Millimeter-Wave Antenna Array on Silicon With Embedded Cavity-Backed Structure Hong Phuong Phan, Manh Ha Hoang
Gustavo Rehder
Department of Telecommunications Engineering Ho Chi Minh City University of Technology Ho Chi Minh City, Vietnam
[email protected],
[email protected]
Laboratory of Microelectronics University of São Paulo São Paulo, Brazil
Tan-Phu Vuong IMEP-LAHC, MINATEC, INP Grenoble Grenoble, France Abstract— In this paper a new silicon-based antenna structure with an embedded back cavity is presented. The basic antenna with an optimized geometry has good impedance matching with return loss of -40 dB and gain of 7.6 dBi at the resonant frequency of 60 GHz. The linear arrays of four antennas with the corporate feed network have been built with three different forms of rectangular back cavity to compare its impact on the directivity and total efficiency of the antenna. As a result, the array with closed-box cavity gives the best performance with directivity of 16.51 dBi with total efficiency of –1.22 dB. Keywords— millimeter-wave antenna; cavity-backed antenna; silicon technology; antenna array
I.
INTRODUCTION
With increasing requirements for data rate, communications systems at 60 GHz such as Wireless Personal Area Networks (WPAN) have intensively been developed for the last two decades. Thus, the problem of antenna design to integrate into millimeter-wave transceivers has become essential but challenging due to the small dimensions, limitations of fabrication technology and careful testing. Besides, gain enhancement for millimeter-wave antennas is an important issue discussed in a series of publications. Bo Pan et al. proposed a SIW-based horn antenna structure, with rows of metalized pillars, and reached a gain of 14.4 dBi [1]. Some other authors [2,3] used periodic structures on dielectric as superstrates to design high gain antennas at 60 GHz. In this paper the authors present a new antenna structure based on silicon technology with an “embedded” back cavity and the possibility of building antenna array with different forms of the back cavity for gain enhancement. The antennas are CPW-fed and considered to be integrated into 60 GHz transceivers. They can be realized and measured using the facilities at the laboratory of IMEP-LAHC, Grenoble, France.
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Constantine A. Balanis School of Electrical, Computer and Energy Engineering Arizona State University Tempe, Arizona II.
ANTENNA STRUCTURE ON SILICON TECHNOLOGY
The proposed basic design of this paper is based on the well-known back cavity principle stated in a series of papers [4]–[6] and in textbooks [7]. The main differences are firstly that the "cavity" walls are made of silicon (instead of metal) and secondly, the CPW feeding method that facilitates the testing process and integration with millimeter-wave transceivers. The proposed structure is shown in Fig. 1(a). The antenna is to be fabricated on a 550-µm-thick low resistivity (1-10 Ω⋅cm) silicon substrate. The silicon substrate is covered (front and back) with a 4-µm-thick low-stress silicon dioxide film deposited by PECVD (relative permittivity of 4 and loss tangent of 0.001). A thin (~20 nm) titanium film is used for adherence of the also thin (~20 nm) copper film that is used as a seed layer for the electrodeposition of the radiating element. Conventional photolithography is used to define the geometry of the patch antenna prior to the electrodeposition of copper. A commercial electroplating solution from EPI is used to produce low stress films with 3 µm thickness. After the electrodepositon process, the copper seed and titanium adherence layers are removed from the unwanted areas. The backside silicon dioxide is patterned by lithography in order to expose the silicon substrate where the cavity will be inserted. The cavity is created by etching the silicon substrate in KOH solution (28% at 80°C) until the silicon is completely removed from underneath the patch antenna. Finally, the silicon substrate is bonded to a copper plate to create a reflector in order to increase the antenna gain. For the design of the radiating element, a circular patch is chosen with approximately half-wavelength radius as according to our experience for this structure, circular-form CPW-fed antennas give better radiation patterns than other basic forms. In our simulations, the form of the cavity is assumed to be rectangular.
Optimization process in both simulation tools Ansoft HFSS and CST Microwave Studio gives similar results indicating that for L=4800um, W=4600um, LS=3460um, WS=3160um, Rpat=1200um and slot=30um, good impedance matching is obtained with return loss of -40dB at 60 GHz and gain of 7.6 dBi [Fig. 1(b)] and [Fig. 1(c)]. Based on the structure presented above, a linear array of four radiating elements has been built to improve the antenna directivity (Fig. 2). For the simple rectangular back cavity a directivity of 10.18 dBi is obtained [Fig. 3(a)]. However, the low total efficiency of –3.8 dB causes a considerable reduction of the antenna gain that can be explained by the coupling effect of the radiating elements and the impact of the feed network. The silicon “walls” are added to the structure in [Fig. 3(b)] increase the directivity to 13.78 dBi and the total efficiency to –1.82 dB. Finally, the array structure with “closed-box” back cavity under each radiating element enables to prevent the coupling between separate antennas and gives the best radiation characteristics with directivity of 16.51 dBi and total efficiency of –1.22 dB [Fig. 3(c)].
III.
An antenna structure is proposed based on silicon technology with an “embedded” back cavity that can reduce surface waves and improve the total efficiency. In addition, the linear arrays of four radiating elements with three different forms of the back cavity have been built to compare the effect of the cavity on the antenna gain. Among them, the array structure with “closed-box” back cavity under each radiating element gives the best directivity of 16.51 dBi and total efficiency of –1.22 dB. ACKNOWLEDGMENT This work is supported by Rhone-Alpes region, France through the CMIRA 2013-2015 project and Vietnam Education Foundation (VEF). The authors would like to thank Dr. James Aberle, Arizona State University for his valuable advice in the design process. REFERENCES [1]
[2]
(b)
[3]
[4]
[5] (a)
(c)
Fig. 1. The basic antenna structures (a); (b) Return loss; (c) Total gain
CONCLUSIONS
[6]
[7]
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Fig. 2. 3D structure of the linear antenna array
(a) (b) (c) Fig 3 The antenna arrays with different forms of the rectangular back cavity and there beam patterns
