Excellent Temperature Coefficient Of Frequency Research Articles

Temperature characteristics of a thickness shear mode quartz crystal resonator bonded to a support substrate

Thickness shear mode (TSM) resonators consisting of metal films and quartz plates are widely used for sensor applications such as film thickness monitoring, force sensors, and odor sensors. However, the current sensor geometry prevents further improvements in its sensitivity and stability. Thinning the plate is necessary for high sensitivity, and advanced fabrication technologies are required for their commercialization. The solution is to use a support substrate to increase the mechanical strength, which can guide the transmittance of the electric field. Herein, we report a TSM resonator bonded to a support substrate. An AT-cut quartz resonator with a floating electrode on the top side was bonded to the support substrate. Two excitation electrodes were placed under the substrate. The support substrates evaluated in this study included borosilicate glass, Z-cut quartz crystals, and AT-cut quartz crystal plates. The quartz crystal resonator (QCR) bonded to the AT-cut quartz crystal plate and positioned at 90° to the crystallographic x-axis shows an excellent temperature coefficient of frequency of −60 ± 14 ppb/°C for a temperature range 11–40 °C. The proposed method reduces temperature sensitivity to 1/4 or less compared to that without a substrate. Furthermore, the resonator could be used as a quartz crystal microbalance. The proposed method may inspire further high-frequency QCR-based biochemical chips or various sensor applications with TSM resonators.

Surface Acoustic Wave Resonators With Hetero Acoustic Layer (HAL) Structure Using Lithium Tantalate and Quartz.

This article reports a new concept of surface-acoustic-wave (SAW) resonator, which uses shear horizontal (SH) wave confined in a thin LiTaO3 (LT) layer supported by a quartz (Qz) substrate. The LT layer is 35-50°YX LT, and the quartz substrate is 35-60°Y90°X Qz. A negative temperature coefficient of frequency (TCF) of the SH SAW in the LT layer is compensated by the quartz substrate, which shows a wide range of positive TCF depending on the crystalline orientation. Excellent TCFs of 2 and -10 ppm/°C were measured for the series and parallel resonance frequencies, respectively. The strong confinement of the SH SAW in the LT layer results in the best level of resonance characteristics ever reported. The measured impedance ratio reached 84 dB. On the other hand, spurious waves other than the SH SAW are not confined in the LT layer due to the unique properties of quartz, which results in spurious-free characteristics throughout a wide frequency range.

Improvement of surface acoustic wave characteristics of ZnO film/interdigital transducer/glass substrate using alumina film interlayer

ZnO films with c-axis (0002) orientation have been successfully grown by RF magnetron sputtering on interdigital transducer/Al2O3/glass substrates. Alumina films were deposited on glass substrates by electron beam evaporation. The crystalline structure and surface roughness of the films were investigated by X-ray diffraction and atomic force microscopy, respectively. The phase velocity and coupling coefficient of surface acoustic wave (SAW) device apparently increased when we increased the thickness of the alumina films. In addition, an excellent temperature coefficient of frequency of the SAW device was obtained by increasing the thickness of the alumina films. This experimental result is beneficial for enhancing the performance of ZnO thin-film SAW devices on inexpensive glass substrates.

Temperature Effects on the Propagation Characteristics of Love Waves along Multi-Guide Layers of Sio2/Su-8 on St-90°X Quartz

Theoretical calculations have been performed on the temperature effects on the propagation characteristics of Love waves in layered structures by solving the coupled electromechanical field equations, and the optimal design parameters were extracted for temperature stability improvement. Based on the theoretical analysis, excellent temperature coefficient of frequency (Tcf) of the fabricated Love wave devices with guide layers of SU-8/SiO2 on ST-90°X quartz substrate is evaluated experimentally as only 2.16 ppm.

Enhancement of characteristics of ZnO thin film surface acoustic wave device on glass substrate by introducing an alumina film interlayer

ZnO films with c-axis (0002) orientation have been successfully grown by RF magnetron sputtering on Al2O3/glass substrates. The alumina films were deposited on glass substrates by electron beam evaporation. The crystalline structure and surface roughness of the films were investigated by X-ray diffraction and atomic force microscopy, respectively. The phase velocity and coupling coefficient of SAW device were apparently increased when we increased the thickness of alumina film. Besides, an excellent temperature coefficient of frequency of SAW device was obtained with increasing the thickness of alumina film. The experimental result is beneficial to improve the performance of the ZnO thin film SAW devices on cheap glass substrate with easier process and lower cost.

Surface Acoustic Wave Duplexer Composed of SiO2/Cu Electrode/LiNbO3 Structure Having Convex and Concave Portions

The transition bandwidth of 20 MHz between the transmission (Tx: 1850–1910 MHz) and the receiving (Rx: 1930–1990 MHz) bands of personal communication service (PCS) handy phones in the United States (US) is very narrow compared with those of other systems. We have already realized surface acoustic wave (SAW) duplexers with sizes of 5.0×5.0×1.7 and 3.0×2.5×1.2 mm3 for PCS handy phones in the US with an excellent temperature coefficient of frequency (TCF) by using a shear horizontal (SH) wave on a flattened SiO2/Cu electrode/36–48° YX-LiTaO3 structure and a Rayleigh wave on a SiO2/Cu electrode/120–128° YX-LiNbO3 structure. Although the surface of the above-mentioned structures is flattened SiO2, we have also studied the shape of the SiO2 surface. As a result, in addition to increasing the stop-band width, which corresponds to the reflection coefficient, the TCF and power durability have been improved by forming convex portions on the surface of the SiO2 over the interdigital transducer (IDT) gaps.

