Interconnects are the heart of the plumbing on a complex integrated circuit, regardless of whether that's an ASIC, ASSP or SoC.
Interconnects are one of the most difficult steps in the manufacturing process, particularly at advanced process nodes where there are more metal layers to connect, both internally and to the outside world. Interconnects were made of aluminum prior to 130nm. After that, they have been predominantly copper. Copper wires have about 40% less resistance than aluminum, which yields a 15% increase in processor speed.
But as wires continue to shrink, so does concern over RC (resistance-capacitance) delay. Pushing electrons through increasing thin wires and often longer wires is approaching its limitations, which has prompted discussions about high-mobility materials. In just one example of the complexities of the interconnect, the total length of the wires in a device has roughly doubled, from about 2,000 kilometers per square centimeter in 2009 to about 4,000 today, according to Imec. By 2024, that figure could reach 14,000. The measurement only includes wires in the first five metal layers.
To help solve the RC problem, chipmakers are moving toward new materials, such as cobalt (Co) and ruthenium (Ru) in the interconnect. The move also may require new fab tool technologies. But it’s still unclear if the new solutions can halt a troubling trend.