Raman spectroscopic and atomic force microscopic study of graphite ablation at 193 and 248 nm

Abstract

Experimental results are presented for the excimer laser ablation of highly oriented pyrolytic graphite at 193 and 248 nm for both single pulses and pulse trains in the fluence range of ∼1–15 J/cm 2. The morphology and the depth of the ablated pits are monitored by atomic force microscopy, while the material characterization is performed by micro-Raman spectroscopy. A shift from ∼1.12 to ∼2.23 J/cm 2 laser fluence is found in the single shot ablation threshold for the 248 nm laser wavelength compared to that at 193 nm. Broad D and G peaks in the Raman spectra indicate the formation of amorphous carbon layers as a result of laser irradiation with 193 and 248 nm pulses. This amorphous layer is present at lower fluences (several J/cm 2) and after the very first shots. The modified layer created at 193 nm, compared to 248 nm, consists of optically denser material having more turbostratical/glassy character. The spectra do not show significant changes for fluences exceeding 6–7 J/cm 2. A several hundred nanometers-high ring-like structure can be observed around the ablated pits. For laser fluences in excess of the estimated threshold at ∼6 J/cm 2 (close to the aforementioned limit), the diameter of this structure increases with laser fluence. One hypothesis to explain the ring formation and the saturation of the Raman spectra supposes that the graphite melts and squirts on the laser irradiation. The ring, debris material and the amorphous layers disappear after heat treatment of the samples at 650°C, most probably by oxidative etching.