Glyoxylic Acid as Reducing Agent for Electroless Copper Deposition on Cobalt Liner

Cobalt has recently attracted interest as interconnect liner, because it can serve as nucleation layer for Cu deposition [1,2]. Nevertheless, in order to apply it in combination with a wet Cu deposition process, the dissolution of Co has to be taken into account since less-compatible with acidic and more negative redox potential than Cu.

  Using an electroless deposition (ELD) of Cu chemistry with alkaline pH has the advantage of less-dissolution of Co and, therefore, a thinner Co liner can be used. This would be beneficial for small feature filling. Nevertheless, formaldehyde, which is conventional reducing agent of Cu, did not show catalytic effect on Co surface [3]. In this case, the nucleation mechanism on Co is a replacement reaction, during which some cobalt is consumed during ELD-Cu reaction that is in competition with ELD-Cu deposition reaction in the initial stage of the Cu film growth.

  In this paper, we investigate minimization of Co consumption by analyzing each component of glyoxylic acid based ELD Cu.

     Figure 1 shows polarization analysis of glyoxylic acid and formaldehyde on Co surface. In case of formaldehyde, there is no anodic oxidation. On the other hand, glyoxylic acid showed anodic oxidation on Co. It indicates that glyoxylic acid shows catalysis on Co.

    Figure 2 shows linear sweep voltammetry of Cu complex solutions. When higher concentration of complexing than Cu ion was added, oxidation occured without reduction of Cu.  In general, enough amount of complexing agent is added in the ELD solution to avoid generation of Cu hydroxide ions. However, the complexing agent also makes Co-complex. On the other hand, when the same amount of complexing agent was added (i.e. no ‘free’ complexing agent in the bath), there was no Co dissolution, which was deduced from electrochemical measurements.

  Figure 3 shows the cross-sectional TEM images and TEM-EDX images of a 3 ×  50 µm TSV after ELD-Cu on CVD-Co (45 nm)/ALD-TiN (12 nm). The Co thickness was 45 nm on top and 10 nm at the bottom. The ELD solution contained the identical concentrations of complexing agent and Cu ion.  Glyoxylic acid was used as reducing agent. A continuous Cu seed was obtained in the TSV along the sidewall. Neither delamination nor voids were seen after ELD. Furthermore, the Co thickness in entire TSV did not changed. It is due to the optimization of complexing agent and reducing agent inside Cu bath.

The filling of the TSVs was performed by electrodeposition on the ELD-Cu seed. The continuous ELD-Cu seed layer promoted a bottom-up filling of the TSV afterwards.

References

[1] S. Armini et al., J. Electrochem. Soc., 158 (2) (2011) H160

[2] T. Nogami, et al., Proc. of IEEE IITC (2013) 11-1

[3] I. Ohno, et al., J. Electrochem. Soc.,132 (10) 2323 (1985)