Laser-induced Front Side Etching Research Articles

Laser-induced frontside etching of silicon using 1550-nm nanosecond laser pulses

High-quality, low-defect laser machining of dielectric materials remains challenging due to the high energy densities required for inducing the physical processes of laser ablation. Therefore, experiments were performed to demonstrate the laser-induced frontside etching (LIFE) of silicon by nanosecond laser pulses at a wavelength of 1550 nm, utilizing a thin chromium layer as the absorber. Etching was found at a fluence range from 200 to 700 mJ/cm², where the etched surfaces were smooth in the center (5 to 80 nm rms). The etching process was accompanied by incubation as well as saturation effects. At a fluence of 280 mJ/cm², an averaged etching rate of about 0.05 nm/pulse was determined for a high number of laser pulses (N > 1000). At the edges where the fluence was below the etching threshold of about 200 mJ/cm², chromium film melting and the formation of different kinds of laser-induced periodic surface structures (LIPSS) were observed.The LIFE was accompanied by different surface modifications. The optical absorption of silicon likely increased due to the incorporation of chromium into the silicon and the formed defects near the surface. Further, LIPSS were observed at fluences near the threshold of laser etching. In particular, the formation of regular linear ripples with a period of approximately 1.52 µm was observed. The morphological characteristics of the ripples suggest that the mechanism was related to surface scattering at surface features or small ripples and the hydrodynamic instabilities of the molten silicon. In addition, outside the etching track, periodic but nonlinear LIPSS were observed in the chromium film. These hexagonal arranged dot-like patterns had a period of around 440 nm. It is suggested that the hexagonal dot-like patterns resulted from the multi-pulse irradiation and the sequential stress relaxation in the chromium film below the melting point of chromium during spot movement.

Structuring of glass fibre surfaces by laser-induced front side etching

The fabrication of sub-μm structures on glass fibre surfaces poses a big challenge for the laser processing. However, the laser-induced front side etching (LIFE) method has a great potential for the fast, nm-precision, and cost-effective production of surface structures. LIFE is a method for laser etching of transparent materials using thin absorber layers with a high absorption coefficient like metal layers. The LIFE process of the front surface of a fused silica wafer as well as of a glass fibre is studied in dependence on the laser parameters. A KrF excimer laser with a wavelength of 248nm and a pulse duration of 25ns was used. The resultant structures were analysed with microscopic methods (white light interferometry, scanning electron microscopy (SEM)). The analysing of the surface structures presented that the LIFE methods allow the fabrication of well-defined periodic sub-μm structures. Furthermore, the structuring process was simulated by a thermodynamic equation including an approach of the laser–plasma interaction. The theoretically predicted results presented a good agreement with the experimental results.

Laser-induced fabrication of randomly distributed nanostructures in fused silica surfaces

The nanostructuring of dielectrics is a big challenge for laser patterning methods. In this study a novel laser structuring method for the fabrication of randomly distributed nanostructures, called laser-induced front side etching using in situ pre-structured metal layers (IPSM-LIFE), is presented. The pulsed laser irradiation of a thin metal film deposited onto a dielectric substrate with fluences below the ablation threshold results in the formation of randomly distributed metal structures by self-assembly processes. Further pulsed laser irradiation of these metal structures with higher or equal laser fluences causes the formation of complex patterns at the surface of the dielectric due to localized ablation and melting processes of the dielectric surface induced by the absorption of the laser energy by the metal structures and the local energy transfer into the dielectric surface.

Laser-induced front side etching of CaF2 crystals with KrF excimer laser

The laser-induced front side etching (LIFE) of amorphous materials like fused silica was manifold studied and the LIFE process was sufficient optimized for the fabrication of well-defined etching trenches with a very low surface roughness. The LIFE process is an indirect laser-induced ablation process, the – for the used laser wavelength – transparent substrate was covered by a highly absorbing material and the absorbing process causes a transfer of the laser energy into the substrate and, finally, to an ablation process of the substrate surface.However, the structuring of crystalline materials like CaF2 is a great challenge for the LIFE process. The properties of CaF2(111) and CaF2(001) surfaces etched by KrF excimer laser pulses (pulse duration Δtp=25ns, wavelength λ=248nm) were analysed by white light interferometry (WLI) as well as scanning electron microscopy (SEM). The surface morphologies of laser etched CaF2 surfaces depend on the laser parameters and on the crystal orientation and are frequently characterized by microcracks and flake spallation. The most probable reasons therefore are laser-induced thermal stress or laser-induced shock waves.

Laser-induced front side etching of fused silica with femtosecond laser radiation using thin metal layers

Laser-induced front side etching (LIFE) is a method for laser etching of transparent materials using thin absorber layers. In this study, chromium layers were applied as absorber for etching trenches in fused silica with femtosecond Ti:sapphire laser radiation (λ=780nm, Δtp=150fs, f=1kHz). The etching process of fused silica is studied in dependence on the laser fluence, the laser pulse numbers as well as metal absorber layer thicknesses. Especially the influence of the varied parameters on the surface morphology, the etching depth, and the surface modification is presented and discussed. The etched trenches were analyzed with white light interferometry. A fluence window from 0.5 to 2.5J/cm2 for the LIFE process of fused silica with a single laser pulse using a metal layer with a thickness from 5 to 50nm was found. The measured maximum etching depth of approximately 100nm suggests that the LIFE process is appropriated for precision machining.The attempt to simulate the fs laser LIFE process by a thermodynamic model was moderate successfully and exhibits significant differences within the experimental results.

Laser-Induced front Side Etching: An Easy and Fast Method for Sub-μm Structuring of Dielectrics

Abstract Laser-induced front side etching (LIFE) is a method for the nanometer-precision structuring of dielectrics, e.g. fused silica, using thin metallic as well as organic absorber layer attached to the laser-irradiated front side of the sample. As laser source an excimer laser with a wavelength of 248 nm and an pulse duration of 25 ns was used. For sub-μm patterning a phase mask illuminated by the top hat laser beam was projected by a Schwarzschild objective. The LIFE process allows the fabrication of well-defined and smooth surface structures with sub-μm lateral etching regions (Δx 350 nm) and vertical etching depths from 1 nm to sub-mm.

Laser-induced front side etching of fused silica with XeF excimer laser using thin metal layers

Laser-induced front side etching (LIFE) is a method for laser etching of transparent materials using thin absorber layers, e.g., for precision engineering or even optical applications.Aluminium, chromium, molybdenum, silver as well as titanium with various layer thicknesses (5–100nm) were applied as absorber for etching trenches in fused silica with nanosecond XeF excimer laser radiation. The sample surfaces were processed at laser fluences up to 10J/cm2 and laser pulse numbers from 1 to 10 pulses.A linear growth of the etching depth at rising laser fluence was found. The film thickness dependency is more complex and mostly influenced by the optical properties of the thin metal films.The influence of the laser fluence, the number of pulses, the absorber material as well as the absorber layer thickness on the etching process, the etching depth, and the surface modification were presented and discussed.A simple model is given that allows the discussion of the etching depth in dependency on the laser fluence and the metal film thickness.The measurements represented a good agreement with the calculated results by a thermal model.The LIFE method allows nm-precision etching of fused silica with etching depths up to 150nm.