MEMS Millimeterwave Components - Semantic Scholar

1999 IEEE MTT-S International Microwave Symposium

MEMS Millimeterwave Components Jung-Chih Chiao, Yiton Fu, Debabani Choudhury*, Lih-Yuan Lin** Department of Electrical Engineering, University of Hawaii – Manoa * HRL Laboratories, Malibu, CA **AT&T Research Laboratory, Red Bank, NJ

ABSTRACT New microelectromechanical system (MEMS) components that enable reconfigurable MEMS millimeterwave transceivers were presented with a focus on the device architectures for system integration. The architectures for reconfigurable antennas, planar impedance tuners, microswitches and variable capacitors were demonstrated using the three-polysilicon-layer process.

INTRODUCTION Recent progress in monolithic millimeterwave active devices has made it possible for implementation of chipscale integrated millimeterwave systems. However, due to the low output power of solid-state sources and high losses in tuning and switching elements, achievement of high-power or high-sensitivity systems is still a challenge. Efforts toward reduction of insertion losses in transmission lines and increasing of Q-factors in passive elements have shown significant improvement and great promise [1-3]. To further develop a complete millimeterwave system, reconfigurable components with low losses and high Q-factors are also needed. Microelectromechanical system (MEMS) devices, which can provide the advantages of fast actuation, low losses and high Qs, become an attractive option for RF systems [4]. As frequencies increase to millimeterwaves, the antenna and component sizes reduce to the scales of MEMS architectures, which provide for the possibility of fast actuation with low power dissipation by microactuators

Figure 1: The architecture of a MEMS reconfigurable transceiver.

[5,6]. Two- and three-dimensional reconfigurable radiation and wave-guiding structures can be constructed by using metal-coated polysilicon, metal-to-metal contacts and sacrificial materials on the surface of the devices. This allows the waves to propagate and be manipulated in the free space and reduces the dielectric losses and parasitic reactance in the substrate. It also increases the Q-factors due to the low series resistance of metal. In this paper, we investigated several key components for a reconfigurable transceiver. Fig. 1 shows a systemlevel schematic drawing including a reconfigurable Veeantenna, planar impedance tuners, microswitches and variable capacitors. In the architecture, flip-chip integration of monolithic millimeterwave integrated-circuits with MEMS devices is proposed. The flip-chip approach separates the fabrication of the MEMS chips and electronic circuits (RF and IF), allowing for integration of many different substrate materials, increasing of yield rates and reduction of costs. The main structures of all components were fabricated using the three-polysilicon-layer surface micromachining process offered by the Microelectronics Center of North Carolina (MCNC). Post-processing including deposition and etching of dielectric material, metal and silicon was done at the University of Hawaii - Manoa.

RECONFIGURABLE ANTENNA Figure 2 shows the concept and a photo of a MEMS reconfigurable Vee-antenna. The arms of the Vee-antenna are moveable through pulling or pushing by microactuators. Each antenna arm can be controlled independently with forward- or backward-moving biases. When both antenna arms move in the same direction with a fixed Vee-angle, the antenna can be used to steer the radiation beam. When the Vee-angle changes, the radiation beam shape can be adjusted. The reconfigurability and antenna performance have been demonstrated [7]. Beam-steering at 30o and 45o and beam-shaping for Vee-angles of 75o, 90o and 120o have been demonstrated for a 17.5-GHz MEMS Vee-antenna.

MEMS Millimeterwave Components


losses. For a membrane-based switch, it requires a tall air gap when the switch is open and a strong bending of Figure 3 shows the concept and a photo of the planar the membrane diaphragm when the switch is closed. To impedance tuner. The operation principle is similar to achieve high isolation and a good connection, we prothe one for mechanical waveguide backshort tuners used posed a derrick-type microswitch. Figure 5 shows the in metal waveguides [8]. A sliding planar backshort plate on top of a planar transmission line forms a moveable short-circuit over a useful bandwidth. It allows a variation for the electrical length of the transmission line by varying the position of the sliding planar backshort. The planar backshort has a cascade of several sections of short-circuit separated by quarter wavelengths on the sliding plate. Lubecke demonstrated a 3-dB improve(a) ment of detector response over the untuned response at 100GHz using a quasi-optical approach and a 5-section Actuators planar backshort without electrically-controlled actuaVee Antenna Contact Pads tors [9]. Planar tuners on coplanar strips (CPS), shown Moveable Rotation in Fig. 3(b), and coplanar waveguide (CPW) transmisHinges Fixed sion lines have been designed and fabricated to investiRotation Hinge gate the tuning characteristics and the integration of (b) sliding planar backshort plates with microactuators.

