PORTLAND, Ore. Traditional silicon circuitry is lightning fast, but can't be deposited on flexible polymer substrates because of the high temperatures used to process CMOS wafers. Conversely, silicon inks and organic circuitry can print electronics onto flexible substrates at low temperatures, but their speed is dismal compared to CMOS.
Now, researchers claim to have invented a method that combines the best of both worlds--CMOS circuits on flexible substrates--by transferring silicon circuits from a wafer onto the flexible polymer substrate.
"We use a flexible, stretchable polymer as our substrate, but the devices themselves are made with traditional silicon processing technology," said Yonggang Huang, a professor at Northwestern University (Evanston, Ill.). "We fabricate our devices on a silicon wafer, then transfer them to the soft polymer substrate."
By using a donor silicon-on-insulator wafer processed in a traditional fab, the researchers combined the speed of CMOS circuitry with the flexibility of polymer substrates. Because the donor wafer has a thick coat of insulating silicon dioxide on its surface, the circuits fabricated can be removed and the wafer reused many times (until the silicon dioxide coating is used up).
"Initially, our silicon devices are sitting on a sacrificial layer of silicon dioxide, which we etch away so we can remove the devices and transfer them to the polymer substrate," said Huang.
If that was all, however, the brittle silicon circuitry would fracture and break whenever the flexible substrate was deformed. However, the researchers claim to have solved that problem with a two-step transfer process that preflexes the substrate, pops up interconnects, then encapsulates the entire assembly in soft, transparent plastic.
"The trick is that we first stretch out the polymer substrate, then we transfer the circuitry from the donar wafer," said Huang. "Then when we relax the stretch, and the metal interconnections between the silicon islands pop up, suspended over the substrate's surface. Finally, we apply a surface coating to encapsulate and protect the circuitry."
The researchers claim that the resulting flexible circuits are as fast as traditional CMOS, but even more flexible than using silicon ink or organic materials on polymer substrates. The extra flexibility comes from using a very soft polymer that can not only bend, like printable circuitry, but can also be twisted and stretched by as much as 40 percent. The researchers claim that stretching and twisting is essential for some applications, such as wearable devices.
So far, they have attempted to prove the concept by fabricating a ring oscillator using both n- and p-type transistors with channel lengths of 13 microns. The researchers claim their technique should work with sensors, transmitters, photovoltaic cells and microfluidic devices for use in medical and athletic applications.
Funding was provided by the National Science Foundation and the U.S. Energy Department.
The results were published online in the Proceedings of the National Academy of Sciences.