Reaction Of Thin Films Research Articles

In-Situ Studies of Silicide Formation in Ti-Ta Bilayer Thin Films on Poly-Si

AbstractWe have studied the formation of titanium silicides in the presence of an ultra-thin layer of Ta, interposed between Ti and Si. In-situ x-ray diffraction (XRD), resistance measurements and elastic light scattering were used to study the thin film reactions in real time during ramp anneals to 1000°C. On poly-Si substrates the Ta thickness was varied from 0 to 1.5 nm while the Ti thickness was held constant at ∼27 nm. The time-resolved XRD shows that the volume fraction of C40 and metal-rich silicide phases grows with increasing Ta layer thickness. Increased Ta layer thicknesses also delay the growth of the C49 disilicide phase to higher temperatures. Among the Ta thicknesses we examined, 0.3 nm is the most effective in lowering the C49-C54 transformation temperature. Films with Ta layers thicker than 0.5 nm do not completely transform into the C54 phase. The texture of the C54 phase is also sensitive to the Ta thickness. The C54 disilicide film is predominantly (010) textured for the Ti / 0.3 nm Ta sample. The final C54 texture is significantly different for Ta layers thinner or thicker than the optimal 0.3 nm. This suggests that the most effective thickness for lowering the C54 formation temperature is related to the development of a strong (010) texture. The possibility of a template effect by the C40 or metal-rich Ti5Si3 phases is also discussed on the basis of texture considerations.

Stability improvement at high emission densities for gold thin film photocathodes used in advanced electron beam lithography

Multi-electron beam lithography has been proposed as a promising approach to achieve high throughput for mask writing and direct wafer writing. Laser driven photocathodes represent an attractive candidate for multiple beam, high brightness sources. Thin film gold photocathodes that can be handled in air are of particular interest because of their potential for practical sources. In this article we present a study of the degradation mechanisms that change photocurrent yield for thin film gold photocathodes. Two general degradation mechanisms were studied: microstructural changes of the gold thin film and surface reactions. Observed microstructural changes included loss of gold coverage of the sapphire substrate, gold grain growth and an increase in surface roughness. A titanium adhesion layer was shown to stabilize coverage and proved stable to 700 °C by in situ transmission electron microscopy (TEM) experiments on planar sections of the Au/Ti/sapphire thin film stack photocathode. Reactions at the surface included physisorption and chemisorption of species and the subsequent reactions that occur when they diffuse to the laser illumination/electron emission site. X-ray photoelectron spectroscopy detected multiple monolayers of hydrocarbons on the surface of the Au following deposition and exposure to air. A product layer at the illumination sites as thick as 28 nm was detected using scanning electron macroscopy, atomic force microscopy and TEM following long term (1000–1200 min.) photoyield stability tests. Photoyield versus time plots for all the Au/Ti/sapphire cathodes were similar with an initial high photoyield, a drop to a minimum, then a rise to a second maximum followed by a slow 2%/h decay. Desorption of weakly bound physisorbed and chemisorbed species and growth of a reaction product layer within the electron emission zone are proposed as mechanisms to explain the photoyield behavior. Similar photoyield stability behavior was observed when Pt/Ti/sapphire photocathodes were tested. The photoyield degradation mechanisms observed in these tests are likely to be active on all thin film transmissive photocathodes subjected to high intensity illumination and will add to changes which occur when more reactive thin film photocathodes are used such as cesium telluride. Stability was improved to less than 1%/h for 16 h within the region of slow decay following the initial transient by illuminating the emission site on the cathode with high intensity UV light with a high O2 partial pressure. Emission current densities in excess of 6.0 A/cm2 were measured with a Faraday cup. Given these results, this work suggests that higher current densities and stability can be achieved.

Electron-induced reactions in thin solid films of cyclopropane

The electron-induced reactions of cyclopropane in the condensed phase have been induced and monitored simultaneously using a conventional high-resolution electron energy loss (HREEL) spectrometer. The reaction rate is fast enough for the reaction to be followed even at the sub-nanoampere currents typical of an HREELS experiment. The major initial reaction product is propene, as identified from characteristic vibrational bands. Subsequent reactions possibly lead to the formation of longer saturated chains. The reaction rate in the multilayer is considerably faster than at monolayer coverage. The findings are compared to literature data from recent studies on electron-induced reactions in self-assembled monolayers and adsorbed cyclopropane monolayers. The relatively fast reaction suggests that cyclopropane derivatives may be interesting candidates for use in surface patterning by electron beam techniques.

