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Abstract
In this study, we investigate the direct-writing properties of Cu-based microstructures on both glass and Cu-thin-film-coated glass substrates used in femtosecond laser reductive sintering of Cu2O nanospheres. The conductivity of the micropatterns fabricated on the bare glass substrates are evaluated. The patterns exhibit high electrical conductivity. Such highly conductive patterning in the ambient environment is useful for the wiring of electrical devices and drawing electric circuits. In contrast, two types of microstructures are fabricated from multi- and single- layered Cu2O nanospheres at high and low pulse energies, respectively, only on the Cu-thin-film-coated glass substrates. The microstructures fabricated from the multi- and the single-layered Cu2O nanospheres consist mainly of Cu and Cu2O, respectively. A two-step structure is fabricated by internal writing in a Cu2O nanosphere solution film without feeding materials or piling up the layers. This direct-writing technique of Cu-based microstructures is promising for fabricating microsensors and actuators in printable electronics.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The laser direct writing of two- and three-dimensional microstructures is a promising technique for fabricating microdevices in printed electronics and additive manufacturing. Femtosecond laser microfabrication via multi-photon absorption has drawn attention because the microstructures are directly written inside the materials without feeding the raw powders and resins, similar to that in conventional additive manufacturing. This technique was initially developed for fabricating three-dimensional polymeric structures [1,2]. When the femtosecond laser pulses are focused onto a photosensitive resin, multi-photon absorption occurs only around the focal point. Therefore, arbitrary three-dimensional polymeric microstructures can be formed by scanning the focal spot inside the photosensitive resin. In addition, the polymer and metal composite microstructures can be fabricated using multi-photon polymerization and subsequent deposition of thin metallic films. To date, three-dimensional magnetic microstructures which consist of polymers coated with Fe films have been developed for fabricating micromotors [3,4]. However, photosensitive polymers formed the core of these microstructures.
Recently, three-dimensional microstructures based on noble metals such as Au and Ag were also fabricated via multi-photon absorption [5–10], with the noble metal ion solutions being used as raw materials. The photoreduction and precipitation of the noble metals are induced at the focal spot of the femtosecond laser pulses, and two- and three-dimensional microstructures with electrical conductivity were fabricated in this manner. However, metal microfabrication via multi-photon absorption is limited for noble metals, such as Au and Ag, because of the absorption properties of their ions.
Cu-based microfabrication is advantageous because Cu is a cost-effective and highly conductive material. The laser direct writing of micropatterns in Cu via single-photon absorption has already been reported [11,12]. The Cu micropatterns are formed from the thermochemical reduction of CuO nanoparticles, which are mixed with ethylene glycol as a reductant and polyvinylpyrrolidone (PVP) as a dispersant. When nanosecond and femtosecond laser pulses are focused and scanned on the surface of the CuO nanoparticle solution film coated on a glass substrate, a thermochemical reaction occurs to reduce the CuO nanoparticles, thereby forming Cu micropatterns. The Cu2O nanoparticles mixed with PVP and 2-propanol as reductants are also used as raw materials in this technique [13]. However, because of the band gaps of CuO (1.2 eV) and Cu2O (2.1 eV) as well as the laser wavelength, single-photon absorption occurs in the laser-irradiated region [11–13].
We focus herein on the absorption properties of Cu2O. High third-order optical susceptibility has been reported in Cu2O thin films [14]. To achieve this, Cu2O nanospheres prepared by the polyol method [15] and coated on glass substrates are irradiated by focused near-infrared femtosecond laser pulses [16]. The nonlinear optical properties and direct writing properties via multi-photon absorption-induced reductive sintering of Cu2O nanospheres have already been investigated, and the micropatterns fabricated in this process are electrically conductive. However, Cu2O nanospheres were not mixed with the solution which induced thermal reduction. Therefore, Cu2O nanospheres were thermally reduced using only a small amount of PVP surrounding the nanospheres as the reductant. As a results, the conductivity is as low as ∼0.36 S/m, which is more than 10−9 times lower than that of bulk Cu.
In this study, to achieve high electrical-conductive patterning, we prepare a Cu2O nanosphere solution containing Cu2O nanospheres, PVP, and 2-propanol to investigate the patterning properties via multi-photon absorption. In particular, the patterning properties on the glass substrates and Cu-thin-film-coated glass substrates are examined by considering by wiring the electrical pads of the microdevices and drawing electrical circuits. Finally, we report the fabrication of three-dimensional Cu-based microstructures.
