Electromigration-induced Drift Research Articles

Sn-mediated transformations on Si(111) surface: Reconstructions, Electromigration, Homoepitaxy

We have studied Si mass transport and morphological transformations on the Si(111) surface during (√3 × √3)-Sn reconstruction formation and following Si homoepitaxy in 600–825°C interval by in situ ultrahigh vacuum reflection electron microscopy (UHV REM) and ex situ atomic force microscopy (AFM). We have shown that the formation of a metallic α-(√3 × √3)-Sn phase during Sn deposition at T < 650 °С leads to Si 2D island nucleation on > 1 μm terraces, while the formation of a mosaic γ-(√3 × √3)-Sn phase at T > 650 °С is followed by monatomic step shift in the step-up direction and provides Sn/Si(111) interface with wide atomically flat terraces. The step shift value has been measured as functions of substrate temperature and Sn deposition rate to determine emitted by the steps Si coverage participating in γ-phase formation. The electromigration-induced drift of disordered “1 × 1”-Sn domains has been shown to entail enhanced noncompensated Si mass transport followed by the Si(111)-(√3 × √3)-Sn surface roughening. The kinetics of 2D island nucleation and growth near monatomic steps and on wide (up to 10 μm) terraces on the Si(111)-γ-(√3 × √3)-Sn surface during Si deposition at 700°C is limited by surface diffusion only, and critical nucleus i consists of ∼ 20–100 Si atoms.

Impurity-induced island pinning during electromigration

We study the electromigration-induced drift of monolayer Ag islands on Ag(111) which contain one Cu atom. For this purpose a three-dimensional self-learning kinetic Monte Carlo model was extended, and a realistic many-body potential was used. The only free parameters of the model are the effective valences of the Ag and Cu atoms. Due to the impurity, the island drift is significantly reduced, especially for small islands. This is traced back to sequential pinning and depinning events, which are analyzed in detail. Surprisingly, this phenomenon is qualitatively independent of the impurity's effective valence, as long as the impurity does not detach from the island edge. How strongly the drift velocity is reduced depends on the effective valence.

Anisotropy of electromigration-induced void and island drift

By means of our novel self-learning kinetic Monte Carlo model (Latz et al 2012 J. Phys.: Condens. Matter 24 485005) we study the electromigration-induced drift of monolayer voids and islands on unpassivated surfaces of single crystalline Ag(111) and Ag(001) films at the atomic scale. Regarding the drift velocity, we find a non-monotonic size dependence for small voids. The drift direction is aligned with the electromigration force only along high symmetry directions, while halfway between, the angle enclosed by them is maximal. The magnitude of these directional deviations strongly depends on the system parameter, which are investigated in detail. The simulation results are in accordance with void motion observed in experiments performed on Ag(111).

On the atomistic details of electromigration-induced drift

Electromigration drift studies have been performed inside a SEM. Based on the observed drift mode, a grain boundary grooving model is proposed to account for the atomistic details of current-induced motion. The role of grain boundary structure in determining the local direction of groove propagation is illustrated.

Microtexture and electromigration-induced drift in electroplated damascene Cu

In this work, the electromigration (EM) performance of electroplated damascene Cu is investigated by drift experiments on Blech-type test structures in both polycrystalline and bamboo microstructures. For the first, microtexture data were obtained from electron backscatter diffraction as well. While both bonding areas and 10 μm wide lines were found to have a predominantly random grain orientation, the drift studies indicated the importance of strongly segregating impurities in controlling Cu grain-boundary EM. For the bamboo lines, the impact of different barrier layers has been investigated, comparing Ta, TaN, and TiN. Drift was shown to proceed in all cases at the metallic Cu barrier interface, but faster for the Ta as compared to the TaN and TiN barriers. Cu drift data were finally compared to available literature results and to our previous drift studies on Al(Cu).

