Iridium Thin Films Research Articles

Nucleation Behavior and Growth Characteristics of Atomic-Layer Deposited Iridium Thin Films with Ticp and Oxygen

Platinum group elements (Pt, Ru, Pd, Rh, Ag, Au, and Ir) have excellent properties of high thermal and chemical stability, and low electrical resistivity. Due to the attractive characteristics, these materials are also expected for applications in the semiconductor industries. In particular, iridium (Ir) has attracted interests as a potential electrode for memory devices because of its high work function and low resistivity (bulk: 4.7 μΩ cm).Despite the advantages of Ir, the deposition of Ir thin films for the next-generation memory applications faces critical challenges. Since metal has a high surface energy, it is difficult to adsorb on a surface with a lower surface energy. So, formation of metal nuclei is delayed in the initial growth of atomic layer deposition (ALD), and island growth is preferred. During the ALD, sparsely grown nuclei cause an increase in surface roughness which deteriorates the leakage current properties of capacitors.In this study, we attempted ALD of Ir films from newly synthesized tricarbonyl (1,2,3-η)-1,2,3-tri(tert-butyl)-cyclopropenyl iridium and oxygen. The growth behavior and film properties of impurity contents and bulk resistivity were investigated in terms of the growth temperature. Also, we examined the changes in the surface morphology of the iridium film with the film thickness. Consequently, we achieved the growth of continuous and smooth Ir films even below 4 nm by forming nucleation-promoting layer at high temperature and followed by ALD of Ir at low temperature. In addition, the capacitor device characteristics were also investigated using Ir as a bottom electrode.

Effect of Surface Pretreatment to Reduce the Incubation Period of Iridium Thin Films Grown by ALD on the Oxide Surface

Iridium (Ir) has low figure of merit(×λ) and high melting temperature properties, so it has been recently spotlighted as a very important copper (Cu) alternative interconnect material in next-generation semiconductor devices. In particular, in the case of depositing a thin film using atomic layer deposition (ALD) technology, it can be expected to deposit extremely thin film with conformal, uniform and excellent step coverage characteristics even in a very complex structure or a trench structure of several nm dimension due to the inherent self-limiting characteristic of ALD. In this regard, ALD-Ir is considered as one of the most suitable metallization process for the application of advanced semiconductor interconnects. Therefore, considerable efforts have been conducted to develop a reliable ALD-Ir process having improved film qualities. And, ALD-Ir process with excellent thin film properties such as low electrical resistivity and negligible oxygen impurities was recently reported using Tricarbonyl (1,2,3-η)-1,2,3-tri(tert-butyl)-cyclopropenyl iridium (C18H27IrO3 or TICP) precursor and oxygen [1]. However, in the case of this TICP precursor, it was difficult to deposit extremely thin and continuous Ir films on the hydroxyl-terminated oxide layer due to its long incubation delay.Therefore, in this study, a method for depositing a very thin, uniform and continuous ALD-Ir thin film with low resistivity even on oxide materials was explored by reducing the incubation period and promoting nucleation using various surface pretreatment conditions. In addition, the nucleation behavior as well as film properties of ALD-Ir on the oxide material were systemically compared and analyzed according to the surface pretreatment conditions, and finally, ALD-Ir thin film with excellent properties on the oxide surface was obtained.

Solid-state reactions between iridium thin films and silicon carbide in the 700 °C to 1000 °C temperature range

The solid-state reactions between iridium thin films (50 nm) and 6H‐SiC in the 700–1000 °C temperature range were investigated. The microstructure and surface morphology of the Ir thin films on 6H‐SiC were examined using Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM), Raman spectroscopy and grazing incidence X-ray diffraction (GIXRD). The electron beam deposited Ir films had a polycrystalline structure with the (111) preferred orientation and their crystallinity increased with annealing temperature. The Ir films were stable and did not interact with the SiC substrate after annealing at temperatures below 800 °C. The RBS results indicate that the initial reaction at 800 °C was very fast and almost the entire 50 nm film had reacted leading to the formation of Ir-rich silicides, Ir3Si2 and IrSi. After annealing at 900 and 1000 °C, the RBS profiles XRD results showed that the solid-state reactions proceeded further and Ir3Si2 was converted to IrSi. At the final annealing temperature of 1000 °C the predominant phase was IrSi. Thermodynamic analysis of the reaction phases confirmed the formation and sequence of the phases observed.

