Pulsed Laser Deposition
Pulse laser deposition is a physical deposition (PVD) method. In this method, the high-power pulsed laser beam focuses on the target that is inside the vacuum chamber. The target material is vaporized by a laser beam in the form of a Plasma Plume and deposit on the substrate as a thin film. This process can be performed in a high vacuum environment or in an environment with background gases such as oxygen. Oxygen is usually used for the oxide deposition to completely oxygenate the deposited thin film during the PLD process. Figure 1 shows the PLD mechanism of operation.
While the equipment needed to perform the deposition in this manner is roughly similar to other deposition methods (such as spattering), the physical interaction between the laser beam and the target material and the formation of the thin film is very complex. When the laser pulse is absorbed by the target, its energy is first converted to electron excitation and then thermally, chemically and mechanically, resulting in vaporization, ablation, plasma formation and exfoliation of the target material. The particles separated from the Target surface are distributed in a vacuum environment as a plume. These energetic particles include atoms, molecules, ions, electrons, and molten globules that will be deposited on the substrate.
Figure 1: Schematic of the PLD device
The precise mechanism of the PLD process is complex, including the process of target material ablation by laser irradiation, the creation of plasma plume with energetic ions, electrons, atoms, molecules, and the charge-free particles and process of the crystalline growth on the substrate. In general, the PLD process can be divided into four parts:
- Laser absorption at the target material surface, its ablation, and plasma formation.
- Plasma dynamic
- Deposition of the material separated from the target on the substrate.
- Nucleation and growth of the thin film on the substrate surface.
Each of these steps will have a significant impact on the crystallinity, uniformity, and stoichiometry of the thin film. Figure 2 schematically shows the formation of a plasma plume.
Figure 2: Different stages of PLD
The separation of atoms from the bulk of matter is accomplished by evaporation at the surface under unbalanced conditions. The laser pulse penetrates the surface of the target material to a specific penetration depth. Depth of penetration depends on laser wavelength and refractive index of target material at laser wavelength. For most materials, this depth is about 10 nm. The strong electric field created by the laser beam causes the electrons to be separated from the bulk. This process is performed in a pulse of 10 picoseconds or nanoseconds and occurs due to nonlinear processes such as multi-photon ionization. These nonlinear processes increase due to surface microscopic cracks, holes and nodes, and increase the electric field intensity. Free electrons oscillate in the electromagnetic field of the laser light and collide with the atoms of the bulk of matter and transfer energy to the atomic lattice of the target material’s surface. As a result, the surface temperature of the target material increases and vaporizes.
Secondly, due to the Colombian repulsion and recoil from the target material surface, the evaporated material expands into the substrate perpendicular to the target material surface. The spatial expansion of the created plasma plume depends on the vacuum chamber pressure. The dependence of the shape of the plasma plume on the pressure can be described in three stages:
- The vacuum phase, where the plasma plume is very narrow and forward. Almost no scattering with background gases occurs.
- The middle region where the separation of high-energy ions from low-energy species can be observed.
- High pressure region where we find a more diffusion-like expansion of the ablated material. Naturally this scattering is also dependent on the mass of the background gas and can influence the stoichiometry of the deposited film
The most important consequence of increasing background pressure is the slowing of energetic species in the expanding plasma plume. It has been shown that particles with kinetic energy of about 50 eV can re-sputter the film deposited on the substrate. This reduces the deposition rate and changes in the stoichiometry of the deposited thin film.
The third step of the PLD process is very important in determining the quality of the deposited thin film. High-energy particles are ejected from the target and bombard the substrate surface. This process can damage the substrate surface and sputter the atoms on the surface. It may also cause a defect in the deposited film. The particles scattered from the substrate and the particles emitted from the target material form a collision zone that acts as a source of particle condensation.
When the density is high enough, thermal equilibrium is created and the thin film grows on the substrate surface at the expense of the direct flow of ablation particles and the thermal equilibrium obtained. The process of nucleation and growth kinetics of the thin film depends on several growth parameters, including:
- Laser Parameters Factors such as laser fluence (Joule/cm2), laser energy and degree of ionization of the material separated from the target surface are effective in film quality, stoichiometry, and rate of deposition.
- Surface Temperature: Surface temperature has a great influence on the nucleation density. In general, the nucleation density decreases with increasing temperature. Surface warming can be caused by the use of a Co2 laser.
- Substrate Surface: Nucleation and growth are affected by surface preparation. Surface roughness due to processes such as etching has an adverse effect on the process.
- Background Pressure: Oxygen is usually used as background gas in the oxide deposition to ensure proper stoichiometry of the thin film. If the oxygen pressure in the chamber is low, the nucleation, stoichiometry and quality of the deposited thin film will be affected.
During the laser pulse in the PLD process, the substrate surface is saturated. Depending on the laser features, the pulse takes about 10 to 40 microseconds. The saturation of the surface results in a higher density of nucleation in this method than in the sputtering method, which results in the smoothness of the deposited thin film.
The PLD method has significant advantages over other deposition methods such as:
- The stoichiometric transferability of materials from Target to the substrate, the precise chemical composition of a complex material such as YBCO, can be reproduced in the deposited film.
- Deposition rate is relatively high (usually 100 angstroms per minute). Also, the thickness of the deposited thin film can be controlled simultaneously with the deposition process only by turning the laser on and off.
- The fact that a laser is used as an external energy source results in a very clean process without heat filaments.
Despite these important advantages, the industrial use of PLD is low and to date most applications have been limited to the research environment. There are basically three main reasons for this:
- The plasma plume created from the material separated from the target surface by the laser beam is in the forward direction as a result the thickness of the particles accumulated on the substrate being non-uniform and the composition of the created thin film may change extend along it and away from the center of accumulation. The resulting film area will also be small (approximately 1 cm 2).
- The created plasma plume also contains melt globules with an average diameter of 10 μm. Reaching these particles to the substrate will reduce the quality of the thin film.
- The processes, which occur in the plasma produced by the laser, are not fully understood. As a result, new material deposition usually involves a period of experimental optimization of coating parameters.
Using the Target Manipulator’s laser point movement technique on the surface of the target material, or the substrate motion during the coating, the first two problems are largely solved and the PLD method can be applied to deposit films with a uniform thickness and composition.
The Vaccoat Co. PLD device is equipped with the Target Manipulator system. This PLD system is capable of producing thin films of uniform composition and thickness in all substrate areas. It also has a vacuum thermal evaporation with three heat evaporation sources to perform thermal evaporation coating. For more information visit the site of Vaccoat Company.
- Pulsed Laser Deposition of Thin Films, edited by Douglas B. Chrisey and Graham K. Hubler, John Wiley & Sons, 1994 ISBN 0-471-59218-8
- Vaziri, M R R (2010). “Microscopic description of the thermalization process during pulsed laser deposition of aluminium in the presence of argon background gas”. Journal of Physics D: Applied Physics. 43 (42): 425205.
- Ohnishi, Tsuyoshi; Shibuya, Keisuke; Yamamoto, Takahisa; Lippmaa, Mikk (2008). “Defects and transport in complex oxide thin films”. Journal of Applied Physics. 103 (10): 103703–103703–6. Bibcode:2008JAP…103j3703O
- May-Smith, T. C.; Muir, A. C.; Darby, M. S. B.; Eason, R. W. (2008-04-10). “Design and performance of a ZnSe tetra-prism for homogeneous substrate heating using a CO2 laser for pulsed laser deposition experiments, doi:10.1364/AO.47.001767