BST铁电薄膜压控微波器件

作为一种典型的钙钛矿结构铁电体,BST具有极强的非线性介电性质,其介电常数随外加直流偏压变化,同时BST薄膜易于与微带共面波导等电路集成,能有效减小电路体积,因此可广泛应用于压控变容器、滤波器、移相器、振荡器等微波器件中。该类器件调谐速度极快,损耗低,工作频带宽,功率承载能力强,可调性好。介绍了BST薄膜的非线性介电性质与其在微波应用中的国外研究现状。     

BST薄膜非线性特性及机理研究

钛酸锶钡(BaxSr1-xTiO3,简称BST)薄膜具有非线性强、漏电流小、居里温度可调等特点,因而在介质移相器、压控滤波器等方面有着广泛的应用前景。笔者对BaxSr1-xTiO3(x=0.45~0.90)系列陶瓷的晶体结构和介电性能、膜厚均匀性控制、BST薄膜的微结构(包括组成、晶体结构和电畴等)和介电非线性尺度效应、铁电薄膜介电非线性模型、薄膜型介质移相器的设计和制作等重要问题进行了研究,取得了以下研究结果:通过研究靶基中心不重合的磁控溅射系统,建立了平面磁控溅射膜厚分布的数学模型,提出了采用T=∫Ld(x,y)ds=∫0td(ξ(t),ψ(t))ξ'2(t)+ψ'2(t)dt来描述靶基中心不重合的平面磁控溅射的膜厚分布情况。在靶基距为72mm、公自转转速比为5.3的条件下,采用自行设计的射频磁控溅射设备和φ100mm靶材制备的BST薄膜膜厚均匀性偏差为2.8%。采用压电力显微镜(PFM)研究了BST薄膜中电畴的类型和尺寸。不仅证实BST薄膜中存在90°铁电畴,而且确定了BST薄膜由多畴转变为单畴结构的晶粒临界尺寸(单畴临界尺寸)为31nm左右。通过研究BST薄膜的介电非线性尺度效应,发现晶粒尺寸和膜厚对薄膜的介电非线性具有重要的作用。随着晶粒尺寸和膜厚的增加,BST薄膜的介电系数、介电系数变化率都逐渐增大。晶粒尺寸对单畴态薄膜的介电系数电压变化率和矫顽场强影响特别显著,而对多畴态薄膜的介电系数电压变化率和矫顽场强影响不明显。在上述实验研究的基础上,对铁电材料的介电非线性机理进行了研究。首先,从简单、实用的角度出发写出表征P(E)和ε(E)非线性的数学多项式,根据介电偏压特性曲线和电滞回线的特征值和连续性原理,给出边界条件和初始条件,解出表达式中的各项系数,从而建立了铁电晶体的介电非线性模型。然后,采用理想的晶粒–晶界几何模型,画出铁电陶瓷材料的等效电路,引入晶粒和晶界的大小,从而建立了铁电陶瓷的介电非线性模型。再采用理想二极管等效界面势垒,得到薄膜型平板电容器等效电路,引入膜厚和界面厚度两个尺度变量,从而建立了铁电薄膜的介电非线性模型。最后,利用文献的数据和曲线对模型进行了验证,模拟得到的曲线与实验得到的曲线变化趋势基本一致,表明该模型可以较好地解释铁电材料(包括陶瓷和薄膜)的介电非线性特性及其随晶粒大小、膜厚和界面厚度等尺度变化的规律。对薄膜型介质移相器的设计、制作和测试进行了研究。采用ADS和HFSS软件进行仿真,设计出了分布式电容负载型铁电薄膜介质移相器的电路结构和各部分的尺寸;采用改进的剥离工艺,制作出了电极线条整齐的铁电薄膜介质移相器;采用矢量网络分析仪,在频率为35GHz、控制电压为40V条件下,测得介质移相器的移相度为180°,插损为9.7dB。     

Optical properties of CdSe<Subscript>x</Subscript>Te<Subscript>1−x</Subscript> epitaxial films studied by spectroscopic ellipsometry

We have determined the dielectric constants for a series of CdSe_xTe_1−x thin films grown on Si substrates using a rotating-analyzer spectroscopic ellipsometer. Initially, the alloy concentration and the sample quality were determined using x-ray diffraction. A standard inversion technique was then used to obtain the dielectric constants from the measured ellipsometric spectra. Using these calculated absorption spectra, we were able to estimate the fundamental bandgap for these CdSe_xTe_1−x alloys. In addition, we also determined the dispersion of the indices of refraction as well as the critical points related to the higher-order electronic transitions for this alloy.     

