M. A. Korhonen

The microstructure of Sn in near-eutectic Sn–Ag–Cu alloy solder joints and its role in thermomechanical fatigue

During the solidification of solder joints composed of near-eutectic Sn-Ag-Cu alloys, the Sn phase grows rapidly with a dendritic growth morphology, characterized by copious branching. Notwithstanding the complicated Sn growth topology, the Sn phase demonstrates single crystallographic orientations over large regions. Typical solder ball grid array joints, 900 μm in diameter, are composed of 1 to perhaps 12 different Sn crystallographic domains (Sn grains). When such solder joints are submitted to cyclic thermomechanical strains, the solder joint fatigue process is characterized by the recrystallization of the Sn phase in the higher deformation regions with the production of a much smaller grain size. Grain boundary sliding and diffusion in these recrystallized regions then leads to extensive grain boundary damage and results in fatigue crack initiation and growth along the recrystallized Sn grain boundaries.

The effect of current density and stripe length on resistance saturation during electromigration testing

Resistance saturation as a function of current density and stripe length has been investigated for a two‐level structure with Ti/TiN/AlCu/Ti/TiN stripes and interlevel W stud vias. A simple model relating the resistance change at saturation to the current density and stripe length is formulated for structures with short stripe lengths and blocking boundaries at both ends. Experimental results for stripe lengths of 25, 50, or 100 μm are in good agreement with the model predictions.

3-D finite element simulator for migration effects due to various driving forces in interconnect lines

Migration of matter due to various driving forces determines the lifetime of interconnect systems in microelectronics. Recalling the fundamental mechanism behind these failure modes, which is solid state diffusion, a system of models has been set up that yielded in a comprehensive 3-D simulator for migration of matter in interconnect lines, utilizing a commercial FEM package without modifying its code. The simulator has been validated by checking against experimentally backed analytic solutions and has been shown to be an appropriate tool for detailed studies on failure mechanisms in advanced interconnect systems.

Three-Dimensional Finite Element Simulation of Electro and Stress Migration Effects in Interconnect Lines

A tool for 3-D modeling of EM and SM in interconnect lines has been developed based on a commercial finite element code. After detailing the approach, we focus on the verification of the simulator by comparing the results of 1-D analytic and FEM simulations, and then we apply the simulator to interconnect line segments with a specified grain structure.

Cluster interactions and stress evolution during electromigration in confined metal interconnects

In narrow metal interconnects used in advanced integrated circuits, electromigration flux divergences occur at the intersection between polycrystalline cluster segments (where grain boundaries offer a fast diffusion path), and bamboo segments (where there are no grain boundaries along the line length). In confined, passivated metal interconnects, these flux divergences are linked to the evolution of significant mechanical stresses in the metal. A quasisteady state stress distribution builds up quickly in the cluster segments and remains unchanged until the stress profiles between cluster segments begin to overlap, and the clusters begin to ‘‘interact.’’ A significant increase in stress above the quasisteady state can result from cluster interactions, increasing the potential for electromigration and stress‐induced damage. If the cluster separation is small, this stress increase can occur on a time scale which is short compared to the stress evolution of the interconnect line as a whole.

Stress and current induced voiding in passivated metal lines

Thermal stress and current induced damage processes may well determine the ultimate limit for the achievable line widths in microelectronic circuits. According to our current understanding, thermal stresses cause void nucleation during cool-down from high temperature process steps. The voids may continue to grow during subsequent storage. The application of an electrical current may lead to further void growth, as well as migration and coalescence of voids. A recently developed model provides the framework for improved reliability assessments, including the effects of statistics, as well as for the development of remedies.

Stress‐induced voiding and electromigration

Thermal stress induced voiding provides an effective nucleation mechanism for the electromigration induced damage and failure of narrow, passivated metal lines. Small voids are trapped at grain and phase boundaries, where they grow under a current. Growth rates are determined by the local flux divergencies and the current induced stress distribution. After reaching a critical size, some voids begin to migrate and coalesce, eventually leading to line failure. A model is outlined, which is capable of explaining a large number of experimental observations and offers a basis for the prediction of failure statistics.

Under bump metallizations for lead free solders

Several under bump metallization (UBM) schemes using CuNi alloys as the solderable layer were investigated. The nickel slows down the dissolution of the UBM into the solder and the formation of intermetallics during reflow. Ni containing UBMs were fabricated and reflowed with eutectic SnAg solder balls. The solder/UBM interfaces were analysed with SEM to find out how the Ni concentration affects the reaction, and how much Ni is needed to obtain a sufficiently slow reaction rate. Reflows were also made on top of bulk substrates to study the reaction when there is an unlimited amount of CuNi available. To determine the rate of dissolution of the substrate material into solder, CuNi foils of different concentrations were immersed in pure Sn and eutectic PbSn solder baths for soldering times ranging from 30 seconds to 30 minutes. Since nickel metallizations often have high stresses, stress in the UBMs was measured by the wafer curvature method. Stress vs. Ni content plots show that while stresses increase somewhat with the Ni content, the adhesion layer under the CuNi layer has a much larger effect on the stress.

The Effect of Cluster Interactions on Electromigration Induced Stress Evolution in Confined Metal Lines

In near-bamboo interconnect lines used in advanced integrated circuits, electromigration flux divergences occur at the intersection between polycrystalline cluster segments (where grain boundaries offer a fast diffusion path), and bamboo segments (where there are no grain boundaries along the line length). For confined, passivated metal interconnects, these flux divergences are linked to the evolution of significant mechanical stresses in the metal. A quasisteady state stress distribution builds up relatively quickly in the cluster segments and remains unchanged until there is significant diffusion into the bamboo segments. The stress profile of a given cluster then becomes dependent on neighboring clusters as well as the diffusivity and flux in the separating bamboo segments. Previous analyses of electromigration failure in interconnect lines have focused on the distribution of cluster lengths and the stress build up in isolated cluster segments. In this paper, we show that the bamboo length distribution can strongly affect the interaction between clusters and the evolution of stresses in a near-bamboo interconnect during electromigration. We present simulation results, using a ratio of cluster to bamboo diffusivity Dc/Db=100, which show greater interactions and larger maximum stresses in cluster segments as the average bamboo segment length decreases and as the bamboo segment length distribution widens.

Effect of CU and SI in Aluminum on Stress Change and on TiAl 3 Formation in Al Alloy/TI Bilayer Films During Annealing

Interconnect metallizations used in advanced integrated circuits typically use an Al-alloy sputterdeposited onto a Ti barrier layer. The Ti and Al react above ∼ 400°C to form TiAl 3 , which affects the stress evolution of the metal stack during thermal cycling. This paper describes results of thin film experiments performed on Ti/Al-alloy bilayer films. Two Al alloys were studied: Al-I%Cu and Al-0.5%Cu-1%Si. The rate of TiAl 3 formation at 430°C was determined for both alloys and used to relate TiAl 3 formation to the stress evolution of the film stacks during thermal cycling. The dominant effect of the TiAl 3 intermetallic formation on stress arises from a change in the stress-temperature behavior of the film stack, due to a change in the yield behavior, effective modulus, and thermal expansion coefficient of the stack. The presence of Si in the Al-alloy markedly reduces both the rate of TiAl 3 formation and the resulting change in composite stress.