Development of techniques for fracture characterization of thin films for microelectronics

Resumen

The development of new materials and processes, along with the continuous shrinkage of the dimensions in the Integrated Circuit (IC) industry, leads to the need of new methods for characterizing the physical behavior of materials at such scales. The goal of this work is to develop techniques to assess the fracture behavior of thin and very thin films used at the interconnect level. For this purpose, brittle polymeric and ceramic ILDs, with thicknesses from 100 to 750 nm, and ductile Cu films, with thicknesses from 0.5 to 4.5 m, have been studied. This work introduces a new technique for the fracture characterization of very thin films. The technique is called Double Tip Indentation (DTI) and is based on the top indentation of a brittle film with an especially designed tip to provoke controlled and shallow cracking on its surface. DTI experiments are covered in full, from the tip design and machining to the interpretation of the records, and FEM models are developed to extract intrinsic fracture properties. The technique is proven useful to calculate the fracture stress of very thin brittle films. Along with DTI, two other techniques have been explored: microbeam testing and membrane testing. Microbeams are machined using a Focused Ion Beam (FIB) and tested in a nanoindenter. Different beam geometries, simple and composed, are explored and the best suited ones for the different purposes are chosen. The use of notched beams is presented as an alternative for the calculation of fracture energy of ductile films. Beams without notches are used to study fracture of brittle materials and to extract information on the constitutive behavior of all kinds of films through FE modeling. Membranes are prepared by wet etching and tested in a nanoindenter. Brittle materials are taken to fracture and ductile films are tested up to the maximum displacement allowed by the equipment to explore the plastic behavior of the materials. Again, FE models to extract intrinsic information on the materials are presented, allowing for the determination of residual stresses, fracture stress of brittle films and plastic behavior of the ductile ones. A sensitivity analysis of the fracture stress to different inputs and geometric parameters demonstrates the robustness of the technique. Fracture stresses obtained for brittle ILDs with the proposed techniques show the same trends with materials and thickness, and differences in the calculated values are related to the volume tested in each case. For each material, no significant differences in the fracture stress are observed, except for 100 nm films, which show a lower strength. The expected size effect is observed in Cu films: flow stress increases and toughness decreases as the thickness is reduced. For the particular case of tests involving small volumes (beams) the influence of crystallography has been observed and quantified.