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Sputtering is a Physical Vapor Deposition method, utilizing argon ions for bombarding a cathodically connected target, made of the coating material.
Atoms of the target are knocked out by the high energy ions and deposit on the substrate surface.
Sputtering process scheme is shown in the picture:
DC sputtering is abasic spattering method using a constant (DC) voltage between the substrate (anode) and the target (cathode).
Commonly argon (Ar) ions are used as the particles bombarding the target surface.
Argon atoms are introduced into the chamber vacuumed to a very low pressure of about 1-10 mTorr. A DC voltage (0.5-5 kV) ionizes the argon atoms forming an ionized gas (plasma).
The positively charged argon ions accelerate toawards the cathode (target), bombard its surface and break the target atoms out.
The atoms travel at various directions and settle on the substrate surface forming a dposited layer.
DC sputtering is used for deposition of conductive materials (metals). The coating deposited on the anodically connected substrate does not change the anode conductivity. Non-conductive materials can not be deposited by DC sputtering since the non-conductive coating on the substrate prevents the electron flow through the anode.
The main disadvantage of the basic DC sputtering method is too low density of argon ions producing a low deposition rate (sputtering yield).
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Magnetron sputtering is the method of enhancing DC sputtering by crossed magnetic and electrical fields generating a high density plasma confined in the region adjacent to the cathode surface.
The magnetic field is produced by permanent magnets installed just behind the target.
The electrons in plasma travel along the magnetic field lines at a spiral trajectory. Along their way the electrons collide with argon atoms producing much more ions than in the basic DC sputtering method.
Higher density plasma provides more frquent colliding with the target which results in a higher rate of deposition.
Since the plasma is confined near the cathode, argon ions can not reach the substrate therefore magnetron sputtering does not damage the substrate and provides lower heating of its surface.
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AC sputtering uses alternating (periodically reversed) voltage between the electrodes (target and substrate).
The frequency of the alternating voltage is 10-100 kHz.
AC sputtering method allows deposition of electrically non-conductive materials (e.g. oxide films). Obtaining non-conductive coatings by DC sputtering is impossible because of the effect of disappearing anode.
AC spattering technique is used in Dual Magnetron configuration in which the AC voltage is applied between two targets. Their surfaces are periodically coated and cleaned according to the alternating potential, which prevents the effct of disappearing anode.
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RF sputtering operates at the alternating voltage of radio frequency (0.5-30 MHz). The Federal Communication Commision allows the only frequency 13.56 MHz for industrial purposes.
Non-conductive coatings derived from non-conductive targets may be obtained by the RF sputtering method. The potential of the target changes according to the voltage frequency. When it is positive the electrons of the plasma travel to the electrode, when the target turns negatve the argon ions accelerate to the its surface. Thus the target acts as the cathode during a part of each cycle, whereas the rest of the cycle it act as the anode.
Since the mobilities of the electrons and the ions are different due to the different masses (an ion is much heavier than an electron) the times are also different. The electrons reach the target surface much faster therefore the duration of the target as the anode is much shorter than the time when it is negative (the cathode).
Thus the plasma acts as a rectifier producing a negative DC voltage (self-bias) at the target accelerating the argon ions to the target where they knock out its molecules.
The molecules settle on the substrate surface and build the coating.
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Reactive sputtering uses chemical reactions between the atoms settlin on the substrate surface and the gas introduced into the vaccum chamber.
Reactive sputtering is a combination of a conventional sputtering process with Chemical Vapor Deposition (CVD).
Usually Oxygen or Nitrogen serve as the reactive gases. The target is metallic. The metal atoms reach the substrate surface where the reaction occurs. Oxide or nitride molecules deposit on the substrate surface forming the coating.
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