D. Jawarani

Statistical analysis of early failures in electromigration

The detection of early failures in electromigration (EM) and the complicated statistical nature of this important reliability phenomenon have been difficult issues to treat in the past. A satisfactory experimental approach for the detection and the statistical analysis of early failures has not yet been established. This is mainly due to the rare occurrence of early failures and difficulties in testing of large sample populations. Furthermore, experimental data on the EM behavior as a function of varying number of failure links are scarce. In this study, a technique utilizing large interconnect arrays in conjunction with the well-known Wheatstone Bridge is presented. Three types of structures with a varying number of Ti/TiN/Al(Cu)/TiN-based interconnects were used, starting from a small unit of five lines in parallel. A serial arrangement of this unit enabled testing of interconnect arrays encompassing 480 possible failure links. In addition, a Wheatstone Bridge-type wiring using four large arrays in each...

IN SITU TRANSMISSION ELECTRON MICROSCOPY STUDY OF PLASTIC DEFORMATION IN PASSIVATED AL-CU THIN FILMS

Plastic deformation in passivated Al-1 wt %Cu thin films was studied in situ using a straining device in the transmission electron microscope. Both edge and screw dislocations were found to have caused slip on inclined {111} planes. Multiple slip was frequently observed as two or more sets of intersecting slip traces. Microstructural investigations of both unpassivated and passivated Al-1 wt %Cu films indicate that grain size and encapsulation by passivating layers are major contributors to strength of a thin film with a particular thickness. Additional strengthening is also provided by interactions between dislocations on multiple slip systems. The roles of grain orientation and precipitates in plastic deformation are also discussed.

Electromigration early failure distribution in submicron interconnects

The early failure issue in electromigration (EM) has been an unresolved subject of study over the last several decades. A satisfying experimental approach for the detection and analysis of early failures has not yet been established. In this study, a new technique utilizing large interconnect arrays in conjunction with the well-known Wheatstone bridge is presented. A total of more than 20,000 interconnects were tested. The results indicate that the EM failure mechanism studied here follows log-normal behaviour down to the 4 sigma level.

In situ transmission electron microscopy study of plastic deformation and stress-induced voiding in Al–Cu interconnects

Plastic deformation in submicron wide Al-1 wt %Cu interconnects was studied in situ using a straining device in the transmission electron microscope. Dislocation motion occurred readily in unpassivated lines but was nonexistent in passivated lines due to the presence of stiff oxide sidewalls. Instead heterogeneous void nucleation occurred on straining to a critical limit. The void morphology was always near hemispherical and the nucleation always took place at the line edges. Further stretching of the lines led to a rupture of the sidewalls away from the lines, resulting in immediate dislocation motion. Void nucleation, cross slip, and operation of dislocation sources at line edges were all recorded on video. It was noted that dislocations almost parallel to the plane of the lines are rarely observed and furthermore, their movement is sluggish. Based on the dislocation configuration observed in these lines, a generalized geometrical model was arrived at in order to determine the significance of grain orientation on yield stress of passivated lines with columnar, bamboo grains. Frequent occurrence of twinning within the grains indicated that plastic deformation was indeed restricted in confined metal lines.

Detection and analysis of early failures in electromigration

The early failure issue in electromigration (EM) has been an unresolved subject of study over the last several decades. A satisfying experimental approach for the detection and analysis of early failures has not been established yet. In this study, a technique utilizing large interconnect arrays in conjunction with the well-known Wheatstone Bridge is presented. A total of more than 20 000 interconnects were tested. The results indicate that the EM failure mechanism studied here follows lognormal behavior down to the four sigma level.

Electromigration failure model : its application to W plug and Al-filled vias

Abstract For Al–Cu VLSI interconnects at tungsten (W) plug contact/via areas, a new electromigration (EM) failure model has been established. A series of experiments was performed to verify the proposed model using novel test structures. This paper discusses the lifetime extrapolations using the model and experimental data which predicts that lifetimes of Al–Cu interconnects at use conditions are dominated by Cu drift, or incubation times. This paper also discusses EM experimental results obtained for the Al-filled via which is a promising candidate for replacing W plug vias due to requirements of process simplification and cost reduction in multilevel metallizations. EM failure distributions from Al-filled vias show large standard deviations. This observation is explained through extensive failure analysis of EM-failed specimens. The application of the established EM model to Al-filled vias is discussed.

