RuO2 Electrodes Research Articles

Effect of RuO2 electrode on polarization fatigue resistance of PbZr0.52Ti0.48O3 thin films

Sol-gel derived lead zirconate titanate (PbZr0.52Ti0.48O3, PZT) films of thickness 300 nm were deposited on RuO2/SiO2/Si and Pt/Ti/SiO2/Si substrates. Either a ruthenium dioxide (RuO2) or platinum (Pt) top electrode was deposited on each film. The ferroelectric properties and the fatigue and current characteristics of the films were investigated. The film has a higher fatigue resistance but also a higher leakage current when one or both of the electrodes are made of RuO2. The RuO2/PZT/RuO2 film exhibits negligible polarization fatigue, while the polarization of the RuO2/PZT/Pt and Pt/PZT/RuO2 films decreases by about 40% after 109 switchings. This suggests that the polarization fatigue in the PZT films is controlled not only by the top RuO2/PZT interface but also by the bottom PZT/RuO2 interface.

Effect of bottom electrodes on the structural and electrical characteristics of lanthanum doped lead titanate thin films

Ferroelectric Pb0.90La0.15TiO3 (PLT15) thin films were deposited by sol-gel method on Pt, Pt/Si, ruthenium oxide (RuO2)/Si and RuO2/Pt/Si bottom electrodes. X-ray diffraction, and micro-Raman spectroscopy techniques were used for structural characterization of these films. PLT15 films deposited on RuO2 electrodes show (100) preferred orientation of growth and result in larger crystallites. Films on Pt electrode exhibited higher dielectric constant (1300 at 100 kHz) and high values of Pm, Pr values, 68 and 46 μC/cm2 respectively. AC field dependence of dielectric permittivity at sub-switching fields was fitted using the Rayleigh law. The reversible polarization components estimated from the CV and quasi-static hysteresis measurements for films on Pt/Si (24%), was larger than that of Pt (11%) bottom electrodes. The observed results were correlated with the domain wall pinning at the disturbed film-electrode interface.

The role of electrode material and polarization fatigue on electron emission from ferroelectric Pb(ZrxTi1−x)O3 cathodes

Electron emission from bulk Pt/PZT/Pt and RuO2/PZT/RuO2 ferroelectric cathodes is reported. The total emission current (0.8 A) and polarization fatigue are high for ferroelectric cathodes with Pt electrodes, while emission current (0.18 A) and polarization fatigue are significantly lower for cathodes with RuO2 electrodes. The data correlate with earlier results from research on ferroelectric memories, which suggest that oxygen vacancies near the electrode/ferroelectric interface and through the bulk of the ferroelectric layer play a major role in controlling the polarization switching of PZT-based capacitors and consequently in controlling the electron emission process in PZT cathodes. The strong correlation between polarization fatigue and emission observed in the present investigation provides evidence for the role of polarization inversion on the electron emission phenomenon. In addition, the data provide evidence for the role that oxygen vacancies play in controlling electron emission from PZT cathodes.

Coulometric micro-titrator with a ruthenium dioxide pH-electrode

A coulometric diffusional titrator with a ruthenium dioxide pH-sensing electrode for the end-point detection in acid–base titrations is presented. The device was prepared by galvanic deposition of noble metals onto electrodes etched on a copper clad printed circuit board. Five electrodes were employed: three gold electrodes (two actuator and one counter) for the coulometric production of the titrating protons or hydroxide ions, and a RuO 2-electrode and Ag/AgCl pseudo-reference electrode pair for the end-point detection. The RuO 2-electrode for pH-measurements showed a linear response to the variation of the pH-value in the range from 2 to 12 pH-units and was useable for a period of 3 months. The device was found to yield linear relationships between the analyte concentration and the equivalence time for concentration ranges of acetic acid and ammonia between about 0.1 and 100 mM.

