**3. Piezoelectric MEMS in RF circuits**

#### **3.1. Switches and matrixes**

This section describes MEMS switches. These devices use mechanical movement to achieve an open or short circuit in the RF transmission line. RF MEMS switches are the specific micro‐ mechanical switches designed to operate at RF‐to‐millimeter wave frequencies (0.1–100 GHz). The forces required for the mechanical movement can be obtained using electrostatic, mag‐ netostatic, thermal or piezoelectric designs. Even if most MEMS RF switches are electrostatic‐ type switches [29], new switch topologies, based on piezoelectric topology, are now available.

The first piezoelectric material used to design switches has been AlN. Fully integrated RF MEMS switches [30] with piezoelectric actuation have been designed, fabricated and charac‐ terised using silicon bulk technology. First a resistive switch having a pure gold metal contact. The measured isolation and insertion loss of the resistive switch are −62 and −0.8 dB at a frequency of 5 GHz, respectively, for an actuation voltage of 3.5 V. A second capacitive switch exhibits an isolation of −18 dB and an insertion loss of −0.7 dB at a frequency of 5 GHz. The isolation curve of the capacitive switch is very flat over a very wide frequency range, from 0.5 to 30 GHz.

Another fully integrated RF MEMS switch with piezoelectric (PZT) actuation has been proposed and characterised by Lee et al. [31]. This switch is composed of a piezoelectric cantilever actuator with a floated contact electrode and isolated CPW transmission line suspended above the silicon substrate. The measured insertion loss and isolation are −0.22 and −42 dB at a frequency of 2 GHz, respectively. The main innovation of this switch is its very low actuation voltage of 2.5 V, instead of around 6–7 V, for more efficient electrostatic switches.

Another paper described a higher frequency switch that uses piezoelectric actuators [32]. This switch is based on a thin film spun of PZT deposited onto a high‐resistivity silicon substrate with coplanar waveguide transmission lines (**Figure 22**). Actuation voltages less than 10 V with switch operation demonstrated as low as 2 V. The series switch exhibits better than 20 dB isolation from DC up to 65 GHz and an insertion loss less than 1 dB up to 40 GHz. A new design from the same research team improves PZT switch performances at 10 V with better than 30 dB isolation and an insertion loss less than 0.5 dB from DC to 50 GHz [33].

**Figure 22.** PZT series switch.

Another feature of great interest in this switch technology is the opportunity to report on wafer‐ level transfer technologies to integrate PZT‐based RF MEMS switches into CMOS, as described by Guerre et al. [34]. Such heterogeneous integration can overcome the incompatibility of PZT materials with back‐end‐of‐the‐line (BEOL) CMOS technology. Switch characterisation draws out an insertion loss of less than 0.5 dB and an isolation better than 30 dB for the 0.4–6 GHz frequency range with 15 V actuation voltage.

In addition, these low loss and low actuation voltage piezoelectric switches can be correlated to realise a more complex commutation matrix: SP2T (Single Pole Two Throw) and SP4T (Single Pole Four Throw). For example, in the work of Chung et al. [35], as shown in **Fig‐ ure 23**, from DC to 50 GHz, the overall performance of the switches shows better than 20 dB isolation up to 50 GHz when the MEMS switches are in the off or zero‐volt state. When the switches are actuated with 7 V, the SP2T shows less than 1.8 dB of insertion loss while the SP4T shows less than 2 dB of insertion loss, on average, up to 40 GHz.

Some original application associated piezoelectric filters and switches together. Hummel et al. [36] demonstrated an innovative technology platform based on the monolithic integration of AlN resonators and PCM switches that is capable of delivering highly reconfigurable RF components, enabling new radio architectures with enhanced spectrum coverage.

**Figure 23.** Layout of the SP4T junction with PZT RF MEMS switches.

#### **3.2. Phase shifters**

Another paper described a higher frequency switch that uses piezoelectric actuators [32]. This switch is based on a thin film spun of PZT deposited onto a high‐resistivity silicon substrate with coplanar waveguide transmission lines (**Figure 22**). Actuation voltages less than 10 V with switch operation demonstrated as low as 2 V. The series switch exhibits better than 20 dB isolation from DC up to 65 GHz and an insertion loss less than 1 dB up to 40 GHz. A new design from the same research team improves PZT switch performances at 10 V with better

Another feature of great interest in this switch technology is the opportunity to report on wafer‐ level transfer technologies to integrate PZT‐based RF MEMS switches into CMOS, as described by Guerre et al. [34]. Such heterogeneous integration can overcome the incompatibility of PZT materials with back‐end‐of‐the‐line (BEOL) CMOS technology. Switch characterisation draws out an insertion loss of less than 0.5 dB and an isolation better than 30 dB for the 0.4–6 GHz

In addition, these low loss and low actuation voltage piezoelectric switches can be correlated to realise a more complex commutation matrix: SP2T (Single Pole Two Throw) and SP4T (Single Pole Four Throw). For example, in the work of Chung et al. [35], as shown in **Fig‐ ure 23**, from DC to 50 GHz, the overall performance of the switches shows better than 20 dB isolation up to 50 GHz when the MEMS switches are in the off or zero‐volt state. When the switches are actuated with 7 V, the SP2T shows less than 1.8 dB of insertion loss while the SP4T

Some original application associated piezoelectric filters and switches together. Hummel et al. [36] demonstrated an innovative technology platform based on the monolithic integration of AlN resonators and PCM switches that is capable of delivering highly reconfigurable RF

components, enabling new radio architectures with enhanced spectrum coverage.

than 30 dB isolation and an insertion loss less than 0.5 dB from DC to 50 GHz [33].

**Figure 22.** PZT series switch.

220 Piezoelectric Materials

frequency range with 15 V actuation voltage.

shows less than 2 dB of insertion loss, on average, up to 40 GHz.

Recent advances in piezoelectric actuated RF MEMS switches allow for the design of more complex functions and at present highly required phase shifters. A 2‐bit MEMS phase shifter incorporating PZT switches has been presented by Polcawich et al. [33]. A picture of the layout of the 2‐bit phase shifter based on PZT shunt switches is detailed in **Figure 24(a)**. The phase parameters have been measured against the frequency for the four configurations and are

**Figure 24.** (a) A 17 GHz 2‐bit reflection‐type phase shifter incorporating PZT shunt switches. (b) Measured normalised phase response.

presented in **Figure 24(b)**. In addition, this phase shifter has an average insertion loss of 2.96 dB using PZT shunt switches operating at 15 V.

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‐ ing level.

**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) Measured normalised phase response of the 3D phase shifter.

#### **3.3. RF varactors**

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

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 capacitance ratio.

**Figure 26.** (a) Top view of the variable capacitor. (b) *V* hysteresis curve with *V*piezo values of 2.5 and 0 V during pull‐in and pull‐out, respectively.
