Search

In this work reactive pulsed laser deposition of molybdenum- and tungsten-nitride thin films is investigated. Metallic targets were ablated in low-pressure (1, 10 and 100 Pa) nitrogen atmosphere by KrF excimer laser pulses (fluence ∼6.5 J/cm2). Films were deposited on silicon wafers heated to ∼25, 250 and 500 °C. The characteristics of the films strongly depend on the N2 pressure. By increasing N2 pressure, the nitrogen content increases in the films, which leads to a monotonous increase of the electrical resistivity. Deposition rate decreases at 100 Pa as indicated by Rutherford backscattering spectrometry. At this pressure, hardness of the films significantly decreases also, as shown by microhardness measurements. X-ray diffractometry shows that films crystallinity is improved by increasing the substrate temperature. In addition, atomic force microscopy (AFM) and scanning electron microscopy (SEM) were applied for visualising the film surface.

In the present work we focus our attention on the structural modifications induced by repeated absorption/desorption cycles on Mg–Nb layers. Samples consisting of a 30 μm thick pure Mg or Mg–5 at.% Nb doped films, coated with a 20 nm thick Pd layer were submitted to repeated H 2 sorption cycles in a volumetric apparatus. Isothermal desorption analysis at 350 °C was performed to evaluate the amount of absorbed hydrogen. X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and electron microscopy techniques (SEM and TEM) were used for the structural characterisation of the samples. Analyses show a deep modification of the material upon cycling. The presence of Nb enhances the structural modifications and induces an initial lattice contraction of the Mg matrix that tends to decrease on cycling via the formation of Nb nanoparticles (with average size of ∼10 nm). SEM and TEM observations performed in cross section evidenced the formation of a porous structure.

Direct metal laser sintering (DMLS) is a technique to manufacture complex functional mechanical parts from a computer-aided design (CAD) model. Usually, the mechanical components produced by this procedure show higher residual porosity and poorer mechanical properties than those obtained by conventional manufacturing techniques. In this work, a Co–Cr–Mo alloy produced by DMLS with a composition suitable for biomedical applications was submitted to hardness measurements and structural characterization. The alloy showed a hardness value remarkably higher than those commonly obtained for the same cast or wrought alloys. In order to clarify the origin of this unexpected result, the sample microstructure was investigated by X-ray diffraction (XRD), electron microscopy (SEM and TEM) and energy dispersive microanalysis (EDX). For the first time, a homogeneous microstructure comprised of an intricate network of thin e (hcp)-lamellae distributed inside a γ (fcc) phase was observed. The e-lamellae grown on the {111} γ planes limit the dislocation slip inside the γ (fcc) phase, causing the measured hardness increase. The results suggest possible innovative applications of the DMLS technique to the production of mechanical parts in the medical and dental fields.

We report experimental results obtained for electrical and structural characteristics of yttria-stabilised zirconia (YSZ) thin films deposited by pulsed laser deposition (PLD) on Si substrates at room temperature. Some samples were submitted to thermal treatments in different ambient atmospheres (vacuum, N 2 and O 2 ) at a moderate temperature. The effects of thermal treatments on the film electrical properties were studied by C – V and I – V measurements. Structural characteristics were obtained by X-ray diffraction (XRD), X-ray reflectivity (XRR) and transmission electron microscopy (TEM) analyses. The as-deposited film was amorphous with an in-depth non-uniform density. The annealed films became polycrystalline with a more uniform density. The sample annealed in O 2 was uniform over all the thickness. Electrical characterisation showed large hysteresis, high leakage current and positive charges trapped in the oxide in the as-deposited film. Post-deposition annealing, especially in O 2 atmosphere, improved considerably the electrical properties of the films.

Experimental evidence of the anisotropy in the hot-hole drift velocity v d of Si between 300 and 430°K has been theoretically proved to be strongly correlated to the warped shape of the heavy valence band. The comparison between theory and experiments permits a valuable check on the competitive role played by the strong nonparabolicity of the Si valence band and the lattice scattering mechanisms.

The growth kinetics of NiSi has been evaluated by depositing a Ni film on and Si substrates. X-ray diffraction and MeV4He+ backscattering have been used to identify the compounds formed and to measure their thicknesses. Carbon and oxygen impurities have been detected with Auger electron spectrometry with Ar sputtering. During thermal annealing Ni2Si is formed first, while NiSi appears only after the total exhaustion of the Ni film. It has been found that, on both Si substrates, the thickness of NiSi increases with the square root of time. Different growth rates have been observed for the two different Si orientations. The activation energy in the (300÷370)°C temperature range is 1.83 eV for NiSi grown on and 1.23 eV for NiSi on Si. The two competing mechanisms of Ni diffusion through NiSi by grain boundary and by substitutional processes are correlated with the different microcrystalline structure of the NiSi silicide layers grown on different Si-oriented substrates.

