Back-end-of-line (BEOL) denotes the second portion of IC fabrication where the individual devices (transistors, capacitors, resistors, etc.) get interconnected with wiring on the wafer. BEOL generally begins when the first layer of metal is deposited on the wafer. It includes contacts, insulating layers (dielectrics), metal levels, and bonding sites for chip-to-package connections.
After a FEOL step there is a wafer with isolated transistors (without any wires). In BEOL part of fabrication stage contacts (pads), interconnect wires, vias and dielectric structures are formed. For modern IC process, more than 10 metal layers can be added in the BEOL.
The process used to form DRAM capacitors creates a rough and hilly surface, which makes it difficult to add metal interconnect layers and still maintain good yield. In 1998, state-of-the-art DRAM processes had 4 metal layers, while state-of-the-art logic processes had 7 metal layers. [1]
As of 2002, 5 or 6 layers of metal interconnect are common.[2]
As of 2009, typical DRAM devices (1 Gbit) use 3 layers of metal interconnect, tungsten on the first layer and aluminum on the higher layers.[3][4]
As of 2011, many gate arrays are available with a 3-layer interconnect.[5] Many power ICs and analog ICs use a 3-layer interconnect.[6]
The top-most layers of a chip have the thickest and widest and most widely-separated metal layers, which make the wires on those layers have the least resistance and smallest RC time delay, so they are used for power distribution and clock distribution. The bottom-most metal layers of the chip, closest to the transistors, have thin, narrow, tightly-packed wires, used only for local interconnect. Adding layers can potentially improve performance, but adding layers also reduces yield and increases manufacturing cost. [7]
The AMD Athlon Thunderbird has 6 interconnect layers, the AMD Athlon Palomino has 7 interconnect layers, the AMD Athlon Thoroughbred A has 8 interconnect layers, and the AMD Athlon Thoroughbred B has 9 interconnect layers.[8] The Intel Xeon Dunnington has nine copper interconnect layers.[9]
Steps of the BEOL:
Before 1998, practically all chips used aluminum for the metal interconnection layers. [10] The four metals with the highest electrical conductivity are silver with the highest conductivity, then copper, then gold, then aluminum.
As of 2011, many commercial processes support 2 or 3 metal layers; the most layers supported on a commercial process is 11 layers, and 12 layers are expected to be supported soon.[11]
After BEOL there is a "Backend process" (also called post-fab), which is done not in the cleanroom, often by different company. It includes wafer test, wafer backgrinding, die separation, die tests, IC packaging and final test.