After 20 years of exploring the esoteric nature of liquid helium when it is cooled to ultra-low temperatures in zero gravity, physicist John Lipa suddenly finds that his work could have important ramifications for the miniaturization in the microelectronics industry.
His latest experiment scheduled to launch in the space shuttle Columbia on Nov. 19 is called the Confined Helium Experiment (CHeX). Its purpose is to determine what happens to a material when it is confined to such narrow dimensions that it begins to behave as if it has only two dimensions, rather than three. Lipa is the principal investigator on the experiment. His co-investigators are Ulf Israelsson, Talso Chui and Frank Gasparini at the Jet Propulsion Laboratory.
In most materials, this confinement effect surfaces at extremely small dimensions, thicknesses of a few atomic widths. It arises from the fact that fundamental particles have a dual nature, acting sometimes like solid objects and sometimes like a packet of waves. A particle contained within a layer that is so thin that the waves associated with it come in contact with both edges is restricted to moving in only two dimensions. This constraint can change the physical properties of the material. If the particle in question is an electron, for example, then the electrical properties of the material are affected.
"The size of the transistors in today's integrated circuits is about two tenths of a micron. Intel and the other semiconductor manufacturers are talking about reducing this by a factor of 10 or more in the next decade," Lipa said. "That is about the size where we expect these confinement effects to appear in metals and semiconductors. The preliminary indications are that this effect tends to have a depressing effect on properties like electrical conductivity, so it looks as if it might present a roadblock to the miniaturization process."
Such a roadblock could have serious consequences for the microelectronics industry. The ability to continually miniaturize the circuitry printed on silicon chips has been the primary reason that the industry has been able to simultaneously reduce the cost and increase the performance of everything from computers to telephones. If the confinement effect proves to be relatively small, and reduces the conductivity of silicon only slightly, then the process of miniaturization can continue until some other factor intervenes. If the confinement effect is large, however, it could slow or block further size reductions. In that case, the industry will be forced to develop a new technology to reach smaller size scales, Lipa said.
Scientists have several competing theories for how the confinement effect might work, but there is little direct evidence of its exact nature and magnitude. That is where helium comes in. It has some unique qualities that make it an ideal substance in which to observe this effect. It is the only substance that remains a liquid at absolute zero, a temperature of 273 degrees Celsius below zero. At about 2 degrees Celsius above absolute zero, helium becomes a superfluid, a material without resistance to current flow.
As helium is cooled to the point where it turns from an ordinary liquid into a superfluid, its confinement effect increases by a factor of 10,000 or more. As the effect increases, the distance at which helium atoms sense boundaries increases from a few atomic widths to thousands of atomic widths. This makes it possible for Lipa and his colleagues to measure the confinement effect cleanly and directly with current technology.
Lipa and his colleagues designed an experiment that consists of more than 400 silicon wafers. The thin wafers, which are two inches in diameter, are stacked together in a column. The surface of each wafer contains a micromachined recess with a depth of 50 microns, about twice the width of a human hair. When the column is immersed in about two cubic inches of chilled helium, the liquid forms thin layers between the wafers.
Confinement is expected to affect a number of a material's physical properties. The specific manifestation that CHeX is designed to measure is its impact on helium's heat capacity. Heat capacity is the amount of heat it takes to raise the temperature of the substance by a set amount. To make these measurements, the scientists have developed some of the world's most precise thermometers. They can measure temperature changes in liquid helium of less than a billionth of a degree. As a result, they can record changes in energy as small as a fly's landing on a table.
Lambda Point Experiment
The thermometers and much of the other hardware originally were developed for an experiment that flew on the shuttle in 1992. Called the Lambda Point Experiment, the original investigation determined the way in which bulk helium's heat capacity changes as the material makes the transition from normal to superfluid state. As they cool the confined helium to the superfluid transition point, the researchers expect its heat capacity to diverge from that of the three-dimensional helium. The direction and magnitude of that divergence will provide them with a direct measurement of the strength and nature of the confinement effect. The experiment must be done in zero gravity. On earth, the variations in pressure caused by gravity are enough to obscure the divergence.
There are three leading theories that attempt to predict the confinement effect: renormalization group theory by Volker Dohm of the University of Aachen in Germany; a Monte Carlo-based theory by Efstratios Manousakis at the University of Florida; and a vortex ring dynamics theory by Gary Williams at the University of California-Los Angeles. Each makes slightly different predictions for the size of the effect and how it varies in different materials. The results of the CHeX experiment should help refine these theories, Lipa said.
Conducting an experiment that has some important economic implications has provided an added element of excitement, the physicist acknowledged. "Several months ago, I read a newspaper story about Intel's plan to invest $250 million in a plant to reduce the size of the transistors by a factor of 10. Here I've been, sitting in the ivory tower doing esoteric science, and now these guys are getting down to sizes that are relevant to what we're measuring."