ABSTRACT Theoretical methods to calculate band gaps and the linear optical response function (com... more ABSTRACT Theoretical methods to calculate band gaps and the linear optical response function (complex refractive index) have evolved from empirical pseudopotentials (first local, then non-local) to local-density functional theory and are now correcting for many-body effects using the GW approximation and the Bethe-Salpeter equation to obtain increasingly accurate results. At the same time, experimental methods have improved as well. The spectral range of commercial spectroscopic ellipsometers now includes the near-infrared and vacuum ultraviolet regions (0.74 to 9.0 eV). The use of a computer controlled Berek waveplate allows accurate measurements even for non-absorbing materials. We present experimental results (unprecedented in accuracy and spectral range) for a variety of semiconductors and insulators, including Si, GaAs, Ge, 4H and 6H SiC, SrTiO_3, SiO_2, and HfO_2. These results are not only useful for thin-film thickness measurements in the semiconductor industry, but also place stringent tests on new theoretical methods. Fitting the second derivative of the complex dielectric function with respect to photon energy using analytical lineshapes allows an accurate determination of critical-point energies.
A method for deriving the optical constants of organic powdered materials in the mid-infrared spe... more A method for deriving the optical constants of organic powdered materials in the mid-infrared spectral range is introduced using both variable angle spectroscopic ellipsometry and transmission spectroscopy. The approach uses pressed pellets of the powder and is applied to organic solids, which have both strong and weak infrared absorption features. Many powders have significant voids and do not press into smooth, homogenous pellets. To account for pellet non-idealities and to accurately measure both n and k, three different forms of pellets were pressed and measured: A pure analyte pellet, a mixed analyte/KBr pellet with a large analyte percentage, and a KBr transmission pellet with only a small analyte percentage. Using all three pellets in a multi-sample analysis involving both ellipsometric and transmission data, the complex refractive index (n/k) values can be derived for many organic compounds. This method is illustrated to calculate the optical constants for anhydrous lactose from 6000-400 cm-1. The transmission measurements improve the spectral fitting of weak absorption features, and the multi-sample analysis enables a better determination of the significant void space that is present in the pure pellet, leading to lower values for both n and k if not properly accounted for in the multi-oscillator model used to fit the ellipsometric data.
Using nylon-6 and polystyrene as prototypical materials with organic functional groups, we have d... more Using nylon-6 and polystyrene as prototypical materials with organic functional groups, we have demonstrated that for silicon or gold substrates, features due to the molecular vibrations can be seen in the ellipsometric spectra for films as thin as 50Å with signal-to-noise ratios ranging from 2 to 10. It is expected that with some additional optimization, this could be improved by
Oxides with perovskite crystal structure are important candidates for ultrathin gate oxides becau... more Oxides with perovskite crystal structure are important candidates for ultrathin gate oxides because of their large DC dielectric constant. Whenever we are interested in the properties of thin films, it is usually a good idea to start with studying the bulk. Following Jellison, we measured the dielectric function of bulk SrTiO3 from 0.74 to 6.6 eV. We also performed measurements
Mueller matrix (MM) measurements offer the ability to quantify the depolarization capability of a... more Mueller matrix (MM) measurements offer the ability to quantify the depolarization capability of a sample. Depolarization can be estimated using terms such as the depolarization index or the average degree of polarization. However, these calculations require measurement of the complete MM. We propose an alternate depolarization metric, termed the Jones matrix quality factor, QJM, which does not require the complete MM. This metric provides a measure of how close, in a least-squares sense, a Jones matrix can be found to the measured Mueller matrix. We demonstrate and compare the use of QJM to other traditional calculations of depolarization for both isotropic and anisotropic depolarizing samples; including nonuniform coatings, anisotropic crystal substrates, and beetle cuticles that exhibit both depolarization and circular diattenuation.
It is a well-known challenge to determine refractive index (n) from ultra-thin films where the th... more It is a well-known challenge to determine refractive index (n) from ultra-thin films where the thickness is less than about 10 nm [1,2]. We discovered an interesting exception to this issue while characterizing spectroscopic ellipsometry (SE) data from isotropic, free-standing polymer films. Ellipsometry analysis shows that both thickness and refractive index can be independently determined for free-standing films as thin as 5 nm. Simulations further confirm an orthogonal separation between thickness and index effects on the experimental SE data. Effects of angle of incidence and wavelength on the data and sensitivity are discussed. While others have demonstrated methods to determine refractive index from ultra-thin films [3,4], our analysis provides the first results to demonstrate high-sensitivity to the refractive index from ultra-thin layers.
