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Anisotropic multi-step etching for large-area fabrication of surface microstructures on stainless steel to control thermal radiation

View Article: PubMed Central - PubMed

ABSTRACT

Controlling the thermal radiation spectra of materials is one of the promising ways to advance energy system efficiency. It is well known that the thermal radiation spectrum can be controlled through the introduction of periodic surface microstructures. Herein, a method for the large-area fabrication of periodic microstructures based on multi-step wet etching is described. The method consists of three main steps, i.e., resist mask fabrication via photolithography, electrochemical wet etching, and side wall protection. Using this method, high-aspect micro-holes (0.82 aspect ratio) arrayed with hexagonal symmetry were fabricated on a stainless steel substrate. The conventional wet etching process method typically provides an aspect ratio of 0.3. The optical absorption peak attributed to the fabricated micro-hole array appeared at 0.8 μm, and the peak absorbance exceeded 0.8 for the micro-holes with a 0.82 aspect ratio. While argon plasma etching in a vacuum chamber was used in the present study for the formation of the protective layer, atmospheric plasma etching should be possible and will expand the applicability of this new method for the large-area fabrication of high-aspect materials.

No MeSH data available.


Process flow of the proposed multi-step etching.
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Figure 1: Process flow of the proposed multi-step etching.

Mentions: In general, dry etching techniques, called Bosch processes [16], are used to fabricate deep micro-holes, because high-accuracy and high-aspect microstructures can be fabricated by this anisotropic etching process, although the size of the fabrication area is limited by vacuum chamber size. On the other hand, wet etching techniques are suited for large-area and low-cost fabrication, but deep microstructures cannot generally be wet etched. To fabricate deep microstructures over a large area, we developed the multi-step wet-etching process as shown in figure 1. First, a patterned resist mask was fabricated by three-beam interference lithography using Mach–Zehnder type interferometer [17]. Because exposure masks and complicated expensive machines are not required for interference lithography, this technique is suitable for large-area fabrication of periodic microstructures [18–20]. Stainless steel SUS304, which is a general austenitic stainless steel, was used as the substrate after polishing. In the lithography process, TDMR-AR80 (Tokyo Ohka Kogyo Co., Ltd) was used for the resist mask, and a Nd:YAG laser with an emission wavelength of 355 nm was used for the exposure. The exposure time was 100 s for an area with a diameter of 5 cm at a laser power of 100 mW. After the fabrication of the resist mask, the first electrochemical wet etching was performed using dilute aqueous 3% oxalic acid, and an applied voltage of 3 V generated with a direct current (dc) power supply. By monitoring the dc between the sample and the counter electrode, the total etched volume was controlled. Throughout the wet etching process, the temperature of the etchant was maintained at 25 °C. After etching, a side wall protective layer was formed via Ar plasma etching (NLD, ULVAC) at a chamber pressure of 0.3 Pa. As shown in figure 1, resist canopies were formed on the tops of the micro-holes due to the isotropic nature of wet etching. These canopies appeared to play a key role in forming the side wall protective layer by preventing the spread of the resist and the oxidized layer of stainless steel residue to the outside of the holes. Thus, due to the presence of the wall protection, the second wet etching process deepened the micro-holes without widening them. Consequently, deep micro-holes could be fabricated by repeating the etching and protection processes. Finally, the resist mask and accumulated inert residue were removed with acetone. As mentioned in section 1, a vacuum plasma etching process was applied in this study. However, it should be possible to use an atmospheric plasma etching process. Therefore, by applying role-to-role processing, this multi-step etching process should be applicable to the large-area fabrication of periodic microstructures.


Anisotropic multi-step etching for large-area fabrication of surface microstructures on stainless steel to control thermal radiation
Process flow of the proposed multi-step etching.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC5036480&req=5

Figure 1: Process flow of the proposed multi-step etching.
Mentions: In general, dry etching techniques, called Bosch processes [16], are used to fabricate deep micro-holes, because high-accuracy and high-aspect microstructures can be fabricated by this anisotropic etching process, although the size of the fabrication area is limited by vacuum chamber size. On the other hand, wet etching techniques are suited for large-area and low-cost fabrication, but deep microstructures cannot generally be wet etched. To fabricate deep microstructures over a large area, we developed the multi-step wet-etching process as shown in figure 1. First, a patterned resist mask was fabricated by three-beam interference lithography using Mach–Zehnder type interferometer [17]. Because exposure masks and complicated expensive machines are not required for interference lithography, this technique is suitable for large-area fabrication of periodic microstructures [18–20]. Stainless steel SUS304, which is a general austenitic stainless steel, was used as the substrate after polishing. In the lithography process, TDMR-AR80 (Tokyo Ohka Kogyo Co., Ltd) was used for the resist mask, and a Nd:YAG laser with an emission wavelength of 355 nm was used for the exposure. The exposure time was 100 s for an area with a diameter of 5 cm at a laser power of 100 mW. After the fabrication of the resist mask, the first electrochemical wet etching was performed using dilute aqueous 3% oxalic acid, and an applied voltage of 3 V generated with a direct current (dc) power supply. By monitoring the dc between the sample and the counter electrode, the total etched volume was controlled. Throughout the wet etching process, the temperature of the etchant was maintained at 25 °C. After etching, a side wall protective layer was formed via Ar plasma etching (NLD, ULVAC) at a chamber pressure of 0.3 Pa. As shown in figure 1, resist canopies were formed on the tops of the micro-holes due to the isotropic nature of wet etching. These canopies appeared to play a key role in forming the side wall protective layer by preventing the spread of the resist and the oxidized layer of stainless steel residue to the outside of the holes. Thus, due to the presence of the wall protection, the second wet etching process deepened the micro-holes without widening them. Consequently, deep micro-holes could be fabricated by repeating the etching and protection processes. Finally, the resist mask and accumulated inert residue were removed with acetone. As mentioned in section 1, a vacuum plasma etching process was applied in this study. However, it should be possible to use an atmospheric plasma etching process. Therefore, by applying role-to-role processing, this multi-step etching process should be applicable to the large-area fabrication of periodic microstructures.

View Article: PubMed Central - PubMed

ABSTRACT

Controlling the thermal radiation spectra of materials is one of the promising ways to advance energy system efficiency. It is well known that the thermal radiation spectrum can be controlled through the introduction of periodic surface microstructures. Herein, a method for the large-area fabrication of periodic microstructures based on multi-step wet etching is described. The method consists of three main steps, i.e., resist mask fabrication via photolithography, electrochemical wet etching, and side wall protection. Using this method, high-aspect micro-holes (0.82 aspect ratio) arrayed with hexagonal symmetry were fabricated on a stainless steel substrate. The conventional wet etching process method typically provides an aspect ratio of 0.3. The optical absorption peak attributed to the fabricated micro-hole array appeared at 0.8 μm, and the peak absorbance exceeded 0.8 for the micro-holes with a 0.82 aspect ratio. While argon plasma etching in a vacuum chamber was used in the present study for the formation of the protective layer, atmospheric plasma etching should be possible and will expand the applicability of this new method for the large-area fabrication of high-aspect materials.

No MeSH data available.