Zero Temperature Coefficient Of Frequency Research Articles

Near-zero TCF 11.6-GHz lamb wave resonator based on 128°Y-cut LiNbO3

In this paper, a high-frequency lamb wave resonator (LWR) with near zero temperature coefficient of frequency (TCF) is designed, fabricated, and measured. The reported resonator is of a bi-layer structure consisting of lithium niobate (LiNbO3 or LN) and silicon dioxide (SiO2), and lithographically patterned aluminum (Al) inter-digitated electrode fingers on top of the bi-layer structure. By adjusting the thickness ratio of LN and SiO2 layers, both the electromechanical coupling (k2) and the TCF, including both TCF at resonant frequency (TCFr) and TCF at anti-resonant frequency (TCFa), of the thickness shear (TS) type LWR are optimized. Experimental results, which are in excellent agreement with theoretical analysis, show that the fabricated 11.6-GHz LWR achieves a k2 of 12.2%, with TCFr and TCFa being −4.2 and −5.4 ppm/K over a temperature range from 30 °C to 85 °C, respectively, demonstrating huge potential in applications for future wireless communication systems above 10 GHz.

Temperature dependent behaviors of electro-elastic properties of NdCa4O(BO3)3 crystal with monoclinic symmetry

Rare-earth calcium oxyborate crystals (RECa4O(BO3)3, RECOB, RE: rare-earth elements) are a kind of multifunctional crystal materials. In this work, the temperature dependent behaviors of the electro-elastic constants of NdCOB crystal were investigated over the temperature range of –80–200 ℃, and their temperature coefficients were evaluated. It is found that NdCOB crystal possesses minimal variation of relative dielectric permittivities (<3%). The temperature coefficient of frequency for ZY cut with width shear vibration mode is in the order of 0.07 × 10−4/℃. The temperature coefficients of the elastic compliances are obtained to be in the range of −33.0 × 10−4/℃–32.2 × 10−4/℃. Particularly, the s44 and s66 were found to show low temperature coefficients of the elastic compliances, i.e. 1.0 × 10−4/℃ and −0.4 × 10−4/℃, respectively, indicating the existence of zero temperature coefficient of frequency crystal cut. Furthermore, the electromechanical coupling factors and piezoelectric coefficients as a function of temperature were studied. The electromechanical coupling factor k26 and piezoelectric coefficient d26 are determined to be ∼30.8% and ∼15.2 pC/N at room temperature, respectively. The large piezoelectric response and zero temperature coefficient of frequency indicate the potential usage of NdCOB crystal for piezoelectric frequency devices over a wide temperature range.

Enhanced Performance of ZnO/SiO2/Al2O3 Surface Acoustic Wave Devices with Embedded Electrodes.

With the advent of the 5G era, surface acoustic wave (SAW) devices with a larger bandwidth and better temperature stability are strongly required, meanwhile the dimensions of devices are continuously scaling down. In this work, a new layout of ZnO/SiO2/Al2O3 SAW devices with embedded electrodes was developed, and with the help of the finite element method (FEM), the propagation characteristics were simulated. Through adopting embedded electrodes, a large electromechanical coupling coefficient (K2) of 6.6% for the Rayleigh mode can be achieved (5 times larger than that of the conventional ZnO/Al2O3 structure), feasible for wideband SAW devices, and a low acoustic velocity (Vp) of 2960 m/s is exhibited simultaneously, which benefits the miniaturization of SAW devices. The dramatic enhancement of K2 is mainly attributed to the more efficient excitation of SAW in piezoelectric films. Furthermore, a SiO2 overlay is added on the top of the structure to gain an excellent zero temperature coefficient of frequency (TCF). Experimentally, we successfully fabricated SAW one-port resonators based on the proposed structure and good characteristics of high K2, low Vp, and small TCF as simulated were confirmed. Our results show that the proposed structure provides a viable route to design SAW devices with a large bandwidth, small size, and robust temperature compensation for practical use.

