IR Ellipsometry Research Articles

Determination of the mid-IR optical constants of water and lubricants using IR ellipsometry combined with an ATR cell

The mid-IR bulk optical constants for the lubricants Fomblin, Demnum S200, 2001A and Krytox 16256N, as well as water, are determined by infrared ellipsometric analysis using an Attenuated Total Reflection (ATR) cell. The water optical constants closely match those found in other studies. In addition, the real and imaginary parts of the dielectric functions for all the fluids are self-consistent under Kramers–Kronig transformation.

IR ellipsometry studies of polymers and oxygen plasma-treated polymers

An infrared variable angle spectroscopic ellipsometer (IR-VASE) was used to study organic polymers in the infrared (2.5–14 μm wavelength) spectral region. For the analysis of thin film polymers IR spectroscopic ellipsometry has greater sensitivity over traditional FTIR spectroscopy providing an exciting way to characterize these materials optically. The IR-VASE used in this study is of high accuracy, rotating polarizer, rotating compensator ellipsometer that uses an FTIR spectrometer as a light source. The IR-VASE was used to measure the infrared optical constants of various polymers in both solid and liquid form. These optical constants were then used to model the percentage of water in a thin film of gelatin and the percentage of residual solvent in a thin film of silicone. In addition, the IR-VASE provided a sensitive measurement of silicone chemistry and chemical changes caused by exposure to an oxygen plasma.

Adhesion improvement of plasma-deposited silica thin films on stainless steel substrate studied by x-ray photoemission spectroscopy and in situ infrared ellipsometry

Adhesion of plasma-deposited SiO2 on stainless steel is shown to be enhanced after Ar, N2, and NH3 plasma pretreatments of the substrate. Adhesion is related to the chemical bonding at the interface. Therefore, it is studied by x-ray photoemission spectroscopy (XPS) and in situ IR ellipsometry performed on very thin films (≈30 Å thick). IR ellipsometry reveals the removal of adsorbed hydrocarbons on the metallic surface by all plasma treatments. XPS measurements show the removal of NOx species, related to the sample electropolishing, using N2 and NH3 plasmas; in contrast, Ar has practically no effect in this case. Plasma-induced modifications are not limited to surface cleaning. Some nitrogen is incorporated in the substrate after N2 and NH3 plasmas. In particular, nitrogen is found bonded to Si and to Cr after NH3 and N2 treatments, respectively, we interpret the adhesion enhancement by hardening of the substrate surface region in the case of N2 plasma, while, in the case of NH3, it may be related to the formation of silicon nitride. Cr–N–Si linkages between the substrate and the film may also contribute to adhesion improvement. We also observed changes in the early stages of the film growth. A higher sticking coefficient of dissociated species on the surface is revealed after plasma pretreatment. Narrowing of the Si 2p peak in XPS spectra observed in pretreated samples is attributed to a better ordered SiO2 structure at Si sites in terms of bond angles and/or lengths. In contrast, both IR ellipsometry and XPS show no change at O sites.

Carbon nitride thin films prepared by reactive r.f. Magnetron sputtering

Nitrogenated amorphous carbon thin films (CN x ) were prepared by reactive r.f. magnetron sputtering from a graphite target in a nitrogen containing plasma (up to 4.4 at%) on Si substrates. All main controllable deposition conditions (such as base and deposition pressure, power at target, target-substrate distance, substrate bias voltage) were kept constant throughout the series of experiments and the only variable parameter was the nitrogen flux (0–2 sccm). In situ spectroscopic ellipsometry (SE) in the energy region 1.5–5.5 eV in combination with Bruggeman's effective medium theory analysis was used per iterative deposition layer to monitor the film thickness and diamond character. The C:N chemical bonding characteristics of the films were also examined with the new Fourier transform IR ellipsometry technique. It was found that the films have a mixed character composed of constituents (phases) of sp 3 and sp 2 type bonding. The film composition in terms of sp 3, sp 2 and void concentration as a function of film thickness and nitrogen concentration in the plasma was determined. The plasma nitrogen concentration seems to directly affect the film deposition rate and diamond character of the films by favoring sp 3 bond formation.

