New Facility Will Merge Biotech and Nanotech

John Toon, Georgia Institute of Technology, Atlanta -- Semiconductor International, 4/1/2007

The fabrication of ICs takes advantage of the fusion of bottom-up nanotechnology with top-down construction. For example, wafer processing starts when a tiny "seed" crystal is dipped into a crucible of molten silicon. Around the seed, untold numbers of silicon atoms perfectly align themselves with the seed's crystalline structure, creating a self-assembled single-crystal ingot of silicon. Of course, this is combined with the top-down processes — lithography patterning, depositing insulating and conductive layers, etc. — to make ICs.

But with the end of traditional silicon scaling in sight, scientists now ask which technology will be at the heart of the next generation of technology innovation? Jim Meindl doesn't know exactly, but he believes it will involve the fusion of another set of top-down and bottom-up technologies — this one involving the basic mechanisms that govern living creatures. As director of Georgia Tech's Nanotechnology Research Center , Meindl leads the development of an $80M facility that will support a vision for a new kind of technology based on the merger of biological and physical sciences at the nanometer scale.

"Plants, animals and people are the most stunning examples of self-assembly that anyone can point to," Meindl noted. "I believe it is going to take another more elegant, clever and spectacular fusion of bottom-up and top-down nanotechnology to get the breakthrough we need to move from silicon to whatever is next."

One possibility involves nanowires that combine semiconducting and piezoelectric properties to possibly create new types of nanodevices (see "Nano-Piezotronics: Zinc Oxide Nanowires Allow New Class of Nanoelectronics"). For instance, a nano-piezotronic sensor could determine blood pressure within the body by measuring the current flowing through a nanostructure.

Performing this type of research requires a different type of cleanroom. Traditional microelectronics cleanrooms operate under positive pressure to keep dust out and limit humidity. Life sciences cleanrooms work under negative pressure to keep microbes in. "I'm not aware of another facility in the world that has been designed to do this integration from the beginning," Meindl added. The new Marcus Nanotechnology Building under construction on the northern part of the Georgia Tech campus (Figs. 1 and 2 ) will have 20,000 ft² of cleanroom devoted to traditional nanotechnology next to 10,000 ft² of cleanroom devoted to biologically based nanotechnology. It is scheduled to open in mid-2008.

1. Professor Jim Meindl directs Georgia Tech's Nanotechnology Research Center. He is shown at the construction site for the $80M Marcus Nanotechnology Building. (Source: Gary Meek)

With its collaborators at Emory University (Atlanta) and other leading institutions, Georgia Tech's nanotechnology and nanoscience program has already demonstrated the potential for merging the disciplines. Three major research initiatives totaling more than $40M are funding nanotechnology research to develop new ways of fighting cancer and repairing DNA damage, for example.

2. Philanthropist Bernie Marcus (left), Georgia Tech President Wayne Clough (center), and University System Chancellor Erroll Davis discuss the details of the Marcus Nanotechnology Building. (Source: Rob Felt)

"In nanomedicine, we have combined a top engineering school with top medical schools, and we are now in a unique position to be able to move into nanomedicine very effectively," said Charles Liotta, Georgia Tech's vice provost for research and graduate studies. "Nanotechnology and nanoscience are platform technologies that impact many other areas of science and technology. We aim to take advantage of what happens at the boundaries between these disciplines."

The nanomedicine efforts build on a nanotechnology program already ranked among the top 25 in the United States for the dollar volume of research. A recent study ranked Georgia Tech third in the nation for the number of nanotechnology researchers that are "highly cited" in peer-reviewed publications, and in the top 10 for the number of first authors publishing in such journals.

Liotta sees more collaboration ahead and benefits for industrial companies, including those based in Georgia. "No one university can do everything on its own," he said. "Nobody has all the intellectual capital or the facilities to meet the needs of interdisciplinary research today." He points to Oak Ridge National Laboratory (Oak Ridge, Tenn.), Imperial College (London) and the National Nanotechnology Infrastructure Network (NNIN, Ithaca, N.Y.) as examples of Georgia Tech's collaborative approach.

Today, Georgia Tech is part of the Focus Center Research Program, supported by the Defense Advanced Research Projects Agency (DARPA) and the semiconductor industry itself. Other universities involved include Stanford (Stanford, Calif.), The Massachusetts Institute of Technology (MIT, Cambridge, Mass.), the University of California at Berkeley and the University of Texas (Austin, Texas).

Meindl expects the new 160,000 ft² facility to attract industrial companies wanting to share in the university's vision. More than two dozen industrial companies use the cleanroom facilities already. The facility includes state-of-the-art equipment, including an electron-beam lithography tool able to produce feature sizes of 5–10 nm.

3. Ph.D. student Ram Krithivasan examines a SiGe chip inside a cryogenic test station. The chip operates at 500 GHz at cryogenic temperatures (350 GHz at room temperature). (Source: Gary Meek)

Georgia Tech is already known for its semiconductor work, addressing such key issues as interconnects, cooling, power supply and packaging, Meindl noted. An example of its more traditional semiconductor accomplishments include its collaboration with IBM (East Fishkill, N.Y.) to demonstrate the first silicon-germanium transistor able to operate at frequencies above 500 GHz. "For the first time, Georgia Tech and IBM have demonstrated that speeds of half a trillion cycles per second can be achieved in a commercial silicon-based technology using large wafers and silicon-compatible low-cost manufacturing techniques," said John D. Cressler, Byers Professor in Georgia Tech's School of Electrical and Computer Engineering and a researcher in the Georgia Electronic Design Center (GEDC) at Georgia Tech. "This work redefines the upper bounds of what is possible using silicon-germanium nanotechnology techniques."

