A. Shvetsov

Theoretical and experimental investigation of langasite as material for wireless high temperature SAW sensors

Different cuts of langasite were investigated theoretically and experimentally as candidates for high temperature sensors utilizing SAW resonators and reflecting SAW tags. Single port SAW resonators with central frequencies close to 170, 200 and 433 MHz with W, Ir and Pt electrodes of different thickness were fabricated and their characteristics were measured as functions of temperature. An approach to SAW device encapsulation, which helps to avoid stress development during heating to high temperatures, was suggested and tested. The damage of the langasite surface during heating to high temperature is strong in the cut family with Euler angles (0°, 138.5°, ψ), while orientations with Euler angles (0°, 90°, ψ) and (0°, 22°, ψ) were found to be more resistant to heating. In the latter cut family, two cuts, (0°, 22°, 31.5°) and (0°, 22°, 90°) can provide zero power flow angle and sufficient piezoelectric coupling. COM parameters of SAW resonators with W, Ir and Pt electrodes, including complex reflection coefficient, were calculated as functions of electrode thickness and measured in the wide temperature range. The frequency responses were described by COM equations and the fitted COM-parameters were compared to their simulated values. Resonators show high Q and their performance is perfectly described by COM equations

Packageless SAW Devices with Isolated Layer Acoustic Waves (ILAW) and Waveguiding Layer Acoustic Waves (WLAW)

We report on two novel layered structure concepts and their acoustic wave properties for efficient, mechanically isolated, temperature compensated, and technologically attractive packageless SAW device applications as RF filters and duplexers. The piezoelectric substrate is covered with a layer of SiO2 or Pyrex that is in turn covered by a material of higher acoustical impedance creating an isolated layer acoustic wave (ILAW). Otherwise the wave inside a relatively low acoustic velocity waveguiding layer is confined by the higher velocity topmost layer made with high SAW velocity material thus ensuring the creation of waveguiding layer acoustic waves (WLAW). Modeling of acoustical properties of these waves (ILAW and WLAW) is confirmed by experimental results while useful figures of coupling, reflection and temperature stability are obtained. Extensive experiments with in-situ monitored depositions of multiple layers were performed and very good acoustical isolation of the waves was achieved.

One-port SAW resonators using natural SPUDT substrates

We discuss the design of one-port surface acoustic wave (SAW) resonators using substrates with a partial or total degree of directivity, that is, the natu- ral single-phase unidirectional transducer (N-SPUDT) ef- fect. A general design method gives a resonance when all three frequencies (the required resonance frequency and the Bragg frequencies) are different. A second method has been derived from the resonance condition for a symmetrical sub- strate. Two further methods incorporate 4 gaps. The ca- pacitance ratio is presented as a function of the phase of the electrode reflection coefficient. The simulations use data for the N-SPUDT orientation of langasite. The reflection coefficient for Al electrodes has been calculated from finite element modeling (FEM) analy- sis. The approximate perturbation theory is found to agree well for small film thickness ( 2%). The phase of the reflection coefficient is typically 150 , not quite the ideal value of 180 . Measurements on resonators using Al and Cu films are presented.

3D finite element modeling of real size SAW devices and experimental validation

We report on several successful examples of using finite elements based software package for modeling SAW structures with dimensions above 100 wavelengths in the propagation direction including all the electrodes and spaces. The approach easily incorporates additional layers and elements with arbitrary 3D shapes that are common in SAW devices. Temperature behavior of the frequency response, SAW beam shape, bulk wave related data and other relevant information is stored in the solution and this helps determining the sources of response peculiarities. The modeling results for infinite periodic structures are validated against the proven FEMSDA software. Personal workstation is used for modeling. This work demonstrates the capabilities of finite elements based software running on contemporary personal computers to solve practical problems of SAW device design.

Choice of quartz cut for sensitive wireless SAW temperature sensor

We look for materials with good SAW resonator properties and with the largest difference between temperature responses of resonators in a pair on a single substrate. We have identified several cuts of quartz having useful properties with TCF difference up to 140 ppm/°C for a pair of resonators on a single substrate. As a rule, placing of such resonators on a single substrate requires rotation by up to 90° relative to each other thus increasing the die size. The limited range of cuts presents a unique opportunity to place both resonators along the X+90° direction with one resonator using BGS waves (with electrodes placed along X axis) and the other one (with electrodes inclined by about ±10° to X axis) using Rayleigh waves. These cuts are close to the 70°Y-cut where high TCF difference is reached together with acceptable characteristics of resonators. Resonators were designed for all useful cuts (including the 70°Y-cut) and tested. The use of different periods in reflectors and IDT together with individual choice of gaps between reflectors and IDT allowed obtaining low spurious content in resonator responses. The quality factors reached values up to 4000 at central frequencies around 915 MHz for both BGS and Rayleigh types of waves. The measured difference of TCF is about 138 ppm/°C on 70°Y-cut that is close to the calculated value.

