The technology for nanomaterial-based device fabrication is still at its infancy, despite the surge in research publications over the past decade. As a matter of fact, the controllable fabrication of complex, three-dimensional nanoscale structures remains a difficult challenge; it will still take some time before a commercial product can be realized. Although researchers are experimenting with a wide range of nanotechnology fabrication techniques, the lack of reproducibility in nanomaterials synthesis and the absence of a realistic high-throughput fabrication scheme pose the biggest challenges.
Current fabrication technologies for nanoscale devices include deep-UV or electron-beam (e-beam) lithography. Both of these techniques involve successive deposition of metal or insulating layer and a resist layer, which is patterned using a UV source or a scanning electron beam. The process needs to be repeated for each layer of the architecture while the sample is taken out of the high vacuum chamber. Thus, multilayer lithography processes seriously compromise throughput and cost. In addition, the resolution is limited in the sub-10 nm regime.
Researchers at the Institute of Bioengineering and Nanotechnology (IBN) in Singapore have now successfully demonstrated, for the first time, a lithography-free, direct-write technique for fabricating discrete field-effect transistors, as well as digital logic gates on a single nanowire.
In this novel direct-write fabrication process, a focused electron beam or ion-beam is scanned over the sample in the presence of a precursor gas, causing the metals or insulators to be deposited directly onto the sample and with nanometer resolution. While there is some known research on the ion-beam induced deposition of materials, the IBN researchers have made significant progress by translating this technique into a viable way for fabricating nanoscale logic circuits.
This is another step of bringing nanofabrication processes closer to mass production.
"With conventional silicon-CMOS – complementary metal oxide semiconductor – transistors reaching their extreme scalable limit, a strategic shift from pure down-scaling to a hunt for new functional materials and hybrid technology platforms is critical," Somenath Roy, a research scientist at IBN, explains to Nanowerk. "Over the past decade, consistent efforts have been made to develop novel device architectures on quasi-1D materials such as carbon nanotubes and semiconducting nanowires (or nanorods). Due to the quantum transport phenomena, the nanomaterial-based devices exhibit some interesting properties, which are unprecedented for silicon. Nevertheless, the lack of controlled assembly, fabrication intricacies and low throughput are posing persistent challenges to advance from a single device level to the functional circuit level."
In a recent online issue of Nanotechnology ("Direct-write fabrication of a nanoscale digital logic element on a single nanowire "), Roy and his IBN colleague Zhiqiang Gao report on the direct-write fabrication and electrical characteristics of a nanoscale logic inverter, integrating enhancement-mode (E-mode) and depletion-mode (D-mode) field-effect transistors (FETs) on a single-crystalline zinc oxide nanowire.
"Our single-step fabrication technique obviates the time-consuming and labor-intensive lithography process, and enhances the fabrication accuracy and yield," says Roy. "With a higher level of precision and throughput, it can offer a powerful method for rapid prototyping of futuristic nanoelectronic circuits."
One immediate potential application of the direct-write approach is for fabricating nanoelectronic devices and logic circuits. Logic gates are the basic building blocks for computer processors and memory chips used in the information technology sector. Functionalized nanoelectronic devices are also highly promising for futuristic medical diagnostics and therapeutic applications.
Roy notes that, with further optimization, the direct-write fabrication technique could pave the way for scalable nanoscale integrated circuit fabrication.
"To achieve that goal, it is of paramount importance to realize the assembly of homogeneous nanomaterials at a predefined position on the substrate. Once achieved, it will obviate the need for registration and rationalize the argument of scalability."