IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 17, NO. 2, FEBRUARY 2007
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High Aspect Ratio Vertical Cantilever RF-MEMS Variable Capacitor David M. Klymyshyn, Member, IEEE, Darcy T. Haluzan, Student Member, IEEE, Martin Börner, Sven Achenbach, Jürgen Mohr, and Timo Mappes
Abstract—An electrostatically actuated, microwave microelectromechanical system variable capacitor fabricated using deep X-ray lithography is presented. A single exposure has been used to produce the novel high aspect ratio microstructure, which includes a thin, vertically oriented, movable nickel cantilever beam and a 40:1 vertical aspect ratio capacitance gap. The 0.8-pF capacitor operates in the 1–5 GHz region and has -factors of 36 at 4 GHz and 133 at 2 GHz. The variable capacitance ratio is 1.24:1 over a 20-V tuning range at 4 GHz. Index Terms—Capacitors, microelectromechanical devices, micromachining, microwave devices, microwave switches, varactors, X-ray lithography.
I. INTRODUCTION
T
HE need for improved components is a critical challenge for future radio frequency (RF) and microwave systems, not only struggling with higher frequency applications but also with increasingly difficult performance requirements in the context of size and cost constraints. RF microstructures and microelectromechanical systems (MEMS) devices can offer advantages including improved isolation, lower power dissipation, and reduced size and weight [1], and are poised to replace conventional microwave devices in various applications. A promising device for tunable impedance and frequency control applications is the RF-MEMS variable capacitor [2], [3]. However, it has been difficult to consistently achieve high quality ( ) factor performance, particularly in the 3–10 GHz range. Lithographic processes for fabricating these and other RF–MEMS devices to date have been decidedly planar, generally producing thin layers of a few microns and constraining the devices to 2-D geometries. However, it is expected that significant advancements in RF–MEMS performance and integration will be achieved through the development of novel technologies for realizing 3-D structures [4]. One fabrication technology receiving increased attention for RF microstructures is deep X-ray lithography (XRL) [5]–[7], which is also part of the complete LIGA process [8]. Deep XRL is a microfabrication technology for precisely structuring thick polymers and is complementary to silicon-based technologies. High energy X-rays are used to Manuscript received April 27, 2006; revised October 17, 2006. This work was supported by the National Science and Engineering Research Council of Canada and by the Canadian Space Agency. D. Klymyshyn, D. Haluzan, and S. Achenbach are with TRLabs and the Department of Electrical Engineering, University of Saskatchewan, Saskatoon, SK S7N 5A9 Canada (e-mail:
[email protected]). M. Börner, J. Mohr, and T. Mappes are with the Institut für Microstrukturtechnik, Karlsruhe D-76021, Germany. Digital Object Identifier 10.1109/LMWC.2006.890338
pattern polymer resist layers with a typical thickness from 1–3000 m. The patterned polymers are subsequently used as electroforming templates for fabricating metal structures with optical quality sidewall smoothness and large vertical to lateral aspect ratios. These capabilities enable vertical geometries (up to millimetres) and also precise features (sub-micron). In particular, the use of the vertical dimension has a number of potential advantages including greater power handling capability, reduced physical size, reduced electrical size, larger coupling capacitances, and transmission structures with very low loss and dispersion due to separation of the electromagnetic fields from the substrate. Although static RF microstructures [5], [6] have already demonstrated some of these performance advantages, to the authors’ knowledge no actuated RF–MEMS device fabricated using deep XRL and operating at microwave frequencies has been reported. This paper presents first actuation and microwave measurements for a high aspect ratio vertical cantilever variable RF–MEMS capacitor fabricated using deep XRL. This new approach to microwave frequency RF–MEMS devices demonstrates not only a device with impressive structure, but also high factors at several GHz and low actuation voltages. II. VARIABLE CAPACITOR STRUCTURE AND FABRICATION A scanning electron microscope (SEM) image of the top view of the RF–MEMS variable capacitor is shown in Fig. 1. The capacitor is a 100- m tall, high aspect ratio nickel structure on an alumina substrate, and was designed for a nominal capacitance value of 0.8 pF in the 1–5 GHz range. The capacitor is composed of four structures, an actuator electrode, a capacitance electrode, and two ground structures, configured to facilitate testing with standard ground-signal-ground (GSG) wafer probes. The most unusual feature of this device is the thin “vertical” cantilever situated between the two electrodes as opposed to traditional planar cantilever structures which are parallel to the substrate. This implies that the bulk of the electric field is parallel to and largely unaffected by the supporting, and potentially lossy, substrate. The capabilities of deep XRL allow tall structures with very small capacitance gaps and almost unlimited control of cantilever and gap geometries. The cantilever is anchored to one ground structure and is the only portion of the device that is released from the substrate and is free to move. Applying a dc bias voltage between the actuator electrode and the grounded cantilever causes an electrostatic force between the two, which results in a deflection of the cantilever toward the actuator electrode. This decreases the capacitance between the cantilever and the capacitance electrode. The cantilever could
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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 17, NO. 2, FEBRUARY 2007
Fig. 1. Top view of RF–MEMS variable capacitor built with deep XRL.
