以四面体非晶碳为布拉格反射栅高声阻抗材料的固贴式薄膜体声波谐振器性能仿真分析

为了提高固贴式薄膜体声波谐振器(SMR)的电学和声学品质,实现四面体非晶碳(ta-C)在体声波器件领域的新应用,建立了以ta-C为布拉格反射栅高声阻抗材料的SMR模型,利用MathCAD仿真研究布拉格反射栅层数对该SMR的谐振特性的影响以及ta-C中sp3杂化含量和高/低声阻抗层厚度偏差对SMR的品质因子(Q值)的影响。结果表明层数的增加提高了SMR的品质;ta-C薄膜sp3杂化含量越高,达到饱和Q值所需层数越少,当含量为80%时,至少需要6层(3对)布拉格反射层使SMR达到优异Q值;距离压电堆越近的高/低声阻抗层,其厚度偏差对Q值的影响越大,从而实现了高频率(8GHz)低损耗的SMR的设计。     

Synthesis and bulk acoustic wave properties on the dual mode frequency shift of solidly mounted resonators

This study focused on the fabrication and the theoretical analysis of solidly mounted resonators (SMR) concerning dual-mode frequency responses and their frequency shift of bulk acoustic wave (BAW) resonance. For this device fabrication, RF/DC magnetron sputtering and photolithography were employed to constitute the required multilayer structure. For the theoretical analysis, the dualmode frequency shift was characterized by the Sauerbrey's formula, and a modified formula was carried out following the trend for the large frequency shift. In the fabrication of the SMR device, Mo/SiO2 was chosen to construct the Bragg reflector as the high/low acoustic impedance materials, respectively, and aluminum nitride (AlN) was used as a piezoelectric layer. To investigate the characteristics of BAW on the dual-mode frequency shift, the c-axis tilted angle of AlN was altered as well as the various mass loading on the SMR. Based on the experimental results, the dual-resonance frequencies showed a nonlinear decreasing trend with a linear increase of the mass loading. Therefore, a modified formula was carried out. Furthermore, the ratio of the longitudinal-resonant frequency to the shear-resonant frequency remained at a range around 1.76 despite the various c-axis tilted angles of AlN and gradual mass loading on the SMR. The electromechanical coupling coefficient, k2(eff), of the shear resonance rose with the increase of the c-axis tilted angle of AlN.     

Fabrication and frequency response of solidly mounted resonators with 1/4λ mode configuration

A solidly mounted resonator (SMR) consists of a multilayered structure and requires material interfaces that confine waves to resonate as standing waves in order to avoid wave energy loss. The selection of high or low acoustic impedance for the first layer beneath piezoelectric layer results in 1/4λ mode and 1/2λ mode resonance configurations.In this study, Mo/SiO₂ is chosen to construct the Bragg reflector as the high/low acoustic impedance materials, respectively, and aluminum nitride (AlN) is used as the piezoelectric layer. For application at frequency of 2.5 GHz, the specific thicknesses of Mo, SiO₂, and AlN are considered individually in the deposition processes. AlN of 1/4λ thickness is deposited on a seven-layer Bragg reflector. The 1/4λ mode SMR shows the distinct resonant characteristics at 1.3 GHz (shear mode) and 2.4 GHz (longitudinal mode). The coupling coefficient K_eff² of 6.9% is in agreement with the theoretical analysis.     

The AlN based solidly mounted resonators consisted of the all-metal conductive acoustic Bragg reflectors

The AlN based solidly mounted resonators utilizing the all-metal conductive Bragg reflectors have been demonstrated. The devices with different reflectors of Mo/Ti, W/Ti and AlN/Mo pairs have been fabricated and the frequency responses have been compared. The bottom electrode is incorporated into the conductive Bragg reflector, thereby reducing the parasitic resistance and the damping of the acoustic energy. The experimental results reveal that Mo/Ti and W/Ti reflectors exhibit excellent properties of frequency selection and clamping the acoustic wave. The devices show the distinct resonant phenomenon near 2.4 GHz with the excellent suppression of the shear mode and the sidelobe mode. Compared to the conventional device consisted of AlN/Mo reflector, the devices consisted of the conductive reflectors demonstrate the higher resonance frequency and obviously improved performance.     

