SrBi2Ta2O9 Capacitors Research Articles

Reduction of Process-induced Damage and Improvement of Imprint Characteristics in SrBi2Ta2O9 Capacitors by Postmetallization Annealing

We investigated the effect of process damage induced after capacitor etching procedures in the process of ferroelectric random access memory (FeRAM) fabrication for SrBi2Ta2O9 capacitors. We found that this damage was suppressed by postmetallization annealing (400 °C, 30 min in O2) and that imprint characteristics were improved by the annealing, because active elements such as hydrogen and water induced during contact hole formation on tungsten plugs and first-metal formation are adsorbed effectively by the annealing before they penetrate into these capacitors.

Imprint mechanism in integrated Bi-rich SrBi2Ta2O9 capacitors: Influence of the temperature-dependent polarization

In this work, we characterize the imprint properties of three-dimensional Sr0.8Bi2.2Ta2O9 (SBT) ferroelectric capacitors integrated in 0.18μm technology using metal-organic chemical vapor deposition of SBT. We investigated the dependence of hysteresis loops on the storage temperature and duration after poling the capacitors. Also, the influence of unipolar and bipolar voltage cyclings, respectively, during and after the imprint stress was studied. We show that the imprint-induced shift of the hysteresis is limited for all the investigated stress conditions, and we associate this low shift mainly to a pronounced polarization decrease with the temperature, deviating from the Landau-Devonshire law. Our results also suggest that emission within the SBT layer is dominant over injection from the electrode in the imprint mechanism at 0V bias. On the other hand, dynamic imprint stress results suggest an increased contribution of carrier injection. Regarding the switchable polarization, it was observed to be higher after an imprint stress due to a wake-up of ferroelectric domains, which may be thermally driven but was evidenced to be prominently field driven. The wake-up and the imprint are discussed in terms of trapping/detrapping mechanisms.

Characteristics of Fatigue Induced by Distribution of Defect Charges in SrBi2Ta2O9 Capacitors

Pt/SrBi2Ta2O9 (SBT)/Pt capacitors have been successfully fabricated using a metalorganic decomposition (MOD) technique. The switchable polarization of SBT capacitors showed a tendency to increase with the number of switching cycles at first and then decrease after 2×109 switching cycles. During the switching process, a competition existed between the depinning behavior of domains and the aggregation of defect charges at the interfaces between SBT thin films and electrodes. It was this competition that determined the fatigue characterization for SBT capacitors. Higher external voltages facilitated the recovery of polarization suppression, which suggests that more domains in the SBT thin films were easily depinned by a higher external field and the pinning depth due to defect charges was different.

Effects of bottom electrode conditions on ferroelectric properties of SrBi2Ta2O9 thin films

The effect of adhesion layers under the bottom electrodes on ferroelectric characteristics of SrBi2Ta2O9 (SBT) thin films synthesized by sol-gel method was investigated. The remanent polarization (2Pr) of SBT films was found to be smaller for the tantalum oxide (TaOx) adhesion layer than for the titanium oxide (TiOx) adhesion layer, whereas the coercive voltage (2Vc) was larger for the TaOx adhesion layer than for the TiOx adhesion layer. The difference was due to a larger c-axis distribution of crystals in the SBT film on Pt with the TaOx layer than with the TiOx layer. Moreover, the imprint characteristics of SBT capacitors on the TaOx layer were better than those on the TiOx layer because the 2Vc in SBT capacitors was larger for the TaOx layer than for the TiOx layer, and the polarization stability against the hysteresis shift was better for the TaOx layer than for the TiOx layer.

Effects of Storage Conditions on Imprint Characteristics in SrBi2Ta2O9 Capacitors

The effects on the imprint characteristics of SrBi2Ta2O9 capacitors of various storage conditions were investigated. It was observed that the hysteresis shifts due to imprint degradations were smaller in the case in which the both electrodes of the capacitors were connected after increasing the temperature of the capacitors that were polarized at room temperature than in the case in which the electrodes were not connected. The acceleration ratio (Racc) of the slope of the hysteresis shift in the capacitors without connected electrodes to that with connected electrodes is 1.35 at 125 °C in device-size capacitors, and the ratio is much less dependent on temperature than the hysteresis shift. Moreover, it was also found that Racc was smaller in the case in which the process-induced damage located near the interfaces was larger.