Epitaxial growth of Sb thin film and chemical reaction of Sb-induced surface reconstruction on Si(1 1 3)3×2

Antimony layers deposited on a Si(1 1 3)3×2 surface are investigated with low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). No long-range order is obtained in the room temperature grown Sb layer. Post-growth annealing at about 100°C restores long-range order and produces two-domain sixfold symmetric hexagonal structure. The tilted angle between two domains is 33.6°, which corresponds to the angle between [ 7 ̄ 4 1] and [4 7 ̄ 1] direction on bulk-terminated Si(1 1 3) plane. Sb (MNN) and Si (LMM) AES peak-to-peak intensities as a function of Sb deposition time at 100°C indicate that the overlayer structure grows a layer-by-layer fashion on the Si(1 1 3) surface at least up to two full molecule layer. Simple overlayer structure model, which consists of Sb n ( n=1–4) molecules, is suggested. Depending on the substrate temperatures above 300°C and Sb coverages, the 1×1, 2×2, and 2×5 reconstructions are formed successively. Oxygen exposure shows quite different surface chemical reactivities depending on the surface structures. The 1×1 and 2×2 surfaces are strongly passivated, on the other hand, the 2×5 surface reacts with oxygen relatively well. The different chemical reactivity is explained by the existence of the Sb lone pairs and Si dangling bonds in the unit cells.

Nanoscale characterization of interfacial reactions in SrRuO3 thin film on Si substrate

Nanoscale characterization of interfacial reactions in SrRuO3 thin film on Si substrate

Exploring Thin-Film Reactions by Means of Simultaneous X-Ray Surface Roughness and Resistance Measurements

Exploring Thin-Film Reactions by Means of Simultaneous X-Ray Surface Roughness and Resistance Measurements

Numerical solution of coupled systems of ordinary and partial differential equations. Application to the study of electrochemical insertion reactions by linear sweep voltammetry

Modelling of some electrochemical reactions leads to systems combining ordinary differential equations (ODE) and partial differential equations (PDE). Using the Laplace transform method, the mass-transfer equations can be solved for thin-layer materials to give theoretical expressions for the related transfer functions containing terms such as tanh τs / τs or coth τs / τs or a combination of these terms, depending on the mass-transfer boundary conditions. Using transfer functions makes it possible to transform the PDEs into systems of ODEs of infinite size which can be solved numerically after truncation. This method is used here to study electrochemical insertion reactions in thin films of host materials by linear sweep voltammetry (LSV).

Mechanism of ion desorption reaction of PMMA thin film induced by core excitation

Abstract Auger electron spectra of PMMA (poly-methylmethacrylate) which show remarkable site-specific ion desorption are calculated using ab initio molecular orbital (MO) calculation and compared with the results of Auger electron-photoion coincidence experiments. Methyl isobutyrate and its trimer are used as model molecules of PMMA. The calculation is based on the single configuration state function and limited configuration interaction methods, and Auger transition probabilities are estimated by overlap between core and valence MOs. The calculated normal Auger spectra obtained on specific atoms reproduce well the experimental resonant Auger spectra for both the carbon and oxygen sites. The careful analysis of the calculated Auger spectra indicates that the desorption of CHn+ ions is strongly concerned with the Auger final states which have holes in the O–CH3 bonding orbitals.

Epitaxial CoSi 2 by solid phase reaction of Co/Ti and Co/Hf bilayers on Si(001)

TEM and RBS studies of the solid phase reaction of Co/Ti- and Co/Hf/Si(001) layer systems are reported. In addition to conventional annealing procedures a special thermal treatment was applied to investigate the intermediate stages of the reaction during heating up. With rising temperature a complex Co–Si-phase sequence was observed which finally results in the growth of epitaxial CoSi 2. The emerging silicide phases were proved to grow with preferred orientation relations to the substrate. The observed growth behaviour is compared for both the systems and discussed considering aspects of energy minimization and material supply under the conditions of a thin film reaction.

