IBM creates first graphene based integrated circuit

 

Schematic illustration of a graphene mixer circuit. Image credit: Yu-Ming Lin/Science, 10.1126/science.1204428

Taking a giant step forward in the creation and production of graphene based integrated circuits, IBM has announced in Science, the fabrication of a graphene based integrated circuit on a single chip. The demonstration chip, known as a radio frequency "mixer" is capable of producing frequencies up to 10 GHz, and demonstrates that it is possible to overcome the adhesion problems that have stymied researchers efforts in creating graphene based IC's that can be used in analog applications such as cell phones or more likely military communications.

IBM and others had previously demonstrated that it was possible to create a graphene based transistor; this latest research takes that technology one step further by marrying the transistor and other electronics on a single chip to create a full-fledged integrated circuit.

The graphene circuits were started by growing a two or three layer graphene film on a silicon surface which was then heated to 1400°C. The graphene IC was then fabricated by employing top gated, dual fingered graphene FET?s (field-effect transistors) which were then integrated with inductors. The active channels were made by spin-coating the wafer with a thin polymer and then applying a layer of hydrogen silsequioxane. The channels were then carved by e-beam lithography. Next, the excess graphene was removed with an oxygen plasma laser, and then the whole works was cleaned with acetone. The result is an integrated circuit that is less than 1mm² in total size.

The chip produces output signals with mixed frequencies, hence its name, and while the prototype developed is not expected to be used in an actual device, future chips using the new technology are expected to be used in wireless communications. Since a major source of the funding for the research has been provided by the U.S. Defense Advanced Research Projects Agency (DARPA) the first uses of such a chip in an application is likely to be for secretive communications between airborne military pilots; the ultra high frequencies generated make it ideal for such secure applications.

Getting the graphene to adhere to other electronic components was by all accounts the most difficult part of the whole process, and apparently took the team, led by Phaedon Avouris, all of a year to accomplish; but it was clearly well worth the effort as research teams the world over continue to search for an eventual replacement to silicon in integrated chips, which by most accounts will hit its limits within the next few years.

More information: Wafer-Scale Graphene Integrated Circuit, Science 10 June 2011: Vol. 332 no. 6035 pp. 1294-1297. DOI: 10.1126/science.1204428

ABSTRACT
A wafer-scale graphene circuit was demonstrated in which all circuit components, including graphene field-effect transistor and inductors, were monolithically integrated on a single silicon carbide wafer. The integrated circuit operates as a broadband radio-frequency mixer at frequencies up to 10 gigahertz. These graphene circuits exhibit outstanding thermal stability with little reduction in performance (less than 1 decibel) between 300 and 400 kelvin. These results open up possibilities of achieving practical graphene technology with more complex functionality and performance.

 

Researchers develop biological circuit components, new microscope technique for measuring them

 

Rendering of protein assemblies under an atomic force microscope. Credit: Reprinted with permission from "Direct Probe of Molecular Polarization in De Novo Protein-Electrode Interfaces," Kendra Kathan-Galipeau, Sanjini Nanayakkara, Paul A. OBrian, Maxim Nikiforov, Bohdana M. Discher, Dawn A. Bonnell, ACS Nano, Copyright 2011 American Chemical Society

Electrical engineers have long been toying with the idea of designing biological molecules that can be directly integrated into electronic circuits. University of Pennsylvania researchers have developed a way to form these structures so they can operate in open-air environments, and, more important, have developed a new microscope technique that can measure the electrical properties of these and similar devices.

The research was conducted by Dawn Bonnell, Trustee Chair Professor and director of the Nano/Bio Center, graduate students Kendra Kathan-Galipeau and Maxim Nikiforov and postdoctoral fellow Sanjini Nanayakkara, all of the Department of Materials Science and Engineering in Penn?s School of Engineering and Applied Science. They collaborated with assistant professor Bohdana Discher of the Department of Biophysics and Biochemistry at Penn?s Perelman School of Medicine and Paul A. O?Brien, a graduate student in Penn's Biotechnology Masters Program.

