AVS 50th International Symposium
    Workshop on Sputtering Sunday Sessions
       Session WS-SuM

Invited Paper WS-SuM4
Shallow Implantation as a Mechanism for Target Poisoning in Reactive Sputtering

Sunday, November 2, 2003, 11:00 am, Room Constellation C, Hyatt Regency

Session: Workshop on Sputtering (Morning Session)
Presenter: R. De Gryse, University Ghent, Belgium
Authors: R. De Gryse, University Ghent, Belgium
D. Depla, University Ghent, Belgium
J. Haemers, University Ghent, Belgium
G. Buyle, University Ghent, Belgium
Correspondent: Click to Email
Up to now, reactive sputtering and in particular the target poisoning effect has been described in terms of gettering and chemisorption. It is modelled by a set of linear differential equations@footnote 1@ which predict the non linear poisoning behaviour as a function of the mole fraction of the reactive gas (RG). From this picture it also follows that a decrease in sputter rate as well as a decrease in absolute target voltage (ATV) is expected. The expected decrease in ATV relies on the fact that it is widely accepted that the ion induced secondary electron emission coefficient (ISEE) of compounds is larger as compared to the ISEE of the corresponding metal. However, the experiment shows that several combinations of metal - (R.G.) give rise to an increase in ATV upon poisoning. In systems such as Nb/O@sub 2@@footnote 2@; Sn/O@sub 2@@footnote 2@;Si/N@sub 2@; etc. the ATV is reported to increase when poisoning occurs. Recently it has been suggested that the poisoning instability is not always due to the chemisorption effect but can also be ascribed to the combined effect of target etching, preferential sputtering of metal vis a vis compound and shallow implantation of reaction gas into the target near surface region. This D.R.@footnote 3@ model also leads to a poisoning instability without any need of wall gettering and also two levels in sputtering speed depending on the fraction of (RG) i.e. a high sputtering speed for low mole fractions and a low sputtering speed for higher mole fractions. This behaviour has been simulated by means of the TRIDYN code.@footnote 4@. The transition between metallic and compound or poisoned regime can be predicted as a function of an experimental parameter which contains quantities such as pumping speed, wall area, discharge current, sputter efficiency etc. In this model it is assumed, and shown experimentally, that non bonded RG can be present in a shallow surface layer. It is also shown that this non bonded RG is a component which can give rise to an increase in ATV upon poisoning. Also chemisorption, if present, can give rise to an increase in ATV. Reality will probably be best modelled by a combination of the gettering model and the D.R. model.@footnote 5@ In metallic mode, the magnetron discharge can be described quite accurately and several tools are at our disposal varying from Analytical models over Fluid models, Boltzmann models, Monte Carlo models/Particle in cell (MC-PIC) models to Hybrid models (MC-Fluid). All these models are in some or other way a trade off between speed and accuracy. However in pure metallic sputtering the accuracy and speed of the analytical approach is surprising.@footnote 6@ Modelling of the magnetron discharge in poisoned or compound mode requires the correct picture of the poisoning mechanism. This will allow to predict over the full range of reactive gas flows quantities such as number densities, energy and directivity of the different material fluxes towards the substrate. This in turn will give an estimate of the expected deposition speeds, coating homogeneity, target consumption and will eventually predict the growth mechanism of the coating. The ultimate goal is to develop for every particular application a stable running magnetron. @FootnoteText@ @footnote 1@S. Berg et al., J. Vac. Sc. Technol. A5(2), 1987, p. 202. @footnote 2@"Sputter Deposition" by W. Westwood ISBN 0-7354-0105-5. @footnote 3@D. Depla et al., Vacuum 66 (2002) p. 9. @footnote 4@Z.Y. Chen et al., Nucl. Instr. Meth. In Physd. Res. B: in press. @footnote 5@D. Depla, R. De Gryse, submitted for publication in Surface and Coatings Technology. @footnote 6@G. Buyle et al., J. Vac. Sci. Technol., A21(4), July/August 2003.