Effect of the Crystallinity on the Electromigration Resistance of Electroplated Copper Thin-Film Interconnections

Abstract

Dominant factors of electromigration (EM) resistance of electroplated copper thin-film interconnections were investigated from the viewpoint of temperature and crystallinity of the interconnection. The EM test under the constant current density of 7 mA/cm2 was performed to observe the degradation such as accumulation of copper atoms and voids. Formation of voids and the accumulation occurred along grain boundaries during the EM test, and finally the interconnection was fractured at the not cathode side but at the center part of the interconnection. From the monitoring of temperature of the interconnection by using thermography during the EM test, this abnormal fracture was caused by large Joule heating of itself under high current density. In order to investigate the effect of grain boundaries on the degradation by EM, the crystallinity of grain boundaries in the interconnection was evaluated by using image quality (IQ) value obtained from electron backscatter diffraction (EBSD) analysis. The crystallinity of grain boundaries before the EM test had wide distribution, and the grain boundaries damaged under the EM loading mainly were random grain boundaries with low crystallinity. Thus, high density of Joule heating and high-speed diffusion of copper atoms along low crystallinity grain boundaries accelerated the EM degradation of the interconnection. The change of Joule heating density and activation energy for the EM damage were evaluated by using the interconnection annealed at 400 °C for 3 h. The annealing of the interconnection increased not only average grain size but also crystallinity of grains and grain boundaries drastically. The average IQ value of the interconnection was increased from 4100 to 6200 by the annealing. The improvement of the crystallinity decreased the maximum temperature of the interconnection during the EM test and increased the activation energy from 0.72 eV to 1.07 eV. The estimated lifetime of interconnections is increased about 100 times by these changes. Since the atomic diffusion is accelerated by not only the current density but also temperature and low crystallinity grain boundaries, the lifetime of the interconnections under EM loading is a strong function of their crystallinity. Therefore, it is necessary to evaluate and control the crystallinity of interconnections quantitatively using IQ value to assure their long-term reliability.

Enhancement of Electromigration Reliability of Electroless-Plated Nanoscaled Copper Interconnects by Complete Encapsulation of a 1 nm-Thin Self-Assembled Monolayer

“…The activation energies of Cu (SAM) and other sealed Cu interconnects were included for comparison. [28][29][30][31] the Cu(SAM) and Cu interconnects, along with those of the Cu interconnects sealed by conventional sealing layered materials, such as Ta, SiCN and CoWP, [28][29][30][31] are included in this figure as a bar graph for comparison.…”

“…29,32,33 Using a bamboo-like microstructure or a capping layer to block the fastest (interfacial/surface) diffusion routes further strengthens EM reliability of Cu interconnects and elevates the activation energies to typically 0.7-0.8 eV, characteristics of grain-boundary diffusion. 28,33,34 Moreover, adhesion is regarded as another important factor that decides the reliability of a metal interconnect line under EM testing. A weakened adhesion renders the deposited Cu to debond from a substrate, and accelerates Cu diffusion via free surface or substrate interface, also giving reduced values of activation energy for EM.…”

Effects of crystallinity on mechanical properties of electroplated gold thin films used for three-dimensional electronic packaging

“…The crystallinity of the sample was evaluated by our proposed evaluation method of analyzing the IQ value obtained from the EBSD analysis. 14,22,29,30) In the EBSD method, a small area of a sample (several nm in diameter) is irradiated by a focused EB and some inelastic scattered electrons fulfilling the Bragg's diffraction condition produce the so-called Kikuchi pattern consisting of sets of two parallel lines (Kikuchi lines). The IQ value is the average sharpness of obtained Kikuchi lines.…”

“…In addition, the crystallographic quality of grain boundaries plays an important role on the long-term reliability of micro bumps when the volume ratio of grain boundaries increases with the decrease of the size of bumps, because a grain boundary is an important source of electron scattering and a bypass for atomic diffusion under the application of high current density (electromigration) and high mechanical stress (stress-induced migration). [22][23][24] It is, therefore, essential to clarify the mechanism of the fluctuation of mechanical properties of electroplated bumps in order to assure the reliability of the products.…”

Gallium-Enhanced Aluminum and Copper Electromigration Performance for Flexible Electronics

“…Both Al and Cu are largely used in the electronic industry for interconnecting various parts of circuitry. − Their low electrical conductivity and cost define their technological relevance. Mixing both metals in search for more stable interconnects was studied in both bulk and thin film forms, and no exceptional enhancements of ability to withstand electromigration emerged. − An increase in the electrical resistivity of Al–Cu alloys raised concerns requiring a trade-off between resistivity and electromigration reliability to be found .…”