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[Takayoshi Oshima](https://orcid.org/0000-0001-8550-9735), [Yuichi Oshima](https://orcid.org/0000-0001-8293-4891)
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[Fabrication of β-Ga 2 O 3 /air-gap structures on (001) β-Ga 2 O 3 using HCl gas etching](https://mdr.nims.go.jp/datasets/7d1b2663-babf-4d4a-85bc-ce280f55564e)
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Fabrication of β-Ga2O3/air-gap structures on (001) β-Ga2O3 using HCl gas etchingScience and Technology of Advanced Materials: MethodsISSN: 2766-0400 (Online) Journal homepage: www.tandfonline.com/journals/tstm20Fabrication of β-Ga2O3/air-gap structures on (001)β-Ga2O3 using HCl gas etchingTakayoshi Oshima & Yuichi OshimaTo cite this article: Takayoshi Oshima & Yuichi Oshima (2025) Fabrication of β-Ga2O3/air-gap structures on (001) β-Ga2O3 using HCl gas etching, Science and Technology of AdvancedMaterials: Methods, 5:1, 2554046, DOI: 10.1080/27660400.2025.2554046To link to this article: https://doi.org/10.1080/27660400.2025.2554046© 2025 The Author(s). Published by NationalInstitute for Materials Science in partnershipwith Taylor & Francis GroupPublished online: 03 Sep 2025.Submit your article to this journal Article views: 21View related articles View Crossmark dataFull Terms & Conditions of access and use can be found athttps://www.tandfonline.com/action/journalInformation?journalCode=tstm20https://www.tandfonline.com/journals/tstm20?src=pdfhttps://www.tandfonline.com/action/showCitFormats?doi=10.1080/27660400.2025.2554046https://doi.org/10.1080/27660400.2025.2554046https://www.tandfonline.com/action/authorSubmission?journalCode=tstm20&show=instructions&src=pdfhttps://www.tandfonline.com/action/authorSubmission?journalCode=tstm20&show=instructions&src=pdfhttps://www.tandfonline.com/doi/mlt/10.1080/27660400.2025.2554046?src=pdfhttps://www.tandfonline.com/doi/mlt/10.1080/27660400.2025.2554046?src=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1080/27660400.2025.2554046&domain=pdf&date_stamp=03%20Sep%202025http://crossmark.crossref.org/dialog/?doi=10.1080/27660400.2025.2554046&domain=pdf&date_stamp=03%20Sep%202025https://www.tandfonline.com/action/journalInformation?journalCode=tstm20Fabrication of β-Ga2O3/air-gap structures on (001) β-Ga2O3 using HCl gas etchingTakayoshi Oshima and Yuichi OshimaResearch Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba, JapanABSTRACTβ-Ga2O3/air-gap structures were fabricated on (001) substrates via crystallographic etching with HCl gas. Etching at 650 °C under an HCl partial pressure of 250 Pa resulted in a vertical etch rate of 0.10 μm/min on the (001) plane and a lateral etch rate of 0.70 μm/min along the < 010 > direction. This high orthogonal etching anisotropy enabled the formation of β-Ga2O3/air-gap structures – such as cantilevers and air bridges – without the need for wafer bonding or transfer processes. This straightforward technique, compatible with commonly used (001) substrates, holds promise for the integration of β-Ga2O3-based microelectromechanical systems (MEMS) and power electronic devices.IMPACT STATEMENTβ-Ga2O3/air-gap structures were directly fabricated on (001) orientation β-Ga2O3 substrates using crystallographic HCl gas etching, offering strong potential for monolithic integration of β-Ga2O3-based microelectromechanical systems (MEMS) and power electronic devices.ARTICLE HISTORY Received 30 May 2025 Revised 18 July 2025 Accepted 24 August 2025 KEYWORDS β-Ga2O3; cantilever; air bridge; MEMS; crystallographic etching; HCl gas etching1. Introductionβ-Ga2O3 is an emerging wide bandgap semiconductor with great potential for a wide range of applications. It offers both a high critical breakdown field of nearly 8 MV cm− 1 and compatibility with melt growth techniques for producing high-quality, scalable wafers [1,2]. Owing to these two major advantages over conventional wide bandgap semiconductors – such as SiC, GaN, and diamond—β-Ga2O3 is increasingly regarded as a promising material for next-generation low-loss, high-voltage power devices [3]. In addition, β-Ga2O3 demonstrates native solar-blind photoresponsivity and gas sensitivity, which are beneficial for developing filterless solar-blind photodetectors and gas sensors, respectively [4,5]. It also possesses favorable mechanical properties, including a high Young’s modulus (~261 GPa) and a high acoustic velocity (~6623 m