Mario Mosbacher

Local field enhancement effects for nanostructuring of surfaces

We report on a method that allows the nanostructuring of surfaces with intense laser pulses. For this purpose isolated polystyrene spheres with diameters in the order of the laser wavelength were deposited on a silicon or glass surface. Illumination with short and ultrashort laser pulses produced holes underneath these particles. Calculations of the field near the particles make clear that geometrical optics, that is, focusing by a spherical lens, as well as near‐field effects, contribute to the size and shape of these holes. This technique can be utilized for the parallel structuring of large surface areas with a single laser shot.

Imaging optical near-fields of nanostructures

We present a method for imaging the optical near-fields of nanostructures, which is based on the local ablation of a smooth silicon substrate by means of a single, femtosecond laser pulse. At those locations, where the field enhancement due to a nanostructure is large, substrate material is removed. The resulting topography, imaged by scanning electron or atomic force microscopy, thus reflects the intensity distribution caused by the nanostructure at the substrate surface. With this method one avoids a possible distortion of the field distribution due to the presence of a probe tip, and reaches a resolution of a few nanometers. Several examples for the optical near-field patterns of dielectric and metallic nanostructures are given.

Imprinting the Optical Near Field of Microstructures with Nanometer Resolution

Control over the optical near field is a pillar stone of material processing, microscopy, and biosensing at the submicrometer scale. The same applies to scanning probe techniques, which produce an impressive spatial resolution, and to colloidal lithography for casting large periodic nanostructure arrays. However, imaging near-field distributions with subwavelength detail remains a challenge in this context. Here we demonstrate imaging of complex two-dimensional (2D) near-field patterns imprinted on photosensitive films, resulting from interference between laser light and light scattered by dielectric microspheres. We achieve control over the resulting patterns by varying the illumination conditions and the size and arrangement of the particles.Using chalcogenide films to record the near field, the imprint produces optical, electrical, and topographical contrast and allows for the writing of erasable features as small as 10 nm. Our technique is directly applicable to any typeof scatteringparticle (size, shape, and material), thus providing a simple way of imprinting its near field. The optical near field in the vicinity of a microor nanoparticle illuminated by laser light has a spatial distribution that depends on the complex interplay between the properties of the scattering particle, the laser beam, and the substrate. Specifically, the local field enhancement induced by individual particles or sharp tips has been recently identified as a powerful means for nanopatterning applications, opening the possibility to perform subwavelength surface carving of a

Optical near field effects in surface nanostructuring and laser cleaning

We present a method for directly imaging the undisturbed near field of a particle resting on a surface. A comparison with numerical computations shows good agreement with the results of our experiments. These results have important consequences for laser-assisted particle removal where field enhancement may cause local surface damage and is one of the physical key processes in this cleaning method. On the other hand, the application of near fields at particles allows structuring of surfaces with structure dimensions in the order of 100 nm and even below.

Laser cleaning of silicon wafers: mechanisms and efficiencies

We report on experiments on the underlying physical mechanisms in the Dry-(DLC) and Steam Laser Cleaning (SLC) process. Using a frequency doubled, Q-switched Nd:YAG laser (FWHMequals8 ns), we removed polystyrene (PS) particles with diameters from 110-2000 nm from industrial silicon wafers by the DLC process. The experiments have been carried out both in ambient conditions as well as in high vacuum (10-6mbar) and the cleaned areas have been characterized by atomic force microscopy for damage inspection. Besides the determining the cleaning thresholds in laser fluence for a large interval of particle sizes we could show that particle removal in DLC is due to a combination of at least three effects: thermal substrate expansion, local substrate ablation due to field enhancement at the particle and explosive evaporation of absorbed humidity from the air. Which effect dominates the process is subject to the boundary conditions. For our laser parameters no damage free DLC was possible, i.e. whenever a particle was removed by DLC we damaged the substrate by local field enhancement. In our SLC experiments we determined the amount of superheating of a liquid layer adjacent to surfaces with controlled roughness that is necessary, in good agreement with theoretical predictions. Rough surfaces exhibited only a much smaller superheating.

