Filtern
Erscheinungsjahr
Dokumenttyp
- Zeitschriftenartikel (129)
- Vortrag (68)
- Posterpräsentation (31)
- Beitrag zu einem Tagungsband (29)
- Buchkapitel (10)
- Beitrag zu einem Sammelband (4)
- Monografie (1)
- Video (1)
Schlagworte
- Laser-induced periodic surface structures (LIPSS) (70)
- Femtosecond laser ablation (40)
- Femtosecond laser (32)
- Surface functionalization (31)
- Laser processing (23)
- Laser ablation (19)
- Laser-induced periodic surface structures, LIPSS (12)
- Silicon (12)
- Nanostructures (11)
- Applications (10)
- Oxidation (10)
- Friction (9)
- Radiation protection (9)
- Tribology (9)
- Wear (9)
- Ablation (7)
- Femtosecond laser processing (7)
- Laser-induced X-ray emission (7)
- Laser-induced periodic surface structures (7)
- Microstructures (7)
- Steel (7)
- High-speed optical techniques (6)
- Indium phosphide (6)
- Laser beam effects (6)
- Atomic force microscopy (5)
- Dielectrics (5)
- Electromagnetic scattering (5)
- Raman spectroscopy (5)
- Silicon compounds (5)
- Capillary waves (4)
- Chalcopyrite (4)
- Double-pulse experiments (4)
- Titanium (4)
- Ultrafast optical techniques (4)
- Ultrashort laser material interaction (4)
- Ultrashort laser pulses (4)
- Biofilms (3)
- Damage threshold (3)
- Fluid transport (3)
- Free electron laser (3)
- Glass (3)
- Hard X-ray photoelectron spectroscopy (HAXPES) (3)
- Industrial applications (3)
- LIPSS (3)
- Mach-Zehnder interferometer (3)
- Oberflächenfunktionalisierung (3)
- Optical properties (3)
- Polycarbonate (3)
- Polymer (3)
- Refractive index (3)
- Scanning electron microscopy (3)
- Semiconductor (3)
- Surface morphology (3)
- Surface plasmon polaritons (3)
- Surface structure (3)
- Time-resolved scattering (3)
- Titanium alloy (3)
- Titanium nitride (3)
- Ultrafast laser processing (3)
- Ultrashort pulsed laser (3)
- Wetting (3)
- XANES (3)
- Anodization (2)
- Auger electron spectroscopy (2)
- Cell adhesion (2)
- Chemical analysis (2)
- Cribellate spiders (2)
- Cu(InGa)Se2 (2)
- Direct laser interference patterning (DLIP) (2)
- Doping (2)
- Elemental semiconductors (2)
- Femtosecond (2)
- Femtosecond laser irradiation (2)
- Femtosecond laser patterning (2)
- Finite-difference time-domain calculations (2)
- Fused silica (2)
- Glasses (2)
- Incubation (2)
- Laser (2)
- Laser damage (2)
- Laser-Materialbearbeitung (2)
- Laser-induced forward transfer (2)
- Laser-induced forward transfer (LIFT) (2)
- Laser-induced oxide layer (2)
- Laser-induced x-ray emission (2)
- Light concentration (2)
- Lizard (2)
- Magnesium compounds (2)
- Metals (2)
- Micro solar cell (2)
- Micro solar cells (2)
- Micro-concentrator solar cell (2)
- Microscopy (2)
- Microstructure (2)
- Modelling (2)
- Nanosecond laser (2)
- Nanosecond laser ablation (2)
- Nanostrukturen (2)
- Niobium (2)
- Optical breakdown (2)
- Pacemaker (2)
- Photovoltaics (2)
- Polaritons (2)
- Potassium compounds (2)
- Pump-probe experiments (2)
- Scanning force microscopy (2)
- Secondary hazard (2)
- Selective emitter (2)
- Self-organization (2)
- Semiconductors (2)
- Silica (2)
- Spectroscopic imaging ellipsometry (2)
- Surface modification (2)
- Surface plasmons (2)
- Surface texture (2)
- Surface wetting (2)
- TeO2 (2)
- Tellurium dioxide crystals (2)
- Ti6Al4V alloy (2)
- Time-of-flight mass spectroscopy (2)
- Time-of-flight secondary ion mass spectrometry (ToF-SIMS) (2)
- Time-resolved measurements (2)
- Transmission electron microscopy (2)
- Ultra-short pulse laser processing (2)
- Ultrashort laser processing (2)
- Ultrashort pulse laser processing (2)
- X-ray (2)
- (LIPSS) (1)
- 100Cr6 (1)
- 532 nm wavelength (1)
- ASTM (1)
- Ab initio calculations (1)
- Absorber optimization (1)
- Additive (1)
- Additives (1)
- Advanced applications (1)
- Aluminium (1)
- Amorphization (1)
- Analysis (1)
- Anodic oxidation (1)
- Antiadhesive surfaces (1)
- Antibacterial surfaces (1)
- Antibakterielle Oberflächen (1)
- Antimicrobial (1)
- Application (1)
- Bacteria (1)
- Bacteria-repellent surfaces (1)
- Bacterial adhesion (1)
- Bacterial adhesion tests (1)
- Bariumalumoborosilicate glass (1)
- Bessel-Strahlen (1)
- Biofilm (1)
- Biofilm formation (1)
- Biofilm growth (1)
- Biofilme (1)
- Bioinspiration (1)
