Imprint Behavior Research Articles

Monte Carlo simulations of imprint behavior in ferroelectrics

In this letter, Monte Carlo simulation methods were used to investigate the influence of the defect orientation and concentration on the hysteresis loop in ferroelectric thin films. The hysteresis loops were calculated by an existing Monte Carlo model. For a certain type of defect orientation, the simulations revealed an asymmetric hysteresis loop behavior, similar to hysteresis curves recorded by imprint measurements. Though these results may not directly offer a new explanation for the imprint mechanism in ferroelectric thin films, they still provide insight information about the often observed phenomenon of imprinted hysteresis loops of as-prepared thin-film capacitors.

Polarization behaviors of (Bi3.15Nd0.85)Ti3O12 thin films deposited by radio-frequency magnetron sputtering

Ferroelectric (Bi3.15Nd0.85)Ti3O12 (BNdT) thin films of random orientation have been successfully deposited on Pt∕Ti∕SiO2∕Si(001) by radio-frequency magnetron sputtering, followed by rapid thermal annealing at 700 °C. They exhibit a remanent polarization 2Pr of 23.2μC∕cm2 and a coercive field EC of 112kV∕cm at an applied voltage of 10 V. These BNdT films also show a switchable polarization (ΔP=Psw−Pns) of 12.9μC∕cm2 at 5 V, together with an almost fatigue-free behavior up to 1.4×1010 switching cycles at both 100 and 150 °C. They demonstrate desirable retention and imprint behaviors at 100 °C, while a further increase in temperature up to 150 °C led to an acceleration in both retention loss and imprint behavior, which can be accounted for by the thermally assisted redistribution of defects and space charges. Studies of the ac dependence of relative permittivity suggest the occurrence of domain-wall pinning, which is a commonly observed phenomenon in oxide ferroelectric thin films.

IMPRINT RELATED FATIGUE BEHAVIOR OF Pt/SrBi2Ta2O9/Pt CAPACITORS

ABSTRACT Pt/SrBi2Ta2O9(SBT)/Pt ferroelectric capacitors with the film thickness of 300 nm were successfully fabricated using metalorganic decomposition (MOD) technique. The fatigue and imprint behaviors were studied. It is found that the switchable polarization of SBT capacitors had the increasing trend in the first stage, and after 2× 109 switching cycles, the switchable polarization began to decrease. The domains in SBT thin films were weakly pinned by defect charges and they were easily depinned when applying an external field, which resulted in the increase of switchable polarization. Furthermore, the depinned defect charges moved to the electrode interfaces and aggregated there, then resulted in the polarization suppression at last. During the switching process, both the behavior of fatigue and imprint existed simultaneously. A competition relationship existed between the depinning behavior of domains and the aggregation of defect charges at the electrode interfaces. It was the competition that decided the fatigue characterization for SBT capacitors. The higher the external voltage, the more suppressed polarization could be recovered, which suggested that more domains in SBT thin films were easily depinned under a higher external field, and the pinning depth by defect charges was different.

An Interneuronal Chemoreceptor Required for Olfactory Imprinting in C. elegans

Animals alter their behavioral patterns in an experience-dependent manner. Olfactory imprinting is a process in which the exposure of animals to olfactory cues during specific and restricted time windows leaves a permanent memory ("olfactory imprint") that shapes the animal's behavior upon encountering the olfactory cues at later times. We found that Caenorhabditis elegans displays olfactory imprinting behavior that is mediated by a single pair of interneurons. To function in olfactory imprinting, this interneuron pair must express a G protein-coupled chemoreceptor family member encoded by the sra-11 gene. Our study provides insights into the cellular and molecular basis of olfactory imprinting and reveals a function for a chemosensory receptor family member in interneurons.