Photoinduced Surface Wettability Conversion of ZnO and TiO2 Thin Films

The photoinduced surface wettability conversion reactions of ZnO and TiO2 thin films were investigated by means of water contact angle measurement and X-ray photoelectron spectroscopy. Before ultraviolet (UV) illumination, ZnO and TiO2 films exhibited water contact angles of ∼109 and ∼54°, respectively. UV illumination turned both surfaces to highly hydrophilic with water contact angles smaller than 10°. Storage in the dark reconverted the highly hydrophilic films to their original states. Reversible surface wettability conversion reactions were achieved by alternate UV illumination and storage in the dark on both the films. The similar behaviors of wettability conversion observed on ZnO and TiO2 surfaces suggest that they follow a similar conversion mechanism. Preferential adsorption of water molecules on the photogenerated surface defective sites is ascribed to the formation of highly hydrophilic ZnO and TiO2 surfaces. Achievement of highly hydrophilic ZnO and TiO2 surfaces by high-temperature annealing and Ar+ sputtering provided supporting evidence for the explanation that surface defective sites play crucial roles in causing the surface wettability conversion reactions.

Laboratory simulation of tunneling reactions in interstellar ices

The reactions of the solid thin films of C 2H 2, C 2H 4 and CO deposited on the silicon substrate at 10 K with cold H atoms (∼27 K) sprayed over the solid films were studied by thermal desorption mass spectrometry and Fourier transforn (FT)-IR spectroscopy. In the reaction of H with C 2H 2, C 2H 6 was found to be the major product but C 2H 4 could not be detected. This finding is in accord with the observation of comets Hyakutake and Hale–Bopp, i.e., C 2H 2 and C 2H 6 but no C 2H 4 were detected in the comae of these comets. In the reaction of H with CO, HCHO was found to be the major product but no CH 3OH was detected. In all the reactions investigated, the product yields increased drastically with decrease of temperature from ∼50 to 10 K. This may be mainly due to the increase of the sticking probability of H atoms on the solid films. The increase of the rate constant for the tunneling reaction H+C 2H 2 with decrease of temperature has been suggested.

Enhanced Magnetoresistance of La1-xSrxMnO3 Thin Films by Reaction with Substrates.

Magnetic and magnetotransport properties of polycrystalline La1-xSrxMnO3 (LSMO) thin films annealed at high temperatures were investigated. The reaction of the LSMO thin films with YSZ substrates effectively induces enhancement of low-field MR up to 13.8 % at 78 K for the film annealed at 1350°C. The results are consistently explained by the formation of potential barriers at grain boundaries by the annealing.

Processing of CdTe thin films by the stacked elemental layer method: compound formation and physical properties

Cadmium telluride (CdTe) thin films have been deposited using the stacked elemental layer (SEL) technique. This process consists of sequentially depositing tellurium and cadmium layers and then annealing the stacks in order to synthesize the compound. The films were characterized using X-ray diffraction (XRD), optical transmittance and reflectance, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The evolution of the thin film reaction and compound formation were studied using X-ray data. The results show that the growth is diffusion-controlled and the activation energy is (82±2) kJ/mol. The effect of the conventional post-synthesis CdCl 2 heat treatment on the physical properties of the films produced is also reported.

The formation of CdTe thin films by the stacked elemental layer method

Cadmium telluride thin films have been produced using the Stacked Elemental Layer technique. The films were characterized using X-ray diffraction, optical transmittance and reflectance, and atomic force microscopy. The evolution of the thin film reaction and compound formation were studied using X-ray data. The results show that the growth is diffusion controlled and the activation energy is 81.8 kJ/mol. In addition, some physical properties of the films produced are reported.

Comment on “Dominant diffusing species during cobalt silicide formation” [J. Appl. Phys. 79, 153 (1996)

The report that during the formations of the isomorphous CoSi2 and NiSi2 the dominant diffusing species are different is questioned. Not only does this contradict previous results obtained with inert gas markers, but it is based on experiments carried out with Ta markers that are not inert. The problem of using markers in determining the dominant diffusing species during thin film reactions is briefly analyzed.

Effect of plasma-induced damage on interfacial reactions of titanium thin films on silicon surfaces

Solid-state reactions of titanium thin films on Si surfaces damaged and amorphized by CHF3/O2 plasma treatment were investigated. The formation temperature of the C49–TiSi2 phase increased due to plasma-induced damage and contamination from decomposed gas elements. The presence of a plasma-induced damage layer suppressed the interdiffusion of Ti and Si. The Ti5Si3 phase was formed initially at an annealing temperature of 400 °C when a plasma-induced damage layer was present at the interface. The increase of the C49–TiSi2 formation temperature may be caused by the transformation of the Ti5Si3 phase into C49–TiSi2 rather than by the direct nucleation of the C49–TiSi2 phase with a clean interface.

