M.P. da Cunha

Recent measurements of material constants versus temperature for langatate, langanite and langasite

Measurements, experiments and extracted parameters of the material constants of langatate (LGT), langanite (LGN), and langasite (LGS) versus temperature are presented from work conducted at the University of Central Florida. The data presented is from material grown by Crystal Photonics,, Inc. and is part of a contract on growth, characterization and device implementation of these materials. Parameters presented include the material expansion and mass density coefficients, stiffness constants, and dielectric constants. A paper in this proceedings by R. Smythe, et. al., presents the determination of the piezoelectric constants. Discussion of the experimental technique using swept frequency scattering parameter data (S/sub 11/) for obtaining pulse echo data, and capacitance measurements are presented. Comparison of results to preciously published and current data on BAW and SAW parameters is provided. Important parameter contour plots for LGS and LGN slow-shear mode are given. TCF comparisons for several BAW cuts are presented.

High temperature stability of langasite surface acoustic wave devices

High temperature acoustic wave (AW) devices capable of operating above 600degC and in hostile environments have opened potential applications for monitoring industrial processes, power plants, and aerospace systems. The authors have reported on the development of thin film electrodes and protective ceramic layers to allow surface acoustic wave (SAW) device operation up to 800degC on langasite (LGS) crystals. This success motivated further study of the electrode material and protective ceramic overlayer, as well as investigations of long term performance, temperature cycling and shock behavior, which are reported in this work. Among the results reported are: behavior of a co-deposited Pt/Rh/ZrO2 composite electrode structure up to 1000degC; investigation of oxygen rich and nitrogen rich SiAlON protective ceramic layers; long term (4080 hours, or about 5frac12 months) operation of a two-port SAW resonator at 800degC; cyclical thermal tests between room temperature and 850degC; and thermal shock tests of crystals between 700degC and room temperature.

Investigation on recent quartz-like materials for SAW applications

Recent progress in growing and characterizing quartz-like materials of the trigonal system class 32 has been reported by several groups. The promising perspective for bulk acoustic wave frequency control applications indicates the potentiality of employing these materials for SAW applications as well. This paper reports results of investigations focused on SAW orientations of langasite (LGS), gallium phosphate (GaPO/sub 4/), and langanite (LGN), both singly and doubly rotated cuts. Among the characteristics explored, major attention is paid to the temperature coefficient of delay (TCD), the electromechanical coupling coefficient (K/sup 2/), and the power flow angle (PFA). Contour graphs are plotted based on our calculated results and show the regions in space in which low TCD and high K/sup 2/ can be obtained; they also exhibit the associated PFA and phase velocity characteristics. The influence of different sets of material constants is addressed. The spatial investigation performed shows that there are promising orientation regions in these materials at which zero or reduced TCD (<10 ppm//spl deg/C) and PFA are obtained. Additional attractive characteristics for SAW applications have been observed: values of K/sup 2/ a few times higher than the K/sup 2/ of quartz ST-X, thus finding applications in larger bandwidth devices; variation of the TCD with respect to temperature, which is comparable to the variation found for quartz ST-X and less than that for zero TCD Li/sub 2/B/sub 4/O/sub 7/ cuts like 45/spl deg/X-Z and (0/spl deg/ 78/spl deg/ 90/spl deg/); and phase velocity values circa 13 to 26% smaller than the phase velocity of quartz ST-X thus allowing a reduction in size for intermediate frequency device applications.Recent progress in growing and characterizing quartz-like materials of the trigonal system class 32 has been reported by several groups. The promising perspective for bulk acoustic wave frequency control applications indicates the potentiality of employing these materials for SAW applications as well. This paper reports results of investigations focused on SAW orientations of langasite (LGS), gallium phosphate (GaPO/sub 4/), and langanite (LGN), both singly and doubly rotated cuts. Among the characteristics explored, major attention is paid to the temperature coefficient of delay (TCD), the electromechanical coupling coefficient (K/sup 2/), and the power flow angle (PFA). Contour graphs are plotted based on our calculated results and show the regions in space in which low TCD and high K/sup 2/ can be obtained; they also exhibit the associated PFA and phase velocity characteristics. The influence of different sets of material constants is addressed. The spatial investigation performed shows that there are promising orientation regions in these materials at which zero or reduced TCD (<10 ppm//spl deg/C) and PFA are obtained. Additional attractive characteristics for SAW applications have been observed: values of K/sup 2/ a few times higher than the K/sup 2/ of quartz ST-X, thus finding applications in larger bandwidth devices; variation of the TCD with respect to temperature, which is comparable to the variation found for quartz ST-X and less than that for zero TCD Li/sub 2/B/sub 4/O/sub 7/ cuts like 45/spl deg/X-Z and (0/spl deg/ 78/spl deg/ 90/spl deg/); and phase velocity values circa 13 to 26% smaller than the phase velocity of quartz ST-X thus allowing a reduction in size for intermediate frequency device applications.

