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.     

Sputter deposition of ferroelectric thin films

Ferroelectric films are typically deposited by a variety of techniques, the two most common being chemical methods (sol-gel, metalorganic decomposition) and sputtering. In this paper we briefly review the sputtering techniques, and then discuss ion beam sputter deposition in greater detail. In particular, ion beam sputter deposition of epitaxial lead zirconate titanate (PZT) films is described. It is shown that the films with compositions close to the morphotropic boundary typically show well-developed ferroelectric hysteresis loops, Pmax around 45 μC/cm2, and Pr around 20 μC/cm2. In comparison with typical polycrystalline sol-gel PZT films, however, coercive fields of thin epitaxial films are large (120-200 kV/cm for 95 nm films). The pulsed fatigue behavior is remarkably similar to a polycrystalline non-oriented sol-gel PZT film investigated for comparison. The similarities suggest that the aging behavior may be dominated by the electrodes, which were Pt in both systems.     

GHZ polarization dynamics in ferroelectric thin films

Abstract

We present results from experiments which measure the local dielectric response of ferroelectric thin films driven by microwave-frequency electric fields. The repetition rate of a mode-locked Ti:Sapphire laser is used to generate a microwave drive signal that is phase-locked to an optical probe pulse and applied to the ferroelectric thin film. The induced polarization change in the ferroelectric film is measured stroboscopically via the electro-optic effect. Polarization images are acquired by scanning the laser beam across the sample in a confocal geometry. Time resolution is achieved by changing the delay between the electrical pump and the optical probe. Initial results show large local phase shifts in the ferroelectric response of closely separated regions of a Ba_0.5Sr_0.5TiO₃ thin film. This new experimental technique may help to understand the physical mechanisms of dielectric loss in these materials.     

Adaptive-learning neuron circuits using ferroelectric thin films

Abstract

Adaptive-learning neuron circuits are reviewed, in which a pulse frequency modulation (PFM) system is used and the interval of output pulses is changed through the learning process. Key devices of the circuits are MFSFETs (Metal Ferroelectric Semiconductor Field Effect Transistors). They are used for representing the synaptic weights of neurons and the polarity of the films is gradually changed by applying input pulses to the gates. In order to produce PFM signals, circuit using a UJT (unijunction transistor) is discussed. As a preliminary experiment for realization of MFSFETs, the electrical properties of metal ferroelectric metal (MFM) capacitor using sol-gel derived PZT films are discussed.     

Short-time switching of polarization in thin ferroelectric films

Abstract

The peculiarities of repolarization in thin-film metal-PZT-metal structures with varioùs areas of electrodes have been investigated. The nonclassical mechanism of polarization switching was determined to take place at short durations of voltage pulses. The repolarization was shown to carry out according to the mechanism of formation and lateral widening of domains at long times.     

Formation of ferroelectric thin films on ruthenium electrodes

Abstract

Sr_0.8Bi_2.4Ta₂O₉ (SBT) and Bi_3.25+xLa_0.75Ti₃O₁₂ (BLT) films were prepared on Ru electrodes by a sol-gel spin-coating method. It was found in the case of SBT/Ru that the oxidation of Ru was suppressed by annealing at temperatures lower than 650°C in O₂ atmosphere or by annealing lower than 700°C in N₂. However, the remanent polarization values of the films were as low as 2.3 μC/cm2 (2Pr) for the former case and 4.4 μC/cm2 for the latter case. On the other hand, the BLT/Ru samples crystallized at 650°C showed good crystallinity without oxidation of the Ru electrode. It was found that the ferroelectric properties depended on the amount of excess-Bi sensitively, and the film prepared under the optimum Bi composition showed an excellent P-V hysteresis loop with 2 Pr of 25 μC/cm2.     

Preparation of ferroelectric thin films for Si-based devices

Abstract

Lead titanate (PbTiO₃) thin films have been prepared on titanium dioxide coated silicon wafers by chemical vapor deposition (CVD). The pure PbTiO₃ thin films were deposited by controlling the experimental conditions. The gas phase reaction of TiO₂ occurred by exceeding the critical value of titanium input fraction at constant oxygen partial pressure. Strontium titanate (SrTiO₃) thin films have been prepared on p-type silicon wafers by radio frequency (RF) magnetron sputtering. The SrTiO₃ thin film was polycrystalline and the Sr/Ti ratio of this film was 0.91. The SrTiO₃ thin films contain three regions, an external surface layer, a main layer and an interfacial layer. The stoichiometric SrTiO₃ thin film was obtained by using the SrO excess target. The SrTiO₃ film annealed at 600[ddot]C has an ideal capacitance-voltage (C-V) curve and maximum effective dielectric constant.     

