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Electromigration (Page 2 of 2)

                                   

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The acceleration effect of high temperature on electromigration becomes emphasized only when a void has started to form in the metal line.  Prior to any void formation, the metal can still be under uniform thermal distribution.  Once a void forms, however, the current density at the section where the void is present increases as a result of the reduced cross-sectional area of the conductor, leading to current crowding around the void. 

       

 

The higher current density around the void results in localized heating that further accelerates the growth of the void, which again increases the current density.  The cycle continues until the void becomes large enough to cause the metal line to fuse open.

        

Electromigration may be modeled by the following equation, which is known as Black's Equation:

     

t50 = CJ-ne(Ea/kT)                                                                           

                                                          

where:    

t50 = the median lifetime of the population of metal lines subjected to electromigration;

C = a constant based on metal line properties;     

J = the current density;                

n = integer constant from 1 to 7; many experts believe that n = 2;

 

 

T = temperature in deg K;

k = the Boltzmann constant; and                                                  

Ea = 0.5 - 0.7 eV for pure Al.

     

Electromigration failures take time to develop, and are therefore very difficult to detect until it happens.  Thus, the best solution to electromigration problems is to prevent them from taking place.

                      

Electromigration can be prevented by: 1) proper design of the device such that the current densities in all parts of the circuit are practically limited;  2) increasing of the grain sizes of the metal lines such that these become comparable to their widths (whereby bamboo structure is achieved); and 3) good selection and deposition of the passivation or thin films placed over the metal lines in order to limit extrusions caused by electromigration.

         

Electromigration must not be confused with EOS-induced metal reflow, which is a different phenomenon.  Electromigration occurs gradually whereas EOS-induced metal reflow is gross and abrupt. 

     

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See Also:   Die FailuresFailure AnalysisReliability Models

     

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