Sergei Zhgoon

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

AlN/ZnO/diamond structure combining isolated and surface acoustic waves

In order to generate surface acoustic waves (SAW) and waveguiding layer acoustic waves (WLAW) simultaneously, a multilayer structure of AlN/ZnO/diamond has been proposed. This structure has been investigated theoretically (two-dimensional finite element method) and experimentally. The nature of the excited modes and their order were identified by modeling and confirmed experimentally by measuring the frequency response of the device in the air and in contact with the liquid. The demonstrated structure can be used to realize a packageless sensor or resonator, using the WLAW alone. A temperature compensated gas or liquid sensor can also be realized by combined usage of the SAW and the WLAW.

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.

Packageless AlN/ZnO/Si Structure for SAW Devices Applications

The possibility to perform a packageless structure for acoustic wave sensors applications based on AlN/interdigital transducer/ZnO/Si structure was investigated. The effect of thicknesses of AlN and ZnO thin films on structure performance was simulated by 2-D finite element method. Theoretical predictions were confirmed by in-situ measurements of frequency, insertion loss, and thickness during deposition of AlN layer on ZnO/Si.

AlN/ZnO/diamond waveguiding layer acoustic wave structure: Theoretical and experimental results

We present a theoretical calculation and experimental results for a waveguiding layer acoustic wave (WLAW). The experimental device is modeled by the finite element method (FEM) for the AlN/ZnO/diamond structure. It was found that the AlN thickness must be at least larger than 3λ/2 to obtain negligible surface displacement. In the same way, the ZnO thickness for a fixed value of AlN thickness at 2λ must be larger than λ/4 to confine the acoustic wave. The electromechanical coupling of the wave presents an optimum around λ/2 for the ZnO layer thickness. A first experimental AlN/ZnO/diamond device has been developed and shows the WLAW at 412 MHz.

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.

Characterization of langasite for application in high temperature SAW sensors

Experimental SAW resonator structures with iridium electrodes were fabricated on Y-cut of langasite with few different propagation directions and their characteristics were measured in a wide temperature interval, up to 700°C, and compared with simulations. Between two sets of material constants reported by Bungo one set was selected as providing much better agreement with experiments, for the analyzed orientations. Further improvement of simulation accuracy was achieved via after-correction of the calculated temperature characteristics using auxiliary function extracted from experimental data. This approach was used to predict resonator characteristics in orientation with Euler angles (0°, 22°, 90°), in which Bluestein-Gulyaev wave propagates, and provided excellent agreement between experiments and simulations, in a wide temperature interval.

Density of states distribution in AlN films measured by CPM and DLTS

Abstract Aluminium nitride films are prepared by radio frequency magnetron sputtering of pure aluminium target in argon–nitrogen–hydrogen mixture at the different value of the nitrogen–hydrogen flow rates. The argon flow rate and partial pressure are fixed. Nominal substrate temperature is equal to 275°C, glass and silicon wafers are used as substrates. Constant photocurrent method (CPM) and deep level transient spectroscopy (DLTS) are used for the density of electron states (DOS) distribution parameters investigations in the energy ranges which include nearly all AlN band gaps. The CPM is applied for experimental investigations of optical absorption coefficient and the DOS spectra in the 1.4–5.0 eV range. The DOS peaks are found at the energies of 2.2–2.7 and below 1.8 eV from the conduction band bottom. The second peak is attributed to the N Al antisite defects, whereas data on the first one has not been previously reported. Capacitance DLTS technique is used for the N(E) investigation in the energy range up to 0.85 eV below the conduction band bottom. Relatively narrow (15–150 meV halfwidth) peaks are found in the energy range. These peaks appearance is attributed to the nitrogen vacancy triplet. The conduction band tail slope characteristic energy is estimated as 25±10 meV.

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.