Pulsed excimer laser deposition of ferroelectric thin films

Abstract

Pulsed UV excimer laser ablation was employed to deposit multi-axial, bi-axial and uni-axial ferroelectric compositions of PZT, bismuth titanate and lead germanate respectively. In general, a fluence lower than 2 J/cm² caused a preferential evaporation of volatile components, resulting in stoichiometric imbalance. However, the fluences beyond 2 J/cm² enabled the deposition of stoichiometric thin films of multi-component oxide systems. The intrinsic bombardment due to the energetic ablated species during the thin film deposition seemed to influence the composition, structure, orientation and the electrical properties. The electrical characterization of ferroelectric films indicated a dielectric constant of 800–1000, a P_r of 32μC/cm² and E_c of 130KV/cm for polycrystalline PZT films and the corresponding quantities were measured to be 150, 7 μC/cm² and 20 KV/cm for in-situ crystallized c-axis preferred oriented bismuth titanate films. Lead germanate thin films oriented along c-axis (003) showed a dielectric constant of 30, a P_r of 2.5 μC/cm² and E_c of 55 KV/cm.     

Dielectric and ferroelectric properties of BTFCO thin films

Journal of Electroceramics - Single phase Bi3.25La0.75Ti2.5Nb0.25Fe0.125Co0.125O12 (BTFCO) thin films were deposited on Pt/TiO2/SiO2/Si substrates by RF-magnetron sputtering. Ferroelectric domain...     

Characterization of MOCVD Pt electrode for ferroelectric thin films

Abstract

Platinum thin films were deposited by low pressure chemical vapor deposition (LPMOCVD) on SiO₂/Si and (Ba, Sr)TiO₃/Pt/SiO₂/Si substrates using Pt-hexafluoroacetylacetonate at various deposition temperatures. The shiny mirror-like Pt thin films of a high electrical conductivity were obtained, when the deposition temperature is between 325°C and 350°C, whereas above 375°C Pt thin films showed rough surface as well as poor adhesion property to oxide substrate. Pt thin films had a good step coverage of 90%. The results indicate that LPMOCVD Pt thin films can be applied for the top electrode of high dielectric thin film, which is thought to be one of the best candidate materials for a capacitor of ULSI DRAM.     

Dielectric response of ferroelectric thin films on non-metallic electrodes

The dielectric response of thin films of perovskite ferroelectrics fabricated on electrodes such as indium tin oxide is calculated as a function of frequency, film & barrier layer thickness, and dielectric constant using an equivalent circuit model based on properties of the bulk, the electrode and a series resistance. Using reasonable estimates of parameters it is shown that measurements of the dielectric constant and loss by conventional dielectric bridges can lead to results which vary in a complex fashion and which may bear little relation to the true dielectric properties of the film. In particular, observation of a Curie temperature in thin films can often be masked by circuit or electrode effects. Results are given in terms of direct frequency plots and as impedance spectra.     

Recent advances in the deposition of ferroelectric thin films

Abstract

Recent developments in ferroelectric thin film deposition involving plasma based approaches, are described, which include a) multi-magnetron sputter deposition, b) Multi-ion-beam reactive sputter (MIBERS) deposition, c) Pulsed excimer laser ablation and d) ECR (Electron cyclotron resonance) plasma assisted deposition. These methods commonly prevailed intrinsic low energy ion bombardment during the growth process, which may be used for the control over composition, crystallization temperature and microstructure. A low energy (60–75 eV) ion bombardment of the ferroelectric Pb(Zr, Ti)O₃ thin films indicated a reduction in the phase formation/crystallization temperature, improved the electrical properties, microstructure and the surface smoothness. Discussion is presented exphasizing the effects of low energy bombardment in different deposition processes. Recent findings using rapid thermal annealing process are also described.     

Excimer laser ablation processed ferroelectric and antiferroelectric thin films

Abstract

The dielectric and electrical properties of excimer laser ablated processed paraelectric (Ba_0.5, Sr_0.5)TiO₃, ferroelectric Bi-layered SrBi₂(Ta_0.5Nb_0.5)₂O₉, and antiferroelectric (PbZrO₃) thin films have been investigated. The effect of processing parameters on the microstructure of the films and the functional properties has been presented in detail. Some of the recent studies of stress induced effects, dielectric, hysteresis and ac and dc electrical properties have been highlighted in conjunction with microstructures of the films.     

Comparing macroscopic and microscopic properties of seeded ferroelectric thin films

The comparison of macroscopic and microscopic properties of ferroelectric thin films in the systems of lead zirconate titanate (PZT) and strontium bismuth tantalate (SBT) with and without seeds is carried out. Microscopic properties were studied by piezo-response force microscopy (PFM). The local piezoelectric properties with and without seeds are compared with their macroscopic electric properties measured by conventional techniques. Previous microstructure analysis of PZT thin films showed that an intermetallic Pt _x Pb layer between PZT and Pt, formed during the annealing process, was reduced and even eliminated in seeded PZT films. In SBT films, the addition of SBT seeds suppressed the interdiffusion of Pt and film components. Hence, the interfaces of PZT/substrate and SBT/substrate are modified by the presence of seeds, and their electrical properties are improved. In both PZT and SBT films, the remanent polarization values are higher in seeded films than in unseeded ones. Similarly, local piezo-response signal of single grain showed higher longitudinal piezoelectric coefficient d ₃₃ in seeded films than in unseeded ones. The critical voltage in which the ferroelectric domain starts to switch is lower in seeded films than in unseeded ones. The analysis of nanoscale switching in PZT and SBT films by PFM is presented and related to the corresponding macroscopic electric properties.     

