Laser ablation | EPFL Graph Search

Laser ablation or photoablation (also called laser blasting) is the process of removing material from a solid (or occasionally liquid) surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimates. At high laser flux, the material is typically converted to a plasma. Usually, laser ablation refers to removing material with a pulsed laser, but it is possible to ablate material with a continuous wave laser beam if the laser intensity is high enough. While relatively long laser pulses (e.g. nanosecond pulses) can heat and thermally alter or damage the processed material, ultrashort laser pulses (e.g. femtoseconds) cause only minimal material damage during processing due to the ultrashort light-matter interaction and are therefore also suitable for micromaterial processing. Excimer lasers of deep ultra-violet light are mainly used in photoablation; the wavelength of laser used in photoablation is approx

Scanning excimer laser ablation of poly(ethylene terephthalate) (PET) and its application to rapid prototyping of channels for microfluidics

Frank Rüdiger Wagner

Excimer Laser Ablation is a micro-machining method which has undergone an important breakthrough in the last twenty years. In contrast with most literature studies, dealing with Static Ablation, this PhD work focuses on the study of Scanning Ablation of poly(ethylene terephthalate) (PET) at 193 nm, using relatively high fluences of about 1 J/cm2. Here, Static Ablation means that the substrate is fixed with respect to the laser beam. In this way, the size of the machined area per set of pulses does not exceed a few mm2. Moving the substrate under the laser beam during the irradiation, i.e. using Scanning Ablation, enables to machine more easily and with a better quality bigger surfaces. This is particularly interesting for micro-channel prototyping in PET (typical cross-section of the channels: 40×40 µm2, length: >1.5 cm), which was the aimed application of this work. The surface properties of the ablated micro fluidic channels were studied, comparing Static Ablation to Scanning Ablation with different parameters. For this purpose SEM, TEM, XPS, water condensation experiments and electroosmotic flow measurements in laminated micro-channels were carried out. The properties of the ablated surfaces were shown to be modified, in terms of chemical properties and in terms of morphology. In order to explain the modified surface properties after Scanning Ablation, the angle between the irradiated ramp, forming under the beam, and the non irradiated surface was shown to be one key-parameter, the shape of the laser spot on the substrate being the other one. The chemical composition of the ablated surfaces depends on the nature and amount of redeposited debris. There are two types of debris: Indirect debris, which is produced by collision of the ejected material with the ambient atmosphere. After ablation in air, this debris is hydrophilic due to the oxygen and nitrogen content. The geometry of the debris covered area, and thus the amount of indirect debris in the micro channels, depends on the geometry of the irradiated area. Direct debris, which is produced by a mechanism very similar to Pulsed Laser Deposition, is only possible in Scanning Ablation. It is directly ejected in the direction of the channel surface and it is less influenced by the surrounding atmosphere. Condensation experiments proved it to be more hydrophobic than indirect debris. In Static Ablation, only indirect debris is present, and the debris affected surfaces are therefore hydrophilic. In Scanning Ablation, the amount of direct and indirect debris vary differently with the ramp angle and the shape of the irradiated spot. When increasing the ramp angle from 0° to 45°, a maximum electroosmotic flow velocity is measured around 4.3°, which is explained by a maximum of the ratio indirect / direct debris for this angle. Electroosmotic flow velocity measurements in channels of the same width but with different depths (polyethylene laminated), combined with a numerical implementation of an analytical formula for the flow velocity profile, enabled to calculate the ζ-potentials of the ablated the surfaces for different ramp angles and the ζ-potential of the lamination. Besides the chemical composition, Scanning Ablation also changes the morphology of the ablated surfaces. At high ramp angles, the surface structure of the channel floor is determined by the direct debris redeposition leading to a nanometric roughness. The micrometric roughness, which is known to develop in stretched polymer substrates upon Laser Ablation, depends on the ramp angle θ during channel fabrication. As the ramp is the only irradiated surface in scanning ablation, it was studied in detail. Three different structures were observed on ramps in stretched polymer substrates, each in one of three distinct ranges of ramp angles. On the basis of a vector decomposition of the fabrication induced stresses in the polymer, the different structures can be understood as a step wise suppression of the Static Structure formation on the ramps. The stress component normal to the irradiated surface acts as an inhibitor of the Static Structure formation. At ramp angles lower than 10°, the structure on the ramps is identical to the structure on statically ablated surfaces. At 10° < θ < 30° the structure changes but is still present. At ramp angles higher than 30° the ramps are smooth. As the micrometric structure on the channel floor is given by the structure on the lower part of the irradiated ramp, the channel floor structure is of the same type as the structure on the ramp. However, it is not possible to obtain a smooth channel floor. The angle ranges for the different structures do not depend on the laser fluence but vary from one substrate material to another. The presence and the orientation of the micrometric structure only have negligible influence on the electroosmotic flow in the micro channels. In conclusion, the message of this work is that Scanning Ablation modifies drastically the ablated surface properties. This must be taken into account when thinking about fabricating devices for which properties such as wettability, adhesion, ζ-potential or micro/nano-roughness are important.