Polarization and Leakage Degradation of Pt/SrBi2Ta2O9/Pt Capacitors and Their Recovery

Water (H2O) vapor-induced degradation of SrBi2Ta2O9 (SBT) capacitors has been investigated. SBT capacitors with no passivation layer were maintained in the atmosphere at 20°C for one-half year after their preparation. The capacitors showed pronounced degradation of polarization and increased leakage currents. A recovery process based on H2O vaporization from the SBT films was examined. We succeeded in completely recovering the polarization, fatigue and leakage current characteristics using thermal annealing at 300°C in a vacuum.

Dependences of Process-Induced Damage on Imprint Characteristics in SrBi2Ta2O9 Capacitors

We investigated hysteresis shifts in SrBi2Ta2O9 capacitors during high-temperature storage in an attempt to understand the mechanism underlying imprint degradation. We found that the activation energy (Ea) of imprint degradation is derived from the temperature dependence of the hysteresis shift, but is almost independent of the size of the capacitors (about 0.2 eV). On the other hand, Ea is strongly dependent on the size of contact areas on the upper electrodes of the capacitors (0.1 to 0.2 eV). This result suggests that imprint degradation is mainly due to the characteristics of ferroelectric-electrode interfaces, and is not due to the edges of the capacitors. We reached this conclusion because the dependence on the contact areas is related to process-induced damage near the upper electrode.

Time and temperature dependencies of imprint characteristics in SrBi2Ta2O9 capacitors

The temperature dependence of imprint characteristics in SrBi2Ta2O9 capacitors has been investigated by monitoring changes in switching charges (ΔQ) and hysteresis shifts (ΔV). Activation energies Ea and Ea′ were obtained from ΔQ and ΔV, respectively, Ea=0.34 and Ea′=0.14eV. The origin of this discrepancy is due to the fact that the relationship between ΔQ and ΔV is not linear, and because the dependence of ΔQ on temperature is overestimated because of the seeming dependence. It is concluded that a detailed investigation of hysteresis shifts is necessary for precise discussions of imprint mechanisms in ferroelectric capacitors.

Spacers Alternatives for INTEGRATION OF (3D) STACKED SBT FeCAPs

The thermal and mechanical stability of Ir and Ir\\Pt metals spacers deposited on top of Ti(Al)N\\Ir\\IrO2 patterned structures has been investigated in pseudo 3D stacked SrBi2Ta2O9 (SBT) capacitors. Their stability was compared to standard TEOS spacers. The high compressive stress at the edge of patterned electrodes, as a consequence of the high thermal expansion mismatch between the metals used in the electrode and TEOS, make the system mechanically unstable at the SBT crystallization conditions (700°C for 1 hour). The mechanical problems could be overcome if the same noble metals used in the electrode are incorporated as spacers. However, thermal stability during the SBT crystallization conditions is still an issue. For the case of Ir, surface oxidation decreases the SBT polarization values. In the case of Ir\\Pt, Ir diffuses through Pt and oxidizes, leading to unstable patterned structures and to the oxidation of the Ti(Al)N layer.

Imprint Characteristics of Ferroelectric Thin Films at Elevated Storage and Operation Temperatures

The imprint characteristics of SrBi2Ta2O9 (SBT) and Bi4-xLaxTi3O12 (BLT) thin films have been studied at elevated storage and operation temperatures for the application to ferroelectric random access memories (FeRAMs). After the storage at 125°C, a significant charge shift occurs as a function of the logarithmic time due to the thermally induced voltage shift. At an elevated operation temperature, the voltage shift divided by coercive voltage increases leading to the enhanced imprint degradation of polarization. Since the BLT thin film has larger remanent polarization and smaller voltage shift divided by coercive voltage at an elevated temperature than the SBT thin film, it has higher imprint endurance. In order to investigate the imprint characteristics on a device level, the sensing margin of the FeRAM cells using 0.8 µm complementary metal-oxide-semiconductor (CMOS) technology with SBT and BLT capacitors is also detected.

Room temperature aging behavior of thermally imprinted Pt/SrBi2Ta2O9/Pt ferroelectric thin film capacitors

Metalorganic-decomposition-derived SrBi2Ta2O9 (SBT) ferroelectric thin films, sandwiched between Pt electrodes, were imprinted by heat treatment at 130 °C in air. The aging behavior at room temperature of these imprinted SBT capacitors was observed. The voltage shifts of imprinted P–V hysteresis loops increased with the increase of aging time, which indicated the strengthening of the internal bias created by the thermal treatment. With the strengthening of the internal bias, asymmetry of data loss was observed to increase with increasing aging time. The mechanism of the internal bias strengthening, the resultant asymmetric data loss and possible function failure due to this aging behavior were briefly discussed.

