Interfacial characteristic of (Ba,Sr)TiO3 thin films deposited on different bottom electrodes

Barium strontium titanate (Ba_1?x Sr _x )TiO₃ (BST) thin films were deposited on Pt, Ru, RuO₂, and Pt/RuO₂ electrodes by radio frequency magnetron sputtering. The interfacial structure characteristic of the BST films deposited on various electrodes was investigated. X-ray photoelectron spectroscopy investigations showed that the interfacial diffusion layer in BST/Pt and BST/Ru are approximately 6 and 10 nm, respectively. The BST films are short of Ba and O elements comparing with the stoichiometry Ba_0.65Sr_0.35TiO₃ in the interface region. Dielectric measurement of the BST films with thickness ranging from 70 to 400 nm revealed that the BST films deposited on Pt and Pt/RuO₂ bottom electrodes have similar dielectric property, the BST films deposited on Ru have the highest bulk dielectric constant, and the thickness dependence of dielectric constant on the BST film deposited on RuO₂ electrode can be neglected. The interfacial layer dielectric constant of BST films deposited on Pt and Ru electrodes are estimated to be about 34.5 and 157.1, respectively. The effect of interfacial dead-layer on the dielectric constant could be eliminated through selecting appropriate bottom electrodes.     

Preparation and characterization of Ba1 − xSrxTiO3 thin films deposited on Pt/SiO2/Si by sol-gel method

BST thin films have been investigated as potential candidates for use in frequency agile microwave circuit devices. Stoichiometric (Ba_1 − xSr_x)TiO₃ (BST) thin films have been prepared on Pt/SiO₂/Si substrates using sol-gel method. The BST films were characterized by X-ray fluorescence (XRF) spectroscopy analysis, X-ray diffraction (XRD), scanning electron microscope (SEM) and electrical measurements. The relationships of processing parameters, microstructures, and dielectric properties are discussed. The results show that the films exhibit pure perovskite phase through rapid thermal anneal at 700 °C and their grain sizes are about 20-40 nm. The dielectric constants of BST5, BST10, BST15 and BST20 are 323, 355, 382 and 405, respectively, at 80 kHz.     

Large area Ba1 − xSrxTiO3 thin films for microwave applications deposited by pulsed laser ablation

Large area Ba_1 − xSr_xTiO₃ (BST) thin films with x = 0.4 or x = 0.5 were deposited on 75 mm diameter Si wafers in a pulsed laser deposition (PLD) chamber enabling full-wafer device fabrication using standard lithography. The deposition conditions were re-optimized for large PLD chambers to obtain uniform film thickness, grain size, crystal structure, orientation, and dielectric properties of BST films. X-ray diffraction and microstructural analyses on the BST films grown on Pt/Au/Ti electrodes deposited on SiO₂/Si wafers revealed films with (110) preferred orientation with a grain size < 100 nm. An area map of the thickness and crystal orientation of a BST film deposited on SiO₂/Si wafer also showed (110) preferred orientation with a film thickness variation < 6%. Large area BST films were found to have a high dielectric tunability of 76% at an electric field of 400 kV/cm and dielectric loss tangent below 0.03 at microwave frequencies up to 20 GHz and a commutation quality factor of ~ 4200.     

The influence of YSZ interlayer on microstructures and dielectric properties of BST thin films prepared by RF magnetron sputtering

(Ba_{1 − x} Sr _x )TiO₃ (BST) thin films were deposited on Pt/Ti/SiO₂/Si and YSZ/Pt/Ti/SiO₂/Si substrates by radio frequency (RF) magnetron sputtering. The influence of YSZ interlayer on microstructures and dielectric properties of BST thin films were investigated by X-ray diffraction, atomic force microscopy, scanning electron microscopy and dielectric frequency spectra. It was found that the preferred orientation of BST thin films could be tailored by insertion of YSZ interlayer and adjusting the thickness of YSZ interlayer. The BST thin films deposited on YSZ interlayer exhibited a more compact and uniform grain structure than that deposited directly on Pt electrode. Dielectric measurement revealed that the BST thin films deposited on 10 nm YSZ interlayer have the largest dielectric constant and a low dielectric loss tangent. The enhanced dielectric behavior is mainly attributed to the YSZ interlayer which serves as an excellent seeding layer to enhance the crystallization of subsequent BST films layer, and a smaller thermal stress field built up at the interface between YSZ interlayer and BST film layer.     

