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.     

Preparation of PZT Thin Films by Liquid-Source MOCVD Using of Cyclohexane Solvent

Pb(Zr, Ti)O₃ thin films were grown on 8-inch Ir(111)/SiO₂/Si substrate by a MOCVD system aiming at application utilizing high-density ferroelectric memory (FRAM). Two types of solvents, THF and cyclohexane were used for liquid source delivery. It was found that the ferroelectric properties of the MOCVD-PZT films using cyclohexane solvent were better than them using THF solvent. By choosing cyclohexane as solvent, the MOCVD-PZT thin films showed strong ?111? preferred orientation and the Pt/PZT/Ir capacitors exhibited promising ferroelectric performances, for instance, large switching charge (Qsw) of 56.4 uC/cm².     

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.     

Characteristics of PZT films with SRO/Pt stack electrodes for high density mbit feram devices

Abstract

Reliable PZT capacitors have been developed by using stable PZT sputtering technique and Pt/thinSRO(SrRuO₃) stack electrodes. Introduction of SRO electrodes with no leakage current degradation is a key to realize reliable and scalable PZT capacitors. Roles of top electrode (TE) SRO and bottom electrode (BE) SRO were investigated respectively from reliability and process damage points of view. The SRO works as hydrogen resistant electrodes, fatigue free interfaces and nucleation sites for perovskite formation. Relationship between SRO crystallinity and PZT electrical properties was elucidated. Templates made of thin SRO were found to function as barrier layers against diffusion of Pb and Ru from BE resulting in new possible cell structures.     

Interaction of Ir and IrO2 thin films with polysilicon,W and WSIx

Abstract

Ir and IrO₂ thin films have been identified as potential electrode materials for ferroelectric capacitors. These electrodes have shown excellent electrical characteristics. The integration of ferroelectric capacitors into memory cell requires the bottom electrode material to be placed directly over a contact plug. This paper studies the interaction of Ir and IrO₂ with commonly used plug materials such as polysilicon, tungsten (W), and tungsten silicide (WSi_x) after a post-deposition annealing at 800°C. Film properties such as composition, resistivity, crystallinity, adhesion, and micro-structure have been examined before and after anneal. The results show that W is a possible plug material for Ir electrode; while polysilicon and WSi_x are potential candidates if IrO₂ electrodes are used.     

Lead zirconate titanate ferroelectric capacitors produced on sapphire and gallium arsenide substrates

Abstract

Pb(Zr_xTi_1?x)O₃ (PZT) based ferroelectric capacitors have been produced at 600°C on R-cut sapphire and Si-doped (100) GaAs substrates using a pulsed laser ablation deposition (PLAD) technique. La_0.5Sr_0.5CoO₃ (LSCO) conducting electrodes deposited using PLAD serve as top and bottom electrodes. X-ray diffraction results show that the PZT film is polycrystalline and phase-pure in both cases. Electrical characterization of the films show remanant polarization in excess of 20 μC/cm₂. Results of long term properties show that these capacitors: are very tolerant to extremely large numbers of switching cycles; retain charges over very long periods of time; and do not show a strong tendency for their dipoles to be imprinted in a preferred direction.     

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.     

Etching effects on ferroelectric capacitors with multilayered electrodes

Abstract

Dry etching of PZT thin film capacitors with RuOx/Pt multilayered electrodes was studied to examine the etching effects. PZT films were deposited on RuOx/Pt/Ti/SiO₂/ Si substrates by sol-gel process and Pt films were prepared by DC magnetron sputtering. PZT and Pt thin films were etched with Cl₂/C₂F₆/Ar gas combination in an Inductively Coupled Plasma (ICP) by varying the etching parameters such as coil RF power, DC bias to wafer susceptor, and gas pressure.

