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Understanding and Controlling Cu Protrusions in 3D TSV Processing
Keywords: TSV fill, Cu protrusions, Cu electro-plating
Cu-filled through silicon vias (TSVs) are an essential building block of 3D and 2.5D integrated chips. In the TSV- mid flow, which has emerged as an industry mainstream, one or more on-chip wiring levels are formed after the TSV Cu fill is completed, which exposes the fill to peak temperatures up to 400 C. Due to the higher coefficient of thermal expansion (CTE) of Cu than the surrounding silicon and dielectrics, the TSV fill protrudes above the level previously defined by the TSV CMP step. Even after returning to room temperature, the top of the TSV will not occupy the same space due to plastic deformation during the temperature treatment. This phenomenon is known as Cu protrusions, pumping, or popping. Excessive amounts of Cu protrusion present a risk to yields and reliability because the expansion can damage the dielectrics and wiring above or adjacent to the TSVs. In this study, we report on underlying mechanisms as well as process remedies for Cu protrusions. We define a standard inspection and metrology approach to quantify the occurrence of protrusions. The SEMATECH 5 um x 50 um TSV process and learning vehicle are used to conduct process split experiments comparing various plating chemistries, plating recipes, and post-plating anneal conditions. Stresses in the surrounding silicon are studied by micro-Raman, and a distinct signature of acceptable versus unacceptable Cu protrusion behavior is identified. Because Cu material properties such as contamination, grain size, grain orientation, and strain may also influence the protrusion behavior, these factors are studied by micro-SIMS, EBSD, and X-ray diffraction. Our results show that the choice of plating chemistry and an optimized post-plating anneal are the main factors in suppressing Cu protrusions. Our best known method is able to reduce Cu protrusions after typical thermal loads to about 15 nm.
Klaus Hummler, Manager 3D Interconnect Integration
Albany, NY

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