Etching-Free Wafer-Level Fabrication of Suspended Mesh-Type Nanoheaters
Authors: Kim Taejung , Cho Wootaek , Shin Heungjoo 
Published: September 2023

Recently, various strategies have been demonstrated to reduce the operating power of electrothermal devices by implementing a miniaturized Joule-heating-based heater. To operate the heaters with low power, the miniaturized heating element must be well-insulated by spacing it apart from the substrate. The conventional microfabrication methods for implementing small-size (e.g., sub-micrometer to micrometer scales) and suspended heater architecture have relied on the etching process [1], which requires accurate and delicate process control. Moreover, the etching process becomes more complicated to prevent damage to the delicate heater structures, hindering yield and reliability. Therefore, a trade-off has occurred between the reduction in power consumption (i.e., heater size reduction) and production cost (e.g., cost, time, and high-tech development).

Prior to this research, we demonstrated the fabrication of suspended 1D nanoheaters by depositing a thin Au layer selectively on top of a suspended carbon nanowire and their applications as ultralow-power gas sensors [2, 3]. Although the suspended nano-sized carbon heater body was patterned without an etching process, the Si substrate had to be etched in an isotropic manner to form a built-in shadow mask to facilitate batch nanofabrication.

Here, we present a novel wafer-level fabrication method of a suspended sub-micrometer-sized mesh-type heater without using any conventional etching process. The mesh structure is separated from the substrate by 7–8 μm through two-step UV exposure on a negative photoresist layer: the first exposure is executed with large energy for post-forming, and the second exposure forms a suspended structure at the surface of the photoresist with small UV energy. This monolithic polymer structure is then pyrolyzed and converted to a suspended glassy carbon mesh. The suspended mesh consists of sub-micrometer-sized grid-shaped lines (width 200~500 nm, grid size ~ 10 μm). A thin (~ 50 nm) metal film is deposited on the top of the mesh for use as a heater layer by a directional evaporation method. During the directional deposition, the metal deposited on the substrate in the shape of the suspended mesh. Therefore, as long as the photomask size of the metal coating is smaller than the entire mesh size, the metal layer on the substrate is disconnected. Consequently, suspended mesh-type heaters can be facilitated without the etching process. Furthermore, various heater shapes can be engraved according to the photomask patterns of the mesh structure and metal layer, enabling customized mass production. At this conference, the fabrication of the various types of mesh-type nanoheaters and their applications as gas sensors will be presented.

Reference

[1] Choi, K. W.; Lee, J. S.; Seo, M. H.; Jo, M. S.; Yoo, J. Y.; Sim, G. S.; Yoon, J. B. Batch-fabricated CO gas sensor in large-area (8-inch) with sub-10 mW power operation. Sens. Actuators, B Chem.
2019, 289, 153–159.

[2] Kim, T.; Cho, W.; Kim, B.; Yeom, J.; Kwon, Y. M.; Baik, J. M.; Kim, J. J.; Shin, H. Batch Nanofabrication of Suspended Single 1D Nanoheaters for Ultralow‐Power Metal Oxide Semiconductor‐Based Gas Sensors. Small
2022, 2204078.

[3] Cho, W.; Kim, T.; Shin, H. Thermal conductivity detector (TCD)-type gas sensor based on a batch-fabricated 1D nanoheater for ultra-low power consumption. Sens. Actuators, B Chem.
2022, 371, 132541.