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

Experimental results are presented for the excimer laser ablation of highly oriented pyrolytic graphite at 193 and 248 nm for both single pulses and p ulse trains in the fluence range of similar to 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 spectrosc opy. A shift from similar to 1.12 to similar to 2.23 J/cm(2) laser fluence is found in the single shot ablation threshold for the 248 nm laser wavelen gth compared to that at 193 nm. Broad D and G peaks in the Raman spectra in dicate the formation of amorphous carbon layers as a result of laser irradi ation 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 lay er created at 193 nm, compared to 248 nm, consists of optically denser mate rial having more turbostratical/glassy character. The spectra do not show s ignificant changes for fluences exceeding 6-7 J/cm(2). A several hundred na nometers-high ring-like structure can be observed around the ablated pits. For laser fluences in excess of the estimated threshold at similar to 6 J/c m(2) (close to the aforementioned limit), the diameter of this structure in creases with laser fluence. One hypothesis to explain the ring formation an d the saturation of the Raman spectra supposes that the graphite melts and squirts on the laser irradiation. The ring, debris material and the amorpho us layers disappear after heat treatment of the samples at 650 degrees C, m ost probably by oxidative etching. (C) 2000 Elsevier Science B.V. All right s reserved.