Accelerated electromigration study in cobalt by novel in-situ tem technique

Abstract

Electromigration is a phenomenon in which electronic conduction can result in mass transport of the conducting material, resulting in the eventual failure of a device. Momentum transfer from the conducting species to the metal lattice introduces a directional bias to random thermal diffusion, and the net flux of ions and vacancies to opposing ends of the conducting line can cause the spontaneous formation of voids or hillocks. These instances of electromigration damage can render devices non-functional by causing open or short circuits, or just by increasing the line resistance or RC delay beyond acceptable limits. As continued miniaturization of silicon based devices shrinks interconnect dimensions, increasing current density in those interconnects poses increased challenges to system reliability due to an enhanced rate of electromigration. Additionally there is a need to identify and test alternatives to the traditional copper/barrier interconnects as the line resistances of Cu based structures increase rapidly with decreasing line widths. As such, the current electromigration issue is twofold: to extend understanding in existing systems as they are modified to adapt to decreasing sizes, and to test novel systems for reliability as material replacements. In-situ electromigration testing in the transmission electron microscope (TEM) provides great potential for relating the information which can be gathered by wafer-level testing to features of the sample and failure mechanisms on the nanometer length scale. TEM imaging of electromigration test structures combines the high temporal and spatial resolution of the TEM, control over current loading and temperature, and real time imaging and electrical measurements. This can provide a bridge in understanding between the failure statistics which can be extracted from large scale wafer-level testing and the microstructural features of the metal. However, in-situ experimentation poses challenges due to the necessity of an electron transparent sample and large amounts of restively generated heat. These challenges are exacerbated when the materials being studied are more resistant to electromigration than traditional systems such as copper or aluminum, as they require more extreme values of temperature and current density to cause electromigration damage in a similar amount of time. This thesis demonstrates the development and implementation of an in-situ electromigration testing platform suitable for electromigration resistance metals. Through a combination of finite element modeling and experimentation, using titanium as a test system, a robust device structure was developed. The structures were then adapted to a cobalt metal system which allowed for in-situ testing at a high current density (up to $5\times 10^6 A/cm^2$) over a controllable range of temperatures (50-925{\textdegree}C). Electromigration damage is observed in both the titanium and cobalt systems; titanium lines exhibited the formation of voids which did not lead to failure of the line during the time frame of the experimentation, and cobalt exhibited voids which caused failure with different modalities depending on line width and with different directions depending on the line temperature. The failure mechanism in cobalt was dependent on the line width, with narrower lines typically failing when a void along the line edge reached an impassable barrier and grew to span the width of the line. Wider lines more often formed voids in the volume of the line between the edges (including the upper and lower interfaces), and failed when several voids grew large enough to interact and span the width of the line. The orientation of electromigration, either electron-mediated or hole-mediated, was observed to depend on the line temperature, with hole-oriented electromigration observed above 685{\textdegree}C and electron-oriented electromigration observed below 625{\textdegree}C.