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 parti cle-free, thin solid films using very high repetition rate lasers. The proc ess involves evaporation of the target by low energy laser pulses focused t o 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 demo nstration of the process by creating ultrasmooth, thin, amorphous carbon fi lms using high repetition rate Nd: YAG lasers. Both a 10 kHz, 120 ns Q-swit ched Nd: YAG laser, or a 76 MHz 60 ps mode-locked Nd: YAG laser were used i n the experiments. The number of particles visible with an optical microsco pe on the carbon film deposited using the mode-locked laser was less than o ne particle per mm(2). Scanning electron microscopy images demonstrated tha t the deposited film had a very fine surface texture with nanoscale irregul arities. Atomic force microscopy surface microroughness measurements reveal ed a saturation-like behavior of the root-mean-square roughness at <12 nm o ver the whole deposited surface area for 10 kHz Q-switched laser evaporatio n; 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 sp(3) and sp(2) bonded amorphous carbon. The thic kness of the amorphous carbon film deposited simultaneously on two 4 in. si licon wafers varied by only +/-5% over an area of similar to 250 cm(2). The deposition rate was similar to 2-6 Angstrom/s at a distance of similar to 150 mm from the target, which is 10 to 25 times higher than that achieved w ith conventional high energy low repetition rate nanosecond lasers. (C) 199 9 American Institute of Physics. [S0021-8979(99)07407-1].