Faculty Profiles - OKA Chiemi

4Dナノインプリントに向けた高成形性形状記憶合金マイクロ構造体の作製 Open Access

泉光暢, 柴垣竜資, 岡智絵美, 秦誠一, 櫻井淳平

精密工学会学術講演会講演論文集 Vol. 2024A ( 0 ) page： 146 - 146 2024.8

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：公益社団法人精密工学会

高成形性形状記憶合金（HFSMA）は，非晶質状態で金属ガラスの特性を，結晶化後に形状記憶特性を示す．熱ナノインプリントによりHFSMA製繊毛構造体の加工を行い，ナノ繊毛アクチュエータの作製を検討している．本研究では，リソグラフィによりマイクロホールのSiモールドを作製し，熱ナノインプリントによってHFSMAマイクロ柱状構造体を作製した．今後は，高アスペクト比のナノ柱状（繊毛）構造体の作製を目指す．

DOI： 10.11522/pscjspe.2024a.0_146

Effect of Noise on Accuracy of Grain Size Evaluation by Magnetic Barkhausen Noise Analysis Open Access

Omae Kanna, Yamazaki Takahiro, Sano Kohya, Oka Chiemi, Sakurai Junpei, Hata Seiichi

International Journal of Automation Technology Vol. 18 ( 4 ) page： 528 - 536 2024.7

Language：English Publishing type：Research paper (scientific journal) Publisher：Fuji Technology Press Ltd.

Magnetic Barkhausen noise (MBN) is a magnetic signal caused by domain wall motion and is used for non-destructive testing and evaluation of ferromagnetic materials because of its sensitivity to both mechanical and magnetic properties. Recently, machine learning models have been employed to evaluate materials based on MBN; however, the application of material evaluation to low-volume targets is challenging because of their low signal-to-noise ratio, which is due to their low volume. Therefore, understanding the influence of the signal-to-noise ratio is important, particularly for low-volume objects. However, very few reports have quantitatively assessed the influence of noise in MBN analysis. In this study, we focused on noise to improve the accuracy of MBN analysis using machine learning, investigated its impact on the prediction accuracy of machine learning models, and explored methods to mitigate its effects. A method for grain size evaluation based on MBN analysis was adopted and performed for Fe-Co alloy wires with different grain sizes. After the measurement of MBN, the relationship between the extracted features from the analysis of MBN by fast Fourier transform and grain size was learned using a gradient boosting decision tree to create a grain size evaluation model, and the coefficient of determination was used to evaluate the prediction accuracy of the grain size evaluation. The machine learning model demonstrated high prediction accuracy (R2 = 0.926) across the entire grain size range. Using the model to assess the effect of signal-to-noise ratio, experiments were also conducted using MBN time-series data with artificially applied Gaussian noise. Additionally, from the insight of the distribution of predicted grain sizes, we confirmed that a noise reduction method by averaging the MBN prediction results can improve the prediction accuracy by reducing the effect of noise as expected. This research will lead to the adoption of MBN applications, which are simple and practical methods of material evaluation, for the micro–nano discipline.

DOI： 10.20965/ijat.2024.p0528

Prototype of Parallel Plate Type Fast Atom Beam Source and its Improvement of Irradiation Characteristics Open Access

Kato Taisei, Morisaki Ryo, Yamazaki Takahiro, Oka Chiemi, Sakurai Junpei, Hata Seiichi

International Journal of Automation Technology Vol. 18 ( 4 ) page： 513 - 520 2024.7

Language：English Publishing type：Research paper (scientific journal) Publisher：Fuji Technology Press Ltd.

Fast atom beam (FAB) source is used for surface activated bonding. This process is attracting attention as an essential process for next-generation semiconductor manufacturing. Traditional wafer direct bonding processes require annealing or cannot be directly bonded at room temperature. Therefore, there are restrictions on the materials that can be bonded and the combinations of materials that can be bonded. However, surface activated bonding has made it possible to directly bond dissimilar materials at room temperature. This technology is expected to be applied to the manufacturing of various MEMS and three-dimensional stacking of semiconductors. This bonding process involves bombarding the wafer surface with fast argon atom beam in a vacuum chamber. Irradiation removes oxide layer and contaminants, exposing dangling bonds. By pressing the wafers together, the dangling bonds are bonded together, and a strong bond is achieved. The device that generates this fast argon atom beam is FAB source. This device has been of the type that generates a saddle field electric field. However, this FAB source had a narrow beam irradiation area and was unable to support the recent increase in wafer diameter. Therefore, it was necessary to perform irradiation using multiple FAB sources. At production sites, there is a need to develop new FAB sources that can irradiate large areas. In this study, we developed FAB source in which the beam is generated by parallel plate electrodes. We performed a comparison with the saddle field type FAB source and found that the performance was inferior in initial experiments. Next, we improved the design to operate at higher voltages and increased the aperture area. Through these improvements, we have achieved performance superior to the saddle field type FAB source.

