Ellipsometric Angles Research Articles

Spectroscopic ellipsometer based on direct measurement of polarization ellipticity

A polarizer-sample-Wollaston prism analyzer ellipsometer is described in which the ellipsometric angles ψ and Δ are determined by direct measurement of the elliptically polarized light reflected from the sample. With the Wollaston prism initially set to transmit p- and s-polarized light, the azimuthal angle P of the polarizer is adjusted until the two beams have equal intensity. This condition yields ψ=±P and ensures that the reflected elliptically polarized light has an azimuthal angle of ±45° and maximum ellipticity. Rotating the Wollaston prism through 45° and adjusting the analyzer azimuth until the two beams again have equal intensity yields the ellipticity that allows Δ to be determined via a simple linear relationship. The errors produced by nonideal components are analyzed. We show that the polarizer dominates these errors but that for most practical purposes, the error in ψ is negligible and the error in Δ may be corrected exactly. A native oxide layer on a silicon substrate was measured at a single wavelength and multiple angles of incidence and spectroscopically at a single angle of incidence. The best fit film thicknesses obtained were in excellent agreement with those determined using a traditional null ellipsometer.

Evanescent-Wave Cavity Ring-Down Ellipsometry.

We introduce the new technique of evanescent-wave cavity ring-down ellipsometry (EW-CRDE), used for the measurement of ellipsometric angles of samples at a solid-gas or solid-liquid interface, and achieve phase-shift measurements with precision of ∼0.01°. We demonstrate the technique by measuring the time-dependent refractive index of methanol-water mixtures and thin films at the liquid/fused-silica interface, showing that the monitoring of monolayers on microsecond time scales using EW-CRDE should be achievable.

Optical Properties of Ultrathin Copper Thin Films Sandwiched between Nb-Doped TiO2 Films Studied with Spectroscopic Ellipsometry

Cu thin films sandwiched between Nb-doped TiO2 (NTO) thin films were grown on glass substrates using tilted-dual target DC magnetron sputtering deposition. The thicknesses of the top and bottom NTOs were nominally 30 nm, and the thicknesses of the Cu films (t) varied between 1.5 nm and 50 nm. We measured the ellipsometric angles (Ψ,Δ) of the NTO/Cu/NTO/glass by using spectroscopic ellipsometry and estimated the thicknesses and dielectric functions of the Cu films by using a multi-layer model analysis. Transmission electron microscopy measurements showed that the Cu layers evolved from aggregates of Cu nanoparticles to coalesced Cu thin films as the Cu film thickness increased. Sheet resistance data also showed that the t = 8 nm film was near the percolation threshold, suggesting that films thinner than 8 nm were aggregations of Cu nanoparticles that were not well-connected. The films thicker than 8 nm were above the percolation threshold. From the Drude model, we estimated the plasmon frequency (ωp) and the electron relaxation time (τ), which were found to increase with increasing film thickness. By applying standard critical point analysis to the second derivatives of the dielectric function spectra, we found several peaks near 1.5, 2.1, 2.5, 3.5, and 4.3 eV, and attributed to interband transitions. The peak energies (except 1.5 eV) matched to the band structure calculations of bulk Cu found in the literature.

Dehydrogenation process of Mg–Ni based switchable mirrors analyzed by in situ spectroscopic ellipsometry

The dehydrogenation process of hydrogenated switchable mirrors using magnesium–nickel alloy thin film was studied in situ using spectroscopic ellipsometry. Ellipsometric angles Ψ and Δ of the switchable mirrors varied drastically as a result of dehydrogenation, which is a transformation from transparent to reflective states. The process was analyzed by dividing into the following three phases. The first phase was the dehydrogenation process of a thin Mg4Ni layer with several nanometers at a hydrogenated Pd/Mg4Ni interface. The second phase was the dehydrogenation processes of the hydrogenated Mg4Ni layer, which proceeded from the Pd/Mg4Ni interface to the substrate. The final phase was the desorption process of hydrogen, which was absorbed in Mg4Ni as solid solution and the dehydrogenation process was terminated.

Probing adsorption of sodium poly(acrylate) at the calcite–water interface by ellipsometry

Null ellipsometry was used to investigate the adsorption of poly(acrylic acid) (PAA) in the form of sodium poly(acrylate) (NaPA) on polished calcite crystals in aqueous electrolytes at pH 9. The ellipsometric angles were converted into adsorbed mass with an optical layer model, which treats the surface roughness by introducing an additional surface layer. One finds that the adsorbed mass saturates near 4 mg/m 2. Higher values of specific adsorbed mass were also observed, and they can be explained with the precipitation of polyelectrolyte with calcium ions.

