3D-Printed Polymer/Metal Hybrid Microstructures with Ultraprecision for 3D Microcoils

Design of 3D microcoil

The proposed TPS-D can be used for the facile fabrication of polymer/metal hybrid microstructures that are composed of polymer structure and metal-coated polymeric structure. Such 3D electrical structures are difficult to fabricate with conventional processes. The resonance frequency of the 3D microcoil allows having flexibility according to the design of 3D microcoil. In this article, we designed the 3D microcoil with a diameter of 200 μm, a height of 60 μm, and 5 turns.

The inductance and self-capacitance of a coil can be calculated from its dimensions, forming the number of turns according to the parallel equivalent circuit model using equations [Eqs. (1–3)].^20,21 \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} em L ( nH ) = { \frac { { D^2 } { N^2 } } { 0.45D + H } } \tag { 1 } \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} C ( pF ) = { \frac { D { \varepsilon _r } } { 11.45 { { \cosh } ^ { - 1 } } \left[ { \frac { { ( N - 1 ) ( s - d ) } } { d } } \right] } } \tag { 2 } \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} em { f_0 } ( Hz ) = \frac { 1 } { { 2 \pi \sqrt { LC } } } \tag { 3 } \end{align*} \end{document}

Here L is the inductance (nH), N is the number of turns, D is the loop diameter (mm), H is the height (mm), C is the self-capacitance (pF), ɛ_r is the dielectric constant, s is the turn spacing (mm), d is the coil diameter (mm), and f₀ is the resonance frequency (Hz). Using these equations, the theoretical inductance and self-capacitance values for the coil can be calculated. The ideal inductance and self-capacitance are about 6.67 nH and 0.0062 pF, respectively, and the expected electromagnetic resonance frequency is 24.79 GHz. This result shows that the proposed 3D microcoil can be used for high-performance electrical components such as a wireless power transmission device.

3D-printed microcoil

We previously reported various techniques for the realization of complex 3D polymer microstructures using the TPS process.^11,22–25 In addition, we developed a dual-stage femtosecond laser scanning process for fabricating aligned hybrid microstructures that constitute both polymer and metal-coated polymer microstructures. A schematic diagram of the experimental setup is presented in Figure 1. The 3D microcoil was fabricated by TPS-D and selective electroless plating processes.

A silver-coated coiled structure was fabricated in three distinct steps. At first a cylindrical structure was fabricated with SU-8 (MicroChem Co.) and its surface was modified to make it suitable for silver coating. The second step involved creating coiled spacer patterns on the cylindrical structure fabricated from urethane/acrylate photoresists (SCR 500). In the final step, silver was selectively coated on the SU8 part of the microstructure by immersing the whole structure in a silver bath. These processes are described in detail below. As shown in Figure 2a, micropatterns that act as electrical pads for the connection with external measuring equipment were patterned by conventional UV photolithography with SU-8 resin. A drop of SU-8 resin sensitized with a two-photon chromophore was placed on a cover glass substrate, as shown in Figure 2b. A drop of SU-8 was prebaked at 95°C for 15 min. With the developed TPS-D with a laser power of 300 mW and laser scanning speed of 100 μm/s, the SU-8 microstructure aligned with the prefabricated patterns on the substrate was fabricated, as shown in Figure 2c. This structure acted as a support for the spiral coil structure and the metal-coated part. After the laser scanning process, the sample was postbaked at 95°C for 15 min. The remaining resin was washed away with propylene glycol methyl ether acetate (PGMEA) as a solvent, as shown in Figure 2d. The surfaces of the polymer microstructures were functionalized with amine groups by treating them with 1,3-diaminopropane [NH₂(CH₂)₃NH₂] in the presence of butyl lithium for 5 min, after which they were rinsed thoroughly in ethanol, as shown in Figure 2e. ²⁶ The amine groups undergo a Michael addition reaction with unreacted surface epoxides. This reaction creates surface-bound amine-terminated amides. ²⁷ Amines are capable of reducing gold ions from solutions of gold salts. Gold nanoparticles were generated on the surfaces of the SU-8 structures by immersing in aqueous AuCl₄⁻ (5.3 × 10⁻⁴ M aqueous HAuCl₄ for 1 h) and reducing with NaBH₄ (0.1 M aqueous NaBH₄ for 5 min). These gold nanoparticles are capable of acting as seeds that can initiate further metal deposition on the polymeric surfaces under favorable conditions. For fabrication of the second structure, which was a spiral insulating coil, modified SCR-500 (Japan Rubber Co.) resin was used, as shown in Figure 2f. Unlike structures fabricated with SU-8 resin, the structures fabricated from SCR-500 resin can be clearly distinguished from the unpolymerized resin owing to the refractive index change in the polymerized structures. This enables the real-time monitoring of the fabrication process with a CCD camera. The real-time monitoring capability facilitates the alignment of different structures (surface-functionalized polymer structures for metal-coated structures and polymer structures for insulation) with other structures. As shown in Figure 2g, the center of the spiral coil structure must be aligned exactly with the center of the prefabricated supporting structure. Generally, the use of a view-finder in a CCD camera system with an index line is sufficient to align the patterns with each other. However, the focused image varies depending on the condition, such as the amount, position, and different optical properties of the dropped resin in this system. Therefore, for a precise alignment of structures, a pattern-to-pattern comparison method was used to realize the possibility of direct patterning during the TPS-D process.

