Spectroscopic Phase-modulated Ellipsometer Research Articles

Optical properties of TiO2 films made by air oxidation of Ti

AbstractIn this work spectral behaviour of complex refractive index of thermal oxidized titanium films deposited onto Si (100) substrate were investigated by spectroscopic ellipsometry in the range of 1.5‐4.77 eV. The films were subjected to annealing in air at temperature 750 °C for 7 and 25 hours. As it follows from our studies, air oxidation of titanium resulted in coexisting of different phases in the film, and dependence of their optical and structural properties upon z‐coordinate (upon film depth). The spectral dependences of complex refractive index and the changing of film thickness due to cumulative annealing were determined from pseudodielectric function (PDF) ‐ ε(E) spectra, for which data acquisition was performed by Spectroscopic Phase Modulated Ellipsometer (SPME). Complicated phase composition of films was revealed by using multi‐layers conception of investigated film and applying oscillator's model and EMA approximation for description of spectral dependence of dielectric function of film or its separate layers. Raman spectra were used to distinguish phase components in the films. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Vacuum ellipsometry as a method for probing glass transition in thin polymer films

A vacuum ellipsometer has been designed for probing the glass transition in thin supported polymer films. The device is based on the optics of a commercial spectroscopic phase-modulated ellipsometer. A custom-made vacuum chamber evacuated by oil-free pumps, variable temperature optical table, and computer-based data acquisition system was described. The performance of the tool has been demonstrated using 20-200 nm thick poly(methyl methacrylate) and polystyrene films coated on silicon substrates at 10(-6)-10(-8) torr residual gas pressure. Both polymers show pronounced glass transitions. The difficulties in assigning in the glass transition temperature are discussed with respect to the experimental challenges of the measurements in thin polymer films. It is found that the experimental curves can be significantly affected by a residual gas. This effect manifests itself at lower temperatures as a decreased or even negative apparent thermal coefficient of expansion, and is related to the uptake and desorption of water by the samples during temperature scans. It is also found that an ionization gauge--the standard accessory of any high vacuum system--can cause a number of spurious phenomena including drift in the experimental data, roughening of the polymer surface, and film dewetting.

Optical characterizations of complete TFT–LCD display devices by phase modulated spectroscopic ellipsometry

A commercially available phase modulated spectroscopic ellipsometer (PMSE) has been used to characterize a full thin film transistor–liquid crystal display (TFT–LCD) structure, including the glass substrates coated with transparent conducting indium tin oxide (ITO) layers, the twisted liquid crystal (LC) layer sandwiched in between, and the amorphous silicon (a-Si) TFT device which controls the luminance of a pixel. Due to its unique optical design, PMSE presents an unparallel capability to measure very accurately ultra thin films on transparent substrates as often found in display applications. Results show that the ITO layer is inhomogeneous in depth, corresponding to a graded microstructure. In addition, strong uniaxial anisotropy was determined for the liquid crystal device over the entire measured spectral range. Finally, doping effects on the optical properties of the a-Si layer of the TFT device were also measured.

Characterization of TFT-LCD and OLEDs Devices by Phase Modulated Spectroscopic Ellipsometry for Display Applications

ABSTRACTSpectroscopic ellipsometry is one of the most accurate and reliable optical technique to characterize polymers, liquid crystals (LCs) and organic light emitting diodes (OLEDs). Because these devices are formed by complex structure including optical anisotropy, absorbing and graded materials, the correct use of spectroscopic ellipsometer required a combination of the proper choice of hardware and the appropriate ellipsometric model. In this work, we presents ellipsometric results obtained by a commercially available phase modulated spectroscopic ellipsometer (PMSE) on a full TFT-LCD structure characterized from UV to NIR. As expected, strong anisotropy and inhomogeneous optical properties were found respectively on LCs and ITO materials. We also presents measurements of film thickness and optical constants of each layer constituing an OLED structure

Spectroscopic ellipsometry of powdered CuInS 2 with nanowires

The powdered samples of CuInS 2 obtained by a chemical synthesis method and contained either thread-like or belt-like nanowires were studied by a spectroscopic phase-modulated ellipsometer in the photon energy range from 0.8 to 6 e V at room temperature. The samples with thread-like and belt-like nanowires were found to have different pseudodielectric function, which was closer to that of bulky single crystalline CuInS 2 in the case of belt-like nanowires. The same value of energy gap revealed by incoherent ellipsometric approach to bulky samples and samples with nanowires was accounted for the presence of some fraction of bulky phase in the nanostructured samples. Reliability of the obtained data and relationship between the ground state shape of the nanowires and the electronic spectrum were discussed. It was substantiated that the obtained data provide a first realistic glimpse on electronic spectrum of nanostructured CuInS 2.