be actuated toward either electrode to further increase the capacitance tuning range, as was done in [2], but this would involve more complicated RF and control signals and is not presented here. If the cantilever is actuated toward one electrode only, “pulling away” from the capacitance electrode results in a larger tuning range than “pulling toward,” provided the actuator gap is at least 1.24 times larger than the capacitance gap [9]. This increase in tuning range comes with some increase in tuning voltage, but RF and control signals can remain separate. X-ray exposure was done using the 2.5-GeV electron storage ring ANKA and beamline Litho-2 at Forschungszentrum Karlsruhe. A 150- m thick polymethyl-methacrylate (PMMA) photoresist foil (GS 233) was glued on a 1-mm thick alumina wafer coated with a 3- m oxidized titanium (Ti/TiO ) seed layer. This sample was exposed to X-rays through a mask consisting of 20- m thick gold absorbers on a 2.7- m thick titanium membrane. The bottom dose deposition was 3.5 kJ/cm . Following irradiation, megasonic-supported development was performed for 150 min in GG developer at room temperature. The Ti/TiO coating on the wafer was used as a plating base for 100- m thick nickel microelectroplating. The structure was then exposed to X-ray flood irradiation, allowing the remaining PMMA to be removed with another step of development. The structure was then descummed in oxygen plasma and wet etched with 5% HF acid for 2 min to remove the seed layer, electrically isolating the capacitor structures. This isotropic etching also selectively released the thin beam, while still providing good adhesion of the larger metal parts. This allows the plating base to function as a time-controlled etch sacrificial layer, releasing the cantilever beam without requiring an additional lithographic process. Figs. 2 and 3 show the tip of the cantilever beam (right side of Fig. 1) in greater detail. To support the processing of the long and narrow beam and gaps, auxiliary structures were added. The periodic widening of the cantilever beam and the triangular voids in the electrodes add strength to the long and thin capacitance gap and actuator gap resist walls as well as the cantilever beam. They also facilitate seed layer etching in the narrow gaps during the release of the cantilever beam. The design is meant to add maximum strength while least affecting the overall capacitance. Slight rounding of sharp corners can also be observed
Fig. 2. Detailed view of the tip of the cantilever beam.
Fig. 3. End view of the tip of the cantilever beam and air gaps.
which was done to prevent photoresist cracking during processing. Fig. 3 shows the end view of the tip of the cantilever beam and air gaps. The beam width is 11.0 m, the capacitance gap is 2.5 m, and the actuator gap is 7.3 m. The aspect ratio of the small capacitance gap is 40:1 in 100– m nickel and 60:1 in 150- m PMMA prior to resist stripping. Fig. 3 demonstrates not only the large aspect ratios of the cantilever and gaps, but also the excellent verticality and precise feature geometry obtained using deep XRL and metal electroforming. III. MEASUREMENT RESULTS The capacitors were tested using an Agilent 8722ES vector network analyzer connected using coaxial cables terminated with 150- m pitch Cascade ACP40-W-GSG-150 tungsten microprobes. DC control voltage was provided to the actuation electrode through one microprobe, while calibrated microwave one-port impedance measurements were obtained using the other microprobe connected to the capacitance electrode. Calibration of the test setup was performed using a Cascade 005–016 calibration impedance standard substrate.
KLYMYSHYN et al.: HIGH ASPECT RATIO VERTICAL CANTILEVER RF-MEMS VARIABLE CAPACITOR
Fig. 5.
Fig. 4. Static capacitance and impedance of the RF–MEMS variable capacitor versus frequency.
