Thin Film Diffusion Research Articles

Study of nano-scale diffusion in thin films and multilayers

It is shown that x-ray reflectivity and x-ray fluorescence under standing wave conditions can be used to study atomic diffusion with an accuracy of a fraction of a nanometer. Both the techniques can be made isotope selective by making use of nuclear resonance scattering from a Mossbauer active isotope. The techniques have been used to study self-diffusion of Fe in amorphous and nano-crystalline alloys of FeZr and FeN. The observed correlation of the activation energy E with pre-exponent factor D0 confirms that in amorphous FeZr alloy diffusion takes place via collective motion of a group of atoms. Even in nanocrystalline alloys it is found that atomic diffusion occurs mainly through grain boundaries which are amorphous or highly disordered in nature.

Anisotropic oxygen diffusion properties in epitaxial thin films of La2NiO4+δ

We report on the development and validation of a new methodology for the determination of anisotropic tracer diffusion and surface exchange coefficients of high quality epitaxial thin films in the two perpendicular directions (transverse and longitudinal), by the isotopic exchange technique. Measurements were performed on c-axis oriented La2NiO4+δ films grown on SrTiO3 (100) and NdGaO3 (110) by pulsed injection metal organic chemical vapour deposition (PIMOCVD), with different thicknesses ranging from 33 to 370 nm. The effect that the strain induced by the film–substrate mismatch has on the oxygen diffusion through the film was evaluated. Both tracer diffusion coefficients, along the c-axis and along the ab plane, were found to increase with film thickness, i.e., as the stress of the film decreases, while the thickness seems to have no effect on the tracer surface exchange coefficient. Best fits were obtained when considering the thickest films composed by two regions with different c-axis tracer diffusion coefficient values, a higher and constant D* close to the film surface and a variable decreasing D* closer to the substrate. As expected, the tracer diffusion and surface exchange coefficients are thermally activated and are approximately two orders of magnitude higher along the ab plane than along the c-axis. The low activation energies of D* compared with bulk values for both directions at low temperatures seem to confirm the contribution of a vacancy mechanism to the ionic conduction.

Photoacoustic measurement of thermal properties of TiN thin films

Titanium nitride (TiN) thin films were prepared by reactive DC magnetron sputtering under different nitrogen flow rates and at constant substrate temperature as well as at constant nitrogen flow rate and at different substrate temperatures. Photoacoustic measurement of the thermal properties of the films revealed that the thermal diffusivity and thermal conductivity of the TiN thin films are significantly lower than the bulk values and that the grain size of the films has substantial influence on the thermal properties of TiN thin films. The thermal conductivity of the films decreases with increasing nitrogen flow rates and increases with increasing substrate temperature. The above opposing behaviour in the thermal properties is found to be related to the microstructure, especially, the grain size of the films.

Programmable Metallization Cell 응용을 위한 Ag-doped 칼코게나이드 박막의 전기적 저항 변화 특성

We have demonstrated new functionalities of Ag doped chalcogenide glasses based on their capabilities as solid electrolytes. Formation of such amorphous systems by the introduction of silver via photo-induced diffusion in thin chalcogenide films is considered. The influence of silver on the properties of the newly formed materials is regarded in terms of diffusion kinetics and Ag saturation is related to the composition of the hosting material. Silver saturated chalcogenide glasses have been used in the formation of solid electrolyte which is the active medium in programmable metallization cell (PMC) devices. In this paper, we investigated electrical and optical properties of Ag-doped chalcogenide thin film on changed thickness of Ag and chalcogenide thin films, which is concerned at Ag-doping effect of PMC cell. As a result, when thickness of Ag and chalcogenide thin film was 30 nm and 50 nm respectively, device have excellent characteristics.

Diffusion-induced recrystallization in silver–palladium layers

The interdiffusion of Pd–Ag thin film diffusion couples is investigated by transmission electron microscopy and X-ray diffractometry at a temperature of 723 K. By comparison with calculated diffractograms, it is shown that a diffusion-induced recrystallization takes place, which produces a first generation of new grains with a dominant composition of 24.5 at.% Pd inside the diffusion zone.

Aluminum diffusion and reaction in thin films of perylene-3,4,9,10-tetracarboxylic dianhydride: Depth profiles and time-dependent diffusion coefficients

Diffusion and reaction of aluminum metal species (Al) vacuum deposited on perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) thin films were investigated using angle resolved x-ray photoemission spectroscopy. The acquired data were analyzed assuming that the diffusion of Al is described by a one-dimensional diffusion equation with a time-dependent diffusion coefficient. Depth profiles and diffusion coefficients are obtained for reacted and metallic Al separately. The results show that the metallic Al diffuses rapidly during the deposition while the metal diffusion continues at a lower rate even after the deposition at room temperature. On the other hand, the reacted Al does not diffuse further into the PTCDA layers.

