Magnetron sputtering depends on the presence of a magnetic field above the target’s surface. Ferromagnetic targets (Fe, Ni, Co, Permalloy, etc) of normal thickness have high magnetic permeabilities which trap the flux from the source’s normal magnet set. However, reducing the target’s thickness causes it to saturate allowing the magnetic field to penetrate. A nickel target’s permeability is such that a target thickness of 0.100 (or less) saturates. An iron target, with its much higher permeability, must be approximately 0.002 to saturate in the normal magnet set’s field. Sputtering very thin targets brings its own problems: first the power must be very low to prevent target melting and burn-through; second, thin targets tend to bend away from the cooling well’s surface exacerbating the chances of melting and burn-through; and third there may be no obvious signs when burn-through occurs and the cooling well’s surface starts to sputter. For these reasons, SAM strongly recommends the customer use a high-strength magnet set when sputtering ferromagnetic materials.
