The trick is to send short, very high-power pulses through the magnetron at a low repetition rate. When the voltage is high enough, the ion current does not fall off as the gas is depleted but instead, sustained by self-sputtered metal ions, jumps to a new, much higher level. Self-sputtering continues as long as the power supply can deliver a high-voltage discharge current.
The result: energetic electrons are propelled far from the target and produce a dense plasma of metal ions, even in a vacuum. When the plasma is target metal rather than gas, a higher proportion of metal ions reaches the substrate, insuring that the substrate is coated with a uniform, voidless film with improved properties, such as the ability to penetrate into narrow, nanoscale cavities in intricate semiconductor circuits.
Sustained self-sputtering: how it works
A basic magnetron sputterer is characterized by a strong electric field between the target disk (the cathode) and a grounded anode nearby. The substrate to be coated, which carries a small to moderate negative bias, is positioned at some distance from the target. In the simplest case, a nonreactive gas like argon flows into the chamber and is ionized to create a plasma, a mix of positive ions (atoms missing an electron or two) and free electrons.
The electric field created by the negative bias of the target disk accelerates positive ions in the plasma, which strike the disk with enough force to liberate (sputter) metal atoms. Most of the acceleration happens in a thin boundary layer called the sheath, a layer where the electric field is concentrated between the target surface and the plasma. The sheath field not only accelerates ions from th
|Contact: Paul Preuss|
DOE/Lawrence Berkeley National Laboratory