Modeling of optical properties of nanogratings (sub-wavelength gratings) is of scientific and technological interests
for (i) application of nanogratings as new artificial effective materials with unusual optical properties and
(ii) application in non-destructive optical testing of nanogratings using optical spectroscopic ellipsometry and
polarimetry. This paper deals with anisotropic lamellar nanogratings described by Effective Medium Approximation
(EMA). Analytical formulae for effective medium optical parameters of nanogratings from arbitrary
anisotropic materials are derived using approximation of zero-order diffraction mode. The method is based on
Rigorous Coupled Wave Analysis (RCWA) combined with proper Fourier factorization method. Good agreement
between EMA and the rigorous model is observed, where slight differences are explained by the influence
of higher Fourier harmonics in the nanograting. Analytical spectral formulae for ordinary and extraordinary
effective optical functions are derived for nanogratings consisting of material described by Sellmeier, damped
harmonic oscillator, and Drude formulae. Spectral origin for birefringence of dielectric nanogratings and linear
dichroism for absorbing ones is discussed.
The spectroscopic Mueller polarimetry in conical diffraction was applied for the metrological characterization of
the one-dimensional (1D) holographic gratings, used for the fabrication of nanoimprint molding tool. First we
characterized the master grating that consists of patterned resist layer on chromium-covered glass and then we
studied replicated diffraction grating made of nickel. The experimental spectra of Mueller matrix of both samples
taken at different azimuthal angles were fitted with symmetric trapezoidal model. The optimal values of gratings
critical dimensions (CDs) and height were confirmed by atomic force microscopy (AFM) measurements. The
calculated profiles of corresponding master and replica gratings are found to be complementary. We showed that
Mueller polarimetry in conical diffraction, as a fast and non-contact optical characterization technique, can provide
the basis for the metrology of the molding tool fabrication step in the nanoimprint technique.
We used full Mueller polarimetry in conical diffraction geometries to characterize 1D holographic optical gratings etched in bulk silica with a patterned photoresist layer. We studied four different samples corresponding to different stages of etching, with a Mueller polarimeter based on ferroelectric liquid crystals, operated in the visible. Two samples were also characterized by standard spectroscopic ellipsometry (SE) in the UV-VIS range (300-800 nm). The measured spectra were fitted with a Rigorous Coupled Wave Analysis code with different models of grating profiles. With the Mueller spectra the model adequacy could be assessed from the stability of the optimal values of the fitting parameters when the azimuthal angle was varied. The conclusions were found to be in agreement with AFM images of the sample, while the fits of the SE data were too poor to provide any information in this respect. A key issue for process control is resist-silica interface localization, a difficult task due to the low index contrast for these two materials. In fact, strong correlation occurs between resist and silica thicknesses when SE spectra, taken in the usual planar diffraction geometry, are fitted. Our approach clearly reduces such parameter correlations, leading to a reliable localization of this interface.
We designed and built Matrix Distributed ECR (MDECR) PECVD reactor dedicated for dielectric filters deposition and equipped it with multiple sensors for process control. Planar matrix geometry of plasma source is based on electron cyclotron resonance effect at 2.45 GHz microwave frequency and provides scalability of the deposition on large area substrates. High (up to 5 nm/sec) deposition rate obtained due to high dissociation efficiency and careful design of the gas injection system. Optical emission spectroscopy, quadrupole mass-spectrometry and spectroscopic and multi-channel kinetic ellipsometry are installed for in-situ studies and control of the film deposition. We performed studies of the nature of high-density plasma discharge in silane, oxygen and nitrogen mixture and correlated its properties with optical and physical properties of deposited materials. To demonstrate the capabilities, a wide band gradient index antireflection coating on glass was realized by deposition of SiOxNy alloy thin films. The predefined variation of an index in a profile is obtained by changing the flows of precursors. Real-time control is performed with multi-channel kinetic ellipsometry.
Spectroscopic Ellipsometry is widely used to characterize 1D gratings in the usual planar diffraction geometry, with grating grooves perpendicular to the incidence plane. The Jones matrix of the grating is then diagonal, and it can be measured properly by any conventional ellipsometer. On the other hand, if the grating is measured at different azimuthal angles, i.e. in conical diffraction geometries, its Jones matrix is no longer diagonal. As a result, additional information is available about the grating groove shape, but a generalized ellipsometer, or a Mueller polarimeter, are necessary to retrieve this information completely. We used this approach with photoresist gratings deposited on Si for nominal CDs down to 70 nm. The instrument was an original Mueller polarimeter based on ferroelectric liquid crystals, and operated in the visible. The measured spectra were fitted with a RCWA code suitable for conical diffraction, with rectangular and trapezoidal profiles. This approach proved to greatly reduce the parameter correlations observed with standard SE for very small CDs. The relevance of the trapezoidal profile can be established, and top and bottom CDs can be "disentangled" much more easily than with standard planar diffraction geometry.