Effect of RuO2 growth temperature on ferroelectric properties of RuO2/Pb(Zr, Ti)O3/RuO2/Pt capacitors

We present a promising method for obtaining Pb(Zr, Ti)O3(PZT) layers with excellent endurance and pulse-switching properties on RuO2 electrodes using the sol–gel method. As the substrate temperature during reactive sputtering of the RuO2 bottom electrode layer is reduced, the (111) PZT texture component becomes more pronounced, an effect attributed to the change from columnar to granular RuO2 film morphology. Reducing the residual PZT (100) and (101) texture components was found to be a necessary condition for obtaining optimal pulse switching and endurance properties of the layers. Highly (111)-oriented PZT layers, obtained on RuO2 grown at 150 °C exhibit a net switched charge of >60 μC/cm2 during pulse measurement and <10% degradation after 1011 fatigue cycles.

Thermal Stability of a RuO2 Electrode Prepared by DC Reactive Sputtering

RuO2 films were deposited by reactive DC sputtering, and the formation process and the thermal stability were investigated. In the RuO2/Ti/SiO2 structure, the Ti surface was preferentially oxidized at the initial stage of RuO2 growth. Insertion of Ru and TiN layers between the RuO2 and Ti layers can suppress the oxygen diffusion toward the Ti layer during the film growth. However, when the RuO2/Ru/TiN/Ti/Si structure was heat-treated, the RuO2 was partially reduced to form RuO2-X and the TiN was oxidized to TiO2 while generating N2 and O2 gases, resulting in voids formation at the RuO2/Ru interface, regardless of the heat-treatment atmosphere. We found this was caused by internal oxygen diffusion within the stacked layers. This reaction is governed by the transference of oxygen to the TiN and Ti layers by the difference in the Gibbs' formation free energy, and by the oxygen diffusion enhancement.

Hydrogen reduction of a RuO2 electrode prepared by DC reactive sputtering

RuO2 thin films deposited by reactive DC sputtering were heat-treated in a 0.3%-H2 atmosphere at 200°C in order to investigate the reduction property of RuO2. The films were selectively reduced (starting from the interface) because of insufficient oxidation. When the as-deposited RuO2/Si structure was hydrogen-reduced, the RuO2 film was broken into fragments because of poor mechanical strength. On the other hand, the RuO2 film heat-treated once in O2 at 700°C cracked into a star shape as a result of vapor generation and volume shrinkage. The star-shape cracking was still observed even when the RuO2 film was covered with a 90-nm-thick BST film. The mechanical strength and the adhesion of RuO2 were improved to some extent by post-oxidation at high temperatures. However, a way to prevent the morphological destruction could not be found because of the thermodynamic equilibrium during the hydrogen reduction process.

Study of pulsed laser deposition of RuO 2 and SrRuO 3 thin films

In this study, the pulsed laser ablation of RuO2 and SrRuO3 (SRO) is investigated by observing the fluorescence from excited atoms in the plume by using a framing streak camera. Vaporization, phase explosion and boiling are suggested to play the main roles in the processes for the interaction between the laser beam and target. Collisions and adiabatic expansion are also suggested before particles move forward with shifted Maxwellian spatial distribution. In O2 pressure, numerous collisions between fast and slow atoms occur and result in the exchange of speeds. The structural and electrical (conductivity and work-function) properties of RuO2 and SRO thin films are measured. Epitaxial SRO growth was obtained at growth temperatures down to 350 °C. Ferroelectric and high- dielectric thin-film capacitors with RuO2 or SRO thin film electrode are also studied.

Pt and RuO2 Bottom Electrode Effects on Pb(Zr,Ti)O3 Memory Capacitors

This study examines the effects of bottom electrodes for metal ferroelectric metal (MFM) capacitor applications. We investigated the following parameters of bottom electrodes and Pb(Zr0.53Ti0.47)O3 (PZT) thin films: substrate temperature, rf power, gas flow rate, Ar/O2 ratio, electrode material, and post-annealing effect. Bottom electrodes grown at 300°C for Pt and 200°C for RuO2 exhibited a film resistivity of 10-4 Ω·cm, had a surface roughness of approximately 55 Å and a preferred crystal orientation. Rapid thermal annealing (RTA) treatments on a Pt electrode at 600°C for 30 s improved the resistivity to 5×10-6 Ω·cm and generated the (111) preferred crystal orientation. PZT films exhibited a strong PZT (101) peak for an optimized Pt bottom electrode and (111), (200), (112) planes without preferred PZT orientations for the RuO2 electrode. A well-fabricated Pd/PZT/Pt capacitor showed a leakage current density in the order of 6×10-5 A/cm2, a dielectric constant (εr) of 365, a remanent polarization (Pr) of 27 µC/cm2, and a coercive field (Ec) of 50.5 kV/cm. This paper discusses the bottom electrode properties as well as their recommended conditions in memory device applications of thin-film PZT capacitors.

