We have studied the reactions at metal-metalloid interfaces using high resolution transmission electron microscopy, including in situ observation, and differential scanning calorimetry. There is contrasting behavior depending on the affinity for interaction or segregation. For reactive systems, compound formation ultimately results, but this can be preceded by solidstate amorphization. For non-reactive systems, crystallization of the metalloid is often achieved with nucleation and growth mediated by the metal phase.

We report structural properties of PtFe and PtCo intermetallic compounds with perpendicular magnetic anisotropy and a preferred c-axis orientation perpendicular to the film plane, formed from initially epitaxial multilayers. These materials represent particular cases of a more general class of anisotropic magnetic compounds with the CuAu(1) natural superlattice structure. They possess high magneto-optic Kerr rotations and magnetizations, suggesting them as likely candidates for magneto-optic and perpendicular magnetic recording media.

Transmission electron microscopy was performed on epitaxial Fe/Pt and Co/Pt [001] oriented multilayers that were sputter deposited onto single crystal [001] MgO. Films were examined as-deposited and after annealing in vacuum. Before annealing, the films had a magnetic easy axis in the plane while after annealing the easy axis was out of the plane. Perpendicular anisotropies of 8× 106 erg/cm3 and 1.9x 107 erg/cm3 were measured for the Fe/Pt and Co/Pt post-annealed films. Large magneto-optic Kerr rotations were also measured, 0.6° and 0.4° for the Fe/Pt and Co/Pt annealed films, respectively. Changes in the magnetic properties were a direct result of the transformation of the multilayer into c axis oriented FePt and CoPt intermetallic compounds upon annealing.

Thin films of silicon carbide have been formed on silicon field emitters by chemical reaction with ethylene (C2H4) at temperatures of 850 to 950°C using ethylene gas pressures up to 5×10-3 Torr. By controlling the reaction time and temperature, we have made SiC coatings of from ∼20 A thickness to complete transformation of tips to SiC (1-2 μm). The electron diffraction pattern of the SiC layers show the expected 20% lattice mismatch with silicon and, for those emitters completely transformed, a polycrystalline 3C-SiC polytype was identified. The small radius of curvature was maintained for both the coated and completely transformed tips, although some defects and surface roughness was introduced during the treatments.

The initial stages of the reaction between thin Al grains with an average diameter of 2 × 10−5mand sputter deposited, nanocrystalline Cu films has been studied in plan view using in situ transmission electron microscopy. At high reaction rates, the phase transformation in the Al grain resulting from the interdiffusion process is found to exhibit metastable growth morphologies characterized by negative curvature at the growth interface. The crystal structure of the initial phase formed in the Al grain under relatively high reaction rates is a metastable, orthorhombic distortion of the equilibrium body centered tetragonal θ phase. The distortion is found to vary with Al grain surface orientation. The degree of metastability can be experimentally correlated with the kinetics of the interdiffusion process as controlled by diffusion barrier thickness.

Thick (10 to 25 μm), free-standing, equal layer thickness, Copper(Cu)-304 Stainless Steel(SS) multilayer foils, having periods of lnm to 100 nm, synthesized by magnetron sputter deposition, have been examined by plan view and cross-sectional transmission electron microscopy. Multilayer growth morphology, individual layer structure and crystallographic phase orientation relationships were characterized in this study. Electron Energy Loss filtered imaging of a 20 nm period multilayer cross-section was also performed and showed that nickel had diffused into the Cu layers from the SS during synthesis. X-ray powder diffraction scans were performed and analyzed. A pure deposit of 304SS was synthesized and had a metastable BCC structure. Multilayer samples having periods of 20 nm were found to have a coherent layered Cu(FCC)- SS(FCC) structure. At larger periods (50 & 100 nm) a bimodal Cu(FCC)-SS(FCC & BCC) structure was formed. These observations show that the 304SS will grow with a metastable BCC structure when sputter deposited. When layered with Cu(FCC) the 304SS has its equilibrium FCC structure at layer thicknesses up to 10nm as a result of epitaxy with the copper. At larger SS layer thicknesses the SS appears to locally transform to the metastable BCC structure during synthesis, refining the grain structure of the depositing SS layer and the subsequent Cu layer. This transformation significantly increases the strength of the larger period multilayer.

