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A scanning-capacitance-microscope (SCM) mode was implemented by using an atomic force microscope (AFM) operated in intermittent contact and by measuring the tip-to-sample capacitance change at the tip vibration frequency. The intermittent-contact-mode SCM was able to image and determine the overlay separation of metal lines buried under a 1-μm-thick, planarized dielectric layer. Modeling of the intermittent-contact SCM signal across buried metal lines was consistent with the experimental results. This hybrid intermittent-contact AFM and SCM has the potential to measure the lithographic overlay between metal lines located at consecutive levels beneath dielectric layers in an integrated circuit.

The thermodynamic equilibria established in fused silica oxidation tubes operated at 1000°C are analyzed. Transparent fused silica tubes used for thermal oxidation of silicon contain about 10 ppm sodium impurity. At oxidation temperatures sodium diffuses in fused silica, evaporates into the oxidation ambient, and reacts with residual water contaminating the oxidation atmosphere. During the oxidation cycle enough sodium is incorporated into the growing oxide film to be detected later by capacitance measurements in metal oxide semiconductor (MOS) structures. Reactions taking place during currently used in situ furnace cleaning procedures are analyzed. Calculations indicate that the amount of sodium removed from the fused silica tube wall through chlorine or hydrogen chloride cleaning is substantial. The reaction rate is regulated by diffusion of sodium in the fused silica. The use of iodine and hydrogen iodide as cleaning agents is discussed.