DOI： 10.20965/ijat.2024.p0513

Insights into thermally-induced disruption of magnetic-nanoparticle agglomerates Open Access

Junpei, B; Sakurai, J; Hata, S; Oka, C

ALEXANDRIA ENGINEERING JOURNAL Vol. 96 page： 72 - 81 2024.6

Language：English Publishing type：Research paper (scientific journal) Publisher：Alexandria Engineering Journal

We investigated the temperature dependence of agglomeration systems consisting of interacting single-domain magnetic nanoparticles (MNPs). In our study, we utilized electron spin resonance (ESR) experiments to measure how temperature-induced change in agglomerate state affects the peak-to-peak width of ESR spectra (Hpp) and validated these observations through simulations. Micromagnetic simulations were conducted by combining the classical magnetic dipolar Metropolis Monte Carlo method with the semi-classical macro-spin Landau-Lifshitz-Gilbert finite element method, and the results were compared with experimental findings. Our research elucidates the behavior of thermally-induced disruption of MNP agglomerates. We discovered that the temperature at which MNPs are completely dispersed might correspond to one at which the temperature dependence of Hpp disappears.

DOI： 10.1016/j.aej.2024.03.101

Soft magnetostrictive materials: Enhanced magnetostriction of Fe-based nanocrystalline alloys via Ga doping Open Access

Sano, K; Yamazaki, T; Morisaki, R; Oka, C; Sakurai, J; Hata, S

SCRIPTA MATERIALIA Vol. 242 2024.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Scripta Materialia

Soft magnetostrictive materials, having a combination of high soft magnetic properties and substantial magnetostriction, are highly sought for a wide range of applications. Nonetheless, a trade‐off relationship exists between soft magnetic properties and magnetostriction in single‐phase crystalline alloys due to their inherent magnetocrystalline anisotropy. In this study, we introduce the soft magnetostrictive material based on conventional Fe‐based nanocrystalline soft magnetic alloys whose structure quenches magnetocrystalline anisotropy, infused with Ga. All the samples exhibited nanocrystallization behavior and solid‐solution α‐Fe(Ga) was precipitated as a nanocrystalline phase. The net magnetostriction increased with increasing Ga content, concurrently maintaining high magnetic sensitivity with or without Ga addition. As a consequence of this compatibility of properties, the Ga‐doped sample exhibited a magnetostriction susceptibility 1.77 times higher than that of the base alloy. The soft magnetostrictive materials proposed in this study are poised for potential deployment in vibration energy harvesters and strain sensors.

DOI： 10.1016/j.scriptamat.2023.115956

One-dimensional assemblies of magnetic iron-oxide nanoparticles Open Access

Shiojima, T; Sakurai, J; Hata, S; Oka, C

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 63 ( 3 ) 2024.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

Although high-aspect-ratio iron-oxide nanoparticles (IONPs) are known to have higher heating efficiency than spherical and cubic IONPs and focused in cancer treatment areas, their synthesis methods require high temperatures, vacuum, reduction conditions, and substantial time. In this study, we proposed and established a facile manufacturing method for one-dimensional assemblies of IONPs, expected to increase heating efficiency similar to high-aspect-ratio IONPs. We investigated how the fabrication conditions affect the length of the assemblies and found that the average length of the one-dimensional assemblies increased with the extension of magnetic-field-application time. This result demonstrates that the length could be controlled by adjusting the duration of the magnetic field application.

DOI： 10.35848/1347-4065/ad26bd

The effects of periodic textured substrate to control diffusion angle on the conversion efficiency of dye-sensitized solar cells Open Access

Kimura, R; Oka, C; Hata, S; Sakurai, J

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 63 ( 3 ) 2024.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

