Characterization of Ni/Ce Co-Doped BST Thin Film Grown on LSCO/Pt Electrodes Prepared by Pulsed Laser Deposition

Perovskite (Ba_0.6Sr_0.4)TiO₃ (BST) thin films doped with Ni, Ce and Ni/Ce codopants were prepared on LSCO/Pt/SiO₂/Si substrates by pulsed laser deposition method. In this study, La_0.5Sr_0.5CoO₃ (LSCO) bottom electrodes were used to improve the crystallinity and dielectric properties of BST films. Single ion doped(1 mol%Ni doped,1 mol%Ce doped) BST films showed more improved crystallinity, smoother surface, and smaller grain size than that with 1 mol%Ni/1 mol%Ce. The dielectric constant and loss of Ni/Ce co-doped BST films were about 298 and 1.8%, respectively. In addition, tunability and figure of merit of co-doped BST films showed minimum values of approximately 9.3% and 5, respectively. With 1% Ni-doped BST thin films, results gave a tunability of 54.2% and a loss tangent of 1.8% while a figure of merit was 30. Correlation of the material properties with dielectric and tunable properties suggests the 1 mol%Ni-doped BST films are effective potential candidate for tunable device applications.     

Magnetron sputter-deposited multilayer (BaxSr1x)Ti1+yO3+z thin films for passive and active devices

Abstract

High permittivity (Ba_xSr_1?x)Ti_1+yO_3+z(BST) thin films are being investigated for integration into charge storage dielectrics and electric-field tunable elements for high frequency devices. For the latter application, it is desirable to have BST capacitors with high tunability and low losses. Therefore, we investigated the use of multilayer BST thin films consisting of very low dielectric loss BST/electrode interfacial layers ((Ba+Sr)/Ti = 0.73) sandwiching a high tunability, high permittivity primary BST layer ((Ba+Sr)/Ti = 0.9). BST capacitors with multiple layers of controlled composition can be effectively produced insitu by magnetron sputter deposition, using a single stoichiometric target and controlling the layer composition by changing the total process gas (Ar+O<₂) pressure. The layered BST film capacitors exhibit simultaneous low loss (tan Δ = 0.005), high tunability (76%), high charge storage energy density (34 J/cm³), low leakage, and high dielectric breakdown (>2.8 MV/cm).     

DESIGN,FABRICATION AND MATERIAL PROPERTIES OF TEMPERATURE STABLE PERFORMANCE CONSISTENT TUNABLE DEVICES

ABSTRACT

The premier candidate active material for tunable microwave phase shifter devices is single composition, paraelectric BaSrTiO₃ (BST). However, there is concern that in practical applications the device performance will be compromised due to the temperature dependence of the BST based device capacitance. We report a device design which controls the magnitude and the sign of the temperature coefficient of capacitance (TCC) via a multilayer paraelectric BST/buffer layer/ferroelectric BST coplanar device structure. To realize this multilayer device structure we have designed, fabricated, and optimized a 10 mol% Al doped Ta₂O₅ barrier layer with low loss (tan δ = 0.004), moderate permittivity (?_r = 42.8), low TCC (?20 ppm/°C), and a low bias stability of capacitance (0.4%). The thin film integration of the barrier layer with the BST layers was optimized for structure, microstructure, interfacial/surface morphology, and dielectric properties as a function of Al doping concentration, annealing temperature, material growth and integration process parameters.     

Frequency and Voltage Dependent Dielectric Properties of Ni-doped Ba0.6Sr0.4TiO3 Thin Films

Highly (100) preferred undoped and 1–5% Ni-doped Ba_1–xSr_xTiO₃ (BST) thin films were deposited onto MgO (100) single crystal substrate at 750°C using pulsed laser deposition. BST thin film-based interdigital capacitors (IDC) were prepared by standard photolithography process. The microwave properties of BST films were measured at 10 GHz. Ni-doped BST films showed better dielectric properties by exhibiting improved dielectric Q while retaining an appropriate capacitance tuning compared to undoped BST films. 1% Ni-doped BST film showed the maximum figure of merit of 2896.1. It is suggested that 1 mol% Ni doped BST film is an effective candidate for high performance tunable device applications.     