Deformation sensitive cuts of quartz for torque sensor

Single crystal quartz is a common substrate used in sensors for measurement of pressure, stress and torque. We report on the calculation of the torque sensitivity of SAW resonators arranged on all possible orientations of quartz substrate. The calculation is based on third order material constants and it has predicted useful orientations combining good sensitivity (3–3.5 times better than that of the ST-quartz) with practical wave propagation properties. The wafers with selected cuts of quartz have been fabricated; four resonator layouts were designed in order to work with arbitrary phase of reflection coefficient from electrodes. Resonators were tested and measurements prove the validity of the calculations. An important feature of torque sensors is the temperature dependence of the torque sensitivity that cannot be predicted in calculations with third order constants and that may be prohibitively large. This feature has been measured and relatively weak temperature dependence has been found for some cuts, notably for Y+27° cut with the propagation direction of 50° to the X axis.

SAW temperature sensor on Quartz

For biomedical applications, narrow temperature range and high sensor accuracy requirements define the need for high temperature sensitivity. Wireless SAW sensors connected to antennas need a reference element to account for changes in electromagnetic coupling between the transmitter and receiver antennas. A pair of sensors with different temperature sensitivities may serve as a self-referenced sensor assembly. This justifies the need for materials with useful SAW resonator properties and with the largest difference between temperature coefficients of frequency (TCF) for a resonator pair on a single substrate. We have identified several cuts of quartz having useful properties with a TCF difference up to 140 ppm/°C for a pair of resonators on a single substrate. As a rule, placing such resonators on a single substrate requires their rotation by up to 90° relative to each other. The limited range of cuts presents a unique opportunity to place both resonators along the X+90° direction with one resonator using Bleustein-Gulyaev-Shimizu (BGS) waves (with electrodes placed along the x-axis) and the other one (with electrodes inclined by about ±10° to the x-axis) using quasi-Rayleigh waves. These cuts are close to the 70°Y cut where a high TCF difference is reached together with acceptable characteristics of the resonators. Resonators were designed for all useful cuts (including the 70°Y cut) and tested. The use of different periods in reflectors and interdigital transducer (IDT) together with individual choice of gaps between reflectors and IDT meant achieving low spurious content in resonator responses. The quality factors reached values up to 3500 at central frequencies around 915 MHz for both BGS and quasi-Rayleigh types of waves. The measured difference of the TCF is about 138 ppm//spl degC on 70°Y cut that is close to the calculated value.

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.

P2N-3 Single Port SAW Resonators Design for Arbitrary Reflection Phase

This work deals with some aspects of resonator performance improvement on unidirectional substrates with arbitrary reflection angle. A design concept is proposed based on the use of resonators with slight (by about 1-2%, depending on the material properties) frequency increase (or decrease, for opposite phase) of the IDT pitch in the transducer center for bi-directional substrates. For naturally unidirectional orientations properly placed linear chirp transducers with pitch variation of the same order of magnitude are found to give practically useful results, while for arbitrary phase angles in between of these two extreme cases the position of the transducer pitch maximum (minimum for opposite phase) is shifted from the center to the corresponding side of the transducer in accordance with the phase. Relevant the results are compared with experimental data on langasite substrates

P3I-2 Cavityless Wafer Level Packaging of SAW Devices

Original implementations of the solution to cavityless WLP by means of isolation of waves are discussed. Variants of waveguiding layer acoustic wave (WLAW) (similar to boundary and interface waves) and isolated layer acoustic wave (ILAW) together with Bragg mirror-like additional acoustical isolation are compared in modeling and experimentally. The structures include metal electrode patterns and subsequent layers. The first among these layers is a dielectric layer, usually SiO2 (or Pyrex) that possesses temperature compensating properties, while the outer layers are formed with either metals or dielectrics. In order for the wave to be confined into the SiO2 layer the stack of the outer layers may be formed in different ways. For implementation of the WLAW concept, the main feature of subsequent layers is the increased acoustical velocity in comparison to the SiO2 layer. Thus the wave attenuates exponentially in the structure on both sides of the SiO2 waveguiding core. The ILAW concept is based on the application of high acoustical impedance materials providing abrupt change in boundary conditions between the layers. Further improvement of acoustical isolation in this approach is effectuated by means of alternating several layers with low and high acoustical impedance.