Effect of laser intensity on the properties of carbon plasmas and deposited films

The effect of laser intensity on the deposition of diamondlike carbon (DLC) films has been studied using an ArF (193 nm) pulsed excimer laser. Our results are found to be distinct from other studies using Nd-YAG infrared or excimer 248-nm lasers. Two issues concerning the growth mechanism of the films are discussed: (1) the dynamics of the laser-induced plasma and (2) the dependence of the nature of the deposited films on laser intensity. To address the first issue, time-integrated optical emission spectroscopy has been carried out to investigate the carbon plasma induced by the ArF (193 nm) laser. Instead of molecular carbon bands (${\mathrm{C}}_2$ ), monoatomic neutral (${\mathrm{C}}_{\mathrm{I}}$ ) and ionic (${\mathrm{C}}_{\mathrm{II}}$ ) emission lines are found to dominate the spectra. The emissions of (${\mathrm{C}}_{\mathrm{I}}$ ) and (${\mathrm{C}}_{\mathrm{II}}$ ) have been studied as a function of laser intensity. For low laser intensity, the laser irradiation removes the target surface material primarily through thermal evaporation. When the laser intensity is above a threshold of (3.7–4)×${10}^8$ W/${\mathrm{cm}}^2$ , the evaporated species are also ionized. The observed phenomenon can be attributed to higher multiphoton ionization and inverse bremsstrahlung rate as the laser intensity is increased. For the second issue, films deposited at various intensities have been characterized by ellipsometry. Results show that films deposited at low intensity are found to have excellent optical transparency (${\mathrm{E}}_{\mathrm{g}}$ =2.3 eV), which implies a considerable amount of ${\mathrm{sp}}^3$ bonds. However, films deposited at higher intensities are found to be more graphitic. The damage threshold has also been located at 3.7×${10}^8$ W/${\mathrm{cm}}^2$ . A qualitative structural analysis based on the effective-medium approximation has been performed on the deposited films to investigate the influence of laser intensity on their microstructures.