3D printing high performance RF sensors
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Researchers in China have developed a groundbreaking method to build high-aspect-ratio 3D microstructures with sub-10 micron resolution for RF sensors.
The technique achieves deep trenches with a 1:4 width-to-height ratio, while also achieving precise control over resonance properties and significantly enhancing performance. This hybrid technique not only improves the quality factor (Q-factor) and frequency tunability of RF metastructures but also reduces device footprint by up to 45%.
This paves the way for next-gen applications in sensing, MEMS, and RF metamaterials.
Traditional lithography techniques such as electron beam lithography and nanoimprinting have struggled to meet the demand for ultra-fine, high-aspect-ratio structures. Poor thickness control, uneven sidewalls, and material limitations have constrained performance and scalability.
The technique combines two-photon polymerization (2PP), electroplating, and dry etching. However, integrating 2PP with robust metallization for functional RF components remained elusive due to process incompatibilities.
The researchers from Bilkent University and Nanyang Technological University introduced a novel fabrication process that uses 2PP to create intricate deep trenches, which are then filled with copper via electroplating and refined through dry etching. The result is ultra-compact RF resonators with tunable frequencies between 4–6 GHz, a 1:4 aspect ratio, and high Q-factors within a sub-10 µm resolution framework.
The process begins with 2PP to define high-aspect-ratio trenches in a photoresist layer. These voids are then filled with thick copper—up to 8 µm—through electroplating. Subsequent dry etching removes seed layers, yielding freestanding metal structures with flat, vertical sidewalls and remarkable dimensional accuracy. The team demonstrated microstructures as narrow as 2–3 µm in width and over 10 µm in height.
Rapid annealing is used to strengthen copper bonds, addressing thermal and mechanical challenges. Scanning electron microscopy (SEM) verified the high fidelity of the structures, confirming their robustness and manufacturability.
The fabrication method starts with a Spin-coated AZ-4562 positive photoresist over ITO-coated glass, b placing the prepared substrate on the sample holder of the 3D printing system and exposing light to obtain the desired pattern, c developing the exposed part of the photoresist, d thick film deposition of copper metal over ITO seed layer along the line of the given pattern, e spin-coated the protecting layer, f cutting the substrate into smaller pieces with a dicing saw, g removing the photoresist, h dry etching of the ITO seed layer with ICP, and i thermal annealing to strengthen copper structure.
Increasing the metal thickness can improve the Q-factor six to sevenfold, and resonance frequencies shifted by up to 200 MHz, allowing precise tailoring for specific RF applications. Compared with conventional PCB-fabricated resonators, the 3D-printed versions maintained performance while reducing the footprint by 45%.
“This work bridges a critical gap between 3D printing and functional RF devices,” said Prof. Hilmi Volkan Demir at Bilkent. “By achieving sub-10 micron resolution in high-aspect-ratio metal structures, we’ve unlocked new design freedoms for miniaturized, high-performance components. The ability to tune resonance frequencies and Q-factors through geometric control offers exciting opportunities for next-generation sensors and communication systems.”
The technique can be used for smaller RF sensors with improved, or implantable or wearable micro-devices for diagnostics and therapy. Integrated with MEMS, it could be used to create on-chip antennas and signal processors for IoT networks.
Future developments include integrating other functional materials or building multi-layer structures to expand the capabilities of the RF sensors.
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