Etching effects were investigated in terms of etch rate, etch selectivity, etch profiles, and electrical properties of etched capacitors. Quantitative analysis of the etching damage was obtained by calculating the shift of the coercive field and the switchable polarization in hysteresis loops. Finally, the etching damage mechanism was discussed and the optimization of etching processes for the fabrication of PZT capacitors was attempted to minimize the etching damage to ferroelectric capacitors.     

Modification of PZT nucleation and growth using oxide layer in multi-layered electrodes

Abstract

The ferroelectric properties of PZT on RuO₂ electrodes were compared to those on RuO₂/Pt electrodes. The better hysteretic properties were obtained from Pt/RuO₂/PZT/RuO₂/Pt ferroelectric capacitors. The enhancement of ferroelectric properties is likely attributed to the modification in the microstructure of PZT film. The interfacial modification would be affected by the factors such as surface roughness, stress, and porosity of RuO₂ film. As the result of the interfacial modification, better quality PZT films are produced, thereby resulting in better ferroelectric properties. We made an effort to understand the relationship between the grain size and the coercive voltage in terms of the domain formation and the domain pinning in connection with defects like grain boundaries.     

Flat-band voltage and breakdown in single-grained ferroelectric thin film capacitors

Abstract

Single-grained ferroelectric Pb(Zr, Ti)O₃(PZT) thin films, with thickness varying from 0.1 to 0.7μm, were prepared on Pt-coated silicon substrates by pulsed laser deposition combined with rapid thermal annealing method. The current-voltage characteristics of the Pt/PZT/Pt capacitors, with various thickness of PZT, were measured. Two important turning points in J-V curves, namely flat-band voltage and breakdoiwn voltage according to the totaly minority-carrier injection model, were discussed. The electrical strength of the film was also studied by applying the square test pulses with different pulser duration.     

Study on property variation due to the interface between PZT and electrode

Abstract

The ferroelectric capacitors are fabricated using RuO₂/Pt electrode to examine the electrode effect on ferroelectric properties. PZT films are prepared by metalorganic decomposition (MOD) on sputter deposited electrodes. In particular, inductively coupled plasma(ICP) etcher is used to minimize the etching damage. In addition, TiO₂ reaction barrier layer is also employed to retard the degradation of ferroelectric properties due to the reaction between a passivation layer and PZT film. The better hysteretic properties were obtained from Pt/RuO₂/PZT/RuO₂/Pt ferroelectric capacitors. The enhancement of ferroelectric properties is likely attributed to the modification in the microstructure of PZT film. The interfacial modification would be affected by the factors such as surface roughness, stress, and porosity of RuO₂ film. The result implies RuO₂/Pt would be a good electrode for a nonvolatile memory application.     

Integration of H2 barriers for ferroelectric memories based on SrBi2Ta2O9 (SBT)

Abstract

In this study, integration of an hydrogen barrier into a FeRAM process flow is investigated. It is reported in the literature that ferroelectric properties can be maintained after hydrogen annealing by using IrO_x as a top electrode [16][17][18]. Advantage of materials like IrO_x is less catalytic activity compared to Pt. However, we found that IrO_x is not a promising candidate for top electrode barrier. (Pt)/IrO_x/SBT/Pt capacitors are prone to shorting or exhibit high leakage. IrO_x films are very easily reduced by reducing ambient which will result in peeling off. Also, IrO_x films tend to oxidize Ti or TiN layers immediately. Therefore, other barrier materials or layer sequences like Ir/IrO_x have to be considered.

For protection of the entire capacitor an Encapsulation Barrier Layer (EBL) is required. In this study, LPCVD SiN is used. LPCVD SiN is a standard material in CMOS technology. Production tools are available and it is well known as hydrogen barrier. By modifying the deposition process and using a novel process sequence, no visual damage of the capacitors after SiN-deposition and FGA is seen. Also, no degradation of electrical properties after capacitor formation as well as after SiN-deposition and FGA is observed. However, after metal 1 and metal 2 processing, 2P_r values at 1.8V are reduced from 12μC/cm²to 2μC/cm². Polarization at 5.0V is not affected.     

