New Polymer Could Improve Semiconductor Manufacturing, Packaging

Daily Science Journal (Feb. 2, 2008) — Researchers at Rensselaer Polytechnic Institute and Polyset Company have developed a new inexpensive, quick-drying polymer that could lead to dramatic cost savings and efficiency gains in semiconductor manufacturing and computer chip packaging.

Researchers from Rensselaer Polytechnic Institute's Department of Physics and Center for Integrated Electronics have developed a new inexpensive, quick-drying polymer that could lead to dramatic cost savings and efficiency gains in semiconductor manufacturing and computer chip packaging. In this series of scanning electron microscope images of the new PES polymer in a UV-imprint lithography application, the well-defined pattern indicates the material’s potential for use in next-generation chip making techniques. (Credit: Rensselaer Polytechnic Institute)

Along with allowing enhanced performance and cost savings for conventional photolithography processes, the new material, called polyset epoxy siloxane (PES), should also enable a new generation of lower-cost, on-chip nanoimprinting lithography technology, according to the researchers.

"With this new material, chip manufacturers will be able to trim several steps from their production and packaging processes, and in turn realize a cost savings," said Toh-Ming Lu, the R.P. Baker Distinguished Professor of Physics at Rensselaer, who oversaw the study. "PES is cheaper and more reliable."

The widely adopted technique of photolithography involves using a mix of light and chemicals to generate intricate micro- and nano-scale patterns on tiny areas of silicon. As part of the process, a thin polymer film -- called a redistribution layer, and crucial to the effectiveness of device -- is deposited onto the silicon wafer, in order to ease the signal propagation delay and to protect the chip from different environmental and mechanical factors.

The new PES material developed by Lu's group and Polyset Company is one such thin polymer film, and it offers several advantages over the incumbent materials typically used in the semiconductor manufacturing industry. In addition, their new PES material can also be used as a thin polymer film for ultraviolet (UV) on-chip nanoimprinting lithography technology, which is still in the early phases of development. The consistency of using PES in conventional technology, and then continuing to use PES while academia and industry test and gradually migrate to the next generation of devices, should help ease the transition, Lu said.

"Having the ability to use one material -- our new PES -- for both photolithography and imprint will be very attractive to manufacturers," Lu said. "At its core, our project is basic research, but it also has important industry implications. It's very exciting."

Manufacturers today typically use benzocyclobutene and polyimide as polymers for redistribution layers, because of their low water absorption, thermal stability, low curing temperature, low thermal expansion, low dielectric constant, and low leakage current. Lu said PES offers significant advantages to these materials, particularly in the areas of cure temperature and water uptake.

PES cures, or dries and hardens, at 165 degrees Celsius, about 35 percent cooler than the other two materials. The need for less heat should translate directly into lower overhead costs for manufacturers, Lu said. Another advantage of PES is its low water uptake rate of less than 0.2 percent, less than the other materials. Additionally, PES adheres well to copper and can easily be made less brittle if needed. All of these attributes make PES a promising candidate for redistribution layer application and UV imprint lithography.

"The results demonstrate that PES is feasible to be used as UV-curable resist for both the redistribution application for electronic packaging and micro/nano imprint lithography," said Rensselaer Research Associate Pei-I Wang, co-author of the new paper, published recently in the Journal of Vacuum Science and Technology B.

Along with photolithography and on-chip nanoimprinting lithography, PES holds the potential for applications in other optical devices, flat-panel display, biotechnology devices, and microelectromechanical systems, Wang said.

In addition to Lu and Wang, co-authors on the paper include Rensselaer materials science and engineering professor Omkaram Nalamasu, who is also chief technical officer of Applied Materials Inc. in Santa Clara, Calif.; Rajat Ghoshal and Ram Ghoshal of Polyset Co. Inc. in Mechanicville, N.Y.; Charles Schaper of Transfer Devices Inc. in Santa Clara, Calif.; and Andrew Li of Applied Materials.

The project was funded through the New York State Foundation for Science, Technology and Innovation.

Adapted from materials provided by Rensselaer Polytechnic Institute.

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New Method For Creating Self-Assembling, Nanoscale Materials

Daily Science Journal (Feb. 2, 2008) — While biomedical, electronics, and other branches of research are marching steadily into the realm of the smaller-than-small nanometer scale, building needed materials at this scale has been problematic.

