Sputtering is the mechanism used to deposit thin films of target material onto a substrate. This process involves ejecting gaseous ions into the source material, thus sputtering gas atoms, ions, and molecules out from the target surface. These emitted particles contain certain kinetic energy, allowing metal ions to increase surface mobility.
DC sputtering (or Direct-Current Sputtering) is a thin-film physical vapour deposition coating technique that uses a direct current as a power source. DC sputtering offers numerous advantages for metal deposition. It’s popular in many manufacturing processes, such as creating metalized packaging plastics and metal coatings on watches and jewellery.
This guide will provide a comprehensive overview of DC sputtering, how it works, and its distinction from RF sputtering.
What Is DC Sputtering?
Direct current (DC) sputtering is a thin film deposition technique that uses ionised gas molecules to vaporise (sputter) molecules off the target material into plasma. DC sputtering is the preferred technique for electrically conductive target materials because of its low cost and high level of control.
The DC magnetron sputtering process involves a vacuum chamber containing the target material parallel to the target substrate. The vacuum chamber contains a high purity inert gas such as argon that becomes charged when exposed to a pulsed DC current.
The metal target material acts as the negative node (cathode) while the substrate acts as the positive pole (anode). A DC current passes through the system, which causes the argon gas to ionise and results in a forceful collision of the ions with the negatively-charged source metal.
These collisions knock off metal ions off the surface of the target into the plasma (a mixture of ionised gas ions and electrons). The positively-charged substrate attracts the negatively-charged plasma, which condenses on the surface of the substrate, forming a thin film coating of the neutral source.
While DC sputtering is a common process for conductive metals, it doesn’t work as well for dielectric target materials. These target atoms can take on a charge, leading to arcing and other disruptions to electron densities, resulting in an uneven deposition rate. The accumulation of these positive ions can even cause the entire sputtering pathway to cease functioning, resulting in the need for a reset.
Check a breakdown of this deposition technique below.
How Does DC Sputtering Work?
Like any type of sputtering deposition effect, DC magnetron sputtering requires a vacuum chamber. It also requires DC power, positively charged sputtering gas atoms, a target material, and a substrate.
This is the process DC magnetron sputtering systems use to deposit metal coating materials onto substrates:
- The target or coating material to be used as the thin film is placed in a vacuum chamber.
- The vacuum chamber is positioned parallel to the desired substrate.
- The vacuum chamber removes water, air, hydrogen, and argon with a chamber pressure between 1 and 100 mTorr.
- The chamber then fills with inert process gas ions, such as argon ions.
- The system applies a DC voltage to the target surface.
- The target coating material becomes the cathode, and the substrate becomes the anode.
- The neutral argon atoms become ionized when they collide with the negatively charged target and then eject into high-density plasma, which is generated through neutralization.
- The now-ionized gas ions remain in the vacuum and break out the target atoms.
- The ionized gas molecules drive into the substrate.
- The positive ions condense and form thin films on the substrate.
- The magnetic field traps electrons over the sputtering targets, preventing ion bombardment and increasing the deposition rate.
Altogether, DC magnetron sputtering is a relatively simple technique with a high deposition rate, allowing manufacturers to deposit large quantities of surface materials onto substrates quickly, economically, and effectively. This process is essential in a range of commercial applications, such as depositing films of ZnO on glass substrates [1].
Korvus’s HEX series has the power and specialised mechanisms necessary to deploy DC sputtering depositions. Additionally, combining the HEX series with the Fission DC/RF sputtering series module allows users to alternate between DC and RF sputtering without downtime or specialised equipment.
DC vs. RF Sputtering
Direct current sputtering is one of several magnetron sputtering methods. Another is radiofrequency sputtering or RF sputtering.
The primary distinguisher between these two processes is in their applications. DC sputtering is suitable for conductive materials and magnetic materials. However, RF sputtering can deposit conductive and non-conductive materials, such as oxide films.
DC power is also distinct from RF power, impacting the voltage behind the sputtered atoms. Whereas DC sputtering uses a direct charge voltage, RF sputtering alternates charges, necessitating a more complex, expensive manufacturing process.
What Is RF Sputtering?
The radiofrequency (RF) sputtering process is an evolution of DC sputtering that aims to address unwanted charge build-up that happens with some sputtering target materials. This charge build-up can be very disruptive and may result in quality control concerns during film formation.
