BST Thin Films Research Articles

Low-temperature crystallization and electrical properties of BST thin films using excimer laser annealing

(Ba,Sr)TiO 3 (BST) thin film with a perovskite structure has a potential for the practical applications in various functional devices. Normally, the BST thin films derived from sol–gel and sputtering are amorphous or partially crystalline when processed below 700 °C. For the purpose of integrating BST thin film directly into system-on-package (SoP), it is necessary to process the BST film below 350 °C. The electrical properties of low-temperature crystallized BST film were studied in this paper. The BST thin films have been crystallized at 300 °C by excimer laser annealing (ELA). The BST films were exhibited in a single perovskite phase and have well-defined electrical properties such as high dielectric constant, low dielectric loss and low leakage current density. As a result, we demonstrated crystallized BST thin film which has permittivity of 143, dielectric loss of 0.028 and leakage current density of 0.9 μA/cm 2 below 300 °C.

Microstructure and properties of highly elongated (Ba 0.5Sr 0.5)TiO 3 thin films by applied electric field annealing

This study examined the effect of applied electric field annealing on the microstructure and microwave properties of Ba 0.5Sr 0.5TiO 3 (BST) thin films grown on LaAlO 3 substrates. The ultraviolet–visible transmission spectra and micro-Raman spectra of these BST thin films revealed the band-gap and lattice vibration to be strongly dependent on annealing with an applied electric field. Based on the structural analyses and microwave properties, the variation in the band-gap, the chemical binding states, and the tunability of the films were attributed to the combined effects of applied electric field annealing in highly elongated BST thin films.

Electrical and Structural Diagnostics of Barium Strontium Titanate (BST) Thin Films

ABSTRACTThe properties of radio frequency, rf magnetron sputtered Barium Strontium Titanate (Ba1-xSrxTiO3), BST, thin films were investigated and compared with BST thin films deposited by sol-gel method with the aim of determining relationships between the oxide deposition parameters, the film structure, and the electric field dependence. This work presents noncontact electrical characterization of BST films using Corona Kelvin metrology (C-KM) which has been employed earlier only in the silicon industry. The films were structurally characterized using thickness profilometer, X-ray diffraction (XRD) and atomic force microscopy (AFM) techniques. The use of sol-gel technique to fabricate small area metal-insulator-metal (MIM) structures is found to be beneficial from the point of saving fabrication time and production costs.

ZrO2 Layer Thickness Dependent Electrical and Dielectric Properties of BST/ZrO2/BST Multilayer Thin Films

ABSTRACTRecently, high K materials play an important role in microelectronic devices such as capacitors, memory devices, and microwave devices. Now a days ferroelectric barium strontium titanate [BaxSr1-xTiO3, (BST)] thin film is being actively investigated for applications in dynamic random access memories (DRAM), field effect transistor (FET), and tunable devices because of its properties such as high dielectric constant, low leakage current, low dielectric loss, and high dielectric breakdown strength. Several approaches have been used to optimize the dielectric and electrical properties of BST thin films such as doping, graded compositions, and multilayer structures. We have found that inserting a ZrO2 layer in between two BST layers results in a significant reduction in dielectric constant, loss tangent, and leakage current in the multilayer thin films. Also it is shown that the properties of multilayer structure are found to depend strongly on the sublayer thicknesses. In this work the effect of ZrO2 layer thickness on the dielectric, ferroelectric as well as electrical properties of BST/ZrO2/BST multilayer structure is studied. The multilayer Ba0.8Sr0.2TiO3/ZrO2/Ba0.8Sr0.2TiO3 film is deposited by a sol-gel process on the platinized Si substrate. The thickness of the middle ZrO2 layer is varied while keeping the top and bottom BST layer thickness as fixed. It is observed that the dielectric constant, dielectric loss tangent, and leakage current of the multilayer films reduce with the increase of ZrO2 layer thickness and hence suitable for memory device applications. The ferroelectric properties of the multilayer film also decrease with the ZrO2 layer thickness.