Sputtered SiO2 as low acoustic impedance material for Bragg mirror fabrication in BAW resonators

In this paper we describe the procedure to sputter low acoustic impedance SiO₂ films to be used as a low acoustic impedance layer in Bragg mirrors for BAW resonators. The composition and structure of the material are assessed through infrared absorption spectroscopy. The acoustic properties of the films (mass density and sound velocity) are assessed through X-ray reflectometry and picosecond acoustic spectroscopy. A second measurement of the sound velocity is achieved through the analysis of the longitudinal ?/2 resonance that appears in these silicon oxide films when used as uppermost layer of an acoustic reflector placed under an AlN-based resonator.     

Optimized reflector stacks for solidly mounted bulk acoustic wave resonators

The quality factor (Q) of a solidly mounted bulk acoustic wave resonator (SMR) is limited by substrate losses, because the acoustic mirror is traditionally optimized to reflect longitudinal waves only. We propose two different design approaches derived from optics to tailor the acoustic mirror for effective reflection of both longitudinal and shear waves. The first one employs the stopband theory in optics; the second one takes advantage of the periodic nature of reflection spectra in a Bragg reflector: the diffraction grating design approach. The optimized design using stopband theory reaches a calculated minimum transmission of 25 dB and 20 dB at resonance frequency for longitudinal and shear waves, respectively, for various practical reflector material combinations. Using the diffraction grating approach, a near quarter-wave performance is maintained for longitudinal waves, whereas shear waves reach minimum transmission below 26 dB. However, this design does necessitate relatively thick layers. The experimental results show good agreement with finite element models (FEM). The extracted 1-D Q for the realized shear optimized devices was increased to around 3300.     

Raman spectroscopy and mechanical properties of multilayer tetrahedral amorphous carbon films

In order to take the tetrahedral amorphous carbon (ta-C) films as the high acoustic impedence layer in a Bragg reflector isolating acoustic wave from the substrate in solidly mounted resonator, the multilayer films consisting of sp2-rich layers and sp3-rich layers were deposited from a filtered cathodic vacuum arc by adjusting the substrate bias. The microstructure of the films was evaluated using a visible Raman spectroscopy. The stress was calculated according to the changed curvature of the coated and bare substrate. The hardness, modulus and scratching were measured using a nanoindenter. It has been shown that the multilayer structure maintaining high tetrahedral content, high hardness and high elastic modulus is still characterized with lower intrinsic stress and better adhesion.     

Platinum/carbon multilayer reflectors for soft-x-ray optics

We have fabricated platinum/carbon (Pt/C) multilayer reflectors with 2d spacaings between 50 and 200 ?, using an electron-beam evaporator. We investigated the effects of 2d values, the number of layer pairs, substrate temperature, coatings, and the long-term stability on the reflectivity performance by using characteristic x rays and monochromatized synchrotron radiation in the 0.8-8-keV region. In this study we show that Pt/C multilayers with 10-20 layer pairs exhibit high and stable soft-x-ray reflectivity. The interfacial roughness was measured in the range of 5 ? and becomes lower for structures deposited at liquid-nitrogen temperatures. Coating these reflectors with a 100-?-thick platinum layer increased the grazing angle reflectivity without significantly lowering the Bragg peak reflectivity.     

