Self-assembled screen-printed microwave inductors

Introduction: With traditional radio frequency (RF) circuits, planar inductors lie on their host substrate and, as a result, suffer from the unwanted effects of low unloaded Q-factors and low first self-resonant frequencies. The former restricts the use of these inductors to poor selectivity and lossy applications, while the latter presents a limitation on the useful bandwidth of operation. This combined degradation in performance is particularly problematic when the substrate has a high loss tangent and dielectric constant, respectively. As a way of limiting these problems, either surface or bulk micromachining can be employed to spatially separate the current carrying inductor from the substrate [1]. Self-assembly is one method among a number of solutions. Examples of self-assembled inductors on lossy silicon substrates, fabricated using surface micromachining with conventional microfabrication processing, have been reported that exploit surface tension within solder hinges [2, 3]. Traditional screen printing methods have long been used for the manufacture of low-cost planar RF circuits. In recent years, 3-D structures normally associated with thin-film microfabrication processing have been realised by thick-film screen printing. This offers considerable benefits, including higher package densities, being able to fabricate over larger areas and lower costs. For example, a miniature high performance 60 to 80 GHz dielectric-filled metal-pipe rectangular waveguide was demonstrated using screen printing [4]. Screen printing technology has already been applied to conventional microfabrication for non-RF microelectromechanical systems (MEMS) applications, e.g. the printing of piezoelectric material onto a silicon membrane [5]. In contrast, an RF MEMS fabrication technology gap has been recently identified that can be filled by applying conventional microfabrication technology to screen printing [6]. To this end, an entirely new fabrication concept emerges. This Micromachined Screen Printing (MaSPrint) technology [6] offers the potential for substantial reductions to the cost of manufacturing micromachined and MEMS components. Also, it makes such technologies accessible to small manufacturing companies that do not have their own, prohibitively expensive, clean room facilities. This Letter describes the fabrication and measured performance of the first RF micromachined component to be realised using MaSPrint technology.