- Biomimetic (1)
- Biomimetic surfaces (1)
- Biomometics (1)
- Bionic materials (1)
- Bond lengths (1)
- Bone screws (1)
- Bug (1)
- Bulk metallic glasses (1)
- Bulk temperature (1)
- CIGS (1)
- CIGSe (1)
- CIGSe micro solar cells (1)
- Calamistrum (1)
- Carbon fibers (1)
- Carrier excitation (1)
- Carrier plasmas (1)
- Cell appendages (1)
- Cell-repellent surfaces (1)
- Ceramic matrix composites (1)
- Chalkopyrite (1)
- Chemical analyses (1)
- Coefficient of thermal expansion (1)
- Coherent scattering (1)
- Conduction bands (1)
- Copper indium gallium diselenide (CIGSe) (1)
- Copper-indium-gallium-diselenide (1)
- Crystallization (1)
- Cu(In,Ga)Se2 (1)
- CuInSe2 (1)
- D7755-11 (1)
- Darkfield microscopy (1)
- Debris (1)
- Dentistry (1)
- Diffusion (1)
- Direct laser interference patterning (1)
- Double-pulse (1)
- Drude model (1)
- Dünnschichtsolarzellen (1)
- EDX (1)
- Editorial (1)
- Electrical properties (1)
- Electrochemical treatment (1)
- Electromagnetic radiation (1)
- Electromagnetic theories (1)
- Electron backscattering diffraction (EBSD) (1)
- Electron microscopy (1)
- Electrospinning (1)
- European funding strategies (1)
- F pili (1)
- FS-laser ablation (1)
- Femtosecond laser modifications (1)
- Femtosecond laser pulse (1)
- Femtosecond laser-processing (1)
- Femtosecond phenomena (1)
- Femtosecond pulse laser (1)
- Femtosekunden-Laserablation (1)
- Femtosekundenlaser (1)
- Fluid-flow (1)
- Force distance curve (1)
- Fourier transforms (1)
- Free electron laser (FEL) (1)
- Free electron lasers (1)
- Fs-Laser (1)
- Fs-laser patterning (1)
- Fs-time resolved microscopy (1)
- Functional properties (1)
- Glass structure (1)
- Glass transformation temperature (1)
- Glow-discharge optical emission spectroscopy (1)
- Grain orientation (1)
- Grating (1)
- Heat affected zone (1)
- Heat diffusion (1)
- Hexagonally-arranged nano-protrusions (1)
- Hierarchical structures (1)
- Hydrodynamics (1)
- Implant material (1)
- Indium (1)
- Indium Phosphide (1)
- Indium islands (1)
- Industrial application (1)
- Interferometer (1)
- Ionic liquid (1)
- Isotopes (1)
- LIFT (1)
- Large area structuring (1)
- Laser Machining (1)
- Laser Processing (1)
- Laser induced periodic surface structures (1)
- Laser induced periodic surface structures (LIPSS) (1)
- Laser irradiation (1)
- Laser material processing (1)
- Laser materials processing (1)
- Laser materials-processing applications (1)
- Laser nanostructuring (1)
- Laser radiation effects (1)
- Laser safety (1)
- Laser scanning microscopy (1)
- Laser scribing (1)
- Laser technology (1)
- Laser-induced Periodic Surface Structures (LIPSS) (1)
- Laser-induced amorphization (1)
- Laser-induced damage threshold (1)
- Laser-induced micro- and nanostructures (1)
- Laser-induced nanostructures (1)
- Laser-induced periodic surface strctures (LIPSS) (1)
- Laser-induced periodic surface strcutures (LIPSS) (1)
- Laser-induced periodic surface strcutures, LIPSS (1)
- Laser-induzierte periodische Oberflächen-Nanostrukturen (1)
- Laser-induzierte periodische Oberflächenstrukturen (1)
- Laser-modified surface (1)
- Lasers (1)
- Laserschutz (1)
- Laserstrukturierung (1)
- Light transmission (1)
- Literature survey (1)
- Lizards (1)
- Lubrication (1)
- Material sciences (1)
- Materialbearbeitung (1)
- Matter reorganization (1)
- Matter reorganization theories (1)
- Medical implants (1)
- Melting (1)
- Michelson interferometer (1)
- Micro raman spectroscopy (1)
- Microbial adhesion tests (1)
- Microbial adhesions (1)
- Microconcentrator solar cell (1)
- Micromachining (1)
- Mikrostrukturen (1)
- Molybdenum (1)
- Molybdenum substrate (1)
- Monochromators (1)
- Multicrystalline silicon (1)
- Multiphoton absorption (1)
- Nanofibers (1)
- Nanometrology (1)
- Nanosecond laser irradiation (1)
- Nanosecond pulses (1)
- Nanostrcutures (1)
- Nanostructuring (1)
- Native oxide layer (1)
- Neodymium (1)
- Networks (1)
- Nonlinear lithography (1)
- Nonlinear refractive index (1)
- Oberflächenmodifikation (1)
- Ophthalmology (1)
- Optical coatings (1)
- Optical components (1)
- Optical harmonic generation (1)
- Optical microscopy (1)
- Optical pulse shaping (1)
- Optical scattering (1)
- Optical storage (1)
- Optics at surfaces (1)
