ELECTRON-ENERGY DISTRIBUTION FUNCTION IN DC MAGNETRON AXIALLY-SYMMETRICAL DISCHARGES - EVIDENCE OF SPATIAL ANISOTROPY

Experimental measurements of the second derivative d2i/dv2 of the probe current i versus probe voltage v were performed in an axially symmetric magnetron sputtering gas discharge plasma in order to determine the electron energy distribution function. The measurements were performed using a cylindrical Langmuir probe oriented in two directions, along and across the discharge axis. The experiments were carried out at a constant argon gas pressure of 3 mTorr and at a constant discharge current of 1 A to a molybdenum cathode. Three different magnetic field configurations in the discharge region were investigated and these were achieved by superimposing an external magnetic field on the original magnetron magnetic field. By varying the magnetic field distribution and, in particular, the respective axial component of the magnetic induction B(z) at the probe position, the second derivative d2i/dv2 was significantly altered. Certain differences in d2i/dv2 Were also established with the probe oriented in radial and axial directions. A spherical spatial plasma isotropy of the electron energy distribution function was thus found for the opposed superimposed magnetic field (B(z) = -31 G) while for both the original (B(z) = 36 G) and the supporting (B(z) = 103 G) magnetic field configurations, a plasma anisotropy was established. The degree of anisotropy was quantified by comparison of the respective average electron energies [E] evaluated for each of the three magnetic field configurations. It was found that assumption of spherical isotropy during determination of the electron energy distribution function leads to overestimation of [E] by up to 10% under the experimental conditions used.