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.
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.
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.
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.
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.
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.
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.