ELECTROMIGRATION IN AL(CU) 2-LEVEL STRUCTURES - EFFECT OF CU AND KINETICS OF DAMAGE FORMATION

The electromigration characteristics and kinetics of damage formation for Al(Cu,Si) line segments on a continuous W line and Al(Cu)/W two-le vel interconnect structures have been investigated. The mass transport as a function of temperature was measured using a drift-velocity tech nique. The flux divergence at the line/stud contact was found to be re sponsible for formation of open failure in the interconnect structure, as shown by a direct correlation observed between mass depletion at t he contact and resistance increase of the line/stud chain. The depleti on of Al at the stud contact is preceded by an incubation period durin g which Cu is swept out a threshold distance from the cathode of the l ine. This leads to a damage formation process which is controlled by b oth Cu electromigration along grain boundaries and dissolution of the Al2Cu precipitates. This is distinctly different from single-level int erconnects measured using a conventional electromigration test site. M easurements of the mean failure lifetime in the two-level interconnect yield an activation energy of 0.58 eV for Al, in contrast to 0.78 and 0.83 eV for Al(0.5 wt % Cu) and Al(2 wt % Cu), respectively. The acti vation energies of the electromigration drift velocity were found to b e 0.86 and 0.68 eV for Cu and Al in Al(2 wt % Cu, 3 wt % Si), respecti vely. These results enable one to infer that the kinetic process is co ntrolled by electromigration of Cu along grain boundaries instead of b y dissolution of the Al2Cu precipitates.