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The ferroelectricity in fluorite-structure oxides such as hafnia and zirconia has attracted increasing interest since 2011. They have various advantages such as Si-based complementary metal oxide semiconductor-compatibility, matured deposition techniques, a low dielectric constant and the resulting decreased depolarization field, and stronger resistance to hydrogen annealing. However, the wake-up effect, imprint, and insufficient endurance are remaining reliability issues. Therefore, this paper reviews two major aspects: the advantages of fluorite-structure ferroelectrics for memory applications are reviewed from a material's point of view, and the critical issues of wake-up effect and insufficient endurance are examined, and potential solutions are subsequently discussed.
Material development has played a crucial role in modern civilization and IT. The importance of high-density and high-performance memory in modern computer systems and IT is ever increasing. This trend will be more obvious as computational architectures shift from being processing-centric to memory- (or data-) centric. The need for emerging and new memory technologies with nonvolatility and low power-consuming performance is rapidly increasing, while improvements in current dynamic random-access memory and NAND flash are being pursued. In both new and current memories, material innovation is of central importance. In this issue of MRS Bulletin, recent improvements in these two critical fields are reviewed with a focus on emerging and novel materials for the disruptive memory concept. Recent progress in scanning probe-based memory devices is also described.
The synthesis, structure, and electrical performances of titanium dioxide (TiO2 and also doped TiO2) thin films, a capacitor dielectric for dynamic random access memory (DRAM) and a resistance switching material in resistance switching RAM (ReRAM), are reviewed. The three-dimensionality of these structures and the extremely small feature sizes (<20 nm) of these memory devices require the synthesis method of TiO2-based layers to exhibit high degree of conformality. Atomic layer deposition is, therefore, the method of choice in respect of film growth for these applications. The unique arrangement of the TiO6-octahedra in the rutile structure, which results in the value for dielectric constant of the dielectric layer, εr (>100), makes the material especially attractive as the capacitor dielectric layer in DRAM. Removing some of the oxygen ions from the rutile structure and arranging the resulting oxygen vacancies on a specific crystallographic plane results in the so called Magnéli phase materials, which show distinctive conducting semiconductor or metallic characteristics. External electrical stimuli can cause the repeated formation and rupture of conducting channels that consist of these Magnéli phase materials in the insulating TiO2 matrix, and this aspect makes the material a very feasible choice for applications in ReRAM. This article reviews the material properties, fabrication process, integration issues, and prospect of TiO2 films for these applications.
This article reviews recent progress in understanding the resistive switching (RS) behavior and improvements in device performance of RS metal oxide (MO) thin-film systems and devices. The diverse RS MO materials are classified according to their switching mechanisms and characteristics. For each category, some representative materials are selected, and their characteristics are discussed. In addition, other factors such as the device structure, which also plays a crucial role in determining the device properties, are discussed as well. When applied in a real circuit (e.g., in a crossbar structure), there are device features/characteristics that need to be considered, including the bias polarity for switching, the current-voltage relationship, reliability, and scaling issues. Since nonvolatile RS in many MO materials is primarily associated with localized conduction channels, understanding the nature and the dynamic change of the current path structure is crucial and therefore is reviewed at length here. Guidelines for the choice of materials and access devices and their fabrication methods will also be provided. Finally, this review concludes with the outlook and challenges of MO-based resistance change devices for semiconductor memories.
Pb(DPM)2 and Pb(TMOD)2 dissolved in ethylcyclohexane were evaluated as precursors for future atomic layer deposition (ALD) of Pb(Zr,Ti)O3 films. PbO films were deposited by a liquid injection atomic layer deposition on Pt-covered Si substrates at different deposition temperature and precursor volume per cycle. Pb(DPM)2 and Pb(TMOD)2 started thermal decomposition at deposition temperature of around 270°C and 320°C, respectively. Against increasing Pb(DPM)2 injection at 240°C, the deposition rate of PbO films saturated at around 1 Å/cycle, but kept increasing at 300°C, which is above the thermal decomposition temperature. The deposition rate of PbO films at 240°C dropped to a constant value with enough purge time after precursor injection and reactant supply. A saturated deposition rate of PbO films was also observed for Pb(TMOD)2 below the thermal decomposition temperature. However, the saturation behavior observed for Pb(TMOD)2 was slower and the saturated growth rate was higher comparing to Pb(DPM)2. In addition, the film thickness of the PbO films had an apparent gradient over the substrates. These results indicate that Pb(DPM)2 shows more reactive and stable chemisorption comparing to Pb(TMOD)2 for the self-limiting growth rate.
