Traditional spectroscopic ellipsometry (SE) in the ultraviolet, visible, and near infrared is a well-established metrology technique for thin film characterization [1,2]. It offers a non-destructive method to accurately measure film thickness and optical constants. The power of SE is derived from its spectroscopic nature (range and number of wavelengths): in this area, bigger is considered better! Wavelength coverage from 190 nm in the ultraviolet to 1.7 m in the near infrared has been commercially available for over a decade. These instruments can precisely measure the thickness and composition of compound semiconductor devices. Variation in the alloy concentration of a material introduces a shift in optical constants. For instance, the fundamental bandgap of AlxGa1-xAs increases with increasing Al concentration. SE can be used to measure any material property that influences the optical constants within the measured wavelength range. Recent hardware innovations have expanded commercial spectroscopic ellipsometry into both the infrared (IR) and vacuum ultraviolet (VUV). The IR and VUV are at opposite SE wavelength extremes and thus provide new and distinctive information. The IR is sensitive to free-carrier concentrations, lattice vibrations, phonon absorptions, and molecular bond absorptions. At the other end of the spectrum, the VUV enhances sensitivity to ultrathin films and can characterize electronic transitions at higher energies. This article reviews both IR and VUV measurement capabilities, as they relate to semiconductor and insulator materials. What is Ellipsometry? Spectroscopic Ellipsometry measures the interaction of polarized light and a smooth surface. The light can either reflect or transmit from the surface, but is incident at an oblique angle, as shown in . The change in polarization that occurs due to interaction with the sample is measured as two parameters, amplitude (C) and phase (), for each wavelength/angle combination. These terms are related to the complex Fresnel reflection coefficients [3]: This equation illustrates the advantages inherent in an ellipsometry measurement. First, C and are measured from a ratio, which allows ellipsometry to remain very accurate and precise. Second, ellipsometry measures both C and : two parameters are better than one, and the phase information happens to be highly sensitive to the surface condition of a material. Researchers have capitalized on this fact, as thin films reduce to a few nanometers thick, hence the popularity of SE for thin gate measurements. Like other optical measurements, SE does not directly measure the material properties. Instead, they are determined from a model-dependent regression of the experimental data. In this manner, ellipsometry has been used to characterize film thickness, optical constants, alloy concentration, doping concentration, crystallinity and many other properties. The spectral range of an ellipsometer determines its suitability for a particular application. We will highlight key applications of the new IR and VUV wavelength ranges. Extending Ellipsometry to the Infrared Although Beattie [4] developed an infrared spectroscopic ellipsometer (IR-SE) in 1955, the dispersive-monochromator s low throughput severely limited the instrument s capability. It was the advent of Fourier Transform Infrared (FTIR) spectrometers in the 1970 s, with their high throughput and multiplex advantage, which made a practical IR-SE instrument possible. Rseler was the first to combine ellipsometry with FTIR [5,6]. In recent years, commercial instruments, including the IR-VASE (J.A. Woollam Co., Inc.), have been developed with sufficient accuracy and precision to quantitatively determine the infrared optical response of bulk and multilayer semiconductors and dielectrics films. The two most important infrared phenomena associated with semiconductor and dielectric materials are free carrier interactions and lattice vibrations (phonons). Thus, many new applications of the IR-VASE involve the semiconductor industryepitaxial layer characterization, doping concentration, doping profiles, and phonon absorptions. The interaction of free carriers with light is described by the Drude equation [7]. From that equation, we can determine the carrier concentration and mobility of semiconductors and metals, provided the effective mass is known. Resistivity can be directly determined from the Drude equation, without any independent knowledge of