Denny Richter

Material and resonator design dependant loss in langasite bulk acoustic wave resonators at high temperatures

Single crystalline langasite (La3Ga5SiO14) resonators exhibit piezoelectrically exited bulk acoustic waves up to temperatures close to its melting point at 1470 °C. The loss observed in bulk acoustic wave devices depends on the materials properties and the resonance frequency. Anticipated operation at extremely high temperatures requires the understanding of both influences and enables tailoring of both properties to reduce the loss. Electrical impedance spectroscopy and diffusion runs using stable isotopes are the key methods used to study the atomistic transport processes and the electromechanical properties of langasite. At elevated temperatures, electrical as well as mechanical loss contributions are found. In particular, oxygen vacancies are responsible for strong losses which can be, however, suppressed by light donor doping. Above 650 °C, the impact of the conductivity related loss becomes pronounced. Further, the coupling of mechanical and electrical properties due to the piezoelectric effect causes a loss maximum at the dielectric relaxation frequency. Doping of langasite modifies the electrical conductivity and shifts, thereby, the dielectric relaxation frequency. Consequently, the choice of appropriate dopants and/or of the resonance frequency far off the latter frequency minimizes the loss. The concept is demonstrated and leads to an improved performance of resonant sensors at high temperatures.

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.

Optimization of wafer orientation and electrode materials for LGS high-temperature SAW sensors

Resonators with different designs and different resonance frequencies were fabricated on LGS. Resonators were studied at a temperature of 650°C. Annealing of LGS sensors was carried out in a furnace during 1000 hours. Dependences of the resonators behavior versus temperature were obtained. It was found that the first 100-300 hours of thermal annealing lead to the largest change in resonator properties, while further changes over the next hours are much smaller. Pt/Ti based resonators and Ir based resonators showed slow ageing trends, while Pt/Ti resonators passivated with an Al2O3 thin film with a thickness of 160 nm demonstrated steeper initial frequency changes. In spite of the obvious damage to electrodes, resonator properties remained reasonably robust to such kinds of degradation and clearly proved the possibility of long-term measurements with LGS based SAW sensor structures. The results showed that initial annealing of high temperature sensors is required to stabilize their ageing behavior. Using the XRD technique, we observed the process of oxidation of Ir and Pt electrodes of SAW-resonators during the first 100 hours of thermal annealing.

Solid State Sensors for Selective Gas Detection at High Temperatures—Principles and Challenges

To satisfy demands for in-situ monitoring of high-temperature processes, ever more sophisticated sensors are required. Advances in achieving improved sensitivity and selectivity are expected from miniaturization and integration of active electronic components thereby enabling new sensor principles or simultaneous application of different sensor principles. First, conventional resistive and potentiometric sensors are reviewed with respect to their stability and gas selectivity at temperatures up to 1000°C. Limitations caused by e.g. atomistic transport processes such as mixed ionic conduction are summarized. Subsequently, the improvement of resonant gas sensors by miniaturization and related material issues are discussed. The technology used here to prepare miniaturized structures enables to accommodate arrays of resonators in a single device and to reduce cost. For exemplification, miniaturized bulk acoustic wave resonators based on single crystalline piezoelectric langasite (La3Ga5SiO14) are machined by wet chemical etching and coated with gas sensitive films. Those devices could be operated up to at least 1000°C and are demonstrated to be selective in-situ gas sensors for carbon monoxide and hydrogen. The underlying concept includes monolithic structures in order to minimize thermal stress. For example, locally doped areas showing high electrical conductivity are used as electrodes at high temperatures. Further, field emission diodes are prepared and demonstrated to be operational. The radius of the langasite tips is estimated to be as low as 30 nm. Those diodes and other active electronic elements are intended to process e.g. sensor signals already close to the sensor element.

Miniaturized resonant gas sensors for high-temperature applications

The application of bulk acoustic resonators as gravimetric sensors enables high-temperature gas sensing provided that (1) the materials used withstand high temperatures and (2) the transducers are highly sensitive and selective. The former is realized by application of the high-temperature stable piezoelectric material langasite (La3Ga5SiO14). The focus of this work is the improvement of the sensitivity and selectivity by application of microsystems engineering methods to machine membranes. First, appropriate designs to achieve high resonator quality factors and low thermal stress are developed and realized by machining of biconvex membranes and monolithic electrodes, respectively. Further, the thickness of membranes is as little as about 25 mum thereby leading to high mass sensitivities. The concept is proven by successful operation of langasite membranes at temperatures up to 900degC. Membrane arrays wearing different sensor films are shown to improve the gas selectivity at 600degC.

