Dip Pen Nanolithography Heats Up

As we march along the inexorable path toward nanotechnology, there is a seemingly endless array of ideas about how to get there. One increasingly popular idea is dip pen nanolithography (DPN), which uses atomic force microscopy (AFM) probes to write nanometer-scale lines directly onto a substrate. Now researchers from the Georgia Institute of Technology (Atlanta) and the Naval Research Laboratory (NRL, Washington) have improved upon the technique, using heat to achieve some important new capabilities. Dubbed thermal dip pen nanolithography (tDPN), the new technique overcomes previous limitations that have been encountered by dozens of research groups.

One limitation has been DPN's inability to turn off the writing function; if the probes are in contact with the surface, they're applying ink. The problem here is that the probes are meant not only to write new patterns, but also to sense existing surface patterns. This means that if an inked probe touches a pattern for sensing purposes, it inevitably corrupts that pattern. Other developers have overcome this shortcoming by using separate cantilevers for sensing and for writing, but this complicates the procedure and makes it more time-consuming.

The Georgia Tech and NRL researchers, on the other hand, use a heated AFM cantilever tip to control the deposition of the ink, heating it to its melting point to apply the material, and turning off the heat to stop the flow. This capability has some additional benefits as well. For one, it improves deposition control (including control of post-deposition diffusion). Some previous research has shown an ability to control deposition through global control. However, in this case, the deposition rate and diffusion are entwined — increasing the global temperature increases the deposition rate, but it also increases the amount of subsequent diffusion. Local control enables quicker deposition rate while also maintaining sharp features.

Another benefit is that tDPN, unlike conventional DPN, can be used in a vacuum environment. The liquid inks of conventional DPN would evaporate in a vacuum. But the solid materials associated with the thermal variant bond to surfaces.

tDPN also opens research up to a variety of new ink materials, enabling the use of materials that would typically be immobile at room temperature. Previously, to be used in DPN, molecules had to have sufficient mobility in ambient conditions to transfer from the probe tip to the substrate. In their experimentation with tDPN, the researchers have been using octadecylphosphonic acid (OPA) as the ink. It has a melting temperature near 100°C, and can self-assemble on mica, metals such as stainless steel and aluminum, and metal oxides such as TiO₂ and Al₂O₃.

A topographic image shows the results of a mica surface scanned with a heated AFM cantilever tip, at four different temperature levels. No deposition is observed at the two lowest temperatures. At 98°C, near OPA’s melting temperature, there is light deposition; a full monolayer is deposited well above the melting point. (Source: Georgia Tech, NRL)

After depositing two monolayers of OPA onto the cantilever, the researchers experimented with various temperatures to confirm the deposition variance. The Figure shows results based on four temperature levels — 25, 57, 98 and 122°C. In each case, the coated tip of the AFM probe was rastered over a 500 nm square for 256 seconds. At 25 and 57°C, the cantilever produced no pattern in the squares. At 98°C, near OPA's melting temperature, light deposition resulted. Finally, when the tip was raised to 122°C, it created a pattern 2.5 nm high, indicative of a full monolayer.

Experiments have been done primarily on mica, but the researchers theorize that other substrates, such as silicon, could offer better throughput times. As an example, OPA deposition on mica continued for about 2 minutes after the cantilever was no longer heated. Experiments with ultrathin polymer layers on a silicon substrate, however, showed that deposition stopped within 1-10 µsec. The lower thermal conductivity of silicon could lessen the amount of residual heat from the substrate, thereby minimizing delays in cooling and improving actual production time.

This research produced linewidths of 98 nm, but this is not an optimized number. In theory, tDPN's resolution is limited only by the ability to make heated cantilevers with sharper tips. These experiments relied on a cantilever whose tip radius was ~100 nm, but comparable cantilevers with tips sharper than 20 nm have been fabricated. Now cantilevers are being fabricated directly by Georgia Tech — William King, an assistant professor in the School of Mechanical Engineering, and graduate student Tanya Wright. Such optimization could ultimately enable the printing of features as small as 10 nm, according to King.

Given the technique's ability to work in the vacuum environments typically found in semiconductor manufacturing, its potential for linewidths beyond production photolithography, and the presumed ability to reach reasonable throughput times, tDPN could be well suited for a future in chip fabrication. Using organic materials, the Georgia Tech and NRL researchers hope to produce a working semiconductor device by the end of this year.

For additional information on lithography, go to www.semiconductor.net/lithography