Pascal Nicolay

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.

Enhanced Sensitivity of SAW-Based Pirani Vacuum Pressure Sensor

The authors have already presented a novel surface acoustic wave device for vacuum pressure measurement by employing a piezoelectric substrate that has a high value of temperature coefficient of frequency. Frequency-shift measurements as a function of vacuum pressure can be used to extract information about the pressure sensitivity of the device. In this paper, we report that the deposition of an aluminum thin-film layer and rise in the sensors operating temperature significantly improve the sensitivity of the device. The results are crucial for improving the lower limit of the vacuum pressure measurement, which currently stands around 10-3 Pa.

Fast full-FEM computation of COM parameters. Application to multilayered SAW structures

We consider a problem of waves propagation in a multi-layered piezoelectric substrate under a periodic electrode grating utilizing a full-FEM approach for a unit cell with periodic boundary conditions. The perfectly matched layer is employed in order to truncate the computational domain. The simulations are done for the structures containing one- and two-layered piezoelectric substrates. The COM parameters obtained by the proposed model are compared with those extracted using a FEM/BEM technique.

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.

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].

AlN/IDT/AlN/Sapphire SAW Heterostructure for High-Temperature Applications

Recent studies have evidenced that Pt/AlN/Sapphire surface acoustic wave (SAW) devices are promising for high-temperature high-frequency applications. However, they cannot be used above 700°C in air atmosphere as the Pt interdigital transducers (IDTs) agglomerate and the AlN layer oxidizes in such conditions. In this paper, we explore the possibility to use an AlN protective overlayer to concurrently hinder these phenomena. To do so, AlN/IDT/AlN/Sapphire heterostructures undergo successive annealing steps from 800°C to 1000°C in air atmosphere. The impact of each step on the morphology, microstructure, and phase composition of AlN and Pt films is evaluated using optical microscopy, scanning and transmission electron microscopy (SEM and TEM), X-ray diffraction (XRD), and secondary ion mass spectroscopy (SIMS). Finally, acoustical performance at room temperature of both protected and unprotected SAW devices are compared, as well as the effects of annealing on these performance. These investigations show that the use of an overlayer is one possible solution to strongly hinder the Pt IDTs agglomeration up to 1000°C. Moreover, AlN/IDT/AlN/Sapphire SAW heterostructures show promising performances in terms of stability up to 800°C. At higher temperatures, the oxidation of AlN is more intense and makes it inappropriate to be used as a protective layer.

4E-2 Theoretical and Experimental Study of the Differential Thermal Expansion Effect on the TCD of Layered SAW Temperature Sensors Application to Aluminum Nitride Based Layered Structures

In this paper, we show that the stress and strain fields induced in a layered SAW structure by the thermal expansion of the different layers must be taken into account to compute the global structure temperature coefficient of delay (TCD). Experimental and numerical results are provided. The numerical model is described. It is based at the same time on the well- known Campbell and Jones method, the Bolotin equation and a simple way to approximate the strain field in a double layer structure. The model is then applied to test three sets of temperature coefficients for A1N thin film elastic constants. The comparison between AIN/Sapphire already published experimental data and theoretical results leads to the selection of one of the three sets. The recently released A1N 3ld order elastic constants are used here to compute the thermal strain effect. Once chosen, the set is used to compute the TCD of AIN/Diamond structures. The theoretical results are compared with new experimental data. The observed discrepancies are discussed.

A promising method to derive the temperature coefficients of material constants of SAW and BAW materials. first application to LGS

Langasite (LGS) is a promising material for SAW applications at high temperature. However, the temperature coefficients of LGS material constants are not accurate enough to perform reliable simulations, and therefore to make good use of available design tools, above 300°C. In the first part of the paper, we describe a new possible way to derive these coefficients in a wider temperature range. The method is based on Simulated Annealing, a well-known optimization algorithm. The algorithm converges toward a set of optimized temperature coefficients of the stiffness constants which are used to perform accurate simulations up to at least 800°C. In the second part, a deeper analysis of the algorithm outputs demonstrates some of its strengths but also some of its main limitations. Possible solutions are described to predict and then improve the accuracy of the optimized coefficient values. In particular, one solution making use of additional BAW target curves is tested. A promising solution to extend the optimization to the temperature coefficients of piezoelectric constants is also discussed.

An optimized set of temperature coefficients for LGS

LGS is a promising material for SAW applications at high temperature. However, the temperature coefficients of LGS material constants are not accurate enough to perform reliable simulations above 300°C. In the first part of the paper, we describe a new possible way to derive these coefficients in a wider temperature range. The calculated coefficients allow for accurate predictions up to at least 800°C. In the second part, a deeper analysis of the algorithm outputs demonstrates some of its main limitations. Possible solutions are described to predict then improve the accuracy of the optimized coefficients values.

Analysis of the sensitivity to pressure and temperature of a membrane based SAW sensor

ABSTRACT This paper presents a FEM analysis of a membrane-based Surface Acoustic Wave (SAW) sensor. The sensor is a 2.45GHz Reflective Delay Line (R-DL) based on Lithium Niobate (LiNbO3). As the wave propagation time is much smaller than the typical time constant of the phenomena to be monitored (deformation, temperature change etc.), the analysis can be performed in three successive steps. First, a static FEM study of the complete sensor (housing included) is carried out, to compute the temperature, stress and strain fields generated in the sensitive area by the measured parameters (pressure, temperature, etc.). Then, a dynamic electro-mechanical study of the R-DL is performed. The simulation takes the previously computed fields into account, which makes it possible to compute the sensor sensitivity to the measured parameters. The model takes advantage of the periodicity of the components of the R-DL to compute phenomenological parameters (Coupling-of-Mode parameters), which can later on be used to compute the electrical response of the sensor (step 3). In this paper, we focus on the first two steps. The COM parameters are extracted, under simultaneous thermal and mechanical stresses. Especially, the sensor sensitivity is obtained from the evolution of the velocity, under various stress configurations.