Copper Diffusion Characteristics in Single Crystal and Polycrystalline TaN
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N6.11.1
Copper Diffusion Characteristics in Single Crystal and Polycrystalline TaN H. Wang1, Ashutosh Tiwari1, X. Zhang2, A. Kvit1, and J. Narayan1 Department of Materials Science and Engineering, North Carolina State University,
1
Raleigh, NC 27695-7916 2
Materials Science & Technology Division, Los Alamos National Laboratory,
Los Alamos, NM 87544 ABSTRACT TaN has become a very promising diffusion barrier material for Cu interconnections, due to the high thermal stability requirement and thickness limitation for next generation ULSI devices. TaN has a variety of phases and Cu diffusion characteristics vary with different phases and microstructures. We have investigated the diffusivity of copper in single-crystal (NaClstructured) and polycrystalline TaN thin films grown by pulsed laser deposition. The polycrystalline TaN films were grown directly on Si(100), while the single crystal films were grown with TiN buffer layers. Both of poly and single-crystal films with Cu overlayers were annealed at 500oC, 600oC, 650oC, and 700oC in vacuum to study the copper diffusion characteristics. The diffusion of copper into TaN was studied using STEM-Z contrast, where the contrast is proportional to Z2 (atomic number), and TEM. The diffusion distances (2 Dτ ) are found to be about 5nm at 650oC for 30 min annealing. The diffusivity of Cu into single crystal TaN follows the relation D = (160 ± 9.5) exp[−(3.27 ± 0.1)eV / k B T ]cm 2 s −1 in the temperature range of 600oC to 700oC. We observe that Cu diffusion in polycrystalline TaN thin films is nonuniform with enhanced diffusivities along the grain boundary.
INTRODUCTION Interests in structural and electrical properties of tantalum nitride thin films have been stimulated by their promising applications as diffusion barriers in ultra-large-scale integration (ULSI) of Si integrated circuit.1 Recently, Cu has been widely used as interconnect material due to its low resistivity, high electromigration and stress migration resistance, that are superior to Al and Al alloy interconnects.2 However, Cu is very easy to diffuse into SiO2 dielectric and subsequently into silicon region during device fabrication, which is deleterious to device operation. Various diffusion barriers for Cu diffusion have been extensively studied.3-5 But all of these diffusion barriers were around 100nm in thickness. For next generation ULSI devices, the thickness of barrier layers is expected to be within 10 nm, which should prevent Cu diffusion at temperatures above 600oC for 30mins. TaN barrier layers become promising candidates.6 In previous studies, polycrystalline TaN films have been deposited by a variety of techniques: Metal-organic chemical vapor deposition,7 radio-frequency sputtering method 8, DC magnetron sputtering9 and ionized metal plasma10. TaN has different stable phases such as solidsolution α-Ta(N), hcp-γ-phase, and hexagonal ε-phase and metastable phases include bcc β-TaN, hexagonal δ-phase TaN, hexagonal WC structure θ-TaN and B1 NaCl-structured TaN.11,12 These stable and metastable phase
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