Novel Multi-Capacitor Architecture for BST Thin Film Varactors

Novel Barium Strontium Titanate (BST) thin film varactors with parallel capacitor architecture were designed fabricated and tested. Varactors with 2–6 parallel capacitors were experimentally verified using large area processed bst thin films. The multicapacitor varactor shunt switches provided improved isolation at the expense of higher insertion loss. The multicapacitor varactors yielded reduced overall parasitic series inductance and resistances, resulting in higher quality factor Q. This is the first time such a device is reported.

Dielectric Properties of Nanocrystalline Ba0.6Sr0.4TiO3 Thin Films Prepared by a Modified Sol-Gel Process

Barium strontium titanate (Ba0.6Sr0.4TiO3) nanocrystalline thin film fabricated by a modified sol-gel process was crystallized at low temperature 550°C. The microstructure displayed a polycrystalline nanostructure. The BST thin film annealed at 550°C show good dielectric properties. The room temperature dielectric constant and dielectric loss were, respectively 335 and 0.0447 with no bias, while 263 and 0.0379 with dc bias of 10 V at 100 kHz. The film shows typical frequency dispersion at low frequency. The paraelectric nature of the BST thin film was indicated by non-linear field dependence capacitance-voltage curve with the absence of butterfly-shaped C-V curve. The tunability and figure of merit of the BST thin film are calculated to be 21.44% and 5.66 respectively, at an applied voltage of 10 V.

Low Temperature Deposited BST Thin Films for RF MEMS Switch

BST thin films of optimized target composition of Ba0.6Sr0.4TiO3 have been deposited on Cr-Au coated Si substrate by Pulsed Laser Deposition technique with low substrate temperature of 100°C using different laser energies (250 mJ, 300 mJ and 350 mJ). The structural, morphological and electrical studies have been carried out for all the films. The value of dielectric constant and dielectric loss for the film deposited with different laser energies were in the range of 18–20 and 0.04–0.07 respectively. The thickness of the film was 0.4 μm. All the films show higher resistivity (∼107Ω-cm) and high dielectric breakdown strength (∼500 kV/cm). The observed results suggest that the low temperature deposited BST thin films are suitable for RF MEMS Switch application.

Co-doped BST thin films for tunable microwave applications

Ba 0.5Sr 0.5Ti 1.0O 3.0 ferroelectric thin films with different concentrations of Co ion dopant (undoped, 0.25 mol%, 0.5 mol% and 1.0 mol%) were deposited on sapphire < 0001> substrates by the chemical solution deposition (CSD) method. The scanning electron microscopy (SEM) showed that increasing Co doping concentration would cause a decrease in grain size while the thin film surface remains crack-free. According to the X-ray diffraction results, all sample films were composed of cubic perovskite structures. In order to characterize the dielectric properties of the films, coplanar waveguides (CPW) were fabricated on top of the thin films using the photolithography process. The dielectric properties of the Co-doped BST films were found to be strongly dependent on the Co concentration. As the Co content increases, the dielectric loss tangent (tan δ) of the BST films at 12 GHz decreases from 0.17 to 0.038. The improvement of dielectric loss could be related to the decrease in the concentrations of intrinsic oxygen vacancies, thus lowering the electron concentration. However, the increase of Co concentration also resulted in the decrease of BST's dielectric constant ( ε r ) and tunability, leading to a larger space consumption of BST thin film based microwave devices.

Linearity and temperature dependence of large-area processed high-q barium strontium titanate thin-film varactors [Correspondence

Ba(0.6)Sr(0.4)TiO(3) (BST) thin-films with large dielectric tunability as high as 4:1 were obtained using a large-area pulsed laser deposition process, with low loss-tangents below 0.01 at zero-bias and 10 GHz. This paper summarizes experimental results obtained on large-area processed BST thin films on 100-mm-diameter sapphire substrates characterized using a varactor shunt switch test structure. Varactors with 0.25-mumthick BST films exhibited large dielectric tunability, the relative dielectric permittivity at zero bias of 990 tuned to 250 at an electric field of 320 kV/cm. The leakage current through the BST film was below 2 nA up to 6 V dc bias. The quality factor (Q) exceeded 300 at relatively low 6 V dc bias for the BST varactors at 1 GHz. These results confirm that large-area processed BST thin films are ready to compete with semiconductor varactors for commercial applications at RF, microwave, and millimeterwave frequencies.