High-Performance Iridium Thin Films for Water Splitting by CVD Using New Ir(I) Precursors.

Thin films of iridium can be utilized in a wide range of applications and are particularly interesting for catalytic transformations. For the scalable deposition of functional Ir thin films, metalorganic chemical vapor deposition (MOCVD) is the method of choice, for which organometallic precursors that embody a high volatility and thermal stability need to be specifically tailored. Herein, we report the synthesis, analysis, and evaluation of new volatile Ir(I)-1,5-cyclooctadiene complexes bearing all-nitrogen coordinating guanidinate (N,N'-diisopropyl-2-dimethylamido-guanidinate (DPDMG)), amidinate (N,N'-diisopropyl-amidinate (DPAMD)), and formamidinate (N,N'-diisopropyl-formamidinate (DPfAMD)) ligands. The amidinate-based Ir complex [Ir(COD)(DPAMD)] together with O2 was implemented in MOCVD experiments resulting in highly crystalline, dense, and conductive Ir films on a variety of substrate materials. The Ir deposits achieved outstanding electrochemical performance with overpotentials in the range of 50 mV at -10 mA·cm-2 for catalytic hydrogen evolution reaction (HER) in acidic solution. The ability to deposit Ir layers via MOCVD exhibiting promising functional properties is a significant step toward large-scale applications.

Investigation of iridium, ruthenium, rhodium, and palladium binary metal oxide solid solution thin films for implantable neural interfacing applications

Metal oxide solid solution thin films containing binary mixtures of iridium, ruthenium, rhodium, and palladium were combinatorially synthesized by DC reactive magnetron sputtering to investigate the compositional dependence of properties relevant to neural interfacing applications. The binary metal oxides studied included Ir(1-x)MxOy where M = Pd, Rh, and Ru. The properties of the binary metal oxides are compared to that of their single metal oxide endmembers which include palladium oxide (PdOx), ruthenium oxide (RuOx), rhodium oxide (RhOx), and iridium oxide (IrOx). The binary metal oxides exceed the electrochemical performance of their respective single metal oxide endmembers (as measured by cyclic voltammetry) at film thicknesses generally greater than 700 nm. The binary metal oxide concentrations which produce robust microstructures and exceptional electrochemical performance have been identified to be but are not limited to x ≥ 0.5 for Ir(1-x)RhxOy, x ≥ 0.34 for Ir(1-x)RuxOy, and x ≥ 0.14 for Ir(1-x)Pdx Oy. XPS analyses suggest that the electrochemical enhancements observed are at least partly due to the presence of higher oxidation states in the binary metal oxides when compared to that of the corresponding single metal oxide endmembers.

Influence of Substrate Materials on Nucleation and Properties of Iridium Thin Films Grown by ALD

Ultra-thin metallic films are widely applied in optics and microelectronics. However, their properties differ significantly from the bulk material and depend on the substrate material. The nucleation, film growth, and layer properties of atomic layer deposited (ALD) iridium thin films are evaluated on silicon wafers, BK7, fused silica, SiO2, TiO2, Ta2O5, Al2O3, HfO2, Ru, Cr, Mo, and graphite to understand the influence of various substrate materials. This comprehensive study was carried out using scanning electron and atomic force microscopy, X-ray reflectivity and diffraction, four-point probe resistivity and contact angle measurements, tape tests, and Auger electron spectroscopy. Within few ALD cycles, iridium islands occur on all substrates. Nevertheless, their size, shape, and distribution depend on the substrate. Ultra-thin (almost) closed Ir layers grow on a Ta2O5 seed layer after 100 cycles corresponding to about 5 nm film thickness. In contrast, the growth on Al2O3 and HfO2 is strongly inhibited. The iridium growth on silicon wafers is overall linear. On BK7, fused silica, SiO2, TiO2, Ta2O5, Ru, Cr, and graphite, three different growth regimes are distinguishable. The surface free energy of the substrates correlates with their iridium nucleation delay. Our work, therefore, demonstrates that substrates can significantly tailor the properties of ultra-thin films.

