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
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.
Available from: lanl.gov
- "Further, the literature on LGT acoustic wave material properties contains many discrepancies, some of them large, in particular for the values of the elastic and piezoelectric constants and their temperature coefficients as discussed in detail in . It is not presently clear to what extent these discrepancies in the reported material constants and temperature coefficients arose from different or imprecise measurement techniques, variations in the crystal composition and structure in materials provided by different growers, or yet some other reason to be determined   . In an attempt to re-examine the LGT material constants and temperate coefficient, a carefully determined set of elastic and piezoelectric constants and temperature coefficients has recently been extracted by pulse echo overlap and combined resonance techniques . "
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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.
Available from: Edward Andrew Payzant
- ". The LGX crystals have received significant attention for acoustic wave (AW) device applications due to several attractive properties, which include: (1) piezoelectric coefficients between two to three times higher than quartz, which result in stronger bulk acoustic wave (BAW) and surface acoustic wave (SAW) electromechanical coupling   , and allow the design of larger bandwidth filters; (2) existence of temperature compensated orientations     ; (3) high density, with potential for reduced vibration and acceleration sensitivity; (4) single crystals reported in the literature indicated consistent growth up to several inches and high quality factor for fabricated resonant devices  ; and (5) operation at high temperatures, since the LGX crystals do not present phase changes up to their melting point above 1400°C   . "
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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
Available from: Clemens C.W. Ruppel
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ABSTRACT: The properties of Rayleigh waves operating both at radio frequency (RF) and at high temperatures (HT) on langasite crystal cuts with Euler angles of (0°, 138.5°, 26.6°) and (0°, 30.1°, 26.6°) are investigated. By evaluating the frequency response of surface acoustic wave (SAW) test devices the SAW velocity, the coupling factor, and the propagation attenuation have been determined experimentally in a temperature range from 20°C to 500°C. The SAW devices could be operated up to frequencies of 3 GHz. However, considering a future sensor application a limiting factor is a strong increase of the acoustic losses in conjunction with frequency and temperature.
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