Single-Atom Manipulation Breakthrough Boosts Nanotech Possibilities
Alexander E. Braun, Senior Editor -- Semiconductor International, 2/22/2008 6:45:00 AM
IBM scientists at the Almaden Research Center (San Jose) have demonstrated, for the first time ever, the capability to exactly measure how much force is required to move different individual atoms. This enhanced ability to manipulate matter at the atomic level is a significant milestone in the semiconductor industry’s ongoing effort to attain circuits, by building them from the atom up, that are orders of magnitude smaller than anything that can be produced using today’s most advanced technology.
Listen to an interview with the researchers (Runtime: 12:40)
The different forces are measured via tiny changes in the frequency of a small-quartz tuning fork on an atomic force microscope (AFM). The AFM was used to quantify the forces required to pull individual adsorbates along a surface. Among other factors, it was determined that moving cobalt on Pt(111) requires a lateral force of 210 pN. Unexpectedly, it was also discovered that this force is independent of the vertical force. The lateral force can vary substantially depending on the underlying surface’s chemical nature, as it is only 17 pN for cobalt on Cu(111). For both surfaces, the force on the tip caused by the cobalt atom is nearly spherically symmetric. For manipulating a carbon monoxide molecule, the forces are more complex, markedly deviating from spherical symmetry.
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| AFM tip measuring the force that is required to move a cobalt atom on a crystalline surface. (Source: IBM) |
This is another important benchmark on work that IBM has been engaged in for over two decades. “That was a spectacular time for the scanning tunneling microscope,” said Andreas Heinrich, manager of the low-temperature scanning tunneling microscope (STM) effort at Almaden, recalling how, in 1990, Don Eigler first moved individual atoms to create what has since become an iconic illustration: IBM written across a substrate using individual xenon atoms. “For many, this was the true beginning of nanotechnology because it really demonstrated that it’s possible to do engineering at the atomic scale,” Heinrich said.
Although IBM has accomplished many breakthroughs in various technology areas such as magnetic anisotropy, some of the basic mechanisms involved in these atomic manipulations — such as how much force is required to move a single atom on a surface — were unknown. “We have considerable experience in atomic manipulation, and worked with Professor Franz Giessibl of the Institute of Experimental and Applied Physics at the University of Regensburg (Regensburg, Germany), who is a leader in AFM technology, the technique of choice if you need to measure very small forces,” Heinrich said. Three years ago, Giessibl suggested to IBM that they combine their efforts to build a tool that allows both the measure of these extremely tiny forces and atomic manipulation. “From the very beginning, we set out to produce the tool necessary to do this work.”
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| Andreas Heinrich, Chris Lutz (a coworker in the atom manipulation project) and Markus Ternes. (Source: IBM) |
“To do this level of manipulation, we must be able to work in a very clean environment,” said Markus Ternes, a postdoctoral scientist who works with Heinrich. “All the experiments are done in ultrahigh vacuum, because disturbances by air molecules and dust particles would impede the work. We also had to attain very low temperatures, ~5 K; otherwise, Brownian motion would have shifted the atoms around.” Although ultrahigh vacuum and low temperatures are well-established techniques, the IBM group had to be able to measure these forces on a scale smaller than that of the atoms themselves, requiring them to resort to picometer (pm) lengths — one millionth of a millionth of a meter.
“While the various items that went into producing the tool were already available, we had to go through a considerable period of trial and error just to attain the required stability and then determine how to perform the measurement,” Ternes said. “We knew we could measure forces and manipulate atoms, but we were unsure of exactly what it was we needed to measure. It took us about two years to acquire the necessary experience to operate the equipment and exactly determine what we needed.”
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| The AFM’s miniature 'tuning fork.' It measures the interaction between the microscope’s tip and atoms on a surface. When positioned close to an atom on the surface, the tuning fork's frequency slightly changes. This change is analyzed to determine the force exerted on the atom. (Source: IBM) |
Heinrich pointed out that the near-term Semiconductor Roadmap ends in 2015. “Up to a point, we’re certain that we can scale down existing technologies. Right now, beyond that year, the field is blank and completely open — not a good situation for us to be in. We must drive the science forward and determine what is possible. We’re open to many options. We’re not set on developing a new transistor, we want to find something that does logic operations and whether it is a transistor or not is completely irrelevant. Thus, we are prepared to do weird things. Back in 2002, we showed that it was possible to build a mechanical computing device on the scale of single atoms. AFMs and STMs are great tools to explore all these intriguing possibilities.”
As an example of this multi-directional effort, the group is working with magnetic atoms. “If you place magnetic atoms next to each other, then we can design magnetic properties for data storage applications,” Heinrich said. “We believe that with two or five such atoms, we can come up with something that can store information in a non-volatile memory arrangement. This would be a huge improvement over what we have today.”
The specific work that enables the measurement of the force needed to move atoms is not directly implementable in an application. It provides fundamental properties that must be known to construct anything at the nanoscale level. “At nanoscale, one of these fundamentals is how firmly an atom is affixed to the surface. For example, a ‘sticky’ atom would be used for something that should not move, like a base that must remain in one place, while a ‘slippery’ atom would be used to build an on/off switch, which should move with ease. These are properties that we need to know to be able to develop true nanotechnology.”
Heinrich added that because they work in an industry lab, the science pursued must be driven by IT applications. “We’re not after publishing a paper that shows how much force is needed to move different atoms over different surfaces. What we’ve come up with is a proof of principle, we’ve shown that this is possible and how to do it. We are specifically interested in using the AFM to work on insulating surfaces. So far, we have built most of our atomic structures on metal surfaces. This is good for some things but not for others, because the metal has many conduction electrons which influence magnetic properties. We want to build things like spin structures on thicker insulator — this is what drives our research. I think that we’ll probably be able to do this over the next one to two years.”
