Microwave dielectric properties of the (BaxSr1−x)TiO3 thin films on alumina substrate

Barium strontium titanate, (Ba_xSr_1?x)TiO₃ (BST) thin films have been prepared on alumina substrate by sol–gel technique. The X-ray patterns analysis indicated that the thin films are perovskite and polycrystalline structure. The interdigital electrode with 140 nm thickness Au/Ti was fabricated on the film with the finger length of 80 μm, width of 10 μm and gaps of 5 μm. The temperature dependence of dielectric constant of the BST thin films in the range from ?50 °C to 50 °C was measured at 1 MHz. The dielectric properties of the BST thin films were measured by HP 8510C vector network analyzer from 50 MHz to 20 GHz.     

Tunable microwave devices using low-sintering-temperature screen-printed barium strontium titanate (BST) thick films

A tunable microwave phase shifter composed of a low-sintering-temperature, screen-printable barium strontium titanate (BST) film and silver metallization was fabricated on an alumina substrate and co-fired at 900 °C for 3 h. The dielectric properties of the films were characterized in a frequency range of 0.8–8 GHz using scattering parameter measurements and a quasi-static coplanar waveguide transmission line model. The temperature dependency of the films was measured through capacitance measurements in a frequency range of 0.5–2.5 GHz. The figure of merit (phase shift/dB of insertion loss) of the phase shifter was found to be 14.6 at 3 GHz with an applied bias field strength of 2.5 V/μm. The performance of the phase shifter is briefly discussed and compared with other phase shifters fabricated by direct screen-printing of BST films.     

A low-loss BST thin film on initial nucleation layer for micro and millimeter wave tunable phase shifter

Barium strontium titanate, (Ba,Sr)TiO₃ (BST), thin films have been deposited on (1 0 0)-oriented MgO substrate by combining preparation of initial layer by Pulsed Laser Deposition and main layer by Metal-Organic-Decomposition method. Films with an initial layer of 20, 30 and 40 nm thickness and final thickness of 400, 650 and 800 nm have been obtained. Physical and dielectric properties of the BST thin films have been characterized from the viewpoint of frequency-agile micro and millimeter circuit applications. The results reveal that Ba_0.6Sr_0.4TiO₃ thin films have a good crystallinity with characteristic orientation that is affected by the deposition conditions of the initial layer. Interdigital capacitor with a gap of 10 μm has been characterized and the dielectric loss and tunability are as low as 0.002–0.004 and 12%, respectively, at frequency of 1 MHz for the applied voltage from ∓40 to ±40 V. At microwave frequencies, classic-shaped coplanar waveguide lines formed on BST/(1 0 0) MgO were investigated. A differential phase shift of 18° was obtained at 20 GHz with insertion loss of about −2 dB at 60 V for Au/Cr interconnection. Finally, a three-stage LC-ladder-type phase shifter with variable capacitors of BST thin film has been fabricated considering the experimental results obtained for the classic-shaped coplanar waveguide lines and a maximum phase shift of 40° is obtained at 20 GHz and 60 V.     

Low-temperature fabrication of Ba1−xSrxTiO3 thin films with good dielectric properties on platinized silicon substrates

Ba_0.7Sr_0.3TiO₃ (BST) thin films 500 nm in thickness were prepared on technologically desirable Pt/TiO₂/SiO₂/Si(1 0 0) substrates by ion beam sputtering (IBS) and post-deposition annealing method. The effect of annealing temperature on the structural and dielectric properties of BST thin films was systematically investigated. A sharp transition in their tunable dielectric behaviours was observed in good agreement with the evolution of crystal structure from amorphous to crystalline phase. It was demonstrated that the perovskite phase could crystallize in BST films at a very low temperature, around 450 °C. The lowering of perovskite crystallization temperature in the BST films was explained in terms of the high energetic process nature of IBS technique. A high dielectric tunability of 42% at E (electric field intensity) = 500 kV/cm and a low loss tangent of 0.013 at zero bias were both obtained in the 450 °C-annealed film, thereby resulting in the highest figure-of-merit factor among all the different temperature annealed films. Moreover, the 450 °C-annealed film showed superior leakage current characteristics with a low leakage current density of about 10^?4 A/cm² at E = 800 kV/cm.     

