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Abstract
In this study, we developed an in-process sintering method for laser-assisted electrophoretic deposition (LAEPD) using an additional laser to sinter Au particles and improve the Young’s modulus of the microstructures fabricated using LAEPD. Thus, in addition to the laser (λ = 488 nm) that traps nanoparticles, another laser (λ = 785 nm) was installed to effectively absorb and sinter the deposited nanoparticles. Deposition was performed via LAEPD and laser sintering alternatively during fabrication. A Young's modulus of 28.2 GPa was achieved for the Au pillar fabricated with a sintering laser irradiation time of 1000 ms/cycle.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, three-dimensional (3D) microfabrication techniques for metals have been developed owing to the increasing demand for microdevices, such as microelectromechanical systems (MEMS) and metamaterial devices. Additive manufacturing (AM) [1–4] is a method for fabricating microstructures with complex 3D shapes. Various AM technologies with unique characteristics have been developed for metal microfabrication. FluidFM [5–9] can be used to fabricate microstructures using hollow cantilevers. The hollow cantilever is filled with a solution of metal ions, and the metal ions are locally dispensed in an electrochemical cell for electroplating. Electrohydrodynamic printing (EHDP) [10–17] is a microprinting technology that uses a strong electric field to drive printed liquids. The Coulombic force of the ions deformed the meniscus at the tip of the nozzle into a conical shape and ejected the droplets toward the substrate. Polycrystalline metal structures can be fabricated without postprocessing. Direct ink writing (DIW) [18–22] is used to fabricate 3D microstructures by directly depositing gel ink onto a sample. Each technology has its own advantages. However, clogging is often a challenge because of the use of nozzles.
In the absence of nozzles, focused electron beam-induced deposition (FEBID) [23–26] uses a focused electron beam in a vacuum chamber to decompose the precursors adsorbed on the surface of substrates and deposit the metal. By controlling the electron beam irradiation, 3D-shaped microstructures can be manufactured. However, a vacuum environment is required to irradiate the beam, making the equipment expensive.
Laser-based metal deposition methods have been developed as microprinting technologies that do not require vacuum. A laser-induced forward transfer ink (LIFT) [27–30] was used to fabricate 3D structures by transferring molten metal droplets onto a substrate. This process is simple and practical, and an improvement in surface roughness is desired. Direct laser writing (DLW) [31,32] was used to fabricate 3D structures using a pulsed laser with multiphoton absorption. This method is based on the modeling of a photosensitive resin; thus, it is difficult to process metals, and additional processing, such as metal coating or plating, is required when using the modeled resin as a template.
We have developed laser-assisted electrophoretic deposition (LAEPD) [33–36], a 3D microfabrication technique that combines laser trapping [37–41] and electrophoretic deposition [42,43]. Laser trapping is a technique in which nanoparticles are trapped near the focal point of a laser beam. This technique is widely used for the noncontact manipulation of nanoparticles in solution. However, it is difficult to deposit trapped nanoparticles on a substrate because of the repulsion caused by the electric double layer formed on the surfaces of the colloidal particles and substrate. Electrophoretic deposition is a technique for depositing charged nanoparticles on an electrode surface by applying an electric field to a colloidal solution, thereby overcoming repulsion. By combining these two techniques, LAEPD can be used to selectively deposit the nanoparticles trapped by laser trapping. This technology uses a CW laser of a few milliwatts, which allows for a low-cost setup and provides an advantage over other laser 3D printing technologies that use expensive pulsed lasers. However, the microstructures fabricated using LAEPD exhibit significantly lower Young's moduli than the bulk value of the same materials. This is because the shape is maintained by the cohesive forces between the deposited nanoparticles, and the protective agent of the colloidal solution is simultaneously deposited, resulting in a low bonding strength of the deposited material. Annealing after deposition to increase the mechanical strength often causes the structure to shrink and deform. Therefore, in-process annealing, rather than post-annealing, is considered more effective in avoiding structural deformation.
In this study, we describe an in-process sintering method for the deposition of nanoparticles using a laser for local heating. Localized surface plasmon resonance [44,45], which is generated on the surface of nanoparticles by light irradiation, has been used for local sintering. Owing to the size effect [46,47], nanoparticles can be sintered more readily than the bulk of the same material. To avoid degradation of the colloidal solution caused by heating, we propose a method of depositing nanoparticles while a fresh colloidal solution is injected and flowing. We investigated the relationship between the laser sintering time and the Young's modulus of a pillar structure fabricated using LAEPD. In addition, cross-sections of the fabricated pillars were evaluated using field-emission scanning electron microscopy (FE-SEM) and electron backscattering diffraction (EBSD) to investigate the crystallization of the deposited Au structure.
