T.B. Pollard

Recent advances in harsh environment acoustic wave sensors for contemporary applications

There is a significant need for wireless sensor systems capable of operation up to 1100°C and beyond, in abrasive or corrosive harsh environments, in particular for the energy, steel, aerospace, oil and gas exploration industries. These environments and applications preclude the use of batteries and normally require wireless and multiple sensor interrogation. The University of Maine and Environetix Technologies have successfully responded to these needs by researching and developing surface acoustic wave (SAW) sensors based on the langasite family of crystals and co-deposited Pt/Rh/ZrO2 thin-film electrode technology. This paper reports on the recent achievements, which include: long term operation in furnace and technology validation in jet-engine static and rotating parts up to 53,000 gs; stable and repetitive wired and wireless responses of temperature sensors; multiple wireless sensor interrogation; and associated packaging (tests run in the 200°C to 1000°C range).

Wireless acoustic wave sensors and systems for harsh environment applications

This paper reviews current progress in the area of wireless microwave acoustic sensor technology, and discusses advances in wireless interrogation systems that can operate in harsh environments. The use of wireless, battery-free, low maintenance surface acoustic wave (SAW) sensors has been successfully demonstrated in applications including high temperature turbine engines and inflatable aerospace structures. Wireless interrogation of multiple sensors up to 910°C has been established and sensor tests in gas turbine engine are reported. This paper elaborates on several aspects of the technology, including: high-temperature thin-film electrode and sensor development, temperature cycling, thermal-shock behavior, testing in turbine engine environments, sensor packaging and attachment, wireless operation, and adaptation to energy and industrial applications.

LGX pure shear horizontal SAW for liquid sensor applications

This paper reports predicted and measured properties of the pure shear horizontal (SH) mode for the LGX family of crystals, which includes langasite (LGS), langanite (LGN), and langatate (LGT). These crystals are of the trigonal class 32 group, as quartz, and they exhibit the SH symmetry type uncoupling for the Euler angles (0/spl deg/, /spl theta/, 90/spl deg/). This surface acoustic mode, also known as surface transverse wave (STW), is especially attractive for liquid sensing due to the moderate damping observed in liquid or viscous environments. Numerical and experimental propagation data presented for the SH mode on LGX (0/spl deg/, /spl theta/, 90/spl deg/) includes phase velocity (v/sub p/), electromechanical coupling coefficient (K/sup 2/), temperature coefficient of delay (TCD), fractional change in frequency with respect to temperature (/spl Delta/f/fo), penetration depth, metal strip reflectivity, and excitation of spurious plate modes as a function of /spl theta/. High electromechanical coupling and zero temperature coefficient of delay (TCD) along LGX Euler angles (0/spl deg/, /spl theta/, 90/spl deg/), /spl theta/ between 10/spl deg/ and 25/spl deg/, with penetration depths comparable to surface acoustic wave (SAW) devices are disclosed. In particular, along LGT (0/spl deg/, 13.5/spl deg/, 90/spl deg/), the experimental results reported with resonators and delay line structures verify the high electromechanical coupling (0.8%) for a SH SAW mode, about 10 times stronger than the 36/spl deg/ Y rotated quartz SH orientation, and the existence of zero TCD around 140/spl deg/C. The phase velocity of 2660 m/s is within 0.2% of the calculated value, which is about 55% below the phase velocity of 36/spl deg/ Y quartz, thus leading to smaller STW devices. The penetration depth of 6.5 wavelengths is eight times more shallow than 36/spl deg/ Y quartz, thus providing significant SH mode energy trapping close to the surface. With such positive predicted and measured coupling and propagation characteristics, these orientations are appropriate for the fabrication of high coupling, zero TCD, smaller, and highly sensitive STW devices for filtering, frequency control, and liquid sensor applications.

