Effect of Cr additions on the electrical properties of Ni–BaTiO3 ultra-thin multilayer capacitors

Multilayer ceramic capacitors based on BaTiO₃ dielectric compositions and Ni inner electrodes have complex interfacial reactions that impact the continuity of the inner electrode microstructure. Previously we demonstrated that through the addition of Cr to Ni, a significant improvement in the continuity of ultra-thin Ni electrodes in Ni–BaTiO₃ multilayer capacitors could be achieved. Here, the effect of the Cr addition to the nickel electrode pastes is studied with regard to the electrical properties. Low-field electrical measurements demonstrate no major differences between Cr doped Ni and undoped Ni. However, high-field measurements show a significant decrease to the total capacitor resistance. Under a critical electrical bias the conductivity significantly increases due to a Fowler–Nordheim tunneling conduction though the interfacial Schottky barrier at the dielectric–electrode interface; the onset voltage of this conduction is much lower than with the undoped nickel. Based on these results, we evaluate criteria for the selection of an appropriate refractory metal in order to improve the Ni electrode continuity.     

Solubility of Ni in BaTiO3 during co-firing

Nickel is now frequently used as internal electrodes for multilayer ceramic capacitors (MLCC). In order to evaluate the effect of the presence of metallic Ni on the properties of BaTiO₃-based dielectrics, nano-sized Ni particles are mixed thoroughly with BaTiO₃ powder and subsequently sintered at 1,330 °C in nitrogen. The oxygen partial pressure in the nitrogen is slightly higher than the equilibrium oxygen partial pressure for the Ni→NiO reaction. Under such a condition, an upper limit of 5,400 ± 540 ppm for the solubility of Ni in BaTiO₃ is detected. The Ni solute acts as an acceptor to BaTiO₃, and its presence increases the reduction resistance of BaTiO₃ significantly.     

Microstructure analysis of the Y5V multilayer ceramic capacitors based on BaTiO3

The Y5V-1206 base-metal electrode (BME) multilayer ceramic capacitor (MLCC) chips have been characterised for crystalline phases using X-ray diffractometry (XRD), and microstructure using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microstructure features found in the Ni electrode and the BaTiO₃ dielectric layer are discussed in terms of the tensile backstresses induced upon firing due to constrained sintering heterogeneously. The chemical compositions containing BaO-excess and additives CaO and ZrO₂, determined for BaTiO₃ grains by energy-dispersive spectroscopy (EDS) equipped in the TEM are also reported. However, no rare-earth oxides were found in the grains. The representative microstructure of BaTiO₃ grains containing dislocations is the “solid-solution” type distinctive from the “core-shell” of the X7R compositions. The fact that no ferroelectric domains were detected suggests that the BaTiO₃ grains are pseudo-cubic with the c/a ratio ≈ 1.0.     

Dysprosium Doped Dielectric Materials for Sintering in Reducing Atmospheres

Substitution of Dy rare earth ions was studied in Ba(Ti,Zr)O₃ dielectric materials, using thermogravimetry, X-ray diffraction and dielectric measurements. Dy³⁺ ions enter both the A- and the B-sites of the perovskite structure, however, the solubility on B-sites is up to 9 mol %, whereas it is only 2.5 mol% on A-sites. Dy can be easily shifted from A- to B-sites and back, using Ba or Ti excess in the material. Dy³⁺ on B-sites is a strong electron acceptor. Dy doped dielectric materials are cofired with Ni electrodes in reducing atmosphere to highly insulating BME multilayer capacitors.     

