Marianne SAGNARD « Design and development of surface elacstic wave components...

PhD work : « Design and development of surface elacstic wave components dedicated to passive and wireless sensors and to radiofrequency filtering »
This work was performed in the context of a CIFRE DGA agreement in which Frecnsys and FEMTO-ST were associated

Abstract :

This thesis aims at designing innovative, passive and wireless surface acoustic waves (SAW) sensors and filters, dedicated to harsh environments. Several types of SAW components are consequently studied. The main characteristics, such as insertion losses or relative bandwidth, of usual structures (resonators, delay lines, LCRF, ladder filters...) are known by the experts. However, to design a SAW device that respects specific requirements, the definition of the proper behavior of each device must be established before the manufacturing. For this purpose, numerical models are developed. Not only they include the possibility to analyse he beha-vior of systems with complex geometry (ladder filters, apodised transducers) but they take into account disturbing phenomena (transverse modes, losses due to the intrinsic nature of the materials). The comparison between computations and measures points out the match between experimental results and calculations.

The implementation of these tools allows the development of innovative SAW sensors and filters thanks to a fast and reliable numerical analysis of their behavior. Thus, the design of resonators and sensors dedicated to a use at temperatures exceeding 700°C is studied. It is demonstrated that despite its inhomogeneity, Ba2TiSi2O8 is suitable for the manufacturing of SAW devices subject to high temperatures and in a frequency range from 300 MHz to the GHz. Furthermore, a structure composed of a three electrodes per wavelength transducer is used to produce re-sonators that are not subject to directivity effects when the temperature changes. This configuration offers the possibility to design sensors that use a single resonator (versus at least two until now). This last point makes smaller components possible and solves the question of a differential aging of the structures.

A second type of sensors, also passive and wireless, dedicated to humidity measurements, based on the use of a single SAW, is studied. In this new configuration, a LCRF is used as a transponder and the sensitive area is outsourced. The mode sensitivity (of more than a MHz) to the variation of a capacitance or a dipole antenna is numerically brought to light. In practice, the device manufacturing showed a differential variation of the resonances of several kHz depending on the electric condition applied to one of the ports.

This work highlights the ability to predict the behavior of SAW structures thanks to the development of dedicated software. Moreover, the analysis and the manufacturing of innovative sensors and filters pave the way to new functionalities.

The jury members are :

Victor Plessky, Professor, GVR Trade SA, Reporter
Anne-Christine Hladky, Professor, ISEN, Reporter
Yann Kersalé, Professor, Université de Bourgogne Franche-Comté, Examiner
Sami Hage Ali, Assistant Professor, Institut Jean Lamour, Examiner
Alexandre Reinhardt, Ingénieur de Recherche, CEA Leti, Examiner
Jean-Michel Friedt, Assistant professor, Université de Bourgogne Franche-Comté, PhD Director
Thierry Laroche, frecn|n|sys, PhD Co-director

Location : Amphithéâtre Jean-Jacques Gagnepain, Temis Sciences, 15 avenue des Montboucons 25000 Besançon.