We prepared several photochromic composite ®lms based on a hybrid organic±inorganic matrix in whi... more We prepared several photochromic composite ®lms based on a hybrid organic±inorganic matrix in which metal heteropolyoxometallates are entrapped. Infrared Fourier transform and UV-Visible spectroscopic ellipsometry methods have been used for the ®rst time to study the coloring/bleaching process of the composite ®lms. Data on all samples were acquired using a J.A. Woollam Co. VASE instrument and WVASE software. The optical absorption of these spin-coated ®lms changed markedly by a reversible process, in the presence or absence of UV irradiation. The VIS-NIR transmission data showed that the ®lms containing tungsten heteropolyoxometallate (HPOM) had strong absorptions at about 720 nm, 1108 nm and the ®lms containing molybdenum HPOM had a strong absorption at 720 nm. We have also extracted for the ®rst time the visible and infrared optical functions of the ®lms. For the colored ®lms, the Lorentz and Gaussian oscillators were used to describe the optical behavior in the 400±1700 nm wavelength range and 700±4000 cm 21 wavenumber range, respectively. The composite ®lms containing tungsten heteropolyoxometallate (HPOM) showed faster coloration and bleaching than pure tungsten oxide ®lms. The composite ®lms with molybdenum HPOM showed faster coloration and much slower bleaching than tungsten HPOM.
Silicon carrier concentration profiles were determined by resistivity measurements acquired optic... more Silicon carrier concentration profiles were determined by resistivity measurements acquired optically using infrared ellipsometry combined with anodic oxidation sectioning. The method is based upon a technique developed in the early 1960's, except that the resistivity is measured optically using infrared ellipsometry instead of direct electrical contact to the sample. Infrared ellipsometry is a non-contact optical technique that can simultaneously determine the oxide thickness and the resistivity profile of the underlying substrate (via the free-carrier Drude effect). This eliminates the need for special sample geometries, implanted ohmic contacts, repeated hydrofluoric acid etches and separate measurements of oxide thicknesses. In this study the optical constants of anodic oxides are determined and a carrier profile of an As implanted sample is characterized using infrared ellipsometry.
Using nylon-6 and polystyrene as prototypical materials with organic functional groups, we have d... more Using nylon-6 and polystyrene as prototypical materials with organic functional groups, we have demonstrated that for silicon or gold substrates, features due to the molecular vibrations can be seen in the ellipsometric spectra for films as thin as 50 Å with signal-to-noise ratios ranging from 2 to 10. It is expected that with some additional optimization, this could be improved by a factor of 2 to 4. For films on a gold substrate, due to the high reflectivity, one would expect to be able to see these features for 10 Å or thinner. For films on the glass substrate (or any other substrate that has an index of refraction near that of the film in the nonabsorbing region), IR ellipsometry provides thickness information that cannot readily be obtained from traditional ellipsometry. For all of the substrates, IR ellipsometry provides molecular structure information.
Spectroscopic ellipsometry is an important thin film metrology technique. It offers non-destructi... more Spectroscopic ellipsometry is an important thin film metrology technique. It offers non-destructive and noninvasive materials characterization, including measurement of the complex refractive index and other microstructural properties that affect the material optical response. It has been applied to many important ultra large scale integration (ULSI) thin-film metrology needs; as it is applicable to semiconductors, dielectrics, metals, organic films, and more. Recent advances have broadened the impact of ellipsometry for many applications. New instruments are extending the spectral range to both shorter and longer wavelengths. High-speed spectroscopic measurements can be accomplished with diodearray technology. In addition to hardware advances, new data analysis techniques have extended the capabilities of spectroscopic ellipsometry. The advanced capabilities of spectroscopic ellipsometry will be reviewed, including the importance of new spectral ranges in the vacuum ultraviolet (VUV) and infrared (IR). The merit of fast measurements for in-line and in-situ measurements is covered. New data analysis methods are discussed, with their possible applications toward ULSI thin film metrology. In short, this paper reviews the progress of hardware and data analysis innovations and their impact on ULSI thin film metrology.