We suggested improving the conversion efficiency of dye-sensitized solar cells (DSSCs) by the micro-nano periodic textures to control the diffusion angle of the incident light for certain absorbed wavelengths of the used dye. A periodic texture (Prd-Tx) was designed to enhance the light path of the wavelength of DSSCs’ dye absorption with a wide process window by optical simulation (pitch: 1400 nm, pillar diameter: 460-560 nm, pillar height: more than 500 nm). The Prd-Tx was fabricated by photolithography processes and nanoimprinting (pitch: 1400 nm, pillar diameter: 500 nm, pillar height: 1000 nm). The Prd-Tx increased the DSSCs’ conversion efficiency (η of 3.13%), surpassing our previous best result (refabricated W-Tx, η of 3.08%). It was considered that the ohmic loss was suppressed owing to the Prd-Tx enhanced electrical conductivity at the interface between the transparent electrode, F-doped tin oxide (FTO), and TiO2

DOI： 10.35848/1347-4065/ad2417

色素増感太陽電池の変換効率への周期型回折テクスチャ基板の効果 Open Access

木村龍太郎, 岡智絵美, 秦誠一, 櫻井淳平

精密工学会学術講演会講演論文集 Vol. 2024S ( 0 ) page： 713 - 714 2024.2

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：公益社団法人精密工学会

本研究では，微細テクスチャの回折効果を利用してDSSCsの変換効率を向上させることを目指した．光学シミュレーションからDSSCs使用色素に適した回折テクスチャ形状を設計し，ナノインプリント等の微細加工を用いて作製した．回折テクスチャを適用したDSSCsは変換効率を向上させたが，これは回折テクスチャ上に成膜された透明電極FTOの広い表面積により，FTO/TiO2界面の導電性が向上したことに起因すると考えられた．

DOI： 10.11522/pscjspe.2024s.0_713

Automation of particle emission measurement in surface activated bonding

KATO Taisei, MIYOSHI Yuki, OSHIMA Kyosuke, OKA Chiemi, SAKURAI Junpei, HATA Seiichi

The Proceedings of Mechanical Engineering Congress, Japan Vol. 2024 ( 0 ) page： J114p-05 2024

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Japan Society of Mechanical Engineers

Surface activated bonding is a technology used in semiconductor manufacturing. This technology enables wafers to be bonded directly at room temperature by irradiating them with a fast atom beam. The fast atom beam removes oxides and contaminants from the wafer, thereby activating the surface. A device that emits a fast atom beam is called a fast atom beam source, which generates the beam using plasma from a carbon electrode. However, after several hundred uses, carbon particles are emitted from the electrode surface. These carbon particles cause voids at the wafer bonding interface. In a previous study, a method for in-process quantitative evaluation of carbon particle emissions was proposed. However, hundreds of experiments and measurements were performed manually in the previous study, resulting in widely varying data accuracy. Therefore, in this study, we developed the methods used in previous study and automated all experiments and measurements. The operation of the fast atom beam source and the measurement of particles were automated using “LabVIEW” software. Particle measurement includes photography with a camera and image processing. By automating these processes, the frequency and reliability of measurement improved compared to previous methods. As a result, it was confirmed that particles begin to be emitted after approximately 400 uses, and then continue to be emitted at a constant rate. This behavior is consistent with what has been empirically recorded in mass production sites, confirming the effectiveness of this study.

DOI： 10.1299/jsmemecj.2024.j114p-05

Post-Process Measurement Device for Internal Stress in Thin Films: Part 1

TAKASE Shun, ASHIKARI Yuki, OKA Chiemi, SAKURAI Junpei, HATA Seiichi

The Proceedings of Mechanical Engineering Congress, Japan Vol. 2024 ( 0 ) page： J221-05 2024

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Japan Society of Mechanical Engineers

Internal stress control is an important technical issue for MEMS composed of thin film structures. In our previous study, we proposed a device to measure the internal stress of thin film structures directly and precisely in post-processing. The device directly measures the strain of a thin film sample due to internal stress as displacement and calculates the internal stress by connecting a thin film sample with springs in series with fixed beams at both ends. However, the measurement resolution of the strain displacement is insufficient. Therefore, we proposed a high precision measurement method using a vernier scale. In this method, line and space patterns are fabricated as a scale by Si etching. The resolution can be adjusted according to the pitch of the line and space, and in this study, patterns with L/S= 2 μm were fabricated to achieve resolutions of 5 MPa.

