se_methods_for_thin_absorbing_coatings

se_methods_for_thin_absorbing_coatings - Spectroscopic...

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© 2008 Society of Vacuum Coaters 505/856-7188 511 51st Annual Technical Conference Proceedings, Chicago, IL, April 19–24, 2008 ISSN 0737-5921 ABSTRACT Thin absorbing films are becoming common in optical coating applications. Spectroscopic ellipsometry characterization of such films requires proper techniques to insure unique results for both thickness and optical constants. This paper provides a review of ellipsometry methods to characterize thin absorbing layers. All methods benefit from either reducing unknown sample properties or increasing measurement information. While both thickness and optical constants for thin absorbing layers can be determined, mea- surement sensitivity depends on method and implementation. The advantages and limitations of each method are described, along with examples from the optical coatings field. INTRODUCTION Spectroscopic Ellipsometry (SE) is routinely used to mea- sure thickness and refractive index of transparent thin films. Absorbing films are more difficult to characterize because the optical constants ( n and k ) are often correlated with film thickness. This calls into question the uniqueness of the results. When properly implemented, SE measurements can simultaneously and uniquely determine thin absorbing layer thickness and optical constants. Optical measurements of absorbing films are increasingly common for two reasons. First, many applications now include thin absorbing coatings as a critical element of the optical design. For example, a thin Ag layer is added for low-emis- sivity (low-e) coatings. In electrochromic layers, the exchange of ions can vary the absorption within a layer, changing the optical constants with applied voltage. Second, many coatings are transparent at their design wave- lengths, but absorbing in other spectral regions. It may be beneficial to measure the coating at absorbing wavelengths to better understand material properties or monitor coating quality and performance. For example, transparent conductive oxides are intentionally transparent at visible wavelengths, but absorb longer infrared light. The optical response at infrared wavelengths is a measure of the film conductivity. Figure 1 shows the variation in extinction coefficient for an indium Spectroscopic Ellipsometry Methods for Thin Absorbing Coatings J.N. Hilfiker and R.A. Synowicki, J.A. Woollam Company, Lincoln, NE; and H.G. Tompkins, Consultant, Chandler, AZ tin oxide (ITO) coating at various stages of processing. The increase in near infrared absorption (larger extinction coef- ficient) indicates improved conductivity. Figure 1: ITO optical constants measured during processing. Increased absorption at long wavelengths is due to free-carriers, indicating improved conductivity from the annealing step. SPECTROSCOPIC ELLIPSOMETRY
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This note was uploaded on 03/15/2011 for the course FISICA 101 taught by Professor Chavez during the Spring '11 term at ASU.

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se_methods_for_thin_absorbing_coatings - Spectroscopic...

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