench Couplers Xuguo Huang 1 and Stepan Lucyszyn 1 1 Department of Electrical and Electronic Engineering Imperial College London, UK Email: xuguo.huang07@imperial.ac.uk Abstract — A low loss silicon RFIC UWB bandpass filter is reported. The filter exploits a novel metalized coupled trench structure, fabricated by advanced silicon bulk micromachining. The couplers can achieve more than -2 dB coupling, with good even-odd mode phase velocity balance, while having practical dimensions. The CPW filter is designed using well-known interdigital filter design equations. The measured fractional bandwidth is 115%, with only 1 dB mid-band insertion loss at 6.4 GHz and better than -11 dB return loss across the whole passband. The measured differential-phase group delay is less than 200 ps from 2.6 to 10.1 GHz. Being a monolithic filter, it has a compact area of only 3.5 x 5.5 mm 2 . Index Terms — Metalized coupled trench, bandpass filters, UWB, interdigital filter. I. INTRODUCTION Ultra-wideband (UWB) has been widely investigated by academia and industry since the US Federal Communications Commission (FCC) approved the unlicensed use of the frequency spectrum from 3.1 to 10.6 GHz. UWB techniques have the advantages of a higher transmission data rate and lower power consumption. It has already been employed in applications like positioning, imaging, short-range high-data-rate communications systems and wireless personal area networks [1]. In an UWB transmitter and receiver, the UWB bandpass filter is a key subsystem, as well as a challenge to the designer. It not only requires an ultra-wide passband, to cover the target spectrum, but also maintain high selectivity to reject out-of-band signals. There have been a few reports on the realization of UWB filters using PCB and ceramic substrates, employing multiple mode resonators (MMRs) [2], hybrid microstrip/CPW [3] and cascaded high-/low- pass filters [4]. A quasi-lumped-element UWB filter using silicon micromachining was recently reported, demonstrating a compact tunable UWB notch filter [5]. Quarter-wavelength coupled microstrip lines in air have been employed for implementing high performance narrow passband filters, using relatively complicated bulk- micromachined silicon microfabrication processing [6]. However, in contrast, for wide passbands tight coupling is required, which requires extremely small gaps between coupled microstrip/CPW lines when a single metal layer is used. Moreover, the coupling coefficient becomes more sensitive as separation distances decrease [7-9]. Therefore, a challenge exists to obtain tight coupling when the coupled transmission lines employ a substrate. An alternative approach is to use multi-layers, whereby a combination of edge and broadside coupling is employed to increase the level of capacitive coupling between the lines, as demonstrated in GaAs monolithic microwave integrated circuit (MMIC) technology. Examples of this have been demonstrated in microstrip at 7.7 GHz [10], being later employed in a 10 GHz cascaded-match reflection-type phase shifter with 2-octave bandwidth [11], and in CPW within a 24 GHz MMIC serrodyne frequency translator [12]. In this paper, a coupled trench structure, fabricated by advanced silicon micromachining technology is proposed for the first time, without the need for multiple metal/dielectric layers. The structure can achieve tight coupling without extremely small separation distances, while allowing for a certain tolerance of fabrication imperfection. An interdigital UWB bandpass filter employing the coupling structure is demonstrated with excellent performance and compact size. The filter is in CPW form and, therefore, suitable for subsequent assembly or monolithic integration to make compact UWB RF front-end modules realizable. II. TRENCH COUPLING STRUCTURE DESIGN AND FABRICATION In order to achieve tighter coupling, a traditional 2D parallel coupled microstrip/CPW line is extended into 3D. A CPW trench structure is proposed, as shown in Fig. 1. The ground plane is assumed to be infinitely wide. The two coupled trenches, with identical width w and depth t, are separated by a distance s. The gap distance between Fig.1 The schematic of an ideal CPW parallel trench s g w t 978-1-4673-2141-9/13/$31.00 ©2013 IEEE