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.     

Recent progress of (Ba,Sr)TiO<Subscript>3</Subscript> thin films for tunable microwave devices

The (Ba_1−xSr_x)TiO₃ (BST) ferroelectric thin films exhibit outstanding dielectric properties, even at high frequencies (>1 GHz), and large, electric-field dielectric tunability. This feature makes them suitable for developing a new class of tunable microwave devices. The dielectric properties and dielectric tuning property of BST thin films are closely related to the film compositions, substrate types, and post-deposition process. The successful implementation of BST films as high-frequency dielectrics in electrically tunable microwave devices requires a detailed understanding of both their processing and material properties. This paper will review the recent progress of BST thin films as active dielectrics for tunable microwave devices. The technical aspects of BST thin films, such as processing methods, post-annealing process, film compositions, film stress, oxygen defects, and interfacial structures between film and substrate, are briefly reviewed and discussed with specific samples from the recent literature. The major issues requiring additional investigations to improve the dielectric properties of BST thin films for tunable microwave applications are also discussed.     

Electrical properties of sputtered (Ba, Sr)TiO3 thin films prepared by two-step deposition method

The influence of two-step deposition on the electrical properties of sputtered (Ba,Sr)TiO₃ thin films was investigated. BST thin films with thickness 40 nm were deposited by a simple two-step radio frequency-magnetron sputtering technique, where the BST thin film consisted of a seed layer and a main layer. The dielectric constant was strongly dependent on the thickness of seed layer, but there was no dependence on deposition temperature of the seed layer. For a 2 nm seed layer, the dielectric constants were higher by about 29% than those of single-step BST thin films due to higher crystallinity and the leakage current was nearly the same as that of a single-step sample in bias voltage from −2 to 2.5 V. However, an improvement of the dielectric constant was not observed for samples having above 4 nm thick seed layers. A 40 nm thick BST film with 2 nm thick seed layer deposited by a two-step method exhibited a SiO₂ equivalent thickness of 0.385 nm and a leakage current density of 2.74 × 10⁻⁸A/cm²at+1.5V after post-annealing under an atmosphere of flowing N₂ for 30 min at 750°C.     

Thermo-Sensitive Ba0.64Sr0.36TiO3 Thin Film Capacitors for Dielectric Type Uncooled Infrared Sensors

Ba_0.64Sr_0.36TiO₃ (BST) thin films are prepared on Pt/Ti/SiO₂/Si₃N₄/SiO₂/Si substrates by a sol-gel method. Thermo-sensitive BST thin film capacitors with a Metal-Ferroelectrics-Metal (M-F(BST)-M) structure are fabricated as the active elements of dielectric type uncooled infrared sensors. XRD are employed to analyze the crystallographic structures of the films. AFM observations reveal a smooth and dense surface of the films with an average grain size of about 35 nm. Rapid temperature annealing (RTA) process is a very efficient way to improve crystallization quality. The preferable annealing temperature is 800°C for 1 min. The butterfly shaped C-V curves of the capacitors indicate the films have a ferroelectric nature. The dielectric constant and dielectric loss of the films at 100 kHz are 450 and 0.038, respectively. At 25°C, where the thermo-sensitive capacitors work, the temperature coefficient of dielectric constant (TCD) is about 5.9 %/°C. These results indicate that the capacitors with sol-gel derived BST thin films are promising to develop dielectric type uncooled infrared sensors.     

Integration of Ba/sub x/Sr/sub 1-x/TiO/sub 3/ thin films with AlGaN/GaN HEMT circuits

Ba/sub x/Sr/sub 1-x/TiO/sub 3/ (BST) thin films have large dielectric constants that can be varied by as much as a factor of 3 with an applied field, making them attractive for radio frequency (RF) circuits as small-area ac bypass/dc blocking capacitors, or high-power varactors. However, BST must be deposited at relatively high temperatures in an oxidizing environment, presenting significant integration challenges for MMIC applications. This letter describes the successful integration of BST films on AlGaN/GaN high electron-mobility transistor (HEMT) monolithic microwave integrated circuits on sapphire substrates. A sacrificial SiO/sub 2/ buffer layer is used to protect the underlying AlGaN during the RF magnetron sputtering of the BST film at an elevated temperature, with a carefully controlled heater ramp rate to avoid degradation of the ohmic contacts on the HEMT.     