Ž. Generalized Jones matrix ellipsometry is gaining considerable interest because of its ability ... more Ž. Generalized Jones matrix ellipsometry is gaining considerable interest because of its ability to determine properties of Ž. anisotropic samples. Here, the strong uniaxial anisotropy of calcite calcium carbonate was investigated using generalized Ž. ellipsometry. The wavelength range from 0.73 to 6.5 eV 190 nm to 1.7 m was covered using a standard variable angle Ž. spectroscopic ellipsometer; from 0.089 to 0.68 eV 1.8᎐14 m , using a similar instrument based on a Fourier transform spectrometer. Measurements were made on a single air᎐calcite interface for which the optic axis lay nominally in the plane of the surface. To determine the optical constants and orientation of cut, both the angle of incidence and rotation of the sample about its surface normal were varied. Properties of the sample were arrived at by optimizing the parameters of a material model such that the calculated normalized Jones matrix elements best matched the measured ones. Localized spectral regions of absorption due to the internal vibrational modes of the carbonate ions were observed in the infrared at energies which differed for the ordinary and extraordinary optical constants. The certainty to which sample properties could be determined was maximized by judicious choice of measurement configurations.
Ž. Fourier transform infrared variable angle spectroscopic ellipsometry 0.089᎐0.620 eV was used t... more Ž. Fourier transform infrared variable angle spectroscopic ellipsometry 0.089᎐0.620 eV was used to investigate undoped and Ž. Ž. Te doped epilayers of In Ga As Sb on 100 GaSb substrates. The onset of absorption at the bandgap 0.55 eV was 0.15 0.85 0.17 0.83 observed in the data for the undoped epilayer. The data below the bandgap for the doped epilayer displayed free carrier characteristics as well as interference effects. They were simulated by including a Drude term in the epilayer and substrate dielectric functions of a multilayer model. This model produced a good fit to the data and provided the epilayer thickness and Ž. free carrier plasma frequency. SE measurements above the gap 0.75᎐5.4 eV showed only minor effects due to doping. ᮊ 1998 Elsevier Science S.A.
The shift towards shorter wavelengths in the lithographic process has been the driving force for ... more The shift towards shorter wavelengths in the lithographic process has been the driving force for development of a commercial spectroscopic ellipsometer covering the vacuum ultraviolet (VUV) spectrum. Understanding the optical properties of thin films and substrate materials at short wavelengths (193 nm, 157 nm, and shorter) is necessary to develop the lithographic process. Lithography applications include optical coatings, photoresists, AR films, and masks. We present results for both single and multi-layer structures important to this industry. A complete description of the thin film optical properties can be used to track process changes or variations in sample structure. As an example, we compare the measured index for three oxynitrides that were subjected to different process conditions. In addition to lithography applications, VUV ellipsometry has many other potential applications. A variety of other materials have been characterized, often incorporating generalized ellipsometry to investigate anisotropic optical properties. The ordinary and extraordinary dielectric functions for bulk 4H SiC were measured over the wide spectral range from 0.73 to 8.5 eV.
We have studied the optical properties ͑complex dielectric function͒ of bulk SrTiO 3 and thin fil... more We have studied the optical properties ͑complex dielectric function͒ of bulk SrTiO 3 and thin films on Si and Pt using spectroscopic ellipsometry over a very broad spectral range, starting at 0.03 eV ͓using Fourier transform infrared ͑FTIR͒ ellipsometry͔ to 8.7 eV. In the bulk crystals, we analyze the interband transitions in the spectra to determine the critical-point parameters. To interpret these transitions, we performed band structure calculations based on ab initio pseudopotentials within the local-density approximation. The dielectric function was also calculated within this framework and compared with our ellipsometry data. In the FTIR ellipsometry data, we notice a strong lattice absorption peak due to oxygen-related vibrations. Two longitudinal optic ͑LO͒ phonons were also identified. In SrTiO 3 films on Si, the refractive index below the band gap decreases with decreasing thickness because of the increasing influence of the amorphous interfacial layer between the SrTiO 3 film and the Si substrate. There is also a decrease in amplitude and an increase in broadening of the critical points with decreasing thickness. In SrTiO 3 films on Pt, there is a strong correlation between the crystallinity and texture of the films ͑mostly aligned with the Pt pseudosubstrate͒ and the magnitude of the refractive index, the Urbach tail below the bulk band edge, and the critical-point parameters. FTIR reflectance measurements of SrTiO 3 on Pt ͑reflection-absorption spectroscopy͒ show absorption peaks at the LO phonon energies, a typical manifestation of the Berreman effect for thin insulating films on a metal. The Urbach tail in our ellipsomety data and the broadening of the optical phonons in SrTiO 3 on Pt are most likely caused by oxygen vacancy clusters.