Investigation of boron carbide and iridium thin films, an enabling technology for future x-ray telescopes.

We present an experimental examination of iridium and boron carbide thin-film coatings for the purpose of fabricating x-ray optics. We use a combination of x-ray reflectometry and x-ray photoelectron spectroscopy to model the structure, composition, density, thickness, and micro-roughness of the thin films. We demonstrate in our analyses how the two characterization techniques are complementary, and from this we derive that an overlayer originating from atmospheric contamination with a thickness between 1.0-1.6 nm is present on the surface. The magnetron sputtered iridium films are measured to have a density of 22.4g/cm3. The boron carbide film exhibits a change in chemical composition in the top ∼2nm of the film surface when exposed to the ambient atmosphere. The chemical reaction occurring on the surface is due to an incorporation of oxygen and hydrogen present in the ambient atmosphere. Lastly, we present a correlation between the absorption edges and the emission lines exhibited by the thin films in an energy range from 50-800 eV and the impact on the reflectivity performance due to contamination in thin films.

Effect of microstructure on the optical properties of sputtered iridium thin films

The refractory metal iridium has many applications in high performance optical devices due to its high reflectivity into X-ray frequencies, low oxidation rate, and high melting point. Depositing Ir via magnetron sputtering produces high quality thin films, but the chamber pressure and sputter conditions can change Ir film microstructure on the nanoscale. Film microstructure is commonly examined through microscopy of film cross-sections, which is both a destructive characterization method and time consuming. In this work, we have utilized a non-destructive characterization technique, spectroscopic ellipsometry, to correlate the optical properties of the metal films with their structural morphologies, enabling large-scale inspection of optical components or the ability to customize the metal refractive index for the application at hand. The optical properties of Ir thin films deposited at chamber pressures from 10 mTorr to 25 mTorr are reported and compared to microscopy and resistivity results. The measurements were conducted with films deposited both on a bare wafer and on a titanium sublayer.

Galvanostatic Electrodeposition of Thin-Film Ir–Ni Electrocatalyst on Copper Foam for HER Performance in Alkaline Electrolyte

Iridium–nickel (Ir–Ni) film is of great interest for catalytic and corrosive environment applications. Ir–Ni thin films as an electrocatalyst for hydrogen evolution reaction (HER) were galvanostatically electrodeposited on copper (Cu) foam from an electrolyte containing 13.5 mM sodium hexabromoiridate(III) and 40.5 mM Ni sulphate hexahydrate, simultaneously compared with electrodeposited Ir and Ni thin films. The top surface morphology of the film was characterized by scanning electron microscopy. The chemical composition of the film was determined by energy-dispersive spectroscopy and X-ray photoelectron spectroscopy. The electrocatalytic performance was performed by linear sweep voltammogram and cyclic voltammetry. The results showed that Ir–Ni thin film adhered to Cu foam and the surface appeared much rougher than the surface of Ni film. The chemical composition of Ir in the deposit was 80 ± 1.2 at.%. The film was composed of nanograins. The top surface of as-deposited film was mainly composed of metallic state. However, the top surface of the film consisted of oxides states, such as Ni oxides or Ni(OH)2, and Ir oxides after electrochemical measurements. As-deposited Ir–Ni thin film with large real active area exhibited high efficient electrocatalytic activity for HER, and achieved a current density of 10 mA cm2 at an overpotential of 60 mV and a Tafel slope of 40 mV dec−1, which is superior to pure Ir and Ni thin films. The remarkable increase in electrocatalytic activity for Ir–Ni film was ascribed to both increased surface area of active centers due to relatively rough and electrocatalytic synergism of Ir and Ni for the HER. Ir–Ni thin film electrodeposited on a foam copper electrode was used as an electrocatalyst for HER. The surface of as-deposited film was composed of metallic state. As-deposited Ir–Ni thin film with large real active area exhibited high efficient electrocatalytic activity for HER, and achieved a current density of 10 mA cm2 at an overpotential of 60 mV and a Tafel slope of 40 mV dec−1, which is superior to pure iridium and Ni thin films. As-deposited film possessed a good stability by accelerated degradation studies.

