Cross-sectional elastic imaging and mechanical defect detection in low-k dielectrics in integrated circuit interconnect test structures

A novel cross-sectional characterization technique for nanomechanical profiling of low-k integrated circuit interconnect structures has been developed based on ultrasonic force microscopy. So-called CS-UFM has been demonstrated on silicon-based, spin-on dielectrics (SOD) used for gap-fill in 150 nm trenches in a SiO₂ test structure. The SiO₂ trench walls were coated with a thin (∼24 nm) PECVD Si₃N₄ layer. CS-UFM imaging clearly differentiated the SOD, SiO₂ and Si₃N₄ on the basis of elastic modulus. Calibration of this variation was carried out to determine the relative moduli of the three materials. Localized variations in the elastic uniformity of the SOD were also observed in the narrowest trench features and tentatively identified as mechanical defects. These defect regions possessed no topographic signature and are presumed to result from compositional variations due to incomplete SOD curing.