Improvement of Shape Factor and Loss of Surface Acoustic Wave Resonator Filter Composed of SiO2/High-Density-Electrode/LiTaO3

Radio frequency (RF) filters in high frequencies using surface acoustic waves (SAWs), such as MediaFLOTM, time division synchronous code division multiple access (TD-SCDMA) in China's handy phone system, and the global positioning system (GPS) in cars, require a narrow bandwidth. Thus, the SAW substrates for their RF filters also require an excellent temperature coefficient of frequency (TCF) and an optimum electromechanical coupling factor. The authors reported an RF SAW filter for MediaFLOTM using a shear horizontal (SH) leaky SAW (LSAW) on a flattened SiO2 film/high-density metal electrode/36–48°Y·X-LiTaO3 substrate. Although it had a good TCF and a large attenuation out of the pass band, it had a slightly large loss at the pass band only at room temperature compared with that of the conventional Al-electrode/42°Y·X-LiTaO3 in the previous report. In this study, calculation using the coupling-of-modes (COM) theory showed the effect of a new phase inverse method of obtaining a steep slope at the right side of the filter frequency characteristic, although the previous paper showed only the measured frequency characteristics. In addition, an RF SAW filter with a lower loss at the pass band and a better TCF than that of the previous report has been realized.

High-Frequency Surface Acoustic Wave Resonator Filter Composed of ZnO/High-Density Electrode/Quartz Structure

In this paper, we describe the development of a radio frequency (RF) narrow-band surface acoustic wave (SAW) resonator filter composed of a ZnO/high-density metal electrode/quartz structure. First, theoretical analyses were performed to obtain a sufficient reflection coefficient and an appropriate electromechanical coupling factor, where ZnO thickness, type of electrode material, electrode thickness, and metallization ratio were used as variables. Second, one-port SAW resonators were fabricated to confirm the results of the above calculation, and a good agreement between the calculation and measurement results was obtained. Next, an RF narrow-band SAW resonator filter consisting of ZnO/Au interdigital transducer (IDT)/32° YX quartz was fabricated, and a minimum insertion loss of 2.08 dB, a -3 dB bandwidth of 3.9 MHz (0.054%), and an excellent temperature coefficient of frequency (TCF) of 4.0 ppm/°C were simultaneously achieved.

Radio Frequency Surface Acoustic Wave Filter Having Narrow Bandwidth and Excellent Temperature Characteristic

An RF filter in the high-frequency range such as that used in MediaFLOTM and time division synchronous code division multiple access in a Chinese handy phone system requires a narrow bandwidth and also a very excellent temperature coefficient of frequency (TCF). A flat SiO2/high-density metal electrode/36–48° YX-LiTaO3 structure has been applied to this system by adjusting the optimum thickness of SiO2 and the metal electrode to realize a very excellent TCF. As a result, the RF filter with a very excellent TCF of -1.01 ppm/°C, a good insertion loss, and a large outband rejection has been realized.

SiO2/Grooved Al Electrode/LiTaO3 and Edge-Reflection Surface Acoustic Wave Structures Having Large Reflection Coefficient, Large Coupling Factor, and Excellent Temperature Characteristic Even If Al Electrodes Are Used

A surface acoustic wave (SAW) substrate for a duplexer, such as a Personal Communication Service handy phone system in the U.S.A, is required to have a good temperature stability, an optimum electromechanical coupling factor, and a large reflection coefficient. The conventional method of depositing a SiO2 film on a thick Al electrode/LiTaO3 substrate to improve its temperature coefficient of frequency (TCF) is not effective for SAW resonator-type devices, because good frequency characteristics are not obtained due to large convex portions on the surface of SiO2 film. However, resonators having a large reflection coefficient, an excellent TCF, and good frequency characteristics were realized using the following two new structures: (I) SiO2 film/grooved Al electrode/LiTaO3 structure and (II) reflection of a shear horizontal (SH) wave at free edges on a SiO2 film/thin Al electrode/LiTaO3 structure substrate.

