Surface Transverse Wave (STW) resonators on langasite (original) (raw)

Fabrication of high temperature surface acoustic wave devices for sensor applications

Sensors and Actuators A: Physical, 2005

Surface acoustic devices have been shown to be suitable not only for signal processing but also for sensor applications. In this paper high temperature surface acoustic wave devices based on gallium orthophosphate have been fabricated, using a lift-off technique and tested for high frequency applications at temperatures up to 600 • C. The measured S-parameter (S 11 ) has been used to study the mass loading effect of the platinum electrodes and turnover temperature of GaPO 4 with a 5 • cut. The analysis of these results shows that the mass loading effect can be used to predict the desired resonant frequency of the SAW devices. Also two different adhesion layers for Pt metallisation were studied. Our results show that Zirconium is a more suitable under layer than Titanium.

Multilayered Structure as a Novel Material for Surface Acoustic Wave Devices: Physical Insight

InTech eBooks, 2011

Since 70-ies, when the first delay lines and filters employing surface acoustic waves (SAW) were designed and fabricated, the use of SAW devices in special and commercial applications has expanded rapidly and the range of their working parameters was extended significantly (Hashimoto, 2000; Ruppel, 2001, 2002). In the last decade, their wide application in communication systems, cellular phones and base stations, wireless temperature and gas sensors has placed new requirements to SAW devices, such as very high operating frequencies (up to 10 GHz), low insertion loss, about 1 dB, high power durability, stable parameters at high temperatures etc. The main element of a SAW device is a piezoelectric substrate with an interdigital transducer (IDT) used for generation and detection of SAW in the substrate. The number of single crystals utilized as substrates in SAW devices did not increase substantially since 70ies because a new material must satisfy the list of strict requirements to be applied in commercial SAW devices: sufficiently strong piezoelectric effect, low or moderate variation of SAW velocity with temperature, low cost of as-grown large size crystals for fabrication of 4-inch wafers, long-term power durability, well developed and non-expensive fabrication process for SAW devices etc. Today only few single crystals are utilized as substrates in SAW devices: lithium niobate, LiNbO 3 (LN), lithium tantalate, LiTaO 3 (LT), quartz, SiO 2 , lithium tetraborate, Li 2 B 4 O 7 (LBO), langasite, La 3 Ga 5 SiO 14 (LGS) and some crystals of LGS group (LGT, LGN etc.) with similar properties. The SAW velocities in these single crystals do not exceed 4000 m/s, which limit the highest operating frequencies of SAW devices by 2.5-3 GHz because of limitations imposed by the line-resolution technology of IDT fabrication. The minimum achievable insertion loss and maximum bandwidth of SAW devices depend on the electromechanical coupling coefficient, which can be evaluated for SAW as k 2 ≈2ΔV/V, where ΔV is the difference between SAW velocities on free and electrically shorted surfaces. The largest values of k 2 can be obtained in some orientations of LN and LT. Ferroelectric properties of these materials are responsible for a strong piezoelectric effect. As a result, k 2 reaches 5.7% in LN and 1.2% in LT, for SAW. For leaky SAW (LSAW) propagating in rotated Y-cuts of both crystals, the coupling is higher and can exceed 20% for LN and 5% for LT. However, LSAW attenuates because of its leakage into the bulk waves when it propagates along the crystal surface. As a www.intechopen.com Acoustic Waves-From Microdevices to Helioseismology 422 result, insertion loss of a SAW device increases. Attenuation coefficient depends on a crystal cut and IDT geometry. For example, in 36º to 48º rotated YX cuts of LT and in 41º to 76º YX rotated YX cuts of LN, high electromechanical coupling of LSAW can be combined with low attenuation coefficient via simultaneous optimization of orientation and electrode structure (Naumenko & Abbott, US patents, 2003, 2004). When these substrates are utilized in radiofrequency (RF) SAW filters with resonator-type structures, low insertion loss of 1dB or even less can be obtained. Today such low loss filters are widely used in mobile communication and navigation systems. The main drawback of these devices is high sensitivity of the characteristics to variations of temperature because the typical values of temperature coefficient of frequency (TCF) vary between-30 ppm/ºC and-40 ppm/ºC for LT and between-60 ppm/ºC and-75 ppm/ºC for LN. Contrary to LN and LT, quartz is characterized by excellent temperature stability of SAW characteristics but low electromechanical coupling coefficient, k 2 <0.15%. Hence, even in resonator-type SAW filters with very narrow bandwidths, about 0.05%, where the loss of radiated energy is minimized due to the energy storage in a resonator, the best insertion loss achieved in a SAW device with matching circuits is only 2.5-4 dB. In some orientations of LBO, LGS and other crystals of LGS group, zero TCF is combined with a moderate electromechanical coupling coefficient. However, these crystals have limited applications in commercial SAW devices because low SAW velocities restrict highfrequency applications on LGS and LBO dissolves in water and acid solutions, which prohibits application of conventional wafer fabrication processes to this material and finally results in an increased cost of SAW devices. Hence, none of available single crystalline materials provides a combination of large piezoelectric coupling, zero TCF and high propagation velocity. A strong need in such material exists today, especially for application in SAW duplexers and multi-standard cellular phones, where the temperature compensation is the key issue because of necessity to divide a limited frequency bandwidth into few channels with no overlapping allowed in a wide range of operating temperatures. As an alternative to conventional SAW substrates, layered or multilayered (stratified) materials were studied extensively since 80-ies but only in the last decade some of these structures found commercial applications in SAW devices, due to the recent successes of thin film deposition technologies and development of robust simulation tools for design of SAW devices on layered structures.

