Sputtering

Sputtering a vital and prominent process for thin film depositions. In this process, a substrate to be coated is placed in a vacuum chamber containing an inert gas – usually Argon – and a negative bias is applied to the target causing the plasma to form in the vacuum chamber. So, the target is attached to the cathode and substrate to the anode. Ions within plasma are accelerated towards cathode (target) where they hit the target with sufficient energy to dislodge some atoms/ molecules. Individual atoms escape from the target, moves to the substrate and condense as a thin film. Sometimes, reaction gas (e.g., oxygen or nitrogen) is added to the Argon gas. As with the argon gas, ions of the reaction gas are formed, which react with the sputtered layer atoms in the vacuum chamber. The resulting reaction products are then deposited on the substrate surface.

This process is termed as reactive sputtering. The sputtering deposition has become a generic name for a variety of sputtering processes and target configurations. In a planar cathode configuration, the cathode is mounted directly above the substrate. In a confocal cathode configuration, multiple cathodes are positioned confocally to the substrate. This is the preferred method for applications requiring co-sputtering of multiple materials during thin film deposition.

Figure 1: Typical Magnetron sputtering system showing the placement of target, magnet, and substrate (Figure 1 DOI: https://doi.org/10.1088/1361-6595/ac7413)

(Figure 1: DOI: https://pubs.acs.org/doi/10.1021/acscombsci.9b00123)

Magnetron Sputtering

By placing a number of magnets behind the target, the free electrons are trapped in the magnetic field of these magnets near the target surface. As a result, these electrons do not reach and bombard on substrate but collide with the background gas resulting into the formation of high-density plasma. The increased number of ions expediate the rate of deposition. Magnetron sputtering offers several advantages like

• High deposition rates and better adhesion
• High purity films
• Excellent uniformity
• Low temperature
• Low damage to the substrate film as no high energy electron bombard the substrate
• All materials can be deposited regardless of their melting temperatures
• Films of alloys and compounds can be deposited while maintain the source composition

Magnetron sputtering deposition systems come in a variety of configurations depending on the choice of power source such as such as direct current (DC) magnetron sputtering, radio frequency (RF) magnetron sputtering and pulsed DC. Magnetron sputtering each has a different working principle and application objects.

DC magnetron sputtering: Magnetron sputtering using DC power is an effective and economical method to deposit conducting materials like metals and transparent conducting oxides. Only electrical conductors can be sputtered, as otherwise an opposing field builds up and the sputtering process stops. The other limitation is that only low sputtering rates are achieved since plasma density is low.

RF magnetron sputtering: The main advantage of RF (mostly 13.56 MHz) magnetron sputtering system is that it does not require the target to be a conducting material. These systems are used to deposit insulating/ dielectric target materials. RF field set up the oscillations of electrons with the applied frequency increasing their ionizing efficiency and create a high-density plasma. Consequently, the process can be operated at lower pressures with higher growth rate.

Pulsed DC sputtering: It is seen that reactive sputtering of insulators suffers some difficulties like charging of the target and arcing, stoichiometry control and poisoning in reactive sputtering systems. By pulsing the magnetron discharge in low to medium frequency region (10-350 kHz), it is possible to overcome these issues while achieving high deposition rate.

High Power Impulse Magnetron Sputtering (HIPIMS)

This is the most advanced technology in sputtering process. It combines the magnetron sputtering with a high voltage pulsed power source. By pulsing the target with high peak voltages and very high peak powers for low duration, give a low average power to the target and allows a large fraction of the sputtered material to be ionized. The advantage is that this generates a high- density plasma of target material without overheating the target as the target get chance to cool in the “pulse Off” time. This produces high performance dense coatings with good adhesion that are extremely smooth.