Growth and microstructures of sputtered ferroelectric PbTiO3 thin films

Abstract

PbTiO₃ thin films, 5–200 nm in thickness, were epitaxially growth on miscut (001) SrTiO₃ substrates by planar magnetron sputtering for understanding of film growth mechanism and their ferroelectricity. The surface of the miscut substrates with miscut angle of 1.7 degree contains periodic step lines and terraces; the step height is 0.4 nm and terrace width is 14 nm. The surface structures of PbTiO₃ films comprised periodic striped patterns which was reflected in the initial surface of the substrate. It was found that under a stoichiometric film composition the film growth was governed by Frank-van der Merwe type and resultant epitaxial films showed extremely smooth surface. Deposition on a miscut substrate under a stoichiometric condition is essential to making the uniform ferroelectric thin films.     

Electric field dependence of piezoelectric coefficient in ferroelectric thin films

A simple model is developed to explain the hysteretic electric field dependence of the piezoelectric coefficient in ferroelectric thin films. The nonlinear susceptibility and hysteretic electrical polarization behavior can explain the piezoelectric hysteresis characteristics. An empirical model introducing a weighting factor is utilized to represent the electric field dependence of the nonlinear susceptibility. Experimentally, the magnitude of the maximum of the piezoelectric coefficient measured in the backward direction of the piezoelectric hysteresis loop is usually larger than that of the maximum measured in the forward direction. The proposed model shows that the weighting factor in modeling the nonlinear susceptibility may account for this observed phenomenon. The nonlinear susceptibility may also explain the peaked shape observed in piezoelectric coeffieint-electric field hysteresis loops. The approach in this work does not require the “hard ferroelectric approximation” of the polarization characteristics, and provides a method of predicting the piezoresponse from measured capacitance and polarization properties of the ferroelectric thin film.     

Texturing and electrical characteristics of sol-gel derived ferroelectric thin films

Abstract

Highly oriented PZT, PLZT and PMN thin films were fabricated on various substrates using sol-gel method. The thin films having different orientation were fabricated by different drying conditions for pyrolysis. The preferred orientations of the PZT, PLZT and PMN thin films were observed using XRD(X-ray diffraction). The hysteresis loops, capacitance-voltage, and fatigue characteristics of the films were investigated using an RT66A standardized ferroelectric test system. The dielectric constant and current-voltage characteristics of the films were investigated using impedance analyzer and pA meter, respectively. The films oriented in particular direction showed better electrical characteristics to the randomly oriented films.     

Sol-gel processed ferroelectric and multiferroic thin films for integrated devices

Abstract

Ferroelectric barium titanate and multiferroelectric bismuth ferrite thin films have been fabricated by using sol-gel processing technique. The starting materials for fabrication of were barium 2-ethyl hexanoate and titanium (IV) isoproposxide. Bismuth nitrate and ferric nitrate were the precursors for the fabrication of thin films. The as-deposited films were annealed at higher temperature for crystallization. The X-ray diffraction study on the films showed that the as-grown films were found to be amorphous that crystalized to proper phases by annealing at 550?°C in air for one hour. All the samples showed high optical transparencies in the visible frequency range. The room temperature dielectric constant and loss tangent of barium titanate thin films at 1?kHz frequency were found to be 400 and 0.01 respectively. Both the dielectric constant and loss tangent showed small dispersion in the frequency range of 0.10–1000?kHz range. The ferroelectricity in barium titanate thin films was confirmed by the presence of bell shaped capacitance-voltage (C-V) butterfly loop and saturated polarization-field (P-E) hysteresis loop. The as-grown bismuth ferrite thin films were also found to be amorphous that crystallizes after annealing at 500?°C. These films also showed high optical transparencies in the visible region. The bismuth ferrite thin film samples showed saturated hysteresis loop and magnetic polarization-magnetic field hysteresis loop as well, confirming the multiferroic nature of the samples.     

Recent progress in sputtering PZT thin films for ferroelectric memories

Abstract

This paper describes amorphous Pb(Zr, Ti)O₃ (PZT) thin films deposited by cosputtering Pb(Zr_0.5 Ti_0.5)O₃ and PbO targets. By optimizing the amount of the excess Pb and the deposition temperature, PZT thin films with a single perovskite phase were obtained successfully on Ir substrates and Pt substrates at 520°C. 250-nm-thick PZT films crystallized by rapid thermal annealing (RTA) at 600°C for 20 s exhibited excellent ferroelectric properties: a coercive voltage of 1.0 V, a remanent polarization density of about 40 μC/cm², and a polarization switching endurance over 1x109 cycles. Although a heat treatment in a reductive ambient causes degradation of ferroelectric properties of PZT thin films, their degraded ferroelectric properties can be easily recovered from by a 1-min RTA in an oxygen at 400°C.