s−1), both comparable to those of Si, rendering it well- suited for microelectromechanical systems (MEMS) applications [6]. These properties suggest that β-Ga2 O3 could serve as a highly versatile platform for future electromechanically coupled and tunable devices in electronics, optoelectronics, and advanced sensing. Potential applications include radio-frequency MEMS components in high-power, high-frequency systems for defense, wireless infrastructure, satellites, and resonance-enhanced solar-blind UV photodetectors and gas sensors [6].For the monolithic integration of β-Ga2O3-based MEMS components into the aforementioned β-Ga2 O3-based systems, it is preferable to fabricate mechanically suspended β-Ga2O3/air-gap structures – such as cantilevers and air bridges – directly on β-Ga2 O3 substrates. However, previously reported MEMS devices from other groups have employed β-Ga2O3 /air-gap structures formed by manually exfoliating narrow flakes and membranes from bulk single crystals, followed by their transfer onto foreign substrates [7–12]. Even when transferred onto pre-patterned β- Ga2O3 substrates, such a low-reproducibility, human- CONTACT Takayoshi Oshima OSHIMA.Takayoshi@nims.go.jp Research Center for Electronic and Optical Materials, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanSCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS: METHODS 2025, VOL. 5, NO. 1, 2554046 https://doi.org/10.1080/27660400.2025.2554046© 2025 The Author(s). Published by National Institute for Materials Science in partnership with Taylor & Francis Group This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) or with their consent.http://orcid.org/0000-0001-8550-9735http://orcid.org/0000-0001-8293-4891http://www.tandfonline.comhttps://crossmark.crossref.org/dialog/?doi=10.1080/27660400.2025.2554046&domain=pdf&date_stamp=2025-09-03dependent method is unsuitable for practical applications. The advanced ion-cutting technique could potentially offer a solution to this reproducibility issue [13]. Nevertheless, the process is inherently complex, involving hydrogen ion implantation, wafer bonding, low-temperature annealing to induce blistering, and layer splitting, followed by chemical mechanical polishing and high-temperature annealing to recover crystallinity. Therefore, an alternative reproducible method that avoids such complexity while enabling direct air-gap formation on β-Ga2O3 substrates is highly desirable.As a simpler and more direct approach, we propose a crystallographic gas etching technique. Since 2023, we have demonstrated anisotropic etching of single- crystal β-Ga2O3 substrates with various orientations – including (001), (010), (�102), (011), and (100) – using N2-diluted HCl gas and forming gas, without the need for plasma excitation [14–20]. Unlike conventional plasma-based dry etching, these etching methods do not cause plasma-induced damage to the crystal. The resulting etched structures were bounded by crystal facets with lower surface energy densities. Among these, the (100) plane, known to possess the lowest surface energy density [21], exhibited the highest etch resistance and consistently emerged across all substrate orientations. Leveraging this strong anisotropic etching behavior, we recently succeeded in forming air-gap structures to form air bridges on (100) substrates [20]. This achievement was enabled by a highly orthogonal etching rate ratio: the lateral etch rate along the [010] was approximately ten times greater than the vertical etch rate on the (100) plane.However, the (100) plane is unsuitable for homoepitaxial growth due to the formation of twin lamellae due to the double positioning of adatoms – a phenomenon where adatoms may occupy symmetrically equivalent sites leading to twin boundaries [22]. Although this issue can be mitigated by using miscut substrates or applying surfactant-assisted growth within a narrow growth window [22–24], research on (100) substrates still significantly lags behind that on (001) and (010) substrates. The latter orientations naturally support single-domain homoepitaxy without the need for additional techniques. In particular, for the (001) orientation, 4-inch single-crystal wafers with lightly doped, thick homoepitaxial layers are commercially available, facilitating the fabrication of vertical power devices with superior performance [3,25]. Thus, the formation of β-Ga2O3/air-gap structures on (001) substrates