Effect of Metal Thickness and Pattern Size on Micro-Scale Metal-Assisted Chemical Etching on Silicon
Authors: Choi Keorock , Song Yunwon , Oh Jungwoo 
Published: February 2020

Until now, the best way to use metal-assisted chemical etching was to apply it to the fabrication of nanowires and nanostructures. Yet the properties of metal-assisted chemical etching in micro-scale patterns have not been the subject of much research. When the catalyst is too broad, metal-assisted chemical etching becomes difficult because of catalyst bending [1]. If micro-scale metal-assisted chemical etching is more extensively studied, it may be applied in MEMS.
To understand the mechanism of micro-scale metal-assisted chemical etching, we need to examine two mechanisms for the mass transfer of solution and byproducts by defining the diffusion length, either long or short. Although a study exists that explains the two mechanisms using deposition rate [2], our study will demonstrate the two mechanisms using catalysts with varying degrees of thickness and size. The mechanism for metal-assisted chemical etching is determined by the thickness of the catalyst. For thick catalysts, a long diffusion length mechanism is used and for thinner catalysts, a short diffusion length mechanism is used. In a long diffusion length mechanism, the catalyst is thick. Since pin-holes are not generated on thick catalysts, the etching solution cannot penetrate the catalyst to dissolve silicon. Because silicon is etched from the edge of the catalyst, the points the etching solution reaches are of different lengths. As a result, this makes the etching process unstable. If etching occurs over areas that are too large, catalyst begins to bend, caused by etching from the edge of the catalyst. In a short diffusion length mechanism, the catalyst is thin; therefore, pin-holes are generated for mass transportation. Consequently, in a large area, the sample is stably etched with anisotropic properties. This means that a thin film catalyst for metal-assisted chemical etching is independent of the pattern.
To understand the effects of catalyst thickness and size, Au films 10, 20, 30, and 40 nm thick were deposited by thermal evaporation on Si substrates. As metal-assisted chemical etching was best carried out with 20 nm thick catalyst, bar patterns 2, 4, 6, 8, and 10 µm wide and dot patterns of 2, 4, and 6 µm in diameter were used on 20 nm thick catalyst. Results were analyzed by scanning electron microscopy (SEM). Before deposition, native oxide was removed using a BOE solution. On 10 and 20 nm thick catalysts, metal-assisted chemical etching was successfully done by mass transfer due to pin-holes that were generated in the thin catalyst. On the other hand, on 30 and 40 nm thick catalysts, the etching rate was significantly reduced. The etching rate was unaffected by the size of the catalysts on the 20 nm thick catalyst. This shows that micro-scale metal-assisted chemical etching is independent of pattern size on thin catalysts.

Acknowledgements

This research was supported by the Ministry of Science, ICT and Future Planning (MSIP), Korea, under the IT Consilience Creative Program (NIPA-2014-H0201-14-1002) supervised by the National IT Industry Promotion Agency (NIPA). This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2013-8-0884).

Reference

[1] N.Geyer, B.Fuhrmann, Z.Huang, J.de Boor, H.S.Leipner and P.Werner, J Phys Chem C. 116 (2012) 13446
[2] M.Zahedinejad, S.D.Farimani, M.Khaje, H.Mehrara, A.Erfanian and F.Zeinali, J Micromech. Microeng.23 (2013) 055015