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Guru plumbs nanotech in all dimensions

Posted: 16 Mar 2005 ?? ?Print Version ?Bookmark and Share

Keywords:nanotechnology? stanford university? yoshio nishi? silicon oxide? cmos?

Nishi: Nano research activities are not that futuristic; NanoMEMS is a promising field.

Yoshio Nishi keeps adding new facets to a sparkling career. Nishi spent 20 years at Toshiba Corp., where he did pioneering work on applying electron-spin resonance techniques to study the silicon-oxide interface. He then jumped the Pacific, eventually heading up silicon technology development at Texas Instruments Inc. Today, he is director of two centers at Stanford University: the Stanford Nanofabrication Facility and the Center for Integrated Systems. Nishi sat down recently with EE Times to talk about nanoelectronics research.

EE Times: There seems to be a certain tension between the traditional silicon people, who claim they are doing nanoscale devices, and those developing alternative types of logic and memories.

Nishi: We can divide this into evolutionary and revolutionary nano.

Evolutionary nano includes silicon-based CMOS with new materials, such as metal gates and high-k, and non-silicon materials for the active semiconducting part, such as germanium. But the basic principles are the same.

Revolutionary nano can be divided into zero-dimensional, 1D and even 2D devices. Zero-dimensional includes quantum dots or nanodots. There are some people who are working on the replacement of chemical dyes, so that red, blue or yellow can be achieved just by adjusting the diameter of a dot.

Nanowires or nanotubes are almost 1D-conduction devices, with reasonably well-understood physics and chemistry. Today, we can use carbon nanotubes to make nmos transistors with very high current densities. That is the good news. What is missing in 1D nano is the capability to grow those wires or tubes in very specific spatial locations and with the needed characteristics. That will require more of an engineering breakthrough for carbon nanotube (CNT) circuits to become a reality.

What about 2D nanotechnology?
2D nano is a little bit premature. I include molecular electronic devices in that category. Organic devices have non-volatility and are used mostly for memory functions, but unfortunately they cannot switch fast enough. Then there are spintronic devices, which have been experimentally verified at low temperatures. Theory shows that they could be operable at room temperatures. I also put the single-electron devices into the 2D category.

Will any of these alternative devices work out?
In a broad sense, nanoelectronics and nanotech have created a lot of excitement within the research community. There is some hype, but there is a lot of real meat inside.

Some people tend to focus on very futuristic stuff, such as nanorobots that move through the body to provide drugs at certain areas, perhaps enabling a new form of chemotherapy.

For the most part, nano research activities are not that futuristic. NanoMEMSnanoscale electromechanical systemsis a promising field.

What is going on in nanoMEMS that is interesting?
NanoMEMS research is mostly in biological applications. One field of research is to measure extremely small forces, such as from the muscle of an ant, where we need to have a high-sensitivity sensor.

People are working on ways to separate a protein, depending on the molecular weight or size. There are structures with many posts standing on a silicon surface, where a molecule will be accelerated either by the density gradient or by the electric field. And then, depending on the molecular size or weight, you can separate one from another.

Similar techniques are being used to do DNA sequencing because once DNA is separated, it resembles a large molecule. Those techniques are being developed for use in drug design and research, but they still have some ways to go.

What is the difference between these kinds of research and, say, chemistry?
Nano research is interdisciplinary. It is difficult to separate what is chemical, biological and medical. We can manipulate macromolecules by different means, including micromachines. Microvalves and chemical sensors require and use those nanostructures.

Cross-disciplinary collaboration is a characteristic feature of Stanford and other institutions of a similar caliber. Stanford's initiative for nanoscale materials and processes involves two professors from the EE department, three professors of materials science and engineering, two chemical engineers, one mechanical-engineering professor, one professor of applied physics, and others.

Give us a progress report.
We have made excellent progress in terms of finding the metal work function control mechanism for the metal bilayer structure for the MOS gate. We have developed germanium MOSFETs in both the n- and p-channels, developed a quantum chemical understanding of the atomic-layer deposition (ALD) precursor mechanism and figured out how to do area-selective ALD. And we are working on a hafnium dioxide-based high-k gate stack and physical and electrical characterization leading to better design principles for such structures.

Do you think carbon nanotube-based devices will work out practically?
CNTs certainly come under the category of revolutionary nanoelectronics. The good thing is that CNT transistors have much higher hole mobility, especially good for p-type, depletion-mode devices. CNTs can carry five to 10 times more current, per unit of cross-section, compared with silicon.

Some people believe that too much nano research money goes to CNTs and not enough to other areas.
At Stanford, only two or three faculty members are growing CNTs. It is probably the same at Harvard. The equipment needed is inexpensive, so I don't believe we are spending too much on CNT research.

Do you have any suggestions about how the nano research money could be well spent? Are there bottlenecks?
We need some way to update the infrastructure at universities. Stanford is fortunate; there are many companies supporting us. But if I look at the national level, not every university has the infrastructure needed. If we could update that ability to support mostly experimental research, then society could get a greater returnand students would be even better trained.

- David Lammers
EE Times

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