Sputter Deposition of Thin Films: Introduction

Sputtering, in its simplest form, is the ejection of atoms by the bombardment of a solid or liquid target by energetic particles, mostly ions. It results from collisions between the incident energetic particles, and/or resultant recoil atoms, with surface atoms. One of the major advantages of this process is that sputter-ejected atoms have kinetic energies significantly larger than evaporated materials. The growing film is subjected to a number of energetic species from the plasma. Figure 1 shows a comparison of the energetics of thermal evaporation and sputtering processes for Cu [1]. Figure 2 shows the general sputtering process. In this process, atoms or molecules of a solid material (a target or sputtering source) are ejected into a gas form or plasma due to bombardment of the material by energetic gas ions and deposited on a substrate above or to the side of the target. A vacuum is required to initiate a plasma whose ions bombard the target. The sputtering process is essentially a momentum exchange, shown in Figure 3 [2], between the gas ions; the more intense and concentrated the plasma in the region of the target, the higher the atom removal rate (or deposition rate). The number of atoms ejected per incident ion is called the sputtering yield, and is dependent on the energy of the incident ion. A measure of the removal rate of surface atoms is the sputter yield Y, defined as the ratio between the number of sputter ejected atoms and the number of incident projectiles. The mass of the ion is important compared to the mass of the atoms in the target. Momentum transfer is more efficient if the masses are similar. Inert gases are used to generate the plasma. The most common (and cheapest) sputtering gas is argon (Ar), followed by krypton (Kr), xenon (Xe), neon (Ne), and nitrogen (N2). There will be negligible sputtering with light atomic weight gases such as hydrogen and helium. Reactive gases typically used are oxygen (O2), nitrogen, fluorine (F), hydrogen (H2), and hydrocarbons (methane, butane, etc.).

Figure 1. Comparison of thermal energy distributions for Cu evaporated at 1300 K and energy distribution of sputtered Cu [1]

The major sputtering techniques are diode, planar magnetron, cylindrical magnetron, high power impulse magnetron, and ion beam sputtering. All these methods have a number of variations. Diode sputtering, shown in Figure 2, is the simplest configuration of this family. Both RF (poorly conductive targets) and DC (conductive targets) power is applied to the sputtering target. This type of sputtering is typically performed at higher chamber pressures than its cousins, 0.5 to 10 Pa (5 X 10-3 – 0.1 torr). Substrate heating is often required to obtain high quality adherent films. RF sputtering has the advantage of higher deposition rates and a wider range of materials that can be deposited. Both methods, however, can be used for reactive deposition. As with all sputtering processes, deposition rate depends on a number of factors, such as chamber pressure, power to the target and substrate target spacing.

Figure 2. Diagram of the sputtering process.

 

Figure 3. Diagram showing momentum exchange in the target during the sputtering process [2].

As the name implies, planar magnetron sputtering in its simplest form utilizes a flat sputtering target in a cathode enclosure. Magnetron targets can be as small as 1 inch or as large as several meters. Figure 4 shows the geometry of a basic planar magnetron cathode. Magnets (called magnetics) are placed under the target in various configurations to confine the plasma or spread the plasma above the region of the target. The magnetic lines of force focus the charged gas atoms (ions). The stronger the magnetic field, the more confined the plasma will be (consequences are discussed below). The magnetics focus the plasma at the surface of the target and an erosion pattern, commonly called a “racetrack”, is formed as sputtering proceeds. Magnet configuration is different for planar circular, planar rectangular, external and internal cylindrical magnetrons. Figure 5 shows an erosion pattern of a conventional planar magnetron cathode. This pattern is narrow and materials usage was poor, but second and third generation magnetrons use target material much more efficiently. Target utilization of cylindrical magnetrons can be as high as 80%. Other designs such as a rotating circular magnetrons and full face erosion magnetrons also have high target utilization.

Figure 4. Magnet geometry in a planar magnetron cathode.

 

Figure 5. Erosion racetrack in a magnetron sputtering target.

 

The advantages of magnetron sputtering are:

  • Wide range of materials can be deposited
  • DC and RF reactive processes possible
  • Lower chamber pressures
  • Higher deposition rates than diode sputtering processes
  • Dense, high quality thin films
  • Large areas can be covered
  • Good thickness uniformity
  • Substrate heating not required in many cases
  • Amenable to a wide range of substrates, including plastics
  • Deposition conditions easily controlled
  • Several cathode configurations possible

 

Disadvantages are:

  • Poor materials usage (this is improving with modern cathode designs)
  • Lower deposition rates than electron beam evaporation, ion beam sputtering, cathodic arc and CVD processes

 

The next Blog will address other magnetron designs and applications.

 

Reference:

  1. Handbook of Deposition Technologies for Films and Coatings, 3rd Ed., P M Martin Ed., Elsevier (2009).
  2. Handbook of Deposition Technologies for Films and Coatings, R. Bunshaw Ed., Noyes (1994).