# A Feasibility Study of a Novel Piezo MEMS Tweezer for Soft Materials Characterization
## 2 The Adopted Piezo-Mems Microgripper  
The purpose of this paper is to develop a theoretical model of a piezo-MEMS microgripper, which can be used to characterize a grasped sample tissue. Therefore, the actual fabrication process of this instrument will not be herein considered. However, for the sake of completeness, a selection of possible materials and technological processes is briefly recalled. In the last decades, several actuation methods have been proposed to induce motion in MEMS devices such as electrostatic, thermal and piezoelectric. The electrostatic devices are widely adopted but the piezoelectric MEMS's offer some attractive advantages: lower power consumption, broader bandwidth and approximately ten times lower voltages to obtain the same given displacement [64]. Furthermore the piezoelectric materials can be manufactured using the same MEMS conventional technologies and for this reason these materials have been preferred to develop piezo-MEMS devices in the last decades. Typical applications include vibration energy harvesters [65; 66], resonators [67], capacitors [68], micro sensors/actuators [69; 70], micromachined ultrasonic transducer [71], gyroscopic sensors [72], microlens [73; 74], 1D and 2D micro-scanners [75; 76]. The piezoelectric materials can be gathered into two groups: ferroelectric (lead zirconate titanate, PZT compounds) and non-ferroelectric (ZnO and AlN). The piezoelectric characteristics (piezoelectric coefficient, Q factor, dielectric constant, etc.) rely on the crystalline structure. In fact ZnO and AlN thin-films show wurtzite structure that entails lower piezoelectricity if compared to PZT materials (perovskite structure). Nevertheless ZnO and AlN exhibit large mechanical stiffness, high \(Q\) factor and do not require a polarization process so they can be attractive for sensors applications. PZT thin-films provide high piezoelectric properties, lower cost and stability against temperature but require a poling process before using the piezo-MEMS device [77]. The electric field poling direction depends on the functional configuration of the piezo-MEMS. Usually the piezoelectric MEMS actuators/sensors are based on cantilever structures and the number of active layers identifies their working configuration:* [noitemsep,topsep=0 pt]
* unimorph (one piezoelectric layer coupled with an inactive structural layer) or
* bimorph (a structural layer sandwiched by two active layers). When a PZT bimorph bending beam and the transverse piezoelectric effect (\(d_{31}\)) are considered, the PZT layers must be poled in opposite directions in order to maximize the bending action. Then the electrodes of the PZT layers in contact with the structural layer share the same electric potential, whereas the outer electrodes share the same opposite sign potential (see Section 3). The design and fabrication process of piezo-MEMS devices have been extensively explored for the above mentioned unimorph and bimorph cantilever configurations using the aforementioned materials:  
* [noitemsep,topsep=0 pt]
* AlN [67; 68; 71];
* ZnO [70]; and
* PZT [66; 69; 75; 78]. Various technologies can be applied to deposit piezoelectric thin-films such as  
* [noitemsep,topsep=0 pt]
* pulsed laser deposition (PLD);
* chemical vapour deposition (CVD);
* screen printing;
* sol-gel and
* radio frequency sputtering. Usually the sol-gel and sputtering methods are the preferred ones both for research and commercial production because they allow the piezoelectric thin-film to be homogeneously deposited on large Si wafers [79]. The beam structural layer can be metal-based or silicon-based and the etching processes could be accomplished relying on conventional techniques (RIE, D-RIE, ECR). A schematic view of the target piezo-MEMS tweezer is qualitatively depicted in Figure 1. The system can be obtained by using a multilayer wafer that can be built by using the above mentioned techniques. At the end of the process, the whole microsystem is composed of two bimorph beams (**a**)-(**b**)-(**c**) (see Figure 1).