Nanoscale Cu Interconnects

The exponential increase in the use of consumer goods such as handheld PCs, cellular phones, high-graphic computer games and everyday home appliances drives high demands for increased chip performance. To accommodate faster chip speeds, the microelectronic industry is constantly working to improve interconnect speeds, which account for as much as half of the circuit delay. As a result, Cu has replaced Al as the material of choice for interconnects due to its superior electrical resistivity, thermal conductivity, melting temperature, and coefficient of thermal expansion (CTE). While novel interconnect materials such as carbon nanotubes may be used as future interconnects, this replacement is unlikely in the present decade. As demand persists, the response of the microelectronics industry to increase device speeds is to downsize the scale of interconnects to nanoscale dimensions (current 100 nm nodes, operating in most modern computers, are scheduled to be reduced to 45 nm nodes by the year 2010).  In this context, a crucial question arises: As the structural scale reduces from the micro to nanometer range, and the microstructural features of Cu approach the dimensions of the defects, how do mechanical properties and electromigration behavior change and what are the operating mechanisms?  To address this essential question, this research work has the following objectives:

• Identify the effect of interconnect line-width on the grain structure and defect substructure of nanoscale Cu interconnects. These studies will allow us to understand the underlying structure present in Cu interconnects of different widths, which will serve as the platform for establishing the mechanisms associated with thermal stresses and applied electrical currents.
• Determine the effect of interconnect line-width on the thermal stress behavior and electromigration behavior of nanoscale Cu interconnects, in particular the role played by defects, such as dislocations and voids, local texture and grain structure.
• Characterize the effects of thermal stresses and electrical current on nano Cu interconnects by performing in-situ TEM observations. The use of this technique is critical in establishing a direct link between microstructural evolution and properties. In this context, the use of in-situ TEM is an essential tool to enhance the reliability analysis of Cu interconnects, as devices are downscaled.

Nano/Submicron Stainless Steels

Austenitic stainless steels (SS) are frequently selected in applications where good corrosion properties and aesthetic considerations are important. For example, austenitic stainless steels (SS) are being considered for use in railcar and truck bodies due to the fact that stainless steel components exhibit lower maintenance costs and better recyclable characteristics when compared to commonly used galvanized steel parts. However, actual austenitic SS lack adequate strength when compared to their galvanized/painted steel counterparts, which limits their widespread use in aforementioned applications.

Furthermore, in cases where an austenitic SS sheet needs to be subjected to cold forming, such as deep-drawing, stamping, it exhibits a large variation in yield strength (above 50%), and undesirable yield strength in regions absent of deformation. In these areas the material can be easily scratched, dented and deformed in service and thus its corrosion properties and appearance strongly deteriorate. This is crucial for applications, such as cell phone covers, kitchenware, automobile parts, decorative items, architectural facades and appliances.

In this context, the research that we have been carried out in collaboration with the University of Oulu, Finland and Outokumpu Stainless, Finland has focused on developing metastable austenitic SS grades with nano/sub-micron grain sizes. These materials exhibit high strength, but also good ductility and formability characteristics. Specifically, these materials are being processed by first heavy cold-rolling metastable austenitic SS to produce deformation induced martensite (a’). Subsequently, upon annealing, the martensite formed during cold-rolling reverts to nano/sub-micron grained austenite.  In this regard, this work has the following objectives:

• To study the effects of alloy composition, and annealing history on the microstructure, austenite/martensite ratio, average grain size, texture and mechanical properties (hardness, stress/strain behavior) of metastable SS.
• To develop annealing treatments, to allow the local control of the microstructure, in order to obtain nano/submicron grain sizes, with outstanding mechanical properties.
  

