Measurement of the stress-strain behavior of freestanding ultra-thin films

The stress-strain behavior of freestanding ultra-thin films (<100 nm) is characterized using residual-stress-driven structures. The proposed technique does not require an external sample loading setup and uses standard fabrication and metrology techniques that are available in the clean-room. A uniaxial load is applied to the freestanding test films using the residual stress of a well-characterized reference material. Highly cross-linked SU-8, a negative photoresist, is proposed as the reference loading material. The test wafer includes structures for in-situ calibration of the properties of the loading material. The geometry of the structures is varied to measure strains as high as 3.7%. The displacements of the freestanding test films are measured using digital image processing and used to measure the stress-strain response. Measurements on 63 nm thick passivated copper film stack show a slightly lower elastic modulus and a much higher yield strength than observed in bulk materials, and are in-line with the expectations from a nanocrystalline film with passivation. The properties of SU-8 and copper measured here are in good agreement with those measured using an alternate technique by the authors. As such, the proposed technique of residual-stress-driven structures is easy-to-replicate and serves as a useful in-line metrology tool that can be seamlessly implemented on coatings used in a semiconductor device production line.