Their work was published in the journal. The development involves artificial proteins, bundles of peptide helices with a photoactive molecule inside. These proteins are arranged on electrodes, which are common feature of circuits that transmit electrical charges between metallic and non-metallic elements. When light is shined on the proteins, they convert photons into electrons and pass them to the electrode.

?It's a similar mechanism to what happens when plants absorb light, except in that case the electron is used for some chemistry that creates energy for the plant,? Bonnell said. ?In this case, we want to use the electron in electrical circuits.? 

Similar peptide assemblies had been studied in solution before by several groups and had been tested to show that they indeed react to light. But there was no way to quantify their ambient electrical properties, particularly capacitance, the amount of electrical charge the assembly holds.

?It?s necessary to understand these kinds of properties in the molecules in order to make devices out of them. We've been studying silicon for 40 years, so we know what happens to electrons there,? Bonnell said.  ?We didn?t know what happens to electrons on dry electrodes with these proteins; we didn't even know if they would remain photoactive when attached to an electrode.? 

Designing circuits and devices with silicon is inherently easier than with proteins. The electrical properties of a large chunk of a single element can be measured and then scaled down, but complex molecules like these proteins cannot be scaled up. Diagnostic systems that could measure their properties with nanometer sensitivity simply did not exist.

The researchers therefore needed to invent both a new way of a measuring these properties and a controlled way of making the photovoltaic proteins that would resemble how they might eventually be incorporated into devices in open-air, everyday environments, rather than swimming in a chemical solution.  

To solve the first problem, the team developed a new kind of atomic force microscope technique, known as torsional resonance nanoimpedance microscopy. Atomic force microscopes operate by bringing an extremely narrow silicon tip very close to a surface and measuring how the tip reacts, providing a spatial sensitivity of a few nanometers down to individual atoms.  

?What we've done in our version is to use a metallic tip and put an oscillating electric field on it. By seeing how electrons react to the field, we?re able to measure more complex interactions and more complex properties, such as capacitance,? Bonnell said.

Bohdana Discher?s group designed the self-assembling proteins much as they had done before but took the additional step of stamping them onto sheets of graphite electrodes.  This manufacturing principle and the ability to measure the resulting devices could have a variety of applications.  

?Photovoltaics ? solar cells ? are perhaps the easiest to imagine, but where this work is going in the shorter term is biochemical sensors,? Bonnell said. 

Instead of reacting to photons, proteins could be designed to produce a charge when in the presence of a certain toxins, either changing color or acting as a circuit element in a human-scale gadget.  

Largest biochemical circuit built out of small synthetic DNA molecules

 

A wiring diagram specifying a system of 74 DNA molecules that constitute the largest synthetic circuit of its type ever made. The circuit computes the square root of a number up to 15 and rounds down to the nearest integer (the discrete square root of a four-bit integer). Credit: Caltech/Lulu Qian

In many ways, life is like a computer. An organism's genome is the software that tells the cellular and molecular machinery?the hardware?what to do. But instead of electronic circuitry, life relies on biochemical circuitry?complex networks of reactions and pathways that enable organisms to function. Now, researchers at the California Institute of Technology (Caltech) have built the most complex biochemical circuit ever created from scratch, made with DNA-based devices in a test tube that are analogous to the electronic transistors on a computer chip.

Engineering these circuits allows researchers to explore the principles of information processing in biological systems, and to design biochemical pathways with decision-making capabilities. Such circuits would give biochemists unprecedented control in designing chemical reactions for applications in biological and chemical engineering and industries. For example, in the future a synthetic biochemical circuit could be introduced into a clinical blood sample, detect the levels of a variety of molecules in the sample, and integrate that information into a diagnosis of the pathology.

"We're trying to borrow the ideas that have had huge success in the electronic world, such as abstract representations of computing operations, programming languages, and compilers, and apply them to the biomolecular world," says Lulu Qian, a senior postdoctoral scholar in bioengineering at Caltech and lead author on a paper published in the June 3 issue of the journal Science.

