Wet cleaning and surface characterization of Si1−xGex virtual substrates after a CMP step

New reactants such as ozone dissolved in ultra-pure water have been widely used the last few years instead of the original Radio Corporation of America (RCA) cleaning (which is a combination of the Standard Cleaning 1 (SC1) and the Standard Cleaning 2 (SC2)). In a first part of the study (Microelectron. Eng. 83 (2006) 1986), we had quantified the efficiency of a new cleaning sequence (that calls upon HF and H₂O/O₃ solutions) on polished Si_1−xGe_x virtual substrates (x = 0.2-0.5). We are discussing here the surface morphology and wetability together with the oxide thickness and structure typically obtained after this so-called “DDC-SiGe” wet cleaning. Flat surface morphologies are found after cleaning whatever the Ge content (from 20 to 50%). Typical root mean square roughness is around 0.4 nm. We have used X-ray Photoelectron Spectroscopy to determine the characteristics of the surface termination after this “DDC-SiGe” cleaning. An oxide mainly composed of SiO₂ is formed, with a low fraction of Ge sub-oxide and GeO₂. The distribution of chemical species is not that different from the one obtained after the use of a SC1 cleaning. However, the chemical oxide formed is slightly thicker. Such a HF/O₃ cleaning leads, when used on thick Ge layers grown on Si, to the formation of a really thin Ge sub-oxide. Our oxidation model assumes a competition in O₃ solutions between the oxidation rates of Si and Ge atoms (faster for Si) and the dissolution of the Ge oxide formed in solution. This mechanism, which implies the formation of a slightly porous oxide, is different from the one seeming to occur in SC1-based solutions. Indeed, the addition of surfactant in a SC1 solution modifies the oxidation rate compared to standard SC1 or O₃-based solutions, suggesting a diffusion of reactants towards the interface between the SiGe and the oxide in formation, assisted by the reactions of species within the cleaning solutions.