Magnetron Sputtering: Basics

The last Blog introduced magnetron sputtering and the importance of low pressure operation for improved energy of ejected species. Magnets and their configuration with respect to the target determine many of the operating parameters and conditions. The strength of the magnetics affects the voltage at which the cathode operates. Operating voltage decreases with increased magnetic field strength. The operating voltage also decreases with erosion through the target because the surface of the target gets closer to the magnets (increasing magnetic field strength). For this reason there is a limit to target thickness that can practically be used. Thicker nonmagnetic targets can be used with stronger magnetics. Ferromagnetic materials and ferrites present a problem with magnetrons and tend to shunt the magnetic field and reduce magnetic field at the surface. Hence very high target voltages result if the target can be lit at all. Unless the magnetics are exceptionally strong, sputtering targets of magnetic materials must be much thinner than nonmagnetic materials (typically < 0.16 cm).

Both direct current (DC) and radio frequency (RF) power sources can be used with most magnetron cathodes. Pulsed DC and low and mid frequency power supplies are also used in magnetron sputtering. New concepts such as DC with RF overlay are also being developed for a number of coating applications (TCO’s for example).

Magnetron sputtering is particularly well suited for reactive deposition. Virtually any oxide, nitride and carbide thin film material can be deposited. Fluorides present a problem due to reflected neutrals off the target. The operating reactive gas partial pressure depends on deposition rate (target power), target material, chamber pressure and substrate target spacing. Two techniques are used to tune the reactive gas partial pressure, a hysteresis curve that plots target voltage against percentage of reactive gas and target voltage against reactive gas partial pressure [1,2]. Figure 1 shows a typical hysteresis plot for an oxide coating. Target voltage remains constant or increases slightly with increased oxygen, and then plummets at a transition concentration. The surface of the target is oxidized for percentages higher than the transition voltage and in the “metal” mode below the transition. Metallic and substoichiometric coatings result on the left side of the transition. If the percentage oxygen is too high, the target will become “poisoned” and an insulating crust will form on its surface. Deposition rate will decrease significantly and arcing may occur, introducing particulates into the growing film. The resulting coating may be less dense and or poor quality. The operating point is just on the cusp of the transition. The coating is fully stoichiometric and the deposition rate is optimum. Note that if the oxygen is decreased, there will be a hysteresis effect due to cleaning up of the surface oxide. The other technique actually determines the partial pressure of reactive gas. As shown in Figure 2. Reactive gas pressure is plotted against reactive gas flow. The operating point occurs just before the partial pressure increases with flow. The operating point is the flow at which the reactive gas is no longer “reacting” with the target material. There is also a hysteresis effect for this technique.

Whatever method one uses to determine the operating reactive gas pressure, it should be monitored as the target ages. Generally less reactive gas will be needed with increased erosion of the target. It should be noted that magnetron power supplies are available that incorporate reactive gas flow control.

Figure 1. Hysteresis plot of target voltage versus percentage of oxygen.
Figure 2. Plot of reactive gas partial pressure against gas flow. The operating point is the flow at which the reactive gas is no longer “reacting” with the target material [2].

Magnetron sputtering is also particularly well suited for deposition of multilayer thin film structures, including optical designs, electrochromic coatings, nanolaminates, superlattices, tribological coatings and barrier coatings. High quality thin film optical materials have been deposited since the 1970’s. A wide range of optical thin film materials has been developed. The list includes most oxides, nitrides, carbides, transparent conductive materials, semiconductors and many polymers. Fluorides are best left to evaporation processes and hybrid processes. The main advantage is that these materials can be deposited using both nonreactive and reactive processes with excellent control of composition, layer thickness, thickness uniformity, and mechanical properties.

Additionally, deposition of multilayer structures, including combinations of the materials listed above, is straight forward using multiple cathodes and reactive gases. A small sampling of the optical materials deposited by planar magnetron sputtering is listed in Table 1. Deposition of alloys, oxinitrides, alloy oxinitrides, carbon nitrides is straightforward. Use of multiple cathodes and reactive gases also makes deposition of multilayer structures possible. It is possible to deposit virtually any type of optical design, from the simplest antireflection coating to complex beam splitters and multiple cavity filters. Also, since sputtering is a nonequilibrium process, alloys and compounds not found in binary and ternary phase diagrams can be deposited.

Optical and quartz crystal monitoring are used to deposit the exact thickness; however, the process is often so reproducible that many layers can be deposited by simply timing them.

Magnetron sputtering is the most widely used deposition process to achieve high quality thin films and coatings. Modern cathodes offer very good target utilization and high deposition rates. This type of sputtering is used extensively in large area deposition. Large area single and multilayer coatings are deposited by magnetron sputtering using large cathodes and arrays of large cathodes. Magnetron cathodes are also used in in-line and roll-to-roll processes (addressed in a future Blog). Figure 3 shows an array of magnetron cathodes used for deposition of large area optical coatings and Figure 4 shows an array of solar concentrator panels with magnetron sputtered high reflection coatings.

Figure 3. Array of large magnetron cathodes used for large area optical coatings.
Figure 4. Array of solar concentrator panels with magnetron sputtered high reflection coatings.

Advanced techniques such as HPPMS, HPIMS, and dual magnetron sputtering are breaking new ground in thin film deposition and will be addressed in a future Blog.

Reference:

  1. Handbook of Deposition Technologies for Films and Coatings, P M Martin Ed., Wiley/Scrivener (2009).
  2. J A Thornton, Ann. Rev. Mater. Sci, 7 (1977) 239.
  3. W D Sproul and B E Sylvia, SVC 45th Annual Technical Conference Proceedings (2002) 11
  4. D A Glocker et al., SVC 47th Annual Technical Conference Proceedings (2004) 183.
  5. W D Sproul, SVC 35th Annual Technical Conference Proceedings (1992) 236.