Highly Adhesive Displacement Plated Cu Seed on Cowb Barrier for All-Wet TSV Fill Process

Fabrication  of  TSV is one of key technology for 3D-ICs.  For realizing  conformal barrier layers against Cu diffusion with a low temperature process is important to achieve highly reliable TSVs. Electroless plated metals such as Co-alloy and Ni-alloy are  considered to have a high potential as a barrier layer.

We  succeeded in conformal electroless plating of CoWB on SiO2,  using DMAB  as a reducing agent after forming self-assembled monolayer (SAM) of silane coupling   agent (APTES) on  SiO2  [1-2],    assisted with Pd nanoparticle catalyst.   The adhesion strength of CoWB decreased with an increase in film thickness [1], which might be due to an increase in film  intrinsic stress.  Electroless Cu plated film using reducing agent on the CoWB layer showed a very poor adhesion property. In this study, we studied to form electroless Cu seed layer with a high adhesion strength by displacement reaction without reducing agent, and furthermore examined the barrier and seed formation into a high aspect ratio TSVs.

The electroless CoWB plating bath composed of cobalt sulfate and tungstic acid as a metal salt, citric acid as a complex agent, DMAB as a reduction agent. The Cu displacement plating bath composed of sulfuric acid and copper sulfate, and its pH was 3.5.  The adhesion strength was  evaluated  by the  stud-pull  test. Table 1 shows the adhesion strengths of various films such as CoWB (70nm) only, displacement plated Cu on CoWB, electroless Cu plated with reducing agent (glyoxylic acid) on CoWB. The adhesion  strength  of the 70 nm CoWB film was 20MPa.  That of displacement plated Cu/CoWB was 50 MPa. The adhesion strength of electroless plated Cu with reducing agent was very poor. We succeeded in obtaining a high adhesion strength by Cu displacement plating.

Figure 1 shows the displacement plated Cu seed layer on 80 nm electroless CoWB layer in TSV. The diameter is 3 μm and aspect ratio is 10.  Displacement plated Cu film was formed almost uniformly from top to bottom of TSV. The sidewall and bottom coverages of  Cu/CoWB bilayer in TSV was 70 to 80 % against that of  the flat area. At the bottom of TSV, Cu(40nm)/CoWB(40nm) bilayer was formed succesfully without delamination.

We succeeded in the formation of highly adhesive electroless plated Cu seed layer on CoWB layer.  Further study of complete fill of TSV with ECP Cu is in progress. 

 References

[1] F. Inoue, T. Shimizu, H. Miyake, R. Arima, T. Ito, H.Seki, Y. Shinozaki, T. Yamamoto, and S.Shingubara, Microelectronic Engineering, 106 (2013) 164–167.

[2] F.Inoue, T.Shimizu, T. Yokoyama, H. Miyake, K. Kondo, T. Saito, T. Hayashi, S. Tanaka, T. Terui and S. Shingubara, Electrochimica Acta, 56-17 ,6245-6250, (2011).