Balanced and Unbalanced Magnetron Sputtering
Sputtering was first introduced in 1852 and was used by a person named Groe. He could using an electrical discharge deposited the metal film on a cold cathode. At first, sputtering was used for deposition of the refractory metals film, whose deposition was not possible by thermal evaporation, but gradually, using radio frequency waves (RF Sputtering), the possibility of dielectric films was also provided by the sputtering deposition method.
Sputtering is in fact the process of transferring the momentum of the particles (usually the ions of neutral gases) to the surface of the target. Parameters such as energy, angles, and masses of the incident particles, as well as the binding energy between atoms, affect its efficiency. The colliding ions to the target surface are either absorbed or reflected. As the energy increases, ions penetrate into the target material atomic network and begin to degrade the surface. When the amount of energy reaches the threshold, the atoms begin to escape from the surface.
The minimum energy required to throw an atom from the target material’s surface is the amount of energy that it refers to as primary energy. Usually, the initial energy value is about 3 to 4 times greater than the binding energy of the surface target atoms. By creating the potential difference between the cathode (where the target material is placed) and the anode (which is actually the wall of the chamber and the substrate attached to the electrically ground), the neutral gas in the chamber (i.e. argon) is ionized and the plasma is ignited.
The generated positive ions accelerate towards the cathode, which is located at a negative potential. If the energy transferred from the incident ions to the surface of the target material (which is located in the cathode) is greater than the initial energy, the removed atoms collide with other atoms, and the energy is cascaded. The atoms removed from the material’s surface after a few collisions, reach the substrate surface with their remaining energy. Secondary electrons are other products of target ion bombardment, which play an important role in the plasma stability.
Some of the limitations of basic sputtering as defined are low deposition rate, low ionization efficiencies in plasma, and heating the substrate, which are almost eliminated by utilizing magnetron sputtering method and recently unbalanced magnetron sputtering technique.
Magnetron Sputtering
In magnetron sputtering deposition the motion of secondary electrons is limited to the vicinity of target surface by the use of magnetic fields parallel to the target surface, created by magnets with different shape and sizes placed behind the cathode. By this magnetic field, secondary electrons can be trapped near the surface of the target material which causes ionization of more argon atoms prior to re-absorption.
This causes the formation of plasma at lower pressures and plasma stability. Magnets are arranged in such a way that one pole is located at the central axis and the other is formed by a ring of magnets on the outer edge of target (Figure 1). This dipole is created using rows of permanent magnets. The placement of magnets is such that the S-N-S or N-S-N combinations are formed and the central magnet (internal) has the opposite polarity due to the ring magnet.
Figure 1. A Cross Section of Circular Planar Magnetron
Higher ionization efficiency in magnetron sputtering system results in denser plasma, increasing ion bombardment of the target, hence higher sputtering and deposition rate. Also, increasing ionization efficiency helps maintain the plasma at lower chamber pressure (10-3 mbar compared to 10-2 mbar) and in lower bias voltage (~ -500 compared to -2 to -3 kV) compared to conventional sputtering.
The use of Magnetron Sputtering is done in two ways that relates to the configuration of the magnets. The usual Balanced Magnetron Sputtering (BM) and another is Unbalanced Magnetron Sputtering (UBM).
Balanced Magnetron Sputtering
If all the magnets have the same power, all the field lines out of the external magnets pass through the central magnet. This configuration is called Balanced Magnetron Sputtering, in which the magnetic field lines are closed and the electrons cannot escape from the magnetic field, and as a result, the plasma remains only around the cathode (Figure 2). Plasma extends to a 60 mm region from the target surface, where the substrate’s structure and properties is highly affected by the concurrent ion bombardment.
If the substrate is placed out of this region, the ion flux receiving on the substrate is not enough (< 1 mA/cm2) for surface modifications. Higher energy ions bombarding the substrate can be supplied by applying bias voltage on the substrate, but it results in more defects and stress in the layer and in general, deteriorates layer properties [3].