Ultraviolet optical near-fields of microspheres imprinted in phase change films

We report an experimental method for directly imaging optical near-fields of dielectric microspheres upon illumination with ultraviolet nanosecond laser pulses. The intensity distribution is imprinted in chalcogenide films leaving behind a characteristic fingerprint with features below 200 nm in size, which we read out with high-resolution field emission scanning electron microscopy. The experimental results are well matched by a rigorous solution of Maxwell’s equations. Compared to previous works using infrared femtosecond laser pulses, the use of ultraviolet nanosecond pulses is identified to be superior in terms of minimum recordable features size and surface roughness of the imprint.

Dynamic particle removal by nanosecond dry laser cleaning: theory

A model for ns dry laser cleaning that treats the substrate and particle expansion on a unified basis is suggested. Formulas for the time-dependent thermal expansion of the substrate, valid for temperature-dependent parameters are derived. Van der Waals adhesion, the elasticity of the substrate and particle, as well as particle inertia is taken into account for an arbitrary temporal profile of the laser pulse. Time scale related to the size of the particles and the adhesion/elastic constants is revealed. Cleaning proceeds in different regimes if the duration of the laser pulse is much shorter/longer than this characteristic time. Expressions for cleaning thresholds are provided and compared with experiments on the cleaning of Si surfaces from spherical SiO2 particles with radii between 200 and 2585 nm in vacuum with 248 nm KrF excimer laser and 532 nm frequency doubled Nd-YAG laser. Large discrepancies between the experimental data and theoretical results for KrF laser suggest that ns dry laser cleaning cannot be explained on the basis of thermal expansion mechanism alone.

OPTICAL RESONANCE AND NEAR-FIELD EFFECTS IN DRY LASER CLEANING

Optical problems, related to the particle on the surface, i.e. optical resonance and near-field effects in laser cleaning are discussed. It is shown that the small transparent particle with size by the order of the wavelength may work as a lens in the near-field region. This permits to focus laser radiation into the area with the sizes, smaller than the radiation wavelength. It leads to 3D effects in surface heating and thermal deformation, which influences the mechanisms of the particle removal.

Femtosecond-resolved ablation dynamics of Si in the near field of a small dielectric particle

Summary In this work we analyze the ablation dynamics of crystalline Si in the intense near field generated by a small dielectric particle located at the material surface when being irradiated with an infrared femtosecond laser pulse (800 nm, 120 fs). The presence of the particle (7.9 μm diameter) leads to a strong local enhancement (ca. 40 times) of the incoming intensity of the pulse. The transient optical response of the material has been analyzed by means of fs-resolved optical microscopy in reflection configuration over a time span from 0.1 ps to about 1 ns. Characteristic phenomena like electron plasma formation, ultrafast melting and ablation, along with their characteristic time scales are observed in the region surrounding the particle. The use of a time resolved imaging technique allows us recording simultaneously the material response at ordinary and large peak power densities enabling a direct comparison between both scenarios. The time resolved images of near field exposed regions are consistent with a remarkable temporal shift of the ablation onset which occurs in the sub-picosend regime, from about 500 to 800 fs after excitation.

Laser cleaning of particles from silicon wafers : capabilities and mechanisms

Introduction The preparation of surfaces free of particle contamination is one of the crucial prerequisites for a further increase in the integration density of ICs and for the progress in nanotechnology. Therefore the removal of sub-micron sized particles from silicon wafers is of great interest. For this purpose a variety of cleaning methods is currently under investigation. In semiconductor industry an ideal cleaning technique should be capable of removing particles with a diameter considerably below 100 nm, while a damage to the substrate has to be strictly avoided. Furthermore the process should be environmentally friendly and cost-effective. A promising approach which meets these requirements is called Laser Cleaning. So far two major approaches can be distinguished. The first one is called Dry Laser Cleaning (DLC), where the surface to be cleaned is simply irradiated by a short laser pulse [1,2,3]. In Steam Laser Cleaning (SLC) a thin layer of a liquid energy transfer medium is deposited on the surface prior to the laser pulse [1,2,4,5]. In this paper we will present a short overview of these techniques followed by some of our recent results. The emphasis lies on the underlying mechanisms and an industrial applicability of both cleaning methods.