IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 50, NO. 3, MARCH 2002

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Millimeter-Wave Fermi Tapered Slot Antennas on Micromachined Silicon Substrates Jad B. Rizk, Student Member, IEEE, and Gabriel M. Rebeiz, Fellow, IEEE

Abstract—This paper presents 90-GHz Fermi-type tapered slot antennas (TSA) on micromachined 100- m-thick silicon substrate ( = 11 7) and for comparison purposes, 90-GHz Fermi-type TSA on 150- m-thick quartz substrate ( = 3 78). A 100- m -thick wafer is chosen because it is compatible with 90–100-GHz low-noise amplifier circuits on GaAs–InP substrates. The effective thickness of the substrate was reduced by selectively micromachining holes in the silicon wafer using deep reactive ion etching (deep RIE). The radiation patterns of the micromachined antennas were significantly better than the nonmicromachined version and had similar radiation patterns to the quartz design. The etched hole diameter was changed from 300 to 750 m with minor effect on the radiation patterns. This shows that the predominant reason for the improved patterns lies in the reduced effective dielectric constant and not in substrate-mode suppression effects. This type of antenna is well suited for millimeter-wave imaging arrays. Index Terms—Imaging arrays, millimeter-wave, silicon micromachining, tapered slot antennas.

42 m. This results in a mechanically fragile antenna and therefore is not practical in large arrays. Another approach is to micromachine holes in the substrate, resulting in a lower quasi-static (effective) dielectric constant. This technique has been applied successfully to microstrip antennas at 12 GHz [5] and to TSA on high dielectric constant ) and silicon substrates at 10–30 GHz [6]. Duroid ( Also, it was shown in [7] that the same technique can be used at 94 GHz for constant-width TSAs on low dielectric constant ). Also, Vowinkel et al. [8] have selectively Duroid ( removed large portions of the dielectric substrate inside the slot of the TSA with good results at 10 GHz. However, this method is not practical at W-band frequencies. The goal of this paper is to demonstrate the same technique at 90–100 GHz on silicon substrate and to do a careful experimental study on the effect of the hole diameter on the radiation patterns.

I. INTRODUCTION

T

APERED-SLOT antennas (TSAs) are desirable for millimeter-wave applications such as phased arrays and focalplane imaging systems due to their compact design, wide bandwidth, and endfire radiation pattern. TSAs were first developed by Gibson [1] and a major advance in their design was done by Yngvesson et al. [2], [3]. Another improvement was introduced by Sugawara et al. [4] by using a Fermi-type tapering that reduces the side lobes. Corrugated edges were also introduced to reduce the width of the TSA without degradation in radiation patterns. The main limitation of the TSA comes from its sensitivity to the thickness and dielectric constant of the supporting substrate. An effective thickness, which represents the electrical thickness . An of the substrate, has been defined as accepted range for good operation of a TSA has been experimentally determined by Yngvesson et al. [2] to be . For substrate thickness above the upper bound, unwanted substrate modes degrade the performance of the TSA, while antennas on thinner substrates suffer from decreased directivity. The upper bound on the effective thickness necessitates mechanically thin substrates for millimeter-wave applications, especially for high dielectric-constant substrates – ). In fact, to opsuch as silicon, GaAs, or InP ( erate at 90 GHz, the maximum allowable thickness on silicon is Manuscript received February 16, 2001; revised May 17, 2001. The authors are with the Radiation Laboratory, Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI 48109-2122 USA (e-mail: [email protected]; [email protected]). Publisher Item Identifier S 0018-926X(02)04570-2.

