Development of A Novel Bidirectional Electrothermal Actuator and its Application to RF

MEMS switch

Yan-qing Zhu, Lei Han*, Ming Qin, Qing-an Huang, and Ming-xia Jiang Key Laboratory of MEMS of Ministry of Education, Southeast University, Nanjing, 210096, China

*Corresponding Author: Lei Han, hanlei@seu.edu.cn

Abstract—This paper presents the design and testing results of an electrothermally driven MEMS (microelectromechanical systems) actuator. Different from conventional uni-directional thermal actuators, this in-plane bi-directional electrothermal actuator is capable of producing displacements in two directions as a single device. The RF MEMS switch driven by this cascaded electrothermal actuator is also proposed. Due to this bi-directional actuator, the proposed switch can not only realize the OFF-state to ON-state shifting but also provide an additional deep OFF-state. At the deep OFF state, the isolation better than -30 dB can be achieved at the whole frequency range of 0 ~ 40 GHz.

Keywords—bidirectional; electrothermal actuator; low voltage; RF MEMS switch;

I. INTRODUCTION Micro-actuators are essential components in MEMS

devices for producing physical motion at the micro and nanometer scales. A large amount of micro-actuators have been reported in the literature using different operation principles such as electrostatic, electrothermal, electro-magnetic, and piezoelectric effects [1]. Among them, the electrostatic actuators are most commonly used due to their high energy efficiency, large response bandwidth, and bidirectional motion capability. However, to achieve large displacements and high force simultaneously, a rather large actuator footprint is required, which may not practical in applications in which space is limited. Furthermore, the yield of large overlapping areas and compliant suspension structures could be very low due to stiction and/or failure between two electrostatic plates [2]. On the contrary, electrothermal actuators have been demonstrated to be compact, stable, large displacement and high-force devices under low actuation voltages [3]. Electrothermal actuation usually relies on thermal expansion caused by non-uniform Joule heating. In order to achieve motion, device components must thermally expand to a different extent, causing the device to warp or deform. The deform is small but the displacement can be mechanically amplified, for example, by using V-shaped or Chevron shaped beams [4-5]. This type of electrothermal actuator has been widely used due to its small footprint, high force (on the order of mN), and reasonable large motion (on the order of 10 µm). However, most of conventional in plane electrothermal micro-actuators are only capable of producing deflections along a single direction. A mono-directional electrothermal actuator

has limited operational motion range, which is half of its bidirectional counterpart. And it is not practical and stable enough for some applications, e.g. actuation of a cell manipulator [6]. A bidirectional electrothermal actuator has been reported in [7]. However, to achieve bidirectional operation, it must be placed in a magnetic field.

In this paper, we developed a new type of electrothermal actuator, which has bidirectional capability. Two in-plane bi-directional U-beam electrothermal actuators cascaded with a V-shaped beam actuator were used to achieve bidirectional actuation without buckling. Experimental results illustrate that this bidirectional actuator can move from - 8 µm to + 10 µm at least when applying 0.3 - 0.5 V actuation voltage. This bidirectional actuator was also applied into a RF MEMS switch. The test results show that this electrothermally-driven switch has much lower insertion loss and higher isolation, which are better than 0.5 dB and 30 dB at the frequency range of 0 - 40 GHz, respectively. The high performance of the proposed switch reveals that this bidirectional thermal actuator could be very useful for microsystem design.

Fig. 1. SEM photo of the fabricated bidirectional actuator.The proposed microactuator is symmetrically designed, allowing in-plane displacements along two directions.

This work is supported by the National Natural Science Foundation of China (61106114).

978-1-4799-4727-0/14/$31.00 © 2014 IEEE

II. DESIGN AND MODELING

A. Design and principle As shown in Fig. 1, the proposed microactuator is sym-

metrically designed, allowing in-plane displacements along two directions. When an actuation voltage is applied at PadA, and both PadB and PadC are grounded, the outer ‘hot’ arm has a higher current density while the middle ‘cold’ arm and the inner ‘hot’ arm have a lower current density. Because of its lower resistance, the majority of electrical current travels through the ‘cold’ arm to the anchors. Correspondingly, the inner ‘hot’ arm exhibits a very low current density which results in low Joule heating. Thus, U-beam actuators both bent toward the inside, as shown in Fig. 2(a). Both of them give a push force to the cascaded bent beam. As a result, the bent beam moves forward. Similarly, due to symmetry, when the connections for PadA and PadC are swapped, the device bends backward, as shown in Fig. 2(b). Fig. 2 illustrates the actuator’s motion along two directions, as obtained from finite element simulation using ANSYS software.

