Cu Displacement Plating on Electroless Plated CoWB Layer on SiO2 and Its Adhesion Property

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 CoW-alloy and NiW-alloy are widely studied as a  capping layer on Cu, and they are also  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  on  SiO2  [1-3],    assisted with Pd nanoparticle catalyst.   Electroless  Cu  plating  on CoWB using  reducing  agent  showed insufficient adhesion  strength.  The adhesion strength of CoWB decreased with an increase in film thickness [1], which might be due to an  increase of  film  stress with increasing film  stress. In this study, we tried to form electroless Cu seed layer by displacement plating without reducing agent, and investigated improvement of adhesion property.

 The  Cu plating bath composed of sulfuric acid and copper sulfate , and its pH were varied between 1.5 to 3.5.  Fig.1  shows  cross-sectional SEM view after the Cu displacement  plating  in the case of  pH 2.5  on CoWB film. Initially CoWB thickness was 70 nm (Fig.1-a), and  60 nm Cu layer was formed on the CoWB, while  the  thickness was decreased to 25 nm due to replacement reaction.  Oxidation of Co atoms and reduction of Cu ions  occurred concomitantly by displacement plating.

 The  adhesion  strength were  evaluated  by a  stud-pull  test.   Table 1  shows  the  adhesion  strengths of various films such as CoWB (70nm), displacement plated Cu/CoWB, electroless Cu plated with reducing agent 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 reducing agent was very poor. We were able to  get  enough  adhesion  strength with displacement plating.

 We are going to examine Cu displacement plating  of various pH conditions.  Furthermore, application of this process to TSV structure 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).

[3]W. J. Dressick, C. D. Dukey, J. H. Georger. Jr, G. S. Carabrese, and J. M. Calvert, J Electrochem. Soc., 141 (1) 210 (1994)