Tuesday, February 21, 2006
IBM Squeezes More into Microchips
Integrated circuits have been effectively made by the same process for the last 30 years, photolithography. In this process the feature size of the components of the circuit are determined by the ability of a light source to harden a photoresist material laid on a metal surface, a few nanometers thick, which itself is laid on the silicon substrate. The entire surface is then exposed to a chemical etching agent, which removes all except the hardened photoresist, that not exposed to light, and the metal underneath. The photoresist that remains after the first etching is then itself removed using a different chemical treatment, and all that remains is a layer of metal in the same shape as the components required. To reduce the size of the components, it is simply necessary to reduce the wavelength of the light that is used to mark the pattern, and hence reduce the diffraction of the light that results from its wave nature.
Present fabrication techniques rely on light in the ultraviolet region, allowing the production of feature sizes ~100 nm, and this is about as far as present technology can take the technique; the next step down the wavelength chart takes us into the domain of x-rays. The problems here arise from the difficulty associated with finding sources of x-rays, e.g. synchrotrons, and the even greater challenge of focusing and reflecting the beams, the cost, even by the standards of the electronics industry, would be huge with predictions of a fabrication facility costing in the region of $200 billion by 2015.
IBM squeezes more into microchips is the title of an article appearing on todays Technology news on the BBC website (the story also appears on several other sources today). The technology giant claims that they now possess techniques that will help overcome these fundermental problems, without resorting to costlier and unproven chip-making methods. They say they have been able to etch circuits on silicon wafers that are a third of the width of those produced using existing technology. The methods used by the scientists at IBM's Almaden Research Center in San Jose, California, uses a method called deep-ultraviolet optical lithography. "Our goal is to push optical lithography as far as we can so the industry does not have to move to any expensive alternatives until absolutely necessary," said Dr Robert D Allen, manager of lithography materials at IBM's Almaden Research Center.
The IBM scientists have created the tiniest, high-quality line patterns ever made using deep-ultraviolet optical lithography, a technology currently used to "print" circuits on chips.
The distinct and uniformly spaced ridges are only 29.9 nanometres wide.
This is less than one-third the size of the 90-nanometre features now in mass production and below the 32 nanometres that industry consensus held as the limit for optical lithography techniques. According to the researchers, "This result is the strongest evidence to date that the industry may have at least seven years of breathing room before any radical changes in chip-making techniques would be needed."
Does this provide a death blow for molecular electronics? On the surface of it, it would appear that this is nothing more than a short extension to the dominance of silicon components, with molecular electronics still possessing the strongest case for succession. But with work continuing at beathtaking pace in the field of quantum computing (something i intend to talk about in the future), this delay could well provide the time required for chip technology to 'skip a generation' and consign molecular electronics to the great what if bin of science and technology.
Wednesday, June 29, 2005
The Aviram and Ratner Rectifier
The simplest component of the integrated circuit that it is necessary to try and reproduce on the molecular scale is the rectifier, also known as the diode or semiconductor p-n junction. A rectifier can be defined as one or more diodes arranged for converting alternating current (AC) to direct current (DC). When only a single diode is used to rectify AC (resulting in only half wave rectification) the difference between the term diode and the term rectifier is merely one of semantics.
The first molecular rectifier was proposed in 1974 by Aviram and Ratner (original paper available here free), who proposed a model consisting of a donor π-system and an acceptor π-system, separated by a σ-bonded tunnelling bridge. This system could then, theoretically, be attached at both ends to metallic electrodes thereby completing an electric circuit. Aviram and Ratner predicted that, for the molecular rectifier to work, its properties must be equivalent to those of the bulk p-n junction.
In the donor-insulator-acceptor (D-σ-A) structure, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are confined to two different parts of the rectifier, D and A respectively. The insulating σ bridge prevents the orbitals from “spilling over” to the other part. If such a molecule were to be constructed and placed between two metal electrodes, the current-voltage characteristic of the junction would be expected to be highly asymmetric, i.e. molecular rectification can be defined as the absence of inversion symmetry,
I(V)=-I(-V) in the system, where I and V are the measured current and the applied voltage.
