High Isolation X-Band RF MEMS Shunt Switches on Groove Etched Substrates
Authors: Zhang Li , Yao Jun , Wa Dajia , Rao Qing , Zhong Hongsheng , Schmoll Jurgen 
Published: June 2009

Developments in RF MEMS switches have demonstrated great potential at low-loss microwave application. MEMS shunt switches have a few advantages compared to the FET or p-i-n diode counterparts due to their characteristics of low intermodulation distortion or harmonics, low DC power consumption, low insertion losses and high isolation [1][2]. RF MEMS shunt capacitive switches has shown excellent performance from Ka-band to W-band, however, they fail to perform the same in X-band for the low isolation in this frequency range. Various approaches have been introduced to address this shortcoming, such as applying high-impedance transmission line [3], using strontium titanate oxide (SrTiO3) as high relative dielectric constant material [2], etc. Aimed at X-band applications, this paper reports a novel design of a high isolation RF MEMS shunt capacitive switch which is fabricated on a groove etched substrate. Fig. 1(a) and Fig. 2(a) show the schematics of the MEMS capacitive switch. The switch is constructed on a coplanar waveguide (CPW) transmission line. When the switch is up, the switch presents a small shunt capacitance to ground, presenting an RF open. When the switch is pulled down to the center conductor by electrostatic force, the shunt capacitance increases remarkably, presenting an RF short. In this work, a short high-impedance section of transmission line is designed between the MEMS bridge and the ground plane. This increases the series inductance of the switch so as to lower the resonant frequency. The length of this line is designed to put the series resonant frequency into the frequency range of X-band. Two grooves are etched into the substrate along the center conductor between the transmission line and the ground plane. For the desired characteristic impedance, a wider center conductor width can be obtained by increasing the groove depth accordingly. Thus the CPW with grooves potentially has a lower attenuation due to conductor losses [4]. Moreover, as center conductor gets wider, the down-state shorting-circuit capacitance increases which helps to gain a higher isolation. The mechanical and RF performances of this switch have been analyzed by FEA software, IntelliSuite and HFSS. As shown in Fig. 1(b), the actuation voltage of the planar switches is 26V. The RF characteristics of the switch at down state are obtained through HFSS. In Fig. 1(c), the down state isolation reaches −54.6dB at its self-resonate frequency of 13.5GHz. Compared with the non-grooves counterpart, the designed grooves optimize the isolation performance by 7dB. The insertion loss is less than 0.2 dB from 5 to 30 GHz. Fig. 2(a) shows the serpentine folded suspension switch, its actuation voltage is 14V, shown as in Fig. 2(b). The RF response in Fig. 2(c) demonstrates that the series resonant frequency is down to 11GHz due to the inductance introduced by serpentine folded suspensions. The down state isolation is −42.8dB at 11GHz. However, it is demonstrated that the substrate grooves did not help to optimize isolation performance. This is due to the higher resistance and inductance introduced by serpentine folded suspension. This research is supported by “Hundreds Scholar Program”, Chinese Academy of Sciences.