"Three quantities determine the self-sputtering threshold," Anders explains. "One is the probability that a sputtered atom gets ionized. Another is the probability that the new ion returns to the target. Finally, there's the actual yield of atoms from self-sputtering. Multiply these together and you get the self-sputtering parameter, which is symbolized by the Greek letter pi" Π "When pi equals unity, you reach a new steady state," provided, that is, "that the power supply can keep up."
Which is why, says Anders, "we use a special power supply, up to 500 kilowatts peak power. If the system wants power, we give it power!"
Using a copper target in their HIPIMS system, Andersson and Anders found that the ion current to the collector increased exponentially as the discharge voltage was increased. Far above the threshold of self-sputtering, the ion current to the substrate greatly exceeded the discharge current a result that came as a surprise to a number of their peers.
"But this really doesn't require any new physics," Anders says. "The ions are generated by the energy invested, not by the current. We provide both high voltage and high current, the product of which is power, so we give the self-sputtering system enough power" energy per time "to generate a large amount of ions. It's perfectly compatible with energy conservation or any other law of physics."
In their experiment Andersson and Anders used no process gas at all, but instead kick-started their system with very short arc pul
|Contact: Paul Preuss|
DOE/Lawrence Berkeley National Laboratory