Small and low-loss IF SAW filters using zinc oxide film on quartz substrate

It was previously reported that a Rayleigh wave propagating on a zinc oxide film (ZnO)/ST-cut 35 degrees X propagation quartz substrate structure has the characteristics of an excellent temperature coefficient of frequency (TCF) and a large electromechanical coupling factor k(s). This substrate was applied to various intermediate-frequency (IF) stage filters. During the filter development, it was clarified that a spurious response due to the Love wave was generated. In this study, a new quartz substrate has been developed with a specific cut and propagation angle, that has the same values of the TCF and the coupling factor as the above-mentioned ones. In addition, it does not have the spurious response due to the Love wave. The combination of this specific-cut-angle quartz and ZnO film has been applied to IF filters for wideband code division multiple access (W-CDMA) and narrow-band CDMA (N-CDMA) systems. The insertion losses of their IF filters were 3-5 dB better and their TCF was superior (deltaf/f = 0.37 ppm/degrees C: one-third) compared with the conventional surface acoustic wave (SAW) filters.

Small and Low-Loss Intermediate Frequency Surface Acoustic Wave Filters Using Zinc Oxide Film on Quartz Substrate

A ZnO/quartz substrate is expected to be applied to a intermediate frequency (IF) surface acoustic wave (SAW) filter for which a small size, a low insertion loss, and a good temperature coefficient of frequency (TCF) are necessary because it has a large ks and an excellent TCF. However it was clarified that the SAW filter consisting of a single solid electrode or single-phase unidirectional transducer (SPUDT) on ZnO/quartz substrate often has a large spurious response due to the Love wave. The new cutting angle and the propagating direction (26–27°Y-X) for quartz were developed in order to suppress the spurious response. The developed W-CDMA IF 380 MHz filter was mounted on a small package of 3.2×5.0 mm2. It had a low loss of 5.8 dB, and high adjacent channel selectivity (ACS) of 36.1 dB/35.0 dB. Then, the developed N-CDMA IF 183 MHz filter was mounted on a small package of 5×7 mm2. It had a low loss of 4.3 dB. In both cases, the frequency shift per 1°C was less than 0.37 ppm/°C.

Properties of ZnO-Based Love Mode Acoustic Filters on 90° Rotated ST-Cut Quartz Substrates

Love wave acoustic devices are very suitable for high frequency filters because of their high phase velocity. An experimental study of Love wave devices based on ZnO/90° rotated ST-cut quartz with different thickness of ZnO films is presented. ZnO films with c-axis orientated crystalline structure have been successfully grown on quartz by r.f. sputtering technique. Phase velocity, electromechanical coupling coefficient, and temperature coefficient of frequency have been studied as a function of layer thickness. In this research, ZnO/quartz structure based Love wave acoustic filter not only has higher phase velocity and bigger electromechanical coupling coefficient than quartz substrate, but also keeps the excellent temperature coefficient of frequency.

Properties of Leaky, Leaky Pseudo, and Rayleigh Surface Acoustic Waves on Various Rotated Y-cut Langasite Crystal Substrates

A leaky surface acoustic wave (LSAW) and a leaky pseudo SAW (LPSAW) of various rotated Y-cut langasite substrates grown by the Czochralski method were measured by an ultrasonic microscope. The measured results agreed well with the values calculated using Ilyaeve's material constants of five kinds of material. Rayleigh SAW transversal filters, having various aluminum (Al) thicknesses of interdigital transducers (IDT), were constituted on (0°, 140–150°, 24–25°) and (12°, 150–153°, 36–37°) plates. A temperature coefficient of frequency (TCF) of the Rayleigh SAW filters was measured by measuring their center frequency. It was clarified that (0°, 142°, 24.5°) and (0°, 143°, 24°) plates at normalized Al-IDT thickness (HAl/λ) 0.0125, and (12°, 153°, 37°) plates at HAl/λ = 0.0625 showed turnover temperatures of 25°C, 30°C and 28°C and excellent TCFs (ΔF/F/°C = 1.83, 1.63, and 1.95 ppm/°C). Measured results of electromechanical coupling factor on their substrates showed similar values (ks2 = 0.41, 0.42 and 0.46%) to the theoretical ones. The (12°, 153°, 37°) plate having the largest coupling factor is the most suitable one for SAW devices. Furthermore, the velocity, the coupling factor, and the propagation loss of LSAW on air/various Y-X langasite plates (0°, θ, 0°) were also calculated.

Surface Acoustic Wave Characteristics of a ZnO/Quartz Substrate Structure Having a Large Electromechanical Coupling Factor and a Small Temperature Coefficient

A substrate having a suitable electromechanical coupling factor (k s) value and a good temperature coefficient of frequency (TCF) is required for an applications of resonator surface acoustic wave (SAW) filters and rudder-type SAW filters requiring a specific bandwidth, because their bandwidth largely depends on the coupling factor of the substrate. Although a quartz substrate for SAW devices provides a good TCF, the electromechanical coupling factor obtained is small. The author speculated that an excellent TCF and an appropriate electromechanical coupling factor could be realized by combination of a quartz substrate having a positive TCF and a ZnO film having a negative TCF. As a result of theoretical and experimental investigations performed using a 29°45' rotated Y and a 42°45' rotated Y 35°X propagating quartz substrate, an excellent TCF (≈0 ppm/°C) and an appropriate coupling factor (k s=0.1–0.11; k s2=0.01–0.0121) were realized with a zero degree power flow angle on the structure of IDT/ZnO/quartz or IDT/ZnO/shorted-plane/quartz for the first time.