Ion Milling Research Articles

Narrow‐Gap‐Integrated Ring Arrays (GRA) as Ultrahigh Field‐Enhancement Optical Resonators

AbstractGold nanoring nanoantenna are increasingly being utilized in biological and chemical sensing owing to their favorable spatially expanded hotspot distribution and distinctive field focusing capabilities. However, it is extremely challenging to further elevate the local electric field of nanorings for higher sensitivity without trading off the two aforementioned features. Here, a new nanoantenna design integrates conventional nanorings with nanogaps is introduced: in conjunction with metal sputtering and ion milling on a nanopillar template, narrow‐gap‐integrated ring arrays (GRA) are successfully nanofabricated featuring monolithic integration of nanoring arrays with sub‐5 nm nanogaps on‐chip. This new GRA nanophotonic structure simultaneously preserves the intrinsic benefits of the nanoring and amplifies the local electric field, achieving a field enhancement factor of up to 104 magnitudes. Using both top‐down and bottom‐up thin‐film processes, the GRA shows high degree‐of‐freedom in nanoantenna geometry, leading to tunable resonances throughout visible and near‐infrared wavelengths. These results combined showcased the potential of using GRA's tunable resonance, high e‐field enhancement, and wide hotspot‐distribution features in the creation of a universal molecular sensing platform for enhanced fluorescence.

Exploring Platinum Thin Films as Electrodes for High‐Temperature and ‐Pressure Electrochemical Studies in Aqueous Systems

ABSTRACTThis work reports a methodology for the fabrication of Pt thin‐film electrodes for electrochemical studies in hydrothermal systems. The research process was meticulous, with particular attention paid to the multilayer Ti/Pt/Ti/Al2O3 film structure and annealing conditions that are expected to impact the morphology of the films, surface composition and electrochemical response. The findings of this study are significant, as they provide valuable insights into the behaviour of Pt thin‐film electrodes in various media and conditions. Two different approaches were adopted for the preparation of the electrodes: in one case, the Ti/Pt/Ti/Al2O3 films deposited on sapphire wafers were exposed to rapid thermal annealing at 900°C under argon for 5 min, followed by argon ion milling to etch the final electrode pattern (Pt‐RA(900)), while in the other case, uncapped Pt films were annealed, after etching, in a tubular oven under argon at specific temperatures between 200°C and 900°C (Pt‐TO). Rapid annealing at 900°C on capped films resulted in the formation of a Pt3Ti intermetallic alloy with remarkable mechanical and chemical stability even after 10 h of immersion in deionised water, acid (0.1 M H2SO4) and alkaline media (0.1 M KOH) conditions at temperatures up to 150°C, despite the dissolution of the Al2O3 top layer at 150°C and long immersion times (> 10 h). In the case of uncapped Pt films, diffusion and oxidation of Ti through the Pt film at high temperature resulted in the formation of TiO2 on the surface of Pt. The results were confirmed by using a comprehensive suite of ex‐situ characterisation techniques to follow changes in the Pt electrode surface morphology and composition before and after immersion in H2O, 0.1 M H2SO4 and 0.1 M KOH solutions under argon. Ex‐situ electrochemical characterisation studies were also conducted to correlate the changes in the electrode surface properties, including the electrochemical surface area, with different annealing conditions and after various hydrothermal treatments in neutral, alkaline and acidic media.

All Oxide-Based Magnetic Tunnel Junctions Fabricated by Focused Ion Beam Nanomachining Technique

This study offers a comprehensive investigation into the fabrication of all-oxide-based magnetic tunnel junctions (MTJs) utilizing the focused ion beam (FIB) nanomachining method. The oxide thin films of SrTiO3 (STO), La[Formula: see text]CaxMnO3 (LCMO) and La[Formula: see text]SrxMnO3 (LSMO) were grown using pulsed laser deposition (PLD). Magnetization measurements at 5K revealed a notable difference in coercivity between LCMO and LSMO films, highlighting their suitability as electrodes in MTJ device fabrication. The deposited trilayer structure of LCMO/STO/LSMO exhibited heteroepitaxial growth, enabling the independent magnetic switching of electrode layers in the MTJ with the STO barrier layer. Conventional photolithography and ion milling were used to create a series of 4[Formula: see text][Formula: see text]m Hall bar tracks in the deposited films. Nanopillar devices, sized 250[Formula: see text]nm[Formula: see text]×[Formula: see text]250[Formula: see text]nm, were successfully fabricated on these 4[Formula: see text][Formula: see text]m tracks by optimizing the FIB nanomachining technique. The fabricated MTJs demonstrated a high tunnel magnetoresistance (TMR) response, leading to the conclusion that the FIB nanomachining technique holds significant promise for the development of high-performance oxide-based spintronic devices.

