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Volume 58 (Aug 23, 2006)
Overview of Papers Presented at IITC 2006 (3/3)
A Novel CoWP Cap Integration for Porous Low-k/Cu Interconnects with NH3 Plasma Treatment and Low-k Top (LKT) Dielectric Structure
Increased leakage current between interconnects has been a persistent problem for CoWP metal cap copper interconnects. Leakage current is effectively suppressed with a combination of post-cap NH3 plasma treatment and a low-k top (LKT) dielectric interlayer structure.
As integrated circuit devices evolve toward smaller features, there is apprehension that the reliability of copper interconnects will begin to deteriorate at 45 nm node geometries and beyond. The conventional copper damascene interconnect structure covers the sides and bottom of the copper with Tantalum (Ta) or other barrier metal, while applying silicon carbonitride (SiCN) to the top. The problem is that copper adheres less well to dielectric than it does to metal, so the copper atoms are prone to diffuse from the top of the interconnect. This causes voids to form, which are cause of device failure (Figure 1). Many processes have been investigated in pursuit of a solution that would enhance the reliability of interconnects. One approach that does indeed improve the reliability of copper interconnects is to apply a cobalt tungsten phosphide (CoWP) cap layer, an alloy providing excellent adhesion for both copper and dielectric, on just the top surface of the copper interconnect, and we believe this will be an effective way to extend the usefulness of copper interconnect processes for some generations to come. We have now confirmed that this technology effectively extends the electromigration (EM) life of devices more than ten-fold (Figure 2).
The main shortcoming of CoWP-based metal-cap technology has been its tendency to increase leakage current between interconnects. This is attributed to the fact that if CoWP electroless plating is done after copper chemical mechanical planarization (CMP), impurities left by the plating at the dielectric interlayer become leakage paths when the CoWP is deposited. We have addressed this problem by adopting an NH3 plasma treatment after the CoWP electroless plating, a step that improves the leakage by more than two orders of magnitude. We found that the time-dependent dielectric breakdown (TDDB) life of devices is also markedly improved by this approach compared to when the NH3 plasma treatment is not adopted (Figure 3).
Observing the difficulty of retaining CoWP plating solution due to the hydrophobicity of the low-k film surface, we focused our attention on a low-k top (LKT) dielectric structure using porous low-k silicon oxycarbide (SiOC) for the top layer. Test results show that, by using this LKT structure, we can effectively reduce leakage to the same level as when the CoWP metal cap layer is not applied. And by combining the LKT structure with the NH3 plasma treatment, we obtain the same leakage current and current versus electrical field characteristics between interconnects as when the CoWP metal cap layer is not applied (Figure 4).
NEC Electronics is committed to pursuing this promising approach using metal-cap processes to improve the reliability of copper interconnects for application to 45 nm node processes and beyond.
Volume 59
