Electromechanical and process design of a 3 axis piezoelectric MEMS gyro in GaAs

This paper deals with the conception and realization of a 3 axis gyrometer in GaAs known as CVG (Coriolis Vibrating Gyro), designed thanks to MEMS (Micro Electro Mechanical Systems) technologies. This original 3 axis gyroscope [1] can measure angular rate along the 3 sensing axes using the Coriolis inertial force effect on a single vibrating structure and is particularly well suited for applications where high miniaturization/integration and low consumption are required as attitude control of micro-UAVs (micro-Unmanned Aerial Vehicles) for instance. The proposed design shows important sensibilities along the 3 orthogonal directions. In automotive, aerospace and aeronautical industries, there is an increasing demand for accurate detection of acceleration and angular rate. Usually, objects are in motion in a three-dimensional space. To determine with precision the motion of an object, it is necessary to detect acceleration and angular rate in each of the three different space directions. A common way for multi-axial detection is to use a set of three different mono-axial sensors aligned with each of the three axial directions. Although miniaturized single axis gyroscopes have now reached high performances, 3 axis gyroscopes based on the combination and alignment of such single gyroscopes usually have lower performances than a fully integrated triaxial device because of alignment accuracy. In addition the alignment procedure is typically expensive. That is why, since several years, there is a rising need for monolithic 3 axis gyroscopes for angular rate detection. In this paper we investigate a 3 axis monolithic GaAs CVG structure that allows a single “driving mode” for the three “sensing modes”. This allows: — a reduction of mechanical coupling between the “driving mode” and the “sensing modes” — an easier fabrication owing to a simplified electrode network. — a less complex ASIC (Application Specific Integrated Circuit) design having a smaller size and a lower power consumption. Various designs of 3 axis gyro have been proposed in the literature but few of them show good and equivalent sensitivities along the three sensing axis. In the first section we will examine two interesting designs of CVG selected among published works which also use a single driving mode and three sensing modes. In the second section we described a design that we proposed previously and that has potentially much higher performances. Then we will investigate how GaAs piezoelectricity can be used to build a complete piezoelectric transduction system for this CVG. In the last section, we will give an overview of the transduction system design and of its fabrication process. Finally we will report results of the development of GaAs Deep Reactive Ion Etching (DRIE) which is major step of this fabrication process.