Statistical evaluation of device-level electromigration reliability

A new test structure for electromigration failure analysis of via-interconnect metallization schemes was developed. The new ensemble of via-interconnect structures can be wire-bonded within the test chip using different configurations thus selecting the number of vias under test. The correlation of the experimental data for different numbers of vias with the results of numerical simulation allows a better insight on the statistical properties of the failure mechanism. The new testing procedure allows extrapolation to operating conditions at the device level with tighter confidence intervals. Furthermore, critical length effects were investigated using many-via chain test structures with different interconnect lengths in drift-velocity type experiments. At interconnect lengths close to the critical Blech-length, resistance saturation effects were encountered and used for calculations of the critical current⋅length product, (jl)*.

Thermal stresses in Cu damascene submicron line structures

Thermal stress in passivated blanket Cu and in passivated 0.2 μm wide damascene Cu lines have been characterized using x-ray diffraction and compared with that measured for 0.2 μm AlCu line structures. The microstructure plays an important role in controlling the thermal stress behavior, particularly for Cu due to its elastic anisotropy. Microstructural analysis of the blanket Cu revealed a strong (111) fiber texture. The damascene Cu lines had a strong (111) texture along with a weak in-plane (110) texture component that can be explained by surface energy considerations. The grain size of damascene Cu is constrained by the linewidth. The observed stress behavior of blanket Cu films shows stress hysteresis and plastic deformation. Under thermal cycling, the 0.2 μm wide Cu lines show a nearly hydrostatic triaxial stress state with linear elastic behavior. Compared with 0.2 μm wide AlCu lines, the principal stresses are generally lower in Cu lines and the difference can be attributed to the thermoelastic pr...

Characterization of Electromigration Failures Using a Novel Test Structure

Electromigration failures occur at locations where Al mass transport flux divergences exhibit a maximum. In VLSI circuits, these locations correspond to tungsten plug contact / via areas. Characterizing and understanding electromigration failures at tungsten plug contact / via areas are essential in assuring the reliability of 0.5 μm and smaller devices. In this study, a novel test structure with four vias connecting metal 1 and metal 2 levels was used to separate two stages in the Al-Cu interconnect electromigration process. These stages are the incubation time during which Cu in the Al-Cu alloy is swept away from the via area and an Al drift time which leads to interconnect failure. By the use of this structure, a non-destructive method has been established to measure incubation times and Al drift times separately, monitoring discrete increases in line resistance corresponding to a sequential depletion of all four vias. Activation energies for both processes have been determined. The temperature dependence of the incubation time is characterized by a thermal activation energy of Q=1.08 ± 0.15 eV, indicative of Cu grain boundary diffusion through Al grains. Al depletion was found to occur with an activation energy of Q=0.72 ± 0.12 eV. We therefore conclude that Al self-grain boundary diffusion constitutes the Al depletion mechanism. The application of this novel test structure enables distinct determination of dominating failure mechanisms under operating conditions and therefore provides a realistic reliability assessment of actual on-chip interconnect structures.

A Comparative Study on Electromigration Failure Mechanism Between Near-Bamboo and Bamboo Al(Cu) Two-Level Structure

As the interconnect line width reduces to the sub-micron range, the grain distribution of Al(Cu) alloy lines reaches a bamboo structure. Understanding of the electromigration resistance of near-bamboo and bamboo structures is essential to the sub-micron reliability issue. In this study, the mass transport and failure mechanism of 1 μm wide Al(1%Cu) two-level lines with near-bamboo and bamboo structures were investigated by a drift velocity technique [1]. Our results show that interfacial (A10 X /A1 interface) diffusion coupled with grain boundary diffusion determines the failure mechanism of bamboo lines.