Self Forming Barrier Layers from CuX Thin Films Shamon Walker, Erick Nefcy, Samir Mehio Dr. Milo Koretsky, Eric Gunderson, Kurt Langworthy Sponsors: Intel, Oregon Metals Initiative, ONAMI A1 B1 C1 TEM Sample – B1 CuTi (5.6 at. % Ti) Barrier Layer Reaction Study Figure 1. Scanning Electron Microscope (SEM) image of a CuTi (5.6 at.% Ti) alloy annealed for 2 hrs at 500 o C in O 2. Fig 3. SEM image of a CuTi (5.6 at.% Ti) alloy annealed for 2 hrs at 500 o C in UHV w/ pre-anneal air exposure of 30 days. Figure 2. SEM image of a CuTi (5.6 at.% Ti) alloy annealed for 2 hrs at 500 o C in UHV with NO pre-anneal air exposure. Anneal ambient significantly affected CuTi structure. A CuTi phase change was observed for B1 but not for C1. The only difference between B1 and C1 was pre-anneal air exposure. Water molecules from the air adsorbing to the films surface could be the reason why no CuTi phase change was observed in C1. CuX Resistivity Study (X= Ge, Ni, Mn, Ti) Mathematical Modeling: Diffusion of X through Cu CuTi SiO 2 CuTi SiO 2 Cu 7 Ti 2 CuTi SiO 2 Pt CuTi Barrier Layers CuTi SiO 2 Ti x Si Ti y O 2 Possible Oxide Barrier Layer Possible Oxide Barrier Layer Possible Oxide Barrier Layer A simple model for diffusion of X through Cu was formulated using symmetrical boundary conditions. The model was created by combining a material balance and Fick’s 1 st law of Diffusion for a thin slab. The governing equation can be seen below: Interdiffusion of metals into SiO 2 is often observed upon annealing metal overlayers supported on thin SiO 2 films (Dallaporta et al.). It is believed that a large chemical potential gradient facilitates Ti diffusion to the interface where it reduces SiO 2 and forms a titanium oxide compound. Pretorius et al. found that Ti, Zr, Hf, V, and Nb react with SiO 2 to produce oxides and silicides at the interfaces of metal/SiO 2 /Si films. They suggested a direct reaction between a silica film and metal overlayer (M) as follows: The solution to the model can be found below: Project Background Ultra thin diffusion barriers (between Cu and SiO 2 ) are required for super computing. The current deposition orientation for laying interconnect material on an integrated circuit uses a tantalum nitride (TaN)/tantalum barrier to keep SiO 2 separate from Cu: Si/SiO 2 /TaN/Ta/Cu. Project Requirements: 1)4-10 nm barrier layer 2)Film resistivity < 3.0 μΩ-cm 3)No detectable interdiffusion between Cu and SiO 2. Sputtering Process The lower the Gibbs energy of reaction…the larger the spontaneity of the reaction. The data sets with < 0 have a high affinity to reduce SiO 2 and form a metal oxide of their own. All of the Ti oxidation reactions have < 0. Figure 6. A plot of the Gibbs energy of reaction vs. reaction temperature for an assumed redox reaction of M + SiO 2 → M x O y + Si. CuTi and CuMn have a minimum resistivity of 6.9 μΩ-cm and 3.02 μΩ- cm, respectively. These values, however, did not quite satisfy the project requirements of < 3.0 μΩ-cm. However, CuMn was very close. Prolonged pre-anneal air exposure is believed to be the reason behind the peculiar shape of the curves in both figures. Oxygen and water molecules adsorb to the films surface during exposure and eventually form a surface oxide. These ultra thin oxides are highly resistive, and increase the value of the measured resistivity (seen above). Figure 7. A plot showing the affect of anneal time on thin film resistivity of CuMn. One group of samples was exposed to air before annealing and the other was not exposed to air before annealing. All films were 7 at. % X and annealed at 500 o C in Ar at 30 mTorr.. Figure 8. A plot showing the affect of anneal time on thin film resistivity of CuTi. One group of samples was exposed to air before annealing and the other was not exposed to air before annealing. All films were 7 at. % X and annealed at 500 o C in Ar at 30 mTorr.. Figure 9. Plot of Ti concentration in Cu as a function of anneal time and distance from each edge in a 430nm CuTi film. Figure 10. Plot of Ti concentration in Cu as a function of anneal time and distance from each edge in a 430nm CuTi film. Center of Film Center Profile Edge Profile Figure 5. TEM image of CuTi/SiO 2 interface. The CuTi (5.6 at. % Ti) film was annealed for 2 hrs at 500 o C in UHV with NO pre-anneal air exposure. Figure 4. Transmission Electron Microscope (TEM) image of a CuTi (5.6 at.% Ti) alloy annealed for 2 hrs at 500 o C in UHV with NO pre-anneal air exposure. Current Industrial Method: Proposed Method: Pre-anneal deposition orientation: Si/SiO 2 /CuX Post-anneal deposition orientation may be: Si/SiO 2 /Metal Oxide/Cu Pre-anneal air exposure No pre-anneal air exposure In sputtering, a film is grown by the ejection of material from a solid surface following the impact of energetic ions. Modeling of Ti diffusion through Cu CuTi (5.6 at. % Ti) annealed at 500 o C A1 B1 C1 Self forming barrier layers using CuX (where X = Mg, Mn, Ge, Ni, Ti, and Al) AJA Orion IV Sputtering System RF Magnetron 300W dual power supply Two mass-flow controllers (Ar and O 2 ) Maximum substrate temperature: 850 o C Base Pressure ≈ 1E-08 Torr Substrate Rotation Three magnetron sputtering guns References Qiang Fu, Thomas Wagner, Interaction of nanostructured metal overlayers with oxide surfaces, Surf. Sci. 62 (2007) R. Pretorius, J.M. Harris, M.A. Nicolet, Reaction of thin metal films with SiO 2 substrates, Solid State Electron. 21 (1978) H. Dallaporta, M. Liehr, J.E. Lewis, Silicon dioxide defects induced by metal impurities, Phys. Rev. B 41 (1990) 5075 Pre-anneal air exposure No pre-anneal air exposure