(a)

(b)

Fig. 2. Simulated results of the deformation of the actuator under 1.2 mN: (a) push forward state, (b) pull back state

B. Modeling Fig. 3 shows the schematic diagram of the actuator. The

dimensions of actuator are shown in Table 1. An optimized actuator design was obtained through numerical simulation using finite element modeling. The devices were fabricated using the metal (nickel) by electroplating process. The key factor that limits the performance of an electrothermal microactuator is temperature. It was reported that the temperature of a nickel device should be kept under 1273 K (1000 oC) to avoid thermal failure and permanent damage. Thus, design optimization was conducted in this study at an

equal maximum allowable temperature that was chosen to be 973 K. During finite element simulation, input power was adjusted to maintain the equal maximum temperature throughout the optimization process. Physical and material parameters used in simulations are summarized in table 2. Fig. 4 shows the simulation displacements and temperature of the actuator versus applied voltage. The simulated actuation spring constant and release spring constant are both 103 N/m.

Fig. 3. Schematic diagram of the actuator.

TABLE 1 DIMENSIONS OF ACTUATED STRUCTURE IN SIMULATION

Parameter Dimension

Cold arm width ( 1W ) 60 µm

Gap between hot and cold arm ( 2W ) 10 µm

Hot arm width ( 3W ) 10 µm

1Pad width ( 4W ) 150 µm

cPad width ( 5W ) 120 µm

Folded spring width ( tW ) 10 µm

Hot arm length ( 1L ) 410 µm

Distance between cold arm and PAD ( 2L ) 50 µm

2Pad length ( 3L ) 160 µm

Each single bent beam length ( 4L ) 520 µm α 8.90 β 74.60

TABLE 2 MATERIAL PARAMETERS USED IN FINITE ELEMENT SIMULATION

Parameter Dimension

Electrical resistivity 8e-8 m⋅Ω Young’s modulus 210 Gpa Poisson ratio 0.312 Thermal expansion coefficient 12 ppm/K Thermal conductivity 91 W/ km ⋅

Fig. 4. Simulated displacements and temperature of the actuator versus applied voltage.

III. APPLICATION Compared with conventional semiconductor switches, radio

frequency-microelectromechanical systems (RF-MEMS) switches have a better high-frequency characteristics, low power consumption and high linearity [8-9], which makes them become one of the most attractive devices for RF and microwave applications. In this paper, we propose a new type RF MEMS switch, which is actuated by the novel thermal actuator that can produce a bi-directional motion.

A. Switch design Fig. 5 shows a SEM photograph of the designed RF MEMS

switch. The switch is based on a CPW transmission line and a parallel configuration with two resistive gold - gold contacts. A silicon nitride structure is placed between the actuator and the mobile part of the contact for electrical and thermal isolation purposes. Fig. 6 shows three state of the proposed RF MEMS switch. The initial state of the switch is at OFF state owing to the contact plate does not contact the signal line or the ground line when no dc voltage is applied. When the bent beam moves backward, the contact plate will become farther away from the signal line and contact the ground lines. This situation is defined here as the deep OFF state since the isolation of this state will be larger than the initial OFF state, as shown in Fig. 6(b). When the actuator deforms oppositely, the contact plate will contact with the signal line and farther away from the ground lines so that the switch turns to ON state, as shown in Fig. 6(c).

