Introduction of Excellent FOM epitaxial lead zirconate titanate (epi-PZT) Thin Film development

Sumitomo Precision Products Co., Ltd. (SPP) Group contributes to the realization of MEMS in modern society by providing in-house MEMS devices such as high-precision gyroscopes (Fig. 1), MEMS foundry services by Silicon Sensing Systems Ltd, and MEMS semiconductor manufacturing equipment such as silicon deep etching by SPP Technologies Co., Ltd.

Fig. 1 Piezoelectric MEMS gyroscope (courtesy: Silicon Sensing Systems Ltd)

Generally, MEMS electromechanical converters utilize electromagnetic, electrostatic, piezoelectric, and magnetostrictive phenomena. Lead zirconate titanate (PZT) based piezoelectric converters enable small, high power devices. Due to its combination of excellent dielectric, ferroelectric piezoelectric, and pyroelectric properties, PZT is the most commonly desired material for electromechanical and thermoelectric transducers that are incorporated in a variety of MEMS devices, including mirrors, pumps, inkjets, actuators, microphones, speakers, ultrasonic sensors, and infrared sensors.

Our PZT thin films manufactured for piezoelectric MEMS gyroscopes achieve an excellent figure-of-merit (FOM) and high reliability. In addition, in 2020, our MEMS Foundry Services business expanded its 8-inch lineup of piezoelectric MEMS and now provides services worldwide.

Successful MEMS manufacturing relies on three primary elements, materials, manufacturing equipment, and manufacturing processes. Building a successful MEMS business additionally requires precise, reliable control of these elements on a large-diameter wafer process line that enables cost reduction.
The ICT Equipment & Devices Development Department of SPP has enhanced its in R & D and commercialization of next-generation key technologies, such as PZT thin films, by merging its MEMS equipment and process development efforts.

We deposit PZT thin films by sputtering at temperatures above 500℃. The crystalline structure of our PZT thin films are perovskite (ABO3) with Pb2+ in the 12-fold oxygen coordinated A-site and Ti4+ and Zr4+ in the 6-fold oxygen coordinated B-site. During film deposition at high temperature, the PZT structure has cubic symmetry, Upon cooling to room temperature after the deposition is completed, the PZT structure distorts to display either tetragonal or rhombohedral symmetry depending on the ratio of Zr to Ti. Compositions with more than 48% Ti exist in the tetragonal phase with unit cell distortion along the c-axis direction. The c-axis distortion and a spontaneous polarization results from the relative displace of the cation and anion unit cell sublattices where the Ti4+ and Zr4+ (B-site) ions exhibit the largest displacements.. When an electrical field is applied in the c-axis direction, the applied field induces a change in polarization primarily due to displacement of the Ti4+ and Zr4+ ions and this allows the PZT to function as an electromechanical conversion element. (Fig. 2). Conversely an applied force in the c-axis direction can induce a displacement that causes a change in the net polarization resulting in conversion from mechanical to electrical energy.

The FOM for piezoelectric thin films used in sensor devices (conversion of mechanical to electrical energy) is derived from the piezoelectric coefficient d31 and the relative permittivity εr, and is proportional to the square of d31 and inversely proportional to εr. In general, FOMs of commercial PZT thin films are from 20 to 30 GPa. In contrast, our c-axis oriented polycrystalline PZT thin film, which has been optimized to achieve a high FOM, has achieved the highest value of 40 GPa (Fig. 3, Conventional).

The relative dielectric constant, of a material is its permittivity expressed as a ratio relative to the vacuum permittivity. A piezoelectric material with a high dielectric constant exhibits increased capacitance for sensor and actuator devices, but a reduced capacitance is often desired to improve device performance. In piezoelectric PZT materials, part of the high dielectric constant results from ferroelectric domain wall motion. By reducing the density of domain walls the dielectric constant can be reduced. Therefore producing single crystal PZT films without grain boundaries and with a single domain is desirable for obtaining a reduced dielectric constant. Unfortunately, a reduced density of domain walls also lowers the piezoelectric d31 response. Additionally, the stress relief afforded by grain boundaries and correlated crystal lattice tessalation can be reduced, thus resulting in catastrophic device failure by cracking. In order to maintain high d31 and high mechanical strength, SPP engineers its epitaxial PZT films to include a low density of grain boundaries that define moderately sized grains of consistent crystal orientation. These engineered, epitaxial, PZT films provide an optimized device FOM because they exhibit both a high d31 piezoelectric response and a low dielectric constant.

We have built a specially designed deposition system in-house and constructed a new epitaxial process to develop an epi-PZT thin film with FOM≒50 (Fig. 3, Novel A, Novel B). The new epi-PZT thin film development does not simply pursue the formation of low defect density single crystals, but rather modifies the epitaxial crystal structure to realize high-FOMs and reliability and therefore, results in optimized PZT films for MEMS electromechanical transducers (Fig. 4).

SPPs multi-faceted commercialization offering of epi-PZT to MEMS industry customers includes supply of thin film deposition services, MEMS device design and fabrication services, foundry service facilitation, and thin film deposition equipment and processes.

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