High-Temperature Acoustical and Electrical Properties of LGS, LGT and CTGS Resonators

Acoustic characteristics and electrical conductivity of CTGS, LGT and LGS bulk acoustic wave resonators operated at the fundamental mode in the temperature range of 20-1470°C are studied. It is shown that LGS and CTGS resonators can be excited piezoelectrically up to 1470 and 1270°C, respectively, which is close to their melting temperatures. The electrical conductivity of CTGS is found to be by at least two and three orders of magnitude lower than that of LGS and LGT, respectively, over the temperature range 400-1000°C. Measurements of temperature dependent electromechanical losses show, that they are at least by two orders of magnitude lower in CTGS comparing to that in LGS within the measured temperature range.

Fast impedance analyzer interface with direct-sampling-technique for highly damped resonant gas sensors

In order to acquire multiple information about highly damped resonant gas sensors, such as frequency shift and damping figure, a compact impedance analyzer interface with direct-sampling-technique (DST) has been developed. DST allows accurate signal detection, good noise rejection by digital signal processing and very short measurement periods. The compact impedance analyzer is well suited for process control applications and can be employed for any sensor systems based on impedance spectroscopy by adapting the input stage of the interface. As an example, a CeO2 coated langasite BAW resonator acting as high temperature gas sensor is presented, which acts as microbalance for measuring changes in conductivity and mechanical properties of the metal oxide sensor film

A comparative study of LGS coefficient set accuracy assessed by experiments

Recent works have shown the capability of surface acoustic wave (SAW) sensors built on Langasite (LGS) to withstand temperature in excess of 700°C. This emphasizes the need for effective material coefficients allowing for designing SAW resonators with a high level of confidence. The prediction of the device electrical response and more of its temperature coefficient of frequency (TCF) reveals now mandatory. In the proposed paper, the accuracy of the published data sets for LGS and more specifically their capability to predict measured TCF for several crystal cuts supporting Rayleigh and surface transverse waves is assessed. Data from Kaminskii (1983), Ilyaev (1986), Silvestrova (1993), Sakharov (1995) and Bungo (1999) have been used to systematically evaluate their capability to predict first and second order TCF. Considering the experimental data set used to assess the tested coefficients, the authors finally propose a combination of data set allowing for accurate prediction of the device behavior at room and elevated temperature.

High-Temperature Gas Sensors

High-temperature processes in the field of, e.g., energy conversion or chemical technologies require sophisticated process monitoring and control to ensure high-efficiency, low pollutant emissions, and safe operation. These objectives can only be achieved by in-situ control of the processes. The increasing combustion of biofuels, organic waste, wood, etc., tightens the demand for process control even more. Properties to be monitored include temperature, gas composition, pressure, torque, mechanical integrity, and state of functional components. In this chapter, an overview about current gas sensor principles for operation temperatures above 500°C is given. Thereby, the related range of measurement, the selectivity, the sensitivity, the response time, and the long-term stability are presented along with application examples. Since the selection of sensor materials plays a crucial role at high temperatures, material aspects are an essential part of the chapter. The discussion of solid-state sensor principles includes potentiometric, amperometric, resistive, and resonant sensors.

P1.2.2 Variation of the Vibration Profile of Piezoelectric Resonant Sensors with Different Electrode Conductivity at High Temperatures

Langasite based piezoelectric resonators coated with metal oxide films are used as gas sensors for high-temperature applications. The influence of changes in electrical and mechanical properties of those films on the vibration behavior of the resonator is investigated with a laser Doppler interferometer. For this purpose, two different electrode geometries are used, leading to a microbalance mode and a conductivity mode. Thereby, the shift of the resonance frequency caused by a change of the film mass as well as the modification of the vibration profile can be distinguished. Resonators used in the conductivity mode provide information about the oxygen partial pressure. By using both effects simultaneously, a higher gas selectivity of those sensors is achieved.