Recently, however, a team from The Scripps Research Institute unveiled a novel approach to the problem that yields a material with novel properties, which some might find reminiscent of Flubber. The material is produced using naturally occurring proteins as templates for uniform, self-assembled, nano-scale construction.

The material, an organic polymer described in today's early, online edition of the Proceedings of the National Academy of Sciences (PNAS), could one day find application in everything from screening for disease to microelectronics.

Using Nature's Tricks

Nature is replete with examples of molecules such as DNA that self-assemble with uniform patterns on the nanoscale, but until now researchers have had limited success duplicating such processes. The new study, however, provides one synthetic method that has effectively mimicked the templating strategy used in nature for nanoscale construction in the lab.

To create the new material, the Scripps Research team, led by Scripps Research President Richard Lerner and Assistant Professor Tobin Dickerson, began with a natural nanoscale product, a bacterial virus or phage. A nanometer is one billionth of a meter, or the width of a few average atoms.

Specifically, the product the team worked with was a phage known as M13. If scaled up, the phage is proportionally equivalent to a 4-foot-long pencil, with the tip and eraser roughly representing the active parts of the phage that infect bacteria. Other proteins that are biologically inert and analogous to the wood body of the pencil provide the filamentous phage's structure.

Having worked with phage extensively in other applications, the team decided to explore the possibility of using those structural proteins as a potential template for nanoscale construction. To do so, the team chemically modified molecular protrusions on the proteins so they would attract and bind with the components needed to form strands of polyacrylamide, a common polymer used to make laboratory gels.

The resulting polymer-phage combination, which twists into helices like DNA and RNA molecules, takes on the shape of a comb with the polymers as the teeth. These teeth in turn interlock to form a strangely resilient, rubbery solid.

Shocking Flexibility

Once the new material, known as a protein-polymer bioconjugate, was created, the group was shocked to find that it was almost impossible to break a sample apart. It could be sliced, but no matter how hard researchers compressed or squeezed it, it always bounced back to its original state, because the stable phage proteins act like rebar in concrete to provide strength.

Further analyses uncovered additional important characteristics. The combs do not grow completely uniformly—some combs grow more teeth than others, for instance, before interlocking with a nearby comb. But the combs can only be a prescribed distance apart for the chemical interlocking to occur, which leads to uniform size for the pores between the comb teeth. The pores proved to be about 4 nanometers wide and greater than 1 micrometer (one millionth of a meter) in length.

This uniformity is in stark contrast with experiments which showed that simply mixing the phages with polymers without the templating procedure produces a chaotic hodge podge at the molecular level.

Interestingly, after creating the material, the group discovered that a British scientist had done theoretical calculations about how flexible rods—like the phages—would pack together using the least amount of energy.

"It was extremely gratifying to see that the mathematics had exactly predicted what we were observing," says Dickerson

Putting It to Use

Although the work was intended mainly as a proof of concept for using the phages as templates, the researchers have a number of potential applications in mind. The phage can be produced easily and cheaply in very large quantities, and the polymer components are readily available and simple to combine to create the final material. These characteristics would make commercial application possible.

"In essence, bacteriophage-derived materials are a renewable resource," says Dickerson.

The defined channels established by the pores in the material might be adapted as pathways for electrons for use in microelectronics. Or, the pores might be used as a filter for certain molecules, for example to test blood samples for proteins whose presence is tied to particular diseases. More complex potential uses might include altering the biologically active portions of the phage to attract specific molecules, forcing them into the polymers' pores, or to block others.

"These tools can be visualized like Tinkertoys ® or Legos ®," says Dickerson of the possibilities. "You can think about this really in engineering terms using macroscopic analogs such as baskets, or lids, or holes."

To add to the list of potential applications, the team has already begun exploring additional materials that might be created using the basic phage construction scheme.

In addition to Dickerson and Lerner, authors on the paper, entitled "Biologically templated organic polymers with nanoscale order," were Bert Willis, Lisa Eubanks, Malcolm Wood, and Kim Janda, all of Scripps Research. See http://www.pnas.org/cgi/content/abstract/0711308105v1.

