Sony Chooses CuAg as Prime Candidate for Reliable Interconnects

Reliability enhancement for copper interconnects is central to their continued use at the 45 nm and future technology generations. Reliability issues such as electromigration and stress migration are enhanced with further geometric scaling and tighter aspect ratios of copper wires. In particular, stress-induced voiding is often at its worst at the junction between long copper lines and underlying vias, where thermal stress gradients induce vacancies to gather and form voids. However, Sony researchers have found that, by using light doping of silver (1% CuAg) in the copper seed, stress-induced voiding can be significantly suppressed while maintaining excellent electrical results in two-level copper interconnects. They concluded it is the most feasible candidate material for reliability enhancement for the 45 nm node and beyond.

Atsunobu Isobayashi and coworkers from Sony Corp. and Sony Computer Entertainment (Kanagawa, Japan) will report on their findings at the IEEE International Electron Devices Meeting this month in a paper titled, "Thermally Robust Cu Interconnects With Cu-Ag Alloy for sub 45 nm Node." They will demonstrate how the CuAg, deposited by PVD, provided only a 3% increase in film resistivity and tight distribution of resistivity, while delivering similar adhesion to the tantalum barrier as copper, slightly smaller grain size and less softening of the copper films.

The process integration scheme used two-level interconnects with 0.28 µm pitch, SiOC interlevel dielectric (k=3.0) and SiCN etch stop layer (k=5.0). The 60 nm CuAg seed was deposited by PVD (1 wt% Ag) and SIMS depth profiling was performed after CMP and annealing at 370°C for 60 minutes. SIMS analysis showed the silver diffused to the copper wiring from the seed layer, but mostly segregated at the interface with the barrier metal.

The Figure shows the grain size and distribution of the copper and CuAg films after annealing. Presence of the silver did suppress grain size growth slightly, which is quantified in the Table . The Sony researchers also noted that the CuAg films had improved softening temperature of the film to 330°C, relative to 220°C for pure copper.

Grain size distribution of pure copper (left) and CuAg films (right) shows the suppression of grain growth. Both films 25 nm thick on tantalum showed no agglomeration after 450°C for 2 hour annealing, and demonstrated similar adhesion to the barrier metal.

Various test patterns were used to examine the electrical properties. Any alloy of copper will increase resistivity. For instance, the increase for CuAl can be as high as 19%. Silver has a relatively small impact. Measured rise in sheet resistance at overall linewidth was <3%.

The I-V characteristics and line-to-line leakage measurements suggest that CuAg is compatible with pure copper at the point of CMP copper, and that no migration of silver into the dielectric layer occurred. The 90 nm node test patterns used 0.14 µm open/short patterns (9.7 m) for sheet resistance and line-to-line leakage. Via resistance was measured using 2.9 m chains.

Stress-induced voiding generally forms at the interface between copper and the barrier metal in the via. Stress-induced voiding test results of single via structures of wide upper metal lines after 225°C and 504 hours of annealing showed no degradation after thermal stress relative to the copper seed control. The resistance shifts of the wiring with CuAg seed showed superior results. The control showed yield drops at the patterns even without design rule restrictions. The dimensions were M1 = 1, 2 or 5 µm, and M2 = 25 or 35 µm.

The researchers found that CuAg has excellent creep resistance. The mechanism by which the silver acts to reduce stress-induced voiding phenomena is believed to be related to the segregation of silver at the interface with the barrier metal and low concentrations (~0.1 wt% Ag) that migrate into the via, thereby inhibiting copper migration to the upper wider lines.

A great advantage to the CuAg seed approach is its ease of integration into the standard manufacturing flow. CuAg's high creep resistance enables suppression of stress-induced voiding that is not possible with pure copper seed. This process modification holds great promise as researchers investigate various approaches to improving the reliability of copper interconnects for future device generations.