• saw device aluminum nitride fabrication

During the last decades, sensors technology has been increasingly developed.
The widespread use of sensors and acoustic devices in everyday technologies is linked to the growing piezoelectric materials research. In the last ten years, the sputter deposition technology has paved the way to obtain very thin (μm) layers of piezoelectric crystals. This technology allows creating devices with higher resonant frequency (reaching the GHz) and consequently higher sensitivity.
This master thesis work is focused on the fabrication of a Rayleigh Surface Acoustic Wave (R-SAW) device using aluminum nitride (AlN) as the piezoelectric material. Usually, R-SAW devices are realized using lithium niobate (LiNbO3) as the piezoelectric material. AlN has higher transverse and longitudinal sound velocities compared to LiNbO3. This allows reaching a higher resonant frequency than LiNbO3. AlN has good mechanical properties and can withstand high temperatures. For example, AlN can maintain good piezoelectric properties in the air up to 700 ◦C or in an inert atmosphere up to 1000 ◦C. Therefore, AlN potentially enables the development of acoustic devices operating at higher frequency and power, with improved sensitivity and performance at high temperature and in harsh environments. AlN deposition and fabrication are fully compatible with Complementary Metal Oxide Semiconductor (CMOS) processes, making AlN a suitable active material for a large scale and low cost devices production.
The aim of this research is to acquire the fabrication procedures to develop a device able to induce an R-SAW within the AlN piezoelectric layer. The employed microcircuit is a delay line circuit composed of two interdigital transducers. This kind of circuit allows measuring the piezoelectric coupling of the substrate with a vector network analyzer and a laser doppler vibrometer. The fabrication procedure consists of various steps, and each of them is reproduced multiple times with a careful inspection of the substrate after each step. The idea behind this method is to progressively introduce small changes in fabrication steps and record the results with an optical microscope and a scanning electron microscope. This method allowed finding the parameters of the procedure lead1 ing to a complete microcircuit fabrication suitable for sputtered AlN film on silicon wafer. The substrate used in this research is a 2 μm thick sputtered AlN film on a 0.5 mm thick silicon wafer. The design of the microcircuit has been planned with the aid of finite element simulations. The microcircuit response was simulated in order to predict the resonant frequency and the piezoelectric coupling.
This work is divided in five chapters. Chapter 1 provides a theoretical introduction about the physics of piezoelectricity and state-of-the-art sensors. Chapter 2 illustrates the instruments and materials used in this research. Chapter 3 contains a detailed record of the tested procedures and their results. Chapter 4 contains the discussion and explanation of the obtained results. Chapter 5 is dedicated to the conclusions and perspectives for future research.