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Materials & Manufacturing (MM) | ac - sputtering

Sputtering is a widely used and highly versatile vacuum coating system used for the deposition of a variety of coating materials.  Sputtering deposition systems use high energy particles as a way of transferring kinetic energy to a target in order to remove material for deposition.  For sputtering, the energized particles are present as a glow diffuse plasma.  A plasma is a partially ionized gas consisting of positively charged particles (cations), negatively charged particles (electrons and anions), and neutrals.  The overall charge of a plasma is neutral.  In a plasma, the ionization of the gas generally occurs between a cathode and an anode.  In sputtering systems the cathode, which is the electron emitter, is the target material.  The anode, which is the electron receiver, is usually the vacuum chamber wall or the substrate (component to be coated) [14, 15]. 

Sputter deposition systems are generally categorized by the power source used to generate the plasma and how the plasma is manipulated within the coating chamber.  The most common sputtering systems are the magnetron systems.  In recent years, research on sputtering methodology has developed hybrid sputtering systems which combine sputtering with other thin film technologies such as pulsed laser deposition and ion plating in order to better control the film growth properties [16, 17].  An important note is that multiple targets (co-sputtering) can be used simultaneously in order to create complex compositional coatings.  Single targets comprised of more than one element can also be used, but the composition of the coating may not be the same as the target due to differences in sputter yield, the relative ability of the materials to stick to the substrate, the deposition temperature and the relevant percent of sputtered elements to reach the substrate without being scattered from the plasma [15, 18].

Primary Sputter Deposition Technologies

  • DC Diode Sputtering
  • Magnetron Sputtering
    • Unbalanced Magnetron
    • Balanced Magnetron
  • RF Sputtering
  • Reactive Gas Sputtering
  • Ion Beam Sputtering
  • Pulse DC/AC Sputtering

Direct Current Diode Sputtering
The simplest sputtering system is the direct current diode system (dc diode).  Figure 1 illustrates a simple planar diode system.  The plasma is sustained by the breakdown of an inert gas.  Argon is the most widely used working gas in sputtering due to the larger mass as compared to neon and helium (higher mass correlates to more energetic collisions with the target material) and the much lower cost of argon compared to xenon and krypton.  The efficiency of the inert gas for sputtering the target is defined by the sputter yield, S, which is a proportional to the ratio of the mass of the inert gas compared to the mass of the target: 
Only when the target atom or molecule has a much different mass than the argon ion is a heavier or lighter gas than argon necessary for efficient sputtering due to momentum transfer [14, 19].  Several conditions must first be met in order to create a plasma for sputtering.  The sputter deposition chamber is generally evacuated to a base pressure of approximately 10-7 Torr.  The low base pressure will aid in minimizing the impurity concentrations in the thin film.  Once a satisfactory base pressure is obtained, the working gas is introduced into the chamber to attain deposition pressure of approximately 100 mTorr.After the desired partial pressure of working gas in the chamber is achieved, a negative dc potential of a few hundred volts is applied to the cathode.  In order to ignite a plasma, an electron must be accelerated with enough energy towards the anode that a collision with an argon atom will cause ionization:
Formula 2
Generally, for a working gas of argon, the amount of energy that must be transferred to the argon atom from the electron is approximately 15.7 eV according to the gas ionization potential for argon [15, 19].

After ionization, the argon cation accelerates towards the cathode.  Upon colliding with the cathode several events can occur.  The majority of species ejected from the target material are neutrals, but cations and ions can also be sputtered.  A secondary source of electrons is created by the collision of the argon ions with the target.  These electrons increase the overall ionization rate of the argon gas and contribute to creating a stable and self sustaining plasma.  Other forms of energy can also be generated at the substrate including X-rays and photons, although these have a small effect on the deposition [19].

Magnetron Sputtering
The original diode sputtering systems have mostly been replaced in ceramic coating research by magnetron sputtering systems.  A basic magnetron sputtering system uses a magnetic field to trap secondary electron emission near the surface of the target.  The trapping of the secondary electrons allows the magnetron sputtering system to surpass traditional diode sputtering systems in many regards including: increased deposition rate, decrease impurities in coatings, and achieve depositions at lower substrate temperatures [14, 15, 18, 19].

The setup of the magnetron sputtering system is very similar to the diode sputtering system.  Analogous to diode sputtering, the chamber walls, substrate or substrate holder act as the anode and the target material acts as the cathode for a diffuse glow-discharge plasma.  The secondary electrons are trapped close to the surface of the target by a magnetic field that is perpendicular to the electric field.  This magnetic field is used to control the density and location of the secondary electrons above the target.

