In-situ Ion Beam Analysis: Magnetron-Target

Reactive Magnetron Sputtering is a popular and well established technique to fabricate thin film coatings like oxides, nitrides and carbides. Within the plasma assisted deposition process Argon ions originating from a gas discharge are accelerated towards a metallic target. This ion impact causes series of collisions between the atoms of the target and leading to the ejection of these atoms. The ejection process is known as sputtering. In reactive deposition mode, small amounts of reactive gases like Oxygen or Nitrogen may add to the process, which react with the sputtered material and form the desired compound layer on the substrate.

Since this chemical reaction takes place at all surfaces inside the sputtering system, a compound is also formed at the surface of the sputter target. This is the so called “target poisoning”, which limits the deposition process since it results in a significantly reduced sputter yield and, thereby, reduced deposition rate.  

Process parameters like sputter yield, reactive gas partial pressure and hence, the stoichiometry of the compound thin film at the substrate may deviate extremely and show a non-linear, hysteresis dependence on the reactive gas flow into the deposition system. (see Fig. 1)

Fig.1: Target poisoning as well as reactive gas partial pressure are sensitively connected to the reactive gas flow into the system. Both process factors increase rapidly, starting at a critical reactive gas flow value of around 0.6 sccm. The hysteresis behaviour causes instabilities of the deposition process.

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Fig. 2 shows the experimental setup, which allows to investigate the complex, physical developments at the sputter target in-situ and during magnetron operation. The target surface can be characterized locally resolved by means of ion beam analysis (NRA and RBS) and by mass spectrometry of sputtered particles. Furthermore the mass spectrometer is used to characterize plasma properties.

Fig. 2: Experimental setup for the in-situ analysis of a magnetron sputter target. Height and angle of the magnetron, which is placed in the centre of the vacuum chamber, can be changed, enabling various investigations of the target by means of ion beam analysis and mass spectrometry.

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Previous experiments brought basic findings concerning the plasma – target interaction. A compound layer of around 3nm thickness was found at the target surface, which could only be attributed to reactive gas ion implantation and recoil implantation of adsorbed reactive gas [1]. The measured lateral variation of layer thickness gave conclusions about the dominating processes during target poisoning [2]. Existing models, describing the reactive deposition process, could be improved and advanced by inserting these results and by combining them with global plasma modelling [3,4].

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References:

[1]  Mechanisms of target poisoning during magnetron sputtering as investigated by real-time in situ analysis and collisional computer simulation  (Güttler et al. Appl. Phys. Lett. 85, (2004))

[2]  Lateral variation of target poisoning during reactive magnetron sputtering (Güttler et al. Appl. Phys. Lett.90, 263502 (2007))

[3]  Laterally resolved Energy distributions of sputtered and plasma species from reactive magnetron sputtering of Titanium (D.Güttler and W. Möller, submitted (2007))

[4]  Modelling of plasma-target interaction during reactive magnetron sputtering of TiN (W. Möller and D. Güttler J. Appl. Phys., submitted (2007)