Studies of a PT/PZT/PT sandwich structure for feram applications using sol-gel processing

Abstract

A silicon-based PbTiO₃/Pb(Zr,Ti)O₃/PbTiO₃ (PT/PZT/PT) sandwich structure with Pt electrodes are prepared by a sol-gel method. Effects of the PT buffer layers to the phase formation of the PZT films are studies. Dielectric and ferroelectric properties of the sandwich structure are measured. Maximum dielectric constant of about 900 is obtained at the coercive field 20 kV/cm. The leakage current density is less than 5 × 10^?9 A/cm2 below 200 kV/cm, which is much lower than that of PZT/PT structure. And there is almost no fatigue even after 5 × 109 read/write cycles for the PT/PZT/PT sandwich structure.     

Ferroelectric properties of heterolayered lead zirconate titanate thin films

Heterolayered Pb(Zr_{1 − x} Ti _x )O₃ thin films consisting of alternating PbZr_0.7Ti_0.3O₃ and PbZr_0.3Ti_0.7O₃ layers were successfully deposited via a multistep sol-gel route assisted by spin-coating. These heterolayered PZT films, when annealed at a temperature in the range of 600–700^∘C show (001)/(100) preferred orientation, demonstrate desired ferroelectric and dielectric properties. The most interesting ferroelectric and dielectric properties were obtained from the six-layered PZT thin film annealed at 650^∘C, which exhibits a remanent polarization of 47.7 μC/cm² and a dielectric permittivity of 1002 at 100 Hz. Reversible polarization constituents a considerably high contribution towards the ferroelectric hysteresis of the heterolayered PZT films, as shown by studies obtained from C-V and AC measurement.     

Dielectric properties and morphotropic phase boundaries in the xPb(Zn1/3Nb2/3)O3−(1−x)Pb(Zr0.5Ti0.5O3 pseudo-binary system

Ceramics in the xPb(Zn_1/3Nb_2/3)O₃−(1−x)Pb(Zr_0.5Ti_0.5)O₃ [xPZN–(1−x)PZT] solid solution system are expected to display excellent dielectric, piezoelectric, and ferroelectric properties in compositions close to the morphotropic phase boundary (MPB). The dielectric behavior of ceramics with x = 0.1−0.6 has been characterized in order to identify the MPB compositions in this system. Combined with X-ray diffraction results, ferroelectric hysteresis measurements, and Raman reflectivity analysis, it was consistently shown that an MPB exists between x = 0.2 and x = 0.3 in this binary system. When x ≤ 0.2, the tetragonal phase dominates at ambient temperatures. In the range of x ≥ 0.3, the rhombohedral phase dominates. For this rhombohedral phase, electrical measurements reveal a profound frequency dispersion in the dielectric response when x ≥ 0.6, suggesting a transition from normal ferroelectric to relaxor ferroelectric between 0.5 ≤ x ≤ 0.6. Excellent piezoelectric properties were found in 0.3PZN–0.7PZT, the composition closest to the MPB with a rhombohedral structure. The results are summarized in a PZN–PZT binary phase diagram.     

Properties of PbZrxTi1−xO3/CeO2/SiO2/Si structure

Abstract

We report the crystalline quality and electrical properties of PbZr_xTi_1?xO₃ (PZT) films on n-type Si(100) substrates with CeO₂/SiO₂ dual buffer layers. PZT films and CeO₂ buffer layers were prepared by pulsed laser deposition technique, and SiO₂ buffer layers were formed by thermal dry oxidation. It was found that CeO₂/SiO₂ dual buffer layers effectively prevented Si and Pb interdiffusion between PZT and Si substrates. Furthermore, the capacitance-voltage (C-V) characteristics of the PZT/CeO₂/SiO₂/Si heterostructures demonstrated ferroelectric switching properties, showing a memory window as large as 2.7 V at 1 MHz.     

Bilayered Pb(Zr,Ti)O3/(Bi,Nd)4Ti3O12 thin films

Bilayered ferroelectric thin films consisting of Pb(Zr_0.52Ti_0.48)O₃ (PZT) and (Bi_3.15Nd_0.85)Ti₃O₁₂ (BNT) have been successfully synthesized on Pt/Ti/SiO₂/Si substrates, via a combined sol–gel and rf-sputtering route. Their ferroelectric and dielectric properties are critically dependent on the phases present, film texture and in particular layer and film thicknesses. Due to the coupling of PZT and BNT bilayers, there requires an optimized thickness combination of the two ferroelectric layers, in order to give rise to the wanted ferroelectric and dielectric properties, while the phenomenon can not be accounted for by the simple series connection model.     

