Sputtering is one of the methods by which physical vapor deposition (PVD) is achieved. PVD is a process where a thin film of material is deposited on the surface of a substrate. In sputter deposition, high-energy particles are utilized to remove or eject atoms or molecules from the surface of a target material. The ejected atoms are removed from the target material and deposit on a substrate as a thin film. Sputtering is one of the most common methods used to deposit various thin metallic films on wafer substrates, with gold, platinum, aluminum, and tungsten among the common target materials.
PVD by sputtering can be described as a series of four steps. High-energy particles, which are ions of a gas, are generated and directed at a target material. When the high-energy particles collide with the target material, atoms of the target material are sputtered from the target. These sputtered atoms travel through a region of reduced pressure toward the substrate and then condense on the surface of the substrate to form a thin film of the sputtered target material. The high energy particles used in the sputtering process are usually ions of an inert sputtering gas, such as neon. Neon is used to sputter light elements, while krypton or xenon is preferred when sputtering heavier elements. However, reactive gases can also be used. The high-energy ions utilized in PVD sputter deposition are produced via glow discharges, which is a type of self-sustaining plasma generated by applying an RF field to a pressurized sputtering gas. There are many types of sputter deposition, such as ion-beam sputtering, gas-flow sputtering, high-power impulse magnetron PVD sputtering, reactive sputtering, ion-assisted deposition, and high-target-utilization sputtering.
PVD sputtering deposition offers several advantages as compared to other deposition techniques. One of this is that materials with very high melting points can be easily sputtered, while the evaporation of such materials in a Knudsen cell or a resistance evaporator is very difficult. Sputtered films also have better adhesion on the wafer as compared to evaporated films and their composition is also close to that of the target material. PVD sputtering also simplifies the deposition of thin films over large wafers as it can be used on large-size target materials. PVD sputter deposition is a complex process due to the availability of many parameters, but this also enables a large degree of control over factors such as film thickness, film growth, alloy composition, and film microstructure. The substrate can also be sputter cleaned in vacuum before film deposition is performed. Finally, sputter deposition avoids device damage that can happen in electron beam evaporation due to the X-rays generated.
However, sputtering also has its disadvantages, one of which is the high capital expenses required to start and run a sputtering facility. PVD sputtering also has a higher tendency to introduce impurities in the substrate as compared to deposition via evaporation. Other materials are also incompatible with sputtering deposition particularly when they are easily degraded by ionic bombardment, such as organic solids. Finally, some important materials also have relatively low rates of deposition via sputtering, such as SiO2.
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