Researchers at MIT have developed a technique that uses graphene as a kind of “copy machine”, to transfer intricate crystalline patterns from an underlying semiconductor wafer to a top layer of identical material.
As a great deal of money is spent in the semiconductor industry on wafers that serve as the substrates for microelectronics components, which can be turned into transistors, light-emitting diodes etc., this method may help reduce the cost of wafer technology and enable devices made from more exotic, higher-performing semiconductor materials than conventional silicon.
The MIT team designed carefully controlled procedures to place single sheets of graphene onto a wafer. The researchers 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. In addition, graphene 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 strongly bonded to the semiconductor and it is almost impossible to separate without damaging both layers. This new MIT method, however, can use graphene as an intermediate layer, allowing 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, the team says this opens opportunities for choosing more varied semiconductor materials.
The team states that its technique, referred to as “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.
The group’s graphene-based peel-off technique may also advance the field of flexible electronics. In general, wafers are very rigid, making the devices they are fused to similarly inflexible. With graphene, semiconductor devices could be made to bend and twist. In fact, the group demonstrated this possibility by fabricating a flexible LED display, patterned in the MIT logo, using their technique.
“Let’s say you want to install solar cells on your car, which is not completely flat — the body has curves,” the team says. “Can you coat your semiconductor on top of it? It’s impossible now, because it sticks to the thick wafer. Now, we can peel off, bend, and you can do conformal coating on cars, and even clothing.”
Going forward, the researchers plan to design a reusable “mother wafer” with regions made from different exotic materials. Using graphene as an intermediary, they hope to create multifunctional, high-performance devices. They are also investigating mixing and matching various semiconductors and stacking them up as a multimaterial structure.