Plasma is formed in sputtering through a process called gas ionization, which involves creating a low-pressure gas environment inside a vacuum chamber and introducing a gas such as argon. A high voltage is then applied to the gas, which ionizes the atoms and creates a plasma.
Detailed Explanation:
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Vacuum Chamber and Gas Introduction: The process begins by evacuating a chamber to create a vacuum. This is crucial as it reduces the number of air molecules and other contaminants that could interfere with the sputtering process. Once the desired vacuum level is achieved, a noble gas, typically argon, is introduced into the chamber. The pressure of the gas is maintained at a level that supports ionization, usually not exceeding 0.1 Torr.
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Gas Ionization: After the argon gas is introduced, a high voltage, either DC or RF, is applied to the gas. This voltage is sufficient to ionize the argon atoms, knocking off electrons and creating positively charged argon ions and free electrons. The ionization potential of argon is about 15.8 electron volts (eV), which is the energy required to remove an electron from an atom. The application of voltage in the presence of the gas facilitates the formation of a plasma, a state of matter where electrons have been stripped from atoms.
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Formation of Plasma: The ionized gas, now a plasma, contains a mixture of neutral gas atoms, ions, electrons, and photons. This plasma is in a near-equilibrium state due to the dynamic interactions between these particles. The plasma is sustained by the continuous application of voltage, which maintains the ionization process and keeps the plasma active.
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Interaction with Target Material: The plasma is positioned near a target material, which is typically a metal or ceramic. The high-energy argon ions in the plasma are accelerated towards the target material due to the electric field. When these ions collide with the target, they transfer their energy, causing atoms from the target to be ejected or "sputtered" into the gas phase. These ejected particles then travel and deposit on a substrate, forming a thin film.
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Control and Enhancement of Plasma: The quality and rate of sputtering can be controlled by adjusting parameters such as the gas pressure, voltage, and the position of the substrate. Techniques like glow discharge and the use of secondary electrons can enhance the ionization of the plasma, leading to more efficient sputtering rates.
In summary, plasma in sputtering is formed by ionizing a gas like argon in a vacuum chamber using a high voltage, creating a plasma that interacts with a target material to eject and deposit particles onto a substrate. This process is fundamental to the deposition of thin films in various industrial applications.
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