Groundbreaking, waterless approach to microchip making

 The tiny, high-speed computer chips found in every modern electronic device bear little resemblance to their bulky, slow ancestors of decades ago. Different materials, new designs and new production techniques have ensured successive generations of integrated circuits offer ever more performance at lower cost.

Moore's Law -- the observation by Intel co-founder Gordon E. Moore that the number of transistors on a chip, and as a consequence the processing power, doubles approximately every two years -- has been accurate for more than half a century. Today, we carry more computing power in the mobile phones in our pockets than could fit into a house-sized computer back then. But in order to squeeze more transistors into a smaller space -- and ensure Moore's Law continues to hold true -- chip developers have to be increasingly innovative as chip components are shrinking into the 'nano' scale.

Sometimes they have to think completely outside the box. That was the approach taken by the 'Copper interconnects for advanced performance and reliability' (Copper) project, in which researchers from eight organisations -- companies, research institutes and universities -- in four countries solved a key problem of chip manufacturing. In the process, they have opened the door to an entirely new field of research in the semiconductor industry.

Supported by EUR 3.15 million in funding from the European Commission, the researchers focused on the methods and materials used to interconnect the billions of tiny transistors on a modern microchip. Specifically, the Copper team developed a process that enables reactive metals to be used directly as a barrier between copper interconnects and the silicon wafer of the chip by using non-aqueous solvents instead of water-based ones -- a world first in the semiconductor industry.

'As the number of transistors on a chip increases, so too do the lengths of interconnects between transistors. Because interconnects have a certain resistance, this increase in length causes an increase in the time delay in communication between transistors -- it's an impediment to chip performance,' explains Jan Fransaer, a researcher in the Department of Metallurgy and Materials Engineering (MTM) at the Katholieke Universiteit Leuven in Belgium.

If the interconnects can be made smaller, chip performance improves. But now that chip features have reached the 22nm scale -- around 3,000 times smaller than the width of a human hair -- there are new obstacles to further reductions in length.

The problem in a nutshell

In grossly simplified terms, the problem goes something like this:

Until the mid 1990s aluminium was the metal of choice to fill the interconnect 'vias', the small trenches in the silicon that carry electrons between the transistors. Aluminium was sufficiently conductive to meet the performance requirements of the transistors -- then numbered in the millions on each chip -- and unlike other more conductive metals such as copper, silver and gold it did not diffuse into the silicon, a process that over time would ultimately destroy the circuitry.

But as chips got smaller and their transistor counts increased toward the billions, faster interconnect performance was needed. A more conductive metal had to be used. Hence, semiconductor manufacturers switched to copper as an interconnect material. This in turn required that they do something to prevent the copper diffusing into the silicon, a problem they solved by adding something known as a 'diffusion barrier' -- a layer of another metal that protects the silicon from the copper. The diffusion barrier of choice is a metal called tantalum.

So far, so good: the tantalum diffusion barrier now protects the silicon from the copper in the interconnect vias.

The deposition of the copper interconnects is done by a process called 'electrodeposition' in which an electric current is passed through a solvent solution to coat metal ions onto the vias. An aqueous (i.e. water-based) solution is the usual solvent.

But there's another problem: tantalum oxidises immediately in water, so until now manufacturers have had to first coat the tantalum diffusion barrier with copper -- a so-called seed layer that protects the tantalum from the water just as the tantalum protects the silicon from the copper.

The seed layer is applied using a 'chemical vapour deposition' (CVD) process.

'Why can't we just use the seed layer for the interconnects? Because CVD is a line-of-sight process: it lays down enough copper to coat the tantalum but not enough to make continuous interconnects. So we still have to do electrodeposition on to the copper seed layer to fill the vias with enough copper to make the interconnects,' Prof. Fransaer explains.

In essence, chip manufacturers have been playing 'Russian dolls' at the nanometre scale.

'It sounds stupid -- solving one problem generates another problem -- but this fix has worked ok until now,' Prof. Fransaer notes.

So what has changed? Scale. The copper seed layer is 5nm to 10nm thick, so at scales of less than 22nm that layer -- which serves no other purpose than to protect the tantalum diffusion barrier from oxidisation during chip production -- ends up taking up way too much space.

The answer? 'Change the solvent,' says Prof. Fransaer.