2. Experimental procedure
2.1 In-process sintering in LAEPD
Figure 1 illustrates the method of fabricating 3D structures using LAEPD via in-process sintering. The solution cell consisted of a cover glass coated with indium tin oxide (ITO) and a conductive substrate with a spacer between them. The cell was filled with a colloidal solution of monodisperse Au nanoparticles (Tanaka Kikinzoku Kougyo K.K., Au 3 nm diameter, 0.2 wt%). The nanoparticles in the colloidal solution were deposited by LAEPD, which combines laser trapping and electrophoretic deposition. A CW laser (λ= 488 nm) was used for LAEPD (hereafter referred to as deposition laser). For sintering the deposited particles, another CW laser (λ = 785 nm) (hereafter referred to as the sintering laser) was used. Aggregated Au nanoparticles deposited in various sizes and shapes have broad absorption wavelengths up to the near-infrared region [48–51]. Thus, a laser with a wavelength of 785 nm, which was expected to be efficiently absorbed by the aggregated and deposited nanoparticles, was used for sintering. The procedure for fabricating the structures is as follows.
Fig. 1. Schematic of the method for fabricating 3D structures by LAEPD using in-process sintering. (a) Nanoparticles in the colloidal solution are deposited by LAEPD. (b) The structure is heated using a sintering laser. Concurrently, the deposition laser is turned off, and the electric field application is also turned off. (c) By turning on the deposition laser and reapplying the electric field, nanoparticles are deposited again on the top of the sintered structure. Concurrently, the sintering laser is turned off. (d) By turning on the sintering laser, the structure is sintered. Concurrently, the deposition laser is turned off, and the application of the electric field is turned off. These cycles are repeated alternately during the deposition process.
First, by irradiating the focused laser for deposition on the substrate surface and simultaneously applying an electric field between the conductive cover glass and the substrate, nanoparticles were collected at the focused laser spot and deposited on the substrate (Fig. 1(a)). Next, the deposition laser and electric field were turned off and the sintering laser was turned on to heat the deposited nanoparticles (Fig. 1(b)). By repeating this procedure while moving the cell downward, particle deposition and in-process sintering were alternated to fabricate 3D structures, as shown in Fig. 1(c) and (d). In this study, deposition and sintering were performed in one cycle with the solution cell moving 370 nm downward at a rate of 0.37 µ/s during deposition and pausing during sintering. This cycle was repeated until the travel distance reached the pre-determined height of the pillar. In this study, pillars were set at a height of 18 µm and were created by repeating 49 cycles. Therefore, when creating pillars without sintering, the stage moved continuously at the speed of 0.37 um/s, and the pillar height of 18 µm was reached in 49 s. On the other hand, in the sintering process, the irradiation time of the laser for sintering was added to each cycle. When the irradiation time was 1000 ms/cycle, the deposition and sintering processes took 49 s each, so it took 98 s as a total time to create a sintered pillar of 18 µm height.
During the sintering process, the colloidal solution around the laser spot was heated, causing the suspended nanoparticles to intensify their Brownian motion and aggregate. This resulted in a degraded solution state in which microdebris consisting of agglomerated particles were suspended around the spot. In this state, controlling the deposition process was difficult. Therefore, a microflow system with a syringe pump was used to keep the solution flowing and replace it with fresh solution, thereby preventing degradation. The solution was injected into the cell using the syringe pump at 0.04 ml/min, thus, approximately 65 µl of the colloidal solution was consumed in the 98 s required to produce the 18 µm high sintered pillar.