Kinetics characterization of the oxidation of Cu thin films at low temperature by using sheet resistance measurement

Film sheet resistance measurement was developed to obtain the oxidation kinetics of Cu thin films in the nanometer range. Cu thin films with smooth and homogenous surfaces were prepared by vacuum evaporation. Oxidation behaviour of Cu thin films at low temperatures from 180 °C to 260 °C in dry air has been studied. The results show that oxidation of Cu thin film follows a parabolic rate law with a considerable low activation energy of 0.57 eV, which is much lower than that reported for bulk Cu and the thick film. The observed rapid oxidation phenomenon may be attributed to the fast diffusion mechanism dominated by the defect-related grain boundary diffusion in thin films.

Electrochemical impedance investigation of proton exchange membrane fuel cells experienced subzero temperature

Polarization losses of the fuel cells with different residual water amount frozen at subzero temperature were investigated by electrochemical impedance spectroscopy (EIS) taking into account the ohmic resistance, charge transfer process, and oxygen mass transport. The potential-dependent impedance before and after eight freeze/thaw cycles suggested that the ohmic resistance did not change, while the change of the charge transfer resistance greatly depended on the residual water amount. Among the four cells, the mass transport resistance of the cell with the largest water amount increased significantly even at the small current density region. According to the thin film-flooded agglomerate model, the interfacial charge transfer process and oxygen mass transport within the agglomerate and through the ionomer thin film in the catalyst layer both contributed to the high frequency impedance arc. From the analysis of the Tafel slopes, the mechanism of the oxygen reduction reaction (ORR) was the same after the cells experienced subzero temperature. The agglomerate diffusion changed a little in all cells and the thin film diffusion effect was obvious for the cell with the largest residual water amount. These results indicated that the slower oxygen diffusion within the catalyst layer (CL) was the main contributor for the evident performance loss after eight freeze/thaw cycles.

Laptop photothermal reflectance measurement instrument assembled with optical fiber components

In this article, we propose a laptop photothermal reflectance measurement instrument assembled with optical fiber components. The primary feature of this instrument is that all of the optical routes for the pumping and probing beams, as well as the beam sources using a laser diode, are composed of optical fiber and optical fiber components. With this configuration, the problems related to the technical shortcomings of the conventional instrument can be solved completely. Our proposed instrument is also appropriate for in situ measurement of the thermoproperties of thin film. The dimensions of our instrument's case are 400 mm wide, 250 mm deep, and 60 mm tall, and its weight is approximately 1 kg, containing the power supply for driving the laser diode of the pumping beam and electronics for the detection of photothermal reflectance. These are at least 120 and 150 smaller than the volume and weight of the conventional commercial instrument, respectively. Nevertheless, it is only necessary to prepare a synchronous detection instrument for signal recovery (e.g., lock-in amplifier) with our instrument. To evaluate our instrument's thermoproperty measurement capability, we measured the thermal diffusivity and thermal conductivity of Au thin film. The thermal diffusivity of 1.5-microm-thick Au film measured by our instrument matched previously reported values within a margin of error of a few percent.

Probing Chemical Interactions at the Single-Molecule Level in Mesoporous Silica Thin Films

Single-molecule fluorescence spectroscopy is used to study dye molecule diffusion and surface interactions in mesoporous silica thin films. Three different dyes (nile red, DiI, and a sulfonated perylene diimide (SPDI)), selected for their hydrophobicity and charge characteristics, are employed. As-synthesized (surfactant-containing) and calcined (surfactant-free) mesoporous films that are either dry or have been hydrated by exposure to high-humidity environments are studied. Fluorescence correlation spectroscopy (FCS) is used to assess the relative importance of diffusion and reversible binding in each material, to measure the dye diffusion coefficients, and to determine the duration of reversible adsorption events. In CTAB-containing as-synthesized materials, SPDI is found to be immobile under all conditions explored, while Nile Red is always mobile. The mobility of DiI is dramatically dependent on the level of film hydration. In SDS-containing materials, DiI is immobile in dry films while SPDI and Nile ...