Thermal emission of localized surface phonons (LSPhs) from nanostructures of polaritonic material... more Thermal emission of localized surface phonons (LSPhs) from nanostructures of polaritonic materials is a promising mechanism for tuning the spectrum of near-field thermal radiation. Previous studies have theoretically shown that thermal emission of LSPhs results in narrow-band peaks in the near-field spectra, whose spectral locations can be modulated by changing the dimensions of the nanostructure. However, near-field thermal emission of LSPhs has not been experimentally explored yet. In this study, we measure the spectrum of near-field thermal radiation from arrays of 6H-silicon carbide (6H-SiC) nanopillars using an internal-reflection-element based spectroscopy technique. We present an experimental demonstration of thermal emission of the
We have measured the dielectric function of bulk nitrogen-doped 4H and 6H SiC substrates from 700... more We have measured the dielectric function of bulk nitrogen-doped 4H and 6H SiC substrates from 700 to 4000 cm Ϫ1 using Fourier-transform infrared spectroscopic ellipsometry. Photon absorption by transverse optical phonons produces a strong reststrahlen band between 797 and 1000 cm Ϫ1 with the effects of phonon anisotropy being observed in the region of the longitudinal phonon energy ͑960 to 100 cm Ϫ1 ͒. The shape of this band is influenced by plasma oscillations of free electrons, which we describe with a classical Drude equation. For the 6H-SiC samples, we modify the Drude equation to account for the strong effective mass anisotropy. Detailed numerical regression analysis yields the free-electron concentrations, which range from 7ϫ10 17 to 10 19 cm Ϫ3 , in good agreement with electrical and secondary ion mass spectrometry measurements. Finally, we observe the Berreman effect near the longitudinal optical phonon energy in nϪ/nϩ homoepitaxial 4H SiC and hydrogen implanted samples, and we are able to determine the thickness of these surface layers. ͓S0163-1829͑99͒00240-4͔
ABSTRACT Theoretical methods to calculate band gaps and the linear optical response function (com... more ABSTRACT Theoretical methods to calculate band gaps and the linear optical response function (complex refractive index) have evolved from empirical pseudopotentials (first local, then non-local) to local-density functional theory and are now correcting for many-body effects using the GW approximation and the Bethe-Salpeter equation to obtain increasingly accurate results. At the same time, experimental methods have improved as well. The spectral range of commercial spectroscopic ellipsometers now includes the near-infrared and vacuum ultraviolet regions (0.74 to 9.0 eV). The use of a computer controlled Berek waveplate allows accurate measurements even for non-absorbing materials. We present experimental results (unprecedented in accuracy and spectral range) for a variety of semiconductors and insulators, including Si, GaAs, Ge, 4H and 6H SiC, SrTiO_3, SiO_2, and HfO_2. These results are not only useful for thin-film thickness measurements in the semiconductor industry, but also place stringent tests on new theoretical methods. Fitting the second derivative of the complex dielectric function with respect to photon energy using analytical lineshapes allows an accurate determination of critical-point energies.
A method for deriving the optical constants of organic powdered materials in the mid-infrared spe... more A method for deriving the optical constants of organic powdered materials in the mid-infrared spectral range is introduced using both variable angle spectroscopic ellipsometry and transmission spectroscopy. The approach uses pressed pellets of the powder and is applied to organic solids, which have both strong and weak infrared absorption features. Many powders have significant voids and do not press into smooth, homogenous pellets. To account for pellet non-idealities and to accurately measure both n and k, three different forms of pellets were pressed and measured: A pure analyte pellet, a mixed analyte/KBr pellet with a large analyte percentage, and a KBr transmission pellet with only a small analyte percentage. Using all three pellets in a multi-sample analysis involving both ellipsometric and transmission data, the complex refractive index (n/k) values can be derived for many organic compounds. This method is illustrated to calculate the optical constants for anhydrous lactose from 6000-400 cm-1. The transmission measurements improve the spectral fitting of weak absorption features, and the multi-sample analysis enables a better determination of the significant void space that is present in the pure pellet, leading to lower values for both n and k if not properly accounted for in the multi-oscillator model used to fit the ellipsometric data.
