X-ray Reflectivity Technique Research Articles

Energy dispersive x-ray reflectivity technique to study thermal properties of polymer films

A method to study temperature dependent thickness variation of thin films has been developed based on angle and energy dispersive x-ray reflectivity (EDR) techniques. The data analysis scheme developed for this method combines both energy and angle dispersive x-ray reflectivity (ADR) profiles. The ADR technique provides the electron density profile on an absolute scale and the EDR technique facilitates rapid collection of reflectivity data as a function of temperature. We demonstrate the utility of the developed method with results of a study of thermal expansion of single and bilayer polymer films. We could detect a change of the thickness of the film in angstroms and could demonstrate interplay of negative and positive thermal expansions of dissimilar polymer layers in determining density profiles of polymer–polymer interface as a function of temperature.

Early growth stages of sputter deposited Ag on native SiO 2

We have presented the kinetic study of the very initial growth stages of an ultrathin film (40–150 Å) of Ag sputter deposited on a Si(0 0 1) substrate, covered by native oxide, using grazing incidence X-ray reflectivity (GIXR) technique and atomic force microscopy (AFM). We observe that the films consist of mounds with the presence of voids. The thickness ‘ d xray’ and the packing fraction ‘ η’ of the film as a function of growth time ‘ t’ follow a simple power law, d xray∼ t m and η∼ t n with the exponent m=0.58 and n=0.4 respectively. We have quantitatively determined that the voids between the mounds decrease at the initial growth stages with the increase in mound size. The mound size increases mainly by coalescence. The mound size increases radially as ∼ t z as a function of time. The radial growth exponent z crosses over from z>0.5 to 0.25 indicating two growth regimes. GIXR measurements reveal sublinear dependence of η on d and the AFM measurements show a cross over of the radial growth exponent. Both these indicate that at the initial stages, the lateral growth of the mound is dominant over the vertical growth hence reducing the voids.

Characterization of PdAu thin films on oxidized silicon wafers: interdiffusion and reaction

Plasma-deposited thin films prepared at room temperature, ranging from 46 to 250Å of PdAu on ∼45–50Å Si-oxide and Si-oxynitride films grown on Si wafers were studied. Grazing incidence X-ray diffraction, X-ray reflectivity, and XPS depth profile techniques were used to characterize the thin films. A reactive interface involving Pd- and Au-silicides is formed, linking the thin film to the Si-oxide and Si-oxynitride films: a small fraction of Pd and Au atoms from PdAu migrate into the Si substrate, first penetrating the oxide layer, and the small fraction of Si atoms from the oxide layer migrate into the PdAu film and form a silicide interlayer consisting of a reactive interface made up of mixtures of Au- and Pd-silicides interspersed within the matrix of PdAu and substrate. The concentration profiles of these silicides have a maximum at the interface with decay on both sides. The density and the face-centered cubic (fcc) lattice parameter of the film are determined to be ∼13±1g/cm3 and ∼4.004±0.014Å, respectively. The ideal film density is expected to be ∼15.5g/cm3, which suggests substantial defect density and light material mixture, causing more than 13% reduction in the mass density of the film.

Buried hydrophobic silicon bonding studied by high-energy x-ray reflectivity

The non-conventional technique of high-energy x-ray reflectivity is used to investigate buried Si|Si interface obtained by hydrophobic wafer bonding. In this experiment the well-collimated beam is transmitted through the sample and is reflected by the bonding interface described by its density and width. Transmission x-ray reflectivity curves are fitted using this two-parameter model with a Gaussian or exponential profile to analyse the evolution of the bonding interface as a function of temperature from 250°C to 1100°C.

Pulsed excimer laser ablation deposition of YSZ and TiN/YSZ thin films on Si substrates

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.

