Integrated sensors based on Lamb waves

Research Topic

Lamb waves are acoustic modes travelling along a finite thickness plate. The fundamental symmetric mode S0 has a predominant longitudinal polarization for a limited plate thickness range: the longitudinal particle displacement component U1 predominates over the shear vertical and shear horizontal U2 and U3 components (U1>>U2, U3,) and its normalized value is almost constant (U1=1) along the plate depth for a limited plate thickness range; moreover its phase velocity is close to that of the longitudinal bulk acoustic wave travelling in the same direction. Higher order quasi longitudinal modes, while showing the three components U1, U2 and U3 comparable inside the plate, have U1=1 and U2, U3=0 on the plate sides, thus allowing the design and fabrication of acoustic wave sensors suitable for operation in liquid environments. When the Lamb modes propagate along a composite plate (a bi-layered plate consisting in a piezoelectric and a non-piezoelectric layer), the plate symmetry around the mid plane of the plate is lost and the condition U1=1 and U2, U3=0 may be verified only on one plate side. This type of structure offer the advantage to show a high electroacoustic coupling coefficient as the IDTs can be positioned on the free surface of the piezoelectric layer or at the interface between the two layers, with or without a floating metal electrode onto the opposite piezoelectric film surface.
The project exploits the use of Lamb waves for advanced sensing functionalities including, in particular, sensing in liquid environments and for the development of zero-group-velocity (ZGV) devices.


Goals

The main goal of the project is 1. the design and 2. fabrication of a liquid sensor based on Lamb waves travelling along thin suspended membranes. The first part of the project, already developed, focuses on the choice of the type of waves, materials type, crystallographic orientation and layers thickness to achieve suitable sensor for liquid application with optimum sensitivity and working at high frequency. The propagation of Love modes, Shear Horizontal Acoustic Plate Modes, Lamb waves and thickness shear and longitudinal modes along multilayers was theoretically studied and enhanced-coupling-coefficient configurations and high-sensitivity sensors were designed for different materials including glass, AlN, SiC, BN, ZnO, and GaPO4. Once designed the sensor device, all the technological processes required to fabricate the sensors for liquid environment applications have been optimized, such as the metal (Mo, Pt and Al) and piezoelectric thin film deposition (AlN and ZnO c-axis oriented; the AlN and ZnO thin films with the c-axis tilted up to 30°) by rf reactive magnetron sputtering, the interdigital transducer patterning by electron beam lithography (EBL), the Cr/Au film chemical etching or lift-off technique, and silicon micromachining in KOH: works are in progress to optimize each process parameters and to solve some issues especially related to the stresses that affect the piezoelectric thin suspended membrane. Some prototypes of TFBARs based on c-AlN thin layers have been fabricated and tested. IDTs have been successfully patterned on thin (200 nm) Si3N4 suspended membranes with EBL technique. Very low stressed AlN thin films have been sputtered onto bare and Pt-covered thin Si3N4 suspended membranes; bi-layered aSiC/AlN membranes have been sputtered onto the Si3N4 membranes in view of the fabrication of ZGV Lamb mode devices.

Fellow

Muhammad Hamidullah

Host: CNR – Rome

Scientific Background

I studied microelectronic and MEMS microsystem for my bachelor and master degree. My research of interest is in the application of MEMS devices for sensor and actuator, more specifically the application of acoustic wave for liquid sensing.