University of Southampton – Sensors

The University of Southampton’s School of Electronics and Computer Science (ECS) is attempting to combine nanomachining and single-electron transistors to make sensitive sensors. Project leader Professor Hiroshi Mizuta said “This is the first time that anyone has combined these two nanotechnologies to develop a smart sensor”, also “The traditional CMOS approach has many limitations, so we needed to find a new approach. Mizuta and his team are apart of the three-year  European FP&-funded NEMSIC (Nanoelectromechanical System Integrated Circuits) project, which means it could make these devices possible. “As well as being the smallest sensor on the market to date, it will have extreme low power consumption” said the university.

The details of the precise structure of the proposed will remain a secret until the intellectual property is protected. The team has been publishing details of a single-electron transistor whose channel is an under-etched bridge of silicon with air-spaced gates either side. Etched bulges, or embedded silicon nanocrystals, on the bridge act as quantum dots to confine electron. Mizuta was speaking to EW (Electronic Weekly) about the sensor and he said “We will develop two kinds of nanosensor devices on silicon-on-insulator substrates. A common key structure used for both devices is a very thin suspended silicon bridge” 

One of the sensors will work by detecting charge from molecules of interest. “In the first nanosensor device, the suspended nanobridge, with or without an integrated silicon quantum dot cavity, is used as a channel of a transistor with side gates,” Mizuta said. “Small charge transfer caused by the molecules captured on the surface of the nanobridge channel is detected electrically as a change in the conductance.”

The other sensor is mechanically closer to the classic vibrating cantilever micro-machine sensor. “In the second nanosensor device, the suspended nanobridge is used as a movable gate of a nanoscale transistor,” said Mizuta. “A small mass change due to the molecules captured on the nanobridge gate surface is detected electrically as a change in the resonant frequency of the gate.” Why a single-electron transistor and not a conventional mosfet? “Our nanobridge transistor with a suspended quantum dot works as an extremely sensitive charge detector, which in principle senses charge transfer of even a single electron occurring on the surface of the quantum dot. This cannot be achieved by using a convectional mosfet structure,” said Mizuta. “The single-electron transistor combined with nanomachine device technology reduces power consumption both “on” and “off” states of the sensor. Standby power is reduced to zero by having a complete sleep with a mechanical switch when it is off”

Nanomachining will receive a boost at Southampton as its new electron beam lithography machine comes on stream when the £55m rebuilt Mountbatten building, which burned down in 2005, opens this month. The European NEMSIC project is headed by Professor Adrian Ionesco of Ecole Poly-technique Federale de Lausanne.

Other partners are : Delft University of Technology, Stitching IMEC Nederland, Commissariat a L’Energie Atomique – Laboratoire d’Electronique de la Technologie de L’Information, SCIPROM Sarl, Interuniversity Micro-electronics Center, Honeywell Romania SRL – Sensors Laboratory Bucharest, and Universite de  Geneve

Source: 23-29 JULY - ELECTRONICS WEEKLY MAGAZINE (PAGE 7)

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