Effect of Metal-Dielectric Interfaces on Tunability in Barium Strontium Titanate Thin-Film Capacitor

We investigated an influence of interface layer on a tunability of parallel plate (Ba, Sr)TiO3 thin film capacitors. BST thin film capacitors with top electrode of Pt, Au and Ag were fabricated. BST films had thickness of 40, 60, 80 and 120nm. The tunability increased with increasing the BST film thickness. Considering the interfaces between BST films and electrodes as Schottky junctions, depletion layers were formed in the interfaces depending on the difference of the work function of metal electrodes. Larger external bias voltages were applied to the depletion layer than interior BST film, because the permittivity in the depletion layer was smaller than that in interior BST film. Therefore, the depletion layer lowered the tunability. Tunability decreased with increasing the thickness of the depletion layers.

Characterization of doped BST thin films deposited by sol-gel for tunable microwave devices

BST thin films with various dopants were grown by the sol-gel method on platinized silicon and MgO substrates. Their dielectric properties were investigated at low frequency (up to 1 MHz) on silicon with parallel-plate capacitors and at high frequency (up to 15 GHz) with interdigitated capacitors on MgO substrate. The results depend on the nature of the dopant and show that Mg is a very good candidate to reduce dielectric losses. On the other hand, K is a good candidate as dopant of BST thin film to drastically increase the tunability.

Preparation and characterizations of Ba(Zr,Ti)O 3/(Ba,Sr)TiO 3 heterostructures grown on (LaAlO 3) 0.3(Sr 2AlTaO 6) 0.35 single crystal substrates by pulsed laser deposition

Epitaxial perovskite (Ba 0.7Sr 0.3)TiO 3 (BST), Ba(Zr 0.2Ti 0.8)O 3 (BZT), Ba(Zr 0.2Ti 0.8)O 3/(Ba 0.7Sr 0.3)TiO 3(BZT/BST) and Ba 0.85Sr 0.15Zr 0.1Ti 0.9O 3 (BSZT) thin films were grown on (LaAlO 3) 0.3(Sr 2AlTaO 6) 0.35 (LSAT)[001]single crystal substrate coated with conducting oxide LaNiO 3(LNO) by pulsed laser deposition. X-ray diffraction, atomic force microscopy and electrical measurements were used to characterize the thin films. The θ–2 θ linear scan indicated that the thin films with LNO layer show (001) orientations perpendicular to the substrate. Smooth and dense surface with grain size of 30 nm–80 nm were observed. The relative permittivity of the BST, BZT/BST, BZT and BSZT thin films are 1101, 799, 584 and 1784 respectively at 10 kHz. The tunability of the thin films are 39%, 32%, 44% and 60%. The results show that BSZT film has the highest tunability.

Tunable thin film bulk acoustic wave resonator based on BaxSr1-xTiO3 thin film

A tunable membrane-type thin film bulk acoustic wave resonator (TFBAR) based on a Ba(0.3)Sr(0.7)TiO(3)(BST) thin film has been fabricated. The resonance and antiresonance frequencies of the device can be altered by applying a dc bias: both shift down with increasing dc electric field. The resonance and antiresonance frequencies showed a tuning of -2.4% and -0.6%, respectively, at a maximum dc electric field of 615 kV/cm. The electromechanical coupling factor of the device increased up to 4.4%. We demonstrate that the tuning of the TFBAR is nonhysteretic. The Q-factor of the device showed some variation with dc bias and is about 200. The tuning of the TFBAR is caused by the dc bias dependence of the sound velocity and the intrinsic electromechanical coupling factor of the BST layer. We apply our recently developed theory on the electrical tuning of dc bias induced acoustic resonances in paraelectric thin films to successfully model the tuning behavior of the TFBAR. The modeling enabled us to de-embed the intrinsic electromechanical properties of the BST thin film. We show that the mechanical load of our device does not significantly degrade the tuning performance of the BST layer. The performance of the TFBAR is compared with the available data on varactor tuned TFBARs.