The work was supported by the Skaggs Institute for Chemical Biology, Worm Institute for Research and Medicine, and a National Institutes of Health Kirschstein National Research Service Award.

Adapted from materials provided by Scripps Research Institute.

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Researchers Create Gold Aluminum, Black Platinum, Blue Silver Using Tabletop Laser

Daily Science Journal (Feb. 2, 2008) — Using a tabletop laser, University of Rochester optical scientists have turned pure aluminum, gold. And blue. And gray. And many other colors. And it works for every metal tested, including platinum, titanium, tungsten, silver, and gold.

Gold Aluminum, Blue Titanium, Gold Platinum. (Credit: Richard Baker, University of Rochester)

Chunlei Guo, the researcher who a year ago used intense laser light to alter the properties of a variety of metals to render them pitch black, has pushed the same process further in a paper in today's Applied Physics Letters. He now believes it's possible to alter the properties of any metal to turn it any color—even multi-colored iridescence like a butterfly's wings.

Since the process changes the intrinsic surface properties of the metal itself and is not just a coating, the color won't fade or peel, says Guo, associate professor of optics at the Institute of Optics at the University of Rochester. He suggests the possibilities are endless—a cycle factory using a single laser to produce bicycles of different colors; etching a full-color photograph of a family into the refrigerator door; or proposing with a gold engagement ring that matches your fiancée's blue eyes.

"Since the discovery of the black metal we've been determined to get full control on getting metals to reflect only a certain color and absorb the rest, and now we finally can make a metal reflect almost any color we wish," says Guo. "When we first found the process that produced a gold color, we couldn't believe it. We worked in the lab until midnight trying to figure out what other colors we could make."

Guo and his assistant, Anatoliy Vorobeyv, use an incredibly brief but incredibly intense laser burst that changes the surface of a metal, forming nanoscale and microscale structures that selectively reflect a certain color to give the appearance of a specific color or combinations of colors.

The metal-coloring research follows up on Guo's breakthrough "black metal" discovery in late 2006, when his research team was able to create nanostructures on metal surfaces that absorbed virtually all light, making something as simple as regular aluminum into one of the darkest materials ever created.

Guo's black metal, with its very high absorption properties, is ideal for any application where capturing light is desirable. The potential applications range from making better solar energy collectors, to more advanced stealth technology, he says. The ultra-brief/ultra-intense light Guo uses is produced by a femtosecond laser, which produces pulses lasting only a few quadrillionths of a second. A femtosecond is to a second what a second is to about 32 million years. During its brief burst, Guo's laser unleashes as much power as the entire electric grid of North America does, all focused onto a spot the size of a needlepoint.

The intense blast forces the surface of the metal to form nanostructures—pits, globules, and strands that response incoming light in different ways depending on the way the laser pulse sculpted the structures. Since the structures are smaller than the wavelength of light, the way they reflect light is highly dependent upon their specific size and shape, says Guo. Varying the laser intensity, pulse length, and number of pulses, allows Guo to control the configuration of the nanostructures, and hence control what color the metal reflects.

Guo and Vorobyev also achieve the iridescent coloring by creating microscale lines covered with nanostructures. The lines, arranged in regular rows, cause reflected light of different wavelengths to interfere differently in different directions. The result is a piece of metal that can appear solid purple from one direction, and gray from another, or multiple colors all at once.

To alter an area of metal the size of a dime currently takes 30 minutes or more, but the researchers are working on refining the technique. Fortunately, despite the incredible intensity involved, the femtosecond laser can be powered by a simple wall outlet, meaning that when the process is refined, implementing it should be relatively simple.

The new process has worked on every metal Guo has tried, and the results are so consistent that he believes it will work for every metal known. His team is currently working to find the right tuning to create the rest of the rainbow for the solid-colored metal, including red and green.

Adapted from materials provided by University of Rochester.

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New Kind Of Transistor Radios Shows Capability Of Nanotube Technology

Daily Science Journal (Jan. 30, 2008) — Carbon nanotubes have a sound future in the electronics industry, say researchers who built the world's first all-nanotube transistor radios to prove it.