Using nylon-6 and polystyrene as prototypical materials with organic functional groups, we have d... more Using nylon-6 and polystyrene as prototypical materials with organic functional groups, we have demonstrated that for silicon or gold substrates, features due to the molecular vibrations can be seen in the ellipsometric spectra for films as thin as 50Å with signal-to-noise ratios ranging from 2 to 10. It is expected that with some additional optimization, this could be improved by
Oxides with perovskite crystal structure are important candidates for ultrathin gate oxides becau... more Oxides with perovskite crystal structure are important candidates for ultrathin gate oxides because of their large DC dielectric constant. Whenever we are interested in the properties of thin films, it is usually a good idea to start with studying the bulk. Following Jellison, we measured the dielectric function of bulk SrTiO3 from 0.74 to 6.6 eV. We also performed measurements
Mueller matrix (MM) measurements offer the ability to quantify the depolarization capability of a... more Mueller matrix (MM) measurements offer the ability to quantify the depolarization capability of a sample. Depolarization can be estimated using terms such as the depolarization index or the average degree of polarization. However, these calculations require measurement of the complete MM. We propose an alternate depolarization metric, termed the Jones matrix quality factor, QJM, which does not require the complete MM. This metric provides a measure of how close, in a least-squares sense, a Jones matrix can be found to the measured Mueller matrix. We demonstrate and compare the use of QJM to other traditional calculations of depolarization for both isotropic and anisotropic depolarizing samples; including nonuniform coatings, anisotropic crystal substrates, and beetle cuticles that exhibit both depolarization and circular diattenuation.
It is a well-known challenge to determine refractive index (n) from ultra-thin films where the th... more It is a well-known challenge to determine refractive index (n) from ultra-thin films where the thickness is less than about 10 nm [1,2]. We discovered an interesting exception to this issue while characterizing spectroscopic ellipsometry (SE) data from isotropic, free-standing polymer films. Ellipsometry analysis shows that both thickness and refractive index can be independently determined for free-standing films as thin as 5 nm. Simulations further confirm an orthogonal separation between thickness and index effects on the experimental SE data. Effects of angle of incidence and wavelength on the data and sensitivity are discussed. While others have demonstrated methods to determine refractive index from ultra-thin films [3,4], our analysis provides the first results to demonstrate high-sensitivity to the refractive index from ultra-thin layers.
We prepared several photochromic composite ®lms based on a hybrid organic±inorganic matrix in whi... more We prepared several photochromic composite ®lms based on a hybrid organic±inorganic matrix in which metal heteropolyoxometallates are entrapped. Infrared Fourier transform and UV-Visible spectroscopic ellipsometry methods have been used for the ®rst time to study the coloring/bleaching process of the composite ®lms. Data on all samples were acquired using a J.A. Woollam Co. VASE instrument and WVASE software. The optical absorption of these spin-coated ®lms changed markedly by a reversible process, in the presence or absence of UV irradiation. The VIS-NIR transmission data showed that the ®lms containing tungsten heteropolyoxometallate (HPOM) had strong absorptions at about 720 nm, 1108 nm and the ®lms containing molybdenum HPOM had a strong absorption at 720 nm. We have also extracted for the ®rst time the visible and infrared optical functions of the ®lms. For the colored ®lms, the Lorentz and Gaussian oscillators were used to describe the optical behavior in the 400±1700 nm wavelength range and 700±4000 cm 21 wavenumber range, respectively. The composite ®lms containing tungsten heteropolyoxometallate (HPOM) showed faster coloration and bleaching than pure tungsten oxide ®lms. The composite ®lms with molybdenum HPOM showed faster coloration and much slower bleaching than tungsten HPOM.
Silicon carrier concentration profiles were determined by resistivity measurements acquired optic... more Silicon carrier concentration profiles were determined by resistivity measurements acquired optically using infrared ellipsometry combined with anodic oxidation sectioning. The method is based upon a technique developed in the early 1960's, except that the resistivity is measured optically using infrared ellipsometry instead of direct electrical contact to the sample. Infrared ellipsometry is a non-contact optical technique that can simultaneously determine the oxide thickness and the resistivity profile of the underlying substrate (via the free-carrier Drude effect). This eliminates the need for special sample geometries, implanted ohmic contacts, repeated hydrofluoric acid etches and separate measurements of oxide thicknesses. In this study the optical constants of anodic oxides are determined and a carrier profile of an As implanted sample is characterized using infrared ellipsometry.
