Integrating Solution-Derived 3D PZT Structures on Si Mems Platform for RF and Biomedical Applications

Extensive research has been done on integrating PZT thin films (2D) on Si platform to actuate MEMS devices for RF and microfluidic applications. These typically involve PZT thin films solgel deposited or sputtered on Si to piezoelectrically actuate the device. Emerging applications need improved piezoelectric actuation than what can be designed with 2D PZT structures. For example, majority of MEMS designs use flexural mode resonators with transverse actuation (d31) of PZT films though d31 is inferior to d33 resulting in lower electromechanical performance. Integrating vertical PZT structures in Si can lead to much improved electromechanical coupling. The feasibility of filling vertical trenches with a piezoelectric material also enables fabrication of vertical BAW resonators with different widths on a single substrate. Therefore, various frequency resonators could be realized on a single chip, which is not possible with 2D integration. Vapor deposition to make 3D structures has several constraints because of the smaller deposition rates and line-of-sight deposition. Solution derived PZT structures is a cheaper alternative. This paper will describe the solution microcasting of PZT on micromachined Si. The solution deposition process was modulated for two geometries: 1. Conformal solution coating on high-aspect ratio trenches (aspect ratio of 2; 2 micron wide and 4 micorn deep) to yield high surface area capacitors with 2 times improvement capacitance density. 2. Filled 2 micron wide and 6 micron deep trenches in Si with solgel derived PZT, with a compatible process to longitudinally actuate the Si beam resonators. Control of pyrolysis was found to be a very critical step during the process. The pyrolysis and annealing profiles are designed to prevent film cracking, embedded porosity and improved properties. This paper details the process development results, potential applications and the projected performance improvements with this technology