Abstract
The introduction of two-photon polymerization (TPP) into the area of Carbon Micro Electromechanical Systems (C-MEMS) has enabled the fabrication of three-dimensional glassy carbon nanostructures with geometries previously unattainable through conventional UV lithography. Pyrolysis of TPP structures conveys a characteristic reduction of feature size—one that should be properly estimated in order to produce carbon microdevices with accuracy. In this work, we studied the volumetric shrinkage of TPP-derived microwires upon pyrolysis at 900 °C. Through this process, photoresist microwires thermally decompose and shrink by as much as 75%, resulting in glassy carbon nanowires with linewidths between 300 and 550 nm. Even after the thermal decomposition induced by the pyrolysis step, the linewidth of the carbon nanowires was found to be dependent on the TPP exposure parameters. We have also found that the thermal stress induced during the pyrolysis step not only results in axial elongation of the nanowires, but also in buckling in the case of slender carbon nanowires (for aspect ratios greater than 30). Furthermore, we show that the calculated residual mass fraction that remains after pyrolysis depends on the characteristic dimensions of the photoresist microwires, a trend that is consistent with several works found in the literature. This phenomenon is explained through a semi-empirical model that estimates the feature size of the carbon structures, serving as a simple guideline for shrinkage evaluation in other designs.
Highlights
Carbon micro-electromechanical systems (C-MEMS)have emerged as a promising technology for the fabrication of miniaturized functional devices that used to be based almost exclusively on silicon[1]
We examined the degree of shrinkage through direct measurement using scanning electron microscopy (SEM), and studied the quality of the obtained carbon material through energy dispersive X-ray spectroscopy (EDS) and Raman spectroscopy
The presented analysis showed how the volumetric reduction in the carbon nanowires can be directly linked to the increase of surface-to-volume ratio in the structures
Summary
Carbon micro-electromechanical systems (C-MEMS)have emerged as a promising technology for the fabrication of miniaturized functional devices that used to be based almost exclusively on silicon[1]. C-MEMS are made up of carbon microelectrodes of a desired geometry, which are derived by patterning of a polymer precursor through photolithography, followed by thermal degradation using pyrolysis in an inert environment or vacuum. The thermal decomposition of ultraviolet (UV)-cured resins at high temperatures[2] (~900 °C) is typically accompanied by a large volumetric shrinkage of up to 90%3, leading to smaller carbon structures with high aspect ratio. Rich form of carbon known as glassy carbon, which exhibits high isotropy, a wide electrochemical stability window, biocompatibility, superior chemical resistance, and under certain conditions, semiconductor-like electrical properties[2,4]. These characteristics are advantageous for the manufacturing of sophisticated microdevices, which span electrochemical nanosensors[5] to mechanical metamaterials[4].
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