Experiments shed new light on the limits of laser-writing method
Last year at ARCNL, Lorenzo Cruciani made a surprise discovery of a new method to directly write patterns of ruthenium with a laser. Now, he and his co-authors have published a new research paper, highlighted as an Editor’s Pick in Optical Materials Express, diving deeper into how the underlying materials and the laser beam thickness affect the “islands” this method creates.
A serendipitous finding
In a project sponsored by the University of Amsterdam, Lorenzo Cruciani studied laser-induced terahertz emission from layers of ruthenium. ‘One day, I accidentally exposed the ruthenium to a high laser power, and to my surprise, the amount of terahertz emission increased much more than expected.’ To check if anything weird happened to the substrate, the researcher removed the ruthenium with supermarket bleach. Where all of the unexposed ruthenium was dissolved as expected, the exposed part was not removed.
It turned out that the laser light had locally oxidized the surface of the ruthenium, creating a tiny “island” of ruthenium oxide. Unlike the original metal, the oxidized region is not dissolved by bleach, but it can still conduct electricity. The oxidation and thus the writing is the result of a thermal process, group leader Paul Planken explains. ‘The energy from our pulsed laser at the center of the laser spot gives rise to a temperature that is high enough to locally oxidize the material.’
This was a very exciting finding, as the laser-writing method may avoid having to use photoresist in the lithography process, requiring fewer and less cumbersome processing steps. Additionally, they were able to write structures four times smaller than the diffraction limit, raising the question of how small they could go. Excited to further explore the possibilities with this new method, Lorenzo and his co-authors Marnix Vreugdenhil, Dries van Oosten, Klaasjan van Druten and Paul Planken got back to work in the lab.
Face to face
One of the factors that could affect the island formation is the material beneath the ruthenium film, or the “substrate.” Is there an interaction between the substrate and the oxidized ruthenium film, or between the substrate and the laser itself, that changes how the oxidation patterns form?
The researchers tested samples of ruthenium films layered on top of three different substrates: glass, sapphire, and silicon. They found that even though sapphire and silicon conduct heat differently, the same amount of laser light, or fluence, was required to heat the ruthenium film enough to oxidize it. This suggests that heat conductance at the ruthenium-substrate interface has a strong effect on the speed with which the ruthenium cools.
The thickness of the ruthenium film also plays a role. After testing with different film thicknesses, they found that for ruthenium films in the range of 20-50 nanometers thick, the fluence needed for laser writing scales with film thickness, making it nicely predictable.
Writing with a fine-point laser
You would think that if your laser beam is smaller, you should be able to write smaller features with it, the way a fine-point marker writes more precisely than a larger one. However, because heat moves faster through metals than ink through paper, it’s harder to maintain a high enough temperature in the smaller laser spot.
Earlier experiments with this method showed that a focused laser spot 2 micrometers wide could be used to write features as small as 500 nanometers, four times smaller than the diffraction limit. In their latest experiments, the researchers tested a smaller laser spot, 0.8 micrometers wide. Instead of writing even smaller features, however, they found that the illuminated surface cools down more quickly, requiring higher fluences for the ruthenium to oxidize that are closer to the damage threshold.
Based on their calculations, with such small laser spots the method becomes limited by the loss of heat via the thin ruthenium layer itself, radiating outward from the illuminated spot. This “in-plane” heat diffusion has a larger effect on the overall temperature when the beam is small because it becomes faster than the heat diffusion down into the substrate. Future attempts to write with sub-micrometer laser beams will need to take this in-plane heat diffusion into account, as it may limit the level of detail achievable with this method.
Implications
These new insights into laser-writing directly onto ruthenium may be of interest for the semiconductor manufacturing industry. Due to its electrical properties at nano scales, ruthenium is a promising candidate for use in developing even smaller computer chips. The research paper published with these results has been highlighted in Optical Materials Express as an Editor’s Pick.
For more of the latest findings in light-matter interaction, visit the research group webpage: Light-Matter Interaction – ARCNL
Publication
Lorenzo Cruciani, Marnix Vreugdenhil, Dries van Oosten, Klaasjan van Druten, and Paul Planken, Effect of substrate and beam diameter on direct laser patterning of ruthenium thin films, Opt. Mater. Express 15, 1005-1018 (2025).
https://doi.org/10.1364/OME.559351