Barium strontium titanate nanocrystalline thin films prepared by soft chemical method

Barium strontium titanate (Ba_0.65Sr_0.35TiO₃) nanocrystalline thin films, which were produced by the soft chemical method, were crystallized at low temperature using a domestic microwave oven. A SiC susceptor were used to absorb the microwave energy and rapidly transfer the heat to the film. Low microwave power and short time have been used. The films obtained are crack-free, well-adhered, and fully crystallized. The microstructure displayed a polycrystalline nature with nanograin size. The metal-BST-metal structure of the thin films treated at 700 °C show good electric properties. The ferroelectric nature of the BST35 thin film was indicated by butterfly-shaped C–V curves. The capacitance–frequency curves reveal that the dielectric constant may reach a value up to 800 at 100 kHz. The dissipation factor was 0.01 at 100 kHz. The charge storage density as function of applied voltage graph showed that the charge storage densities are suitable for use in trench type 64 Mb (1–5 μC/cm²) and 265 Mb (2–11 μC/cm²) DRAMs.     

Thickness effects on microwave properties of (Ba,Sr)TiO3 films for frequency agile technologies

We fabricated (Ba_0.6Sr_0.4)TiO₃ (BST) thin films of various thicknesses on sapphire (−1 1 2 0) substrates using metal-organic decomposition method. These films showed grain growth from 160 to 650 nm with an increase in the thickness from 90 to 1050 nm. At microwave frequencies, the measured capacitances of the planar capacitors decreased with the film thickness because the electro-magnetic field propagates across high permittivity BST films to the low permittivity sapphire substrate. However, we found that the BST-thin film permittivity remained large up to 90 nm thick, based on electro-magnetic field analysis using the finite element method. On the other hand, the BST thin film tunability decreased with the film thickness.     

Fabrication of patterned Ba0.71Sr0.29TiO3 thick film on Si substrate by tape casting method

Patterned barium strontium titanate (BST) thick films are fabricated in the grooved silicon substrate using tape casting method and sintered from 800 to 1250 °C for 2 h. The slurry used for the tape casting is from sol-precipitation method, and the crystallization of the as precipitated BST powders is improved by hydrothermal treatment at 200 °C for 5 h. The patterned BST thick films have a size of 800 × 300 μm and homogeneous thickness of 30 μm. After sintering below 1000 °C, the obtained BST thick films have a dielectric abnormality at about 30 °C and the dielectric loss is about 0.02.     

Impedance spectroscopic studies of sol–gel derived barium strontium titanate thin films

In the present work electroceramic thin films of barium strontium titanate (Ba_1?xSr_xTiO₃ – BST) were deposited on stainless steel substrates by sol–gel technique. Homogeneous Ba_0.6Sr_0.4TiO₃ thin films as well as spatially inhomogeneous BST thin films exhibiting artificial gradients in composition normal to the growth surface were deposited. Both up- and down-graded BST films were fabricated by depositing successive layers with Sr mole fraction x ranging from x = 0.5 to x = 0.3. In the present study the tool of impedance spectroscopy has been used to study the dielectric properties of BST thin films at room temperature. To analyze the impedance spectroscopy data the Nyquist (Z″ vs. Z′) plots as well as the simultaneous representation of the imaginary part of impedance and electrical modulus (Z″, M″) vs. frequency were used. Experimental data were fitted using the CNLS fitting method. Agreement between experimental and simulated data was established. The data indicated that the thin film samples fabricated can be represented by an equivalent circuit with two relaxation frequencies.     