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。     

The microwave properties of Li doped 0.7(Ba,Sr)TiO3-0.3MgO thick film interdigital capacitors on the alumina substrates

The crystalline and electrical properties of Li doped 0.7(Ba,Sr)TiO₃-0.3MgO thick film interdigital capacitors have been investigated. Screen printing method was employed to fabricate Li doped 0.7(Ba,Sr)TiO₃-0.3MgO thick films on the alumina substrates. (Ba,Sr)TiO₃ materials have high dielectric permittivity (>500 @ 1 MHz) and low loss tangent (0.01 @ 1 MHz) in the epitaxial thin film form. To improve dielectric properties and reduce sintering temperature, MgO and Li were added, respectively. 10 μm thick films were screen printed on the alumina substrates and then interdigital capacitors with seven fingers of 200 μm finger gap were patterned with Ag electrode. Current-voltage characteristics were analyzed with elevated temperature range. Up to 50 °C, the thick films showed positive temperature coefficient of resistivity (dρ/dT) of 6.11 × 10⁸ Ω cm/°C, then film showed negative temperature coefficient of resistivity (dρ/dT) of −1.74 × 10⁸ Ω cm/°C. From the microwave measurement, the relative dielectric permittivity of Li doped 0.7(Ba,Sr)TiO₃-0.3MgO thick films interdigital capacitors were between 313 at 1 GHz and 265 at 7 GHz.     

Coplanar Phase Shifters Based on Ferroelectric Thin Films

Barium strontium titanate (Ba_0.6Sr_0.4TiO₃, short for BST) thin films were prepared by RF-magnetron sputtering. The dielectric tunability, loss tangent, remanent polarization (2Pr) and coercive electric field (Ec) of the BST films are respectively 29.5%, 0.013, 2.29 μC/cm² and 22.27 kV/cm at 1 kHz and 20 V. The designed coplanar waveguide (CPW) phase shifter consists of 56 same sections. It is shown that the fabricated lines of electrodes are smooth and the widths at the tip of bottom electrodes are about 5 μm. At the central frequency of 28 GHz and a bias voltage of 20 V, the figure of merit is approximately 13°/dB.     

溶胶-凝胶法制备外延Ba1-xSrxTiO3薄膜及其结构与性能研究

应用溶胶-凝胶技术在Pt/MgO(100)衬底上成功地制备了Ba0.65Sr0.35TiO3外延薄膜.XRD和SEM分析结果表明该薄膜在O2气氛中650℃热处理1h后,其(001)面是沿着Pt(100)和MgO(100)面外延取向生长的;薄膜表面均匀致密,厚度为260nm,平均晶粒大小为48.5nm.当测试频率为10kHz时,BST薄膜的介电常数和损耗因子分别为480和0.02.介电常数-温度关系测试结果表明sol-gel工艺制备的Ba0.65Sr0.35TiO3薄膜其居里温度在35℃左右,且在该温度下Ba0.65Sr0.35TiO3薄膜存在扩散铁电相变特征.当外加偏置电压为3V时,BST薄膜的漏电流密度为1.5×10-7A/cm2.该薄膜可作为制备新型非制冷红外焦平面阵列和先进非制冷红外热像仪的优选材料.     

