Sequential 3D Integration is stacking sequential silicon layers via epitaxy and bonding on single substrate for monolithic 3D circuits — true integration without post-CMOS assembly. Layer Growth silicon epitaxy deposits crystalline silicon atop first layer dielectric. Epitaxy Temperature silicon growth ~600-800°C; below copper reflow (~800°C). Process selection critical. Silicon Quality epitaxial silicon may have defects; gettering and annealing improve. Gate Sequencing gate-first: pattern gates, subsequent layers. Gate-last: maximum flexibility, higher complexity. Oxide Interface Si-SiO₂ interface quality critical (Dit, Nss); engineering improves interface. Via Metallurgy tungsten or copper vias connect tiers. Silicide formation, contact resistance important. Planarization CMP after each layer growth before next deposition. Interconnect Distribution traditional single-tier multiple metals → distributed across tiers. Fewer layers per tier. Via Density sub-100 nm vias possible at advanced nodes. Lithography each layer requires alignment (~50 nm overlay tolerance). Defect Impact first-layer defects foundational; yield concern. Manufacturing Variation temperature, pressure, contamination affect yield. Stress Migration metal EM reduced from shorter wires. Benefit. Thermal Dissipation stacked transistors generate heat; lower tiers dissipate upward. Seq-3D enables monolithic stacking with integrated sequential layer processing.