2.2 Experimental setup for the fabrication of 3D microstructures
Figure 2 shows the experimental setup for LAEPD with in-process sintering. A laser beam from a CW laser (Coherent, Sapphire488 NX SF 488-50, λ = 488 nm) for deposition was expanded via a beam expander and focused onto the substrate with an objective lens (Olympus, LUMFLN60XW, × 60, numerical aperture (NA) = 1.10, working distance (WD) = 1.5 mm). The diameter of the deposition laser spot is 0.54 µm, calculated from the Airy disk (1.22 λ/NA), which determines the minimum deposition size [35]. The laser beam from the CW laser (Omicron, LuxX785-200, λ = 785 nm) used for sintering was focused onto the substrate with the same optical axis as that of the deposition laser using a polarization beam splitter. The diameter of the sintering laser spot is 0.87 µm (Airy disk), which determines the area to be efficiently heat treated. Optical shutters were placed after the laser and positioning stage (Suruga Seiki, KY1040C-L) for each axis, and the electric field applied between the cover glass and substrate was controlled with a personal computer using a data acquisition system (DAQ) (National Instruments, USB-6001). The substrate was positioned using a positioning stage and a z-axis piezoelectric actuator (Cedrat Technologies, APA60S) was used to move the substrate and fabricate a pillar-shaped deposit. A colloidal solution flow system consisting of a syringe pump, a silicone hose, and a pipe was connected to the cell to keep the solution fresh and prevent degradation. The state of deposition on the substrate was monitored using a CCD camera. After deposition, the fabricated pillars on the ITO substrate were observed using scanning electron microscopy (SEM).
Fig. 2. Schematic of the experimental setup for LAEPD with in-process sintering. The deposition and sintering lasers are alternately irradiated during 3D fabrication to repeat deposition and heat treatment. The irradiation of each laser is controlled by opening and closing the respective shutter.
2.3 Methods for evaluating the stiffness of the fabricated pillars
The Young’s moduli of the fabricated pillars were also evaluated using a commercial microcantilever (Olympus, OMCL-RC800PSA) of an atomic force microscope (AFM). The spring constant of the cantilever was calibrated to be 0.07 N/m. This value was calculated from the Young's modulus of the material and the length and width of the cantilever measured by SEM, while the thickness was determined from the resonance frequency measured by a network analyzer. A micromanipulator developed in our laboratory [52] was operated to position the cantilever in the vacuum chamber of the SEM. The spring constants of the fabricated pillars were obtained from their deflection values by applying a calibrated load to the pillars using the cantilever in the SEM vacuum chamber. Young's modulus was evaluated for the pillars prepared by varying the irradiation time of the sintering laser.
3. Experimental results
3.1 Fabrication of pillars through in-process sintering
We present a problem in the case of post-annealing of structures deposited by LAEPD. In this study, micropillars were fabricated by moving the z-axis piezo stage downward while depositing nanoparticles on a substrate using LAEPD. Figures 3(a) and 3(b) show the as-deposited and post-annealed pillars, respectively. The pillars were fabricated using a deposition laser intensity of 1.8 mW and an applied voltage of 2 V for electrophoresis. The pillars shown in Fig. 3(b) were annealed in an electric furnace at 300 °C for 1 h after deposition. A significant change in the shape of the post-annealed pillars was confirmed by comparison with as-deposited pillars. The deformation of the structure was possibly due to shape shrinkage caused by desorption of the protective agent and migration of the material in the structure during annealing. Therefore, the structures must be annealed during the deposition in colloidal solutions. Figure 4 shows the effects of the in-process sintering and solution flow. Cases with and without solution flow are also shown. As a reference to the proposed process, Fig. 4(a) shows a pillar deposited without sintering or solution flow for comparison with the method proposed in this study. The intensity of the laser used for the deposition was 1.6 mW, which is the minimum intensity required to fabricate 3D structures. The applied voltage for the electrophoretic deposition was 2 V. During deposition, the solution cell was moved 18 µm downward at a velocity of 0.37 µm/s using the z-axis piezo actuator to create the pillar structure. Figure 4(b) shows the pillar deposited via in-process sintering without solution flow. The laser intensity for deposition, applied voltage, moving velocity, and displacement of the z-axis piezoactuator were the same as those used for the fabrication of the pillar deposited without sintering (Fig. 4(a)). The laser power used for sintering was 3.8 mW, which is the maximum intensity required to heat the colloidal solution in the cell without generating bubbles. The deposition laser (λ = 488 nm) and the sintering laser (λ = 785 nm) were irradiated alternately every 1000 ms. Voltage application for electrophoresis and movement of the z-axis piezoactuator were performed only when the laser for deposition was irradiated. As shown in Fig. 4(b), the diameter of the pillar is large (1.9 µm) and the surface of the pillar is rough. To avoid the deposition of agglomerated particles, deposition and annealing were performed under solution flow. Figure 4(c) shows the pillar deposited via sintering and solution flow. Excluding the conditions with solution flow, the laser intensity, applied voltage, displacement, and movement velocity of the z-axis stage were the same as those of the pillars deposited by sintering without solution flow (Fig. 4(b)). The colloidal solution flowed at a velocity of 4 µm/s around the pillar during deposition.
