1. Introduction

Two-photon microfabrication using a femtosecond-pulsed laser beam is attracting considerable attention for its ability to make three-dimensional (3-D) microstructures with close to 100-nm resolution [1–3]. Two-photon microfabrication with photopolymers is applied in a broad range of fields, including photonic crystals [4], drug delivery devices [5] and micromachines such as microstirrers [6] micropumps [7, 8] and microtweezers [9, 10]. However, since 3-D microstructures made of photopolymers are non-conductive, their use in microelectronic devices and MEMS is limited. To overcome this limitation, some groups have studied 3-D direct laser writing of metallic microstructures employing two-photon processes [11–15]. These methods are based on photoreduction of metallic ions such as silver and gold. Sample preparations for two-photon-induced photoreduction are classified into liquids and polymeric films. Tanaka et al. used liquid samples to produce silver microstructures [11]. In their method, conductive 3-D microstructures with low resistivity, close to that of bulk metal, can be fabricated. However, although a method for improving their resolution has been reported [12], the surface roughness is relatively high owing to nonuniformity of the density of reduced silver ions caused by thermal diffusion that results from irradiation with a femtosecond-pulsed laser beam. Other groups have used polymeric films containing metal ions for making metallic microstructures [13–15]. Since polymeric films make metal ions disperse uniformly, preventing harmful diffusion of metal ions, the surface of the resultant structure is likely to be smoother than that of structures produced from liquid samples. In addition, film samples are easily prepared and feasible for application to electrical circuits and MEMS devices.

In this study, we demonstrate that both two-dimensional (2-D) continuous metallic patterns and 3-D metallic microstructures can be produced from polymeric films containing silver ions. We apply a method reported by Baldacchini et al. to prepare film samples for making silver structures, since their method uses readily available materials [15]. Although the resultant structures demonstrated in their previous work were not continuous solids but nonconductive aggregated nanoparticles, we succeeded in producing continuous 2-D and 3-D silver structures. The process reported here has the potential for widespread use in 3-D electric wiring and for the fabrication of 3-D metallic microstructures, since the materials used can be easily obtained commercially.

2. Experimental procedures

2.1 Sample preparation

In our experiments, polyvinylpyrrolidone (PVP) is used as a polymer matrix to disperse silver ions. PVP has been used in previous investigations of single-photon and two-photon photoreduction of silver ions [16]. Sample preparation and the fabrication process are shown in Fig. 1. First, we dissolved 1.25 grams of PVP (ISP Japan Inc., PVP K-30, molecular weight: 40,000–80,000) in 50 ml of ethanol. We prepared two types of sample solutions with different silver nitrate densities (3.8 wt% and 7.3 wt%). For making the sample solution containing 3.8wt% of silver nitrate, for example, 2.0 grams of silver nitrate were dissolved in 10 ml of deionized water. The PVP solution and the silver nitrate solution were mixed and stirred for 30 minutes in the dark at room temperature. While stirring, the initially colorless solution changed first into a dark orange color and then gradually to dark green in several ten hours. To make a thin film containing silver ions, the mixed solution is spread onto a cover glass or stored in a well attached to the cover glass depending on the thickness needed for the desired structure. The cover glass coated with the mixed solution was baked in an air circulation-type oven at 110 °C for 10 minutes. After baking, the thickness of the cast film was less than 0.5 µm. The thickness of the stored films ranged from 10 µm to 20 µm.

 

Fig. 1. Experimental procedure for sample preparation and 3-D laser drawing of metallic microstructures.

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2.2 Fabrication process of silver microstructures

In our two-photon microfabrication system, a Ti:sapphire laser is used as a light source to trigger two-photon-initiated photoreduction. The laser is operated at a wavelength of 752 nm, a pulse width of 200 fs and a repetition rate of 76 MHz. The laser beam passes through a variable ND filter, and then it is collimated with a beam expander of ten magnifications. The collimated laser beam is focused on the sample via an objective lens whose numerical aperture (NA) is 1.25. The thin-film sample was placed on a sample stage and then scanned using a 3-D piezoelectric translation stage (Physik Instrumente GmbH & Co. KG, P-563.3CD). The 3-D piezoelectric translation stage was scanned according to the 3-D computer-aided design (CAD) data of the desired 3-D microstructure or 2-D pattern. After fabricating the 3-D microstructure, the sample was soaked in ethanol for 30 minutes to remove the polymer matrix, then rinsed with deionized water.