Concentration-Dependent Formation Mechanisms in Mesophase Silica−Surfactant Films

The development of surfactant-templated mesoporous films grown at the air/water interface was investigated using specular and off-specular X-ray reflectivity techniques. The samples were prepared in acidic conditions using cetyltrimethylammonium bromide (CTABr, 0.075 M) and different concentrations (0.27−0.88 M) of tetramethyloxysilane (TMOS). At CTABr/TMOS molar ratio between 0.277 and 0.093, the final films exhibited at least three orders of diffraction, corresponding to a real space repeat distance of 46 A. This long-range order was not observed when the TMOS content was increased to 0.88 M at constant CTABr concentration. In each case, the induction period prior to film formation was monitored using diffuse X-ray scattering. The intensity of the specular reflectivity peak, first and second-order diffraction peaks, and Yoneda wing were recorded as a function of time. The reflected intensity of the specular peak oscillated during the lengthy induction period, indicating that at some concentrations the f...

Structural Evolution of Surfactant−Silica Film-Forming Solutions, Investigated Using Small-Angle Neutron Scattering

Time-resolved small-angle neutron scattering has been used to investigate the evolution of micelles in the subphase of surfactant-templated silica film-forming solutions. Two samples have been prepared using cetyltrimethylammonium bromide (CTABr), as the structure-directing agent, and different amounts of tetramethoxysilane (TMOS) (CTABr/TMOS molar ratio 0.093, 0.139). The solutions have been reproduced at three different H2O/D2O contrasts and allowed to develop until a precipitate formed in the bulk. The surfactant initially forms micelles in solution while the hydrolyzed TMOS is dispersed uniformly through the aqueous phase. The micelles appear to have an ellipsoidal structure immediately after mixing, composed of a surfactant core surrounded by a thin, low-density silica film. The ellipsoidal micelles are typically 32 Å in length and with time elongate and become “rodlike” silica-coated cylinders with length in excess of 150 Å. This value is significantly larger in the composition containing more TMOS (≈1100 Å). In contrast, the radius of the core exhibits limited growth, swelling from 15.6 to ≈19 Å during the induction period. At long times the CTABr/TMOS = 0.139 sample exhibits a first-order diffraction peak at Q = 0.133 Å-1, corresponding to cooperative scattering from an ordered mesostructure. An analogous peak is not observed in the CTABr/TMOS = 0.093 solution. This is discussed in terms of different film formation mechanisms and compared with previous data measured by X-ray reflection techniques at the air/water interface.

Fixed-angle, energy-dispersive x-ray reflectivity measurement of thin tantalum film thickness

In this work, we employed a fixed-angle, energy-dispersive x-ray reflectivity technique to obtain the thickness of thin tantalum films (examples >9 nm) within seconds using a conventional, low-energy x-ray copper or chromium source (20 kV/20 mA/400 W). We compared this fixed-angle, energy-dispersive result with more conventional fixed-energy (monochromatic source), angular-dispersive x-ray reflectivity to establish the validity of the energy-dispersive method. This x-ray technique may be particularly useful for the metrology of growing thin-barrier layers, atomic-layer passivation, and seed-layer measurements in future microelectronics applications. Currently, thickness precision is limited by modeling assumptions and the energy discrimination available from x-ray detectors.

Surface and interface characterization of GaN/AlGaN high electron mobility transistor structures by x-ray and atomic force microscopy

We report on the characterization of GaN/AlGaN high electron mobility transistor (HEMT) structures grown by molecular beam epitaxy and metal-organic chemical vapor deposition techniques on semi-insulating SiC substrates. We have exploited the surface and interface sensitivity of the grazing incidence x-ray reflectivity (GIXA) technique and combined it with atomic force microscopy (AFM) and x-ray diffraction to obtain an accurate evaluation of the surfaces and channel interfaces for HEMT structures. The presence of smooth interfaces is responsible for the observation of intensity oscillation in GIXA, which is well correlated to step flow observation in AFM images of the surface. While GIXA provides a macroscopic average of the surface and interface roughness, the AFM provides us with a microscopic view of surface roughness. Further, the x-ray diffraction results have been combined with reflectivity data to evaluate the layer thickness and Al mole fraction in the AlGaN layer. The capacitance–voltage (C–V) technique is used to determine the electrical properties of two-dimensional electron gas depth and hence the AlGaN layer thickness. The C–V results are in good agreement with the x-ray data.