- PAG (1)
- Phase mask (1)
- Phase-contrast microscopy (1)
- Photoelektron spectroscopy (1)
- Photovoltaik (1)
- Physical vapor deposition (1)
- Picosecond (1)
- Plasma formation (1)
- Plasma processing (1)
- Polarisation (1)
- Polyethylene (1)
- Polyethylene terephthalate (1)
- Polyimide (1)
- Polymethylmethacrylate (1)
- Potassium-magnesium silicate glass (1)
- Protection housing (1)
- Pulse Laser (1)
- Pulse duration (1)
- Pulse shaping (1)
- Pump-probe (1)
- Quantitative phase-contrast microscopy (1)
- Quarzglas (1)
- Reactive oxygen species (1)
- Reflectivity measurements (1)
- Refractive index engineering (1)
- Reibung (1)
- Reibungsreduktion (1)
- Ripples (1)
- Röntgenemission (1)
- SEM (1)
- Saphir (1)
- Sapphire (1)
- Scanning Auger electron microscopy (1)
- Scanning Force Microscopy (1)
- Scanning/atomic force microscopy (SFM/AFM) (1)
- Scattering (1)
- Second harmonic generation (SHG) (1)
- Second-generation high temperature superconductor technology (1)
- Sekundärstrahlung (1)
- Self-induced transparency (1)
- Semiconductor materials (1)
- Silicate glass systems (1)
- Silicate glasses (1)
- Silicon nitride (1)
- Silicon oxide (1)
- Silicon solar cell (1)
- Simulation (1)
- Sliding (1)
- Solar cell (1)
- Solid lasers (1)
- Solid-state plasma (1)
- Spatial light interference microscopy (1)
- Sputter deposition (1)
- Stiffness (1)
- Strain (1)
- Structural relaxation (1)
- Superconductivity (1)
- Surface (1)
- Surface chemistry (1)
- Surface engineering (1)
- Surface oxidation (1)
- Surface plasmon polariton (1)
- Surface processing (1)
- Surface reflectivity (1)
- Surface structures (1)
- Surface superconductivity (1)
- Theoretical modelling (1)
- Thin films (1)
- Thin-film solar cells (1)
- Threshold of silicon (1)
- Ti6Al4V alloys (1)
- TiAl64V (1)
- Time-of-flight mass spectrometry (TOF-MS) (1)
- Time-offlight secondary ion mass spectrometry (ToF-SIMS) (1)
- Time-resolved analysis (1)
- Time-resolved coherent XUV scattering (1)
- Time-resolved coherent scattering (1)
- Time-resolved microscopy (1)
- Time-resolved reflectivity measurements (1)
- Titanium alloys (1)
- Titanium nitride films (1)
- ToF-SIMS (1)
- Transmission measurements (1)
- Tribologie (1)
- Ultrafast laser excitation (1)
- Ultrafast microscopy (1)
- Ultrafast optics (1)
- Ultrafast phenomena (1)
- Ultrafast scattering (1)
- Ultrakurze Laserpulse (1)
- Ultrakurzpuls-Laser (1)
- Ultrakurzpuls-Laserbearbeitung (1)
- Ultrashort (1)
- Ultrashort lasers (1)
- VUV nanosecond laser ablation (1)
- Verschleiß (1)
- Verschleißreduktion (1)
- X-ray emission (1)
- X-ray emission hazards (1)
- X-ray energies (1)
- X-ray photoelectron spectroscopy (1)
- X-ray spectrum (1)
- XUV scattering (1)
- a-Si/my-Si (1)
- lossy materials (1)
- plasmon lifetime (1)
- second laser (1)
- surface plasmon polaritons (1)
Organisationseinheit der BAM
- 6 Materialchemie (106)
- 6.2 Material- und Oberflächentechnologien (106)
- 9 Komponentensicherheit (16)
- 9.5 Tribologie und Verschleißschutz (16)
- 6.1 Oberflächen- und Dünnschichtanalyse (9)
- 4 Material und Umwelt (8)
- 4.1 Biologische Materialschädigung und Referenzorganismen (8)
- 6.7 Materialsynthese und Design (4)
- 6.6 Physik und chemische Analytik der Polymere (3)
- 5 Werkstofftechnik (1)
Paper des Monats
- ja (1)
Some applications of polymer films require the microstructuring of partly uneven substrates. This cannot be achieved by conventional photolithography, usually performed with ultraviolet short-pulse lasers (excimer, fourth harmonic Nd:YAG). When processing thermally sensitive or undoped polymers with low optical absorption, the use of femtosecond laser pulses can improve the ablation precision, also reducing the heat-affected zone. Therefore, a Ti:sapphire laser system was employed to perform ablation experiments on polyimide (PI). The irradiated areas were evaluated by means of optical and scanning electron microscopy. Highly oriented ripple structures, which are related to the polarization state of the laser pulses, were observed in the cavities. The relationship between the ablation threshold fluence and the number of laser pulses applied to the same spot is described in accordance with an incubation model.
