New advance in nanofabrication techniques, 'significant'

Teams from Lund University in Sweden and the University of New South Wales say that the transistor the design simplifies the fabrication and has the unique ability to tune the length of the wrap gate via a single wet etch step. The research was undertaken to address the problems caused by packing higher densities of transistors into a chip – such as the difficulty of switching a current on and off due to the reduced overlap between the semiconductor channel and the metal gate. The resulting 'Fin Field Effect Transistor' (FinFET) has the silicon either side of the channel etched away to create a raised mesa structure. This allows the gate to fold down around the sides of the channel, improving the switching without increasing the chip space needed by the device. According to the researchers, even better control can be obtained by wrapping the gate all the way around the channel, although getting metal underneath the channel without compromising the device is difficult when using conventional top down silicon microfabrication techniques. This has resulted in significant interest in self assembled nanowires for computing applications. The tiny semiconductor needles, around 50nm in diameter and up to several microns in length, are grown using chemical vapour deposition and stand vertically on a semiconductor substrate. This, claim the researchers, makes it possible to deposit an insulator and gate metal around the nanowire's entire outer surface. The coated nanowires can be made into fully functioning transistors in the vertical orientation, however, the process to achieve this is very involved. In many cases, it is more desirable to have the nanowire transistor lying flat on the substrate, as with conventional silicon transistors. The challenge was to make nanowire transistors with an all around metal wrap gate that lay flat on a semiconductor substrate. In work published in NanoLetters the team not only demonstrated the first such horizontal wrap gate nanowire transistors, but they demonstrate that they can be made using a remarkably simple process that allows them to precisely set the wrap gate's length using a single wet etch step, without any need for further lithography. Their approach exploits the etchant solution's ability to undercut the resist and etch along the nanowire, producing gates that range in length from slightly less than the contact separation to as low as 100nm, simply by tuning the etchant concentration. The resulting devices are said to have excellent electrical performance and can be produced reliably with high yield. The devices could open new avenues for research into one dimensional quantum transport in semiconductors, where phenomena such as electron crystallisation and spin charge separation may be observed. The strong gate channel coupling combined with an exposed gold wrap gate surface also offers the potential for sensing applications by using the established chemistry for binding antibodies and other polypeptides to gold surfaces.