Influence of irradiation on the switching behavior in PZT thin films

Spatially nonuniform imprint behavior induced by X-ray synchrotron, electron and neutron irradiation has been investigated in sol–gel Pb(Zr,Ti)O 3 thin films. The analysis of the switching current data reveals the strong influence of irradiation on the switching current shape. The obtained effects have been explained as a result of acceleration of the bulk screening process induced by irradiation. It was shown that the spatial distribution of the internal bias field is determined by the domain structure existing during irradiation. The changes in the structural characteristics during fatigue cycling have been reveled by high resolution synchrotron X-ray diffraction experiments on (1 1 1)-oriented PZT-based capacitors with a composition in the morphotropic region. From both ex situ and in situ measurements, microstructural changes with cyclic switching during fatigue have been evidenced and correlated with the evolution of the switching characteristics.

Interface-oxygen-loss-controlled voltage offsets in epitaxial Pb(Zr0.52Ti0.48)O3 thin-film capacitors with La0.7Sr0.3MnO3 electrodes

Epitaxial Pb(Zr0.52Ti0.48)O3 (PZT) thin-film capacitors with La0.7Sr0.3MnO3 (LSMO) electrodes have been grown on (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (001) substrates by pulsed-laser deposition. The process-induced imprint behavior in the ferroelectric capacitors was examined by in situ and ex situ annealing at various conditions. It was found that for the capacitors in situ annealed at reduced oxygen pressures, where the LSMO electrodes are stable, voltage offsets in the polarization-electric field hysteresis loops were observed only for those treated at temperatures higher than the Curie temperature. At lower temperatures, the oxygen loss may be suppressed by stresses arising primarily from the paraelectric-to-ferroelectric transformation. However, for the capacitors ex situ annealed at the same low temperature, large voltage offsets were induced due to the oxygen instability of the LSMO electrodes. We show evidence that the imprint is caused by oxygen loss at the PZT∕LSMO interface, and closely related to the variation of the PZT structure.

A chick model for the mechanisms of mustard gas neurobehavioral teratogenicity

The chemical warfare blistering agent, sulfur mustard (SM), is a powerful mutagen and carcinogen. Due to its similarity to the related chemotherapy agents nitrogen mustard (mechlorethamine), it is expected to act as a developmental neurotoxicant. The present study was designed to establish a chick model for the mechanisms of SM on neurobehavioral teratogenicity, free of confounds related to mammalian maternal effects. Chicken eggs were injected with SM at a dose range of 0.0017–17.0 μg/kg of egg, which is below the threshold for dysmorphology, on incubation days (ID) 2 and 7, and then tests were conducted posthatching. Exposure to SM elicited significant deficits in the intermedial part of the hyperstriatum ventrale (IMHV)-related imprinting behavior. Parallel decreases were found in the level of membrane PKCγ in the IMHV, while eliciting no net change in cytosolic PKCγ. The chick, thus, provides a suitable model for the rapid evaluation of SM behavioral teratogenicity and elucidation of the mechanisms underlying behavioral anomalies. The results obtained, using a model that controls for confounding maternal effects, may be replicated in the mammalian model and provide the groundwork for studies designed to offset or reverse the SM-induced neurobehavioral defects in both avian and mammals.

Convergent Effects on Cell Signaling Mechanisms Mediate the Actions of Different Neurobehavioral Teratogens: Alterations in Cholinergic Regulation of Protein Kinase C in Chick and Avian Models

Although the actions of heroin on central nervous system (CNS) development are mediated through opioid receptors, the net effects converge on dysfunction of cholinergic systems. We explored the mechanisms underlying neurobehavioral deficits in mouse and avian (chick, Cayuga duck) models. In mice, prenatal heroin exposure (10 mg/kg on gestation days 9-18) elicited deficits in behaviors related to hippocampal cholinergic innervation, characterized by concomitant pre- and postsynaptic hyperactivity, but ending in a reduction of basal levels of protein kinase C (PKC) isoforms betaII and gamma and their desensitization to cholinergic receptor-induced activation. PKCalpha, which is not involved in the behaviors studied, was unaffected. Because mammalian models possess inherent confounding factors from maternal effects, we conducted parallel studies using avian embryos, evaluating hyperstriatal nucleus (intermedial part of the hyperstriatum ventrale, IMHV)-related, filial imprinting behavior. Heroin injection to the eggs (20 mg/kg) on incubation days 0 and 5 diminished the post-hatch imprinting ability and reduced PKCg and bII content in the IMHV membrane fraction. Two otherwise unrelated agents that converge on cholinergic systems, chlorpyrifos and nicotine, elicited the same spectrum of effects on PKC isoforms and imprinting but had more robust actions. Pharmacological characterization also excluded direct effects of opioid receptors on the expression of imprinting; instead, it indicated participation of serotonergic innervation. The avian models can provide rapid screening of neuroteratogens, exploration of common mechanisms of behavioral disruption, and the potential design of therapies to reverse neurobehavioral deficits.