Science and Technology of High Dielectric Constant Thin Films and Materials Integration for Application to High Frequency Devices

Thin films of Ba_1?x Sr _x Ti_1+y O_3+z (BST), were fabricated using both by RF-magnetron sputtering and MOCVD to demonstrate application to high frequency devices. Precise control of composition and microstructure is critical for the production of (Ba _x Sr_1?x )Ti_1+y O_3+z (BST) dielectric thin films with the large dependence of permittivity on electric field, low losses, and high electrical breakdown fields that are required for successful integration of BST into tunable high frequency devices. Here we review results on composition-microstructure-electrical property relationships of polycrystalline BST films produced by magnetron sputter deposition that are appropriate for microwave devices such as phase shifters. BST films with a multilayer structure were also developed with different Ti-elemental ratio in each layer to minimize losses and leakage current. Interfacial contamination from C and H species was studied and implications on electrical properties are highlighted. Finally, York's group at the University of California-Santa Barbara successfully integrated our BST films onto phase shifter arrays. The results show potential of BST films in such applications. Results from initial work on the integration of Cu-electrodes with BST films are also presented.     

The effects of the LaNiO3 interlayer in the preferential orientation and the dielectrical properties of Ba x Sr1− x TiO3 thin films

Ba _x Sr_1?x TiO₃ (BST) thin films were grown on different substrates with or without LaNiO₃ (LNO) layer by a modified sol–gel process. The BST thin films obtained have full perovskite phase with dense and crack-free surface. BST thin films on different substrates with LNO layer show (100) preferential orientation, the texture coefficient (TC) value is calculated to be about 42.7%, whereas those without LNO layer show a fairly reduced preferential orientation, the TC value is just about 24.8%. It is considered that the preferential orientation is induced by the interface stress between LNO and BST. Electrical property measurements showed that BST thin film with a LNO interlayer has lower capacitance and larger dielectric loss, which is due to smaller grains of the thin film.     

Low dielectric dissipation and enhanced tunability of Ba0.6Sr0.4TiO3 thin films by the modified composition and multilayer structure

Ba_0.6Sr_0.4Ti_1+yO₃ (BST, y?=?0.1, 0.15, 0.2, 0.25, 0.3) thin films were fabricated on Pt-coated silicon substrates by modified sol-gel techniques. It was found that the tunability of BST thin films and dissipation factor decreased with the increase of Ti content. The multilayer structure of Ba_0.6Sr_0.4Ti_1+yO₃(200 nm)/Ba_0.6Sr_0.4TiO₃(100 nm)/Ba_0.6Sr_0.4Ti_1+yO₃ (200 nm; y?=?0.1, 0.2, 0.25) was designed to enhance the tunability. Our results indicated that the modified composition and multilayer structure were beneficial to lowering the dielectric dissipation and enhancing the tunability simultaneously. The tunability of 26.7% and dielectric dissipation of 0.013 were achieved for modified BST thin films.     

Microwave Tunable Properties of Ni-Doped (Ba0.5Sr0.5)TiO3 Thin Films Grown by Pulsed Laser Deposition

(Ba_0.5Sr_0.5)TiO₃ (BST) thin films were deposited by pulsed laser deposition (PLD) and investigated as a function of Ni dopant concentration in low and high frequency regions. In low frequency region (<10 MHz), the Ni-dopant concentration in BST films has a strong influence on the material properties including dielectric and tunable properties. Ni-doped (≤3 mol%) BST films showed denser, smoother morphologies and smaller grain sizes than those with 6 and 12 mol% Ni. Dielectric constant and loss of 3 mol% Ni-doped BST films were about 980 and 0.3%, respectively. In addition, tunability and figure of merit of 3 mol% doped BST films showed maximum values of approximately 39% and 108, respectively. In high frequency region (>1 GHz), the frequency tunability range at center frequency of undoped BST and 3 mol% Ni-doped BST coplanar waveguide (CPW) resonators showed 102 and 152 MHz, respectively at 30 V dc bias. The Ni-doped BST thin films are possible in applications of microwave tunable capacitors.     