Dielectric properties of low loss Ba<Subscript>6−3x</Subscript>Nd<Subscript>8+2x</Subscript>Ti<Subscript>18</Subscript>O<Subscript>54</Subscript> thin films prepared by pulsed laser deposition for microwave applications

Ba_6−3xNd_8+2xTi₁₈O₅₄ with x=0.25 (BNT-0.25, or simply, BNT) dielectric thin films with a thickness of 320 nm have been prepared on Pt-coated silicon substrates by pulsed laser deposition (PLD) at the substrate temperature of 650°C in 20 Pa oxygen ambient. X-ray analysis showed that the as-deposited films are amorphous and the films remain amorphous after a postannealing at 750°C for 30 min. The dielectric constant of the BNT films has been determined to be about 80 with a low loss tan δ of about 0.006 at 1 MHz. The capacitance-voltage (C-V), capacitance-frequency, and capacitance-temperature characteristics of a BNT capacitor with Pt top electrode were measured. A low leakage-current density of 4×10⁻⁶ A/cm² at 6 V was measured, and a preliminary discussion of the leakage-current mechanism is also given. It is proposed that amorphous BNT-0.25 thin films will be a potential dielectric material for microwave applications.     

BST薄膜铁电移相器研究进展

钛酸锶钡(BST)薄膜因其具有高的介电调谐量,相对低的损耗tgδ和快的开关速度,在微波移相器的应用中显示出巨大优势。介绍了改善BST薄膜的介电性能的有效方法,衬底材料的选择,以及BST薄膜铁电移相器的结构类型和研究进展。     

Comparison of Microwave Dielectric Properties of Ba0.6Sr0.4TiO3 Thin Films Grown on (100) LaAlO3 and (100) MgO Single-Crystal Substrates

The microwave properties of barium strontium titanate (Ba_0.6Sr_0.4TiO₃) thin films grown on (100) LaAlO₃ (LAO) and (100) MgO single-crystal substrates through the sol–gel technique were investigated. The interdigital capacitor (IDC) technique was used to measure the nonlinear dielectric properties in the frequency range from 1 GHz to 10 GHz. The results show that the Curie temperature, capacitance, and tunability of the films are strongly dependent upon the substrate. The film fabricated on the LaAlO₃ substrate has a higher tunability of 16.77% than that grown on the MgO substrate (～8.38%), measured at 10 GHz with an applied voltage of 35 V. The loss tangent is a linear function of the frequency in the microwave range, and the film grown on the MgO substrate has a lower loss tangent than that grown on the LAO substrate. This work reveals the great potential of Ba_0.6Sr_0.4TiO₃ (BST) films for application in tunable microwave devices.     

Crystallization behavior and ferroelectric properties of PbTiO<Subscript>3</Subscript>/Ba<Subscript>0.85</Subscript>Sr<Subscript>0.15</Subscript>TiO<Subscript>3</Subscript>/PbTiO<Subscript>3</Subscript> sandwich thin film on Pt/Ti/SiO<Subscript>2</Subscript>/Si substrates

A PbTiO₃/Ba_0.85Sr_0.15TiO₃/PbTiO₃ (PT/BST15/PT) sandwich thin film has been prepared on Pt/Ti/SiO₂/Si substrates by an improved sol-gel technique. It is found that such films under rapid thermal annealing at 700°C crystallize more favorably with the addition of a PbTiO₃ layer. They possess a pure, perovskite-phase structure with a random orientation. The polarization-electric field (P-E) hysteresis loop and current-voltage (I-V) characteristic curves reveal that a PT/BST15/PT film exhibits good ferroelectricity at room temperature. However, no sharp peak, only a weak maximum, is observed in the curves of the dielectric constant versus temperature. The dielectric constant, loss tangent, leakage current density at 20 kV/cm, remnant polarization, and coercive field of the PT/BST15/PT film are 438, 0.025, 1.3 × 10⁻⁶ Acm⁻², 2.46 μCcm⁻², and 41 kVcm⁻¹, respectively, at 25°C and 10 kHz. The PT/BST15/PT film is a candidate material for high sensitivity elements for uncooled, infrared, focal plane arrays (UFPAs) to be used at near ambient temperature.     