Solving the solvent issue

Instead of using water, the Copper project team developed an innovative process using non-aqueous solvents such as liquid ammonia and ionic liquids. These do not cause tantalum to oxidise, hence allowing electrodeposition to occur without the need for the copper seed layer. The result is that because the interconnect vias can be smaller, chip size can be further reduced, transistor count increased and chip performance greatly improved.

'Electrodeposition using liquid ammonia and ionic liquids has been done before, but this is the first time that this process has been used in the semiconductor industry,' Prof. Fransaer says. 'This technique will certainly help enable a continuation of Moore's Law at least for a few more generations.'

To develop the process, the team studied different wafer materials and electrolyte ingredients for the non-aqueous solution, investigated their physical properties and used analytical and simulation techniques to determine the best approach. They then used micro-modelling of the process before building a proof-of concept demonstrator.

'We were really delving into terra incognita. It was totally uncharted territory, as prior to the Copper project not a single paper had been published on using non-aqueous solutions in the semiconductor industry,' the project manager notes.

Unsurprisingly, the project generated considerable interest from chip makers when the team presented their results at international conferences.

'There was definitely a lot of interest, though we can't say for sure if anyone has used our research as a basis to use this process commercially. Nonetheless, I think it's only a matter of time before non-aqueous solutions start being used now that we've shown it can be done,' Prof. Fransaer says.

Though ammonia -- which needs to be pressurised to stay in liquid form -- or ionic liquids are less abundant and more expensive than water, the cost of using them is a 'non-issue' in the multi-billion euro semiconductor industry, Prof. Fransaer points out.

'Moving from aqueous to non-aqueous solutions would have only a miniscule impact on cost in the grand scheme of things,' he says.

Perhaps even more significantly, the team's research has opened people's eyes to other possibilities, not just with tantalum but also other metals and not just for semiconductor applications.

For example, members of the project consortium are planning a follow-up project using elements of the Copper project's research to work on improving heat dissipation for power electronics, of the sort that will be needed in the smart electricity grids now being rolled out in Europe and elsewhere.

'A lot of elements -- among them all the so-called noble metals -- can be electroplated from water, but a lot can't: aluminium, silicon, germanium etc. We have shown that by using a non-aqueous solution, some of these can also be electroplated. That opens up a whole new range of applications that probably weren't thought possible before,' Prof. Fransaer says.

Copper received research funding under the European Commission's Seventh Framework Programme (FP7).

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Toward a test strip for detecting TNT and other explosives in water

The sensor also has potential uses in detecting water pollution involving TNT, according to Yu Lei, Ph.D., and Ying Wang, who developed the sensor. Such contamination can occur from production, obsolete storage facilities and other sources. TNT contamination of drinking water carries a risk of serious health disorders.

Wang, a graduate student in Lei's laboratory at the University of Connecticut, said there has been a long-standing need for a fast, simple, accurate way to detect so-called "nitroaromatic compounds" in salt water, fresh water and other liquids. That family of compounds includes 2,4,6-trinitrotoluene — TNT — which is so widely used in construction, agriculture and military applications that it has become the standard for measuring explosive force, even for nuclear weapons.

"Law enforcement or homeland security officials concerned about the presence of TNT in a harbor at docks need an answer quickly so they can take steps to protect people and property," Wang pointed out. "That's not easy with traditional testing methods."

Those tests involve taking a sample of water and shipping it to a full-scale laboratory. The sample must be concentrated because water currents dilute the explosive, leaving only minute amounts in the sample. And water samples must be prepared in other ways before analysis with expensive laboratory instruments.

"Our new sensor promises to provide answers on-the-scene almost immediately," Wang added, noting that it is based on a color change that occurs when a sensing molecule in the device attaches to an explosive. Lei explained that the device can detect very small amounts of TNT, as well as larger amounts. The broad sensing range, high sensitivity and dual action make this new sensor unique among those that work on water-based samples, he noted.

So far, Lei and Wang have been able to detect concentrations of explosives, such as TNT, ranging from about 33 parts per trillion (equivalent to one drop in 20 Olympic-sized swimming pools) to 225 parts per million.