Using nylon-6 and polystyrene as prototypical materials with organic functional groups, we have d... more Using nylon-6 and polystyrene as prototypical materials with organic functional groups, we have demonstrated that for silicon or gold substrates, features due to the molecular vibrations can be seen in the ellipsometric spectra for films as thin as 50 Å with signal-to-noise ratios ranging from 2 to 10. It is expected that with some additional optimization, this could be improved by a factor of 2 to 4. For films on a gold substrate, due to the high reflectivity, one would expect to be able to see these features for 10 Å or thinner. For films on the glass substrate (or any other substrate that has an index of refraction near that of the film in the nonabsorbing region), IR ellipsometry provides thickness information that cannot readily be obtained from traditional ellipsometry. For all of the substrates, IR ellipsometry provides molecular structure information.
Spectroscopic ellipsometry is an important thin film metrology technique. It offers non-destructi... more Spectroscopic ellipsometry is an important thin film metrology technique. It offers non-destructive and noninvasive materials characterization, including measurement of the complex refractive index and other microstructural properties that affect the material optical response. It has been applied to many important ultra large scale integration (ULSI) thin-film metrology needs; as it is applicable to semiconductors, dielectrics, metals, organic films, and more. Recent advances have broadened the impact of ellipsometry for many applications. New instruments are extending the spectral range to both shorter and longer wavelengths. High-speed spectroscopic measurements can be accomplished with diodearray technology. In addition to hardware advances, new data analysis techniques have extended the capabilities of spectroscopic ellipsometry. The advanced capabilities of spectroscopic ellipsometry will be reviewed, including the importance of new spectral ranges in the vacuum ultraviolet (VUV) and infrared (IR). The merit of fast measurements for in-line and in-situ measurements is covered. New data analysis methods are discussed, with their possible applications toward ULSI thin film metrology. In short, this paper reviews the progress of hardware and data analysis innovations and their impact on ULSI thin film metrology.
Ž. Generalized Jones matrix ellipsometry is gaining considerable interest because of its ability ... more Ž. Generalized Jones matrix ellipsometry is gaining considerable interest because of its ability to determine properties of Ž. anisotropic samples. Here, the strong uniaxial anisotropy of calcite calcium carbonate was investigated using generalized Ž. ellipsometry. The wavelength range from 0.73 to 6.5 eV 190 nm to 1.7 m was covered using a standard variable angle Ž. spectroscopic ellipsometer; from 0.089 to 0.68 eV 1.8᎐14 m , using a similar instrument based on a Fourier transform spectrometer. Measurements were made on a single air᎐calcite interface for which the optic axis lay nominally in the plane of the surface. To determine the optical constants and orientation of cut, both the angle of incidence and rotation of the sample about its surface normal were varied. Properties of the sample were arrived at by optimizing the parameters of a material model such that the calculated normalized Jones matrix elements best matched the measured ones. Localized spectral regions of absorption due to the internal vibrational modes of the carbonate ions were observed in the infrared at energies which differed for the ordinary and extraordinary optical constants. The certainty to which sample properties could be determined was maximized by judicious choice of measurement configurations.
Ž. Fourier transform infrared variable angle spectroscopic ellipsometry 0.089᎐0.620 eV was used t... more Ž. Fourier transform infrared variable angle spectroscopic ellipsometry 0.089᎐0.620 eV was used to investigate undoped and Ž. Ž. Te doped epilayers of In Ga As Sb on 100 GaSb substrates. The onset of absorption at the bandgap 0.55 eV was 0.15 0.85 0.17 0.83 observed in the data for the undoped epilayer. The data below the bandgap for the doped epilayer displayed free carrier characteristics as well as interference effects. They were simulated by including a Drude term in the epilayer and substrate dielectric functions of a multilayer model. This model produced a good fit to the data and provided the epilayer thickness and Ž. free carrier plasma frequency. SE measurements above the gap 0.75᎐5.4 eV showed only minor effects due to doping. ᮊ 1998 Elsevier Science S.A.
