In 2016, annual global semiconductor sales reached $339 billion worldwide. In that same year, the semiconductor industry spent about $7.2 billion worldwide on wafers that serve as the substrates for microelectronics components. These ultimately turned into quintillions of transistors, light-emitting diodes, and other electronic and photonic devices.
Going forward, a new technique developed by MIT engineers may vastly reduce the overall cost of wafer technology and enable devices made from more exotic, higher-performing semiconductor materials than conventional silicon.
The new method, reported recently in Nature, uses graphene as a sort of "copy machine" to transfer intricate crystalline patterns from an underlying semiconductor wafer to a top layer of identical material. Graphene is a single-atom-thick sheet of graphite, with miraculous properties, which looks like chicken-wire at the nano-scale.
Since graphene's discovery in 2004, researchers have been investigating its exceptional electrical properties in hopes of improving the performance and cost of electronic devices. The MIT group took an entirely new approach to using graphene in semiconductors. Instead of focusing on graphene's electrical properties, the researchers looked at the material's mechanical features.
Graphene, with its ultrathin, Teflon-like properties, can be sandwiched between a wafer and its semiconducting layer, providing a barely perceptible, nonstick surface through which the semiconducting material's atoms can still rearrange in the pattern of the wafer's crystals. The material, once imprinted, can simply be peeled off from the graphene surface, allowing manufacturers to reuse the original wafer.
The team found that its technique, which they term "remote epitaxy," was successful in copying and peeling off layers of semiconductors from the same semiconductor wafers. The researchers had success in applying their technique to exotic wafer and semiconducting materials, including indium phosphide, gallium arsenenide, and gallium phosphide - materials that are 50 to 100 times more expensive than silicon.
They are also investigating mixing and matching various semiconductors and stacking them up as a multimaterial structure. Exotic materials should become more practical to use, because you don't have to worry about the cost of the wafer. With this graphene copy machine, engineers can grow a semiconductor device, peel it off, and reuse the wafer.
The MIT engineers worked out carefully controlled procedures to place single sheets of graphene onto an expensive wafer. They then grew semiconducting material over the graphene layer. They found that graphene is thin enough to appear electrically invisible, allowing the top layer to see through the graphene to the underlying crystalline wafer, imprinting its patterns without being influenced by the graphene.
Graphene is also rather "slippery" and does not tend to stick to other materials easily, enabling the engineers to simply peel the top semiconducting layer from the wafer after its structures have been imprinted.
In conventional semiconductor manufacturing, the wafer, once its crystalline pattern is transferred, is so strongly bonded to the semiconductor that it is almost impossible to separate without damaging both layers. So, you end up having to sacrifice the wafer - it becomes part of the device.
But with the group's new technique, manufacturers can now use graphene as an intermediate layer, allowing them to copy and paste the wafer, separate a copied film from the wafer, and reuse the wafer many times over. In addition to saving on the cost of wafers, this opens opportunities for exploring more exotic semiconductor materials.
The industry has been stuck on silicon, and even though we've known about better performing semiconductors, we haven't been able to use them, because of their cost. This gives the industry freedom in choosing semiconductor materials by performance and not cost.