2. Experimental methods
The entire process of Cu-based microfabrication using femtosecond laser reductive sintering of Cu2O nanospheres is conducted in the ambient environment. First, Cu2O nanosphere solution including Cu2O nanospheres and reductants was spin-coated on substrates. Next, femtosecond laser pulses were focused onto the surface of the solution and scanned to form micropatterns. Finally, non-irradiated Cu2O nanospheres were removed by rinsing the substrates in ethanol. The details of the process and of methods to evaluate the fabricated micropatterns are described below.
2.1 Preparation of Cu2O nanosphere solution film
The Cu2O nanosphere solution consisted of Cu2O nanospheres, 2-propanol, and PVP. Cu2O nanospheres were synthesized by the polyol method [15,16]. Cu(NO3)2·2.5H2O (0.20 g) and PVP (Mw ∼55000, 0.39 g) were dissolved in each 20 mL-ethylene glycol. Next, the two solutions were simultaneously added at 90 mL/h to ethylene glycol (20 mL) that had been previously heated to 140°C for 30 min. Stirring the mixed solution at 140°C for 25 min produced a suspension solution containing Cu2O nanospheres. The Cu2O nanospheres were next separated and washed by using a centrifugal separator at 14 500 rpm, and then dispersed into a mixture of 2-propanol (3.6 mL) and PVP (0.09 g). The solution was spin-coated onto two types of substrates; bare glass and Cu-thin-film-coated glass substrates, following which the solution film was baked at 80°C to solidify it. The glass substrates were 1 mm thick. A 200-nm-thick Cu thin film was then deposited by a radio-frequency magnetron sputtering. The process Ar gas pressure was 1 Pa, and radio-frequency power was 30 W. The roughness Ra of the Cu thin film was approximately 20 nm.
2.2 Femtosecond laser direct writing system
A femtosecond laser writing system (Photo Professional GT, Nanoscribe) was used for Cu-based microfabrication by reductive sintering of Cu2O nanospheres. The pulse duration, wavelength, and repetition frequency of the femtosecond laser pulses were 120 fs, 780 nm, and 80 MHz, respectively. The laser pulses were focused onto the surface of the Cu2O nanosphere solution films by using an objective lens with the numerical aperture of 0.75. The focal-spot diameter was estimated to be 1.3 µm. The sample substrate was scanned by using an xyz piezo stage to write micropatterns. The scanning speed was less than 1000 µm/s.
2.3 Methods to evaluate Cu2O nanosphere solutions and micropatterns
The shape and size of the Cu2O nanospheres were imaged by using a field emission scanning electron microscope (FE-SEM, SPG-724, JEOL). The absorption properties of the Cu2O nanosphere solution film on glass substrates was evaluated by using an ultraviolet-to-visible spectrometer (UV2600, Shimadzu).
Crystal structures of the Cu2O nanospheres and the fabricated micropatterns were examined by using x-ray diffraction (XRD) analysis (RINT RAPID-S, Rigaku), and the morphology of the micropatterns was imaged by using the FE-SEM and atomic force microscopy (AFM, Dimension3100, Digital Instruments). The width of the line patterns was measured from the FE-SEM images. The electrical conductivity of the micropatterns was determined by using the two-probe method and a multimeter (Truevolt, Keysight).
3. Results and discussion
3.1 Preparation of Cu2O nanosphere solution
Cu2O nanospheres were synthesized by using the polyol method. Figure 1(a) shows the FE-SEM image of the Cu2O nanospheres. Monodispersed nanospheres with approximately 100 nm diameters were prepared and were consistent with the previous reports [15,16]. The Cu2O nanosphere solution containing the Cu2O nanospheres, 2-propanol, and PVP was spin-coated on a glass substrate, following which the Cu2O nanosphere solution film was baked to solidify it. Figure 1(b) shows a photograph of the film on the glass substrate. A yellow-colored film with high transparency in the visible region was obtained. The absorption spectrum of the film was shown in Fig. 1(c). Although it is highly transparent at the femtosecond laser wavelength of 780 nm, an intense absorption appears below half this wavelength (390 nm). We have evaluated nonlinear optical absorption properties of Cu2O nanospheres without the mixture of reductant using an open-aperture z-scan method [16]. When only two-photon absorption assumed to be caused by femtosecond laser pulses, the two-photon absorption coefficient was estimated as ∼70 GW/cm2 by fitting the open-aperture z-scan curve which was theoretically obtained. In addition, the absorption of the reductant solution containing 2-propanol and PVP was negligible in the range of 300 nm to 800 nm. By considering to those absorption properties of Cu2O nanospheres and the mixed solution, the Cu2O nanosphere solution is expected to exhibit mainly two-photon absorption. However, three-photon absorption may occur by Cu2O nanospheres.
