State-of-the-art nanolithography machines employ extreme ultraviolet (EUV) light to pattern nanometer-scale features on silicon wafers for the production of integrated circuits. This radiation is generated in a laserproduced plasma formed on tin microdroplet targets. In this contribution, we give an overview of our recent experimental and theoretical studies on the properties of tin plasmas driven by short-wavelength lasers and the subsequent tin fluid dynamics. First, we will present a comprehensive characterization of the properties of laserproduced tin plasmas driven by lasers with wavelengths in the 1–10 µm range. Second, we present absolutely calibrated, charge-state-resolved measurements of the ion kinetic energy distribution recorded under multiple detection angles. Through extensive radiation-hydrodynamic simulations of the plasma formation, growth and expansion, we demonstrate that a single-fluid approach accurately reproduces the angular dependence of the ion energy distribution. Moreover, we identify the origin of a high-energy peak in the distribution as a high-speed shell generated at early times in the expansion. Finally, we show that the time evolution of the droplet target morphology is entirely determined by the early-time plasma-driven pressure impulse on the droplet.
We have investigated the permanent refractive index changes inside fused silica glass after laser inscription of waveguides using ultrashort laser pulses at different wavelengths. To this end the laser frequency was detuned using an optical parametric amplifier (OPA) combined with a confocal arrangement (for cleaning the laser Gaussian spatial profile). The suitability for waveguiding and the refractive index changes were inspected by measuring the near- and farfield output profiles of the laser-machined lines using a continuous wave laser at 660 nm. Raman spectroscopy was performed both for tracks of damage and for good optical waveguides. The structural changes were inferred from peak shifts and relative intensity fluctuations associated with representative Raman bands. In fused silica, changes in the 605 cm-1 peak, which is due to 3-membered Si-O ring structures, were monitored. Since the laser photon energy rules the order, k, of the multiphoton absorption in dielectrics, the role of the laser wavelength for waveguide fabrication will be discussed in terms of different powers of the laser intensity (I(r,z)k(λ)) and the Keldysh formalism for strong field ionization (WSFI(λ)).
The permanent refractive index change induced by ultrashort laser pulses in zinc phosphate glasses has been investigated both at the surface and in bulk. At the sample surface, irradiations have been performed by using loosely focused single fs-laser pulses at different energies. Optical microscopy images of the irradiations illustrate an interferometric pattern in form of concentric Newton rings due to the laser induced multilayer system (unmodified glass, thin laser-modified layer, air). This experimental reflectivity modulation along with simulations based on Abeles theory for multilayer optical systems allows retrieving laser-induced refractive index changes on the order of Δns= -10-3. In bulk, fs-laser written waveguides have been generated by translating the sample with respect to a tightly focused laser beam. The so-produced waveguides have been characterized by studying the optical near field of the TEM00 guided mode at 660 nm and using white light microscopy. The optical changes linked to the inscribed waveguides have been characterized by measuring the far field output profiles yielding values of approximately Δnb= +3·10-4. The laser-modified optical properties in bulk and at the surface will be linked to the glass structural changes as well as discussed in terms of the role of the incubation effects for multi-pulse processing.
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