Laser ablation with femtosecond pulses (130 fs, wavelength 800 nm, repetition rate 2 Hz) was compared with nanosecond-pulse ablation (10 ns, wavelength 266 nm, repetition rate 2.5 Hz) of bariumalumoborosilicate glass in air using the direct focusing technique. Different ablation thresholds and heat-affected zones were observed. The lateral and vertical machining precision was evaluated. Single nanosecond laser pulses in the far UV resulted in a bubble or a circular hole in the centre of the illuminated spot, depending on the applied fluence. The ablation behaviour in the case of near-IR femtosecond pulses contrasted to this. Bubble formation was not detected. It needed repeated pulses at the same spot to modify the surface until material removal could be observed (incubation). Cavity dimensions of less than the beam diameter were achieved in this case.
Physical and chemical phenomena resulting from irradiation of silicon and indium phosphide with ultrashort laser pulses (~100fs) were investigated
with respect to the difference between single- and multiple-pulse treatment. In the single-pulse case, several processes were identified: modification,
recrystallization and ablation. All processes exhibit a distinct treshold behaviour. A two photon-absorption coefficient can be determined from a single
spatial ablation profile. Accumulation effects were observed for multi-pulse illumination. Different morphological features like bubbles, rippels and
microcolumns were found.
Multiple pulse investigations of 130-fs Ti:sapphire laser-induced damage of a high reflecting mirror consisting of alternating ?/4-layers of Ta2O5 and SiO2 and a single 500-nm Ta2O5 film were performed. In both cases, fused silica served as the substrate. For a fixed number of 1000 laser pulses per spot, a decrease in the damage threshold fluence of the mirror by a factor of two was observed by changing the repetition rate from 10 Hz to 1 kHz. A single 500-nm Ta2O5 film shows higher damage resistance compared to the mirror. The mirror and the Ta2O5 film samples were partially coated with a 300-nm-thick aluminium layer. The aluminium coating does not influence the damage threshold of the dielectrics underneath.
Structure formation on the surface of indium phosphide irradiated by femtosecond laser pulses
(2005)
Laser-induced periodic surface structures (LIPSS; ripples) with different spatial characteristics have been observed after irradiation of single-crystalline indium phosphide (c-InP) with multiple linearly polarized femtosecond pulses (130 fs, 800 nm) in air. With an increasing number of pulses per spot, N, up to 100, a characteristic evolution of two different types of ripples has been observed, i.e., (i) the growth of a grating perpendicular to the polarization vector consisting of nearly wavelength-sized periodic lines and (ii), in a specific pulse number regime (N = 530), the additional formation of equally oriented ripples with a spatial period close to half of the laser wavelength. For pulse numbers higher than 50, the formation of micrometer-spaced grooves has been found, which are oriented perpendicular to the ripples. These topographical surface alterations are discussed in the frame of existing LIPSS theories.
Multi-shot investigations of Ti:sapphire laser (wavelength (lambda) approximately equals 800 nm) induced damage were performed in three different laboratories (BAM, Berlin; LZH, Hannover; UNM, Albuquerque). The ablation behavior of a high reflecting mirror consisting of alternating (lambda) /4- layers of Ta2O5 and SiO2 was studied. Fused silica served as substrate. The influence of the pulse duration ((tau) equals 13 - 130 fs), the pulse number (30 - (infinity) ) and the repetition rate (10 Hz - 100 MHz) on the damage threshold will be discussed.
The effects of single femtosecond laser pulse irradiation (130 fs pulse duration, 800 nm center wavelength) on the structure of binary lithium silicate glasses of varying chemical compositions were investigated by micro-Raman spectroscopy. Permanent modifications were generated at the surface of the glass samples with varying laser fluences in the ablative regime and evaluated for changes in the corresponding Raman band positions and bandwidths. For increasing laser fluences, the position of certain Raman bands changed, indicating an increase in the mass density of the glass inside the irradiated area. Simultaneously, the widths of all investigated bands increased, indicating a higher degree of disorder in the glass structure with respect to bond-angle and bond-length variations.
The formation of laser-induced periodic surface structures (LIPSS) in different materials (metals, semiconductors, and dielectrics) upon irradiation with linearly polarized fs-laser pulses (τ~30–150 fs, λ~800 nm) in air environment is studied experimentally and theoretically. In metals, predominantly low-spatial-frequency-LIPSS with periods close to the laser wavelength λ are observed perpendicular to the polarization. Under specific irradiation conditions, high-spatial-frequency-LIPSS with sub-100-nm spatial periods (~λ/10) can be generated. For semiconductors, the impact of transient changes of the optical properties to the LIPSS periods is analyzed theoretically and experimentally. In dielectrics, the importance of transient excitation stages in the LIPSS formation is demonstrated experimentally using (multiple) double-fs-laser-pulse irradiation sequences. A characteristic decrease of the LIPSS periods is observed for double-pulse delays of less than 2 ps.
