cnse.albany.edu Background: Statistical Electromigration Analysis: Constant Current vs Constant Voltage Jonathan Gentile, Andrea Hubbard, Dr. Lloyd College of Nanoscale Science and Engineering, SUNY, Albany NY, 12203 Objective: Compare and contrast statistical electromigration (EM) data retrieved from Constant Current and Constant Voltage accelerated electromigration tests in order to better predict reliability lifetimes of Cu interconnects. Motivation Results Conclusion References [1] Breu, F., Guggenbichler, S. & Wollmann, J., 2008. Grain size dependance of narrow EM failures. Vasa. Available at: http://medcontent.metapress.com/index/A65RM03P4874243N.pdf [Accessed December 5, 2013]. [2] Hau-Riege, C.S., 2004. An introduction to Cu electromigration. Microelectronics Reliability, 44(2), pp.195–205. Available at: http://linkinghub.elsevier.com/retrieve/pii/S0026271403004475 [Accessed December 5, 2013]. [3] Hauschildt, M. et al., 2006. Analysis of electromigration statistics for Cu interconnects. Applied Physics Letters, 88(21), p.211907. Available at: http://link.aip.org/link/APPLAB/v88/i21/p211907/s1&Agg=doi [Accessed December 5, 2013]. [4] Hauschildt, M., 2005. Statistical analysis of electromigration lifetimes and void evolution in copper interconnects. Available at: http://adsabs.harvard.edu/abs/2005PhDT ........75H [Accessed December 5, 2013]. [5] Ho, P.S., Uehling, T. & Ramanathan, L.N., 2006. Electromigration Lifetime Statistics for Pb-Free Solder Joints with Cu and Ni UBM in Plastic Flip-Chip Packages. 56th Electronic Components and Technology Conference 2006, pp.650–656. Available at: http://ieeexplore.ieee.org/lpdocs/epic03/wrapper.htm?arnumber=1645719. [6] Laboratories, S.N., 1997. E l e c t r o m i g r a t i o n : a review. , 37(7), pp.1053–1072. [7] Li, B. et al., 2006. Threshold electromigration failure time and its statistics for Cu interconnects. Journal of Applied Physics, 100(11), p.114516. Available at: http://link.aip.org/link/JAPIAU/v100/i11/p114516/s1&Agg=doi [Accessed December 5, 2013]. [8] Lloyd, J. & Clement, J., 1995. Electromigration in copper conductors. Thin solid films, 262, pp.135–141. Available at: http://www.sciencedirect.com/science/article/pii/0040609094058067 [Accessed December 5, 2013]. [9] Schafft, H. & Lechner, J., 1988. Statistics for electromigration testing. … 1988. 26th Annual …. Available at: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=23449 [Accessed December 5, 2013]. What is electromigration?: • Definition: The transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. (Figure 1) • Affects devices with high current densities (Nanoelectronics). • Typical current density in a lamp cord ~ • Typical current density in copper interconnects ~ • Voids are formed typically near the via, killing your devices. (Figure 2) Accelerated Electromigration Testing: • We need to be able to get 10 year lifetimes in a matter of hours. • In order to do that, we test specific test structures and elevate the temperature and voltage applied to the structure. (Figure 3) • We take the data from test structures containing a single failure element and we need to relate that to the failure of a chip with perhaps 100,000 to 1,000,000 failure elements. • We plot lognormal failure distributions and extrapolate fitting parameters in order to get useful information. • Electromigration is the main reason of why high performance nanoelectronic devices fail over time. • The reliability of nanoscale electronics is very important to industry and corporations to bring down RMA and return expenses. The customer and Manufacturer are happier as a result. • Electromigration has been difficult to treat in the past. This is mainly due to the difficulties in testing large sample populations. 