Surface characterization by spectroscopic infrared ellipsometry

Summary For ellipsometry in the infrared range the advantages of Fourier-transform spectroscopy can be exploited when the experimental procedure is based on photometry. The measured intensities are interpretable in terms of the so-called ellipsometric parameters which describe the polarization state of the radiation after having been reflected from the sample. These parameters in turn are correlated with the optical constants of the sample such as the dielectric function or the complex refractive index as well as with its geometrical structure. The potential of spectroscopic infrared ellipsometry for the characterization of surfaces and surface films is demonstrated by examples including compounds with spectral intervals of strong absorption and assemblies of semiconductor films.     

Infrared optical properties and AFM of spin-cast chitosan films chemically modified with 1,2 Epoxy-3-phenoxy-propane

Chemical modification of spin-cast chitosan films has been performed. This modification involves the attachment of 1,2 Epoxy-3-phenoxy-propane, commonly known as glycidyl phenyl ether (GPE), to the amine group of the chitosan molecule. Optical properties of modified films have been determined in the infrared region of the spectrum using spectroscopic ellipsometry, and are reported in this paper. Special attention is paid to the infrared region where the index of refraction and extinction coefficients from 750 to 4000 cm(-1) were determined. Difference plots of IR optical data before and after chemical modification were generated to confirm that modification had occurred. Optical modeling of infrared spectroscopic ellipsometry (IRSE) data with respect to chemical bond vibrations has also been performed. This modeling involved curve fitting of resonant chemical bond absorptions using Lorentz oscillators. These oscillator models allow for comparison of modified chitosan to unmodified chitosan. The purpose of this research was to determine infrared optical constants of chemically modified chitosan films This work shows that surface chemistry of biomaterials can be studied quite sensitively with spectroscopy ellipsometry, detecting as little as 100 ng/cm(2) of GPE.     

Study of the properties and structure of nanosize membranes for acousto-optic devices

Polyimide films on BK7 glass, which are used to produce matrices for infrared detectors, have been synthesized. When these systems are investigated by monochromatic zero ellipsometry, the least difference between parameters of the incident light polarization, which are determined from suppression angles of polarizer and analyzer and calculated within a certain model, is obtained via introducing air between mounting glass and the film. Validity of the solution was verified by holding the film in water vapor; then, solution of the inverse problem of ellipsometry indicated the appearance of liquid water between the film and mounting glass. Data of the electron microscopy study were used to suggest a water penetration mechanism and develop a physical model of the system polyimide film/mounting glass.     

红外椭圆偏振光谱及其应用

阐述了红外椭圆偏振光谱的原理，技术的发展，对国外ＳＩＲＥ的应用做了介绍并给出部分实例，共收集文献２９篇。     

Infrared ellipsometry of interdiffusion in thin films of miscible polymers

A new application of infrared ellipsometry is reported. Specifically, the interdiffusion between thin films of miscible polymers—poly(methyl methacrylate) and poly(vinylidene fluoride)—is detected in a non‐invasive measurement. A novel technique of data analysis for interdiffusion was developed and is described. The validity of the approach is supported by simulations of diffusion in a bilayer. The onset of extensive interdiffusion over a time period of 15 min occurs at a temperature of 160 °C. At a temperature of 190 °C, the data show that complete mixing of a bilayer (850 nm thick) occurs within 30 s, which is consistent with previously reported values of the mutual diffusion coefficient. Infrared ellipsometry is non‐invasive, applicable at elevated temperatures and relatively fast and sensitive. Although, in these measurements, it was unable to determine a concentration profile at the interface, infrared ellipsometry was used successfully to detect when interdiffusion had occurred. Hence, it is a useful means for screening polymer pairs for miscibility. Copyright © 2004 John Wiley & Sons, Ltd.     

Quantitative chemical composition of thin films with infrared spectroscopic ellipsometry: application to hydrated oxide films on aluminium

A method to quantify the composition of thin films using infrared spectroscopic ellipsometry (IRSE), supplemented by visible spectroscopic ellipsometry (VISSE), is proposed. Because ellipsometry measures the thickness and optical constants of a surface layer simultaneously, the absorption coefficient of the film as a function of wavelength can be obtained. Using values of the absorption coefficients for the pure components of the film, the percentages (mol.% or wt.%) of each component in the film can be calculated. The method is demonstrated in a study of the hydration of oxide films on electropolished aluminium and the anodically formed barrier oxide film. The IRSE technique shows that hydration of the films by immersion in boiling water results in the conversion of aluminium oxide to pseudoboehmite. Copyright © 2003 John Wiley & Sons, Ltd.     

Structural analysis of organic interfacial layers by ellipsometry

Ellipsometry has ‘come of age’ as a technique for the analysis of problems related to colloid and interface science. It has advanced far beyond applications of measuring film thickness or optical constants — although these remain important uses. Studies of the structure of polymers at the solid/liquid interface have been advanced significantly by the realisation of Fourier transform ellipsometry. Another important achievement has been the calibrated measurement of the dynamic surface excess at the flowing surface of a liquid jet. The uses of ellipsometry to study critical adsorption in binary liquids and to measure the width of liquid/liquid interfaces are also noteworthy. An important development is the use of infrared — rather than visible — light, which opens up numerous possibilities for the simultaneous structural and chemical interrogation of interfaces non-invasively.     