The interaction of Ni film deposited on Si has been studied. Growth kinetics of NiSi on (111) and (100) silicon single crystals are reported. Activation energies for NiSi are found to be 1.23 eV for (100) and 1.83 eV for (111).

Silicide formation is strongly affected by the presence of oxygen contained in the metal film. A study has been performed by heating at various times and temperatures samples constituted by a thin Pt film sputter deposited on a silicon substrate. Four different oxygen concentrations were implanted in the Pt film ranging from 1 to 0.1 at.%. The presence of oxygen slows down the growth rate of Pt2Si. At 315 °C in the sample with an oxygen concentration of 1 at.%, about 1000 A of Pt2Si are formed after 20 min of annealing. During the silicide formation, the oxygen originally contained in the platinum films segregates at the Pt2Si/Pt interface. This accumulation acts as a barrier for the transport of Pt and a new phase, PtSi, can form at the Pt2Si/Si interface. The dose necessary to stop the Pt2Si reaction is about (2–4)×1015 at./cm2 at 315 °C.

The drift velocity of electrons and holes in silicon has been measured in a large range of the electric fields (from 3 . 102to 6 . 104V/cm) at temperatures up to 430 K. The experimental data have been fitted with a simple formula for the temperatures of interest. The mean square deviation was in all cases less than 3.8 percent. A more general formula has also been derived which allows to obtain by extrapolation drift velocity data at any temperature and electric field.

During investigations of failure mechanisms and failure avoidance we have studied the kinetics and the mechanisms responsible for the growth of the Au-Al compounds. Au4Al, Au5Al2, Au2Al, AuAl, AuAl2have been observed and the growth kinetics of the Au2Al, AuAl, AuAl2follow a(time)1/2dependence. The activation energy for the growth of Au2Al is 1 eV and 1.2 eV was found for AuAl and AuAl2. The kind of compound formed depends on the samples preparation and the results indicate that the kinetics of the mixing is the main mechanism responsible for the sequence of formation of the various compounds.

The drift velocity of electrons in silicon has been measured over a wide range of electric fields (from 3×102 to 6×104 V/cm) at temperatures of up to 430 K. The experimental data have been fitted to a simple formula for the temperatures of interest. The mean-square deviation was, in all cases, less than 3·8%.

We report on the deposition of thin Yttria-stabilized zirconia (YSZ) films and TiN/YSZ bilayers on Si substrates at room temperature by pulsed KrF excimer laser ablation deposition (PLD). YSZ films were deposited onto (1 0 0) Si substrates by ablating an YSZ target in vacuum and in low-pressure oxygen atmosphere. TiN/YSZ/Si bilayers were deposited in two steps without exposure to the air using a multitarget system: first, an YSZ film was deposited onto a Si substrate by ablating an YSZ target in oxygen ambient gas, then a Ti target was ablated in nitrogen ambient gas. Morphological and structural characteristics were investigated by Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray reflectivity (XRR) techniques.

The interface reactions in Co/W multilayers (ML) with Co thickness of 1 and 2 nm and W thickness of 1, 2 and 5 nm in as-deposited state and under KrF laser irradiation with fluences F=50–200 mJ/cm2 were studied by different X-ray methods. Superlattice satellites around W (1 1 0) direction were found in as-deposited and irradiated multilayers. The layered structure persists up to the highest applied fluence. Co3W compound is formed at the interfaces. However, the intermixing is low.

Ag/Co bilayers and Co/Ag/Co trilayers were studied in order to extend our previous results on the excimer laser-induced grain boundary diffusion in Ag/Co layered structures, used in giant magnetoresistance devices. The e-beam deposited structures were processed by KrF excimer laser at the fluences F =0.1–0.25 J cm −2 with 1–1000 pulses. The temperature and depth of melting vs. time evolutions were computed for the first laser pulse. Samples were studied by X-ray reflectivity (XRR), X-ray diffraction (XRD), grazing incidence XRD, Rutherford backscattering spectrometry (RBS) and resistometry. The intermixing phenomena at the Ag/Co solid–solid, liquid–solid and liquid–liquid interfaces were quantitatively characterized. It was shown that in the immiscible Ag/Co combination of metals the interfaces could become sharper under laser processing, probably due to the back-diffusion effect.