Ag diffusion in ZnS thin films prepared by spray pyrolysis

ZnS thin films were deposited by spray pyrolysis method on glass substrates. Diffusion of Ag in ZnS thin films was performed in the temperature range 80–400 °C under a nitrogen atmosphere. The diffusion of Ag is determined with XRF, and the obtained concentration profile allows to calculate the diffusion coefficient. The temperature dependence of Ag diffusion coefficient is determined by the equation D = 8 × 10 − 9 exp(− 0.10 eV / kT). It was found that the as-grown undoped high resistive n-type ZnS thin films were converted to the p-type upon Ag doping with a slight increase in resistivity only by rapid thermal annealing at 400 °C in N 2 atmosphere. In addition, the band gap of the p-type film was decreased as compared with the undoped sample annealed under the same conditions. The results were attributed to the migration of Ag atoms in polycrystalline ZnS films by means of both along intergrain surfaces and intragrain accompanied by interaction with native point defect.

Stress Evolution in Thin Films; Diffusion and Reactions

After a brief discussion of possible mechanisms of stress generation in thin film diffusion/reaction couples, two recent experimental examples are reviewed: (i) Thin film diffusion couples (Pd-Cu, individual layer thicknesses: 50nm) prepared by DC-magnetron sputtering on silicon substrates. The microstructural development, phase formation and the stress evolution during diffusion annealing have been investigated employing Auger-electron spectroscopy in combination with sputter depth profiling, transmission electron microscopy, in-situ wafer-curvature measurements and ex-situ and, in particular, in-situ X-ray diffraction measurements. (ii) Tin layers on copper substrates (layer thicknesses of some microns) prepared by electrodeposition. Upon storage at ambient temperatures, Cu diffuses into the Sn layer and forms the intermetallic phase η’- Cu6Sn5. The phase formation is accompanied by a volume expansion and as a consequence, compressive residual stresses can be generated in the Sn layers. These compressive residual stresses may drive the formation of Sn whiskers on the Sn surface. The microstructural development, phase formation and the stress evolution during diffusion annealing have been investigated employing scanning electron and focused ion beam microscopy, metallography and ex-situ and, in particular, in-situ X-ray diffraction measurements.

First study of manganese diffusion in Cr2O3 polycrystals and thin films by SIMS

Chromia layers are formed on many industrial alloys and act as a protective barrier against the corrosion of the materials by limiting the diffusion of oxygen and cations. Most of these alloys contain manganese as an impurity, and manganese oxides are often found at the outer surface of the oxide films. In order to clarify the oxidation mechanism and to check if chromia acts as a barrier, manganese diffusion in chromia was studied in both polycrystals and oxide films formed by oxidation of Ni–30Cr alloy in the temperature range 700–1100°C at an oxygen pressure of 10−4 atm. After deposition of Mn on the chromia surface and a diffusing treatment, the manganese penetration profiles were established by secondary ion mass spectrometry. In all cases, the diffusion profiles showed two domains. For the first domain, using the solution of Fick's law for diffusion from a thick film into a semi-infinite medium, bulk diffusion coefficients were determined. For the second domain, the Le Claire model allowed the grain boundary diffusion parameter (αD gbδ) to be obtained. Manganese diffusion does not vary significantly according to the nature and microstructure of chromia. The activation energy of grain boundary diffusion is not far from that obtained for bulk diffusion, probably on account of segregation phenomena. Manganese diffusion was compared to cationic self-diffusion and iron diffusion, and related to the protective character of chromia.

Study of copper diffusion through a ruthenium thin film by photoemission electron microscopy

Photoemission electron microscopy is used to study copper diffusion through a ruthenium thin film. The photoemission electron microscopy images display a large contrast between Cu and Ru due to the differences in work function, making photoemission electron microscopy an ideal methodology to study thin film diffusion in real time. Between 175 and 290°C, Cu mainly diffuses through defect sites in the thin Ru film. Uniform diffusion of Cu through the Ru film begins at approximately 300°C. The results are confirmed by x-ray photoemission spectroscopy depth profiling and scanning electron microscopy–energy dispersive x-ray spectroscopy analysis.

Method to Predict Phase Formation and Specific Capacity for Lithium in Codeposited Silicon-Transition Metal Thin Films

A modified version of the effective heat of formation (EHF) model of Pretorius et al. is presented to predict phase formation in codeposited silicon-transition metal films. The EHF model predicts that the first Si–M (transition metal) phase to form in thin-film diffusion couples is the phase with the most negative EHF at the composition of the lowest temperature eutectic (i.e., at the growth interface). Combinatorial thin-film libraries of codeposited Si–M produced using the modified composition spread method of Dahn et al. consist of intimately mixed Si and M atoms. We propose the entire film can be considered as the growth interface and that the Si–M phase with the most negative EHF at a given composition is present. Film nanostructure as a function of composition can then be directly determined from EHF diagrams (assuming regions of Si are amorphous). This information, when combined with the assumption that all Si–M phases are inactive, can be used to predict the specific capacity of all Si-(transition or rare-earth metal) systems in cells as a function of composition.

