Principles, Applications, And The Future Of Piezoelectric MEMS

Piezoelectricity is a property of certain materials to become electrically polarized under strain and stress. This phenomenon has been studied extensively since it was first discovered in the mid-18^th century. Piezoelectric materials can generate an electric charge in response to an applied mechanical stress and can also generate mechanical stress upon an applied electrical charge. These materials provide a direct 2-way transduction of a signal from the mechanical domain to the electrical domain, and vice-versa.

The challenges of piezoelectric material production

Processing and manufacturing piezoelectric materials is challenging. In fact, stoichiometry (chemical reaction material control) and morphology (physical structure control) are critical parameters during piezoelectric material production, and accurate control of the piezoelectric crystallization process is required. In addition, commonly used piezoelectric material processes often include materials incompatible with standard CMOS technology. This has previously made it difficult to integrate piezoelectric elements into larger CMOS electronic circuits. The development of low cost, high quality deposition processes to produce piezoelectric materials has made significant progress during the last few decades. Chemical solution deposition and patterning of ferroelectric lead zirconate titanate (PZT) films, for example, has made it possible to integrate piezoelectric materials into MEMS-based transducers.

Piezoelectricity and MEMS

Many commercially successful MEMS devices rely on the displacement of mechanical parts by electrostatic fields to operate, including a number of MEMS-based motion sensors. Piezoelectric-based MEMS provide a more direct and linear actuation and detection mechanism than conventional electrostatically based actuation. There are a large variety of piezoelectric-based MEMS products, including microphones, speakers, gyroscopes, energy harvesters, analog mirrors, and others. These piezoelectric applications of MEMS technology can each be designed and studied using CoventorMP.

CoventorMP is a highly parametrizable design platform dedicated to multiphysics simulation. Its large collection of linear and non-linear analyses capabilities enable a complete study of piezoelectric devices. CoventorMP also includes a MATLAB Simulink interface for system design, along with a Cadence interface for IC design, so that the operation of the piezoelectric MEMS device can be studied within its surrounding system and electronic circuit.

Energy harvester design example

Figure 1 illustrates the study of a piezoelectric energy harvester using CoventorMP. In fabricating a MEMS-based energy harvester, the piezoelectric transducer is designed to maximize the generation of electrical energy. In the example shown in figure 1, a floating mass is connected to an anchored mass using a suspension beam (similar to a diving board at a pool). A piezoelectric film and two electrode layers have been deposited on top of the suspension beam. Physical deformation of the beam (through movement) will generate a current at the 2 electrodes deposited on top of the beam, due to the presence of the piezoelectric film. In figure 1 (far left), the 1^st resonance mode of the MEMS-based energy harvester is shown. The resonant mode occurs when the piezoelectric stack stretches and pulls back at a frequency close to the natural resonance of the beam structure, so that a large displacement (and more energy) can be efficiently obtained. In the center of figure 1, a reduced-order model of the MEMS device (the large block) is shown inside a Cadence circuit model. The electrical response of the piezoelectric device within its larger circuit is shown on the far right. CoventorMP can be used to design a MEMS-based energy harvester and predict its electrical response prior to silicon-based testing.

Fig. 1: From left to right. Left: 1st resonance mode of energy harvester. Center: Circuit design including MEMS component. Right: Transient analysis of voltage at output resistance.

Micromirror design example

Some MEMS-based piezoelectric transducers are used to actuate (or move) a microscale structure. As an example, MEMS-based micromirrors use piezoelectric-force to move micron-scale mirrors. These microscale mirrors are employed in optical collection and display applications and can be found in products such as video display projectors and LIDAR sensors used in autonomous vehicle control. The micromirror design shown below (figure 2) is constructed by placing a square mirror on top of a rotor. Thick beams on both sides of the mirror have a piezoelectric layer on top and are used to apply a moment (force) to the rotor. Due to this applied force, the mirror will rotate in a range that can be precisely determined and controlled.

Fig. 2: Piezoelectric µ-mirror.

What’s next?

In summary, piezoelectric materials are high energy density materials that scale very favorably upon miniaturization. There is a growing interest in using piezoelectric films for MEMS applications. However, significant design and material processing issues need to be overcome to increase the efficiency of piezoelectric MEMS devices. New and advanced piezoelectric materials will enable the design of novel piezoelectric MEMS products and provide greater integration with CMOS circuitry and into advanced low-power applications.

Reference:

1. Ulrich Schmid and Michael Schneider, “Editorial for the Special Issue on Piezoelectric MEMS”, Micromachines 2018, 9(5), 237; https://doi.org/10.3390/mi9050237
2. S Tadigadapa1 and K Mateti, “Piezoelectric MEMS Sensors: State-of-the-Art and Perspectives”, Meas. Sci. Technol. 20 092001, July 2009; https://iopscience.iop.org/article/10.1088/0957-0233/20/9/092001