We present a multispectral polarimetric imaging system well suited for complete Mueller matrix microscopy. The source is a spectrally filtered halogen light bulb, and the image is formed on a fast CCD camera The light polarization is modulated before the sample and analyzed after the sample by using nematic liquid crystal modulators.. The whole Mueller matrix image of the sample is typically measured over 5 seconds for a good signal-to-noise ratio. The instrument design, together with an original and easy-to-operate calibration procedure provides a high polarimetric accuracy over wide ranges of wavelengths and magnifications. Mueller polarimetry provides separate images of scalar and vector retardation and dichroism of the sample, together with its depolarizing power, while all these effects do contribute simultaneously to the contrasts observed in standard polarized microsopy. Polarimetric images of several samples, namely an unstained rabbit cornea, a picrosirius red stained hepatic biopsy, and a rat artery specifically stained for collagen III are shown and discussed
We present a first evaluation of the interest of the degree of polarization (DOP) imaging technique for early detection of cervical dysplasia. A set of ten ex vivo samples of cervix have been examined, just after surgical extraction, with an DOP imaging system comprising two linear polarizers, a liquid crystal based polarization rotator, and fast CCD camera. Routine histological examination revealed that for all samples but one, dysplasia was present only in the inner part of the cervix, which cannot be imaged with our current setup. On the other hand, the only sample exhibiting dysplasia in its outer region did show nonzero DOP with, however, a somewhat loose correlation between the DOP and the histological mapping of the dysplasia.
We present a new polarimetric imaging system based on liquid crystal modulators, a spectrally filtered white light source and a CCD camera. The whole Mueller matrix image of the sample is measured in around 5 seconds in transmission mode. The instrument design, together with an original and easy-to-operate calibration procedure
provides a high accuracy (better than 1.5% for the normalized Mueller matrix) over a wide spectral range. The data can be processed with different algorithms. Results on hepatic biopsies with different grades of fibrosis are presented.
Here is presented a new method to detect and classify microorganisms from their polarimetric response in the mid-infrared range (2-12 μm) measured by FTIR ellipsometry. Apart from the ellipsometric measurements, the performance of the methods also stands on the simplicity of sample preparation and on the data analysis. Spectroscopic ellipsometry is a non-invasive optical characterization technique sensitive to the polarization of the light reflected or transmitted by a sample. The extreme sensitivity of ellipsometry allows the detection of minute changes on the sample surface, even at the monomolecular layer level. In the mid-infrared range each molecule exhibits a characteristic absorption fingerprint, thus making ellipsometry chemically selective. FTIR ellipsometry is used here for the first time to analyze bacteria grown in culture media. Sample preparation is extremely simple and consists of the evaporation of a droplet of an aqueous suspension of microorganisms on a planar surface. Ellipsometric measurements are performed on the solid residue left on the surface after the evaporation of the droplet. Data analysis can be divided in two steps. First, simplification of the measured spectra by Principal Component Analysis (PCA), which is one of the existing multivariate statistical techniques commonly used to eliminate redundant information. Second, classification of the simplified spectra using a standard clustering method. As a result, we show how this method can be employed to discriminate and identify bacteria at the species level. The results of this experiment are very promising for the application of ellipsometry for analytical purposes in biochemistry and in medicine.
The Spectroscopic Ellipsometry and the Time Resolved Microwave Conductivity (TRMC) are efficient tools for in-situ non invasive characterizations during the growth of semiconductors and interfaces. From ellipsometry, one estimates the optical absorption, structural composition of the material in the bulk and near the interface. The TRMC measures the transient microwave reflectivity induced by carriers photogenerated by a pulsed laser. From TRMC, one may estimate the mobility of the carriers in a thin film or in bulk materials, the carrier lifetime in the bulk or near the surface. Particularly, we characterize microcrystalline silicon: electron and hole mobility, electron mobility inside the grain, trapping. We also analyze the semiconductor/dielectric interface, particularly for c-Si/SiO2. Using various UV laser fluxes, we can characterize the surface recombination, estimate the interface field and compare with the density of states obtained from capacitance measurement. The results are compared with simulation.
