Via Last using Polymer Liners and their Reliability
Authors: Tezcan Deniz Sabuncuoglu , Majeed Bivragh , Civale Yann , Soussan Philippe , Beyne Eric 
Published: October 2015

This paper discusses the use of polymer dielectrics as insulating liner for realizing two types of 3D-WLP through Si vias (TSV's) in Si wafers with a thickness of respectively 50 and 100μm. These TSV technologies are based on thinning first, via last approach where 3D interconnects are implemented on the backside of thinned IC fabricated device wafers. Key aspects of the TSV processes are: i) fabricating the TSV from the backside of the wafer and contacting to BEOL M1, hence it is independent of the BEOL interconnect stack. ii) use of thick polymer as via isolation is a differentiator from most classical approaches using thin CVD oxide or nitride. The result is a significantly reduced capacitance, and hence, improvement of electrical performance. Another advantage of using thick polymer is that it can absorb some of the stress induced by CTE mismatch between the Cu in via and surrounding Si.
This paper presents IMEC's 3D-WLP TSV flavors developed using spin-on dielectric polymers as isolation layer and the reliability of these TSVs' while undergoing thermal cycle tests. For each wafer thickness a 3-mask process sequence is implemented to fabricate the TSVs. Different process flows are used for the different Si thicknesses. For the 100μm thick substrate, a low aspect ratio TSV is used. A Si-via is etched from the wafer backside with a bottom diameter of about 70μm. The top side of the via is chamfered and has a top diameter of about 100μm. As a dielectric liner, a photo-sensitive spin-on polymer is conformally coated on the wafer. At the via bottom, a contact hole with approximately 30μm diameter is realized by photo patterning. Next, conformal Cu plating is used as TSV metallization. For the 50μm thick substrates, a different approach is used enabling a higher aspect ratio TSV. First, a 5μm wide ring trench is etched in the Si. Subsequently, these trenches are filled with a spin-on polymer. After filling the ring trenches, the central Si pillar is etched out of the via (second litho step) and the oxide at the via bottom is removed to provide access to the lowest chip metal level. Next, bottom-up Cu via fill is used to form the TSV metallization. This via has a Cu TSV diameter of 25μm, and an overall diameter with isolation of 35μm [1]. All processes employed in the fabrication of the TSVs are performed at low temperature (&lt;200°C) for maximum post CMOS compatibility
The test vehicle used to develop the TSV technologies include TSV daisy chains of various lengths connecting different number of vias to determine the TSV yield and resistance. For both TSV approaches, yielding electrical measurements are done on fabricated TSV wafers. For 50μm thick silicon, up to 95% electrical yield is achieved and the single TSV resistance is measured to be ~10–15mΩ on bonded stacks through Kelvin structures. Thermal cycling from −40 to +125°C of TSV dies are also done, and after 1000 thermal cycles no electrical failure is observed. For 100μm thick silicon TSV approach, single TSV resistance is measured to be ~4–20mΩ with good yield. Test die are subjected to 1000 thermal cycles and results show limited yield loss.