The Aviram and Ratner model can be thought of as occurring in two distinct stages:
1: At a particular value of applied voltage in the positive direction, the Fermi level of the electrode on A (cathode) aligns with the LUMO allowing electron tunnelling from the Fermi level to the LUMO. Simultaneously, the D side aligns with the HOMO, resulting in electron transfer from the HOMO to the Fermi level of the electrode on D (anode). In terms of the charge, the molecule has gone from D-σ-A to D+-σ-A-.
2: At this voltage, the current rises sharply because the electrons can now be loaded onto the LUMO, then tunnel inelastically through the σ-bridge to the HOMO and then escape into the second electrode. This returns the molecule from its excited, ionised state to the ground state of the system D-σ-A. In the opposite direction, a similar process does not occur until a much higher applied voltage.
The Aviram and Ratner model has now become the standard explanation for observed rectification from a molecular monolayer. In my next post I will discuss some of the evidence that has arisen over the last decade in support of the model, including my own paper (subscription required) that has been published recently in the Journal of materials chemistry, by the Royal Society of Chemistry.
Friday, May 27, 2005
Biology meets Molecular Electronics
But now the world of biology is encroaching into the field of molecular electronics, a branch of nanotechnology that had previously been teh domain of physicists and organic chemists. Bob Willett and colleagues at Bell Labs have measured how different amino acids adhere to the various types of materials that are used to make electronic devices (Proc. Natl. Acad. Sci. to be published). The Bell Labs team then went on to design an inorganic nanostructure that was capable of selectively binding to a particular sequence of amino acids.
"Our results demonstrate a surprisingly large range of adhesion interactions," says Willett. "The adhesion maps are an empirical tool for attempting to understand certain molecular interactions with inorganic surface states and, perhaps more importantly, provide an empirical guide for building nanostructures that are hybrids of peptide-based materials and inorganics."
Friday, April 22, 2005
Moore magazines please
Moore's Law and Molecular Electronics
There are strong indications that we are approaching the time when this dramatic increase, which has done so much to shape the way the world’s economy has changed, is coming to an end. This is due to the inherent susceptibility of silicon (the primary component of integrated circuits) to information leakage (i.e. quantum tunnelling of electrons from the bulk material near to the transistor surface causing a corruption in the data being transferred), at small scales.
In order to continue the trend of Moore’s law we need smaller electronic components to occupy integrated circuits, components that instead of being hindered by the effects of quantum mechanics, take advantage of them. The ultimate prospect for miniaturisation is in the form of single molecule electronics.
Molecules fulfil several of the required criteria: they are small, reproducible in large quantities, take advantage of quantum effects instead of being hindered by them; in molecules electron energies are quantize, unlike the bulk silicon case. Another reason for choosing to work with individual molecules is that molecules can be pi conjugated suggesting that the conductance can be controlled by changing the molecular conformation, this is an effect that is still relatively un-investigated in the field of molecular electronics and also the focus of my personal research. The final advantage of using molecules is the fact that some may be able to self assemble (something I intend to discuss in greater detail at a later time), allowing the, relatively, rapid fabrication of large scale structures.
Moore’s law is no longer just a law for the computing industry, it is a major component of modern economical growth. Economic growth, for a company for example, is achieved by having an advantage over your competition, whether this advantage is in the form of a new product, market or most commonly a technological edge. This edge is often due to superior computing power, either in a theoretical role or practical application, an edge achieved through the continuation of Moore’s law. Molecule electronics are still along way from practical applications, but with incessant march of forward for ever superior computing power, by increased transistor density, they are will be an essential part of the future world of both computing and global economics.
Thursday, April 21, 2005
"epistemological, historical,professional, ethical, legal, and societal implications of nanotechnology and converging technologies."
Although most of the articles are relatively short, they do make for some interesting reading. Written in part by non-scientists they make, I believe, a refreshing addition to the web-based Nanotechnology debate.
Wednesday, April 20, 2005
I am currently a research student in the nanomaterials group at the University of Cranfield, UK. My main interests are in the field of single molecule electronics, specifically there creation from Langmuir Bloggett films and molecular self assembly. I also take a wider interest in Nanotechnology in general, specifically its social and ethical implications, as well as its persecption by the general public. I intend to post here my thoughts issues arising in the world of nanotechnology, as well as some details about my current work.
Thanks for reading