PLANAR IMPEDANCE TUNER

Chiao 1998

MICROSWITCHES Low insertion losses and low power dissipation for micromachining membrane-based switches, such as singlepole cantilever switches and air-gap switches, have been demonstrated. The results are very promising and have initiated many millimeterwave applications. Practical application issues for the membrane-based switches include stiction of metal or membrane, stress control and reliability of the membrane [10]. Therefore, we demonstrated new MEMS architectures for microswitches using polysilicon to construct rigid structures and microhinges [11] for mechanical movement to eliminate the bending of membrane. Figure 4 shows the concept and a photo of a see-saw-bar switch. A see-saw bar, with bias electrodes on both ends, is held by hinges on the sides of the bar. One end of the bar is attached to a metal contact pad to make connection between contacts. With biases on different bias electrodes, the metal contact can be pulled up or pushed down to open or close the connection. The rigid polysilicon bar helps prevent stiction and degradation associated with the thin-film membrane. A 4-µm air-gap was achieved with an actuation voltage of 20v. The DC series resistance is negligible and RF performance is currently under investigation. For a transceiver, it is necessary to have switches in the T/R module with high RF isolation and low insertion

Figure 2: (a) The concept and (b) top view of a MEMS reconfigurable Vee-antenna.

(a) Sliding plate

Actuator

Coplanar Stripline

(b) Figure 3: (a) The concept and (b) a prototype for a sliding backshort impedance tuner.

(a) Actuation Bias Contact

50µm Chiao 1998

Hinge

(b) Figure 4: (a) The concept and (b) a photo for a seesaw-bar switch.



concept and a photo for a derrick-type switch. The microactuators pull away or push forward the pull/push arms which are connected on the sides of the support bar with moveable hinges. With one end of the support bar held by fixed hinges on the substrate, the moveable hinges on both ends of the pull/push arms translate the lateral movement of the actuators to a vertical movement of the support bar. A metal contact pad, attached in the end of the support bar, will be pulled up to open, or pushed down to close the interrupted transmission line. In the design, the air gap between the metal contact and the transmission line can be as high as 100µm. The measured switching time for a 100-µm gap is 150µs. The strong actuation force of scratch drive actuators [6] can provide a firm contact between metal pads. The RF performance and reduction of switching time is currently under study.

capacitor is more than 200v, where the measurement was limited by the availability of high-voltage sources. A different architecture for MEMS parallel-plate capacitors is also under investigation. In some applications, the variable capacitors are not required to have a large dynamic range, but a linear operation and compact configuration. Fig. 8 shows a circular parallel-plate variable capacitor. The coupling area between the top and bottom plates can be varied by the microactuators

(a)

PARALLEL-PLATE VARIABLE CAPACITORS Figure 6(a) shows the concept of a MEMS parallel-plate variable capacitor consisting of two parallel metal plates with variable gap spacings. The microactuators pull or push the support arms. The microhinges on both ends of the support arms translate the lateral movement of the microactuators to a vertical motion in order to vary the gap spacing between two metal plates. With the actuators on both sides moving toward the plate at the same time pushing the support arms up, the gap spacing is increased. With the actuators moving away from the plate, the gap spacing is reduced. The form of the device is similar to the ones in [12,13] but with different mechanical and RF design. The variable capacitance should follow the formula, C(x) = εoA/x + Cp, where A is the plate size, x is the gap spacing, εo is the dielectric constant of air and Cp is the parasitic capacitance. The actuators can be programmed so that not only the height of the gap is variable, but the coupling area (A) between two metal plates can also be varied for a linear operation of variable capacitance. Several parallel-plate capacitors, with plate sizes in the ranges from 100µmx100µm to 2mmx2mm, were designed and fabricated, as shown in Fig 6(b). The gap spacings can be varied between 1µm and 100µm, with an increment of 20nm, giving a dynamic range of 1:100. Fig. 7 shows the SEM pictures for the gap spacings. Preliminary results found a maximum capacitance of 35pF and a minimum capacitance of 0.5pF, measured by a low-frequency capacitance meter. The breakdown voltages of the MEMS variable capacitors should be much higher than diode varactors. The measured breakdown voltage of the MEMS

(b) Switch Contact

Moveable Hinges

Pull/push Arm

Actuator

Fixed Hinge

(c) Figure 5: (a) The concept, (b) the cross section and (c) a photo of a derrick-type switch.