Effects of ILD & IMD characteristics on ferroelectric properties of fram devices

Abstract

We have deposited SiO₂ using plasma-enhanced TEOS-based (PE-TEOS) CVD method and USG and PSG using atmosphere-pressure CVD method on Pb(Zr, Ti)O₃(PZT) capacitors. The ferroelectric and dielectric properties of the SiO₂ covered PZT capacitors were characterized. SIMS (secondary ion mass spectroscopy) was utilized to obtain hydrogen concentration in the deposited ILD and IMD materials. The concentration of hydrogen in the PE-TEOS-derived SiO₂ was lower than that in the PSG and the USG. Internal stress was low tensile at room temperature and the behavior of thermal stress hysteresis was nearly similar for all SiO₂ materials. Remnant polarization (Pr) of the PE-TEOS covered PZT capacitors was severely degraded as compared to that of as-deposited capacitors. From these results, we have concluded that the degradation of ferroelectric characteristics of PZT capacitors associated with the ILD and IMD processes was closely related to the plasma-induced damage.     

Sputtered ferroelectric thin films for dynamic random access memory applications

Abstract

A low thermal budget (with 550°C annealing) process with Ti-compensation for sputtered ferroelectric PZT thin films has been developed. PZT films with a composition near the morphotrophic phase boundary (Zr/Ti = 53/47) annealed at 550°C for 1 hr in a N₂ ambient exhibits high charge storage density and low leakage current density, which are the important requirements of dielectric materials for ULSI DRAM cells. It was also found that Ti compensated films show good fatigue endurance in comparison with non-Ti compensated films.     

Fatigue properties of Sb doped PZT thin films deposited by dc reactive sputtering

Abstract

Sb doped reactive sputtering-derived Pb(Zr, Ti)O₃ (Zr/Ti=48/52) thin films were investigated with the intention of improving ferroelectric properties. Also, the atomic valence of Sb in PZT thin film was confirmed as trivalent cation (Sb³⁺) by x-ray photoelectron spectroscopy (XPS). According to the tolerance factor t, Sb³⁺ tends to occupy the B-site of ABO₃ perovskite structure and acts as an acceptor that generates oxygen vacancies and holes. Transmission Electron Microscope(TEM) was used to observe the structural changes of PZT thin films by Sb addition. The leakage current densities and P_r of PZT thin films increased as the Sb contents increased. 0.7at% Sb doped PZT(PZST07) thin films exhibited improved fatigue properties (about 10% degradation of the remanent polarization after 10¹⁰ switching cycles).     

MOCVD of Pb(Zr x Ti1-x )O3 thin films on MgTiO3/Si substrates and their electrical properties

Abstract

Pb(Zh _x , Ti_1-x )O₃(PZT) thin films were deposited on Si substrates using MgTiO₃ as the buffer layer and the electrical properties of those MFIS structures were investigated. PZT and MgTiO₃ films were made by MOCVD using ultrasonic spraying technique. Perovskite PZT films have been succesfully made at the substrate temperature of 550 to 600°C only when using MgTiO₃ buffer layer. AES depth profile analysis and RBS analysis revealed that there is no remarkable interdiffusion and no formation of reaction layer between PZT and MgTiO₃ and/or between MgTiO₃ and Si substrate. The capacitance-voltage (C-V) curves of the MFIS structure which were made with PZT and MgTiO₃buffer layer have shown the hysteresis resulted from the ferroelectric switching of the PZT films.     

Effects of temperature on the ferroelectric properties of PbZr0.53Ti0.53O3-based capacitors prepared by pulsed laser-ablation deposition

Abstract

The effects of elevated temperature on the long and short term ferroelectric properties of La_0.5Sr_0.5Co₃(LSCO)/PbZr_0.53 Ti_0.47O₃(PZT)/La_0.5Co₃(LSCO) heterostructure capacitors are presented in this paper. Capacitors are evaluated in the temperature range of 25°C to 100°C. Polarization values (switched and non-switched), coercive voltages, and PZT resistivity are measured as a function of temperature. Rate of fatigue, tendency to imprint, and retention loss are also investigated in the 25–100°C temperature range. An activation energy for the process responsible for loss of remanent polarization is determined from the fatigue data.     

Conducting oxide electrodes for ferroelectric films

Abstract

Suitability of oxide electronic conductors [e.g. ruthenium oxide (RuO_x) and indium-tin-oxide (ITO)] as contact metallization for ferroelectric films (e.g. PbZr_xTi_1?xO₃) was investigated using techniques such as Rutherford backscattering spectrometry, x-ray diffraction and electron spectroscopy for chemical analysis. Thin films of RuO_x and ITO were deposited on Si substrates by reactive sputtering. Either PbO or PZT (x = 0.53) films were deposited onto the conducting oxides and the specimens were annealed at various temperatures between 400°C and 700°C. Less intermixing was found in Si/RuO_x/ PZT films when compared to Si/ITO/PZT under similar processing conditions. The ferroelectric properties of PZT films on RuO_x electrodes are compared to those on Pt electrodes. The PZT films show improved fatigue properties on RuO_x electrodes.     