Spectroscopic Ellipsometry for Characterization of Thin Films of Polymer Blends

Morphology, composition, miscibility, interdiffusion, and interactions at interfaces are important quantities of polymer blends. Many of these parameters can be probed with spectroscopic ellipsometry. Ellipsometry in the visible spectral range is very suitable for determination of thicknesses and the high frequency refractive indices of thin organic films. However the spectral contrast is low for many polymers in comparison to infrared spectroscopic ellipsometry (IRSE) where specific contributions of the molecular vibrations are probed. In the presented study the infrared optical constants of a double layer (206.6 nm in total) of poly(n-butyl methacrylate) (PnBMA) and poly(vinyl chloride) (PVC) and of the films of the single compounds have been determined with optical simulations using layer models. The multiple layer model served for simulation of the ellipsometric spectra taken after an annealing induced mixing process in a polymeric double layer. The ellipsometric spectra of a not completely mixed sample could be fitted in a three-layer model, in which a mixed interphase in between the two layers of the polymers is formed due to interdiffusion.     

Physically self-assembled monolayers (PSAMs) of lecithin lipids at hydrophilic silicon oxide interfaces

A new method of making physically self-assembled monolayers (PSAMs) on hydrophilic solid surfaces is presented. This method uses a mixture of a nonpolar solvent, such as hexane, and a strong polar solvent, such as ethanol, to dissolve the lipids. The deposition of two lecithin lipids, dipalmitoylphosphatidylcholine (DPPC) and dilauroylphosphatidylcholine (DLPC), has been studied. These lipids physically self-assemble, or adsorb, onto hydrophilic silicon oxide/silicon surfaces when such surfaces are in contact with the lipid solution. The adsorbed layers were probed with ex-situ attenuated total reflection infrared (ATR-IR) spectroscopy, ellipsometry, contact angle measurements, and atomic force microscopy (AFM). The thicknesses of the adsorbed monolayers are about 2.8 +/- 0.2 nm for DPPC and 2.0 +/- 0.2 nm for DLPC, as determined by ellipsometry and AFM. Smooth, uniform monolayers of controlled surface density are formed. The surface density of adsorbed layers is comparable to those of close-packed lipid monolayers, as calculated from the ellipsometry and ATR-IR results. Producing controlled-thickness monolayers has applications in boundary lubrication, biomaterials, sensor technologies, and electronics. The method can be used for depositing many biological surfactants or lipids without the need to modify these surfactants chemically to form chemical bonds with the surfaces, as required by the usual chemical SAMs. Moreover, the new method has several advantages compared to the Langmuir-Blodgett (LB) method.     

Preparation and properties of thin films of hyperbranched polyesters with different end groups

Hyperbranched polyesters (HBP) with different end groups were prepared as thin films. They were characterized with regard to their chemical composition, thickness, optical constants and morphology using infrared spectroscopy, spectroscopic ellipsometry, and atomic force microscopy. The surface properties of the films were determined by zeta‐potential and contact angles measurements. The differences in the molecular structure and surface energetic and acid‐base properties between HBP materials with carboxylic, hydroxy and acetoxy end groups result in differences in their swelling behavior in atmospheric humidity. The swelling behavior at different atmospheric humidity was observed in situ using spectroscopic ellipsometry and reflectometric interference spectroscopy. From the results it can be concluded that HBP films can be used potentially as sensoric materials.     

Oligo(vinylidene fluoride) Langmuir-Blodgett films studied by spectroscopic ellipsometry and the density functional theory

Thin films of amphiphilic vinylidene fluoride oligomers prepared by Langmuir-Blodgett deposition on silicone substrates were investigated by comparing experimental and theoretical mid-infrared (IR) spectra. The experimental spectra were obtained using infrared spectroscopic ellipsometry. Theoretical spectra were calculated using density functional theory. Excellent correspondence of major IR bands in both data sets shows that the molecular backbone is oriented with the long axis normal to the substrate plane. This is in contrast to poly(vinylidene fluoride) LB films, in which the polymer chains are parallel to the substrate.     

Analysis of biosensors by chemically specific optical techniques. Chemiluminescence-imaging and infrared spectroscopic mapping ellipsometry

The standard methods currently used to read out microarrays are fluorescent and chemiluminesent imaging techniques. These methods require labeling of a component with a marker and, usually, only the concentration of the marker molecule is detected. A label-free imaging method that also enables quantitative spectroscopic analysis of the composition and component interaction would be of great advantage. In this article it is shown for the first time that IR mapping ellipsometry enables label-free imaging of a biochip before and after incubation with peptide solution. The measurements prove that IR ellipsometry is a sensitive tool for laterally resolved identification of the different materials and determination of the composition of a biochip. The lateral resolution required was achieved by using radiation from an infrared synchrotron beamline.      

