Filtern
Erscheinungsjahr
Dokumenttyp
- Beitrag zu einem Sammelband (31) (entfernen)
Referierte Publikation
- nein (31)
Schlagworte
- Ablation (5)
- Femtosecond pulse laser (3)
- Micromachining (3)
- Femtosecond-Pulse Laser (2)
- Laser processing (2)
- Semiconductors (2)
- Silicon (2)
- Acoustical measurement (1)
- Aluminium oxide (1)
- Auger electron spectroscopy (1)
Micromachining experiments were performed with Ti:sapphire laser pulses (130 fs - 150 fs, 800 nm, approximately 10 Hz) in air. Employing the direct focusing technique, highly absorbing titanium nitride (TiN) and weakly absorbing polyimide (PI) and polymethylmethacrylate (PMMA) served as target materials. The lateral and vertical precision of the laser ablation and morphological features were characterized by scanning force (SFM), scanning electron (SEM) and optical microscopy. For TiN, incubation can be observed, i.e. the single-pulse surface damage threshold (0.26 J/cm2) is by a factor of two greater than the threshold for 100 pulses. Ablation rates below 10 nm per pulse can be achieved. The evolution of sub-wavelength ripples is presented in dependence on pulse number and laser fluence, respectively. The incubation behavior of the polymers can be described by an accumulation model as for TiN. Experiments on PI with varying focal lengths result in the same modification thresholds. Different polarization states of light (linear, circular) lead to a variation of the ablation rate and to various morphological patterns in the ablation craters (wavelength ripples, cones). Swelling of PMMA occurred at fluences below the ablation threshold.
Ablation of indium phosphide wafers in air was performed with 130 fs laser pulses at a wavelength of 800 nm at a low repetition rate of 10 Hz. In order to evaluate the role of the incubation effects, the relationship between the number of laser pulses used for the ablation and the threshold fluence was studied. Particular attention was paid to the chemical composition, surface morphology and structural variations of the ablated area.
Summary form only given. Machining investigations of crystalline silicon have been performed with laser pulses at a wavelength of 780 nm in the range between 5 fs and 400 fs. Applying 100 pulses per spot, surface damage thresholds were determined by the measurement of the damage diameter. In this pulse duration regime, the threshold fluences were nearly constant. Single-pulse investigations with 5 fs pulses yielded a value of about 0.15 J cm-2 identical to the multi-pulse experiment. This is in contradiction to the behaviour of dielectrics where incubation effects alter the optical properties down to the 5 fs pulse regime. Employing laser pulses with a duration of 130 fs at a wavelength of 800 nm, single-pulse ablation thresholds of 0.23 J cm-2 and 0.16 J cm-2 were determined for Si and InP in air, respectively. The threshold fluence was calculated from the linear relation between the square of the diameters versus the logarithm of the laser fluences.
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.
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.