SPUTTERING
Two categories of techniques—plasma vapour deposition (PVD) and chemical vapour deposition (CVD)—are most often used to prepare thin films. Â
The essence of the PVD technique is that the solid source material – target (typically metal (e.g. aluminum, titanium), or alloy (titanium-aluminum) or semiconductor (e.g. silicon), or a dielectric (e.g. oxide) usually in the form of a flat disk or rectangle mounted on a holder within the deposition chamber), is converted into its gaseous state and sent through a vacuum chamber at a pressure of units of Pa to the material to be coated – substrate, where it solidifies as a thin film. Among the most well-known PVD techniques are sputtering, thermal evaporation, ARC evaporation, laser ablation, etc. Its main advantages include the ability to form thin films with different chemical compositions at low deposition temperatures (at the level of hundreds of °C), as well as the ability to modify the film’s structure as it is being deposited by controling key deposition parameters. PVD techniques found their application in various industrial spheres for depositing thin films of diverse types of materials onto a substrate.Â
Sputtering, in general, is a phenomenon, in which miscroscopic particles of a target are ejected from its surface, after the exposion to bombardment of energetic particles, most often from plasma of inert gas – fig. 1 (left). The high-energy ion beam or plasma is generated inside the deposition chamber – fig. 1. (right).
A sputtering system typically consists of a vacuum chamber, vacuum system, a target material, a substrate, and a power supply. Before deposition process, the chamber is evacuated to create a low-pressure environment, usually in the range of 10-4 to 10-3 Pa after heating, to minimize the presence of gas molecules that could interfere with the sputtering process.
More specifically, in the sputtering process, the ions of working gas from the plasma are accelerated in the electric field and attracted to target and collide with the atoms in the target. The mass and energy of the impact ions determine the behaviour of the target atoms after collision. Release of one or more atoms takes place if the impact ion particle has enough energy (between 100 eV and 50 keV) to break bonds between surface atoms. The sputtered atoms travel in a straight-line trajectory through the vacuum chamber and eventually reach the substrate. Upon reaching the substrate surface, the atoms condense and form a thin film. Properties of the deposited film, such as thickness, composition, morphology and structure, can be controlled by adjusting various parameters, including the sputtering power, gas pressure, substrate temperature, and deposition time. Monitoring and controlling these parameters ensure the desired film characteristics are achieved. Therefore the sputtering process offers advantages such as good film uniformity, high deposition rates, excellent adhesion, and the ability to deposit materials with high melting points. It also allows for precise control over film thickness and composition.Â
Overall, sputtering is an important technique in materials science and technology, enabling the production of thin films with diverse properties for a wide range of applications including the deposition of metal, semiconductor, and dielectric films for electronic and optical devices. It is widely used in the manufacturing of hard films for cutting applications, integrated circuits, flat-panel displays, solar cells, magnetic storage media, and decorative coatings, among others.
MAGNETRON SPUTTERING
Magnetron sputtering is a specific variant of the sputtering process that utilizes magnetic fields to enhance the efficiency and control of the sputtering process.
In magnetron sputtering, a magnetron (typically of a flat or cylindrical structure, in single, dual – fig. 2 (left) or multiple arrangement) is employed as a cathode in the sputtering system. Within the magnetron, set of permanent magnets or electromagnets are arranged in a specific configuration to generate a magnetic field fig. 2 (right). The magnetic field lines are typically parallel to the target surface and can be arranged in various patterns, such as circular, rectangular, or racetrack-shaped configurations.Â
When a voltage is applied to the cathode, a plasma is generated in the vicinity of the target. The magnetic field forms closed loops or closed electron trajectories near the target surface and confines the electrons within the closed electron trajectories. This trapping effect increases the probability of electron collision with neutral gas atoms leading to enhanced ionization of the sputtering gas (usually argon), denser and more energetic plasma compared to conventional sputtering. Â
Therefore, the magnetic field does allow to ignite the plasma discharge at lower pressures compared to cathodic sputtering (without magnets). As a result, high deposition rates can be achieved and samples with very low impurity concentration can be prepared.Â
The magnetic field strength, orientation, and configuration in magnetron sputtering can be tailored to influence the plasma density, energy distribution, and ion-to-atom arrival ratio. By adjusting these parameters, it is possible to control the stoichiometry and composition of the deposited thin films. This capability is advantageous for applications requiring specific film compositions or precise control over film properties.
