**4. Conclusion and prospects**

presented in **Figure 24(b)**. In addition, this phase shifter has an average insertion loss of 2.96

A second topology (**Figure 25**) of a 2‐bit phase shifter based on two SP4T switch with piezoelectric MEMS switches with a compact 3D passive design on a liquid crystal polymer (LCP) organic substrate was described by Chung et al. [37]. A picture of the complete layout is shown in **Figure 25(a)**. The block diagram presented in **Figure 25(b)** describes the princi‐ ple based on two SP4T switch matrixes. The transmission phase measured results are detailed in **Figure 25(c)**. The multilayer LCP process allows a low‐cost and lightweight circuit that can easily be integrated with other RF front‐end components, such as an antenna, at the packag‐

**Figure 25.** Proposed 3D phase shifter. (a) Layout with size comparison with a phase shifter with its footprint on a sin‐ gle layer. (b) Corresponding block diagram and measured normalised phase response of the 3D phase shifter. (c)

RF MEMS devices are competitive for handling high‐frequency microwave signals. In comparison with semiconductor devices, the performance of RF MEMS devices is highly

A 3 V operation RF MEMS variable capacitor using hybrid actuation of piezoelectric and electrostatic forces was presented by Ikehashi et al. [38] and Ikehashi et al. [39]. An image of this varactor is presented in **Figure 26(a)**. The measured capacitance ratio is *C*max/*C*min = 14. The hybrid actuation and the optimised bending enabled 2.6 V pull‐in voltage, with the pull‐out voltage as high as 2.0 V, as shown in **Figure 26(b)**. The piezoelectric actuator, which uses thin film PZT, enabled low voltage actuation, while the electrostatic actuator realised a large

dB using PZT shunt switches operating at 15 V.

Measured normalised phase response of the 3D phase shifter.

linear, minimising signal distortion.

ing level.

222 Piezoelectric Materials

**3.3. RF varactors**

capacitance ratio.

The need for piezoelectric materials for RF applications will continue to grow. Some important features must be taken into account in the future: Lead‐free materials are necessary, as lead must be removed from almost all devices. Future devices will be tunable in order to reduce the number of components and to give the possibility of using several standards, particularly in radio transmission systems. As the number of devices to fabricate is important, future materials must be compatible with fabrication processes of micro technologies. The control of the thickness of piezoelectric layers is a great challenge because the cost of the control with good accuracy of this parameter remains high. Finally, testing piezoelectric devices for high‐ volume manufacturing remains to be explored.

Owing to new piezoelectric materials, innovative RF devices have been designed from a simple resonator or switch to a more complex architecture, such as a phase shifter on a tunable filter. MEMS piezoelectric devices offer promising performances for RF applications. For example, switches allow actuation voltage levels two times smaller than in electrostatic switches and varactors a large capacitance ratio (*C*max/*C*min = 14).