Dielectric enhancement of BaSrTi1.1O3/BaSrTi1.05O3/BaSrTiO3 multilayer thin films prepared by RF magnetron sputtering

BaSrTi_1.1O₃/BaSrTi_1.05O₃/BaSrTiO₃ multilayer (BSTM) thin films and BaSrTiO₃ (BST) thin films were deposited on LaNiO₃ (LNO)/SiO₂/Si substrates by radio frequency (RF) magnetron sputtering at substrate temperature of 400 °C, respectively. X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) investigations revealed that all the films have uniform and crack-free surface morphology with a perovskite structure. The dielectric constant of the BSTM thin films was increased and dielectric loss was decreased compared with those of uniform BST thin films. The dielectric constant of 420, dielectric loss of 0.017, and dielectric tunability of 38% were achieved for the BSTM thin films.     

Microwave dielectric properties of bismuth zinc niobate thin films deposited on alumina by pulsed laser deposition

Bi₂Zn_2/3Nb_4/3O₇ thin films were prepared on Al₂O₃ substrates by pulsed laser deposition. The phase compositions and microstructures were characterized by X-ray diffraction and atomic force microscopy. The as-deposited films were all amorphous in nature. All films were crystallized after the post annealing at the temperature range of 700–900 °C for 30 min in air. The texture characteristics change with annealing temperature. A split post dielectric resonator method was used to measure the microwave dielectric performance at the resonant frequencies of 10, 15 and 19 GHz. For the films annealed at 900 °C, the preferential orientation is similar to the monoclinic BZN bulk. The microwave dielectric constants at 10, 15 and 19 GHz are 69.4, 58.9 and 47.9, respectively, which are closer to these of the monoclinic BZN bulk.     

Anti-reduction of Ti4+ in Ba4.2Sm9.2Ti18O54 ceramics by doping with MgO,Al2O3 and MnO2

The anti-reduction of Ti⁴⁺ ions in Ba_4.2Sm_9.2Ti₁₈O₅₄ (BST) ceramics at high sintering temperature over 1300 °C was investigated. MgO, Al₂O₃ and MnO₂ were added separately to suppress the reduction of Ti⁴⁺ ions so as to improve the microwave dielectric properties of BST ceramics. The microstructure of BST ceramics was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) was used to study the electroconductivity of BST ceramics and valency changes of Ti ions. The results showed that MgO or Al₂O₃, when acting as an acceptor, could effectively suppress the reduction of Ti⁴⁺ ions and significantly improve the Q × f values of BST ceramics at the cost of dielectric constant. Meanwhile, MnO₂ as an oxidant had also improved the Q × f values but with no decrease in dielectric constant. Excellent microwave dielectric properties were achieved in Ba_4.2Sm_9.2Ti₁₈O₅₄ ceramics doped with 0.2 wt.% Al₂O₃ sintered at 1340 °C for 3 h: ?_r = 76.9, Q × f = 10,120 GHz and τ_f  = ?22.7 ppm/°C.     

Elimination of parasitic effects due to measurement conditions of SrTiO3 thin films up to 40 GHz

Effective use of electromagnetic simulation software for evaluation of the microwave properties of dielectric thin films was demonstrated. The reliability of the high-frequency dielectric properties extracted from the measured S₁₁ reflection coefficients with the aid of the electromagnetic simulation software is mainly limited by how accurately the measured parasitics are simulated. The need to correct the parasitic differences between the simulation and measurement was shown by the significant dependence of probe contact position on the obtained dielectric properties. The parasitic differences were represented by series and parallel correction admittances connected to the measured admittance and were effectively eliminated. The high-frequency dielectric properties of a highly crystalline SrTiO₃ (STO) thin film were investigated up to 40 GHz by using the measurement techniques developed. The permittivity (relative dielectric constant) of the STO thin film remained substantially constant at 265 up to 40 GHz, and the dielectric loss value was about 0.03 at 40 GHz.     

