This study describes the numerical implementation of accurate and fully coupled physical models in order to investigate the sensitivity of Surface Acoustic Wave (SAW) devices using the magnetoelastic interaction with an external magnetic field. The model was first validated using experimental data previously published by Kadota et al. obtained with SAW resonators based on quartz substrates and nickel Inter Digital Transducers (IDTs). The model was then used to optimize the geometry of a new magnetostrictive-piezoelectric layered structure (Ni/ZnO/IDT/LiNbO3), regarding its sensitivity to the magnetic field intensity. The optimized structure was designed and fabricated and experimental results show a good correlation with the numerical modeling. Simulations also show that if alumina is used instead of ZnO, the Ni/Al2O3/IDT/LiNbO3 structure exhibits a sensitivity that is 9 times higher than the one based on ZnO. (C) 2016 Elsevier B.V. All rights reserved.

In this paper we implement full model using Comsol Multiphysics software, leading to study both SAW structures, resonator and delay line. A general finite element method (FEM) of two-dimensional piezo-magnetic devices is used. In magnetostrictive thin film, a coupling between AC/DC module and structural mechanics module was implemented by adding appropriate terms to the subdomain variables for strain and magnetic field. The methodology was first validated using SAW resonator based on Quartz substrate and Nickel IDT. Variation of resonance frequency, quality factor and electromechanical coupling coefficient were investigated versus magnetic field intensity and direction and compared with published experimental results. The model was then considered to study original structures and predict their behaviors and sensitivities to magnetic fields. Thus both SAW structures Ni/ZnO/IDT/LN-Y128 and Ni/Al2O3/IDT/LN-Y128 were investigated. The first one simulated and realized experimentally shows a moderate sensitivity 0.65 ppm/mT when the fundamental SAW is considered (159 MHz) and 2.1 ppm/mT when considering the third pseudo-harmonic (460 MHz). For the second structure, the sensitivity was enhanced by a factor 9 using comparable operating frequency while a sensitivity of 31.5 ppm/mT is obtained when considering device operating at 815 MHz.

This study describes the implementation of accurate and fully coupled physics models leading to investigate Surface Acoustic Wave (SAW) devices sensitivity to an extern applied magnetic field. The model was firstly validated using experimental published data of SAW resonator based on Quartz substrate and Nickel Interdigital Transducers (IDT). Original layered structures, Ni/ZnO/IDT/LiNbO3 and Ni/Al2O3/IDT/LiNbO3 were then investigated and optimized to enhance their sensitivity to magnetic field intensity. Results show that Ni/Al2O3/IDT/LiNbO3 exhibits a sensitivity of 10ppm/mT, 14 times higher than the structure with ZnO one.

1. Background, Motivation and Objective : Demand for small, sensitive, accurate and wireless magnetic sensor record an important increase. Accurate modeling of SAW magnetic sensors including magnetostrictive materials requires to take into account the coupling between electrical, magnetic, mechanical, and acoustic domains. The aim of this work is to implement a full model using the Comsol multiphysics software, leading to predict the piezomagnetic sensor sensitivity to the direction and intensity of an external applied magnetic field. Both resonators and delay lines will be considered and the model accuracy will be validated experimentally. 2. Statement of the Contribution/Methods : A general finite element method (FEM) of two-dimensional piezomagnetic devices is used. In the magnetostrictive thin film, a coupling between the AC/DC module and the structural mechanics module was implemented by adding appropriate terms to the subdomain variables for the strain and the magnetic fields. The coupled equations for the mechanical and magnetic systems allowing the determination of the dependence between the elastic constants and the applied magnetic field are detailed in Ref [1], A heterostructure Ni/ZnO/IDT/LN-Y128 SAW delay line was fabricated using conventional photolithography and two mask levels. The S21 frequency response was measured using a network analyzer at different steps of fabrication (IDT/LN, ZnO/IdT/LN, Ni/ZnO/IdT/LN) and compared to the simulated results. The final and packaged structure was then characterized under magnetic field.

The effects of the thermoelastic and piezoelectric strain exerted by an active polymer matrix on a Ni nanowire (NW) are studied at the nanoscale by measuring the inverse magnetostriction of single-contacted Ni NWs. The reorientation of the magnetization is measured by anisotropic magnetoresitance. In the absence of strain, the Ni NW exhibits a typical uniform rotation of the magnetization as a function of the external field. When piezoelectric or thermoelelastic strain is present in the polymer matrix, the hysteresis loop becomes strongly modified by the inverse magnetostriction of Ni. It is shown that the ferromagnetic NW plays then the role of a mechanical probe that allows the effects of the mechanical strain to be characterized and described qualitatively and quantitatively. Moreover the stress exerted by the polycarbonate matrix on the NW is found to be isotropic while the one produced by the PVDF matrix is anisotropic.

