When IBM was first manufacturing metallic thin film interconnects, a component required to make integrated circuits in the microelectronics industry, one of the earliest materials of choice was aluminum. But this material is a poor conductor of electricity, and in late 1997 the computer giant announced it would begin using copper film interconnects in its computer chips.
The fast pace of technology brought another announcement on Aug. 4, 1998. Big Blue reported in the Wall Street Journal that it had “perfected a long-sought process to make computer chips that are up to 35 percent faster.” IBM added that these chips would use much less power, consequently extending the battery life of laptops, cell phones, and other products. This process, called silicon on insulator or SOI, acts as a sheath for the millions of transistors on a chip, reducing molecular-level leaks that often act as a drag on the chip’s performance.
But, says Rick Claus, Virginia’s Outstanding Scientist for 2001, room for improvement still remains. Most thin films formed by conventional processes still “have lots of little defects, mostly pinholes and impurities. These defects cause resistance, and when the resistance is high in an integrated circuit, you lose power and sacrifice speed.”
That’s why Claus, a Virginia Tech professor of electrical and computer and materials science engineering, turned to the burgeoning field of nanotechnology to manufacture thin films. Using nanolevel processing, a material is created molecule by molecule, where each molecule is nanometers in dimension, hence the prefix nano.
Claus says he and his colleagues have perfected a new modified self-assembly process where the molecules are ordered in such a way that they create “nearly perfect materials.” And with this process, they are beginning to manufacture ultra-thin films with electrical resistance that is “100 times better than that of bulk materials like copper and aluminum.” The Virginia Tech professor and his colleagues hold nine patents covering the new technique, called modified electrostatic self-assembly (MESA).
By assembling materials layer by layer, the researchers are able to arrange the molecules in nanometer-size grains that become the essential building blocks of the new materials. As they synthesize the materials, they are able to design them with particular large-scale characteristics in mind.
To date, Claus, the director of the Fiber & Electro-Optics Research Center and president of NanoSonic Inc. of Blacksburg, Va., says they have created a “library” of these self-assembled materials. Many of them are based on MESA processing, and they have demonstrated the synthesis of more than 2,000 individual material layers, or thicknesses, of tens of microns.
“The idea is that we are making materials from the ground up, not the top down. As we build up from the molecular level, we can make devices that are smaller, cheaper, and with multiple functions. At the nano level, particles are more energetic,” Claus explains.
“These new small particles have remarkable electronic, optical, mechanical, and other properties in comparison to larger bulk materials (i.e. metals and semiconductors) of the same molecular composition,” Claus explains. “The trick in making them useful is to collect very large numbers of the nanoparticles and then be able to form them into larger physical systems, with control over their structure at the molecular level.”
One of the keys to Claus’ success is the method his group is using to synthesize these new materials. MESA processing involves the simple dipping of a chosen substrate into alternate aqueous solutions containing positively and negatively charged ionic (anionic and cationic) materials such as: polymer complexes; metal and oxide nanoclusters; cage-structured molecules such as fullerenes; proteins and other biomolecules.
A significant advantage of the MESA process is that manufacturing can be done at room temperature, resulting in great energy savings and in a more environmentally benign process. The difference between MESA and conventional manufacturing can be pictured in this way. A jet engine ceramic fan blade once needed to be sintered (heated without melting) in a huge oven at a temperature greater than 1500 degrees C. However, nanophase ceramic materials similar to those that Claus is developing can be put together in a room not very different from your own kitchen and heated to much lower temperatures, or not at all.
To the folks who inhabit Silicon Valley, MESA processing also means that the already tiny microelectronic circuits available today will become even smaller. “Part of the impact over the long term,” Claus says, “is that electronic devices will become much, much smaller.... Currently microelectronic circuits are micron size and a nanoparticle is a thousand times smaller than a ‘micro’.”
Richard Feynman, the Nobel prize-winning physicist, is credited with getting scientists to think at the nano level when he wrote the scientifically acclaimed article, “There is room at the bottom,” some four decades ago. Claus, who has presented a paper, “Self assembly and nanoelectronics — There is still more room at the bottom,” even goes so far as to describe the current technology for manufacturing microchips as “brute force.” The conventional photolithographic technique used to make today’s microelectronic circuits is analogous “to digging a strip mine,” he says.
“Lightning bugs are like slow biologically self-assembled circuits,” says Claus. “Our MESA design just tells the molecules where to go, and the structure gets the signals in and out fast.” For example, this design would greatly speed up the processing of Internet messages.
“A lot of electronics are not that complicated,” adds Claus, who holds an endowed professorship. “Our job is to put nanotechnology to work.”