Fig. 1. (a)FE-SEM image of Cu2O nanospheres, (b)photograph of Cu2O nanosphere solution film, and (c)absorption spectrum of solution film.
Cu-based microstructures are expected to be formed via multi-photon absorption-induced reductive sintering of Cu2O nanospheres as follows: first, femtosecond laser pulses are absorbed by Cu2O nanospheres via multi-photon absorption. Then, photo-thermal conversion and heat accumulation cause thermal reduction of Cu2O nanospheres to Cu nanoparticles using formic acid as a reductant which is generated by thermochemical reduction of 2-propanol and PVP [13]. The generated Cu nanoparticles absorbed femtosecond laser pulses and are sintered to form Cu-based structures. When the reduction of Cu2O nanospheres are not enough by low femtosecond laser pulses, Cu2O nanospheres are just sintered without reduction.
3.2 Evaluation of line patterns
To investigate how the substrate affects the patterning, a glass substrate partially coated with a Cu-thin film was prepared. Figure 2 shows the line pattern formed on this substrate. The pulse energy and the scanning speed were 0.312 nJ and 300 µm/s, respectively. A finer line forms on the Cu-thin film surface than on the bare glass surface. Due to the thermal conductivity of glass and of the Cu-thin-film-coated glass substrates, thermal energy is considered to be highly dispersed in the Cu-thin film. As a result, the maximum temperature of the Cu2O nanospheres on the Cu-thin film is less than that on the bare glass substrate. In contrast, thermal energy accumulates on the bare glass substrate because of its low thermal conductivity, which heats the Cu2O nanospheres to higher temperature than those on the Cu-thin film, thereby forming the wide line pattern on the glass surface.
We now discuss the width of the line on the glass substrate. The minimum pulse energy for which the line patterns survive after removing the non-irradiated Cu2O nanospheres was 0.312 nJ. Therefore, the scanning speed was varied while using a pulse energy of 0.312 nJ. Figure 3(a) shows the FE-SEM image of the line patterns fabricated with a scanning speed of 30 µm/s. Both a molten zone and a heat-affected zone (HAZ) appear in the line. The width of the molten zone and of the HAZ zone is shown in Fig. 3(b). The line pattern did not survive on the glass substrate at a scanning speed of 1000 µm/s. Although the width of the molten zone is less than the focal-spot diameter of the femtosecond laser pulses at scanning speeds from 10 to 100 µm/s, the actual line width including the HAZ is greater than the focal-spot diameter for scanning speeds. These results indicate that it is difficult to obtain line patterns beyond the diffraction limit on the glass substrates. The increase in line width at high scanning speeds of 100 and 300 µm/s is attributed to the sputtering of Cu2O nanospheres without proper sintering and to insufficient irradiation energy, which is consistent with the results obtained in previous reports [16,17].
Fig. 3. (a)FE-SEM image of line pattern on bare glass substrate at scanning speed of 30 µm/s and pulse energy of 0.312 nJ. Line widths of HAZ as function of scanning speed at a pulse energy of 0.312 nJ.
We also investigate the line width on Cu-thin film coated glass substrates. Figures 4(a) and 4(b) show FE-SEM images of typical two line patterns on the Cu-thin-film-coated glass substrate. The scanning speed was fixed at 100 µm/s, and the pulse energies used to fabricate the line patterns shown in Figs. 4(a) and 4(b) were 0.078 and 0.023 nJ, respectively. The multi-layered Cu2O nanospheres seemed to be sintered at 0.078 nJ. In contrast, the single-layered Cu2O nanospheres seemed to remain on the thin Cu film at the lower pulse energy of 0.023 nJ. In addition, at low pulse energy, the Cu2O nanospheres remained their shape without sintering and melting. Figure 4(c) shows the line width as a function of scanning speed at various pulse energies. The widths of all the line patterns fabricated from single-layered (multi-layered) Cu2O nanospheres are less than (greater than) the focal-spot diameter. These results suggest that the line patterns fabricated at high pulse energy are well-sintered and melted around the focal position.
Fig. 4. (a-b)FE-SEM images of the line patterns with multi- and single-layered Cu2O nanospheres, and (c)the relationship between line width and scanning speed.