The formation of laser-induced periodic surface structures (LIPSS) on two different silica polymorphs (single-crystalline synthetic quartz and commercial fused silica glass) upon irradiation in air with multiple linearly polarized single- and double-fs-laser pulse sequences (τ = 150 fs pulse duration, λ = 800nm center wavelength, temporal pulse separation Δt < 40 ps) is studied experimentally and theoretically. Two distinct types of fs-LIPSS [so-called low-spatial-frequency LIPSS (LSFL) and high-spatial-frequency LIPSS (HSFL)] with different spatial periods and orientations were identified. Their appearance was characterized with respect to the experimental parameters peak laser fluence and number of laser pulses per spot. Additionally, the 'dynamics' of the LIPSS formation was addressed in complementary double-fs-pulse experiments with varying delays, revealing a characteristic change of the LSFL periods. The experimental results are interpreted on the basis of a Sipe-Drude model considering the carrier dependence of the optical properties of fs-laser excited silica. This new approach provides an explanation of the LSFL orientation parallel to the laser beam polarisation in silica—as opposed to the behaviour of most other materials.
Some nonlinear optical properties such as the nonlinear refractive index and the nonlinear effective absorption, as well as the laser-induced single-pulse ablation threshold are characterized for a series of binary and ternary silicate glasses upon irradiation with near-infrared femtosecond laser pulses (800 nm, 130 fs). The laser-induced ablation threshold varies from 2.3 J/cm² in case of potassium silicate glass up to 4.3 J/cm² in case of Fused Silica. Nonlinear refractive indices are qualitatively similar within the range 1.7-2.7×10-16 cm²/W. Complementary optical and physico-chemical properties like band gap energy and the glass transformation temperature have been measured for all the glasses.
The formation of laser-induced periodic surface structures upon irradiation of titanium, silicon, and fused silica with multiple irradiation sequences consisting of parallel polarized Ti:sapphire femtosecond laser pulse pairs (pulse duration 50–150 fs, central wavelength ~800 nm) is studied experimentally. The temporal delay between the individual near-equal energy fs-laser pulses was varied between 0 and 5 ps with a temporal resolution of better than 0.2 ps. The surface morphology of the irradiated surface areas is characterized by means of scanning electron microscopy (SEM). In all materials a decrease of the rippled surface area is observed for increasing delays. The characteristic delay decay scale is quantified and related to material dependent excitation and energy relaxation processes.
The irradiation of ~0.9-µm-thick hydrogenated amorphous carbon (a-C:H) layers deposited on silicon substrates with single femtosecond (fs) laser pulses (35 fs pulse duration, 790 nm centre wavelength) in air is studied experimentally. Irradiation spots have been generated with different peak fluences and subsequently investigated by optical topometry, micro Raman spectroscopy and microscale mechanical indentation in order to evaluate their microscopic, topographical, structural and mechanical properties (e.g. elastic modulus). By this multi-method approach, a clear separation of different effects (delamination and graphitisation) becomes possible. The joint application of mechanical and spectroscopic techniques provides unique insights into the effects of the fs-laser radiation on the carbon layer.
The selective emitter is a well-known technology for producing highly doped areas under the metallization grid to improve the solar cell performance. In this work, the influence of laser irradiation on phosphoric acid coated multicrystalline silicon PV-wafers on the wafer surface structure, the phosphorous depth distribution and the electrical contact resistance within the laser treated area as well as the electrical series resistance of laserprocessed solar cells was evaluated. Different laser processing settings were tested including pulsed and continuous wave (cw) laser sources (515 nm, 532 nm, 1064 nm wavelength). Complementary numerical simulations using the finite element method (FEM) were conducted to explain the impact of the laser parameters on the melting behavior (melt duration and geometry). It was found that the melt duration is a key parameter for a successful laser Doping process. Our simulations at a laser wavelengths of 515 nm reveal that low-repetition rate (<500 kHz) laser pulses of 300 ns duration generate a melt duration of ~0.35 µs, whereas upon scanning cw-laser radiation at 532 nm prolongates the melt duration by at least one order of magnitude. Experimentally, the widely used ns-laser pulses did not lead to satisfying laser irradiation results. In contrast, cw-laser radiation and scan velocities of less than 2 m/s led to suitable laser doping featuring low electrical resistances in the laser treated areas.
Towards an industrial laser doping process for the selective emitter using phosphoric acid as dopant
(2011)
Different laser supported approaches have already been realized, proving the great potential of laserdoped selective emitters (LDSE). However, it is challenging to establish a low-cost process by using pulsed laser tools. So far a single-step process only leads to satisfying results utilizing cw-lasers. In this paper we have examined a two-step process to produce laser-doped selective emitters on multicrystalline textured standard silicon photovoltaic wafers (90-Ω/sq-Emitter, SiN-antireflection coating (ARC)). The precise ARC removal by near-infrared fs-laser pulses (30 fs, 800 nm), and the doping of uncoated silicon wafers by ns-laser pulses (8 ns, 532 nm) were systematically investigated. In the fs-experiment, optimum conditions for ARC removal were identified. In the nsexperiments under suitable conditions (melting regime), the phosphorous concentration underneath the wafer surface was significantly increased and the sheet resistance was reduced by nearly a factor of two. Moreover, electrical measurements on fired metallization fingers deposited on the laser processed wafers showed low contact resistances. Hence, wafer conditioning with combined fs-laser- and ns-laser-processes are expected to be a promising technology for producing selective emitters.