Effect of excess-PB and prenucleation layer on properties of Pb(ZryTi1-y)O3 thin films prepared by mod process

Abstract

A modified metal-organic decomposition process, MOD has been successfully utilized to improve the ferroelectric properties of Pb(Zr_0.52Ti_0.48)O₃, PZT, thin films. Multilayer PZT/Pt(Si) films, which contain 0.12 μm layer of spin coated PbO-excess (10 mol%) precursors on top of 0.12 μm layer of stoichiometric PZT precursors, exhibit superior ferroelectric properties (P_r=14.2 μC/cm²; E_c=62 kV/cm) to the single layer PZT/Pt(Si) films of the same thickness (0.24 μm), which are either stoichiometric or 10 mol% Pb-enriched. The ferroelectric properties are further improved when the PZT films were synthesized using a thin pulsed laser deposited (PLD) prenucleation layer (0.06 μm). The subsequently MOD-prepared PZT films posses high remanent polarization (P_r=23.2–26.6 μC/cm²) and low coercive field (E_c=62.9–69.0 kV/cm).     

Ferroelectric PbZr0.2Ti0.8O3 thin films on epitaxial Y-Ba-Cu-O

Abstract

Using a combination of pulsed laser deposition and sol-gel processing, we have fabricated epitaxial PbZr_0.2Ti_0.8O₃/YBa₂Cu₃O₇-x heterostructures on single crystalline [001] LaAlO₃. Rutherford back-scattering studies show the composition to be the same as the nominal starting composition. Transmission electron microscopy shows the existence of a randomly oriented polycrystalline microstructure in the PZT layer with a grain size of about 500–1000Å. Microscopic pores were also observed in the PZT layer. The PZT film exhibits ferroelectric hysteresis with a saturation polarization of 22–25μC/cm² (at 7.5V, 1kHz), a remanence of 5–6μC/cm² and a coercive field of about 40kV/cm.     

Preparation of BaPbO3 Electrode Thin Films by RF Magnetron Sputtering

For electrode materials of Pb(Zr,Ti)O₃ (PZT) thin films in ferroelectric random access memory (FeRAM), various materials have been studied. As new electrode material with which the polarization and fatigue properties are improved, we take notice of barium metaplumbate BaPbO₃ (BPO). Because the BPO contained lead (Pb) and oxygen is conductor that adopted same perovskite structure as PZT. BPO thin films were prepared by rf magnetron sputtering on various substrates. (SiO₂/Si, MgO, Al₂O₃ and Pt-coated substrates), and influence of growth conditions (sputtering gas, rf power, the substrate-heating temperature and post anneals) on crystallization and conductivity were investigated. In case of post anneal after sputtering at room temperature, perovskite single phase was obtained above 400°C. In case of substrate heating while sputtering, without post anneal, perovskite single phase was obtained at 350–500°C on SiO₂/Si substrates (110) preferred orientation BPO films obtained at low temperature, and resistivity of the films decreased at decreasing sputtering temperature. Resistivity of the film at substrate temperature 350°C was 3 × 10^?3 Ω cm. In the case of single crystal substrate, the BPO films were epitaxially grown. Orientation of the films was varied with the sputtering condition. The epitaxial PZT thin films were also grown on the BPO, revealing that PZT(111)[011] //BPO(111)[011] //Pt(100)[011] //MgO(100)[011] and PZT(111)[011] //BPO(111)[011] //Pt(111)[011] //Al₂O₃(001)[100] structures were obtained, and their ferroelectric properties were also evaluated.     

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.     

Investigation on various insulator layers for MFIS capacitors

Abstract

Effects of insulator layers of Metal(Au)/Ferroelectrics(PZT)/Insulator/Si (MFIS) structure capacitors are investigated for non-destructive type non-volatile memory device applications. Various high dielectric oxide layers such as Al₂O₃, Ta₂O₅, TiO₂ and ZrO₂ were fabricated by reactive sputtering as insulating layers. The oxide insulators give significant impacts on the morphologies of PZT layer and the properties of capacitors. It is noted that the oxide layers with small thermal expansions (<6x10^–6/°C) coefficient caused cracks on PZT films during PZT crystallization annealing. The effects of insulators as a diffusion barrier are also comparatively studied using Auger electron spectroscopy. In addition, the characteristics of high dielectric solid solution, such as titanium oxide-zirconium oxide, are also studied.     