We report on the deposition of thin vanadium nitride films by ablating vanadium targets in low-pressure N 2 atmosphere, and on their characterization. The targets were vanadium foils (purity 99.8%). 3 in. Si(1 1 1) wafers were used as substrates. Film characteristics (composition and crystalline structure) were studied as a function of N 2 pressure (0.5–200 Pa), KrF laser fluence (4.5–19 J/cm 2 ), substrate temperature (20–750 °C) and target-to-substrate distance (30–70 mm). Vanadium nitride is already formed at low N 2 ambient pressures (1 Pa) and laser fluences (6 J/cm 2 ) on substrates at room temperature. At the N 2 pressures of 1–10 Pa, the prevalent phase is VN. At higher pressures (100 Pa) and at relatively high laser fluences (16–19 J/cm 2 ), the dominant phase is V 2 N. The crystallinity of the films improves by increasing the substrate temperature. Well-crystallized films are obtained on substrates heated at 500 °C.

Titanium carbide films were deposited on silicon wafers by XeCl (308 nm) excimer laser ablation of titanium in low pressure (10 -6 -10 -1 mbar) methane atmosphere. Series of 10 000 pulses at the repetition rate of 8 Hz were directed to the target surface. The fluence was set at 4.5 and 5.5 J/cm 2 . Pulse duration was about 30 ns. The deposited films were charcterized by different techniques (Rutherford backscattering spectrometry, X-ray diffractometry and scanning electron microscopy). Results show that at both fluences and at pressures below ∼ 10 -5 mbar the deposited films are mainly formed of metallic titanium. In the range 10 -3 -10 -1 mbar, at the fluence of 4.5 J/cm 2 , the deposits are formed of titanium carbide (TiC) with only traces of oxides. At the fluence of 5.5 J/cm 2 the amount of ablated material is too large to undergo complete reaction and mostly metallic titanium is deposited.

Silicon-nitride films were deposited on silicon waters by XeCl (308 nm) excimer-laser ablation of silicon in low-pressure (0.05–5 mbar) ammonia atmospheres. Series of 10 000 pulses at the repetition rate of 8 Hz were directed to the target surface. The fluence was set at about 5 J/cm2. Pulse duration was about 30 ns. The deposited films were characterized by different techniques (X-ray diffraction, X-ray photoelectron spectroscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry, scanning electron microscopy, profilometry). Silicon-nitride films with thickness close to 1 μm were obtained under specific experimental conditions.

Titanium and silicon nitride films were deposited on silicon wafers by XeCl (308 nm) excimer laser ablation of titanium and silicon, respectively, in low pressure (10 −3 −5 mbar) nitrogen-containing atmospheres. Series of 10 000 pulses at a repetition rate of 10 Hz were directed to the target surface. The fluence was set at about 5 J/cm 2 . Pulse duration was about 30 ns. The deposited films were characterized by different techniques (X-ray diffraction, scanning and transmission electron microscopy, etc.). Good quality films with a thickness exceeding 1 μm were obtained under specific experimental conditions.

In this paper the microstructural characterisation of yttria-stabilised zirconia (YSZ) thin films deposited by laser ablation is reported for the as-deposited sample as well as for samples submitted to thermal treatments in different atmospheres (vacuum, N 2 and O 2 ) at a moderate temperature (500 °C). Results obtained by different characterisation techniques such as grazing incidence X-ray diffraction, X-ray reflectivity and transmission electron microscopy evidenced the formation of the cubic YSZ phase after the annealing treatments. On the contrary, the as-deposited sample is amorphous with nanocrystals of the cubic YSZ phase dispersed inside. It also exhibits a difference between the density of the surface region and the region of the interface with the substrate. This latter effect has been attributed to the loss of oxygen atoms during the deposition. The annealing treatments are able to recover the density unhomogeneity present inside the as-deposited sample, the degree of recovering depends on the ambient atmosphere.

The growth of high-mobility modulation-doped Si/SiGe heterostructures on silicon-on-insulator (SOI) substrates is demonstrated. The structural and electrical properties of the samples are comparable with those of similar samples grown on standard Si substrates. Electron mobilities as high as 2800 cm 2 V −1 s −1 at room temperature and 8.2×10 4 cm 2 V −1 s −1 at 4 K are obtained.

Carbon nitride films deposited on Si (001) substrates at room temperature by XeCl laser ablation of graphite targets in low pressure (1, 5, 10 and 50 Pa) N2 atmosphere at fluence of 12 J/cm2 (∼0.4 GW/cm2) have been investigated by x-ray diffraction and transmission electron microscopy in order to study the structure of the films. The results showed that the samples are constituted of a continuous amorphous film inside which microcrystals of a coherently grown CNx phase are dispersed. This phase is monoclinic with the following lattice parameters: a=b=0.384 nm, c=0.302 nm, α=γ=90°, and β=96.5°. The CNx phase grows with the (001) plane coincident with the (001) plane of the Si substrate and with the [010]CNx direction parallel to the [110]Si one.