Low-Temperature Sintering of Ba0.5Sr0.5TiO3-SrMoO4 Dielectric Tunable Composite Ceramics for LTCC Applications

A sintering-aid system using melting of B-Li glass for barium strontium titanate (BST)-based compositions to be used in low-temperature cofired ceramic (LTCC) layers is introduced. The effects of the sintering aid on the microstructure, dielectric properties, and application in LTCC were investigated. The composition Ba_0.5Sr_0.5TiO₃-SrMoO₄ with 3 wt.% B-Li glass sintered at 950°C exhibits optimized dielectric properties, including low dielectric constant (368), low dielectric loss (0.007), and moderate tunability (13%, 60 kV/cm) at 10 kHz. At 1.44 GHz, it possesses a dielectric constant of 218 and Q value of 230. LTCC multilayer ceramic capacitors fabricated by the tape-casting process have steady relative tunability of 12% at 300 V, suggesting that BST50-SrMoO₄-B-Li glass composite ceramic is a promising candidate for electrically tunable LTCC microwave device applications.     

Bi_(1.5)Zn_(1.0)Nb_(1.5)O_7/Ba_(0.5)Sr_(0.5)TiO_3复合薄膜介电性能的频率特性研究

采用射频磁控溅射法在蓝宝石基片上制备了Bi1.5Zn1.0Nb1.5O7(BZN)/Ba0.5Sr0.5TiO3(BST)双层复合薄膜,并研究了该薄膜在100 kHz~6 GHz频率范围内的介电性能。研究结果表明,BZN/BST复合薄膜的介电性能具有良好的频率稳定性。该复合薄膜的介电常数在研究的频率范围内基本与频率无关;其介电损耗在频率低于1 GHz时与频率无关,在频率高于1 GHz时随频率的上升而略微增大;薄膜在研究的频率范围内具有稳定的介电调谐率。     

Fabrication and Properties of Novel Polyetheretherketone/Barium Titanate Composites with Low Dielectric Loss

Dielectric, thermal, and microhardness properties of high-performance barium titanate (BaTiO₃)-filled polyetheretherketone (PEEK) composites were studied. BaTiO₃ was varied from 0 vol.% to 67 vol.% in the PEEK matrix. The dielectric constant of the composites measured at 1 MHz increased approximately 14-fold. There was no dispersion in the dielectric constant with frequency between 10 kHz and 15 MHz. The Lichtenecker equation and modified Lichtenecker equation agreed well with the experimental data. The dissipation factor of the composites varied from 0.0056 to 0.0096. Scanning electron microscopy showed uniform dispersion of BaTiO₃ in the matrix. The microhardness of the composites increased by more than 2.5-fold compared with pure PEEK. The coefficient of thermal expansion measured below and above the glass-transition temperature was reduced by up to 56%. These results make these composites promising candidate high-temperature organic substrates.     

BST/LSCO异质结构的研究

利用脉冲激光沉积法在STO(001)基片上外延生长了La0.5Sr0.5CoO3(LSCO)导电氧化物薄膜,研究了基片温度对LSCO薄膜结构和电性能的影响,并制备了Ni-Cr/BST/LSCO多层膜结构。XRD谱发现,沉积温度在450~700℃均能得到高度(00 l)取向的LSCO薄膜,LSCO(002)峰的半高峰宽FWHM=0.1°~0.2°;在LSCO薄膜上制备的BST介质膜具有良好的c轴取向和较高的表面平整度,其εr约为470,tgδ为0.036~0.060。     

Thermal-sensitive BST thin film capacitors for dielectric bolometer prepared by RF magnetron sputtering

We have been developing a monolithic microbolometer technology for uncooled infrared focal plane arrays (Uncooled IRFPAs) along the route from fabricating pixels of thin-film dielectric bolometers on micromachined silicon substrates. In the paper, the thermal-sensitive barium strontium titanate (BST) thin film capacitors for that objective prepared by Radio-Frequency Magnetron sputtering have been investigated focusing on the condition of fabrication of BST thin films. Capacitor-Temperature properties of the thermal-sensitive BST thin film capacitors have been measured with impedance analyzer. According to the Capacitor-temperature curves, these indicated that the temperature coefficient of dielectric constant (TCD) within the ambient temperature region highly depended on the Radio-Frequency Magnetron sputtering condition of fabrication of BST thin films. BST thin film capacitors with TCD-value more than 21%/K have been prepared on the optimized condition. That is a good base for preparation of dielectric bolometer mode of uncooled IRFPAs.     