An advanced method for investigation of multicomponent systems is proposed. Thin wedged-shape films of pure components are subsequently deposited to form a multilayered structure with continuously-varying composition as a function of sample location, providing a large number of differently composed samples in one deposition run. Each sample is then subjected to various heat treatments, and phase content as well as the microstructure formed is under investigation.

In this study an Au-Ag-Cu multilayered structure was sputtered at a room temperature onto 55 Formvar-coated Mo grids. The satellite-like X-Ray Diffraction (XRD) patterns of these samples revealed the formation of an artificial composition-modulated ternary superlattice, complete destruction of which was observed during heat treatments, where phase formation according to the ternary Au-Ag-Cu phase diagram took place.

Several aspects of phase formation were analyzed using XRD, Electron Probe for Micro- Analysis (EPMA) in Scanning Electron Microscopy (SEM), and Scanning Transmission Electron Microscopy (STEM) combined with Selected Area Electron Diffraction (SAED), Digital X-Ray Mapping (DXM), Secondary Electron Mapping and EPMA methods.

A metal-insulator-metal (MIM) voltage programmable link (VPL) has been fabricated that relies on the breakdown of a thin nitride - oxide - nitride (N-O-N) plasma deposited insulator stack. The metal electrodes were prepared by depositing a titanium layer (400 Å) on top (or bottom) of 7500 Å Al ( 1% Si). The resulting metallization resists hillock formation during thermal anneal steps and thus preserves the integrity of the thin link dielectric. The breakdown voltage is typically 10 volts for a 100 Å-150 A-100 Å NON stack. Sintering at temperatures up to 425°C results in a negligible increase in breakdown voltage. After sinter at 455°C, the breakdown voltage drops to 6 volts and continues to decrease with further sintering. Auger depth profiles of the as-deposited sample indicate the N-O-N stack is intact and the titanium has also not reacted with the aluminum film. After sinter at 455°C no distinct interfaces are observed in the auger depth profile. Angle-resolved x-ray photoelectron spectroscopy (ARXPS) of the nitride film after deposition and sinter indicate that the nitride film reacts with the titanium to form titanium nitride or titanium silicide. Angle resolved measurements indicate the titanium concentration has increased by a factor of 5 throughout the film as compared to the as-deposited sample. These results explain the observed drop in breakdown voltage since the insulator is being consumed and being converted into conducting titanium nitride or titanium silicide.

In order to arrive at a model for nucleation in the reaction of polycrystalline thin films, we have made use of a transport model that combines atom transport across interface reaction barriers with transport along grain boundaries. Through this transport model, the boundary chemical potential, μIi, and a characteristic length Li for each specie are defined. Li and the ratio of grain size to Li determine the spatial variation and the time evolution of the boundary chemical potential respectively. Nucleation of the product phase is modeled as a process whose driving force is determined by these position dependent (and time dependent) boundary chemical potentials. Thus thin film reactions become similar to precipitation from bulk homogeneous supersaturated solid solutions. Numerical calculations, however, show that boundary diffusion results in low “effective” driving forces for nucleation which can lead to heterogeneous nucleation of even the first phase. The model provides a new approach to phase selection by re-evaluation of the driving force and considers the effect of product and reactant grain structure to be fundamental to the reaction process.