水下刚性角反射器散射特性

为了分析水下角反射器的声散射特性,提出了一种计算水下凹面目标散射声场的方法。采用ANSYS软件构建水下角反射器实体模型,再利用声学分析软件SYSNOISE对其远场散射声场进行仿真。计算了二面角反射器的目标强度,并与声束弹跳法和板块元法的计算结果进行对比,验证了该方法的适用性和精确性。对三面角反射器的散射特性进行了分析,得到了不同声波入射角度下的目标强度、目标强度随频率的变化规律以及散射方向图,结果表明,正方形角反射器的目标强度最大,三角形角反射器的散射宽度最大,角反射器目标强度不存在明显的频率效应。计算了八面角反射体的方向图,计算结果表明角反射器组合可以有效增大目标强度和散射宽度。     

GZO厚度对非晶硅电池中复合背电极的影响

在非晶硅太阳能电池中加入复合背电极是提高非晶硅太阳能电池光电转换效率和稳定性的有效手段.本文利用磁控溅射技术在非晶硅薄膜太阳能电池上制备了ZnO :Ga(GZO)/Al复合背电极,研究了GZO厚度对GZO薄膜光电性质及非晶硅电池中GZO/Al复合背电极性能的影响.研究表明：随着GZO层厚度的增加,GZO薄膜的光电性质均表现出较高水平,适合制备GZO/Al复合背电极;相较于单层Al背电极的非晶硅太阳能电池,具有GZO/Al复合背电极的太阳能电池性能大幅提高.当GZO层厚度为100 nm时,太阳能电池的短路电流(I_SC)、开路电压(V_OC)和填充因子(FF)分别达到8.66 mA,1.62 V和54.7%.     

Modeling of bulk acoustic wave devices built on piezoelectric stack structures: impedance matrix analysis and network representation

The fundamental electro-acoustic properties of a solid layer are deduced in terms of its impedance matrix (Z) and represented by a network for modeling the bulk acoustic wave devices built on piezoelectric stacked structures. A piezoelectric layer is described by a three-port equivalent network, a nonpiezoelectric layer, and a short- or open-circuit piezoelectric layer by a two-port one. Electrical input impedance of the resonator is derived in terms of the Z-matrix of both the piezoelectric layer and an external load, the unique expression applies whether the resonator is a mono- or electroded-layer or a solidly mounted resonator (SMR). The loading effects of Al-electrodes on the resonating frequencies of the piezoelectric ZnO-layer are analyzed. Transmission and reflection properties of Bragg mirrors are investigated along with the bulk radiation in SMR. As a synthesizing example, a coupled resonator filter (CRF) is analyzed using the associated two-port equivalent network and by calculating the power transmission to a 50Omega-load. The stacked crystal filter is naturally included in the model as a special case of CRF. Combining a comprehensive matrix analysis and an instructive network representation and setting the problem with a full vectorial formalism are peculiar features of the presented approach.     

Study of cellular adhesion with scanning acoustic microscopy

A mechanical scanning acoustic reflection microscope was applied to living cells (e.g., osteoblasts) to observe their undisguised shapes and to evaluate their adhesive conditions at a substrate interface. A conditioned medium was collected from a bone-metastatic breast cancer cell line, MDA-MB-231, and cultured with an immature osteoblast cell line, MC3T3-E1. To characterize the cellular adhesion, MC3T3-E1 osteoblasts were cultured with or without MDA-MB-231 conditioned medium for 2 days, then assayed with the scanning acoustic reflection microscope. At 600 MHz the scanning acoustic reflection microscope clearly indicated that MC3T3-E1 cells cultured with MDA-MB-231 conditioned medium had both an abnormal shape and poor adhesion at the substrate interface. The results are compared with those obtained with laser scanning confocal microscopy and are supported by a simple multilayer model.     

