Portland, Ore. -- IBM Corp. has found a way to electrically control the deposition rate of materials--the ink--from a dip-pen lithography system without lifting it from the substrate on which it is writing, thereby enabling molecular-scale nanolithography.
"Our approach works like an inkjet," said IBM fellow H. Kumar Wickramasinghe. "Every time you pulse it, you deposit a metered amount of material--in principle you could put down single molecules. Right now we are at 10 molecules, but we have not really pushed it yet." Wickramasinghe performed the work with researchers Kerem Unal and Jane Frommer at IBM's Almaden Research Center (San Jose, Calif.).
Dip-pen lithography harnesses an atomic-force microscope (AFM) to ink virtually any chemical compound onto a substrate with nanometer control. The method has proven "very useful," Wickramasinghe said, "but there was no way to control the deposition rate, because the molecules were deposited on a surface by diffusion--like a quill pen." The only thing that could be controlled was the speed at which the tip was scanning, and the only way to stop and start deposition was to lift and touch the pen to the substrate--a lengthy and error-prone procedure.
IBM's trick was to add a reservoir to the AFM's tip at the place where it connects to a cantilever. Thus, the new cone-shaped tip directly ties into a material reservoir that can convey molecules up or down to a substrate with an electric field of a million volts per meter. By changing the strength and duration of the field, IBM has demonstrated millisecond-level control of deposition--a thousandfold faster than today's best methods.
Like all dip-pen approaches, IBM's electronically controlled, direct-writing meth- od uses AFM positioning accuracy to define complex patterns in a variety of materials with features down to 10 nm. That's five times smaller than today's e-beam lithography equipment and 10 times smaller than photolithography. The twist lies in adding control by an electric field, and in setting up the right conditions to make that work.
"If you apply an electric field normally, nothing happens--that's why no one has thought of this before," said Wickramasinghe. "But we found that if you put a dip pen in a humid environment, so you have a few monolayers of water on the tip's surface, then you can make molecules go up and down it with an electric field. You can suck up material and put it in a reservoir at the base where the tip meets the cantilever, then move the tip to where you want it and pulse the material back out in metered amounts."
Besides use in nanoscale lithography for electronic circuits, IBM predicts the method will enable nanoscale-size microfluidic devices, such as those that perform electrophoresis assays for everything from DNA fingerprinting to routine blood tests. Electronic control and faster processing speeds, enabled by the nanoscale dimensions of devices, could speed any medical test that depends on the separation of biological molecules with electrophoresis, according to IBM. Measured electrophoresis tests were found to perform in milliseconds what traditional electrophoresis does in hours, since IBM used a million-volt/ meter electric field to propel molecules up or down the 11.2-micron length of the AFM tip, rather than thousands of volts along the millimeter-wide, centimeter-long tubes used in a traditional electrophoresis test.
"IBM's method works like normal electrophoresis, just unbelievably smaller," said David Garfin, president of the American Electrophoresis Society. "Today, electrophoresis tubes are millimeters wide and typically take minutes or even hours to work. IBM's very clever use of atomic-force microscopy has the potential to provide much, much faster results that are also ultrasensitive."
IBM's approach also holds the potential of directly writing to wafers with parallel millipede-like arrays of hundreds or thousands of AFM tips attached to a single cantilever (go to www.eetimes.com and search for article ID: OEG20000913S0061). The massively parallel tip arrays on a single cantilever resemble their namesake, the multilegged insect.
Massively parallel arrays of tips posed a problem for traditional dip-pen lithography, since they required a corresponding array of nanoscale inkwells to hold each tip's material. But with electrical control of ink from an external supply, IBM can pump in materials from a single reservoir to all of the tips on a cantilever, then directly write arrays of devices on a wafer simultaneously.
"Something we want to do next is have an ink supply come in from the cantilever side with several reservoirs containing different materials, which you then turn on and off electrically as you like," said Wickramasinghe.
Parallel arrays of tips also have the potential to speed traditional medical testing and even gene sequencing. "Next we want to characterize how many DNA bases we can go to," said Wickramasinghe. "Today we have only done 16 bases, but a typical gene might have a thousand bases."
To demonstrate its dip-pen system's current capabilities, IBM wrote a 3.3 x 8.8-micron logo in DNA using lines that measured 49 to 79 nm wide--enabling more than 300 to be fit on a line the width of a single human hair. IBM has traditionally drawn its logo as small as possible to prove its prowess with atomic-force microscopy, which it invented and continues to develop (see story below). Wickramasinghe himself is credited with the technique now universally used by atomic-force microscopy worldwide.
"The work I did with my team [at IBM's T.J. Watson Research Center] on the vibrating-mode AFM is the second most widely cited in the scanning-probe field, after the work of [Nobel prize winners] Gerd Binnig and Heinrich Rohrer," said Wickramasinghe.