Hans-Ulrich Krebs

Calculations and experiments of material removal and kinetic energy during pulsed laser ablation of metals

Numerical calculations are presented, which describe the processes of target heating and ablation of pure metals (for instance Fe) during irradiation by 30 ns laser pulses at 248 nm. The following effects are taken into account: the absorption of the laser radiation and the heat conduction within the target, the evaporation of material from the target surface, the cooling of the target surface by the heat of evaporation and the partial absorption of the incident laser beam in the evaporated material. As results of the calculations, the temperature profile in the target and the ablated material can be obtained. Furthermore, by energy balance, an energy of the ablated material of up to some 100 eV is obtained. Experiments performed concerning the threshold energy of ablation, the removed mass from the target and the kinetic energy of the deposited ions (by time-of-flight measurements) are in good agreement with the performed calculations.

Pulsed laser deposition of thin metallic alloys

The pulsed KrF excimer laser ablation was applied for the preparation of thin metallic alloys. Above an ablation threshold of about 5 J/cm2, an explosive evaporation of the target material occurs leading to high deposition rates of up to 3 nm/s and a stoichiometry transfer between the target and the deposited film. The surfaces of the grown amorphous and polycrystalline films are smooth except for a small number of droplets. The pulsed laser ablation was found to be an attractive alternative to other film deposition techniques, not only for high‐temperature superconductors, semiconductors, and insulators, but also for metallic alloys.

Sub-5 nm hard x-ray point focusing by a combined Kirkpatrick-Baez mirror and multilayer zone plate

Compound optics such as lens systems can overcome the limitations concerning resolution, efficiency, or aberrations which fabrication constraints would impose on any single optical element. In this work we demonstrate unprecedented sub-5 nm point focusing of hard x-rays, based on the combination of a high gain Kirkpatrick-Baez (KB) mirror system and a high resolution W/Si multilayer zone plate (MZP) for ultra-short focal length f. The pre-focusing allows limiting the MZP radius to below 2 μm, compatible with the required 5 nm structure width and essentially unlimited aspect ratios, provided by enabling fabrication technology based on pulsed laser deposition (PLD) and focused ion beam (FIB).

Island growth and surface topography of epitaxial Y‐Ba‐Cu‐O thin films on MgO

High‐quality epitaxial Y1B2Cu3Ox thin films (Tc,o ≥ (R18) 90 K, jc (77 K)≥ (R18)3 ×106A/cm2) were in situ grown on MgO by KrF excimer laser ablation. The combination of in situ resistance measurements, x‐ray diffraction experiments, Tc measurements, scanning electron microscopy and scanning tunneling microscopy gives clear indications for an island growth on these substrates and shows growth steps and spirals at the film surface.

Resputtering during the growth of pulsed-laser-deposited metallic films in vacuum and in an ambient gas

To determine the effective sputter yield during pulsed-laser deposition a method by measuring the deposition rate on tilted substrates is proposed. Under vacuum conditions, sputter yields of up to 0.17 and 0.55 were found at a laser fluence of 4.5 J/cm2 for Fe and Ag, respectively. These strong resputtering effects are induced by the large fraction of energetic ions occurring during deposition. With decreasing laser fluence or increasing Ar gas pressure, the sputter yields are reduced due to a decrease of the kinetic energy of the ions. For the deposition of stoichiometric films, an optimum Ar partial pressure of about 0.04 mbar exists, where the deposition rate is highest and the sputter yield is reduced.

Grain orientation in thick laser‐deposited Y1Ba2Cu3O7−δ films: Adjustment of c‐axis orientation

Y1Ba2Cu3O7−δ films with a thickness between 0.5 and 5 μm were grown on Si covered with an amorphous SiO2 layer, on Zr foils, and on a single‐crystalline MgO substrates by KrF laser ablation. The influence of film thickness and substrate temperature on the structure, texture, and microstructure of the as‐grown films was investigated by x‐ray diffraction and scanning electron microscopy. At an appropriate, substrate‐dependent temperature, on all three substrate materials, the films grow c‐axis oriented up to a thickness of about 2 μm (critical thickness), followed by a sharp transition to a‐axis‐oriented growth occurring within about 100 nm. Similar changes could be observed by lowering the substrate temperature by 120 °C. Therefore, the hypothesis was propounded that the thickness dependence of the growth orientation of the film is due to a decrease of the surface temperature. To prove this the influence of raising the substrate temperature during the growth process was investigated. It could be shown that...

Determination of elastic constants in thin films using hydrogen loading

By measuring stress and strain that build up in thin films during hydrogen absorption, the elastic constants of the films can be determined, if a one-dimensional elastic behavior occurs only. This will be demonstrated for hydrogen absorption in Nb films. The in-plane stress is determined from the substrate curvature that is measured by using a two-beam laser setup. The out-of-plane strain is measured via x-ray diffraction. Furthermore, this method allows us to distinguish whether the film is plastically or elastically deformed by checking the reversibility of the stress–strain curve. In the case of a 250-nm-thick Nb film, the elastic constants obtained are similar to that of bulk Nb.

Pulsed laser deposition of metals in low pressure inert gas

The influence of an ambient Ar gas on the pulsed laser deposition (PLD) of metallic systems (Ag, Fe, Fe/Ag) is examined. Time-of-flight (TOF) measurements and measurements of the deposition rate are presented showing a reduction of particle energy with increasing Ar pressure and a reduction of resputtering and interface mixing. The determined Ar pressure for significant changes of the effective sputter yield is about 0.04 mbar. The experimental results are explained by scattering of a dense cloud of ablated material in a diluted gas.

Quantification of resputtering during pulsed laser deposition

During pulsed laser deposition in ultrahigh vacuum, the deposited material consists of a large fraction of ions with kinetic energies in the range of 100 eV. In many cases, these energetic particles induce resputtering at the film surface and lead to composition deviations. For Fe–Ag the resputtering effects are quantified by monitoring the deposition rate during growth of elementary and multilayer films. It is shown that preferential resputtering and, therefore, composition deviations of alloy films can be reduced at laser fluences close to the ablation threshold or even better under Ar gas pressure of about 0.05 mbar. The experimental results are described by a model, which includes atom deposition on the film surface, implantation of energetic ions below the surface and resputtering of atoms from the top monolayer.During pulsed laser deposition in ultrahigh vacuum, the deposited material consists of a large fraction of ions with kinetic energies in the range of 100 eV. In many cases, these energetic particles induce resputtering at the film surface and lead to composition deviations. For Fe–Ag the resputtering effects are quantified by monitoring the deposition rate during growth of elementary and multilayer films. It is shown that preferential resputtering and, therefore, composition deviations of alloy films can be reduced at laser fluences close to the ablation threshold or even better under Ar gas pressure of about 0.05 mbar. The experimental results are described by a model, which includes atom deposition on the film surface, implantation of energetic ions below the surface and resputtering of atoms from the top monolayer.

Structure of laser deposited metallic alloys

The structure of metallic alloys grown by pulsed KrF laser deposition is examined by x‐ray diffraction and compared with results from literature on sputtered and evaporated films. In general, the phases and structures obtained are similar, while laser deposited films often exhibit unusually enlarged lattice parameters in direction of growth, small grain sizes, larger extensions of the single‐phase regions and sometimes better alignment of the grains. These differences can be explained by the approximately 105 times higher instantaneous deposition rate of the pulsed laser ablation process and the high kinetic energy of the deposited particles of up to more than 100 eV.