Etching Characteristics and Mechanism of Ba0.7Sr0.3TiO3 Thin Films in an Inductively Coupled Plasma

The inductively-coupled-plasma (ICP) etching behavior of Ba0.7Sr0.3TiO3 (BST) thin films has been characterized with Cl2/CF4, Cl2/SF6 and Cl2/Ar gas mixtures. CF4 and SF6 were found to impede the etch process, presumably due to competition between plasma deposition and etching. A chemically assisted etch of BST was obtained under various Cl2/Ar gas mixtures. The etch profile along with etch anisotropy was observed as a function of etching parameters by scanning electron microscopy. The surface morphologies after etching were analyzed by atomic force microscopy. A smooth surface (roughness ∼1.8 nm) with no residue was observed under 30%Cl2 in Ar/Cl2, ICP power of 1 kW, substrate bias of 500 V, and 10 mTorr. To clarify the etching mechanism, the surface reaction of the BST thin films was investigated by X-ray photoelectron spectroscopy. It was found that Ba was mainly removed by chemically assisted physical etching (possible products such as BaClx). Physical bombardment is more effective than Cl chemical reaction for removing Sr, while Ti can almost be removed by chemical reaction (such as TiClx). The etching results described correlate well with the thermochemical calculations.

Study of the Reactions of H and D Atoms with Solid C2H2, C2H4, and C2H6at Cryogenic Temperatures

The reactions of the solid thin films of C2H2, C2H4, and C2H6 deposited on the silicon substrate at 10 K with cold H and D atoms (~27 K) sprayed over the solid films were studied systematically by thermal desorption mass spectrometry and Fourier transform (FT)-IR spectroscopy. In the reaction of H with C2H2, C2H6 was found to be the major product but C2H4 could not be detected. This is because the first-step addition reaction H + C2H2 ? C2H3 is the rate-controlling process, and the following reactions to form the final product C2H6 proceed much faster than the initial one. This finding is in accord with the observation of comets Hyakutake and Hale-Bopp, i.e., C2H2 and C2H6 but no C2H4 were detected in the comas of these comets. In all the reactions investigated, the product yields increased drastically with decrease of temperature from 50 to 10 K. This is most likely to be due to the increase of the sticking probability of H and D atoms on the solid films at lower temperatures. This finding led us to conclude that the chemical evolution taking place on the dust grains via H-atom tunneling reactions becomes efficient only at cryogenic temperatures. The abstraction reaction of D with C2H6, D + C2H6 ? DH + C2H5, was found to proceed much slower than the addition reactions of D with C2H4 and C2H2. The rates of reactions of solid C2 hydrocarbons with H atoms sprayed over the solid films were in the order of C2H4 C2H2 C2H6. The reactivity of D with C2H2 was considerably lower than that of H. Such a remarkable isotope effect was not found for C2H4.

Real-time RBS of solid-state reaction in thin films

Selected examples of real-time RBS are shown to underline the strengths of the technique. Due to the simultaneous depth and compositional profiling capabilities of RBS it is possible in combination with a linearly ramped heat treatment, to determine both the pre-exponential factor and the activation energy of phase formation from a single sample. Other in situ, real-time techniques also use linearly ramped temperature anneals but require several different temperature ramps to obtain the same parameters. In inert marker experiments the movement of a thin, inert layer is used to determine the atomic transport during growth. The example shown highlights the advantages of the real-time RBS technique where slight movements of the marker layer are significant. Linear reaction kinetics is shown for CrSi 2 formation. Two linear regimes were found. The second regime coincided with unintentional oxidation of the Cr surface. An added advantage of kinetic analysis by real-time RBS is the fact that in those cases where sample preparation is difficult and at times irreproducible, e.g. due to experimental details, the fact that the sample kinetics can be measured from a single sample provide far better results. This is illustrated by considering growth kinetics of Ni–Si samples, where the Si had been intentionally doped with oxygen.

Solid-phase reactions, self-propagating high-temperature synthesis, and order-disorder phase transition in thin films

The results of experimental studies of self-propagating high-temperature synthesis in double-layer Cu/Au thin-film systems are presented. It is shown that the synthesis initiation temperature for a Cu/Au film is determined by the order-disorder phase-transition temperature in the Cu-Au system. The order-disorder transition temperature for thin films is found to be lower than for the bulky samples. It is assumed that the temperatures of initiation of solid-phase reactions in thin films can be associated with the structural phase-transition temperatures.