Lei and Wang explained that the sensor is already easy to use, but they plan to make it even more user-friendly by incorporating it into a paper strip, similar to the test strips used to test for pregnancy. That way, an explosives expert or airport screener would simply dip the filter paper into a sample of ocean water or other liquid, and put that filter paper into a machine that would read the fluorescence and detect the presence of explosives in real time. The sensor also could be used to detect TNT that leaches into the environment, in streams or rivers near munitions testing sites and manufacturing facilities.

More information:
Abstract
Sensitive and selective detection of explosives is crucial for homeland security and civilian safety. The fluorescence-based methods for explosives hold much promise to satisfy all the requirements as an effective platform for trace detection of explosives. In this work, a novel fluorescence polymer is synthesized by facile side-chain reaction in one synthetic step with high conversion, and characterized by 1H NMR, 13C NMR, UV-vis and fluorescence spectroscopy. In aqueous solution, this new polymer forms a highly emissive hydrogel and undergoes a dramatic fluorescence quenching when nitroaromatics (i.e., 2,4,6-trinitrotoluene (TNT)) molecules bind to the polymer through fluorescent resonance energy transfer (FRET)-amplified effects. This amplification was demonstrated by the quantitative analysis of the fluorescence titration profiles. Down to ppt level of TNT could be discriminated, representing the most sensitive response among the reported fluorescence sensors for nitroaromatics. Moreover, this polymer gives a fast response to nitroaromatics and could be used in real-time detection.

Provided by American Chemical Society (news : web)

More economical way to produce cleaner, hotter natural gas

 New technology is offering the prospect of more economical production of a concentrated form of natural gas with many of the advantages -- in terms of reduced shipping and storage costs -- of the familiar frozen fruit juice concentrates, liquid laundry detergents and other household products that have been drained of their water, scientists reported in San Diego on March 27.

They told the 243rd National Meeting & Exposition of the American Chemical Society (ACS), the world's largest scientific society, that this "super natural gas" would burn hotter than the familiar workhorse fuel and occupy about 40 percent less space in pipelines, railroad tank cars and storage chambers. But its potential benefits range beyond that -- to making natural gas a better source of hydrogen for use in fuel-cell-powered cars in the much-discussed "hydrogen economy" of the future, according to Mohammad G. Rabbani, Ph.D., who reported on the study and is a research scientist in the team of Hani El-Kaderi, Ph.D.

"Natural gas has a reputation among the public as a clean-burning fuel, and that is true," said El-Kaderi. "Compared to coal and oil, burning natural gas releases small amounts of pollutants and less carbon dioxide, the main greenhouse gas. People may not realize, however, that natural gas straight from the well often is contaminated with carbon dioxide and other undesirable gases, such as sulfur dioxide and nitrogen oxides, that are highly corrosive, increase its volume and decrease its heating value. Our new porous polymers, which have exceptionally high thermal and chemical stabilities, remove that carbon dioxide and do it better than any other solid, porous material."

Scientists are searching for such new solid materials to purify natural gas, and the quest has grown more intense as reserves of high-quality gas, lower in contaminants and higher in heating value, grow scarcer, and concerns about global warming due to carbon dioxide continue. The traditional process for purifying natural gas, performed for decades, uses liquids to capture and separate the carbon dioxide and other gases. The process, however, is far from ideal, and scientists are seeking solids and other materials that can more efficiently capture the carbon dioxide, and then can be purged of the gas more economically, recycled and reused time and again. "Our theoretical studies on variable mixtures of CO2/CH4 have actually indicated that these polymers would have high selectivities for CO2," said Thomas E. Reich, Ph.D., who performed these theoretical investigations as a part of his doctoral research in the El-Kaderi group.

El-Kaderi's group developed purely organic polymers termed benzimidazole-linked polymers (BILPs) that are riddled with nano-engineered, minute, empty chamber-like pores so small that thousands would fit on the period at the end of this sentence. When exposed to streams of natural gas, the pores absorb and trap its carbon dioxide. When all of the pores are full of carbon dioxide, the BILPs can be run through a low-pressure processing unit to remove the carbon dioxide and prepare them for reuse.

Rabbani, who is in El-Kaderi's team at Virginia Commonwealth University in Richmond, noted that the new material is as effective at capturing carbon dioxide as monoethanolamine, the most common nitrogen-based liquid used for carbon dioxide scrubbing. Amine liquids have a disadvantage, however, in that they must be purged of carbon dioxide by heating, which requires more energy than the BILPs, which are regenerated under low-pressure conditions.

