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Graphene Used as “Copy Machine” for Semiconductor Wafers
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engineers to simply peel the top semi- conducting layer from the wafer after its structures have been imprinted. Jeehwan Kim, the Class of 1947
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Career Development Assistant Pro- fessor in the departments of mechan- ical engineering and materials sci- ence and engineering, says that in conventional semiconductor manu- facturing, the wafer, once its crys- talline pattern is transferred, is so strongly bonded to the semiconductor that it is almost impossible to sepa- rate without damaging both layers. “You end up having to sacrifice
the wafer —it becomes part of the de- vice,” says Kim. With the group’s new technique,
Kim says 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, Kim says 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 semiconduc- tors, we haven’t been able to use them, because of their cost,” says Kim. “This (development) gives the industry free- dom in choosing semiconductor mate- rials by performance and not cost.”
Graphene Shift Since graphene’s discovery in
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2004, researchers have been investi- gating its exceptional electrical prop- erties in hopes of improving the per- formance and cost of electronic de- vices. Graphene is an extremely good conductor of electricity, as electrons flow through graphene with virtually no friction. Researchers, therefore, have been intent on finding ways to adapt graphene as a cheap, high-per- formance semiconducting material. “People were so hopeful that we
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might make really fast electronic de- vices from graphene,” Kim says. “But it turns out it’s really hard to make a good graphene transistor.” In order for a transistor to work,
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it must be able to turn a flow of elec- trons on and off, to generate a pat- tern of ones and zeros, instructing a device on how to carry out a set of computations. As it happens, it is very difficult to stop the flow of elec- trons through graphene, making it an excellent conductor but a poor semiconductor. Kim’s group took an entirely new approach to using graphene in
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semiconductors. Instead of focusing on graphene’s electrical properties, the researchers looked at the materi- al’s mechanical features. “We’ve had a strong belief in
graphene, because it is a very robust, ultra-thin material and forms strong covalent bonding between its atoms in the horizontal direction,” Kim says. “Interestingly, it has very weak Van der Waals forces, meaning it doesn’t react with anything vertical- ly, which makes graphene’s surface very slippery.”
Copy and Peel The team now reports that
graphene, with its ultra-thin, Teflon- like properties, can be sandwiched between a wafer and its semicon- ducting layer, providing a barely per- ceptible, nonstick surface through which the semiconducting material’s atoms can still rearrange in the pat- tern of the wafer’s crystals. The ma- terial, once imprinted, can simply be peeled off from the graphene surface, allowing manufacturers to reuse the original wafer. The team found that its tech-
nique, which they call “remote epi- taxy,” was successful in copying and peeling off layers of semiconductors from the same semiconductor wafers. The researchers had success in ap- plying their technique to exotic wafer and semiconducting materials, in- cluding indium phosphide, gallium arsenide, and gallium phosphide — materials that are 50 to 100 times more expensive than silicon. Kim says that this new tech-
nique makes it possible for manufac- turers to reuse wafers, made of sili- con and other higher-performing ma- terials, “conceptually, ad infinitum.”
An Exotic Future The group’s graphene-based
peel-off technique may also advance the field of flexible electronics. In general, wafers are very rigid, mak- ing the devices they are fused to sim- ilarly inflexible. Kim says now, semi- conductor devices such as LEDs and solar cells can be made to bend and twist. In fact, the group demonstrat- ed this possibility by fabricating a flexible LED display, patterned in the MIT logo, using their technique. “Let’s say you want to install so-
lar cells on your car, which is not completely flat — the body has curves,” Kim 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 coat- ing on cars, and even clothing.” Going forward, the researchers
plan to design a reusable “mother wafer” with regions made from differ- ent exotic materials. Using graphene as an intermediary, they hope to cre- ate multifunctional, high-perform- ance devices. They are also investigat- ing mixing and matching various semiconductors and stacking them up as a multi-material structure. “Now, exotic materials can be
popular to use,” Kim says. “You don’t have to worry about the cost of the wafer. Let us give you the copy ma- chine. You can grow your semicon- ductor device, peel it off, and reuse the wafer.” r
June, 2017
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