NiCr Alloy As Both Absorption Layer and Top Electrode onto Pb(Zr0.3Ti0.7)O3 Thin Films for Infrared Sensors

NiCr alloys prepared by dc magnetron sputtering are considered to apply simultaneously both the absorption layer and the top electrode on PZT thin films for infrared sensors. NiCr alloys deposited with dc powers of Ni 80 and Cr 50 W showed the most stable oxidation resistance even at 600°C in an oxygen ambient. They have a resistivity of approximately 70 μΩ-cm and a rms roughness of 2.0 nm in samples annealed at 600°C for 5 min in O₂. The NiCr/PZT/Pt capacitors showed a well-saturated hysteresis loop having a remanent polarization of 20 μC/cm². Ultra-thin NiCr alloys showed a possibility as a top electrode for infrared sensors.     

Dielectric behavior of Bi2/3Cu3Ti4O12 ceramic and thick films

The paper reports on synthesis, sintering and microstructure of Bi_2/3Cu₃Ti₄O₁₂, a lead-free, high-permittivity material with internal barrier layer capacitor behavior. Complex impedance and capacitance of the ceramic and thick films were studied as a function of frequency (10 Hz–2 MHz) and temperature (−170 to 400°C). Dc electrical conductivity of the samples was measured in the temperature range 20–400°C. Broad and high maxima of dielectric permittivity versus temperature plots were observed reaching 60,000 for ceramic and 5,000 for thick films. The maxima decrease and shift to higher temperatures with increasing frequency. Two arcs ascribed to grains and grain boundaries were found in the plots of imaginary part versus real part of impedance. Analysis of the impedance spectra indicates that Bi_2/3Cu₃Ti₄O₁₂ ceramic could be regarded as electrically heterogeneous system composed of semiconducting grains and less conducting grain boundaries. The developed thick film capacitors with dielectric layers based on Bi_2/3Cu₃Ti₄O₁₂ exhibit dense microstructure, good cooperation with Ag electrodes, high permittivity up to 5,000 and relatively low temperature coefficient of capacitance in the temperature range 100–300°C. Broad maxima in the dielectric permittivity versus temperature curves may be attributed to Maxwell–Wagner relaxation.     

Co-Doping Effect of Mn and Y on Charge and Mass Transport Properties of BaTiO3

BaTiO₃-based multilayer-ceramic capacitors (MLCC) using base metal (Ni) electrodes normally contains Mn and Y each approximately on the order of 0.5 mol%. It is only empirically known that the co-doping of Y and Mn facilitates sintering with the base-metal electrodes as well as improves the device performance and life time. In order to understand the effect of the co-doping, we have measured the electrical conductivity and chemical diffusivity on polycrystalline BaTiO₃ that is co-doped with Y and Mn each by 0.5 mol% against oxygen partial pressure at elevated temperatures. It is found that while the n-type conductivity in reducing atmospheres (e.g., Po₂ < 10^{– 6} atm at 1000C) remains similar to that of undoped or acceptor-doped BaTiO₃, its p-type conductivity in oxidizing atmospheres (e.g., Po₂ > 10^{– 6}at 1000C) is remarkably suppressed compared to the latter. The chemical diffusivity is also similar to that of the latter in magnitude (e.g., 10^{– 2}–10^{– 5} cm²/s at 1000C), but its trend of variation with oxygen partial pressure is rather opposite. These variations of the conductivity and chemical diffusivity are mainly attributed to Mn ions changing their valence from +2 to +3 to +4 with increasing oxygen partial pressure. It is explained from a defect-chemical view why the codoping of fixed-valent donor (Y) and variable-valent acceptor (Mn) has been practiced in MLCC processing.     

Sintering Behavior and Interfacial Analysis of Ni/Cu Electrode with BaTiO<Subscript>3</Subscript> Particulates

The sintering behavior of Ni electrode alloyed with Cu and the interfacial structure between Ni/Cu to BaTiO₃ (BT) have been investigated. The quantitative properties, which include thermal shrinkage, thermal expansion, wetting behaviors of Ni/Cu alloys on BT sheet, and composition distribution were measured by several thermal analysis techniques (TGA/DTA/TMA) and microstructural techniques (SEM/TEM/ HRTEM) with energy-dispersive spectroscopy (EDS). The shrinkage of the Ni/Cu/ BaTiO₃ composite tested in 5%H₂/N₂ atmosphere showed strong influence by the addition of Cu, and retarded slightly due to the addition of the BT particulates. The Cu alloyed with Ni improves the continuity of the electrode and does not trigger mutual reaction between Ni and BT.     