Illustration of surface crosslinking of different polymers treated in argon plasma

AbstractThe influence of argon plasma on polycarbonate (PC) and poly(propylene) (PP) has been studied in terms of structural changes and reaction mechanisms. In situ UV‐visible ellipsometry reveals formation of a surface layer with a higher refractive index than the untreated polymer. The increase in the refractive index is attributed to polymer densification, which is assigned to crosslinking. However, a decrease in the average molecular weight of the PC is also observed and two populations of macromolecules of different size are detected by light scattering measurements, revealing a competition between crosslinking and degradation. The reaction mechanisms were investigated using Nuclear Magnetic Resonance (1H NMR) and in situ IR ellipsometry. Degradation is caused by carbonate bond breaking, while crosslinking seems to be related to a decrease in methyl groups. The ellipsometry results are correlated with amorphous phase extraction and 1H NMR analysis of modified PP. The crosslinking mechanism involves the elimination of methyl groups, and also the formation of exomethylenic bonds. Spectroscopic ellipsometry appears to be a valuable tool to study the interaction between a plasma and a polymer, as UV‐visible ellipsometry is sensitive to structural changes in the polymer, while IR ellipsometry detects the appearance and disappearance of chemical groups in a surface layer.

Study of the SiO 2 layer growth by the ML-ALE method

The investigation of the molecular layering (ML)-ALE synthesis of the SiO 2 nanolayers on the Si substrate was performed by use of IR multiple transmission spectroscopy, ellipsometry and contact angle measurements. SiCl 4 and H 2O were used as precursors. The correlation between structure of the native oxide on the Si surface, contact angle and the coverage of the surface by the SiO 4\\2 tetrahedrons was established. The affect of the composition and the thickness of the SiO 2 layer, as well as its hydroxylation, on the kinetics of the SiO 2 layers growth was found.

In situ spectroellipsometry study of the crosslinking of polypropylene by an argon plasma

The exposure of polypropylene to an argon plasma is studied by in situ spectroscopic ellipsometry in the UV-visible and in the IR range. The results are correlated with amorphous phase extraction and proton nuclear magnetic resonance (NMR) analysis. UV-visible real-time ellipsometry reveals the formation of an overlayer with higher refractive index, which is attributed to a densification of the polymer, and thus to crosslinking at the surface. This assumption is confirmed by the decrease in the extracted mass after treatment. The crosslinking mechanism involves the elimination of methyl groups, and also the formation of exomethylenic bonds, as shown by NMR and IR ellipsometry. Spectroscopic ellipsometry appears as a valuable tool to study the interaction between a plasma and a polymer, as UV-visible ellipsometry is sensitive to structural changes in the polymer, while IR ellipsometry can detect the appearance and disappearance of chemical groups in a surface layer.

In situ IR ellipsometry study of the adhesion and growth of plasma deposited silica thin films on stainless steel substrates

The adhesion and growth of plasma deposited a-SiO 2 films on stainless steel substrate were studied. The adherence, measured by the microscratch test, is shown to be enhanced by N 2, NH 3 and O 2 plasma treatments prior to deposition of SiO 2. In situ IR ellipsometry suggests that these plasmas remove hydrocarbon contaminants from the surface, resulting in better adhesion. During the growth of a-SiO 2 thin films a vibrational mode near 1180 cm −1 is observed, in addition to the usual mode near 1070 cm −1. This effect is discussed and correlated with the viriations of the position of the longitudinal optical mode as a function of the film thickness. An interpretation related to the presence of interface stress or disorder in the layer is proposed. More generally, IR ellipsometry is found to be a good tool to analyse plasma surface pretreatment and growth of thin films.

