Optical Plasma Diagnostics During Reactive Magnetron Sputtering

Abstract

Magnetron plasmas are used to sputter metal in pure Ar gas and in Ar–N2, Ar–O2 gas mixtures for reactive sputtering to synthesize pure metal thin films or their corresponding nitride and oxide compounds. Since the 70s, several plasma diagnostics tools have been developed [1–3]. Amongst them, optical diagnostic methods are playing a very important role to characterize magnetron plasmas. Optical diagnostics are “in situ” experimental methods which allow studying the plasma composition as excited and metastable atoms, ions, molecules and radicals (from dissociated reactive molecules such as oxygen or nitrogen). Sputtered metal atoms and their respective ionic populations are analysed as well to estimate the ionization rate of the metallic vapour. The later is a very important parameter for thin film growth. These diagnostic tools also offer the advantage of being non-intrusive, easy to use, and generally inexpensive. The optical emission spectroscopy (OES) of magnetron plasmas is first described in Sect. 9.2. However, by OES, only the radiative plasma species are detected. These species only represent a small fraction (~10−8) of the total density of the species considered [4]. Therefore, to study the whole plasma population, we will show how to interpret the measured emission intensities by means of kinetic equations involving ground and radiative states. In Sect. 9.3, we show how to measure the absolute atom densities thanks to atomic absorption spectroscopy (AAS) and, in Sect. 9.4, thanks to laser induced fluorescence (LIF). The effect of the amplification of a DC magnetron plasma by means of a secondary inductively coupled plasma produced by a RF coil superimposed to the magnetron cathode is also presented. In the case of the well-known transition from metal to poisoned regime in reactive sputtering of titanium nitride, we demonstrate that there is a correlation between neutral titanium density, as determined by AAS, and the Ti+ signal obtained from glow discharge mass spectroscopy (GDMS).