High-temperature 434 MHz surface acoustic wave devices based on GaPO/sub 4

IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, 2006

Research into surface acoustic wave (SAW) devices began in the early 1970s and led to the development of high performance, small size, and high reproducibility devices. Much research has now been done on the application of such devices to consumer electronics, process monitoring, and communication systems. The use of novel materials, such as gallium phosphate (GaPO4), extends the operating temperature of the elements. SAW devices based on this material operating at 434 MHz and up 800 C, can be used for passive wireless sensor applications. Interdigital transducer (IDT) devices with platinum/zirconium metallization and 1.4 m finger-gap ratio of 1:1 have been fabricated using direct write e-beam lithography and a lift-off process. The performance and long-term stability of these devices has been studied, and the results are reported in this paper.

High-Temperature SAW Resonator Sensors: Electrode Design Specifics

IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, 2018

Surface acoustic wave (SAW) sensors are steadily paving the way to wider application areas. Their main benefit consisting in the possibility of wireless interrogation with the radio frequency (RF) interrogation signal being the only energy source for the re-radiated signal. This feature is getting more and more attractive with the growing demand in monitoring multiple industrial objects difficult to access by wired sensors in harsh environments. Among such wider applications the possibility of making measurements of temperature, deformation, vibrations and some other parameters at temperatures in the range of 300-1000 °C look quite promising. This paper concentrates on specific features of the SAW resonator based sensors operation at this temperature range. High temperature influences the material choice and thus the properties of SAW resonators design peculiarities intended for use at high temperature. It is suggested that preferable designs should use synchronous resonators with relatively thick electrodes (10% of wavelength) based on Ir or Pt alloys while benefiting from the possibilities of specific designs that could reduce the negative impact of thick electrodes on the manufacturing in quantity. This solution benefits from lower resonance frequency scatter because of the automatic compensation of SAW velocity decrease due to electrode metallization ratio increase. This compensation originates from the resonance frequency increase that is related to the decrease of the Bragg bandwidth defined by the reflection. It is shown in modeling examples that the value of metallization ratio at which this compensation occurs is close to 65-70%.

Surface Acoustic WaveAmmonia Sensors Based on ST-cut Quartz under Periodic Al Structure

Sensors, 2009

Surface acoustic wave (SAW) devices are key components for sensing applications. SAW propagation under a periodic grating was investigated in this work. The theoretical method used here is the space harmonic method. We also applied the results of SAW propagation studied in this work to design a two-port resonator with an Al grating on ST-cut quartz. The measured frequency responses of the resonator were similar to the simulation ones. Then, the chemical interface of polyaniline/WO 3 composites was coated on the SAW sensor for ammonia detection. The SAW sensor responded to ammonia gas and could be regenerated using dry nitrogen.