Properties of reactively sputtered IrOx for PZT electrode applications

Abstract

The use of IrO_x for electrodes in PbZr_xTi_1?xO₃ (PZT) capacitors for ferroelectric memory applications has proven to be advantageous in several respects. In comparison with Pt, IrO_x often exhibits improved fatigue and provides resistance to hydrogen induced degradation at elevated temperatures. Since IrO_x is often produced by sputtering in an oxygen containing environment, several forms of IrO_x can be produced depending on the process conditions. This work concentrates on an analysis of the DC reactive sputtering of IrO_x from an Ir metal target. As with other oxidizable metals, Ir exhibits a transition between metal and oxide mode sputtering when sputtered in oxygen containing atmospheres. Variations in the Ar/O₂ gas flow ratio were used to produce Ir and IrO_xfilms on both sides of the metal-to-oxide mode reactive sputtering transition. Changes in the IrO_x film properties were quantified by using a combination of metrics including X-ray diffraction, sheet resistance, and stress. It was found that the, IrO_x crystalline structure and other IrO_x properties could be directly related to the ferroelectric switching performance of PZT capacitors with IrO_xtop electrodes. A relationship between IrO_x deposition processes and resistance to etch induced damage of the ferroelectric properties was also observed.     

The dependence of ferroelectric and fatigue behaviors of PZT films on annealing conditions

Films of PZT about 0.2 μm thick with the composition PbZr_0.53Ti_0.47O₃ were prepared using the metalloorganic decomposition (MOD) process. The amorphous films produced by pyrolysis at 350°C were annealed at 550, 575, 600 and 650°C for 10 minutes, 1 hour or 4 hours. Films annealed at temperatures below 550°C showed no ferroelectric behavior while others annealed above 650°C showed signs of loss of ferroelectric behavior. Most films demonstrated satisfactory ferroelectric properties such as low switching voltage and high polarization values. Some PZT films also demonstrated fatigue life-time of more than 10⁹ switching reversals. The performance of the films was dependent on the annealing time and temperature. It was found that films with better initial polarization values did not necessarily demonstrate better fatigue behavior. The causes of film degradation as a result of switching based on the pinning of domains at grain boundaries triggered by the migration of pores is discussed.     

Preparation and characteristics of fine-grained ferroelectric glass-ceramic composites via a modified hybrid route at low temperature sintering

Dense, homogeneous and fully developed fine-grained ferroelectric Pb(Zr_0.53Ti_0.47)O₃?Cbased glass-ceramics have been successfully prepared at a low-sintering temperature of 850?C900°C by a modified hybrid process in air. The influence of the PbO-B₂O₃-SiO₂ (abbreviated as PBS) glass-gel content on the microstructure, dielectric, and ferroelectric properties of such glass-ceramics has been investigated. The temperature dependence of the dielectric constant indicated that the fine-grained Pb(Zr_0.53Ti_0.47)O₃ (abbreviated as PZT(53/47)) based glass-ceramic shows the characteristic dispersion at the Curie point. Ferroelectric hysteresis loop analyses have been performed to manifest the ferroelectric nature of the highly crystallized PZT(53/47) phase prepared by this modified novel hybrid process despite containing higher wt% glass-gel contents. The best dielectric and ferroelectric properties in a typical sample with 5% by weight of glass-gel content were found to have dielectric constant and loss tangent of 920 and 0.02 at 1 kHz, respectively. The saturation polarization (P _s ), and remanent polarization (P _r ) as well as the coercive field (E _c ) are 21.9 ??C/cm², 10.8 ??C/cm² and 2.19 kV/mm, respectively.     

The dependence of ferroelectric and fatigue behaviors of PZT films on microstructure and orientation

Microstructure of PbZr_0.53Ti_0.47O₃ films about 0.2 μm thick was studied as a function of annealing temperature and time using TEM and SEM. The films were heat treated in the temperature range 550–650°C for times from 10 minutes to 4 hours. The features characterized were crystal structure of individual PZT particles, PZT grain size and shape, porosity, and grain boundaries. The films were prepared by the metallo-organic decomposition (MOD) process which produced films amorphous to x-rays after pyrolysis. A growth mechanism of PZT particles from the amorphous film is discussed. It was found that at mild annealing conditions (i.e., low temperature and short annealing times), agglomerates of microcrystallites of perovskite, and probably pyrochlore, were formed. As the temperature was increased, the microcrystallites grew into porous submicron perovskite single crystals. With excessive heating, intraparticle pores were found to migrate and became pinned at the grain boundaries, causing widening of boundaries which probably was responsible for the degradation in ferroelectric properties observed in such films. The dependence of ferroelectric and fatigue behaviors on grain orientation was also investigated. It was found that PZT films oriented in the [111] direction demonstrated better fatigue behavior but not necessarily higher polarizations.