(Invited) Plasmonic Nanofabrication of Chiral Nanodisk Ensembles

Chiral plasmonic nanostructures attract attention because those could be applied to enantioselective chemical sensors, optical isolators, light sources for circularly polarized light (CPL), metasurfaces, and metamaterials. In many cases, chiral plasmonic nanostructures are fabricated by a top-down method such as electron beam lithography (EBL). Since EBL is time-consuming and expensive, we have developed photoelectrochemical methods in which a site-selective reaction is driven by optical near field generated around anisotropic metal nanoparticles under right- or left-CPL. As anisotropic metal precursors for preparation of chiral nanoparticles, we have used gold or silver nanocuboids, nanorods, nanocubes, and triangular or hexagonal nanoplates so far.1-5 In the present work we demonstrate that laterally isotopic, circular nanodisks can also be used as precursors for the preparation of chiral nanostructures.We prepared ensembles of circular gold nanodisks on a glass plate by a nanosphere lithography method. A glass plate was coated with a gold thin film, and an array of polystyrene spheres (500 nm diameter) was prepared on the gold film. The polystyrene spheres were etched by oxygen plasma, and the exposed gold film was etched by argon ion milling. As a result, an ensemble of gold nanodisks (~200 nm diameter) was obtained. Each nanodisk was capped with a polystyrene cone, and the cap was removed by sonication in toluene if necessary. We used both capped and uncapped nanodisk ensembles. The ensemble was irradiated with visible right- or left-CPL in the presence of silver ions and sodium citrate. The obtained Au-Ag nanostructures exhibited circular dichroism (CPL), indicating that the nanostructures have chirality. Initial deposition at arbitrary site may break the geometric symmetry, resulting in the growth into chiral structures. Saito, K.; Tatsuma, T. Nano Lett. 2018, 18, 3209–3212.Morisawa, K.; Ishida, T.; Tatsuma, T. ACS Nano 2020, 14, 3603–3609.Shimomura, K.; Nakane, Y.; Ishida, T.; Tatsuma, T. Appl. Phys. Lett. 2023, 122, 151109.Homma, T.; Sawada, N.; Ishida, T.; Tatsuma, T. ChemNanoMat 2023, 9, e202300096.Ishida, T.; Isawa, A.; Kuroki, S.; Kameoka, Y.; Tatsuma, T. Appl. Phys. Lett. 2023, 123, 061111.

Influence of Xe+ and Ga+ milling species on the cathodoluminescence of wurtzite and zincblende GaN

III-nitride materials, such as GaN and its alloys, are essential for modern microelectronics and optoelectronics due to their unique properties. Focused ion beam (FIB) techniques play a crucial role in their prototyping and characterization at the micro- and nanoscale. However, conventional FIB milling with Ga ions presents challenges, including surface amorphization and point defect introduction, prompting the exploration of alternative ion sources. Xenon-based inductively coupled plasma or plasma FIB has emerged as a promising alternative, offering reduced damage and better sample property preservation. Despite extensive research on FIB-induced damage in GaN, systematic comparisons between Ga and Xe ion milling on the luminescence characteristics of GaN remain limited. This study aims to fill this gap by evaluating and comparing the extent of FIB-induced damage caused by Ga and Xe ions in wurtzite and zincblende GaN through cathodoluminescence measurements. Our findings indicate that Xe ion milling yields higher integrated intensities compared to Ga ion milling, attributed to shallower implantation depths and reduced lattice disorder. We also observe a decrease in integrated intensity with increasing ion beam acceleration voltage for both wurtzite and zincblende GaN layers. This study provides valuable insights into optimizing FIB-based sample preparation techniques for III-nitride materials, with implications for enhancing device performance and reliability.

