Monday, May 08, 2006

"CHIPS: Dip-pen technique drives molecular-scale litho"

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. Dip-pen lithography harnesses an atomic-force microscope (AFM) to ink virtually any chemical compound onto a substrate with nanometer control. Unfortunately, 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 method 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. 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.