Gravitational Effects on the Initial Stage of Cu Electrodeposition

The electrochemical processing has been favorably reviewed in the microelectronics industry. It is essential to control the nucleation and crystal growth in the initial stages of electrodeposition process. For this purpose many effects of additive have been examined.  However, few quantitative examinations has been reported on the coupling phenomena of ionic mass transfer rate and microstructural/morphological variations through nucleation and crystal growth.

This work discusses various gravitational levels as new parameters into the nonequilibrium electrochemical processing in order to create or tailor the unique physical properties in micrometer space field. It is fruitful in terms of not only physical chemistry but also industrial applications to comprehensively understand the coupling phenomena. In fact, it has been confirmed that the gravitational levels as well as the magnetic field introduce the intensive effects to the coupling behavior different from that under 1 G normal environment.

Here is a report to analyze the effect of gravitational level on the initial stages of Cu electrodeposition on TaN or TiN substrate installed in a cavity-type electrolytic cell. In case of Cu on TaN substrate, cathode, anode and reference electrode are TaN substrate, copper sheet and copper wire, respectively. Electrolyte solution is 0.05 M CuSO4 – 0.05 M H2SO4. Two kinds of electrolytic cell configuration are used; one is a cathode facing downward over anode horizontally (C/A), and another is an anode over a cathode (A/C). A centrifugal field is applied to introduce a high gravitational field.

Time dependence of nucleus number density shows that the nucleation of Cu on TaN is “progressive” nucleation. Under 1 G, saturated nucleus number density (N0) increases and mean diameter (dmean) becomes smaller as the electrode potential decreases. At any potential, N0 in A/C configuration is higher and dmean is smaller than C/A. The current transient shows the difference caused by two cell configurations and macroscopic natural convection due to the concentration difference should be inevitably induced with time progressed in a cavity in the A/C configuration. The image analysis indicates the nucleus distribution is considerably deviate from the Poisson distribution as long as the nearest-neighbor distance concerns. The existence of “exclusion zone” around the growing nuclei in nucleation phenomena has been found. Under high G level field, N0 becomes slightly increased and is saturated as G level increases. The effect of G level on the current transient is quite large. It is almost saturated under G level over 100 G. It is moreover found that the nucleation is saturated under high G level earlier than under low G level. The effect of ionic mass transfer rate due to the concentration difference becomes significant to enhance the nucleation rate knN0.