Damage and ablation of large bandgap dielectrics induced by a 46.9 nm laser beam

Periodic surface structure of 4H-SiC by 46.9 nm laser

This paper presents an experimental study on the laser-induced atomic and close-to-atomic scale (ACS) structure of 4H-SiC using a capillary-discharged extreme ultraviolet (EUV) pulse of 46.9 nm wavelength. The modification mechanism at the ACS is investigated through molecular dynamics (MD) simulations. The irradiated surface is measured via scanning electron microscopy and atomic force microscopy. The possible changes in the crystalline structure are investigated using Raman spectroscopy and scanning transmission electron microscopy. The results show that the stripe-like structure is formed due to the uneven energy distribution of a beam. The laser-induced periodic surface structure at the ACS is first presented. The detected periodic surface structures with a peak-to-peak height of only 0.4 nm show periods of 190, 380, and 760 nm, which are approximately 4, 8, and 16 times the wavelength. In addition, no lattice damage is detected in the laser-affected zone. The study shows that the EUV pulse is a potential approach for the ACS manufacturing of semiconductors.

Characterizing the Grating-like Nanostructures Formed on BaF2 Surfaces Exposed to Extreme Ultraviolet Laser Radiation

Monocrystalline barium fluoride (BaF2) slab targets were irradiated by focused 46.9-nm laser radiation at various fluence levels above the ablation threshold. Well-developed ablation patterns with sharp edges were studied by AFM (atomic force microscopy). Their inner surfaces were uniformly covered by periodic structures. The spatial period of the ripples depends on the laser fluence. When the sample is rotated by 45°, the orientation of the grating-like structure changes accordingly. Thus, the grating vector of the periodic structure seems to be coupled to the crystallographic planes of the single crystal. This means that the XUV-laser induced ripples reported here differ from LIPSS (laser-induced periodic surface structures) associated with interference phenomena occurring on illuminated surfaces. Therefore, other mechanisms are discussed to explain the formation of the periodic nanostructures reported in this article.

Soft x-ray laser beamline for surface processing and damage studies

We have developed a soft x-ray laser (SXRL) beamline equipped with an intensity monitor dedicated to ablation study such as surface processing and damage formation. The SXRL beam having a wavelength of 13.9 nm, pulse width of 7 ps, and pulse energy of around 200 nJ is generated from Ag plasma mediums using an oscillator-amplifier configuration. The SXRL beam is focused onto the sample surface by the Mo/Si multilayer coated spherical mirror. To get the correct irradiation energy/fluence, an intensity monitor composed of a Mo/Si multilayer beam splitter and an x-ray charge-coupled device camera has been installed in the beamline. The Mo/Si multilayer beam splitter has a large polarization dependence in the reflectivity around the incident angle of 45°. However, by evaluating the relationship between reflectivity and transmittance of the beam splitter appropriately, the irradiation energy onto the sample surface can be derived from the energy acquired by the intensity monitor. This SXRL beamline is available to not only the ablation phenomena but also the performance evaluation of soft x-ray optics and resists.

Si and Cu ablation with a 46.9-nm laser focused by a toroidal mirror

Si and Cu targets were ablated by a capillary-discharge 46.9-nm pumped laser beam, focused by a toroidal mirror at grazing incidence. The peak power density of the focal spot was ~2 × 10⁷ W/cm². Clear ablation patterns on the surfaces of Si and Cu targets were obtained, with shapes consistent with simulations.

A YAG

Ce scintillator (cerium-doped YAG crystal) was used to image the variations of the laser spots. We discuss the shape and damage mechanics of the measured patterns. Melting of the target material was observed in the ablation region on Cu, but not on Si.

LiF, an underestimated supercontinuum source in femtosecond transient absorption spectroscopy

We present supercontinua generated in LiF and CaF(2) revealing that LiF is advantageous especially in the near UV region since it pushes the cut-off wavelength about 17 nm towards lower wavelengths and the occurrence of color centers, which has been considered as a drawback up to now, is not a limitation for its applicability in femtosecond transient absorption spectroscopy. Even though the color centers occur within a short time of illumination, they do not influence the supercontinuum generation significantly and they can furthermore, if desired, be eliminated from the substrate simply by heating. Thus LiF is a promising substrate for broad band measurements in the UV/vis range.