The sputter-anneal cleaning process is one of the most common methods of producing clean, defect free, crystalline Si surfaces. However the effects of small amounts of residual Carbon, probably the most insidious surface contaminant, and of Inhomogeneities in the sputter profile have not been investigated in any detail.
Using a very high resolution Kelvin probe coupled with Auger Electron Spectroscopy (AES), we have followed the changes in work function, together with surface contaminants, throughout the cleaning process. We show that very small amounts of surface carbon contamination, at or about the AES detection limit, lead to a significant increase in work function of the “Clean surface”. Further as little as 0.6% Carbon substantially influences the initial stages of oxidation of the Si(111) 7×7 surface by (i) hindering formation of the elementary dipole layer and (ii) enhancing oxygen penetration through the surface layer into deeper layers.
We have also measured work function profiles of a Si(111) 7×7 sample during sputtering. Inhomogeneities in the sputter profile, particularly around the periphery, are clearly evident and produce variations in work function of some 200 meV across the specimen. Repeated sputter-anneal cycles lead to a degree of disorder in these regions and this is reflected in a different oxidation behaviour.
We conclude that work function measurements are both simple to perform and can be utillsed to determine the presence of surface contamination and roughness. As the oxide layer thickness in MOS devices continues to diminish the detection and elimination of residual Carbon contamination will play an ever increasing role in determining device performance. The Kelvin probe can be used at elevated temperatures and pressures and has potential applications in alternative surface cleaning methods including flash annealing and chemical preparations.