Surface Acoustic Wave Devices Based on High Quality Temperature-Compensated Substrates

We report surface acoustic wave (SAW) devices based on two kinds of novel and optimized high quality piezoelectric substrates. SAW devices with excellent temperature stability and electromechanical coupling coefficient using the layered structures LiTaO3 (LT)/Si and LiNbO3 (LN)/SiO2/Si as the substrates are fabricated. These structures exhibit superior temperature performance than the conventional LT, LN substrates and reported related works with the SiO2 interlayer and Si substrate providing temperature compensation and thermal diffusion block, respectively. SAW device fabricated on the optimized LT/Si substrate shows a parabolic frequency–temperature relation with near zero temperature coefficient of frequency (TCF) in a low temperature range. SAW device fabricated on the novel LN/SiO2/Si substrate shows excellent performance with near constant TCF about −18 ppm/°C and electromechanical coupling coefficient ( $K^{2}$ ) near 18.69%. Theoretical analysis and performance comparison with reported works are presented to elucidate the temperature compensation mechanism of the proposed layered structures.

SAW Characteristics of AlN/SiO2/3C-SiC Layered Structure With Embedded Electrodes.

A layered structure of aluminum nitride (AlN)/silicon dioxide (SiO2)/cubic silicon carbide with embedded electrodes, which enables the growth of high-quality AlN thin films, is proposed and studied. The phase velocity, coupling factor, and temperature coefficient of frequency (TCF) of surface acoustic waves in the proposed structure have been investigated using the finite-element method. The simulation results show that a high velocity of 5485 m/s and a large effective coupling factor ( K2<sup/>) of 1.45% can be simultaneously obtained for the first mode. The dramatic enhanced K2 of 10.5% is also obtainable on the proposed structure employing Sc0.4Al0.6N thin film. Besides, the excellent zero TCF is also achieved without deteriorating the coupling factor by adding an amorphous SiO2 overlay.

Turnover Temperature Point in Extensional-Mode Highly Doped Silicon Microresonators

This paper demonstrates the existence of a local zero temperature coefficient of frequency (i.e., turnover point) in extensional-mode silicon microresonators, fabricated on highly n-type-doped substrates and aligned to the [100] crystalline orientation. It is shown through both theoretical analysis and finite-element simulation that the turnover point in thin-film piezoelectric-on-silicon (TPoS) resonators is a function of doping concentration and orientation. Moreover, the turnover point can be adjusted by changing the thickness ratio of Si and the piezoelectric film (e.g., AlN) in the resonant structure. In order to experimentally validate this result, similar resonators are fabricated on silicon-on-insulator substrates, and the temperature variation of frequency is measured. An overall temperature-induced frequency variation of less than 245 ppm is measured over the range of -40 °C-85 °C for an ~ 25-MHz TPoS resonator aligned to the [100] plane. This is more than a 15-times reduction with respect to the uncompensated conventional silicon resonators reported before. This work is a significant step toward strengthening silicon's position as an alternative resonator technology in the quartz-dominated stable oscillator market.

Transverse-Mode Spurious Suppression Technique for Surface Acoustic Wave Resonator with Zero Temperature Coefficient of Frequency on a SiO2/Al/LiNbO3 Structure

In this paper, we describe a suppression technique of transverse-mode spurious responses for a surface acoustic wave (SAW) resonator with a near zero temperature coefficient of frequency (TCF) on a SiO2/Al/LiNbO3 structure. We investigated the thinning of SiO2 on the dummy electrode region and studied how the transverse-mode responses change with remaining SiO2 thickness h on the dummy electrode region. As the results, we clarified that the remaining SiO2 thickness h on the dummy electrode region has an optimum value and could suppress the transverse-mode spurious responses completely when H and h are set at 0.35 λ and 0.20 λ, respectively. It was demonstrated that the selective SiO2 removal technique is effective to suppress transverse-mode spurious responses for SAW resonators employing the SiO2/Al/LiNbO3 structure for a wide range of SiO2 thicknesses, provided that the SiO2 thickness at the dummy electrode region is adjusted properly.