The thermal stability of e-beam deposited multilayers (MLs) for soft X-ray reflection optics was studied under XeCl laser processing in vacuum. MLs with five Co/Si/W/Si periods, each 13.5 nm (MLS1) or 18.4 nm (MLS2) were deposited onto oxidized Si and irradiated at the fluences of 0.075–0.62 J cm −2 by 1 or 100 pulses. The samples were analyzed by X-ray diffraction, hard X-ray reflectivity and sheet resistance measurements. The layered structure of our samples persists up to 0.62 J cm −2 per 1 pulse for MLS2 and 0.62 J cm −2 per 100 pulses for MLS1 irradiations. The thermal stability of MLS1 is even better than for W/Si MLs studied previously. In laser irradiated samples the Co 2 Si 3 phase which is normally formed under high pressures (>4 GPa) was found. It has not been reported in the film couples so far. The high thermal stability and Co 2 Si 3 formation in MLS1 are explained by complex Co–Si silicide formation conditions and compressive stress parallel to the surface of irradiated samples.

Silicon carbide films were deposited on Si wafers (at room temperature and 150°C) by XeCl laser (fluence 5 J/cm2, pulse duration 30 ns, repetition rate 10 Hz) ablation of Si in low pressure (0.25–6 mbar) CH4 atmosphere. The deposits were characterized by different diagnostic techniques (XRD, XPS, RBS, etc.). Polycrystalline stoichiometric silicon carbide films ∼ 1 μm thick were obtained under specific conditions after ∼ 104 laser pulses.

Titanium carbide films were deposited on silicon wafers by XeCl (308 nm) excimer laser ablation of titanium in low pressure (10 −3 mbar) methane atmosphere. Series of 10 000 pulses at the repetition rate of 8 Hz were directed to the target surface. The fluence was set at about 5 J cm −2 . Pulse duration was about 30 ns. The deposited films were characterized by different techniques (X-ray diffraction, scanning electron microscopy and Rutherford backscattering spectrometry).

Silicon nitride films were deposited on silicon wafers by XeCl (308 nm) excimer laser ablation of silicon in low pressure (1 × 10 −3 −1 mbar) ammonia atmospheres. Series of 10000 pulses at the repetition rate of 8 Hz were directed to the target surface. The fluence was set at about 5 J/cm 2 . Pulse duration was about 30 ns. The deposited films were characterized by different techniques (X-ray diffraction, infrared spectrophotometry, X-ray photoelectron spectroscopy, Auger electron spectroscopy, scanning electron microscopy, profilometry). Deposition of stoichiometric silicon nitride films with thickness close to 1 μ m (deposition rate ∼ 0.1 nm/pulse) was obtained when ablation was performed in ammonia at the pressure of 1 mbar.

We have succeeded in simultaneously conducting two chemical reactions resulting in the formation of titanium nitride and titanium silicide in well-shaped clearly separated layers as an effect of multipulse excimer laser treatment of a Si wafer with a thin Ti coating in an ambient N 2 gas. We demonstrate by a numerical analysis that the parallel evolution of the two reactions in a one-step process is a consequence of the melting of both the Ti coating and of a thick layer of Si bulk in conditions that completely avoided substance expulsion.

Lasers provide appropriate approaches for the synthesis of materials of interest in modern technology. Laser chemical vapour deposition [ l ] and synthesis of compounds under direct laser i~radiation [2] are among the most successful methods. Also, congruent ablation deposition of compounds (mainly high critical temperature superconductors, but also titanium nitride) was successfully investigated [3, 4 3 . A different approach is now emerging [5]: laser reactive ablation (LRA) deposition of thin films. In the LRA deposition method a solid support collects the material ablated by the laser irradiation of the target and reacted with a low-pressure ambient gas. We deposited titanium and silicon nitride films on silicon wafers at room temperature and at moderate temperature (

The formation of tungsten silicide induced by XeCl laser irradiation of W(150 nm)/Si and W(500 nm)/Si samples was studied at laser fluences ranging from 0.6 to 1.8 J/cm 2 . Up to 300 subsequent laser pulses were directed to the same irradiation site. After irradiation the samples were examined by different diagnostic techniques: Rutherford backscattering spectroscopy, X-ray scattering, resistometry and surface profilometry. Complete reaction of the 150 nm W film with silicon was obtained between 30 and 100 laser pulses at the fluence of 1.2 J/cm 2 , and after 30 laser pulses and at the fluence of 1.5 J/cm 2 . The sheet resistance of these silicides was 5–10Ω. At the used fluences, only the onset of silicide synthesis at the W-Si interface was observed for the 500 nm W film. Numerical computations of the evolution and depth profiles of the temperature in the samples as a consequence of a laser pulse were performed and compared to the experimental results.