Stress effects of the inter-level dielectric layer on the ferroelectric performance of integrated SrBi2Ta2O9 capacitors

The thermal stress effects of the inter-level dielectric (ILD) layer on the ferroelectric performance of integrated Pt/SrBi2Ta2O9(SBT)/Pt capacitors were investigated. Two different thin film materials, pure SiO2 grown at 650 °C and B- and P-doped SiO2 grown at 400 °C by chemical vapor deposition techniques, were tested as an ILD layer. The ILD layer encapsulated the SBT capacitor array. During high temperature thermal cycling (up to 800 °C) after ILD deposition, which is used for both densifying the ILD and curing of the various damage imposed on the SBT capacitors, a large thermal stress occurred in the bottom Pt layer due to the thermal expansion mismatch between the various layers. In particular, the pure SiO2 ILD layer between the capacitors did not allow thermal expansion of the Pt layers, which led to a large accumulation of compressive stress in the layer. This resulted in hillock formation in the bottom Pt layer and eventual capacitor failure. However, the B- and P-doped SiO2 ILD layer contracted during thermal cycling by removing residual impurities, which allowed greater expansion of the Pt layer. Therefore, compressive stress accumulation did not occur and excellent ferroelectric properties were thus obtained from the integrated capacitor array.

Effect of interfacial layers on dielectric properties in very thin SrBi2Ta2O9 capacitors

The effect of interfacial layers on the dielectric properties in very thin SrBi2Ta2O9 (SBT) capacitors has been investigated using static measurements. Total permittivity (εt) decreased as the film thickness was reduced in both Pt/SBT/Pt and Ir/SBT/Pt capacitors. The contribution of the interfacial capacitance (Cint) and bulk capacitance to the total capacitance indicates that Cint of the Ir/SBT/Pt structure was lower than that of the Pt/SBT/Pt structure, while the bulk permittivity (εb) was essentially the same. The dispersion of all capacitors followed the power law, while the Ir/SBT/Pt capacitor showed a larger dispersion of Cint. These results suggest that the Pt/SBT/Pt capacitor is preferred for obtaining the high performance with less effect of the interfacial layers on the dielectric properties.

Fabrication of ferroelectric SrBi2Ta2O9 capacitor films using plasma-assisted metalorganic chemical vapor deposition and their electrical properties

The fabrication of SrBi2Ta2O9 (SBT) films using plasma-assisted metalorganic chemical vapor deposition (P-MOCVD) has been investigated. Optimizing the process conditions under plasma environment, amorphous SBT films were successfully deposited at a substrate temperature below 300 °C, suggesting that the P-MOCVD process effectively utilizes plasma energy to promote the reaction and decomposition of metal organic source molecules. The amorphous SBT films were crystallized to the bilayered perovskite SBT films by a postannealing at 725 °C. Thin SBT capacitors fabricated using P-MOCVD showed a good step coverage and the excellent ferroelectric properties including endurance. Low voltage operation below 1.5 V was successfully achieved using a 75 nm SBT capacitor, in which the signal level derived from the hysteresis curve suggests the feasibility of application to a 64 Mbit ferroelectric random access memories.

Improvements in electrical properties of hydrogen-treated SrBi2Ta2O9 capacitors with chemical vapor deposited Pt top electrode

The ferroelectric property and leakage current of metalorganic chemical vapor deposition (MOCVD)-Pt/SrBi2Ta2O9 (SBT)/Pt and dc-sputtered Pt/SBT/Pt capacitors are evaluated with the microstructures of Pt top electrodes before and after hydrogen forming gas anneal. The SBT films with MOCVD-Pt top electrodes of large grain size and dense structure show a surprising decrease of leakage current density and still exhibit ferroelectric properties after hydrogen forming. On the other hand, SBT films with dc-sputtered Pt top electrodes of small grain size show an increase of leakage current density and then polarization switching properties cannot be measured due to fairly high leakage current. The microstructures of Pt top electrodes play an important role in improving the ferroelectric and leakage current characteristics after forming gas anneal. MOCVD-Pt top electrodes can prevent the complete loss of ferroelectricity and improve the leakage current properties during forming gas treatment.