High coupling, zero TCD SH wave on LGX

This paper reports predicted and measured properties of the pure shear horizontal (SH) mode for the LGX family of crystals, which includes langasite (LGS), langanite (LGN), and langatate (LGT). Our results show high electromechanical coupling and zero temperature coefficient of delay (TCD) along LGX Euler angles (0/spl deg/, /spl theta/, 90/spl deg/), /spl theta/ between 10/spl deg/ and 25/spl deg/, with penetration depths which are comparable to SAW devices. In particular along LGT (0/spl deg/, 13.5/spl deg/, 90/spl deg/), experimental results are disclosed with resonators and delay line structures which verify the high electromechanical coupling (0.8%), about 10 times stronger than the 36/spl deg/ Y rotated quartz SH orientation, and zero TCD around 140 *C. The penetration depth of 7 wavelengths is about eight times shallower than 36/spl deg/ Y quartz. The phase velocity of 2660 m/s is within 0.2% of the calculated value, which is about 55% below the phase velocity of 36/spl deg/ Y quartz, thus leading to smaller Surface Transverse Wave (STW) devices. With such positive predicted and measured coupling and propagation characteristics, these orientations suggest the fabrication of high coupling, zero TCD, and smaller STW devices for liquid sensor, filtering, and frequency control applications.

Pure SH-SAW propagation, transduction and measurements on KNbO/sub 3/

Potassium niobate (KNbO/sub 3/) supports the electromechanically active pure shear horizontal surface acoustic wave (SH-SAW) mode along Z-axis cylinder orientations, Euler angles: (/spl phi/, 90/spl deg/, 0/spl deg/), in which two uncoupled wave solutions exist: a purely mechanical sagittal Rayleigh SAW and a piezoelectrically stiffened pure SH-SAW. Within this family of cuts, a maximum electromechanical coupling coefficient for the pure SH-SAW, K/sup 2/ = 53%, is observed along (0/spl deg/, 90/spl deg/, 0/spl deg/). This pure SH-SAW orientation also has the maximum value of electromechanical coupling observed along rotated Y-cut X propagation directions, Euler angles (0/spl deg/, /spl theta/, 0/spl deg/). The use of the pure SH-SAW mode is attractive for liquid-sensing applications because the SH-SAW is modestly attenuated by the adjacent liquid, unlike the generalized SAW (GSAW), which has particle displacement normal to the surface. This work investigates propagation and excitation properties of the SH-SAW and the shear horizontal bulk acoustic wave (SH-BAW) on single crystal KNbO/sub 3/, Euler angles (0/spl deg/, 90/spl deg/, 0/spl deg/). Interdigital transducer (IDT) arrays are analyzed using boundary element method (BEM) techniques, addressing IDT properties such as: power partitioning between the SH-SAW and SH-BAW, SH-BAW radiation as a function of wave vector direction and radiation angle, and overall IDT impedance. The percentage of SH-SAW power to total input power is above: 98% for IDTs containing 1.5 to 5.5 wavelengths of active electrodes with surrounding metalized regions. For nonmetalized regions outside the IDT, the ratio drops to between 1 and 2%, showing the importance of an energy trapping structure for efficient SH-SAW excitation and propagation along this orientation. Simulated and experimental IDT admittance results are compared, verifying the validity of the analysis performed. The reported measurements on the frequency variation with temperature indicate that the orientation considered is temperature compensated at about 8/spl deg/C. The surface of the SH-SAW devices fabricated have been loaded with deionized water arid showed additional 1.6 dB transmission loss with respect to the unloaded surface, verifying the suitability of the pure SH-SAW mode on KNbO/sub 3/ for liquid sensor applications.

Capacitively coupled IDT for high temperature SAW devices

Harsh environment surface acoustic wave (SAW) sensors are being researched and developed jointly by the University of Maine (UMaine) and Environetix Technologies Corp. for wireless and wired sensor applications, such as those found in gas turbines and power plant combustors. One goal of this work is to extend the operational temperature range of SAW sensors above 1000°C, potentially up to near the melting point of piezoelectric langasite crystals at 1400°C. To achieve stable performance at 1000°C and above, UMaine has developed nanocomposite thin film electrode materials, such as PtRh/HfO2, and protecting capping layers, such as SiAlON and Al2O3. However, these protective top layers, which aid in extending the life of the electrodes, are electrical insulators that prevent direct bonding to the electrodes. The UMaine team also found evidence of accelerated thin-film degradation close to the SAW interdigital transducer (IDT) bond pad welds at these extreme temperatures. This paper introduces a high temperature capacitive approach to electrically couple to the IDT, thus allowing electrical access to the SAW device. The capacitive coupling approach also avoids premature failure of the nanocomposite film caused by interdiffusion between the bond wires and the SAW IDT bond pads. The technique has been successfully implemented and SAW device operation at 1000°C has been achieved.

