Vertical and Deep Micro- and Nanofabrication Technique of SiO₂ Using HF Gas and Catalyst

Here, we report a new SiO₂micro- and nanofabrication technique, advanced gas-phase chemical reaction for anisotropic catalyst etching (AGC-ACE) using a catalyst and HF gas. SiO₂ is used in optical components such as metalens owing to its high chemical durability and transparency. Reactive ion etching (RIE) is a typical SiO₂ nanofabrication technique used to fabricate optical components. Because SiO₂ has a lower selectivity ratio with the mask than Si, RIE has the drawbacks of tapered etching shapes and difficulty in deep etching. The taper etching has caused the silica glass metalens to deviate from the optical design. In addition, a phase difference of 2π has not been achieved because the height of the meta-atom cannot be increased (<i>Nano Lett.</i> 2019, <b>19</b>, 8673-8682). Therefore, if a vertical and deep nanofabrication technique for SiO₂ is realized, it will be possible to fabrication of optical components made of SiO₂ with higher performance.<br/>Although the etching rate of SiO₂with HF is very low in an anhydrous atmosphere, we have previously shown that the silanol groups on the SiO₂ surface catalyze the reaction (<i>J Am Ceram Soc.</i> 2023, <b>106</b>, 4052-4060). Therefore, we hypothesized that if a material containing OH groups was patterned on a SiO₂ substrate and treated with HF, selective SiO₂ etching could be achieved. As a result, we developed a novel microfabrication method for SiO₂ substrates by combining a polymer, such as a photoresist, with HF (<i>ACS Appl. Mater. Interfaces</i> 2024, <b>16</b>, 22657-22664). After patterning an i-line resist on an SiO₂ substrate, the hydroxyl (OH) groups in the resist acted as catalysts upon treatment with HF, selectively etching the resist-coated area. This technique is the opposite of the conventional RIE technique in that the area to be etched is the area where the resist is located. Additionally, highly vertical processing is enabled because the catalyst will continue to retain its shape without being consumed. In fact, a trench structure with a width of 5 μm and a depth of 20 μm could be fabricated with a taper angle of &lt;1 °. Another feature of this etching technique is that the etching depth is capable of much deeper than RIE. Deep holes with a diameter of 10 μm and a depth of 100 μm could be fabricated with this process. Furthermore, the etching depth and rate can be controlled by the processing time and HF concentration. The AGC-ACE etching rate at 20% HF was approximately 1 μm/min, several times faster than that of RIE, and is advantageous in terms of throughput.<br/>AGC-ACE enables the vertical and deep etching of SiO₂, which is not possible with conventional methods. SiO₂ pillar structures were fabricated by AGC-ACE for application in optical components, such as metalenses. The i-line resist was patterned into a 500 nm hole and treated with HF. High aspect ratio pillars with a diameter of 500 nm and a height of 25 μm (aspect ratio 50) were achieved, which is impossible with conventional techniques. Extending the HF processing time also produced a structure with an aspect ratio exceeding 100 with a height of 80 μm. Even with this high-aspect-ratio structure, the pillar diameters were nearly uniform. Further, density functional theory (DFT) calculations of the catalytic effect of the photoresist showed that polar functional groups, such as hydroxyl groups in the polymers, enhanced the dissociation of HF and activated F^-. This suggests that resins with carboxylic acid and amino groups may also act as catalysts. In fact, it has also been experimentally confirmed that AGC-ACE is possible in principle with electron beam resist and nanoimprint. AGC-ACE is expected to be applicable to many optical components, including highly functional visible-light metalenses that achieve an ideal phase difference. Now, we plan to create SiO₂ metalens using AGC-ACE.