Effect of lattice-misfit strain on the process-induced imprint behavior in epitaxial Pb(Zr0.52Ti0.48)O3 thin films

The effect of lattice-misfit strain on the process-induced imprint behavior in Pb(Zr0.52Ti0.48)O3 (PZT) capacitors with Pt (top), and SrRuO3, La0.7Sr0.3MnO3 or LaNiO3 (bottom) electrodes has been studied. With the different oxide electrodes and by changing the deposition oxygen pressure, various lattice-misfit strains in the epitaxial PZT films have been produced. It was found that after in situ annealing at reduced oxygen pressures, the capacitors showed an increased voltage offset in the polarization-electric field hysteresis loops with increasing the misfit strain, irrelevant to the oxide electrodes employed, while lattice disorder at the bottom interface can effectively eliminate the voltage shift. Our results suggest that the imprint behavior is caused by oxygen loss via dislocations generated by the misfit strain relaxation at the growth temperature.

Long-term effect of in vitro culture of mouse embryos with serum on mRNA expression of imprinting genes, development, and behavior.

The long-term developmental and behavioral consequences of mammalian embryo culture are unknown. By altering the culture medium with the addition of FCS, we wanted to determine whether mouse embryos cultured under suboptimal conditions develop aberrant mRNA expression of imprinting genes at the blastocyst stage and whether fetal development, growth, and behavior of adult mice are affected. One-cell embryos obtained from superovulated female B6CBAF(1) mice were cultured for 4 days in K(+)-modified simplex optimized medium in the presence of either 10% FCS or 1 g/liter BSA. After embryo transfer, born animals were submitted to several developmental and behavior tests. The mRNA expression of some imprinting genes was significantly affected in blastocysts cultured in the presence of FCS. Two of the eight measures of preweaning development and some specific measures of neuromotor development, such as the walking activity, were delayed in the group originated with FCS. After 34 weeks, the weight of female mice cultured in vitro in the presence of FCS was significantly higher than controls. In addition, the locomotion activity of mice was altered at 5 and 15 months. Anatomopathological and histological analysis of animals at 20 months of age showed some large organs and an increase in pathologies. We have found that mice derived from embryos cultured with FCS exhibited specific behavioral alterations in anxiety and displayed deficiencies in implicit memories. Our data indicate that long-term programming of postnatal development, growth, and physiology can be affected irreversibly during the preimplantation period of embryo development by suboptimal in vitro culture.

Quantifying the Role of Electronic Charge Trap States on Imprint Behavior in Ferroelectric Poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) Thin Films

ABSTRACTPoly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) ferroelectric thin films are a potentially promising material for sensors or non-volatile memories. Imprint, the time-dependent resistance to polarization reversal, is a key material property that limits applications and is poorly understood. Based on experimental time and temperature dependences, we propose and investigate the link between imprint and charge trap states. A novel fast-ramp thermally stimulated current (TSC) measurement was developed to quantify and characterize the traps in an appropriate time-frame.Thin films of P(VDF-TrFE) on oxidized Si substrates were characterized following controlled initialization, fatigue, polarization, and imprint. Trap states were thermally filled/emptied by temperature cycling between 20–100 °C, using heating and cooling rates between 1 and 5 °C/s. Dynamics of this fast-ramp TSC indicate the presence of not only trap states, but also reversible and non-reversible charge accumulation. The presence of electrically active traps were verified by measurements over 1–10 s imprint times. Trapped charge directly correlated with the log of the imprint time, with a rate of ∼0.12 /μC/cm2/decade.