(a) Support Arms

Parallel Plates

Actuators

(b) Figure 6: (a) The architecture of a parallel-plate variable capacitor. (b) A photo of a parallel-plate variable capacitor with 1mmx1mm parallel plates.



moving in a circular motion. The top plate is held by a rotation hinge, which is electrically isolated from the bottom plate, and actuated by the circular scratch drive actuators on the edges [6]. The actuators on the opposite sides move in the opposite directions (clockwise or counter-clockwise movements), which allow the top plate to rotate ±90o. The sizes of the parallel plates are reduced to eliminate parasitic capacitance. The gap between the parallel plates is 2µm. The coupling area can be varied by an amount corresponding to a 0.5o increment in rotation angle.

CONCLUSIONS We investigated several MEMS millimeterwave components for a reconfigurable transceiver, including reconfigurable Vee-antennas, planar impedance tuners, microswitches and variable capacitors. In this paper, we focused on demonstration of architectures with emphasis on integration issues. A layout database for designing various components is being established. Many of these components, such as MEMS microswitches and variable capacitors, can also be used at lower frequencies such as microwaves and UHF.

ACKNOWLEDGMENTS The authors greatly appreciate the support and donation by the HRL Laboratories and the current support for Yiton Fu by the U.S. Army Research Office under Grant No. DAAG55-98-1-0475.

[3] K. Herrick, T. Schwarz and L. Katehi, “Si-Micromachined Coplanar Waveguides for Use in High-Frequency Circuits,” IEEE Trans. Microwave Theory and Tech., Vol.46, No.6, pp.762, 1998. [4] C. Nguyen, L. Katehi and G. Rebeiz, “Micromachined Devices for Wireless Communications,” Proc. IEEE, Vol.86, No.8, 1998. [5] L. Fan, Y.-C. Tai, R. Muller, “Integrated Moveable Micromechanical Structures for Sensors and Actuators,” IEEE Trans. of Electron Devices, Vol.35, No.6, 1988. [6] T. Akiyama, D. Collard and H. Fujita, “Scratch Drive Actuator with Mechanical Links for Self-Assembly of Three Dimensional MEMS,” J. MEMS, Vol.6, No.1, pp.10, 1997. [7] J.-C. Chiao, Y. Fu, I. Chio, M. DeLisio and L. Lin, “MEMS Reconfigurable Vee Antenna,” Proceedings of the 1999 IEEE Intl. Microwave Symposium, TH2B-3, 1999. [8] W. McGrath, T. Weller and L. Katehi, “A Novel Noncontacting Waveguide Backshort for Submillimeterwave Frequencies,” J. of IR & MM, Vol.16, No.1, pp. 237, 1995. [9] V. Lubecke, W. McGrath and D. Rutledge, “A 100GHz Coplanar Strip Circuit Tuned with a Sliding Planar Backshorts,” IEEE Microwave and Guided Wave Letters, Vol.3, No.12, pp.441, 1993. [10] E. Brown, “RF-MEMS Switches for Reconfigurable Integrated Circuit,” IEEE Trans. Microwave Theory Tech., Vol.46, No.11, pp.1868, 1998. [11] K. Pister, M. Judy, S. Burgett and R. Fearing, “Microfabricated Hinges,” Sensors and Actuators, Part A, Vol.33, pp.249, 1992. [12] L. Fan, R. Chen, A. Nespola and M. Wu, “Universal MEMS Platforms for Passive RF Components: Suspended Inductors and Variable Capacitor,” MEMS’98, 1998. [13] H. Wu, K. Harsh, R. Irwin, W. Zhang, A. Mickelson, Y. Lee and J. Dobsa, “MEMS Designed for Tunable Capacitors,” Digest of MTT-S’98, pp.127, 1998.

REFERENCES [1] C. Chi and G. Rebeiz, “Planar Microwave and Millimeterwave Lumped Elements and Coupled-Line Filters Using Micro-Machining Techniques,” IEEE Trans. Microwave Theory and Tech., No.4, pp.730, 1995 [2] S. Robertson, L. Katehi and G. Rebeiz, “Micromachined W-Band Filters,” IEEE Trans. Microwave Theory and Tech., No.4, pp.598, 1996. (a) Hinges

Rotation Hinge

Hinges

Top Plate Top Plate

100µ µm Bottom Plate Support Arm 100µm Actuator

Bottom Hinges Plate

1µ µm

(a) (b) Figure 7: SEM photos for a parallel-plate variable capacitor with a gap spacing of (a) 100µm and (b) 1µm.

Bottom Plate

Top Plate Circular Actuator

(b) Figure 8: (a) The concept and (b) a photo of a circular parallel-plate variable capacitor.