Analytical techniques for characterization of organic molecular assemblies in molecular electronics devices

The analytical techniques used for the physical characterization of organic molecular electronic-based devices are surveyed and discussed. These protocols include methods that are used to probe molecular assemblies such as single wavelength ellipsometry, water contact angle goniometry, cyclic voltammetry, infrared spectroscopy, and X-ray photoelectron spectroscopy, and methods used to measure charge transport properties of devices such as scanning tunneling microscopy, and inelastic electron tunneling spectroscopy. Examples from our laboratory and the literature are given for each of these analytical techniques.     

Grafted self-assembled monolayers derived from naturally occurring phenolic lipids

Self-assembled monolayers grafted onto silicon surfaces were obtained from the hydrosilylation products by trialcoxysilanes of naturally occurring phenolic lipid allyl ethers. The as-obtained materials were characterized by various physical and physicochemical methods. Thus, contact angles of water drops showed that they possess very high hydrophobicity. Their excellent regularity was corroborated by AFM microscopy. The frequencies of the stretching CH2 infrared modes indicate the presence of alkyl chains mainly in the trans/trans conformation. Additionally, optical ellipsometry and quartz microbalance measurements enabled us to estimate the thickness of the films. The results, as a whole, are in good agreement with the formation of densely packed monolayers.     

Ellipsometric studies related to surface-enhanced infrared absorption

Infrared ellipsometry has been applied to determine the refractive index n and the absorption index k as well as the thickness of metal island films causing surface enhanced infrared absorption (SEIRA). The results from numerous films prepared in several campaigns are presented. For films of a nominal thickness of 6 nm, k is found to range from 0 to 3, while n ranges from 5 to 8. Among these two optical constants a close correlation is observed. Layers suitable for SEIRA exhibit an unusual spectral feature whose origin is explained.     

Self-assembled monolayers of alkylsiloxanes on SrTiO3 substrates

The formation and structure of alkyltrichlorosilanes on several types of SrTiO₃ substrates have been studied. The silanes adsorb spontaneously from a hexadecane solution and form monolayers on all the substrates used. Characterization has been performed by atomic force microscopy, wettability, angle resolved X-ray photoelectron spectroscopy, reflection absorption infrared spectroscopy, and spectroscopic ellipsometry. It was found that highly ordered and densely packed monolayers were formed below a certain temperature.     

Condensation and Densification Mechanism of Sol-Gel TiO2 Layers at Low Temperature

Titanium dioxide films were prepared by the sol-gel process at low temperature under dry and humid conditions. The effect of solution parameters on the solution reactivity was studied. Layers deposited on silicon wafers as well as on organic substrates were characterized by Fourier Transform infrared spectoscopy and ellipsometry. Their mechanical quality was also tested. Results reported in this paper demonstrate the influence of the temperature and humidity during post-deposition treatment on the network development and densification. A suitable balance between both mechanisms led to hard and flexible coatings compatible with the thermal expansion of polymer substrates.     

Multilayer assembly and patterning of poly(p-phenylenevinylene)s via covalent coupling reactions

Poly(p-phenylenevinylene)s with amines and pentafluorophenyl esters on side chains were synthesized and assembled on solid substrates by sequential layer-by-layer (LBL) deposition. This approach enables the creation of robust multilayer thin films via in-situ covalent coupling reactions between successive layers. The buildup of the multilayers was followed by UV/vis absorption spectroscopy and ellipsometry. The observed complex assembly behavior suggests that both covalent and hydrogen-bonding interactions are involved in the formation of multilayer films. The organized structure and surface morphology of resultant multilayers were investigated by reflectance Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. This covalent LBL method was further applied to generate conjugated polymer micropatterns using microstamped self-assembled monolayers as templates.     

Porogen approach for the fabrication of plasma-polymerized nanoporous polysiloxane films

Nanoporous polysiloxane films were fabricated by plasma polymerization of hexamethyldisiloxane mixed with cyclohexane under different conditions. The pores were generated through the elimination of carbonaceous aggregates (porogen) by annealing at 600 degrees C. Results of spectroscopic ellipsometry, Fourier transform infrared spectroscopy, and positron annihilation lifetime spectroscopy suggest that not only film porosity but also average pore size depends on the amount of the decomposable porogen. The pore size was controllable in a range between 0.6 and 1.0 nm in radius by proper selection of the substrate temperature and precursor composition.     

Infrared-optical properties of vapour-deposited metal films

Spectroscopic infrared ellipsometry was applied to determine the optical constants of thin metal layers deposited on dielectric substrates such as glass or CaF(2). The layers were produced by evaporating gold or silver in a vacuum, and the coverage, that is the deposited mass per area, was chosen in the range 80-1200 mg m(-2) for gold, which refers to thicknesses in the lower nanometer range; in the case of the specifically lighter silver, about half the coverage was applied. At low coverage a metal island structure is obtained, which gives rise to surface-enhanced infrared absorption (SEIRA). Depending on the coverage, the deposited films exhibit either dielectric or metallic optical properties. Atomic force microscopy and conductivity measurements complement the spectroscopic observation.