Evidence of swelling of SiO2 upon thermal annealing

Ultrathin SiO2 film was thermally grown on Si(001) substrate by dry oxidation and wet oxidation processes. The films were then subjected to thermal annealing (TA) at 1000 °C for 30 min. The structural characterization of the as-grown and the TA samples was carried out using the grazing incidence x-ray reflectivity technique. The analysis of the x-ray reflectivity data was carried out by using a model independent formalism based on the distorted wave Born approximation for obtaining the electron density profile (EDP) of the film as a function of depth. The EDP of both films show a decrease in the electron density as well as an increase in their thickness when the films are subjected to TA. It has also been observed that the total number of electrons is conserved in the oxide film after TA. Our analysis of the x-ray reflectivity data indicates that the SiO2 film swells and its interface with the substrate modifies upon TA.

Temperature stability of sputtered niobium–oxide films

The effect of annealing temperature on the structural and optical properties of sputtered niobium–oxide films has been investigated. The temperature dependence of structure, density, and optical constants has been studied by Rutherford backscattering, x-ray diffraction, x-ray reflection, optical spectroscopy, and variable angle spectroscopic ellipsometry techniques. Rutherford backscattering measurements show no variation in the stoichiometry of the films upon annealing of amorphous Nb2O5 films, while amorphous NbO is oxidized to Nb2O5. X-ray diffraction studies show that as-deposited films are amorphous and only crystallize at around 500 °C. X-ray reflectivity studies reveal a continuous increase of film density with increasing annealing temperature. Optical spectroscopy and spectroscopic ellipsometry confirm that the refractive index n and the band gap Eg increase upon increasing annealing temperature. The variation of the refractive index with density of the niobium–oxide films is observed to follow the Clausius–Mossotti relation and the molecular electronic polarizability has been deduced from the straight line fit of Lorentz–Lorentz law.

Real-time observation of surface morphology of indium phosphide MOVPE growth with using X-ray reflectivity technique

The results of real-time X-ray reflectivity measurements of MOVPE grown indium phosphide surface are presented. At the low growth temperature of 450°C, large decreases of reflectivity were observed, suggesting the formation of indium islands. At higher growth temperature of 550°C, only small changes were observed at high growth rate, indicating the step-flow growth mode. Oscillations longer than mono-layer growth were also observed at 500°C and 550°C, and roughness changes obtained from these oscillations were less than 0.01-nm, suggesting small islands formation on the terrace or step-edge fluctuation during the growth.

Large electronically mediated sputtering in gold films

Electronically mediated sputtering in thin gold films, bombarded with 200-MeV Ag ions, has been observed by ex-situ thickness measurements of the film using x-ray reflectivity technique. The observed sputter yield depends upon the film thickness and is about 410 atoms per incident ion for films of thickness 150 \AA{} and 235 atoms per incident ion in 450-\AA{}-thick film. This sputtering rate is a few orders of magnitude higher as compared to that normally encountered in the regime of elastic collisions. Reduced mobility of the electrons due to scattering from the surface and the grain boundaries plays an important role in enhancing the effects of electronic excitations. Sputtering is accompanied by a significant smoothening of the film surface and smearing of the boundaries between the grains.

Structures of Free-Standing Vertical Thin Films of Hydrophobically Modified Poly(sodium acrylate)s

Our investigation of free-standing vertical films of hydrophobically modified poly(acrylic acid) sodium salt by the X-ray reflectivity technique shows that the organization of these films strongly depends on the molecular weight and hydrophilic−hydrophobic monomer unit ratio of the copolymers. Long polymer chains with a low amount of hydrophobes form films composed of two layers of coils facing each other. In contrast, short polymer chains with a higher degree of hydrophobic modification form films with a well-defined semiordered structure whose properties are closely linked to the properties of the corresponding solution. We show that this structure is of electrostatic origin. Finally, the comparison of films made of the copolymers in their charged and neutral forms shows that the dominant force within the two types of film is respectively the counterion osmotic pressure and the energy of adsorption of the hydrophobic monomers.

