Flow cell design for metal deposition at recessed circular electrodes and wafers

Abstract

In this study we have designed and tested a vertical flow channel which can be used to electrodeposit microelectronic devices, where industry standards demands require good uniformity and low roughness of deposited structures. At first, a theoretical analysis was carried out to determine when mass transfer to the electrode was controlled by forced convection. The analysis revealed that a forced convection flow velocity of 0.12 m s −1 , a channel gap (or inter-electrode distance) of 0.1 m , and an entry length of 0.44 m were sufficient to ensure that mass transfer was fully governed by forced convection. Based on the analysis, a design for a rectangular flow channel was proposed. The performance of the flow cell was assessed by electrodepositing copper and gold. The cathodes used in these experiments were either rectangular, placed flush against the channel wall or circular, located in a slight recess. A series of limiting current experiments with CuSO 4/H 2SO 4 electrolyte and Reynolds number ranging between 450 and 4500 was carried out to obtain a Sherwood–Schmidt–Reynolds number correlation. It was found that mass transfer at electrodes that were flush with the channel wall was governed by laminar forced convection. Mass transfer at electrodes placed in a recess was controlled by turbulent forced convection. Copper and gold deposits obtained in the flow cell were found to have thickness variation of <12% and roughness of <1.6%.