The effects of oxygen partial pressure on BST thin films deposited on multilayered bottom electrodes

Ba_0.5Sr_0.5TiO₃ (BST) thin films have been deposited by r.f. magnetron sputtering on silicon and platinum-coated silicon substrates with different buffer and barrier layers. BST films deposited on Si/SiO₂/SiN/Pt and Si/SiO₂/Ti/TiN/Pt multilayer bottom electrode have been used for the fabrication of capacitors. XRD and SEM studies were carried out for the films. It was found that the crystallinity of the BST thin film was dependent upon oxygen partial pressure in the sputtering gas. The role of multilayered bottom electrode on the electrical properties of Ba_0.5Sr_0.5TiO₃ films has been also investigated. The dielectric properties of BST films were measured. The results show that the films exhibit pure perovskite phase and their grain sizes are about 80–90 nm. The dielectric properties of the BST thin film on Si/SiO₂/Ti/TiN/Pt electrode was superior to that of the film grown on Si/SiO₂/SiN/Pt electrode.     

Different microstructure and dielectric properties of Ba1−xCaxTiO3 ceramics and pulsed-laser-ablated films

A comparative study of the microstructure and dielectric properties between Ba_1−xCa_xTiO₃ (BCT) ceramics and films were performed in the whole Ca concentration range of x = 0-1. The ceramics were prepared by conventional solid-state reaction technique and the films by the method of pulsed-laser deposition. X-ray diffraction (XRD) study of the BCT ceramics exhibited a pure tetragonal phase for x = 0-0.25, a tetragonal-orthorhombic diphase for x = 0.25-0.85 and a pure orthorhombic phase for x = 0.90-1.00. And the dielectric phase transition temperature from tetragonal to cubic was marginally affected by the Ca doping into BaTiO₃. However, BCT films deposited on Pt/Si/SiO₂/Si substrates showed a different microstructure and dielectric properties. Tetragonal-orthorhombic diphase was not found in the BCT films for x = 0.25-0.85, and a large decrease of the Curie point and diffuse phase transition were observed in the BCT films. Based on the compositional analysis, such phenomena were ascribed to the occupancy of some Ca²⁺ to the Ti⁴⁺ sites in the BCT films.     

Electrical properties of r.f. magnetron sputtered BaxSr1−xTiO3 films on multi-layered bottom electrodes for high-density memory application

Ba_xSr_1–xTiO₃ (BST) thin films have been deposited by r.f. magnetron sputtering on silicon and platinum-coated silicon substrates with different buffer and barrier layers. Electrical properties of BST films have been evaluated using both metal–insulator–semiconductor (MIS) and metal–insulator–metal (MIM) structures. MIS capacitor C–V and G–V characteristics have been utilized to determine the fixed charge density, interface trap density and the trap distribution in the silicon bandgap. BST films deposited on Si/SiO₂/SiN/Pt and Si/SiO₂/Ti/TiN/Pt multilayer bottom electrodes have been used for the fabrication of MIM capacitors. The role of bottom electrode, processing temperature and Ba to Sr ratio on the electrical properties of Ba_xSr_1–xTiO₃ films have been investigated. Current–voltage behavior has indicated an ohmic nature at lower voltages and Poole–Frenkel conduction at higher voltages. Deposited films have shown an excellent time-dependent dielectric breakdown under constant-current stressing.     

Structural and dielectric properties of Ba0.7Sr0.3TiO3 thin films grown on thin Bi layer-coated Pt(111)/Ti/SiO2/Si substrates

In this work, the growth and study of dielectric properties of Ba_0.7Sr_0.3TiO₃ (BST) thin films grown on thin Bi layer coated Pt(111)/Ti/SiO₂/Si substrates, depending on thin Bi layer thickness is reported. The BST thin film (thickness 180 nm) grown on 10-nm-thick Bi layer exhibited more improved structural and dielectric properties than that grown on bare Pt(111)/Ti/SiO₂/Si substrate. The 10-nm-thick Bi layer in optimum configuration was effective for the grain growth of BST phase and suppressed the formation of the oxygen-deficient layer at the interface between the BST thin film and bottom electrode, which resulted in an increase in dielectric constant and a decrease in leakage current density of the Pt/BST thin film/Pt capacitor.     

