Laser-induced nanopatterning by means of microspheres

The project suggests a novel approach which combines the advantages of single-step laser materials patterning with focused beams with the high throughput of large-area parallel processing. The first part concentrates on investigations of light focusing and interference effects from quartz microspheres arranged in a closely packed hexagonal monolayer. The microspheres shall be attached to a transparent quartz plate by natural adhesion forces. Laser light illuminates the substrate to be processed through this support. Each individual sphere acts as a lens. At certain distances behind the focus nontrivial interference patterns with high energy concentration arise. They are useful for laser material processing. The dependence of both the individual foci and the higher order interference patterns on the diameter of microspheres and laser wavelength shall be investigated. The optimization of optical parameters shall be supported by model calculations. In the second part of the project, these radiation patterns shall be applied for parallel processing of a large number of nano-structures and for applications in nano-science and technology via ablation, etching, deposition, and structural modification. We want to employ the present technique for single-step nano-patterning on a sub-100 nm scale. Usually, feature sizes obtained in optical experiments cannot be smaller than the wavelength of light. However, certain physical and chemical nonlinearities, permit one to overcome this limit. In particular, with femtosecond pulses nonlinear absorption concentrates deposited energy into sub-wavelength regions. Arrays of microspheres automatically provide a combination of small regular spacing, and large numerical aperture in each of the tightly packed "microlenses". Large numerical apertures are necessary for tight focusing and efficient usage of nonlinear optical effects. Arrays of objectives with such spacing and such high numerical apertures are simply unavailable. The investigations shall lead to the development of a flexible and expandable laser single-step processing tool, which meets the industrial demands for easy handling and high throughput desired in applications.

Laser-induced patterning of material surfaces has actual and potential applications in micromechanics, sensor technology, micro- and optoelectronics, chemcial engineering, biotechnology and medicine. In most of these cases, patterning is performed by "direct writing", by projection of the laser light via a mask, by employing a contact mask or by laser-beam interference. Within the present project on LASER-INDUCED NANOPATTERNING BY MEANS OF MICROSPHERES, we have developed a new method that permits to generate with a single laser pulse up to several billions of micro- / nanostructures. The technique employs a regular, two-dimensional (2D) lattice of microspheres. Such lattices are formed by well- known self-assembly processes from colloidal suspensions. In contrast to earlier investigations we use such 2D lattices not as lithographic masks for consecutive processes but as an array of microlenses either on a transparent support or directly on the substrate. In the present investigations, we mainly employed microspheres of fused quartz and of a polymer (polystyrene). The microspheres focus the incident laser radiation onto the substrate. By this means, surface patterning by local laser-induced ablation, etching, deposition and surface modification has been demonstrated. Depending on the diameter of spheres, thousands, millions or even billions of structures can be generated with a single laser shot. By employing interference effects, the number density of features can significantly exceed the number density of microspheres. Due to optical, physical and/or chemical non-linearities and near-field effects, features with submicrometer- and nanometer-dimensions can be produced. Among the examples are hexagonal periodic arrays of nanoholes, nanobumps, nanocones, and different types of chemical/physical surface modifications on various different substrate materials. Among those are metal films, silicon wafers, different types of oxides, polymer foils, etc. By changing the angle of incidence between the laser beam and the substrate, patterns with different shapes can be generated. The experimental results can be described semiquantitatively on the basis of local enhancements of the electromagnetic field and non-linear laser-matter interactions.