Ferroelectric Properties of La0.5Sr0.5CoO3/Pb(Zr0.4, Ti0.6)O3/RuO2 Capacitors

As a continuation of the previous studies on Pt/La0.5Sr0.5CoO3/Pb(Zr0.4, Ti0.6)O3/RuO2/SiO2/Si(100) capacitors, the effect of post-annealing conditions of the sputtered-deposited RuO2 bottom electrodes upon crystallization of the PZT(40/60) films and their ferroelectric properties was examined. The switchable polarization (dP=P*-P∧) in the fatigue tests under comparatively low driving voltage amplitudes of 2.0–2.5 V of the PZT films grown on O2- or Ar-annealed RuO2 electrodes continued to be larger than 10 µC/cm2 up to 1010 switching cycles. The fatigue profile generally exhibited gradual degradation with increasing cycles. However, the rates of polarization degradations depended on post-annealing conditions of the bottom electrodes; in the case of O2 annealing, the degradation rate was the lowest (20%) when the annealing temperature was the lowest in the range of 700–800°C. On the other hand, in the case of Ar annealing, the opposite trend was noted. The leakage current density was in the range of 10-6 A/cm2 or less at 2 V, which appeared to be independent of the postannealing condition of the bottom electrode. A possible interpretation of the effectiveness of using the La0.5Sr0.5CoO3 top electrode for the Pb(Zr0.4, Ti0.6)O3 film in suppressing the polarization fatigue has been presented.

Structural properties of Ba0.6Sr0.4TiO3 thin films on epitaxial RuO2 electrodes

For the first time, we have demonstrated that epitaxial Ba0.6Sr0.4TiO3 thin films can be grown on the epitaxial RuO2 electrodes by pulsed laser ablation. The Ba0.6Sr0.4TiO3 films, deposited on epitaxial (200) oriented RuO2 grown on (100) yttria-stabilized zirconia, exhibit single out-of-plane orientation as well as in-plane alignments. The detailed epitaxial relationships between the Ba0.6Sr0.4TiO3 films and the RuO2 electrodes have been analyzed by X-ray diffraction and transmission electron microscopy.

Full chemical fabrication of SrBi2(Ta,Nb)2O9 ferroelectric thin film capacitors

SrBi2(Ta,Nb)2O9 ferroelectric thin film capacitors with RuO2 electrodes were prepared for the first time by a full chemical route. Stable sols of SrBi2(Ta,Nb)2O9 precursors were obtained from mixtures of niobium (tantalum) ethoxide, bismuth and strontium 2-ethylhexanoates. Ruthenium dioxide precursors was prepared by dissolving an aqueous solution of ruthenium nitrosylnitrate into 2-methoxyethanol. Capacitors were fabricated by sequential spin coating the precursors on silicon wafers according to the sequence Si/RuO2/SrBi2(Ta,Nb)2O9/RuO2. Fully crystallized crack-free materials were obtained by annealing at 700°C for 2 h. Hysteresis loops (3–10 V) are similar to those observed using platinum electrodes.

Stoichiometry and Interdiffusion in PZT Thin Films Studied by Transmission Electron Microscopy

