Magnetron sputtering is a technique used for thin film deposition, an effective process for various materials science applications, including coating microelectronics, altering properties of materials, and adding decorative films to products.
The deposition process involves ejecting material from a target onto the surface of a substrate, such as a silicon wafer. Magnetron sputtering is unique because it uses a magnetic field and negatively charged cathode to trap electrons near the target materials.
This article will explain the magnetron sputtering deposition process, how it works, and its benefits over other common thin film deposition methods.
Magnetron Sputtering: An Overview
Magnetron sputtering is one of many vacuum deposition methods for depositing layers of material onto a surface, molecule-by-molecule. Vacuum deposition uses either a chemical source (chemical vapour deposition) or a solid or liquid source (physical vapour deposition). Magnetron sputtering is a physical vapour deposition process. [4]
What Is Magnetron Sputtering?
The phenomenon of sputtering occurs when energetic particles of a gas or plasma (incident ions) bombard a material, also known as the target. [1] The incident ions cause collision cascades in the target materials, and if one exceeds the surface’s binding energy, an atom breaks free. Incident ions can originate from many sources, including plasma, constructed ion sources, and particle accelerators.
It was initially observed in the 1850s but was not commercially relevant until the 1940s when diode sputtering found use as a coating process. However, the method had severe disadvantages due to its low deposition rate and high cost.
In 1974, the magnetron sputtering method appeared as an enhanced alternative to diode sputtering, surpassing its predecessor in deposition rate. [2] Compared to other vacuum coating methods, the magnetron sputtering process has significant advantages.
Magnetron sputtering utilises a magnetic field and an electric field to confine particles near the surface of the target, increasing the ion density and resulting in a high rate of sputtering. The process has multiple variations, including direct current (DC) magnetron sputtering, pulsed DC sputtering, and radio frequency (RF) magnetron sputtering.
RF magnetron sputtering does not require the target surface to be electrically conductive like DC magnetron sputtering, widening the range of materials that work in the sputtering process. However, RF sputtering requires costly supplies and specialised equipment.
Magnetron sputtering is effective for the deposition of metallic coatings that enhance the substrate with specific properties like scratch resistance, conductivity, and durability.
How Does Magnetron Sputtering Work?
Magnetron sputtering is a physical vapour deposition (PVD) method, a class of vacuum deposition processes for producing thin films and coatings.
The name “magnetron sputtering” arises from the use of magnetic fields to control the charged ion particles’ behaviour in the magnetron sputter deposition process. The process requires a high vacuum chamber to create a low-pressure environment for sputtering. The gas that comprises the plasma, typically argon gas, enters the chamber first.
A high negative voltage is applied between the cathode and the anode to initiate the ionisation of the inert gas. Positive argon ions from the plasma collide with the negatively charged target material. Each collision of high energy particles can cause atoms from the target surface to eject into the vacuum environment and propel onto the surface of the substrate.
A strong magnetic field produces high plasma density by confining the electrons near the target surface, increasing the rate of deposition and preventing damage to the substrate from ion bombardment. Most materials can act as a target for the sputtering process since the magnetron sputtering system does not require melting or evaporation of the source material.
The type of sputtering gas depends mainly on the substrate, specifically its atomic weight. Lighter substrates benefit from the use of neon, while heavier substrates would work better with elements like xenon or krypton. [3] Introducing gases such as oxygen or nitrogen into the chamber will result in reactive sputtering.
Magnetron sputtering has many applications, one of the earliest being the production of computer hard disks. It is common in the semiconductor industry to deposit thin films of materials for integrated circuit processing. The method is also common in optics, microelectronics, textiles, and machining.
Learn more: The HEX Sputtering System
Magnetron Sputtering and Thin Film Deposition
The thin film deposition process involves applying light coatings of materials, usually metals, to various surfaces. Sputtering is one technique for depositing thin films and requires a controlled gas flow, usually argon, into a vacuum chamber. An electrically charged cathode, the target surface, is the material from which sputtered atoms will eject and coat the substrate to create a thin film.
To achieve deposition, the negative electrical potential from the cathode attracts target atoms inside the plasma, which have a positive charge. The collisions inside the plasma lead to energetic ions accelerating into the target with enough kinetic energy to dislodge molecules from the material. The sputtered material crosses the vacuum chamber and coats the substrate.