Platinum and palladium high-temperature transducers on langasite

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.

High velocity pseudosurface waves (HVPSAW)

Current interest in low-loss UHF filters for use in mobile and personal communication systems has led to a number of pseudo-SAW (PSAW) and SH mode based SAW devices. These filters can operate at higher frequencies than SAW-based ones for a given line-width because PSAW and SH mode velocities can be significantly higher than corresponding SAW velocities. Furthermore for certain orientations the attenuation of the PSAW is acceptably small. In this paper the existence of an independent high-velocity pseudo-SAW (HVPSAW) mode is discussed. The HVPSAW is shown to have a phase velocity close to the longitudinal bulk wave velocity and to be quasi-longitudinally polarized. The nature of this HVPSAW mode is described for both electrically open-circuited and layered metallic short-circuited conditions for several piezoelectric materials. Numerical and experimental data are presented which discuss the existence and properties of these high-velocity pseudosurface waves.<<ETX>>

P4L-1 Enabling Very High Temperature Acoustic Wave Devices for Sensor & Frequency Control Applications

The introduction of piezoelectric crystals capable of acoustic wave (AW) excitation at high temperatures (> 600degC) has opened new possibilities for harsh environment applications, such as combustion engines, industrial processes, and gas/oil extraction. Significant remaining challenges are the fabrication of electrode thin films as well as appropriate packaging capable of withstanding such harsh environments. Thin film electrodes utilizing platinum over zirconium (Pt/Zr) developed by the University of Maine research team for surface acoustic wave gas sensors proved to be inappropriate for long term operation above 700degC, due to the de-wetting phenomenon of thin film Pt. In this paper the fabrication and testing of thin film electrodes and AW devices for longer term operation (from a few hours to months) in high temperature environments (up to 1000degC) have been investigated. The techniques used to overcome the problem of AW device electrode failure at temperatures above 600degC include: multilayered film architectures, alloy compositions, high temperature processing, and protective ceramic overlay films. In particular Pt, zirconium (Zr), ZrO2, Pt/Rhodium, and Pt/Au films have been examined alone or in combinations as the electrode materials, and ultra-thin SiAlON coatings have been used to extend electrode lifetime and to provide device protection in harsh environments. It has been found that the combination of layered and alloy electrodes retarded or prevented de-wetting of the Pt film, and extended the long-term AW device operation from 600degC to at least 950degC. These results indicate the feasibility of very high temperature AW device operation, and open up new opportunities for AW device applications in harsh environments.

A lateral-field-excited LiTaO 3 high-frequency bulk acoustic wave sensor

The most popular bulk acoustic wave (BAW) sensor is the quartz crystal microbalance (QCM), which has electrodes on both the top and bottom surfaces of an AT-cut quartz wafer. In the QCM, the exciting electric field is primarily perpendicular to the crystal surface, resulting in a thickness field excitation (TFE) of a resonant temperature compensated transverse shear mode (TSM). The TSM, however, can also be excited by lateral field excitation (LFE) in which electrodes are placed on one side of the wafer leaving a bare sensing surface exposed directly to a liquid or a chemi/bio selective layer allowing the detection of both mechanical and electrical property changes caused by a target analyte. The use of LFE sensors has motivated an investigation to identify other piezoelectric crystal orientations that can support temperature-compensated TSMs and operate efficiently at high frequencies resulting in increased sensitivity. In this work, theoretical search and experimental measurements are performed to identify the existence of high-frequency temperature-compensated TSMs in LiTaO3. Prototype LFE LiTaO3 sensors were fabricated and found to operate at frequencies in excess of 1 GHz and sensitively detect viscosity, conductivity, and dielectric constant changes in liquids.

