When the frequencies are less than about 50 kHz, electrons and ions in plasma are mobile and both follow the switching of the anode and cathode. This is equivalent to DC sputtering of both surfaces. At frequencies above about 50 kHz, since ions (heavy) can no longer follow the switching and electrons can neutralize positive charge build up sputter deposition occurs as shown in figure. And it is easy to keep plasma going under these conditions. RF sputter can operate at lower Ar pressures (1-15 mTorr) and the fewer gas collisions lead to more line of sight deposition.

The Magnetron Sputter Deposition

The Magnetron Sputter Deposition can be used with DC or RF. The goal is to increase ionization of Ar. This leads to higher sputter rates at lower Ar pressures (down to 0.5 mTorr) and more line of sight depositions. The probability of electrons striking Ar is increased by increasing electron path length using a crossed electric and magnetic fields. This is achieved by placing magnets (200 Gauss) behind target. The basics electromagnetic interactions are shown in Fig. 3.32. This leads to traps electrons near cathode resulting more ionization near cathode (10 times) and fewer electrons reach substrate (less heating).