Photostructurable polymers such as SU-8 have a tremendous impact on the development of the field of micro/nano-electromechanical systems (MEMS/NEMS), by allowing simple fabrication of plastic MEMS/NEMS elements with nearly vertical sidewalls and high aspect ratios using straightforward photolithographic procedures. However, materials such as SU-8 do not present any inherent functionality (as electrical conductivity) and can only be used to fabricate passive elements. By doping the material with nanoparticles, new functionalities can be added. However, the resulting material can have low photolithographic performance: usually aggregates of nanoparticles are formed, which can be difficult to remove or also incompatible with the polymer developer. In this case, it is necessary to use alternative methods to the UV exposure: soft lithography methods offer the possibility to structure polymers in simple steps.
In this work we report the combination of two soft lithography methods, micromolding in capillaries (MIMIC) and microtransfer moulding (μTM) to pattern carbon doped SU-8 layers. The former method consists of a PDMS stamp in contact with the substrate. Capillarity action fills the channels with polymer precursor. The Carbon SU-8 is exposed to UV light and the stamp is removed. Considering that the PDMS stamp is obtained from standard SU-8 master, high aspect ratios are obtained. Nevertheless, we have found that the carbon particles clusters can block the channels, preventing its further filling. This situation does not occur using microtransfer moulding, where a drop of the carbon SU-8 is placed on the PDMS stamp, pressed over a substrate and cured with UV light. While the resolution achievable with UV lithography was limited to 20 μm, it was enhanced to 1 μm when using soft-lithography.
As an application, carbon-doped AFM tips have been fabricated. Inverted pyramids were etched on a silicon wafer using TMAH and are used as a mould and a sacrificial silicon oxide layer has been grown. Then, the moulds have been filled by microtranfer moulding with Carbon SU-8. Afterwards, the silicon oxide has been removed by HF etching and the tips are released. Tips of 20 nm radius have been obtained, which validates the process proposed in this work.
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