We report a study of the formation of nitride surface layers on titanium foils by multipulse (100 to 2500) XeCl excimer laser (λ = 308 nm) irradiation in N2 and NH3 atmosphere. After irradiation the samples were analyzed by Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA) and then examined by electron microscopy (SEM, TEM). The nitridation process was shown to be very efficient with both gases. The amount of nitride has been observed to depend on the number of subsequent laser pulses, on the pulse repetition rate and on the nature of the ambient gas.

Thin chromium films, 60 nm thick, were deposited onto single-crystal silicon wafers. The samples were irradiated with 30 ns single pulses from a Nd: glass laser at fluences ranging from 0.4 to 2.25 J/cm2. Rutherford backscattering spectrometry, transmission electron microscopy and electron diffraction measurements evidence the formation of CrSi2 layers at the Cr/Si interface. The silicide thickness depends on the laser fluence.

Polycrystalline SiC layers were obtained through pulsed annealing of thin (100 nm) carbon films deposited on single-crystalline silicon wafers. The samples were submitted to electron beam irradiation (25 keV, 50 ns) at various current densities in vacuum (∼10 −4 mbar) and to XeCl excimer laser pulses (308 nm, 15 ns) in air. Rutherford backscattering analysis showed that in the e-beam annealed samples mixing of the elements at the interface starts at current densities of about 1200 A/cm 2 . The mixed layer thickness increases almost linearly with current density. At current densities higher than 2400 A/cm 2 ablation of the C film was always observed. Using the XeCl excimer laser, a good intermixing of the deposited C film with the Si substrate was observed after a single 0.3 J/cm 2 pulse. From the RBS spectra a composition of the intermixed layers close to the SiC compound was deduced. Transmission electron microscopy and electron diffraction studies clearly evidenced the formation of SiC polycrystals.

Summary form only. The prediction of a covalently bound carbon nitride solid /spl beta/-C/sub 3/N/sub 4/ with characteristics (hardness, bulk modulus) comparable to or even better than those of diamond, stimulated many attempts to synthesise and deposit thin films of this material. Pulsed laser ablation is among the most promising technique to achieve this result.

Carbon nitride films were deposited at 20, 250 and 500°C on 〈111〉 Si substrates by XeCl laser ablation of graphite in low pressure (1–50 Pa) N2 atmosphere at fluences of 12 and 16 J/cm2. Different diagnostic techniques (SEM, TEM, RBS, XPS, XRD) were used to characterize the deposited films. Films resulted plane and well adhesive to their substrates. N/C atomic ratios up to 0.7 were inferred from RBS measurements in films deposited at 20°C and 16 J/cm2. Nitrogen concentration increases with increasing ambient pressure and laser fluence. The N 1s peak of the XPS spectra indicate two different bonding states of nitrogen atoms to C atoms, while the C 1s peak, apart from the two bonding states to nitrogen atoms, indicates one bonding state with regard to carbon atoms. XRD and TEM analyses point to an oriented microcrystalline structure of the films. Heating of the substrate results in a lower nitrogen concentration in respect of films deposited at 20°C in otherwise identical experimental conditions. Optical emission studies of the laser plasma plume indicate a correlation between the emission intensity of the CN radicals in the plume and the nitrogen atom concentration in the films.

Carbon nitride films, deposited on Si(111) substrates at room temperature by XeCl laser ablation of graphite targets in low pressure (1, 5, 10, and 50 Pa) N2 atmosphere at the fluence of 12 J/cm2 (∼0.4 GW/cm2), have been submitted to accurate x-ray diffraction and transmission electron microscopy investigations in order to study the structure of the films. Results showed that the samples are constituted of a continuous amorphous film inside which microcrystals of a new coherently grown CNx phase are dispersed. This new phase has a triclinic crystallographic cell with lattice parameters a=b=0.384 nm, c=0.438±0.007 nm, α=110±1°, β=105±1°, and γ=120°. It coherently grows on the (111) Si plane with the following orientation relationships: (001)CNx∥(111)Si, [100]CNx∥[1-10)]Si, and [010]CNx∥[01-1)]Si.