The electrical conduction mechanisms of approximately 8-nm-thick Ta2O5 films grown by metalorganic chemical vapor deposition were investigated by measuring the current density–voltage characteristics at various temperatures. The Ta2O5 films were grown in two steps with or without intermittent annealing at 450 °C under an O3 atmosphere with ultraviolet light radiation (UV-O3 treatment). High-resolution transmission electron microscopy of the films after post-deposition annealing at 750 °C under an O2 atmosphere showed that the intermittent UV-O3 treatment improved the crystallization of the film during post-annealing. Auger electron spectroscopy of the variously treated samples showed that the improvement in crystallization was due to the increase in the oxygen concentration of the Ta2O5 films by the UV-O3 treatment. The Ta2O5 film without the UV-O3 treatment mostly exhibited a Poole–Frenkel conduction behavior with the electron trap level of 0.62 eV from the conduction band edge. The whole layer UV-O3 treated Ta2O5 films also showed a Poole–Frenkel conduction behavior with an almost identical electron trap level and a reduced density. The partially UV-O3 treated Ta2O5 films exhibited a direct tunneling behavior in a relatively low voltage region by the tunneling through the thin (∼3.8 nm) UV-O3 treated surface layer. However, these films showed a Poole–Frenkel conduction behavior in the high-voltage region. In general, the UV-O3 treatment was an efficient method to reduce the leakage current of the high-dielectric Ta2O5 films.
Pt thin films of various thicknesses (30 nm ∼ 200 nm) were deposited on Si wafers with SiO2, Ti, TiO2, or IrO2 buffer layers at various temperatures (room temperature ∼200 °C) by a direct current magnetron sputtering process. The Pt films showed a strong (111)-preferred texture irrespective of the thickness, under-layer, and growth temperature. The authors previously reported [J-E. Lim, D-Y. Park, J.K. Jeong, G. Darlinski, H.J. Kim, and C.S. Hwang, Appl. Phys. Lett. 81, 3224 (2002)] that the films were composed of three kinds of grains with slightly different (111) lattice parameters (bulklike, 1.0% and 2.1% larger). This study details the microstructural variations of the Pt films according to the variations of experimental parameters. The different deposition conditions produced slightly different crystalline structures, but the three different (111) lattice parameters were always found. Epitaxial (200) Pt films on a (200) MgO substrate and a highly (111) textured Au thin film on a SiO2/Si did not show the same splitting in the lattice parameter. The grains with 1.0% and 2.1% larger (111) lattice parameter almost disappeared after postannealing at 1000 °C. However, surface chemical binding of the Pt film before and after annealing was unchanged. Therefore, it is believed that the lattice parameter splitting in the (111) textured Pt film originated from the interfacial grains with the distorted crystal structure due probably to growth stress.
The oxidation behavior of sputtered TiAlN thin-film barrier layers was studied by cross-section transmission electron microscopy. Bare 100-nm-thick TiAlN films on SiO2/Si began to oxidize from the surface after annealing in air for 10 min from about 550 °C. Annealing at 700 °C oxidized half of the layer thickness. A 100-nm-thick Pt overlayer on the barrier layer retarded macroscopic oxidation at 650 °C. However, a 10-nm-thick Pt overlayer accelerated oxidation as a result of the catalytic dissociation of O2 molecules to form O atoms, which oxidized the barrier layer at 550 °C to the same extent as without the thin Pt overlayer at 650 °C. The effects of other thin metal overlayers, such as Ru and Ir, were also investigated. Ru and Ir did not accelerate TiAlN oxidation due to the absence of catalytic activity.
The metalorganic chemical vapor deposition of very thin (<50 nm) Pb(Zr,Ti)O3 (PZT) thin films was performed for high density (>32 mega bit) ferroelectric memory devices. The growth temperatures were set between 450 and 530 °C to obtain a smooth surface morphology and prevent damage to the underlying reaction barrier layer. The average grain size of a 50-nm-thick film on a Pt electrode was about 34 nm with a size distribution (σ2) of 11 nm. These values are much smaller than the sol-gel-derived PZT films (55 and 25 nm, respectively). Very thin films with a thickness of approximately 30 nm were prepared at wafer temperatures ranging from 500 to 525 °C. Even with the very small thickness, the films showed good ferroelectric properties with a typical remanent polarization from 10 to 15 μC/cm2 and an extremely low coercive voltage of 0.3 V. However, the leakage current density was rather high resulting in nonsaturating polarization versus voltage curves. Even though good ferroelectric properties were obtained, the formation of PtxPby alloys on top of the Pt electrode was consistently observed. This precludes the reliable control of film composition and electrical performance. The adoption of an Ir electrode successfully eliminated intermetallic alloy formation and resulted in better and reproducible process control. A 50-nm-thick PZT film on an Ir/IrO2/SiO2/Si substrate also showed a reasonable ferroelectric performance.
