Publication
Each one of the billions of smartphones on the planet contains some acoustic filter for signal routing and processing, commonly made with Surface Acoustic Wave (SAW) or Bulk Acoustic Wave (BAW) resonators. The frequency of a SAW resonator can be easily set by changing the pitch of its electrodes, allowing for better integration of multiple filtering units in the same chip; the frequency of a BAW resonator usually depends on the film thickness, requiring further processing to achieve multiple operation frequencies in the same chip. A solution is using suspended Lamb wave resonators (LWR), where the frequency of a Bulk wave is usually defined by the pitch of the electrodes. The challenges for commercial adoption of high-Sc-doping Lamb Wave resonators come from the deposition of a good textured film and a suitable process for mass production. The first objective of the thesis is to fabricate MEMS resonators that employ Al0.6Sc0.4N as the piezoelectric material, achieving large coupling and high frequency of operation. An Al0.6Sc0.4N film with good piezoelectric properties was deposited using pulsed-DC Reactive Sputtering on heated substrates and biased with RF power for film densification. A platinum bottom layer serves to promote the AlScN growth and as electrode to have a vertical field. Good AlScN growth can be achieved if the metal is deposited at high temperature, meaning that the electrode must be patterned after deposition using Ion Beam Etching. Stripping the photoresist mask with a three-step process results in a clean metal surface, limiting the growth of Abnormally Oriented Grains. The piezoelectric film quality is confirmed by fabricating a batch of resonators and comparing their performances with simulations. The second objective of the thesis is the development of a process flow for the wafer-level fabrication of suspended LWR. Resonant frequencies of 5 GHz and above with a lateral vibration mode require an electrode width of 250 nm. To achieve such a small critical dimension and good alignment, a process using Deep Ultraviolet (DUV) lithography has been developed. The use of off-axis alignment allows alignment on all the layers of the process flow from the silicon substrate to reflective metals and transparent dielectrics. The result is a standard deviation of the resonance frequency on the wafer surface of 1 %. A simplified fabrication process, derived from the one used for the suspended resonators, has been used to fabricate hybrid SAW/BAW resonators on a SiC substrate. In this type of resonator an acoustic wave propagates in the piezoelectric AlScN without leaking in the substrate due to the mismatch in phase velocities between the guided mode and the bulk wave. By using the Coupling of Modes model we show how a clean response can be achieved by suppressing the unwanted Rayleigh mode without affecting the Sezawa mode. At the end of the thesis work, combining the deposition of a high-coupling piezoelectric material with a scalable p