Transition Layer Thickness Control in Additive-Assist Electroplated Nanotwin Copper

Abstract

Unidirectional (111) nanotwin copper has shown exceptional performance such as thermal stability, mechanical strength, fatigueless behavior, electromigration resistance, electric conductivity, low bonding temperature, and ultra-large grain growth. These unique properties make it a promising solution to several IC applications including fine line RDL in fan-out WLP, copper direct bond in 3D IC vertical interconnect, high conductivity Cu interconnect for VLSI etc. Both electrochemical deposition and physical vapor deposition are able to form such microstructure, and electrochemical deposition is considered the suitable production method in terms of process flexibility, throughput, and overall cost. Direct current was successfully applied to nanotwin copper formation in many reports, but the common feature of these tests is using low or no acid electrolyte with pH around 1. This approach greatly hinders the industrial implementation because of poor deposit uniformity from low electrolyte conductivity. Pulse plating with low duty cycle was also tried, and able to produce nanotwin in high acid electrolyte. However, throughput and hardware cost are the downside of this method. Another approach is DC plating with additives of crystal plane adsorption selectivity, and then electrolyte acid concentration over 100g/L could be used to form nanotwin copper. During the preferred orientation grain development, a micrometer transition layer between non-nanotwin substrate and columnar nanotwin grain is formed. The transition layer thickness is found sensitive to substrate copper grain orientation as well as electrolyte acid concentration, and may dominate the material property when overall deposition thickness reduces in fine line RDL application. In order to reduce the transition layer thickness, two additive modification approaches are demonstrated in this paper. The first one is to increase columnar grain nucleation density by reducing nucleation overpotential, and the second one is to increase lateral grain growth rate by increasing nucleation overpotential. The prior approach results in smaller columnar grain diameter, and the later one results in larger columnar grain diameter. Both approaches are effective in reducing transition layer thickness. Figure Caption: Transition layer boundary represented by green dash line on TiW/Cu substrate. (a) thick, irregular transition layer with nt-Cu additive. (b) thin transition layer with modified nt-Cu additive increasing substrate nucleation sites. Figure 1