High-k Dielectrics for Application in Broadband Radio Frequency-Microelectromechanical System Capacitive Shunt Switch

Abstract

The performance of RF MEMS capacitive shunt switch is dominantly determined by the insulator layer. Thin high-k dielectric is preferred for achieving high down-state capacitance, which in particularly determines the switch performance at low frequency range. It is also well known that the reliability of RF capacitive switches is mainly determined by the charging of the insulator layer if package is hermetic and near-hermetic. SiO 2 and Si 3 N 4 are still the most-commonly used insulator materials because of their technology maturity. Switches of SiO 2 and Si 3 N 4 have limited isolation performance due to their low dielectric constant. As a result, much of recent effort has been devoted to seeking new high-k dielectrics. SrTiO 3 (k = ~ 200), Ta 2 O 5 (k = ~ 22), and (Ba, Sr)TiO 3 (k= ~ 29) dielectric have been investigated. It is worthy to note that HfO 2 -based dielectrics, which are leading candidate materials for next generation of high-k gate dielectric, have been being ignored in RF MEMS capacitive switch. High isolation performance had been achieved for RF MEMS capacitive switch using 250 nm-thick PZT/100 nm-thick HfO 2 stack dielectric in our previous work. It would be interesting to examine application of single HfO 2 dielectric layer in RF-MEMS capacitive switch. In this work, π-type RF-MEMS capacitive shunt switches of PZT/HfO 2 stack and single HfO 2 dielectric would be developed and characterized for broadband application. Figure 1 illustrates schematic structure of the π-type RF-MEMS capacitive shunt switch, which was fabricated using 5 masks in this work. The insulator layer is 100 µm square.Polycrystalline PZT layer in 130 nm thick was achieved after annealing at 500 o C using the sol-gel procedure. HfO 2 dielectric was prepared by sputtering. The PZT film had the dielectric constant of about 1185 at 1k Hz and the remanent polarization Pr of 9.86µC/cm 2 . The HfO 2 had the dielectric constant of about 17and the dielectric strength of about 24 MV/cm. Figure 2 shows the measured RF performance of the prepared RF MEMS switches. All the prepared switches had insertion loss better than -0.75 dB in the whole measured frequency range (1~35 GHz). The switches exhibited excellent isolation performance in a broadband range (1~35 GHz). The switch of HfO 2 has the isolation performance better than -40 dB in the sweep frequency range of 5 ~ 35 GHz. The PZT/HfO 2 stack dielectric-based switch had the isolation better than -20 dB within the whole sweep frequency range. The PZT layer had complicated effects. 130 nm-thick lead zirconate titanate(PZT)/45 nm-thick HfO 2 stack and single 45 nm-thick HfO 2 dielectric film were utilized as insulator layer in π-type radio frequency (RF) capacitive shunt switches for achieving high isolation performance in broadband application. Thin PZT film in perovskite structure mainly with (1 1 1) orientation was successfully prepared at low temperature (500 o C) using sol-gel method. The thin PZT film exhibited excellent ferroelectric properties and high dielectric constant (k ≈ 1...