Electron energy distribution function in a dc magnetron sputtering discharge

Abstract

The electron energy distribution function (EEDF) of a dc magnetron sputtering discharge ignited in either pure argon gas or a mixture of Ar and 4.7% methane is measured in the substrate vicinity (7 cm from the cathode) using ac differentiation of a cylindrical Langmuir probe characteristic. The results show the presence of two energy groups at a small discharge current l = 0.1 A for two values of argon gas pressure p = 0.4 and 1.07 Pa. The high-energy electron group is more pronounced at the lower pressure p of the sputtering gas and disappears with increasing l. The small amount of methane gas in the argon does not affect significantly EEDF at the location of measurement, if compared with that obtained in pure argon. Fitting the measured EEDF to the equation f(E) = a·E 0.5 exp (−b·E x) we find values of x in the range 1.43–1.70, which correspondsto a distribution between those of the Maxwellian (x = 1) and Druyvesteyn (x = 2) ones. The mean electron energies 〈 E〉 M calculated from the measured EEDF are in the range of 4.8–7.1 eV, depending on the discharge conditions. For our conditions of operation, the comparison of 〈 E〉 M values with those obtained by means of the standard Langmuir technique [〈E〉 L = 1.5( kT e )] shows a reasonable agreement (〈E〉 L = 4.4–6.0 eV).