El-Kaderi said the BILPs seem well-suited for removing the traces of carbon dioxide that remain in hydrogen produced with existing technology. "Carbon dioxide can account for 20 percent of the finished product, the hydrogen, and if we consider a true hydrogen economy of the future, we should be removing that carbon dioxide. Our technology appears to be an excellent candidate."

The scientists acknowledged funding from Virginia Commonwealth University and the U.S. Department of Energy, Basic Energy Sciences.

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New catalyst promises cheaper, greener drugs

A chemistry team at the University of Toronto has discovered environmentally-friendly iron-based nanoparticle catalysts that work as well as the expensive, toxic, metal-based catalysts that are currently in wide use by the drug, fragrance and food industry.

"It is always important to strive to make industrial syntheses more green, and using iron catalysts is not only much less toxic, but it is also much more cost effective," said Jessica Sonnenberg, a PhD student and lead author of a paper published this week in the Journal of the American Chemical Society.

The research, which was directed by Robert Morris, chair of the Department of Chemistry, involved several steps. Suspecting the existence of nanoparticles, the team first set out to identify the iron catalysts. They then conducted investigations using an electron microscope to confirm that the iron nanoparticles were actually being formed during catalysis. The next step was to ensure that the iron nanoparticles were the active catalytic agents. This was done with polymer and poisoning experiments which showed that only the iron atoms on the surface of a nanoparticle were active.

But a further challenge remained. "Catalysts, even cheap iron ones developed for these types of reaction, still suffer one major downfall," explained Sonnenberg. "They require a one-to-one ratio of very expensive organic ligands -- the molecule that binds to the central metal atom of a chemical compound -- to yield catalytic activity. Our discovery of functional surface nanoparticles opens the door to using much smaller ratios of these expensive compounds relative to the metal centres. This drastically reduces the overall cost of the transformations."

The research team included Neil Coombs at U of T's Centre for Nanostructure Imaging and Imagetek Analytical Imaging Inc., and Paul Dube of the Brockhouse Institute for Materials Research at McMaster University in Hamilton, Ontario. Funding was provided by the Natural Sciences and Engineering Research Council of Canada as a Discovery grant to Morris and as an Alexander Graham Bell Canada Graduate Scholarship and Vanier Scholarship to Sonnenberg.

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Journal Reference:

Jessica F. Sonnenberg, Neil Coombs, Paul A. Dube, Robert H. Morris. Iron Nanoparticles Catalyzing the Asymmetric Transfer Hydrogenation of Ketones. Journal of the American Chemical Society, 2012; 120326165122000 DOI: 10.1021/ja211658t

New material cuts energy costs of separating gas for plastics and fuels

 A new type of hybrid material developed at the University of California, Berkeley, could help oil and chemical companies save energy and money -- and lower their environmental impacts -- by eliminating an energy-intensive gas-separation process.

Today, to separate hydrocarbon gas mixtures into the pure chemicals needed to make plastics, refineries "crack" crude oil at high temperatures -- 500 to 600 degrees Celsius -- to break complex hydrocarbons into lighter, short-chain molecules. They then chill the gaseous mixture to 100 degrees below zero Celsius to liquefy and divide the gases into those destined for plastics and those used as fuel for home heating and cooking.

"Cryogenic distillation at low temperatures and high pressures is among the most energy-intensive separations carried out at large scale in the chemical industry, and an environmental problem because of its contributions to global climate change," said Jeffrey Long, a professor of chemistry at the UC Berkeley and a faculty researcher at Lawrence Berkeley National Laboratory.

Long and his UC Berkeley colleagues now have created an iron-based material -- a metal-organic framework, or MOF -- that can be used at high temperatures to efficiently separate these gases while eliminating the chilling.

"You need a very pure feedstock of propylene and ethylene for making some of the most important polymers, such as polypropylene, for consumer products, but refineries dump a lot of energy into bringing the high temperature gases down to cryogenic temperatures," Long said. "If you can do the separation at higher temperatures, you can save that energy. This material is really good at doing these particular separations."

"The research conducted by the Long group exemplifies the potential of MOF-based materials relative to olefin/paraffin separations," said chemist Peter Nickias, a Dow Fellow at Dow Chemical Company in Michigan who was not involved in the research. "More specifically, the ability of the reported iron-based MOF to separate a variety of unsaturated hydrocarbons from saturated species not only shows the versatility of the iron-MOF system, but also clearly reveals the potential of MOFs as alternative adsorbents."

