Making memory smaller

The long-term trend that has persisted in the computer industry since the invention of the integrated circuit in 1958 has been for the hardware to become smaller and smaller.

Following Moore’s law, the number of transistors that can be accommodated on an integrated circuit of given size has increased exponentially, doubling roughly every two years.

This has been a crucial characteristic because almost every measure of the performance of computers and other digital devices is linked closely to physical size, including memory capacity as well as processing speed.

The result is that Moore’s law has led to an increase in the usefulness of digital electronics in a range of applications as well as computing.

But scientists believe that conventional miniaturisation processes could soon reach their fundamental limits.

As transistors approach nanoscales, their operation is disrupted by quantum phenomena such as electrons tunnelling through the barriers between circuit elements.

And as handheld devices — from mobile phones and cameras to music players and laptops — get more powerful, the race is on to develop memory formats that can satisfy the ever-growing demand for information storage in tiny spaces.

Researchers at the University of Nottingham in England are now exploring ways of exploiting the properties of carbon nanotubes to create a low-cost and compact memory cell that uses little power and writes information at high speeds.

Up unil now the miniaturisation of computer devices has involved continual improvement and shrinking of their basic element, the transistor.

Present memory technologies fall into three groups:

• Dynamic random access memory (DRam) — the cheapest method;
• Static random access memory (SRam) — the fastest memory, although DRam and SRam require an external power supply to retain data;
• Flash memory — non-volatile; it does not need a power supply to retain data but has slower read-write cycles than DRam.

Carbon nanotubes — tubes made from rolled graphite sheets only one carbon atom thick — could provide the answer.

If one nanotube sits inside another (slightly larger) one, the inner tube will 'float' within the outer, responding to electrostatic, van der Waals and capillary forces.

Passing power through the nanotubes allows the inner tube to be pushed in and out of the outer tube. This telescoping action can either connect or disconnect the inner tube to an electrode, creating the 'zero' or 'one' states required to store information using binary code.

When the power source is switched off, van der Waals force, which governs attraction between molecules, keeps the inner tube in contact with the electrode. This makes the memory storage non-volatile, similar to flash memory.

Researchers from across the scientific disciplines will be working on the 'nanodevices for data storage' project led by Dr Elena Bichoutskaia in Nottingham’s School of Chemistry.

In collaboration with colleagues from the university’s schools of chemistry, physics and astronomy, pharmacy and the Nottingham Nanotechnology & Nanoscience Centre, she will examine the methods and materials required to develop this new technology, as well as exploring other potential applications for the telescoping properties of carbon nanotubes.

These include drug delivery to individual cells and nano-thermometers that could differentiate between healthy and cancerous cells.

“The electronics industry is searching for a replacement of silicon-based technologies for data storage and computer memory,” said Dr Bichoutskaia.

“Existing technologies, such as magnetic hard discs, cannot be used reliably at the sub-micrometre scale and will soon reach their fundamental physical limitations.

“In this project, a new device for storing information will be developed, made entirely of carbon nanotubes and combining the speed and price of dynamic memory with the non-volatility of flash memory,” she added.

The research is being funded by Britain’s Engineering & Physical Sciences Research Council.

University of Nottingham