For example for some technological purposes, it is more practical to have the plasma expanded farther into deposition chamber – closer to the substrates, rather than having it close to the target. The higher percentage of ions close to the substrates can be then used to modify the growing film. Then, we are discussing an a magnetron with an unbalanced magnetic field (unbalanced magnetron) – Fig. 3 (right).
REACTIVE MAGNETRON SPUTTERING
Reactive magnetron sputtering is a variation of magnetron sputtering in which a reactive gas (nitrogen, oxygen, acetylene, …) is introduced into the sputtering process along with the inert gas used for plasma generation (typically argon). The reactive gas chemically interacts with the target material during sputtering, allowing the deposition of compound thin films or the formation of thin film coatings with specific chemical properties.
More detaily, In reactive magnetron sputtering, two types of gases are introduced into the vacuum chamber: an inert gas (typically argon) and a reactive gas (nitrogen, acetylene, oxygen, …). The inert gas acts as the sputtering gas, while the reactive gas is responsible for the chemical reactions during film deposition. The reactive gas is usually a precursor gas that contains the desired elements for compound thin film formation.Â
A high-voltage power supply is applied to the magnetron cathode, creating an electric field that ionizes the gases in the chamber. The inert gas is easily ionized in low-pressure environment and forms a plasma. The magnetic field given by the magnet configuration confines the plasma near the target surface.
The high-energy ions from the plasma are accelerated towards the target (cathode). The collisions between the ions and the target atoms cause sputtering, resulting in the ejection of target atoms from the cathode.Â
At the same time, the reactive gas molecules are also ionized by collisions with the electrons and ions in the plasma. The ionized reactive gas species are chemically reactive and can react with the sputtered target atoms or other reactive species in the plasma. The reactive gas may undergo dissociation, ionization, or other chemical reactions to form compounds that will deposit on the substrate.
The sputtered target atoms and the reactive gas species combine in-flight or upon reaching the substrate surface, forming a thin film with a desired composition. The film can be a compound (e.g., metal oxides, nitrides, carbides) or a mixture of the target material and the reactive gas species.
The composition of the deposited film is influenced by several factors, including the partial pressure of the reactive gas, the flow rate ratio between the reactive gas and the inert gas, and the sputtering rate of the target material. By adjusting these parameters, it is possible to control the stoichiometry and chemical composition of the thin film.Â
Reactive magnetron sputtering requires careful control of various parameters, such as gas flow rates, gas partial pressures, power input, substrate temperature, and target-to-substrate distance. As a chemical reaction occurs on the substrate, there may be circumstances where the deposition rate would be lower (in the case of a critical high concentration of reactive gas in the chamber). The reactive ingredient causes an undesirable layer of compound to form on the surface. The target is referred to be poisoned. One of the potential signs is a rise in the chamber’s overall pressure and a fall in the deposition rate. In poisoned regime, it is necessary to first remove any undesired film from the target surface before making the reverse transition from the poisoned to the metallic state. Only when the reactive gas concentration is lower than the critical one does, the metal mode transition takes place. The system reports hysteresis behavior. As deposited films might have a variety of physical characteristics depending on the mode of deposition, this phenomena is undesirable, and technologist must be aware of it. The deposition parameters should be optimized to achieve the desired film composition, structure, and properties.
Reactive magnetron sputtering is widely used in various industries and applications. It is employed for depositing compound thin films with tailored properties, such as hard corrosion-resistant coatings, transparent conductive coatings (e.g., indium tin oxide), semiconductors, optical coatings, and more.Â
DC AND RF SPUTTERING
Direct current – DC (cathodic or magnetron) sputtering and RF (Radio Frequency) sputtering are two different variants of the sputtering process used in thin film deposition.Â
The primary difference between DC sputtering and RF sputtering lies in the power source used to generate the plasma. In DC sputtering, a low-voltage DC power supply is employed, typically in the range of a few hundred volts. On the other hand, RF sputtering utilizes a high-frequency RF power supply, typically in the range of 13.56 MHz.Â
Therefore in DC sputtering, the plasma is generated by applying a constant voltage between the target (cathode) and the substrate (anode). This voltage creates an electric field that accelerates ions towards the target, leading to the sputtering of material. In RF sputtering, the RF power supply generates an oscillating electric field. The RF energy is applied to the target electrode, creating a high-frequency plasma that sputters the target material. RF sputtering typically produces a higher plasma density compared to DC sputtering. The high-frequency oscillating electric field in RF sputtering promotes a more effective ionization of the sputtered atoms, resulting in a higher density of ions in the plasma.Â
The choice between DC sputtering and RF sputtering can impact the properties of the deposited films. RF sputtering is known for preparation of low-conductivity films with improved density, uniformity, and smoothness due to higher ionization and more efficient target utilization. DC sputtering may be more suitable for applications that require lower ion energies or for depositing materials that are sensitive to plasma/heat damage.