RF sputtering alternates the potential of the current in the vacuum environment by using a high-voltage RF source. This alternating current avoids the build-up of charged plasma particles and gas ionisation, essentially “cleaning up” the charge build-up after every cycle. If the first cycle uses a negatively-charged target material, the ionised gas ions remain at the anode at the end of the cycle.
The next cycle uses reverse polarisation to change the charge on the target and substrate, resulting in positively-charged sputtering gas atoms that move towards the negatively-charged substrate.
The main drawback of RF sputtering compared to pulsed DC sputtering is a significantly lower deposition rate and a much higher power requirement. The deposition rate is improvable with a strong magnetic field along the power source to keep the charged plasma discharge closer to the metallic target surface. The technique also draws a lot of power, which is why most RF power supplies can provide up to 1,000V to generate the correct signal.
Another drawback is that any RF sputtering source needs an impedance matching network between the vacuum chamber and the RF power supply equipment. This network prevents interference from the RF discharge that may reduce the overall sputtering rate.
Most RF sputtering takes place using a frequency of 13.5 MHz. Power supplies often use 300 W and 1,000 to generate this frequency.
The Difference Between DC Sputtering and RF Sputtering
Although DC and RF sputtering are both sputtering deposition techniques, these processes impact the material morphology of their sputtering targets differently [2].
One of the primary differences between DC and RF sputtering is their power sources. As the name suggests, DC sputtering uses a direct current as a power source. Meanwhile, RF sputtering alternates its electrical charge to prevent the charge from building up on the target material.
RF sputtering is sometimes known as AC sputtering because of the alternating current within RF power supplies.
DC sputtering also has a higher deposition rate than RF sputtering [3]. While DC sputtering is suitable for depositing large quantities onto large substrates, RF is more effective within smaller substrates.
DC and RF sputtering can deposit different types of target materials. While DC sputtering can deposit electrically conductive target materials, an RF sputtering method is suitable for various sputtering targets, including conductive and non-conductive materials.
Finally, DC and RF sputtering vary in their complexities and price points. DC sputtering is more affordable overall, as it uses less specialised processes. RF sputtering involves a more complex process, yet its versatility and excellent campaign length make up for its high price point.
The Advantages of DC Sputtering for Thin Film Manufacturing
DC sputtering is the simplest and most economical process among PVD metal deposition methods. This process offers numerous advantages over RF sputter deposition within thin-film manufacturing.
Manufacturing companies that utilise DC reactive magnetron sputtering can save money on their equipment and processes, creating wider profit margins. DC power requires simple, affordable configuring compared to other methods of sputter deposition. This process can also be adapted for use with magnetic sputtering targets, such as iron, nickel, and cobalt.
DC magnetron sputter deposition also offers greater control within the manufacturing process. Using DC as a power source allows for more precision and versatility in the chamber pressure vacuum. This process is applicable to numerous thin-film deposition materials.
Additionally, DC sputtering has an impressive sputtering rate compared to other methods. This advantage allows DC sputtering systems to process large substrates quickly.
However, DC sputtering also has limitations. For example, as we’ve mentioned, this process is only suitable for conductive materials. When attempting to deposit non-conductive insulating materials using DC sputtering, the materials may take on an electric charge, harming the target material.
Notably, this limitation is overcome by other sputtering processes for deposited films, such as radiofrequency sputtering and reactive sputtering.
Final Thoughts
Altogether, DC magnetron sputtering is a highly advantageous physical vapour deposition technique for depositing thin layers of films onto conductive substrates. This process offers benefits in terms of complexity and affordability, making it suitable for a range of commercial applications.
If you’re interested in implementing DC sputtering within your lab or research facility, we can help. Our Fission DC and RF Sputtering System support sputtering processes for a range of materials, including all solid metals. This system even supports high-power impulse magnetron sputtering (HiPIMS).
Contact us today to learn more about the Fission module of the HEX Sputtering System.
References
[1] Banerjee, A.N. et al. (2005). “Low-temperature deposition of ZnO thin films on PET and glass substrates by DC-sputtering technique.” Thin Solid Films, 496(1), 112-116.
[2] T Bui, Mathieu Halbwax, Jean-Pierre Vilcot. (2021). “Comparison of DC and RF Sputtering of ZnSnN2 : Effect on Structural, Optical and Electrical Properties.” Journées Nationales du PhotoVoltaïque, Fédération de recherche du PhotoVoltaïque, FedPV.
[3] Kim, Jaemin et al. (2020). “Effect of IGZO thin films fabricated by Pulsed-DC and RF sputtering on TFT characteristics.” Materials Science in Semiconductor Processing, 120.