Thin Film Electrodes for High Temperature Surface Acoustic Wave Devices

Procedia Engineering, 2011

Wireless surface acoustic wave devices based on langasite allow sensor operation in high-temperature environments. While langasite shows piezoelectric behavior up to temperatures close to its melting point of 1470 • C, the stability of thin film electrodes for excitation of acoustic waves limits the application temperature. Different metal and ceramic based thin films are tested regarding their applicability as electrode materials for wireless surface acoustic wave devices at temperatures above 600 • C. The devices should withstand temperatures of 800 • C for at least several hours. The high-temperature stability of platinum, platinumrhodium, iridium as well as lanthanum strontium manganite (LSM) based electrodes is characterized using electrical measurements, x-ray diffraction (XRD) measurements and scanning electron microscopy (SEM) imaging. SAW devices with Ti/Pt thin film electrodes are operated up to temperatures of 800 • C. LSM/Pt films exhibit a good stability up to temperatures of 800 • C. Above 600 • C its resistance is comparable to platinum films .

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°, 138.5°, 26.6°) and (0°, 30.1°, 26.6°) 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 °C to 500 °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.

Surface Acoustic Wave (SAW) Sensors: Physics, Materials, and Applications

Sensors

Surface acoustic waves (SAWs) are the guided waves that propagate along the top surface of a material with wave vectors orthogonal to the normal direction to the surface. Based on these waves, SAW sensors are conceptualized by employing piezoelectric crystals where the guided elastodynamic waves are generated through an electromechanical coupling. Electromechanical coupling in both active and passive modes is achieved by integrating interdigitated electrode transducers (IDT) with the piezoelectric crystals. Innovative meta-designs of the periodic IDTs define the functionality and application of SAW sensors. This review article presents the physics of guided surface acoustic waves and the piezoelectric materials used for designing SAW sensors. Then, how the piezoelectric materials and cuts could alter the functionality of the sensors is explained. The article summarizes a few key configurations of the electrodes and respective guidelines for generating different guided wave patterns ...

Thermal characterization of Surface Acoustic Wave devices

2013

Reliability of micro-electronic devices is one of the most important issues in mobile communication systems and is significantly influenced by the thermal behavior of the components. This study presents different schemes for thermal characterization of a half-section ladder-type Surface Acoustic Wave (SAW) filter which is acoustically passivated with a thick SiO2 layer. Unitarity violation quantifies the entire power loss in the device but is unfeasible regarding correlation to each resonator. The Temperature Coefficient of Frequency (TCF) characterizes thermally induced frequency shifts and has the potential to investigate the resonators' temperatures separately in first order. However, uncertainties arise using this indirect approach as soon as other effects causing a frequency shift play a role. Thermographic techniques such as Infrared Thermography (IRT) and Liquid Crystal Thermography (LCT) serve as direct measurement schemes eliminating inaccuracies inherent to TCF based evaluations and show good agreement with simulation results. Moreover, LCT and IRT provide spatially resolved temperature measurements of the component.

Langasite surface acoustic wave gas sensors: modeling and verification

IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 2000

We report finite element simulations of the effect of conductive sensing layers on the surface wave velocity of langasite substrates. The simulations include both the mechanical and electrical influences of the conducting sensing layer. We show that three-dimensional simulations are necessary because of the out-of-plane displacements of the commonly used (0, 138.5, 26.7) Euler angle. Measurements of the transducer input admittance in reflective delay-line devices yield a value for the electromechanical coupling coefficient that is in good agreement with the threedimensional simulations on bare langasite substrate. The input admittance measurements also show evidence of excitation of an additional wave mode and excess loss due to the finger resistance. The results of these simulations and measurements will be useful in the design of surface acoustic wave gas sensors.