Mix it!

Pulsed laser deposition (PLD) is nothing new. A laser source — generally an excimer laser — fires at what is known as a ‘target’. The laser pulses vaporize material from the target and this material then accumulates as a thin film on the workpiece. The major advantage of PLD is that neither the ‘substrate’ (the workpiece) nor the target need to be magnetic or electrically conductive.

Scanner systems are also nothing new in industrial applications: the small, lightweight mirrors incorporated in these systems are used to move the focus spot over the surface of workpieces at extremely high speed. But now scientists at the Max Planck Institute for Solid State Research have created something that really is new: a successful technique that combines the PLD concept with scanner technology. Their ‘Combining Laser Deposition (CLD)’ method substitutes scanner optics for the fixed focus of the PLD technique.

Pico-scale lunar landscape

In the PLD process, the usable area on the target is very small, and it gets rougher each time a laser pulse hits it, forming a pico-scale lunar landscape from which the laser blasts out increasingly uneven ‘chunks’. Larger particles begin to be deposited on the substrate as droplets. Since the target can only supply the material for a single coating layer, multilayered coatings are produced in multiple process steps.

Alternatively, the laser can be focused on alternating targets in a single process step, though each change of target is reflected as an unevenness in the coating. Coatings combining multiple elements can either be produced using a ‘pre-mixed’ target or by quickly alternating between individual targets consisting of the pure elements. However, in both these cases the elements often accumulate in a slightly different ratio to that at which they are ablated from the targets.

The CLD process

In the CLD process, one target supplies all the components for all the layers of a coating system. In addition, the technique generates composite layers during the process. This is achieved by arranging the elements for the coating system in a suitable geometry.

In the CLD process, the scanner directs the laser pulses over the target line-by-line. This lets the process use the entire target surface while simultaneously coating large areas of the substrate with a continuous and complete coating system. It is even possible to form gradients with evenly increasing or decreasing concentrations of the coating components within a single layer.

The Max Planck Institute investigated and developed this new technique as part of a project to coat sapphire substrates with an aluminum-titanium-niobium coating system. The target they used consisted of a titanium disk into which wedge-shaped segments had been inserted and a niobium disk in the center. The experiments were performed at target temperatures of between 25 and 500 degrees Celsius employing a femtosecond laser

The laser struck the target at a wavelength of 516 nanometers, pulse energy of between 0.3 and 0.6 millijoules, and pulse frequency of one kilohertz. The researchers subsequently examined the ablation of the target and the deposition and composition of the coating layers on the substrate samples.

No droplet formation

This series of experiments showed that one of the biggest difficulties of PLD — droplet formation — does not even occur. Overall, the new CLD technique transfers more material, thanks to the higher pulse frequency of the femtosecond laser. Yet the ultrashort pulses ablate smaller quantities of material moreevenly with each pulse.

In addition, the focus spot migrates across the target instead of constantly firing at the same point, which results in the surface actually being smoothed by the laser.The substrates used in the experiments featAir Jordan VII 7.5 Ture Flight