Ellipsometric Measurements Research Articles

An ellipsometric method for determining the optical parameters of thin-film coatings with a complex structure

A method for taking into account the complex structure of single-layer thin-film coatings, which is formed due to the effect of interfaces between the film and surrounding media, for solving the inverse problem of ellipsometry by a modified Malin—Vedam method is proposed and the features of its application are described. As an example, this method is used to interpret the results of ellipsometric measurements of a close-to-stoichiometric polycrystalline La0.7Sr0.3MnO3 film on a crystalline Al2O3 (LSMO/AO) substrate.

Control of Friction Distribution on Stainless Steel Surface in Sodium Dodecyl Sulfate Aqueous Solution by Bipolar Electrochemistry

It is demonstrated that bipolar electrochemistry can be used to create friction distribution on stainless steel surface, measured by ball-on-disk friction test. Stainless steel samples (AISI 316L), acting as bipolar electrode, were immersed in sodium dodecyl sulfate (SDS) aqueous solution and exposed to an electric field to establish a potential gradient along the surface. In this way, a molecular gradient of adsorbed SDS on stainless steel was created dependent on the potential gradient, acting as boundary lubrication film to form friction distribution. In conjunction with ellipsometric measurement, the potential-induced molecular gradient on stainless steel surface was further visualized. The friction coefficient at the cathodic pole is higher than that at the other pole, as the adsorbed SDS molecules are repelled by electrostatic repulsion. The distribution profile of friction can be easily altered by adjusting the applied current in solution. The morphological differences in the worn surface of different positions were given, in accordance with the friction distribution. This novel approach of active control enables rapid simultaneous control of friction distribution on a conducting surface.

Adsorption mechanisms of L-Glutathione on Au and controlled nano-patterning through Dip Pen Nanolithography.

Dip Pen Nanolithography technique has been employed for patterning L-Glutathione tripeptide (l-y-glutamyl-l-cysteinyl-glycine) nanostructures at specific locations on metallic Au(111) substrate. The formed supramolecular architectures were designed through straight lines and dots serving as precursors for building blocks assemblies in nano-bio-electronics applications or as template structures for functionalized particles in the form of host-guest networks. Tween 20 polyoxyethylene surfactant concentrations ranging from 0.005 to 0.1% (v/v) into initial l-Glutathione tripeptide (2 mg mL(-1)) ink solutions were sequentially tested for the improvement of the ink delivery process and to assure an optimum uniformity and homogeneity over the patterned space. A strong relationship was found between the coated atomic force microscope (AFM) cantilever within the highly effective Tween 20 activator adjuvant and the molecular diffusion along concentration gradients. An increase in the driving force for ink transport from the AFM tip has been demonstrated within the highest 0.1% (v/v) TW 20 surfactant concentration, favoring the patterning of GSH molecules routinely with sub-100 nm resolution. Self-assembled monolayers of GSH were also fabricated and characterized in the light of X-ray photoemission spectroscopy (XPS) and ellipsometric optical measurements. Adsorption from water of l-Glutathione to the gold substrate is proven to be made by the thiol group of cysteine. Theoretical DFT approaches were applied for quantum chemical studies dedicated to electronic processes underneath molecular GSH/Au(111) systems.

Electronic and optical properties in ZnO:Ga thin films induced by substrate stress

The effects of biaxial stress in ZnO:Ga thin films on different substrates, e.g., sapphire(0001), quartz, Si(001), and glass have been investigated by X-ray diffraction, atomic force microscopy, and electrical transport and ellipsometric measurements. A strong dependence of orientation, crystallite size, transport, and electronic properties upon the substrate-induced stress has been found. The structural properties indicate that a tensile stress exists in epitaxial ZnO:Ga films grown on sapphire, Si, and quartz, while a compressive stress appears in films grown on glass. The resistivity of the films decreased with increasing biaxial stress, which is inversely proportional to the product of the carrier concentration and Hall mobility. The refractive index n was found to decrease with increasing biaxial stress, while the optical band gap E0 increased with stress. These behaviors are attributed to lattice contraction and the increase in the carrier concentration that is induced by the stress. Our experimental data suggest that the mechanism of substrate-induced stress is important for understanding the properties of ZnO:Ga thin films and for the fabrication of devices which use these materials.

