What is the sputtering?

Sputtering is a technique used to deposit thin films of a material onto a surface (a.k.a. “substrate”). By first creating a gaseous plasma and then accelerating the ions from this plasma into some source material (a.k.a. “target”), the source material is eroded by the arriving ions via energy transfer and is ejected in the form of neutral particles – either individual atoms, clusters of atoms or molecules. As these neutral particles are ejected they will travel in a straight line unless they come into contact with something – other particles or a nearby surface. If a “substrate” such as a Si wafer is placed in the path of these ejected particles it will be coated by a thin film of the source material.

Although SPUTTERING as described above seems relatively intuitive, familiarization with the following terms/concepts will give a more comprehensive understanding of this process:

Sometimes described as the “fourth state of matter” (the first three being solid, liquid, gas), a gaseous plasma is actually a “dynamic condition” where neutral gas atoms, ions, electrons and photons exist in a near balanced state simultaneously. An energy source (eg. RF, DC, MW) is required to “feed” and thus maintain the plasma state while the plasma is losing energy into its surroundings. One can create this dynamic condition by metering a gas (e.g. Ar) into a pre-pumped vacuum chamber and allowing the chamber pressure to reach a specific level (eg. 0.1 Torr) and introducing a live electrode into this low pressure gas environment using a vacuum feedthrough.

1. Ever present “free electrons” will immediately be accelerated away from the negatively charged electrode (cathode). These accelerated electrons will approach the outer shell electrons of neutral gas atoms in their path and, being of a like charge, will drive these electrons off the gas atoms. This leaves the gas atom electrically unbalanced since it will have more positively charged protons than negatively charged electrons – thus it is no longer a neutral gas atom but a positively charged “ion” (e.g. Ar +).

2. At this point, the positively charged ions are accelerated into the negatively charged electrode (a.k.a. “cathode”) striking the surface and “blasting” loose electrode material (diode sputtering) and more free electrons by energy transfer. The additional free electrons feed the formation of ions and the continuation of the plasma.

3. All the while, free electrons find their way back into the outer electron shells of the ions thereby changing them back into neutral gas atoms. Due to the laws of conservation of energy, when these electrons return to a ground state, the resultant neutral gas atom gas gained energy and must release that same energy in the form of a photon. The release of these photons is the reason the plasma appears to be glowing.

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