Rotating compensator sampling for spectroscopic imaging ellipsometry

In this work, a rotating compensator sampling for spectroscopic imaging ellipsometry (SIE) is presented and demonstrated by characterization of a SiO2 nanofilm pattern on Si substrate. Experiment results within spectrum of 400–700nm show that the rotating compensator sampling is valid for SIE to obtain the ellipsometric angle distributions ψ (x, y, λ) and Δ (x, y, λ) over the thin film pattern, the sampling times of ψ (x, y) and Δ (x, y) with 576×768pixels under each wavelength is less than 8s, the precision of fitting thickness of SiO2 is about 0.2nm and the lateral resolution is 60.9μm×24.6μm in the parallel and perpendicular direction with respect to the incident plane.

Spectroscopic null ellipsometer using a variable retarder

We describe a polarizer-compensator-sample-analyzer (PCSA) null ellipsometer in which a variable retarder is used as the compensator and either the polarizer or the analyzer is held at a fixed azimuthal angle. Ellipsometric angles ψ and Δ are determined directly from the azimuth of the rotating component and the compensator delay, respectively. A Soleil-Babinet compensator with quartz plates is used as the variable delay element and the delay at any wavelength is calculated from the independently measured delay at 632.8 nm and a knowledge of the dispersion of quartz. The thicknesses of thin silicon dioxide films on a silicon wafer were determined both spectroscopically and at a single wavelength and show excellent agreement with those determined using a traditional single wavelength PCSA null ellipsometer.

Ellipsometric study of optical switching processes of Mg–Ni based switchable mirrors

Optical switching processes of Mg–Ni based switchable mirrors were studied “in situ” using spectroscopic ellipsometry. Ellipsometric angles Ψ and Δ of the switchable mirrors varied drastically as a result of hydrogenation and dehydrogenation. These variations are due to transformation between metal and semiconductor of Mg–Ni layers. We have clarified the switching processes by analyzing these variations using the following two steps. First, we evaluated dielectric functions of five materials: metal, hydrogen-solid-solution, hydride states of Mg–Ni alloy, and metal and hydride states of Pd. Using these dielectric functions, we then varied thickness and concentration of each layer.

Optical properties of thin films of mixed Ni–W oxide made by reactive DC magnetron sputtering

Thin films of NixW1−x oxides with x=0.05, 0.19, 0.43 and 0.90 were studied. Films with thicknesses in the range 125–250nm were deposited on silicon wafers at room temperature by reactive DC magnetron co-sputtering from targets of Ni and W. The films were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), and spectroscopic ellipsometry (SE). XRD spectra and SEM micrographs showed that all films were amorphous and possessed a columnar structure. The ellipsometric angles Ψ and Δ of as-deposited films were measured by a rotating analyzer ellipsometer in the UV–visible-near infrared range (0.63–6.18eV) and by an infrared Fourier transform rotating compensator ellipsometer in the 500–5200cm−1 wavenumber range. SE measurements were performed at angles of incidence of from 50° to 70°. Parametric models were used to extract thicknesses of the thin films and overlayers of NixW1−x oxide at different compositions, band gaps and optical constants. Features in the optical spectra of the NixW1−x oxides were compared with previous data on tungsten oxide, nickel oxide and nickel tungstate.

Influence of active admixtures onto tellurite glass refractive index

Influence of active admixtures onto tellurite glass refractive indexThe goal of this work was to investigate the influence of rare earth ions such as Tm3+, Yb3+on physico-chemical properies of tellurite glass from the TeO2-WO3-PbO-PbF2-Na2O system. The thermal characteristic of tellurite glass Tm3+, Yb3+doped have been presented. The effect of the glass crystallization on thermal stability of the glass and crystallizing phases formed upon heat treatment were investigated by DTA/DSC/, XRD methods. The spectral dependence of ellipsometric angles of the tellurite glass samples, have been studied. The influence of ions of rare earth elements, i.e. Tm3+and Yb3+, onto changes of refractive index of glass P1 (without RE admixture) were examined. The optical measurements were conducted on Woollam M2000 spectroscopic ellipsometer, in spectral range of 190-1700 nm.

Optical characterization of SnO2:F films by spectroscopic ellipsometry

Abstract Spectroscopic ellipsometry (SE), which is a non-destructive and a non-contact optical technique used in characterization of thin films, is widely used to determine thickness, microstructure and optical constants. In this work, the effect of F incorporation on optical properties of SnO2 films grown by ultrasonic spray pyrolysis technique (USP) is presented. The reflections, refractive indices and thicknesses of the films were investigated using room temperature spectroscopic ellipsometry. The optical constants and the thicknesses of the films were fitted according to Cauchy–Urbach model, and ellipsometric angle Ψ was used as source point for optical characterizations. Besides, transmittance spectra of the films were taken from UV spectrometer, and the optical method was used to determine the band gaps. Also, band tailing resulting from defects or impurities was investigated. From the results obtained from the optical analyses, the application potential of SnO2:F films for solar cell devices was searched.

