Investigations of electromigration failure by electrical measurement and scanning probe microscopy with additional simulation

A set of 10, 5 and 2 mu m wide, 500 mu m long and 0.7 mu m thick Al/Si/Cu lines (1% Si, 0.5% Cu) was investigated under different stress conditions. Typical stress conditions were current densities of 2 to 6 MA/cm(2) and ambient temperatures of 125 to 225 degrees C. For additional mathematical simulation it was important to observe the complete history of the resistance development from the beginning until the break down of the samples. Depending on the line width a number of different resistance developments and times to failure occured. Especially at smaller line widths, i. e. near to bamboo structures, the differences of the resistance development and times to failure are wide-ranging. This fact must be taken into acount for the simulation. Furthermore unpassivated samples were used to enable investigations of the structures by SEM and SPM. Measurements of the size of hillocks and voids has been carried out by AFM. Thereby twin-crystals were observed. This fact indicates that there are special strains, which could be estimated. Another aspect of investigations was the exact measurement of the temperature in the break down area. For that purpose a special test structure with diodes underneath the stressed line was developed. These diodes are used to deduce local temperatures.