Development of RF MEMS Switches for High Power Applications Joo-Young Choi, Jun-Su Lee and Stepan Lucyszyn Dept. of Electrical & Electronic Engineering, Imperial College London, UK
[email protected] Abstract This paper introduces a new concept in RF MEMS switches intended for high RF signal power applications at millimetre-wave operation. The novel SPST switch architecture employs electrothermal hydraulic microactuators. Out-of-plane silicon levers, having spring action and latching mechanisms, have been designed to create an OFF-state gap separation distance of 250 μm between ohmic contact electrodes. Having simple assembly, many of the inherent problems associated with the more traditional suspension bridge and cantilever beam type architectures can be overcome. In principle, low ON-state insertion loss, high OFF-state isolation and high RF signal power handling characteristics can be achieved with careful design. Using Ansoft`s HFSS software, a 40 to 60 GHz prototype was developed. The simulated ON-state insertion loss and return loss are 1.2 dB and 23.8 dB, respectively, at 50 GHz; while OFF-state isolation is 30.1 dB at 50 GHz. Preliminary small-signal measurements, from DC to 8.5 GHz, show encouraging results.
I. INTRODUCTION RF MEMS switches offer many advantages over solid-state switches, in terms of low loss, high isolation, low power consumption and high linearity [1]. However, RF MEMS switches have reliability problems that are linked to RF signal power level. Generally, RF MEMS switches are based on designs that employ electrostatic actuation. These switches can be classified as capacitive membrane or metal-to-metal ohmic contact switches [2,3]. The capacitive membrane switches have large contact areas, separated by a dielectric layer to provide DC isolation. Their transmission lines include electrodes to which DC bias is applied. The metal-to-metal ohmic contact switches can have small contact areas, and separate pull-down electrodes are employed for biasing the switch positions. In terms of the mechanical structure, traditional switches can be divided into those that have architectures based on suspension bridges or cantilever beams [2]. With the suspension bridge structure, there are two anchors at both ends of a membrane suspended over an electrode, and the membrane contacts the electrode at its center. The membrane of the cantilever structure has one fixed end at the anchor, and the other free end has electrodes for position biasing. Switches can also be designed to have either series of shunt configurations within the circuit. Whatever their type may be, traditional RF MEMS switches have inherent limitations of one sort or another to RF signal power handling (e.g. due to self-actuation, stiction, etc.). These problems can result from the membrane structure itself, which is very thin (typically 0.5 to 2 μm) and having very small gap separation distances (typically 1 to 5 μm) to an electrode. Because the power handling capacity varies with many variables associated with the switch architecture, there has been a lot of diverse efforts to improve RF signal power handling capacity. For example, the addition of a top electrode, above the membrane, to pull the membrane up [4,5]; an array of a number of switching elements, to increase isolation and reduce current density [6,7]; and an increase in the width and thickness of the membrane [7]. However, these require an increase in the complexity of their design and fabrication, and are not fundamental solutions because a membrane-based architecture is still employed. Using a previously established electrothermal hydraulic microactuator technology [8,9], this paper proposes a novel RF MEMS switch architecture that, in principle, can overcome the limitations associated with membrane-based architectures that employ electrostatic actuation. Section II describes the design and fabrication of the single-pole single-throw (SPST) switch and Section III presents simulation and preliminary measurement results.
II. DESIGN AND FABRICATION A. Characteristics of Paraffin Wax Phase change material (PCM) characteristics can be exploited to realize electrothermal hydraulic microactuators. As a PCM, paraffin wax shows a volumetric expansion of ~15% when it melts, and shrinks back to the initial volume on cooling, as illustrated in Fig. 1. This technology has previously been investigated and employed within a number of non-RF MEMS application demonstrators [8,9].