A Data-driven Deep Learning Approach for Remaining Useful Life in the ion mill etching Process

Prognostics and Health Management (PHM) is regarded as an essential element in the scope of intelligent manufacturing. Precise forecasting of the remaining useful life (RUL) of an ion mill is crucial in order to enhance the overall efficiency of the ion mill etching (IME) procedure. This paper proposed a Data-driven Deep Learning (DL) framework that integrates a Temporal Convolution Network (TCN), Long Short-Term Memory (LSTM), and self-attention mechanism to improve the accuracy of RUL prediction in the ion mill etching Process. Initially, sensor input data is divided into two parallel paths - one with TCN blocks for capturing long-range dependencies, and the other with LSTM layers for extracting temporal patterns. The outputs from both paths are then merged and input into an LSTM layer for enhanced learning, followed by a self-attention mechanism to highlight important features then fully connected layer for predicting RUL. The efficacy of this suggested model was assessed through the utilization of the 2018 PHM Data Challenge Dataset and juxtaposed against various Deep Learning models to demonstrate its efficacy. The results from the experiments indicate that ATCN-LSTM serves as a robust option for estimating the RUL in the ion mill etching Process as it outperformed all other models that were compared. The source code is publicly accessible at https://github.com/ion-mill-etching-Process.

Effect of Angle of Incident on Taper Angle in Femtosecond Laser Machining for Fabrication of Cross Section Analysis Specimen

Focused ion beam (FIB) technology is one of the most widely used methods for fabricating crosssectional analysis specimens because of its high precision and characteristics that minimize the occurrence of defects. Demand for large cross-sectional area analysis is increasing to improve product reliability in various industries, but is limited by the low milling speed of FIB. Other potential techniques such as Ar ion milling and plasma FIB have been adopted, but low milling speed for large areas still remains a problem. A promising solution to this issue involves laser machining prior to FIB milling. In laser machining a laser beam is irradiated to remove materials from the target. This technique can provide several orders of magnitude higher material removal rate than FIB, however, tapering of the machined surface and laser induced damage can occur. Removing these defects leads to increased FIB milling time. In this study, the laser parameters including angle of incident (AOI) were optimized to achieve a vertical like sidewall and minimize laser induced defects. Before applying AOI, laser machining parameters were optimized to reduce the angle of the machined sidewall. The taper angle of 2.5° was fabricated using the optimized parameters and application of AOI. Raman spectroscopy, SEM, and EDS analysis were used to measure not only the geometry of the laser machined sidewalls, but laser induced residual stress and defects. These results were then used to calculate the volume of FIB milling required to remove the laser induced damages and achieve vertical sidewalls. The application of AOI can significantly reduce the processing time in the FIB milling compared to the processing time when AOI is not applied.

Shaping single crystalline BaTiO3 nanostructures by focused neon or helium ion milling

The realization of perovskite oxide nanostructures with controlled shape and dimensions remains a challenge. Here, we investigate the use of helium and neon focused ion beam (FIB) milling in an ion microscope to fabricate BaTiO3 nanopillars of sub-500 nm in diameter starting from BaTiO3 (001) single crystals. Irradiation of BaTiO3 with He ions induces the formation of nanobubbles inside the material, eventually leading to surface swelling and blistering. Ne-FIB is shown to be suitable for milling without inducing surface swelling. The resulting structures are defect-free single crystal nanopillars, which are enveloped, on the top and lateral sidewalls, by a point defect-rich crystalline region and an outer Ne-rich amorphous layer. The amorphous layer can be selectively etched by dipping in diluted HF. The geometry and beam-induced damage of the milled nanopillars depend strongly on the patterning parameters and can be well controlled. Ne ion milling is shown to be an effective method to rapidly prototype BaTiO3 crystalline nanostructures.

Impact of Sample Preparation Approach on Transmission Electron Microscopy Investigation of Sputtered AlNi Multilayers Used for Reactive Soldering

This work presents studies of sputtered Al/Ni reactive multilayers by transmission electron microscopy. They are prepared for these analyses by three methods (both Ga‐ and Xe‐based focused ion beam, FIB, and tripod polishing plus Ar+ ion milling in precision ion polishing system, PIPS) to check their impact on these materials. Every sample shows polycrystalline and mostly chemically pure Al/Ni layers. They also hint existence of intermetallic compounds, especially the tripod‐prepared sample. These intermetallics first originate from the sputtering process. The layers are increasingly rough along the growth direction. Other remarkable findings can be highlighted. First, the heating operations applied during the tripod polishing preparation lead to recrystallization and blurred Al/Ni interfaces due to increased metals reactions, although additional contributions by roughness and preparation thickness to the current compositional uncertainties must be distinguished by optimizing future sample fabrications and preparations. Second, Xe‐based FIB leads to lamellae with seemingly low contamination, although Ga‐FIB is a potentially good alternative. Finally, FIB is better to study cross‐section preparations of pristine Al/Ni multilayers, whereas the present tripod polishing procedures are unsuitable for this purpose but allow to observe the beginning of Al/Ni transformations upon heating, which is interesting for the technological optimization of these materials.

