Researchers at the University of Wisconsin-Madison have found a way to produce thin films of strained mono-crystal silicon, then attach them to flexible substrates.
“We first grow epitaxially layers that produce strain in one of the layers, then we release this multilayer layer structure,” Professor Max Lagally told Electronics Weekly. “This produces elastically tensile strained Si, without the usual dislocations and defects when the strain becomes shared by all the layers.”
Starting with a silicon-on-insulator wafer with a 10nm top Si layer, epitaxial growth is used to build this up to 50nm, then add 150nm of Si0.83Ge0.17, and a further 45nm of Si - with the SiGe introducing strain into the sandwich.
Photolithography is used to divide the wafer surface into 1mmx20µm strips, and the strips are released by etching away the underlying oxide with HF. As the oxide goes, the strips weakly re-bond to the substrate below and strain evens out in the sandwich.
A flexible substrate made from PET plastic is prepared by adding a layer of ITO, which becomes the transistor gates. This is spin-coated with a polymer bonding agent, which doubles as the gate insulator.
The original wafer, with strips still clinging, is inverted over the PET substrate, pushed down, then pulled away, leaving the Si-SiGe-Si strips stuck to the PET structure.
Ti contacts are deposited on top for drain and gate connections.
The resulting ITO-gate transistors, with a 3µm long gate, have standard FET characteristics: conducting 50µA with a 1.4µm bonding layer; 12V on the gate; and a VDS of 4V.
In addition to these, conventional transistors can be added before releasing the strips, and into the top surface of the final structure.
“We can and have actually done both, it is very powerful,” said Lagally. “After separation we simply place the membrane on a support - which can be another Si wafer - and it goes through processing readily. We can also make the circuitry on both sides of the membrane, which has not been done before.”
The process has also been applied to non-strained silicon. “Strained Si has a higher charge carrier mobility than normal Si, and the fact that there are no dislocations or threads makes it much better than the old way of producing strained silicon,” said Lagally. “So this way we can make faster devices and we can put them on both sides of a membrane, and we retain all the good properties of using SoI in the first place, and additionally it is flexible.”