The Native Oxide Effects on the Optical Properties of TbFe2

The native oxide effects on the optical properties of a polycrystalline TbFe2 were studied using spectroscopic ellipsometry and the grazing-incident X-ray reflectivity technique. The spectra of the dielectric functions for the TbFe2 sample exposed to air at room temperature were measured, as the sample was oxidized. The spectra of the reflectance, optical conductivity, and skin depth were obtained in a 12 h interval. As the thickness of the oxide layer gets larger, the magnitude of the optical conductivity decreases. The thickness of the native oxide layer for the fully oxidized surface was estimated using an X-ray reflectivity measurement.

The Native Oxide Effects on the Optical Properties of TbFe2

The native oxide effects on the optical properties of a polycrystalline TbFe2 were studied using spectroscopic ellipsometry and the grazing-incident X-ray reflectivity technique. The spectra of the dielectric functions for the TbFe2 sample exposed to air at room temperature were measured, as the sample was oxidized. The spectra of the reflectance, optical conductivity, and skin depth were obtained in a 12 h interval. As the thickness of the oxide layer gets larger, the magnitude of the optical conductivity decreases. The thickness of the native oxide layer for the fully oxidized surface was estimated using an X-ray reflectivity measurement.

Influence of the electrolyte viscosity on the structural features of porous silicon

The influence of an increase in the electrolyte viscosity, obtained by reducing the chemical bath temperature, replacing a volume fraction of ethanol with the heavier glycerol and increasing the anodization current density, was studied in porous silicon layers formed on p-type silicon. Synchrotron X-ray reflectivity and cross-sectional transmission electron microscopy techniques were used for 300-nm-thick porous silicon layers. For greater thicknesses (10μm), the temperature effect was investigated by multicrystal X-ray diffraction with a laboratory source. It was found that higher average porosities and lower roughnesses at the interface between porous silicon and substrate are obtained when the electrolyte viscosity increases. Moreover, the formation of flatter porosity depth-profiles is favored by lowering the electrolyte temperature. The importance of these effects on the performances of optical devices based on porous silicon is underlined.

X-ray reflectivity and nanotribological study of deposition-energy-dependent thin CNx overcoats on CoCr magnetic films

Using a combination of high-resolution x-ray reflectivity (XRR), nanotribological, and x-ray photoelectron spectroscopy (XPS) techniques, a thorough description of the structural parameters of ultrathin CNx overcoats on CoCr magnetic films is obtained. In addition, the tribological performance as a function of deposition energy is shown to correlate well with trends found by XRR and XPS.

Self-assembled inorganic/organic composite superlattice thin films with photochromic properties

A multicomposite WO 3/4,4′-BPPOBp self-assembled multilayer (SAM) film was fabricated by the consecutive self-assembly of WO 3 and 4,4′-BPPOBp on 3-aminopropyl-triethyoxysilane (APS) modified quartz or silicon substrate by a polyelectrolyte (PE) approach. A well-ordered superlattice structure, d-spacing 0.694 nm, was identified by X-ray diffraction and X-ray reflection techniques. The SAM film shows fine photochromic properties.

X-ray reflectivity studies of very thin films of silicon oxide and silicon oxide–silicon nitride stacked structures

Oxide/nitride/oxide films were deposited onto bare silicon substrates by low-pressure chemical vapor deposition (LPCVD) of silicon nitride and thermal oxidation. The X-ray reflectivity technique has been used to study the influence of the growth conditions and of different cleaning procedures of the silicon substrate on the structure and morphology of the deposited multilayers. The results revealed how, from an analysis of the X-ray reflectivity data performed by using Fresnel equations for multilayers modified to account for the interface imperfections, we determine, in a non-destructive manner, structural parameters such as density, thickness, roughness, and interface structure of the whole dielectric layers, giving us more information and greater sensitivity respect to cross-section transmission electron microscopy (TEM) and ellipsometric measurements.