1.00 1000.00 10.00 100.00 1.00 5.00 10.00 50.00 99.00 ReliaSoft's Weibull++ 6.0 - www.Weibull.com Probability - Lognormal Time, (t) Unreliability, F(t) 4/9/2014 12:05 IBM Jim Lloyd Lognormal EAKOF4 L2 MLE - SRM MED F=11 / S=4 CB[FM]@60.00% 2-Sided-B [T2] 1.00 1000.00 10.00 100.00 1.00 5.00 10.00 50.00 99.00 ReliaSoft's Weibull++ 6.0 - www.Weibull.com Probability - Lognormal Time, (t) Unreliability, F(t) 4/9/2014 12:10 IBM Jim Lloyd Lognormal 27SEE0 L2 MLE - SRM MED F=13 / S=1 CB[FM]@60.00% 2-Sided-B [T2] 1.00 1000.00 10.00 100.00 1.00 5.00 10.00 50.00 99.00 ReliaSoft's Weibull++ 6.0 - www.Weibull.com Probability - Lognormal Time, (t) Unreliability, F(t) 3/5/2014 12:45 IBM Jim Lloyd Lognormal 09TMG5 L2 RRX - SRM MED F=11 / S=4 CB[FM]@60.00% 2-Sided-B [T2] 1.00 1000.00 10.00 100.00 1.00 5.00 10.00 50.00 99.00 ReliaSoft's Weibull++ 6.0 - www.Weibull.com Probability - Lognormal Time, (t) Unreliability, F(t) 3/5/2014 12:48 IBM Jim Lloyd Lognormal 58SJB5 L2 RRX - SRM MED F=12 / S=4 CB[FM]@60.00% 2-Sided-B [T2] 10.00 1000.00 100.00 1.00 5.00 10.00 50.00 99.00 ReliaSoft's Weibull++ 6.0 - www.Weibull.com Probability - Lognormal Time, (t) Unreliability, F(t) 3/5/2014 12:50 IBM Jim Lloyd Lognormal 40TMH2 L2 RRX - SRM MED F=10 / S=3 CB[FM]@60.00% 2-Sided-B [T2] Constant Current Data: Lognormal Distributions Constant Voltage Data: Lognormal Distributions Lots of roughly 16 samples were run under accelerated testing conditions on 5 different wafers using Constant Current and Constant Voltage methods. CC CV Temperature 300°C 300°C Voltage ~ 200mV Current 40μA ~ Accelerated Test Conditions: Current Situation and Methods • Traditionally Electromigration (EM) testing is performed using constant current. • However, variations in geometry (line width, thickness, cross section) will produce corresponding variations in current density if the same current is passed through each structure. This contributes to the variation in lifetime, • For a given length of conductor, the electric field along the conductor will be identical at a given applied voltage regardless of the cross sectional area. • Although cross sectional area can vary significantly across a wafer (thickness and conductor width) the length proportionately does not. – Therefore, at constant voltage the current density will be constant regardless of these inherent variations. Figure 1: Electromigration concept diagram Figure 2: Electromigration void nucleation and SEM images Figure 3: Test structure and wirebonding diagrams • Constant Current vs Constant Voltage shows some clear differences. • The standard deviation, which represents the spread of failure times, is showing a decrease in value for all of our Constant Voltage tests up to a factor of 2. • Most T-50’s that we’ve recorded, which is the median time to failure, have been within an order of magnitude difference. We can note that constant voltage on average seems to take less time to fail. This could be due to under-stressing the constant current tests. JGEM03 JGEM04 JGEM05 JGEM06 JGEM07 T50 (h) σ Test # Wafer # CC CV CC CV JGEM03 09TMG5 100.5847 69.17434 1.0762 0.574278 JGEM04 58SJB5 195.6251 59.98551 0.9617 0.661062 JGEM05 40TMH2 111.8994 87.58686 0.6683 0.582221 JGEM06 EAK0F4 37.13277 38.96727 0.9215 0.494421 JGEM07 27SEE0 38.34024 55.49667 1.0995 0.649492