Gudrun Bruckner

Applicability of LiNbO/sub 3/, langasite and GaPO/sub 4/ in high temperature SAW sensors operating at radio frequencies

The applicability of LiNbO/sub 3/, langasite and GaPO/sub 4/ for use as crystal substrates in high temperature surface acoustic wave (SAW) sensors operating at radio frequencies was investigated. Material properties were determined by the use of SAW test devices processed with conventional lithography. On GaPO/sub 4/, predominantly surface defects limit the accessible frequencies to values of 1 GHz. Langasite SAW devices could be operated up to 3 GHz; however, high acoustic losses of 20 dB//spl mu/S were observed. On LiNbO/sub 3/, the acoustic losses measured up to 3.5 GHz are one order of magnitude less. Hence, SAW sensors capable of wireless interrogation were designed and processed on YZ-cut LiNbO/sub 3/. The devices could be successfully operated in the industrial-scientific-medical (ISM) band from 2.40 to 2.4835 GHz up to 400/spl deg/C.

Wireless SAW based high-temperature measurement systems

The paper exemplifies the development of a surface acoustic wave (SAW) temperature measurement system, showing the advantages of wireless data transmission and passive sensor operation. It includes results of research on lithium niobate, langasite and gallium-orthophosphate, three piezoelectrica used as substrate crystals for the SAW devices. Critical parameters, limits and prospects of the materials as well as technological issues concerning the metallization and the lifetime of reflective SAW delay lines are discussed. The packaging and assembly of high temperature (HT) stable transponders is explained. A slot antenna, a patch antenna and a dipole antenna, thus providing maximum flexibility for the sensor geometry, are investigated. A phase matching algorithm, enhancing the measurement resolution by two orders of magnitude compared to a conventional peak detection algorithm, is described. A fully integrated SAW temperature sensor system is presented, basing on a frequency modulated radar operating in the 2.45 GHz ISM band and designed for a world wide legal usage

Properties of radio frequency Rayleigh waves on langasite at elevated temperatures

The properties of Rayleigh waves operating both at radio frequency (RF) and at high temperatures (HT) on langasite crystal cuts with Euler angles of (0/spl deg/, 138.5/spl deg/, 26.6/spl deg/) and (0/spl deg/, 30.1/spl deg/, 26.6/spl deg/) are investigated. By evaluating the frequency response of surface acoustic wave (SAW) test devices the SAW velocity, the coupling factor, and the propagation attenuation have been determined experimentally in a temperature range from 20/spl deg/C to 500/spl deg/C. The SAW devices could be operated up to frequencies of 3 GHz. However, considering a future sensor application a limiting factor is a strong increase of the acoustic losses in conjunction with frequency and temperature.

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.

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.

A wireless SAW-based temperature sensor for harsh environment

A SAW-based temperature sensor system is presented. Much effort was placed in the development of a high temperature resistive transponder compromising of both a SAW device and an antenna stable up to 400 /spl deg/C for at least a few days. The system solution also includes a low-cost reader unit employing the FSCW (frequency stepped continuous wave) principle operating in the ISM band from 2.4 GHz to 2.4835 GHz. Spare space on the SAW device has been employed also to implement an ID for identification purpose. Hence, this solution also allows for tagging of objects exposed to harsh environments.

SAW ID tag for industrial application with large data capacity and anticollision capability

Surface acoustic wave (SAW) RFID tags are promising devices for the identification of work pieces in an industrial environment. Their practicality was tested using different types of ID tags operating in the 2.45 GHz ISM band with a code volume of 220- 236. The tags were produced to withstand temperatures up to 350degC. The performance was assessed by measuring the probability of identification as a function of distance. A reading range of several meters was demonstrated, showing that the tags are appropriate for industrial processing lines. Readout of two tags within one antenna beam (anticollision) was demonstrated.