6 MHz BAW resonators fabricated using new piezoelectric crystals PrCa4O(BO3)3 and NdCa4O(BO3)3

AbstractThickness shear mode Bulk Acoustic Wave (BAW) resonators with frequency of 6 MHz, were fabricated using monoclinic piezoelectric crystals PrCa4O(BO3)3 (PrCOB) and NdCa4O(BO3)3 (NdCOB). Zero temperature coefficient of frequency (TCF) characteristics were achieved over the temperature range of –140 °C to 200 °C for (YXt)–1.5° cut PrCOB and (YXt)15° cut NdCOB, with the turnover temperature at 20 °C. The electromechanical coupling factor k26 and the piezoelectric coefficient d26 were determined to be 30.2% and 15.8 pC/N for PrCOB, 29.0% and 15.1 pC/N for NdCOB resonators, respectively. The temperature independent frequency behavior, large coupling factor, high piezoelectric coefficient, together with noticeable mechanical quality factors (Q &gt; 2,500), make PrCOB and NdCOB crystals good candidates for sensing applications with expanded temperature usage range. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Surface Acoustic Wave Propagation Characteristics and Temperature Effects on Quartz under Periodic Aluminum Structure

In communication applications, surface acoustic wave (SAW) devices should have excellent temperature stability. For this purpose, a numerical procedure for analyzing the Rayleigh SAW propagation and temperature effect was established in this work. The theoretical method used here is the space harmonic method (SHM). The theoretical analysis showed a slightly enhanced reflectivity for the open grating and presented the centers of transduction and reflection as separate at a spatial distance of λ0/4. The computational frequency responses of a two-port resonator were similar to the experimental data. The temperature characteristics of quartz under aluminum grating were also studied in this work. The cut angle of the quartz with zero temperature coefficient of frequency (TCF) decreased with increasing aluminum grating thickness. The coupling-of-modes (COM) parameters of quartz with zero TCF were calculated.

Effects of various oxide fillers on physical and dielectric properties of calcium aluminoborosilicate-based dielectrics

Physical and dielectric properties of LTCC (low temperature co-fired ceramics) materials based on a typical calcium aluminoborosilicate glass and various fillers such as Al2O3, BaTiO3, CaTiO3, TiO2, ZrO2, MgO and SiO2 were investigated. Densification, crystallization and thermal and dielectric properties were found to strongly depend on the type of filler. The XRD patterns of Al2O3, BaTiO3, CaTiO3 and MgO samples demonstrated crystalline phases, CaAl2Si2O8, BaAl2Si2O8, CaTiSiO5 and CaMgSi2O6, respectively, as a result of firing at 850 °C. For the sample containing CaTiO3 filler, specifically, dielectric constant increased drastically to approximately 19.9. A high quality factor of >210 and a high TCE (temperature coefficient of expansion) of >8.5 ppm/°C were obtained for the composition containing MgO or SiO2. Near zero TCF (temperature coefficient of frequency) was obtained for the samples containing TiO2. The purpose of this work is to investigate the effects of various ceramic fillers on physical and dielectric properties and ultimately to provide the technical guidelines for the proper choice of filler in various LTCC systems.

Crystallization behavior and microwave dielectric characteristics of ZnO-(La, Nd)2O3-B2O3-based dielectrics

Crystallizable zinc borate glasses modified with different contents of La2O3 or Nd2O3 were investigated as a potential low loss dielectric with respect to their crystallization behavior and microwave dielectric characteristics. The glasses were admixed with Al2O3 filler and fired at 850°C for 30 min in air to prepare low temperature dielectrics. Crystallization behavior and microwave dielectric properties of the resulting samples strongly depended on the relative content of La2O3 or Nd2O3 in the glass. As a promising result, the composition of 0.15ZnO-0.25Nd2O3-0.6B2O3 exhibited k ∼ 6.5 and Q ∼ 1194 at the resonant frequency of 18.9 GHz. Near zero temperature coefficient of frequency (TCF) was obtained by additional modification of the composition with ∼10 wt.% of TiO2 filler. Crystallization kinetics of the samples was studied based on the differential thermal analysis (DTA) curves obtained with different heating rates. Correlation of the observed dielectric properties to the crystallization behavior is the main subject of this work.

