Origin and avoidance of droplets during laser ablation of metals

During pulsed laser deposition (PLD) of metals (and other materials) one typical problem occurs: beside atoms and ions macroscopic droplets (up to some μm) are expelled from the roughening target and are deposited on the film surface. In the case of metals two kinds of droplets are observed during laser ablation, which have different sizes, angular distributions, velocities, and dependencies on the number of pulses, target roughening and energy density. These experiments indicate different origins for the two types of droplets: expelling from the cones growing out of the target and expelling from the molten surface. Consequences for the reduction and avoidance of these droplets are discussed.