Exploiting Oxygen For 3D Photolithography

Once limited to 2D, photolithography has now been extended to 3D structures by researchers from KAIST.

AsianScientist (Apr. 13, 2015) – Using oxygen diffusion, researchers have developed a photolithographic technology that can be used to create precise patterns in 3D. Their research has been published in Nature Communications and was selected as a featured image.

Photolithography is an optical process for transferring micropatterns on to a substrate by exposing specific regions of the photoresist layer to ultraviolet (UV) light. It is widely used throughout industries that require micropatterns, especially in the semiconductor manufacturing industry.

Conventional photolithography relies on photomasks which protected certain regions of the substrate from the input UV light. Areas covered by the photomasks remain intact with the base layer while the areas exposed to the UV light are washed away, thus creating a micropattern. This technology was limited to a two-dimensional, disc-shaped design as the boundaries between the exposed and roofed regions are always in a parallel arrangement with the direction of the light.

A team of researchers led by Professor Kim Shin-Hyun from the Korea Advanced Institute of Science and Technology (KAIST) has now developed a new photolithographic technology that enables the production of micropatterns with three-dimensional structures.

Photoresist under UV light creates free radicals which initialize a chemical reaction. However, these radicals are eliminated in the presence of oxygen which thus acts as an inhibitor of polymerization. This suggests that the photoresist must be exposed to UV light for an extended time to completely remove oxygen for a chemical reaction to begin.

The research team, however, exploited the presence of oxygen. While the region affected by the UV light lowered oxygen concentration, the concentration in the untouched region remained unchanged. This difference in the concentrations caused a diffusion of oxygen to the region under UV light.

When the speed of the oxygen flow is slow, the diffusion occurs in parallel with the direction of the UV light. When the speed of oxygen flow increases, the diffusion process develops horizontally, outward from the area affected by the UV light.

Kim and his team proved this phenomenon both empirically and theoretically. Furthermore, by injecting an external oxygen source, the team was able to manipulate diffusion strength and direction and thus control the shape and size of the polymer. The use of the polymerization inhibitors enabled and facilitated the fabrication of complex, three-dimensional micropatterns.

“While 3D printing is considered an innovative manufacturing technology, it cannot be used for mass-production of microscopic products. The new photolithographic technology will have a broad impact on both the academia and industry especially because existing, conventional photolithographic equipment can be used for the development of more complex micropatterns,” said Kim.

According to Kim, his technique will enhance the manufacturing process of three-dimensional polymers which were considered difficult to be commercialized.

The article can be found at: Shim et al. (2015) Dynamic Designing of Microstructures by Chemical Gradient-Mediated Growth.

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Source: KAIST.
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