David Eisele

Precise measurements of BAW and SAW properties of Langasite in the temperature range from 25°C to 1000°C

There is a high demand for wireless sensing devices in harsh environments for industrial applications. For temperatures above 250degC, silicon-based sensors cannot be used. In contrast, bulk acoustic wave (BAW) and surface acoustic wave (SAW) devices are still suitable for this purpose. Further high-temperature applications include thermogravimetry on small volumes and gas sensing based on stoichiometry change of thin sensor films. Langasite (La3Ga5SiO14) is a piezoelectric single crystal that preserves its piezoelectric properties and is chemically stable up to its melting point at 1470degC without any phase transition and, therefore, is a promising material for high-temperature devices. Using a resonance-antiresonance method based on bulk oscillations, all components of elastic and piezoelectric tensors of langasite have been determined at temperatures up to 900deg C. Resonance spectra of several langasite samples have been measured and fitted with the impedance calculated from a one-dimensional physical model of piezoelectric bodies vibrating in several modes. In order to extract the electromechanical parameters, different resonator geometries and orientations are used. Also, the results of measurements are presented for SAW devices on langasite at temperatures from 25 to 750deg C. Two cuts with Euler angles (0deg, 138.5deg,26.6deg) and (0deg,30.1deg,26.6deg) have been studied. The devices were fabricated with a platinum (Pt) layer with different heights on a zirconium (Zr) adhesion layer. The main material parameters relevant for SAW devices such as phase velocity vp, propagation loss alpha and coupling coefficient k2 have been obtained. The measured SAW phase velocities compare well with those calculated with the elastic and piezoelectric tensors obtained by bulk-oscillation measurements.

SAW-relevant material properties of langasite in the temperature range from 25 to 750 °C: New experimental results

The aim of this work is to determine the acoustical parameters of langasite up to the highest possible point of temperature. This paper presents measurements of transfer functions for delay lines on langasite at frequencies ranging from 150 MHz to 1 GHz, at temperatures from 25 to 750degC. Two cuts with Euler angles (0deg, 138.5deg, 26.6deg) and (0deg, 30.2deg, 26.6deg) have been studied. The devices were fabricated using langasite as substrate, with two different platinum (Pt) layer heights (45 nm and 75 nm), on a zirconium (Zr) adhesion layer (4 nm). A special signal processing algorithm utilizing cross-correlation was implemented in MATLAB and used for the analysis of measured data. The material parameters relevant for SAW devices, such as phase velocity, propagation loss, and electromechanical coupling coefficient, have been determined as a function of temperature.

P1I-9 Passive 2.45 GHz TDMA based Multi-Sensor Wireless Temperature Monitoring System: Results and Design Considerations

This paper presents a TDMA (time division multiple access) based wireless temperature monitoring system using 2.45 GHz passive surface acoustic wave (SAW) delay line sensors. A three-step resolution refinement scheme using a combined delay and phase evaluation is proposed. Using a transmission power of 2 dBm (1.59 mW) a temperature accuracy of 0.19 K and 0.1 K(6sigma), were achieved for an interrogation distance of 1.4 m, and 1.3 m, respectively. The sensor design is discussed using experimental results concerning the relationship between the SNR (signal to noise ratio) of the sensors and the accuracy in time delay or phase measurement. Also, the direct electron beam writing on chemically-reduced (black) LiNbO3 is described

2.45 GHz Passive Wireless Temperature Monitoring System Featuring Parallel Sensor Interrogation and Resolution Evaluation

We report on the development of a TDMA (time division multiple access) based wireless sensor system for temperature monitoring. The transponders consisting of passive surface acoustic wave delay line sensors were designed for operation in the ISM band at 2.45 GHz and for a temperature range from -20degC to 180degC. A multi-step evaluation scheme using a combined delay and phase analysis, is introduced. A temperature resolution of 0.19 K (6sigma) was achieved at a transmission power of +2 dBm (1.59 mW), when the distance between the transceiver and sensors was about 140 cm. We also report the importance of mounting the sensors during packaging based on experimental results.

SAW-properties of langasite at high temperatures: Measurement and analysis

The measurement range of temperature using SAW-devices is depending on the substrate material used in the measurements. This dependence is related to the temperature point, where the substrate loses its piezoelectric properties. This occurs by the SAW-materials such as quartz and lithium niobate at about 570°C or 350°C, respectively. The piezoelectric material langasite (La3Ga5SiO14) has a thermodynamically stable phase and do not lose its piezoelectric properties up to its melting point at about 1470 °C and can be used for SAW devices at high temperatures. This paper presents the measurements of acoustical parameters of langasite up to 750°C. Langasite is used as substrate in the measurements with Euler angles (0°, 138.5°, 26.6°) and two different platinum (Pt) layer heights (45 nm and 75 nm) top of a zirconium (Zr) adhesion layer (4 nm). The investigated acoustic properties of langasite are group and phase velocity, propagation loss, and electromechanical coupling coefficient as functions of temperature. The measured data is analyzed using a special signal processing algorithm to obtain these acoustic properties of the Langasite.

