OPTICAL SENSING AND ANALYSIS SYSTEM BASED ON POROUS LAYERS Andras Kovacs 1* , Aina Malisauskaite 1 , Alexey Ivanov 1 , Ulrich Mescheder 1 and Rainer Wittig 2 1 Institute for Applied Research and faculty Computer&Electrical Engineering, Furtwangen University, Robert-Gerwig-Platz 1, 78120 Furtwangen, GERMANY 2 Institut für Lasertechnologien in der Medizin und Messtechnik, University of Ulm, Helmholtzstr. 12, 89081 Ulm, GERMANY ABSTRACT Analysis concept of gaseous or liquid media using porous silicon multilayers with optical sensing is presented. The sensor consists of optical filter with λ/4 layers, fabricated by electrochemical etching in HF/ethanol. The optical system is tested with ampicillin and shows a linear dependence with increasing ampicillin concentration. For general sensing applications the influences of porosity and pore size on the optical output are investigated. Additionally, transport processes of gases and fluids in the nanostructured porous layer are investigated to optimize the sensor response time and to evaluate further specific material properties of the investigated media which are needed for analytical information. KEYWORDS: optical sensing, porous multilayers, analysis system INTRODUCTION Si-based microporous or mesoporous layers with pore size in the range 2-40 nm – tunable to the specific application – provide large inner surface (100m 2 /cm 3 - 800m 2 /cm 3 ) [1]. Such materials are quite sensitive for analytical measurements, i.e. material properties changes considerably by filling media into the pores or by attachment of molecules to the pore walls of the large inner surface. The optical output is based on a characteristic spectral shift of multilayers formed by periodically modified porous layers with slightly different porosity and thus effective refractive index. However, the sensor’s optical output provides in this case only integral information on effective refractive index of filled media and on the pore filling state. Further sensor characteristics have to be used to obtain material specific data (e.g. permeance→ viscosity, time response→vapor pressure). THEORY The porous layers are generated by anodization of silicon in a wet electro-chemical process which is a one-step and low-cost method to build porous optical multilayers e.g. as λ/4 layers with well-defined thicknesses and refractive indices (depending on porosity) as shown in Figure 1. The periodic structures of alternating λ/4 layers are defined by the refractive indices and physical thicknesses of the corresponding high (n H ) and low (n L ) refractive layers fulfilling the condition λ c /4=n H ˑd H =n L ˑd L and thus forming a so-called Distributed Bragg Reflector (DBR) with a central peak wavelength of the reflectance spectrum at λ c . Figure 1: Schematic and SEM cross-section of a porous DBR-multilayer optical filter The multilayer’s optical output can be tuned in respect to the needed sensor performance (measurement range, sensitivity, time response). Additionally, the multilayers provide mechanical filter characteristics which can be used to get additional information about the analyzed media. A tight control of pore size and a narrow pore size distribution is essential to provide analytical information when using porous multilayers. Low temperature anodization allows an accurate control of the critical process parameters and thus material properties. The use of tunable optical multilayers have been reported already for different sensor applications [2, 3, 4, 5]. The basic sensor signal is based on a change of the effective refractive index by filling the porous multilayers with the media under investigation (Figure 2) and is therefore in general not material specific. 978-0-9798064-6-9/µTAS 2013/$20©13CBMS-0001 275 17th International Conference on Miniaturized Systems for Chemistry and Life Sciences 27-31 October 2013, Freiburg, Germany