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  • What is Sputtering?
    Dec 13, 2017

    Sputtering is a process whereby particles are ejected from a solid target material due to bombardment of the target by energetic particles, particularly gas ions in a laboratory. It only happens when the kinetic energy of the incoming particles is much higher than conventional thermal energies (1 eV). This process can lead, during prolonged ion or plasma bombardment of a material, to significant erosion of materials, and can thus be harmful. On the other hand, it is commonly used for thin-film deposition, etching and analytical techniques. Sputtering is done either using DC Voltage (DC Sputtering) or using AC Voltage (RF Sputtering). In DC Sputtering, voltage is set from 3-5 kV and in RF Sputtering, power supply is set at 14 MHz. Due to the application of an alternating current, the ions inside the plasma oscillate resulting in an increases in the levels of plasma.

    Physics Of Sputtering

    Physical sputtering is driven by the momentum exchange between the ions and atoms in the target materials, due to collisions.

    The incident ions set off collision cascades in the target. When such cascades recoil and reach the target surface with an energy greater than the surface binding energy, an atom would be ejected, and this process is known as sputtering. If the target is thin on an atomic scale, the collision cascade can reach the back side of the target and atoms can escape the surface binding energy "in transmission". The average number of atoms ejected from the target per incident ion is called the sputter yield and depends on the ion incident angle, the energy of the ion, the masses of the ion and target atoms, and the surface binding energy of atoms in the target. For a crystalline target the orientation of the crystal axes with respect to the target surface is relevant.

    The primary particles for the sputtering process can be supplied in a number of ways: for example by a plasma, an ion source, an accelerator or by a radioactive material emitting alpha particles.

    A model for describing sputtering in the cascade regime for amorphous flat targets is Thompson's analytical model. An algorithm that simulates sputtering based on a quantum mechanical treatment including electrons stripping at high energy is implemented in the program TRIM.

    A different mechanism of physical sputtering is heat spike sputtering. This may occur when the solid is dense enough, and then the incoming ion heavy enough, that the collisions occur very close to each other. Then the binary collision approximation is no longer valid, but rather the collisional process should be understood as a many-body process. The dense collisions induce a heat spike (also called thermal spike), which essentially melts the crystal locally. If the molten zone is close enough to a surface, large numbers of atoms may sputter due to flow of liquid to the surface and/or microexplosions. Heat spike sputtering is most important for heavy ions (say Xe or Au or cluster ions) with energies in the keV–MeV range bombarding dense but soft metals with a low melting point (Ag, Au, Pb, etc.). The heat spike sputtering often increases nonlinearly with energy, and can for small cluster ions lead to dramatic sputtering yields per cluster of the order of 10,000.

    Physical sputtering has a well-defined minimum energy threshold equal to or larger than the ion energy at which the maximum energy transfer of the ion to a sample atom equals the binding energy of a surface atom. This threshold typically is somewhere in the range 10–100 eV.

    Preferential sputtering can occur at the start when a multicomponent solid target is bombarded and there is no solid state diffusion. If the energy transfer is more efficient to one of the target components, and/or it is less strongly bound to the solid, it will sputter more efficiently than the other. If in an AB alloy the component A is sputtered preferentially, the surface of the solid will, during prolonged bombardment, become enriched in the B component, thereby increasing the probability that B is sputtered such that the composition of the sputtered material will ultimately return to AB.