M. Pereira da Cunha

Pure shear horizontal SAW biosensor on langasite

The undetected introduction of pathogens into food or water supplies can produce grave consequences in terms of economic loss and human suffering. Sensitive and selective sensors capable of quickly detecting microbial pathogens are urgently needed to limit the effects of bioterrorist incidents, accidents, or pollution. Shear horizontal surface acoustic wave (SH SAW) devices provide an attractive platform for the design of microbial biosensors that function in liquid media, where Rayleigh-type modes are rapidly attenuated. This paper reports on an exploratory SH SAW delay line designed and fabricated on langasite, La/sub 3/Ga/sub 5/SiO/sub 14/ (LGS), along the novel Euler propagation direction (0/spl deg/, 22/spl deg/, 90/spl deg/). A liquid chamber was fabricated and attached to the top surface, and the device was submitted to liquid and biochemical tests. Moderate (6 dB) additional attenuation of the transmission coefficient, |S/sub 21/|, was consistently observed when the SH SAW delay line was assembled in the test fixture and submitted to the liquid tests, indicating that LGS is an attractive candidate for liquid sensing. Sensor selectivity can be achieved by integrating the LGS SH SAW delay line with a biochemical recognition layer. A test setup was implemented for the characterization of LGS SH SAW-based biosensors. The delay line response to biomolecule binding was shown by detection of sequential binding of proteins to the SH SAW device delay path. The biotinylated sensor was exposed sequentially to biotin-binding deglycosylated avidin, biotin-modified rabbit IgG, and goat anti-rabbit IgG antibody. As each protein was bound to the sensing surface, marked changes in the delay-line phase were recorded. The reported results demonstrate the capability of these devices to act as biochemical detectors in aqueous solutions, and this work represents the first effort using the novel material LGS in SAW-based biosensor technology.

LGX pure shear horizontal SAW for liquid sensor applications

Reports theoretical and experimental properties of the 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 shear horizontal (SH) symmetry type uncoupling for the Euler angles [0/spl deg/ /spl theta/ 90/spl deg/]. This surface acoustic mode, also know as surface transverse wave (STW) is especially attractive for liquid sensing, because the pure horizontal particle polarization characteristic of this wave, with the material particle motion parallel to the surface, is not as severely damped as observed for the general polarized SAW or pure sagittal Rayleigh waves, which have a significant polarization component normal to the surface. The results focus on one of the materials from the LGX family, LGT. The experimental data uncovers a zero temperature coefficient of delay (TCD) propagation direction around 140/spl deg/C on the LGT crystal. Liquid loading experiments with both water and photoresist at the surface of SH wave resonators and delay lines along that orientation have been carried out, showing the expected moderate attenuation of the SH mode.

Surface and pseudo surface acoustic waves in langatate: predictions and measurements

Langatate (LGT, La/sub 3/Ga/sub 5.5/Ta/sub 0.5/O/sub 14/) is a recent addition to materials of the trigonal crystal class 32. In this paper SAW contour plots of the phase velocity (v/sub p/), the electromechanical coupling coefficient (K/sup 2/), the temperature coefficient of delay (TCD), and the power flow angle (PFA), are given showing the orientations in space in which high coupling is obtained, with the corresponding TCD, PFA, and vp characteristics for these orientations. This work reports experimental results on the SAW temperature fractional frequency variation (/spl Delta/f/fo) and the TCD for several LGT orientations on the plane with Euler angles: (0/spl deg/, 132/spl deg/, /spl psi/). The temperature behavior has been measured directly on SAW wafers from 10 to 200/spl deg/C, and the results are compared with numerical predictions using our recently measured temperature coefficients for LGT material constants. This research also has uncovered temperature compensated orientations, which we have experimentally verified with parabolic behavior, turnover temperatures in the 130 to 160/spl deg/C range, and /spl Delta/f/fo within 1000 ppm variation from 10 to 260/spl deg/C, appropriate for higher temperature device applications. Regarding the pseudo surface acoustic waves (PSAWs), results of calculations are presented for both the PSAW and the high velocity PSAW (HVPSAW) for some selected, rotated cuts. This study shows that propagation losses for the PSAWs of about 0.01 dB/wavelength, and phase velocities approximately 20% higher than that of the SAW, exist along specific orientations for the PSAW, thus showing the potential for somewhat higher frequency SAW device applications on this material, if required.

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.

Precise orientation of single crystals by a simple x-ray diffraction rocking curve method

A simple method has been developed for accurately measuring the crystallographic orientation of a single crystal boule, employing a conventional four-circle x-ray diffraction arrangement in the rocking curve mode which relaxes the need for precise instrument and/or reference alignment. By acquiring a total of eight rocking curve measurements at specific orientations about the specimen azimuth, the absolute miscut angle between a crystal surface and the desired crystallographic plane can be resolved to within ±0.01°.

