Laser linking of metal interconnects: analysis and design considerations

Lateral connections between adjacent lines of metallization have been developed in order to achieve high density linking for customization in programmable gate arrays and for additive redundancy in restructurable integrated circuits. Links were formed by focusing a pulsed laser between two same-level aluminum lines. The mechanism of link formation appears to be the nucleation of a fissure, induced by the thermal expansion mismatch between the metallization and the surrounding dielectric (SiO₂) and passivation layer (Si₃N₄); molten aluminum fills the crack. Numerical simulation by the finite element method was carried out using a plane strain model. The probable path for the link-forming fissure, as predicted by the model on the premise that the local maximum tensile stress determines cracking, is shown to be consistent with experimental observations. Parametric analyses were performed to gain insights into the linking processes. It is found that damage in the passivation can be avoided by increasing the thickness of the dielectric between the aluminum and the passivation. Reducing the spacing between the metal lines increases the chance of successfully forming the link. Under certain conditions, the linking propensity can also be increased by reducing the metal width. In addition, the link is much easier to form when symmetric laser heating between the two metal lines can be achieved. These findings can be directly applied to improving the design of the laser linking processes and devices