SAW impedance elements

In the past few years a new class of SAW devices has been under development based on the use of one port SAW resonators as lumped elements with their impedance strongly changing in a frequency range of effective SAW excitation. These `SAW impedance elements' can be connected in ladder type, balanced bridge type, or others types of networks to provide desired filter characteristics. In this paper the admittance characteristics of a single element are analyzed using coupling of mode (COM) model simulations. It is found that resonant type characteristics of a long transducer can be described by very simple formulas. The frequency gap between resonance and anti-resonance, important for filter design, is determined practically by the ΔV/V parameter of the piezoelectric substrate used, while the minimum magnitude of the impedance at the resonant frequency is determined by the SAW attenuation. The internal reflections only shift the resonance to the left edge of the stopband not changing other characteristics. A case of a synchronous resonator comprising a relatively short transducer and two reflectors placed on both sides of it is also analyzed. This type of impedance element has smaller frequency shift between resonance and anti-resonance points     

Compact, high-Q, zero temperature coefficient, TE011 sapphire-rutile microwave distributed Bragg reflector resonators

Some novel new resonator designs based on the distributed Bragg reflector are presented. The resonators implement a TE₀₁₁ resonance in a cylindrical sapphire dielectric, which is confined by the addition of rutile and sapphire dielectric reflectors at the end faces. Finite element calculations are utilized to optimize the dimensions to obtain the highest Q-factors and zero frequency-temperature coefficient for a resonator operating near 0°C. We show that a Q-factor of 70,000 and 65,000 can be achieved with and without the condition of zero frequency-temperature coefficients, respectively     

Distributed bragg reflector resonators with cylindrical symmetry and extremely high Q-factors

A simple non-Maxwellian method is presented that allows the approximate solution of all the dimensions of a multilayered dielectric TE0qp mode cylindrical resonant cavity that constitutes a distributed Bragg reflection (DBR) resonator. The analysis considers an arbitrary number of alternating dielectric and free-space layers of cylindrical geometry enclosed by a metal cylinder. The layers may be arranged along the axial direction, the radial direction, or both. Given only the aspect ratio of the cavity, the desired frequency and the dielectric constants of the material layers, the relevant dimensions are determined from only a set of simultaneous equations, and iterative techniques are not required. The formulas were verified using rigorous method of lines (MoL) calculations and previously published experimental work. We show that the simple approximation gives dimensions close to the values of the optimum Bragg reflection condition determined by the rigorous analysis. The resulting solution is more compact with a higher Q-factor when compared to other reported cylindrical DBR structures. This is because it properly takes into account the effect of the aspect ratio on the Bragg antiresonance condition along the z-axis of the resonator. Previous analyses assumed the propagation in the z-direction was independent of the aspect ratio, and the layers of the Bragg reflector were a quarter of a wavelength thick along the z-direction. When the aspect ratio is properly taken into account, we show that the thickness of the Bragg reflectors are equivalent to the thickness of plane wave Bragg reflectors (or quarter wavelength plates). Thus it turns out that the sizes of the reflectors are related to the free-space propagation constant rather than the propagation constant in the z-direction.     

Normal-incidence efficiencies of 4800-grooves/mm-ruled replica gratings with multilayer and gold coatings in the 125-325-A wavelength region

The normal-incidence efficiencies of two 4800-grooves/mm ruled replica gratings, one with a dual-bandpass molybdenum/silicon multilayer coating and the other with a gold coating, were measured by use of synchrotron radiation in the 125-325-A wavelength region. The peak reflectance of the multilayer coating was 22% in the first Bragg order near 235 A and 28% in the second Bragg order near 126 A. The peak efficiency of the multilayer grating was 2.6% in the first diffraction order near 225 A and 0.3% in the second diffraction order near 125 A. The efficiencies of the multilayer grating were much higher than the corresponding efficiencies of the gold grating. The characterization of the surfaces of the gratings by atomic force microscopy indicated rms microroughness values in the 5-18-mum(-1) frequency range of 12-20 A for the multilayer grating and 22-32 A for the gold grating. Both gratings had bumpy surface features larger than the nominal groove height. The rather large surface roughness and groove irregularities had a detrimental effect on the grating efficiencies.     

