IEC 62415 pdf download Semiconductor devices – Constant current electromigration test
This standard describes a method for conventional constant current electromigration testing of metal lines, via string and contacts.
2 Symbols, terms and definitions
For the purposes of this document, the following symbols, terms and definitions apply:
the maximum current density permitted to flow in a via of a real product
the maximum current density permitted to flow in a line of a real product
the current density in a via of a test structure during electromigration test
the current density in a line of a test structure during electromigration test
time to failure of x % of the population
NOTE The method for calculation of t (50 %) is described in Clause 8.
2.2 Terms and definitions
test element group. This is the test structure used for the test
the line length below which electromigration time to failure increases sharply [1 ] 1
NOTE The drift of metal atoms causes stress build-up in the metal lines, which caused a back flow of atoms.
For short lines the stress gradient is higher than for long lines with the same current density. The forward flow increases more rapidly with current density than the backflow, and consequently the Blech length is inversely proportional to the current density. The Blech length can be determined by using a chain with different line lengths between the vias.
3 Background The background of electromigration testing as described in this procedure is based on the assumption that the entire electromigration failure time distribution stays intact when accelerated. Acceleration can be described by an activation energy and a current acceleration factor, as originally proposed by Black .
4 Sample size 1 5 samples or more are recommended for each test (each test structure, temperature and current density). In some cases, to get a better statistical confidence of the results or to analyze a bimodal distribution, a higher number of samples might be necessary.
5 Test structures 5.1 Lines Electromigration characterization shall be carried out on fully back-end processed samples. The metal line test structure in a 4-terminal (Kelvin) configuration shall be used (see Figure 1 a).The line length is recommended to be at least 800 μm. The use of monitors for opens, inter-layer shorts and optional intra-layer shorts is recommended (see Figure 1 b).The line length is determined by the constraints that short lines are not sensitive to failure and exhibit the Blech effect [1 ], and too long lines require high voltages. For line lengths <200 μm the Blech effect shall be verified.