Improvement of the adhesion of silica layers to polypropylene induced by nitrogen plasma treatment

We investigated the influence of plasma pretreatment in Ar and in an ArN 2 mixture on the adhesion of plasma-deposited silica layers to polypropylene (PP). In situ UV-visible ellipsometry measurements enabled us to detect cross-linking at the polymer surface, probably induced by UV radiation emitted by the plasmas. The cross-linking mechanism involves methyl group abstraction, as recorded by in situ IR ellipsometry. The adhesion of silica to PP, estimated from the micro-scratch test, is improved by both kinds of treatment. In the case of Ar, the improvement is attributed to cross-linking, and possibly to surface activation by radical formation. In addition, a significant nitrogen grafting is observed after ArN 2 treatment; amine and imine vibrational modes were detected using IR ellipsometry, and contact angle measurements show a significant increase in the electron-acceptor and especially electron-donor components of the surface free energy. Better adhession after ArN 2 treatment than after simple Ar treatment may be attributed to CNSi bond formation or to acid-base interactions between the nitrogenated polymer surface and silica.

Surface modifications of polycarbonate (PC) and polyethylene terephtalate (PET) by cold remote nitrogen plasma (CRNP)

Industrial PC and PET specimens have been treated by CRNP with pure N 2 or with N 2O 2 mixtures in order to modify their adhesion and surface properties. Scanning electron microscopy (SEM), IR ellipsometry, XPS analyses and wettability measurements reveal interesting modifications at the surface of these materials which extent mainly depends on the conditions used during the plasma treatment, a key parameter being the amount of O 2 introduced in the mixture. IR ellipsometry gives evidence of the presence of new functional groups after CRNP exposure at 1270 cm −1 (PC and PET) and in the 1650–1800 cm −1 (PC) or 1700–1750 cm −1 (PET) regions. Assignments to vibrational stretching modes of CO, CN and CO bonds are discussed. XPS also allows identification of new surface groups associated with O and N species but when O 2 is present in the plasma, no N species is present on PC. Moreover, during the XPS analyses, the surface of treated PC is progressively modified by desorption of small oxygenated fragments which presence was induced by the plasma treatment. Wettability of treated PC increases initially very sharply with the duration of the CRNP treatment and then reaches a constant value. Presence of 10% O 2 in the N 2O 2 mixture is detrimental for high wettability and also induces surface roughness as detected by SEM.

Adhesion mechanisms of silica layers on plasma-treated polymers. Part I. Polycarbonate

We have studied the influence of plasma treatment in various gases (Ar, NH3, N2) on the surface modification of polycarbonate (PC) and on the adhesion of plasma-deposited silica layers to PC. Surface modification was investigated using in situ IR ellipsometry, X-ray photoelectron spectroscopy, and contact angle measurements interpreted in terms of electron-acceptor/ donor groups. In addition, in situ UV-visible ellipsometry enabled crosslinking analysis. Due to UV photon emission, Ar plasma treatment induced crosslinking as well as photo-Fries rearrangements and related reactions, thus creating phenolic groups. These groups are acidic sites and are likely to react with the electronegative oxygen atoms of silica. Adhesion of silica to PC, as measured by the micro-scratch test, was enhanced by Ar plasma treatment. The improvement is attributed to the crosslinking of PC, on the one hand, and to acid-base interactions and/or covalent bonding between PC and silica, on the other hand. A further improvement in a...

Adhesion mechanisms of silica layers on plasma-treated polymers. Part II. Polypropylene

We have investigated the influence of plasma pretreatment in various gases (Ar, NH3, N2) on the adhesion of plasma-deposited silica layers to polypropylene (PP). UV-visible ellipsometry measurements enabled us to detect crosslinking at the polymer surface. All treatments, and especially the Ar-diluted N2 treatment, induced crosslinking in the surface layer. The adhesion of silica to PP, as measured by the micro-scratch test, was improved by every treatment. In the case of Ar, the adhesion improvement is attributed to crosslinking, and possibly to surface activation by radical formation. In addition, surface functionalization was studied by in situ IR ellipsometry, X-ray photoelectron spectroscopy, and contact angle measurements. N2 plasma treatments induced a significant nitrogen grafting (17.5 at.% concentration after dilute N2 treatment, in the form of amines and imines), leading to an increase in both the electron-acceptor and the electron-donor components of the surface free energy. In contrast, dilute NH3 plasma treatment seemed to result in weak bonding of the nitrogen atoms to the surface, as deduced from a comparison of measurements performed in situ after this treatment, after aging, and after an additional SiH4 plasma treatment. The dilute NH3 treatment, performed after Ar treatment, did not further improve the adhesion of silica to PP, while an additional improvement was obtained with dilute and pure N2 treatments. This may be attributed to C-N-Si bond formation or to acid-base interactions between the nitrogenated polymer surface and silica.

