PORTLAND, Ore.— Researchers at Stanford University recently demonstrated a novel wafer-scale lift-off process for fabricating nanowire-based circuits on reusable silicon wafers, then transferring them to any substrate in any shape.
The research team, led by professor Xiaolin Zheng, claims the flexible circuitry can be used to create anything from paper-thin displays and solar cells to biomedical sensors that attach directly to the tissue being monitored.
"Devices can be transferred without sustaining any damage," said Zheng. "And the detachment process can be done at room temperature in just a few seconds."
The key to the novel process is depositing a sacrificial nickel layer atop a donor silicon wafer which has been pre-coated with insulating silicon dioxide. Next, a flexible polymer just 800 nanometers thick is deposited on the nickel, after which the nanowire circuitry is fabricated which can include field-effect transistors (FETs), diodes and resistors. Once the circuitry is finished, the wafer is submerged into water which detaches the nickel and the circuitry atop its polymer substrate. The whole assembly can then be transferred to nearly any target substrate including paper, plastic, glass or metal.
"The lift-off process only separates the nickel from the silicon wafer," said Zheng. "The nickel can be etched afterwards to leave the polymer only."
After fabrication on a silicon wafer, nanowire-based circuits can be lifted off and transferred to any substrate in any shape.
The silicon wafer is undamaged by the whole process, allowing it to be repeated over-and-over with the same wafer—the donor silicon wafer is merely recoated with nickel before each reuse.
The detached circuitry uses nanowires just two microns long, allowing circuits to be deposited on nearly any shaped target substrate without substantial damage from crimping. The circuits can also be detached and reused, such as for biological sensors deposited on tissue—like the heart—the later removed and reused with other patients.
Zheng performed the work with doctoral candidates Chi Hwan Lee and Dong Rip Kim.