A Non-galvanic D-band MMIC-to-Waveguide Transition Using eWLB Packaging Technology Ahmed Hassona 1 , Zhongxia Simon He 1 , Chiara Mariotti 2 , Franz Dielacher 2 , Vessen Vassilev 1 , Yinggang Li 3 , Joachim Oberhammer 4 , Herbert Zirath 1 1 Chalmers University of Technology, Gothenburg, Sweden 2 Infineon Technologies Austria AG, Villach, Austria 3 Ericsson AB, Gothenburg, Sweden 4 KTH Royal Institute of Technology, Stockholm, Sweden Abstract—This paper presents a novel D-band interconnect implemented in a low-cost embedded Wafer Ball Grid Array (eWLB) commercial process. The transition is realized through a patch slot antenna directly radiating to a standard waveguide opening. The interconnect achieves low insertion loss and good bandwidth. The measured minimum Insertion Loss (IL) is 2 dB and the average is 3 dB across a bandwidth of 22% covering the frequency range 110-138 GHz. In addition, the structure is easy to integrate as it does not require any special assembly nor any galvanic contacts. Adopting the low-cost eWLB process and standard waveguides makes the transition an attractive solution for interconnects beyond 100 GHz. Index Terms—D-band, interconnects, waveguide transition, eWLB, millimeter waves, THz. I. INTRODUCTION The ever increasing advance in semiconductor technologies makes mm-wave technologies very attractive for several wireless applications from telecom to safety, production quality check and several other applications [1]. In this context, one of the biggest challenges that researchers face, is the realization of low-loss and low-cost interconnection and high-level integration. Various approaches are proposed in literature in order to couple the RF signal to the MMIC at mm-wave frequencies. One possibility is the integration of the antenna on chip, which is a very compact solution with the drawback of low antenna efficiency and limited bandwidth since most of the broad-side antennas are resonant structures [2]. Another option is to couple the MMIC directly to a waveguide and hence achieve efficient coupling over broader frequency range. However, most highly integrated circuits are relatively large in size with respect to the wavelength and therefore the integration of MMIC-to-waveguide transitions on chip is hardly possible. In this case, a separate transition is needed not only because of the area limitation but also to prevent waveguide modes from leaking into the circuit cavity. The drawback of this solution is that it requires a bond-wire interface between the waveguide-transition and the MMIC. The use of bondwires is not suitable at such high frequencies since they introduce inductive effects and require special measures to provide reasonable return-loss for the interface [2]. Embedded wafer level ball grid array (eWLB) provides an attractive solution for packaging MMIC into a ball grid array surface mountable module with hundreds of I/O connections. Low frequency I/O has been proposed and studied in [3] using standard eWLB connections. High frequency I/O connection up to 100 GHz has been proposed using antenna-like structures [3]. To the authors’ knowledge, sub mm-wave interconnects on eWLB technology has not been proposed before. In this work, a generic approach for a MMIC-to-waveguide transition based on eWLB process is proposed with the support of experimental results. The choice of eWLB technology is motivated by the need of a low-cost and high- volume process for interconnects that operate at mm-wave frequencies. This has been a major challenge hindering commercialization of mm-wave systems. To the authors’ knowledge, this is the first attempt to fabricate a transition beyond 100 GHz using eWLB process. II. TRANSITION DESIGN The proposed solution consists of an eWLB chip with an embedded MMIC, Ball grid array (BGA) for low frequency I/Os, eWLB antenna for I/Os above 100 GHz, a PCB to support eWLB and a standard air-filled metal waveguide. The complete solution is shown in Fig. 1. The eWLB process provides two metal layers named redistribution layers (RDL), mainly used for I/O connections. In this work, the redistribution layers are also used to realize the antenna that couples the signal into the waveguide avoiding galvanic connection. The dielectric constant of the substrate is 3.2 which is suitable for limiting the leakage into the substrate. The substrate height is 0.45 mm which is close to quarter wavelength at D-band and hence, placing the chip on a conductive surface, would make it act as a backshort leading to better radiation into the waveguide. Fig. 1. Simplified schematic for the proposed solution