Effect of fabrication processes on BaTiO3 capacitor properties

There is an increasing desire to utilize complex functional electronic materials such as ferroelectrics in next-generation microelectronics. As new materials are considered or introduced in this capacity, an understanding of how we can process these materials into those devices must be developed. Here, the effect of different fabrication processes on the ferroelectric and related properties of prototypical metal oxide (SrRuO3)/ferroelectric (BaTiO3)/metal oxide (SrRuO3) heterostructures is explored. Two different types of etching processes are studied, namely, wet etching of the top SrRuO3 using a NaIO4 solution and dry etching using an Ar+-ion beam (i.e., ion milling). Polarization-electric-field hysteresis loops for capacitors produced using both methods are compared. For the ion-milling process, it is found that the Ar+ beam can introduce defects into the SrRuO3/BaTiO3/SrRuO3 devices and that the milling depth strongly influences the defect level and can induce a voltage imprint on the function. Realizing that such processing approaches may be necessary, work is performed to ameliorate the imprint of the hysteresis loops via ex situ “healing” of the process-induced defects by annealing the ferroelectric material in a barium-and-oxygen-rich environment via a chemical-vapor-deposition-style process. This work provides a pathway for the nanoscale fabrication of these candidate materials for next-generation memory and logic applications.

Comparative study of ion milling techniques for the preparation of ceramic fibers in transmission electron microscopy

Sample preparation, essential to any analysis, remains an under-documented step. Preparation methods for transmission electron microscopy (TEM) are complex and lead to artifacts that need to be identified to avoid wrong conclusions about the sample's microstructure. Ion milling techniques are increasingly becoming the reference techniques to prepare thin foils. The possibilities of different ion beam processes for milling samples will be shown and compared, using ceramic carbon/carbide fibers, a material applied in many industrial applications, as a test specimen. This overview of ion milling preparation techniques will enable us to identify the advantages, disadvantages and parameters in order to best prepare these thin samples. It will be carried out for the community, and will also highlight a new preparation method for ceramic fibers to minimize the artefacts inherent in these techniques.

3D porous structure imaging of membranes for medical devices using scanning probe microscopy and electron microscopy: from membrane science points of view.

The evolution of hemodialysis membranes (dialyzer, artificial kidney) was remarkable, since Dow Chemical began manufacturing hollow fiber hemodialyzers in 1968, especially because it involved industrial chemistry, including polymer synthesis and membrane manufacturing process. The development of hemodialysis membranes has brought about the field of medical devices as a major industry. In addition to conventional electron microscopy, scanning probe microscopy (SPM), represented by atomic force microscopy (AFM), has been used in membrane science research on porous membranes for hemodialysis, and membrane science contributes greatly to the hemodialyzer industry. Practical studies of membrane porous structure-function relationship have evolved, and methods for analyzing membrane cross-sectional morphology were developed, such as the ion milling method, which was capable of cutting membrane cross sections on the order of molecular size to obtain smooth surface structures. Recently, following the global pandemic of SARS-CoV-2 infection, many studies on new membranes for extracorporeal membrane oxygenator have been promptly reported, which also utilize membrane science researches. Membrane science is playing a prominent role in membrane-based technologies such as separation and fabrication, for hemodialysis, membrane oxygenator, lithium ion battery separators, lithium recycling, and seawater desalination. These practical studies contribute to the global medical devices industry.

Multi-Scale and Multi-Branch Transformer Network for Remaining Useful Life Prediction in Ion Mill Etching Process

Multi-Scale and Multi-Branch Transformer Network for Remaining Useful Life Prediction in Ion Mill Etching Process

An evaluation of Ar ion milling in TEM sample preparation by energy-filtered TEM technique

Irradiated materials impose unique challenges when it comes to sample preparation for transmission electron microscopy (TEM) analysis. After the preparation of TEM samples by electrolytic polish, Ar ion milling was usually used to remove the surface contamination, as well as to thin the samples to desired thickness. Therefore, it is necessary to evaluate the advantages and limitations of Ar ion milling technique. In this work, the optimum milling condition with minimal surface artifacts was determined by Ar ion milling with various incident angles and beam energies, and the high-resolution TEM (HR-TEM) examinations confirmed the optimum condition to be 4.0keV Ar ions with incident angles of ±4°. In addition, Ar ion milling and subsequent thickness measurement by energy-filtered TEM (EF-TEM) were conducted to determine the thinning rate. It is found that electrolytic polished TEM lamella has a wedge-shape structure with a very sharp edge, and the thinning rates varied from 9.7±1.4nm/minute to 5.0±2.3nm/minute across the TEM lamella for 4.0keV Ar ions. However, the thinning rates for 2keV and 1keV Ar ions were under the examination limit, and they were supposed not sufficient to thin the tungsten samples. Even though the exact thinning rate varies for different materials, the general trends for the Ar ion milling observed in this study can be used as a guidance.