Temperature coefficient of frequency of LiTaO3 single crystals was tuned by changing the Li stoichiometry. For this purpose, congruent 36°Y cut crystals, available commercially, were treated by vapor transport equilibration and temperature coefficient of frequency of surface acoustic waves propagating along X-axis as a function of Li concentration was studied. The temperature coefficient of frequency of -7 ppm/° was achieved for surface acoustic waves propagating in the crystals containing 49.43 mol % of Li2O.

Vapor Transport Equilibration was used to control the Li2O concentration in LiTaO3 single crystals. Li concentration was estimated from Curie Temperature and by means of Raman spectroscopy. Temperature coefficient of frequency TCF was determined from the frequency response of SAW devices measured versus temperature and a correlation between the temperature coefficient of frequency and LiTaO3 non-stoichiometry was studied. Experimental results showed that the temperature coefficient of frequency of treated LiTaO3 crystals was strongly dependent on the Li2O concentration. For example, in these case of the 36°Y cut crystals commonly used in SAW industry, it was moved from -35 ppm/°C for the congruent and commercially available crystal with 48.5 mol% of Li2O to -7 ppm/°C for the treated crystals with 49.43 mol% of Li2O. Moreover, experimental results show that the insertion losses were reduced suggesting the enhancement of the crystalline quality of the treated crystals. Note that temperature coefficient of an almost stoichiometric crystal (49.95 mol% of Li2O) was -13 ppm/°C.

This paper describes the use of a general purpose modified simulator, first developed by K. Hashimoto to determine a priori the device responses of the specify Pt/LGS Y-X structure. The parameters for the analysis are determined as a function of the electrode type (physical constants, thickness) and substrate. The simulation responses are analyzed and compared to the experimental results and we should conclude how effectively the modified simulator is applicable to assessing Pt/LGS Y-X device performances as a function of temperature.

In this paper, we report on the use of tantalum as adhesion layer for platinum electrodes used in high-temperature SAW devices based on langasite substrates (LGS). Tantalum exhibits a great adhesive strength and a very low mobility through the Pt film, ensuring a device lifetime at 900°C of about one hour in an air atmosphere and at least 20 h under vacuum. The latter is limited by morphological modifications of platinum, starting with the apparition of crystallites on the surface, followed by important terracing and breaking of the film continuity. Secondary neutral mass spectroscopy (SNMS), Auger electron spectroscopy (AES), X-ray diffraction (XRD) measurements, and comparison with iridium-based electrodes allowed us to show that this deterioration is likely intrinsic to platinum film, consisting of agglomeration phenomena. Finally, based on these results, we present a solution that could significantly enhance the lifetime of Pt-based IDTs placed in high-temperature conditions.

A new kind of surface acoustic wave (SAW) sensor has been developed to measure sub-atmospheric pressure below 100 mtorr with accuracy better than 0.1 mtorr. It provides an efficient measuring solution in the pressure range inaccessible in past by conventional diaphragm-based SAW sensors. Indeed, because of the small bending force in lower pressure and limited sensitivity, diaphragm-based SAW sensors are only suited to monitor relatively high pressure with a precision hardly better than 0.5 torr. To reach precision level better than 1 mtorr at sub-atmospheric pressure for vacuum technology applications, a radically different SAW-based solution is necessary. Our device aims to measure sub-atmospheric pressure less than 100 mtorr with a threshold resolution better than 0.1 mtorr. The concept is similar to the one used by Pirani pressure gauges. However, it is claimed that a heated and suspended SAW device should have better sensitivity. A theoretical model based on the basic concepts of gas kinetic theory and thermodynamics is presented. The validity of the model is checked by comparison between theoretical and experimental results.

AlN/sapphire layered structure has been investigated as a potential substrate for surface acoustic wave (SAW) devices operating at high temperatures up to 950 °C under air atmosphere. Frequency characterizations of the SAW delay lines based on this structure indicate a slight increase of 2 dB in the insertion losses after annealing for 30 min at 900 °C. Scanning electron and atomic force microscopy as well as x-ray diffraction measurements suggest that theses losses are due to the deterioration of the Pt/Ta electrodes and to a slight oxidation of the AlN film.

