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Airgaps in nano-interconnects: Mechanics and impact on electromigration
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In this study, electromigration (EM) of interconnects (90 nm pitch) with airgaps was investigated using a combination of computational mechanics, analytical modelling, and EM experiments. EM experiments reveal that airgapped Cu lines without dielectric liner (non-capsulated) fail early by voiding in the EM tests due to oxidation and deterioration of interfacial adhesion at Cu
interfaces. Also at high temperature regimes, extrusive failures under thermal compressive stresses were observed in airgapped Cu lines without dielectric liner. Therefore, Cu encapsulation using a conformal dielectric liner of adequate thickness is necessary in order to ensure hermeticity and provide endurance to the thermal and EM induced extrusive stresses. For an airgapped interconnect with a hermetic 5 nm PECVD conformal carbon doped silicon nitride (SiCN) liner deposited at 370 °C, a comparable to that of non-airgapped interconnects (with ultra-low-k dielectric 2.5 inter-layer dielectric) was predicted by the simulations. The process-oriented simulations reveal, however, that the tensile stress in Cu lines increases linearly with the thickness of the SiCN liner. Therefore, increasing the thickness of the dielectric liner beyond the minimum thickness required for hermeticity was found to impact the critical line length adversely.
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