Fig. 3. Fabricated pillars. (a) As deposition. (b) Post-annealing. An electric furnace was used for annealing after the deposition of the pillars. (300 ℃, 1 h)
Fig. 4. SEM images of the fabricated pillars. (a) Without sintering and solution flow. (b) With sintering and without solution flow. (c) With both sintering and solution flow. Schematic of colloidal solution flow mechanism. (d) Deposition without solution flow (e) Deposition with solution flow
The Pillar was straight, upright, thin, with a diameter of 750 nm, and had a smooth surface. During sintering, the colloidal solution around the laser spot was heated and the colloidal nanoparticles aggregated. Without solution flow, large aggregated particles were deposited on the pillar surfaces, resulting in larger diameters and rougher surfaces (Fig. 4(d)). By contrast, with solution flow, the aggregated particles flowed out, and the area near the laser spot was continuously provided with colloidal nanoparticles in the fresh solution, resulting in the formation of a thin pillar with a smooth surface (Fig. 4(e)). Therefore, the proposed solution-flow mechanism is effective for deposition during sintering.
3.2 Evaluation of stiffness of the fabricated pillars
To evaluate the improvement in the mechanical properties owing to sintering, the deflection of the pillar was observed by SEM when the pillar was loaded using an AFM cantilever, as shown in Fig. 5(a). Figure 5(b)-(e) show typical SEM images of the pillars deflected by the AFM cantilever. The laser intensity, applied voltage, and moving velocity of the z-axis piezo actuator used for pillar were 1.6 mW, 2 V, and 0.37 µm/s, respectively. The intensity of the sintering laser was 3.8 mW and the irradiation time was 1000 ms/cycle. Figure 5(c) shows a pillar deflected by the cantilever by applying a loading force. In the figure, deflection is evident. Figure 5(d) shows the superimposed images before (Fig. 5(b)) and after (Fig. 5(c)) deflection. As shown in the figure, the pillar deflection was 1.27 µm. Figure 5(e) shows a superimposed image of the cantilever before and after applying the load force to the fabricated pillar. The deflection of the cantilever was 3.01 µm. This means that the fabricated pillar was subjected to a loading force of 0.22 µN, calculated from the spring constant (0.07 N/m) using Hooke's law. From the deflection of the pillar and the loading force, the spring constant of the pillar was calculated to be 0.17 N/m. From the value of the spring constant k, the Young’s modulus E was calculated as 28.2 GPa using the following equation (E = kL3/3I): Here, the pillar length L(=17.7 µm) and cross-sectional secondary moment I, calculated from the pillar diameter of 0.69 µm, were obtained from detailed dimensions of the pillar shape by SEM observation.
Fig. 5. (a) Schematic of Young's modulus measurement method of the fabricated pillar using the AFM cantilever. SEM images of the stiffness measurement of the pillar fabricated with sintering. (b) Before contact and (c) when the loading force is applied. In the SEM vacuum chamber, a loading force was applied to the fabricated pillars using an AFM cantilever, and the deflection was measured. (d) Superimposed image of the pillar before and after applying the loading force with the cantilever. (e) Superimposed image of the cantilever before and after applying the loading force to the pillar. To measure the deflection, the base of the cantilever was superimposed and the displacement was measured.
Figure 6 shows the dependence of the Young’s moduli of the pillars on sintering time. The pillars were fabricated under the same conditions except for the sintering time per cycle. The intensity of the laser for deposition, intensity of the laser for sintering, and applied voltage were 1.6 mW, 3.8 mW, and 2 V, respectively. To fabricate the pillars, the solution cell was moved 18 µm downward at a velocity of 0.37 µm/s. The colloidal solution flowed at a velocity of 4 µm/s around the pillar during deposition. The pillars were irradiated with the sintering laser at irradiation times of 25, 50, 100, 500, and 1000 ms/cycle. In Fig. 6, the horizontal axis represents the irradiation time per cycle of the sintering laser. The error bars and dots indicate the maximum, minimum, and average measured values for the three pillars created under each condition. As shown in the figure, Young's modulus improved as the irradiation time per cycle increased. The Young's modulus increased rapidly as the irradiation time per cycle increased from 0 to 50 ms, followed by a gradual increase. As the irradiation time increased from 500 ms/cycle to 1000 ms/cycle, the error bars decreased. This indicates that the Young's moduli of the pillars could be reproducibly increased by increasing the irradiation time. At an irradiation time of 1000 ms/cycle, the Young's modulus exceeded 20 GPa for all pillars. The highest Young’s modulus was 28.2 GPa. This value was approximately one-third of the Young’s modulus of bulk gold (78 GPa), indicating that the stiffness of the pillars fabricated using the proposed method was significantly improved.
