Via Superfill with Time Pulse Plating in a Dual Couple Redox System
DC plating, modulating plating by current density variation, has been the historical norm in plating techniques employed for via fill. Time pulse plating has the added advantage in its ability to tune composition, structure and porosity by employing varied combinations of current amplitudes and time pulses.
In a normal via-fill bottom up plating process, local plating rate is proportional to the local cupric ion concentration. The bottom of the via sees lower concentration than the top of the feature leading to pinch off or void formation. We investigated fluid velocity dependent convective transport inside the via. Flow velocities for both large and small vias were found to be very low for wide range of inlet flow velocities. Peclet numbers of < 10-4indicate an overwhelming dominance of diffusion impacting ion transport in the interior of the feature.
When an “off time” step is incorporated into a pulsed plating recipe, it enables the relaxation of the cupric ion gradient inside the via feature by diffusion (unhindered by consumption along the vertical sidewall) while the presence of the Fe2+/ Fe3+redox couple enables preferential etching of the top of the via feature versus the bottom. This exaggerates the bottom-up fill effect and can be tuned to achieve super fill of the vias leading to a robust void free filling.