Pulsed laser deposition (PLD) is one of several physical vapour deposition techniques. This application of surface science uses a high-power laser beam to hit the target surface of the material to be deposited. The laser energy turns solid material particles into a plasma plume, after which the particles land on the substrate surface.
Disclaimer: This article is for educational purposes only. The HEX thin film deposition system does not currently support the pulsed laser deposition process.
A major advantage of the pulsed laser deposition technique is its successful use in complex stoichiometric transfer. PLD works well in depositing compound materials like complex oxide thin films. The controllability, versatility, and consistency of pulsed laser deposition make this method a preferred choice for thin-film fabrication in optical coatings, superconducting thin films, magneto-resistive films, and more.
How does PLD work, what are the pros and cons of this technique, and what are its most prominent applications? Learn all this and more in the following article.
The Pulsed Laser Deposition Process
Like other physical and chemical vapour deposition methods, PLD takes place in a low-pressure or vacuum chamber in deposition systems. The process starts when a high-power UV or infrared laser beam hits the target material. This causes the target’s molecules to break apart, creating high-energy plasma known as an ablation plume.
The plasma plumes consist of positive ions and free electrons. As these ions meet the substrate, they settle on its surface in an atomic layer-by-layer growth process, called thin-film deposition.
The physical, magnetic, and optical properties of PLD-grown thin films depend on the film’s structure – i.e., on the arrangement of atoms or ions in the coating material. Manipulating the material’s crystal structure allows the manufacturer to adjust the film’s properties.
PLD’s precision and flexibility make this technique a superior choice when manufacturers need to grow thin films of complex materials, like buffer layers for microelectronics and optical devices. However, due to high costs and deposition scale limitations, other methods may be preferable in large area deposition.
Below, we will dive deeper into how the PLD process works, the applications of PLD in the coatings industry, and the comparative advantages and disadvantages of this thin film deposition method.
By Adam Andersen Læssøe – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=45333275
A Brief History of Pulsed Laser Deposition
PLD took its first steps [1] shortly after Maiman’s development of the ruby laser in 1960. In 1962, Breech and Cross first used a laser to vaporise solid material atoms. In 1965, Smith and Turner achieved the first successful deposition of thin films using a laser.
However, in those early years, PLD-deposited films could not compete with films produced by methods like a chemical deposition or molecular beam epitaxy. During the 1980s, technological improvements like excimer lasers allowed better results in laser deposition.
Pulsed laser deposition took a leap forward in 1987 when researchers successfully used PLD to deposit a high-quality film of yttrium barium copper oxide (YBCO), a crystalline compound with high-temperature superconductivity. Since then, PLD has gained recognition as an effective method of producing superior-quality crystalline coatings, films of oxides like ITO (indium tin oxide), metallic multilayers, and nitride films.
Laser technology advanced further in the 1990s with the development of high-repetition and short-pulse lasers. This technological progress enabled PLD to evolve into an ultra-precise, competitive thin film deposition method, particularly for complex stoichiometric coatings.
Today, various industries use PLD on a commercial scale for micro-electromechanical systems, coated-conductor applications, and more.
Pulsed Laser Deposition of Thin Films Explained
PLD is a physical vapour deposition technique that enables the production of versatile thin films. In PLD, high-intensity electromagnetic radiation acts as an energy source to evaporate target materials through laser ablation. The process occurs in low gas pressure or vacuum chamber.
Laser Absorption and Laser Ablation
The laser beam interacts with the target surface fixed in place by a substrate holder, creating a plasma plume. The plume consists of molecules, atoms, ions, electrons, and, in some cases, larger particles of ablated material.
The highly energetic plasma moves away from the target surface. The plasma particles will eventually collide with the substrate and form a film.
Plasma Dynamics
The laser pulse length and laser wavelength influence the excitation process in the PLD chamber. A short-pulse laser will cause an electrostatic ablation process based on electronic excitation. Longer laser pulses lead to high temperatures and a mechanism known as thermal ablation, in which the ablation plume absorbs most of the laser energy.
Deposition of the Ablation Material
As the extracted target material evaporates, it collides with the substrate, resulting in film growth. Only a fraction of the bulk material’s top surface converts into a plume with each laser pulse. This results in a slow deposition rate but creates exceptionally high-quality films with a superior level of control over film thickness.
Nucleation (Growth of the Thin Film on the Substrate)
Under optimal conditions, pulsed laser evaporation creates a deposited film layer of the right composition, structure, and uniform thickness. The operators can control the environment in the deposition chamber, such as the gas composition and pressure, to fine-tune film growth.
PLD can occur in different gas environments [2] and under varying background pressure. [3] With metal oxide deposition, oxygen is usually present in the ablation chamber to ensure enough oxygen binds to the metal. Under the same principle, nitrite film deposition may take place in a nitrogen gas environment.
