Thick Copper Layer Growth By Dynamic Hydrogen Bubble Template Using Continuous and Pulse Currents

Abstract

Copper is the most widely used materials for microelectronics applications due to their electrical and thermal conduction properties, and in several appplications, thick and porous layers are requested. Different possibilities still exists such as copper-zinc or copper-manganese electrodeposition followed by an anodic dissolution, but the faster and simpler way resides in electroforming deposition made by Dynamic Hydrogen Bubble Template (DHBT) [1]. This process works in unusual potentials and current densities where the competition between hydrogen discharge (commonly avoided) and copper reduction is very high. In this case, this method achieves thick deposits in very short time (few seconds) with high currents allowing a very fast growth of copper around hydrogen bubbles. Pores and ligaments sizes can be controlled by several operating parameters i.e. current densities, temperature, deposit time as well as by chemical additives [2]. With the use of a copper sulphate acid bath at various concentrations, with or without additives (acetic or hydrochloric acids) and by applying current densities between 100 and 400 A.dm-2, it is possible to obtain thick and porous coatings, with pores diameters between 10 and 500 µm and ligaments length between 10 and 200 µm. An example of a SEM image shows the morphology of the deposits obtained in top view and in cross section (Figure 1). Another possibility to control the 3D growth resides in the use of pulsed currents, with an effect on bubbles size. Furthermore, reverse pulses are useful for edge effects limitation. [1] Abdel-Karim, R.; El-Raghy, S. Chap 4 in Advanced materials and their applications - micro to nano scale. One Central Press, United Kingdom, pp 69–91, 2017 [2] Shin, H.-C.; Liu, M. Chem. Mater. 2004, 16, 25, 5460–5464 doi.org/10.1021/cm048887b Figure 1 : SEM images of a) top view and b) cross section of a DHBT copper deposit Figure 1