the effective mass (see ). We can exploit ellipsometry s high sensitivity to measure thin layers that are identical except for differences in carrier densities. In these instances, the only optical contrast between the layers results from free carrier effects. Examples of this include homoepitaxial layers [7] and surface depletion regions [8,9]. One can also determine carrier concentration profiles in ion-implanted materials [7]. Vibrational modes become infrared-active whenever there is an ionic component to the molecular bonds, which is typical for compound semiconductors. Crystalline materials often have one or more IR-active phonon lattice resonance. Materials containing light elements, such as C, N, and O, are particularly good examples because their phonon spectra contain strong features located at reasonably high infrared energies. Therefore, IR ellipsometry has been particularly successful with wide bandgap semiconductors, such as GaN [9] and SiC [10], high-k gate dielectrics (e.g., SrTiO3[11]), and organic films on semiconductors. Sometimes, second order vibrational resonance effects are detected, including summation bands (two or more phonons interacting with the incoming photon) and localized defect modes [9]. In low dimensional structures such as superlattices, folded modes can be observed. Free carrier/phonon interactions can also produce additional resonant effects, such as LO-phonon-plasmon modes [9] and Fano resonances [8]. Although LO-phonons are not infrared active, various effects occur at or near the LO phonon energy. These include the Berreman effect, surface polaritons, and spectral features related to phonon and free-carrier anisotropy. Interestingly, these effects can have dramatic impact on IR-SE, even with a dielectric response that is featureless. For many studies referenced here, the ellipsometric spectra provide sensitive and reliable measures of the broadening and frequency of LO-phonons and anisotropy of phonon and effective mass. Examples of IR-SE Hexagonal silicon carbide (polytype 4H-SiC) is prototypical of compound semiconductors in that its ellipsometric spectra readily show most of the effects described above. The data shown in were acquired from several bulk, c-plane 4H-SiC samples. The data are centered on the strong planar (782 cm-1) and axial (797 cm-1) TO phonon modes of c-plane 4H-SiC. The data also demonstrate how the screening effects of free carriers alter the ellipsometric response throughout the spectrum. The strong features seen in the region near axial (964 cm-1) and planar (970 cm-1) LO phonon energies are the result of phonon anisotropy. Several two-phonon summation bands are also visible. A careful inspection of C near 838 cm-1 reveals the weak A1 phonon mode, which is IR-active due to zone folding (see inset ). Zone folding effects occur in artificial and naturally occurring superlattices (such as hexagonal SiC). Sensitivity to these effects depends on the orientation of the optic axis (data are from c-plane 4H-SiC only). In this case, a careful study provided the optic axis orientation, ordinary and extraordinary dielectric functions, free-carrier concentration, effective mass or mobility anisotropy, TO and LO phonon energies and broadenings, and 2-phonon summation bands [10]. IR-SE can also measure the thickness of ion-implanted doping profiles [7], carrier depleted surface regions and homoepitaxial layers [9,10]. ) compares free-carrier concentration profiles independently derived from IR-SE and spreading resistance probe (SRP) measurements. Note the excellent agreement between the two curves. As mentioned above, the Drude effect allows us to measure carrier profiles in otherwise homogeneous materials; including ion-implanted materials [8] and carrier depleted surface regions [8,9]. IR-SE has been applied to heteroepitaxial systems as well. These include systems with dielectric substrates such as sapphire. In these situations, ellipsometry can provide sufficient information to determine the parameters of the semiconductor epilayers while accounting for all the optical effects of the substrate [9]. Ultraviolet and Beyond Historically, commercial "tabletop" spectroscopic ellipsometers have been limited to wavelengths above 190 nm, as shorter wavelengths are completely absorbed by the atmosphere. The shift of optical lithography toward 157 nm has encouraged characterization of materials at this wavelength. This motivation led to the rapid development