Transcript Slide 1
Self Forming Barrier Layers from CuX Thin Films Project Background Shamon Walker, Erick Nefcy, Samir Mehio • Ultra thin diffusion barriers (between Cu and SiO2) are required for super computing. • CuTi and CuMn have a minimum resistivity of 6.9 μΩ-cm and 3.02 μΩcm, respectively. These values, however, did not quite satisfy the project requirements of < 3.0 μΩ-cm . However, CuMn was very close. Dr. Milo Koretsky, Eric Gunderson, Kurt Langworthy Sponsors: Intel, Oregon Metals Initiative , ONAMI Current Industrial Method: •The current deposition orientation for laying interconnect material on an integrated circuit uses a tantalum nitride (TaN)/tantalum barrier to keep SiO2 separate from Cu: CuX Resistivity Study (X= Ge, Ni, Mn, Ti) Affects of Pre-anneal Air exposure on Resistivity CuTi (5.6 at. % Ti) Barrier Layer Reaction Study 35 30 Pre-anneal air exposure Resistivity (μΩ-cm) 25 Si/SiO2/TaN/Ta/Cu. 20 Pure Cu 325nm CuMn 15 CuMn (no air exposure) 10 No pre-anneal air exposure CuTi A1 5 0 Possible Oxide Barrier Layer 0 0.5 CuTi Pre-anneal air exposure 45 40 35 Possible Oxide Barrier Layer Cu7Ti2 Resistivity (μΩ-cm) B1 A1 B1 C1 SiO2 30 Pure Cu 325nm 25 CuTi 20 CuTi (no air exposure) 15 No pre-anneal air exposure 10 Figure 2. SEM image of a CuTi (5.6 at.% Ti) alloy annealed for 2 hrs at 500oC in UHV with NO pre-anneal air exposure. 5 0 0 0.5 1 1.5 Possible Oxide Barrier Layer SiO2 Fig 3. SEM image of a CuTi (5.6 at.% Ti) alloy annealed for 2 hrs at 500oC in UHV w/ pre-anneal air exposure of 30 days. TEM Sample – B1 3.5 4 -100 MnO -200 NiO -300 GeO2 -400 MnO2 200 400 600 800 1000 Mn2O3 i 1 g f h f T s f Mn3O4 z0 Temperature (K) C X z,t 2 n 4 CX ,o 1 n z L D XCu t sin e n1 n L • Pretorius et al. found that Ti, Zr, Hf, V, and Nb react with SiO2 to produce oxides and silicides at the interfaces of metal/SiO2/Si films. They suggested a direct reaction between a silica film and metal overlayer (M) as follows: Figure 5. TEM image of CuTi/SiO2 interface. The CuTi (5.6 at. % Ti) film was annealed for 2 hrs at 500oC in UHV with NO pre-anneal air exposure. [Ti] (#/nm3) • Interdiffusion of metals into SiO2 is often observed upon annealing metal overlayers supported on thin SiO2 films (Dallaporta et al.). It is believed that a large chemical potential gradient facilitates Ti diffusion to the interface where it reduces SiO2 and forms a titanium oxide compound. 5 6 4 2 0 315 15 z (nm) Time (hr) Figure 9. Plot of Ti concentration in Cu as a function of anneal time and distance from each edge in a 430nm CuTi film. 4 3 2 1 0 165 3.50 CuTi Center Profile Center of Film 2.00 M SiO2 M x Si M yO2 for n 1,3,5,... • CuTi (5.6 at. % Ti) annealed at 500oC 0.00 0.01 TiyO2 Modeling of Ti diffusion through Cu [Ti] (#/nm3) TixSi • The solution to the model can be found below: 1.00 SiO2 2 CX CX (t ) DXCu 2 z t 0.67 Figure 4. Transmission Electron Microscope (TEM) image of a CuTi (5.6 at.% Ti) alloy annealed for 2 hrs at 500oC in UHV with NO pre-anneal air exposure. • The lower the Gibbs energy of reaction…the larger the spontaneity of the reaction. • The data sets with grxn < 0 have a high affinity to reduce SiO2 and form a metal oxide of their own. • All of the Ti oxidation reactions have grxn < 0. 0.42 AJA Orion IV Sputtering System 0.19 Pt • 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 1st law of Diffusion for a thin slab. The governing equation can be seen below: 0.02 Figure 6. A plot of the Gibbs energy of reaction vs. reaction temperature for an assumed redox reaction of M + SiO2 → MxOy + Si. zL Edge Profile 3.50 TiO2 2.50 0 1.50 TiO 1.00 100 g rxn i g f ,i 0.75 200 MgO 0.58 Ti2O3 0.42 300 0.28 Ti3O5 0.14 400 n 0.02 CuTi Al2O3 500 • 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). 0.01 Gibbs Energy of Reaction (kJ/mol) Barrier Layers •Qiang Fu, Thomas Wagner, Interaction of nanostructured metal overlayers with oxide surfaces, Surf. Sci. 62 (2007) 431-498 •R. Pretorius, J.M. Harris, M.A. Nicolet, Reaction of thin metal films with SiO2 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 3 Mathematical Modeling: Diffusion of X through Cu Gibbs Energy of Reaction for M + SiO2 → MxOy + Si SiO2 References 2.5 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 500oC in Ar at 30 mTorr.. CuTi C1 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. • In sputtering, a film is grown by the ejection of material from a solid surface following the impact of energetic ions. • RF Magnetron 300W dual power supply • Two mass-flow controllers (Ar and O2) • Maximum substrate temperature: 850oC • Base Pressure ≈ 1E-08 Torr • Substrate Rotation • Three magnetron sputtering guns 2 Anneal Time (hours) • • • • Sputtering Process 4 50 0.01 Si/SiO2/Metal Oxide/Cu 3.5 0.00 • Post-anneal deposition orientation may be: 3 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 500oC in Ar at 30 mTorr.. (where X = Mg, Mn, Ge, Ni, Ti, and Al) 1) 4-10 nm barrier layer 2) Film resistivity < 3.0 μΩ-cm 3) No detectable interdiffusion between Cu and SiO2. 2.5 Affects of Pre-anneal Air exposure on Resistivity • Self forming barrier layers using CuX Si/SiO2/CuX 2 Anneal Time (hours) Proposed Method: Project Requirements: 1.5 SiO2 Figure 1. Scanning Electron Microscope (SEM) image of a CuTi (5.6 at.% Ti) alloy annealed for 2 hrs at 500oC in O2. • Pre-anneal deposition orientation: 1 Time (hr) Figure 10. Plot of Ti concentration in Cu as a function of anneal time and distance from each edge in a 430nm CuTi film.