Effects of nitridation treatments for SBT/Ta2O5 stack gate capacitors

Abstract

In this study, effects of ICP nitride treatments on characteristics of ferroelectric gate stack capacitor were investigated for FET type ferroelectric memory applications. Pt/SBT(200nm)/Ta₂O₅(20nm)/ Nitride/Si (MeFINS) structure capacitors show wide ΔV (memory window) of 1.06V under ±3V operation, while Pt/SBT(200nm)/ Ta2O₅(20nm)/Si (MeFIS) capacitors without nitride treatments exhibit memory window of 0.60V. At the same time, an accumulation capacitance of the MeFINS structure device is higher than that of the MeFIS structure capacitor. This result implies that the ICP nitride treatment successfully suppresses a formation of low dielectric constant interfacial SiO_x layer and alleviates a series capacitance problem.     

Synthesis of fine Ca-doped BaTiO3 powders by solid-state reaction method—Part I: Mechanical activation of starting materials

Fine (Ba_0.98Ca_0.02)_1.002TiO₃ powders for high capacitance multilayer ceramic capacitors (MLCCs) application were synthesized by solid state reaction method. The effects of mechanochemical activation using high energy milling and the starting materials properties on the reaction temperature and on the final powder properties were investigated. Previous heavy milling of BaCO₃ and the adoption of fine, anatase-rich TiO₂ phase were effective in decreasing the reaction temperature and in increasing the tetragonality (=c/a). BaCaTiO₃ powders with a tetragonality of 1.0097, an average particle size of 213 ± 43 nm and a specific surface area of 6.30 m²/g were acquired after heat treatment at 985 °C for 2 h. MLCCs utilizing this developed powder showed superior dielectric and temperature characteristics to those with conventional, Ca-free BaTiO₃ powder.     

The Effect of La-Doped Bi4Ti3O12 Buffer Layer on Crystallinity and Ferroelectric Properties of PbZr0.58Ti0.42O3/Bi3.25La0.75Ti3O12 Multilayered Thin Films

PbZr_0.58Ti_0.42O₃ (PZT) ferroelectric thin films with Bi_3.25La_0.75Ti₃O₁₂ (BLT) buffer layer of various thickness were fabricated on Pt/TiO₂/SiO₂/p-Si(100) substrates by rf-magnetron sputtering method. The pure PZT film showed (111) preferential orientation in the XRD patterns, and the PZT/BLT films showed (110) preferential orientation with increasing thickness of the BLT layer. There were no obvious diffraction peaks for the BLT buffer layer, for its thin thickness in PZT/BLT multilayered films. There were the maximum number of largest-size grains in PZT/BLT(30 nm) film among all the samples from the surface images of FESEM. The growth direction and grain size had significant effects on ferroelectric properties of the multilayered films. The fatigue characteristics suggested that 30-nm-thick BLT was just an effective buffer layer enough to alleviate the accumulation of oxygen vacancies near the PZT/BLT interface. The comparison of these results suggests that the buffer layer with an appropriate thickness can improve the ferroelectric properties of multilayered films greatly.     

Barrier mechanism of Pt/Ta and Pt/Ti layers for SrTiO3 thin film capacitors on Si

Abstract

The barrier effect of Pt/Ta and Pt/Ti has been investigated, when used as bottom electrodes for SrTiO₃ thin film capacitors on Si. The Pt/Ta/Si stacks were more stable than the Pt/Ti/Si, both in vacuum and in oxygen annealing. Though the Pt/Ta bilayer was suitable for the SrTiO₃ deposition at 400[ddot]C, its resistivity became slightly higher after the deposition at 600[ddot]C, due to Ta layer oxidation during the SrTiO₃ deposition. This would result in a contact resistance problem for high density dynamic random access memory application.     

