This thesis focuses on the fabrication and characterisation of a thin film bulk acoustic wave resonator (BAW) fabricated on Si/SiO2 with stacked structures of multilayer graphene (MG), ZnO thin film, and gold.
The project aims to fabricate a thin film-based BAW device in which ultrathin multilayer graphene is used as the bottom electrode with ZnO thin films as the piezoelectric layer and gold thin films as the top electrode.
To accomplish the aim, different protection methods are tested to protect the graphene from degradation during the high-temperature growth of ZnO in the presence of oxygen. It was found that coating graphene with a self-assembled monolayer of bis(trimethylsilyl)amine (HMDS) can effectively reduce graphene damage during the high-temperature annealing process. Furthermore, a direct deposition process of multilayer graphene on substrates using pulsed laser deposition (PLD) was adopted and optimised to reduce the fabrication process and eliminate the multiple steps associated with the CVD transfer process. It was found that by tuning laser fluence and cooling rate high conductive and continuous graphene film can be directly deposited on SiO2. The growth of graphene was further optimised on other substrates such as SrTiO3. Apart from the growth of graphene, the deposition process for the growth of ZnO thin films is also studied and optimised. ZnO thin films with a high orientation of (002) and minimum defects using PLD on silicon substrates are realised.
Finally, thin film BAW devices with MG/ZnO/Au are fabricated and characterised. The theoretical working frequency of the device is calculated with modeling simulation and is also consistent with the practical measurement value. The working frequency of this thin film BAW is around 6 GHz without the need for an air cavity or Bragg reflector.