Engineers developing new, environmentally friendly ways to make microchips
provided by University of Arizona
niversity of Arizona engineers are developing new, environmentally friendly processes for making microchips. These processes soon will find their way to fabrication plants, where they will be used to build faster, smaller and more sophisticated electronics at lower cost. This is important because microchips are ubiquitous. They're found in everything from medical imaging equipment to coffee makers, from computers to automobile engines.
Many of the areas we are studying are so new that we're just shedding light on them defining the limits, says Anthony Muscat, assistant professor of Chemical and Environmental Engineering at UA. Muscat is studying the physical characteristics of interfaces where chip materials meet. He's also devising technologies for cleaning chip surfaces during production.
In one research project he is using metal nitride to encapsulate copper. In today's complex microcircuits, the copper wires that connect transistors are packed so tightly that stray copper molecules sometimes ruin the circuitry by diffusing through insulation and shorting adjacent wires. To prevent this, the insulation must be separated from copper with a barrier such as a metal nitride. Future chips will use barrier layers that are only a molecule or two thick. Any thicker, and there won't be enough copper left inside the wire to efficiently transfer signals.
The question is, 'How do we get a layer on the order of one or two molecules thick that uniformly coats a feature that's ten times deeper than it is wide?' Muscat asks.
In addition, copper doesn't readily stick to nitride. So Muscat is modifying the chemical characteristics of the nitride film to help copper stick and react.
In another area, Muscat is studying barrier films. As devices get smaller, these films will be used to isolate transistor gates from silicon. Gates are electronic valves that can be adjusted to increase or decrease the flow of electrons through a transistor. Chip manufacturers currently use silicon dioxide to form gates, but new gate materials will be needed as microchips shrink in size. Undesirable alloys form at the junction of these new gate materials and silicon during some high-temperature processing steps, and the barrier films will retard production of these alloys.
Muscat also has been working on gas-phase cleaning processes. Currently, microchips are transferred to and from vacuum chambers as they're processed because cleaning steps require liquids, while etching and deposition steps use gases. In all-gas-phase processing, the chip stays in a single chamber and is never exposed to the atmosphere. This helps avoid contamination and increases yields. It also promotes water conservation because water rinses are no longer needed.
As part of his gas-phase processing work, Muscat is using supercritical carbon dioxide to clean chip surfaces. Supercritical carbon dioxide combines the best attributes of gases and liquids: it transports contaminants efficiently like a gas, but has a high density of molecules like a liquid. It's created by pressurizing carbon dioxide to about 1,000 pounds per square inch at around 100 degrees Fahrenheit. This packs in a sufficient number of carbon dioxide molecules to dissolve contaminants, but the carbon dioxide still acts like a gas and the contaminants diffuse through it easily.
This could have an enormous environmental benefit, Muscat says. Carbon dioxide is inexpensive and easy to obtain. There are no toxicity problems if it's accidentally released. And since it is under such high pressure, when you decrease the pressure the carbon dioxide goes into the gas phase and the contaminants, which are liquids and solids, drop out easily.
Muscat conducts his research with the help of six PhD students and four undergraduates in the NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing. The center, which is administered from UA, includes researchers from UA, MIT, Stanford, UC Berkeley, Arizona State University, Cornell, University of Maryland, and MIT's Lincoln Lab.
We want to advance technologies in parallel with minimizing their environmental effects and educate a new breed of engineers, Muscat says. This is not an either/or proposition. We're designing completely new chemistries from the ground up, and we take the environment as another design constraint. The researchers want to develop chemistries that use fewer resources while producing few hazardous byproducts.
Our approach to environmentally benign processing is to learn how to engineer the microchip wafer surface on a molecular scale, Muscat adds. Once the researchers understand the basic science involved and devise methods and materials for new processing technologies, microchip manufacturers scale the processes up to production levels and build the tools necessary for mass production.