Spinning deposition of silica and silica-titania optical coatings: A round robin test
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Spinning deposition of silica and silica-titania optical coatings: A round robin test M. Guglielmi and A. Martucci Dipartimento di Ingegneria Meccanica, S. Materiali, Via Marzolo 9, 35131 Padova, Italy
R. M. Almeida and H. C. Vasconcelos INESC, R. Alves Redol 9, 1000 Lisboa, Portugal
E. M. Yeatman, E. J. C. Dawnay, and M.A. Fardad Department of Electrical Engineering, Imperial College, London SW7 2BT, England (Received 1 August 1995; accepted 11 June 1997)
A round robin test has been performed on sol-gel processing for the deposition of silica and silica-titania films on silicon substrates by spin-coating. Three solution preparation processes for silica coatings and three for silica-titania coatings were used to prepare samples at each of the participating laboratories. The films have been characterized mainly by thickness (profilometry and ellipsometry measurements), refractive index, porosity, and optical scattering. Different processes gave different thicknesses. Thickness differences were found in films prepared by the same process and by the same deposition parameters, but in different laboratories, when heat-treated at 500 ±C. Variations were reduced in samples annealed at 1000 ±C. Refractive index and porosity measurements suggest that variations were due to structural differences, particularly porosity. Furthermore, films heat-treated at 500 ±C were not completely stabilized, and showed index and porosity variations after six months.
I. INTRODUCTION Thin oxide films can be easily deposited by the solgel method using techniques such as dip, spin, meniscus, and spray coating; of these, the first three are suitable for obtaining thin layers of optical quality.1 Coating of large glass sheets by the dip-coating process has been commercialized for many years, and many other products are nowadays produced, ranging from a few millimeters to many meters in size, demonstrating the industrial feasibility of sol-gel processing.2,3 However, it is the general opinion of researchers involved in the field that the potential of the sol-gel method still requires technological development.2,3 One important reason for this is the perception that the method is not sufficiently reliable and reproducible. Indeed, there are many parameters affecting the thickness, structure, and properties of sol-gel coatings; the question to be addressed here is whether all the significant parameters are known and can be controlled. Thickness control is very important for optical coatings. A large body of literature exists on the effect of various chemical and processing parameters,4 but very often the results are not comparable because the experimental conditions are not completely specified. The same is true for microstructural control. There are many theoretical studies,4 but few experiments which relate theories to practical results. Therefore it seems that very often microstructural features (the porosity being the most important) cannot be effectively and completely controlled by the chemistr
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