Optical Properties of Silicon Nanopillar Arrays for Biosensing

Abstract: Biosensing is currently a growing research field which is relevant for different applications, for instance in health care. Sensitive and cheap biosensors are required, preferably as simple as possible in their working principle. In this work Si nanopillar structures have been fabricated and used to show the sensing principle by both depositing oxide layers with different thicknesses and by using the biotinstreptavidin model system. Si nanopillars were fabricated by two different surface patterning methods – colloidal lithography and nanoimprint lithography (obtained from a commercial source). For colloidal lithography, a modified drop-coating technique as well as a spin-coating technique is used to make self-assembled silicon dioxide (SiO2) monolayers. It is shown that SiO2 particles with sizes of 0.5 μm and 1.0 μm form even monolayers across areas of ~2 mm2 (sufficient for optical measurements) after optimizing the spin-coating parameters. Particle size reduction is done by using reactive ion etching (RIE) and nanopillars with heights of 1.0 μm to 1.5 μm are etched by inductively coupled plasma RIE (ICP RIE). Spectrally resolved reflectance from the nanopillar arrays, often show distinct reflectance peaks. Depending on the nanopillar geometry, the wavelength position of the reflectance peaks can be sensitive to changes in the refractive index at the nanopillar surface, for example by attached bio-molecules or by a thin dielectric (e.g. silicon-di-oxide) surface layer. In order to simulate the effect of a surface-bio layer on the optical properties of the nanopillar arrays, silicon-di-oxide coated Si nanopillars were investigated experimentally and theoretically. The simulated reflectance spectra, obtained by Lumerical FDTD, show that the spectral shifts of the reflectance peaks grow linearly with the layer thickness. The deposition of the oxide layers is done by plasma-enhanced chemical vapor deposition (PECVD). While this technique is reliable for planar surfaces, pillar structures showed both a much reduced side-wall oxide thickness as well as oxide pile-up on the top of the pillars. However, by thermally driven material reflow it was possible, though not completely, to redistribute the piled-up oxide from the pillar top to the sidewalls. Reflectance from Si nanopillar structures was investigated primarily using UV-vis-NIR spectrophotometer. However, ellipsometry and Fourier transform infrared spectroscopy were also used for comparison. The experimental results of the oxide layer deposition on Si nanopillars show a maximum spectral shift of 4.6 nm per every 10 nm of deposited SiO2. 3 Moreover, the obtained linear behavior of the spectral shift with oxide thickness is similar to the simulated one. In order to use the biotin-streptavidin model system to demonstrate the sensing principle with Si nanopillar structures, a surface functionalization protocol was optimized on the planar SiO2 coated Si surface. As it turns out, both an anhydrous environment and water presence during the surface silanization prior to biotinilation are acceptable and lead to similar results. Further work is necessary for effective surface functionalization of nanopillars. However, preliminary investigations of (test structures) nanopillar arrays surface functionalized by biotinstreptavidin showed spectral shifts. The sensitivity was not sufficient to perform a full assay. Optimization of the nanopillar geometry for high surface sensitivity as well as improvement in the surface functionalization process are required to produce a sensitive biosensor.