High temperature sensing technology for applications up to 1000°C

This work reports on the research and development of high temperature (HT) thin electrodes for use in acoustic wave sensor platforms up to 1000degC. Previously developed thin film platinum (Pt) electrodes limits HT operation due to a de-wetting phenomena, which results in loss of Pt film electrical continuity and device failure above 650mnplus750degC. To address the problem, co-deposition of Pt/rhodium (Pt-Rh) alloys with zirconia (ZrO2) was used to fabricate HT-stable thin film surface acoustic wave interdigitated electrodes on langasite substrates. The resulting devices showed stable operation for over five months at 800degC. As temperature sensors, these devices have a sensitivity of 9.1 KHz/degC @800degC. In addition, ceramic silicon-alumina-nitrogen (SiAlON) overlayers were investigated to protect the sensor surface. The resulting devices enable harsh environment temperature and pressure sensors, for applications such as monitoring of jet engines, atmospheric re-entrance, and space exploration.

Investigation on recent quartz like materials for SAW applications

Recent progress on growing and characterizing quartz like materials of the trigonal system class 32 has been reported by several groups. The promising perspective for bulk acoustic wave frequency control applications indicates the potentiality of employing these materials for SAW applications as well. This paper reports results of investigations focused on SAW orientations of langasite (LGS), gallium phosphate (GaPO/sub 4/), and langanite (LGN), both single and doubly rotated cuts. Among the characteristics explored, major attention is paid to the temperature coefficient of delay (TCD), the electromechanical coupling coefficient (K/sup 2/), and the power flow angle (PFA). Three-dimensional graphics and contour graphics are plotted based on our calculated results, showing the regions in space where low TCD and high K/sup 2/ can be obtained, also exhibiting the associated PFA and phase velocity characteristics. The influence of different sets of material constants is addressed where appropriate. The spatial investigation performed shows that there are promising orientation regions in these materials where zero or reduced TCD (<10 ppm//spl deg/C) and PFA are obtained. Additional attractive characteristics for SAW applications have been observed like: values of K/sup 2/ a few times higher than the K/sup 2/ of quartz ST-X, thus finding applications in larger bandwidth devices; variation of the TCD with respect to temperature which is comparable to the variation found for quartz ST-X and less than that for zero TCD Li/sub 2/B/sub 4/O/sub 7/ cuts like 45/spl deg/X-Z and (0/spl deg/78/spl deg/90/spl deg/); and phase velocity values circa 13 to 25% smaller than the phase velocity of quartz ST-X, thus allowing a reduction in size for intermediate frequency device applications.

High-temperature langatate elastic constants and experimental validation up to 900°C

This paper reports on a set of langatate (LGT) elastic constants extracted from room temperature to 1100°C using resonant ultrasound spectroscopy techniques and an accompanying assessment of these constants at high temperature. The evaluation of the constants employed SAW device measurements from room temperature to 900°C along 6 different LGT wafer orientations. Langatate parallelepipeds and wafers were aligned, cut, ground, and polished, and acoustic wave devices were fabricated at the University of Maine facilities along specific orientations for elastic constant extraction and validation. SAW delay lines were fabricated on LGT wafers prepared at the University of Maine using 100-nm platinum rhodium- zirconia electrodes capable of withstanding temperatures up to 1000°C. The numerical predictions based on the resonant ultrasound spectroscopy high-temperature constants were compared with SAW phase velocity, fractional frequency variation, and temperature coefficients of delay extracted from SAW delay line frequency response measurements. In particular, the difference between measured and predicted fractional frequency variation is less than 2% over the 25°C to 900°C temperature range and within the calculated and measured discrepancies. Multiple temperature-compensated orientations at high temperature were predicted and verified in this paper: 4 of the measured orientations had turnover temperatures (temperature coefficient of delay = 0) between 200 and 420°C, and 2 had turnover temperatures below 100°C. In summary, this work reports on extracted high-temperature elastic constants for LGT up to 1100°C, confirmed the validity of those constants by high-temperature SAW device measurements up to 900°C, and predicted and identified temperature-compensated LGT orientations at high temperature.