Jan Sauerwald

Plano-Convex Shaped Langasite Microbalances for High Temperature Applications

Piezoelectrically actuated plano-convex thickness shear mode (TSM) resonators in lanthanum gallium silicate (langasite) were fabricated. As main fabrication steps a wet etching process and a dry etching process were developed and furthermore graytone-lithography in combination with photoresist melting has been applied. The plano-convex shape is necessary to improve the Q-factor of the devices. The resulting Q-factor was up to two times higher than for simple planar resonators reaching values of more than 60,000. The special characteristics of langasite allow working temperatures of more than 700degC and a sensitive CeO2 coating can be used for gas measurements at high temperatures.

Gas detecting langasite membranes by wet chemical etching

Piezoelectrically actuated membranes in lanthanum gallium silicate (langasite) were fabricated. The membranes have shown operating temperatures up to 850 degC and are applied as high temperature gas detecting devices by means of CeO2 sensor films. The measurements show, that gas sensing with membranes is feasible and results in even higher frequency shifts as for thick plan-parallel resonators. A multi-step wet chemical etching process has been developed and was used to structure the langasite. Furthermore, membranes have been fabricated by using the different etch characteristics of doped and undoped langasite

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.

Displacement Characteristics of Piezoelectric Langasite Transducers at High Temperatures

Piezoelectrically excited langasite (La3Ga5SiO14) transducers are studied by laser doppler vibrometry (LDV) at temperatures up to 860 °C. The scope of our research is to design and realize different resonator concepts based on this material. The properties of langasite tuning forks and cantilevers are subject of our investigations. Room temperature investigations of the displacement for at least three resonance frequencies result in values up to 5 μm for a tuning fork and 27 nm for a cantilever. Vibration nodes are identified using the spatial distribution of displacement. Further, the resonances are assigned to different vibration modes. The electrical impedance of the tuning forks is determined simultaneously in the temperature range from 50 °C to 425 °C. Trends in the temperature dependence of the displacement are consistent with that of the resonator quality factor and indicate increasing losses with increasing temperature. The observation corresponds to earlier studies at elevated temperatures where ...

Design and fabrication of high-Q langasite resonator arrays for high temperature applications

The design of piezoelectrically actuated plano-convex shaped resonators in lanthanum gallium silicate (langasite, LGS) was investigated and improved by finite element simulations. Especially effects of shape and clamping were simulated. Further, devices that demonstrate the working principle have been fabricated. The membrane arrays have shown operating temperatures of more than 700degC and are applied as high temperature gas detecting devices by means of CeO2 sensor films. The special shape of the resonators causes an energy confinement and improves the quality factor at high temperatures. Wet etching, dry etching and masking technologies had to be applied for the fabrication. Simulation results could be confirmed by measurements.

Piezoelectrically driven spherically contoured resonators in LGS for high temperature applications

Piezoelectrically actuated plano-convex thickness shear mode (TSM) resonators in lanthanum gallium silicate (langasite, LGS) were fabricated. As main fabrication steps a wet etching process and a dry etching process were developed and furthermore graytone-lithography in combination with photoresist melting has been applied. The plano-convex shape is necessary to improve the Q- factor of the devices. Simulations have been run to show the influence of the spherically contoured surface. The resulting Q-factor was up to two times higher than for simple planar resonators reaching values of more than 60.000. The special characteristics of langasite allow working temperatures of more than 700degC and a sensitive CeO2 coating can be used for gas measurements at high temperatures.

Miniaturized resonant sensors for harsh environments

Miniaturized active structures for operation temperatures between 500 and 1000 °C are presented. They base on langasite single crystals (La3Ga5SiO14) which exhibit piezoelectrically excited bulk acoustic waves up to at least 1000 °C. Those devices enable new high-temperature sensing approaches. Resonant microbalances are of particular interest since they correlate very small gas composition-dependent mass changes of thin films already deposited onto the resonators with the resonance frequency shift of such devices. Thereby, high-temperature processes as occurring in combustion systems can be monitored in-situ. Miniaturization of those sensor devices improves the sensitivity due to higher operation frequencies. Arrays consisting preferably of miniaturized devices increase the selectivity. Miniaturization of high-temperature devices requires even more stable materials due to the increased effect of e. g. diffusion processes. Further, the resonator design, the arrangement of electrodes and sensor films, the vibration profiles etc. must be reviewed critically in order to take account for their miniaturization. Beside the characterization of the electromechanical properties such as temperature dependent resonance frequency and loss, the specific vibration profiles of devices like membranes of different shape, cantilevers and tuning forks are determined. For this purpose a novel measurement system based on a laser Doppler vibrometer is used to characterize different types of resonant sensor devices in-situ at high temperatures and in different atmospheres. Mapping of the sample surfaces provides the spatial distribution of the mechanical displacement and, thereby, the vibration modes.

Miniaturized piezoelectric structures for application temperatures up to 1000 °C

Miniaturized piezoelectric structures based on the material langasite are fabricated and characterized. Single crystalline langasite is a high-temperature stable material and can be excited using the piezoelectric effect up to temperatures above 1000 °C. The scope of our research is to design and realize small piezoelectric structures to serve as resonant sensors or active electronic components at high temperatures. Wet chemical etch processes are developed which enable isotropic and anisotropic etching with rates up to 90 μm/h. Small structures such as membranes as well as cantilevers are prepared. Electrical and optical characterization provides information about the electromechanical properties of those structures. Resonance properties are determined with electrical impedance measurements in-situ at temperatures close to 1000 °C. The temperature dependent frequency shift and the resonator quality factor are determined for membranes, cantilevers and tuning forks. In addition, investigations with a laser interferometer are obtained. Thereby, the spatial distribution of the vibration displacement is observed for several piezoelectric devices.

Fabrication and test of arrays of langasite microbalances

The design of piezoelectricaly actuated plano-convex shaped resonators has been studied to optimize their Q-factor and signal spectrum at high temperatures. The investigated arrays of thickness-shear-mode (TSM) resonators consist of langasite, a high temperature stable material. As viscoelastic damping and an increasing conductivity decreases the Q-factor at elevated temperatures, design optimizations have to counteract these effects. Two and three dimensional finite element (FE) models have been solved to analyze the resonant behavior and the effects of energy confinement at different temperatures depending on geometry. The separation and suppression of spurious modes, the improvement of the Q-factor and the confinement of the TSM could be shown. The simulated effects of energy confinement could be proofed by impedance measurements.