Improved pure SH SAW transduction efficiency on LGS using finite thickness gratings

Pure shear horizontal surface acoustic wave (SH SAW) orientations exist on trigonal class 32 crystals along (0°, θ, 90°). In particular, this mode has been recently considered on single crystal langasite (LGS) for liquid and biological sensor applications due to reduced propagation loss in the presence of liquid-loaded surface. A limitation of the pure SH SAW orientations for device applications has been the significant excitation of the shear horizontal bulk acoustic wave (SH BAW) by the interdigital transducer (IDT), which compromises the SH SAW transduction efficiency and thus increases SH SAW device loss. Previous work by the authors verified that an infinitesimally thin metallic guiding layer used in the propagation path around the IDT typically increases the SH SAW transduction efficiency ηSHSAW , defined as the ratio of SH SAW power to total IDT input power, from less than 1% (no layer) to 60% (infinitesimally thin metallic layer) on LGS, Euler angles (0°, 22°, 90°) for a 240 split finger electrode IDT. In this work, finite thickness periodic grating guiding structures are explored along LGS (0°, 22°, 90°) to further improve the IDT ηSHSAW. The structure studied consists of a finite number of IDT electrodes bordered by gratings on both sides. The model implemented uses orthogonal Chebyshev polynomial basis functions in conjunction with the finite element method and harmonic admittance (HA) technique to study the SH SAW mode excitation efficiency in the referred structure. The IDT ηSHSAW is examined as a function of: electrode material and normalized thickness (h/λ) where h is electrode thickness and λ is the wavelength; metallization ratio (a/p) where a and p are electrode width and center-to-center spacing, respectively; and number of split IDT finger electrodes. The analysis performed shows a significant difference in ηSHSAW if high density electrode gratings, such as gold (Au) or platinum (Pt) are used instead of low-density gratings, such as aluminum (Al). For instance, using 282 split finger electrode IDT and surrounding gratings composed of Au or Pt electrodes, with h/λ = 1%, results in ηSHSAW = 99%. If Al is used, a reduced ηSHSAW = 54% is achieved for the same h/λ = 1%. Moreover, to achieve ηSHSAW = 99% with Al electrodes, the required value of h/λ is about 8.5%, a nearly ten-fold increase in metallization thickness over Au or Pt electrodes. The effects of metallization ratio a/p on ηSHSAW have also been studied and indicate that higher metallization ratios lead to higher ηSHSAW. Numerical and experimental IDT admittance is compared, validating the analysis performed. The work reported shows that finite thickness gratings can significantly improve the SH SAW IDT performance, reducing the amount of SHBAW excited by the IDT, and thus leading to high efficiency LGS SH SAW devices for biosensor applications.

FEM/BEM impedance and power analysis for measured LGS SH-SAW devices

Pure shear horizontal surface acoustic waves (SH-SAW) exist on rotated Y-cuts, Euler angles (0/spl deg/, 0, 90/spl deg/), of trigonal class 32 group crystals, which include quartz and the LGX family of crystals (langasite, langatate, and langanite). This pure SH-SAW mode has a number of interesting propagation characteristics. Along selected LGX orientations, interdigital transducers (IDTs) used to generate the SH-SAW also excite a significant amount of shear horizontal bulk acoustic wave (SH-BAW) that propagates almost parallel to the surface, with a slightly different phase velocity. We use combined finite and boundary element methods (FEM/BEM) to calculate IDT admittance, while considering both the SH-SAW and SH-BAW contribution. The ratio of transduced SH-SAW power to total input power is analyzed as a function of device geometry and IDT metallization material and thickness. Typically, for a 20/spl lambda/ IDT, the FEM/BEM simulations indicate that about 40% of input power is converted to the SH-SAW when the area outside of the IDT is coated with an infinitesimally thin conducting film. Both our simulations and experimental results consistently indicate that the ratio of SH-SAW to total input power strongly depends on metal type and thickness. The SH-BAW angular plots obtained with the FEM/BEM analysis confirm that the mode excited by the IDTs propagates nearly parallel to the surface. Measured device impedances agree extremely well (within 5%) with the FEM/BEM IDT impedance calculations accounting for both SH-SAW and SH-BAW generation.

