Back end of line

Back end of the line or back end of line (BEOL) is a process in semiconductor device fabrication that consists of depositing metal interconnect layers onto a wafer already patterned with devices.

It is the second part of IC fabrication, after front end of line (FEOL).

BEOL includes contacts, insulating layers (dielectrics), metal levels, and bonding sites for chip-to-package connections.

For modern IC processes, more than 10 metal layers can be added in the BEOL.

Before 1998, practically all chips used aluminium for the metal interconnection layers, whereas copper is mostly used nowadays.

The BEOL process deposits metalization layers on the silicion to interconnect the individual devices generated during FEOL (bottom).
CMOS fabrication process