Texture and interface characterization of iridium thin films grown on MgO substrates with different orientations

Iridium thin films are grown by direct-current plasma magnetron sputtering, on MgO single-crystal substrates with various surface orientations, i.e. (100), (111), and (110). The surface morphology, the crystalline properties of the films, and the substrate–thin-film interface are investigated by atomic force microscopy, X-ray diffraction (XRD), focused ion beam scanning electron microscopy, and high-resolution transmission electron microscopy, respectively. The results reveal that hetero-epitaxial thin films with different crystallographic orientation and notable atomic scale smooth surface are obtained. From the XRD analysis, the following epitaxial relations are obtained: (1) (100)Ir||(100)MgO out-of-plane and [001]Ir||[001]MgO in-plane for Ir grown on MgO(100), (2) (110)Ir||(110)MgO out-of-plane and [1-10]Ir||[1-10]MgO in-plane for Ir grown on MgO(110), and (3) (111)Ir||(111)MgO out-of-plane and two variants for in-plane orientation [1-10]Ir||[1-10]MgO and [1-10]Ir||[10-1]MgO, respectively, for Ir grown on MgO(111). Because of the large misfit strain (9.7%), the thin films are found to grow in a strain-relaxed state with the formation of geometrical misfit dislocations with a ~ 2.8-nm spacing, whereas thermal strain is stored upon cooling down from the growth temperature (600 °C). The best structural characteristics are obtained for the (111)-oriented films with a mosaicity of 0.3° and vanishingly small lattice distortions. The (100)- and (110)-oriented films exhibit mosaicities of ~ 1.2° and lattice distortions of ~ 1% which can be explained by the larger surface energy of these planes as compared to (111).

Reversible Decay of Oxygen Evolution Activity of Iridium Catalysts

Iridium has long been recognized as one of the best oxygen evolution catalysts in terms of activity and stability over a wide range of pH. Despite exhibiting initially high activity for the oxygen evolution reaction (OER), a rapid and reversible activity decay is observed during continuous operation. The potential dependence of recovery of the activity is explored, and 0.0 V vs NHE is found to be an effective potential to recover the initial water oxidation performance. Iridium thin films on rotating disk electrodes are used to show that this OER activity decay is neither explained by progressive oxidation of the iridium nor by reduced mass transport to/from the electrode surface. Careful examination of the time dependence of the activity decay reveals that it is well described by a t−1/4 functional dependence across multiple electrochemical cell geometries. Tafel behavior is analyzed by normal pulse voltammetry, suggesting that, after 10 minutes of activity decay, the catalyst exhibits a five-fold decrease in active site density while the mechanism of water oxidation is not altered. We hypothesize that this decay may result from a loss of active sites capable of forming the Ir(V) = O species, possibly via progressive cross-linking of iridium sites by bridging mu-oxides.

Metallic Iridium Thin-Films as Model Catalysts for the Electrochemical Oxygen Evolution Reaction (OER)—Morphology and Activity

Iridium (Ir) oxide is known to be one of the best electrocatalysts for the oxygen evolution reaction (OER) in acidic media. Ir oxide-based materials are thus of great scientific interest in current research on electrochemical energy conversion. In the present study, we applied Ir metal films as model systems for electrochemical water splitting, obtained by inductive heating in a custom-made setup using two different synthesis approaches. X-ray photoelectron spectroscopy (XPS) and selected area electron diffraction (SAED) confirmed that all films were consistently metallic. The effects of reductive heating time of calcined and uncalcined Ir acetate films on OER activity were investigated using a rotating disk electrode (RDE) setup. The morphology of all films was determined by scanning electron microscopy (SEM). The films directly reduced from the acetate precursor exhibited a strong variability of their morphology and electrochemical properties depending on heating time. The additional oxidation step prior to reductive heating accelerates the final structure formation.

Influence of sputtering pressure on microstructure and layer properties of iridium thin films

Iridium layers with low stress, high density, and low surface roughness find widespread use in different high-technology applications. This paper presents a study of the influence of the sputtering pressure on the properties of iridium thin films and of its effect on the substrate surface microstructure. We analysed the dependence of the microstructure, crystalline structure, electrical resistivity, and deposition rate on the sputtering pressure and surface defects of the substrate. For the latter, plasma etching of the substrate was performed for different processing times and its effect on the surface roughness of substrates and, subsequently, on the grown iridium films, was examined. The sputtering pressure and the substrate plasma etching time both had a strong influence on the microstructure and surface roughness. These microstructural changes are in good agreement with the tendency described in the Thornton Structure-Zone Model for different sputtering pressures and the microstructure phase map of Alvarez. The electrical resistivity, deposition rate, and crystalline structure were highly dependent on the sputtering pressure.

