Stacked Contact Electromigration in advanced SiGe heterojunction bipolar transistors

Contact Electromigration (EM) constraining the performance for a SiGe hetero-junction bipolar transistor (HBT) is investigated. By incorporating a stacked contact structure with Via, the time-to-fail (TTF) increases by ten-fold and contact EM current density (J_{C_CNT_EM}) increases by three-fold. These improvements are shown in experiments incorporating a space between contact and Via, a multi-level stack, and multi-Via stack effects. Also this result can be theoretically explained by a restoring force between contact and Via called Blech effect and unidirectional current flow. Consequently, J_{C_CNT_EM} is no longer limited in a safe operating area (SOA) of a HBT and EM is regulated by only the top metal in a stacked contact structure. This allows us to design an EM enhanced HBT primitive cell without compromising device performance and to eliminate processing problems caused by rectangular contact.