Mechanisms of Pb(zr0.53Ti0.47)O3 thin film etching with ECR/RF reactor

Abstract

Ferroelectric capacitive devices for memory and MEMS applications require patterned ferroelectric thin films with high anisotropic etched features. In this paper, physical and chemical parameters during etching of Pb(Zr_0.53Ti_0.47)O₃ (PZT) by a dual frequency ECR/RF reactor have been investigated. The removal characteristics of blanket films and films with a patterned mask were investigated as a function of gas chemistry (Ar, halogen gases), substrate bias RF power and working pressure (from 5 × 10^?4 Pa to 1Pa). The etch processes were characterized in terms of etch rate, selectivity and mask stability. High etching rate processes (up to 70 nm/min with removable photoresist mask) were obtained and micron scale patterns were demonstrated. The impact of the etch process on the PZT surface layer modification was characterized by AFM, SEM, TEM and XPS. A strong influence of process chemistry and RF bias power on etching selectivity and surface topography (roughness, involatile residues) was observed. No surface damage layer was detected by Transmission Electron Microscopy. However, XPS revealed fluorine (up to 34%) and chlorine radicals (below 10%) in a 10nm thick surface layer.     

Reactive ion etching of sol-gel deposited lead zirconate titanate (pzt) thin films in sf6 plasmas

Abstract

Sol-gel deposited lead zirconate titanate piezoelectric thin films were reactive ion etched in SF₆ plasmas. Etch rates increased approximately linearly with cathode power, reaching 65 nm/min at 300 W, Etch rate was independent of SF₆ flow rate but increased as pressure was reduced, indicating an etch process that is dependent on ion-induced mechanisms. Photoresist masks could be removed subsequent to etching, but mask erosion led to gradually sloped sidewalls. Use of Pt masks provided for anisotropic etching with vertical sidewalls and smooth etched surfaces.     

Effect of etch gases on iridium etching using a hard mask

Abstract

The dry etching of iridium(Ir) and iridium oxide(IrO₂) using a hard mask has been studied in a high density Inductively Coupled Plasma (ICP). The etch rate and etch selectivity have been obtained in terms of gas chemistry including CI₂/O₂/Ar, HBr/O₂/Ar, and C₂F₆/O₂/Ar gases. The etch profile and the etch mechanism have been examined for various etch gases by using field emission scanning electron microscopy (FESEM) and field emission auger electron spectroscopy (FEAES). In addition, the electrical properties of the etched ferroelectric capacitors were measured for each etching gas.     

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.     

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.     

Bottom electrode etching effect on the electrical properties of lead strontium titanate thin film

(Pb_0.4Sr_0.6)TiO₃ thin films were prepared by a modified sol-gel method on Pt/Ti/SiO₂/Si substrates, where lower figure of merit of about 16% was observed in spite of higher tunability above 58%. The electrode surface was etched with different CF₄ and Ar gas ratios to modify the surface roughness. The electrical properties of PST thin films were investigated as a function of etching condition and film thickness. With changing CF₄/(Ar+CF₄) gas composition, the dielectric loss and the figure of merit were apparently affected which can be explained in terms of the surface roughness of Pt bottom electrode. When the Pt electrode surface was etched by using CF₄/(Ar+CF₄) = 20% gas mixture, the improvement above 25–27% in dielectric loss and figure of merit was observed, according to the decreased rms value of Pt surface of ∼30%, from 1.8 to 1.2 nm. The etching effect was found to be dominant for the dielectric loss and the thinner films.     

A novel wet etch for patterning lead zirconate-titanate (PZT) thin-films

Abstract

Little is reported in the literature on the wet etching of PZT and all wet etch recipes described use high concentrations of hydrofluoric acid (HF). Here, a novel recipe which operates in a new regime of extremely low HF concentration is used to wet etch thin film PZT. The recipe provides excellent etch control, minimizes undercut and does not attack photoresist. Unlike most wet PZT etchants it does not leave a lead-rich residue, and the etchant is selective over SiO₂ and ZrO₂. The recipe and preliminary etching results for 5 micron-thick sol-gel deposited PZT films are described in this paper.     

Miniature Microphone with Silicon-Based Ferroelectric Thin Films

A miniature microphone with silicon-based lead zirconate titanate (PZT) thin films has been fabricated and tested. The main structure of the device is composed of Al/Pt/PZT/Pt/Ti/SiO₂/Si₃N₄/SiO₂/Si multi-layer diaphragm. The PZT thin films have been prepared using an improved sol-gel method. Optimized fabrication process of the device has been developed, especially, RIE (reactive ion etching) and IBE (ion beam etching) processes have been used to etch the PZT thin film and electrode metal successfully. The sensitivity of microphone is 16 mV/Pa at 1 KHz and 158 mV/Pa at the resonant frequency of 17.3 KHz. The electrical and thermal reliability of the microphone is satisfactory. This miniature microphone can be widely used in hearing aids, mobile phones, and many other applications.     

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.     