The electrical conduction behavior of sputter-grown (Ba,Sr)TiO3 thin films having IrO2 electrodes were studied under the assumption of a fully accumulated film having a negative space charge density of 1 × 1019 cm−3 at 25 °C. The negative space charge decreased the actual field strength in the film and resulted in a decreasing leakage current with increasing film thickness at a given applied field. The current conduction in a very low field, roughly less than 150 KV/cm, showed a linear current density–voltage (J–V) behavior at 25 °C. From that field to about 420 KV/cm, the bulk-limited Poole–Frenkel mechanism controlled the overall conduction property at room temperature. Under high field strength, from 420 KV/cm to 1 MV/cm, the interface-limited thermionic field emission mechanism was dominant. The dielectric constant obtained from Poole–Frenkel fitting was approximately 300 ± 50 at 25 °C, which was in qualitative agreement with the value obtained from low-frequency capacitance measurements. The detailed mechanisms of the linear and nonlinear field-dependent emission conductions were discussed with reference to the direction of band bending, not to the carrier concentration.
An optimization study on crystallization annealing of (Ba,Sr)TiO3 (BST) thin films, grown by metalorganic chemical vapor deposition at a wafer temperature of 420 °C, was performed. The as-grown film had an amorphous structure with a dielectric constant of about 20. The annealing parameters, including temperature, atmosphere, time, and sequence, were varied considering the limitations imposed by integration processes for ultralarge-scale integrated dynamic random access memory devices. The dielectric constants of the crystallized films were largely determined by the maximum temperature that the films experienced with minor effects from the annealing atmosphere. However, the leakage current densities were quite dependent on the annealing sequence and atmosphere. It was concluded that the major factors which determine leakage characteristics were concentrations of impurities, especially carbon, oxygen vacancies, and interfacial defects caused by top electrode sputtering.
The cocktail source of BST was prepared by mixing of Ba, Sr, and Ti precursor solution with specific mole ratio. This cocktail source was vaporized and delivered into the warm wall reactor by liquid delivery system(LDS) and gaseous source was distributed by simple structure of gas injector instead of showerhead system. The thickness uniformity of BST on 8 inch wafers were less than 3%. The Ti composition uniformity of our films were less than the 1 at%(1σ ) at stoichiometric and near stoichiometric. Their dielectric constant was about 230 and leakage current density was lower than 10-8 A/cm2 under ±1V bias. Excellent step coverage and smooth (haze-free) surface morphology of BST films were obtained by a deposition using a noble dome type reactor. The merit of our warm wall type reactor also will be explained by excellent step coverage and the uniform composition with 3 dimensional structure. Our achievement should be applicable to the capacitor of next generation DRAM.
The degradation behavior of integrated Pt/SrBi2Ta2O9/Pt capacitors caused by hydrogen impregnation during the spin-on glass (SOG)-based intermetal dielectric (IMD) process was investigated. SOG was tested as an IMD since it offers better planarity for multilevel metallization processes compared to other SiO2 deposition methods. It was found that the SOG itself does not degrade the ferroelectric performance. Deposition of an under-layer of SiOxNy by plasma-enhanced chemical vapor deposition (PECVD) using SiH4 + N2O + N2 source gases and a SiO2?x capping layer by another PECVD process using SiH4 + N2O source gases produced hydrogen as a reaction by-product. The hydrogen diffused into the SBT layer and degraded the ferroelectric performance during subsequent annealing cycles. A very thin (10 nm) Al2O3 layer grown by atomic layer deposition before the IMD process successfully blocked the impregnation of the hydrogen. Therefore, excellent ferroelectric performance of the SBT capacitors were maintained after the multilevel metallization process as well as passivation. The adoption of SOG in the IMD process greatly improved the surface flatness of the wafer resulting in a higher capacitor yield with very good uniformity in ferroelectric properties over the 8-in.-diameter wafer.
Pb(Zr, Ti)O3 (PZT) thin films were deposited on Pt/SiO2/Si substrates by metalorganic chemical vapor deposition using solid delivery system. The effects of deposition parameters such as the substrate temperature, the concentration of Pb precursor in the precursor mixtures, and the reactor pressure on the structural and electrical properties of PZT thin films were investigated. To obtain single-phase PZT thin films, the optimal range of the substrate temperature should be between 600 and 650 °C. The PbO content in PZT thin films was proportional to the fraction of Pb in the precursor mixture below 550 °C, but it was independent of the fraction of Pb in the mixture above 600 °C. With the increment of the reactor pressure, Zr contents in PZT thin films were increased, and the Pb/(Zr + Ti) ratio became more stoichiometric so that the ferroelectric properties were improved.