Magnetron sputtering is an exceptionally versatile method for thin film deposition. Other deposition methods are limited in the materials they can use as the sputtering target. The source material can be almost anything with magnetron target sputtering since it does not need to melt or evaporate the material. The sputter-deposited film will have a composition that closely matches the source, and it will adhere to the substrate better than evaporated films.
Advantages of Magnetron Sputtering
Magnetron sputtering dominates over other techniques for thin film deposition because it allows for the preparation of large quantities of films for little cost. Various power systems are available in this process, including the RF magnetron sputtering technique, which does not require a conductive material for sputtering and thin film deposition.
The advantages of magnetron sputtering include:
- High deposition rate
- Thorough coverage of materials
- High purity of films
- Highly-adhesive films
- Uniformity on large-area substrates
- Low temperature
Magnetron Sputtering Deposition Explained
The following is a detailed example of magnetron sputtering deposition for creating high-quality functional films. The process varies depending on the desired conditions and properties of the resulting film. Magnetron sputtering is possible using various power systems, materials, and types of gases.
Pre-Sputtering Preparation
The first step is to prepare the substrate by mounting it on a substrate holder, typically with two to three screws. Securing the substrate within the deposition chamber avoids damage to the materials. Once attached securely, the substrate holder goes into the load lock chamber.
In many systems, the deposition chamber will maintain a vacuum environment at all times. The load lock chamber allows the substrate to move in and out of the deposition area without comprising the vacuum. Once the load lock chamber is a vacuum, the substrate can move into the main disposition chamber through a gate.
The deposition chamber contains the sputter gun with the target material. Behind the target material, strong magnets create the magnetic field from which the process gets its name.
When everything is in place, the next step is to start the argon gas flow. Other gases work for this process, but reactive gases can have undesired results. For example, a reactive gas such as oxygen could cause a chemical reaction with the target material to create aluminium oxide.
Before fully initiating the sputtering process, the power must ramp up from a lower voltage. High voltage DC power flows into the cathode where the sputter gun and target material sit. This pre-sputtering period cleans the target and the substrate before the deposition process.
Sputtering Deposition Process
Free electrons are moving around the deposition chamber. The magnetic field close to the surface of the target captures the electrons, and they spiral around the target. This area is known as the “race track.” As the electrons move through the high-density argon gas, they collide with the argon atoms and create positive ions.
Because of the negative power at the target, the positive ions propel toward it and collide with its surface. These high-energy collisions can cause atoms to break off from the source material. Since the sputtered atoms are neutral, they are not affected by the negative charge or the magnetic field. The target atoms are free to travel across the vacuum chamber toward the substrate.
As more and more atoms break free from the target, the substrate develops a coating of the source material. The growing film is micrometres thick.
While most of the process is not observable, a plasma glow is visible because of the high-energy particle collisions near the target. The rate of sputtering depends on the amount of pressure and power. Magnetron sputtering allows for a high sputter rate with its magnetic field and negative electric charge, producing a high density of molecules and facilitating more ionizing collisions.
Final Thoughts
There are many deposition processes for producing thin films, but magnetron sputtering has numerous advantages over other methods. It is faster, less expensive, and creates a high-purity coating on the substrate.
Korvus Technology offers a series of thin-film deposition systems with unique, modular designs that allow remarkable versatility and customisation. The HEX series is ideal for teaching applications, research facilities, and thin-film production. The HEX series deposition systems are your organisation’s chance to incorporate the latest thin-film technology.
Among the compatible sources of the HEX series of thin film deposition systems, we offer the Fission DF/RF Sputtering Source.
References
[1] Behrisch, R., ed. (1981). Sputtering by Particle Bombardment. Berlin: Springer. ISBN 978-3-540-10521-3.
[2] Suchea, M. P. et al. (2019). Functional Nanostructured Interfaces for Environmental and Biomedical Applications. Amsterdam: Elsevier Press. ISBN 978-0-12-814401-5. DOI https://doi.org/10.1016/C2017-0-01307-1.
[3] Tong, Xingcun Colin (2014). Advanced Materials for Integrated Optical Waveguides. Schaumburg, IL: Springer International Publishing. ISBN 978-3-319-01549-1.
[4] Quintino, Luisa (2014). “Overview of coating technologies,” In Miranda R.M. et al, Eds, Surface Modification by Solid State Processing. Amsterdam: Elsevier Press. pp. 1–24. DOI 10.1533/9780857094698.1. ISBN 9780857094681.