Chemical stability of hydrogen-containing boron nitride films obtained by plasma enhanced chemical vapour deposition

The purpose of this paper is the investigation of the dehydrogenation kinetics of boron nitride films during thermal annealing. BN x :H films on silicon substrates were prepared by remote plasma enhanced chemical vapour deposition at 473 K using a mixture of borazine and helium. IR spectroscopy and ellipsometry were used to characterize the film properties and composition. The films contain a certain amount of hydrogen in BH and NH bonds. The breakage kinetics of these bonds is different. The breakage of NH bonds determines the hydrogen annealing kinetics at 973–1073 K. The low-temperature annealing (673–873 K) of BH bonds is sensitive to the generation of hydrogen from NH bonds. Heat treatment leads to ordering of the films.

Microstructural modifications in diamond-like carbon thin films caused by high energy ion irradiation

The effect of high energy (85 MeV) nickel ions on diamond-like carbon thin films, deposited by microwave discharge of acetylene and methane gases mixed with hydrogen, has been investigated. IR, Raman and spectroscopic ellipsometry studies of irradiated and as-deposited films show large changes in their microstructural features. The effects of irradiation on films prepared from acetylene-hydrogen and methane-hydrogen gas mixtures are very different. Such a behaviour is proposed to be due to the microstructural variations in the as-deposited films.

Surface-near analyses of ultra thin silicon nitride layers by NRA, channeling RBS, FT IR ellipsometry and AFM

Homogeneous ultra thin silicon nitride layers (SiN x layers) close to the surface have been produced by 10 keV 15 N 2 + molecular ion implantation and an ion current density of 10 μA/cm 2 , into single crystal silicon at room temperature. Stoichiometric SiN x layers with thicknesses of about 28 nm (analyzed by NRA) were obtained at fluences of 1.5 x 10 17 at/cm 2 . NRA anlayses of samples annealed by EB-RTA at T = 1150°C for 15 s indicated that the N/Si ratio and the layer thickness did not change drastically. FT IR ellipsometry analyses indicated the existence of Si 3 N 4 bonds in as-implanted specimens. A disordered Si layer (d-Si, typically 15 nm thick) underneath the implantation region caused by the ion implantation was found by channeling RBS analyses. The d-Si layer partly recrystallized during EB-RTA showing a thickness of 6 nm afterwards. The SiN x layers showed no decomposition and detachment after EB-RTA. Due to EB-RTA, however, the smooth surface of the as-implanted specimens changed into a surface with remaining whisker-like structures surrounded by circular recesses as shown by AFM analyses. A model for the growth of these whisker-liker structures caused by low energy ion implantation and EB-RTA is presented on the basis of the thickness of the SiN x layer, the existence of the d-Si layer and the special annealing process.

Magnetron sputtered carbon nitride films

Carbon nitride (CN x ) thin film deposition was undertaken in a closed-field, unbalanced magnetron system. Reactive deposition utilised a graphite target in an argon/nitrogen plasma. The main deposition parameters investigated were magnetron current, total gas pressure and nitrogen gas partial pressure. The films deposited were examined using IR spectroscopy, ellipsometry, and X-ray diffraction (XRD). Analysis of the results yielded the following characteristics: refractive index of nitrogen-containing films was reduced below that for pure carbon films but had little dependence on substrate bias and total gas pressure. It was, however, strongly dependent on the magnetron current at which the film was deposited. IR spectroscopy showed the transmittance of the films for given deposition conditions to be a monotonic function of nitrogen content. The deposition conditions themselves also had a significant effect, in particular the magnetron current. In addition, C N bonding was evident from the absorption bands observed. Structural analysis by means of XRD showed that the amorphous nature of the films with some degree of graphitic content depended more on the deposition parameters than on the nitrogen concentration. Increased graphitic-type structure was observed at low magnetron currents, high system pressure and large (negative) substrate bias.

