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A Study on the Epitaxial Growth of Low-Resistivity Nickel-Cobalt Alloy Films for Advanced Interconnect Applications

  • 성주영 (아주대학교 일반대학원)

초록/요약

The continuous scaling of logic and memory devices has reduced metal interconnect dimensions to the nanometer scale, resulting in a significant increase in resistivity and reliability challenges. Although copper （Cu） has been widely used as the dominant interconnect material because of its low bulk resistivity （ρ0）, its long electron mean free path （EMFP, λ） of 39 nm leads to enhanced surface and grain boundary scattering when interconnect dimensions decrease below 10 nm. Therefore, materials with a short EMFP and low bulk resistivity should be selected to reduce the resistivity size effect in scaled interconnects. In addition, high cohesive energy and process compatibility are required for advanced interconnect applications. This dissertation investigates low resistivity metal alloy thin films as post-Cu interconnect materials, focusing on nickel-cobalt （NiCo） and cobalt-molybdenum （CoMo） alloy systems. First, NiCo alloy thin films were investigated to reduce the resistivity size effect. Density functional theory （DFT） calculations predicted that hexagonal-close-packed （HCP） NiCo has a lower ρ0∙λ value of 5.68×10 -16 Ωm2 than Cu, mainly due to an EMFP of approximately 5 nm. To evaluate the thickness dependence of the resistivity in NiCo films, epitaxial growth with a controlled crystal structure was required. However, face-centered-cubic （FCC） and HCP phases coexist in the NiCo alloy system, which limits the formation of single-phase epitaxial films. Therefore, seed layers were introduced to control the FCC and HCP phases of NiCo. In particular, a Co-rich seed layer reduced the lattice mismatch with the sapphire substrate and enabled the epitaxial growth of single-phase HCP NiCo films. The seed layer approach also enabled the formation of FCC NiCo films on sapphire substrates, confirming that the crystal structure of NiCo can be controlled through substrate and seed layer selection. The resulting HCP NiCo films exhibited a resistivity of 19.83 μΩ∙cm at a thickness of 4.9 nm and showed lower resistivity than Cu below 8 nm. To investigate the origin of the reduced resistivity size effect, their resistivity variation with thickness was analyzed using the Fuchs- Sondheimer （FS） and Mayadas-Shatzkes （MS） models. The fitting results indicated that the EMFP of HCP NiCo was approximately 5 nm, consistent with the value estimated from the DFT-calculated ρ0∙λ. This result indicates that the reduced resistivity size effect of HCP NiCo can be explained by its short EMFP. The NiCo films also showed thermal stability and dry etching feasibility, indicating their process compatibility for advanced interconnect applications. Second, CoMo alloy thin films were investigated as a follow-up material with a lower calculated ρ0∙λ value. DFT calculations predicted that body-centered cubic （BCC） CoMo has a ρ0∙λ value of 4.60×10 -16 Ωm2, which is lower than those of Cu and HCP NiCo. Based on this result, BCC CoMo thin films were expected to show a reduced resistivity size effect. However, single-phase BCC CoMo thin films were difficult to obtain without additional growth control because CoMo thin films grown directly on sapphire substrates remained amorphous. Therefore, a molybdenum （Mo） seed layer was introduced between the sapphire substrate and the CoMo layer to reduce the lattice mismatch with BCC CoMo and promote its crystallization. The Mo seed layer promoted the formation of BCC CoMo thin films by reducing the lattice mismatch with BCC CoMo. However, the high temperature required for crystallization and excessive CoMo thickness caused interdiffusion at the CoMo/Mo seed-layer interface and increased surface roughness. Therefore, the process temperature and film thickness were controlled to reduce these structural challenges while maintaining BCC CoMo crystallization. Under the optimized growth condition, the CoMo thin films showed improved crystallinity and reduced interdiffusion at the interface, resulting in the epitaxial growth of single-phase BCC CoMo thin films on the sapphire substrate. Thickness-dependent resistivity analysis indicated that the EMFP of BCC CoMo was approximately 5 nm, consistent with the calculated ρ0∙λ value. However, the experimentally measured bulk resistivity remained higher than expected. This result was attributed to alloy disorder scattering associated with the electronic structure mismatch between Co and Mo. Overall, this dissertation demonstrates that low resistivity metal alloy thin films with short EMFPs can reduce the resistivity size effect in scaled interconnects. The NiCo alloy system maintained low resistivity even at thicknesses below 10 nm through phase-controlled epitaxial growth, whereas the CoMo alloy system revealed that electronic structure control is also required in addition to crystallinity improvement. These results provide material selection and thin film growth strategies for advanced interconnect applications beyond Cu.

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목차

Chapter 1. Introduction 1
1.1 Interconnect scaling challenges 1
1.2 Resistivity size effect in metallic interconnects 9
1.3 Alternative interconnect materials 16
Chapter 2. Experimental 29
2.1 Thin film deposition 29
2.2 Thin film characterization 31
2.3 Electrical characterization 33
2.4 First-principles calculations 34
Chapter 3. NiCo alloy systems 37
3.1 Selection of Ni-based alloy systems 37
3.2 DFT calculation of FCC and HCP NiCo alloys 39
3.3 Substrate selection for epitaxial growth 45
3.4 Growth condition and phase formation of NiCo thin films 51
3.5 Seed layer strategy for single-phase NiCo thin films 56
3.6 Epitaxial growth and structural analysis of single-phase NiCo thin films 59
3.7 Resistivity size effect and diffusion reliability of NiCo thin films 67
3.8 Conclusion 73
Chapter 4. CoMo alloy systems 75
4.1 Selection of CoMo alloy system 75
4.2 Substrate selection and growth of Mo thin films 77
4.3 Crystallization of CoMo alloy thin films 82
4.4 Interdiffusion, surface roughness, and optimized epitaxial growth 89
4.5 Resistivity size effect and alloy disorder scattering 98
4.6 Conclusion 100
Chapter 5. Summary and Conclusions 102
References 106

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