AbstractElectrode stability, interdiffusion, phase purity and deviation from stoichiometry at the PZT-electrode interface are key issues in PZT thin film integration. This article highlights the use of transmission electron imaging combined with energy dispersive spectroscopy (EDS) for the investigation of these phenomena. The accuracy of the EDS analysis is discussed. It will be shown that using a standard PZT sample and controlled conditions, reliable analysis can be performed.Diffusion mechanisms have been studied for Pt based electrode systems and RuO2-based electrode systems developed for direct integration onto silicon. The materials studied were composed of stacks of silicon-silicon oxide, an adhesion layer (Ti, Ta or TiOx), an electrode (Pt or RuG2) and PZT (45/55). The PZT was deposited by sol gel using the same parameters to allow for comparison of the different electrodes. Four different electrode / adhesion layer materials were compared (Pt/Ti, Pt/TiOx, Pt/Ta and RuO2 / TiO2). In the case of Pt, lead and oxygen diffusion through the electrode is observed. While the initial Ta layer transforms into a homogeneous pyrochlore phase, the Ti adhesion layer is heavily deformed. In the case of TiOx the lead is mainly incorporated at the interfaces with Pt and with SiO2. No lead diffusion to the adhesion layer is observed for the RuO2 electrode. In-situ sputtering and sol-gel deposition of PZT are also compared. The sol-gel films are close to the right stoichiometry for the perovskite while the sputtered films contained an excess of lead. No sign of second phase is found by X-ray diffraction (XRD), by EDS and by high resolution transmission electron microscopy (HRTEM) which suggests that the excess lead is accommodated in the perovskite lattice.

Evaluation of the surface area of RuO2 electrodes by periodic current signals

Abstract The surface area of RuO 2 electrodes was evaluated through the analysis of the potential transients obtained when the equilibrium potential of the quinone/hydroquinone redox couple is perturbed with a periodic current signal of low amplitude. Under these conditions, the behaviour of a smooth electrode is equivalent to that of an RC circuit. The applicability of this method to the RuO 2 electrodes characterised by a highly porous structure was demonstrated. The optimum ranges of frequencies and current amplitudes were determined. The accuracy of the proposed method was verified through the independence of the calculated surface area on the type of current signal as well as on the current amplitudes.

Structure and electrical properties of boron-added (Ba,Sr)TiO3 thin films fabricated by the sol-gel method

Thin films of BaxSr1−xTiO3 (BST, with x=0.5) were fabricated on a RuO2/Ru/SiO2/Si substrate by the spin coating of the multicomponent sol prepared using metal alkoxides. Boron alkoxide was intentionally introduced to establish a better microstructure and to reduce the leakage current. AFM indicated that a crack-free uniform microstructure having a smooth surface was gradually developed with increasing boron content. The relative dielectric permittivity of the 250-nm thick BST thin films fired at 700°C decreased with increasing content of boron, from 420 for the undoped film to 190 for the 10 mol% boron-added film at 1 MHz. This observation was interpreted in terms of a serial capacitance composed of the perovskite BST grain and the interfacial B2O3 glassy phase having a low dielectric permittivity. The leakage current density (J) also decreased with the amount of boron added. The leakage current for the applied voltage greater than 1 V showed a linear variation of logJ with E1/2 at room temperature, suggesting that the interface-controlled Schottky emission was the dominant conduction process for the BST thin films fabricated on the RuO2 electrode.

Preparation of high quality RuO2 electrodes for high dielectric thin films by low pressure metal organic chemical vapor deposition

Pure and conducting RuO2 thin films were deposited on a Si substrate at 250–450 °C using Ru(C11H19O2)3 as a precursor by low pressure metal organic chemical vapor deposition (MOCVD). At a lower deposition temperature, the smoother and denser RuO2 thin films were deposited. The amount of O2 addition did not seriously affect the properties of the RuO2 thin film. The RuO2 thin films which were crack free and well adhered onto the substrates showed very low resistivity of 45–60 μΩ cm. At a lower deposition temperature and a smaller amount of O2 addition, RuO2 thin films showed better step coverage, indicating that MOCVD RuO2 thin films from Ru(C11H19O2)3 can be applied for an electrode of high dielectric thin films for a capacitor of ultralarge scale integrated dynamic random access memory.

Electrical Characterization of Ferroelectric Pb(Zr, Ti)O3 Thin Films Deposited on Pt-Coated RuO2 Electrodes

Ferroelectric Pb(Zr, Ti)O3 [PZT] thin films deposited on Pt-coated RuO2 electrodes are found to have concurrent advantages in both fatigue and leakage current characteristics. Perovskite phase of PZT having a random orientation was obtained on as-deposited and Pt-coated RuO2 bottom electrodes and the crystallinity increased slightly when the PZT films were deposited on Pt-coated RuO2. PZT films with hillock-free and fine-grained microstructures resulted in all the cases. The PZT films on hybrid electrodes (Pt/RuO2) showed a typical P–E hysteresis loop. PZT films in all the cases did not exhibit fatigue until over 1010 switchings regardless of the bottom electrodes. The asymmetric I–V curve of a PZT thin film on Pt/RuO2 exhibited the Schottky behavior, which indicates that blocking contact is formed at the interface between PZT films and hybrid electrodes. The leakage current density of the PZT thin films on as-deposited and 30-nm-thick Pt-coated RuO2 at 75 kV/cm was 4.73×10-6 and 9.55×10-7 A/cm2, respectively.

