Comparative Study of the Surface Roughness of Oxide Thin Films
Reactive low voltage ion plating (RLVIP) is based on plasma assisted electron beam evaporation of metallic starting material that is ionized and reacts with ionized and exited oxygen upon arriving at the surface to be coated. In earlier experiments, the roughness of the coating surface remained in the 0.2 to 0.6 nm root mean square (rms) range even for rather thick multilayer coatings when measured with an optical surface profilometer or a contacting stylus instrument. Contrary to this observation, films made with conventional electron beam evaporation have surface roughness increasing with film thickness. Smooth surfaces are of considerable interest for any optical coating because of low light scattering.
We investigates the surface of one silica and zirconia single layer coating each using atomic force microscopes (AFMs) of the types: WYKO MicroProbe 3D, Park Scientific AutoProbe (at Spectra Physics), and Digital Instruments NanoScope II (at FIU). A contacting stylus-type profiler (Talystep, at NAWC), and a noncontact optical heterodyne surface profilometer (WYKO TOPO 2-D, at NAWC) were used for comparison. First, AFM measurements using an experimental AFM (at WYKO) and the AutoProbe were repeated with the same samples after they were sized to fit the NanoScope II. For the repeating measurements at WYKO, a new MicroProbe 3D was used. The measurements yielded results that were consistent for each individual AFM, but fairly inconsistent in comparison. Talystep measurements compared in part with AFM measurements although different spatial wavelengths are contained in the surface profiles. The WYKO optical profiler averages over areas at least 0.5.tm in diameter and does not resolve short spatial wavelengths, yielding 0.4 nm rms roughness for the substrate and coating surfaces alike.
In conclusion, we compared classical profiling of smooth RLVIP coatings with AFM measurements using three different instruments. Optical profiling renders coating roughness comparable to that of uncoated substrate surfaces, due to limited spatial resolution. Stylus profiling and AFMs include shorter spatial wavelengths, yielding larger coating roughness. This test showed also that different AFMs give different results.
KeywordsOptical Surface Optical Coating Surface Roughness Measurement Bare Substrate Spatial Wavelength
Unable to display preview. Download preview PDF.
- 1.J.M. Bennett, and L. Mattson, “Introduction to Surface Roughness and Scattering,” Opt. Soc. Am., Washington, D.C. (1989).Google Scholar
- 2.J.M. Bennett, Surface finish and its measurement, “Collected Works in Optics,” Vol. 2, Opt.Soc. Am., Washington, D.C. (1992).Google Scholar
- 3.K.H.Guenther, J.A. McCandless, and F.D.Orazio, Jr., Correlation of light scattering measurements and visual ranking of optical surfaces, Appl. Opt. 32 (1993).Google Scholar
- 5.E.L. Church, and P.L. Takacs, Instrumental effects in surface finish measurement, /n:“Surface Measurement and Characterization,” J. M. Bennett, ed., Proc. SPIE 1009, 46–55 ( SPIE - The International Society of Optical Engineering, Bellingham, WA ) (1989).Google Scholar
- 6.ISO 9211, “Optical Coatings,” Draft International Standard (International Standardization Organization, Geneva, Switzerland).Google Scholar
- 9.K.H. Guenther, Optical thin films deposited by energetic particle processes, In:“Thin Films for Optical Systems,” K. H. Guenther, ed., Proc. SPIE 1782, 344–355 (SPIE - The International Society of Optical Engineering, Bellingham, WA).Google Scholar
- 10.K.H. Guenther, Recent advances in optical coating technology: reactive ion plating deposition of Optical coatings, In: “Optical Thin Films and Applications,” R. Herrmann, cd., Proc. SPIE 1270, 211–221 ( SPIE–The International Society of Optical Engineering, Bellingham, WA ) (1990).Google Scholar
- 11.K. H.Guenther, Recent Advances in Reactive Ion Plating Deposition. In:Optical Thin Films III: New Developments,“ I. R. Seddon, cd., Proc. SPIE 1323, 29–39 (SPIE - The International Society of Optical Engineering, Bellingham, WA).Google Scholar
- 12.T. Takagi, “Ionized Cluster Beam Deposition and Epitaxy,” Noyes Publications, Park Ridge, New Jersey (1988).Google Scholar
- 13.D.T. Wei, Ion beam interference coating for ultralow optical loss, Appl. Opt. 28: 2813–2819.Google Scholar
- 14.K.H. Guenther, M.M. Tchrani, and J.M. I3ennett, Coating and substrate surface roughness, In: “OSA Annual Meeting Technical Digest,” Opt. Soc. Am., 58 (1991).Google Scholar
- 15.K.11. Guenther, D. Boon Loo, D. Burns, J. Edgell, D. Windham, and K.-H. Muller, Microstructure analysis of thin films deposited by reactive evaporation and by reactive ion-plating, J. Vac. Sci.Tech. A 7, 1436–1445.Google Scholar
- 16.F.J. Boero, R.A. Chipman, and K.H. Guenther, Smoothing of optical surfaces with dielectric thin films produced by reactive ion plating deposition, In: “Laser Induced Damage in Optical Materials: 1988,” H. E. Bennett, A. H. Guenther, B. E. Newnam, and M. J. Soileau, eds., NISI Spec. l’ubl., U. S. Dept. of Commerce, 775, 339–347 (1989).Google Scholar