Gradient filling of copper in porous silicon using a non-contact electrochemical method

In this study, gradient bottom up filling of Cu in porous silicon substrates has been demonstrated using a non-contact electrochemical method. A radially symmetric current that gradually varied across the radius of the sample area was achieved using a modified cell design, which resulted in gradient filling of the vias. Effect of different deposition parameters such as applied current density, copper sulfate concentration and etching to deposition area ratio has been examined and discussed. Increasing the current density from 10 to 15 mA/cm ² resulted in bottom up deposition with less sharp gradients. Further, the study on the effect of copper sulfate concentration highlighted the importance of mass transfer in this process, as either bottom-up deposition or gradient filling could not be achieved at lower CuSO ₄ concentration (0.1 and 0.25 M). Interestingly, the filling gradient of deposited Cu was obtained with etching to deposition area ratio of 1.6 and 2.7, while a more uniform deposition was observed when the ratio was increased to 3.8. This suggested that the gradient filling may only be accomplished within a certain range of the etching to deposition area ratios.