Improvement of Temperature Coefficient of Frequency in Ba-deficient Ba5Nb4O15 Microwave Dielectrics

Ba-deficient non-stoichiometric compositions of Ba5-xNb4O15-x (x=0 to 0.5) have been investigated for the purpose of improving microwave dielectric properties. While there was no significant variation in dielectric constant, variations in Q×f and TCF (temperature coefficient of frequency) values with the degree of Ba deficiency were prominent. Near zero TCF value was obtained for the composition of Ba4.77Nb4O14.77. The apparent formation of a second BaNb2O6 phase for the Ba-deficient compositions was believed as responsible for the improved TCF. It was interesting to note that the second phase was present more preferably inside of the sample.

Theoretical and Experimental Results of Ultrawide-Band Zero-TCF Ladder-Type SAW Filters and Arbitrary Bandwidth Filters Using High-Coupling SiO2/Y-X LiNbO3

We show ladder type SAW resonator filters using high-coupling SiO2/5°Y-X LiNbO3 with zero temperature coefficients of frequency (TCF). The experimental results showed lower than below 1.6 dB insertion loss with a wide band and zero TCF in the 2 GHz range. Also, we show the narrower band width of ladder-type filters using a new configuration of IDTs and reflectors on high-coupling substrates.

The Improvement of Temperature Coefficient of Frequency in Thin Film Bulk Acoustic Wave Resonator using Secondary Harmonics

ABSTRACTWe have succeeded in making an 870MHz-range thin film bulk acoustic wave (BAW) resonator that has a small temperature coefficient of frequency (TCF) using secondary harmonics. The 870MHz-range BAW resonator has been requested to have nearly zero TCF, because it will be used in an oscillator for remote keyless entry systems. The BAW resonator has composite structure that consists of Al electrodes and ZnO/SiO2. We directed our attention to the fact that ZnO and Al have negative TCF, and SiO2 has a positive one. It is theoretically possible to make zero TCF BAW resonators by optimizing the thickness ratio of ZnO and SiO2. However, using fundamental resonance, TCF is so sensitive to the thickness ratio that it cannot be easily controlled by MEMS techniques. We founds in finite element method simulation and confirmed by experiment that the TCF of secondary harmonics has a local minimum when changing the ZnO/SiO2 thickness ratio. As the result, a nearly zero TCF resonator without strict control of ZnO/SiO2 thickness ratio has been realized by adopting Al/ZnO/SiO2/ZnO/Al/SiO2 structure and combining thermal oxidized Si and sputtered SiO2. The resonator has the TCF of -1.86ppm/degree in the range of −40 to 85 degrees centigrade.

Optimal cuts of langasite, La3Ga5SiO14 for SAW devices.

The results of a theoretical and experimental investigation of the SAW propagation characteristics in an optimal region of langasite defined by the Euler angles phi from -15 degrees to +10 degrees, theta from 120 degrees to 165 degrees, and psi from 20 degrees to 45 degrees are presented. Based on temperature coefficients of the elastic constants derived from experimental data, some optimal orientations of langasite characterized by high electromechanical coupling factor (k2), zero power flow angle (PFA) and low or zero temperature coefficient of frequency (TCF) were found. The SAW velocity in the region of interest is highly anisotropic; this results in a significant amount of diffraction, which must be taken into account in the search for orientations useful for SAW devices. An orientation having simultaneously zero PFA, zero TCF, negligible diffraction, and relatively high piezoelectric coupling has been found and verified experimentally. The experimental results are in excellent agreement with the calculated SAW characteristics. The frequency response of a SAW device fabricated on the optimal cut of langasite is presented and demonstrates that high performance SAW filters can be realized on this optimal cut of langasite.