Multi-stimuli responsive poly(ether amine) (PEA) brush fabricated by “thiol-ene” click chemistry

Multi-stimuli responsive poly(ether amine) (PEA) was successfully grafted on the planar silicon through a convenient and efficient way via “thiol-ene” click chemistry. The whole process for fabrication of PEA brush was carefully traced by XPS, AFM, ellipsometer and water contact angle (WCA). The results indicated that PEA brushes with different thickness can be obtained through both photochemical and the thermal processes. The obtained PEA brush in aqueous solution exhibited its response to temperature and pH value. With the increase of pH and temperature, the PEA chain became less hydrophilic and adopted the coil–globule transition.

Reflection of electromagnetic plane waves in a long-wavelength approximation from a multilayer system of anisotropic transparent films on non-absorbing isotropic medium

The reflection of s- and p-polarised electromagnetic plane waves from an N-layer system of anisotropic dielectric films upon transparent homogeneous substrate is investigated in the long-wavelength approximation. The analytical expressions are obtained for the amplitude reflection (transmission) coefficients, reflectances (transmittances), and ellipsometric angles of an anisotropic multilayer thin-film system. All analytical results are in agreement with the numerical solution of the reflection problem for anisotropic stratified medium. The possibilities of using obtained expressions for resolving the inverse problem for ultrathin anisotropic films upon isotropic substrates are discussed.

Dielectric Function and Electronic Excitations of Functionalized DNA Thin Films

We measure the dielectric functions of organic-soluble, functionalized DNAs bearing functional moieties in the near-infrared, visible, and ultra-violet spectra by using spectroscopic ellipsometry. Natural double-stranded DNA is dissolved in water and reacted with carbazole-based trimethyl ammonium bromide, cetyltrimethylammonium bromide, and chalcone-terminated trimethyl ammonium bromide. The functional DNA products are all precipitated and filtered for washing and drying. We successfully prepare functionalized DNAs that are insoluble in water but soluble in organic solvents. The thin films are fabricated by using the spin coating technique after preparing solutions in either homogeneous or mixed organic solvents. We measure the ultraviolet–visible absorbance spectra of the films. The absorbance spectra show that the optical energy gaps of the functionalized DNAs change little even though the DNAs are connected to the complex molecules by electrostatic interaction. From the measured ellipsometric angles, we estimate the dielectric functions by using parametric optical constant model and layer model analysis. Depending on the nature of the attached complex molecules, the dielectric functions change, new optical structures develop below and above band gaps arising from the side molecules, and the optical energy gaps of the DNAs are altered slightly by weak coupling to the tethered complex molecules.

In situ spectroscopic ellipsometry study of the hydrogenation process of switchable mirrors based on magnesium-nickel alloy thin films

The hydrogenation process of switchable mirrors using magnesium-nickel alloy thin films including a thin palladium cap layer was analyzed by measuring the variation in ellipsometric angles Ψ and Δ using in situ spectroscopic ellipsometry. The process was divided into three phases and each phase was identified as follows. The first phase was the process in which the solid solution was formed because a Mg–Ni alloy in its metal state absorbs hydrogen. The second phase was the hydrogenation processes of the solid solution and the metal Pd layers. The third phase was the hydrogenation process of residual metal Pd in the Pd layer. In the initial state of the second phase, a hydride of the alloy was nucleated at the film/substrate interface as a result of hydrogenation of the solid solution, and a mixture layer of the hydride and solution was formed. With proceeding hydrogenation, the thickness of the mixture layer increased and the homogenous hydride layer was afterwards formed at the film/substrate interface. As a result of further hydrogenation, the Mg–Ni alloy layer was completely hydrogenated. After the alloy layer was completely hydrogenated, the hydrogenation of Pd was terminated.

Optical characterization and determination of carrier density of ultrasonically sprayed CdS:Cu films

In this work, CdS and Cu doped CdS films (at the Cu percentages of 1, 3 and 5) have been deposited onto glass substrates at 350 ± 5 °C by ultrasonic spray pyrolysis technique and their application potential for photovoltaic solar cells have been investigated. Optical properties and thicknesses of the films have been investigated by spectroscopic ellipsometry (SE). Ellipsometric angle ψ was used as the source point for optical characterizations. The optical constants ( n and k) and the thicknesses of the films have been fitted according to Cauchy model. Also, optical properties of the produced films have been analyzed by transmittance and reflectance spectra. Refractive index ( n), extinction coefficient ( k) and reflectance ( R) spectra have been taken by spectroscopic ellipsometer, while transmittance spectra have been taken by UV/vis spectrophotometer. The optical method has been used to determine the band gap type and value of the films. Mott-Schottky (M-S) measurements have been made to determine the conductivity type and carrier concentration of the films. Samples showed n-type conductivity and carrier concentration of undoped CdS sample was found to be 1.19 × 10 19 cm −3. Also, it was concluded that Cu doping has an acceptor effect in CdS samples. From the results of these investigations, the application potential of CdS:Cu films for photovoltaic solar cells as window layer was searched.