The silicon-germanium heterojunction bipolar transistors built by the IBM/Georgia Tech team operated at frequencies above 500 GHz at 4.5 K (-451°F) — a temperature attained using liquid helium cooling. At room temperature, these devices operated at ~350 GHz. Performance measurements were made using a specialized high-frequency test system (Fig. 3 ). Simulations suggest that the technology could ultimately support much higher (near-Terahertz) operational frequencies at room temperature, Cressler said.

Historically, Georgia Tech has not always been well known among other world-class universities in terms of microelectronics research. The university had to play catch up after it initially missed out on much of the technology years ago that is now so important to the world's economy.

At the groundbreaking for the new building, Georgia Tech President Wayne Clough vowed that the institute would be a national leader in nanotechnology, with the new facility fueling rapid growth in Georgia Tech's nanotechnology research. "We had to work really hard to catch up with the microelectronics revolution," he told attendees. "We're not going to miss out on this one."

Nano-Piezotronics: Zinc Oxide Nanowires Allow New Class of Nanoelectronics

Researchers have taken advantage of the unique coupling of semiconducting and piezoelectric properties in zinc oxide nanowires to create a new class of electronic components and devices that could provide the foundation for a broad range of new applications.

So far, researchers have demonstrated field-effect transistors, diodes, sensors and current-producing nanogenerators that operate by bending zinc oxide nanowires and nanobelts. The new components take advantage of the relationship between the mechanical- and electronic-coupled behaviors of piezoelectric nanomaterials, a mechanism the researchers call nano-piezotronics.

"Nano-piezotronics utilizes the coupling of piezoelectric and semiconducting properties to fabricate novel electronic components," said Zhong Lin Wang, a Regents Professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. "These devices could provide the fundamental building blocks that would allow us to create a new area of electronics."

For example, in a nano-piezotronic transistor, bending a 1-D zinc oxide nanostructure alters the distribution of electrical charges, providing control over the current flowing through it. By measuring changes in current flow through them, nano-piezotronic sensors can detect forces in the nano- or pico-Newton range. Other nano-piezotronic sensors can determine blood pressure within the body by measuring the current flowing through the nanostructures. And, an electrical connection made to one side of a bent zinc oxide nanostructure creates a piezotronic diode that limits current flow to one direction.

The nano-piezotronic mechanism takes advantage of the fundamental property of nanowires or nanobelts made from piezoelectric materials: Bending the structures creates a charge separation — positive on one side and negative on the other. The connection between bending and charge creation has also been used to create nanogenerators that produce measurable electrical currents when an array of zinc oxide nanowires is bent and then released.

"The future of nanotechnology research is in building integrated nanosystems from individual components," Wang said. "Piezotronic components based on zinc oxide nanowires and nanobelts have several important advantages that will help make such integrated nanosystems possible." These include:

Zinc oxide nanostructures can tolerate large amounts of deformation without damage, allowing their use in flexible electronics, such as folding power sources.
The large amount of deformation possible permits a large volume density of power output.
Zinc oxide materials are biocompatible, allowing their use in the body without toxic effects.
The flexible polymer substrate used in nanogenerators would allow implanted devices to conform to internal structures in the body.

In comparison to conventional electronic components, the nano-piezotronic devices operate much differently. In conventional field-effect transistors, for instance, an electrical potential — called the gate voltage — is applied to create an electrical field that controls the flow of current between the device's source and drain (S/D). In the piezotronic transistors developed by Wang and his research team, current flow is controlled by changing the conductance of the nanostructure by bending it between the S/D electrodes (Fig. 1 ). The bending produces a "gate" potential across the nanowire, and the resulting conductance is directly related to the degree of bending applied.

1. In the piezotronic field-effect transistor, the current flow is controlled by changing the conductance of the nanostructure by bending it between the source and drain electrodes. (Source: Zhong Lin Wang)

"The effect is to reduce the width of the channel to carry the current, so you can have a tenfold difference in the conductivity before and after the bending," Wang explained.

Diodes, which restrict the flow of current to one direction, have also been created through nano-piezotronic mechanisms to take advantage of a potential barrier created at the interface between the electrode and tensile (stretched) side of the nanowire by mechanical bending. The potential barrier created by the piezoelectric effect limits the flow of current to one direction.

Nanogenerators harvest energy from the environment around them, converting mechanical energy from body movement, muscle stretching, fluid flow or other sources into electricity. By producing current from the bending and releasing of zinc oxide nanowires (Fig. 2 ), these devices could eliminate the need for batteries or other bulky sources for powering nanometer-scale systems. Piezotronic nanosensors measure nano-Newton forces by examining the shape of the structure under pressure. Implantable sensors based on the principle could continuously measure blood pressure inside the body and relay the information wirelessly to an external device similar to a watch, Wang said. The device could be powered by a nanogenerator harvesting energy from blood flow.

2. Two probes are used to bend a single zinc oxide nanowire. The current flow (right) is a direct function of the degree of bending. (Source: Zhong Lin Wang)

Other nanosensors can detect very low levels of specific compounds by measuring the current change created when molecules of the target are adsorbed to the nanostructure's surface. "Utilizing this kind of device, you could potentially sense a single molecule because the surface area-to-volume ratio is so high," Wang said.

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New Facility Will Merge Biotech and Nanotech

John Toon, Georgia Institute of Technology, Atlanta -- Semiconductor International, 4/1/2007

Nano-Piezotronics: Zinc Oxide Nanowires Allow New Class of Nanoelectronics

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