Single crystal growth of KNbO3 and application to surface acoustic wave devices

Potassium niobate (KNbO 3 ) has large piezoelectric constants. The surface acoustic wave (SAW) substrates with high couplings are very important for wide-band SAW filters and SAW devices. Therefore, the large size KNbO 3 single crystals are required for SAW device applications. In this paper, the techniques of a large size of KNbO 3 using the top seeded solution growth techniques (TSSG) with large size of crucible of 60 mm φ platinum are described. The results show that 50×50×15 mm 3 single crystals are obtained. Also, the poling techniques of KNbO 3 crystal are investigated by using the nonlinear scanning dielectric microscope. The propagation characteristics of SAW and piezoelectric leaky surface waves in KNbO 3 single crystal were investigated theoretically. The results show that the electro-mechanical coupling coefficient of the surface wave propagating along the X -axis of the rotated Y -cut plane is extremely large: K 2 =0.53, compared to K 2 of 0.055 for LiNbO 3 . The experimental results of the above KNbO 3 substrates agree with the theoretical ones. Also the KNbO 3 substrates showed experimentally the zero temperature coefficients of frequency (TCF) around 20 °C.

Super high electromechanical coupling and zero temperature coefficient surface acoustic wave substrates in KNbO/sub 3/ single crystal

The propagation characteristics of surface acoustic waves (SAW) and piezoelectric leaky surface waves in KNbO(3) single crystal have been investigated theoretically and experimentally. The results show that the electromechanical coupling coefficient k(2) of the surface wave propagating along the X-axis of the rotated Y-cut plate is very large with k(2)=0.53. This is about 10 times as large as that of LiNbO(3) in the surface wave branch. Experimental results for wideband SAW filters show low loss and temperature stable characteristics. The bandwidth is about 20%, and insertion losses are less than 2 to 6 dB. Simulation results for a ladder type filter using Y-cut KNbO(3) indicate that a bandwidth of 40% should be possible. The KNbO(3) substrates exhibit zero temperature coefficient of frequency (TCF) around 20 degrees C in rotated Y-cut substrates.

Thin-Film Bulk Acoustic Resonators Employing ZnO/Pyrex-Glass Composite Diaphragm Structure

This paper discusses the application of ZnO/Pyrex-glass composite diaphragm structure to thin-film bulk acoustic resonators (FBARs). Analysis of ZnO/Pyrex-glass composite diaphragm structure showed for thickness-longitudinal vibration that zero temperature coefficient of frequency with relatively large electromechanical coupling constant of about 20% is realised for the second- and third-order resonances as well as for the fundamental resonance. An FBAR employing the ZnO/Pyrex-glass composite diaphragm structure was fabricated, and Q values of 970 and 858 were obtained, respectively, for the fundamental and second-order resonances at 298.7 MHz and 497.5 MHz.

Some Physical Properties of Single Crystals of Normal and Deuterated Cesium Dihydrogen Arsenate

The elastic, dielectric, and piezoelectric constants of single crystals of cesium dihydrogen arsenate grown in H2O and of cesium dideuterium arsenate grown in D2O have been measured in the temperature range of 20°–80°C. The shear elastic constants S44 and S66 increase, while the extensional elastic constants S11 and S33 decrease on deuteration. Considerable changes in the temperature coefficients of three constants S11, S33, and S66 are produced by deuteration. Zero temperature coefficients of frequency of extensional and shear resonator plates are obtained in the Z plane, which are useful in electromechanical applications.

High Q, BT-type, quartz oscillator

The design of a BT-type quartz oscillator, operating on the third overtone and having a Q value of 2 × 106 at a frequency of 15 Mc/s, is described. For a crystal element cut to the specification of (yxl)-46°, the zero temperature coefficient of frequency occurred at 24.8° C.