Julien Garcia

Chemical Sensors for the Detection of Chlorine and Nitrogen Trichloride at ppb Level

The control of air quality is of great importance, in particular for the protection of the workers who can be exposed to toxic gas. The present work reports the development of new chemical sensors for the detection of halogenated gases. These sensors are based on the use of nanoporous matrices doped with cationic surfactants and acting as sponges to trap the targeted pollutants. These chemical sensors can be used for the detection of chlorine Cl2 or nitrogen trichloride NCl3 at ppb level.

A comprehensive model of the electrical response of SAW devices submitted to thermal perturbation

A comprehensive model for computing the electrical response of surface acoustic wave (SAW) devices is proposed in this paper. Based on a mixed-matrix analysis, it allows for the prediction of any SAW device admittance/impedance at any temperature. In that purpose, the temperature dependences of the wave velocity of course but also of the reflexion coefficient, directivity, conductance and capacitance are established using polynomial developments. The interest of this approach is illustrated for SAW devices on directive crystal cuts for which dramatic changes of the SAW parameters versus temperature may occur, such as the (YXlt)/48.5°/26.7° Langasite (LGS) cut. First assessment elements are shown to illustrate the importance of accurately designing SAW devices operating on extended temperature range.

A new acoustic resonator concept based on acoustic waveguides using silicon/periodically poled transducer/silicon structures for RF applications

We propose a new acoustic resonator concept based on a periodically poled transducer (PPT) in a piezoelectric substrate (LiNbO3 or LiTaO3), embedded between two guiding substrates in order to create an acoustic waveguide. A resonator operating at 131MHz have been successfully fabricated and used in order to stabilize an oscillator. However the fabricated resonator presents a significant thermal sensitivity. The following experiments have consisted in studying a Si/thinned PPT layer/Si in order to reduce the thermal sensitivity.

2-and-3D Analysis of temperature effects on periodic transducers using a FEA/BEM approach

The thermal sensitivity of acoustic-wave-based devices still is an optimization key-point for various applications (filters, sources, sensors). As the architecture of such devices becomes more and more complex to fit the RF manufacturer requirements, simple analytical models usually exploited to optimize their frequency thermal drift are found obsolete. The optimization of these complex periodic wave-guide thermal sensitivities require the development of simulation tools capable to address the problem. This paper describes a Finite Element Analysis-Boundary Element Method (FEA/BEM) model allowing for the simulation any 2D and 3D periodic wave-guide thermal sensitivity, and more specifically the extraction of the corresponding Temperature Coefficient of Frequency (TCF). Validation of the computation approach first is reported. Its capabilities then are illustrated by computing the TCF of Love wave resonator on quartz.

An acoustic waveguide using doubly-bonded silicon/thinned PPT/silicon structures for RF applications

In this paper, we present new results on the development of piezoelectric transducers based on periodically poled ferroelectric domains in a lithium niobate plate bonded between two silicon wafers. The fabrication of the periodically poled transducers operating in the range 50 – 500 MHz has been achieved on a 3 inches 500 µm thick wafer. These devices then have been bonded on silicon wafers to fabricate a waveguide. Guided elliptic as well as partially guided longitudinal modes are excited. The experimental responses of the tested devices are compared to predicted harmonic admittances, showing a good agreement between both results and allowing for a reliable analysis of the nature of the excited modes. We also show interesting studies of material combinations used to guide ultrasonic waves. Dispersion properties have also been studied for a structure Si/PPT/Si. Operating points corresponding to a specific thickness/period ratio are found. Therefore a new conception with a Si/thinned PPT/Si structure is fabricated.

Simulation of finite acoustic resonators from Finite Element Analysis based on mixed Boundary Element Method/Perfectly Matched Layer

The simulation of finite length Surface Acoustic Wave (SAW) resonators is addressed here. Both the electro-mechanical coupling and the acoustic wave propagation in inhomogeneous space are considered through a Finite Element Analysis (FEA). The homogeneous parts of the space are treated using a Boundary Element Method (BEM) whereas the side edges of the transducer are completed by Perfectly Matched Layer (PML) method. By combining these two boundary methods BEM and PML, we are able to decrease the contributions of the losses due to the diffraction of SAW into Bulk Acoustic Waves (BAW). Thus, we can simulate the effects of real boundary filters (dual mode) on SAW resonators behavior as well as infinite passivation layer laid aver acoustic resonators.