Electromigration-induced drift in damascene and plasma-etched Al(Cu). II. Mass transport mechanisms in bamboo interconnects

We have discussed electromigration (EM)-induced drift in polycrystalline damascene versus reactive ion etched (RIE) Al(Cu) in part I. For polycrystalline Al(Cu), mass transport is well documented to occur through sequential stages : an incubation period (attributed to Cu depletion beyond a critical length) followed by the Al drift stage. In this work, the drift behavior of bamboo RIE and damascene Al(Cu) is analyzed. Using Blech-type test structures, mass transport in RIE lines was shown to proceed both by lattice and interfacial diffusion. The dominating mechanism depends on the Cu distribution in the line, as was evidenced by comparing as-patterned (lattice EM) and RTP-annealed (interface EM) samples. The interfacial EM only occurs at metallic interfaces. In that case, Cu alloying was observed to retard Al interfacial mass transport, giving rise to an incubation time. Although the activation energy for the incubation time was found similar to the one controlling Al lattice drift, for which no incubation time was observed, lattice EM is preferred over interfacial EM because it is insensitive to enhancing geometrical effects upon scaling. When comparing interfacial electromigration in RIE with bamboo damascene Al(Cu), with the incubation time rate controlling for both, the higher EM threshold observed for damascene was shown to be insufficient to compensate for its significantly increased Cu depletion rate, contrary to the case of polycrystalline Al(Cu) interconnects. Two factors were demonstrated to contribute. First, there are more metallic interfaces, intrinsically related to the use of wetting or barrier layers in recessed features. Second, specific to this study, the additional formation of TiAl3 at the trench sidewalls further enhanced the Cu depletion rate, and reduced the rate-controlling incubation time. A separate drift study on RIE via-type test structures indicated that it is very difficult to suppress interfacial mass transport in favor of lattice EM upon TiAl3 formation.

Electromigration-induced drift in damascene and plasma-etched Al(Cu). I. Kinetics of Cu depletion in polycrystalline interconnects

In this work, a direct and quantitative comparison is presented on the rate of electromigration-induced Cu depletion for polycrystalline damascene and reactive ion etched (RIE) Al(Cu). Kinetic data are derived from the incubation time, obtained from drift characteristics of both unpassivated and passivated Blech-type test structures between 155 and 230 °C. Since the incubation time represents the time necessary for the electron wind to deplete the critical length free of Cu, both electromigration (EM)-threshold and the rate of Cu depletion was systematically investigated. For the latter specific microstructural features, related to Al2Cu precipitate morphology and distribution, are discussed. EM-threshold on the other hand is a more intrinsic characteristic of the interconnect, depending on geometrical and mechanical properties (like aspect ratio and encapsulation). For geometrically equivalent, unpassivated structures, it was found that the improvement in incubation time for the damascene implementation is predominantly controlled by a higher critical length caused by its encapsulation inside the dielectric. For passivated structures, the impact of encapsulation on enlarging the maximum elastic stress buildup becomes similar. As a result, the difference in threshold between passivated damascene and RIE was observed to decrease. The incubation time in passivated structures becomes more importantly controlled by the kinetics of Cu depletion. It is demonstrated that it is most effective to maximize the degree of intergranular θ precipitation to retard the Cu depletion rate in polycrystalline structures. Implications on the relative EM performance under operating conditions for damascene and RIE Al(Cu) are also discussed.

The role of grain boundary structure on electromigration-induced drift in pure Al and Al(0.5wt% Cu)

The effect of microstructure on the electromigration performance of aluminum-based metallizations in integrated circuits has been extensively investigated over the past three decades. Besides grain size, grain size distribution and precipitate morphology, crystallographic texture has been documented to have a major impact on the electromigration (EM) lifetime. The authors present here, for the first time, a direct relation between the aluminum microtexture and EM-induced drift velocities, both for different alloys (pure Al vs Al(Cu)) and different metallization technologies (reactive ion etching or RIE vs. damascene).

Electromigration-Induced Drift in Damascene vs. Conventional Interconnects: An Intrinsic Difference

AbstractThe electromigration (EM) behaviour of damascene Al(Cu)-interconnects has been studied for the first time by drift experiments and results directly compared to plasma-etched lines. For the latter EM is known to occur through sequential stages : an incubation period, needed for Cu-depletion, and an Al-drift stage. It is shown that using a damascene implementation, the incubation time significantly increases but the Al-drift velocity does as well. This is demonstrated to be an intrinsic effect, directly related to the technological nature of both metallization schemes. Implications on lifetime prediction at operating conditions are discussed.