Investigation on the dielectric properties of (Ba, Sr)TiO 3 thin films on hybrid electrodes

Ba 0.65Sr 0.35TiO 3 (BST) thin films were deposited on Pt/Ti/SiO 2/Si, RuO 2/SiO 2/Si and RuO 2/Pt/Ti/SiO 2/Si substrates by radio frequency magnetron sputtering technique. The effects of these bottom electrodes on the microstructure and dielectric properties of the BST thin films were investigated by using X-ray diffraction (XRD), atomic force microscope (AFM), transmission electron microscopy (TEM) and electrical measurements. The BST thin films on RuO 2/Pt hybrid bottom electrodes exhibit good crystalline and interfacial structure with a thinner transition layer. Dielectric measurement reveals that the films on RuO 2/Pt hybrid electrodes have comparable dielectric constant and loss tangent with the films on Pt electrode, and the dielectric tunability of BST films on RuO 2/Pt reaches 38.2%, which is higher than that of BST films on a single Pt or RuO 2 electrode. The BST thin films on RuO 2/Pt exhibit lower leakage current density by nearly two orders’ of magnitude than that on RuO 2 electrode. The higher tunability and lower leakage current of the films on RuO 2/Pt hybrid bottom electrodes are mainly attributed to the RuO 2 layer, which facilitates the nucleation and growth of BST films, and inhibits the interfacial diffusion between the BST films and bottom electrodes. The results show a potential for RuO 2/Pt hybrid electrodes replacing Pt electrode in microelectronic device applications.

Investigation of BST thin films deposited by RF magnetron sputtering in pure Argon

Ba 0.5Sr 0.5TiO 3 (BST) thin films were deposited by rf magnetron sputtering using a Ba 0.5Sr 0.5TiO 3 target in pure Argon on two electrodes (Pt and RuO 2) at room temperature. The interface formation between BST and bottom electrode (Pt or RuO 2) was investigated by XPS for thicknesses in the 1 to 50 nm range. The chemical composition of the BST layers can be modified by the electrode nature over the first five nanometers. A 1 h ex-situ annealing, under flowing oxygen at 600 °C, was necessary to obtain crystallized 150 nm thick BST films, as evidenced by XRD and TEM analysis.

Preparation of (Ba&lt;sub&gt;1-x&lt;/sub&gt;, Sr&lt;sub&gt;x&lt;/sub&gt;)TiO&lt;sub&gt;3&lt;/sub&gt; Thin Films on Glazed Alumina Substrate and Improvement of Temperature Dependence of Dielectric Properties

In this study, (Ba1-x,Srx)TiO3 thin films of various composition ratios were deposited on glazed Al2O3 substrates by the CSD method, and the temperature dependence of the dielectric property was analyzed. The dielectric property of the BST (80/20) thin film annealed at C was highly tunable, and the temperature dependence of the dielectric constant was stabilized at around room temperature. However, this BST (80/20) thin film exhibited intense ferroelectricity. Its tan intensified to 0.0511 in the high-frequency range (1 MHz). After the BST thin film was annealed at 600C, the grain size of the BST thin film was reduced to 40 nm and the ferroelectricity was alleviated. The dielectric constant, tan, and tunability of the BST thin film at 100 kHz were 158, 0.0170, and 39.6 (214 kV/cm, 6.0 V), respectively. The change rate of the dielectric constant, which indicates its temperature dependence, was ±7 or less in the range of -55 to 85C. Therefore, this BST thin film was considered to guarantee a wide operation temperature range for microwave tunable devices.

Preparation and characterization of La-doped Ba (1 − x) Sr xTiO 3 powders and thin films