Effect of excess Bi2O3 on structures and dielectric properties of Bi1.5Zn1.0Nb1.5O7 thin films deposited at room temperature by RF magnetron sputtering

To compensate for bismuth loss that occurred during the film deposition process, Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) thin films were deposited at room temperature from the ceramic targets containing various excess amounts of bismuth (0–20 mol%) on Pt/TiO₂/SiO₂/Si substrates by using RF magnetron sputtering technique. The effect of bismuth excess content on the microstructure and electrical properties of BZN thin films was studied. The microstructure and chemical states of the thin films were studied by SEM and XPS. EPMA was employed to assess the film stoichiometry. The X-ray diffraction analysis reveals that the BZN thin films exhibit the amorphous structure in nature. An appropriate amount of excess bismuth improves the dielectric and electrical properties of BZN thin films, while too much excess bismuth leads to deterioration of the properties. BZN thin film with 5 mol% excess bismuth exhibits a dielectric constant of 61 with a loss of 0.4% at 10 kHz and leakage current of 7.26×10^?7 A/cm² at an electric field of 200 kV/cm.     

Effect of substrate temperature on structural and electrical properties of BaZr0.2Ti0.8O3 lead-free thin films by pulsed laser deposition

Barium zirconate titanate (BaZr_0.2Ti_0.8O₃, BZT) 250 nm thick thin films were fabricated by pulsed laser deposition and the influence of the substrate temperature on their preferred orientation, microstructure, morphology and dielectric properties was investigated. Dielectric measurements indicated the (1 1 0)-oriented BZT thin films deposited at 750 °C to show good dielectric properties with high dielectric constant (~500 at 100 kHz), low loss tangent (<0.01 at 100 kHz), and superior tunability (>70% at 400 kV/cm), while the largest figure of merit was 78.8. The possible microstructural background responsible for the high dielectric constant and tenability is discussed. In addition, thin films deposited at 750 °C with device quality factor of 8738 and dielectric nonlinearity coefficient of 1.66×10⁻¹⁰ J/C⁴m⁵ were demonstrated.     

Impact of microwave sintering on dielectric properties of screen printed Ba0.6Sr0.4TiO3 thick films

The influence of 30 GHz microwave sintering compared to conventional sintering has been investigated on polycrystalline Ba_0.6Sr_0.4TiO₃ (BST60) thick films with respect to an application as tunable dielectrics. The BST thick films were prepared as metal–insulator–metal (MIM) capacitors on alumina substrates. The average grain size (440 nm) and the porosity (approx. 30%) of the sintered films are only little affected by the sintering method. However, permittivity, dielectric loss and tunability have been influenced substantially. The dielectric improvement by microwave sintering is interpreted in terms of an increased crystal quality (ξ_S) and/or a decrease of defect concentrations. It is assumed that microwave sintering preferably heats up parts of the film where an increased defect density exists and therefore causes a selective heating process. This may heal up charged defects, inhomogeneities, and structural defects.     

Effects of composition and stacking sequence on dielectric properties of the multilayered (Pb,Sr)TiO3 thin films for tunable device application

Multilayered (Pb_1−xSr_x)TiO₃ (PST(x)) thin films consisted of uniform, PST(x) and heterostructure, PST(x)–PST80 were synthesized by coating the solutions with different Sr contents (50 ≤ x, Sr(mol%) ≤80), respectively. Their structural and dielectric properties were investigated in terms of composition and stacking sequence of each film. Among uniform PST(x) thin films, the PST60 films showed the highest dielectric constant and tunability, while so lower figure of merit which is an important parameter for microwave tunable device application was obtained due to relatively higher dielectric loss. In an effort to bring down the dielectric loss, the PST(x) thin films were alternately coated with PST80 thin layer. Dielectric properties of the heterostructured PST(x)–PST80 films were found to be dependent on the intrinsic dielectric values of each film composition and corresponding phase transition temperature shift effect. Furthermore, surface roughness became smoother by inserting PST80 thin layer, resulting in decrease in dielectric loss. In case of the PST60–PST80 heterostructured film, despite of slight decrease in tunability, the figure of merit on account of lowered dielectric loss was effectively improved (>40%), compared to that of the uniform PST60 film.     

