Diffusion Characteristics of Vacancies in Aluminum Interconnects
- PDF / 825,844 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 33 Downloads / 214 Views
DIFFUSION CHARACTERISTICS OF VACANCIES IN ALUMINUM INTERCONNECTS. A. S. NANDEDKAR Technology Modeling Department, IBM, East Fishkill Laboratory, Z/47C, Hopewell Junction, NY 12533, USA. ABSTRACT Computer simulations were performed to investigate the migration energy of vacancies in Al in the presence of grain boundary, dislocation and various imposed forces (such as stress gradients and electro-migration). The energy values were significantly smaller when the vacancy moved into the grain boundary or a dislocation core than in opposite direction. This makes these defects sinks for vacancies from which they are unlikely to migrate out. The major time required for the movement of vacancies is thus limited to within a single grain. Calculations of migration time from the center of the grain to the grain boundary gives time needed for void formation. Calculations of this time assuming an electrical potential gradient along the metal line gave estimates consistent with experimental observations. The increased values of migration energy under compressive stresses suggest that void formation could be deterred by applying such stresses. INTRODUCTION Integrity of the Aluminum interconnects is vital to the performance of devices. The interconnects are known to fail when large voids spreading through the cross section of the metal line are formed. Therefore, it is of interest to study the nucleation and growth of voids so that their formation could be controlled, if not prevented. The problem of void formation can be divided in to two areas; nucleation and growth. The nucleation involves structural changes at atomic level. Therefore experimental and continuum theoretical models (for example [2 - 10]) are not best suited to investigate it. We have used atomistic computer simulation in which an Aluminum cell containing vacancies was constructed [I]. The position of atoms and vacancies was swapped to obtain minimum energy configurations in the presence of dislocations, grain boundary and various stress levels. Minimum energy configurations were always obtained when the vacancies had migrated in to the grain boundary or to the interface between metal line and passivation, thus forming a void. We concluded that voids may nucleate by agglomeration of vacancies at the grain boundary, external surface and dislocations. This is consistent with experimental observations that also show voids being formed at the grain boundary and external surface [6,9]. The above study was performed using static and Monte Carlo energy minimization techniques and thus did not provide any information about the time and temperature relationships to void formation. Therefore the objective of this study was to investigate the dynamics of vacancy movement. Atomic transport in metal lines is known to be affected by several parameters such as temperature and stress gradients, electro-migration, supersaturation of vacancies, alloying, defects such as grain boundaries and dislocations in the thin metal lines etc. [ 11]. A long range transport of vacancies has been
Data Loading...
