Nanoscale surface modifications of diamond-like carbon films by in situ laser irradiation in atomic force microscope

A combined laser-AFM-Raman setup is applied to study nanoscale surface modifications in DLC (a-C:H) films caused by multipulse laser irradiation at low (sub-threshold) fuences. The beam of a pulsed Nd:YAG laser (wavelength lambda = 532 nm, pulse duration tau = 7 ns, pulse repetition rate f = 1-2.5 kHz) is introduced into the optical path of an atomic force microscope (AFM), matched with that of a Raman confocal microscope. Three laser-induced processes that determine nanoscale surface modifications are found to occur in laser spots: (i) annealing - resulting in subnanometer surface deepening at the spot edges, (ii) graphitization - leading to surface swelling of 5-10 nm height, and (iii) nanoablation (oxidation) - resulting in formation of microcraters of few nm to hundred nm depth in the spot center. Raman spectra confirm the surface graphitization in the spots during formation of nm-height hillocks and microcraters, evidencing a decrease of the graphitized layer thickness at lower fluences. The nanoablation rates of 4 x 10(-5)-4 x 10(-3) nm/pulse are determined for multipulse irradiation of a-C:H films at the fluences of E = 0.045-0.1 J/cm(2). The extremely low rates of laser ablation of the DLC surface are characteristic of pulsed laser-assisted oxidation with the activation energy of similar to 6.3 kcal/mol estimated from the Arrhenius plot of the ablation rate-on-fluence dependence.