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In this paper, a novel electrostatic switch is reported that exhibits an ON/OFF capacitance ratio of 72, an ON capacitance value of 90 pF, and a pull-in voltage of less than 5 V within an area of 750 × 750 μm. These characteristics are achieved by utilizing a slit-shape middle metal layer in-between the top and bottom metal plates. This design makes the capacitance ratio and the ON state capacitance tolerant against fabrication and temperature induced residual stress in the suspended metal membrane, without significantly degrading the electrostatic force between the top and bottom metals. In this three-metal configuration, at the ON state, the ohmic contact between the top membrane and the middle metal layer enables direct charge transfer between these two electrodes. This characteristic is attractive for device applications in solar energy harvesting. As a proof of concept, charging of an external storage capacitor is shown for an exemplary switch, when a constant current is supplied to the top electrode. INTRODUCTION MEMS switches are an attractive alternative to existing switch solutions and have thus been a topic of extensive research in the past few decades [1], [2]. Conventionally, an electrostatic MEMS switch is made using two electrodes separated by an air gap and a suitable dielectric. One of the electrodes is a moving membrane, and it can be pulled onto the fixed electrode with the application of an electrostatic potential. For a capacitive switch with a given device area, the up-state air gap and the down state effective dielectric thickness decide the ON and OFF capacitance, respectively. In many applications where the MEMS device is used as a switched capacitor, the switch is not only required to have a high ON/OFF capacitance ratio but also a sufficient ON capacitance value. Scaling up the device area to achieve a high ON capacitance is not beneficial as larger devices also exhibit an undesirably high OFF capacitance. In addition, the ON capacitance which is defined by the effective insulator thickness of the switch does not scale linearly with the device size because of warping in the movable membrane (caused by residual or thermal stress) and the surface roughness of the contacts [3]. In order to resolve these issues, a modified switch structure is presented that makes use of an additional metal layer to achieve both a high ON/OFF capacitance ratio and a high ON capacitance value (Fig. 1). The three-metal switch design makes the ON capacitance invariant to stress-induced membrane warping. A micromachined capacitive switch exhibiting an ON/OFF capacitance ratio of 72 and an ON capacitance value of 90 pF within an area of 750 × 750 μm is demonstrated. A low pull-in voltage of less than 5 V is realized by designing the middle metal electrode in a slit type configuration ensuring sufficient electrostatic actuation force on the top suspended membrane. With such properties, the proposed threemetal switch design compares favorably with the state of the art MEMS switches [4]. In the ON state, the top metal makes an ohmic contact with the middle metal layer, allowing the transfer of electrostatic charge from the top membrane to the un-biased middle electrode. Such charge transfer can be useful when employing the switch as a solar energy harvester. We investigate this application and demonstrate charging of an external capacitor connected to the middle metal layer. Another unique feature of this three-metal switch when used as a harvester is the ability to deliver high-voltage charging pulses under heavy capacitive load without the need for an electrical boost converter. This is a significant advantage over conventional photovoltaic devices [5]. DEVICE CONCEPT AND FABRICATION The three dimensional model of the proposed switch is shown in Fig. 1. The MEMS switch is actuated by applying an electrostatic potential between the top and the bottom metal layers, whereas the middle metal layer is left floating. When the top membrane pulls in, it makes an ohmic contact with the middle metal layer. Thus, the ON capacitance of the switch is mainly determined by the capacitance between the middle and bottom metal layers, and is unaffected by the curvature of the top membrane. Therefore, a repeatable and large ON capacitance can be achieved even if the top membrane is warped due to the inevitable fabrication and thermally induced residual stresses. Figure 1: Schematic of the device structure with a top electrode section cut out to reveal the middle metal layer. The slit type design of the top and middle electrode can be clearly seen. Figure 2: Cross-section of the proposed switch structure. The floating middle metal layer can be used as an additional terminal. Note that in this configuration, the substrate acts as the ground electrode. To minimally affect the electric field distribution between the top and bottom metal layers, the metal layers are designed in a slit configuration (Fig. 2) and the middle metal