Sputtering (sputter deposition) is a method used to create THIN FILMS and is a type of PHYSICAL VAPOR DEPOSITION. Unlike some other vapor deposition methods, the material does not melt. Instead, atoms from the source material (target) are ejected by momentum transfer from a bombarding particle, typically a gaseous ion.
An advantage of this process is that sputter-ejected atoms have kinetic energies significantly higher than evaporated materials for better adhesion. Sputtering can be performed bottom up or top down. Another advantage of sputter deposition is that even materials with very high melting points are easily sputtered.
In the gaseous ion sputtering configuration, the sputtering process sequence is as follows:
- The substrate that will be coated is set inside a vacuum chamber. Air is removed from the chamber, and an inert gas is pumped in at low pressure. Inert gas is used because it does not react chemically to the target material. The most common (and cheapest) sputtering gas is argon (Ar), followed by krypton (Kr), xenon (Xe), neon (Ne) and nitrogen (N2).
– For efficient momentum transfer, the atomic weight of the sputtering gas should be close to the atomic weight of the target, so for sputtering light elements neon is preferable, while for heavy elements krypton or xenon are used.
– Light atomic weight gases such as hydrogen (H) and helium (He) result in negligible sputtering. - Voltage is applied to the target (the material that will be deposited onto the substrate), making it a cathode (negatively charged). The positive anode is the chamber body, which acts as the ground.
- The voltage causes free electrons to flow from the negatively charged target material and collide with the outer electronic shell of the inert gas atoms. The free electrons drive electrons off the inert gas due to their like charge. The inert gas atoms become positively charged ions.
- The inert gas ions attract to the negatively charged target material. During a collision cascade, this attraction ultimately causes inert gas ions to strike the target at an extremely high velocity.
- The bombarding ions have sufficient force to dislodge and eject (sputter off) atoms from the face of the target. The atoms from the target cross the vacuum chamber and are precisely deposited in a typical line-of-sight cosine distribution on the substrate surface as a thin film of material.
- The glowing “plasma” is created when the inert gas ions recombine with free electrons into a lower energy state. The excess energy is emitted as light.
The basic sputtering methods are planar magnetron, cylindrical magnetron, high-power impulse magnetron, diode and ion beam sputtering.
Sputter deposition is used in the following commercial applications:
- Architectural and anti-reflective glass coating
- Solar technology
- Display web coating
- Automotive and decorative coating
- Tool bit coating
- Computer hard disc production
- Integrated circuit processing
- CD and DVD metal coating
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