# MEMS-Switched Triangular and U-Shaped Band-Stop Resonators for K-Band Operation
## 2 SPDT Design, Technology, and Test  
Two preliminary technological runs were performed to select the more promising configuration for the SPDT switch. The final layout used in this work for the actual devices is shown in Figure 1.  
The MEMS switch actuation mechanism is based on electrostatically actuated fixed-fixed metal bridges suspended over the center conductors of CPW transmission lines. An air gap separates the bridge from the center conductor when no bias is applied. When a voltage is applied between these two metals, the electrostatic force starts to pull the bridge towards the lower conductor. The bias is then increased until a threshold voltage is reached, and, at this point, the bridge's spring force is overcome; the bridge snaps down and contacts the bottom signal line. Two clamped-clamped microswitches are used to send the input signal from port 1 to port 2 or 3. The switch beam is made of gold with a thickness of about 1.8 \(\upmu\)m for the flexible and deformable arms and about 5 \(\upmu\)m for the rigid main body. The switches are electrostatically actuated using high-resistivity polysilicon electrodes underneath the lateral side of the beams. The air gap between the non-actuated switch, the underneath actuation electrodes, and the bottom signal line is 3 \(\upmu\)m.  
The main improvements between the first and the second run were the introduction of three air bridges, also pointed out in Figure 1, and a new configuration of the T-section, where the lines were tapered to provide a better impedance matching of the overall device. A section of the metal beam is shown in the SEM (scanning electron microscope) micrograph in Figure 2, with particular attention on the holes necessary for removing the sacrificial layer below the bridge. The technology fabricating the entire device differed only for the MEMS switch compared to the planar resonators. In the case of the MEMS SPDT, the entire process encompassed a multilayer deposition of dielectric and metal layers, selective electroplating to make the beam more robust in the contact regions, and the final removal of a sacrificial layer to obtain the suspended beams. The complete procedure required an eight-mask process, as described in [24], and it was a simplified and optimized version of the SPDT presented in [25]. The resonators resulted from a more straightforward process using  
Figure 1: Manufactured SPDT switch. The arrows point to the actual details of the device, including the air bridges’ location with the optimized geometry of the T-junction, the individual switches, the actuation lines, and the CPW feeding line. Figure 2: SEM micrograph with detail of the metal beam layout. In this section, the metal beam is reinforced with a double-step electroplating process to strengthen the central part mechanically, closing the interrupted central conductor of the CPW. In contrast, only a single-step electroplating is performed on the anchors to maintain an elastic response during the actuation. As usual, the holes facilitate removing the sacrificial layer underneath the bridge. simple photolithography and electroplating to obtain a 5 \(\upmu\)m thick metal to mitigate the skin depth effects. Considering all the geometrical details, a full-wave electromagnetic simulation preliminarily verified this configuration. The results of the simulation, when the switch at port 3 was considered as not actuated, are plotted in Figure 3, in terms of the matching at the input port (S\({}_{11}\)), transmission to the actuated branch (S\({}_{21}\)), and the isolation between port 1 and 3, and between port 2 and 3, respectively, S\({}_{31}\) and S\({}_{32}\). The reflections at the input port were below \(-20\) dB until 25 GHz, then slowly degraded, but were still acceptable up to 30 GHz (\(<\)\(-15\) dB). Regarding the transmission to port 2, in this case, the chosen active branch of the device, represented by the S\({}_{21}\) parameter, showed an insertion loss above \(-1\) dB.