Sputtering

Sputtering is a common method of physical vapor deposition (PVD) used to produce thin film coatings. Sputtering removes material from a target (or source), and deposits it on a surface, such as a component or other working piece, which is referred to as a substrate. sputter basics

High energy particles are needed to sputter a target. These particles are supplied by creating a plasma above the target. A plasma is a collection of electrons (negative charge) and ions (positively charged particles). A simple example of a plasma is a neon sign, in which the light is generated from the electrons and ions recombining with each other. Different gases release differing amounts of energy when the electrons and ions recombine, which manifests itself as different colors. For example an Argon discharge is lavender/blue, Nitrogen is pinkish-purple, and Oxygen is white-blue.

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Once created, these high energy particles strike the surface of the target material. When they strike the target, these particles can be:

  • reflected from the surface of the target
  • penetrate into the target surface and disrupt the target material atoms
  • free electrons from the target (called secondary electrons)
  • eject material from the target, in which case the ejected material becomes the sputtered flux that is deposited

Requirements to sputter a material

  • Vacuum chamber – low pressures are necessary to generate a plasma, allow effective transport of the sputtered material, and ensure purity of the film created.
  • Power to the target – electricity is passed through the target to sustain the plasma and create the high-energy particles for sputtering.
  • Cooling of the target – most of the power going through the target is converted to heat. The target must therefore be cooled or it will be damaged.
  • Pure gases – either non-reactive gas (for example argon) or reactive gas (nitrogen or oxygen) is needed, depending on the process. Either way, the amount and flow of these gases are controlled very precisely.

The simplest sputter configuration is a planar cathode, which consists of a flat piece of target material in contact with a water cooled body. The water cooled body is supported on insulating blocks that electrically isolate the cathode from the coating system. Power is delivered through a direct connection (often the water line) to the cathode body.

An obvious concern is the water being in direct contact with the electrical power. If the water is conductive, power will be transmitted from the cathode to the water manifold behind the system. This is unacceptable for both safety and process control. This is prevented by using treated, low electrical conductivity water– the low conductivity prevents the passage of power.

To increase the performance of the cathode, a magnet array is placed in the cathode. This confines the electrons within the magnetic field, increasing the density of the plasma thereby increasing the rate at which the target can be sputtered. The electrons are confined to a region of the magnet field that defines a racetrack over the surface of the target.

To “start” a cathode, gas is first flowed into the chamber and then power is applied to the cathode. Initially no plasma is present, but the power creates a very large negative voltage on the target. This negative voltage repels and accelerates any naturally present electrons. These accelerating electrons collide with atoms in the gas, ionizing them and freeing other electrons. These new electrons will also be repelled by the voltage and can collide with and ionize other gas atoms. The result is an avalanche effect which electrically breaks down the gas and generates a plasma. The plasma provides the charged particles (ions and electrons) necessary to allow electric current to flow. At this point the voltage on the cathode drops and current begins to flow. The voltage on the target determines the amount of energy the ions have when they reach the target and the current is a measure of how many ions are striking the surface. The magnetic fields define the region of electron and therefore plasma confinement.

There are several issues or limitations with planar magnetron cathodes, both related to the magnetic profile. Since the electrons are confined to a racetrack region over the target surface only this region is effectively sputtered, and the target gets eroded in this region. The target erosion is very non-uniform and develops a “V” shape. This limits the possible utilization of the target. Well designed planar magnetrons can achieve a utilization of 45% but 20-25% is more common.

Sputtering is used in a wide range of applications to apply thin coatings of materials to alter the appearance or functionality of a surface. Some common examples are:

  • Coating tool bits to make them last longer
  • Decorative coatings (“gold” color on faucets or door handles is often zirconium nitride)
  • CD’s and DVD’s. Some DVD’s are actually made from two discs glued together. One disc is partially transmitting so light can pass through it to read the other (a challenging application)
  • Antireflection coatings for eye glasses
  • Low-E coatings for windows
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