1. Introduction

Microfluidics is the science and technology of systems that process or manipulate small (10⁻⁹ to 10⁻¹⁸ litres) amounts of fluids, using channels with dimensions of tens to hundreds of micrometers [1]. A microfluidic device can be identified by the fact that it has one or more channels with at least one dimension less than 1 mm. Microchannels, reaction reservoirs, microvalves, sensors and other related components are conventional parts in such devices. Microfluidics has the potential to influence subject areas from chemical synthesis and biological analysis to optics and information technology [2–5]. Generally speaking, three aspects must be considered in order to produce a microfluidic device with high quality, such as material properties on performance, tooling and processing methods, and measurements for process control [6]. In recent years, polymer-based microfluidic devices have become increasingly important in biological applications. Many polymers have been developed to construct microfluidic devices, such as polydimethylsiloxane (PDMS), PMMA, SU-8 and polystyrene [7–10]. As an important material for the fabrication of microsystems including microfluidic systems, PDMS has received broad attention due to its many advantages, such as biocompatible, optically transparent to wavelengths down to near 256 nm, elastic and soft nature easy for reversible deformations needed during analytical operations [11]. The tooling and processing of polymer-based microfluidic devices are generally divided into two categories: direct tooling techniques and the mold based processing techniques. The former are suitable for the fabrication of the special features on a mold based device, e.g. through holes, or irregular shapes using some machining tools, such as diamond knife and laser beam, while the mold based processing techniques, suitable for mass production of the polymer microfluidic devices, refer to processes where the final parts are replicated from the molds [6]. However, the possible minimum size formed by direct tooling techniques is limited to several hundreds of micrometers owing to the elasticity of PDMS. Therefore, the development of reliable direct tooling techniques, which is applicable to fabrication of special features with micrometer size, such as through holes, will be an important issue and realize new three-dimensional microstructures of PDMS by combination with the molding-type techniques.

In the present work, a PDMS-based microfluidic channel was presented by the fabrication of synchrotron radiation (SR) stimulated etching utilizing a SR etching beam line consisting of differential pumping and an etching chamber. Experimental results indicate that the SR stimulated etching method is very useful for the fabrication of PDMS-based microfluidic channel owing to its unique features, such as surprisingly high etching rate, low damage to substrates, anisotropy etching and high spatial resolution and aspect ratio because of the short wavelengths.

2. Experimental

The SR etching of PDMS film was carried out at the beam line 4A1 of the SR facility (UVSOR) at the Institute for Molecular Science in Japan. The photon energy region is 50-95 eV and the pulse duration is between 20 ps and 1 ns with a harmonic cavity system. The end station consists of a parabolic focusing mirror chamber and an etching chamber as shown in Fig. 1 . The SR beam from the light source is focused by a Pt coated vertical pre-mirror, and then tightly focussed by another Pt coated vertical parabolic mirror (FM in Fig. 1) to the aperture with 1 mm diameter and 10 mm length. The light finally irradiates onto the surface of the PDMS sample in the etching chamber. The SR beam spot size on the sample surface is about 0.5 mm in diameter. This aperture is very important to keep sufficiently high vacuum (<10⁻⁵ torr) in the focusing mirror chamber when the XeF₂ gas pressure increases up to about 1 torr in the etching chamber. The reaction gas (100% XeF₂) pressure in the etching chamber is controlled by repeating open-close sequence of the valves controlled by the computer with two timers. The PDMS films in the experiments were fabricated from a 10:1 (weight ratio) mixture of Sylgard Silicone Elastomer 184W/C and Sylgard Curing Agent 184 (Dow Corning Corp.).

 

Fig. 1 End station of the beam line containing the etching chamber and the focussing mirror chamber.

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3. Results and discussion

When the PDMS was etched by directly irradiating the SR beam onto the sample surface without mask, a high speed, area selective and anisotropic etching was easily realized. Under the condition of 0.5 torr XeF₂ gas pressure, 150 mA ring current (the photoemission current measured by the Pt detector set at the sample position was about 312 µA) and 10 min irradiation time, the maximum etching depth can reach 500 μm and the hole size was about 0.5 mm. However, some rugged structures were observed at the surrounding of the etched hole due to the heating effect of the SR beam. It is also found that PDMS was not etched by XeF₂ gas without SR irradiation or not etched by SR irradiation without XeF₂ gas. In the following etching experiments, in order to avoid the damage due to the SR beam heating effect, the beam intensity was attenuated using a slit set close to the center of the beam line. The photoemission current was measured to be 30 μA for ring current 110 mA. The following etching was carried out by irradiating the SR beam through three kinds of masks and etching time was 10 min. The pressure of reaction gas XeF₂ was controlled in the range of 0.18 to 0.22 torr. Two copper masks, with width of the square window of 20 or 30 μm, were purchased from Okenshoji Inc. The other mask as shown in Fig. 2 (a) , with width of the square window of 1 μm, was obtained by etching a piece of gold foil with a thickness of 10 μm using focussed ion beam (FIB) technique, which has been used for milling or deposition in the surface of materials in well defined patterns in the micrometer and nanometer scale (micro- and nanoprinting) [12,13].

