Surface scattering optical loss measurements in thin oxide planar waveguide layers I. Szendrő a , Zs. Puskás b , K. Somogyi a, ⁎ , K. Erdélyi a a MicroVacuum Ltd., Kerékgyártó u.: 10, H-1147 Budapest, Hungary b Minvasive Ltd., Goldmann Gy. tér 3., H-1111 Budapest, Hungary Available online 16 April 2008 Abstract There are some typical optical loss problems in the case of optical waveguides, though there are great differences between the fibres and planar waveguides. The optical losses can be significant especially in thin layers and this can remarkably decrease the sensitivity and increase the noise of the measurements when applied in sensors. The thickness of the sensitive oxide layers ranges from nanometers up to several micrometers thus the thickness dependent transparency measured perpendicular to the surface is far from being sufficiently sensitive to characterise losses. In this work, efforts are made to characterize or visualise losses of the incoupled light propagating in the planar waveguide layer. An experimental setup was built for such measurements. The light is incoupled via a surface grating and the light scattered by various centres and scattered out from the layer is studied along the layer. Scattering or reflecting effect of the surface and interface roughness plays decisive role in thin planar waveguides. The scattered light, the position of the scattering centres, etc. are observed and detected by a CCD camera and the corresponding software allows also numerical evaluation of the detected picture. The method is described and first results are presented in this paper. © 2008 Elsevier B.V. All rights reserved. Keywords: Planar optical waveguides; Optical losses; Out of plane light scattering; Transparent conductive oxides; Silicon titanium oxide; Indium tin oxide 1. Introduction Recent development in integrated optics leads to an in- creasing demand also in transparent metal oxides. Planar optical devices require thin oxide layers and an electronic combination of these oxide layers, such as in flat panel displays of various kinds, solar cells, light emitting devices, surface sensors, ther- mally insulating windows etc. A combination of transparency and electrical conduction can be achieved in different types of materials, though these requirements are contradicting in some extent. Thus, investigation of various compositions and of their relevant properties has an increasing importance [e.g. 1–5]. One of the compromises is the use of extremely thin metallic layers. Another layer category can be found among the wide bandgap metal-oxide layers. Though there is a great variety of alternative binary and ternary oxides, the combination of SnO 2 and In 2 O 3 oxides, called as Indium Tin Oxide (ITO) achieved the greatest success both in preparation/deposition technology and applica- tion [6], though the theoretical understanding remained very limited [1]. Application of ITO can be characterised as application in form of thin layers for different purposes. In any application, however, the high transparency remains a property of pa- ramount importance beside the excellent electrical conductivity. As for the transparency, the transmittance measurements per- pendicular to the surface lead to results which lay in the range of the measurements limits, when very thin layers are applied. In this work, basically ITO and Si 1 - x Ti x O 2 (STO) waveguid- ing thin layers of 10–200 nm thickness were studied. These layers are applied for surface sensitive optical waveguide sensors [7,8]. Light scattering and optical losses in general can limit the performance of planar optical circuits as a noise generated in the waveguide layer. Our measurements also showed, however, that the optical losses measured on the ITO coated sensors perpen- dicular to the plane of the layered system are really near to the limit of measurements sensitivity since they normally represent only a few percents (1–2%) at 633 nm wavelength. Consequently, Available online at www.sciencedirect.com Thin Solid Films 516 (2008) 8215 – 8218 www.elsevier.com/locate/tsf ⁎ Corresponding author. E-mail address: karoly.somogyi@microvacuum.com (K. Somogyi). 0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.04.066