A first principles investigation of new cathode materials for advanced lithium batteries

First principles calculations on the crystal and electronic structure of a layered Li(Ni_1/3Mn_1/3M_1/3)O₂ (M = Al, Ti, Cr, Fe and Mo) were undertaken as part of a search for new positive electrode materials for advanced lithium ion batteries. The formal charge of Ni, Mn and M (Ti and Mo) were estimated to be +2, +3 and +4, respectively, from electronic structures and interatomic distances. In the cases of the Al, Cr and Fe substitution, the compounds had trivalent M and tetravalent Mn ions. The solid-state redox reactions of Li(Ni_1/3Mn_1/3M_1/3)O₂ were calculated assuming a Li deinsertion scheme, and the reactions were shown to be Ni²⁺/Ni³⁺/Ni⁴⁺ and M³⁺/M⁴⁺ for the Cr and Fe substitution. Al substitution will lead to higher voltages, as fixed 3+ valence of Al forces more electron exchange with oxygen. The cases of Ti and Mo substitution, Ti and Ni ions do not participate in the redox reactions over the entire range, respectively. The substitutive cation-oxygen bonding has a more covalent character, when the redox energy of Ni is lowered, resulting in an increase in potential. As described above, the voltage profiles are very different because the types of metals are different and participate in electrochemical reactions according to the substituted.     

Properties of Chemical Solution Deposited Polycrystalline Neodymium-Modified Bi4Ti3O12

Thin films of neodymium-modified bismuth titanate Bi_3.44Nd_0.56Ti₃O₁₂ (BNT) were grown on Pt/TiO₂/SiO₂/Si substrates using chemical solution deposition method. The capacitors made by applying top Au electrodes on BNT films showed significantly improved values of the remanent polarization as compared to that using bismuth titanate Bi₄Ti₃O₁₂ (BT) films. The 2P _r value for the BNT capacitors was determined to be equal to 38 C/cm² at an applied voltage of 24 V, whereas, for Bi₄Ti₃O₁₂ (BT) capacitors a value of 20 C/cm² was measured at the same applied voltage. The maximum piezoelectric and pyroelectric coefficients of 22 pm/V and 112 C/m² K respectively, were measured for the BNT thin films.     

Hysteresis caused by defects in buffer layer of metal-ferroelectric-insulator-semiconductor (MFIS) devices

Abstract

We have investigated the roles of buffer layer in the Pt/SBT-Y₂O₃/p-Si (MFIS) capacitors. We found that the insertion of Y₂O₃ buffer layer prevents the charge injection from the Si substrate to ferroelectric layer. However, negative charges with the effective density of 3.21×10¹²/cm² were generated due to the additional process step for Y₂O₃ deposition. We suggested that the asymmetrical increase of a memory window is due to the domain pinning caused by negative charges in buffer layer. In addition, we reported that the mobile positive charges in ferroelectric layer can induce the shift of the hysteresis loops depending on the gate-bias polarity and a ramp rate during the capacitance-voltage (C-V) measurement. Since Y₂O₃ buffer layer minimize the charge injection, the shift of the hysteresis loops was asymmetrical.     

Electrodes for ferroelectric thin films

Abstract

Platinum and ruthenium oxide (RuO₂) deposited by ion beam sputter-deposition are evaluated for use as electrodes for PZT thin film capacitors. The effect of deposition temperature, film thickness, and the presence of oxygen on hillock formation in platinum is discussed. It is shown that the hillock density in Pt/Ti/SiO₂/Si films can be significantly reduced by properly controlling the processing conditions and film thickness. Stress measurements correlate with the experimental observation that depositing thinner platinum films (<800 Å) is an effective means of reducing hillock formation. The use of an intermediate deposition temperature 200–250°C also helps minimize hillock formation. Diffusion of the Ti adhesion layer into and/or through the platinum was significantly reduced by replacing the Ti with a TiO_x adhesion layer. RuO₂ electrodes are compared to Pt in terms of resistivity, surface morphology, microstructure and film orientation.     