Preparation of damage-free kerogen specimen for microscopy: Understanding the damage mechanisms induced by ion milling techniques

Focused ion beam (FIB) milling techniques have been used for more than a decade to prepare organic-rich shale samples for electron microscopy. Nano- and micro-scale imaging of organic pores is used to assess pore volumes and hence estimate gas-in-place. Ion milling natural composites is particularly challenging due to drastic extremes in the physical properties of individual phases such as minerals and organics. Damage to the organic-rich phases occurs due to the large differences in thermal conductivities of minerals and organics that could reach an order of magnitude and the resulting buildup of heat in the organics. Furthermore, radiation damage via ion implantation may also alter the structural characteristics of the shale.Herein, the damage associated with preparing electron-transparent kerogen specimen using low kV FIB, cryo-FIB, and cryomicrotome, a new technique for shale kerogen, is quantified using experimental and modeling techniques. The degree of gallium implantation is quantified using scanning transmission electron microscopy (STEM), selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDX) along with the stopping range of ions in matter (SRIM) model. Interestingly, although the use of cryogenic temperature removes the thermal effect, Ga+ ion implantation is observed in the specimen prepared by cryo-FIB.Furthermore, the magnitude of the thermal damage at a highly localized level was determined using nano-beam electron diffraction (NBED) performed at −196 °C. Time-resolved diffraction patterns indicate that despite the fact kerogen is among the most stable geopolymers, the aromatic components still degraded due to the local temperature increases. On the other hand, the cryomicrotome technique operating at liquid nitrogen temperature is demonstrated as a suitable method to prepare thin kerogen specimen while minimizing structural damage and preserving its native porosity and chemical composition.

Fabrication of V1−x Ti x O2 microbolometers on Si3N4/SiO2 membranes

V1−x Ti x O2 thin films have been prepared on Si3N4/SiO2/Si substrates by the two-step process of (1) growth of VO2/TiO2 thin films by metal-organic decomposition with carbothermal reduction, and (2) the interdiffusion between V and Ti atoms at the interface between the VO2 and TiO2 thin films during the growth of the VO2 thin film. The abrupt resistance change and the thermal hysteresis due to the metal-insulator transition were gradually suppressed with increasing the firing temperature, and they completely disappeared for the V0.65Ti0.35O2 thin film prepared at 650 °C. A relatively high TCR of −4.2%/K was obtained from the film over a wide temperature range of 20 °C–90 °C. V0.65Ti0.35O2 microbolometers with a size of 5 μm × 15 μm were fabricated by photolithography and Ar ion milling, and a Si3N4/SiO2 membrane under the microbolometer was formed with an aqueous KOH solution. The DC sensitivity of the fabricated microbolometer on the membrane was estimated to be 9876 W−1 and 20 times higher than that of the microbolometer on the Si3N4/SiO2/Si substrate. The V0.65Ti0.35O2 microbolometer on the membrane is available for a terahertz detector operating over the wide temperature range.

Characterization and ion-irradiation studies of typical microstructures in nuclear graphite

Nuclear graphite is an important structural material for advanced gas cooled reactors and molten salt reactors. Graphite microstructures are always accompanied with typical pores or cracks which plays an important role in nuclear graphite behaviors. A new surface processing method involving ion milling and fast oxidation is used here to characterize typical microstructures from the large-size calcination cracks in fillers to very small quinoline insoluble particles in NBG-17 and NBG-18 graphites. By using this surface processing method and ion irradiation technique, an attempt is made to explore the microstructural basis of graphite irradiation behaviors. Heating-induced relaxation of graphite microstructures is observed, indicating the presence of residual stresses in nuclear graphite. Creased graphite sheets are identified on ion-milled surface and are believed to accommodate a-axis shrinkage of graphite crystallites at the beginning of irradiation, which is consistent with the initial plateau of slow dimensional shrinking. Compared to NBG-17 graphite, NBG-18 is found to have much more calcination cracks which contract during irradiation, resulting in a deeper volume shrinkage of NBG-18. The quinoline insoluble particles containing chaotic structures are shown to severely contract and separate from surrounding structures, leading to voids in between. Both NBG-18 and NBG-17 have abundant quinoline insoluble particles, which might make a contribution to their smaller volume shrinkage than other medium-grain graphite.

