June 16, 2000 EVANSTON, Ill. -- In a paper to be published in the June 9 issue of the journal Science, researchers at Northwestern University demonstrate an eight-pen nanoplotter capable of simultaneously creating eight identical patterns drawn with tiny lines of molecular ink. Each line is only 30 molecules wide and one molecule high.
This breakthrough transforms dip-pen nanolithography (Science, Oct. 15, 1999) from a serial process into a parallel process, paving the way to making it competitive with other optical and stamping lithographic methods used for patterning large areas on metal and semiconductor substrates, including silicon wafers.
"Our multiple-pen, parallel process nanoplotter gives the nanotechnologist a powerful new tool," said Chad Mirkin, George B. Rathmann Professor of Chemistry. "The miniaturization of the plotter writing technique opens up exciting avenues of doing things differently, better and on a much smaller scale than they are today."
Mirkin and fellow author Seunghun Hong, a postdoctoral researcher at Northwestern, report that the nanoplotter could be equipped with a significantly greater number of pens than a mere eight. The technology should be able to support hundreds, or even a thousand, of tiny nanopens working together at the same time to miniaturize electronic circuits, pattern precise arrays of organic and biomolecules such as DNA and put thousands of different medical sensors on an area much tinier than the head of a pin.
A major limitation of other scanning probe lithography (SPL) methods is that contact between the tip and the substrate (the writing surface) changes the line width and quality of each patterned structure. Therefore, each tip requires a separate feedback system in order to control each line, which means a large amount of expensive and complex instrumentation.
Mirkin's parallel nanoplotter, however, produces consistent line widths with multiple pens and requires only one feedback system for the entire device. The reason for this lies in a tiny drop of water.
In dip-pen nanolithography (DPN), "inks" of organic molecules are applied to an atomic force microscope (AFM), which serves as the writing tool. The molecular ink then is deposited onto an underlying substrate, or "paper," via a tiny capillary in the water droplet that forms naturally at the tip. DPN is a nano-version of the 4,000-year-old quill pen.
When taking the DPN plotter to a parallel process, Mirkin's team made an important scientific discovery. When the writing tips were applied to the substrate using different contact forces, the pens still produced identical dots and lines, with respect to diameter and line width. In other words, with increased pressure, only the water at the AFM tip spreads out, but the width of the nanocapillary, through which the ink flows, remains constant.
This discovery means that only one pen of the multi-pen device needs to be "smart" or have its tip equipped with a feedback system. This pen is called the imaging tip and is used for both imaging and writing. As it patterns an area, sensors in the imaging tip communicate with the customized computer software that drives the nanoplotter. In the case of the eight-pen nanoplotter, the other seven writing tips are passive and follow the lead of the pen with the imaging tip, drawing identical patterns a fixed distance apart.
Mirkin and Hong demonstrated the nanoplotter's parallel writing capability by first drawing two squares using the same ink, then two squares made of two different inks, and finally drawing eight identical patterns -- a set of a dot, a line, an octagon and a square -- made using the same ink. In each demonstration, the patterns were perfectly aligned with respect to each other.
The nanoplotter also can be used in a serial fashion to create nanostructures made up of different inks, one ink being added after another to build the final structure.
In addition to requiring only one feedback system, Mirkin's nanoplotter has other advantages. It can be automated, it uses a relatively inexpensive tool (an atomic force microscope) that is common in the laboratories of companies and universities, and it works under normal atmospheric conditions as opposed to a billion-dollar semiconductor fab line.
"Ideally, we want to have total control over the chemical composition, or architecture, of the nanostructures we build down to the sub-10 nanometer regime," said Mirkin, also director of Northwestern's Institute for Nanotechnology and Center for Nanofabrication and Molecular Self-Assembly. "It's a level of refinement that will open the doors to remarkable scientific discovery and the realization of exciting new technologies. The parallel process nanoplotter takes us closer to our goal."
Mirkin's next step is to expand the current nanoplotter's capabilities. He hopes to have a working 50-pen nanoplotter by the end of next year.
"It soon will be possible to pattern one master plate with thousands of different organic nanostructures, each structure designed to react with a certain disease agent, for example," said Mirkin. "That's what is exciting about this -- no other method exists to do this on such a small scale."
In the case of biomolecules like DNA, it will be possible to generate ultrahigh density combinatorial arrays that could be quite useful in the genomics and medical diagnostics industries. Such arrays are currently generated via techniques with much lower resolution than DPN.
The research was funded by the U.S. Air Force Office of Scientific Research, the Defense Advanced Projects Research Agency and the National Science Foundation-funded Northwestern University Materials Research Center.
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