MEMS Aluminum Nitride Technology for Inertial Sensors
The design and fabrication of MEMS Inertial Sensors (both accelerometers and gyroscopes) made of Aluminum Nitride (AlN) is described in this dissertation.
The goal of this work is to design and fabricate inertial sensors based on c-axis oriented AlN polycrystalline thin films.
AlN is a post-CMOS compatible piezoelectric material widely used for acoustic resonators, such Bulk Acoustic Wave (BAW) and Lamb Wave Resonators (LWR). In this work we develop the design techniques necessary to obtain inertial sensors with AlN thin film technology. Being able to use AlN as structural material for both acoustic wave resonator and sensing elements is key to achieve the three level integration of RF-MEMS components, sensing elements and CMOS in the same chip.
Using AlN as integration platform is particularly suitable for large consumer emerging markets where production costs are the major factor that determine a product success.
In order to achieve a platform integration, the first part of this work focuses on the fabrication process: starting from the fabrication technology used for LWR devices, this work shows that by slightly modifying some of the fabrication steps it is possible to obtain MEMS accelerometers and gyroscopes with the same structural layers used for LWR.
In the second part of this work, an extensive analysis, performed with analytical and Finite Element Models (FEM), is developed for beam and ring based structures. These models are of great importance as they provide tools to understand the physics of lateral piezoelectric beam actuation and the major limitations of this technology.
Based on the models developed for beam based resonators, we propose two designs for Double Ended Tuning Fork (DETF) based accelerometers.
In the last part of the dissertation, we show the experimental results and the measurements performed on actual devices.
As this work shows analytically and experimentally, there are some fundamental constraints that limit the ultimate sensitivity of piezoelectric sensors based on resonating beam structures.
Although the limitations of the structures here considered cannot achieve tactical grade sensitivities, this research proves that it is possible to achieve performances close to those required by large consumer electronics. This work proves that AlN based platforms can be a great opportunity for future developments in IMU and in general for MEMS integrated solutions.