Integrated Modeling of Mechanical and RF

Integrated Modeling of Mechanical and RF

Performance of MEMS Capacitive Shunt Switch

INTRODUCTION

Radio frequency Microelectromechanical System (RF MEMS) is an emerging technology, which is envisaged to play a key role in the development of broadband communications, software radio applications and agile radar systems. MEMS shunt capacitive switches can deliver superior RF performance to existing technologies up to 100GHz [1-2]. RF MEMS switches can be integrated into System-on-Chips, offer wider bandwidth, high isolation, low power consumption, low losses and have an excellent linearity, compared to traditional RF front-end switches [3].

The MEMS capacitive shunt switch which is used to demonstrate the integrated modeling approach, consists of a suspended movable metal bridge, which is mechanically anchored and electrically connected to the ground line of a coplanar waveguide (CPW) transmission line. The simplified circuit diagram for the shunt switch is shown in figure 1.

Figure 1. Simple circuit representation of capacitive shunt switch

In operation, a DC bias voltage and an RF signal are superimposed and applied to the signal line. In the RF-On state, i.e. when the DC bias is zero, the bridge remains up, hence the capacitance is small, and hardly affects the impedance of the line. In the RF-ON state the signal freely passes through. By increasing the DC bias voltage the bridge is pulled down onto a dielectric layer placed on the top of the signal line. The switch capacitance increases, causing an RF short to ground, and the switch is in the RF-OFF state. A high down capacitance and a low up-state capacitance results in high isolation and low insertion loss in the RF-OFF and RF-ON states respectively. This paper describes a method of integrating electromechanical and RF simulations. This allows the device designer to optimize both the mechanical and RF performance of the device and enables RF system designers to include device S-parameters in system simulations. This method is demonstrated with simulations of a novel 'curled cantilever' MEMS capacitive shunt switch.

II. INTEGRATED MODELING METHOD

The integrated modeling method is developed to allow the construction of 3D actuated geometries of MEMS devices, from electromechanical modeling results as input to 3D electromagnetic simulation software.

Figure 2. Process flow of integrated modeling and simulation

The mechanical modeling of the MEMS switch is performed in the Coventorware 3D finite element package.

The simulated profile of the switch in both up and down states is exported in 2D format. This 2D data is imported in spline format into CST Microwave Studio. Measured interferometric profiles can also be imported as 2D data into CST. The curve is translated into 3D structure for electromagnetic simulations. The process flow for integrated modeling is shown in figure 2.

III. CANTILEVER SHUNT SWITCH DESIGN

The CMOS compatible single layer cantilever switch is fabricated in the Tyndall surface micromachining process with a nominal switch gap of 1.5μm. The switch uses a process-induced stress gradient to achieve 30μm air gaps at the tip of a 350μm long cantilever switch. The switch is built on a high-resistivity silicon substrate, on which a 0.5μm thick silicon oxide is deposited as an isolation layer. The

CPW is fabricated from 0.5μm thick sputtered Al/1%/Si. A

140nm thick PECVD silicon oxide dielectric is deposited as passivation layer between the switch electrodes. The sacrificial layer is formed by a layer of 2.5μm thick polyimide. The switch cantilever is formed from cold sputtered aluminium, which has a tensile stress of approximately 35MPa, and is placed over a 700μm long 110/200/110 G/S/G transmission line. The SEM image in figure 3 shows the fabricated beam after release.

Figure 3. SEM Image of the cantilever showing deflection due to stress Gradient

A. Mechanical modeling

The switch uses thin-film residual stress gradient to deflect the switch cantilever to achieve high air gaps, despite the use of a relatively thin sacrificial layer in the fabrication process. The effect of biaxial stress gradient becomes apparent for wide cantilevers causing transverse bending. Experiments from previous fabrication runs have shown the process gradient averages 51MPa/μm in a 1μm thick aluminium film.

The switch design is optimized in Coventorware to minimize unwanted transverse bending by incorporating a number of 2ìm wide slots in the structure. The slots also allow easy removal of the sacrificial layer. This design modification eliminates transverse bending which can be seen from the simulated cantilever deflection shown in figure 4. To ensure uniform movement of the switch, the beams are linked using 5μm tethers at regular intervals.

Figure 4. Coventor simulation of a cantilever switch incorporating 2μm wide stress release slots. This modification eliminates transverse bending and increases tip deflection.

The cantilever switch design presented here has a 350μm x 200μm x 1μm structure, consisting of 10 cantilever beams, each 18μm wide and separated by a series of 2μm slots. The shape of the released switch agrees well with the simulations, and little or no transverse warping is observed. The measured deflection of the cantilever structure shows the tip deflection of 30.7μm which agrees with the simulated deflection of 28.5μm, which can be seen in the surface profile, figure 5.

The slight offset is most likely due to the effects of the beam anchor and errors in the assumption that the stress gradient is linear across the metal thickness.

Figure 5. Measured surface profile shows the tip deflection is 30.7μm, which agrees with the simulated, for a stress gradient of 51MPa/μm.

B. Radio frequency modeling

The RF operation of the switch consists of a grounded metallic beam suspended over a passivated CPW on high resistivity silicon of thickness 500μm. The simulated profile from the mechanical modeling of the deflected cantilever was imported to CST Microwave Studio [6]. The curve was swept to 1μm thickness along the path of profile and the width of 200μm to get the exact 3D structure of the cantilever with stress gradient induced tip deflection after release. The constructed model of the cantilever switch can be seen in figure 6. The 3D structure in the down state can be constructed from the simulated MEMS profile in the down state. The structure is simulated by enclosing it in an air box with imposed radiation boundary conditions.

Figure 6. The up state model of the cantilever built in CST MWS for RF Simulations

IV. MEASUREMENTS AND RESULTS

The RF measurements were performed using an HP8722D Vector Network Analyzer (VNA) by wafer probing on a Cascade station using Ground-Signal-Ground (GSG) coplanar probes. The full two-port Short-Open-Load-Thru (SOLT) calibration was performed using the Impedance Standard Substrate (ISS) prior to the device measurement. The frequency range used during the calibration and measurement was 0.5GHz to 20GHz with 401 points. The bias voltage required for the switch actuation was provided from a Keithley 237 high-voltage source connected to the DC port of the VNA. This enables a DC bias to be superimposed with a -10dBm-power microwave signal that is applied to the device. The two-port S-parameters of the switch in the open and closed state were recorded and then the Insertion Loss and Isolation were calculated from the S21 parameter, respectively. The S21 is defined as a signal transmission coefficient between the input and output port. To actuate the switch a voltage of 24V is used. When no bias is applied and the switch stays in the openstate, the RF signal is almost fully transmitted between the input and output port with the maximum measured and simulated loss of -0.4dB at 20GHz. The insertion Loss of the switch is shown in figure 7.

Figure 7. Measured and simulated Insertion loss of the switch.

However, when a voltage is applied and the switch is closed, the RF signal is attenuated and only a small portion passes between the ports. The switch capability for the RF signal attenuation is called the switch Isolation and the maximum value in our device is -30dB at 12.3GHz for simulation and measurement.

Ideally the switch would have 0dB Insertion Loss and infinite Isolation. However, presented values can be classified as a good switching performance. A very good agreement in the characteristic shape between the measurement and the simulation is seen in case of the Isolation. The characteristics and resonance frequencies (ƒr=12.3GHz) are almost overlapped as shown in figure 8.