layer is shifted laterally (Fig. 3). From the electrostatic simulations shown in Fig. 3, the slit-shaped middle metal layer is seen to have no significant effect on the electric field between the top and bottom metal layers. As a result, the electrostatic force on the suspended top metal remains unaffected. This allows us to design the membrane with a sufficient restoring force to overcome stiction, while still actuating the device with relatively small voltages. The fabrication process flow for the proposed devices is depicted in Fig. 4. Starting with a nominal silicon wafer, a 500 nm thick aluminum (Al)/ chrome (Cr) layer is lift-off patterned as the first metal layer (or ground electrode). A 150 nm thick aluminum oxide (Al2O3) layer is deposited using atomic layer deposition (ALD) as the dielectric layer. This material and its thickness along with the metal area define the ON state capacitance of the device. The dielectric layer is patterned to enable contact to the bottom metal layer. A 100 nm thick gold is evaporated and lift-off patterned as the middle floating electrode with Cr as the adhesion layer. Poly-methyl-methacrylate (PMMA) is spun as the sacrificial layer with a thickness of 1.7 μm which defines the air gap between the top and the floating electrode. Using titanium (Ti) as the etch mask, PMMA is patterned using a low power O2 plasma. Post PMMA etching, the Ti mask is removed and a 10/900 A TiW/silver layer is sputtered as a seed layer for electroplating. The plating mold is created using AZ 9260 photoresist and an 8 μmthick top electrode is subsequently electroplated. In the final step, the devices are released by removing the PMMA sacrificial layer and dried using critical point drying (CPD) to prevent stiction. The SEM images of a fabricated device are shown in Fig. 5. Figure 3: Simulated (a) electric potential and (b) electric field distribution of the proposed electrode layout for a two finger section. From (b) a uniform electric field is seen in between the top and bottom electrodes. The electric field is not disturbed significantly due to the addition of the middle metal layer. The two scale bars plot the range of voltage and electric field. Figure 4: Process flow of the proposed MEMS capacitive switch. SWITCH DEVICE DESIGN There are two primary design objectives in released membrane type electrostatic switches: low pull-in voltage and large restoring force. Both can be controlled by optimizing the stiffness of the springs, and the membrane shape and area. Using the process shown in Fig. 4, the membrane and the spring thickness is constrained to be the same to reduce the fabrication complexity. The spring stiffness is varied using lithographically defined dimensions, as shown in Fig. 6. A crab leg type spring design is implemented to allow self-compensation against fabrication induced in-plane stresses. While such a design is robust against in-plane stresses, the membrane is prone to bending under a vertical stress gradient commonly observed in most electroplated metals [6]. Figure 7 plots the simulated and measured top membrane bow due to a stress gradient in the top electroplated gold layer. Using a close to optimum plating condition, a membrane bow of ~ 2 μm was measured. From finite element simulations, a stress gradient of ~ 10 MPa/μm in the top membrane was found to fit the measured bow. Using other plating conditions, higher stress gradients have been measured. With such values of inevitable stress in the top membrane, designing a traditional MEMS capacitive switch with large capacitance values is challenging [7]. In a typical two electrode switch, the membrane bow necessitates application of a voltage higher than pull-in to flatten the membrane and achieve a full area contact [8]. In the proposed design, the capacitance between the middle metal and the bottom metal layer is set by the layout of the two electrodes and the dielectric material thickness in between. As such, a high ON state capacitance is obtained even when the top membrane is only partially in contact with the middle metal layer. Therefore, although warping of the top membrane makes a uniform contact difficult, the ON capacitance value remains unaffected. This is clearly seen from the measurement results shown in Fig. 8. Figure 6: Modal simulation results for (a) double beam (b) single beam (c) single fold beam and (d) double fold beam designs. The color spectrum plots the total displacement. The frequency shown is a measure of maximum switching speed. (a) (b) (c) (d) f ~ 2.5 kHz f ~ 2.02 kHz f ~ 1.38 kHz f ~ 1.04 kHz (a) (b) Figure 5: (a) Top SEM view of a micro-fabricated MEMS switch and (b) incline view of the top membrane revealing the air gap. Figure 7: (a) Simulated 2 μm top metal membrane bow with a stress gradient of 10 MPa/μm (b) Actual membrane bow measured using optical interferometry. The measured bow is ~2 μm, indicating the stress gradient in gold is ~ 10 MPa/μm.

A HIGH ON/OFF RATIO MEMS CAPACITIVE SWITCH WITH APPLICATIONS IN SOLAR ENERGY HARVESTING