A multi-method study of femtosecond laser modification and ablation of amorphous hydrogenated carbon coatings
Source: BAM
Carbon based coatings are an interesting class of thin layer materials with several properties making them promising candidates for high-tech applications. They can be manufactured on arbitrarily shaped surfaces in a very efficient process with a high level of control over the layer properties. They have interesting mechanical properties, like hardness up to the diamond-like regime, high elastic modulus, biocompatibility. Apart from the mechanical aspect of these materials, there are interesting optical and electrical applications in discussion for carbon layers. The high-performance mechanical properties of carbon layers make them difficult to process post deposition.
Laser materials processing with ultrashort laser pulses provides a potential solution to this problem. This technique can be used to ablate or modify surfaces in a very local way with a minimal size of the heat-affected zone. To understand and optimise this process, it is crucial to understand how laser processing influences materials, especially in the modification regime below the ablation threshold. Precise control over the material properties by laser treatment could open the way to many applications where the exact dielectric, mechanical, and thermal properties of a thin layer coating has to be be controlled with microscopic resolution.
In the present work we used correlative imaging spectroscopy to analyse thin amorphous hydrogen-containing carbon (a-C:H) layers. As the central bridging method, we used spectroscopic imaging ellipsometry (SIE) allowing for the simultaneous determination of the local dielectric function and the layer thickness of a thin layer material with micrometre lateral resolution. We combined this with topographic measurements by means of white-light interference microscopy and profilometry and with Raman spectroscopy. Together, these methods deliver a complete picture of thin film materials treatment with laser light in the ablation and sub-ablation regime. For the first time, we determined the local dielectric function after irradiation of this class of materials, depending on the location relative to the laser beam. We managed to determine thresholds for the ablation and materials modification and to prove for the first time an interference enhancement mechanism depending on the absorption coefficient of the layer material. This work further focusses on the validation of all results by correlating parameters from different methods (hybrid metrology) enabling a very high reliability of the results.
A multi-method study of femtosecond laser modification and ablation of amorphous hydrogenated carbon coatings
Kenta Hirahara, Jörn Bonse, Jörg Krüger, Matthias Weise, Elena Ermilova, Robert Schusterbauer, Andreas-Neil Unterreiner, Andreas Hertwig
Applied Physics A, A 130, 860, 2024