Copper Resistivity Fixable for 45 nm Node

Although copper resistivity will remain a challenge for the semiconductor industry, it appears to be fixable for the 45 nm node, according to participants at an industry workshop sponsored by Sematech and Novellus. At the 45 nm technology node, chip designers are likely to use hierarchical design workarounds to modify the metal for linewidths. The looming challenge with copper is that resistance increases dramatically with scaling as wire cross-sections approach the mean free path of electrons (see "How Real Are Size Effects to Scaled Interconnect Performance? " Semiconductor International, July 2005; and "Unraveling the Mysteries Behind Size Effects in Metallization Systems ," Semiconductor International, May 2005).

Commenting on the recently concluded Copper Resistivity Workshop in Burlingame, Calif., Sematech and Novellus interconnect specialists shared insights on results of the recent meeting: "Due to the fundamental laws of physics, copper resistivity is bound to increase and will result in several critical issues that need to be addressed," said Andreas Knorr, conference co-chair and manager of the advanced materials development program in Sematech's interconnect division. "Various process refinements could alleviate perhaps 5-15% of the problem, provided that chip manufacturers are willing to accept added cost and design complexity."

Below 90 nm linewidths, copper resistivity rises dramatically because of increased electron scattering on grain boundaries and interfaces. These resistivity increases can sharply diminish or wipe out the capacitance benefits of low-k dielectric materials, which have long been an industry focus.

"The increase in resistivity of an ultrathin wire was of academic interest long before the first IC," said Ron Powell, conference co-chair and Novellus fellow. "But we have been so successful at scaling down CMOS devices and wiring that we now have to consider the practical impacts of these 'size effects' as well."

"Ironically, the switch from aluminum to copper wiring has accelerated the problem, since size effects show up in copper at closer-in technology nodes. Regardless of how the situation came about, it is likely to be addressed by a synergistic combination of materials, process and design changes," Powell added. "Novellus and Sematech, therefore, conceived of a cross-functional workshop to raise awareness of the problem and drive a solution."

Knorr and Powell said the workshop highlighted two promising "process fixes" that could moderately mitigate the effects of resistivity:

• Minimize the volume that diffusion barriers occupy by making them ultrathin.
• Enlarge copper grains to diminish boundaries and encourage unimpeded electron flow.

1. This narrow trench from a test structure was used to study copper resistivity. (Source: Novellus)

Figures 1 and 2 show the results of copper resistivity studies that evaluated the impact of copper overburden, the electrofill chemistry and the post-electrofill annealing conditions on copper resistivity.

While participants believed that designers using hierarchical design rules will be able to work around the resistivity increase to reach the 45 nm node, they warned that it will be critical to minimize line resistance differences induced by process variation. These differences originate mostly with lack of adequate critical dimension (CD) control, and dishing and erosion problems caused by chemical mechanical polishing (CMP), resulting in line cross-section variations. Also, workshop experts cautioned that reliability in fine lines will be a critical issue because of generally smaller grain sizes, and higher ratios of metal surface area to metal volume.

"The ultimate solutions will probably come in the form of short lines and a move to three-dimensional interconnect," said Sitaram Arkalgud, Sematech's interconnect director. But he added that, despite copper's inherent problems, the workshop revealed scant support for returning to the metal it replaced several years ago.

2. Resistivity data taken from trenches, such as that shown in Figure 1, as a function of trench width. While the resistivity of the copper line does in fact increase, the increase is probably manageable. (Source: Novellus)

Arkalgud said Sematech will continue to guide member companies and the industry in seeking effective resistivity solutions for the subsequent 32 and 22 nm technology nodes.

For more information on wafer processing, go to www.semiconductor.net/wafer.