Analysis of Processing Damage on a Ferroelectric SrBi2Ta2O9 Capacitor for Ferroelectric Random Access Memory Device Fabrication

For Ferroelectric random-access memory (FeRAM) device fabrication, it is important to control the processing ambience, especially the hydrogen in process, due to the reduction of ferroelectric oxides such as SrBi2Ta2O9 (SBT). We investigated processing damage to SBT capacitors during (1) the deposition of a cover film on SBT capacitors, (2) the fabrication of a contact hole by reactive ion etching (RIE), and (3) the photoresist mask removal. The O3-TEOS chemical vapor deposition (CVD) process for the deposition of a cover film causes less damage due to the presence of O3 a strong oxidizer. However, during the fabrication of contact holes in SBT capacitors, the plasma-gas species strongly affect the damage to the SBT capacitors. A less damaging process has been developed as follows: forming the contact hole by RIE with CF4 plasma gas instead of CHF3, and removing the photoresist by exposure to oxygen-radical downflow to eliminate hydrogen regeneration from the photoresist decomposition in the O2 plasma. After the process, the leakage current density and the remanent polarization are kept below 10-6 A/cm-2 and over 15 µC/cm2, respectively.

Effect of ultra-violet irradiation on SrBi2Ta2O9 thin film capacitors

Significant suppression of the switchable polarization of SrBi2Ta2O9(SBT) capacitors can be induced by ultra-violet irradiation, and the degradation in retained polarization is dependent upon the initial polarization states and the irradiation time. This can be well explained based on the model of weak pinning and mobility of domain walls. Furthermore, repeated cycling of the irradiated capacitor results in substantial recovery of the switchable polarization, and this recovery is applied voltage dependent. Experimental results indicate that the domain walls of SBT thin films can be weakly pinned by charge trapping and depinned by charge detrapping.

Improvement in the electrical properties of ferroelectric SrBi2Ta2O9 capacitors by electron beam irradiation on precursor films

Ferroelectric SrBi2Ta2O9 (SBT) capacitors were fabricated by the Electron-Beam-Induced Patterning process and their electrical properties were evaluated in detail. In this process, the dose of an electron beam irradiated on precursor films had a great influence on the electrical properties of the SBT capacitors. The appropriate electron dose significantly improved the electrical properties of the SBT capacitors although the excess electron dose made severe deterioration in the properties. The SBT capacitors fabricated with the optimum electron dose, 1.5 mC/cm2 for Sr:Bi:Ta = 0.8:2.2:2.0 solutions, exhibited excellent ferroelectric properties: a remanent polarization of 11.5 μC/cm2, a coercive field of 40 kV/cm, a leakage current of 8×10−8 A/cm2@ 1V and fatigue-free up to 1010 cycles. It seems that such improvements were caused by the adjustment of Bi contents in the films and the modification or the decompositon of precursors before heat treatment by electron beam irradiation.

Increase in Switching Charge of Ferroelectric SrBi2Ta2O9 Thin Films with Polarization Reversal

The increase in switching charge of as-grown SrBi2Ta2O9 (SBT) capacitors with polarization reversal was investigated. We call this phenomenon wake-up. The switching charge was increased and the rectangularity of the D-E hysteresis loops was improved with polarization reversal. The switching pulse with larger amplitude, longer width and shorter period generated a larger increase in the switching charge with polarization reversal. At the same time, the capacitors came to exhibit a rapid increase in leakage current at a certain applied field after the switching. Furthermore, the D-E hysteresis loop of the capacitors with wake-up shifted toward the positive- or negative-bias field according to the sweeping direction of the applied voltage. We have concluded that some spontaneous polarizations locked by localized space charges at the interface layer in the wake-up-exhibiting SBT capacitors were freed by the application of numerous switching pulses, and trap levels generated in the interface layer by the switching rendered the rapid increase in leakage current and the voltage shift of D-E hysteresis easy.

Hydrogen barriers for SrBi2Ta2O9-based ferroelectric memories

We report on a hydrogen barrier necessary for a conventional passivation process of integrated SrBi2Ta2O9 (SBT)-based memories. The passivation process significantly degraded electrical properties of the memories, resulting from hydrogen damage in the SBT capacitors. Metallic films (Ti, TiN, and Al) were investigated as a hydrogen barrier during the passivation process. The Ti(>500 Å) hydrogen barrier only showed the electrical properties of memories free from hydrogen damage. The formation of stable hydrides and the suppressed diffusion of hydrogen through the Ti films during the passivation processes resulted in sufficient switching polarization, low leakage current, and good reliabilities at high temperature.