Titania/Silica Sol-Gel Films: Comparison of Techniques for Thin Film Thickness Measurement
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Titania/Silica Sol-Gel Comparison of Techniques for Thin Film Films: Thickness Measurement S. M. Melpolder, A. W. West, M. P. Cunningham and R. Sharma, Eastman Kodak Company, Rochester, New York, 14650-2019. ABSTRACT Two techniques for thin film measurement were compared: an optical method combining ellipsometry and reflectance spectroscopy, and cross-sectional transmission electron microscopy. These techniques were used to measure the absolute thicknesses of titania/silica sol-gel films in the size range 0.1 to 0.8 microns. The relative advantages and disadvantages of these methods will be described in this study. INTRODUCTION Many sol-gel thin film applications require homogeneous defect-free layers with tight tolerances for specific properties, i.e. refractive index, thickness, surface energetics, thermal expansion coefficient, etc. In particular the thin film thickness must be controlled within a few percent in most optical and microelectronic applications.[1-4] For example, the use of sol-9el thin films to produce multi-layer antireflective coatings [4,5] requires that the refractive index and thickness of individual layers be reproducible and of a predetermined value. Lukosz [6] initially reported on the use of sol-gel films of varying composition and refractive index to prepare low loss waveguides. In order to prepare these devices one must predetermine the film properties necessary to achieve the desired spectral response. As mentioned in the previous illustrations, both the refractive index and thickness of the sol-gel film are critical to its successful application. Through experimentation, one must determine if sol-gel processing can produce the required film reproducibility. W. Beier [7] presented a technique for measuring film thickness by determining the substrate's weight gain after applying the sol-gel film. His accuracy for determining thickness was +/- 20 percent. Bel Hadj et. al. [8] used ellipsometry and interference spectra to determine the change in thickness and refractive index of titania and zirconia sol-gel films coated on soda-lime-silica substrates. Sakka et. al. [9] used refractive index data generated in this manner to estimate the residual porosity of the processed films. Various other authors have used the previously mentioned and additional methods for determining the thickness of sol-gel films coated onto silicon or glass substrates. [10-17] EXPERIMENTAL Synthesis and Sample Film Preparation Tetraethyl silicate (TIEOS) and tetraisopropyl titanate (TIPT) sol-gel solutions were prehydrolyzed separately. For the silica prepolymer a 1:10:2 molar ratio of TEOS, ethanol and water was used. In the case of the titania prepolymer TIPT, ethanol, hydrochloric acid (HCl) and water were reacted at 1:11:0.56:1 molar ratio. The TEOS and TIPT prehydrolyzed sol-gels were cooled to room temperature, their respective volumes measured and the molar concentration of each sol-gel calculated. The titania and silica sol-gels were mixed to prepare the desired titania/silica molar ratios of 40/60, 60/40 and
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