(Invited) Vacuum Ultraviolet Photochemical Atomic Layer Deposition of Alumina and Titania Films

Conventional atomic layer deposition (ALD) is a thermo-chemical process where co-reagents are sequentially pulsed in cycles onto a heated substrate. As an alternative to substrate heating, various forms of other “energy – enhanced” ALD processes are being investigated. Herein, the photochemical atomic layer deposition of Al2O3 and TiO2thin films at 60°C is reported using a shuttered vacuum ultraviolet (VUV) light source to excite molecular oxygen as a co-reagent with the metal precursors. The growth mechanisms using trimethyl aluminium (TMA) and titanium isopropoxide precursors, are investigated using in-situ quartz crystal microbalance (QCM) and post-deposition ellipsometric measurements. The figure below illustrates the mass gain / loss processes during a series of ALD cycles of TMA with O2/ UV excitation from a VUV lamp with peak emissions at 125 nm (9.9 eV) and 160 nm (7.7 eV). Further, the figure shows the growth per cycle (GPC) derived from the QCM data and also separately from single wavelength (633 nm) ellipsometry measurements made on silicon (100) samples exposed to the same growth conditions. Both data sets indicate that as the VUV illumination time is extended beyond 5 s to 10 s the overall mass gain per cycle saturates and becomes approximately constant, which is consistent with other self limiting ALD processes. The photochemical process is exploited to achieve area-selective film deposition. Selective area deposition was achieved by projecting the VUV light through a metalized magnesium fluoride photolithographic mask. The selectivity of deposition is a function of the UV exposure time and the mask – substrate separation. The photochemical atomic layer deposition of TiO2 thin films from titanium isopropoxide exhibits similar growth characteristics to the Al2O3 photochemical process, however subtle differences in the growth mechanisms are attributed to the different reactions of the excited oxygen species (O( 3P)) with the Al-CH3 and Ti-OCH(CH3)2 bonds. Figure 1

Spectral Ellipsometry of Cobalt-Ions Implanted Silicon Surface

Monocrystalline silicon wafers implanted by cobalt ions with energy of 40 keV at a fluence range from 6.6×1012 to 2.5×1017 Co+-ion/cm2 were investigated by optical spectroscopic ellipsometry. By comparison of experimental data with modeling it is shown that the ellipsometric measurements are accurate and reliable method for monitoring of a low-dose ion implantation process.

Microsecond-resolved SDR-based cavity ring down ellipsometry.

We present an experimental apparatus that allows microsecond-resolved ellipsometric and absorption measurements. The apparatus is based on an optical cavity containing a Dove prism, in which light undergoes total internal reflection (TIR), while the data acquisition is based on software defined radio technology and custom-built drivers. We demonstrate the ability to sense rapid variations in the refractive index above the TIR interface for arbitrarily long times with a temporal resolution of at least 2μs.

Lanthanide-Assisted Deposition of Strongly Electro-optic PZT Thin Films on Silicon: Toward Integrated Active Nanophotonic Devices.

