Toshiba has developed a quartz-replacing all-CMOS +/-85ppm oscillator, claiming it to be the most accurate in its class.

Presented at the VLSI Symposium in Honolulu, the 24MHz RC oscillator was implemented on 0.18µm CMOS and consumes 2.9mW.

“Smaller CMOS oscillators have been designed, but with a much lower oscillation frequency, due to large temperature dependence,” said the firm.

This is a compensated oscillator, using a digital phase-locked loop to modify the oscillator’s natural output frequency depending on measured temperature.

Instead of an external oven, an on-chip heater is used during initial test to set device junctions at multiple temperatures to extract a calibration curve. The firm claims this approach cuts test cost and improves accuracy.

The calibration data is stored on the chip and automatically applied to correct the oscillator in normal operation.

“High-order temperature coefficients are extracted by employing a carefully designed on-chip heater, so that the frequency deviation due to the temperature variation is accurately estimated, and compensated in digitally by means of the PLL,” said Toshiba.

The PLL is also used to provide adjust the outputs between 2MHz and 40MHz in 40Hz steps.

Plans are underway to shrink the chip to one third the size of a conventional crystal oscillator, and commercialise it in two to three years. It will also be used on microcontrollers and asics.

VLSI Symposium paper 22.5: ‘A 2.9mW, +/-85ppm accuracy reference clock generator based on RC oscillator with on-chip temperature calibration’.

Researchers are attaching chains of carbon atoms to C₆₀ ‘buckyball’ molecules causing them to controllably self-assemble into spheres, wires and sheets. The strings are semiconductive and photoconductive, and the structures could improve doping in organic solar cells.

Once the chains are attached, the resulting molecule is an ‘amphiphile’ (see box below) – its ends have a different affinity to certain solvents, much like washing-up detergent in water.

Detergents are well understood, and are increasingly being exploited to create order in liquid, forming ‘soft’ structures, some of which can used as the basis for solid structures.

“We have applied science that is normally applied to detergent, and applied it to molecules you wouldn’t normally expect it to apply to,” Dr Martin Hollamby of Keele University told Electronics Weekly.

This means that self-assembly techniques developed using detergents can be adapted for C₆₀.

Hollamby is working with Dr Takashi Nakanishi of the Japanese National Institute for Materials Science.

In their experiments, the chains attached to C₆₀ are branched alkanes (a form of hydrocarbon).

When dissolved in a liquid n-alkane (octane is an alkane eight carbon atoms long, so n=8), the new molecules assemble to form a spherical core of C₆₀ molecules within a shell of carbon chain tails (see box below).

“Changing the chemistry of the chains can lead to gels made of bundled C60 wires that have a measureable photoconductivity,” said Hollamby. “By adding pristine C₆₀ in place of the solvent, we instead prepare a sheet-like material now with totally different properties.”

The wires are hexagonally-packed gel-fibres containing insulated C60 nanowires, according to a Nature Chemistry paper on the work (‘Directed assembly of optoelectronically active alkyl–π-conjugated molecules by adding n-alkanes or π-conjugated species‘).

“The assembled structures contain a large fraction of opto-electronically active material and exhibit comparably high photo-conductivities. This method is shown to be applicable to several molecules, and can be used to construct organised functional materials,” said the paper.

Neutron beamline D11 at the Institut Laue-Langevin (ILL) allows scientists to watch molecular self-assembly

Many different structures can be produced by making small changes to the chemical structure and the additives (solvent or C₆₀) used, according to the team, and this level of control over self-assembly in complex molecules such as C60 is not accessible by any other method reported to date.

So what has all this got to do with electronics?

Importantly for electronics, C₆₀ is a strong electron acceptor, and is already used to dope organic solar cells.

However, according to Hollamby, unstructured C₆₀ is an insulator and only becomes a semiconductor if C₆₀s are bought in close proximity, as they are in the amphiphilic wires, for example. And the wires come ready-insulated by being surrounded by the alkane tails.

What Keele and the National Institute for Materials Science have done is put another tool in the toolbox of organic electronics research. Dots, wires and sheets of C₆₀ are now available for experimentation – although currently only in solution.

Already Hollamby is trying to make solar cells and capacitors using the structures.

The neutron scattering facility (Beamline D11, see photo) at the Institut Laue-Langevin (ILL) was used to investigate assembly and resulting structures.

“The light elements that makes up these molecules are easily located by neutrons” said Dr Isabelle Grillo at ILL.”Small-angle neutron scattering which we use at the ILL allows us to characterise the self-assembled systems from the nanometre scale to tenth of micrometres and observes the coming together of C₆₀s into beautiful core structures.”