Thin film aluminum–gold interface interactions

Thin film diffusion couples of 1 μm Au with 0.3 μm and 1 μm Al were annealed at different temperatures and for different times and the resulting interface reactions were tracked by X-ray diffraction and electron microscopy. The phase formation sequence was identified and the interface microstructure was shown to consist of layers of the different phases. Interface tracking by secondary ion mass spectroscopy depth profiling showed that Au is the predominant diffusant in this system. Intermetallic thickness measurements were made and the activation energy calculated.

Low Temperature Self-Diffusion of Iron in the <sup>56</sup>Fe/<sup>57</sup>Fe Stable-Isotope Thin-Film System

Low-temperature diffusion in thin films of the stable isotopes, 56Fe/57Fe, was studied by applying secondary ion mass spectrometry (SIMS). Processing of the concentration profiles obtained was done by using the Hall – Morabitto “median gradient” method. The bulk and intergrain selfdiffusion coefficients were determined.

Ion leaching of a glass-ionomer glass: an empirical model and effects on setting characteristics and strength

The release of ions from a glass-ionomer glass, which in the polyacid matrix effects the cross-linking and setting of a cement, can be modelled and initiated by acid-treatment in a dilute acid. This study examined the effect of time of acetic acid leaching on the working time, setting time, and strength of a model GIC. A reactive fluoride glass was immersed in hot acetic acid for 0 (control), 5, 15, 35, 65, 95 and 125 min, filtered and dried. The glass was mixed with an experimental GI liquid in a capsule system and the mixed pastes assessed for working and initial setting time. Compressive strength testing was undertaken according to ISO9917:2003. Immersion time had a significant effect on both working and setting time of the resultant pastes only up to 65 min of immersion, and corresponded with a thin-film ion diffusion model. Compressive strength did not vary significantly with immersion time. The glass-ionomer setting reaction can be conveniently retarded by immersion of the powder in acetic acid, without affecting strength. A reactivity model was developed, whereby the effects of various changes to the leaching process may be usefully examined.

Thermal Transport Properties of Various Thin Films for MEMS Applications

The thermal properties of thin films, such as thermal conductivity and diffusivity, are important in design and analysis of MEMS (micro electro mechanical systems), particularly in microscale thermal systems and high-power electronic/optoelectronic devices. In the present study, the thermal conductivity and diffusivity of a variety of thin film materials, which are commonly used in MEMS applications, are measured. The samples include Au, Sn, Mo, Al/Ti alloy, AlN, and SiC. The Au sample is deposited by the e-beam evaporation technique while the rest of the metallic samples are deposited by sputtering processes. The AlN and SiC films are also prepared by sputtering processes. In the experiment, the thermal diffusivities of metallic thin films are measured by two independent methods — the AC calorimetric method and photothermal mirage technique. The thermal conductivities of dielectric thin films are measured by the 3 omega technique. The results show that the thermal transport properties of some of the films are significantly smaller than those of the same material in bulk form. Especially, the AlN and SiC thin films exhibit pronounced thermal conductivity reduction because of the size effect. The electrical conductivities of the metallic thin films are measured as well. The results for Au and Sn are consistent with the thermal conductivity, confirming the Wiedmann-Franz law. However, Al/Ti and Mo thin films show considerable deviation from the law. The results are analyzed based on the XRD (X-Ray diffraction) and AFM (Atomic Force Microscope) measurement.

Influences of Elastic Stress and Interfacial Kinetic Barrier on Phase Evolution Paths of Thin-Film Diffusion Couples

An explicit, finite difference scheme was used to examine the effects of coherency stresses and interface kinetic barriers on the phase evolution of a binary, thin-film diffusion couple. Thin-films, initially consisting of alternating layers of two terminal phases, α and γ, were held at a temperature at which the formation of an intermediate phase, β, at α/γ interface was thermodynamically probable. When either the coherency stresses or interface kinetic barriers are present, the interfacial compositions become time-dependent and, thus, the formation of the thermodynamically stable β phase can be kinetically prohibited at the early stage of the evolution. Even if the initial α/γ thin-films have the same overall composition, the coherency constraint can result in different final equilibrium states depending on either the initial compositions of the α and γ phases or the relative magnitudes of the interfacial kinetic barriers. When both the coherency constraint and interfacial kinetic barriers are present, an intermediate phase can repeatedly form and disappear during the evolution.