The plasma flow still exists between the anode and the cathode, but the magnetic field causes the electrons to travel in a cyclic path as they are emitted from the cathode.  The first significant effect of trapping the electrons in cyclic patterns is that the working gas will undergo a far greater rate of ionization compared to diode sputtering.  The path of the trapped electrons is significantly longer than the distance between the electrodes, so the trapped electrons under go many more collisions with the working gas while trapped in front of the target.  The increased number of collisions between the working gas and the secondary electrons produce more electrons for sputtering.  The operating pressure of traditional diode sputtering is relatively high compared to magnetron sputtering because more gas molecules are necessary in order to produce a significant number of ions through electron collisions with electrons.  The magnetron systems therefore have a much more effective use of electrons and can operate at much lower pressures.  A typical operating pressure is 0.1 Pa for magnetron sputtering compared to 3.0 Pa for diode sputtering compared to 3.0 Pa [14, 15, 18, 19]. 

One of the disadvantages of traditional magnetron sputtering is the localization of the plasma over the substrate.  The electrons are trapped in a specific area above the target, so the ionization of the carrier gas tends to be more heavily concentrated in the regions of the trapped electrons.  The concentrated plasma preferentially wears down the source material over the target area where the magnetic lines are parallel to the target face.  The higher plasma density also decreases the overall ion bombardment of the substrate.  In order to increase target utilization, the target material or the magnets are kept in constant motion in order to physically change the sputtering location on the target, and this motion also increases the coating thickness uniformity at the substrate [14, 15, 18, 19].

A simple way to increase the density of charged species impinging on the substrate is by creating a non-uniform or unbalanced magnetic field at the source.  In an unbalanced magnetron, one of the magnets producing the magnetic field above the target is given a decreased field strength compared to the field generated by the other magnets.  The weaker magnetic field can not compensate for all of the field lines resulting in the redirection of some of the field lines towards the substrate.  The stray field allows a greater percent of electrons and charged ions to escape towards the substrate where collisions with the growing film can promote densification through resputtering and re-absorption of the film [15]. 

Radio Frequency (rf) Sputtering
Since the cathode of a diode or magnetron sputtering system is also the target, only electrically conductive materials can be used as targets for deposition in these configurations.  Sputtering from a dielectric or electrically insulating target material can be accomplished by use of an alternating or radio frequency (ac or rf) power supply.  The basis for the use of radio frequency is the large mass difference between the ionized gas particles and the electrons in the plasma.  If the frequency of the alternating frequency is high enough, a plasma can be sustained by continually accelerating and reversing the direction of the electrons through long enough distances that they gain the kinetic energy required to ionize the sputter gas through collisions.  The frequency required to sustain the plasma is generally above one MHz, but rf sputtering systems are generally operated at 13.56 MHz which is the maximum allowed frequency in the United States by the Federal Communications Commission.  This eliminates the need for secondary electrons from the target to sustain the plasma.  The main drawback with rf sputtering is the decrease in deposition rate due to lack of secondary electrons for gas ionization and the expense associated with radio frequency power supplies and the tuning systems required to couple the alternating potential to the plasma [14, 18].

Reactive Gas Sputtering
Instead of sputtering from a dielectric substrate, ceramic coatings are often produced by reactive gas sputtering [20-25].  Reactive sputtering involves reacting a gas with the target material as it travels between the substrate and the target.  Reactive gas deposition can be used with any sputtering method.  The partial pressure of the gas in the chamber will directly impact the stoichiometry of the coating composition.  One benefit of reactive sputtering as opposed to rf sputtering is that high purity targets of one material can be used for deposition.  Another benefit of reactive sputtering is that functionally gradient coatings can be produced by controlling the partial pressure of the sputtering gas and the reactive gas during deposition [15, 18].  One problem with reactive sputtering is that the target can build up a thin layer of dielectric on the surface if the partial pressure of the reactive gas is too high.  This can lead to decreased deposition rates [18]. 

Advantages of the Sputtering Process for Deposition:

  • Production of multilayer coatings through co-sputtering or reactive gas sputtering
  • Functionally gradient coatings through the control of partial pressure of gas
  • Controlled microstructure and good coating uniformity across the substrate

Disadvantages of the Sputtering Process for Deposition of Erosion Resistant Coatings:

  • Difficult to deposit uniformly on complex shapes such as turbine blades
  • High performance thick coatings are hard to produce due to high internal residual stress levels