Pt/RuO2 Hybrid Bottom Electrodes and Their Effects on the Electrical Properties of (Ba, Sr)TiO3 Thin Films

Thin platinum layer of 100 Å thickness was used for the modification of the interface state between (Ba, Sr)TiO3 (BST) thin films and RuO2 bottom electrodes. Pt/RuO2 hybrid bottom electrodes were fabricated with various platinum deposition temperatures by a conventional dc magnetron sputtering method. Although the surface morphology and X-ray diffraction (XRD) patterns of BST thin films were not changed, the electrical properties of BST films deposited on Pt/RuO2 hybrid electrodes were improved compared to the films on RuO2 electrodes. Dielectric constant (εr) and leakage current density of the 1000 Å thick BST thin films on Pt/RuO2 hybrid electrodes prepared at the platinum deposition temperature of 400°C were 498 and 8.6×10-8 A/cm2 at 1.5 V, respectively.

Fundamental characteristics of DRAM capacitor application of PLZT ultrathin films prepared by MOCVD

Lanthanum-modified lead zirconate titanate (PLZT) thin films (50 nm to 200 nm) were deposited on Pt/SiO2/Si substrate by metal-organic chemical vapor deposition (MOCVD). The electrical properties of the films were investigated as a function of the La content or the substrate temperature. Ferroelectric PZT(0/50/50) films were obtained at substrate temperatures as low as 500 °C; their electrical characteristics improved with increasing substrate temperature. La exhibited adequate solid solution in the PZT above 650 °C. PLZT(15/45/55) films with a thickness of 100 nm were found to have good properties for application to the capacitors of dynamic random access memory (DRAM), namely, an effective charge density of 80 fF/μm2, a permittivity of 1000, an SiO2 equivalent thickness of 0.4 nm, and a leakage current density of 5 × 10−8 A/cm. Addition of La to PZT was effective in reducing the leakage current with an increase in the registration rate. RuO2 and/or IrO2 bottom electrodes for ferroelectric PLZT films were also investigated. The RuO2 films were found effective as diffusion barriers for PLZT and MgO. Significant interdiffusion at RuO2/Si and RuO2/SiO2 interfaces occurred during the deposition of PLZT films. Annealing of the RuO2 film considerably depressed interface reactions. © 1998 Scripta Technica. Electr Eng Jpn, 122(1): 25–36, 1998

Effect of Annealing Treatment on PZT Thin Fiilm Properties Using Oxygen 18 Depth Profiling Technique

AbstractLead zirconate titanate (PZT) thin films have been deposited in a reactive argon/oxygen gas mixture from a metallic target of nominal composition Pb1.1(Zr0.4Ti0.6)O3 by rf magnetron sputtering on Si substrates and RuO2/SiO2/Si structures. During plasma deposition, in situ Optical Emission Spectroscopy (OES) measurements clearly show a correlation between the evolution of characteristic atomic emission line intensities and the thin film composition determined by simultaneous Rutherford Backscattering Spectroscopy (RBS) and Nuclear Reaction Analysis (NRA). As a result, the cathode surface state can be monitored by OES to ensure a good compositional transferability from the target to the film and reproducibility of thin film properties for given values of deposition parameters. Electrical properties and crystallization have been optimized with a 90 nm PZT thin film grown on RuO2 electrodes. These PZT films, annealed with a very modest thermal budget (550°C) are fatigue-free and show very low leakage currents (J=2.10−8 A/cm2 at 1 V). The use of a metallic target allows us to control the oxygen incorporation in the PZT thin film and also, using 18O as a tracer, to study the oxygen vacancy migration which plays a key role in fatigue, leakage current, and electrical degradation/breakdown in PZT thin films.