Ba (1 − x) Sr x TiO 3 powders with different Ba/Sr ratios ( x = 0.10, 0.25, 0.40, 0.55, 0.70) and La-doped Ba 0.9Sr 0.1TiO 3·yLa powders ( y = 0.002, 0.004, 0.006, 0.008, 0.010) have been prepared by sol–gel technology using dehydrated barium-acetate, strontium-carbonate, lanthanum-nitrate, and titanium-isopropoxide as raw materials. The experimental results show that the dielectric properties of Ba (1 − x )Sr x TiO 3 powders depend on the Ba/Sr ratios. When the Sr fraction is 0.10, the dielectric constant is relatively higher and the dielectric loss is relatively lower, which are more than 2000 and less than 2.0 × 10 − 2 at 1000 Hz, respectively, the most important is that this kind of powder has better frequency stability. La-doping can increase the dielectric constant distinctly, but the dielectric loss can also be increased. Their dielectric properties at 1.0 × 10 3 Hz are better than those at 1.0 × 10 5 Hz. At 1.0 × 10 3 Hz the dielectric constant is much higher, while the dielectric loss is much lower. The dielectric constant of different La-doping contents is nearly 3.5 × 10 4 and the dielectric loss is less than 0.20 when La fraction is 0.008. The La-doped BST sample also has better frequency stability, especially at high frequency. La-doped BST thin films are successfully deposited on mild steel substrates by using plasma spray system with suspension precursors of Ba 0.90Sr 0.10TiO 3·0.8La powders. The XRD patterns of Ba 0.90Sr 0.10TiO 3 and Ba 0.90Sr 0.10TiO 3·0.8La powders are almost the same. No new peaks appear after La-doping, but the peaks move slightly to a larger degree, which indicates that the element La has entered the lattice of the Ba 0.90Sr 0.10TiO 3 and has made the constant of the crystal cell reduce. The XRD pattern of the thin films is just like that of the Ba 0.90Sr 0.10TiO 3·0.8La powders except a peak corresponding to Fe substrate. The SEM results show that the thin films have a uniform and smooth surface. The morphology of cross-section shows a columnar grain structure indicating smooth surface and uniform thickness of the film. The thickness of the film is about 15 um. The thin films obtained are expected to be prospective material for applications in tunable microwave devices.

Determination of refractive indices and thicknesses of Ba0.7Sr0.3TiO3 films with and without MgO (001) buffer layer on silicon substrate

Ba 0.7 Sr 0.3 TiO 3 (BST30) thin films, deposited on single crystal Si (100) substrates with or without a MgO(001) buffer layer by the sol-gel method, have been investigated for their optical properties with a visible reflection spectrophotometer. An improved simplex numerical optimization method based on the reflectance spectra fitting is utilized to obtain the refractive indices and thicknesses of the thin films. Two models for the refractive index variations of the films (with or without a MgO buffer layer) together with a resemble double-layer structure are used to describe the films. The crystallization and microstructure of the films are characterized by the field emission scanning electron microscopy, which indicates that the MgO(001) buffer layer can significantly hold up the interdiffusion between the BST30 layer and the Si substrate, acuminate the interfaces, and improve the crystallinity of the BST thin film deposited on it. Furthermore, due to the large difference between the refractive index of the BST30 thin film and that of the MgO(001) buffer layer, which results in a strong beam confinement, a desirable waveguide structure could be constructed.

BST Thin Film Deposited on Glass Substrate with TISI Nanowire Electrode by RF-Sputtering Method

Ferroelectric Ba 0.7 Sr 0.3 TiO 3 (BST) thin film is sputtered on the titanium silicide bottom layer with TiSi nanowires on glass substrate at room temperature and annealed at 450°C for 30 minutes. The phase structure, morphology, dielectric properties of the BST thin film are measured by X-ray diffraction, Field emission scanning electron microscopy, and Impedance Analyzer, respectively. The results show that the crystallinity of the BST thin film is perfect, and the capacitance is independent on the frequency between about 100 KHz and 8 MHz. The tunability is 52.1% with 9V applied dc bias voltage and the dielectric loss is below 0.09.

Effect of bottom electrodes on dielectric properties of high frequency Ba 0.5Sr 0.5TiO 3 parallel plate varactor

Three types of Ba 0.5Sr 0.5TiO 3 (BST) thin film parallel plate varactor with different bottom electrodes were fabricated. The bottom electrodes of three types of varactor were perovskite conducting oxide La 0.7Sr 0.3MnO 3 (LSMO), Pt and Au, respectively. Dielectric properties of the BST thin films were characterized in the frequency range from 10 MHz to 15 GHz. The microstructure of the BST thin films was investigated by X-ray diffraction and scanning electron microscope. The microstructural analysis shows that the BST thin films grown on LSMO and Pt bottom electrodes are polycrystalline textured with columnar grains. Dielectric measurement indicates that the BST thin film grown on LSMO bottom electrode has a maximum dielectric constant and a little higher loss tangent.