COMPARATIVE ANALYSIS FOR THE LOCAL PIEZOELECTRIC PROPERTIES OF ION BEAM AND REACTIVE ION BEAM ETCHED Pb(Zr,Ti)O3 THIN FILMS

ABSTRACT

Piezoelectric properties of highly (111)-oriented unetched, ion beam etched (IBE) and reactive ion beam etched (RIBE) PbZr_0.35Ti_0.65O₃ (PZT) thin films have been investigated on the nanoscale by piezoelectric force microscopy. Crystallization of the films was performed before or after etching. Piezoelectric contrasts imaging reveals a clear modification of the domain architecture for all the films etched after crystallization; also, within grains, local piezoelectric hysteresis loops measurements show that coercive voltage increases about 130% (1.0 V to 2.3 V) for ion beam etched films while there is no significant change for reactive ion beam etched films. No evolution of coercive voltage is evidenced for films crystallized after ion beam etching. These results are explained in terms of domain-wall pinning in the film. Piezoelectric activity is shown to be similar for all the etched films; it is only slightly reduced of about 10% in comparison to the one measured for unetched film. On the other hand, theoretical considerations using a simple charge model indicate that the depth of the ellipsoid taken into account at the surface of the film for these measurements is about 20 nm.     

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.     

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.     

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.     

Reactive Ion Etching and Ion Beam Etching for Ferroelectric Memories

The fabrication of the FeRAM requires the development of etching processes for ferroelectric thin films and electrodes as well as deposition processes. But different etching methods have different impacts such as resolution and process-induced damages. In this paper, reactive ion etching (RIE) and ion beam etching (IBE) for ferroelectric thin films and Pt electrodes have been studied in details. Different silicon-based ferroelectric thin films and their corresponding electrodes have been etched successfully. To compensate the possible oxygen loss caused by the etching process, the samples should be annealed in O₂ at 550°C for 30 min after RIE and IBE etching.     

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.     

Effects of Ti/Ir top electrodes of PZT capacitors on the hydrogen related degradation

Abstract

Hydrogen annealing damages on properties of PZT capacitors and a role of Ti/Ir hybrid structure top electrodes on capacitors are investigated in this study. It is demonstrated that the capacitors with Ti/Ir structure top electrodes improve a resistance against hydrogen related degradation. As the thickness ratio of Ti/Ir increases, the capacitors show enhanced endurance against hydrogen damages. Especially, PZT (350nm) capacitors with Ti(80nm)/Ir(20nm) hybrid top electrodes show only 26% decrease in nonvolatile polarizations (P?_r) under ± 7V, while 67% of P?_r of ferroelectric capacitors with Ir top electrodes is reduced after forming gas annealing at 250°C for 10min. Based on the XPS analysis, ferroelectric characteristics of PZT thin film capacitors are degraded by destruction of Pb-O bond into metallic Pb due to hydrogen anneal on the catalytic top electrodes (Ir, Pt).     

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.     

EFFECT OF SILICON CARBIDE PASSIVATION FILM ON THE RESONANCE CHARACTERISTICS OF CANTILEVER

ABSTRACT

We have used silicon carbide (SiC) thin films as an insulating material of the PZT micro cantilevers for electrical and biological passivation. The use of SiC thin films as a passivation layer of the PZT microcantilevers is also seemingly viable to insure the high mass sensitivity as well as the stable passivation. In this study, we report the effect of SiC passivation layer on the performance of the PZT microcantilevers. The micromachined PZT microcantilevers having a structure of SiN_x/Ta/Pt/PZT/Pt were fabricated through MEMS processes. In order to improve the mass sensitivity and the passivation, SiC thin films of the high elasticity material were deposited on the cantilever using plasma enhanced chemical vapor deposition (PECVD) at the temperature of 400°C. Plane-strain modulus of SiC thin film was measured by nanoindentation. We observed that SiC thin films showed higher Young's modulus than Si and SiO₂. Before and after the deposition of SiC thin films, the end-tip deflection and the resonant frequency change of microcantilevers were measured by a confocal microscope and an impedance analyzer. It was confirmed that end-tip deflection of microcantilever was reduced by 13～18% through the deposition of SiC thin films, indicating the stress relaxation of the microcantilevers.     

Fabrication and structural characterization of interdigitated thin film La1 − x Sr x CoO 3 (LSCO) electrodes

For the prospective use as micro-Solid Oxide Fuel Cell (μ-SOFC) cathodes and for the investigation of reaction kinetics, La_{1 − x}Sr_xCoO₃ (LSCO) mixed ionic electronic conducting thin films were deposited by DC and RF sputtering onto a number of different substrate materials and characterized. Standard photolithographic and wet chemical etching methods were utilized to microstructure the LSCO films and XRD, SEM, AFM, WDS, and RBS were used to characterize their structure, topography, and chemistry. Sputtering resulted in very homogeneous and smooth thin crystalline films with Sr deficiency and submicron sized grains. Hydrochloric acid was found to readily etch LSCO with the etching quality strongly dependent on substrate material. LSCO films were most easily etched when deposited directly on silicon substrates, etched at intermediate rates when deposited on Gd:CeO₂ films, and most resistant to etching after deposition onto single crystal yttria stabilized zirconia (YSZ) substrates. Imperfect etching was attributed to interface formation and the presence of impurities.     

Effect of seed layer on electrical properties of Pb(Zr,Ti)O3 films grown by metalorganic chemical vapor deposition

Abstract

Pb(Zr_xTi_1?x)O₃ (PZT) ferroelectric thin films were prepared by metalorganic chemical vapor deposition (MOCVD) on Pt/Ti/SiO₂/Si substrate. Very thin PZT films, which were deposited at a lower temperature and post-annealed at higher temperature for crystallization, were used as a seed layer. PZT films grown on the seed layer exhibited superior characteristics in the crystalline structure and electrical properties, compared to those deposited without seed layer. Depending on the deposition conditions of PZT seed layer, a wide variation of surface morphology and stoichiometry was found between samples, whereas chemical composition was found to be very similar.