Micromachining
Due to the extreme brevity of femtosecond pulses enormous peak powers in the range of megawatts are reached at moderate average power levels. By focusing the light down to focal spots in the micrometer range the ultrafast laser is turned into a high precision tool. When applied to various materials this results in remarkably clean ablation properties due to the ionization and vaporization of the material quickly before thermal effects such as heat diffusion can occur. Even delicate materials can be processed. Further, the high pulse repetition rates of tens or hundreds of megahertz support fast processing speed and uninterrupted operation. Tool deterioration and reproducibility is not an issue, since light is not getting blunt. The ideal tool one might think. However, real world industrial applications of femtosecond lasers are at the very beginning. At present nanosecond and picosecond lasers with repetition rates in the kHz range are the first choice for applications like molding, cutting or marking.
That femtosecond lasers can be an excellent alternative for some of the applications is shown in the lithography system “Photonic Professional”. Based on direct laser writing it offers a new level in precise manufacturing of 3D nano- and microstructures. Together with Nanoscribe GmbH we have engineered a femtosecond fiber laser that is the enabling light tool for the laser writing process offered by Nanoscribe.
Why are femtosecond lasers the perfect tool for this application? The direct laser writing process makes use of laser pulses with energy below the absorption threshold of the photosensitive material. The illuminated material is transparent for the light. Only by focusing the ultrashort light pulses to a small focal spot, multi photon absorption processes in a very localized volume can be triggered. Hence, a chemical modification of this area occurs, which in a subsequent baking process leads to a local polymerization. The process allows engineering almost arbitrary 3-dimensional structures out of various photosensitive materials such as SU-8, Ormocere, PDMS, and chalcogenide glasses. Furthermore, these 3D structures can act as templates for replication (positive - positive) or inversion (positive - negative) processes into other materials like e.g. silica, and silicon. The laser lithography system routinely achieves 150 nm linewidth in a sample volume of 300x300x80 µm. Main applications include the engineering of 3D photonic crystal structures, and the generation of 3D scaffolds for biology, micro- and nanofluidic circuitry.
