BEOL

BEOL stands for Back-End of Line, a phase in semiconductor fabrication that begins after the front-end of line (FEOL) processes have formed the transistor structures. BEOL encompasses all steps needed to realize the metal interconnect network that wires transistors and connects devices to outside terminals. The work typically involves the deposition of dielectric layers, lithography and etching to form vias and trenches, metallization to fill interconnects, and planarization, often by chemical mechanical polishing, to produce a smooth, multi-level interconnect stack. The interconnect stack is wired through multiple metal layers, separated by interlayer dielectrics, with vias and contact plugs creating electrical connections between layers and to the device contacts.

Historically aluminum interconnects transitioned to copper in the 1990s, driven by copper's lower resistivity and better electromigration resistance. Modern BEOL commonly uses copper damascene or dual-damascene processes, diffusion barrier and seed layers (for example titanium or tantalum nitrides as barriers, copper seed), and advanced deposition methods. Low-k dielectric materials reduce capacitance between adjacent lines to improve performance, though they pose mechanical and reliability challenges. BEOL must also manage thermomechanical stress, integration with FEOL transistor structures, and reliability concerns such as electromigration, time-dependent dielectric breakdown, and corrosion or delamination.

Packaging and final test are downstream of BEOL, but increasingly, some BEOL considerations influence package interconnect and 3D integration. BEOL performance is a major determinant of overall circuit speed and power delivery, with scaling driven by interconnect density and material advances.