II. DESIGN OF W-BAND FERMI TAPERED SLOT ANTENNAS W-band Fermi TSAs (Fig. 1) were designed on 100- m-thick silicon substrate and on 150- m-thick quartz substrate. (13.3 mm) long where is the The antennas were 4wavelength in air at 90 GHz. The Fermi tapering follows with and . (2.85 mm) at 90 GHz. The aperture opening 2 is 0.86 The slot-line feed is 20 m wide corresponding to a slot-line impedance of 67 . A 300 400 m pad at the feed of the TSA is used to place the Schottky diode which was mounted and soldered using silver epoxy. Seven pairs of interdigital away from the diode to act as an capacitors were put RF short. Corrugated edges were used to reduce the width of the antenna without degrading the radiation patterns and thus increasing the packaging density of the array. The corrugation (570 m) and the corrugation width length CL is 0.17 , with a period of 2 CW. These dimensions were CW scaled from the work of Sugawara et al. [4]. Five different antennas were fabricated and tested. • One 4- -long TSA on a 100- m-thick silicon wafer with no micromachined holes. (This serves as a reference antenna on silicon to see the improvement due to micromachining.) • Three 4- -long micromachined TSA on 100- m-thick silicon wafer: 90–100- m-deep holes (with same diameter) were etched in the silicon substrate from the back side of the antenna using deep reactive ion etching (deep RIE). The diameters of the holes were 300, 600, and 750 m. • One 4- -long TSA on 150- m-thick quartz wafer for comparison purposes.

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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 50, NO. 3, MARCH 2002

(a)

(b) Fig. 1. Picture of the W-band Fermi TSA fabricated on 100-m-thick silicon substrate with (a) 300-m- and (b) 600-m-wide holes etched from the back side.

The holes’ pattern (Fig. 1) is a square lattice which results in and at 90 GHz (effective dielectric constant and effective thickness, respectively) as per (1) and (2), given below. The effective dielectric constant is a quasi-static value given by the volumetric average dielectric constant of the micromachined substrate. The square pattern makes an angle of 45 with the axis of the antenna. The diameter of the holes is m , m , or chosen to be m , and the spacing between the holes is such that , which is practically the minimum . The effective spacing achievable and results in the lowest dielectric constant does not depend on the hole diameter but only . on the ratio (1) for micromachined silicon wafers, and (2) for micromachined silicon wafers at 90 GHz.

Fig. 2. Measured patterns for TSA 4- -long on 100-m-thick silicon wafer with no holes. (a) E-plane pattern and (b) H-plane pattern, at 90 GHz.

For the reference TSA on silicon without any etched holes, and at 90 GHz. The effective thickness is well above the upper limit of the Yngvesson condition and should result in bad radiation patterns. For the TSA and at 90 GHz. In on quartz, fact, the thickness of the quartz wafer (150 m) was chosen to give an effective thickness (0.042) close to the one of the micromachined 100- m-thick silicon TSA for comparison purposes.

III. FABRICATION A 100- m-thick silicon wafer with 4000 of oxide on both sides is first mounted on a 500- m-thick mechanical wafer, and the holes are patterned using a 5- m-thick photoresist film. The oxide is then etched in the holes using buffered HF and the silicon is exposed to the deep silicon RIE etcher. The etch rate is 4.5–5 m per minute resulting in a total etch time of 20 min for 90–100- m-deep holes. The sidewall etch profile is 84 steep and the width of the etched hole increases by 10–15 m. The wafer is then flipped and mounted again on a 500- m -thick mechanical wafer. The antenna metal layer is patterned using a lift-off process and is 5000 thick. The 100- m wafer is released from the mechanical support wafer and attached at its edge to a glass microslide.

RIZK AND REBEIZ: MILLIMETER-WAVE FERMI TAPERED SLOT ANTENNAS ON MICROMACHINED SILICON SUBSTRATES

Fig. 3. Measured patterns for TSA 4- -long on 100-m-thick micromachined silicon wafer with 300-m-wide holes (dashed line) and on 150-m-thick quartz (solid line). (a) E-plane pattern and (b) H-plane pattern, at 90 GHz.