The Spectroscopic Ellipsometry and the Time Resolved Microwave Conductivity (TRMC) are efficient tools for in-situ non invasive characterizations during the growth of semiconductors and interfaces. From ellipsometry, one estimates the optical absorption, structural composition of the material in the bulk and near the interface. The TRMC measures the transient microwave reflectivity induced by carriers photogenerated by a pulsed laser. From TRMC, one may estimate the mobility of the carriers in a thin film or in bulk materials, the carrier lifetime in the bulk or near the surface. Particularly, we characterize microcrystalline silicon : electron and hole mobility, electron mobility inside the grain, trapping.
A very simple OCT system has been developed, based on a Linnik interferometric microscope with its reference mirror mounted on a piezoelectric translator. The geometrical extension of the optics allows efficient illumination of this device with a low power (3 W) light bulb, yielding full field interferometric images at 50 Hz acquisition rate with a fast CCD camera. Due to the very broad spectral width of the light source and camera response, a longitudinal resolution of 1.5 micrometers is achieved. Tomographic images of cell smears are shown.
Immunohistochemistry contributes to determine the site of origin of the tumor, helps setting the treatment and the prognosis. However, these assays are both time consuming when treatment may be an emergency, and expensive. Instead of the usual biochemistry we are using for the first time for medical issues Fourier-transform IR ellipsometry, a non- invasive optical characterization technique used in the semiconductor field to characterize bare substrates and thin films. Working in the mid-IR makes this technique sensitive to the molecules present inside the cell. Since the ellipsometric analysis can be performed on tissue slides extended on a specular surface, they may provide the physician with a chemical map of the sample. Here we show that ellipsometry can detect and measure physiological concentration of glycogen diluted in water and retrieve the polysaccharide band in a tissue slide. Our objective is to show that ellipsometry can give a quick and comprehensive analysis of the diverse biological markers forming the living matter.
A very simple OCT system has been developed, based on a Linnik interferometric microscope with its reference mirror mounted on a piezoelectric translator. The geometrical extension of the optics allows efficient illumination of this device with a low power (3 W) light bulb, yielding full field interferometric images at 50 Hz acquisition rate with a fast CCD camera. Due to the very broad spectral width of the light source and camera response, an axial resolution better than 2 micrometers is easily achieved. Tomographic images of cell smears are shown.
Mueller matrix ellipsometry measurements are performed on accurately sizes polystyrene latex (PSL) and silicon oxide (SiO2) spherical particles deposited on a crystalline silicon surface. The mean particle diameter ranges from 0.2 μm to 1.5 μm. An argon laser beam (wavelength 515 nm) is impinging on the sample at a fixed near grazing incidence angle. The Mueller matrix of the diffuse light scattered by the particles is measured in the plane of incidence as a function of the scattering angle. Results are presented and compared with exact theory.
Real-time monitoring by multiwavelength phase modulated ellipsometry (PME) of the growth of plasma deposited microcrystalline Silicon ((mu) c-Si) is presented. Several growth models for process-monitoring are reviewed, and in particular the inhomogeneity in the (mu) c-Si layer is treated by allowing graded-index profile in the bulk. By also using the Bruggeman effective medium theory to describe the optical properties of (mu) c-Si, the monitoring of the crystallinity in the upper and lower part of the layer, together with the thickness is demonstrated. The inversion algorithms is very fast, with calculation times within 5 seconds using a standard Pentium computer. This opens up for precise control of surface roughness, bulk thickness, and crystallization of both the top and bottom interfaces of the layer during the elaboration of devices such as solar cells and thin film transistors.
Real time monitoring and control, by multiwavelength phase modulated ellipsometry of the growth of plasma deposited optical structures is presented. The transparent layers consists of SiO2 SiNx and oxynitrides. We present an efficient method for the estimation of the optical parameters based on a sliding window containing the last acquired measurements. This method is used to monitor the deposition of multilayer coatings with homogenous deposition conditions, and is also used in feedback control of such coatings. The sliding window method is further developed to follow slowly time-varying parameters such as the deposition rate. A preliminary study of a fast novel method based on the same principle for real time monitoring of refractive index gradients is described. As an example, the real time monitoring of the growth of a linear gradient index is demonstrated.
