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  • Choosing the Right Polymer Aids Direct-Write Multiphoton Photolithography

Photonics Spectra
Jan 2008
Molecular weight, photon absorption and glass transition temperature each play a role in etchability.

Hank Hogan

Fabricating nanoscale polymer thin-film structures mightbecome easier, thanks to researchers from Kansas State University in Manhattan. Such structures could be used in chemical and biological sensor arrays, as optical elements, for electronic components and in portable nanofluidic devices.


A schematic illustrates the ablative multiphoton etching process. A thin polymer film on a glass substrate is illuminated with near-IR light from a laser (red line). Multiphoton absorption removes the polymer at the focus spot. Arbitrary patterns can be written in the sample by scanning it while controlling the laser illumination. Images courtesy of Daniel A. Higgins, Kansas State University.

In their work, graduate student Shaida Ibrahim, chemistry professor Daniel A. Higgins and assistant chemistry professor Takashi Ito investigated the etching behavior of several commercial polymers used in ablative multiphoton photolithography. They discovered that those polymers with high glass transition temperatures, low molecular weight and no photon absorption in the visible range provided the best etching resolution.

They also found that nothing at all happened until a threshold was reached. “Within the focused laser spot, the intensity can go from being below the etching threshold to above threshold over a very short distance, producing etched regions that have very sharp edges — down to approximately 120 nm,” Higgins said.

An atomic force microscope (AFM) image shows a surface relief grating etched into an 80-nm-thick polymethyl methacrylate (PMMA) film (top). The transition from unetched to etched PMMA regions (the edge sharpness) occurs over <180 nm, as seen in a line profile taken across the first two etched lines (bottom). The actual resolution likely is higher than measured because of the interaction between the surface and the AFM probe.

This thresholding, he added, is why the investigators believe that it will be possible to easily get <100-nm features with the technique. This effect and the best-result polymer characteristics are a consequence of multiphoton absorption by the polymer.

The resulting depolymerization produces fragments that are then vaporized, leaving a hole in the film. By moving a focused laser spot around, the researchers can create a pattern in the film.

The technique has the advantage of being a direct-write method. Thus, no money or time need be spent producing a photomask or using developers or other postprocessing chemicals.

The method has inherent depth resolution, opening up the possibility of producing 3-D patterns within thick films and gray-scale patterns in films <100 nm thick. Finally, the method uses near-IR light, meaning that the optical setup is less expensive and simpler than one that uses a UV or an x-ray source.

Another demonstration of an arbitrary pattern is shown in this false-color image of a PMMA film that has been doped with a fluorescent dye and etched with multiphoton absorption ablation. The etched region shown measures 50 μm2. The dark regions are etched, while the light ones are unetched PMMA.

However, exactly which polymers produce the best results had not been explored, and the etching process itself had to be understood better. The researchers examined a group of commercial polymers of varying characteristics, creating ∼100-nm-thick films of each on coverslips and then mounting them on a scanning stage made by Sifam Instruments Ltd. (now part of Elektron plc of Romford, UK).

They used a Ti:sapphire laser from Coherent Inc. of Santa Clara, Calif., operating at 870 nm and a Nikon optical microscope, focusing the beam down to a diffraction-limited spot roughly 570 nm in diameter at the sample.

They varied the power to help determine how many photons were involved in the etching process. They found that the number ranged from two photons for the conducting polymer PTEBS to as many as six for polymethyl methacrylate (PMMA) and polybutyl methacrylate (PBMA), with polystyrene (PS) falling in between at three to four.

These results coincided with the polymer absorption spectra measured by the group with a Hewlett-Packard spectrometer. The polymers that had an absorption peak of 240 nm required the most photons, whereas PTEBS, which absorbs in the visible, needed the fewest.

Based on these results, the scientists devised guidelines for achieving the highest resolution: The absorption peak should not be in the visible but rather in the ultraviolet; the glass transition temperature should be high, and the molecular weight should be low.

Polymers that have UV absorption peaks must absorb multiple photons simultaneously; therefore, they have a higher etching resolution. Those that have low molecular weights fragment into smaller pieces — again an etching-resolution benefit. Finally, those with low glass transition temperatures tend to flow as a result of laser heating. “This effectively reduces the resolution of the etching,” Ito said.

The group used the technique to create a patterned mold in PMMA, the polymer with the best resolution results, and transferred the pattern by contact to the polydimethylsiloxane. The result appeared good visually, and the edges were sharp, according to measurements that were performed with an atomic force microscope from Veeco Instruments Inc. of Woodbury, N.Y.

The researchers are continuing to explore various polymers and the effect of laser power and focus modulation on the etching process. One goal is better control of the ablation process.

They hope that this will allow the development of gray-scale etching of 3-D surface relief patterns for use with items such as sensors, optical elements and electronic components.

“These methods could also be useful in anticounterfeiting methods and in encryption technologies,” Higgins said.

Langmuir, Nov. 20, 2007, pp. 12406-12412.

A kind of spectrograph in which some form of detector, other than a photographic film, is used to measure the distribution of radiation in a particular wavelength region.  
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