The electro-optical properties of lead zirconate titanate (PZT) thin films depend strongly on the quality and crystallographic orientation of the thin films. We demonstrate a novel method to grow highly textured PZT thin films on silicon using the chemical solution deposition (CSD) process. We report the use of ultrathin (5-15 nm) lanthanide (La, Pr, Nd, Sm) based intermediate layers for obtaining preferentially (100) oriented PZT thin films. X-ray diffraction measurements indicate preferentially oriented intermediate Ln2O2CO3 layers providing an excellent lattice match with the PZT thin films grown on top. The XRD and scanning electron microscopy measurements reveal that the annealed layers are dense, uniform, crack-free and highly oriented (>99.8%) without apparent defects or secondary phases. The EDX and HRTEM characterization confirm that the template layers act as an efficient diffusion barrier and form a sharp interface between the substrate and the PZT. The electrical measurements indicate a dielectric constant of ∼650, low dielectric loss of ∼0.02, coercive field of 70 kV/cm, remnant polarization of 25 μC/cm(2), and large breakdown electric field of 1000 kV/cm. Finally, the effective electro-optic coefficients of the films are estimated with a spectroscopic ellipsometer measurement, considering the electric field induced variations in the phase reflectance ratio. The electro-optic measurements reveal excellent linear effective pockels coefficients of 110 to 240 pm/V, which makes the CSD deposited PZT thin film an ideal candidate for Si-based active integrated nanophotonic devices.

Spectroellipsometric characterization of ZnO films produced by chemical spray pyrolysis

In this study, zinc oxide thin films (ZnO) were produced on glass substrate by using spray pyrolysis. Ellipsometric measurements of ZnO thin films were taken using spectroscopic ellipsometry (SE) technique. An important optical parameters of films such as refractive index (n), extinction coefficient (k) and thicknesses (d) of ZnO thin films were determined variable-angle (50°-60°-70°) by spectroscopic ellipsometry (SE) technique. Variable angle spectroscopic ellipsometry was used for thickness and optical constant calculations. Multiple angle measurements were taken in the most sensitive angle of incidence region. The optical parameters (n, k and d) were then obtained the different two ellipsometric angles (ψ and ) by a fitting procedure were experimental data, which were generated from the Cauchy-Urbach dispersion model, were compared with the experimental ones. Also, the surfaces properties and roughness values of ZnO thin films, is an important factor in the ellipsometric measurements, were examined by atomic force microscopy. In conclusion, the ellipsometric incidence angle and surface properties effects were discussed on the optical parameters of ZnO thin films such as thickness and optical constants (refractive index and extinction coefficient).

Improvements to the Volume Measurement of <sup>28</sup>Si Spheres to Determine the Avogadro Constant

In the determination of the Avogadro constant using the X-ray crystal density method with a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">28</sup> Si-enriched crystal prepared by the International Avogadro Coordination (IAC) project, the volume measurement and surface analysis of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">28</sup> Si spheres play a crucial role. For accurate volume determination, an optical interferometer has been improved taking account of the geometrical shape of the optical components. Furthermore, a new spectroscopic ellipsometer with a sphere rotation system has been developed for accurate surface analysis. The optical interferometer and the ellipsometer have been used for the volume determination and surface analysis of the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">28</sup> Si spheres repolished by the IAC, respectively. On the basis of the preliminary uncertainty estimation, the relative standard uncertainty of the apparent volume measurement has been reduced from 4.4 $\times 10^{-8}$ to 2.7 $\times 10^{-8}$ by the improvement of the interferometer. The number of ellipsometric measurement points on the sphere surface has been increased from 20 to 7782 by the installation of the new ellipsometer, yielding more reliable information on the sphere surface. Details of the improvements and the preliminary results of the measurements of the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">28</sup> Si spheres are given.

Simultaneous ellipsometric and chronoamperometric study of barrier aluminium oxide growth and dissolution in acetate buffer

In situ ellipsometric and chronoamperometic measurements were applied simultaneously to monitor barrier oxide growth and dissolution on aluminium. The steady-state thickness prior to each potential step was determined by electrochemical impedance spectroscopy (EIS) for calibration of the transient data. The growth of the barrier oxide, following a positive potential step, was consistent with the Cabrera-Mott inverse square logarithmic growth law. About 20 % of the measured current resulted in barrier oxide growth. The oxide dissolution rate, following a negative potential step, was controlled by diffusion of aluminium ions into the test solution. The obtained dissolution rate was thus much smaller than the corresponding oxide growth rates. Oxide solubilities calculated from Fick’s second law, by using literature data for the diffusion coefficient of Al3+, were about two orders of magnitude larger than that obtainable from thermodynamic considerations. The methodology developed provides the kinetic and solubility data needed for improving the existing know-how about the growth and dissolution kinetics of the barrier oxide layer in aqueous solution.

