Pulsed Laser Deposition Techniques

In general, PLD can be used to vaporize and to deposit thin films of any material if the abosrbed laser power density is high enough.

Origin of Splashing:
1. Subsurface boiling
It occurs if the time required to transfer laser energy into heat is shorter than that needed to evaporate a surface layer with a thickness on the order of skin depth. Under this condition, the subsurface layer is superheated before the surface itself has reached the vapor phase.
In general, the power densities used in most PLD-grown dielectric materials are lower than the maximum power density that a solid surface could absorb without causing splashing.
2.Expulsion of the Liquid layer by the shock wave recoil pressure
In this mechanism, the force comes from above the liquid layer in a form of recoil pressure exerted by the shock wave of the plume. The signature of this type of splashing is also the formation of micron-sized condensed globules, and therefore, indistinguishable from the true splashing. Its magniture can be reduced by lowering the laser power density at the expense of decreasing the deposition rate.
3. Exfoliation
For most materials, in particular sintered ceramic targets, the surface is eroded by repetitive laser ablation, forming long needle-shaped microstructures of only a few microns in dimension. These microdendritic structures point toward the direction of the incoming laser beam due to shadowing effect. Mechanically, they are very fragile and can be broken by the thermal shock induced during the intense laser irradiation. This process is termed "exfoliation" by Kelly and coworkers (1985). The loose debris are carried toward the substrate by the rapidly expanding plume and condense onto the thin film.

Solutions to Avoid Splashing
The splashing can be redued by adding a particle filter or manipulating plume, improving target surface, doing off-axis deposition.

Surface Segregation
There's a surface modification by pulsed laser on film composition. take YBCO system for example, it is severely modified by cumulative laser irradiation. After laser preconditioning of the target (tens of shots/site or longer), a steady-state composition can be reached.
The major effect of surface modification on PLD is a reduction of the deposition rate. This complicates film thickness control and slows the deposition process. Though both issues inhibits progress greatly, they are not an issure in a research lab because the target can be periodically resurfaced to its original level by sanding or scraping in a lab.

Penetration depth
For metals, the absorption coefficient α decreases with decreasing laser wavelength λ. Hence the laser penetration depth in metals is larger in the UV range than in the infrared (IR) range. For other materials, the variation of the absorption coefficient with the wavelength is more comlex since the various absorption mechanisms, such as lattice vibration, free carrier absorption, impurity centers, or bandgap transition, can take place. For the oxide semiconductor, the penetration depth appears to be larger in the near IR than in the UV, somewhat reverse to that of the metals.
The primary effect of the laser wavelength on particulate generation is most likely due to the difference in the absorption when different laser wavelengths are used.

Elimination of particulates
1.Reduce target surface roughness
2.Reduce the laser power density to below the threshold level that causes the splashing of the molten layer. Blank et al. observed that a simple relation holds for the deposited film thickness, d∝S²E, where S is the laser spot size on the target, and E is the laser energy density. Since the deposited film thickness is directly proportional to the spot size when the laser energy, SE, is fixed, increasing the spot size is the most efficient way to reduce the particulates as well as to increase the throughput of vapor species.

Effects of the high background gas pressure
The gas pressure commonly used for PLD of complex ceramic materials is approximately 300mT, which corresponds to approximately 1x10²⁰ gas atoms per square centimeter per second striking the substrate and the film. This arrival rate compares to typical peak arrival rates of film atoms of 10¹⁸-10¹⁹ atoms per square centimeter and typical average film atom arrival rates of 10¹⁵-10¹⁶ atoms per square centimeter per second. In addition to the obvious effect of promoting stoichiometric film formation for such systems as oxide deposition in a background gas of oxygen, such a high arrival rate of gas atoms could change film and substrate surface energies, possibly even changing the film growth mode.