3D Laser Technology Designs Microscopy Tips at Nanoscale

Mechanically stable atomic force microscopy (AFM) tips with arbitrary shapes can now be added to existing AFM cantilevers using 3D direct laser writing technology based on two-photon polymerization (TPP). TPP, a 3D printing process that provides structuring with extremely high resolution, could offer the ability to design optimal tips and probes with significantly enhanced resolution, leading to more options for analyzing samples.

Researchers at Karlsruhe Institute of Technology (KIT) demonstrated that tips with arbritrary shapes and radii as small as 25 nanometers could be attached to conventionally shaped micromachined AFM cantilevers, using TPP as a tool.

3D direct laser writing based on two-photon polymerization can be used to create custom-designed tips. (a) Schematic drawing of the writing process on the cantilever using two-photon polymerization. The inset shows a scanning electron microscope image of the tip apex. To obtain a sharp and defined tip apex, it was written with optimized (but slower) parameters so that the surface of the tip apex is smoother than the rest of the structure. A large variety of tips can be fabricated in this way. (b) High (20 microns) and thin (5 microns at the base) tip. (c) Extremely long tip with a height of 100 microns. (d) Spherical tip with diameter of 10 microns. (e) Tip in the shape of the famous Swiss mountain "Matterhorn." (f) Protruding tip that is visible from the top during scanning. (g) Shrunk conical tip after carbonization through pyrolysis (decomposition caused by exposure to high temperatures). Courtesy of Karlsruhe Institute of Technology.

"This concept isn't new at the macroscopic scale: you can freely design any shape with your computer and print it in 3-D. But at the nanoscale, this approach is complex," said Hendrik Hölscher, head of the scanning probe technologies group at KIT. "To write our tips, we applied two-photon polymerization with an experimental setup, recently developed at KIT, which is now available from startup company Nanoscribe GmbH." The researchers showed the viability of their approach by directly writing sharp tips (R ≈ 25 nm) in one lithography step. They demonstrated exceptionally tall tips (h > 100 μm), optimal for the imaging of surfaces with high aspect ratio features or within deep trenches. The researchers also demonstrated that rebar structures could be written onto a cantilever to tune its resonance spectrum, for applications such as multifrequency AFM. Long-term scanning measurements showed low wear rates, demonstrating the reliability of the tips.

"We were also able to prove that the resonance spectrum of the probe can be tuned for multi-frequency applications by adding reinforcing structures to the cantilever," Hölscher said.

Two-photon polymerization involves using a tightly focused IR femtosecond laser to expose a UV-light-curable photoresist material, which causes two-photon adsorption that, in turn, triggers a polymerization reaction, allowing freely designed parts to be written at the place of their purpose — even nanoscale objects like AFM tips.

Adding tips to cantilevers is only one application of 3D laser writing, the researchers believe. Any arbitrary structure can be written at any position along the length of the AFM beam. This feature may be useful for tuning the resonance spectra of cantilevers, for multifrequency AFM, where higher eigenmodes of the cantilever are excited.

The researchers expect that two-photon polymerization will become more widely available for nanotechnology researchers. "We expect other groups working within the field of scanning probe methods to be able to take advantage of our approach as soon as possible," Hölscher said. "It may even become an Internet business that allows you to design and order AFM probes via the web."

The group will continue to optimize their approach and apply it to research projects ranging from biomimetics to optics and photonics.

The research was published in Applied Physics Letters (doi: org/10.1063/1.4960386).