Magnets are placed behind the target in sputtering to enhance the ionization of the sputtering gas and increase the rate of deposition, while also protecting the substrate from excessive ion bombardment. This is achieved through the interaction of the magnetic field with the electric field, which alters the path of electrons, increasing their ionization efficiency and directing them away from the substrate.
Enhancing Ionization and Deposition Rate: In magnetron sputtering, the addition of a magnetic field behind the target creates a complex interaction with the electric field. This interaction causes the electrons to follow a spiral or cycloid path rather than a straight line. The trapped electrons move in a circuitous path directly above the target surface, significantly increasing their likelihood of colliding with and ionizing neutral gas molecules. This heightened ionization leads to a greater number of ions available to bombard the target material, thereby increasing the erosion of the target and the subsequent deposition of material onto the substrate. The electron density is highest where the magnetic field lines are parallel to the target surface, leading to a localized area of high ionization and sputtering.
Protection of Substrate: The magnetic field also serves to confine the electrons near the target surface, reducing their ability to reach and potentially damage the substrate. This confinement not only protects the substrate but also concentrates the ionization process near the target, optimizing the sputtering efficiency. The ions, due to their larger mass, are less affected by the magnetic field and thus continue to strike the target directly below the area of high electron density, leading to the characteristic erosion trenches seen in magnetron sputtering.
Use of Permanent Magnets: Modern sputtering systems typically use a system of permanent magnets located behind the target. These magnets help contain the secondary electrons generated by the collision of ions with the target surface. These electrons, held close to the target surface by the strong magnetic field, further enhance the ionization of the sputtering gas and sometimes even ionize some of the target adatoms. The rapid movement of these electrons along the magnetic field lines increases their ionization efficiency, contributing to the overall effectiveness of the sputtering process.
In summary, the placement of magnets behind the target in sputtering is crucial for enhancing the ionization of the sputtering gas, increasing the deposition rate, and protecting the substrate from ion bombardment. This is achieved through the complex interaction of the magnetic and electric fields, which alters the path of electrons and concentrates the ionization process near the target surface.
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