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FIG. 2.

Schematic diagram of the fabrication process flow. (a) Polymer (SU-8) micropatterns for electrical pads were fabricated by conventional UV photolithography. (b) Modified SU-8 resin was dropped onto the prefabricated micropatterns. (c) Microfabrication by TPS-D. (d) Removal of the remaining SU-8 resin with PGMEA (propylene glycol monomethyl ether acetate) as a solvent. (e) Polymer surface treatment to improve the metal coating. (f) Modified SCR resin was dropped onto the structure. (g) Microfabrication by TPS-D. (h) Removal of the remaining SCR resin with ethanol. (i) Silver coating by electroless plating. TPS-D, two-photon stereolithography process using a dual-stage scanning process. Color images are available online.

Figure 3 shows the pattern-to-pattern comparison method used in this process. Several primary mark patterns were fabricated, as shown in Figure 3a and b. After developing the remaining primary resins (SU-8), another polymer resin (SCR-500) was dropped. Then, it was possible to fabricate the secondary mark pattern in the center of the prefabricated primary mark pattern using a CCD vision system, as shown in Figure 3d. The errors of the x and y axes compared with the primary mark pattern and the secondary mark pattern can be measured using the CCD vision system and piezoelectric stages as shown in Figure 3c. Using this method, the x and y axes on the stages can be aligned to the original positions, and the aligned primary and secondary mark patterns can be fabricated, as shown in Figure 3e and f. With this pattern-to-pattern comparison method, the insulating coil structure with the modified SCR-500 resin could be fabricated in alignment with the prefabricated supporting structure made by SU-8 resin. After the residual SCR-500 resin is washed away with ethanol, the polymer spiral structure acts as insulation for the 3D metallic microcoil, as shown in Figure 2h. Finally, silver nanoparticles are selectively deposited onto the functionalized surface of the polymer structure using an electroless coating process, as shown in Figure 2i. The electroless silver coating solution was prepared by mixing 12 mL of 33 wt% aqueous gum Arabic, 1 mL of 1.5 M citric acid, 1 mL of 0.5 M trisodium citrate, 3 mL of 37 mM aqueous silver lactate, and 3 mL of 0.52 M aqueous hydroquinone. ¹⁸ The silver coating time was 60 min to achieve a thickness of 100 nm. The silver-coated samples were rinsed with deionized water and isopropanol. The cleaned silver-coated sample was annealed at 200°C for 30 min. The measured electrical resistivity was 2.0 × 10⁻⁵ Ω·cm. Although this resistivity is nearly 12.5 times higher than that of bulk silver (1.59 × 10⁻⁶ Ω·cm), this result signifies that electroless silver-coated structures are good electrical conductors that can be readily used in electronic devices. The discrepancy between the resistivity of the silver-coated pattern and the bulk resistivity value could be mainly due to the electron scattering caused by the surface roughness (R_rms ∼ 10 nm) and irregular aggregated silver nanoparticles. Figure 4 shows a fabricated 3D microcoil with a diameter of 200 μm, a height of 60 μm, and 5 turns. Silver coating occurs selectively on the SU-8 surface functionalized with gold nanoparticles, as shown in Figure 4d. The total fabrication time for the 3D microcoil is about 5 h, including TPS-D and selective metal coating process.

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FIG. 3.

Pattern-to-pattern comparison method. (a, b) Primary mark patterns. (c, d) Errors of the x and y axes compared with the primary mark pattern and secondary mark pattern can be measured using a CCD vision system and piezoelectric stages. (e, f) After compensation for errors on x and y axes, aligned primary and secondary mark patterns can be fabricated. CCD, charge-coupled device. Color images are available online.

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FIG. 4.

(a–c) SEM images of the fabricated 3D microcoil structure with a diameter of 200 μm, a height of 60 μm, and 5 turns. (d) Partially magnified SEM image (area A in (c)) of the 3D microcoil structure showing the hybrid microstructure consisting of conductive metal-coated polymer and nonconductive polymer structures. 3D, three dimensional. Color images are available online.

To compare the measured resonance frequency with theoretical calculations (24.79 GHz), the frequency characteristics of the fabricated 3D microcoil were measured using a network analyzer (Agilent, E8364B). Figure 5 shows the measured |S₁₁| value of the fabricated 3D coil. A resonance frequency was detected at 25.4 GHz. The measured return loss drops to almost −30.03 dB at the resonance frequency. These results show that the measured resonance frequency (25.4 GHz) is a reasonable value when considering errors, such as skin effect and the metal-coated surface roughness (R_rms ∼ 10 nm). This result shows that the demonstrated 3D coil can be operated as a small antenna that can be readily used for high-performance electrical components such as low-power transfer applications. The new concept of microdevices for electronic, optoelectronic, or electromechanical applications can be studied using the wirelessly transferred electrical signal.

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FIG. 5.

This graph shows the measured |S₁₁| values of the 3D microcoil as a function of frequency.