A hydroxyapatite [HAp; Ca10(PO4)6(OH)2] coating layer was formed on a Ti-based alloy by the electron-beam deposition method. When pure HAp was used as a target for the deposition, an amorphous layer was formed on the metal substrate. By heat treatment in a vacuum at 630 °C, the layer was crystallized into tricalcium phosphate [Ca3(PO4)2]. The crystallization improved the dissolution rate of the layer remarkably; however, at the same time, it deteriorated the bond strength with the substrate. When extra CaO (up to 25 wt%) was added to the target and processed under the same conditions, a layer compositionally close to crystalline HAp was deposited. Before the heat treatment, even though the layer was in amorphous state, the dissolution rate in the physiological solution was extremely low. Furthermore, the bond strength increased remarkably compared to the layer formed by the pure HAp target. Compositional and structural resemblance of the layer with the crystalline HAp was attributed to these improvements in properties.
Structure and composition of the ferroelectric Pb(Zr, Ti)O3 layers in a capacitor of the ferroelectric random-access memory (FeRAM) device having a density of 64 k were investigated by transmission electron microscopy (TEM) together with the energy-dispersive spectroscopy (EDS) technique. The 250 nm thick PZT layer derived by the sol-gel route showed a 2–3% Pb-deficient, 3–4% Ti-deficient, and 5–7% Zr-excess composition at the top electrode interface compared to the bulk composition when they were as-fabricated. The local compositional nonuniformity became more critical as the integration process proceeded, which seriously degraded the ferroelectric hysteresis and the device yield. The major cause of the compositional variation was the outward diffusion of Pb through the capping barrier TiO2 layer during annealing at 650 °C. The AlN capping barrier layer was also not effective in suppressing the diffusion of Pb. However, the Al2O3/TiO2 double capping layer was very effective in suppressing the outward diffusion of Pb, and excellent ferroelectric characteristic was expected.
Pt-coated silicon substrates with strong (111) Pt texture were annealed in an oxidizing atmosphere at temperatures from 500 °C to 750 °C. BaTiO3 thin films were deposited by pulsed laser ablation on the substrates. Observation by transmission electron microscopy showed that the substrate anneal caused the formation of TiO2 in the Pt layer, accompanied by the formation of a high density of faceted protrusions on the Pt surface, particularly at the higher anneal temperatures. The Pt protrusions had (111) facets, parallel to the substrate surface, on which (100)-oriented BaTiO3 grains were observed. BaTiO3 grains with an epitaxial relationship to the Pt lattice were observed on inclined facets of the Pt protrusions [which were not (111) planes], and also on the nonplanar regions of the Pt surface. These epitaxial BaTiO3 grains had (111) preferred orientation relative to the substrate surface. Thus, the BaTiO3 films displayed bimodal growth behavior, with both (100) texture and (111) epitaxy. We propose a model for this behavior based on surface energy considerations.
BaTiO3 thin films were deposited by metal-organic chemical vapor deposition at 840 °C on two differently treated (100) MgO single crystal substrates. One MgO substrate was only mechanically polished and the other substrate was polished and then annealed at 1100 °C for 4 h in oxygen. Observation by transmission electron microscopy showed that the BaTiO3 thin film deposited on the unannealed substrate was fine-grained and that the whole film was epitaxial (100) in nature. In contrast, the film deposited on the annealed substrate consisted of large, (100)-oriented, epitaxial grains within which were distributed (110)-oriented grains with random in-plane orientations. These differences in BaTiO3 films deposited on differently treated substrates are discussed with reference to the surface structure of the MgO substrate and nucleation kinetics of BaTiO3 thin films on MgO.
Cross-sectional and plan-view transmission electron microscopy were used to characterize a BaTiO3 thin film deposited on a (100) MgO single-crystal substrate at 1000 °C. The major observations were as follows: interdiffusion between the film and substrate; a large number of antiphase boundaries in the BaTiO3; a two-phase microstructure in the film composed of perovskite BaTiO3 and a second nonperovskite phase, Ba2MgTi5O13 (2:1:5); and a well-defined orientation relationship between the 2 : 1 : 5 and BaTiO3 phases. We propose a mechanism for the formation of the 2 : 1 : 5 phase based on the similarities between the crystal structure of this phase and the structure of (210) antiphase boundaries in BaTiO3.
ZrO2 thin films were deposited at 1 atm on Si substrates by oxidation-assisted thermal decomposition of zirconium-trifluoroacetylacetonate in the temperature range of 300–615 °C. Above a deposition temperature of 400 °C, the deposited thin films have a columnar grain structure, where each grain is perpendicular to the substrate surface with a c-axis preferred crystallographic orientation, and have poor electrical characteristics as a dielectric thin film. But the thin film deposited at 350 °C has a fine equiaxed microcrystalline structure and has superior electrical characteristics of a breakdown field of 1 MV/cm and a relative dielectric constant of 27.