AC conductivity,dielectric relaxation and modulus behavior of Sb2S2O new kermesite alloy for optoelectronic applications

In this work, we present some physical properties of Sb₂S₂O thin films obtained through heat treatment of Sb₂S₃ thin films under an atmospheric pressure at 350 °C. The electrical conductivity, dielectric properties and relaxation model of these thin films were studied using impedance spectroscopy technique in the frequency range from 5 Hz to 13 MHz at various temperatures from 350 °C to 425 °C. Besides, the frequency and temperature dependence of the complex impedance, AC conductivity and complex electric modulus has been investigated.     

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.     

Characterization of pulsed laser deposited Ba0.6Sr0.4TiO3 on Pt-coated silicon substrates

The high dielectric constant (Ba,Sr)TiO₃ (BST) films have been widely used to realize capacitors integrated on silicon with a high value of capacitance. The multilayer Pt/Ti/SiO₂/Si is one of the most currently used bottom electrodes for the integration of BST on silicon. However, the crystal orientation and the dielectric properties of ferroelectric thin films are greatly influenced by the underlying Pt/Ti metallization, and particularly by the out-diffusion of titanium towards the platinum surface during thermal treatments. In this study, we show that the heating stage of the Pt/Ti/SiO₂/Si substrate before the BST pulsed laser deposition is of primary importance in both favoring the (1 1 1) growth of the BST material within a wide range of oxygen deposition pressure and in reducing drastically the loss tangent values of Al/BST/Pt capacitors because of the oxygen saturation of platinum. Electrical measurements indicate the existence of an interfacial layer degrading the capacitance. They support the presence of an interfacial depleted layer. The dramatic increase of the loss tangent under positive polarities applied on the platinum electrode is attributed to the ohmic contact of the BST/Al interface. Except this increase, all of the electrical properties are very promising in view to realize capacitors with high capacitance value and low dispersion.     

Preparation and characterization of thin films of MgO,Al2O3 and MgAl2O4 by atomic layer deposition

MgO, Al₂O₃ and MgAl₂O₄ thin films were deposited on silicon substrates at various temperatures by the atomic layer deposition (ALD) method using bis(cyclopentadienyl)magnesium, triethylaluminum, and H₂O and were characterized systematically. High-quality polycrystalline MgO films were deposited for a substrate temperature above 500°C, and amorphous thin films were deposited around 400°C. The deposited Al₂O₃ and MgAl₂O₄ thin films were characterized as amorphous in structure. Applicability of ALD to complex oxides is discussed.     

Effects of O2 plasma treatment on the electric and dielectric characteristics of Ba0.7Sr0.3TiO3 thin films

The effects of the O₂ plasma treatment on the electric and dielectric characteristics of Ba_0.7Sr_0.3TiO₃ (BST) thin films were investigated. As a result of the exposure of the as-deposited or the annealed BST films to the O₂ plasma, the leakage current density of the BST films can be improved. Typically, the leakage current density can decrease by three orders of magnitude as compared that of the non-plasma treated sample at an applied voltage of 1.5 V. It is found that the plasma treatment changes the surface morphology. The capacitance of the BST films was reduced by 10%30%. The improvement of the leakage current density and the reduction of a dielectric constant for the plasma treated samples could be attributed to the reduction of carbon contaminations of BST thin films. The 10 year life time of the time-dependent dielectric breakdown (TDDB) studies indicates that all the samples have a life time of over 10 years of operation at a voltage bias of 1 V.