is highly desirable to ensure compatibility with the most advanced device platform.In a previous study, we investigated the HCl gas etching characteristics on the (001) plane using etching masks with various etching geometries, and systematically varied key process parameters, including the HCl partial pressure (PHCl = 25–250 Pa) and process temperature (TP = 548–949 °C) [15]. We observed that the lateral-to-vertical etch rate ratio increased with increasing PHCl and decreasing TP, with lateral etching being most pronounced along the < 010 > direction at lower TP. These findings suggest that air-gap formation with crystallographic etching is possible even for (001) substrates.In this study, therefore, we conducted HCl gas etching on (001) β-Ga2O3 substrates under conditions of high PHCl of 250 Pa and low TP of 650 °C. The resulting lateral-to-vertical etch rate ratio reached as high as 7, enabling the fabrication of cantilevers and air-bridge structures, which are commonly employed in MEMS applications.2. ExperimentalWe conducted etching experiments based on the processes and measurement methods described below. The SiO2 mask layer was deposited on the β-Ga2O3 surface using plasma-enhanced chemical vapor deposition (PECVD) with tetraethoxysilane (TEOS) and O2 as precursors. Conventional capacitively coupled plasma reactive ion etching (CCP- RIE) with fluorine chemistry (CHF3 and N2) and inductively coupled plasma reactive ion etching (ICP-RIE) with chlorine chemistry (BCl3 and Ar) were used to etch the SiO2 mask and β-Ga2O3 substrate, forming etching windows and mesa structures, respectively. Laser lithography with a 375-nm semiconductor laser was employed to define the CCP-RIE and ICP-RIE regions. The photoresist mask was removed by cleaning with organic solvents (N-methyl-2-pyrrolidone and isopropyl alcohol) and oxygen plasma treatment in a parallel-plate plasma system. HCl gas etching was carried out in a custom-built halide vapor phase epitaxy/etching system. The etching process was performed in an N2-diluted HCl gas flow under atmospheric pressure, with relatively high PHCl of 250 Pa and low TP of 650 °C. The gas flow was directed perpendicular to the substrate, which was mounted on a rotating holder. The SiO2 thickness was measured by ellipsometry. The SiO2 mask was removed using buffered hydrofluoric acid (BHF). The etched structures were observed using differential interference contrast (DIC) microscopy and scanning electron microscopy (SEM). A focused ion beam (FIB)-SEM hybrid system was also employed for cross-sectional imaging, with a carbon layer deposited prior to FIB milling to preserve the surface structure. A low acceleration voltage of 2.0 kV was used for all SEM observations to enhance material contrast. Etch depths were measured with a stylus profilometer or the FIB-SEM system.Sci. Technol. Adv. Mater. Meth. 5 (2025) 2 T. OSHIMA AND Y. OSHIMA3. Results and discussion3.1. Crystallographic etching on (001)First, HCl gas etching was performed on a planar (001) β-Ga2O3 substrate. The process sequence is illustrated in Figure 1. A 151-nm-thick SiO2 layer was deposited by PECVD and patterned using laser lithography and CCP-RIE to form etching windows. The window shapes included a rectangle and two types of wagon-wheel patterns. The size of the rectangular window was 100 μm × 200 μm. The wagon-wheel patterns consisted of line- shaped windows, each measuring 50 μm in length and 0.64 μm in width. One configuration included 36 windows arranged at 10° intervals. The other configuration featured windows aligned along specific crystallographic directions, which are parallel to the nearly vertical low Miller-index oxygen sublattice planes of (100), (010), (310), and (�310). HCl gas etching of the masked substrate was then carried out for 10 min.The etched structures were examined to investigate the in-plane anisotropic etching nature. By measuring the etched depth within the rectangular window, the vertical etch rate was determined to be 0.10 μm min−1. In contrast, the side etching exhibited a strong dependence on in-plane orientation, as observed in the DIC images of the two wagon- wheel patterns Figure 2(a) and 2(b), where the side-etched regions are visualized by distinctive colors, in contrast to the gray-toned masked areas. The magnified DIC image of the line-shaped window aligned along the [100] direction clearly shows the color distinctions