Jochen Bardong

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.

In situ high-temperature characterization of AlN-based surface acoustic wave devices

We report on in situ electrical measurements of surface acoustic wave delay lines based on AlN/sapphire structure and iridium interdigital transducers between 20 °C and 1050 °C under vacuum conditions. The devices show a great potential for temperature sensing applications. Burnout is only observed after 60 h at 1050 °C and is mainly attributed to the agglomeration phenomena undergone by the Ir transducers. However, despite the vacuum conditions, a significant oxidation of the AlN film is observed, pointing out the limitation of the considered structure at least at such extreme temperatures. Original structures overcoming this limitation are then proposed and discussed.

Iridium interdigital transducers for high-temperature surface acoustic wave applications

Iridium is investigated as a potential metal for interdigital transducers (IDTs) in SAW devices operating at high temperatures. SAW delay lines based on such IDTs and langasite (LGS) substrate are fabricated and electrically characterized. The results show reliable frequency responses up to 1000°C. The strong increase of insertion losses beyond this temperature, leading to the vanishing of the signal between 1140 and 1200°C, is attributed to surface transformation of the LGS crystal, consisting of relevant gallium and oxygen losses, as evidenced by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and secondary ion mass spectroscopy.

Experimental and theoretical investigations of some useful langasite cuts for high-temperature SAW applications

Passive high-temperature sensors are a most promising area of use for SAW devices. Langasite (La₃Ga₅SiO₁₄; LGS) has been identified as promising piezoelectric material to meet high-temperature SAW challenges. Because it is necessary to know the material behavior for an accurate device design, the frequency-temperature behavior of Rayleigh SAW (R-SAW) and shear-horizontal SAW (SH-SAW) LGS cuts is investigated on delay line and resonator test structures up to 700°C by RF characterization. In the range of the 434-MHz ISM band, the (0°, 22°, 90°) SH-SAW cut shows thermal behavior similar to the (0°, 138.5°, 26.7°) R-SAW cut. Associated with the (0°, 22°, 31°) cut, in which SAWs present mixed types of polarization, the (0°, 22°, 90°) SH-SAW orientation might allow differential measurements on a single substrate. In the temperature range of 400 to 500°C, delay line test devices using the SH-SAW cut show a considerable drop of signal. Theoretical analysis indicates that this newly described behavior might be a result of anisotropy effects in this cut, occurring in case of any slight misorientation of electrode alignment.

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.

Characterisation setup of SAW devices at high temperatures and ultra high frequencies

A HT stable measurement setup for SAW devices operating at UHF has been developed. It is based on a ceramic chip carrier clamped on a metal ground plate that can be inserted into a tube oven and interrogated via coaxial cables. All interconnections except for the SAW device itself are clamped with metallic springs. Thus, the chip carrier can easily be inserted and removed from the measurement setup. The selected geometry and materials allow a simultaneous operation up to frequencies of 3.5 GHz and temperatures of 1000°C. To introduce the coaxial cables into the quartz glass tube, a vacuum tight feed through is used. So, influences of different atmospheres like laboratory air, inert gases or vacuum on the behavior of the SAW devices can be studied. Four similar test units can be inserted into the tube allowing in-situ measurements of up to four two port devices. The design aspects of the setup and first measurements of SAW test devices on langasite are presented.

Influence of packaging atmospheres on the durability of high-temperature SAW sensors

Surface acoustic wave (SAW) devices are a technology of choice for passive, radio-interrogable sensor applications operating under extreme conditions. Suitably designed SAW devices can withstand e.g. temperatures exceeding 400°C. At high temperatures (HT), thermal energies reach values corresponding to the activation energies of reactions between gas components and the crystals substrate elements and/or the metallisation elements, respectively. Thus, the atmosphere in the hermetic packaging becomes a crucial factor for the SAW devices stability. This work investigates the influence of various potential packaging atmospheres on SAW devices at temperatures up to 650°C. The SAW test structures consist of two delay lines with different lengths, which have been processed with Pt - based thin films. Substrate materials were either langasite (LGS), lithium niobate (LN) or stochiometric lithium niobate (sLN). The devices were annealed in a tube oven equipped with a HT-stable radio frequency (RF) measurement system in different atmospheres at several temperature levels up to 650°C. Afterwards, the SAW surfaces were characterised microscopically.

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.

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.