Fabrication and properties of composites from BST and polypropylene-graft-poly(styrene-stat-divinylbenzene)

In this work, ceramic–polymer composites were fabricated from barium strontium titanate powder (BST) and polypropylene-graft-poly(styrene-stat-divinylbenzene) (ER) using a twin-screw extruder. The compounding process was characterized by rheological measurements. The effects of volume loading of BST on dielectric and mechanical properties were investigated. The dielectric properties were measured as a function of frequency and BST loading. For example, the relative permittivity and loss tangent (tan δ) of the BST–ER composites at 1 GHz were gradually increased from 2.4 and 0.0001 to 28.5 and 0.0085, respectively, as the loading was increased from 0 to 50.5 vol.%. Stearic acid (StA) was used as a surface-modifier of the BST. With an approximate surface coverage of 83%, an improvement in processability and a slight increase of the permittivity was observed, while tan δ remained low. The excellent dielectric characteristics of these composites, with high permittivity and low tan δ, make them attractive novel electronic materials for high frequency applications.     

Precise measurement of the dielectric properties of BaxSr1−xTiO3 thin films by on-wafer through-reflect-line (TRL) calibration method

Polycrystalline Ba_xSr_1−xTiO₃ (x = 0.3, 0.4, 0.5) (BST) thin films with a thickness of 200 nm were deposited on r-cut sapphire substrates by rf sputtering method. The permittivity and loss tangent of the films were successfully observed in the range of 1–3 GHz, by utilizing the on-wafer through-reflect-line (TRL) calibration method although the estimated relative permittivity depended on an applied power to waveguides and the loss tangent had the dispersion around 1 GHz even in the case of 2 μm-thick aluminum. Finally, we concluded that the BST thin film with x = 0.4 is the most suitable for microwave tunable devices because it had the lowest loss tangent and relatively high permittivity.     

Measurements of the surface resistance and conductivity of thin conductive films at frequency about 1 GHz employing dielectric resonator technique

A dielectric resonator technique has been developed for measurements of conductivity and surface resistance of thin metal films deposited on a dielectric substrate. This technique allows for measurements of films having surface resistances that are smaller than 5 Ω without requiring the need to perform measurements of the substrate thickness. The uncertainty of the surface resistance measurements is about 2–3% for both thin films and bulk materials. The accuracy of the conductivity measurements of the thin films is similar to the accuracy of the measurements of their thickness. Several samples have been measured having thicknesses that range from 66 nm to 50 μm.     

Dielectric properties and electrical behaviors of tunable Bi1.5MgNb1.5O7 thin films

The Bi_1.5MgNb_1.5O₇ (BMN) thin films were prepared on Au-coated Si substrates by rf magnetron sputtering. We systematically investigated the structure, dielectric properties and voltage tunable property of the films with different annealing temperatures. The relationships of leakage current and breakdown bias field with annealing temperature were firstly studied and a possible explanation was proposed. The deposited BMN thin films had a cubic pyrochlore phase when annealed at 550 °C or higher. With the increasing of annealing temperature, the dielectric constant and tunability also went up. BMN thin films annealed at 750 °C exhibited moderate dielectric constant of 106 and low dielectric loss of 0.003–0.007 between 10 kHz and 10 MHz. The maximum tunability of 50% was achieved at a bias field of 2 MV/cm. However, thin films annealed at 750 °C had lower breakdown bias field and higher leakage current density than films annealed below 750 °C. The excellent physical and electrical properties make BMN thin films promising for potential tunable capacitor applications.