Mechanisms for microstructure evolution in electroplated copper thin films near room temperature

Abstract

We present a model which accounts for the dramatic evolution in the microstructure of electroplated copper thin films near room temperature. Microstructure evolution occurs during a transient period of hours following deposition, and includes an increase in grain size, changes in preferred crystallographic texture, and decreases in resistivity, hardness, and compressive stress. The model is based on grain boundary energy in the fine-grained as-deposited films providing the underlying energy density which drives abnormal grain growth. As the grain size increases from the as-deposited value of 0.05–0.1 μm up to several microns, the model predicts a decreasing grain boundary contribution to electron scattering which allows the resistivity to decrease by tens of a percent to near-bulk values, as is observed. Concurrently, as the volume of the dilute grain boundary regions decreases, the stress is shown to change in the tensile direction by tens of a mega pascal, consistent with the measured values. The small as-deposited grain size is shown to be consistent with grain boundary pinning by a fine dispersion of particles or other pinning sites. In addition, room temperature diffusion of the pinning species along copper grain boundaries is shown to be adequate to allow the onset of abnormal grain growth after an initial incubation time, with a transient time inversely proportional to film thickness.

Rapid trench initiated recrystallization and stagnation in narrow Cu interconnect lines

“…In the overburden over 48 nm lines, the defect density was found to be $1 Â 10 12 cm À2 , which is similar to defect densities typically associated with ECD Cu films. 5 The defect density in the 1000 nm lines was uniform across the trench with a value of $9 Â 10 11 , essentially the same as the overburden, consistent with the expectations for overburden-dominated transformation which has been widely reported for wide lines. 10 In the narrow lines, assuming they had a defect density similar to the overburden, each 48 nm trench should have 90 to 100 dislocations per 100 nm in length (approximately the thickness of the TEM specimen).…”

“…Historically, this uncontrolled process was circumvented using a modest thermal anneal to encourage grain coarsening, yielding large columnar grains in the overburden. In wide (>100 nm) lines, this process resulted in large bamboo-type grains, 5,6 but in narrow lines (<100 nm) the process stagnates, leaving the polygranular microstructure which is detrimental to performance. This dilemma has contributed to a re-evaluation of production targets, resulting in less aggressive scaling of back end of the line (BEOL) feature sizes than had been predicted just a few years ago.…”

Thermomechanical characterization and modeling for TSV structures

“…The nonlinear curvature-temperature relation during heating of the first cycle is mainly attributed to the nonlinear stress relaxation caused by grain growth. Similar curvature behavior due to grain growth has been observed for Cu thin films [14,15]. As grain growth proceeds to eliminate grain boundaries and reduce the excess volume, it is favored when the average stress in the Cu vias is compressive during heating [16].…”

Increasing the mean grain size in copper films and features

“…It is assumed here that the driving force for grain growth is a summation of a term related to the grain boundary energy and mobility, a term related to the difference in elastic or plastic strain energy between neighbouring grains due to external forces and dislocations, a term related to stress anisotropy due to dislocation arrays, and a term related to the difference between surface and interface energies of neighbouring grains. It is usually assumed that a "pinning force" caused by particles or impurities that are present at the grain boundaries (Zener pinning) may oppose all of the aforementioned driving forces [44]. The texture then develops in such a way that the energy of the system can be minimized.…”

“…Several mechanisms that could drive such a transformation have been proposed, including defect, grain-boundary, and surface energies, but also chemical gradients in the material that can cause diffusion-induced grain-boundary migration [84]. During the last decade, microstructural evolution or self-annealing has been observed in electroplated copper films that are stored at room temperature [44,[85][86][87][88][89]. Although this behaviour of electroplated copper is intriguing, the kinetics are hard to study, since the as-deposited grain structure and impurity content of the film strongly depend on the electroplating bath chemistry.…”