Ellipsometry method application in optics of inhomogeneous media

Physico-mathematical methods of modeling and measurements of polarization optical characteristics of “inhomogeneous surface layer – inhomogeneous medium” reflective system have been improved based on generalization of the polarized light reflection theory in the Born approximation from inhomogeneous media interface. Theoretical and experimental justification for simultaneous application of multi-angular and scanning ellipsometry methods is given for gradient optics elements in inhomogeneous layer presence on the surface of the optical detail. The measurement of the main ellipsometric parameters – linear reduced polarization azimuth Ψand phase difference Δbetween mutually orthogonal p– and s– components of a reflected polarized light beam – was carried out on a dual-channel installation, in which the scheme of measurements of polarization parameters Ψand Δby balanced (null) ellipsometry method is used in the first channel; the photometric scheme of measurements of parameters Ψand Δby Fourier method of output detection is used in the second channel. Such separation of channels gives the possibility for measuring of polarization parameters of reflected light beam separately and, thereby, improving the accuracy of ellipsometric measurements in relation to observable physical quantity of measurements object. The values of refractive index n(r) and absorption index k(r) were calculated by axial section of selfoc on the basis of measurements of polarization parameters Ψand Δwith angle of incidence of the light beam φ=45о within the limits of scanning ellipsometry method and a model of geometrically flat interface of homogeneous media. Empirical data indicated the model incorrectness of the geometrically flat interface for polarization measurements of refractive index distributionn(r) and absorption index k(r) in gradient optical engineering elements. For the reflective system “inhomogeneous surface layer – inhomogeneous medium” the refractive index n(r) change by axial section of an element r is proportional to phase difference Δ(r) between mutually orthogonal components of a reflected light beam. At the same time the linear reduced polarization azimuth Ψ(r) in the section gradient optical element varies slightly. The main proportions for calculation of refractive index nсand thickness dсof superficial layer in each local point r of selfoc are adduced on the basis of the ellipsometry equation, obtained within the framework of the polarized light reflection Born theory. Empirical data of experimental researches make it possible to give quantitative assessment of surface processing quality for optical engineering elements. Elaborated ellipsometric techniques are recommended in technological production of optical base assemblies with gradient and fiber optics elements.

Characterization of molecular organization in pentacene thin films on SiO2 surface using infrared spectroscopy, spectroscopic ellipsometry, and atomic force microscopy

Thin films of pentacene of 32 and 100nm thickness obtained by organic molecular beam deposition (OMBD) in high vacuum conditions onto silicon/native silica (Si/SiO2) and fused silica substrates were examined. Alignment, anisotropic optical properties and morphology were studied in ambient conditions using infrared (IR) transmission and polarized grazing angle attenuated total reflection (GATR) techniques, variable angle spectroscopic ellipsometry (VASE), UV–VIS absorption, and atomic force microscopy (AFM). For the first time dichroic GATR IR spectra were recorded for such thin films and conclusions on pentacene orientation were deduced on the basis of dichroic ratio of the IR-active vibrations. The symmetry assignment of the vibrational transitions is also discussed. The films exhibit continuous globular texture with uniaxial alignment of pentacene molecules and strongly anisotropic optical properties evidenced in the ellipsometric measurements. The results revealed that there are some quantitative differences in the orientation and in the dielectric properties between the two pentacene films of different thickness.

