Homogeneity of Langasite and Langatate wafers for acoustic wave applications
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: There is a pressing need for the fabrication of surface acoustic wave (SAW) devices capable of operating in harsh environments, at elevated temperature and pressure, or under high power conditions. These SAW devices operate as frequency-control elements, signal-processing filters, and pressure, temperature , and gas sensors. Applications include gas and oil wells, high-power duplexers in communication systems, and automobile and aerospace combustion engines. Under these high-temperature and power-operating conditions, which can reach several hundred degrees centigrade, the typically fabricated aluminum (Al) thin film interdigital transducer (IDT) fails due to electro and stress migration. This work reports on high temperature SAW transducers that have been designed, fabricated, and tested on langasite (LGS) piezoelectric substrates. Platinum (Pt) and palladium (Pd) (melting points at 1769/spl deg/C and 1554.9/spl deg/C, respectively) have been used as thin metallic films for the SAW IDTs fabricated. Zirconium (Zr) was originally used as an adhesion layer on the fabricated SAW transducers to avoid migration into the Pt or Pd metallic films. The piezoelectric LGS crystal, used as the substrate upon which the SAW devices were fabricated, does not exhibit any phase transition up to its melting point at 1470/spl deg/C. A radio frequency (RF) test and characterization system capable of withstanding 1000/spl deg/C has been designed and constructed. The LGS SAW devices with Pt and Pd electrodes and the test system have been exposed to temperatures in the range of 250/spl deg/C to 750/spl deg/C over periods of up to 6 weeks, with the Saw devices showing a reduced degradation better than 7 dB in the magnitude of transmission coefficient, |S/sub 21/|, with respect to room temperature. These results qualify the Pt and Pd LGS SAW IDTs fabricated for the above listed modern applications in harsh environments.IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 05/2005; · 1.82 Impact Factor
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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 measuredInternational Frequency Control Symposium and Exposition, 2006 IEEE; 07/2006
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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.IEEE transactions on ultrasonics, ferroelectrics, and frequency control 03/2010; 57(3):533-9. · 1.80 Impact Factor