Martin Ehrhardt

Ultra-short laser processing of transparent material at the interface to liquid

Similarly to laser-induced backside wet etching (LIBWE) with nanosecond ultraviolet (ns UV) laser pulses, the irradiation of the solid/liquid interface of fused silica with sub-picosecond (sub-ps) UV and femtosecond near infrared (fs NIR) laser pulses results in etching of the fused silica surface and deposition of decomposition products from liquid. Furthermore, the etch threshold is reduced compared with both direct ablation with an fs laser in air and backside etching with UV ns pulses. Using 0.5 M pyrene/toluene as absorbing liquid, the thresholds were determined to be 70 mJ cm−2 (sub-ps UV) and 330 mJ cm−2 (fs NIR). Furthermore, an almost linear increase in the etch rate with increasing laser fluence was found. The roughness of surfaces backside etched with ultra-short pulses is higher in comparison with ns pulses but lower than that obtained using direct fs laser ablation. Hence a combination of processes involved in fs laser ablation and ns backside etching can be expected. The processes at the ultra-short pulse laser irradiated solid/liquid interface are discussed, considering the effects of ultra-fast heating, multi-photon absorption processes, as well as defect generation in the materials.

Crystallization of Ge2Sb2Te5 thin films by nano- and femtosecond single laser pulse irradiation

The amorphous to crystalline phase transformation of Ge2Sb2Te5 (GST) films by UV nanosecond (ns) and femtosecond (fs) single laser pulse irradiation at the same wavelength is compared. Detailed structural information about the phase transformation is collected by x-ray diffraction and high resolution transmission electron microscopy (TEM). The threshold fluences to induce crystallization are determined for both pulse lengths. A large difference between ns and fs pulse irradiation was found regarding the grain size distribution and morphology of the crystallized films. For fs single pulse irradiated GST thin films, columnar grains with a diameter of 20 to 60 nm were obtained as evidenced by cross-sectional TEM analysis. The local atomic arrangement was investigated by high-resolution Cs-corrected scanning TEM. Neither tetrahedral nor off-octahedral positions of Ge-atoms could be observed in the largely defect-free grains. A high optical reflectivity contrast (~25%) between amorphous and completely crystallized GST films was achieved by fs laser irradiation induced at fluences between 13 and 16 mJ/cm2 and by ns laser irradiation induced at fluences between 67 and 130 mJ/cm2. Finally, the fluence dependent increase of the reflectivity is discussed in terms of each photon involved into the crystallization process for ns and fs pulses, respectively.

Change of electrical properties of CIGS thin-film solar cells after structuring with ultrashort laser pulses

Low-damage laser scribing of thin films to perform series interconnection (external and integrated) of thin-film CIGS solar cells for module fabrication is still a challenge. In consequence, the influence of laser scribing parameters on the electrical characteristics of thin-film CIGS solar cells must be studied in addition to standard analytical techniques for imaging and spectroscopy. Hence, CIGS solar cells were scribed with ultrashort Ti:Sapphire laser pulses with a wavelength of 775 nm and a pulse length of 150 fs. The I-V curves with the open circuit voltage, parallel, and series resistance were measured directly after the laser-scribing process and were compared with initial cell parameters. Apart from studying the influence of laser fluence etc. also various laser-scribing geometries were examined. The most significant effect of the laser-scribing procedure can be found for the parallel resistance. Laser ablation and laser-induced material modifications during scribing results in (i) alterations of the material properties of the films, e.g. the CIGS, and (ii) material modifications outside of the laser scribe, where the interfaces, e.g. p-n junction, primarily are effected; both effects are leading to the sudden decrease in parallel resistance. Morphology, topography, geometry and material modifications of the laser-scribed areas were analyzed by scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDX) and focused ion beam (FIB) cross sectioning. The results of the laserscribing induced alterations are discussed in relation to the applied scribing parameters. A model is introduced to improve the understanding of the physical reasons of the measured solar cell degradation while scribing.

Simulation of laser-induced backside wet etching of fused silica with hydrocarbon liquids

The mechanism of laser-induced backside wet etching (LIBWE) is important for the optimization of application processes but is still ambiguous. Extremely high surface absorption coefficients of more than 40×104 cm−1 at λ=248 nm that decay exponentially within less than 25 nm were measured for LIBWE-etched fused silica surface. Therefore, the resulting laser-induced temperatures quickly exceed the boiling point and result in surface erosion of the modified material. Numerical calculations of the temperature considering the measured absorption and phase transitions have been used to evaluate the etching depth of fused silica with a pyrene/toluene solution that agrees well with the measured rates well. A model of LIBWE is proposed that bases on the laser ablation of the high-absorbing modified fused silica as the dominating erosion process.

Laser-induced front side etching of fused silica with short and ultra-short laser pulses

The patterning or figuring of fused silica, e.g. for optical components, requires sophisticated methods. The usage of laser radiation enables a fast as well as high-quality machining of transparent materials. In particular, the laser-induced front side etching (LIFE) method has an excellent potential for nm-precision structuring of dielectrics with a high surface quality. At the LIFE process the laser beam interacts with an absorber layer on top of the front side of the dielectric surface to be machined. Here, the LIFE of fused silica is studied by using laser radiation with a wavelength from ultraviolet to infrared with pulse durations from nanosecond to femtosecond. With all investigated laser sources a well-defined, nm-precise etching of fused silica by the LIFE process is possible. A linear dependence of the etching depth on the laser fluence can be found whereas etching depths up to 300 nm can be achieved. The optimal laser fluence ranges as well as the achievable etching depths are dependent on the laser radiation used for the LIFE process.

