Conference Paper

Homogeneity of Langasite and Langatate wafers for acoustic wave applications

Vienna Univ. of Technol., Austria;
DOI: 10.1109/ULTSYM.2003.1293365 Conference: Ultrasonics, 2003 IEEE Symposium on, Volume: 1
Source: IEEE Xplore

ABSTRACT Variations in the concentration of the chemical constituents of Langasite (La3Ga5SiO14) and its homeotype Langatate (La3Ta0.5Ga5.5O14) have been found by the means of X-ray methods and selective crystal etching. To determine their influence on the acoustic properties SAW resonators have been designed and processed on 3" wafers from Langasite and Langatate crystal boules of different suppliers. Compositional changes on a short range scale according to growth striations and on a long range scale as well could be distinguished leading to variations of the SAW velocity up to 1000 ppm within a wafer. If a device covers several growth striations a superposition of different propagation characteristics occurs leading to a multimode behavior and thereby degrading the effective Q-value of the SAW resonators. Wafers from recently grown boules, however, reveal frequency shifts with a standard deviation of 50 ppm only and maximum Q-values of up to 15000, thus demonstrating the progress in crystal growing.

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    ABSTRACT: Among the langasite family of crystals (LGX), the three most popular materials are langasite (LGS, La3Ga5SiO14), langatate (LGT, La3Ga5.5Ta0.5O14) and langanite (LGN, La3Ga5.5Nb0.5O14). The LGX crystals have received significant attention for acoustic wave (AW) device applications due to several properties, which include: (1) piezoelectric constants about two and a half times those of quartz, thus allowing the design of larger bandwidth filters; (2) existence of temperature compensated orientations; (3) high density, with potential for reduced vibration and acceleration sensitivity; and (4) possibility of operation at high temperatures, since the LGX crystals do not present phase changes up to their melting point above 1400degC. The LGX crystals' capability to operate at elevated temperatures calls for an investigation on the growth quality and the consistency of these materials' properties at high temperature. One of the fundamental crystal properties is the thermal expansion coefficients in the entire temperature range where the material is operational. This work focuses on the measurement of the LGT thermal expansion coefficients from room temperature (25degC) to 1200degC. Two methods of extracting the thermal expansion coefficients have been used and compared: a) dual push-rod dilatometry, which provides the bulk expansion; and b) x-ray powder diffraction, which provides the lattice expansion. Both methods were performed over the entire temperature range and considered multiple samples taken from <001> Czochralski grown LGT material. The thermal coefficients of expansion were extracted by approximating each expansion data set to a third order polynomial fit over three temperature ranges reported in this work: 25degC to 400degC, 400degC to 900degC, 900degC to 1200degC. An accuracy of fit better than 35ppm for the bulk expansion and better than 10ppm for the lattice expansion have been obtained with the aforementioned polynomial fitting. The percentage d- ifference between the bulk and the lattice fitted expansion responses over the entire temperature range of 25degC to 1200degC is less than 2% for the three crystalline axes, which indicates the high quality and growth consistency of the LGT crystal measured
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    ABSTRACT: This paper reports on the assessment of langatate (LGT) acoustic material constants and temperature coefficients by surface acoustic wave (SAW) delay line measurements up to 130 degrees C. Based upon a full set of material constants recently reported by the authors, 7 orientations in the LGT plane with Euler angles (90 degrees, 23 degrees, Psi) were identified for testing. Each of the 7 selected orientations exhibited calculated coupling coefficients (K(2)) between 0.2% and 0.75% and also showed a large range of predicted temperature coefficient of delay (TCD) values around room temperature. Additionally, methods for estimating the uncertainty in predicted SAW propagation properties were developed and applied to SAW phase velocity and temperature coefficient of delay calculations. Starting from a purchased LGT boule, the SAW wafers used in this work were aligned, cut, ground, and polished at University of Maine facilities, followed by device fabrication and testing. Using repeated measurements of 2 devices on separate wafers for each of the 7 orientations, the room temperature SAW phase velocities were extracted with a precision of 0.1% and found to be in agreement with the predicted values. The normalized frequency change and the temperature coefficient of delay for all 7 orientations agreed with predictions within the uncertainty of the measurement and the predictions over the entire 120 degrees C temperature range measured. Two orientations, with Euler angles (90 degrees, 23 degrees, 123 degrees) and (90 degrees, 23 degrees, 119 degrees), were found to have high predicted coupling for LGT (K(2) > 0.5%) and were shown experimentally to exhibit temperature compensation in the vicinity of room temperature, with turnover temperatures at 50 and 60 degrees C, respectively.
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    ABSTRACT: In the past few years there has been an increasing interest in the langasite family of crystals (LGX) for surface acoustic wave (SAW) applications in communications, frequency control, and sensors. LGX has several interesting properties including: up to about six times higher electromechanical coupling than quartz ST-X; existence of temperature compensated cuts with zero power flow angle and minimal diffraction; up to 26% reduction in phase velocities with respect to quartz ST-X, which allows the fabrication of smaller devices; and the absence of a crystal phase transition up to the crystal melting point (around 1177K). Due to this absence of crystal phase transition, bulk acoustic wave (BAW) and SAW devices have been explored as temperature sensors at temperatures up to several hundred °C. In addition to temperature and pressure sensors, a need exists for high temperature sensors capable of detecting target gases. Hydrogen (H<sub>2</sub>) detection, in particular, is of paramount importance in applications such as hydrogen fuel cells, fuel leakage from jet engines, and power plants. This paper reports on a dual LGS SAW device configuration for the detection of H<sub>2</sub> gas at 250°C using original all palladium (Pd) electrodes. The Pd electrodes are used for the SAW transduction and reflection functions and to detect H<sub>2</sub>. The frequency differences between two identical all Pd SAW resonators have been tracked. The dual configuration scheme has been used to minimize temperature cross interference, since the LGS (0°, 138.5°, 26.6°) orientation selected in this work is not temperature compensated at 250°C. The detection of H<sub>2</sub> gas produces a 6-8 KHz differential frequency shift with respect to the reference for H<sub>2</sub> gas concentrations of 100 ppm, 250 ppm, 500 ppm, and 1000 ppm. The SAW resonators respond to the presence of H<sub>2</sub> in a matter of seconds and become stable between 25 to 75 minutes later. The devices have been continuously operated at 250°C for a period of sixteen weeks, with less than three dB of degradation in the |S21| response. The dual configuration high temperature LGS SAW devices and experiments reported in this work prove the capability of these crystal- s to withstand prolonged exposure to high temperatures (250°C) and to perform as appropriate high temperature H<sub>2</sub> gas sensors.
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