Challenges for plasma etch integration of ferroelectric capacitors in FeRAM's and DRAM's

Abstract

The plasma etch process requirements are different for etching 2μm ferroelectric capacitor structures in FeRAM's (SRAM) vs. the smaller capacitor sizes (0.2–0.5 μm) of DRAM's. Plasma etch integration of ferroelectric capacitors presents three major differences between FeRAM's and DRAM's. The first difference is in the ferroelectric capacitor structure. FeRAM's use planar capacitors with top side metal contacts to vias while DRAM's use vertical capacitor structures with bottom side contact to a poly post structure. The second major difference is in material selected and thickness of layers. FeRAM's use thicker electrodes of Pt or Ir and a thicker PZT or Y1 dielectric layer. FeRAM's use a thick bottom electrode (and a thin top electrode) consisting of Pt, Ru or Ir and a thin BST dielectric layer. The third major difference is the plasma etch process requirements for the two devices. FeRAM's require a clean etch process and no corrosion. Profile is not critical but should be maintained at greater than 60° for 2μm bottom post electrode. An HRe^? (Highly Density Reflected Electron) etch system is used to develop process trends for ferroelectric capacitor applications.     

Fabrication and characteristics of piezoelectric PZT cantilever for high speed atomic force microscopy

Abstract

A PZT actuator integrated onto cantilever structure was fabricated for high speed atomic force microscopy (AFM). Five different electrodes were used to investigate the effect of top electrodes on the adhesion and the electrical properties in the PZT capacitors. The PZT capacitors with RuO₂ top electrodes exhibited the best characteristics of five electrodes. The tip deflection and the resonant frequency of the PZT actuator were 11 μim at 10 V and 79 kHz, respectively. The PZT actuator provided much better AFM image quality and imaging speed than those done by using the conventional bulk PZT tube scanner. The creep distortion in the AFM image was greatly improved by using the high speed PZT film actuator.     

Rapid Thermal Annealing of Oxide Electrodes for Nonvolatile Ferroelectric Memory Structures

We report on the properties of a ferroelectric stack comprising (La_0.5Sr_0.5)CoO₃ (LSCO)/Pb(Nb,Zr,Ti)O₃ (PNZT)/LSCO deposited on 4 inch diameter platinized Si wafers (Pt/Ti/SiO₂/Si). The LSCO electrodes were deposited at room temperature by pulsed laser ablation and the ferroelectric layer was deposited by the sol-gel technique. Rutherford backscattering was performed to confirm the uniformity in composition, thickness and stoichiometry of LSCO across the wafers. Conventional furnace or rapid thermal annealing was performed to crystallize the electrodes. The oxidation resistance of the conducting barrier layers, Pt/Ti, was found to be dependent on the annealing procedure adopted for the bottom electrode. In the case where the bottom LSCO was crystallized by rapid thermal annealing, Rutherford backscattering analysis and transmission electron microscopy studies revealed that there was no oxidation of the Pt/Ti conducting barrier composite. This is in contrast to the observations for in-situ deposition or conventional furnace annealing of the bottom electrode. The resistivity, coercive field and polarization of the ferroelectric stack were uniform across the 4-inch wafers. The ferroelectric capacitors showed no fatigue up to 10¹¹ cycles and no imprint at 100°C. The ferroelectric properties were independent of the annealing procedure used for crystallizing the electrodes.     

Enhancement of Ferroelectric Properties of Pb(Zr,Ti)O3 Films Grown by Metal-Organic Chemical Vapor Deposition on Noble-Alloy Bottom Electrodes

Pb(Zr,Ti)O₃ (PZT) films grown on Ir electrodes by a metal-organic chemical vapor deposition (MOCVD) have suffered from high leakage and rough surface. We sputtered Pt and Ir simultaneously onto Ti/SiO₂/Si substrates and formed Ir-Pt alloy bottom electrodes with various compositions. With an optimal composition of Ir and Pt, PZT films grown by MOCVD on this substrate showed smoother surface and suppressed leakage via the bottom interface. At the specific composition of Ir and Pt, two different phases seemed to be acquired. They constituted the electrodes and affected the PZT grain nucleation independently so that the grains with different origins grew and restrained the vicinal grains, and finally soothed the faceted-grain-formation. No fatigue was observed even in PZT on Ir-Pt alloy with much Pt content.     