Backside etching of fused silica with ultra-short laser pulses at the interface to absorbing liquid

The etching of fused silica substrates by employing the process of laser-induced backside wet etching (LIBWE) using the laser radiation of a 248nm, 500fs excimer laser and a 775nm, 130fs Ti:sapphire is presented here for the first time. Etched volume results are presented in combination with topological investigations of the etched areas performed by SEM scans, revealing new aspects of the nature and products of the process.

Nanosecond laser-induced back side wet etching of fused silica with a copper-based absorber liquid

Cost-efficient machining of dielectric surfaces with high-precision and low-roughness for industrial applications is still challenging if using laser-patterning processes. Laser induced back side wet etching (LIBWE) using UV laser pulses with liquid heavy metals or aromatic hydrocarbons as absorber allows the fabrication of well-defined, nm precise, free-form surfaces with low surface roughness, e.g., needed for optical applications. The copper-sulphatebased absorber CuSO4/K-Na-Tartrate/NaOH/formaldehyde in water is used for laser-induced deposition of copper. If this absorber can also be used as precursor for laser-induced ablation, promising industrial applications combining surface structuring and deposition within the same setup could be possible. The etching results applying a KrF excimer (248 nm, 25 ns) and a Nd:YAG (1064 nm, 20 ns) laser are compared. The topography of the etched surfaces were analyzed by scanning electron microscopy (SEM), white light interferometry (WLI) as well as laser scanning microscopy (LSM). The chemical composition of the irradiated surface was studied by energy-dispersive X-ray spectroscopy (EDX) and Fourier transform infrared spectroscopy (FT-IR). For the discussion of the etching mechanism the laser-induced heating was simulated with finite element method (FEM). The results indicate that the UV and IR radiation allows micro structuring of fused silica with the copper-based absorber where the etching process can be explained by the laser-induced formation of a copper-based absorber layer.

Nanostructuring of sapphire using time-modulated nanosecond laser pulses

The nanostructuring of dielectric surfaces using laser radiation is still a challenge. The IPSM-LIFE (laser-induced front side etching using in-situ pre-structured metal layer) method allows the easy, large area and fast laser nanostructuring of dielectrics. At IPSM-LIFE a metal covered dielectric is irradiated where the structuring is assisted by a self-organized molten metal layer deformation process. The IPSM-LIFE can be divided into two steps: STEP 1: The irradiation of thin metal layers on dielectric surfaces results in a melting and nanostructuring process of the metal layer and partially of the dielectric surface. STEP 2: A subsequent high laser fluence treatment of the metal nanostructures result in a structuring of the dielectric surface. At this study a sapphire substrate Al2O3(1-102) was covered with a 10 nm thin molybdenum layer and irradiated by an infrared laser with an adjustable time-dependent pulse form with a time resolution of 1 ns (wavelength λ = 1064 nm, pulse duration Δtp = 1 – 600 ns, Gaussian beam profile). The laser treatment allows the fabrication of different surface structures into the sapphire surface due to a pattern transfer process. The resultant structures were investigated by scanning electron microscopy (SEM). The process was simulated and the simulation results were compared with experimental results.

Pattern transfer, self-organized surface nanostructuring, and nanodrilling of sapphire using nanosecond laser irradiation

Nanostructures have a widespread field of applications. The structuring of sapphire assisted by a nanosecond laserinduced self-organized molten molybdenum layer deformation process was studied. At low laser fluence the irradiation of a thin metal layer on dielectric surface results in a melting and nanostructuring of the metal layer and partially of the dielectric surface. Furthermore, a subsequent high laser fluence treatment of the metal nanostructures results in different features: (i) pattern transfer, (ii) self-organized surface nanostructuring, and (iii) nanodrilling. (i) Pattern transfer: The irradiation of the pre-structured metal layer with high laser fluences allows the transfer of the lateral geometry of the metal nanostructures into the dielectric surface. (ii) Self-organized surface nanostructuring: The multi-pulse irradiation of the metal layer/dielectric system with moderate laser fluences results in a selforganized nanostructuring of the dielectric surface. (iii) Nanodrilling: The multi-pulse low laser fluence irradiation of the metal layer results in the formation of metal droplets and a further high fluence irradiation of the laser-generated metal droplets results in a stepwise evaporation of the metal and in a partial evaporation of the dielectric and, finally, in the formation of cone-like holes. The resultant structures were investigated by scanning electron microscopy (SEM).

Nanosecond laser-induced nanostructuring of thin metal layers and dielectric surfaces

Nanostructuring of dielectric surfaces has a widespread field of applications. In this work the recently introduced laser method validates this novel concept for complex nanostructuring of dielectric surfaces. This concept combines the mechanism of self-assembly of metal films due to laser irradiation with the concept of laser-assisted transfer of these patterns into the underlying material. The present work focuses on pattern formation in fused silica near the border of the laser spot, where distorted nested ring-like patterns were found in contrast to concentric ring patterns at homogeneous laser irradiation. For the experiments a lateral homogeneous spot of a KrF excimer laser (λ = 248 nm) and a Gaussian beam Yb fiber laser (λ = 1064 nm) was used for irradiation of a thin chromium layer onto fused silica resulting in the formation of different ring structures into the fused silica surface. The obtained structures were analysed by AFM and SEM. It is found that the mechanism comprises laser-induced metal film melting, contraction of the molten metal, and successive transfer of the metal hole geometry to the fused silica. Simulations taking into account the heat and the Navier-Stokes equations were compared with the experimental results. A good agreement of simulation results with experimental data was found. These first results demonstrate that the variation of the laser beam profile allows the local control of the melt dynamics which causes changes of the shape and the size of the ring patterns. Hence, a light-controlled self-assembly is feasible.