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Surface Micromachining Polymer-Core-Conductor Approach for High-Performance Millimeter-Wave Air-Cavity Filters Integration
Bo Pan, Student Member, IEEE, Yuan Li, Student Member, IEEE, Manos M. Tentzeris, Senior Member, IEEE, and John Papapolymerou, Senior Member, IEEE
Sep 29, 2008:
Abstract—This paper presents a novel approach to integrate high-performance millimeter wave filters on top of wafers. The proposed method eliminates the dielectric loss by elevating cavity-based filters into the air with the aid of the polymer-core conductor surface micromachining technology. The electrical fields of the cavity are thus entirely in air. A coplanar waveguide input and output interface is designed for easy integration with other planar electronics. Several 60-GHz ( V-band) air-cavity filters with superior performances, including two two-pole filters and one four-pole transmission zero filter using a novel capacitive coupling scheme, are developed and characterized to demonstrate advantages of the proposed technology. The filters exhibit excellent performances. Insertion losses as low as 1.45 dB for a two-pole filter and 2.45 dB for a four-pole transmission-zero filter have been observed at 60 GHz. Design curves and parametric analyses are included to help readers better understand key factors in optimizations. The proposed technology is capable of integrating high-performance millimeter-wave cavity filters on top of wafers, while providing easy integration with other electronic components.
Index Terms—Cavity resonator filter, millimeter wave, surface micromachining, transmission zeros, V-band.
I. Introduction
High-performance millimeter-wave filters play very important roles including filtering, diplexing, and multiplexing in emerging communication systems [1]. In the millimeter-wave regime, loss from the substrate has become a dominant factor that limits filters’ performances [2].Waveguide filters have been used for millimeter-wave applications for years because of their excellent insertion loss, power-handling capability, and frequency selectivity [3], [4]. In the millimeter-wave regime, sizes of waveguide filters become smaller and silicon wafers can be etched, metallized, stacked, and bonded together to implement millimeter waveguide filters [5]–[7]. Great fabrication accuracy can be achieved, which is critical for millimeter-wave and terahertz applications. However, most of these filters use standard waveguide input/output interfaces that can limit their applications. Another technology, called stereo-lithography, uses a laser to make various complicated 3-D structures [8]. However, it is hard to directly integrate those structures with planar components. In [9], a silicon micromachined filter with an integrated transition from a coplanar waveguide (CPW) to a rectangular waveguide was reported. A great performancewas demonstrated, except for the requirement of a bulk-micromachineable substrate. As an alternative method, the approach proposed in this paper is substrate independent and can theoretically be implemented on any substrate.
Another idea is a substrate integrated waveguide (SIW) (or called an electromagnetic bandgap (EBG)/magnetic bandgap (MBG) cavity, laminated waveguide, and post-wall waveguide in some literature) [10]–[14]. It is compatible with planar component integration. This concept has been implemented using various substrates such as printed circuit boards (PCBs) [14] and low-temperature co-fired ceramic (LTCC) [15], [16]. Great performances were reported from these designs. One limit is that the technology requires a low-loss substrate and the via-hole technology. A substrate integrated image waveguide/resonator is associated with the similar issue [17], [18]. Reference [19] reported a surface micromachined approach to implement the SIW. It uses a photodefinable dielectric to form the waveguide on top of the substrate. It still requires a low-loss dielectric.
In this paper, a waveguide cavity filter is moved onto the top of the substrate using polymer-core conductor surface micromachining technology [20]. The entire cavity/waveguide filter is on top of the substrate. The requirement for a low-loss substrate is no longer necessary since air fills the cavity/waveguide. In our previous research, this technology has been used to build other millimeter-wave components such as a W-band monopole, a Yagi–Uda array, an elevated patch antenna, and an elevated coupler [21]–[23]. We have also reported research results on a 30-GHz cavity resonator using this technology [24]. In this paper, we will focus on design, fabrication, and characterization of 60-GHz ( V-band) filters with superior measurement results using this new integration method, including two all-pole filters and a novel transmission-zero filter. Insertion losses as low as 1.42 dB for a two-pole filter and 2.45 dB for a four-pole quasi-elliptical type filter have been observed.