3.3 Evaluation of the internal structure of the fabricated pillars
The internal structures of the pillars used to measure mechanical strength were observed. Figure 7 shows the internal structure of the pillars. First, the structures were cut using a focused ion beam (FIB; JEOL JIB-4500). The pillars were cut at an angle of 70° to the substrate to expose the cross-sections. Cross-sections of the cut pillars were observed using FE-SEM (JEOL, JSM-7001F). Figure 8(a)–(f) show the internal structures of the pillars deposited at sintering times of 0, 25, 50, 100, 500, and 1000 ms/cycle, respectively. The internal structure varies depending on the sintering time. As shown in Fig. 8(a)–(c), fine pores and boundaries of agglomerates were observed from 0 to 50 ms/cycle. By contrast, when the sintering time exceeded 100 ms/cycle, the agglomerate boundaries disappeared, and a smooth surface appeared (Fig. 8(d)). Furthermore, in the image with a sintering time of 1000 ms/cycle (Fig. 8(f)), pores of approximately 200 nm were not observed, as in the images with sintering times of 100 and 500 ms/cycle. This is possibly due to shrinkage of the material caused by annealing. Figure 9 shows the dependence of pillar diameter on sintering time. The pillars used in the evaluation were the same as those used in the evaluation of the Young’s modulus shown in Fig. 6. The error bars and dots indicate the maximum, minimum, and average measured values for the three pillars created under each condition. The pillars became thinner with increasing irradiation time, suggesting that sintering progressed because of the longer heating.
Fig. 7. (a) SEM image of the pillar before FIB cutting. (b) SEM image of the pillar after FIB cutting. (c) Schematic of the cutting method using FIB for observation of the internal structure using FE-SEM.
Fig. 8. Internal structure of the cut pillars fabricated with and without sintering using the sintering laser. (a) Without sintering, (b) with sintering (25 ms/cycle), (c) with sintering (50 ms/cycle), (d) with sintering (100 ms/cycle), (e) with sintering (500 ms/cycle), and (f) with sintering (1000 ms/cycle). Scale bars: 200 nm
3.4. EBSD analysis
The crystal orientations of the cut pillars were analyzed using EBSD (JEOL, JSM-7001F). As shown in Fig. 6, the pillars deposited by sintering exhibit greater mechanical strength than those deposited without sintering. According to these results, the laser-sintering process crystallized the material deposited via LAEPD. Therefore, the crystallinities of the pillars deposited via sintering were investigated. The pillars were cut parallel to the substrate for the EBSD analysis.
Figures 10(a), 10(b), and 10(c) show color-coded inverse pole figures (IPFs) superimposed on the SEM images of the pillars deposited at sintering times of 0, 500, and 1000 ms/cycle, respectively. The IPF images were obtained on a sample surface tilted at 70° at an acceleration voltage of 15 kV with a step size of 15 nanometer-per-pixel. As shown in the figure, there was a protrusion at the center of the sample surface that remained after cutting using the FIB. EBSD analysis was performed to avoid protrusions near the center of the sample. No crystal domains are generated in the pillars deposited without sintering (Fig. 10(a)). By contrast, the IPF maps of the pillars deposited via sintering (Fig. 10(b) and 10(c)) show crystallized domains with the same crystallographic orientation. In the pillar deposited at a sintering time of 500 ms/cycle, the crystallized domains that grew to approximately 100 nm were partially distributed. In the pillars deposited with a sintering time of 1000 ms/cycle, crystallized domains that grew to several hundred nanometers or larger occupied almost the entire pillar cross section. Therefore, without sintering, the deposited structure consisted of particle aggregates, whereas sintering improved the mechanical strength of the deposited nanoparticles by crystallizing them.
Fig. 10. IPF maps superimposed on the SEM images. (a) Cut surface of the pillar deposited without sintering. (b) Cut surface of the pillar deposited with sintering (500 ms/cycle). (c) Cut surface of the pillar deposited with sintering (1000 ms/cycle).