In contrast, only the Cu2O nanospheres located immediately above the Cu-thin film were connected to the film. It is considered that the localized surface plasmon resonance enhanced the electromagnetic field between the Cu2O nanospheres and the Cu-thin film without linear optical absorption of the femtosecond laser pulses from the surface of the Cu2O nanosphere solution film. Therefore, only the single-layered Cu2O nanospheres remained on the film after rinsing the sample substrate to remove the non-irradiated Cu2O nanospheres. We thus conclude that electromagnetic enhancement is localized by plasmon resonance between the rough surface of the Cu-thin film at Cu2O nanospheres at low pulse energy. However, the localization effect seems to got behind the sintering and melting of Cu2O nanospheres when using large pulse energy.
By comparing with the patterning on the glass and the Cu-thin-film-coated glass, the minimum pulse energy on the glass was 0.312 nJ which is greater than that on the Cu-thin-film-coated glass substrates (0.023 nJ). These results support the assumption that electromagnet enhancement by localized surface plasmon resonance locally heats at between the Cu2O nanospheres and the Cu thin film, whereas and only Cu2O nanospheres are heated just above the Cu thin film connected to the film.
3.3 Evaluation of micropatterns on bare glass substrates
Micropatterns, not line patterns were generally used to fabricate various microdevices. Therefore, we evaluated the crystal structures and electrical conductivity of the micropatterns on the glass substrates. The micropatterns were formed by raster scanning of the focal spot of the femtosecond laser pulses. Figures 5(a) and 5(b) show FE-SEM images of the micropatterns fabricated at the raster scan pitch of 1 and 2 µm, respectively. The scanning speed and the pulse energy were 100 µm/s and 0.312 nJ, respectively, which were the condition for which multi-layered Cu2O nanospheres were well-sintered and melted, as discussed in Fig. 3(a). A relatively smooth surface of the micropatterns was obtained at the raster scan pitch of 1.0 µm. However, a rough surface of the micropatterns, which consisted of molten and sintered Cu2O nanospheres, resulted from using a raster scan pitch of 2.0 µm. Figure 5(c) shows XRD spectra of the micropatterns produced with various raster scan pitches. Although Cu micropatterns form at the scan pitch of 1.0 and 1.5 µm, Cu and Cu2O composite micropatterns are fabricated at the scan pitch of 2.0 and 5.0 µm. These results suggest that the molten and sintered zones of the micropatterns are composed of Cu and Cu2O, respectively, based on the raster scan pitch and the focal-spot diameter of the laser pulses (∼1.3 µm).
Fig. 5. FE-SEM images of micropatterns with raster scanning pitch of (a)1.0 and 2.0 µm, respectively. (c)XRD spectra of micropatterns fabricated by using various raster scan pitches.
The electrical conductivity of the micropatterns was also investigated. Figure 6(a) shows the relationship between the electrical conductivity of the micropatterns and scanning speed. The pulse energy was 0.312 nJ, which was the minimum energy to obtain line patterns. The raster scan pitch was 1.0 µm, as determined by considering the line width. The resistance of the micropatterns was measured by using the two-probe method. First, Cu thin film electrodes were formed on the glass substrate by using semiconductor technology such as lithography and sputtering deposition. Next, a Cu2O nanosphere solution was spin-coated on the glass substrate with the Cu thin film electrodes. Finally, micropatterns were formed to connect the Cu-thin film electrodes, following which the non-irradiated Cu2O nanospheres were removed by rinsing in ethanol. The electrical conductivity was obtained by using the cross sections and the resistance of the micropatterns. The electrical conductivity of the micropatterns increased with increasing scanning speed. By considering the XRD spectra in Fig. 6(b), the low electrical conductivity of the micropatterns fabricated at low scanning speeds was caused by Cu2O. This femtosecond laser direct writing was carried out in air. Therefore, rapid heating and cooling is important to prevent the reductively-generated Cu from being reoxidized by oxygen in air. However, the generated Cu was reoxidized when the micropatterns are formed at lower scanning speed because rapid cooling cannot be achieved and keeping the reduced materials at high temperature allows to reoxidized them. As a result, the electrical conductivity was lower when the scanning speed was lower than 30 µm/s. In contrast, the electrical conductivity slightly decreased at high scanning speed of 300 µm/s, which was caused by lack of reduction of Cu2O nanosphere due to the lower irradiated energy. The maximum conductivity was approximately 4.1×106 S/m at a scanning speed of 200 µm/s, which was 1/10 of the Cu bulk conductivity. It is considered that the difference between the resistivities of the bulk and the fabricated micropatterns was induced by copper oxide and porosities in the micropatterns. However, the resistivity is higher as the Cu micropatterns fabricated by direct writing in air [18]. Such high electrical conductivity is actually promising for wiring of microdevices and for drawing electrical circuits.