Schematic exploded view of a radio-frequency transistor that uses parallel, aligned arrays of carbon nanotubes for the semiconductor. (Credit: Images courtesy John Rogers)

The nanotube radios, in which nanotube devices provide all of the active functionality in the devices, represent "important first steps toward the practical implementation of carbon-nanotube materials into high-speed analog electronics and other related applications," said John Rogers, a Founder Professor of Materials Science and Engineering at the University of Illinois.

Rogers is a corresponding author of a paper* that describes the design, fabrication and performance of the nanotube-transistor radios, which were achieved in a close collaboration with radio frequency electronics engineers at Northrop Grumman Electronics Systems in Linthicum, Md.

"These results indicate that nanotubes might have an important role to play in high-speed analog electronics, where benchmarking studies against silicon indicate significant advantages in comparably scaled devices, together with capabilities that might complement compound semiconductors," said Rogers, who also is a researcher at the Beckman Institute and at the university's Frederick Seitz Materials Research Laboratory.

Practical nanotube devices and circuits are now possible, thanks to a novel growth technique developed by Rogers and colleagues at the U. of I., Lehigh and Purdue universities, and described last year in the journal Nature Nanotechnology.

The growth technique produces linear, horizontally aligned arrays of hundreds of thousands of carbon nanotubes that function collectively as a thin-film semiconductor material in which charge moves independently through each of the nanotubes. The arrays can be integrated into electronic devices and circuits by conventional chip-processing techniques.

"The ability to grow these densely packed horizontal arrays of nanotubes to produce high current outputs, and the ability to manufacture the arrays reliably and in large quantities, allows us to build circuits and transistors with high performance and ask the next question," Rogers said. "That question is: 'What type of electronics is the most sensible place to explore applications of nanotubes"' Our results suggest that analog RF (radio frequency) represents one such area."

As a demonstration of the growth technique and today's nanotube analog potential, Rogers and collaborators at the U. of I. and Northrop Grumman fabricated nanotube transistor radios, in which nanotube devices provided all of the key functions.

The radios were based on a heterodyne receiver design consisting of four capacitively coupled stages: an active resonant antenna, two radio-frequency amplifiers, and an audio amplifier, all based on nanotube devices. Headphones plugged directly into the output of a nanotube transistor. In all, seven nanotube transistors were incorporated into the design of each radio.

In one test, the researchers tuned one of the nanotube-transistor radios to WBAL-AM (1090) in Baltimore, to pick up a traffic report.

"We were not trying to make the world's tiniest radios," Rogers said. "The nanotube radios are a demonstration, an important milestone toward building the technology into a form that ultimately would be commercially competitive with entrenched approaches."

*The paper has been accepted for publication in the Proceedings of the National Academy of Sciences, and is to be published in PNAS Online Early Edition in the first week in February, 2008.

The work was funded by the National Science Foundation and the U.S. Department of Energy.

Adapted from materials provided by University of Illinois at Urbana-Champaign.

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You Thought Your Nano Was Small: Nano-sized Detector Turns Radio Waves Into Music

Daily Science Journal (Oct. 19, 2007) — Researchers in California report development of the world's first working radio system that receives radio waves wirelessly and converts them to sound signals through a nano-sized detector made of carbon nanotubes.

The "carbon nanotube radio" device is thousands of times smaller than the diameter of a human hair. The development marks an important step in the evolution of nano-electronics and could lead to the production of the world's smallest radio, the scientists say.

Peter Burke and Chris Rutherglen developed a carbon nanotube "demodulator" that is capable of translating AM radio waves into sound. In a laboratory demonstration, the researchers incorporated the detector into a complete radio system and used it to successfully transmit classical music wirelessly from an iPod to a speaker several feet away from the music player.

Although other researchers have developed nano-sized radio wave detectors in the past, the current study marks the first time that a nano-sized detector has been demonstrated in an actual working radio system, the scientists say. The study demonstrates the feasibility of making other radio components at the nanoscale in the future and may eventually lead to a "truly integrated nanoscale wireless communications system," they say. Such a device could have numerous industrial, commercial, medical and other applications.

Their findings appeared online October 17 and are scheduled for publication in the Nov. 14 print edition of ACS' Nano Letters.

Adapted from materials provided by American Chemical Society, via EurekAlert!, a service of AAAS.

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