Stable electrodes and ultrathin passivation coatings for high temperature sensors in harsh environments

Sensor operation in harsh environments up to 1000degC requires robust packages including stable electrodes and protective coatings. We have developed nanostructured ultra-thin (< 100 nm) Pt-10%Rh / ZrO2 electrode structures grown by e-beam co-evaporation that operate at temperatures approaching 1000degC. X-ray diffraction (XRD), resistivity, and electron microscopy (EM) studies indicate incorporation of ZrO2 within the film delays recrystallization, maintaining a stable morphology. We have also developed ultra-thin (< 50 nm) SiAlON passivation coatings that mechanically protect the sensor surfaces, yet allow interaction with the environment. Different SiAlON stoichiometeries were produced by rf magnetron sputtering of Al and Si targets in O2/N2/Ar mixtures. The SiAlON films are amorphous and extremely smooth (< 1 nm rms) and remain so even after extended annealing at 1000degC. Our results are applicable to a wide range of high temperature sensor configurations.

CDMA and FSCW surface acoustic wave temperature sensors for wireless operation at high temperatures

This paper reports on the successful operation of surface acoustic wave (SAW) devices at high temperature, including wireless measurements of SAW devices up to 750°C. Two types of SAW devices and corresponding systems were investigated: (1) 15-bit code-division multiple-access (CDMA) SAW tags, interrogated using a device-specific direct sequence spread spectrum (DSSS) binary phase shift key (BPSK) code; and (2) multi-track reflective delay lines, interrogated using a frequency-stepped continuous-wave (FSCW) radar signal. The SAW devices used in this work were fabricated on langasite (LGS, La3Ga5SiO14) using Pt/Rh/ZrO2 electrodes, which can withstand temperatures up to 1000°C. A FEM/BEM simulation package using both the Pt/Rh/ZrO2 thin film and LGS properties was used to simulate the interdigital transducers (IDTs) used in the fabricated structures. The two classes of SAW sensors were first directly wired and characterized at high temperature. The FSCW system was then used to test the wireless operation of LGS SAW devices up to 750°C.

Electrode optimization for a lateral field excited acoustic wave sensor

Lateral field excited (LFE) acoustic wave sensors have a distinct advantage over the standard, AT-cut, quartz crystal microbalance (QCM) in which the thickness shear mode (TSM) is excited by electrodes on the sensing and reference surfaces. The TSM in LFE sensors is excited by placing electrodes only on the reference surface of the crystal, leaving the sensing surface free. Due to the lack of a shielding electrode on the sensing surface, the TSM electric field is able to penetrate into an analyte selective film or an adjacent liquid thus enabling the detection of mechanical and electrical property changes in the film or liquid. Several electrode designs on the reference surface are examined and evaluated by measuring each sensors admittance and resonant frequency when exposed to a variety of analytes and liquids. These results are compared to the results obtained using a standard QCM.

Thin films and techniques for SAW sensor operation above 1000°C

High temperature (300°C to 1400°C) wireless sensors have applications in energy exploration and generation, harsh environment industrial processing, and aerospace engineering. Existing technology developed at the University of Maine allows the fabrication of surface acoustic wave (SAW) langasite (LGS) sensors with Pt-Rh/ZrO2 electrodes that can deliver long-term stable operation up to 850°C. Since LGS remains piezoelectric up to its melting point of ~1400°C, it is desirable to extend the current SAW sensor temperature range of operation. In addition, it is desirable to diminish the SAW interdigital transducer (IDT) electrode dimensions to increase the wireless frequency of operation towards the GHz range. In this work, new thin film electrode materials have been investigated to allow the operation of SAW one-port resonators up to 1000°C and beyond. In particular, alternative Pt/Al2O3 and Pt-Rh/HfO2 thin film electrode compositions are presented, which yield operation of SAW resonator sensors up to 1100°C. In addition to a previously used capping layer, an interfacial layer has been added between the LGS and the electrodes to delay any interdiffusion between the materials and extend the temperature and/or time of sensor performance. Finally, it is also reported in this work that exposure of untreated SAW device electrodes with 120 nm thick and 2μm wide Pt-Rh/ZrO2 co-deposited IDT fingers to temperatures above 850°C can create long platinum-rich nano-whiskers. These structures short-circuit the SAW interdigital (IDT) fingers, rendering the device unusable. The short-circuit problem was solved by the use of multilayered electrode structures and the used of the capping layer.

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.