SAW parameters for langasite at high temperatures

For three cuts of langasite with Euler angles (0°,138.5°,27°), (0°,22°,31°), and (0°,22°,90°) the temperature dependencies of SAW parameters - velocities, reflection coefficients and propagation loss were determined experimentally up to 800°C. For this, the test structures - delay lines and resonators, operating in the frequency range from 200 MHz up to 600 MHz, were designed and fabricated using Pt metallization. The influence of a passivation layer of Al₂O₃ was investigated. The perspective for the use of the STW cut is discussed.

Mass Sensitive Thin Film Bulk Acoustic Wave Resonators

The mass sensitive effect of RF-filter technology based film and a solidly-mounted acoustic bulk wave single resonator (FBARs and SBARs) is investigated for metrological use. For a high resolution of a sensor system all systematical errors like cross temperature sensitivity must be eliminated. A high quality factor Q is needed for stable oscillating for minimizing statistical errors. Experiments using FBAR and SBAR single resonators have been carried out in a vapor deposition facility and thermal chamber to analyze mass and temperature sensitive effects. The usage of more than one resonant mode with distinct polarization might allow the compensation of cross sensitivities

High-precision signal processing algorithm to evaluate SAW properties as a function of temperature

This paper presents a signal processing algorithm which accurately evaluates the SAW properties of a substrate as functions of temperature. The investigated acoustic properties are group velocity, phase velocity, propagation loss, and coupling coefficient. With several measurements carried out at different temperatures, we obtain the temperature dependency of the SAW properties. The analysis algorithm starts by reading the transfer functions of short and long delay lines. The analysis algorithm determines the center frequency of the delay lines and obtains the delay time difference between the short and long delay lines. The extracted parameters are then used to calculate the acoustic properties of the SAW material. To validate the algorithm, its accuracy is studied by determining the error in the calculating delay time difference, center frequency, and group velocity.

Investigations of SAW delay lines on c-plane AlN/sapphire at elevated temperatures

Aluminum nitride (AlN) on sapphire is a promising substrate for SAW (surface acoustic wave) sensors operating at high temperatures and high frequencies. To get an experimental measure of the suitability and temperature stability of such devices, several samples of SAW delay lines were fabricated on 2″ c-plane (0001) sapphire substrates with 1 µm c-plane AlN layer on top. Time- and frequency responses were recorded during annealing treatments at temperatures up to 850°C and the signals were analyzed afterwards.

The k-model - green's function based analysis of surface acoustic wave devices

We have derived a model for the analysis of surface acoustic wave devices based on relating the surface potential adherent to surface acoustic waves propagating on a piezoelectric substrate and applied transducer potentials. Device structures are analyzed on the basis of the Greens function, including end effects, electrical loading, mass loading effects, electrode resis- tance, dispersion, and propagation loss, leading to a 3-port matrix representation of single electrode cells. Cascading these matrices leads to the final sought after device response. The coupling to bulk waves is currently not taken into account. The model has been used to simulate and design 2.45 GHz SAW devices for wire- less sensor applications, showing close agreement of simulation and measurement of fabricated devices. Results for a single-electrode interdigital transducer (IDT) with a designed cen- tre frequency of 2441.75 MHz are compared and discussed. The measured equivalent circuit elements at the centre frequency for the SAW transducers fabricated on 128°YX-LiNbO 3 compared to simulated results in brackets yielded 53.3 Ω (49.8 Ω), 0.76 pF (0.78 pF), and 5.57 nH (5.45 nH) for the input impedance, suscep- tance (static capacitance and acoustic susceptance), and computed parallel matching inductance. The deviation of simulated and measured centre frequency was found to be 0.14 %. I. Introduction The precise simulation of SAW devices operating at UHF concerning their frequency characteristics and input impedance, require the consideration of higher order effects, as their contributions increase with frequency, or rather the reduced acoustic wavelength, in the order of several micrometers. An extensive overview of currently applied simulation models based on equivalent circuits (ECM), coupling of modes (COM), or the P-matrix model (PMM) is given by Ruppel et al. (1). Our intention was to work with a model closely related to the underlying SAW physics, at the same time being capable of modelling higher order effects, and requiring only a minimum of empirical data, as found necessary to account for propagation loss, dispersion, and mass loading effects. Preliminary measurements of fabricated test structures to extract simulation parameters were to be avoided. Although the PMM seems to offer these qualities, it has unfortunately not been published in detail. Our approach to derive a model with the capabilities mentioned above is also based on a mixed matrix representation and formulations similar to the P-matrix introduced by Tobolka (2). However, in contrast to the P-matrix, the K-model relates the surface potentials of propagating surface waves a i and b i , of unit Volt and not acoustic power amplitudes of unit . The 3-port matrix representation relating surface potentials of propagating surface waves, transducer potential