The shift towards shorter wavelengths in the lithographic process has been the driving force for ... more The shift towards shorter wavelengths in the lithographic process has been the driving force for development of a commercial spectroscopic ellipsometer covering the vacuum ultraviolet (VUV) spectrum. Understanding the optical properties of thin films and substrate materials at short wavelengths (193 nm, 157 nm, and shorter) is necessary to develop the lithographic process. Lithography applications include optical coatings, photoresists, AR films, and masks. We present results for both single and multi-layer structures important to this industry. A complete description of the thin film optical properties can be used to track process changes or variations in sample structure. As an example, we compare the measured index for three oxynitrides that were subjected to different process conditions. In addition to lithography applications, VUV ellipsometry has many other potential applications. A variety of other materials have been characterized, often incorporating generalized ellipsometry to investigate anisotropic optical properties. The ordinary and extraordinary dielectric functions for bulk 4H SiC were measured over the wide spectral range from 0.73 to 8.5 eV.
We have studied the optical properties ͑complex dielectric function͒ of bulk SrTiO 3 and thin fil... more We have studied the optical properties ͑complex dielectric function͒ of bulk SrTiO 3 and thin films on Si and Pt using spectroscopic ellipsometry over a very broad spectral range, starting at 0.03 eV ͓using Fourier transform infrared ͑FTIR͒ ellipsometry͔ to 8.7 eV. In the bulk crystals, we analyze the interband transitions in the spectra to determine the critical-point parameters. To interpret these transitions, we performed band structure calculations based on ab initio pseudopotentials within the local-density approximation. The dielectric function was also calculated within this framework and compared with our ellipsometry data. In the FTIR ellipsometry data, we notice a strong lattice absorption peak due to oxygen-related vibrations. Two longitudinal optic ͑LO͒ phonons were also identified. In SrTiO 3 films on Si, the refractive index below the band gap decreases with decreasing thickness because of the increasing influence of the amorphous interfacial layer between the SrTiO 3 film and the Si substrate. There is also a decrease in amplitude and an increase in broadening of the critical points with decreasing thickness. In SrTiO 3 films on Pt, there is a strong correlation between the crystallinity and texture of the films ͑mostly aligned with the Pt pseudosubstrate͒ and the magnitude of the refractive index, the Urbach tail below the bulk band edge, and the critical-point parameters. FTIR reflectance measurements of SrTiO 3 on Pt ͑reflection-absorption spectroscopy͒ show absorption peaks at the LO phonon energies, a typical manifestation of the Berreman effect for thin insulating films on a metal. The Urbach tail in our ellipsomety data and the broadening of the optical phonons in SrTiO 3 on Pt are most likely caused by oxygen vacancy clusters.
Thermal emission of localized surface phonons (LSPhs) from nanostructures of polaritonic material... more Thermal emission of localized surface phonons (LSPhs) from nanostructures of polaritonic materials is a promising mechanism for tuning the spectrum of near-field thermal radiation. Previous studies have theoretically shown that thermal emission of LSPhs results in narrow-band peaks in the near-field spectra, whose spectral locations can be modulated by changing the dimensions of the nanostructure. However, near-field thermal emission of LSPhs has not been experimentally explored yet. In this study, we measure the spectrum of near-field thermal radiation from arrays of 6H-silicon carbide (6H-SiC) nanopillars using an internal-reflection-element based spectroscopy technique. We present an experimental demonstration of thermal emission of the
We have measured the dielectric function of bulk nitrogen-doped 4H and 6H SiC substrates from 700... more We have measured the dielectric function of bulk nitrogen-doped 4H and 6H SiC substrates from 700 to 4000 cm Ϫ1 using Fourier-transform infrared spectroscopic ellipsometry. Photon absorption by transverse optical phonons produces a strong reststrahlen band between 797 and 1000 cm Ϫ1 with the effects of phonon anisotropy being observed in the region of the longitudinal phonon energy ͑960 to 100 cm Ϫ1 ͒. The shape of this band is influenced by plasma oscillations of free electrons, which we describe with a classical Drude equation. For the 6H-SiC samples, we modify the Drude equation to account for the strong effective mass anisotropy. Detailed numerical regression analysis yields the free-electron concentrations, which range from 7ϫ10 17 to 10 19 cm Ϫ3 , in good agreement with electrical and secondary ion mass spectrometry measurements. Finally, we observe the Berreman effect near the longitudinal optical phonon energy in nϪ/nϩ homoepitaxial 4H SiC and hydrogen implanted samples, and we are able to determine the thickness of these surface layers. ͓S0163-1829͑99͒00240-4͔
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Papers by Tom Tiwald