Structuring of thin-film photovoltaic modules requires basic knowledge of the laser – thin-film interaction in order to adapt the accessible laser parameters, like wavelength, power, repetition rate and scribing speed whilst taking into account the specific material properties of the layer. We have studied the nanosecond laserablation behavior of corresponding layers (i) of silicon based thin-film solar cells with a-Si/µc-Si tandem absorber type and (ii) of back contact and absorber layer of CIGSe solar cells. The respective ablation threshold fluences were determined as integrative parameters describing the specific laser – material interaction. For the threshold determination we used two different methods and developed a new analytical approach taking into account scribing through the glass substrate as it is preferred for most structuring processes. This was done by analyzing the thin film ablation results by means of optical microscopy, profilometry, scanning electron microscopy (SEM). Moreover, we determined the incubation coefficient of the regarded material layers which allows us to predict quantitatively the influence of the spot overlap on the scribing threshold.
Manufacturing of CIGSe thin film solar modules involves typically one laser structuring step (P1) and two mechanical structuring steps (P2 and P3) for serial interconnection. In our approach, complete laser structuring is successfully demonstrated by application of short nanosecond laser pulses (<10 ns) with a single, visible wavelength of 532 nm. The P1 and the P3 trenches are scribed by induced and direct ablation, respectively. For the P2 scribe, the thermal input of the ns laser pulses is used to transform the CIGSe absorber layer locally into a highly conductive compound to provide proper electrical interconnection. These findings promise further simplification and flexibility to thin film solar cell production.
Ein wesentlicher Vorteil der Dünnschichtphotovoltaik gegenüber der waferbasierten Photovoltaik liegt in der monolithischen Serienverschaltung. Bei der Herstellung von Chalkopyrit(CIGSe) -Dünnschicht-Solarmodulen erfolgen dafür typischerweise ein Laser-Strukturierungsschritt (P1) und zwei mechanische Strukturierungsschritte (P2, P3). In diesem Beitrag wird gezeigt, dass die Strukturierung von CIGSe-Solarmodulen vollständig mit kurzen Laserpulsen (<10 ns Pulsdauer) und einer einzigen Wellenlänge (532 nm) möglich ist. Der P1- und P3-Schnitt erfolgen durch direkte induzierte Ablation. Für den P2-Schnitt wird gezielt der hohe Wärmeeintrag der ns-Laserpulse genutzt, um die CIGSe-
Absorberschicht lokal aufzuschmelzen und strukturell so zu verändern, dass eine elektrisch gut leitende Verbindung zwischen Front- und Rückkontakt entsteht.
Femtosecond diffraction dynamics of laser-induced periodic surface structures on fused silica
(2013)
The formation of laser-induced periodic surface structures (LIPSS) on fused silica upon irradiation with linearly polarized fs-laser pulses (50 fs pulse duration, 800?nm center wavelength) is studied experimentally using a transillumination femtosecond time-resolved (0.1 ps-1 ns) pump-probe diffraction approach. This allows to reveal the generation dynamics of near-wavelength-sized LIPSS showing a transient diffraction at specific spatial frequencies even before a corresponding permanent surface relief was observed. The results confirm that the ultrafast energy deposition to the materials surface plays a key role and triggers subsequent physical mechanisms such as carrier scattering into self-trapped excitons.
Near-IR femtosecond (fs) (pulse duration = 150 fs, wavelength = 775 nm, repetition
rate 1 kHz) and VUV nanosecond (ns) (pulse duration = 20 ns, wavelength = 157 nm,
repetition rate 1 to 5 Hz) laser pulse ablation of single-crystalline TeO2 (c-TeO2 ) surfaces
was performed in air using the direct focusing technique. A multi-method
characterization using optical microscopy, atomic force microscopy and scanning
electron microscopy revealed the surface morphology of the ablated craters. This allowed
us at each irradiation site to characterize precisely the lateral and vertical dimensions of
the laser-ablated craters for different laser pulse energies and number of laser pulses per
spot. Based on the obtained information, we quantitatively determined the ablation
threshold fluence for the fs laser irradiation when different pulse numbers were applied to
the same spot using two independent extrapolation techniques. We found that in the case
of NIR fs laser pulse irradiation, the ablation threshold significantly depends on the
number of laser pulses applied to the same spot indicating that incubation effects play an
important role in this material. In the case of VUV ns laser pulses, the ablation rate is
significantly higher due to the high photon energy and the predominantly linear
absorption in the material. These results are discussed on the basis of recent models of the
interaction of laser pulses with dielectrics. In the second part of this chapter, we use time-
of-flight mass spectrometry (TOFMS) to analyze the elemental composition of the
ablation products generated upon laser irradiation of c-TeO2 with single fs- (pulse
duration ~200 fs, wavelength 398 nm) and ns-pulses (pulse duration 4 ns, wavelength
355 nm). Due to the three order of magnitude different peak intensities of the ns- and fs
laser pulses, significant differences were observed regarding the laser-induced species in
the plasma plume. Positive singly, doubly and triply charged Te ions (Te+, Te2+, Te3+) in
the form of many different isotopes were observed in case of both irradiations. In the case
of the ns-laser ablation, the TeO+ formation was negligible compared to the fs case and
there was no Te trimer (Te3+) formation observed. It was found that the amplitude of Te
ion signals strongly depends on the applied laser pulse energy. Singly charged oxygen
ions (O+) are always present as a byproduct in both kinds of laser ablation.
