Effects of microstructure of electrodeposits (Sn, Cu and Ni) on the reliability of three-dimensional interconnections

Abstract

Three-dimensional (3D) interconnection technology with flip-chip bump and through silicon vias (TSVs) has emerged as a solution to increase wiring density and improve package form factor and performance, allowing chip technology to continue to evolve and meet the demand for future electronic products. However, due to the scale shrinkage of packaging modules, reliability issues have become more and more serious. In the 3D interconnection process, solder bump/ Ni diffusion barrier/Cu interconnects are produced in an electroplating process that has the merits of productivity, accuracy, and cost effectiveness. The mechanical/electrical properties of electrodeposits highly depend on its microstructure, which is determined by the electroplating conditions. To overcome the reliability issues, therefore, it is necessary to investigate a direct correlation between the reliability failures of 3D interconnects and the electroplating conditions. This study reports on the effects of the microstructure of electrodeposits on the reliability of Pb-free Sn-based solders for flip chip and three-dimensional (3D) interconnects. This dissertation is organized into following parts: 1) Effects of microstructure of solder electrodeposits formed with a degradation of electroplating bath on the collapsing failure of Sn-2.3Ag (wt%) solder capped Cu pillar flip-chip bump, 2) Effects of microstructure of Ni diffusion barrier controlled by the organic additives on the intermetallic compound (IMC) formation and the its joint reliability. In the first part, the effect of the degradation of a methanesulfonic acid (MSA) based electroplating bath on the microstructure of a solder bump was investigated. The study examined the degradation mechanism of methanesulfonic acid (MSA) based electroplating bath used for the electrodeposition of Sn-Ag alloy solder bump, and its effects on the collapse failure of a flip-chip solder bump. To examine the degradation behavior of the electroplating bath, a degraded electrolyte was prepared by accelerated aging treatment. In the presence of dissolved oxygen and Ag+ ions in the electrolyte, the chemical oxidation of $Sn^{2+}$ ions to $Sn^{4+}$ and the precipitation of $SnO_2$ nanoparticles with a diameter below 100 nm were promoted by the reduction of $Ag^+$. Under cathodic bias, colloidal $SnO_2$ particles are adsorbed on the surface of the Sn-Ag solder bumps via electrophoresis, and incorporated into the layer by the electrodeposition layer of Sn-Ag. The presence of oxide layer mainly composed of $SnO_2$ on the surface of bumps significantly reduced the friction coefficient of the solder surface by hardening the electrodeposits and deteriorated the solderability of the solder bumps, which leads to collapse failures during the solder reflow. Cyclic Voltammetry stripping analysis of the degraded bath demonstrated that tin dioxide/hydroxide species are heavily formed on the solder surface and deep subsurface. Substantial changes of the electrochemical characteristics provide meaningful information regarding the relationship between the degradation of the electroplating bath and the formation of stannic oxide particles. The second part of this dissertation considers the effects of the microstructure of the nickel electrodeposits on the growth of Sn-Ni intermetallic compound (IMC) and on the electro-migration (EM) reliability. The crystal structure of the Ni diffusion barrier can be controlled by addition of additives in electroplating bath. When both coumarin and cis-2-butene-1,4-diol were contained in the bath, extremely fine grain Ni electrodeposits were deposited by the synergistic effects of the additives. The growth rate of $Ni_3Sn_4$ IMC formed on the interface between the extremely fine grain structured Ni layer and Sn solder is higher than that of the IMC formed on the interface between the large grain structured Ni layer and Sn solder. The EM failure stage corresponds to large void propagation at the $Sn-Ni_3Sn_4$ interface due to volume consumption of Sn for IMC formation. The results of STEM analysis in an early stage of the heat treatment of the solder demonstrated that substantial quantities of Ni atoms were dominantly dissolved into the liquid Sn layer. $Ni_3Sn_4$ IMC was mainly formed on the finely grained Ni layer due to the high dissolution of Ni atoms into the solute Sn during the reflow process. IMC growth was suppressed at the interface between the large and fine grain structured area. This study suggests a robust concept of the vertical structure of Ni diffusion barrier having a multi layered Ni microstructure, which can reinforce an adhesion property as well as diffusion barrier properties