Electrochemical Plating System Development of Nanotwinned Cu for Multiple WLP Features

Abstract

We develop a series of electrochemical plating (ECP) baths and processes, generating nano-twinned Cu (nt-Cu) in wafer-level packaging (WLP) applications to fulfill the expanding market demand for copper-to-copper direct bonding and hybrid-bonding. This newly developed system shows a high density of nt-Cu (close to 100%), controllable grain size (0.10 to 1.0 micron), vertical columnar structure (through the full height), and minimal transition layer between Cu seed and nt-Cu initial position (dozens of nanometers). These systems have expansive operating windows which produce high-density nt-Cu films: from the inorganic electrolyte concentrations, including copper, acid and chloride, to the ECP current density, ECP current waveform, agitation speed and additive concentrations. The new ECP system consists of one, two or three additives, depending on the applications, and we demonstrated that this sytem could fill multiple WLP features with a high density of nt-Cu and a flat surface. Unlike the features without recess, there exist more challenges for the components with PI-recess. The balance between conformal and bottom-up ECP must be well-controlled to achieve a flat surface without affecting the nt-Cu formation. To do so, we used two or three additives in the system, including the one that promotes bottom-up fill. Although having an additional additive in the electrolyte weakens the formation of nt-Cu to a certain level, we achieved the balance successfully by adjusting electrolyte compositions and ECP parameters for a series of WLP features. The working window of ECP parameters for generating nt-Cu structure is relatively wide. To achieve a uniform and flat surface of a high density of nt-Cu for specific WLP features, the additive concentrations and plating current waveform should be well-controlled. They are more critical than other parameters for controlling the ECP nt-Cu performance. We evaluated numerous waveforms, such as constant current, multistep current, ramping current ECP and other waveforms to generate nt-Cu films. At the current stage, both within die (WID) and within feature (WIF) of ECP nt-Cu for WLP features tested can be as low as 5.00 % with high density, columnar orientation, and less transition layer nt-Cu. Contrary to some published literature, our ECP process can produce nt-Cu films with Cu(111) orientation irrespective of the direction of the base substrate. Our ECP nt-Cu process does not appear to exhibit such substrate-dependent preference. We can generate high-density nt-Cu films using Cu (111) dominating seed layer and non-Cu (111) dominating substrates and even using stainless steel substrates. This paper will attempt to discuss additive functions, the mechanism of the balance between conformal and bottom-up ECP process, agitation speed, plating current waveform, multi-bath plating, electrolyte compositions, and substrate influence.