The effects of femtosecond laser pulse irradiation on the glass structure of alkaline silicate glasses were investigated by x-ray absorption near edge structure spectroscopy using the beamline of the Physikalisch-Technische Bundesanstalt at the electron synchrotron BESSY II in Berlin (Germany) by analyzing the magnesium Κ-edge absorption peak for different laser fluences. The application of fluences above the material modification threshold (2.1 J/cm²) leads to a characteristic shift of ~1.0 eV in the Κ-edge revealing a reduced (~3%) mean magnesium bond length to the ligated oxygen ions (Mg-O) along with a reduced average coordination number of the Mg ions.
The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica with multiple irradiation sequences of parallel polarized Ti:sapphire femtosecond laser pulse pairs (160 fs pulse duration, 800 nm central wavelength) was studied experimentally. For that purpose, a Michelson interferometer was used to generate near-equal-energy double-pulse sequences allowing the temporal pulse delay between the parallel-polarized individual fs-laser pulses to be varied between 0 and 40 ps with ~0.2 ps temporal resolution. The surface morphologies of the irradiated surface areas were characterized by means of scanning electron and scanning force microscopy. In the sub-ps delay range a strong decrease of the LIPSS periods and the ablation crater depths with the double-pulse delay was observed indicating the importance of the laser-induced free-electron plasma in the conduction band of the solids for the formation of LIPSS.
The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica and silicon with multiple (NDPS) irradiation sequences consisting of linearly polarized femtosecond laser pulse pairs (pulse duration ~150 fs, central wavelength ~800 nm) is studied experimentally. Nearly equal-energy double-pulse sequences are generated allowing the temporal pulse delay Δt between the cross-polarized individual fs-laser pulses to be varied from -40 ps to +40 ps with a resolution of ~0.2 ps. The surface morphologies of the irradiated surface areas are characterized by means of scanning electron and scanning force microscopy. Particularly for dielectrics in the sub-ps delay range striking differences in the orientation and spatial characteristics of the LIPSS can be observed. For fused silica, a significant decrease of the LIPSS spatial periods from ~790 nm towards ~550 nm is demonstrated for delay changes of less than ~2 ps. In contrast, for silicon under similar irradiation conditions, the LIPSS periods remain constant (~760 nm) for delays up to 40 ps. The results prove the impact of laser-induced electrons in the conduction band of the solid and associated transient changes of the optical properties on fs-LIPSS formation.
The formation of laser-induced periodic surface structures (LIPSS) on titanium upon irradiation with linearly polarized femtosecond (fs) laser pulses (τ = 30 fs, λ = 790 nm) in an air environment is studied experimentally and theoretically. In the experiments, the dependence on the laser fluence and the number of laser pulses per irradiation spot has been analyzed. For a moderate number of laser pulses (N < 1000) and at fluences between ~0.09 and ~0.35 J/cm², predominantly low-spatial-frequency-LIPSS with periods between 400 nm and 800 nm are observed perpendicular to the polarization. In a narrow fluence range between 0.05 and 0.09 J/cm², high-spatial-frequency-LIPSS with sub-100-nm spatial periods (~λ/10) can be generated with an orientation parallel to the polarization (N = 50). These experimental results are complemented by calculations based on a theoretical LIPSS model and compared to the present literature.
We investigated the initial modification and ablation of crystalline silicon with single and multiple Ti:sapphire laser pulses of 5 to 400 fs duration. In accordance with earlier established models, we found the phenomena amorphization, melting, re-crystallization, nucleated vaporization, and ablation to occur with increasing laser fluence down to the shortest pulse durations. We noticed new morphological features (bubbles) as well as familiar ones (ripples, columns). A nearly constant ablation threshold fluence on the order of 0.2 J/cm2 for all pulse durations and multiple-pulse irradiation was observed. For a duration of ,100 fs, significant incubation can be observed, whereas for 5 fs pulses, the ablation threshold does not depend on the pulse number within the experimental error. For micromachining of silicon, a pulse duration of less than 500 fs is not advantageous.
The ultrafast laser ablation of silicon has been investigated experimentally and theoretically. The theoretical description is based on molecular dynamics (MD) simulations combined with a microscopic electronic model. We determine the thresholds of melting and ablation for two different pulse durations =20 and 500 fs. Experiments have been performed using 100 Ti:Sap-phire laser pulses per spot in air environment. The ablation thresholds were determined for pulses with a duration of 25 and 400 fs, respectively. Good agreement is obtained between theory and experiment.
Laser ablation of single-crystalline indium phosphide (InP) was performed in air by means of linearly polarized Ti : sapphire femtosecond pulses (800 nm, 130 fs, 10 Hz). As a result of the irradiation with a variable number of laser pulses per spot (N /spl les/ 5), several morphological changes (crater formation, rim formation, ripple structures, and cones) were observed. These effects were explored using force modulation microscopy (FMM), a technique based on scanning force microscopy, allowing the simultaneous imaging of both topography and local stiffness at a high lateral resolution. The first laser pulse induces the formation of a protruding rim (height < 20 nm, width /spl ap/ 300 nm) bordering the ablated crater. A Fourier analysis of the multipulse generated topographies reveals the formation of wavelength-sized periodic ripples (modulation depth < 100 nm) with an orientation perpendicular to that of the electric field vector of the laser radiation. Besides these morphological alterations, material modifications were also observed in the irradiated regions by means of the FMM technique. Within the ablated craters, local stiffness variations were found revealing an inhomogeneous material composition/structure as a consequence of the femtosecond pulse laser treatment.
