Nanoindentation for quality control of ULK films

The structural integrity of ultra low-k (ULK) dielectrics is highly dependent on their mechanical properties, e.g. fracture toughness, adhesion and elastic modulus. For materials and process development, the ULK interfacial adhesion can be determined applying four-point-bending (FPB) and double-cantilever-beam (DCB) methods in out-of-fab labs. On the other hand, the characterization of interfacial adhesion is also very important for process control and quality assurance in-line/at-line. The ability to detect a defective ULK films at an early processing step could potentially save processing and material cost. Therefore, the development of the current experimental methodology for the purpose of in-line/at-line mechanical properties evaluation is of great interest for semiconductor industry and tool suppliers. In this paper, a wedge indentation method particularly suitable for ULK quality control is described. The indentation experiment can be conducted on an as-deposited ULK film, without further sample preparation. Furthermore, a simple analysis and a straight forward experimental methodology are introduced, allowing a short “time-to-data”. Interfacial delaminations of the ULK films are observed after the wedge indenter has been penetrated into the film at a critical penetration depth. The indentation stress that induced the interfacial delamination (σ_o) was found to be proportional to the indentation volumetric strain. Therefore, assuming that the energy release rate is equivalent to the interfacial adhesion, the interfacial adhesion of the ULK films is given as Γ=(1-ν_f²)tσ_o²/2E_f. The interfacial adhesion values as concluded from this analysis have been validated through a sufficient number of indentation experiments on several ULK materials and film thicknesses. In addition to the wedge indentation experiment, several independent simulation studies and- - four-point-bending experiments have shown consistent interfacial adhesion values, compared to the results from the analysis mentioned above. The wedge indentation method developed in this study has a strong potential for semiconductor industry application, especially as a quality control technique.