Application of nickel alloy electrode to characterization of crystal oriented PbTiO3 thin films

A new electrode material made of nickel alloy has been developed that can be used in place of platinum electrodes, which have commonly been utilized in conjunction with PbTiO₃ and Pb(Zr, Ti)O₃ thin films. This alloy electrode is composed of nickel, chromium, iron and aluminium. When heated in an oxidation atmosphere, an alumina film layer is generated on the surface of the alloy that prevents the electrode from oxidizing and reacting with PbO. The remanent polarization of the PbTiO₃ film on the alloy electrode oriented in the (111) direction was 25 μC/cm², a value which is approximately equal to that of ceramics. The breakdown strength of the film reached the relatively large value of 190 kV/cm at room temperature. In addition, the as-grown film was already polarized with a degree of polarization estimated to be half that of fully polarized bulk ceramics.     

Kinetics of (Fe,Cr)7C3/Fe dense ceramic layer prepared by in situ synthesis

Abstract

In this study, the (Fe,Cr)₇C₃/Fe dense ceramic layer was produced by in situ synthesis method with subsequent heat treatment. According to the results of thermal analysis, the as-cast specimen was subjected to heat treatment at 1186, 1208, 1230?°C for 1–5?h in argon atmosphere respectively. The microstructures of the compound region were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and the kinetics of the (Fe,Cr)₇C₃ dense ceramic layer were analyzed. The results show that a-Fe and (Fe,Cr)₇C₃ are the predominant crystalline phases in the composite obtained. Depending on the layer process time and temperature, the thickness of the reaction layer formed on the substrate ranged from 12.3 to 32.6?mm. The kinetics of dense ceramic layer by the in situ synthesis method was also studied and the activation energy for the diffusion process was estimated to be 292?kJ/mol. Moreover, an attempt was made to investigate the possibility of predicting the iso-thickness of chromium carbide layer variation and to establish some empirical relationships between process parameters and dense ceramic layer thickness.     

Microwave Properties of Thin BSTO Films Based Varactors for High Frequency Applications

The influence of film thickness on electrophysical properties of planar varactors based on ion-plasma deposited BaSrTiO₃ films is investigated. Planar capacitors with copper electrodes were used for tunability and loss factor investigation. Microwave parameters of varactors based on BaSrTiO₃ films were measured at (1 ÷ 8) GHz frequency range and (170 ÷ 370) K temperature range. The tunability and loss factor was found to be strongly depended on film thickness. The optimal thickness of BaSrTiO₃ film from high tunability and low loss factor point of view was estimated as 1 μm approximately. Dielectric losses at frequency range (1 ÷ 8) GHz (tan δ ～0,01 at 1 GHz, tan δ ～ 0,014 at 8 GHz and zero bias) and high tunability (Cmax/Cmin ～ 2) at room temperature were observed.     

Electrical characterizations of MgTiO3 thin films grown on Si

Abstract

We have analyzed MgTiO₃ thin films grown on the Si substrate with/without SiO₂ using pulsed laser deposition (PLD). We find that MgTiO₃ thin films start to crystallize at 600°C, causing electrical instabilities in the MIS capacitors above this temperature. Detailed analysis by XRD technique reveals that structural differences of MgTiO₃ thin films were not obvious below 600°C, whereas the electrical characteristics changes as a function of deposition temperature and the presence of thermally grown SiO₂. We observe that the decrease of deposition temperature results in the increase of leakage current and anomalous positive charge (APC) density. These drawbacks were effectively suppressed by growing 100A SiO₂ layer on the Si substrate prior to the deposition of MgTiO₃ thin films.