HfO2/SiO2 multilayer enhanced aluminum alloy-based dual-wavelength high reflective optics

Laser durable multiband high reflective optics can be attained by depositing HfO2/SiO2 stacks on diamond-turned and optically polished aluminum alloy substrates. HfO2 and SiO2 single layers were prepared using modified plasma-ion assisted deposition. Ellipsometric measurements were performed using two types of variable angle spectroscopic ellipsometry with a combined spectral range of 150nm to 14μm. Optical constants were generated in the entire spectral range. Scatter loss as a function of surface roughness was calculated at 1064nm, 1572nm, and 4.1μm, representing a primary wavelength, a secondary wavelength, and a middle wave infrared band selected for a dual-wavelength laser beam expander, respectively. The surface requirement of the aluminum alloy substrates was determined. Calculated and measured spectral reflectances were compared. Laser-induced damage threshold tests were performed at 1064nm, 20ns, and 20Hz. A laser-induced damage threshold of 47J/cm2 was determined. Post-damage analysis suggests that nodule defects are the limiting factor for the laser-induced damage threshold. Surface modification of the aluminum alloy was identified as a potential technical solution that may further increase the laser damage resistance of the dielectric enhanced dual-wavelength reflective optics.

Cell proliferation on modified DLC thin films prepared by plasma enhanced chemical vapor deposition.

Recently, diamondlike carbon (DLC) thin films have gained interest for biological applications, such as hip and dental prostheses or heart valves and coronary stents, thanks to their high strength and stability. However, the biocompatibility of the DLC is still questionable due to its low wettability and possible mechanical failure (delamination). In this work, DLC:N:O and DLC: SiOx thin films were comparatively investigated with respect to cell proliferation. Thin DLC films with an addition of N, O, and Si were prepared by plasma enhanced CVD from mixtures of methane, hydrogen, and hexamethyldisiloxane. The films were optically characterized by infrared spectroscopy and ellipsometry in UV-visible spectrum. The thickness and the optical properties were obtained from the ellipsometric measurements. Atomic composition of the films was determined by Rutherford backscattering spectroscopy combined with elastic recoil detection analysis and by x-ray photoelectron spectroscopy. The mechanical properties of the films were studied by depth sensing indentation technique. The number of cells that proliferate on the surface of the prepared DLC films and on control culture dishes were compared and correlated with the properties of as-deposited and aged films. The authors found that the level of cell proliferation on the coated dishes was high, comparable to the untreated (control) samples. The prepared DLC films were stable and no decrease of the biocompatibility was observed for the samples aged at ambient conditions.

Fabrication of Interdigitated Electrodes using Molecular Beam Epitaxy and Optical Lithography

ABSTRACTThis study reports the fabrication of interdigitated electrodes using molecular beam epitaxy and photolithography techniques. Sandwich-based 10 nm silver and 50 nm gold films were deposited on glass substrates using magnetron sputtering and molecular beam epitaxy deposition. The thickness and roughness were determined by use of a suitable mathematical model and ellipsometric measurements. The gold film (111) orientation was determined by X-ray diffraction. Optical microscopy measurements were made for the lateral resolution determination of the fabricated electrodes and the quality was similar to that of the photomask. Atomic force microscopy measurements offered information regarding the topography of the sample and the possible application of the fabricated system for molecular detection.

The effects of thermally-induced biaxial stress on the structural, electrical, and optical properties of Cu2O thin films

The effects of biaxial stress in Cu2O thin films grown by rf magnetron sputtering at different growth temperatures were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), electrical transport, and ellipsometric measurements. A predominant diffraction peak in the XRD patterns corresponds to the (111) plane direction of the Cu2O phase. Biaxial tensile stress was induced by thermal mismatch between the film and substrate and increased with increasing growth temperature. As the growth temperature and biaxial stress increased, the electrical resistivity decreased, while the carrier concentration and Hall mobility both increased. The ellipsometric data were fit using an optical dispersion model, and the decrease in refractive index was attributed to contraction of the lattice parameter with increasing biaxial stress. The optical absorption peaks shifted slightly toward higher energy with increasing stress. Our experimental data suggest that the mechanisms of stress are important for understanding the properties of Cu2O thin films and for the fabrication of devices using them.

