Fourier transform infrared spectroscopic observation of the surface reactions during electroless copper deposition in formaldehyde based electrolyte

The reaction mechanism of copper-electroless deposition (Cu-ELD) under various electrolyte conditions was investigated by in-situ and ex-situ Fourier transform infrared spectroscopy (FTIR) analyses using an attenuated total reflection-liquid cell. In the analysis of the surface reaction, we found that gem-diol (CH₂(OH)O), which is the hydrated form of formaldehyde, was oxidized to HCOO^- by the Pd catalyst, thereby generating H₂O at room temperature. In this case, copper nuclei were generated around the Pd catalyst and developed to form a dense Cu film, which may have resulted in "bottom-up growth" in the via filling process. At 60 °C, however, the spontaneous oxidation of gem-diol was observed in our FTIR analysis, even without a Pd catalyst. Therefore, Cu-ethylenediaminetetraacetate was spontaneously reduced in the electrolyte, and then precipitated onto the Si surface. In this case, the surface morphology of the as-grown copper film was very coarse and it exhibited poor surface adhesion. Furthermore, when the Cu-ELD was performed on Pd/Si at 60 °C, the surface reaction catalyzed by the Pd particles occurred simultaneously with the spontaneous reaction in the electrolyte. Based on these results, we concluded that optimization of the Cu-ELD process requires the suppression of this spontaneous reaction by either modifying the electrolyte or using additives.