Mechanical stress effect on imprint behavior of integrated ferroelectric capacitors

Stress-induced changes in the imprint and switching behavior of (111)-oriented Pb(Zr,Ti)O3 (PZT)-based capacitors have been studied using piezoresponse force microscopy. Visualization of polarization distribution and d33-loop measurements in individual 1×1.5-μm2 capacitors before and after stress application, generated by substrate bending, provided direct experimental evidence of stress-induced switching. Mechanical stress caused elastic switching in capacitors with the direction of the resulting polarization determined by the sign of the applied stress. In addition, stress application turned capacitors into a heavily imprinted state characterized by strongly shifted hysteresis loops and almost complete backswitching after application of the poling voltage. It is suggested that substrate bending generated a strain gradient in the PZT layer, which produced asymmetric lattice distortion with preferential polarization direction and triggered polarization switching due to the flexoelectric effect.

Spatial inhomogeneity of imprint and switching behavior in ferroelectric capacitors

Piezoresponse force microscopy has been used to perform nanoscale characterization of the spatial variations in the imprint and switching behavior of (111)-oriented Pb(Zr,Ti)O3-based capacitors on Pt electrodes. Mapping of polarization distribution in the poled capacitors as well as local d33–V loop measurements revealed a significant difference in imprint and switching behavior between the peripheral and inner parts of the capacitors. It has been found that the inner regions of the capacitors are negatively imprinted (with the preferential direction of the normal component of polarization upward) and tend to switch back after application of the positive poling voltage. On the other hand, switchable regions at the edge of the integrated capacitors generally exhibit more symmetric hysteresis behavior. Application of an ac switching voltage, contrary to what was expected, resulted in an increase of the negatively imprinted regions. The observed effect has been explained by incomplete or asymmetric switching due to the mechanical stress conditions existing in the central parts of the capacitors.

Non-Linear Imprint Behavior of PZT Thin Films

Ferroelectric thin film capacitors are promising candidates for non-volatile ferroelectric Random Access Memories (FeRAM's) as they exhibit a switchable polarization. There are three important failure-mechanisms influencing the performance of these capacitors and disturbing the long-term stability and reliability under operation conditions fatigue, retention, and imprint. The imprint effect of lead zirconate titanate (PZT) thin films was investigated in this paper. Establishing and maintaining a polarization state leads to a shift of the hysteresis loop on the voltage axis and also to a loss of polarization. The voltage shift as well as the loss of polarization can cause a failure of the ferroelectric memory cell (read and write failure). The experimental results obtained on PZT films are discussed in view of the predictions of imprint models proposed in the literature.

Influence of Asymmetric Oxide Electrode Structures on the Interface Capacity and the Failure Mechanisms in PZT Thin Films

The influence of symmetric and asymmetric electrodes including SRO and IrO2 within the electrode structure on the imprint and fatigue behavior in sputtered and CSD derived PbZrxTi1 − xO2 (PZT) thin films is investigated. It is found that SRO buffer layers are needed within top and bottom electrode to improve the fatigue behavior. However, for improvement of the imprint behavior, only one SRO buffer layer within either top or bottom electrode is sufficient, whereas IrO2 reveals no improvement of the imprint behavior. Furthermore the direction dependence of the imprint behavior is examined. To examine the influence of the interface capacity on the fatigue mechanism in more detail, the capacity is measured for different PZT film thicknesses during fatigue treatment. It is shown that there is hardly any difference in interface capacity during fatigue.

Nanoscale observation of photoinduced domain pinning and investigation of imprint behavior in ferroelectric thin films

Piezoresponse force microscopy has been used to investigate the nanoscale mechanism of imprint behavior of ferroelectric PbTiO3 thin films by studying the photoinduced changes in the hysteresis loops of individual grains. Illumination of the film with UV light resulted in a voltage shift opposite to that observed in ferroelectric thin film capacitors. This effect is attributed to the generation of an electric field within the surface dielectric layer as a result of the interaction between photoinduced charges and polarization charges. Application of a small nonswitching bias to the film with simultaneous UV illumination resulted in domain pinning in the grains where the polarization direction coincided with the direction of the applied field, in agreement with the proposed model. Domain pinning was also observed in grains with polydomain structure suggesting that charge entrapment at the existing domain boundaries in the bulk of the film contributes to the suppression of switchable polarization. However, a symmetric character of hysteresis loops observed in such grains implies that charge entrapment in the bulk of the film does not cause the voltage shift. It has been suggested that a thin high-dielectric interfacial layer can improve the imprint behavior of ferroelectric capacitors.

