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Eingeladener Vortrag
- nein (25)
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.
The present study concerns the cleaning of matenats using pulsed laser irradiarton and it summarises the most recent results obtalned by the collaborative research of different European groups, within the kamework of a European program for tralnlng and mobillrv of researchers. A series of pulsed lasers, which emit at various wavelengths (liom UV to IR) wit11 short duration of pulse (few nano-, pico- or fernto-seconds), is used for the removal of metallic, ceramic and organic pollutants kom contaminated solid surfaces of different natures. The scientific results obtained so far are focused on the laser cleaning of silicon wafers from sub-micrometer particles, tile theoretical modelling of particles removal mechanism during dry laser cleaning, the removal of oxide layers from oxidised metals and alloys, as well as on the development of laser imaging as a diagnostrc tool for the estimation of the efficiency of the proposed cleaning technique.
A complementary study of composition and bonding of sputtered a-C films, deposited on Si(001) substrates with various bias voltages (Vb) applied to the substrate during deposition is presented. The sp3 and sp2 fractions in the films were calculated by deconvolution of the X-ray photoelectron spectroscopy (XPS) C1s peak and studied by the differential auger electron spectroscopy (AES) CKLL peak signal. The results of this analysis are compared with the estimation of sp3 fraction calculated by spectroscopic ellipsometry (SE) and validated using density measurements by X-ray reflectivity. It was observed a considerable increase of sp3 content in films deposited with negative Vb. The respective sp3 and sp2 fractions and Ar concentration with respect to the Vb and the depth profile analysis give valuable information on the deposition mechanism of the sputtered a-C films. XPS valence band spectra provided the electron density of states in the a-C films valence band. The characteristic broad p band of diamond was prominent in most of the films. The valence band structure of the films was correlated with their optical response measured by SE.
A study on the bonding structure and mechanical properties of magnetron sputtered CNx thin films
(2001)
Carbon nitride (CNx) films have been deposited by reactive (RF) magnetron sputtering, in order to investigate the effect of the energetic ion bombardment during deposition (IBD), in terms of applied Vb, on their bonding structure. Fourier Transform IR Ellipsometry (FTIRE) and X-ray photoelectron spectroscopy (XPS) were used for the investigation of the films bonding structure, while their mechanical properties were evaluated by nanoindentation measurements. At films grown with low negative Vb, (low energy IBD) the N atoms are distributed homogeneously in substitutional sites in graphitic rings through both sp2 and sp3 bonds and in linear chains, through sp2 bonds. In contrast, the high negative Vb (high energy IBD) has been suggested to promote the non-homogeneous N distribution at localized regions in the films where the formation of sp3 C---N bonds is favored. This behavior was also evidenced by the C1s and N1s XPS peak components, assigned to the sp3 and sp2 carbonnitrogen bonds. Also, high energy IBD films revealed increased values of hardness and elasticity, while hardness values up to 45 GPa were measured at localized regions.
Zusammenfassung
The surface chemical modification of polyurethane (PU) films was performed by an UV laser-induced chemical reaction in a polysaccharide solution. This process may be applicable as hydrophilic packaging of implantable medical devices and in vivo sensors. When a PU film in contact with an aqueous alginic acid (AAC) solution was irradiated with a XeCl laser, the PU film turned hydrophilic. Contact angles of water on the film were reduced from 110° to 60°. Since light absorption of the AAC solution at 308 nm was negligibly small, reactive sites were generated solely on the PU surface. There, AAC could be immobilized by chemical bonds thus allowing for a nanometer-scaled grafting of this biomolecule. The mechanism was investigated by surface analyses with Fourier-transform infrared spectroscopy (FT-IR), dye staining, ultraviolet-visible (UV-VIS) spectroscopy, and scanning electron microscopy (SEM) techniques. A one-photon photochemical process could beidentified.
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.