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Paper des Monats
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Zusammenfassung
Laser pulses in the 10-fs domain provide a quality of micromachining of fused silica and borosilicate glass that is unobtainable with longer pulses in the range of several 100 femtoseconds up to picoseconds. The shortening of the pulses reduces the statistical behavior of the material removal and the ablation process thus attains a more deterministic and reproducible character. The improved reproducibility of ablation is accompanied by significantly smoother morphology. This offers the potential for lateral and vertical machining precision of the order of 100 nm and 10 nm, respectively.
Ablation of indium phosphide wafers in air was performed with low repetition rate ultrashort laser pulses (130 fs, 10 Hz) of 800 nm wavelength. The relationships between the dimensions of the craters and the ablation parameters were analyzed. The ablation threshold fluence depends on the number of pulses applied to the same spot. The single-pulse ablation threshold value was estimated to be fth(1)=0.16 J/cm2. The dependence of the threshold fluence on the number of laser pulses indicates an incubation effect. Morphological and chemical changes of the ablated regions were characterized by means of scanning electron microscopy and Auger electron spectroscopy.
Abstract Femtosecond laser micromachining has excited vivid attention in various industri-
al fields and in medicine owing to the advantages of ultrashort laser pulses compared to
long-pulse treatment.These are mainly the reduction of the laser fluence needed to induce
ablation and the improvement of the contour sharpness of the laser-generated structures.
Recently,special attention was paid to femtosecond laser experiments on nonabsorbing in-
organic dielectrics.This is due to the fact that optical damage in dielectric optical elements
limits the performance of high-power laser systems.Despite the fact that a large variety of
organic polymers can be machined with excimer lasers successfully,the involvement of
thermal processes can lead to an unsatisfactory quality of the structures.Ultrashort,fs-laser
pulses might be an alternative for the treatment of polymers.Therefore,femtosecond laser
machining investigations of dielectrics and polymers are reviewed in this paper.Similarities
and differences of the ablation behavior of both material classes are discussed.The influ-
ence of the bandgap on the ablation threshold in dependence on the pulse duration,the en-
hancement of the machining precision with a shortening of the pulse duration,incubation
phenomena,and morphological features appearing on the surface after femtosecond laser
treatment are mentioned.Possible applications,e.g.,in medicine and biosensors,are de-
scribed.
We present surface micro-modifications of bariumalumoborosilicate glasses with a high transformation temperature near 700 °C. Laser processing of the glass substrate was realized by using ultra-short laser pulses at 800 nm. The morphological conversion of the laser-treated surfaces was characterized by scanning force microscopy and nano-indentation. A hardness increase by at least a factor of six on the laser ablation crater edge relative to illuminated non- ablated areas shows that the material in the crater walls underwent drastic morphological and mechanical changes. In this heat- and shock-affected zone, the material became more elastic as a result of increased stress.
Femtosecond-pulse laser ablation of dental hydroxyapatite and single-crystalline fluoroapatite
(1999)
Laser microdrilling of healthy human enamel and dentine using 300 fs pulses at a wavelength of 615 nm and 3 Hz repetition rate leads to an enhanced structuring quality in comparison with nanosecond-laser results. Microcracking and damage to neighboring tissue can be reduced. Ablation threshold fluences for 100 laser pulses of 0.3 J cm-2 (human dentine), 0.6 J cm-2 (human enamel) and 0.8 J cm-2 (single crystalline fluoroapatite) could be determined. Ablation depths per pulse below 1 7m were observed.
Zusammenfassung
The threshold fluences for laser-induced damage of fused silica with single 5-fs pulses from a Ti:sapphire laser system were determined by extrapolating the ablated volume to zero. These thresholds are about 4 times as high as the values previously obtained from multi-shot experiments. This result is interpreted in terms of an irreversible modification of the original material below the single-shot threshold (incubation).
Ablation experiments in several glasses with single and multishot irradiation by laser pulses in the 10-fs pulse duration domain are presented; physical and technological implications are discussed. We demonstrate that these short pulses offer the potential for lateral and vertical machining precision of the order of 100 nm.
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.
Damage experiments of absorbing filters (Schott BG18 and BG36) were performed with Ti:sapphire laser pulses with durations from 30 fs to 340 fs (800 nm, 1 kHz) in air. The direct focusing technique was employed under single- and multi-pulse irradiation conditions. Ablation threshold fluences were determined from a semi-logarithmic plot of the ablation crater diameter vs. laser fluence. The damage threshold fluence decreases for shorter pulse durations. In the investigated pulse duration range, the measured multi-pulse ablation threshold fluences are practically similar to those of undoped glass material (~1 Jcm-2). That means that the multi-pulse ablation threshold is independent on the doping level of the filters. For more than 100 pulses per spot and all pulse durations applied, the threshold fluence saturates. This leads to technically relevant damage threshold values in the femtosecond laser pulse duration domain.
