Dry oxidation mechanisms of copper in trenches

The effect of trace oxygen on the annealing of Cu/Ta(N)/SiO2/Si(001) damascene structures was studied. The dry oxidation of copper was investigated by annealing the wafers at 420 °C for 20 min in N2 ambients with oxygen concentrations ranging from 0 to 2500 ppm in a Rapid Thermal Processing (RTP) system. Electron Backscattered Diffraction (EBSD) mapping (also called 'Orientation Imaging Microscopy' (OIM)) and high resolution Scanning Electron Microscopy (SEM) were used to determine the structure, texture and chemical composition of the annealed copper. For low oxygen contents, the oxidation of the surface in bonding areas is initiated at the grain boundaries and on (111) oriented grains. The oxidation is selective and depends on the segregation of sulphur at the surface of grains with a specific orientation. For higher oxygen concentrations, the oxidation occurs readily and is dependent on the trench geometry, increasing with decreasing line width. In this case, the dimensions, the distribution of impurities in the trenches, and the microstructure of the copper modify the oxidation rate in the small features.