IV. ANTENNA PATTERN MEASUREMENTS AT W-BAND A Schottky diode (Siemens BAT 14-0775, Agilent 1GG5-4002) was mounted using silver epoxy at the end of the tapered slot. A 90-GHz Gunn diode was connected to a radiating W-band horn antenna with gain of 23 dB and AM modulated by a square wave at 1 kHz. The diode was biased at 10 A and connected to a lock-in amplifier. The distance between the transmitting horn and the TSA under test was around 1 m, satisfying well the far-field condition. The signal-to-noise ratio was around 20 dB with 0.3 s of integration time. Figs. 2–5 show the radiation patterns for the five antennas at 90 GHz. It is clear that the TSA on 100- m-thick silicon does not perform well, and micromachining is essential to reduce the effective dielectric constant. It is important to notice the similar radiation patterns of the TSA on 150- m quartz and the micromachined TSAs on silicon with 300- and 600- m-wide holes. These antennas have almost the same effective thickness, and therefore, the improvement in the micromachined silicon TSA is directly related to the effective dielectric constant of the substrate, and not to a

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Fig. 4. Measured patterns for TSA 4- -long on 100-m-thick micromachined silicon wafer with 300-m (dashed line) and 600-m-wide holes (solid line). (a) E-plane pattern and (b) H-plane pattern, at 90 GHz.

substrate-mode suppression phenomena due to the presence of the periodic holes. The 300- m- wide hole antenna and the 150- m-quartz antenna resulted in very similar cross-polarization components. The improvement in the micromachined TSA starts to deteriorate for a hole diameter of 750 m. In fact, in this case, the hole diameter is so large that only two hole periods are defined in the TSA aperture at the edge of the substrate. Therefore, the volumetric average of the dielectric constant cannot be used accurately. Still, the E-plane pattern is quite acceptable, but the H-plane pattern starts to break down. The measured sidelobe at 45 is due to the large diameter holes and moves to 45 if the antenna is flipped. The 3 and 10-dB beamwidths at 90 GHz of all the antennas are tabulated in Table I. The cross-polarization levels are less than 15 dB for all the antennas and are very similar for the quartz TSA and the 300- and 600- m-holes micromachined TSA. No directivity values are given since the 45 -plane co- and cross-polarization patterns were not measured. Compared to published results by Muldavin et al. [7], one can readily see the effect of the corrugated edges in cancelling the sharp sidelobes due to the edge currents on the finite-width ground plane [4].

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Fig. 5. Measured patterns for TSA 4- -long on 100-m-thick micromachined silicon wafer with 300-m (dashed line) and 750-m-wide holes (solid line). (a) E-plane pattern and (b) H-plane pattern, at 90 GHz.

TABLE I

Fig. 6. Measured patterns for TSA 4.67 long on 100-m-thick micromachined silicon wafer with 300-m-wide holes (dashed line) and on 150-m-thick quartz (solid line). (a) E-plane pattern and (b) H-plane pattern, at 105 GHz.

V. CONCLUSION

MEASURED BEAMWIDTHS FOR TSAS AT 90 GHZ

The quartz TSA and the micromachined silicon TSA with 300 m holes were also measured at 105 GHz (Fig. 6). The anlong with an aperture opening of 1 . tenna becomes 4.67 The measured patterns of the quartz and the micromachined TSA are very similar and are much better than the nonmicromachined case (not shown). This shows that the effective dielectric constant model is quite wideband and the design shown in Fig. 1 can easily cover the 80–110-GHz range.

Fermi-type TSAs on micromachined 100- m-thick silicon wafers and on 150- m-thick quartz wafers were built and tested at 90 GHz. The effective dielectric constant on silicon was reduced from 11.7 to 5.1 due to the micromachining approach. The micromachined TSAs resulted in very good patterns similar to the TSA on 150- m-thick quartz substrate. We believe that the improvement in the radiation patterns for the micromachined silicon TSAs is due to an overall decrease in the average volumetric dielectric constant and not from a substrate mode suppression effect: the hole diameter did not have a significant impact on the radiation pattern due to the relatively thin substrate used (100 m at 94 GHz). The micromachining approach should be useful up to 120 GHz for TSAs on silicon or GaAs substrates. However, for higher frequencies it is not possible to get an effective thickness ), close to the range given by Yngvesson ( because it is hard to get a low effective dielectric constant on silicon (less than 4.0) due to the mechanical fragility of the antenna.