Factors Affecting Pulsed Laser Deposition
Thickness, composition, crystallinity, roughness, uniformity, and other deposited film properties depend on the parameters in the PLD deposition chamber, like gas pressure, laser fluence, and laser repetition rate. [4]
The deposited material amount per unit of the surface area also depends on the substrate distance from the target. A greater distance results in less deposited material but may also produce a higher-quality film.
Background gas pressure influences film growth rate, stoichiometry, and crystallinity. More gas leads to lower kinetic energies of the target material particles and enables the creation of high-crystallinity films. Excessively low gas pressure means high-energy particles that may deposit too fast to form a crystalline structure.
Crystallinity also depends on the substrate temperature and the chamber’s thermal equilibrium. A heated substrate aids the creation of crystalline films, while lower surface temperatures may cause an amorphous film structure.
Advantages and Disadvantages of Pulsed Laser Deposition
PLD is the preferred method in many applications of thin films thanks to its unique advantages, such as:
- Convenience. PLD can work with low temperatures and short test periods. The PLD process is simple, flexible, and compatible with many target and substrate materials that won’t tolerate high heat.
- Precision. PLD enables the preparation of highly uniform, multi-component films with an excellent degree of control over the stoichiometric ratio.
- Sustainability. The PLD technique utilises high-energy UV lasers for generating plasma. It is an energy-efficient, non-polluting method of producing thin films.
However, PLD also has some drawbacks. The PLD technique has a slow average rate of deposition and may be less suitable for covering large substrate areas compared to other methods.
Also, depending on the material and deposition parameters, small particles of the already-deposited film may sputter when bombarded by additional high-energy particles in the laser-induced process. This occurrence may compromise film quality.
Overall, PLD’s costs and currently available deposition scale make the method best suited for high-tech applications like sensor technology, optical technology, and microelectronics.
Difference Between Pulsed Laser Deposition and Thermal Evaporation
Although both PLD and thermal evaporation are physical vapour deposition methods, several key differences exist between the two techniques.
While PLD uses UV light as an energy source, thermal evaporation produces thin coatings of vaporised material under high temperatures in a high-vacuum environment. Thermal evaporation deposition is straightforward, cost-effective, and suitable for large-scale applications like solar cells, semiconductors, and ultra-thin metal plating for consumer packaging.
Although thermal evaporation can also produce high-purity metal and nonmetal films, PLD is the method of choice when manufacturers need to grow thin films of complex composition and structure, like organic-inorganic hybrids. [5]
Pulsed Laser Deposition Applications
PLD’s sensitive process control and current comparatively high costs make this technique suitable for a narrow range of applications that require ultra-high-quality complex films. Examples of PLD uses include:
- Diamond-like coatings for optics and other high-precision applications
- Superconducting films for fuel cells and flat-panel displays
- Single-walled nanotubes for sensors, biomedical devices, and wearable electronics
- Graded coatings for corrosion and thermal barriers
- Biocompatible coatings for medical applications
As advanced excimer laser deposition technologies continue to develop, PLD will likely gain a larger foothold in various industries thanks to its ability to produce highly controlled, ultra-pure thin films in on- and off-axis deposition.
Final Thoughts
Pulsed laser deposition is a highly accurate method for producing thin films of superior quality, especially in applications that require coatings of complex materials with precise stoichiometry. PLD uses a straightforward laser-based process that allows for exceptional control over film properties.
PLD’s suitability for creating crystalline films makes this technique highly promising in the development of future applications that require superconductive or diamond-hard coatings.
Would you like to learn more about different coating techniques? Browse other articles by Korvus Technology, a producer of advanced thin film deposition systems.
References
[1] Greer, J.A. (2014). History and current status of commercial pulsed laser deposition equipment. Journal of PhysicsD: Applied Physics, 47(3).
[2] Albu, D.F., J. Lungu, G. Popescu-Pelin, C.N. Mihailescu, G. Socol, A. Georgescu, M. Socol, A. Banica, V. Ciupina, & I.N. Mihailescu. (2022). Thin film fabrication by pulsed laser deposition from TiO2 targets in O2, N2, He, or Ar for dye-sensitized solar cells. Coatings, 12(3), 293.
[3] Martínez-Landeros, V.H., N. Hernandez-Como, G. Gutierrez-Heredia, M.A. Quevedo-Lopez, & F.S. Aguirre-Tostado. (2019). Structural, chemical and electrical properties of CdS thin films fabricated by pulsed laser deposition using varying background gas pressure. Thin Solid Films, 682, 24-28.
[4] Ojeda, A., M. Dobeli, & T. Lippert. (2018). Influence of plume properties on thin film composition in pulsed laser deposition. Advanced Materials Interfaces, 5(18).
[5] Wang, H., Y. Wu, M. Ma, S. Dong, Q. Li, J. Du, H. Zhang, & Q. Xu. (2019). Pulsed laser deposition of CsPbBr3 films for application in Perovskite solar cells. ACS Applied Energy Materials, 2(3), 2305–2312.