Descriptions of metal (Me) diffusion into semiconductor (Se) film by diffusion-kinetic and purely kinetic methods were compared. To analyze thin film diffusion, thin film surfaces were considered as a substantial source/sink of point defects. Conditions of transition from one Me diffusion transport mechanism/regime to another, when Se sample thickness is changed, were found to be practically the same in both considerations. Critical Se film size, dislocation density, and temperature of diffusion mechanisms and regimes coincide, providing a correct interpretation of the experimental results in terms of the corresponding diffusion mechanism.

Thin interconnection layers entirely consisting of intermetallic phases have been produced by joining planar Cu substrates coated with thin films of Sn. The thin layers of intermetallic phases are produced at low temperatures (<300°C) and short reaction times (5-120 s) in a process similar to reflow soldering by a liquid-solid reaction between Sn and Cu. Metallographic methods and X-ray analysis have been used to characterize the initial stages of the formation of the intermetallic phases. The reaction to form intermetallic phases via Cu dissolution and diffusion in the liquid Sn is very fast in the first 10-15 s when the liquid Sn is in contact with the Cu substrate along channels of liquid Sn between grains of the intermetallic phases. The stability of these narrow channels of liquid Sn is discussed in terms of the wettability of different types of grain boundaries in the textured layer of intermetallic phases.

The formation and growth of an amorphous phase or other metastable phases during interdiffusion reactions in metals has been the subject of much study during the past decade [1-4]. The thermodynamic and kinetic factors which control phase selection and growth during the evolution of a binary diffusion couple are of particular interest. Amorphous phase formation in binary A/B diffusion couples has been shown to be favored in systems satisfying several themodynamic and kinetic criteria. First, the free energy of formation of the amorphous phase from the reactants must be negative. This provides the thermodynamic driving force to form the amorphous phase. In general, this condition is best satisfied in cases where the crystalline metals A and B have a negative heat of mixing in the amorphous phase [1,2,4]. On the other hand, the formation of simple crystalline solid solutions of the parent metals should be relatively unfavorable. Thus amorphous phase formation occurs generally in systems for which crystalline terminal solid solutions are restricted. In such binary systems, it is generally true that other crystalline intermetallic phases exist which have lower free energies than the amorphous phase and thus a larger thermodynamic driving force for growth during interdiffusion. As such, additional kinetic constraints must be present which suppress the nucleation and/or growth of the competing crystalline intermetallic phases [2,4].

Multilayers of Gd/Co produced by sputter deposition are observed to be amorphous for a wide range of bilayer periods. This structure is largely the result of an amorphization reaction during the deposition between crystalline Gd and newly arriving Co atoms. We explore this reaction by producing multilayers of Gd/Co/Cr, and examining the resulting microstructure. We observe both interfacial disordering due to a layer thickening mechanism and evidence for growth of the amorphous phase from Gd grain boundaries. The reaction is enhanced by the presence of a large number of crystalline defects in the Gd layers.

The formation of the metastable bcc solid solution and the amorphous phase during interfacial reaction of elemental multilayers has been studied using X-ray diffraction. Different kinds of interfaces have been synthesized in order to investigate the influence of the interfacial structure on phase evolution during annealing. The amorphization is found to be dependent on the structure of the interface and the results are discussed in terms of the kinetics and thermodynamics of the Ti-Fe system which has been calculated by the CALPHAD method.

A study of phase transformations is reported for Cu-rich, Cu-Zr multilayer foils that were synthesized using magnetron sputter deposition and annealed using a differential scanning calorimeter. The foils range in composition from 1.6 at% to 9.0 at% Zr and consist of alternate layers of polycrystalline Cu and Zr. Differential scanning calorimetry, X-ray analysis and electron microscopy were used to examine three distinct reactions in the foils: a mixing and an amorphization of the Cu and the Zr, a crystallization to the metastable intermetallic, Cu51Zr14, and a transformation of the Cu51Zr14 phase into the equilibrium phase, Cu9Zr2. The asdeposited layering remained stable during the first two reactions and then broke down in the third reaction as large grains of Cu9Zr2 encompassed the smaller Cu grains. The heats of the reactions and the activation energies of these reactions are measured and are compared to values reported for bulk samples. The measured heats support the observation that amorphous Cu-Zr alloys phase separate and provide evidence that mixing and short range ordering produce 3.5 times more heat than long range ordering when Cu and Zr react and form Cu51Zr14.