3. Experimental results

3.1 Measurements of line width of silver line patterns

We measured the line width of silver lines drawn with different laser powers and scanning speeds of the laser beam. Figure 2 shows the dependence of line width of silver lines on scanning speed at different laser powers. The laser power was measured at the back of the objective lens. Decreasing the laser power results in decreased line width. Faster laser scanning speeds also result in decreased line width. Figure 3 shows scanning electron microscope (SEM) images of typical drawing lines. The width of the narrowest line was about 200 nm, as shown in Fig. 3(a). Although the narrow line appears to be an aggregation of silver nanoparticles, it was continuous. By contrast, as shown in Fig. 3(b), the silver line whose width was about 1 µm was smooth and continuous. Continuous silver lines could be stably drawn at laser powers over 1 mW. Although no continuous silver lines were formed in the report by T. Baldacchini et al. [15], we were able to make continuous silver structures using a similar experimental procedure and materials. We assume that this difference in the resultant structures results from the photon density at the focus and the density of silver ions in the polymer matrix. In our experimental conditions, the photon density at the focus is several ten times larger than that of Baldacchini’s experiments. When the laser power was low and scanning speed was fast, the resultant line pattern was an aggregation of silver nanoparticles, as shown in Fig. 3(a). This indicates that a sufficient photon density is needed to produce smooth continuous silver structures inside a polymer matrix that contains silver ions.

 

Fig. 2. Laser power dependence of line width of silver lines.

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Fig. 3. Scanning microscopic images of silver lines drawn by two-photon-induced photoreduction. (a) Aggregation of silver nanoparticles at low laser power, (b) Continuous silver line drawn at sufficient laser power

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3.2 Evaluation of conductivity of silver line patterns

To verify the conductivity of the smooth continuous silver line patterns, we measured the current and voltage of a drawn line by four-probe conductivity measurement. As a sample for four-probe conductivity measurement, we made a line pattern with four 250-µm-square electrodes. Gold wires (diameter: 80 µm) were bonded to the four electrodes with silver paste as shown in Fig. 4. The measurement of conductivity was done using an electric power supply (Kenwood Corp., PW18-3AD) and digital multimeters (Hewlett-Packard Development Company, HP34401A), and a variable resistor for fine adjustment of the current. In the experiments, we examined the dependence of electrical conductivity on the density of silver nitrate in the sample solutions.

Figure 5(a) and (b) show the relationship between current and applied voltage for two samples whose densities of silver nitrate were 3.8 wt% and 7.3 wt%. In these results, since the measured current was proportional to the applied voltage, the resultant silver lines were electrically conductive. In the case of the 7.3 wt% density solution, a higher current passed through the silver line pattern than in the 3.8 wt% density solution. This indicates that the density of the silver line generated by photoreduction grows by increasing the density of silver nitrate in the sample solution.

To estimate the conductivity of the resultant silver structure, we measured the feature size of the drawn silver lines using an optical microscope (Keyence Corp., VHX-100F) and a laser scanning optical microscope (Keyence Corp., VK-8710). The silver line shown in Fig. 5(a) proved to be 250.3 µm long, 12.1 µm wide, and 0.4 µm thick. The silver line shown in Fig. 5(b) was 246.2 µm long, 11.0 µm wide, and 0.1 µm thick. Using the measured resistance, length, width, and height of each silver line, the resistivities of 3.8 wt% and 7.3 wt% samples were estimated at 1.81×10^-6 Ωm and 3.48×10^-7 Ωm. The resistivity of the silver line drawn in the sample solution of high-density silver nitrate was about 22 times greater than that of bulk silver (1.59×10^-8 Ωm). Although the maximum current passing through the silver line made from the 3.8 wt% sample was 8 µA, the silver line made from 7.3 wt% allowed a current of over 100 µA to pass. These results indicate that use of liquid samples of high density of silver nitrate can provide a continuous silver structure with high conductivity.