Polarized transmission intensity studies at and below energy gap of wide-gap single crystalline CaGa 2S 4

Polarized transmission intensity studies have been performed on wide-gap single crystalline CaGa 2S 4 at room temperature. The effective value of the partial birefringence in (100) plane is determined to be comparatively large varying from 0.14 to 0.50 with increasing photon energy from 1.9 to 3.5 eV. Four structures, which were preliminary ascribed to the critical points of interband optical transitions are displayed by ellipsometric examination with the aid of a spectroscopic phase-modulated ellipsometer in the spectral range 1.5–6.3 eV. These points, together with band gap singularity, are believed to largely contribute into the optical properties below the energy gap of biaxial CaGa 2S 4. The pseudo values of two major components of dielectric function tensor of CaGa 2S 4 are given for the first time.

Spectroscopic ellipsometry of multilayer dielectric coatings

Multilayer dielectric coatings deposited by e-beam evaporation have been characterised by the phase modulated spectroscopic ellipsometer (PMSE). Measurements have been done on various multilayer thin films devices e.g., high reflectivity mirror, narrow band filter, beam combiner, beam splitter, etc. consisting of several bilayers of TiO 2/SiO 2. Results have been shown here for the first two samples. The measured Ellipsometry spectra are fitted with theoretical spectra generated assuming appropriate models regarding the sample structures. Optical constants of the substrates and the SiO 2 films have been supplied and trial dispersion relations have been used for the optical constants of the TiO 2 layers. The fittings have been done by minimising the squared difference ( χ 2) between the measured and calculated values of the ellipsometric parameters ( ψ and Δ) and accurate information have been derived regarding the thickness and refractive indices of the different layers.

Investigation of a half-wave method for birefringence or thickness measurements of a thick, semitransparent, uniaxial, anisotropic substrate by use of spectroscopic ellipsometry

A half-wave method of measurement of wafer birefringence that is based on interference fringes recorded from a uniaxial wafer by use of a standard phase-modulated spectroscopic ellipsometer is investigated. The birefringence of uniaxial wafers is calculated from the extremal points in the recorded oscillating intensities. A formalism is developed to incorporate the change in birefringence with wavelength as a correction factor. The correction explains the overestimation of the birefringence from previous similar research on thick uniaxial sapphire substrates. The enhanced derivative of the birefringence that is due to polarization-dependent intraconduction band transitions is detected. Furthermore, for well-characterized wafers it is shown that this method can be used in wafer-thickness mapping of 4H-SiC and similar uniaxial high-bandgap semiconductors.

Zone-averaged method to minimize polarizer and analyzer imperfections in a phase-modulated spectroscopic ellipsometer

We have theoretically investigated the effects of polarizer and analyzer imperfections in a phase-modulated spectroscopic ellipsometer adopting a photoelastic modulator. Jones matrices having off-diagonal elements have been introduced to represent the imperfections of the polarizer and analyzer, and the effects of the imperfections were analytically derived to the first-order approximation. It was found that the effects of the imperfections could be completely eliminated by the zone average. Also, the effects of an incorrect setting of the modulation amplitude were found to be largely reduced by the zone average.

Construction and Performance of a Fourier-Transform Infrared Phase-Modulated Ellipsometer for In-Process Surface Diagnostics

A Fourier-transform infrared phase-modulated spectroscopic ellipsometer (FT-IR PMSE) has been constructed by insertion of a grid polarizer as an analyzer in front of an infrared detector in addition to a conventional setup for phase-modulated reflection absorption spectroscopy (PMRAS). Although ellipsometric parameters Ψ and Δ are subject to the effect of mirrors placed between a photoelastic modulator and the analyzer, this can be canceled out in the derivation of optical density of the measured surface. The performance of the ellipsometer has been demonstrated in the characterization of thermally oxidized Si surfaces. It is shown that the imaginary part of the optical density spectrum has a higher signal-to-noise ratio than the real part, which corresponds to the spectrum obtained by PMRAS, and a linear relationship with SiO2 thickness. By this method the presence of a fluorinated layer has been detected on CF4 plasma-treated Si surface.

Real-time monitoring of resonant-tunneling diode growth using spectroscopic ellipsometry

We report in situ growth monitoring of pseudomorphic AlAs/In0.53Ga0.47As/InAs resonant-tunneling diodes (RTDs) using spectroscopic ellipsometry. A phase-modulated spectroscopic ellipsometer was triggered synchronously with 20 rpm substrate rotation of the III–V molecular beam epitaxy system. Postgrowth data analysis yielded layer thicknesses which were compared with the electrical characteristics of the processed RTDs. A sample set of 28 RTDs included designed variations in eight growth variables and provided for a linear model fit to the RTD peak current, peak voltage, and valley current I–V characteristics. The I–V characteristics are sensitive to the thickness of the AlAs, InAs, and InGaAs layers.

Real-Time monitoring of III-V molecular beam epitaxial growth using spectroscopic ellipsometry

We report the real-time monitoring of monolayer thickness changes in AlAs and GaAs layer growth on rotating GaAs substrates using spectroscopic ellipsometry (SE). A phase-modulated spectroscopic ellipsometer was integrated with a III-V MBE system by triggering spectral acquisition synchronously with substrate rotation. Absolute thickness accuracy was verified using ex situ SE measurement. Reasonable agreement was also obtained between in situ growth rate measurements by SE and reflection high energy electron diffraction. The precision and speed of this method appears suitable for real-time control of quantum devices, such as resonant-tunneling diodes.