梯度多层BST薄膜介电性能研究

成分梯度多层钛酸锶钡薄膜具有较好的综合介电性能,包括适中的介电常数、高的介电调谐率、低的介质损耗及低的介电温度系数等,日益成为微波调制器件如移相器、滤波器、谐振器等的重要候选薄膜材料。就国内外近年来取得的重要成果进行了综述,对今后成分梯度多层BST薄膜的研究前景及方向进行了展望。     

铁电（Ba,Sr）TiO3及介电Bi2O3-ZnO-Nb2O5薄膜材料应用于电调谐微波器件研究进展

铁电钛酸锶钡BaxSr1-xTiO3(BST)是一种拥有十分优越铁电/介电性能的材料,在可调谐微波器件方面具有很好的应用前景。本文概括介绍了BST薄膜的研究意义、基本结构、薄膜的制备方法,并针对可调谐微波器件应用需求,详细探讨了通过掺杂、组分梯度变化、纳米铁电多层薄膜以及将铁电BST与新型介电Bi2O3-ZnO-Nb2O5(BZN)薄膜相结合等对铁电薄膜性能进行优化的手段,最后对该领域的前沿问题从材料研究层面作了小结与展望。     

Effect of Rapid Thermal Annealing on Sputtered CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> Thin Films

Calcium copper titanium oxide (CaCu₃Ti₄O₁₂, abbreviated to CCTO) films were deposited on Pt/Ti/SiO₂/Si substrates at room temperature (RT) by radiofrequency magnetron sputtering. As-deposited CCTO films were treated by rapid thermal annealing (RTA) at various temperatures and in various atmospheres. X-ray diffraction patterns and scanning electron microscope (SEM) images demonstrated that the crystalline structures and surface morphologies of CCTO thin films were sensitive to the annealing temperature and ambient atmosphere. Polycrystalline CCTO films could be obtained when the annealing temperature was 700°C in air, and the grain size increased signifi- cantly with annealing in O₂. The 0.8-μm CCTO thin film that was deposited at RT for 2 h and then annealed at 700°C in O₂ exhibited a high dielectric constant (ε′) of 410, a dielectric loss (tan δ) of 0.17 (at 10 kHz), and a leakage current density (J) of 1.28 × 10⁻⁵ A/cm² (at 25 kV/cm).     

High K Capacitors and OFET Gate Dielectrics from Self‐Assembled BaTiO3 and (Ba,Sr)TiO3 Nanocrystals in the Superparaelectric Limit

Nanodielectrics is an emerging field with applications in capacitors, gate dielectrics, energy storage, alternatives to Li‐ion batteries, and frequency modulation in communications devices. Self‐assembly of high k dielectric nanoparticles is a highly attractive means to produce nanostructured films with improved performance—namely dielectric tunability, low leakage, and low loss—as a function of size, composition, and structure. One of the major challenges is conversion of the nanoparticle building block into a reliable thin film device at conditions consistent with integrated device manufacturing or plastic electronics. Here, the development of BaTiO₃ and (Ba,Sr)TiO₃ superparaelectric uniform nanocrystal (8–12 nm) films prepared at room temperature by evaporative driven assembly with no annealing step is reported. Thin film inorganic and polymer composite capacitors show dielectric constants in the tunable range of 10–30, dependent on composition, and are confirmed to be superparaelectric. Organic thin film transistor (TFT) devices on flexible substrates demonstrate the readiness of nanoparticle‐assembled films as gate dielectrics in device fabrication.     

Low-Dielectric-Loss Barium Strontium Titanate Thin Films with MgO Buffer Layer for Tunable Microwave Devices

The dielectric and microwave properties of Ba_0.6Sr_0.4TiO₃ (BST60) thin films with a MgO buffer layer deposited on Al₂O₃ substrates were investigated. Insertion of the MgO buffer layer is demonstrated to be an effective approach to fabricate low-dielectric-loss BST thin films. x-Ray pattern analysis indicates that the thin films exhibit good crystalline quality with a pure perovskite phase and that insertion of the MgO buffer layer does not change the crystal structure of BST. The nonlinear dielectric properties of the BST films were measured by using an interdigital capacitor (IDC). At room temperature, the tunability of the BST films with a MgO buffer layer was 24.1% at a frequency of 1 MHz with an applied electric field of 80 kV/cm. The dielectric loss of the BST thin films is only 0.005 to 0.007 in the frequency range from 20 Hz to 2 MHz, the same as for BST films prepared on single-crystal MgO substrates. The microwave dielectric properties of the BST thin films were also measured by a vector network analyzer from 50 MHz to 10 GHz.     