The contact gap between the contact plate and the signal lines is designed to be 10 μm (the actual gap depend on the thickness of the sputtered gold on the sidewall), while the gap between the contact plate and the ground line is 8 μm. For push forward state (ON-state), when actuation voltages are 0.3 - 0.39 V, the simulated total contact and release force is 0.3 - 1.1 mN and 0.6 mN, respectively. For pull back state (deep OFF-state), when actuation voltages are 0.45 - 0.51 V, the simulated total contact and release force is 0.1 - 0.3 mN and 0.8 mN, respectively.

Fig. 5. The SEM photograph of the proposed switch.

Fig. 6. Three state of the proposed RF MEMS switch

Fig. 7. A structure-based lumped-circuit model of the proposed switch.

B. Modeling In order to evaluate RF performance of the proposed switch,

we have performed small-signal modeling [10-11]. Fig. 7 shows the structure-based lumped-circuit models for the MEMS series switch. The contacts are modeled as resistors or capacitors, depending on the state of the switch. The lateral contact structure has two contact areas which are between the input and output ports. The coupling capacitance contains two series of the Cc in parallel with Cg. Cc refers only to the OFF-state capacitance due to the distance of the overlapping part between the front end of the contact plate and each broken signal line. Cg refers to the signal-line coupling capacitance due to the gap between each broken signal line. Rc is half of the contact resistance when the switch is at ON state. Rd and Cd are the resistor and capacitor between the ground line and the movable structure, respectively. ON state and OFF state is equivalent to switch between Rc and Cc. At the deep OFF state, Cc becomes smaller than its initial state due to the larger distance between the signal line and the movable contact plate. Moreover, Cd is switched to Rd, resulting in the resistor

connection between the movable element and the ground line. These two reasons make larger isolation possible.

C. Fabrication The electrothermal actuator and the switch proposed in this

paper were micro-fabricated using MEMSCAP’s Metal Multi-User MEMS Process (MetalMUMPs) [12]. The process flow of the fabricated switch is given in Fig. 8: (a) A 2 μm thick oxide is grown on the surface of the starting n-type (100) silicon wafer. This is followed by deposition of a 0.5 μm thick sacrificial phosphosilicate glass (PSG) layer (Oxide 1). Wet chemical etching is used to remove the unwanted sacrificial PSG. Oxide 1 also defines the regions in which the silicon trench will be formed. (b) Two 0.35 μm layers of silicon nitride is deposited as mechanical support to link the actuator and the contact plate. (c) RIE etching is performed to remove nitride from the patterned areas where are not needed. (d) A second sacrificial layer (Oxide 2), 1.1 μm of PSG, is deposited and wet etched and a thin metal layer is deposited. (e) The Plating base layer, consisting of 500 nm Cu + 50 nm Ti is deposited (not shown). The wafers are coated with a thick layer of photoresist and patterned to form a patterned stencil for the electroplated Metal layer. (f) Nickel is electroplated to a nominal thickness of 20 μm into the patterned resist stencil. (g) The photoresist stencil is then chemically removed. (h) A 1-3 μm gold layer (Sidewall Metal) is electroplated to provide a low resistance contact. (i) a 49% HF solution is used to remove the PSG sacrificial layers (Oxide 1 and Oxide 2) and the oxide layer over the trench areas. (j) Finally, a KOH silicon etch is used to form a 25 μm deep trench in the silicon substrate in the areas defined by the Oxide 1. The SEM image for the fabricated switch is shown in Fig. 5. As shown in the picture, gap between the signal lines is 100 μm.

Fig. 8. The process flow of the proposed switch. A 25 μm deep trench is etched below the contact area of the switch, so as to reduce the coupling capacitance when the signal go through the substrate, and this further increases the isolation of the switch.

D. Measurement and discussion In order to analyze the performance of the fabricated

switch, we performed the measurements of actuation characteristics and RF characteristics.

Fig. 9. Measurement results of the resistance of the switch.

We firstly performed the actuated voltage measurement by monitoring dynamical response of the switch. When the applied voltage exceeds the threshold voltage of the switch, the switch turns to ON-state. Fig. 9 shows the relation between the contact resistance and applied voltage for the proposed switch. As can be seen from the figure, the switch turned to ON-state at 0.3 V. The contact resistance became lower than 0.1 Ω at 0.36 V, which depends on properties of the contact material(electrical resistivity, hardness, plasticity, roughness, and melting point), contact force [13], real contact area [14], temperature because of current flow through the contact and manufacturing process [15].

