Finding the Ultimate Copper Barrier and Seed
-- Semiconductor International, 7/1/2001
In efforts to extend copper interconnect performance, device manufacturers are evaluating a wide array of barrier/liner materials — including Ti, Ta and W nitrides and ternary compounds, deposited by ionized PVD, CVD or the latest, atomic layer deposition (ALD) methods. At IEEE's 4th annual International Interconnect Technology Conference (IITC) in June, engineers and researchers also investigated the extendibility of copper PVD seed, with CVD and ALD alternatives.The industry is debating the seemingly obvious transition from PVD to CVD for barrier and seed layers, which get thinner with each device generation. Steven Rossnagel of IBM (Yorktown Heights, N.Y.) estimated that today's ionized PVD processes should extend PVD's use for the next two technology nodes, when CVD or ALD may become necessary. The barrier and seed layer challenge lies in obtaining good coverage along high aspect ratio (AR) sidewalls, while coating the bottoms of unlanded vias and limiting feature overhang at the top that prevents good ECD fill. CVD's clear advantage is more conformal coverage of thinner films.
IBM explored ALD, with its self-limiting pulsed deposition (using TaCL5 or TiCl4 precursors and H2), which allows the atomic layer control at moderate temperatures (0.5-2 Å/pulse at <200°C). Challenges to ALD barriers include understanding the reaction of ALD films with various substrates (including porous low-k dielectrics), optimizing the reaction chemistry and controlling Cl-contamination in the film. Researchers from the University at Albany (Albany, N.Y.) added the importance of preventing parasitic CVD reactions in the ALD chamber.
Depending on the process and tooling, today's ionized PVD processes, extended using hollow-cathode magnetron or planar self-ionizing geometries, may hit a wall beyond 0.18 µm vias at 4:1 AR. Researchers from IMEC (Leuven, Belgium) showed process optimization of ionized PVD layers of Ta and TaN that allows good sidewall coverage, limited overhang and no coverage at the feature bottom, an advantage for future device generations.
IBM's Dan Edelstein explained that IBM is currently using an ionized PVD bilayer approach (TaN/b-Ta/Cu). Here, the TaN layer ensures adhesion to SiO2 and the b-Ta layer provides the best adhesion to copper, along with current strapping to improve protection against electromigration. But even ionized PVD may not be capable of filling >4:1 AR, 0.10 µm features. A tool change to MOCVD or ALD is likely because the industry is rapidly approaching the time when displacement of even a single atomic layer of copper could raise via and line resistance by several percentage points, a trend that becomes increasingly severe. Rossnagel expressed reservations with CVD barriers and seeds, due to difficulties in simultaneously optimizing for reaction rate, conformality, nucleation and film purity.
But researchers from Intel (Hillsboro, Ore.) demonstrated success in using MOCVD of copper — using (hfac)Cu(tmvs) — on a variety of barriers (Table) and the determination of the thinnest possible copper seed layer (tcrit) that allows continuous, higher AR fill without pinch-off at the top of the feature. The tests revealed that tcrit as thin as 100 Å can be produced by depositing 50-350 Å copper seeds on an intermediate adhesion layer (PVD Cu or CVD Co), which minimizes tcrit. The copper on a TaN/Co bilayer was continuous at 120 Å. But 100 Å of copper was continuous on a TiN/PVD-Cu bilayer, yielding a total barrier/seed thickness of only 140-210 Å.
| CVD COPPER MICROSTRUCTURAL AND MECHANICAL PROPERTIES | ||||
| CVD Barrier | RMS Roughness (nm) | Cu Texture | Relative F-contamination (cm-2) | Adhesion |
| WN | 4.91 | Weak | 2×105 | Marginal |
| TaN | 5.27 | Random | 3×104 | Fail |
| TaN+Co | 3.13 | Random | 3×104 | Pass |
| TiN | 4.37 | Random | 3×104 | Marginal |
| TiN+PVD Cu | 4.29 | Strong <111> | nd | Pass |
| (Source: Intel) | ||||
The researchers hypothesized that the conductive nucleation layer may act as a catalyst to increase the reactivity of the copper precursor, while improving grain orientation and surface roughness. Only the PVD-Cu nucleation layer led to a strong <111> copper film needed for electromigration resistance and successfully passed adhesion tests.
Interest in TaSiN and TiSiN barriers is also growing, mostly due to the fact that these films may be more robust in manufacturing. Unlike Ta, TaN and TiN, TiSiN and TaSiN adhere well to copper in the presence of thin oxidized layers, the inevitable result of exposure to ambient. Engineers from the University of North Texas (Denton) demonstrated good adhesion of PVD TaSiN to PVD copper and a reduced tendency to copper agglomeration. In a different study, engineers from Dow Chemical (Midland, Mich.) demonstrated the compatibility of TaSiN barrier with sputtered Cu and SiLK low-k dielectric.
— Laura Peters
