Heat-Sensitive Ink, Heated Probe Improve Nanoscale Lithography

An improved method to directly write nanometer-scale patterns onto a variety of surfaces combines thermal control of the ink flow with existing dip pen nanolithography (DPN). "Thermal DPN" (tDPN) extends DPN, an increasingly popular technique that uses atomic force microscopy (AFM) probes as pens to produce nanometer-scale patterns on plastic, metal, silicon, and other surfaces.

In conventional DPN, a probe tip is coated with a liquid ink, which then flows onto the surface to make patterns wherever the tip makes contact. Dozens of research groups are working on DPN applications. But the technique, which uses the AFM tips to both sense surface patterns and write new patterns, has been limited by the inability to turn the ink flow on and off. Existing dip pens apply ink as long as they remain in contact with a surface.

The tDPN scheme unveiled by the Georgia Institute of Technology and the Naval Research Laboratory (NRL) solves this by using easily melted solid inks and special AFM probes with built-in heaters (see the figure). When the heaters are on, the ink melts and flows from the probe onto the surface. When the heaters are off, the ink stays solid and no ink is transferred. Thus, the write process can be turned on and off at will.

"We've created a heated AFM tip that gives us control over the deposition and deposition rate during writing," said William King, assistant professor in Georgia Tech's School of Mechanical Engineering. "So for the first time, we can write in some places and not write in others."

Conventional DPN cannot be used in a vacuum because liquid inks would simply evaporate. Yet the solid materials used in the thermal process bond to surfaces, allowing them to be used in vacuum environments that are part of conventional semiconductor manufacturing. The thermal materials also provide sharper features because they don't spread out like liquid inks.

The tDPN technique could be used to produce features too small to be formed with light-based lithography and as a nanoscale soldering iron for repairing circuitry on semiconductor chips. It also could provide a new tool for studying basic nanotechnology phenomena.

"This technique extends DPN into new sets of materials and provides a higher degree of control," said Lloyd J. Whitman, head of NRL's Surface Nanoscience and Sensor Technology Section. "We also believe this technique will extend DPN into new environments, such as the vacuum environment, that would be more compatible with conventional semiconductor device fabrication."

By combining thousands of individually controlled AFM pens into arrays, a system could write complex semiconductor patterns, said King. The thermal dip pen technique could produce features as small as 10 nm--well beyond the limits of conventional semiconductor patterning processes that depend on light projected through a lithographic mask. So far, researchers have produced lines about 95 nm wide and are optimizing their process to make smaller features.