DOI： 10.1299/jsmemecj.2024.j221-05

Estimation Of Oxide Film Removal For Surface Activated Bonding By Fast Atom Beam

Takamatsu Yuya, OSHIMA Kyosuke, KATO Taisei, OKA Chiemi, SAKURAI Junpei, HATA Seiichi

The Proceedings of Mechanical Engineering Congress, Japan Vol. 2024 ( 0 ) page： J114p-04 2024

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Japan Society of Mechanical Engineers

Surface activated bonding (SAB) is a technique that involves irradiating surfaces with an inert gas as a fast atom beam (FAB) in a vacuum to remove oxide films and adsorption layers, thereby activating the surfaces and enabling bonding at low pressure. This technique allows for the bonding of dissimilar materials and piezoelectric crystals. Recent research has developed an evaluation method for the irradiation performance of FAB sources using plasma simulation to enhance the irradiation performance for large-diameter Si wafers. However, this method is only applicable when the FAB source is directly facing the wafer, necessitating an investigation into the effects of oblique placement in actual bonding scenarios. In this study, simulations and experiments were conducted to investigate the effects of oblique placement.

DOI： 10.1299/jsmemecj.2024.j114p-04

Microstructure and Magnetostrictive Properties of Fe-Ga-based Nanocrystalline Magnetostrictive Materials with High Ga Composition

SANO Kohya, YAMAZAKI Takahiro, OKA Chiemi, SAKURAI Junpei, HATA Seiichi

The Proceedings of Mechanical Engineering Congress, Japan Vol. 2024 ( 0 ) page： J221-02 2024

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Japan Society of Mechanical Engineers

Magnetic materials that exhibit a high reverse magnetostriction effect—soft magnetic properties combined with large magnetostriction—are essential for magnetostrictive vibration energy harvesters. We have proposed Fe-Ga-based nanocrystalline soft magnetostrictive materials that integrate both properties. Previously, we fabricated and evaluated samples with Ga doping of 12 at.% or less, demonstrating that Ga addition enhances magnetostrictive properties. In this study, thin film samples with Ga doping levels up to 30 at.% were fabricated using a radio-frequency magnetron sputtering system. The thin film samples were evaluated for their magnetostrictive curves using the optical leverage method and their microstructure using X-ray diffraction. Results showed that magnetostriction increased with Ga doping below 12 at.% but decreased at 23 at.% or more. This change is attributed to the microstructure. Specifically, the overall magnetostriction may be limited by the precipitation of a compound phase with low magnetostrictive properties or by the magnetostriction of the matrix phase.

DOI： 10.1299/jsmemecj.2024.j221-02

Quantitative evaluation of particle emission in bidirectional magnetic field-applied fast atom beam source

MIYOSHI Yuki, KATO Taisei, OKA Chiemi, SAKURAI Junpei, HATA Seiichi

The Proceedings of Mechanical Engineering Congress, Japan Vol. 2024 ( 0 ) page： J114p-06 2024

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Japan Society of Mechanical Engineers

Surface-activated bonding, also known as room-temperature bonding, uses fast atom beam sources to create high-performance substrates. This technique is used to bond substrates with different coefficients of thermal expansion, such as those in surface acoustic wave filters substrates for next-generation (5G and 6G) communications. This bonding technique is also expected to be utilized for three-dimensional wafer stacking of semiconductors in the future. According to previous studies, it has been confirmed that the irradiation performance of a conventional fast atom beam source can be improved by applying a bidirectional magnetic field. However, a quantitative evaluation of particle emission suppression has not yet been achieved. In this study, we evaluated the quantitative lifetime of the bidirectional magnetic field-applied fast atom beam source, which had not been quantitatively evaluated before by measuring particle emissions from a fast atom beam source in-process. The experimental results confirmed that the lifetime of the bidirectional magnetic field-applied fast atom beam source is at least 1.5 times longer than that of the conventional saddle-field type fast atom beam source. This result indicates that applying of bidirectional magnetic fields to fast atom beam sources is an effective means to suppress particle emission.

DOI： 10.1299/jsmemecj.2024.j114p-06

Yield improvement of self-roll-up process for micro coils

KANEMATSU Daiki, SANO Kohya, OKA Chiemi, SAKURAI Junpei, HATA Seiichi

The Proceedings of Mechanical Engineering Congress, Japan Vol. 2024 ( 0 ) page： J221p-14 2024

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Japan Society of Mechanical Engineers

We have attempted to improve yield by applying the self-roll-up process, which enables forming of three-dimensional structures with fewer processes than conventional processes, to the micro coil fabrication process. Conventional solenoid micro coils are fabricated mainly by layering and patterning thin films, which makes the process more complex due to the increased number of layers. In the self-roll-up process, rolled structures like a coil shape are fabricated by using the difference in internal stress of the deposited thin films. If even one of them is damaged, the entire coil formed by a series of multiple roll structures will cease to function, so reliable roll-up is necessary. However, since breakage occurred frequently in actual fabrication, the strength of the structure was improved by increasing the overall film thickness and creating a smoother shape in the areas where breakage was frequent. In addition, the burden on the sample was reduced by changing the process that placed a load on the structure. As a result, there was no damage to all the fabricated roll-up structures, and the yield improvement was achieved.