Preparation and characterization of PZT ferroelectric thin films by plasma enhanced metalorganic chemical vapor deposition

Abstract

PZT thin films with a uniform distribution of components were prepared by plasma enhanced chemical vapor deposition (PECVD) using Pb(C₂H₅)₄, Z (O-i-C₄H₉)₄, Ti(O-i-C₃H₇)₄, and oxygen. The crystallization of films was occure after annealing in the temperature range between 450 and 550°C under O₂ ambient for 1 hr. The significant change of Pb concentration in PECVD PZT thin films was not observed in the relation to annealing temperature and time. The dielectric constant PECVD PZT thin films increased with the Ti content, showed the maximum value in the vicinity of morphotropic phase boundary (MPB) composition of PZT material, and decreased with the Ti content. The leakage current density of PZT (65/35) thin film of 180 nm in thickness was 3·37 × 10^?7 A/cm² at the applied voltage of 3 V. Remanent polarization increased with increasing of Zr content in the film and coercive field was nearly independent of the composition. The typical values of electrical properties were ε_r = 570, E_c = 90 kV/cm, and P_r = 19 μC/cm² in the PECVD PZT (54/46) thin film of 220 nm in thickness.     

PZT Capacitors on Top of CrTiN/TiN Double Barrier Layers

In this paper we investigated the thermal stability of a novel CrTiN/TiN double barrier, compared to those of TiN and CrTiN single layer barriers. The CrTiN/TiN double layer was stable against heat treatments at higher than 700°C in oxygen ambient for 30 min. However, the double layer showed a severe Pt and Cr interdiffusion issue after crystallization annealing for PZT. We found that pre-annealing treatments prior to deposition of Pt significantly reduced the interdiffusion. In addition, we also demonstrated ferroelectric characteristics of PZT capacitors on top of Pt/CrTiN/TiO₂/Si bottom electrode system for ultra-high density memory applications.     

Improvement of the electrical properties of PZT thin films using TiO2 buffer layer

Abstract

Thin TiO₂ layers were sputter-deposited on Pt/Ti/SiO₂/Si wafers, as buffer layers for PZT thin film capacitors. It was found that TiO2 buffers of less than 4-nm-thickness could assist in obtaining highly uniform PZT thin films with no second phase. The leakage current behaviors of the PZT based capacitor are improved, while retaining the ferroelectric properties of PZT thin films such as remanent polarization and coercive field. In addition, the uniform distribution of oxygen in PZT on TiO₂/Pt indicates that the TiO₂ buffer layer act as a barrier for lead-platinum reaction, as well as for oxygen diffusion.     

EFFECTS OF PZT BUFFER LAYERS ON PZFNT FERROELECTRIC THIN FILMS

ABSTRACT

PZFNT thin films were fabricated on 5-inch Pt/Ti/SiO₂/Si and PZT/Pt/Ti/SiO₂/Si substrates by RF magnetron sputtering method and rapid thermal annealing process. By investigating the two thin films at various annealing temperatures, the results show that the annealing temperature of PZFNT thin films without PZT buffer layers is about 730°C, which is higher than that of PZFNT films with PZT buffer layers. By use of PZT buffer layers, the annealing temperature of PZFNT films is decreased greatly, and the dielectric and ferroelectric properties are improved. In the optimum process, the thin films with PZT buffer layers have a dielectric constant of 1199 and dielectric loss of 3.0% at 1 KHz. The remanent polarization and coercive field of the thin films are 21.1 μC/cm² and 53.5 KV/cm respectively. The films have the significant potential for FRAM and pyroelectric infrared detectors.