Perovskite oxynitride LaTiOxNy thin films: Dielectric characterization in low and high frequencies

Lanthanum titanium oxynitride (LaTiO_xN_y) thin films are studied with respect to their dielectric properties in low and high frequencies. Thin films are deposited by radio frequency magnetron sputtering on different substrates. Effects of nitrogen content and crystalline quality on dielectric properties are investigated. In low-frequency range, textured LaTiO_xN_y thin films deposited on conductive single crystal Nb-STO show a dielectric constant ε′ ≈ 140 with low losses tanδ = 0.012 at 100 kHz. For the LaTiO_xN_y polycrystalline films deposited on conductive silicon substrates with platinum (Pt/Ti/SiO₂/Si), the tunability reached up to 57% for a weak electric field of 50 kV/cm. In high-frequency range, epitaxial LaTiO_xN_y films deposited on MgO substrate present a high dielectric constant with low losses (ε′ ≈ 170, tanδ = 0.011, 12 GHz).     

Structure of post-annealed ferroelectric PbZrxTi1-xO3 and SrBi2Ta2O9 thin films

In order to fabricate good quality ferroelectric thin films, PbZr_xTi_(1-x)O₃ (PZT) and SrBi₂Ta₂O₉ (SBT) films were fabricated on SiO₂/Si(100) substrates and on Pt/Ti/ SiO₂/Si(100) substrates by pulsed laser excimer deposition (PLD). X-ray diffraction, Rutherford backscattering analysis, and atomic force microscopy were used to characterize the structural properties of the samples, which were post-annealed at different temperatures. The results showed that the PZT and SBT films fabricated on Pt/Ti/SiO₂/Si(100) substrates and annealed at 700 °C exhibited optimum properties.     

Investigation of the surface modification for Ru and RuO x films using a post-treatment method for high-dielectric applications

The effect of post-treatment on the surface modification for Ru and RuO _x films was investigated. The surface morphology of Ru film oxidized by N₂O plasma was observed to be much smoother than that by rapid thermal oxidation. In particular, the leakage current property of the Ru film oxidized by N₂O plasma was significantly improved due to suppression of heterogeneous grain growth and the Ru–O bonds formed. After post-treatment, otherwise, oxygen is fully diffused through the RuO _x layer, producing a porous amorphous microstructure. In addition, the barrier properties for the Ru/TiN/poly-Si/Si contact system are better than that for the RuO _x /TiN/poly-Si/Si contact system. Therefore, N₂O plasma oxidation can be suggested as a post-treatment method and the Ru film can be proposed as a bottom electrode to improve the electrical properties of dielectric film at the dielectric film/Ru bottom electrode interface.     

Crystallization and electrical properties of Ba<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>TiO<Subscript>3</Subscript> thin films on SrRuO<Subscript>3</Subscript>/Pt hybrid bottom electrode

Ba_0.7Sr_0.3TiO₃ (BST) thin films were deposited on Pt and SrRuO₃(SRO)/Pt hybrid bottom electrodes by radio frequency magnetron sputtering. X-ray analysis indicated that both films were polycrystalline. Dielectric measurements showed that the films on SRO/Pt hybrid bottom electrode had lower dielectric constant and loss than the films on single Pt and the dielectric properties were frequency-independent. The leakage current density of Ba_0.7Sr_0.3TiO₃ thin films on hybrid bottom electrode was also lower. Leakage mechanism investigations showed that the contact between the electrode-film interfaces of thin films on SRO/Pt hybrid bottom electrode was ohmic. Based on the results, the effects of SRO/Pt hybrid bottom electrode on the crystallization and electrical properties of BST thin films were discussed.     

Preparation,microstructure and dielectric properties of Ba0.5Sr0.5TiO3 thin films grown on Pt/Ti/SiO2/Si substrates by pulsed laser deposition

Barium strontium titanate Ba_0.5Sr_0.5TiO₃ (BST) thin films have been deposited on Pt/Ti/SiO₂/Si substrates by a pulsed laser deposition method. The low-frequency dielectric responses of the BST films, grown at different substrate temperatures (T_s), were measured as functions of frequency in the frequency range from 1 kHz to 1 MHz. With increase of T_s, the grain size of BST thin films became larger and the crystallinity was greatly improved, and then the dielectric permittivity increased, while the dielectric dispersion rose drastically. The origin of the large dielectric relaxation is believed to result from the aggravation of oxygen diffusion at the BST/Pt interface for the BST thin films grown at comparatively higher temperatures. This concept could be further explained by considering the influence of post-annealing in oxygen ambient on the dielectric properties of BST thin films. Our results reveal that the dielectric properties are strongly dependent on the processing conditions and the microstructure of thin films.     