EPFL2000

In-situ Laue diffraction to follow dislocation patterning during fatigue

Ainara Irastorza Landa

The macroscopic strength of metals is determined by the dislocation arrangements that are formed when dislocations slip in the crystal lattice in response to the applied stress. Despite the extensive research carried out, the transition from uniform to non-uniform dislocation structures is not yet fully understood. This information is however essential to support the development of computational models that aim to predict dislocation patterning. Lattice rotation caused by the presence of dislocations is, for instance, a parameter that is often not taken into account in computational models although it plays a role in the development of dislocation ensembles. Experimentally following in-situ dislocation patterning including involved lattice rotations at lengthscales comparable to those in simulations is not straightforward. In fact, the current available techniques that have the necessary spatial and angular resolution are either destructive (3D-EBSD) or very time consuming (3D X-ray Laue diffraction). In this dissertation we present a new experimental approach that allows following lattice rotation and dislocation ensembles time-resolved during deformation. The technique is based on X-ray Laue diffraction scanning in transmission mode, which provides a sub-micron spatial resolution in 2D and statistical information on orientation spread in the third dimension. The in-situ mechanical tests are performed at the microXAS beamline of the Swiss Light Source Synchrotron. The method has been applied to understand dislocation pattering of copper single crystals occurring in the earlier phases of low cycle fatigue. Samples with different crystal orientations (single crystal, coplanar and collinear) have been cyclically deformed up to a maximum of 120 cycles. A dedicated miniaturized shear-fatigue device compatible with Laue diffraction has been constructed for that purpose and a sample preparation based on picosecond laser ablation has been developed. The developed procedure analyzes the evolving dislocation microstructures in terms of lattice rotation, lattice curvature and geometrically necessary dislocation densities at lengthscales similar to those addressable in computational models. It sacrifices on the fully 3D aspect but it brings the time resolution required to validate ongoing simulations. It has been shown that the error made by integration depends on the dislocation network expected. For instance, the technique can be used to validate 3DDD models or density based dislocation dynamics models when applied in deformation geometries where the formation of 2D dislocation patterns are expected (e.g. vein-channel structure in single slip oriented fatigued samples). The greatest advantage of this technique is that the evolving regions are easy to follow due to the high rotational sensitivity and time resolution of the method. What is more, the influence of the initial microstructure can be evaluated and quantitative values can be provided. On the other hand, the principal drawbacks are related to the gauge thickness of the samples. Even if the technique can estimate the orientation spread along the third direction, the approach cannot physically resolve the integrated signal along the thickness. This is source of uncertainty when providing actual values of lattice curvatures and geometrically necessary dislocation densities. Another shortcoming is the sensitivity of the method to the energy distribution of the X-ray beam provided in the beamline, which determines the collected signal - the principal basis of the subsequent analysis and interpretation.

EPFL2016

Développement et étude d'une technique de microsablage à haute résolution

Anne-Gabrielle Pawlowski

We have used powder blasting as a technique to structure brittle materials like glass using an erosion resistant mask. The study of the erosion rate shows that the erosion of glass is maximum at a normal incidence of the powder beam while the erosion rate of metals and elastomers is minimum. A metallic contact mask is very resistant, easy to use, but its resolution as defined by its manufacturing process (laser ablation) is limited. The resolution of the mask depends on the thickness of the metal piece. However due to the stress generated by the powder blasting technnique, the metal mask used must have an important thickness (about 0,5 mm), which limits the resolution (50 μm). The photolithographic masking method we developed allows to increase the resolution. We have demonstrated that the erosion resistance of Polydimethylsiloxane (PDMS), an elastomer very frequently used in microsystems realisation is excellent, and that the use of a photosensitve epoxy resist like SU8 allows to define high-resolution structures. The replication quality is good as the majority of structures defined in PDMS have the same size compared to the features defined by the SU8. In our process, the SU8 is patterned into a mould structure that is filled with PDMS. The SU8, has a similar erosion behavior as glass and is quickly removed by powder blasting. The resolution of the process determined by the particle size ; we have used particles with 10 μm diameter to explore the ultimate limits of powder blasting. The study of the etching by powder blasting shows that the etching rate changes with the width of the channel and the etching time. Furthermore, dimensions of channels or hole structures change during powder blasting, they are increased by the underetching effect. This underetching depends on the depth of the channels and can induce changes in resolution.The erosion mask resistance is function of the thickness of the mask, of the etching time and of the applied pressure. We typically have chosen a PDMS mask thickness of 50 μm. When the applied pressure of the powder jet increases, the PDMS erosion is more important. When an energetic particle impacts on the PDMS, the glass underneath the mask can be damaged and the resolution of the structures is decreased. Experimentally, this PDMS/SU8 mask is resistant and efficient for an applied pressure up to 200 kPa and for a glass erosion depth of 300 μm for large channels. The minimum feature size is 20 μm, but this depends also on the shape ; for example triangular holes are only well defined for a minimum dimension of 60 μm.

EPFL2006