Appl Phys A (2009) 97: 369–374 DOI 10.1007/s00339-009-5218-y Mechanical strength lowering in submicron Cu thin films by moderate DC current R.M. Niu · J. Zhang · Z.J. Wang · G. Liu · G.J. Zhang · X.D. Ding · J. Sun Received: 23 March 2009 / Accepted: 30 March 2009 / Published online: 11 April 2009 © Springer-Verlag 2009 Abstract This letter reports an experimental investigation into the direct current (DC) induced reduction in the yield strength of 60 ∼ 700-nm-thick Cu films. Results show that the larger the current density and the thinner the film, the greater the reduction when the film thickness is below about 340 nm. This reduction could be described on the basis of dislocation buckling, which, caused by the electron wind of the current flow, induces an increase in the dislocation length and a decrease in the critical stress for multiplying the dis- location. PACS 61.05.-a · 62.20.-x · 62.25.-g Lower resistivity and improved electromigration resistance have been putting Cu rapidly forward to replace Al for advanced interconnects in multilevel metallization of ul- tralarge scale integrations. As the dimension of the inte- grated circuits continues to shrink, it is becoming ever more crucial to understand the operational properties of Cu thin films with size in submicron scale [1], in order to ensure the reliability of Cu interconnects and favor the design and fabrication of integrated circuits. Most important proper- ties of the submicron-thick Cu films, such as tensile prop- erties [2, 3], mechanical fatigue [4, 5], thermal or thermo- mechanical fatigue [6], and electromigration behaviors [7], have been elaborately investigated and well revealed. Elec- tromigration is termed the phenomenon of atomic flux in R.M. Niu · J. Zhang · Z.J. Wang · G. Liu · G.J. Zhang · X.D. Ding · J. Sun ( ) State Key Laboratory for Mechanical Behavior of Materials and School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an, 710049, China e-mail: junsun@mail.xjtu.edu.cn metals under an applied current, which could induce a stress gradient and then initiate voids after a somewhat long dura- tion, e.g., several hundreds of hours. For the metals used in the microelectronic interconnects, the required current den- sity that could cause an appreciable electromigration is usu- ally in the range of 10 5 –10 6 A/cm 2 . As one of the major failure mechanisms of the Cu films, the electromigration- induced void formation and growth have attracted extensive attention and some statistically based life-predicted models have been suggested [8–10] that might be referred to as de- sign criteria. However, on another important failure mechanism, the weakening in strength of Cu films under applied electric current, little systematic work has been carried out, which results in a lack of the design criterion on strength require- ment. Because the shrinking in size and resultant increase in current density make the electricity-related mechanical fail- ure of thin films more and more important in integrated cir- cuits, it is urgent to well understand the yield strength of Cu films and its size effect in application of an electric current, which could be helpful for strength design in Cu metalliza- tion. The yield strength of films could be determined by a va- riety of methods, such as tensile testing, indentation, mi- crobeam bending, and bugle testing [11–13]. In compari- son, tensile testing has the advantage of providing mechan- ical parameters directly, without the need of a model-based data analysis. However, the tensile testing of freestanding films, especially with size in submicron scale, is compli- cated and difficult. Recently, thin films deposited on com- pliant polymer substrates have been used to determine the mechanical properties of films [3, 4, 14], because the poly- mers usually have large elastic limits and their response could thus easily be subtracted from the overall measure- ment. On the other hand, many flexible electronics that are