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.
Commercially available absorption filters (Schott BG18 and BG36) were investigated with respect to their single- and
multi-pulse ablation threshold using laser pulses from a Ti:Sapphire laser in the range between 30 and 340 fs. It could be
observed, that the threshold fluence decreases for shorter pulse durations. The similarity of the measured multi-pulse
threshold fluences with those of undoped glass material (around 1 J/cm^2 for a pulse duration of 30 fs) suggests that, for
very short pulses, the threshold is independent on the doping level and therefore, linear absorption does not significantly
contribute to laser-induced damage. For >100 pulses per spot and all pulse durations applied, the threshold fluences
saturate. This independence on the number of applied pulses leads to technically relevant damage threshold values.
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.
KrF excimer laser ablation of polyethylene terephthalate (PET), polyimide (PI) and polycarbonate (PC) in air was studied by an in situ monitoring technique using a microphone. The microphone signal generated by a short acoustic pulse represented the etch rate of laser ablation depending on the laser fluence, i.e., the ablation strength. From a linear relationship between the microphone output voltage and the laser fluence, the single-pulse ablation thresholds were found to be 30 mJ cm-2 for PET, 37 mJ cm-2 for PI and 51 mJ cm-2 for PC (20-pulses threshold). The ablation thresholds of PET and PI were not influenced by the number of pulses per spot, while PC showed an incubation phenomenon. A microphone technique provides a simple method to determine the excimer laser ablation threshold of polymer films.
Near-ablation threshold investigations focusing on the generation of periodic nanostructures and their correlation with physico-chemical
properties of the solid phase such as e.g., the material-dependent surface energy, were conducted. Molecular dynamic modelling in the
sub-picosecond time domain was used to consider ultrafast opto-electronic processes triggering surface reorganization reactions.
Fluid containment of solid interfaces showed strong influence on the resulting micro- and nanostructures due to its drastic reduction of the
surface energy. The phenomena are discussed in respect to the minimization of the surface free energy in dependence of material composition
and interfacial structure.
On the damage behavior of dielectric films when illuminated with multiple femtosecond laser pulses
(2005)
The physical effects reducing the damage threshold of dielectric films when exposed to multiple femtosecond pulses are investigated.
The measured temperature increase of a Ta2O5 film scales exponentially with the pulse fluence. A polarized luminescence signal is observed that
depends quadratically on the pulse fluence and is attributed to twophoton excitation of self-trapped excitons that form after band-to-band
excitation. The damage fluence decreases with increasing pulse number, but is independent of the repetition rate from 1 Hz to 1 kHz at a constant
pulse number. The repetition rate dependence of the breakdown threshold is also measured for TiO2 , HfO2,
Al2O3, and SiO2 films. A theoretical model is presented that explains these findings.
Femto- and nanosecond laser ablation of polymethylmethacrylate (PMMA) and PMMA doped with a linear absorber was
investigated in the infrared spectral region. Ablation thresholds were determined and incubation phenomena were identified. The
degree of incubation was calculated employing a phenomenological model. The influence of the pulse duration on the
machining quality of the polymers was examined. The presence of an absorbing chromophore is not a prerequisite for a
controllable fs-laser structuring in contrast to the ns-treatment. Surface swelling always accompanied ablation.
Femtosecond laser interaction with silicon was investigated in water and in air, with 130-fs laser pulses at 800 nm wavelength. Under water confinement, higher modification thresholds, lower ablation depths and similar incubation factors were found in comparison to the dry experiment. Morphological features of the laser-induced cavities also differed. In contrast to air experiments, debris redeposition was negligible, while the ablated material remained suspended in the water layer phase. Underwater cavities obtained at high fluences and high number of pulses per spot showed anomalous profiles, consistent with a strong spatial deformation of the laser beam coupled into the target. Ripples formed at the edges of the modified area showed varying spacings: f100 and f700 nm for water and air experiments, respectively. Differences to the air experiment were related to a complex combination of fluence-dependent non-linear effects occurring in the water layer and to pulse-number-dependent shielding effects induced by cavitation bubbles and suspended ablated
material.