among the mask, window, and side-etched regions Figure 2(c). Side etching was most enhanced when the windows were • PECVD • Lithography• CCP-RIE• HCl gas etchingSiO2(001) β-Ga2O3Figure 1. Process sequence for selective-area HCl gas etching on the (001) β-Ga2O3 substrate.Figure 2. Summary of the in-plane dependence of side etching characteristics on the (001) β-Ga2O3 substrate. (a)–(c) DIC microscope images of side-etched structures formed beneath the wagon-wheel window patterns. The line-shaped windows were arranged at 10° intervals from the [010] direction in (a), whereas in (b) they were aligned along specific crystallographic directions parallel to the low-index oxygen sublattice planes. (a) and (b) were recorded at the same magnification. (c) Magnified image of the region near the line-shaped window aligned with the [100] direction. (d) Polar plot of side etch rates perpendicular to the wagon- wheel windows shown in (a) and (b). ‘*’ and ‘×’ denotes the reciprocal lattice vector and cross product, respectively. (e) 54° tilted- view SEM image of the cross section of the etched structure formed beneath the line-shaped window in the [100] direction, which corresponds to the structure shown in (c).Sci. Technol. Adv. Mater. Meth. 5 (2025) 3 T. OSHIMA AND Y. OSHIMAaligned with the [100] direction and least pronounced along the [010] direction. It was also suppressed for windows oriented along [130] and [1 �30]. These window directions are parallel to the aforementioned oxygen sublattice planes, suggesting that the oxygen sublattice strongly influences the gas etching process. The side etch rates measured from the wagon-wheel patterns were plotted in polar coordinates, as shown in Figure 2(d). The resulting pattern closely reflects the symmetry of the β-Ga2O3 crystal structure [26], which exhibits mirror symmetry with respect to the (010) plane and two-fold rotational symmetry around the [010] axis. These symmetries explain why right and left halves of the plot display mirror symmetry, while upper and lower halves exhibit quasi-mirror symmetry. The maximum side etch rate of 0.70 μm min−1 in the [010] and [0 �10] directions – which are crystallographically equivalent – is seven times higher than the vertical etch rate of 0.10 μm min−1 on the (001) plane. The cross- sectional profile of the etched trench in the [100]- oriented window, corresponding to that shown in Figure 2(c), was observed using the FIB-SEM system Figure 2(e). The etched trench primarily extended in the [010] and [0 �10] lateral directions, while its depth remained relatively shallow. The high lateral-to-vertical etching selectivity is considered to enable the formation of air gaps using the etching process alone, eliminating the need for wafer bonding and transfer processes.3.2. Fabrication of β-Ga2O3/air-gap structuresNext, cantilevers and air bridges – fundamental MEMS components – were fabricated on a (001) β-Ga2O3 substrate. The process flow is schematically illustrated in Figure 3. First, mesa etching was performed to define the cantilever and air bridge areas via laser lithography and ICP-RIE. The mesas were oriented along the [100] direction so that their sidewalls faced the [010] and [0 �10] directions. Their dimensions were 0.92 μm in height, ~1.7 μm in width, and ~30.0 μm in length. Second, a 119-nm-thick SiO2 layer was deposited on the surface using PECVD. Third, the SiO2 layer was patterned via laser lithography and CCP-RIE to open etching windows for the subsequent HCl gas etching. Fourth, the mesas were undercut through lateral etching with HCl gas for 5 min. Finally, the SiO2 mask was removed by BHF etching.After HCl gas etching, the resulting cantilevers and air bridges were examined by SEM, as shown in Figure 4. Figure 4(a1) and 4(a2) show SEM images of the cantilever and air-bridge structures with the SiO2 mask still in place, respectively. Shadow regions observed beneath these structures suggest the formation of air gaps. In contrast, no such shadowed regions were observed along the etched sidewalls of the anchor regions due to a very low lateral etch rate of 0.07 μm min−1 along the [�100] direction [Figure 2(d)], which is attributed to the high etch resistance of the (100) plane. The presence of air gaps was confirmed by cross-sectional observations at the centers of the cantilever and air bridge, as shown in Figure 4(b1) and 4(b2). Air gaps of approximately 0.5 μm and 0.6 μm