Ge wetting layer increases ohmic plasmon losses in Ag film due to segregation.

We have investigated the influence of the Ge wetting layer on both ohmic and scattering losses of a surface plasmon-polariton (SPP) wave in Ag film deposited on SiO2 substrate with an e-beam evaporator. Samples were examined by means of atomic force microscopy (AFM), spectroscopic ellipsometry (SE), two-dimensional X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and microscopic four-point probe (M4PP) sheet resistance measurements. Ag films of 100 nm thickness were deposited at 180 and 295 K directly onto the substrates with or without a Ge interlayer. In AFM scans, we confirm the fact that the commonly used Ge adhesion layer smooths the surface of Ag film and therefore reduces scattering losses of the SPP wave on surface roughness. However, our ellipsometric measurements indicate for the first time that segregation of Ge leads to a considerable increase in ohmic losses connected with a boost of the imaginary part of Ag permittivity in the 500-800 nm spectral range. Moreover, the trend develops over time, as confirmed in a series of measurements performed over an interval of three months. XPS analysis confirms the Ge segregation to the Ag free surface and most probably to grain boundaries. M4PP measurements show that the specific resistivity in Ag films evaporated on a Ge interlayer at 295 K is nearly twice as high as in layers deposited directly on a SiO2 substrate. The use of an amorphous Al2O3 overlayer prevents Ge segregation to free surface.

Lipid Selectivity, Orientation, and Extent of Membrane Binding of Nonacylated RP2.

Retinitis pigmentosa 2 (RP2) is an ubiquitary protein of 350 residues. The N-terminus of RP2 contains putative sites of myristoylation and palmitoylation. The dually acylated protein is predominantly localized to the plasma membrane. However, clinically occurring substitution mutations of RP2 in photoreceptors lead to the expression of a nonacylated protein, which was shown to be misrouted to intracellular organelles using different cell lines. However, the parameters responsible for the modulation of the membrane binding of nonacylated RP2 (naRP2) are still largely unknown. The maximal insertion pressure of naRP2 has thus been determined after its injection into the subphase underneath monolayers of phospholipids, which are typical of photoreceptor membranes. These data demonstrated that naRP2 shows a preferential binding to saturated phospholipid monolayers. Moreover, polarization modulation infrared reflection absorption spectroscopy has allowed comparison of the secondary structure of this protein in solution and upon binding to phospholipid monolayers. In addition, simulations of these spectra have allowed to determine that the β-helix of naRP2 has an orientation of 60° with respect to the normal, which remains unchanged regardless of the type of phospholipid. Finally, ellipsometric measurements of naRP2 demonstrated that its particular affinity for saturated phospholipids can be explained by its larger extent of insertion in this phospholipid monolayer compared to that in polyunsaturated phospholipid monolayers.

Effect of annealing in a various oxygen atmosphere on structural, optical, electrical and gas sensing properties of MoxOy thin films

Molybdenum oxide thin films were thermally evaporated on a glass substrate and monitored by an annealing process in a variable oxygen atmosphere. The effects of post annealing condition on the microstructural, morphological, optical and electrical properties were investigated using X-ray diffraction, Raman spectroscopy, atomic force microscope, spectroscopic ellipsometry and impedance spectroscopy. As-deposited amorphous films crystallized into tetragonal metastable phase of Mo5O14 on annealing at 500°C in vacuum and air. This structure transformed to stable orthorhombic of MoO3 with annealing in oxygen environment. The optical parameters such as the refractive index, extinction coefficient, optical band gap energy and the Urbach energy were calculated from Cauchy formalism. Ellipsometric measurements reveal that the samples present optical gap located between 3.24 and 3.90eV when the atmosphere becomes rich on oxygen. The variation of the conductivity in terms of the temperature shows an electrical behavior with oxygen environment. Finally, it has been found that MoO3 thin films had high sensitivity to ethanol, which made them as a good candidate for the ethanol sensor.