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

Experimental measurements of the second derivative d2i/dv2 of the prob e current i versus probe voltage v were performed in an axially symmet ric magnetron sputtering gas discharge plasma in order to determine th e electron energy distribution function. The measurements were perform ed using a cylindrical Langmuir probe oriented in two directions, alon g and across the discharge axis. The experiments were carried out at a constant argon gas pressure of 3 mTorr and at a constant discharge cu rrent of 1 A to a molybdenum cathode. Three different magnetic field c onfigurations 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 a nd, in particular, the respective axial component of the magnetic indu ction B(z) at the probe position, the second derivative d2i/dv2 was si gnificantly altered. Certain differences in d2i/dv2 Were also establis hed with the probe oriented in radial and axial directions. A spherica l spatial plasma isotropy of the electron energy distribution function was thus found for the opposed superimposed magnetic field (B(z) = -3 1 G) while for both the original (B(z) = 36 G) and the supporting (B(z ) = 103 G) magnetic field configurations, a plasma anisotropy was esta blished. The degree of anisotropy was quantified by comparison of the respective average electron energies [E] evaluated for each of the thr ee magnetic field configurations. It was found that assumption of sphe rical isotropy during determination of the electron energy distributio n function leads to overestimation of [E] by up to 10% under the exper imental conditions used.