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This paper deals with an innovative via-last TSV polymer filling process dedicated to straight (non-slopped), high aspect ratio (2:1 to 5:1) TSV, partially filled with copper. New polymers were investigated for this purpose. Firstly, the need for a polymer filling process is pointed out. Potential reliability issues added by this new process are developed through finite element modelling. Secondly, the filling process and the optimization phase in order to obtain a void free polymer inside the TSV are presented. A complete integration of polymer filling on electrical test vehicle is then performed. Finally, the impact of polymer filling on the TSV electrical performances is presented. Electrical tests are performed on chains of 10 and 100 TSV in order to check critical parameters such as electrical resistance. A reliability study of TSV polymer filling through JEDEC standard harsh temperature cycling is presented. First results of a comparison of the stack behaviour between a standard RDL polymer insulation and our innovative TSV polymer filling process is performed by means of morphological and electrical characterizations of the TSV before and after cycling.