Synthesis of ALD Iridium Thin Films on Silicon and Monel K-500 Steel Substrates

Iridium (Ir) thin films have been extensively investigated for a variety of applications. Ir is a potential optical material for Fresnel zone X-ray microchannel plates, and inductive grid filters. It is also a noble metal making it well suited to corrosion resistance in harsh environments Atomic Layer Deposition (ALD) is considered a special case of chemical vapor deposition (CVD) in that two different chemical precursor materials are alternately introduced into a vacuum chamber where a chemical reaction occurs on the surface of the substrate. This method of thin film growth is self-limiting and conformal and consequently results in precise layer thickness control, stoichiometry, composition, and uniformity across a large surface area. Advantages that are unique to ALD include the ability to grow conformal thin films across complex geometries, and the relatively low temperatures required to achieve film growth. In this work, iridium metal films were synthesized on silicon (Si) and Monel K-500 alloy substrates by ALD using Iridium (III) acetylacetonate known as Ir(acac)3 or [CH3COCH=C(O-)CH3]3 Ir as precursor 1, and industrial grade O2 as oxidizing reactant. The solid Ir precursor was heated to 150 °C, and the substrate temperature was varied to establish an optimum growth window. The lowest temperature in which Ir growth was found is 200 °C, and the thin film growth rate increased as a function of temperature up to 250 °C. Higher temperatures have not yet been attempted due to equipment constraints. The Monel K-500 steel alloy flat discs were cut from commercially available Monel K-500 bars of ~1 inch diameter. In this study we report on the successful synthesis of Ir films on Si and Monel K-500 substrates. Detailed characterization has been performed using X-ray diffraction (XRD) techniques, atomic force microscopy (AFM), and field emission scanning electron microscopy (FE-SEM) of samples which have been notched with focused ion beam (FIB) techniques for cross-sectional film thickness measurements as revealed in Figure 1. The polycrystalline ALD Iridium film exhibited good chemisorption and adhesion to native oxide covered Si substrates, with hydroxyl surface termination. However even with the absence of a SiO2 surface film, the ALD synthesis of Iridium metal films worked equally well on the Monel K-500 steel alloy flat disc substrates and yielded excellent surface coverage.

Single orientation graphene synthesized on iridium thin films grown by molecular beam epitaxy

Heteroepitaxial iridium thin films were deposited on (0001) sapphire substrates by means of molecular beam epitaxy, and subsequently, one monolayer of graphene was synthesized by chemical vapor deposition. The influence of the growth parameters on the quality of the Ir films, as well as of graphene, was investigated systematically by means of low energy electron diffraction, x-ray reflectivity, x-ray diffraction, Auger electron spectroscopy, scanning electron microscopy, and atomic force microscopy. Our study reveals (111) oriented iridium films with high crystalline quality and extremely low surface roughness, on which the formation of large-area epitaxial graphene is achieved. The presence of defects, like dislocations, twins, and 30° rotated domains in the iridium films is also discussed. The coverage of graphene was found to be influenced by the presence of 30° rotated domains in the Ir films. Low iridium deposition rates suppress these rotated domains and an almost complete coverage of graphene was obtained. This synthesis route yields inexpensive, air-stable, and large-area graphene with a well-defined orientation, making it accessible to a wider community of researchers for numerous experiments or applications, including those which use destructive analysis techniques or irreversible processes. Moreover, this approach can be used to tune the structural quality of graphene, allowing a systematic study of the influence of defects in various processes like intercalation below graphene.

High temperature reactive ion etching of iridium thin films with aluminum mask in CF4/O2/Ar plasma

Reactive ion etching (RIE) technology for iridium with CF4/O2/Ar gas mixtures and aluminum mask at high temperatures up to 350 °C was developed. The influence of various process parameters such as gas mixing ratio and substrate temperature on the etch rate was studied in order to find optimal process conditions. The surface of the samples after etching was found to be clean under SEM inspection. It was also shown that the etch rate of iridium could be enhanced at higher process temperature and, at the same time, very high etching selectivity between aluminum etching mask and iridium could be achieved.

