The first generation of through silicon via (TSV) designs for interposer and 3D die stacking has concentrated on TSV features with aspect ratio (AR) on the order of ten. Typical via sizes are 10 X 100 um for interposer and 5 X 50 um for 3D applications. Ionized physical vapor deposition (IPVD) has been successful in depositing barrier and seed layers in these AR=10 vias that allow efficient “bottom-up” filling by electrochemical deposition (ECD) using available chemistries. While these applications are currently moving to pilot lines and low scale production, research and development has already begun on the next generation of TSV structures for interposer and die stacking. These may be based on via middle or via last designs. They are expected to increase in aspect ratio for denser TSV arrays while maintaining similar wafer thickness. Structures with AR in the range of 15–20 are being designed and produced. For interposer, a typical structure might be 8 X 120 um and 2 X 40 um for 3D stacks. These structures will challenge all TSV formation processes, including etch, dielectric liner deposition, barrier-seed deposition and TSV fill.
This paper will focus on barrier-seed and TSV fill processes. IPVD barrier-seed deposition will be difficult for AR~15–20 and will require much longer deposition times for complete via coverage. Long IPVD times will produce thick overburden and pinch off the opening at the top of vias. Even if successful, IPVD may not be viable economically. More conformal deposition processes, such as atomic layer deposition (ALD), chemical vapor deposition (CVD) and wet processes, like electro-less plating and conformal ECD, may be better alternatives to IPVD. A conformal process only needs to deposit the minimum required seed thickness in the via for successful ECD filling with the overburden being nearly the same thickness. The development of a successful conformal barrier-seed process may even challenge IPVD for AR=10. This paper presents ECD TSV fill results using several conformal barrier-seed processes, demonstrating the feasibility of this approach for structures with AR = 10–20.