Along with Erik Winfree, Caltech professor of computer science, computation and neural systems, and bioengineering, Qian used a new kind of DNA-based component to build the largest artificial biochemical circuit ever made. Previous lab-made biochemical circuits were limited because they worked less reliably and predictably when scaled to larger sizes, Qian explains. The likely reason behind this limitation is that such circuits need various molecular structures to implement different functions, making large systems more complicated and difficult to debug. The researchers' new approach, however, involves components that are simple, standardized, reliable, and scalable, meaning that even bigger and more complex circuits can be made and still work reliably.

"You can imagine that in the computer industry, you want to make better and better computers," Qian says. "This is our effort to do the same. We want to make better and better biochemical circuits that can do more sophisticated tasks, driving molecular devices to act on their environment."

To build their circuits, the researchers used pieces of DNA to make so-called logic gates?devices that produce on-off output signals in response to on-off input signals. Logic gates are the building blocks of the digital logic circuits that allow a computer to perform the right actions at the right time. In a conventional computer, logic gates are made with electronic transistors, which are wired together to form circuits on a silicon chip. Biochemical circuits, however, consist of molecules floating in a test tube of salt water. Instead of depending on electrons flowing in and out of transistors, DNA-based logic gates receive and produce molecules as signals. The molecular signals travel from one specific gate to another, connecting the circuit as if they were wires.

Winfree and his colleagues first built such a biochemical circuit in 2006. In this work, DNA signal molecules connected several DNA logic gates to each other, forming what's called a multilayered circuit. But this earlier circuit consisted of only 12 different DNA molecules, and the circuit slowed down by a few orders of magnitude when expanded from a single logic gate to a five-layered circuit. In their new design, Qian and Winfree have engineered logic gates that are simpler and more reliable, allowing them to make circuits at least five times larger.

Their new logic gates are made from pieces of either short, single-stranded DNA or partially double-stranded DNA in which single strands stick out like tails from the DNA's double helix. The single-stranded DNA molecules act as input and output signals that interact with the partially double-stranded ones.

"The molecules are just floating around in solution, bumping into each other from time to time," Winfree explains. "Occasionally, an incoming strand with the right DNA sequence will zip itself up to one strand while simultaneously unzipping another, releasing it into solution and allowing it to react with yet another strand." Because the researchers can encode whatever DNA sequence they want, they have full control over this process. "You have this programmable interaction," he says.

Qian and Winfree made several circuits with their approach, but the largest?containing 74 different DNA molecules?can compute the square root of any number up to 15 (technically speaking, any four-bit binary number) and round down the answer to the nearest integer. The researchers then monitor the concentrations of output molecules during the calculations to determine the answer. The calculation takes about 10 hours, so it won't replace your laptop anytime soon. But the purpose of these circuits isn't to compete with electronics; it's to give scientists logical control over biochemical processes.

Their circuits have several novel features, Qian says. Because reactions are never perfect?the molecules don't always bind properly, for instance?there's inherent noise in the system. This means the molecular signals are never entirely on or off, as would be the case for ideal binary logic. But the new logic gates are able to handle this noise by suppressing and amplifying signals?for example, boosting a signal that's at 80 percent, or inhibiting one that's at 10 percent, resulting in signals that are either close to 100 percent present or nonexistent.

All the logic gates have identical structures with different sequences. As a result, they can be standardized, so that the same types of components can be wired together to make any circuit you want. What's more, Qian says, you don't have to know anything about the molecular machinery behind the circuit to make one. If you want a circuit that, say, automatically diagnoses a disease, you just submit an abstract representation of the logic functions in your design to a compiler that the researchers provide online, which will then translate the design into the DNA components needed to build the circuit. In the future, an outside manufacturer can then make those parts and give you the circuit, ready to go.

The circuit components are also tunable. By adjusting the concentrations of the types of DNA, the researchers can change the functions of the logic gates. The circuits are versatile, featuring plug-and-play components that can be easily reconfigured to rewire the circuit. The simplicity of the logic gates also allows for more efficient techniques that synthesize them in parallel.

"Like Moore's Law for silicon electronics, which says that computers are growing exponentially smaller and more powerful every year, molecular systems developed with DNA nanotechnology have been doubling in size roughly every three years," Winfree says. Qian adds, "The dream is that synthetic biochemical circuits will one day achieve complexities comparable to life itself."