现代介质薄膜

介质薄膜是一类重要的电子薄膜。现代常用的介质薄膜种类很多，按其成分可分为无机薄膜、有机薄膜和复合薄膜三类。同一种介质薄膜当制造工艺略有不同时，就会出现性能差别较大的结果，且随着膜厚的减小，这种现象表现得越来越明显。现在介质薄膜的进展可归纳为：（１）成膜技术的提高，其工艺特点是精细、程控和准确；（２）薄膜下限厚度的减小，如陶瓷薄膜已降到１μｍ，大量生产的高分子聚合物薄膜已降至１００ｎｍ；（３）涌现出了不少新型介质薄膜。从８０年代以来兴起了对纳米级介质薄膜的研究热潮     

Investigations of Indium Tin Oxide—Barium Strontium Titanate–Indium Tin Oxide Heterostructure for Tunability

This letter reports for the first time the interesting behavior of the interface between indium tin oxide (ITO) as a high resistive electrode and barium strontium titanate tunable paraelectric thin film material. The interface is consisting of barium strontium titanate (BST) thin film dielectric material sandwiched between two ITO high resistive layers, all are integrated above glass substrate. When dc field is applied between the ITO layers, the BST thin film material properties are tuned. It is found that the ITO/BST/ITO heterostructure exhibits a tunable resistor performance. To our knowledge; these results are never reported.     

Microwave Dielectric Properties of ZnO-2TiO<Subscript>2</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> Ceramics with BaCu (B<Subscript>2</Subscript>O<Subscript>5</Subscript>) Addition

The influence of BaCu(B₂O₅) (BCB) addition on the sintering temperature and microwave dielectric properties of ZnO-2TiO₂-Nb₂O₅ (ZTN) ceramic has been investigated using dilatometry, x-ray diffraction, scanning electron microscopy, and microwave dielectric measurements. A small amount of BCB addition to ZTN can lower the sintering temperature from 1100°C to 900°C. The reduced sintering temperature was attributed to the formation of the BCB liquid phase. The ZTN ceramics containing 3.0 wt.% BCB sintered at 900°C for 2 h have good microwave dielectric properties of Q × f = 19,002 GHz (at 6.48 GHz), ε _r = 45.8 and τ _f  = 23.2 ppm/°C, which suggests that the ceramics can be applied in multilayer microwave devices, provided that Ag compatibility exists.     

Structure and dielectric properties of Ba<Subscript>1−x</Subscript>Sr<Subscript>x</Subscript>ZnTi<Subscript>7</Subscript>O<Subscript>16</Subscript> hollandite ceramics

Hollandite-type Ba_1−xSr_xZnTi₇O₁₆ (x=0, 0.2, 0.4, 0.6, 0.8, and 1) ceramics have been synthesized by the conventional solid-state ceramic route. The phase purity and microstructure of these compositions have been characterized using x-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD analysis shows that an increase in strontium concentration in the A-site causes pairing of vacant tunnel sites, and hence the structure becomes unstable due to the collapse of the tunnel walls. The dielectric properties such as dielectric constant (ε_r), loss tangent (tan δ), and temperature variation of dielectric constant (τε_r) have been measured up to the 13 MHz region. The present study shows that zinc hollandites have relatively high dielectric constant and low loss tangent. The temperature variation of dielectric constant studies reveal that Ba-rich compositions have high positive τε_r and Sr-rich compositions have high negative τε_r in the 0–100°C region. Proper tailor making of these compositions has been attempted to arrive at near-zero temperature variation of the dielectric constant.     

A Review on Dielectric Breakdown in Thin Dielectrics: Silicon Dioxide,High‐k,and Layered Dielectrics

Thin dielectric films are essential components of most micro‐ and nanoelectronic devices, and they have played a key role in the huge development that the semiconductor industry has experienced during the last 50 years. Guaranteeing the reliability of thin dielectric films has become more challenging, in light of strong demand from the market for improved performance in electronic devices. The degradation and breakdown of thin dielectrics under normal device operation has an enormous technological importance and thus it is widely investigated in traditional dielectrics (e.g., SiO₂, HfO₂, and Al₂O₃), and it should be further investigated in novel dielectric materials that might be used in future devices (e.g., layered dielectrics). Understanding not only the physical phenomena behind dielectric breakdown but also its statistics is crucial to ensure the reliability of modern and future electronic devices, and it can also be cleverly used for other applications, such as the fabrication of new‐concept resistive switching devices (e.g., nonvolatile memories and electronic synapses). Here, the fundamentals of the dielectric breakdown phenomenon in traditional and future thin dielectrics are revised. The physical phenomena that trigger the onset, structural damage, breakdown statistics, device reliability, technological implications, and perspectives are described.     