HIGH POWER IMPULSE MAGNETRON SPUTTERING (HIPIMS)
HIPIMS is an advanced variant of magnetron sputtering. In traditional magnetron sputtering, a target material is bombarded with ions, typically generated by a low-voltage direct current (DC) power supply. The ions collide with the target surface, causing atoms or molecules to be ejected and deposit on a substrate, forming a thin film. However, this constant and relatively low-power ion bombardment has certain limitations in terms of film quality and properties, therefore is insufficient for certain applications (e.g., Diamond-Like Carbon – DLC coatings).
Figure 4. View on HIPIMS deposition process
HIPIMS, on the other hand, employs high-power pulses to enhance the sputtering process. It uses short, high-energy pulses of power to create a highly ionized plasma near the target surface. These pulses typically have a duration of several microseconds and can reach power densities in the kilowatt range. The intense plasma created during HIPIMS leads to increased ionization of the sputtered material and higher ion energy.Â
The intense ionization allows for a higher proportion of ionized sputtered atoms, resulting in increased film density and improved adhesion to the substrate.
The higher voltage accelerates the ions towards the substrate with greater energy, which can lead to denser and smoother film growth.
The pulsed nature of HIPIMS allows for precise control over the ion bombardment. The pulse parameters, such as pulse frequency, duration, and amplitude, can be adjusted to tailor the energy and flux of the ions. This control enables fine-tuning of film properties, such as composition, thickness, and surface morphology. The advantages of HIPIMS, including higher film density, smoother surfaces, improved film properties, precise control, and compatibility with various materials, have made it an attractive technique in the development of high-performance thin films for applications such as microelectronics, optics, wear-resistant coatings, and corrosion protection.Â
Reactive HIPIMS can be also used deposit films from reactive gases. By introducing a reactive gas into the plasma during HIPIMS, the highly ionized species can react with the gas molecules, resulting in the deposition of compound films. This capability expands the range of materials that can be deposited using HIPIMS.
However, researchers and industries still continue to explore and optimize the parameters and applications of HIPIMS to further enhance the quality and performance of thin films.
REFERENCES
Daniel, R., et al., Nanocomposite coatings, (2004), ISBN 978-9814411172.
Ohring, M., Material science of thin films, NJ, USA, (2001), ISBN 978-0-125-24975-1.
Alfonso, E., et al., Thin film growth through sputtering technique and its applications, Universidad Nacional de Colombia, (2012).
Gleiter, H., Nanocrystalline materials. Progress in materials science, 33, (1989).
Petrov, I. et al., Microstructural evolution during film growth, J. Vac. Sci. Technol., (2003).Â
Pogrebniak, A. D., et al., Hard nanocomposite coatings, their structure and properties, Sumy State University, Sumy Institute for Surface Modification, (2012).
Bunshah, R., Handbook of hard coatings : Deposition technologies, properties
and applications, University of California, Los Angeles, (2000), ISBN 978-0815517535.
Cavaleiro, A., et al., Nanostructured Coatings (Nanostructure Science and Technology), (2006), ISBN 978-0387256429.
Hashmi, S., Comprehensive materials processing, Amsterdam: Elsevier, (2014), ISBN 978-0-08096-533-8.
Edstöm, D., et al., Effects of atomic ordering on elastic properties of TiN and VN-based ternary alloys, Linköping University, (2014).
Lundin D., et al., High Power Impulse Magnetron Sputtering Fundamentals, Technologies, Challenges and Applications1st Edition, (2019), 9780128124550.