In this paper, we report on the use of tantalum and iridium as adhesion layers for platinum electrodes used in high temperature SAW devices based on langasite substrates (LGS). Unlike iridium, tantalum exhibits a great adhesive strength, and a very low mobility through the Pt film, ensuring a device lifetime of at least half an hour at 1000°C. The latter is limited by morphological modifications of platinum, starting by the apparition of crystallites on the surface, and followed by important terracing and breaking of the film continuity. SNMS and XRD measurements allowed us to show that these phenomena are likely intrinsic to platinum film, whatever be the nature of the adhesion layer. Finally, after having outlined a possible scenario leading to this deterioration, we consider some solutions that could replace platinum in order to increase the lifetime of LGS-based SAW devices in high temperatures conditions.

A new kind of surface acoustic wave (SAW) sensor has been developed in order to measure sub-atmospheric pressure below 100 mTorr with accuracy better than 0.1 mTorr. It provides an efficient measuring solution in a pressure range inaccessible in past by conventional diaphragm-based SAW sensors. Indeed, due to the small bending force in low pressure and limited sensitivity, diaphragm-based SAW sensors are only suited to monitor relatively high pressure with a precision hardly better than 0.5 Torr. In order to reach precision level better than 1 mTorr at sub-atmospheric pressure for vacuum technology applications, a radically different SAW-based solution is desired. Our device aims to measure sub-atmospheric pressure less than 100 mTorr with a threshold resolution better than 0.1 mTorr. The concept is similar to the one used by Pirani pressure gauges. However, it is claimed that a heated and suspended SAW device may have better sensitivity. A theoretical model based on the basic concepts of gas kinetic theory and thermodynamics is presented. The validity of the model is checked by comparison between theoretical and experimental results.

High performance Brillouin microscopy has been used as a versatile method in order to characterize the spatial distribution of piezoelectrically induced acoustic fields excited at microwave frequencies in a ZnO film deposited on silicon. Filtering properties and acoustic field distribution emitted by inter-digital transducers as well as propagation losses are investigated by μ-Brillouin spectroscopy. It turns out that the acoustic field intensity decreases dramatically outside the immediate excitation area situated below the inter-digital finger structure.

In this paper we present a new experimental set-up leading to characterize SAW sensor properties in high temperature up to 900degC. The characterization method consists in hanging a small piece of self-warming piezoelectric SAW device in a vacuum chamber. The device is made of the piezoelectric material to be tested equipped with its IDT plus a heating resistance, both in Platinum. The whole system is suspended from a PCB by mean of classical bonding wires. It is therefore thermally isolated from the rest of the experimental set-up. This allows using standard low-cost circuitry, to connect the SAW device to the measurement apparatus (standard coaxial feed-lines and SMA connectors). The warming being localised on the piezoelectric substrate, it also becomes possible to reach very high temperature, quickly and at low energy cost. This allows easy making of temperature cycles to test the aging of materials. In a first step, TCF values for Quartz ST and LiNbO 3 Y-128deg were measured in the range [20-500degC], then compared to calculated ones in order to validate the method. In a second step, one LGS Y-X SAW Delay-Line with Pt/Ta IDT was characterized using this test method in the range [20-900degC].

This work is oriented on the used of effective simulation tools to design high temperature SAW sensor. Physical constants of platinum and LGS and related temperature coefficients recently determined were implemented in FEM/SDA program. Plessky's parameters of the Pt/LGS structure were extracted for various temperatures and injected in COM software to calculate frequency response of SAW devices. A comparison between experimental and theoretical results showed good agreement up to 250degC and the requirement of a new LGS constants set.

An original application for surface acoustic wave (SAW) subatmospheric pressure sensor was developed to measure pressure below 100mTorr with very high precision. The basic operating principles and the most significant experimental results of the sensor are presented here. A simple theoretical model is proposed. This sensor provides an efficient measuring solution in a wide range of subatmospheric pressure, which has been inaccessible in past by conventional diaphragm-based SAW sensors.

A new concept of SAW powered lab on chip able to actuate and to perform accurate sensing is presented. Using IdT/ZnO/IdT/Quartz structure, it is possible to obtain simultaneously Rayleigh and Love waves. The first wave will be used to mix or heat whereas the second one will be use to sense. In order to determine the optimal characteristics, a theoretical approach has been developed. A comparison between our former classical microfluidic device and the new structure is presented. First results are explained.

In this paper, we present the first results of a new measurement method using SAW devices as pressure sensor. This method allowed us to reach a threshold precision better than one mTorr at low pressure. Sensibilities of 28 ppm/mTorr have been commonly achieved in the range [0-10 mTorr]. The stability and the repeatability of the measures have been checked. A patent is pending detailing the basic principles of the method. Thus only experimental results are presented here. A discussion about the industrial applicability of that new sensor concludes the paper.