Structure and dielectric properties of Ca-doped (Ba0.7Sr0.3) TiO3 thin films fabricated by sol–gel method

A wide range of Ca-doped (Ba_0.7Sr_0.3)TiO₃ (BST) thin films (from 0 to 20 mol%) have been prepared on Pt/Ti/SiO₂/Si (100) substrates by sol–gel technique. The structural and dielectric properties of BST thin films were investigated as a function of Ca dopant concentration. The results showed that the microstructure and dielectric properties of the BST films were strongly dependent on the Ca contents. With the Ca dopant concentration increasing, the grain size, dielectric constant and dielectric loss of the BST thin films decreased. As the content of Ca dopant reaches 10 mol%, the dielectric constant, dielectric loss, tunability, the value of FOM and the leakage current density are 281, 0.0136, 16.7%, 12.3 and 5.5?×?10^?6 A/cm², respectively.     

The Effect of Stress on the Microwave Dielectric Properties of Ba0.5Sr0.5TiO3 Thin Films

Single phase, (1 0 0) epitaxial Ba_0.5Sr_0.5TiO₃ (BST) films have been deposited onto (1 0 0) LaAlO₃ and MgO substrates by pulsed laser deposition (PLD). The capacitance and dielectric losses of as-deposited and annealed films have been measured from 1–20 GHz as a function of electric field (0–80 kV/cm) at room temperature. The dielectric properties are strongly affected by the substrate type, post-deposition annealing time (6 h) and temperature (1200°C). For epitaxial BST films deposited onto MgO, it is observed that, after a post-deposition anneal the dielectric constant and the dielectric loss decreases. For epitaxial BST films deposited onto LAO, a post-deposition anneal (1000°C) results in an increase in the dielectric constant and an increase in the dielectric loss. The dc electric field induced change in the dielectric constant tends to increase with the dielectric constant and is largest for as-deposited films on MgO and post-deposited annealed films on LAO. In general, for epitaxial BST films, a large electric field effect is observed in films that have a large dielectric loss and a small electric field effect in films that have a low dielectric loss. High resolution X-ray diffraction measurements indicate that deposited film exhibit a significant tetragonal distortion which is strongly affected by a by a post deposition anneal. The observed differences in dielectric properties of the epitaxial BST films on MgO and LAO are attributed to the differences in film stress which arise as a consequence of the lattice mismatch between the film and the substrate and the differences in the thermal coefficient of expansion between the film and the substrate. A thin amorphous buffer layer of BST has been used to relieve stress induced by the lattice mismatch between the film and the substrate. Unlike epitaxial films, stress relieved films do not show an inverse relationship between dielectric tuning and Q (1/tan) and may be superior materials for tunable microwave devices.     

Multilayer Ceramic Capacitors with Thin (Ba,Sr)TiO3 Layers by MOCVD

Barium strontium titanate ((Ba,Sr)TiO₃; BST) thin films were prepared on platinum-coated MgO substrates at 650°C by metalorganic chemical vapor deposition (MOCVD). Perovskite single phased BST thin films were obtained. Dielectric constant at 1 kHz–100 mV was 1000. Multilayer ceramic capacitor with twelve BST dielectric layers of 0.26 m thick was formed on the MgO substrate. Capacitance and dissipation factor (tan) at 1 kHz–100 mV were 32 nF and 1.5% respectively. Capacitance per unit volume of 33 F/mm³ provided 10 to 20 times larger volumetric efficiency than the conventional multilayer ceramic capacitors. Temperature coefficient of capacitance was –4000 ppm/°C. The leakage current at 1 V was 2.3×10^-9 A that yielded an acceptable CR product of 12.8 M-F. MOCVD was proposed as one of the promising manufacturing technologies for multilayer ceramic capacitors of high performance with sub-micron thick dielectric layers.     

