What is Sputtering? Understanding the Process and Applications

Introduction 

Sputtering, also known as sputter deposition, is a coating technique of PVD processes based on high vacuum conditions. Moreover, it is used as a cleaning method to produce high-purity surfaces. Also, it assists in analyzing the surface’s chemical composition. In short, sputtering is a process where the target’s atom (Solid state) is ejected due to the bombardment with energetic ions entering the gas phase.

Understanding the Sputtering Process

Principle: 

The working principle of sputtering is to eject atoms from the target’s surface using plasma energy and deposit them on the substrate.

Sputtering process:

Sputtering is achieved by creating plasma by ionizing inert gas(Argon) using pulsed DC or electromagnetic excitation. Though plasma production occurs at higher pressures of order 10^-1 to 10^-3 bar, the pressure should be increased gradually and not at once. Due to the applied magnetic field, Ar⁺ ions in the plasma are accelerated and surround the sputtering target. On Striking the target, every ionized atom transfers enough energy to rip an atom and project to the substrate. The sputtering targets can be circular, rectangular, delta, or tubular. The geometry and material type help create several thin layers, including alloys, in one shot.

During the collision of ions on the surface, several effects occur depending on the type of ions used and their kinetic energy.

1. Remove the material from the bombarded target.
2. Incorporate the ions into the target, forming a chemical compound. This effect is known as ion implantation.
3. These ions get deposited on the bombarded substrate, forming a thin layer.

Classification of Sputtering:

1. DC diode sputtering: At a DC voltage of 500-1000V, the low-pressure argon plasma is burned between a target and a substrate. The positive argon ion ejects the target atom and transfers them to the substrate.

2. Radio Frequency sputtering: RF sputtering uses a high-frequency AC field instead of a DC field. The required voltage source, capacitor, and plasma are connected in series. The capacitor acts as a filter that separates the DC component of the current and maintains the plasma in an electrically neutral state.

The AC field accelerates the ions and electrons in both directions. After reaching 50Hz, the ions stop following the AC field due to its small charge-to-mass ratio. The oscillating electrons near the plasma undergo more collisions with the argon atoms. These collisions lead to a high plasma rate and reduction in pressure of about 10^-1 to 10^-2 Pa without altering the sputtering rate. Hence, producing thin layers with various microstructures is possible at high pressures.

3. Magnetron sputtering: Unlike regular cathode sputtering with an electric field, magnetron sputtering has an additional magnetic field at the back of the cathode plate. Thus, the charge carriers move along a spiral path due to the superimposition of electric and magnetic fields.  The electron density is highest where the magnetic field aligns with the target surface. Thus, at this point, the ionization is high.  

4. Reactive sputtering:  A reaction gas such as oxygen or nitrogen is added to the Argon gas. Along with the argon gas, the reaction gas’s ions react with the sputtered atoms in the vacuum chamber. The resultant product gets deposited on the surface of the substrate. This technique is also available as a variant of DC and HF.

Applications

1. It is used to create thin films that are essential for electronic components in the semiconductor industry.
2. It is used to produce anti-reflecting coatings and mirror finishes for optical devices.
3. It is employed in depositing thin films in photovoltaic cells, improving efficiency and reducing costs.
4. It is essential for fabricating magnetic materials used in data storage devices and electronic applications.

Conclusion

Sputtering is a physical process where solid material is vaporized by bombarding it with ion energy. This technique assists in creating thin films, engraving, material erosion, and various analytical methods. Sputtering occurs primarily due to momentum transfer between the ions and the atoms of the target material during collisions. The number of atoms released per incoming ion is the sputter yield, an essential indicator of the sputtering process’s efficiency. Several factors affect this yield, including the energy and mass of the incoming ions, the target atoms’ masses, and the solid material’s bond energy.