IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 56, NO. 5, MAY2009 721 Microfabricated Silicon High-Frequency Waveguide Couplers and Antennas Amy M. Marconnet, Mike M. He, Sean Sengele, Student Member, IEEE, Sung-Jin Ho, Hongrui Jiang, Member, IEEE, Nicola J. Ferrier, Member, IEEE, Daniel W. van der Weide, Senior Member, IEEE, Vidhya Madhavan, Natalie Nelson, and John H. Booske, Fellow, IEEE Abstract—A method has been developed to fabricate waveguide- to-waveguide couplers and tapered dielectric rod antennas for the millimeter-wave regime from microetched silicon. A proof-of- concept study shows that the structures can be realized using relatively simple wet etching and robotic process control. Ex- perimental measurements of the waveguide–waveguide couplers agree in key features with simulations. The results indicate that two-stepped tapers perform nearly as well as smooth linear tapers, but are much easier to fabricate. Coupling transmissivity of better than -1 dB, and peak antenna gain of 8–10 dB are indicated at W-band frequencies. Lateral dimension etch control of 5-μm precision was realized. To solve a challenge of controlling the length of the first step, either an improved masking method or a switch to dry etching processes is required. Index Terms—Dielectric rod antenna, dielectric waveguide, millimeter-wave, terahertz (THz). I. INTRODUCTION E MERGING needs exist for novel sources of high- frequency radiation sources, in the frequency ranges of 30–300 GHz (millimeter-wave) and 300–1000 GHz [terahertz (THz)]. These include high-data-rate communication, high- resolution radar, biomedical imaging, and remote sensing [1]–[6]. To exploit these new high-frequency sources, it is also necessary to develop specialized components, such as low- loss waveguides and antennas. This is particularly important in compact high-frequency vacuum electronic devices where novel methods for input and output coupling are needed that Manuscript received September 24, 2008; revised December 17, 2008. First published March 31, 2009; current version published April 22, 2009. This work was supported in part by the Northrop Grumman Corporation ERAB gift, by an L3 Industrial Affiliates grant, and by the Air Force Office of Scientific Research under Grant FA9550-05-1-0147. The review of this paper was arranged by Editor W. Menninger. A. M. Marconnet was with the Mechanical Engineering Department, Uni- versity of Wisconsin–Madison, Madison, WI 53706 USA. She is now with Stanford University, Stanford, CA 94305 USA (e-mail: amymarco@ stanford.edu). M. M. He was with the University of Wisconsin–Madison, Madison, WI 53706 USA. He is now with the University of California–Berkeley, Berkeley, CA 94702 USA. S. Sengele, S.-J. Ho, H. Jiang, D. W. van der Weide, V. Madhavan, N. Nelson, and J. H. Booske are with the Department of Electrical and Computer Engineering, University of Wisconsin–Madison, Madison, WI 53706 USA (e-mail: booske@engr.wisc.edu). N. J. Ferrier is with the Department of Mechanical Engineering, University of Wisconsin–Madison, Madison, WI 53706 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2009.2015415 are easily integrated with novel microfabricated interaction structures currently under development [6]. There are several methods of fabricating high-frequency waveguide components, both conventional and novel. Conven- tional methods include precision machining (including electric discharge machining) and electroplating. More novel methods use photolithography and either sacrificial polymer mold elec- troplating or LIGA [7] or deep reactive ion etching (RIE) [8]. However, fabrication of high-frequency high-gain antennas presents more challenges. The antennas must mate easily with the high-frequency waveguides. The beam must be well defined in both lateral dimensions. While horned waveguide antennas are possible, they must be flared in both dimensions which is challenging with planar microfabrication processes and can consume a significant amount of lateral space to achieve a highly directional beam. Tapered dielectric rod antennas provide an alternative with highly attractive properties. In principle, they provide an ar- chitecture that is easily integrated with miniature waveguide systems for millimeter-wave and submillimeter-wave (or THz regime) frequencies. Tapered dielectric rod inserts can also be used as an alternative to precision flanges for coupling or transitioning between separate waveguides [9]. Both theoretical [10] and experimental studies [11]–[13] have been previously reported. Dielectric rod waveguide transitions and antennas can be fabricated to provide high transmission and low loss. For good electromagnetic coupling, these components need to be fabricated from dielectric (nonconducting) materials and have precisely dimensioned shapes. In fact, experimental realiza- tions reported to date typically require fabricating the antennas from refractory high-permittivity materials such as ceramics or sapphire, and require precise mechanical grinding of del- icate brittle miniature parts [11]–[13]. While there is much discussion on possible applications and theoretical predictions of electromagnetic properties, there is little discussion as to scalable fabrication approaches. Therefore, the development of a repeatable reliable low-cost approach to fabricate this type of component would be a significant advance. In this paper, we expand on our previous work published in the International Vacuum Electronics Conference digest [14] and describe the results obtained with tapered rectangular cross section dielectric rod waveguide inserts and antennas fabricated by controlled etching of high-resistivity silicon. For this inves- tigation, an easily implemented wet-chemical etch approach was used and stepped-tapers rather than pyramidal-prismatic 0018-9383/$25.00 © 2009 IEEE