Atomic Layer Deposition and Sputtering of Piezoelectric Thin Films for Improved IMU Performance

Abstract

We discuss recent research and development efforts towards low cost, size, weight, and power gyroscopes based on piezoelectric MEMS technology. The potential advantages of piezoelectric and ferroelectric material integration into high performance inertial sensors include the following: significantly increased drive amplitudes for Coriolis Vibratory Gyroscopes relative to electrostatic actuators, dynamically equalized and stabilized Quality Factor mismatch (>>10×) in mode-match gyroscopes, and greatly improved drive amplitude (through 3D, sidewall piezoelectric integration) of mode-split gyroscopes. Piezoelectric thin films with large electromechanical coupling coefficients such as lead zirconate titanate (PbZrxTh1-xO3, PZT) can help to enable these performance benefits. Specifically, we will discuss efforts to optimize PZT thin film deposition using sputtering and new efforts to deposit 3D, conformal PZT using Atomic Layer Deposition (ALD). ALD is a well-established thin film growth technique that may enable conformal deposition of thin films on high aspect-ratio features. ALD of piezoelectric films on high aspect-ratio features, such as arrayed actuator elements, may improve the areal piezoelectric energy density (the total maximum piezoelectric energy projected onto the area of a planar substrate) by up to 200x or more, yielding proportionally larger drive amplitudes for CVGs compared to planar piezoelectric drive elements. However, ALD processes for piezoelectric films such as PZT or AlN are not widely reported. Recent laboratory demonstrations of ALD processes for PZT and a structural isomorph lead hafnate titanate (PbHfxTh1-xO3, PHT) have been successful, yielding piezoelectric perovskite films following a post-deposition anneal. Here, we present a modified process for ALD PHT which has yielded ferroelectric remanent polarizations (2Pr) of 68 μC/cm2, apply the new process to ‘ALL ALD’ PHT film stacks on 3D-MEMS comb structures, and present initial results on the integration of ALD PHT into ‘planar’ MEMS QMG. We will also discuss recent efforts to grow AlN by ALD, which have yielded c-axis oriented polycrystalline films with high dielectric breakdown voltage > 10 MV/cm and e31,f of −0.53 C/m2 on platinized substrates. This paper will also present results of applying piezo- and ferroelectric materials to gyroscopes to improve performance including 50% Q-tuning resulting in a>>10x improved quality factor mismatch.