Nanostructuring of surfaces for optical and visual security applications using lithography techniques
Nowadays, the use of micro- or nano-patterned surfaces is well established in wide fields of science and in numerous applications in the industry. Periodic surface-relief profiles with dimensions in the wavelength range of the light can act as diffractive optical elements but moreover, the regular pattern can also modify physical properties of a material’s surface. In this dissertation, we focus on the patterning of surfaces by lithography techniques. The useful structured areas are several square centimeters (4-100cm2) large and the dimensions of the -periodic surface-relief structures are in the range of hundreds of nanometers to micrometers (= 160nm - 10m). We present different lithographic methods, in particular interference lithography, to originate the structures in photoresist. We introduce novel setups for multiple beam interference, interference in dense media and sequentially exposures with controlled phaseshift. A way to realize arbitrarily profiles is to use the Fourier Synthesis (FS). The desired profile is decomposed in a series of sinusoidal functions having a determined amplitude, phase and harmonic number, which is an integer number of the base period of the desired structure. The sum of these sinusoidal functions, aka Fourier components, gives an approximation of the desired profile. Thus, the more components are added, the better becomes the approximation. But what happens when the harmonic number is not exactly an integer? Through the dissertation, we cover all the aspects, theoretical as well as practical, of the FS. We present a setup that allows to expose sequentially Fourier components and demonstrate the potential of the FS technology by three examples. These consist in saw-tooth profiles, micro-prisms and a fan-out function profile. The profiles were realized with ten Fourier components with structure’s period between 1.8m and 4.7m. We investigate also one- as well as two- dimensionally profiles for Optically Variable Devices (OVDs) for visual security against counterfeiting, which exhibit interesting color and polarization properties. Part of them was realized by using the moiré phenomenon.
Record created on 2011-03-30, modified on 2016-08-09