were clearly observed beneath the cantilever and air bridge, respectively. The slightly narrower air gap under the cantilever is likely attributed to its downward bending compared to the air bridge. The thicknesses of the cantilever and air bridge were reduced to ~0.6 μm from their original mesa heights of 0.92 μm, due to etching from the reverse side of the mesas. Figure 4(c1) and 4(c2) show the cantilever and air bridge after the removal of the SiO2 mask, respectively, demonstrating our state-of-the-art fabrication technique for β-Ga2O3/air-gap structures. Aside from the air-gap structures, the morphology of the etched bottom surface was slightly rough due to HCl gas etching. This implies that the reverse sides of the cantilevers and air bridges may also exhibit similar surface roughness. Therefore, further optimization is required to achieve improved surface smoothness, and this will be addressed in future work.4. SummaryThis study revealed significant anisotropy in the HCl gas etching of (001) β-Ga2O3 with lateral etch rates reaching up to 0.70 µm min−1 along the < 010 > direction-seven times greater than the vertical etch rate of 0.10 µm min−1. Taking advantage of this high lateral- to-vertical etch rate ratio, we demonstrated the direct fabrication of cantilevers and air bridges, fundamental mechanically suspended structures for MEMS applications. Considering that most vertical power devices are fabricated on (001)-orientated substrates, the HCl • Lithography• ICP-RIE• Lithography• CCP-RIE• HCl gas etching(001) β-Ga2O3SiO2[010]001*[100]• BHF etching• PECVDFigure 3. Process sequence for the fabrication of β-Ga2O3/air- gap structures on (001) β-Ga2O3 substrate.Sci. Technol. Adv. Mater. Meth. 5 (2025) 4 T. OSHIMA AND Y. OSHIMAgas etching method presented here is particularly promising for the monolithic integration of MEMS and power devices.Lastly, it is worth noting that a variety of crystallographic anisotropic etching techniques have been reported for β-Ga2O3, including hot phosphoric acid etching [27–29], photoelectrochemical etching [30,31], metal-assisted chemical etching [32,33], hydrogen-environment anisotropic thermal etching [34], forming gas etching [18], Ga flux etching [35], triethylgallium gas etching [36], and tert-butyl chloride gas etching [37]. Although our study focused on HCl gas etching, the air-gap formation demonstrated here might also be achievable using these alternative methods. However, further investigation should be required – particularly to determine the conditions under which the lateral etching rate becomes sufficiently high relative to the vertical etching rate.AcknowledgementsAll experiments, except for HCl gas etching, were conducted using equipment in the Nanofabrication and Electron Microscopy Units at the National Institute for Materials Science (NIMS), under the framework of the Advanced Research Infrastructure for Materials and Nanotechnology (ARIM), supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (No. JPMXP1225NM5079).Disclosure statementNo potential conflict of interest was reported by the author(s).FundingThis work was financially supported by a Grant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science (JSPS), MEXT, Japan [JP24K01368].ORCIDTakayoshi Oshima http://orcid.org/0000-0001-8550- 9735Yuichi Oshima http://orcid.org/0000-0001-8293-4891References[1] Higashiwaki M, Sasaki K, Kuramata A, et al. Gallium oxide (Ga2O3) metal-semiconductor field-effect transistors on single-crystal β-Ga2O3 (010) substrates. Figure 4. Summary of 54° tilted-view SEM images of the fabricated (a1)–(c1) cantilever and (a2)–(c2) air bridge structures on (001) β-Ga2O3 substrates. Images (a1) and (a2) were acquired after etching with HCl gas. Images (b1) and (b2) show cross-sectional views corresponding to (a1) and (a2), respectively. Images (c1) and (c2) were taken after the removal of the SiO2 mask.Sci. Technol. Adv. Mater. Meth. 5 (2025) 5 T. OSHIMA AND Y. OSHIMAAppl Phys Lett. 2012;100(1):013504. doi: 10.1063/1. 3674287 [2] Kuramata A, Koshi K, Watanabe S, et al. High-quality β-Ga2O3 single crystals grown by edge-defined film- fed growth. Jpn J Appl Phys. 2016;55(12):1202A2. doi: 10.7567/JJAP.55.1202A2 [3] Sasaki K. Prospects for β-Ga2O3: now and into the future. Appl Phys Express. 2024;17(9):090101. doi: 10. 35848/1882-0786/ad6b73 [4] Chen H, Li Z, Zhang Z, et al. 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