Anomalous behaviors of plasma parameters in unbalanced direct-current magnetron discharge

Abstract

The pressure dependences of electron distribution functions and plasma parameters are investigated in an unbalanced direct-current magnetron sputtering system. The anomalous behaviors of electron density and electron temperature, and the transition of the electron energy distribution function, which are obtained from the cylindrical probe measurement, from a bi-Maxwellian distribution at low pressures to a Druyvesteyn distribution at relative high pressures with changing pressure can be observed. The planar probe measurement shows that the low-energy electron group in the electron energy distribution function consists of electrons, which are scattered back from the sheath wall formed on the substrate and the population decreases with the decreasing plasma potential as the pressure increases. It then disappears at high pressures above 20 mTorr when the plasma potential drops to almost ground level, resulting in a Druyvesteyn electron energy distribution. These observed results are explained by considering the mechanism of the electron transport in the downstream region and the effect of the sheath boundary, which is determined by the plasma potential with respect to the grounded substrate, on the electron energy distribution, especially the depletion of the low-energy part in the electron energy distribution function.