Charge transport in magnetrons

Abstract

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.

Anomalous electron transport in high power impulse magnetron sputtering

“…Classical theory of diffusion and electrical conductivity using collisions to move electrons across the magnetic field, or Bohm diffusion, usually results in j H /j D values of about 16-35 [9,10]. Thus, a low j H /j D value indicates that electron transport across the magnetic field lines is more efficient in the HIPIMS case than can be explained by standard theory.…”

Cross-field ion transport during high power impulse magnetron sputtering

“…In dcMS, measurements of J ϕ / J D give typical values in the range 8 < ω ge τ c < 30 [26,27], which is often referred to as the Bohm regime. Note that for the upper limit reference 26…”

Transition between the discharge regimes of high power impulse magnetron sputtering and conventional direct current magnetron sputtering

“…It has recently been found that the cross-conductivity, governing the transport of charged species, is much higher for HiPIMS [16,17,18,19] than for DCMS [20,21]. It is therefore expected that there is a transition from an anomalously high HiPIMS crossconductivity to a lower DCMS Bohm conductivity as the current approaches the plateau-level in the pulse.…”