Determination of the composition and thickness of borophosphosilicate glass films by infrared ellipsometry

A quantitative analysis of IR ellipsometry spectra of borophosphosilicate glass (BPSG) thin films deposited on silicon wafers is presented. The film thickness is determined with a standard deviation of 5.8 nm using a fitting procedure based on only two free parameters. A simple linear analysis of the dependence of the ellipsometric measurements upon film composition provides standard deviations of 0.04 and 0.12 wt % on boron and phosphorus content, respectively.

Mueller matrix analysis of infrared ellipsometry

A Mueller analysis has been done of IR ellipsometry performed with imperfect optical components. Equations linking experimental and calculated Fourier coefficients have been derived and consistently solved. Correction routines for permittivity measurements are demonstrated and discussed with gold and SrTiO(3) as examples. It is shown that such effects as interferometer polarization, detector dichroism, transmission, and phase changes in polarizers can be calculated and effectively removed from the spectra. The problems of calibration and multiple reflections between IR polarizers are discussed, and error propagation in permittivity measurements is analyzed.

In situ characterization of plasma-deposited a-C:H thin films by spectroscopic infrared ellipsometry

A computer-controlled rotating polarizer ellipsometer, operating in the infrared spectral region between 3.00 and 3.75 μm, has been developed for in situ characterization of amorphous hydrocarbon (a-C:H) thin films, deposited from methane in a rf plasma-enhanced chemical vapor deposition reactor. Spectroscopic IR ellipsometry permits insight into the chemical bonding structure of a-C:H coatings by the nondestructive detection of infrared stimulated C:H stretch vibrations. It is shown that the sp2CHx/sp3 CHx ratio, the content of bonded hydrogen, the infrared linewidth, and the real refractive index of the films depend on the negative self-bias voltage, which is formed at the samples during the deposition process. A transition from a-C:H films with polymerlike properties to hard a-C:H films was attained at a self-bias voltage of approximately −75 V.

Single layers of inorganic extended lattices formed at Langmuir-Blodgett templates

Zirconium octadecylphosphonate monolayer and multilayered films have been prepared and characterized by Brewster angle microscopy (BAM), attenuated total reflection-Fourier transform IR spectroscopy, ellipsometry, X-ray diffraction and X-ray photoelectron spectroscopy (XPS). BAM data reveal the rigid nature of a Langmuir monolayer of octadecylphosphonic acid compressed on a Zr 4+ ion subphase; therefore a deposition procedure combining Langmuir-Blodgett (LB) and self-assembly techniques has been developed. First, an octadecylphosphonic acid organic template is prepared by the LB technique and then Zr 4+ ions are self-assembled onto the template from solution. Finally, a capping layer of octadecylphosphonic acid is transferred to the zirconated template to complete the bilayer assembly. The asymmetric methylene stretch frequency υ a(CH 2) appears at 2918 cm −1 and possesses a full width at half-maximum of 20 cm −1 for the template layer, a bilayer and a multilayered film. The position and width of the ν a(CH 2) band indicates that close-packed well-ordered films are formed. Ellipsometry demonstrates a layer-by-layer deposition in multilayered films, while X-ray diffraction proves that the films are indeed layered, possessing a d spacing of 52 Å. XPS analysis of the template layer shows a Zr:P ratio of 1:1 within 10% error, while the bilayer and multilayered films possess a Zr:P ratio of 1:2 consistent with the stoichiometry of the bulk zirconium phosphonates.