The RF response of the fabricated switch was measured under atmospheric conditions using the Agilent N5244A PNA-X Vector Network Analyzer (0-43.5GHz range) with 150μm-pitch Cascade Microtech ground-signal-ground coplanar probes. A short-open-load-through (SOLT) standard on-wafer calibration technique was employed. Fig. 10 shows the RF-MEMS switch ON-state characterization with insertion loss better than 0.5 dB in the frequency range 0–40 GHz, whereas that of the return loss was from 39.1 to 18.0 dB. Fig. 10 also shows return and insertion loss obtained using the equivalent circuit model for the ON-state proposed in Fig. 7, where lumped elements values have been selected to fit the measured return and insertion loss curves. The fitted results: Rc = 0.2 ~ 0.4 Ω.

Fig. 11 shows the measured and modeled S-parameter for the fabricated MEMS switch at OFF-state. In the initial OFF-state mode, the measured isolation are from 61.4 to 30.3 dB at 0.5–15 GHz, and higher than 22.5dB up to 40 GHz. The fitted result of the isolation loss agrees well with the measured data, in which Cc is 3.6 fF, Cg is 1.6 fF, Cd is 1.2 fF. When the applied voltage is 0.5V, the contact is poor but the isolation of the switch has been improved by 2-3 dB. At this moment, there exists not only a resistant contact Rd but also a coupling

capacitance between the contact plate and the ground line. dR is fitted as 2500 Ω, while the coupling capacitance is fitted as 6 fF. Presumably, the isolation characteristic at the deep OFF-state can be improved simply with a lower resistance, which can be achieved with a higher applied voltage due to the larger contact force between the contact plate and the ground line. When the actuation voltage increases to 0.55 V, which is high enough, the isolation higher than 30 dB at the whole frequency bands can be achieved. Compared to initial OFF-state, the isolation of deep OFF-state can achieve maximum improvement of -9.5dB. The fitted result shows that the contact resistance Rd is 1 ~ 10 Ω , and the effect of the coupling capacitance can be ignored. The validity of the model is demonstrated by the fact that its RF performances are almost the same as that of the measured data, as shown in Fig. 11. The excellent agreement shows that the model can be used for designing an optimized device with respect to specific applications.

Fig. 10. The ON-state S-parameter measured results and the fitted results by the lumped-circuit model.

Fig. 11. The OFF-state and Deep-OFF-state S-parameter measured results and the fitted results by the lumped-circuit model. Vact is the actuation voltage of the switch at the deep OFF-state. The fitted results: Cc = 3.6 fF, Cg = 1.6 fF, Cd

= 1.2 fF. When Vact is 0.55 V, Rd = 1 ~ 10 Ω.

The actuation/releasing time of the fabricated switch were also measured. The measurement results of the switching time are shown in Fig. 12. It can be seen from this figure that the actuation time is 28.5 ms for the transition from the OFF to the ON state, and the releasing time is 1.9 ms for the transition from the ON to the OFF state. Although the switching time of the proposed switch is longer than its electrostatic counterparts, it could find several applications where the switching time requirement is less stringent such as redundancy networks, multiband receiver band selection networks, or automated test equipment.

Fig. 12. Measurement results of the switching time: The actuation time is 28.5 ms and the releasing time is 1.9 ms.

IV. CONCLUSION This paper presented an electrothermally driven MEMS actuator that is capable of producing in-plane deflections along two directions as a single integrated device. An optimal design was obtained through finite element simulation. The devices were fabricated using the standard MetalMUMPs microfabrication process. In satisfactory agreement with numerical simulation results, testing results demonstrated that the microactuators have a large displacement range. This bidirectional actuator was also applied into a RF MEMS switch. The test results show that this electrothermally-driven switch has much lower insertion loss and higher isolation, which are better than 0.5 dB and 30 dB at the frequency range of 0 - 40 GHz, respectively. The performance of the proposed electrothermal actuator is excellent that it may lead to interesting possibilities in microsystem design.

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