On the effectiveness of high-entropy alloy/nitride thin films as diffusion barrier for copper metallization

• Main Authors: Ming-Hung Tsai, 蔡銘洪
• Other Authors: Jien-Wei Yeh
• Format: Others
• Language: en_US
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/69062958620625376955

博士 === 國立清華大學 === 材料科學工程學系 === 96 === The aim of this study is to test the effectiveness of high-entropy alloy/nitrides as diffusion barrier layer for copper metallization. A new HEA composition, AlMoNbSiTaTiVZr, is specifically designed for this purpose. We first deposited various (AlMoNbSiTaTiVZr)Nx films with different nitrogen concentration, and investigated their structure and mechanical properties. Then three high-entropy alloy/nitride films including AlMoNbSiTaTiVZr, (AlMoNbSiTaTiVZr)50N50, and NbSiTaTiZr were tested to understand their effectiveness as diffusion barriers. In the characterization of (AlMoNbSiTaTiVZr)Nx, we found that the concentration of nitrogen increases rapidly at lower RN and then saturates near the stoichiometric composition at RN = 33% and above. At lower RN (0% and 3%), film structures are amorphous and their cross-sectional microstructures are featureless. At RN of 11%, FCC nitride nanocrystals coexist with amorphous phase. At higher RN ( ≧ 33% ), films exhibit single FCC NaCl-type structure having fine column structures with nanograins. Compared with other published HEA films prepared without bias, AlMoNbSiTaTiVZr exhibits the highest hardness and modulus both in its alloy and nitride states up to now. This confirms the effectiveness of the present alloy design. High-entropy alloy concept could provide another route to enhance mechanical properties in addition to the route of nanocomposite design. As for the effectiveness of high-entropy alloy/nitride as diffusion barrier, standard Cu/barrier/Si test structures were fabricated and annealed at different temperature for 30 min. The results show that a 100 nm AlMoNbSiTaTiVZr alloy film withstands 700 °C anneal. (AlMoNbSiTaTiVZr)50N50 films, 70 nm in thickness prevent the reaction up to 850 °C, which ranks second in all the published barrier materials. NbSiTaTiZr films, 20 nm in thickness is effective up to 800 °C. These results demonstrate that high-entropy alloy/nitride possess superior properties in this regard and have good resistance to the invasions from both sides, namely the silicidation reaction from one side and Cu in-diffusion from the other. High-entropy alloy/nitride have good resistance to silicidation because the atomic pairs in them have large enthalpy of mixing and the multi-element design brings in large entropy of mixing, which both lowers their free energy. As for the resistance against Cu in-diffusion, we suggest that it originates from the marked thermal stability and high packing density of its amorphous structure, which avoids the formation of crystalline phase and thus fast-diffusing grain boundaries. Therefore, high entropy alloy/nitride has excellent performances and thus this multi-element design concept may bring new insights to the design of future barrier materials. Moreover, it was found that on many failed samples there exists many directional silicide line patterns. Such patterns were also reported by many authors, but none has tried to explain such phenomenon. We therefore proposed a model to describe the formation of these silicide patterns. The invasion of Cu begins from discontinuous points of the barrier layer. Cu then diffuses outwards in the Si single crystal from the break point. Due to the diffusion anisotropy in Si, Cu concentration develops faster along Si <110> and <100> and thus new silicides nucleates preferentially on these directions. Directional silicide line segments are therefore formed. When new silicides grow large enough they protrude from the substrate and break the barrier overlayer. Such barrier deformation and cracking offers new pathways for Cu, producing a new Cu source. In this way the growth of directional silicide line segments becomes self-sustaining. Moreover, the existence of directional silicide line segments indicates that the barrier failure is owing to extrinsic defects instead of the intrinsic disfunction of the barrier itself. Therefore, to obtain the intrinsic failure temperature of a barrier material one should pay extreme attention to the uniformity of the barrier film and the oxygen content in the annealing atmosphere.