Development of Glyoxylic Acid Based Electroless Copper Deposition on Ruthenium

High aspect ratio through Si via (TSV) has received much attention as a process integration route to reduce stress build up around the TSV, leading to a small keep-out zone where device performance is impacted. High aspect ratio TSV requires alternative barrier and seed process, because physical vapor deposition (PVD) is facing difficulties yielding continuous layer in such structures. Combining the excellent conformality of ELD in high aspect ratio TSV and atomic layer deposition of barrier and liner, is seen as a potential process route [1].  Glyoxylic acid is a non-toxic reducing agent for ELD-Cu on Ru [2]. In previous work, we found that the nucleation mechanism is catalyst reaction [2]. In this paper, the different anodic reaction of formaldehyde and glyoxylic acid on Ru was compared by polarization analysis. Furthermore, the purity was evaluated after adding stabilizer in the ELD bath. The demonstration of deposition in TSV was performed using 3 × 50 µm TSV.  

  Figure 1 shows polarization analysis of glyoxylic acid and formaldehyde on Ru and PVD-Cu as a reference. All the measurements showed anodic oxidation. It indicates that the driving force of anodic oxidation of glyoxylic acid and formaldehyde is catalysis of Ru and Cu. Although the anodic oxidation of glyoxylic acid on Ru was weaker than formaldehyde, glyoxylic acid showed similar potential on Ru (-0.63 V) and Cu surface (-0.68 V) at -0.01 mA/cm². They were -0.63 V on Ru and -0.83 V on Cu for the case of formaldehyde.

  Furthermore, we found that 2,2’-bipyridyl worked as stabilizer, brightener and suppressor in the glyoxylic acid based bath. Figure 2 shows surface morphology, roughness  and purity of ELD Cu using various concentrations of 2,2’- bipyridyl. The roughness and the purity of copper films improved by adding 2,2’-bipyridyl in the bath. The high purity of Cu might help to reduce void generation during the annealing process after filling. Figure 3 shows cross sectional TEM images of TSV after ELD on Ru. The ELD layer was defect free and worked as a seed layer for electrodeposition of copper. The advantage of depositing a conformal seed layer is that it also reduces the total Cu overburden, which is to be removed by CMP.

Reference

[1] F. Inoue, H. Philipsen, A. Radisic, S. Armini, Y.Civale, S. Shingubara, and P. Leunissen Journal of The Electrochemical Society, 159 (7) D437-D441 (2012)

[2] F. Inoue, H. Philipsen, A. Radisic, S. Armini, Y. Civale, P.  Leunissen, M. Kondo, E. Webb, and S. Shingubaraba, Electrochimica Acta 100 (2013) 203– 211