Atomic Layer Deposition of Iridium Thin Films Using Sequential Oxygen and Hydrogen Pulses

Atomic layer deposition (ALD) is an advanced thin-film deposition method based on self-limiting surface reactions that allows for the controlled deposition of conformal, high-quality thin films of various materials. In this study, we aimed to explore how modifying the deposition chemistry affects the growth and properties of iridium films. We demonstrated a new ALD process using sequential pulses of iridium acetylacetonate [Ir(acac)3], oxygen (O2), and hydrogen (H2) and compared this to the established Ir(acac)3 + O2 process in the wide temperature range of 200–350 °C. A reaction scheme is proposed to explain how both oxygen and hydrogen affect the film growth. Comprehensive information on film properties was obtained for both processes. In particular, the strong (111) texture seen in this study has not been reported before for ALD iridium films. Changes in film properties, especially lowered resistivity, stronger (111) texture, and faster nucleation compared to the Ir(acac)3 + O2 process, should motivate...

Growth of highly textured iridium thin films and their stability at high temperature in oxygen atmosphere

The growth and thermal stability of textured iridium thin films used as bottom electrode in electronic devices based on ferroelectric materials were investigated. The thin films were grown using the dc magnetron sputtering technique. The Ir layers directly deposited on SiO2/Si substrates present a mixed (111) and (200) orientations, while the films grown on Ti seed layers exhibit a strong preferred (111) orientation favoured by good matching with the titanium lattice. The substrate temperature during the growth of iridium/titanium stack has a significant effect on the surface morphology of the iridium layer and its thermal stability. The as-grown surface of 20-nm-thick Ir films is smooth, having a root-mean-square (rms) roughness of 0.7 nm. After thermal annealing the Ir film shows an increased surface roughness due to the formation of agglomerations. The change in the surface morphology of the Ir layer is due to titanium diffusion and its oxidation. Thicker and better crystallised iridium thin films annealed in oxygen atmosphere at 700 °C show a good thermal stability with only a slight modification of the surface morphology. Within the limits of experimental error, there is no change in the electrical resistivity before and after thermal annealing. The rms roughness has not varied significantly and the XPS investigation shows no traces of titanium oxide on iridium surface. Ir/Ti stack deposited under the optimum conditions could be successfully used as electrode in devices based on oxide thin films.

Synthesis and Thermal Decomposition Mechanism Study of a Novel Iridium Precursor

Ir(TFA) 3 (TFA=1,1,1-trifluoro-2,4-pentanedionate) metal complexes with high purity was successfully synthesized. The metal complexes have been characterized by elemental analysis, infrared spectroscopy, nuclear magnetic resonance spectroscopy. The volatility of metal complexes was studied by thermogravimetry analysis. And a possible decomposition mechanism was studied by mass spectroscopic analysis method. The novel iridium complexes can be severed as precursor in metalorganic chemical vapor deposition of iridium films. iridium films were deposited by metalorganic chemical vapor deposition method, and the Ir(TFA) 3 complex was used as precursor. The iridium thin films were characterized by X-ray diffraction and scanning electron microscopy in order to determine crystallinity and surface morphology.

Deposition of Iridium Thin Films on Three‐Dimensional Structures With PE‐MOCVD

Iridium thin films are deposited on sub‐micrometer three‐dimensional trench structures by plasma‐enhanced metal‐organic chemical vapor deposition (PE‐MOCVD). The iridium precursor used in this study is (ethylcyclopentadienyl)(1,5‐cyclooctadiene)iridium [Ir (EtCp)(1,5‐COD)]. Various process conditions at substrate temperatures from 300 °C to 450 °C, with and without plasma enhancement, are investigated and compared. Crystal structure of the deposited iridium films is analyzed by X‐ray diffraction (XRD). Step coverage of the deposited iridium films on three‐dimensional trench structures is analyzed by scanning electron microscopy (SEM). Surface morphology is quantitatively evaluated by atomic force microscopy (AFM) and the electrical resistivity of the deposited Ir films is measured by the four‐point probe method.