Studies of BaTiO3 thin films on different bottom electrode

BaTiO₃ (BT) thin films were prepared on Pt/Ti/SiO₂/Si and Ru/Ti/SiO₂/Si substrates by a modified sol-gel technique. The microstructure of the films was characterized by atomic force microscopy (AFM), X-ray diffraction (XRD) and Raman spectroscopy. The results showed that BT thin films crystallized with perovskite structure. Compared to BT film on Pt/Ti/SiO₂/Si substrate, BT thin film deposited on Ru electrode has similar dielectric constant, while it has higher dielectric loss. C–E curve for BT film on Pt/Ti/SiO₂/Si was more symmetrical around zero-bias field than C–E curve for BT film on Ru/Ti/SiO₂/Si substrate. The tunability was 52.02% for BT film on Pt electrode, which was 33.42% on Ru electrode, at 275 kV/cm and room temperature. The leakage current density of BT on Pt electrode was about an order of magnitude lower than BT film on Ru electrode at the applied electrical field below 150 kV/cm. The leakage conduction mechanism was investigated.     

The effect of Bi1.5Zn0.5Nb0.5Ti1.5O7 cover layer on the dielectric and tunable properties of Ba0.6Sr0.4TiO3/Bi1.5Zn0.5Nb0.5Ti1.5O7 bilayered thin films

Bi_1.5Zn_0.5Nb_0.5Ti_1.5O₇ (BZNT) thin films with different thicknesses as cover layers were deposited on the Ba_0.6Sr_0.4TiO₃ (BST) thin films on the Pt/Ti/SiO₂/Si substrates by radio frequency magnetron sputtering method. The microstructure, surface morphology, dielectric and tunable properties of BST/BZNT heterogeneous bilayered films were investigated as a function of the thickness of BZNT films and the effect of BZNT films on the asymmetric electrical properties of BST/BZNT bilayered films was discussed. It was found that BZNT cover layer significantly improved the leakage current and the dielectric loss, and the dielectric constant and tunability of BST/BZNT bilayered thin films simultaneously decreased with the increasing thickness of BZNT films. The BST/BZNT bilayered thin film with a 50 nm BZNT cover layer gave the largest figure of merit (FOM) of 33.48 with the upper tunability of 55.38%. The asymmetric electrical behavior of BST/BZNT bilayered films is probably related to an internal electric field caused by built-in voltages at Pt/BST and BZNT/Au interfaces.     

Microstructure and electrical properties of Pb(Zr0.52Ti0.48)O3 ferroelectric films on different bottom electrodes

The crystalline quality, dielectric and ferroelectricity of the Pb(Zr_0.52Ti_0.48)O₃ (PZT) films deposited on the LaNiO₃ (LNO), LNO/Pt and Pt bottom electrodes were comparatively analyzed to investigate the possibility for their application. LNO thin films were successfully prepared on Si (100) and Pt(111)/Ti/SiO₂/Si substrates by modified metallorganic decomposition (MOD). The PZT thin films were spin-coated onto the LNO, LNO/Pt and Pt bottom electrodes by sol–gel method. The crystallographic orientation and the microstructure of the resulting LNO films and PZT thin films on the different bottom electrodes were characterized by X-ray diffraction analysis. The dielectric and ferroelectric properties of PZT films on the different bottom electrodes are discussed. The PZT films deposited onto Pt/Ti/SiO₂/Si and LNO/Si substrates show strong (110) and (100) preferred orientation, respectively, while the films deposited onto LNO/Pt/Ti/SiO₂/Si substrates show the peaks of mixed orientations. PZT films on LNO and LNO/Pt bottom electrodes have larger dielectric constant and remnant polarizations compared with those grown on the Pt electrode.     

Improved dielectric properties of CaCu3Ti4O12 thin films on oxide bottom electrode of La0.5Sr0.5CoO3

We report on the effect of La_0.5Sr_0.5CoO₃ (LSCO) bottom electrode to the dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) thin films grown on Ir/Ti/SiO₂/Si substrates. Compared with the films grown directly on Ir/Ti/SiO₂/Si substrates, the dielectric constant has been increased greatly about 100%, and the dielectric loss decreased to lower than 0.2 in the frequency range of 1-100 kHz. The origin has been discussed in details based on the analysis of the X-ray diffraction and impedance spectra measurements. Results of the impedance spectra suggest that the absence of undesired interfacial layer between Ir/CCTO thin films might be one of the major reasons of the improvement of the dielectric properties when the LSCO was introduced as the bottom electrode.     

