Hisao Kawasaki

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.

Realistic electromigration lifetime projection of VLSI interconnects

Abstract Tungsten plug contacts/vias electromigration experiments have been performed using a variety of test structures under different stress conditions. It was found that electromigration failures, failure mechanisms of tungsten plug contacts/vias, were strongly influenced by the test structures and stress conditions. This paper discusses the effect of test structures and stress conditions on electromigration failure for realistic lifetime projection of VLSI interconnects.

Blech effect in single-inlaid Cu interconnects

This work demonstrates that we can prevent electromigration failures in single-inlaid copper during DC electromigration testing by taking advantage of the Blech effect. Electromigration tests were performed on single-inlaid copper metal lines ranging from 5 to 250 /spl mu/m in length at 300/spl deg/C and a stress current density of 1.4/spl times/10/sup 6/ A/cm/sup 2/. Shorter lines showed no resistance increase, while longer lines failed at the same time, independent of line length. The critical product (jl)/sub c/ was calculated to be between 2800 and 3500 A/cm at 300/spl deg/C for single-inlaid copper.

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.

An electromigration failure model of tungsten plug contacts/vias for realistic lifetime prediction

An EM failure model which considers the incubation time and Al drift time separately has been established. The model predicts that the lifetimes of Al-Cu interconnects at the W plug contacts/vias under operating conditions are dominated by Cu diffusion (incubation time) in Al-Cu. For a realistic EM lifetime prediction of VLSI interconnects under operating conditions, the understanding of Cu movement during current stress is essential.

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.

The effect of test structure and stress condition on electromigration failure

A series of electromigration experiments has been performed using various test structures to investigate the effect of the test structure and stress condition on electromigration failures. It was found that electromigration failures were significantly influenced by both test structure and stress condition. Some issues in electromigration lifetime prediction are also discussed.