Femtosecond pulse laser ablation of metallic, semiconducting, ceramic, and biological materials
(1994)
Production of holes and grooves of < 30 micrometers diameter with high aspect ratio value is a delicate task either for mechanical tools, or for conventional nanosecond pulse lasers like e.g. pulsed Nd:YAG or excimer lasers. They later tend to cause microcracks extending from an annular melting zone, or substantial disruption, respectively. Experimental results are presented which demonstrate that the development of intense ultrashort pulse laser systems (>> 1012 W cm-2, (tau) < 1 ps) opens up possibilities for materials processing by cold plasma generation and ablation of metals, semiconductors, ceramics, composites, and biological materials. A femtosecond and a nanosecond dye laser with pulse durations of 300 fs (< 200 (mu) J) and 7 ns (< 10 mJ), and center wavelengths at 612 and 600 nm, respectively, both focused on an area of the order of 10-5 cm2, have been applied either to absorbing substrates, like polycrystalline gold, silicon (111), aluminum nitride ceramics, or transparent materials, like synthetic and human dental hydroxyapatite composites, bone material, and human cornea transplants. The fs-laser generates its own absorption in transparent materials by a multiphoton absorption process, and thus forces the absorption of visible radiation. Because the time is too short (< ps) for significant transport of mass and energy, the beam interaction generally results in the formation of a thin plasma layer of approximately solid state density. Only after the end of the subpicosecond laser pulse, it expands rapidly away from the surface without any light absorption and further plasma heating. Therefore, energy transfer (heat and impulse) to the target material, and thermal and mechanical disruption are minimized. In contrast to heat- affected zones (HAZ's) generated by conventional nanosecond pulse lasers of the order of 1 - 10 micrometers , HAZ's of less than 0.02 micrometers were observed.
Structuring of dielectric and metallic materials with ultrashort laser pulses between 20 fs and 3 ps
(1997)
Laser-micromachining of barium aluminum borosilicate glass, fused silica and stainless steel has been extended down to a pulse duration of 20 fs generated by a Ti:sapphire laser system at a wavelength of 0.8 micrometer. A systematic study shows that, below 100 fs, an enhanced precision and a substantial decrease of the ablation threshold fluence in comparison to pulse laser processing with pulses in the picosecond and nanosecond range could be achieved. The physical mechanism and the technical relevance of this novel microtechnology is discussed.
Femtosecond laser pulses are close to industrial use. The advantages of ultrashort laser pulses for micromachining applications especially in the case of dielectric and biological samples down to pulse durations of 5 fs have been established. The current international standards of laser safety are primarily concerned with CW and pulsed lasers down to the nanosecond range. Therefore, human tissue and laser-protection equipment was investigated with respect to its resistance and protection performance for femtosecond laser illumination down to 30 fs. This included filter glasses for laser protection eyewear, polymer and textile materials used in curtains and guards. Bulk absorber filters can provide enough protection against laser radiation, give a sufficiently broad absorption spectrum. Materials are damaged more easily by femtosecond laser radiation. The need for sufficient spectral broadness as well as the different damage thresholds have to be included in international laser safety standards. This work should trigger the development of novel eye-protection devices that are lighter and ergonomically more acceptable than present commercial models.
In situ optical second-harmonic generation (SHG) on centrosymmetric crystalline semiconductor electrodes opens up a new field of in situ investigations of hurried solid state interfaces and metal front contacts relevant to electronic and photovoltaic devices, which are rarely accessible by other methods. Photoelectrochemical nanosecond- and femtosecond-pulse laser investigations of silicon (111) electrodes show that in situ SHG is feasible in such complex interfacial systems. In a p-p polarization configuration, the azimuthal dependence of the SHG from oxide-covered and bare n-Si (111) electrodes, with and without Ni contact deposits, have been studied. Etching and regrowth of silicon oxides as well as burried interfacial electric field distributions were monitored. In situ SHG is shown to be extremely sensitive to trapped interfacial charge, crystal misorientations and surface step arrays. An advantage of femtosecond-pulses is the fact that illumination fluences that are well below the damage threshold, but still with sufficient power density, can be applied. (Author)
In situ second harmonic generation measurements during the electrodeposition of Ni on n-Si(111)
(1993)
A femtosecond pulse laser in the visible spectral region shows promise as a potentially new powerful corneal sculpting tool. It combines the clinical and technical advantages of visible wavelengths with the high ablation quality observed with nanosecond-pulse excimer lasers at 193 nm. A femtosecond and a nanosecond dye laser with pulse durations of 300 fs and 7 ns, and centre wavelengths at 615 nm and 600 nm, respectively, both focused to an area of the order of 105 cm2, have been applied to human corneal ablation. Nanosecond laser pulses caused substantial tissue disruption within a 30100 m range from the excision edge at all fluences above the ablation threshold of F th60 J cm2 (I th9 GW cm2). Completely different excisions are produced by the femtosecond-pulse laser: high quality ablations of the Bowman membrane and the stroma tissue characterised by damage zones of less than 0.5 m were observed at all fluences above ablation threshold of F th1 J cm2 or I th3 TW cm2 (3×1012 W cm2). The transparent cornea material can be forced to absorb ultrashort pulses of extremely high intensity. The fs laser generates its own absorption by a multiphoton absorption process.
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 laser ablation behaviour of ion-doped glass filter materials under irradiation with femtosecond laser pulses was investigated with respect to spot size and repetition rate. The damage threshold fluence of Schott BG18 depends on the size of the irradiated area. The effect is discussed in terms of a defect-site model.