Pt–Ru alloy electrode for(Ba,Sr)TiO3 capacitor

The Pt–Ru alloy was evaluated as a bottom electrode material in the (Ba,Sr)TiO₃ (BST) capacitor. Surface oxidation producing the RuO₂ layer for Pt–Ru(50 at %) first occurred and then BST film was deposited in the BST sputtering process. The thickness of RuO₂ formed from oxidation by an oxidant gas for BST formation, was thicker for the higher substrate temperature and the RuO₂ film shows the (1 0 1) orientation. The RuO₂ film between BST and Pt–Ru(50 at %) may block the diffusion of oxygen through the bottom electrode. The BST film deposited on Pt–Ru(50 at %) has a rough surface and a stronger intensity of (1 1 0) than that on Pt.     

RuO2/Ru electrode on Si3N4/Si substrate for microelectromechanical systems devices based on Pb(Zr1-xTix)O3 film and surface micromachining

RuO₂/Ru conducting films were deposited on low stress Si₃N₄/Si substrates by reactive r.f. sputtering deposition at a substrate temperature of 400°C to introduce a new bottom electrode for microelectromechanical system devices based on a Pb(Zr_1-xTi_x)O₃ film and a surface micromachining technique with high temperature processes. X-ray diffraction and scanning electron microscopy measurements after heat treatment at 700°C were conducted to investigate structural stability of the RuO₂/Ru films, which showed no silicide and silicon oxide formations by the heat treatment. Interfacial structures of the film with the heat treatment were similar to those of the as-deposited films. The surface of the film with the heat treatment consisted of larger grains than those of the as-deposited film. Rutherford backscattering spectrometry and Auger electron spectroscopy showed no interfacial reactions between the RuO₂/Ru and the Si₃N₄. In order to investigate the feasibility of the RuO₂/Ru as a bottom electrode, Pb(Zr_0.53Ti_0.47)O₃ films were deposited by metalorganic decomposition. After deposition of a Pb(Zr_0.53Ti_0.47)O₃ film at 700°C for 30 min, the interface structure between the RuO₂/Ru and the Pb(Zr_0.53Ti_0.47)O₃ film showed no interface reactions. The electrical properties of the PZT film on the RuO₂/Ru were not changed before and after an HF etching to make an air gap, even though the piezoelectric coefficients on the RuO₂/Ru were lower than on the Pt/Ti. Therefore, the RuO₂/Ru conducting film could be used for a bottom electrode on the Si₃N₄/Si for a microelectromechanical system device based on a Pb(Zr_1-xTi_x)O₃ film and a surface micromachining technique with high temperature processes.     

Enhanced dielectric and tunable properties of barium strontium titanate thin films through introducing Nd(Zn1/2Ti1/2)O3 and adjusting Ba/Sr

Ba_0.6Sr_0.4TiO₃ (BST) and 0.06Nd(Zn_1/2Ti_1/2)O₃–0.94Ba _x Sr_1?x TiO₃ (NZT–BST) thin films with x = 0.6, 0.7, 0.75, and 0.8 were fabricated on Pt/Ti/SiO₂/Si substrates by sol–gel method. The structures, surface morphology, dielectric, and ferroelectric properties, and thermal stability of BST and NZT–BST thin films were investigated as a function of NZT and Ba content. It was found that introducing NZT into BST decreased significantly dielectric loss, however, along with the tunability. On this basis, increasing Ba/Sr in NZT–BST thin films led to the simultaneous increase of dielectric constant and tunability of thin films. As a result, optimized dielectric and tunable properties were obtained for 0.06Nd(Zn_1/2Ti_1/2)O₃–0.94Ba_0.7Sr_0.3TiO₃ thin film with the highest FOM value of 43.22. It awakens us that, for reducing dielectric loss, introducing a certain amount of low permittivity oxides or non-ferroelectrics like NZT into weak ferroelectric perovskite tunable materials, not into paraelectric perovskite tunable materials, may obtain more excellent dielectric and tunable performances.