Microwave Properties of Mn Doped (Ba1 - x,Srx)TiO3Thin Films for Tunable Phase Shifter

Ferroelectric Mn doped Ba_0.5Sr_0.5TiO₃ (Mn-BST) films with/without BaTiO₃ (BT) buffer layer have been grown on (001) MgO substrates by a pulsed laser deposition to investigate electrical tunability at microwave frequencies. Structural properties and surface morphologies of the films were investigated using an X-ray diffractometer and a scanning electron microscope, respectively. Microwave dielectric properties of Mn-BST thin films with BT buffer were studied for reduction of dielectric loss and improvement of electrical tunability. Distributed analog phase shifters have been designed and fabricated on Mn-BST films with/without BT buffer layer to understand microwave dielectric properties. The differential phase shift of the phase shifter fabricated on Mn-BST film was 22° at 10 GHz with 80 V of applied dc bias voltage. In comparison, phase shifter fabricated on Mn-BST/BT multilayers exhibit 41° of differential phase shift at the same condition. This suggests that a BT buffer layer is for microwave tunable device applications. The phase shifter fabricated on Mn-BST/BT multilayers exhibit a low insertion loss (S₂₁) of ?1.1 dB, and a low return loss (S₁₁) of ?14 dB with a bias voltage of 80 V.     

Microwave Dielectric Properties of (111) Oriented Ferroelectric (Ba1 − xSrx)TiO3 Thin Films

Epitaxial (111) oriented ferroelectric (Ba_{1 ? x}Sr_x)TiO₃ (BST) films were deposited on MgO (111) single crystals using pulsed laser deposition. Structural properties of BST films were investigated using X-ray diffractometer. The dielectric properties of BST films were investigated under the dc bias field of 0–40 V using interdigital capacitors (IDT) fabricated by photolithography and etching process. The small signal dielectric properties of BST films were calculated by modified conformal mapping both the measured data using an impedance gain/phase analyzer and the reflection coefficient data measured using a HP 8510C vector network analyzer in 0.05–10 GHz at room temperature. The IDT capacitor based on (111) oriented BST film exhibits about 40% of capacitance change with an dc bias of 40 V which value is somewhat smaller than that of the IDT device based on (001) oriented BST film. But the dielectric quality factor value is about twice that of the device based on (001) oriented BST film.     

The Dielectric Properties for Yttrium Doped SBT Thin Films Prepared by Sol-Gel Method

In this work, Sr_0.5Ba_0.5Ti_1-zY_zO₃ (SBT) thin films were prepared on a Pt/SiO₂/Si substrate by sol-gel process. The microstructures of SBT thin films were examined by IR, XRD and TEM. The influences of Y on the microstructure and dielectric properties of Sr_0.5Ba_0.5Ti_1-zY_zO₃ thin films were studied. It is found that tetragonal perovskite crystal grains existed in SBT thin films. Increasing Y doping has a clear effect on the grain size of SBT. It is found that the proper Y doping content decreases dielectric loss for SBT thin films.     

The influence of microstructure on the electronic properties of thin films of barium strontium titanium oxide composites

Abstract

Microstructural and electrical properties were investigated for barium strontium titanium oxide (BSTO) magnesium oxide doped (0 to 10 wt.%) thin films deposited by the pulsed laser deposition (PLD) method on Pt/TiO_x/SiO₂/Si substrates. The dielectric properties were found to be strongly dependent on composition and microstructure. In cross-section, all films exhibited a columnar polycrystalline microstructure with vertical orientation deviations dictated by the surface roughness of the Pt electrode. A uniform granular microstructure was observed for all films in plan view. The dielectric constant, loss tangent and grain size decreased with increasing MgO concentration.     

Synthesis and characterization of (BaxSr1−x)Ti1+yO3+z thin films and integration into microwave varactors and phase shifters

