Physical Vapor Deposition by Sputtering

Physical Vapor Deposition (PVD) by Sputtering

Physical Vapor Deposition (PVD) is a process by which a thin film of material is deposited on a substrate according to the following sequence of steps: 1) the material to be deposited is converted into vapor by physical means; 2) the vapor is transported across a region of low pressure from its source to the substrate; and 3) the vapor undergoes condensation on the substrate to form the thin film. In VLSI fabrication, the most widely-used method of accomplishing PVD of thin films is by sputtering.

Sputtering is a mechanism by which atoms are dislodged from the surface of a material as a result of collision with high-energy particles. Thus, PVD by Sputtering is a term used to refer to a physical vapor deposition (PVD) technique wherein atoms or molecules are ejected from a target material by high-energy particle bombardment so that the ejected atoms or molecules can condense on a substrate as a thin film. Sputtering has become one of the most widely used techniques for depositing various metallic films on wafers, including aluminum, aluminum alloys, platinum, gold, TiW, and tungsten.

Sputtering as a deposition technique may be described as a sequence of these steps: 1) ions are generated and directed at a target material; 2) the ions sputter atoms from the target; 3) the sputtered atoms get transported to the substrate through a region of reduced pressure; and 4) the sputtered atoms condense on the substrate, forming a thin film.

Sputtering offers the following advantages over other PVD methods used in VLSI fabrication:

1) Sputtering can be achieved from large-size targets, simplifying the deposition of thins with unifrom thickness over large wafers;

2) Film thickness is easily controlled by fixing the operating parameters and simply adjusting the deposition time;

3) Control of the alloy composition, as well as other film properties such as step coverage and grain structure, is more easily accomplished than by deposition through evaporation;

4) Sputter-cleaning of the substrate in vacuum prior to film deposition can be done;

5) Device damage from X-rays generated by electron beam evaporation is avoided.

Sputtering, however, has the following disadvantages too:

1) High capital expenses are required;

2) The rates of deposition of some materials (such as SiO2) are relatively low;

3) Some materials such as organic solids are easily degraded by ionic bombardment;

4) Sputtering has a greater tendency to introduce impurities in the substrate than deposition by evaporation because the former operates under a lesser vacuum range than the latter.

Figure 1. Examples of Sputter Systems

The high-energy particles used in sputter-deposition are generated by glow discharges. A glow discharge is a self-sustaining type of plasma created by applying an RF field to a pressurized gas like argon, creating free electrons within the discharge region. A complete theory on how sputtering occurs has not yet been established due to the complexity of interactions involved, but experts in the field state that sputtering is comparable to billiard ball kinetics in three dimensions.

Sputtering yield, or the number of atoms ejected per incident ion, is an important factor in sputter deposition processes, since it affects the sputter deposition rate. Sputtering yield primarily depends on three major factors: 1) target material; 2) mass of the bombarding particles; and 3) energy of bombarding particles. In the energy range where sputtering occurs (10 to 5000 eV), the sputtering yield increases with particle mass and energy.

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