1080 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010 Integrated Wideband 2-D and 3-D Transitions for Millimeter-Wave RF Front-Ends Amin Rida, Student Member, IEEE, Alexandros Margomenos, Member, IEEE, Jae Seung Lee, Member, IEEE, Paul Schmalenberg, Symeon Nikolaou, and Manos M. Tentzeris, Fellow, IEEE Abstract—This letter reports on broadband 3-D and 2-D tran- sitions on flexible organic liquid crystal polymer (LCP) substrate with return loss below 15 dB for frequencies up to 110 GHz. The presented novel 3-D coplanar waveguide–coplanar waveguide–mi- crostrip (CPW–CPW–MSTRIP) transition features an insertion loss (IL) of 0.45 dB for a 3.3-mm total (longitudinal) length of transition, while the novel 3-D CPW–CPW transition has an IL of 0.25 dB for a 2.8-mm (longitudinal) length of transition. These transitions require strategically placed vias and tapering of the CPW ground planes in order to suppress radiation loss and optimize the performance over a very broad frequency range. This letter also includes a 90 CPW bend that shows a return loss lower than 15 dB up to 100 GHz and an insertion loss of 0.75 dB for a 6.35-mm total length of the transition. All these transitions are simple to realize and are compatible with low-cost substrate fabrication guidelines allowing for the easy integration of ICs in 3-D modules, especially in compact automotive radar applications and beam-steering wideband antenna arrays. An example of the integration of the proposed 3-D transitions with a practical antenna array is presented, and experiments verify the very good performance of the integrated topology up to 100 GHz. Index Terms—2-D transition, 3-D transition, broadband transition, coplanar waveguide (CPW), integration, microstrip, millimeter-wave. I. INTRODUCTION L OW-COST, wideband, and miniaturized RF 3-D and 2-D transitions that are compatible with current state-of-the-art design rules on flexible materials are critical for an effective performance in millimeter-wave frequencies, the use of which is constantly growing to achieve increasingly higher bandwidths or better radar resolutions [1]–[3]. Realizing compact interconnects with low insertion and reflection loss over an ever-increasing frequency range, sometimes reaching Manuscript received September 09, 2010; revised November 03, 2010; ac- cepted November 04, 2010. Date of publication November 11, 2010; date of current version November 29, 2010. This work was supported by the Intercon- nect Focus Center, Semiconductor Research Corporation (IFC-SRC), Atlanta, GA. A. Rida, S. Nikolau, and M. M. Tentzeris are with the Electrical Engineering Department, Georgia Institute of Technology, Atlanta, GA 30332 USA (e-mail: arida@gatech.edu; simos.nikolaou@gmail.com; etentze@gatech.edu). A. Margomenos was with Toyota Research Institute North America, Ann Arbor, MI 48105 USA. He is now with HRL Laboratories, LLC, Malibu, CA 90265 USA (e-mail: admargomenos@hrl.com). J. S. Lee and P. Schmalenberg are with Toyota Research Institute North America, Ann Arbor, MI 48105 USA (e-mail: jae.lee@tema.toyota.com; paul.schmalenberg@tema.toyota.com). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2010.2091714 above 100 GHz, is paramount for high-sensitivity miniaturized RF applications. Such vertical architectures could potentially eliminate bulky high-loss waveguide transitions that are cur- rently used in many broadband millimeter-wave systems, especially in the field of automotive radars [4]. In addition to the 3-D transitions, the planar or 2-D transitions of microstrip and coplanar waveguide (CPW) lines become a critical part of RF modules with large antenna arrays. This is due to the far more stringent spatial requirements on the spacing and separation distances among the subarray antenna elements (typically ) in comparison to the interconnection spacing among the feeding lines to multichannel T/R modules or ICs . The purpose of this letter is to describe a method for the development of easy-to-fabricate low-cost 3-D and 2-D transi- tions on the flexible organic liquid crystal polymer (LCP) [5] for use in millimeter-wave (up to 100 GHz) front-end appli- cations such as broadband high-speed wireless LAN [1], au- tomotive radar, and imaging systems. LCP has recently gained much consideration as a potential high-performance microwave medium with excellent “dual” (substrate and packaging) func- tionality due to its very good electrical, mechanical, and her- meticity properties, not to mention its attractive high-frequency properties for developing millimeter-wave circuitry [5]. In addi- tion, LCP can be easily laminated and micromachined to form 3-D multilayer modules. Due to its near-hermetic characteristic, LCP can be easily used for the integration of embedded bare ICs without the use of additional packaging material—something that allows for the module miniaturization, but also necessitates the development of broadband low-loss 3-D CPW-to-CPW and CPW-to-microstrip (MSTRIP) interconnects. For the purpose of this work, the dielectric characterization of the utilized LCP material was performed using a free-space method [6], and the dielectric constant was measured to be around 3.0 for the frequencies of interest. II. WIDEBAND 3-D TRANSITIONS A recently reported 3-D micromachined transition on Si fea- tures good performance up to 50 GHz [7], but it is very costly due to the tedious formation of cavities. In [8], another broad- band vertical transition has been realized, however with sup- ported measurement results up to 60 GHz and with a four-layer cross section. In [9], an ultrawideband microstrip-to-CPW tran- sition has been reported with a 0.7-dB transition up to 40 GHz. Other reported efforts, such as [10], utilize waveguide-to-mi- crostrip line transitions from one layer to another. However, they are bulky and limited in bandwidth, and they require the use of waveguide-based topologies. In this letter, two simple wideband 1536-1225/$26.00 © 2010 IEEE