Novel Chain Stack Capacitor for 32Mb FeRAM and Beyond

ABSTRACTFor high density FeRAM devices small cell sizes are essential. The combination of the capacitor on plug (COP) structure with the Chain FeRAM™ cell design is used to develop a 32Mb FeRAM. Based on a 0.2 μm standard CMOS process a silicide capped polysilicon plug is used to contact the bottom electrode of the ferroelectric capacitor to the transistor. The barrier contact to the plug is formed by IrO2/Ir and a sputter deposited PZT (40/60) is used as ferroelectric material. The function of SrRuO3 (SRO) layers at the electrode/PZT interfaces is described in more detail. Double sided SRO results in slightly lower coercive voltage and imprint behavior compared to capacitors without SRO. Double sided SRO is essential to achieve excellent fatigue behavior measured up to 1×1011 switching cycles.

Thickness effects on imprint in chemical-solution-derived (Pb, La)(Zr, Ti)O3 thin films

The film thickness-dependent imprinting behavior (voltage shift) of (Pb, La)(Zr, Ti)O3 capacitors was evaluated by a thermal stress process under a remanence bias. The remanent polarization (Pr) was found to be almost independent of the film thickness whereas in the 50–300 nm range the relative dielectric constant (εr) increased linearly with the square root of the film thickness. It was found that the voltage shift, which was attributed to the accumulation of charged defects near the electrode interface, also increased linearly with increasing film thickness. In addition, the charge accumulated thickness varied with the square root of the film thickness. This was established from a simple assumption that the level of charge accumulation is determined by the product of the total amount of charged defects (total film thickness×charged defect density) and the internal field that is generated by the Pr. Therefore, the imprint is much more a bulk-related degradation phenomenon compared to the fatigue.

Imprint in ferroelectric Pb(Zr,Ti)O3 thin films with thin SrRuO3 layers at the electrodes

The imprint behavior of ferroelectric Pb(Zr,Ti)O3 (PZT) films with thin SrRuO3 (SRO) layers at the platinum electrodes was investigated. Different models (defect dipole alignment, bulk screening and interface screening) are discussed which are usually applied to explain the imprint effect in ferroelectric materials. Based on our experimental results for the material under investigation, we suggest that the interface screening mechanism is the dominant mechanism which is responsible for the imprint scenario in ferroelectric thin films.

Protection of SrBi2Ta2O9 ferroelectric capacitors from hydrogen damage by optimized metallization for memory applications

The degradation behavior of integrated Pt/SrBi2Ta2O9/Pt capacitors by hydrogen impregnation during the intermetal dielectric deposition and passivation is investigated. The hydrogen ions generated as a reaction byproduct from the SiH4-based deposition processes of the dielectric films induce reduction in the remanent polarization (Pr) as well as the imprint behavior of the small size capacitors (2×2 μm2). The degree of degradation is quite dependent on the size of the individual capacitors. The smaller capacitors underwent more serious degradation implying that the hydrogen ions impregnate into the SBT layer mainly along the etched side area of the capacitors not through the top Pt electrode. Metallization adopting TiN/Al/TiN/Ti multilayer is very effective in suppressing the hydrogen impregnation. In particular, the Ti layer appears to block the hydrogen penetration. Therefore, the optimized metallization scheme, wider metal lines than the top electrode area by 1 μm, successfully protects the integrated capacitors from the hydrogen damage. 12 μC/cm2 of 2Pr and 1.1 V of 2Vc (coercive voltage) with an imprinting voltage of 0.16 V were obtained from the passivated 2×2 μm2 array capacitors by the optimized metallization.