 

Fig. 4. Silver line attached to four electrodes for the measurement of resistivity.

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Fig. 5. Relationship between current and applied voltage of silver lines. (a) Sample solution of 3.8 wt% of silver nitrate (b) Sample solution of 7.3 wt% of silver nitrate

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3.3 Fabrication of 3-D metallic structures

3-D microstructures were fabricated using a thick film sample. The thick film sample was prepared using a well made from poly (dimethylsiloxane) (PDMS) whose inner diameter was about 6.7 mm. The well was attached to a cover glass to reserve the sample solution. The sample for making a thick film was also baked using the preparation process shown in Fig. 1. Figure 6(a) shows a laser scanning microscopic image of a pyramidal microstructure produced by 3-D scanning of the sample stage. The pyramidal structure was built up from the top layer by raising the piezoelectric translation stage supporting the sample at constant intervals of 0.5 µm, because the prefabricated silver layer prevents the laser beam from focusing tightly. The height of the pyramidal structure was 10 µm. The laser power and scanning speed of the stage were 4 mW and 50 µm/s.

Hybrid polymer and silver microstructure was also demonstrated. In our experiments, polymer lines with different heights were first fabricated on a cover glass using the same fabrication system as for two-photon microfabrication with epoxy-type photopolymer (D-MEC Co. Ltd., SCR-701). After washing out any unsolidified photopolymer with glycol ether ester, thin film for making silver lines was formed on the cover glass, where the polymer lines are drawn, using the sample preparation process shown in Fig. 1. The coated sample was set on the sample stage of the two-photon fabrication system again, and silver lines were drawn by scanning the sample stage. Finally, the thin film, including the resultant silver and polymeric lines was soaked in ethanol for 30 minutes to remove the polymer matrix, then rinsed with deionized water. Figure 6(b) shows a fabricated hybrid microstructure in which the four polymer lines and three silver lines are orthogonal. The length of each line is 100 µm. The polymer lines have heights ranging from 0.3 µm to 3 µm. The silver lines were formed despite the heights of polymer lines, suggesting that direct writing of the over coating thin film will be feasible for 3-D electrical wiring, 3-D microelectronic devices and MEMS devices. As one demonstration of the production of hybrid microdevices with 3-D metallic wiring, for example, Fourkas’s group reported the writing of 3-D electrical circuits on a polymer structure [17] and a 3-D inductor by combining a laser-induced photoreduction process with electroless plating [18]. Our method has a potential ability to make such a 3-D hybrid microstructure covered with metallic lines on a polymer structure. The direct writing of metallic microstructures with polymer thin film is thus potentially useful for rapid manufacturing of hybrid microdevices such as polymer MEMS and microelectronic devices.

 

Fig. 6. 3-D microstructures made by 3-D laser direct writing. (a) Pyramidal microstructure (b) Hybrid microstructure consisting of silver lines and polymer lines.

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4. Conclusions

We have reported a technique for fabricating continuous silver microstructures using a polymer film containing silver ions by photoreduction using a femtosecond-pulsed laser beam. We succeeded in fabricating not only 3-D microstructures such as a pyramidal silver structure to heights of 10 µm, but also hybrid microstructures consisting of silver and polymer line patterns. The resistivity of the resultant silver structures produced by photoreduction attained 3.48×10^-7 Ωm using a sample solution containing high-density silver nitrate. Direct writing processes of electrically conductive metallic 3-D microstructures have potential for application to 3-D electrical wiring of microelectronic devices and production of 3-D MEMS devices. In addition, a submicron line pattern of aggregated nanoparticles was formed by reducing the irradiation of the laser beam. These submicron line patterns are useful for the production of optical components such as gratings [14, 15] and plasmonic devices.