Wetting of copper on α-Al2O3 surfaces depending on the orientation and oxygen partial pressure

The influence of oxygen partial pressure and misorientation on the chemistry and bonding at interfaces between Cu and single crystalline alpha-Al2O3 was studied by analytical electron microscopy with nanoscale resolution. The interfaces were prepared by the sessile drop technique, i.e. by melting pure Cu (99.99%) in a furnace onto alpha-Al2O3 plates. We investigated the chemistry and bonding at the interfaces for two orientations of the single crystalline alpha-Al2O3 substrates with two different surface planes, the basal (B-) plane, (0001), alpha-Al2O3B, and the rhombohedral (R-) plane, (10 (1) over bar 2), alpha-Al2O3R. The oxygen partial pressure (p(Ox)) in the furnace was varied between 10(-4) and 10(-15) bar. This was controlled using a solid state electrolyte cell instrument. Spatially resolved (SR) electron energy-loss spectroscopy (EELS) in a dedicated scanning transmission electron microscope (STEM) was applied to characterize the chemistry and the electronic structure at the interfaces. At low oxygen partial pressure (p(Ox) < 3 x 10(-11) bar), all interfaces fractured during transmission electron microscopy (TEM) specimen preparation, independently of the alpha-Al2O3 surfaces (B- or R-plane). At higher oxygen partial pressures (p(Ox) = [1.5 x 10(-10)...7.8 x 10(-4)] bar) the successful TEM specimen preparation indicates stronger bonding across the interfaces. No changes in the electronic structure at the Cu/alpha-Al2O3B interfaces as compared to the bulk were found by spatially resolved EELS while the electronic structure at the Cu/alpha-Al2O3R interfaces changed strongly. These differences are attributed to the reconstruction of the alpha-Al2O3B plane being neutral compared to the relaxation of the alpha-Al2O3R-plane which is charged and can therefore form ionic bonds with the Cu. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.