Deformation of Nanocrystalline Metals

The study and development of nanometer scale materials (nanomaterials) is one of the most promising fields in science and technology today. At the fundamental level, current research in the field of mechanical behavior demonstrates a broad range of fascinating properties for nanomaterials, namely a significant increase in hardness, yield stress, and ductility. As evidence of these unique behaviors and properties emerge, a crucial question arises: As the structural scale reduces from the micro to nanometer range, and the microstructural features of the materials in question approach the dimensions of the defects, how do mechanical properties change and what are the operating mechanisms?
In pursuit of an answer to this vital question, this research work will investigate:

• The inverse Hall-Petch behavior observed in FCC nanocrystalline metals.
• The ductile-brittle transition behavior in BCC nanoscale metals by in-situ straining-cooling TEM.
• The behavior of individual nanoparticles subjected to in-situ nanoindentation.

The results from this work will be of central importance for designing new bulk nanomaterials with novel mechanical properties and will pave the way for future advancements based upon solid, fundamental knowledge.

Formation of Carbon Nanostructures by Metal Dusting

Carbon nanostructures, in particular carbon nanotubes are promising materials for what has become the main research thrust in the beginning of the 21st century, i.e, the field of nanotechnology. Carbon nanotubes exhibit various unique properties, such the capacity to behave as metallic conductors or semiconductors and also the ability of withstanding large stresses with little elastic deformation (high Young’s modulus). Despite these characteristics, in order to generate a large technological impact, carbon nanotubes must be produced in large scales. Therefore, researchers around the world have been devising methodologies to synthesize carbon nanotubes, such as carbon arc discharge, laser vaporization, catalytic combustion, and chemical vapour deposition (CVD). In addition to these processes, one other route which could become promising in the near future for the generation of carbon nanotubes is the phenomenon of metal dusting. Simply, metal dusting is the disintegration of metallic alloys by corrosion, which is initiated by exposure of the alloys to strongly carburizing atmospheres. The result of the decomposition is a mixture of metal particles and carbon structures.

The research work produced so far has shown that both carbon nanotubes and carbon nanorods, of various diameters and lengths, and metal nanoparticles of various shapes and composition were present in the coke formed during metal dusting. However, despite the research work performed so far, there is very little understanding about the role played by the size and shape of nanoparticles on the growth of carbon nanostructures. This research work will attempt to answer some of these questions.

Characterization of Catalyst Nanoparticles for Proton Exchange Membrane Fuel Cells

Over the past 100 years, the world’s temperature has gone up by about 0.6 C due to severe demands in energy levels. Contrary to previous changes in the world’s climate due to variations in Earth’s rotation angle or its distance from the sun, this time there is a completely different factor involved: man-made “greenhouse gases”. At the current rate of increase, the levels of CO2 and other greenhouse gases will reach 800 ppm by the end of this century. As CO2 stays in the atmosphere for up to 200 years, removing those high concentrations will take a long time. If this scenario continues, the world will witness a rise in sea level, ecosystem changes, and a deleterious effect on public health.

Alternative energy technologies are, therefore, no longer an option but a crucial need for sustained economic growth, energy independence, national security, and ultimately the protection of our planet. Among the various alternative energy technologies, fuel cells are very appealing for a variety of applications including transportation, domestic power supplies for individual residences, electronic devices and waste water treatment plants. Among the various types of fuel cells, proton exchange membrane fuel cells (PEM) offer the advantages of low weight and volume, while delivering high power densities. In addition, they need only hydrogen, oxygen from the air, and water to operate and do not require corrosive fluids like other fuel cells. However, PEM fuel cells have been suffering from 1) the need to use expensive and scarcely available platinum (Pt) nanoparticle catalysts, 2) the difficulty in controlling the shape of Pt nanoparticles and thus their activity for oxygen reduction, and 3) the challenge in maintaining the catalyst activity of Pt nanoparticles during fuel cell operation, due to particle agglomeration, which decreases needed surface area.

In this context, less expensive bi-metallic nanoparticles with higher catalyst activities have been developed. The objective of this proposal is to identify the role played by structure and chemical composition of bi-metallic nanoparticles on the activity of proton exchange membrane fuel cells.