Fig 1. Expansion characteristics of paraffin wax
B. Switch Concept The proposed SPST switch consists of paraffin wax microactuators and silicon levers, as illustrated in Fig. 2. Instead of membranes, silicon levers are designed with springs and latches that make an ohmic contact with the CPW`s signal track. Paraffin wax microactuators control the silicon levers by means of a mechanical push-pull mechanism. The simple latching mechanism can maintain both the OFF-state and ON-state, without continuous DC biasing of any microactuator. Latch Spring
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Fig 2. Basic cross section of the novel RF MEMS switch. (a) OFF-state and (b) ON-state
C. Paraffin Wax Containers The paraffin wax microactuators that have already been established in non-RF MEMS applications [8,9] are now employed here in the novel RF MEMS switch. The structure and design of paraffin wax containers are only briefly described here. The structure of the container is illustrated in Fig. 3. Paraffin wax fills the bulk micromachined silicon containers, and is sealed using an elastic diaphragm of PDMS. When the required DC bias voltage is applied to the integrated microheater, the paraffin wax expands with the associated increase in heat, and is deliberately shaped into a hemisphere, shown in Fig. 4.
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(a) Fig 3. Electrothermal hydraulic microactuator. (a) Exploded view and (b) Cross-sectional view
Fig 5. Side view of the designed silicon lever with springs and latches
Fig 4. Illustration of an actuated hemisphere
D. Silicon Levers The silicon levers are inspired from elements from a microgripper [9]. Two silicon levers are created, one for the ohmic contact and the other for the latching mechanism. With the former, for demonstrating purposes only, gold is selectively deposited onto the lever pins, in order to provide a metal contact with the CPW’s signal line. Fig.5 shows the lever elements and latching mechanisms integrated into one piece of silicon, to simplify assembly. With the silicon wafer having a thickness of ~525 μm, wide ohmic contacts can be created for high power applications. Moreover, these microactuators can easily introduce OFF-state gap separation distances of as much as 1 mm to enhance isolation characteristics. In this first proof-of-concept design, a gap separation distance of 250 μm was introduced. III. PRELIMINARY RESULTS A. Simulation Novel SPST switches were designed and optimized for 40 to 60 GHz (V-band) operation. The optimal dimensions for the silicon levers and CPW lines were established using Ansoft’s High Frequency Structure Simulator (HFSS) software. The simulation results are summarized in Fig. 6. At 50 GHz, the predicted ON-state insertion loss and return loss are 1.2 dB and 23.8 dB, respectively, and OFF-state isolation is 30.1 dB.
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Fig 6. 3D modeling and simulation results of the novel RF MEMS switch. (a) 3D model in ON-state, (b) 3D model in OFF-state, (c) insertion and return losses, and (d) isolation characteristics
B. Measurement Results A photograph of the assembled SPST switch is shown in Fig. 7. Preliminary measurements for the switch could only be undertaken from DC to 8.5 GHz, and at small-signal power levels, as a suitable V-band vector network analyzer was not available. Fig. 8 shows the measured frequency responses. In the ON-state, the measured minimum insertion loss and the maximum return loss were approximately 0.4 dB and 37 dB, respectively. The OFF-state isolation was higher than 23 dB. An applied bias to each actuator was 12 V, and actuation time was 2-3 sec.
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Fig 8. Measurement results of the switch. (a) ON-state insertion and return losses and (b) OFF-state isolation
IV. CONCLUSIONS A novel RF MEMS switch architecture has been proposed, for V-band operation and intended for use in high power applications. It has many inherent advantages, in terms of relatively high RF signal power handling and broadband ON-state insertion loss and OFF-state isolation properties, when compared to the more traditional suspension bridge and cantilever beam architectures. However, all this comes at the price of slow switching speed. A latching mechanism is introduced to minimize power consumption, when switching between states. Clearly, much further work is needed to improve switching speed, reduce DC power consumption during switching operations, undertake measurements at the intended V-band design frequencies and at high RF signal power levels, perform reliability and lifetime testing and then investigate size and packaging issues. However, within this un-funded project, the preliminary results from this first prototype design are encouraging. ACKNOWLEDGEMENTS The authors would like to thank Michael P. Larsson for undertaking the RF measurements. This work is associated with the European Union’s Network of Excellence in RF MEMS (AMICOM). REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9]
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