Ruthenium (Ru) is an emerging interconnect metal for advanced nodes where its properties outperform copper at the smallest dimensions, addressing the resistivity scaling crisis in BEOL. Why Ru: at line widths below ~15nm, Cu resistivity increases dramatically due to electron scattering at grain boundaries and surfaces (mean free path of Cu ≈ 39nm). Ru has shorter mean free path (~6nm), so resistivity increases less at small dimensions. Key advantages: (1) Lower effective resistivity at sub-15nm widths—Ru becomes competitive with or better than Cu; (2) No barrier needed—Ru is a self-barrier (doesn't diffuse into dielectric like Cu), saving barrier volume that otherwise reduces Cu volume; (3) No seed layer—Ru can be deposited directly by CVD/ALD; (4) Better electromigration—higher melting point and stronger bonding. Integration approach: (1) Subtractive etch—deposit Ru blanket film, pattern with hard mask, etch (vs. Cu damascene); (2) Damascene—fill trenches with Ru using CVD, CMP; (3) Hybrid—Ru for narrow lines (local interconnect), Cu for wider lines (semi-global/global). Deposition: CVD or ALD using Ru precursors (carbonyls, amidinates), achieving conformal fill. Etch: Ru etching uses O₂-based plasma chemistry (forms volatile RuO₄). Challenges: (1) Ru deposition cost—expensive precursors; (2) CMP—Ru harder to polish than Cu; (3) Etch—requires new etch chemistry development; (4) Integration—interface engineering with adjacent materials. Industry status: under active development at Intel, TSMC, Samsung for 2nm and beyond—Ru likely for M1/M2 (tightest pitch), Cu retained for upper metals. Part of broader BEOL material transition including molybdenum and alternative barrier approaches.