A new optical instrument allowing photoellipsometric measurements is presented. Photoellipsometry (PE) is a modulation spectroscopy technique which uses ellipsometry in presence of a chopped external light excitation. PE measurements are obtained using a double modulation system, combining spectroscopic phase-modulated ellipsometry (SPME) with a laser pump beam. The experimental system described here takes advantage of the high frequency polarization of SPME (approximately equals 50 kHz). As a consequence the frequency of the pump beam can be varied up to 5 kHz. The field-induced changes in the real and imaginary parts of the bulk dielectric function can be directly measured and analyzed in terms of the pump beam power or the probe beam photon energy. Demonstration of this method is made with measurements, recorded in the band-gap E0 region (approximately equals 1.4 eV), on n-type GaAs sample. In particular, Franz-Keldish oscillations are observed with a very good sensitivity. More generally, PE measurements are compared with a theoretical model. From this preliminary study, it can be concluded that PE appears as a promising technique for semiconductor characterization.
Application of spectroscopic phase modulated ellipsometry (PME) to study both ultrafast and slow processes of interaction of silane (SiH4) with thin film Pd, and to the investigation of the growth kinetics of a-Si:H films deposited by rf glow discharge under UV light irradiation are presented. As compared to other ellipsometric techniques like rotating analyzer ellipsometry (RAE), the phase modulation uses a high frequency of about 50 kHz provided by a photoelastic modulator. Thus, PME allows one to reach 1 - 5 ms time resolution which permits faster real-time measurements than RAE. This remarkable feature of PME makes it particularly suitable for in-situ applications. Changes of optical properties of Pd thin films exposed to SiH4 at different fluxes are monitored by in situ single wavelength ellipsometry in the case of high fluxes which lead to ultrafast process and by in situ spectroscopic ellipsometry at small fluxes and slow kinetics. The study reveals a complicated character of the process which depends on initial flux of silane and leads to formation of Pd disilicide, Pd hydride, and an intrinsic porosity. A qualitative model of the process is proposed.
KEYWORDS: Signal processing, Modulation, Sensors, Signal detection, Digital signal processing, Plasma, Thin films, Polarization, Mirrors, Infrared radiation
A new infrared phase modulated ellipsometer (IRPME) is presented here. The polarization phase modulation technique takes advantage of the high frequency modulation (37 kHz) provided by a ZnSe photoelastic modulator. In order to increase the signal to noise ratio, the conventional globar source was superseded by a cascade arc, which emitted intensity corresponds to that of a blackbody at a temperature > 10,000 K. Ellipsometric measurements can be recorded from 700 up to 4000 cm-1 combining photovoltaic InSb and MCT detectors. A monochromator is used to record spectra, with a 2 - 5 cm-1 spectral resolution, depending on the wavelength domain. The signal acquisition and data processing is based on the use of a numerical electronic system. The improvements of both optical and electronic parts of the ellipsometer result in increased performances by more than one order of magnitude. The precision on (Psi) and (Delta) is now approximately equals 0.01 deg., achieving a submonolayer sensitivity.
Recent applications of spectroscopic phase modulated ellipsometry, from UV to IR, to the study of the growth of plasma deposited thin film semiconductors like amorphous (a-Si:H) and microcrystalline ((mu) c-Si) silicon are reviewed. The high sensitivity of this technique is emphasized. In the UV range, the ability of kinetic ellipsometry, with fast time resolution, to study the complex growth mechanism of (mu) c-Si is illustrated. In particular, the importance of hydrogen etching during (mu) c-Si growth is evidenced. In the IR, the hydrogen incorporation during a-Si:H growth can be precisely investigated. Photoelectronic quality a- Si:H films grow beneath a hydrogen rich overlayer (1-3 monolayers thick) containing SiH2, the hydrogen being bonded as SiH in the bulk material.
Since the last few years there is an increasing interest for materials dealing
with nonlinear optical effects. Besides semiconductors, crystalls and organic
materials specific attention has been paid to silicate glasses in which a
CdSSe1 > microcrystalline phase is thermally developed. These glasses are the
basis for a commercially available set of yellow-to-red "sharp cut" filters.
Although these glasses have not been optimjzd3for nonlinear behaviour by now,
they already show large nonlinear effects.
Recent in situ applications of reflectance anisotropy (RA) to the study of the growth of 111-V materials by low pressure MOCVD are reviewed. These results illustrate the extreme sensitivity of the RA technique. During heterojunction growth the first 1-2 seconds are dominated by the change of group V species. Over the time scale of several minutes the signal exhibits damped oscillations correlated to the growth rate. An optical model is proposed to account for this behaviour. A difference in the optical anisotropy between growing and non-growing AsH3 stabilized InAs surface is observed. Large reflectance anisotropies during the growth of lattice-mismatched semiconductors are also presented. It is shown that these anisotropies are related to 3-dimensional growth. The beginning of the lattice-mismatched growth is quantitatively described by an optical model based on effective medium theories. More generally RA technique appears a very promising new method for in situ monitoring of epitaxial processes.
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