Anders calls these accelerated, "hot" electrons "the engines of the discharge." A circular permanent magnet beneath the target creates magnetic field lines that confine most of them close to the target, causing plasma to concentrate in a donut shape on the target and creating a ring-like ion-impact or sputter erosion zone, often labeled the "racetrack".
Self-sputtering, as noted, occurs when target atoms that have themselves been ionized return to the target to knock out yet more target atoms. Some of the sputtered atoms remain neutral and may fly straight to the substrate; others are ionized and may return to the target, producing yet more ions and yet more free electrons (secondary electrons). At low to moderate power levels the ion current reaches a preliminary maximum, and then as gas temperature increases and sputtered atoms push ionized gas away from the target the ion current quickly returns to a lower equilibrium.
"To get higher deposition rates and generate more ions, you need to increase the power," says Anders. "But at high power you run the risk of heating the system so much the permanent magnets behind the target demagnetize, or the target starts to melt. So the magnet and cathode assembly have to be water-cooled. And commercial sputter guns the size we use are usually limited to an average power of about a thousand watts, one kilowatt."
"Average power" is an important qualification, says Anders. "If the power is supplied in short pulses, each pulse can exceed the average by up to a hundred times. At that kind of power, all processes become stronger." With the right kind of target material, such as copper, this phenomenon is what makes self-sputtering "far above the runaway threshold" possible.
Once self-sputtering gets started, if enough new atoms get ionized and enough new ions return to the targ
|Contact: Paul Preuss|
DOE/Lawrence Berkeley National Laboratory