DOI： 10.1299/jsmemecj.2024.j221p-14

The Fabrication and Evaluation of a Capacitive Pressure Sensor Using Ru-Based Thin Film Metallic Glass with Structural Relaxation by Heat Treatment Open Access

Otsuka, H; Ninoseki, T; Oka, C; Hata, S; Sakurai, J

SENSORS Vol. 23 ( 23 ) 2023.12

Language：English Publishing type：Research paper (scientific journal) Publisher：Sensors

Microelectromechanical systems (MEMS)-based capacitive pressure sensors are conventionally fabricated from diaphragms made of Si, which has a high elastic modulus that limits the control of internal stress and constrains size reduction and low-pressure measurements. Ru-based thin-film metallic glass (TFMG) exhibits a low elastic modulus, and the internal stress can be controlled by heat treatment, so it may be a suitable diaphragm material for facilitating size reduction of the sensor without performance degradation. In this study, a Ru-based TFMG was used to realize a flattened diaphragm, and structural relaxation was achieved through annealing at 310 °C for 1 h in a vacuum. The diaphragm easily deformed, even under low differential pressure, when reduced in size. A diaphragm with a diameter of 1.7 mm was then applied to successfully fabricate a capacitive pressure sensor with a sensor size of 2.4 mm2. The sensor exhibited a linearity of ±3.70% full scale and a sensitivity of 0.09 fF/Pa in the differential pressure range of 0–500 Pa.

DOI： 10.3390/s23239557

Thick film MEMS process using reverse lift-off Open Access

Takase, S; Yamada, K; Nakagawa, Y; Oka, C; Sakurai, J; Hata, S

MICROELECTRONIC ENGINEERING Vol. 281 2023.9

Language：English Publishing type：Research paper (scientific journal) Publisher：Microelectronic Engineering

The lift-off process is a microfabrication technique that has the capability of forming thin film structures. However, several issues, such as structural shape and burr formation, arise when applying this method to the formation of thick film structures. Thick film structures formed through the lift-off process exhibit a mountainous cross-sectional shape and non-uniform film thickness due to variations in the width of the structure. These challenges make it difficult to apply the method to MEMS. While the reverse lift-off process addresses some of the structural shape problems, its initial development using metal molds makes it challenging to directly apply to MEMS without modification. Therefore, this paper proposes an adaptation to the Si process and presents a method to reduce burrs. The microfabrication characteristics are evaluated by comparing the cross-sectional shapes of thick metallic glass structures formed through both the lift-off and reverse lift-off processes. The structures formed through the reverse lift-off process exhibit a rectangular cross-sectional shape, and the film thickness remains consistent regardless of the structure's width. However, burrs persist on the backside edge of the structure, which hinders its application to MEMS. This paper confirms that pre-etching the underlying Si substrate into an inverse tapered shape effectively reduces burrs. As an illustration of the improved reverse lift-off process applied to MEMS, MEMS mirror structures are fabricated, and their resonance frequency closely aligns with the design value.

DOI： 10.1016/j.mee.2023.112081

Stress measurement based on magnetic Barkhausen noise for thin films

Omae, K; Yamazaki, T; Oka, C; Sakurai, J; Hata, S

MICROELECTRONIC ENGINEERING Vol. 279 2023.7

Language：English Publishing type：Research paper (scientific journal) Publisher：Microelectronic Engineering

Non-destructive testing methods based on magnetic Barkhausen noise (MBN) are expected to be applied to thin films. Recently, MBN analysis using machine learning has been performed on bulk materials and used as an in-situ stress and material evaluation method. However, for thin films, it is difficult to measure due to the weak MBN signal intensity, and few examples of its use as a stress and material evaluation method have been reported. This study acquired and analyzed MBN of Fe–Co polycrystalline thin films under bending stress. After pre-processing the acquired MBNs, two representative machine learning algorithms were used to learn the relationship between MBN and stress. By quantitatively comparing the prediction accuracy of each machine learning method, the characteristics of the MBN-based stress evaluation method were discussed from two perspectives: the need for domain knowledge and its applicability to unknown data. This study provides insight into machine learning-assisted MBN analysis as a stress evaluation method for thin films. The extension of MBN-based stress evaluation methods to thin films could be applied to the non-destructive stress evaluation of micro- and nanostructures, where stress state is critical, and could improve process development efficiency.

DOI： 10.1016/j.mee.2023.112057