Mesoporous bragg stack color tunable sensors

Herein we report a novel self-assembly synthesis, structural and optical characterization of mesoporous Bragg stacks (MBS) composed of spin-coated multilayer stacks of mesoporous TiO(2) and mesoporous SiO(2). Investigation of the optical response of MBS to the infiltration of alcohols and alkanes into its pores reveals better sensitivity and selectivity than conventional Bragg reflectors. Furthermore, we demonstrate that the chemical sensing ability can be tuned via layer thickness, composition and surface properties.     

Mode selection for a piezoelectric layer in a bulk-acoustic-wave composite acoustic resonator

An analysis is made of the mode selection for a thin piezoelectric layer in a bulk-acoustic-wave composite layered acoustic resonator by transformation of the impedance using a set of quarter-wave layers. Depending on the number of layers, the impedance of the substrate on which the piezoelectric layer is deposited may be abruptly increased or sharply reduced. This serves to simulate a “fixed” or “free” surface. As a result, a mechanically strong resonator structure is produced, operating at the natural frequencies of a thin piezoelectric layer. A numerical simulation is made of multilayer structures formed by alternating quarter-wave layers of LiNbO₃ and LiTaO₃. It is shown that if these layers are of micron thickness and there is a sufficiently large number of them, the acoustic properties of the substrate do not influence the frequency characteristics and Q factor of the thin piezoelectric vibrating layer in the microwave range. Pis’ma Zh. Tekh. Fiz. 23, 35–41 (October 12, 1997)     

涂层界面刚度的超声数值仿真检测

针对涂层结合界面刚度在非破坏条件下难以精确测量的问题,提出了一种超声检测特征参量表征刚度系数的方法。利用声波在n层各向同性介质中的反射、透射原理,结合界面的弹簧模型,建立了多层介质界面刚度系数的超声检测数学模型。基体选用钢、铸铁、铝合金,表面选用不同喷涂工艺得到的Al2O3陶瓷涂层,获得了不同界面刚度系数的超声反射频谱。仿真结果表明:分离界面和理想界面时,谐振频率都具有周期性,但周期大小不同;弱结合界面时,随着界面的刚度系数逐渐增加,谐振频率逐渐增多,这些谐振频率均向高频方向移动。与频率较高处相比,频率较低处的谐振频率随着刚度系数的增加向高频移动的速度更快。因此建立了第一个谐振频率与刚度系数之间的关系。在同一刚度系数下,由谐振频率与材料的特性阻抗关系获得如下规律:当涂层材料不变时,谐振频率随着基体特性阻抗的增大而增大;当基体材料不变时,谐振频率随着涂层特性阻抗的增大而减小。给出了以指数函数形式拟合的刚度系数与谐振频率的变化曲线。通过对该指数函数参数与材料特性阻抗之间关系的分析,获得了弱界面时谐振频率与刚度系数和材料特性阻抗三者之间的函数表达式。该方法为涂层复合材料弱界面的超声检测提供了理论支持。     

Reflection of ultrasonic Lamb waves produced by thin conducting strips

The reflection of ultrasonic Lamb waves produced by a periodic array of thin conducting strips deposited on a piezoelectric plate was investigated both theoretically and experimentally. A repetitively mismatched transmission line model was used to analyze the performance of the reflector. The reflection coefficient of a single strip is given by R approximately Deltav/v, where Deltav/v is the fractional change in velocity produced by electrical shorting of the propagation surface. An attractive property of Lamb waves is that they can provide a much higher value of Deltav/v than is possible with surface acoustic waves. Therefore, efficient Lamb wave reflectors can be realized with relatively few strips in the reflector. For example, reflection coefficient very close to unity, R approximately 0.98, has been obtained in a Lamb wave reflector consisting of just 12 strips on a Y-X lithium niobate plate. The reflector has been used to realize a unidirectional transducer (UDT). A Lamb wave delay line consisting of two UDTs shows insertion loss of less than 3 dB with fractional bandwidth greater than 7%