In the chemical industry, ethylene and propylene are called olefins, while methane, ethane and propane are called paraffins.

Long and his colleagues at UC Berkeley, the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., and the University of Amsterdam in the Netherlands report their findings in the March 30 issue of Science.

MOFs for natural gas purification

The iron-MOF is also good at purifying natural gas, which is a mixture of methane and various types of hydrocarbon impurities that have to be removed before the gas can be used by consumers. These impurities can then be sold for other uses, Long said.

"MOF compounds have a very high surface area, which provides lots of area a gas mixture can interact with, and that surface contains iron atoms that can bind the unsaturated hydrocarbons," Long said. "Acetylene, ethylene and propylene will stick to those iron sites much more strongly than will ethane, propane or methane. That is the basis for the separation."

Nickias noted that increased supplies of natural gas from shale have provided more opportunity to extract and use ethylene and propylene from natural gas, and a variety of materials and approaches are being examined to cut energy use during the refining and purification of olefins.

"Significant energy savings could be achieved if a non-distillation separation could be implemented, or more realistically, the load on a cryogenic distillation unit can be reduced via upstream modifications to the process," Nickias said.

Petroleum refined for the chemical industry is typically a mix of hydrocarbons, primarily two-carbon molecules -- ethane, ethylene and acetylene -- and three-carbon chains -- propane and propylene. Cryogenic distillation separates these compounds -- all of them gases at room temperature -- by liquefying them at low temperatures and high pressure, which causes them to separate by density. Ethylene and propylene go into plastic polymers, while ethane and propane are typically used for fuel.

The researchers found that when pumping a gas mixture through the iron-based MOF (Fe-MOF-74), the propylene and ethylene bind to the iron embedded in the matrix, letting pure propane and ethane through. In their trials, the ethane coming out was 99.0 to 99.5 percent pure. The propane output was close to 100 percent pure, since no propylene could be detected.

After the ethane and propane emerge, the MOF can be heated or depressurized to release ethylene and propylene pure enough for making polymers.

"Once you saturate the material - with ethylene, for example -- you shut off the valve, stop the feed gas, warm up the absorber unit and the ethylene would come out in pure form as a gas," Long said.

MOFs are like packed soda straws

Through a microscope, Fe-MOF-74 looks like a collection of narrow tubes packed together like drinking straws in a box. Each tube is made of organic materials and six long strips of iron, which run lengthwise along the tube. Analysis by Long's colleagues at the NIST Center for Neutron Research showed that different light hydrocarbons have varied levels of attraction to the tubes' iron. By passing a mixed-hydrocarbon gas through a series of filters made of the tubes, the hydrocarbon with the strongest affinity can be removed in the first filter layer, the next strongest in the second layer, and so forth.

"It works well at 45 degrees Celsius, which is closer to the temperature of hydrocarbons at some points in the distillation process," said coauthor Wendy Queen, a postdoctoral fellow at NIST who worked for six months in Long's UC Berkeley lab. "The upshot is that if we can bring the MOF to market as a filtration device, the energy-intensive cooling step potentially can be eliminated. We are now trying out metals other than iron in the MOF in case we can find one that works even better."

Long and his laboratory colleagues are developing iron-based MOFs to capture carbon from smokestack emissions and sequester it to prevent its release into the atmosphere as a greenhouse gas. Similar MOFs, which can be made with different pore sizes and metals, turn out to be ideal for separating different types of hydrocarbons and for storing hydrogen and methane for use as fuel.

Long's other colleagues are UC Berkeley graduate students Eric D. Bloch and Joseph M. Zadrozny; Rajamani Krishna of the Van't Hoff Institute for Molecular Sciences at the University of Amsterdam; and Craig M. Brown of NIST and The Bragg Institute at the Australian Nuclear Science and Technology Organisation in Menai, New South Wales.

The research is part of the Center for Gas Separations Relevant to Clean Energy Technologies, an Energy Frontier Research Center funded by the Department of Energy that focuses primarily on creating novel materials for capturing and storing carbon dioxide.

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The above story is reprinted from materials provided by University of California - Berkeley. The original article was written by Robert Sanders.

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