Spectroscopic characterisation of DLC films deposited on polycarbonate by pulsed laser ablation

Diamond-like carbon (DLC) films have been synthesised by KrF-excimer pulsed laser deposition (PLD) at a wavelength of 248 nm, with laser fluences between 1 and 37 J cm−2, in vacuum and in a nitrogen atmosphere. The substrates were (100) Si wafers coated with a polycarbonate (PC, LEXAN type) layer a few μm thick. Structural analysis was performed by combined visible and UV Raman spectroscopy. In visible Raman spectra, D and G bands are found; in UV Raman spectra, the blue-shifted G band and the T peak are observed. A further band at approximately 1400 cm−1 is found in spectra from vacuum-deposited films at low fluences. The evolution of Raman fitting parameters with increasing laser fluence shows that in the films deposited in vacuum, a structural transition from mainly disordered graphitic to predominantly amorphous, tetrahedrally bonded carbon (ta-C) has occurred at 10 J cm−2. In the presence of an N2 atmosphere during deposition, important modifications of the Raman spectra, associated with changes in the expansion dynamics of the carbon plasma plume are observed. Fourier-transform infrared (FTIR) spectroscopy on such films gave evidence of bands due to vibrations of the CN, CH and NH groups. Mechanical properties of the films were measured by nanoindentation; the hardness H of vacuum-deposited films with the highest sp3 content was 8 GPa. The hardness of DLC films prepared in a nitrogen atmosphere at the highest fluence was 5 GPa.