Focused ion beams from GaBiLi liquid metal alloy ion sources for nanofabrication and ion imaging

In this work, we present an overview of nanopatterning and imaging applications using newly developed workflows with focused ion beams (FIBs) produced with a GaBiLi liquid metal alloy ion source. The primary beam of this source type contains gallium, bismuth, and lithium as well as cluster ions which can be separated quickly using a Wien filter. Lithium ion milling has been applied to generate heptamer-arranged nanohole arrays in gold films with high resolution. Workflows for two-step bowtie nanofabrication using lithium and bismuth ions from the same source have been established. Furthermore, we present ion beam imaging results that were obtained with lithium ions on various sample materials. Combining the large sputter yield and high depth resolution of heavy bismuth ions with the high lateral imaging resolution of light lithium ions enables 3D nanoscale tomography using different ion species generated from the same source. Sample tilt is not required due to the top-down geometry of the FIB.

The formation and evolutionary characteristics of organic matter and pyrites in the continental shales of the 3rd submember of Chang 7 Member, Yanchang formation, Ordos Basin, China

Through microscopic analyses (e.g., organic macerals, thin section observation, scanning electron microscope (SEM) imaging of fresh bedding planes via argon ion milling, and energy spectrum tests) combined with Rock-Eval analyses, this study systematically investigated the organic matter and pyrites in the continental shales in the 3rd submember of the Chang 7 Member (Chang 73 submember) in the Yanchang Formation, Ordos Basin and determined their types and the formation and evolutionary characteristics. The results are as follows. The organic matter of the continental shales in the Chang 73 submember is dominated by amorphous bituminites and migrabitumens, which have come into being since the early diagenetic stage and middle diagenetic stage A, respectively. The formation and transformation of organic matter is a prerequisite for the formation of pyrites. The Ordos Basin was a continental freshwater lacustrine basin and lacked sulphates in waters during the deposition of the Chang 7 Member. Therefore, the syndiagenetic stage did not witness the formation of large quantities of pyrites. Since the basin entered early diagenetic stage A, large quantities of sulfur ions were released as the primary organic matter got converted into bituminites and, accordingly, pyrites started to form. However, this stage featured poorer fluid and spatial conditions compared with the syndepositional stage due to withdraw of water, the partial formation of bituminites, and a certain degree of compaction. As a result, large quantities of pyrrhotite failed to transition into typical spherical framboidal pyrites but grew into euhedral monocrystal aggregates. In addition, pyrites are still visible in the migrabitumens in both microfractures and inorganic pores of mudstones and shales, indicating that the pyrite formation period can extend until the middle diagenetic stage A.

Effects of anodization conditions of stainless steel on the formation of ordered nanoporous structures with high aspect ratios

The nanoporous structures obtained by the anodization of stainless steel are functional materials with various potential applications. It has been reported that nanoporous structures can be prepared by the anodization of stainless steel in an electrolyte containing fluoride ions. However, under the reported anodization conditions, the control range of the interpore distance of resulting nanoporous structures was narrow. To expand the application fields of the nanoporous structures obtained by the anodization of stainless steel, it is an important challenge to determine the anodization conditions that can control the interpore distance of nanoporous structures over a wide range. In this study, we investigated the effects of the electrolyte composition on the anodization behavior of stainless steel and the interpore distance of the resulting nanoporous structure. As a result, we found that the maximum voltage for the stable anodization of stainless steel increases when a mixture of ethylene glycol and glycerol containing NH4F is used as the electrolyte. Since the interpore distance of nanoporous structures obtained by the anodization of stainless steel is proportional to the anodization voltage, as the voltage range over which stainless steel can be anodized increased, the range of interpore distances of the nanoporous structures obtained also increased. On the basis of these results, ordered nanoporous structures with a large interpore distance (100 nm), which could not be obtained under the previously reported anodization conditions, were fabricated by the anodization of a stainless steel substrate with a depression pattern formed by Ar ion milling using an alumina mask under optimized anodization conditions. The resulting ordered nanoporous structures with controlled interpore distances are expected to be used in various devices such as capacitors and photocatalysts.