Practicing a different type of self-assembly process in the field of nanotechnology is Rick Davis of chemical engineering. He and colleagues Kevin Van Cott in chemical engineering and William Ducker in chemistry are using genetically engineered bacteria to produce synthetic polypeptides, or proteins designed to form self-assembled, nanometer-thin layers at surfaces. The synthetic proteins made by these bacteria can be made with exquisite control of their composition that is essential for a given application.
Proteins are comprised of amino acids strung together like pearls on a necklace to form polymers or chain-like molecules. There are 20 different naturally occurring amino acids from which most proteins are made. The side chains of the 20 naturally occurring amino acids can be hydrophilic, hydrophobic, acidic, basic, and hydrogen bonding. A remarkable feature of proteins is the immense number of possible compositions even for relatively small proteins. For a protein chain just 100 amino acids long, there are 10020 different combinations possible, “creating a marvelous palette that engineers and scientists can draw upon in designing and discovering new materials,” Davis says.
“Biochemists have developed remarkable tools for manipulating DNA and creating novel proteins over the last 25 years. Engineers, chemists, and materials scientists have just recently begun using these tools to make synthetic protein materials for nonbiological purposes,” Davis says. With Van Cott doing the biosynthesis part of the work, Davis and Ducker are investigating how synthetic proteins adsorb on surfaces.
“We are focusing on making a material called a block copolymer that is comprised of an anchor block designed to stick strongly to a surface and a tail block designed to stay in solution and not be absorbed,” Davis says. The result would be a water-soluble polymer molecule that would assemble itself onto a surface and form a layer that, if one could see it in a microscope, would look like a tiny brush several nanometers thick. These brush-like layers can be very useful for modifying surfaces for applications such as adhesion, lubrication, and particle processing.
In particle processing, these tiny brushes, when adsorbed onto particles, generate strong repulsive forces that keep the particles from aggregating. Control of aggregation is important for many particle processes, such as those used to make ceramics, paints, and printing inks.
The practice of dispersing polymers in water to prevent particles from aggregating is not new. The ancient Egyptians and Chinese as far back as 2500 BC made writing ink by dispersing powdered carbon black (obtained from burning oil) in water using naturally occurring polymers, such as proteins from egg whites and milk. In the same regard, nanotechnology is not totally new, but is a “natural evolution of what has been going on in science for a long time,” Davis says. “Scientists have been playing with molecules and learning about their properties and how to use them in reactions for several centuries. What is new is the use of self-assembly techniques in the last two decades to make materials where composition and structure are controlled at the nanometer level with much greater precision than ever before.” An added advantage of the synthetic protein approach is that it is environmentally friendly, a good example of green engineering that has been strongly emphasized at Virginia Tech and elsewhere in recent years. Proteins are inherently biodegradable, unlike most synthetic polymers. In addition, many particle processes are still done using toxic solvents.
“The synthetic proteins under development are designed to work in water so that water could be used in particle processing instead of organic solvents. This could reduce the amount of toxic organic solvents currently used in many particle processes and released into the environment,” Davis says. If the volatile organic compound (VOC) regulations under consideration by the federal government are implemented, then the allowable releases of VOCs would have to be reduced by a factor of five or more by 2008.
With self-assembly processing being the key, Davis is part of a group at Virginia Tech involved in the creation of the Center for Self-Assembled Nanostructures and Devices (CSAND). Chemistry professor Harry Dorn directs CSAND, and Davis serves as an associate director along with Randy Heflin of physics. This sharing in the administration of CSAND provides a truly interdisciplinary outlook.
Davis works with CSAND colleagues Van Cott, Heflin, and Harry Gibson of chemistry on developing new organic materials for the conversion of electrical signals to optical signals. This conversion is necessary whenever voice or computer signals are sent out over fiber optic communications networks. The device used for this conversion step is called an electro-optic (EO) modulator. Current state-of-the-art EO modulators based on inorganic crystalline materials, such as lithium niobate, are relatively expensive (about $2,000 apiece) and are limited to modulation rates of about 10 GHz. EO modulators made with organic materials could, in principle, be at least 10 times faster and much cheaper to build. “There is a joke that ‘www’ now stands for the world wide wait. If EO modulators could be made cheaply enough to be used in someone’s home or office, that could greatly speed up access to the Internet.”
In this project, Davis and his colleagues are using self assembly techniques to make nano-structured polymer films with selected dye molecules that are aligned or oriented. When such a material, typically in the form of a thin film between one and 10 microns thick, is exposed to an electric field, its refractive index can change. The refractive index determines the speed with which a light wave travels through the film. This tunable or switchable refractive index is the basis for changing an electrical signal to an optical signal that would travel through the thin film. “We’re hopeful that our work will lead to improved materials for EO modulators. The prospect of cheaper and faster service with organically based devices is a real possibility,” Davis says.