Unbalanced Magnetron Sputtering Technique
If the center magnet is weaker compared to the ring magnets, all the magnetic field lines will not be caught up by the central magnet and the shape of the resultant magnetic field will change allowing some of the electrons easily escape from plasma towards the substrate, and this condition of magnetron arrangement is called Unbalanced Magnetron Sputtering (Figure 3).
The presence of electrons around the substrate leads to the ionization of inert gas and the penetration of plasma into this region. Ions bombarding the substrate increase the quality of the growing thin film.
As the ion current toward the substrate is related to target current, deposition rate is directly related to the target current; as a result, the ion-to-atom ratio reaching the substrate remains constant with increasing deposition rate.
The unbalanced magnetron can be used in another configuration, where the central magnet is stronger than the ring magnet (type 1) (Figure 4). Here, the unclosed magnetic field lines are pointed toward the chamber walls and the plasma density in the substrate region is low. This configuration is not commonly used on behalf of insufficient ion current on the substrate.
Applications of Unbalanced Magnetron Sputtering Method
Unbalanced magnetron sputtering can be utilized in the deposition of thin films to be used in different application. The most researched areas are as follows:
- Deposition of elemental layers, such as Fe, Mo, Nb, and W
- Deposition of thin films of TiN, Ni/Cr and TiO2 for electronic and optical applications
- Deposition of corrosion resistant coatings, such as Al-Mg alloys
- Deposition of wear resistant coatings, as diamond-like carbon, TiN and other transition metal-based nitrides thin films
One of the most promising applications of unbalanced magnetron sputtering systems is in the deposition of wear resistant coatings of TiN and TiC, which are formed by means of a pure metal target (like Ti) and a reactive gas (like N2).
Cons and Pros of Unbalanced Magnetron Sputtering Method
Despite slight differences in the design of balanced sputtering systems and unbalanced sputtering systems, the difference in their performance is considerable. The greatest advantage of unbalanced sputtering device in layer deposition is the capability of combining high deposition rate with high flux, low energy ion current. Moreover, deposition can occur at greater target-to-substrate distance, hence substrates with varied geometries can be coated in UBM. Also, unbalanced magnetron sputtering method is effective for multi-cathode magnetron sputtering systems with large chamber volumes like DST3 and DST3-T.
The unbalanced magnetron sputtering suffers limitation as well, like higher substrate heating (up to 250 ̊C) and high structure defects as a result of increased ion bombardment on the substrate. Then numerous potential control parameters compared to the balanced magnetron sputtering, optimization of the properties of the films for a specific application is time consuming [4].
Vac Coat Balanced & Unbalanced Magnetron Sputtering Systems
Vac Coat Ltd. is known for designing and manufacturing high-quality and reliable physical vapor deposition and vacuum coating systems. Its products include sputtering systems, carbon coating systems, thermal evaporation systems, and pulsed laser deposition system. All Vac Coat Sputter Coaters deposit thin films by magnetron sputtering method.
The sputter coaters include triple target sputter coaters with a thermal evaporator (DST3 & DST3-T) and magnetron desk sputter coater (DST1-300 & DST1-170).
In addition, desk sputter coater (DSR1) is one of the most popular sputter coaters, and we also produce hybrid sputter coaters, such as turbo pumped or rotary pumped desk sputter and carbon coater (DSCT & DSCR), for SEM sample preparation (SEM Coaters), that can deposit by sputtering and carbon coating method. For more information, visit the website please.
References
- Rossnagel, S.M.; Magnetron sputtering. Journal of Vacuum Science & Technology A 38, 060805 (2020). https://doi.org/10.1116/6.0000594
- Kelly, Peter J., and R. Derek Arnell. “Magnetron sputtering: a review of recent developments and applications.” Vacuum 56.3 (2000): 159-172. https://doi.org/10.1016/S0042-207X(99)00189-X
- Rohde, Suzanne L. “Unbalanced magnetron sputtering.” Physics of thin films. Vol. 18. Elsevier, 1994. 235-288. https://doi.org/10.1016/B978-0-08-092513-4.50008-8
- Window, N. Savvides, J. Vac. Sci. Technol. A 4 (1986) 196
- http://www.gencoa.com/resources/documents/comparison-balanced-unbalancedarray-designs.pdf