Reflection Characterization of Nanoscopically Stratified Surface Structures and Optical Probing of Anisotropic Nanolayers

The reflection of linearly polarized light from a layered nanoscopic system of anisotropic dielectric films on homogeneous isotropic substrate is investigated in the long-wavelength limit. The expressions for the reflectances and ellipsometric angles of an N-layer system of anisotropic ultrathin films are derived. For absorbing substrate mathematical relationships take a relatively simple additive form. In comparison with absorbing substrate the fully transparent system is more complicated and in this case an ultrathin film in multilayer system is sensitive to the presence of other ultrathin films (an interaction between them emerges) and the contribution of each layer to the reflection coefficient is not expressed in a purely additive form. The analytical results are supported by computer-aided analysis made on the basis of general wave propagation theory for anisotropic layered media. The most useful feature of obtained approximate expressions is that they are simply invertible, allowing a direct calculation of the parameters of ultrathin layers. [DOI: 10.1380/ejssnt.2010.167]

Thermo‐optical investigations of NaNbO3 thin films by spectral ellipsometry

AbstractIn this work a spectroscopic ellipsometry was applied to the thermo‐optical investigations of sodium niobate NaNbO3 (NN) thin films at the wide temperature range of 5–820 K. The temperature dependence of complex refractive index dispersions and optical bang energy of the direct allowed electron transitions were evaluated. Additionally dynamic scans of the main ellipsometric angles at the several fixed wave lengths of 300, 400, 500 and 635 nm were performed to acquire more detailed temperature dependences of the refractive index and extinction coefficient. Pronounced minima/maxima and substantial jumps in the temperature dependence of complex refractive index and band gap energy were found and corresponded to the structural phase transition temperatures of NN single crystals and ceramics. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical diagnostics of anisotropic nanoscale films on transparent isotropic materials by integrating reflectivity and ellipsometry

The reflection of s- and p-polarized electromagnetic plane waves from an anisotropic ultrathin dielectric film on transparent isotropic substrate is investigated in the long-wavelength limit. The analytical approximate formulas are obtained for the reflection coefficients and ellipsometric angles that agree with the exact computer solution of the reflection problem for anisotropic systems. The possibilities of using the obtained expressions for resolving the inverse problem for ultrathin anisotropic dielectric films upon isotropic dielectric substrates are discussed. It is shown that a promising technique for determining the optical constants of anisotropic dielectric films on transparent substrates is the integration of ellipsometry and differential reflectivity.

Switchable selectivity for gating ion transport with mixed polyelectrolyte brushes:approaching ‘smart’ drug delivery systems

A pH-responsive mixed polyelectrolyte brush from tethered polyacrylic acid (PAA) and poly(2-vinylpyridine)(P2VP) (PAA:P2VP = 69:31 by weight) was prepared and used for selective gating transport of anions and cationsacross the thin film. An ITO glass electrode was modified with the polymer brush and usedto study the switchable permeability of the mixed brush triggered by changesin pH of the aqueous environment in the presence of two soluble redox probes:[Fe(CN)6]4− and[Ru(NH3)6]3+. The responsive behavior of the brush was also investigated using the in situellipsometric measurements of the brush swelling, examination of the brushmorphology with atomic force microscopy (AFM), and contact angle measurementsof the brush samples extracted from aqueous solutions at different pHvalues. The mixed brush demonstrated a bipolar permselective behavior. AtpH<3 thepositively charged P2VP chains enabled the electrochemical process for the negatively charged redoxprobe, [Fe(CN)6]4−,while the redox process for the positively charged redox probe was effectively inhibited. On the contrary,at pH>6 a reversible redox process for the positively charged redox probe,[Ru(NH3)6]3+, was observed, while the redox process for the negatively charged redox species,[Fe(CN)6]4−, was fully inhibited. Stepwise changing the pH value and recording cyclic voltammogramsfor the intermediate states of the polymer brush allowed electrochemical observation of thebrush transition from the positively charged state, permeable for the negatively chargedspecies, to the negatively charged state, permeable for the positively charged species. Thedata of ellipsometric, AFM and contact angle measurements are in accord with theelectrochemical study. The discovered properties of the brush could be used for thedevelopment of ‘smart’ sensors and drug delivery systems, for example, a smart drugdelivery capsule which could release negatively charged molecules of drugs in acidicconditions, while positively charged molecules of drugs will be released in neutralconditions.