Objective 1: Simulation of membrane supported SAW structures
First simulations will be performed in order to identify the two propagation modes that occur in the membrane supported SAW structures. This task will be completed with mechanical simulations (performed in COMSOL or ANSYS software) of the membrane in order to verify the breaking point when pressure is applied. Static capacity will be extracted vs. pressure and vs. temperature. After the first run is characterized, an optimization of the membrane will be performed. A coupled simulation will be implemented for the first time for membrane supported SAW structures: the FEM (COMSOL) simulation will be coupled with Coupling of Modes (COM) in order to extract the admittance.
____________

Objective 2: Technological development

A. Top side process for the SAW structures
The manufacturing of the single resonator SAW structures will start with the topside process. The first step in the fabrication of the single SAW resonator structure consists of the patterning and deposition of the connection pads to enable on-wafer S parameter measurements. For this, photolithography, e-gun metallization (Ti/Au) and lift-off technique will be used. Using electron beam lithography, the interdigitated transducers (IDT) will be patterned and the lift-off technique will be used for the metallization of the fingers having widths between 80 and 150 nm.

B. Back-side process for the membrane manufacturing of the SAW structures
The structures will have different membrane areas and the IDTs will be placed on different positions on the membrane. A hard mask and dry etching will be used for the backside selective etching of silicon to obtain the proposed 10 to 25 μm GaN/Si membranes and the 0.7 – 1.3 μm GaN membranes. Backside metallization of the membrane supported sensors will be performed in the end by sputtering technique.
____________

Objective 3: Exploiting the two types of membrane supported SAWs as dual pressure and temperature sensing structures
The manufactured SAW sensor structures will be hermetical soldered on a substrate, to create the diaphragm, which will be able to keep quasi-constant the pressure underneath the sensing structures (at atmospheric pressure). The characterization of the proposed structures will be performed in several steps: first, the single resonator SAWs will be measured on-wafer, at room temperature and atmospheric pressure, analyzing the two resonance peaks that correspond to Rayleigh and Lamb propagation modes; then, the measurements will be performed in Nitrogen atmosphere in the range of 1 to 20 Bars, analyzing the resonance frequency shift vs. pressure. The pressure coefficient of frequency and the pressure sensitivity will be extracted from linear fitting. The variation of the resonance frequency vs. temperature, at different pressures and vs. pressure, at different temperatures (in the range 21 – 150ºC) will be investigated and the pressure and temperature sensitivities will be the main outputs of this task.
____________

Objective 4: Embedding of the developed SAW structures
Encapsulation of the MEMS structures is currently one of the most difficult problems. Due to the peculiarities of the structures and of their applications, is not possible to develop a standard method. Usually, encapsulation raises more problems for MEMS comparing to the ICs. For the pressure sensors, the problem of encapsulation comes mainly from the detection: the pressure has to reach the detection element, which is usually a membrane. In this project, we propose a method to embed the sensing structure using a solid silicon cap, but vias will be developed over the structure allowing for pressure and temperature changing detection.