Electrical characteristics and TDDB breakdown mechanism of N2-RTA-treated Hf-based high-κ gate dielectrics

This study investigates the effects of rapid thermal annealing (RTA) in nitrogen ambient on HfO₂ and HfSiO_x gate dielectrics, including their electrical characteristics, film properties, TDDB reliability and breakdown mechanism. The optimal temperature for N₂ RTA treatment is also investigated. The positive oxide trap charges (oxygen vacancies) in HfO₂ and HfSiO_x dielectric films can be reduced by the thermal annealing, but as the annealing temperature increased, many positive oxide trap charges (oxygen vacancies) with shallow or deep trap energy level will be formed in the grain boundaries, degrading the electrical characteristics, and changing the breakdown mechanism. We believe that variation in the number of positive oxide trap charges (oxygen vacancies) with shallow or deep trap energy levels is the main cause of the CV shift and difference in the breakdown behaviors between HfO₂ and HfSiO_x dielectrics. With respect to CV characteristics and TDDB reliability, the optimal temperature for N₂ RTA treatment is in the range 500-600 °C and 800-900 °C, respectively.     

Optical and Dielectric Properties and Microstructures of rf-Magnetron Sputtered ZnO-Doped Zr<Subscript>0.8</Subscript>Sn<Subscript>0.2</Subscript>TiO<Subscript>4</Subscript> Films on Indium Tin Oxide/Glass Substrates

Optical and dielectric properties and microstructures of ZnO-doped (Zr_0.8Sn_0.2)TiO₄ thin films prepared by radiofrequency (rf)-magnetron sputtering on indium tin oxide/glass substrates at different rf powers and substrate temperatures have been investigated. Selected-area diffraction patterns showed that the deposited films exhibited a polycrystalline microstructure. All films exhibited the ZnO-doped (Zr_0.8Sn_0.2)TiO₄ structure with the (111) orientation perpendicular to the substrate surface. The grain size as well as the deposition rate of the film increased with an increase in both rf power and substrate temperature. At an annealing temperature of 700°C, the ZnO-doped (Zr_0.8Sn_0.2)TiO₄ film possessed a dielectric constant of 47 at 10 MHz, a dissipation factor of 0.02 at 10 MHz, a leakage current density of 7.35 × 10⁻⁹  A/cm² at an electrical field of 1 kV/cm, average transmission in the visible range of over 70%, and an optical bandgap of 3.6 eV. This film will allow fabrication of fully transparent semiconductor devices such as a resistive random-access memory (RRAM) and thin-film transistors (TFTs) completely based on ZnO-doped (Zr_0.8Sn_0.2)TiO₄ thin films.     

The indices of refraction of molecular-beam epitaxy-grown Be<Subscript>x</Subscript>Zn<Subscript>1−x</Subscript>Te ternary alloys

We have used a combination of prism-coupling, reflectivity, and ellipsometric techniques to investigate the indices of refraction, n, of a series of Be_xZn_1−xTe thin films grown on InP substrates. After determining the concentrations of each of the Be_xZn_1−xTe alloys using x-ray diffraction measurements, we measured their n at discrete wavelengths using a prism-coupler setup. In addition, we used reflectivity measurements to complement the prism-coupler data and arrive at the dispersion relations of n for the Be_xZn_1−xTe alloys below their fundamental energy gaps. We then employed a rotating analyzer-spectroscopic ellipsometer to measure the complex reflection ratio for each of the films at angles of incidence of 65°, 70°, and 75°. By using the n values obtained from both the prism-coupler and the reflection-spectroscopy techniques to guide the ellipsometric analysis, we were able to obtain accurate results for the dispersion of n for the Be_xZn_1−xTe alloys, not only below their fundamental energy gaps, but also above their energy gaps (up to 6.5 eV) using these three complementary techniques.