Ultrafast ablation with high-pulse-rate lasers. Part II: Experiments on laser deposition of amorphous carbon films

Ultrafast pulsed laser deposition is a novel technique for depositing particle-free, thin solid films using very high repetition rate lasers. The process involves evaporation of the target by low energy laser pulses focused to an optimum intensity to eliminate particles from the vapor. This results in films with very high surface quality while the very high repetition rate increases the overall deposition rate. Here we report an experimental demonstration of the process by creating ultrasmooth, thin, amorphous carbon films using high repetition rate Nd:YAG lasers. Both a 10 kHz, 120 ns Q-switched Nd:YAG laser, or a 76 MHz 60 ps mode-locked Nd:YAG laser were used in the experiments. The number of particles visible with an optical microscope on the carbon film deposited using the mode-locked laser was less than one particle per mm². Scanning electron microscopy images demonstrated that the deposited film had a very fine surface texture with nanoscale irregularities. Atomic force microscopy surface microroughness measurements revealed a saturation-like behavior of the root-mean-square roughness at <12 nm over the whole deposited surface area for 10 kHz Q-switched laser evaporation; and almost at the atomic level (<1 nm) for the 76 MHz mode-locked laser evaporation. Raman spectroscopy of the deposited films indicated that they consisted of a mixture of ${\mathrm{sp}}^3$ and ${\mathrm{sp}}^2$ bonded amorphous carbon. The thickness of the amorphous carbon film deposited simultaneously on two 4 in. silicon wafers varied by only ±5% over an area of ∼250 cm². The deposition rate was ∼2–6 Å/s at a distance of ∼150 mm from the target, which is 10 to 25 times higher than that achieved with conventional high energy low repetition rate nanosecond lasers.