of commercial vacuum ultraviolet SE (VUV-SE). Unlike previous research VUV-SE tools that utilized ultrahigh vacuum and synchrotron radiation [12], the commercial instruments are purged by dry nitrogen gas to avoid atmospheric absorption. Now, VUV-SE can reach wavelengths below 146 nm, or the equivalent to 8.5eV, without the need for synchrotron-based measurements. Lithography remains the primary application of VUV-SE [13]. Each aspect of lithography can benefit from accurate materials characterization. The optical components used in steppers are coated to optimize transmission or reflection at the lithographic wavelength. The thickness and optical constants of the coatings are very important to performance. The photoresists and anti-reflective coatings currently used for 193 nm can be qualified for potential use at 157 nm, as new materials are developed to take their place. VUV-SE is also being used to develop the coatings used for photomasks and pellicles [14]. In each case, accurate optical constants and film thickness are essential and can be obtained from a non-contact, non-destructive spectroscopic ellipsometry measurement. Another application for VUV-SE involves the study of thin gate dielectrics. The short wavelengths of this spectral range provide increased sensitivity as the layer thickness decreases. Alternative materials with high dielectric constant, such as SrTiO3, are likely to replace current gate materials. VUV-SE can be used to study the material properties and provide more insight into its optical behavior [11]. III-Nitrides and other high-bandgap materials are important for both optoelectronic devices and for high-power and high temperature applications. VUV-SE extends the study of critical points in these materials to larger photon energies [15,16]. The optical constants of AlN shown in have been measured from 0.73 to 8.5 eV. Notice that the interesting electronic transitions occur beyond 6.5 eV. The electronic transitions of hexagonal SiC are also located in the VUV. In this case, the optical response is anisotropic (different along the ordinary (o) and extraordinary (e) crystal axis). For in-plane substrates, the dielectric response along each direction can be determined from VUV-SE. presents results from a 4H SiC substrate [17]. The most interesting region for this material occurs in the VUV, where the electronic transitions along the ordinary and extraordinary axis differ radically. Conclusion The expansion of SE into both the IR and VUV is opening new doors for material characterization. Both wavelength extensions provide new information, and thus new applications. IR-SE is sensitive to free-carrier concentrations, lattice vibrations, phonon absorptions, and molecular bond absorptions. VUV-SE enhances sensitivity to ultrathin films and can characterize electronic transitions at higher energies. Both instruments have a good outlook for future semiconductor characterization. References [1] H. Tompkins, and W. McGahan, Spectroscopic Ellipsometry and Reflectometry, John Wiley & Sons, New York (1999). [2] J. A. Woollam, "Variable Angle Spectroscopic Ellipsometry," in Wiley Encyclopedia of Electrical and Electronics Engineering, J. Webster, ed., John Wiley & Sons, New York, (2000) 109117. [3] R. Azzam, and N. Bashara, Ellipsometry and Polarized Light, North Holland Press, Amsterdam, 2nd edition 1987. [4] J. Beattie, Philos. Mag. 46, 235 (1955). [5] A. Rseler, Infrared Phys. 21, 349 (1981). [6] A. Rseler, Infrared Spectroscopic Ellipsometry (Akademie, Berlin, 1990). See also http://berlin.isas-dortmund.de/g_mar [7] T. Tiwald, et al., Thin Solid Films, 313314, 662667 (1998). [8] J. Humlcek et al., Appl. Phys. Lett. 69, 2581 (1996). See also http://www.muni.cz/sci/people/JosephHumlicek. [9] A. Kasic, et al., Phys. Rev. B 62, 7365 (2000). See also http://www.uni-leipzig.de/~hlp/ellipsometrie. [10] T. Tiwald, et al., Phys. Rev. B 60, 1146 411474 (1999). [11] S. Zollner, et al., J. Vac. Sci. Technol. B18 (4) 22422254 (2000). [12] J. Barth et al., "Spectroscopic Ellipsometry in the 6-35eV Region," Chapter 10 in Handbook of Optical Constants of Solids II, ed. Palik, Academic Press, (1991). [13] J. Hilfiker et al., SPIE Proc. 3998, (2000) 390398. [14] R. French, et al. SPIE Proc.4000 (2000) 14911502. [15] J. Hilfiker, et al., Future Fab International, 8 (1999) 243247. [16] T. Wethkamp, et al., Thin Solid Films, 313314, (1998) 745750. [17] J. Woollam, et al., SPIE Proc. July 2000, submitted.