The measured static capacitance and impedance of the capacitor, with no applied actuation voltage, as a function of frequency is shown in Fig. 4. The capacitor was designed to have a nominal reactance of approximately 50 at an operational frequency of 4 GHz. Evident from Fig. 4 is a high quality capacitive reactance, with very low parasitic resistance of around 1 over the operating frequency range. At 4 GHz the capacitance is 0.84 pF and the -factor of the capacitor is 36.0. Also, the -factor increases significantly as the frequency is decreased. For instance, at a frequency of 2 GHz, the capacitance is 0.73 pF and the -factor is 133.0. Fig. 5 is a plot of the tuning characteristic of the capacitor at 4 GHz. As the actuation voltage is increased, the beam bends gradually toward the actuation electrode until the pull-in voltage of approximately 20 V is reached. Increasing the voltage beyond the pull-in voltage causes the beam to collapse onto the actuator electrode. With zero actuation voltage applied, the measured capacitance is 0.84 pF, with minimum -factor of 36.0. Just before 51.8), pull-in, the minimum capacitance value is 0.68 pF ( corresponding to a tuning range of 1.24:1. The measured capacitance after pull-in was 0.59 pF, corresponding to a tuning range of 1.42:1, with maximum -factor of 75.2. IV. CONCLUSION An actuated high aspect ratio vertical cantilever variable capacitor fabricated using deep X-ray lithography and operating at frequencies up to 5 GHz has been presented for the first time.
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C and Q-factor versus V tuning characteristic of the capacitor at 4 GHz.
This device demonstrates that deep XRL is suitable for fabrication of high performance RF-MEMS devices with movable parts. The device features impressive structure, for instance a 100 m tall, 2.5 m wide capacitance gap in nickel, corresponding to an aspect ratio of 40:1, running laterally for several hundred microns. The device has a nominal capacitance of 0.84 pF at an operational frequency of 4 GHz and a tuning ratio of 1.24:1 before pull-in at 20 V. The low series resistance of the thick nickel structural layer and the compact vertical architecture leads to relatively high -factors, for instance a -factor of 133.0 at 2 GHz, 59.9 at 3 GHz, and 36.0 at 4 GHz. Future work includes fabrication of devices with higher conductivity metals, smaller dimensions, and increased tuning ratio. REFERENCES [1] L. P. B. Katehi, J. F. Harvey, and E. Brown, “MEMS and Si micromachined circuits for high-frequency applications,” IEEE Trans. Microw. Theory Tech., vol. 50, no. 3, pp. 858–866, Mar. 2002. [2] A. Dec and K. Suyama, “Micromachined electro-mechanically tunable capacitors and their applications to RF IC’s,” IEEE Trans. Microw. Theory Tech., vol. 46, no. 12, pp. 2587–2596, Dec. 1998. [3] J. Zou, C. Liu, and J. Schutt-Aine, “Development of a wide tuning range two-parallel-plate tunable capacitor for integrated wireless communication systems,” Int. J. RF Microw. CAE, vol. 11, no. 5, pp. 322–329, Sept. 2001. [4] K. J. Herrick, J. G. Yook, and L. P.B. Katehi, “Microtechnology in the development of three-dimensional circuits,” IEEE Trans. Microw. Theory Tech., vol. 46, no. 11, pp. 1832–1844, Nov. 1998. [5] T. L. Willke and S. S. Gearhart, “LIGA micromachined planar transmission lines and filters,” IEEE Trans. Microw. Theory Tech., vol. 45, no. 10, pp. 1681–1688, Oct. 1997. [6] A. Kachayev, D. M. Klymyshyn, S. Achenbach, and V. Saile, “High vertical aspect ratio LIGA microwave 3-dB coupler,” in Proc. Int. Conf. MEMS, NANO, Smart Syst. (ICMENS’03), Jul. 2003, pp. 38–43. [7] D. T. Haluzan and D. M. Klymyshyn, “High-Q LIGA-MEMS vertical cantilever variable capacitors for upper microwave frequencies,” Microw. Opt. Technol. Lett., vol. 42, no. 6, pp. 507–511, Sep. 2004. [8] E. Becker, W. Ehrfeld, P. Hagmann, A. Maner, and D. Münchmeyer, “Fabrication of microstructures with high aspect ratios and great structural heights by synchrotron radiation lithography, galvanoforming, plastic moulding,” Microelect. Eng., pp. 35–36, May 1986. [9] D. T. Haluzan, “Microwave LIGA-MEMS Variable Capacitors,” M.Sc. thesis, Univ. Saskatchewan, Saskatoon, SK, Canada, 2004.
