Electromigration

In thin Cu-based interconnect lines for microelectronic circuits with a high integration level the current densities are very high, and a material degradation due to current-driven migration of metal atoms can occur. Defects like voids or hillocks are observed and can cause interruptions or shortcuts in the lines. This  process is called electromigration (shortly: EM) and is understood as a current induced vacancy diffusion, but the details of vacancy accumulation (void formation), vacancy flow (void movement), and materials extrusion (hillock formation) are not quite clear yet. Especially the role of microstructural details such as high angle grain boundaries or twins is discussed controversially.

In our group, the formation, growth, and movement of such defects in Cu based interconnects are studied with respect to microstructure details like grain boundaries, texture effects and impurities. Furthermore, the influence  of alloying element addition on electromigration behaviour is investigated. To do this, the following experiments are performed:

  • fabrication and microstructural analysis of damascene Cu interconnect structures down to 500 nm in width, varying the Cu deposition conditions and the type of underlying diffusion barrier material
  • EM tests with supervision of electric resistance (lifetime tests)
  • in situ loading experiments in SEM to correlate defects and microstructural details
  • drift velocity estimation with line sections in SEM (so-called BLECH-experiments) to determine the EM activation energy
  • thermal cycling under EBSD observation or with similar layer stress measurements (wafer curvature method)
  • study of alloying effects on electromigration characteristics of CuX lines (X = Ag, Al)

We found that defect formation sites are often connected with discontinuity points in the network of high angle grain boundaries. In very thin lines, some hints for stress concentration at certain types of twins are observed.

Cu lines alloyed with low amounts of Ag (about 1 at-%) reveal increased yield strength and slightly increased lifetime in EM tests. Further work will be conducted to show if this is mainly an effect of improved interfaces or of varied microstructure.

Examples

In situ EM test with prior EBSD mapping, 3.5µm wide ECD Cu line (AMD), tested with 3 MA/cm2 @ 280°C
BLECH test in SEM, Cu-0.8%Ag line 0.8 µm wide, 9 µm long, 1.8 MA/cm2 @290°C
In situ EM test with prior EBSD mapping, 3.5µm wide ECD Cu line (AMD), tested with 3 MA/cm2 @ 280°C
BLECH test in SEM, Cu-0.8%Ag line 0.8 µm wide, 9 µm long, 1.8 MA/cm2 @290°C

References

K. Mirpuri, H. Wendrock, S. Menzel, K. Wetzig,J. SzpunarTexture evolution in Copper film at high temperature studied in situ by electron back-scatter diffraction, Thin Solid Films 496 [2] (2006) 703-717 (doi:10.1016/j.tsf.2005.08.353)

R. Spolenak, H. Wendrock, K. Wetzig
Electromigration in metallization layers, in: Metal Based Thin Films for Electronics, K.Wetzig ; C.M.Schneider (eds.), 231-247 (2006) (ISBN-13:978-3-527-40650-0)

S. Strehle, S. Menzel, H. Wendrock, T. Gemming, K. Wetzig
Effect of film thickness on the thermo-mechanical behavior of unpassivated Cu(Ag) thin films during thermal cycling, 8th International Workshop on Stress-Induced Phenomena in Metallization, Dresden, 12.-14.9.05, in: Stress-Induced Phenomena in Metallization, E. Zschech et.al. (eds.), AIP-Conference Proceedings 817 (2006) 52-58

S. Menzel, S. Strehle, H. Wendrock, K. Wetzig
Effect of Ag-alloying addition on the stress–temperature behavior of electroplated copper thin films, Applied Surface Science 252 [1] (2005) 211-214 (doi:10.1016/j.apsusc.2005.01.120)

M. Pletea, W. Brueckner, H. Wendrock, R. Kaltofen
Stress evolution during and after sputter deposition of Cu thin films onto Si (100) substrates under various sputtering pressures, Journal of Applied Physics 97 [5] (2005) 54908/1-7 (doi: 10.1063/1.1858062)

H. Wendrock, K. Mirpuri, S. Menzel, G. Schindler, K. Wetzig
Correlation of electromigration defects in small damascene Cu interconnects with their microstructure, Microelectronic Engineering 82 [3-4] (2005) 660-664 (doi:10.1016/j.mee.2005.07.077)

S. Strehle, S. Menzel, H. Wendrock, J. Acker, T. Gemming, K. Wetzig
Thermo-mechanical behavior and microstructural evolution of electrochemically deposited low-alloyed Cu(Ag) thin films, Microelectronic Engineering 76 [1-4] (2004) 205-211 (doi:10.1016/j.mee.2004.07.055)

S. Strehle, S. Menzel, H. Wendrock, J. Acker, K. Wetzig
Microstructural investigation of electrodeposited CuAg-thin films, Microelectronic Engineering 70 [2-4] (2003) 506-511 (doi:10.1016/S0167-9317(03)00422-2)

H. Wendrock, K. Mirpuri, S. Menzel, K. Wetzig, G. Schindler
Formation of electromigration defects in small damascene Cu interconnects with respect to the microstructure, in: 7th International Workshop on Stress-induced Phenomena in Metallization, Austin/USA, 14.-16.6.04, AIP Conf. Proc. 741; P. S. Ho; S. P. Baker ; T. Nakamura ; C.A. Volkert (eds.), 173-179 (2004)