Periodically Poled Acoustic Wave-Guide and Transducers for Radio-Frequency Applications

The demand for highly coupled high quality acoustic wave devices for RF signal processing based on passive devices has generated a strong innovative activity, yielding the investigation of new excitation principles and waveguide structures. Among all the tested devices, one can mention thick passivation SiO2-based structures using high velocity modes on lithium niobate (LiNbO3) or lithium tantalate (LiTaO3) (Kando et al, 2006), (Gachon et al. 2010), yielding the definition of interface or isolated-wave-based devices but modes excited on compound substrates (Elmazria et al, 2009), for instance consisting of a piezoelectric layer (AlN, ZnO, single crystal LiNbO3 or LiTaO3, etc.) deposited atop a high acoustic wave velocity material such as diamond-C, silicon carbide, sapphire, silicon, and so on (Higaki et al, 1997), (Iriarte et al, 2003), (Salut & al, 2010). All these devices generally exploit interdigitized transducers (IDTs) operating at Bragg frequency (Morgan, 1985), i.e. exhibiting a mechanical period equal to a half-wavelength of the acoustic propagation. Although passivation allows for an improved power handling compared to IDTS on free surfaces, this feature is still limited by electro-migration and material diffusion phenomena (Greer et al, 1990). An interesting answer to this problem is the use of bulk acoustic waves in thin films exhibiting a high disruptive field material such as AlN (Lakin, 2003), (Lanz, 2005). In that case, the frequency control reveals more difficult than for IDT based devices, as the resonance frequency of the so-called Film Bulk Acoustic Resonators (FBARs) is proportional to the film thickness. As significant progresses were achieved in thin film technologies during the last decade, this did not prevent the use of FBARs for actual low-loss RF filter implementation (Bradley et al, 2000). Nevertheless, it turns out there is still missing capabilities for better controlling the operation frequency of these passive devices, particularly for future generations of telecommunication systems which push toward higher RF bands than those exploited until now. The idea to transfer the transducer periodicity within the substrate has been shared by numerous scientists but it took rather a long term before the first experimental evidence, allowing for a correlation between theory and experiment and hence yielding a satisfying explanation of the corresponding mode distribution and realistic property description.

Fabrication and characterization of acoustic waveguides using Silicon/PPT/Silicon structures and analysis of diffraction effects for various modelings

In this paper, we present new results on the development of a new acoustic waveguide concept using an. acoustic wave excited by a Periodically Poled Transducer (PPT) and guided by guiding layers. Periodically poled transducers have been investigated recently as an alternative to classical inter-digital transducers for the excitation and detection of guided acoustic waves. The fabrication of PPTs operating in the range 50 – 500 MHz has been achieved on 3 and 4 inches 500 µm thick lithium niobate (LiNbO3) and tantalate (LiTaO3) Z-cut wafers. The compact structure proposed allows high frequency operation with a simplified package based on Si/LiNbO3/Si material combination. Dispersion properties have been studied for this structure in order to find operating points corresponding to a specific thickness/period ratio. Two main devices have been fabricated, a Si/500 µm thick PPT/Si structure in order to validate the concept and a Si/20 µm thick PPT/Si structure to excite only one acoustic wave in the purpose of diffracting this wave. The experimental responses of the tested devices are compared to the predicted harmonic admittances, showing a good agreement between both results. The temperature sensitivity of the excited wave of both structures are also been measured and predicted. Finally, we expose different structures with impedance mismatches generating scattering effects.

An acoustic waveguide based on doubly-bonded silicon/PPT/silicon structures

In this paper, we present new results on the development of piezoelectric transducers based on periodically poled ferroelectric domains in a lithium niobate plate bonded between two silicon wafers. The fabrication of the periodically poled transducers operating in the range 50 – 500 MHz has been achieved on a 3 inches 500 µm thick wafer. These devices then have been bonded on silicon wafers to fabricate a waveguide. Guided elliptic as well as partially guided longitudinal modes are excited. The experimental responses of the tested devices are compared to predicted harmonic admittances, showing a good agreement between both results and allowing for a reliable analysis of the nature of the excited modes. We also show interesting studies of material combinations used to guide ultrasonic waves. Dispersion properties have also been studied for a structure Si/PPT/Si.

On the convergence of 2D and 3D finite element/boundary element analysis for periodic acoustic waveguides

Finite Element Analysis/Boundary Element Methods (the so-called FEA/BEM) are a key-tool for the simulation of elastic waveguides and transducers used for the design of RF filters, sources and sensors. We analyze the optimal convergence conditions of corresponding computations, showing the interest of 2nd degree interpolation to improve both the accuracy and the delay of such calculations. We also propose a general and efficient approach to integrate boundary integrals of 3D problems, allowing to minimize the computation delay of the BEM part, the most costly of the whole model.