4. Discussion
Here, the sintering mechanism is discussed. In the in-process sintering method, agglomerates of Au particles deposited by LAEPD are sintered by heating with a near-infrared laser. In LAEPD, nano colloidal particles are gathered in the beam spot by laser trapping, and by applying an electric field inside the solution cell, the trapped nanoparticles are subjected to electrostatic forces and deposited on the substrate surface by electrophoretic deposition [35]. In this process, the material is deposited as an agglomerate of particles of several tens of nanometers or less to form structures, as shown in Fig. 8(a) and Fig. 10(a). Aggregates of Au particles with a size of several tens of nanometer are known to have a broad absorption peak in the near-infrared region around the wavelength of 700-800 nm [48–51]. Therefore, during sintering, the deposited agglomerates were efficiently heated by irradiation with a sintering laser at a wavelength of 785 nm, leading to an increase in the crystallization size. During the crystallization growth process, the pores between aggregates gradually shrink owing to mutual bonding, resulting in a decrease in the volume of the structure and narrowing of the pillar diameter, as shown in Fig. 8 and 9. The sintering laser spot is 0.87 µm in diameter (Airy disc). This size is larger than the pillar diameter (0.7 µm) that was shrunk by sintering, as shown in Fig. 9, indicating that the entire cross section of the pillar could be heat treated during deposition. At a laser sintering time of 1000 ms/cycle, the entire pillar cross section was occupied by crystallized domains that grew to several hundred nanometers or larger, as shown in Fig. 10(c).
The improvement in the stiffness of the structures fabricated using the in-process sintering method is discussed. As shown in Fig. 6, the Young's modulus of the pillars increased with increasing sintering time, and the Young's modulus error bar decreased. When the sintering time was short, for example, 500 ms/cycle or less, the crystallized domains were small, approximately 100 nm or less, and partially distributed in the cross-section, as shown in Fig. 10(b). This variation in the crystallite size and distribution may be responsible for the variation in the Young's modulus of each pillar. As the sintering time increased, the crystallization domains widened over hundreds of nanometers and eventually occupied the entire cross-section as shown in Fig. 10(c), which was expected to improve the Young's modulus of the structure, reduce the variation, and improve the reproducibility of the values.
In this study, the Young's modulus achieved by the in-process sintering method was approximately 24-28 GPa at 1000 ms/cycle, as shown in Fig. 6. However, the values were still lower than the Young’s modulus of bulk gold (78 GPa). The Young's modulus increased as the sintering laser irradiation time increased, but the improvement rate decreased. Therefore, further increasing the irradiation time would be difficult to achieve further significant improvement. As shown in Fig. 10(c), the cross section of the pillar fabricated by the in-process sintering method was widely crystallized, but the size of each crystallized domain was still a few hundred nanometers. With further heat treatment, each domain could grow significantly while aligning the crystallographic orientation [53], and further improvement in the Young's modulus could be expected by crystallizing the entire structure. However, a longer irradiation time for the sintering laser increases the total time required for 3D printing. In addition, the laser intensity could not be increased to avoid boiling of the solution. Post-annealing, which is generally used as a heat treatment at higher temperatures, was not applicable to the structures fabricated with conventional LAEPD because it caused deformation owing to the volume reduction of the structures, as shown in Fig. 3(b). By contrast, the structures fabricated via LAEPD with in-process sintering were denser and stiffer than those fabricated without sintering. For such high-density and rigid structures, a reduction in shape deformation can be expected, even after post-annealing treatment. Therefore, the hybrid heat treatment method combining in-process sintering and post-annealing has the potential to further improve the Young's modulus to the bulk equivalent value while maintaining the shape, and is highly promising as a future challenge.
5. Conclusion
In-process sintering method in LAEPD was developed for fabricating stiffer 3D microstructures using a sintering laser. The mechanical strength of the pillars was evaluated using an AFM cantilever in the vacuum chamber of the SEM. A Young's modulus of 28.2 GPa was achieved for the Au pillar fabricated with a sintering laser irradiation time of 1000 ms/cycle. Cross-sectional observation of the pillars by FE-SEM and EBSD analysis revealed large crystallized domains of several hundred nanometers in the pillars deposited with in-process sintering. These results show that the in-process laser sintering method is effective in improving the mechanical strength of the structures fabricated by LAEPD. Because this laser sintering method can crystallize aggregates of Au nanoparticles in a solution environment, it is expected to be applied to not only fine 3D fabrication but also various micro-and nanotechnologies.
Funding
Japan Society for the Promotion of Science (20H02044, 23H01317).
Acknowledgments
The authors thank Mr. S. Kinoshita of Shizuoka University for assembling the optical system and the staff at the Hamamatsu Center for Instrumental Analysis, Shizuoka University for their helpful support.
Disclosures
The authors declare no conflicts of interest.
Data availability
The data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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