RIZK AND REBEIZ: MILLIMETER-WAVE FERMI TAPERED SLOT ANTENNAS ON MICROMACHINED SILICON SUBSTRATES

REFERENCES [1] P. J. Gibson, “The Vivaldi aerial,” in Proc. 9th Eur. Microwave Conf., Brighton, U.K., June 1979, pp. 101–105. [2] K. S. Yngvesson, D. H. Schaubert, T. L. Korzeniowski, E. L. Kollberg, T. Thungren, and J. F. Johansson, “Endfire tapered slot antennas on dielectric substrates,” IEEE Trans. Antennas Propagat., vol. 33, pp. 1392–1400, Dec. 1985. [3] K. S. Yngvesson, T. L. Korzeniowski, Y. S. Kim, E. L. Kollberg, and J. F. Johansson, “The tapered slot antenna: A new integrated element for millimeter wave applications,” IEEE Trans. Microwave Theory Tech., vol. 37, pp. 365–374, Feb. 1989. [4] S. Sugawara, Y. Maita, K. Adachi, K. Mori, and K. Mizuno, “A mm-wave tapered slot antenna with improved radiation pattern,” in IEEE MTT Int. Microwave Symp., Denver, CO, June 1997, pp. 959–962. [5] G. P. Gauthier, A. Courtay, and G. M. Rebeiz, “Microstrip antennas on low dielectric-constant substrates,” IEEE Trans. Antennas Propagat., vol. 45, pp. 1310–1314, Aug. 1997. [6] T. J. Ellis and G. M. Rebeiz, “Millimeter-wave tapered slot antennas on micromachined photonic bandgap dielectrics,” in IEEE MTT Int. Microwave Symp., San Francisco, CA, June 1996, pp. 1157–1160. [7] J. B. Muldavin and G. M. Rebeiz, “Millimeter-wave tapered slot antennas on synthesized low permittivity substrates,” IEEE Trans. Antennas Propagat., vol. 47, pp. 1276–1280, Aug. 1999. [8] U. K. Kotthaus and B. Vowinkel, “Investigation of planar antennas for submillimeter receivers,” IEEE Trans. Microwave Theory Tech., vol. 37, pp. 375–380, Feb. 1989.

Jad B. Rizk (S’00) received the Diplôme d’Ingénieur in electrical engineering from Saint-Joseph University, Beirut, Lebanon, in 1997, and the M.S. degree in electrical engineering from the University of Michigan, Ann Arbor, MI, in 2000, where he is currently working toward the Ph.D. degree. His research interests are in applying micromachining techniques and MEMS for the development of planar antennas and microwave/millimeter-wave circuits for communication systems.

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Gabriel M. Rebeiz (F’97) received the Ph.D. degree in electrical engineering from the California Institute of Technology, Pasadena, in 1988. He joined the faculty of the University of Michigan, Ann Arbor, in September 1988, and was promoted to Full Professor in 1998. He has held short visiting professorships at Chalmers University of Technology, Gothenburg, Sweden, the Ecole Normale Superieur, Paris, France, and Tohoku University, Sendai, Japan. His research interests include applying micromachining techniques and microelectromechanical systems (MEMS) for the development of novel components and sub-systems for radars and wireless systems. He is also interested in Si/GaAs RFIC design for receiver applications and in the development of planar antennas and microwave/millimeter-wave front-end electronics for communication systems, automotive collision-avoidance sensors, monopulse tracking systems, and phased arrays. Dr. Rebeiz was the recipient of the National Science Foundation Presidential Young Investigator Award in April 1991 and the URSI International Isaac Koga Gold Medal Award for Outstanding International Research in August 1993. He also received the Research Excellence Award in April 1995 from the University of Michigan. Together with his students, he is the winner of best student paper awards at IEEE-MTT (1992, 1994–1999) and IEEE-AP (1992, 1995), and received the JINA’90 best paper award. He received the University of Michigan EECS Department Teaching Award in October 1997 and was selected by the students as the 1997–1998 Eta-Kappa-Nu EECS Professor of the Year. In June 1998, he received the College of Engineering Teaching Award. In October 1998, he received the Amoco Foundation Teaching Award, given yearly to one member of the faculty at the University of Michigan, for excellence in undergraduate teaching. He is the co-recipient (with S. Barker) of the IEEE 2000 Microwave Prize for his work on MEMS phase shifters.