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Ever since it was discovered, scientists have dreamed of using graphene as a key semiconductor component. These efforts have generally been stymied thanks to the difficulty of growing graphene in large quantities and its lack of a band gap — with graphene, the challenge is making it stop conducting to turn a circuit off. New research at Stanford, however, suggests an interesting application for the material that doesn’t rely on adapting it to change some of its most fundamental characteristics. Graphene-coated copper wires have significantly better performance than standard copper wire sheathed in tantalum nitride, and the performance boost actually increases as the wires get smaller.
The perverse mechanics of wire scaling
When the semiconductor industry talks about process nodes, it’s generally assumed that moving to a new, smaller process geometry is a good thing. This continues to hold true even today — companies like Intel, TSMC, Samsung, and GlobalFoundries are required to do much more work to ensure positive results when moving to new geometries, but the net effect is still positive by certain metrics. This general trend does not hold true for copper wires.
The problem is this: Making a wire smaller means you’re reducing the amount of metal that’s available for electrons to flow through. Imagine two pipes — one with a one foot diameter, and one with a 10 foot diameter. At any flow rate (measured in gallons per minute), you have to move water through the smaller pipe at a higher velocity compared to the larger. This increases both friction within the pipe and the turbulence of the water flowing through it. In a wire, pushing the same amount of current a small wire increases the resistance (and the excess heat) compared to a larger wire. (Thanks to reader Sean T for sharpening my original water flow analogy).
Here’s where Stanford’s research comes in. The reason the copper wires are traditionally wrapped in tantalum nitride is to ensure that copper doesn’t migrate into the surrounding area of the chip. One of the team’s findings was that the tantalum nitride layer is roughly 8x thicker than an equivalent layer of graphene that performs the same function. That’s important in and of itself, as it allows wires to be made thinner overall without actually changing the amount of copper (think of this as making a water pipe thinner by reducing the exterior diameter but leaving the interior diameter unchanged.
The second and arguably more important reason is that the graphene effectively acts as a secondary conduction path for the copper itself. Today, the effect is relatively modest — sheathing copper wires in graphene boosts speeds by 4-17% depending on the length of the wire. In future chips, however, the benefits could be more significant — wires might be up to 30% faster while still scaling to smaller sizes. Because wire delays have become one of the most significant performance limiters in modern semiconductor designs, increasing copper wire speeds could improve multiple aspects of chip design and power consumption.
I’m more optimistic about this use of graphene as compared to its use in semiconductors. Using graphene in semiconductors requires changing some of the fundamental properties of the material or changing everything about how we build semiconductors. The one fly in the ointment will be volume production. The researchers talk about growing the graphene layers directly around the wire, but graphene has proven extremely difficult to create in volume. Until we crack that particular problem, we won’t be building anything with graphene in significant quantities.
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