SAW sensors for high temperature applications

Abstract

The technology of surface acoustic wave (SAW) devices allows the integration of signal processing and sensor functions within one product. In the past, SAW sensors have been operated at room temperature or 100 to 200°C at most. Materials related problems become obvious if one attempts to increase this operating temperature to a value as high as 1000°C. First experimental results will be presented based on a variation of the metallization and the use of diffusion barriers. It is expected that the use of these specially taylored materials with particular functional properties will lead to a considerable improvement of the lifetime and reliability of SAW sensors and the development of devices resistant to high temperatures as well as high pressures and chemically aggressive environments. The high-temperature characteristics of such novel devices are investigated by finite element simulation and by experimental deformation analyses. It will also be discussed which assembly, interconnection, and packaging techniques are applicable at 1000°C.

Defects and transport in langasite I: Acceptor-doped (La3Ga5SiO14)

“…This leads to values of E r = 5.7 ± 0.06 eV. Similarly, the activation energy for hole generation (of p-type conductivity) equals 1 2 E o (half of oxidation enthalpy) plus the hole migration energy E μ e . Since we have no independent information, we will assume that holes move through the lattice in a nonactivated manner, and thus the oxidation enthalpy is given as E o = 2.18 ± 0.08 eV.…”

“…According to the defect model for acceptor-doped langasite, the activation energy for electron generation (n-type conductivity) equals 1 2 E r (half of reduction enthalpy) plus the electron migration energy E μ e . From studies on donor-doped langasite, we learned that electrons migrate by an activated mechanism, with an activation energy of 0.15 ± 0.01 eV [10].…”

“…Langasite, which retains its piezoelectric properties to elevated temperatures, has operated as a surface acoustic wave device to temperature as high as 1000 • C [1], greatly surpassing the temperature limitations of other piezoelectric materials [2]. The high temperature capabilities of langasite, for example, make it a prime candidate as a crystal microbalancebased sensor for in-situ monitoring of e.g., automobile exhaust [3][4][5].…”

Sensors and smart electronics in harsh environment applications

“…1b). After cooling down to room temperature, the device and especially the metallization had survived (Mrosk, 1998).…”

Tungsten as a Chemically-Stable Electrode Material on Ga-Containing Piezoelectric Substrates Langasite and Catangasite for High-Temperature SAW Devices

“…Surface acoustic wave (SAW) devices have exhibited promising application as sensors for monitoring and controlling high temperatures above 400 °C and are increasingly demanded due to their functionality provided by their small size, robustness, and unique capability for wireless interrogation under extreme environments [ 1 , 2 , 3 , 4 ]. However, for high-temperature applications above 500 °C, several issues are related to the choice of durable materials for the interdigital transducer (IDT) thin film electrode and the piezoelectric substrate for enabling long-term applicability, such as: Chemical and structural stability of film-substrate composite in harsh environments; Resistance to stress-induced damaging effects in the nanocrystalline thin film electrode, such as agglomeration, delamination, creep, etc.…”