Charge transport in magnetrons

The current conduction processes in a magnetron sputtering diode have been the subject of discussion since the origination of the device. Several techniques have been used to study these conduction processes. Langmuir probe measurements of electron temperature, density, and plasma potential show clear evidence of a positive space-charge sheath. The magnitude of the E×B drift current, determined by measuring the magnetic field induced by the current, is suggestive of Bohm diffusion as the principle means for electron conduction. Sheath thickness measurements, made optically, show a strong functional dependence on the discharge voltage. When the sheath dependences are substituted into Child’s law, the current and voltage are found to follow the earlier, empirical current–voltage relation used with magnetrons: I=kVn. It appears that Bohm diffusion is the appropriate function to describe the impedance of the magnetron plasma, and that conduction across the sheath is space-charge limited. Gas density measurements indicate a significant reduction in the plasma region due to gas heating, which is the primary reason for the increase in voltage with power.