 

Fig. 2 (a), microscope image of mask with 1 μm size formed by FIB technique; (b), top view of the pattern formed on the PDMS film surface covered with 1 μm size mask; (c) the depth profile of pattern measured with noncontact 3-dimension measuring meter

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Figure 2(b) shows the top view of the etched pattern on the PDMS surface covered with the mask of 1μm size at 134 mA storage ring current. Figure 2(c) shows depth profile measured with noncontact 3-dimension profile meter (Mitaka). From the cross-section profile of the etched pattern, it is found that the lateral dimensions of the structure viewed from above after removal of the etching mask were 3.7 μm at the bottom and 10 μm at the top of the hole, respectively. The difference of the mask pattern and the etched pattern should be induced by the diffraction effect due to the uncontrollable gap between the mask and PDMS sample surface. When covering PDMS films using the other two masks, it was found that the lateral dimensions of the pattern were 21- 24 μm for the mask of 20 μm size, while the lateral dimensions of the pattern were 32- 35 μm square for the mask of 30 μm size. Compared with the case of the mask of 1 μm size, the diffraction in the case of the mask of 20 or 30 μm size became less obvious.

Figure 3 shows the dependence of the etched depth on the storage ring current. From the repeated measurements made on the sample a maximum error bar of ± 7% was estimated. As expected, etched depth of pattern on PDMS film was found to increase with storage ring current. However, a nonlinear relationship was provided between etched depth and ring current. This was probably because it was difficult to control the gas flow at low XeF₂ pressure. It was obvious that the etched depth decreased as the mask size became smaller due to the greater attenuation of SR irradiation. Furthermore, the maximum etched depth for the mask of 30 μm was up to 42 μm. Although we did not discuss the etching reaction mechanism in this work, the high spatial resolution, anisotropic and high speed etching at room temperature indicated that the etching should be induced by the electric excitation of PDMS [14–16], since PDMS is a silicon-based polymer. The small pattern size and large etched depth obtained in the etching experiments indicated that the SR etching method will be very useful for the fabrication of microfluidic devices containing special features with micrometer size, such as through holes or irregular shapes.

 

Fig. 3 The dependence of etched depth on the ring current using the masks with different size: (a), 1 μm; (b), 20 μm; (c) 30 μm.

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In order to confirm the feasibility of SR etching for its application in the fabrication of special features, a PDMS-based microfluidic channel containing a through hole was presented by combining SR etching with photolithography. A three-dimensional structure on a piece of round PDMS thin film with 1 mm thickness and 10 mm diameter was fabricated by a molding method. There is a conical shape depression on the upper side surface of PDMS, and a cross road shape microfluidic circuit, which is observed like a wind mill, on the lower side surface as shown in Fig. 4(a) . The circular pattern observed in Fig. 4(a) is the bottom flat area of the conical shape depression with 100 $\mu m$ in diameter. The height of the cross road shaped microfluidic circuit was about 5 $\mu m$. The mold of the conical shape depression was made by machining of acrylic resin, and that of the cross road shape micro fluidic circuit was fabricated by photolithography [17]. The silicon wafer was spin-coated with negative photoresist SU-8 and was exposed to UV light through a photo-mask. The thickness of PDMS film forming the circular bottom of the conical shape depression was about 20 $\mu m$. The micro through hole with about 20 $\mu m$ diameter, which connects the upper and the lower side patterns was fabricated by the XeF₂ assisted SR etching in ultra high vacuum as shown in Fig. 4(a). In this SR experiment, we utilized a gold foil mask with a through hole of 20 $\mu m$ diameter, which was also fabricated by using FIB technique. The mask was carefully set on the PDMS film under a microscope in order to make the through hole as close to the center of the conical shape depression as possible. The SR irradiation time was 20 min at the ring current of 200 −192 mA. Figure 4(b) shows the etched depth profile of the single through hole, and the measured depth was up to 24$\mu m$, which indicated that a through hole was formed. In order to confirm the compatibility of cells and fabricated PDMS-based microfluidic channel, a preliminary experiment is carried out to culture the PC12 cells on the smooth PDMS film surface. Figure 4(c) shows the optical microscope image of the PC12 cells cultured on the smooth PDMS substrate, on which collagen IV is coated as an extra cellar matrix. It can be seen that the cells grow well on the surface of PDMS.

 

Fig. 4 (a), The fabricated PDMS-based microfluidic channel; (b) the etched depth profile of single through hole; (c) the optical microscope image of the PC12 cell incubated on the smooth PDMS film surface.

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4. Conclusion

In conclusion, high spatial resolution, area selective and anisotropic etching of elastic material PDMS film has been demonstrated. Extremely high etching rate (as large as 40-50 $\mu m$ per 10 min) and small patterning size (in order of several micrometers) can be easily realized by using SR etching, which will have potential values in a wide variety of materials and surface chemistries. Utilizing the SR etching technique and photolithography, a microfluidic channel with three-dimensions was fabricated.