Interfacial reactions of ultrahigh vacuum deposited yttrium thin films on atomically clean (111)Si at low temperatures have been studied by both conventional and high resolution transmission electron microscopy, Auger electron spectroscopy and x-ray diffraction. A 10–nm–thick yttrium thin film, deposited onto (lll)Si at room temperature, was found to completely intermix with Si to form an 11–nm–thick amorphous interlayer. Crystalline Y5Si3 and Si were observed to nucleate first within the amorphous interlayer in samples annealed at temperatures lower than 200 °C. Epitaxial YSi2−x was found to be the only phase formed at the interface of amorphous interlayer and crystalline Si in samples annealed at temperatures higher than 250 °C. In as deposited 20– to 60–nm thick Y thin films on silicon samples, crystalline Y5Si3, Si, and YSi and a 2.5–nm–thick amorphous layer were found to be present simultaneously.

We studied crystallization of amorphous germanium (a-Ge) induced in a Ag/a-Ge layered system, using in situ transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Amorphous Ge was found to crystallize at about 270°C with the heat of reaction of 10±lkJ/mol. In situ TEM revealed that Ag grains migrate into the a-Ge phase leaving the crystalline Ge (c-Ge) phase behind. From this observation, we propose a model whereby the Ag provides the fastest path for the Ge atoms to diffuse from a-Ge to c-Ge phases.

The Au-mediated crystallization of a-Si:H has been investigated by in situ electrical measurements and scanning electron microscopy. The a-Si:H structure was prepared by evaporation of Au thin films of different thicknesses (50 - 600 Å) onto intrinsic glow discharge deposited a-Si:H layers (≈ 5000 Å). A rapid increase in the electrical resistance was associated to the nucleation and growth of Si crystals inside the Au film. The growth process promotes the accumulation of Au at the Si grain boundaries forming a metallic network. This network controls the electric resistance at the end of the crystallization reaction. The kinetics of transformation has been studied by isothermal treatments over the 140 - 180 °C temperature range. An activation energy of 2.1 eV was measured. An analysis of resistance data obtained by constant heating rate treatments suggest the rupture of the network interconnections at T ≥ 170 °C. SEM pictures showing the formation of Au clusters confirmed this suggestion.

Amorphous Te3Se4 X (X = Br,I) thin films were prepared by RF sputter deposition where the halide is used to stabilize the amorphous structure. Crystallization in the films was induced by heating with pulsed laser irradiation, with an electron beam, and in a furnace. Crystallization was achieved when the halide was depleted which became more difficult the higher its mass and concentration. As a consequence, crystallization was highly anisotropic and confined to a thin surface layer with a lateral size to thickness ratio of up to 104. We observed various stages of the process. At low temperature, a spherulitic-type growth produced a low-dimensional branching morphology by non diffusive processes such as rotation. Increasing temperature favored the formation of higher dimensions with patterns typical for aggregation processes. At the highest temperatures material flow enabled the formation of three-dimensional structures. In case of the films with Iodine, facetted single crystals grew out of the surface to sizes many times larger than the original film thickness.

Elastic stress arising from differences in lattice parameters between phases is known to alter both qualitatively and quantitatively the characteristics of phase equilibria in coherent systems. One important consequence of misfit or epitaxial strain is the possible existence of several linearly stable equilibrium states: For a given composition, temperature and applied stress, different combinations of volume fraction and corresponding phase compositions render the free energy of the system a minimum. Here, we examine how epitaxial stresses influence phase equilibria in a binary alloy when the system can select from three different phases. In particular, we show the existence of several equilibrium states with different combinations of phases that minimize the system free energy.