Femtosecond time-resolved microscopy has been used to analyze the structural transformation dynamics (melting, ablation, and solidification phenomena) induced by single intense 130 fs laser pulses in single-crystalline (100)-indium phosphide wafers in air on a time scale from ~100 fs up to 8 ns. In the ablative regime close to the ablation threshold, transient surface reflectivity patterns are observed by fs microscopy on a ps to ns time scale as a consequence of the complex spatial density structure of the ablating material (dynamic Newton fringes). At higher fluences, exceeding six times the ablation threshold, optical breakdown causes another, more violent ablation regime, which reduces the energy deposition depth along with the time of significant material removal. As a consequence, ablation lasts longer in a ring-shaped region around the region of optical breakdown. This leads to the formation of a crater profile with a central protrusion. In the melting regime below the ablation threshold, the melting dynamics of indium phosphide has been quantified and subsequent superficial amorphization has been observed upon solidification on the ns time scale leading to amorphous layer thicknesses of the order of a few tens of nanometers.
We report self-assembly of periodic surface structures in a commercial block copolymer (BCP) (Filofocon A) upon irradiation with a few tens of excimer laser pulses (20 ns, 193 nm) at fluences above the ablation threshold. This new type of structures is characterized by much larger periods than those characteristic for Laser-Induced Periodic Surface Structures (LIPSS) and features nanochains instead of ripples. We find a period of 790?nm at 400 mJ/cm², scaling linearly with laser fluence up to a maximum of 1.0 µm. While an entangled random network of nanochains is produced for normal-incidence and non-polarized light, nanochain alignment can be achieved either by irradiation at an angle or by using linearly polarized light, forming a lamella-like structure. In both cases, the nanochains are aligned parallel to the penetrating polarization orientation and their period does not show a dependence on the angle of incidence, as opposed to the general behavior of standard LIPSS. Also, our results show that the chains are not formed by frozen capillary waves. In contrast, we show analogies of the nanochains produced to lamellar structures fabricated on a smaller scale in other BCP. We discuss the origin of the self-assembly process in terms of a combination of chemical (BCP), optical (surface scattering), and thermal (melting, coarsening, and ablation) effects.
Compositional dependent response of silica-based glasses after femtosecond laser pulse irradiation
(2013)
Femtosecond laser pulse irradiation of inorganic glasses allows a selective modification of the optical properties with very high precision. This results in the possibility for the production of three-dimensional functional optical elements in the interior of glass materials, such as optical data storage, waveguide writing, etc. The influence of the chemical glass composition to the response upon ultrashort laser irradiation has not been studied systematically. For that, simple silicabased model glasses composed of systematically varying alkaline- and earth-alkaline components were prepared, irradiated on the surface and in the volume with single fs-laser pulses (~130 fs, 800 nm), and were subsequently analyzed by means of micro-Raman spectroscopy and quantitative phase contrast microscopy in order to account for changes in the glass structure and for alterations of the optical refractive index, respectively. The Raman spectroscopic studies of the laser-irradiated spots revealed no change in the average binding configuration (the so called Q-structure), but local changes of bond-angles and bond-lengths within the glass structure structure. Those changes are explained by structural relaxation of the glass network due to densification caused by a transient laser-induced plasma generation and the following shock wave and other thermal phenomena. Glasses with a low amount of network modifiers show changes in the Si-O network while glasses with a high amount of network modifiers react primarily via variation of the nonbridging oxygen ions. The results are discussed in terms of possible structural response mechanisms and conclusions are outlined regarding glass compositions with technical suitability for fs-laser modifications.
Nonlinear optical properties such as the nonlinear refractive index and nonlinear absorption are characterized by z-scan measurements for a series of silicate glasses upon irradiation with laser pulses of 130 fs duration and 800 nm center wavelength. The stoichiometry of the silicate glasses is varied systematically to reveal the influence of the glass composition on the nonlinear optical properties. Additionally, the thermal properties such as glass–transformation temperature and thermal expansion coefficient are obtained from dilatometric measurements. It is found that the nonlinear refractive index is mainly related to the silica matrix. The nonlinear absorption is increased with the addition of network–forming ions.
The formation of near-wavelength laser-induced periodic surface structures (LIPSS) on silicon upon irradiation with sequences of Ti:sapphire femtosecond laser pulse pairs (pulse duration 150 fs, central wavelength 800 nm) is studied theoretically. For this purpose, the nonlinear generation of conduction band electrons in silicon and their relaxation is numerically calculated using a two-temperature model approach including intrapulse changes of optical properties, transport, diffusion and recombination effects. Following the idea that surface plasmon polaritons (SPP) can be excited when the material turns from semiconducting to metallic state, the 'SPP active area' is calculated as function of fluence and double-pulse delay up to several picoseconds and compared to the experimentally observed rippled surface areas. Evidence is presented that multi-photon absorption explains the large increase of the rippled area for temporally overlapping pulses. For longer double-pulse delays, relevant relaxation processes are identified. The results demonstrate that femtosecond LIPSS on silicon are caused by the excitation of SPP and can be controlled by temporal pulse shaping.