0018-926X (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TAP.2018.2863098, IEEE Transactions on Antennas and Propagation 1 Abstract—A novel W-band (75–110 GHz) surface micromachined Vivaldi antenna fed with a new combination of coplanar waveguide (CPW) and substrate integrated waveguide (SIW) is proposed. The design procedure of the feeding structure and the effects of the antenna’s key structural features on its return loss and gain are studied in detail. The selected exponential taper with optimized feeding parameters results in a significantly higher gain (gain > 10 dBi) compared to similar tapered slot antennas reported in the literature over the entire bandwidth of operation. The antenna is fabricated using microfabrication processes, and high correlations between simulation and measurement results confirm its low- loss performance and symmetric and stable radiation pattern over the entire frequency range of 75–110 GHz. Index Terms—Broadband antennas, micromachining, millimeter waves, tapered slot antenna, W-band tapered slot antenna I. INTRODUCTION Considerable research efforts have been devoted in recent years to the design and implementation of systems operating in the millimeter- wave and sub-millimeter-wave regime such as radars, communication devices, and imaging instruments [1]. The higher data rates and/or precision localization requirements in these applications have instigated a need for radio frequency (RF) front-ends in the W-band (75–110 GHz) and even higher frequencies. On the other hand, to compensate for high propagation losses at millimeter-wave frequencies, high-gain, low-cost, and light-weight antennas are required. A promising antenna candidate for these applications is the tapered slot antenna (TSA) with its planar structure, high gain, wide bandwidth, and moderate integration complexity with active devices and arrays [2]. It also exhibits excellent polarization performance in the millimeter-wave regime [3]. TSAs have been extensively studied with many variations over the past 30 years [4]-[11]. Although TSAs have been actively pursued in the millimeter-wave frequency regime, they are mostly designed at lower side of the spectrum (< 50 GHz). At higher frequencies (> 50 GHz), the performance of TSAs degrades, mainly due to high losses and limited bandwidths of their feeding structures. A summary of TSAs reported in the literature operating at frequencies above 50 GHz is shown in Table I [12]-[15]. As seen, all these antennas have a gain of less than 10 dBi. The feed-lines used in these designs include microstrip line [12], [13], coplanar waveguide (CPW) [14], and substrate-integrated waveguide (SIW) [15]. At frequencies higher than 30 GHz, microstrip lines show high losses, degrading antenna This work is supported by the U.S. National Science Foundation (NSF) under grant number CAREER 1554402. A. Mirbeik-Sabzevari and N. Tavassolian are with the Department of Electrical and Computer Engineering, Stevens Institute of Technology, Hoboken, NJ 07030 (e-mails: {amirbeik, negar.tavassolian}@stevens.edu) efficiencies [16]. On the other hand, direct CPW connections require a CPW-to-slotline transition. Such transitions are generally bandwidth- limited as they incorporate frequency-dependent transition elements, namely ‘baluns’ [17]. The SIW technology has been proposed to replace regular waveguide components up to the W-band [18], [19]. In addition, SIW losses at millimeter-wave frequencies are an order of magnitude lower than those of microstrip lines [16]. However, only a few TSAs have incorporated this technology in the millimeter-wave [15] and microwave [20], [21] regimes. In these cases, the SIW is fed either by a microstrip line resulting in high radiation losses [15], or by a grounded CPW (GCPW) [20], [21] resulting in a limited bandwidth due to the excitation of higher-order (waveguide-type) modes [22]. In this work, we present a novel W-band antipodal Vivaldi TSA, fed with a combination of CPW and SIW. The main innovation of our work is in the feeding structure of the Vivaldi antenna, rather than the slot profile itself. However, a parametric study is also performed which optimizes the slot profile and its taper growth rate for optimal performance characteristics over the entire frequency band of interest. A high gain of 10–12 dBi is achieved over the entire frequency range of interest (75–110 GHz) due to the low loss of the proposed feeding structure. The achieved gain performance is the highest in such a wide impedance bandwidth (75–110 GHz) among all reported millimeter- wave TSAs. SIW components are typically fabricated using the printed circuit board (PCB) technology at lower frequencies. However, PCB fabrication cannot render spacing and via widths which are sufficiently small for operation at millimeter-wave frequencies. Micromachining will therefore be used to address this issue and meet the requirements of small size and high accuracy. The paper is organized as follows. Section II discusses the design procedure of the feeding structure in detail, followed by a parametric study on the slot profile and taper growth rate. Details of the antenna fabrication process are also provided. Section III presents the measurement procedure and results. Section IV presents the S. Li, E. Garay, H. Nguyen, and H. Wang are with the Department of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332 (e-mails: {lisensen, egara001, huythong, hua.wang}@gatech.edu). Amir Mirbeik-Sabzevari, Student Member, IEEE, Sensen Li, Student Member, IEEE, Edgar Garay, Student Member, IEEE, Huy-Thong Nguyen, Student Member, IEEE, Hua Wang, Senior Member, IEEE, Negar Tavassolian, Senior Member, IEEE W-Band Micromachined Antipodal Vivaldi Antenna Using SIW and CPW Structures TABLE I MILLIMETER-WAVE TAPERED SLOT ANTENNAS USING MICROSTRIP (MS), COPLANAR WAVEGUIDE (CPW), AND SUBSTRATE-INTEGRATED WAVEGUIDE (SIW) STRUCTURES Reference Feed-line type Gain [dBi] Bandwidth [GHz] [12] MS 5 – 10 70 – 92 [13] MS 8 75 – 82 [14] CPW 8 – 10 75 – 110 [15] SIW – MS 2 – 5 25 – 55 This work SIW – CPW 10 – 12 75 – 110