Copper Cap Reduces Leakage, Improves Breakdown Behavior

Amorphous SiC-based dielectrics (α-SiC:H) are starting to replace SiN-based films as etch stop and copper capping layers in low-k dielectric stacks because of their lower k value, better etch selectivity with organosilicate glass (OSG) films, copper barrier properties and passivation properties. Recently, companies have begun optimizing these film stacks for specific integration with OSG films.

For instance, researchers from the National Chiao-Tung University (Hsinchu, Taiwan) and Taiwan Semiconductor Manufacturing Corp. (TSMC, Hsinchu) recently determined that a bilayer of α-SiCN (k~5) and α-SiC (k~4) can improve the time-dependent dielectric breakdown (TDDB) lifetime of copper damascene structures. In their latest work, Chiu-Chih Chiang et al. showed that leakage mechanisms depend on the ratio of the nitride to carbide thickness, but the breakdown field and TDDB exhibited little dependence on the thickness ratio and appears to be caused by breakdown of the bulk OSG dielectric.

In the copper damascene structure with varying thicknesses of α-SiCN/α-SiC bilayer, the large leakage current (Frenkel-Poole emission) between copper lines was attributed to the abundance of interfacial defects at the α-SiC/OSG interface. The tests used 0.12/0.12 µm comb capacitors with OSG, PVD TaN liners, ECD copper and SiCN/α-SiC bilayer of 50/2 nm, 45/5 nm, 40/10 nm or 30/20 nm, followed by PECVD of OSG for the M2 layer.

The researchers determined that the dominant leakage path in the copper damascene structure is electronic current through the bulk of OSG and/or the α-SiC/OSG interface (CMP-surface). Since the SiCN has a compressive stress and the OSG and α-SiC have tensile stress, in the case of thicker α-SiC films, the larger tensile force may generate more interfacial defects such as cracks, voids, traps or dangling bonds at the SiC/OSG interface.

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