FEM/BEM impedance and power analysis measured LGS SH-SAW devices

Pure shear horizontal piezoelectrically active surface and bulk acoustic waves (SH-SAW and SH-BAW) exist along rotated Y-cuts, Euler angles (0/spl deg/, /spl theta/, 90/spl deg/), of trigonal class 32 group crystals, which include the LGX family of crystals (langasite, langatate, and langanite). In this paper both SH-SAW and SH-BAW generated by finite-length, interdigital transducers (IDTs) on langasite, Euler angles (0/spl deg/, 22/spl deg/, 90/spl deg/), are simulated using combined finite- and boundary-element methods (FEM/BEM). Aluminum and gold IDT electrodes ranging in thickness from 600 /spl Aring/ to 2000 /spl Aring/ have been simulated, fabricated, and tested, with both free and metalized surfaces outside the IDT regions considered. Around the devices operating frequency, the percent difference between the calculated IDT impedance magnitude using the FEM/BEM model and the measurements is better than 5% for the different metal layers arid thicknesses considered. The proportioning of SH-SAW and SH-BAW power is analyzed as a function of the number of IDT electrodes; type of electrode metal; and relative thickness of the electrode film, h//spl lambda/, where /spl lambda/ is the SH-SAW wavelength. Simulation results show that moderate mechanical loading by gold electrodes increases the proportion of input power converted to SH-SAW. For example, with a split-electrode IDT, comprising 238 electrodes with a relative thickness h//spl lambda/ = 0.63% and surrounded by an infinitesimally thin conducting film, nearly 9% more input power is radiated as SH-SAW when gold instead of aluminum electrodes are used.

GaPO/sub 4/ SAW devices: measured and predicted propagation properties

New piezoelectric crystals capable of providing higher electromechanical coupling than quartz, while maintaining temperature compensated characteristics comparable to quartz, are in demand for communications, sensors, and signal processing applications. In response to this demand, several new crystals have been recently developed and characterized for acoustic wave applications. Gallium Orthophosphate (GaPO/sub 4/) is among these crystals, which include lithium tetraborate, langasite, langanite, langatate, potassium niobate, SNGS (Sr/sub 3/NbGa/sub 3/Si/sub 2/O/sub 14/) and STGS (Sr/sub 3/TaGa/sub 3/Si/sub 2/O/sub 14/). In this work theoretical and measured SAW propagation characteristics for phase velocity (v/sub p/), electromechanical coupling (K/sup 2/), diffraction (/spl gamma/), temperature coefficient of frequency (TCP) and fractional frequency variations (/spl Delta/f/fo) along selected GaPO/sub 4/ orientations are reported. Measured propagation properties are compared to calculated values along the propagation directions Euler angles (0/spl deg/, 76/spl deg/, 0/spl deg/), (0/spl deg/, 90/spl deg/, 0/spl deg/), (0/spl deg/, 110/spl deg/, 0/spl deg/), and (0/spl deg/, 130/spl deg/, 0/spl deg/). The measured phase velocities for these orientations agree with the calculated values to within 0.4% for the propagation directions investigated. The measured TCF values agree with the predicted values showing temperature compensated behavior between 70/spl deg/C and 200/spl deg/C for the selected cuts. Fractional frequency variations of about 15 ppm over a 50/spl deg/C span around the turnover temperatures are predicted and measured, comparable to equivalent Y rotated quartz orientations. Values of K/sup 2/ ranging from 0.3% to 0.5% are calculated and extracted from the experimental data for the selected orientations mentioned, confirming up to four times higher K/sup 2/ of GaPO/sub 4/ with respect to Y rotated quartz orientations.