Abstract

Precise control of composition and microstructure is critical for the production of (Ba_xSr_1?x)Ti_1+yO_3+z (BST) dielectric thin films with the large dependence of permittivity on electric field, low losses, and high electrical breakdown fields that are required for successful integration of BST into tunable high frequency devices. Here we review recent results on composition-microstructure-electrical property relationships of polycrystalline BST films produced by magnetron sputter deposition, that are appropriate for microwave devices such as phase shifters. Films with controlled compositions were grown from a stoichiometric Ba_0.5Sr_0.5TiO₃ target by control of the background processing gas pressure. It was determined that the (Ba+Sr)/Ti ratios of these BST films could be adjusted from 0.73 to 0.98 by changing the total (Ar+O₂) process pressure, while the O₂/Ar ratio did not strongly affect the metal ion composition. Film crystalline structure and dielectric properties as a function of the (Ba+Sr)/Ti ratio are discussed. Optimized BST layers yielded capacitors with low dielectric losses (0.0047), among the best reported for sputtered BST, while still maintaining tunabili-ties suitable for device applications. These BST films were used to produce distributed-cir-cuit phase-shifters, using a discrete periodic loading of a coplanar waveguide with integrated BST varactors on high-resistivity silicon. Phase shifters yielding 30 degrees of phase shift per dB of insertion loss were demonstrated at 20GHz.     

Planar and Parallel Dielectric Properties of Compositionally Graded (Ba,Sr)TiO3 Thin Films for Tunable Microwave Applications

Compositionally graded (Ba_x,Sr_{1 ? x})TiO₃ [BST] ferroelectric thin films have been received much attention in graded ferroelectric devices due to their unique properties, such as large pyroelectric coefficients, large polarization offset and small temperature coefficient of dielectric constant for microwave tunable devices. Compositionally graded BST thin films were deposited epitaxially on LaAlO₃ [LAO] and Nb-doped SrTiO₃ [STO:Nb] substrates by pulsed laser deposition. The planar and parallel dielectric properties of compositionally graded BST epitaxial thin films ware investigated in the frequency ranges of 100 Hz ～ 1 MHz as a function of the direction of the composition gradient with respect to the substrate at room temperature. The dielectric properties of the graded BST films depended strongly on the direction of the composition gradient with respect to the substrate. The graded ST → BT films grown on LAO and STO:Nb substrates exhibited a excellent dielectric properties than the graded BT → ST films.     

Structural properties and dopant-modified bandgap energies of Ba0.5 Sr 0.5 TiO 3 thin films grown on LaAlO 3 substrates

Ba_0.5Sr_0.5TiO₃ thin films doped with different concentration of Ti, Mg and Al dopants were prepared by pulsed laser deposition technique on LaAlO₃ substrates. The crystalline properties of these doped thin films were studied using X-ray diffraction, micro-Raman scattering, atomic force microscopy, and transmission electron microscopy. The bandgap energies of BST thin films are determined from the transmission and absorption measurements by the ultraviolet-visible spectrophotometer. It was found out that the bandgap energies of the doped BST thin films depend strongly on the dopant concentration.     

Microwave Characterization of BST Thin Films on LAO Interdigital Capacitor

The dielectric properties of (Ba_0.5Sr_0.5)TiO₃ (BST) thin films on LaAlO₃ (LAO) single crystals were studied over a wide frequency range. The samples with interdigital electrodes were prepared by microelectronic processing. The dielectric characterizations were carried out in the following steps: 1) the standard calibration of the instrument, 2) the removal of parasitic capacitance and the extraction of the capacitance of the interdigital capacitor (IDC) and 3) the extraction of the dielectric permittivity (?) of BST. It was found that ? of BST has a constant value (about 400) from 50 MHz to 2 GHz; at higher frequencies, ? gradually decreases. The BST interdigital capacitors exhibited good dielectric tunability.     

Structure and electrical properties of Ba0.67 Sr0.33TiO3 thin film fabricated by sol-gel based composite technique

A sol-gel based novel technique is used to fabricate Ba_0.67Sr_0.33TiO₃ (BST) thin films with thickness up to several microns. In this technique, surface-modified fine BST particles are dispersed in a sol-gel precursor solution. The pH value of the precursor solution is modified to achieve high zeta potential for the dispersed powder, hence a very stable and uniform slurry can be produced. The slurry is then spin-coated onto Pt/Ti/SiO₂/Si substrate, pre-heated and annealed as in conventional sol-gel process. The resulting films show well-developed dense polycrystalline structure with uniform grain distribution. The metal-BST-metal structure of the films displays good dielectric properties.