Laurent Bouvot

Surface acoustic wave devices based on AlN/sapphire structure for high temperature applications

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.

Behavior of platinum/tantalum as interdigital transducers for SAW devices in high-temperature environments

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.

Innovative surface acoustic wave sensor for accurate measurement of subatmospheric pressure

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.

Brillouin microscopy on microwave-induced phonons in LiNbO3

High-performance Brillouin microscopy is proposed as a powerful technique to characterize the lobe of microwave-induced acoustic phonons generated by interdigital finger electrodes in a LiNbO3 device. The generation efficiency is compared with the intensity of thermal phonons of the same wave vector and polarization.

Wide vacuum pressure range monitoring by pirani SAW sensor

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.

Study of tantalum and iridium as adhesion layers for Pt/LGS high temperature SAW devices

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.

New measurement method to characterize piezoelectric saw substrates at very high temperature

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 LiNbO3 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].

Theoretical and experimental study of layered SAW magnetic sensor

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.

SAW pressure sensor for vacuum control applications

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.

P1K-5 High Sensitivity Pressure Sensor in the range [0-100mTorr] based on Surface Acoustic Wave Device

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