Fig. 6. (a)Electrical conductivity of micropatterns fabricated at various scanning speed. (b)XRD spectra of micropatterns fabricated at scanning speeds of 30 and 100 µm/s.
3.4 Direct writing of three-dimensional Cu-based microstructures on Cu-thin films
Line patterns with single-layered Cu2O nanospheres were formed on only the Cu-thin film coated glass substrates when the pulse energy was remarkably lower as shown in Fig. 4(b). The technique was applied for direct writing of three-dimensional microstructures because three-dimensional microfabrication is possible by direct writing inside the Cu2O nanosphere solution film without piling up each layer of the three-dimensional structures, as occurs in conventional 3D printing and additive manufacturing.
Figure 7(a) shows schematic illustrations of direct writing of a two-step structure. The first step structure was formed under the condition that the structures were fabricated from single-layered Cu2O nanospheres. The pulse energy was 0.059 nJ, and the scanning speed was 100 µm/s. The second step structure was subsequently formed under the condition that the structures were fabricated from multi-layered Cu2O nanospheres. The pulse energy and the scanning speed were 0.117 nJ and 100 µm/s, respectively. The raster scan pitch was determined to be 0.5 µm by considering the line width shown in Fig. 4(c). FE-SEM images of the two-step structure are shown in Figs. 7(b) and 7(c). The microstructure with different-height steps was fabricated by using the internal writing technique without feeding materials or piling up for each layer. The surface morphologies and the cross-sectional profiles are shown in Figs. 7(d)–7(g). The heights of the first and second steps are approximately 100 and 300 nm, respectively. The height of the first step is consistent with the diameter of the Cu2O nanospheres. The height of the second step is almost the same as the thickness of the initial Cu2O nanosphere solution film. These results are consistent with the results shown in Fig. 4.
Fig. 7. (a)Schematic illustration of direct writing process of two-step microstructure. (b) FE-SEM image of microstructure and (c)of magnified FE-SEM image of (b). (d), (e) AFM images of surface of second and first steps of the microstructure and (f), (g) their cross-sectional profiles.
Finally, we discuss the composite of the microstructures. Figure 8 shows the Raman shift of the first and second steps of the two-step structure. Both the initial Cu2O nanospheres and the first step of the structure are composed of Cu2O. In contrast, no Cu2O is included in the second step of the microstructure. These results indicate that well-sintered Cu2O nanospheres such as the second step are reduced to Cu without Cu2O, which is as same as the micropatterns fabricated at high pulse energy (0.312 nJ) and high scanning speed (100 µm/s) in Fig. 6(b). The first step structure from single-layered Cu2O nanospheres were not affected by thermal processing, and might be connected to the Cu-thin film simply by solid-phase diffusion.
Fig. 8. Raman shift of Cu-thin film on glass substrate, Cu2O nanosphere film on Cu-thin film, the first step and the second step of microstructures.
4. Summary
We report the direct writing of Cu-based microstructures on bare glass and Cu-thin-film-coated glass substrates in air by femtosecond laser reductive sintering of Cu2O nanospheres. The fabrication properties were investigated for two- and three-dimensional microfabrication.
- (1) The micropatterns fabricated on the bare glass substrates have high electrical conductivity. Such highly conductive patterning is useful for the wiring of the electrical pads of microdevices and for drawing electric circuits.
- (2) Two types of microstructures were fabricated from multi- and single- layered Cu2O nanospheres at high and low pulse energies, respectively, only on the Cu-thin-film-coated glass substrates. The microstructures fabricated from the multi- and the single-layered Cu2O nanospheres consisted mainly of Cu and Cu2O, respectively. A two-step structure was fabricated by internal writing in a Cu2O nanosphere solution film without feeding materials or piling up the layers.
Funding
Ministry of Education, Culture, Sports, Science and Technology (MEXT); Shiseido Group (Female Researcher Science Grant); Amada Foundation; Japan Society for the Promotion of Science (JSPS) (JP16H06064).
Acknowledgments
This study was partially supported in part by the Leading Initiative for Excellent Young Researchers (LEADER) and the Nanotechnology Platform Program (Micro-Nano Fabrication) of the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT), the 10th “Shiseido Female Researcher Science Grant”, the Amada Foundation for Metal Working and Laser Work Technology, and JSPS KAKENHI Grant number JP16H06064.
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