Fabrication of three-dimensional microfluidic ...

SIMTech technical reports (STR_V11_N1_03_MTG)

Volume 11 Number 1 Jan-Mar 2010

Fabrication of three-dimensional microfluidic structures inside glass using femtosecond laser Z. K. Wang, H. Y. Zheng, and W. Zhou

are mature and suitable for mass production of polymer-based microlfuidics. However, they are time-consuming and expensive processes requiring alignment, stacking, bonding and assembly of the individual microcomponents [6]. It is preferable to develop new techniques for rapid fabrication in a single procedure. Laser-based techniques are highly attractive for flexible and high-speed manufacturing compatible with a high level of integration of each component [7- 9]. Photostructurable glasses such as Foturan are interesting materials for µ-TAS or lab-on-chip, as they are inherently photosensitive, and hence do not require a photoresist layer for patterning [10-11]. Attempt has been made to study laser machining of Foturan glass [12-14]; however, the researches were carried out using ns laser instead of ultrafast laser. Those researches focused on surface machining only because ns laser is not suitable for subsurface machining. The current study aimed to explore use of ultrafast laser for subsurface machining in Foturan glass. Ultrafast laser with fs pulse duration provides a unique capability for modifying subsurface material properties by depositing energy only near the focal volume because of a nonlinear absorption process [15] and therefore makes it possible to carry out precise micromachining inside the material. Direct fabrication of a three-dimensional (3D) microstructure inside transparent glass using fs laser offers great potential for the manufacture of 3D integrated microchips without the need for the cumbersome alignment and assembly processes of individual components. Direct fabrication of individual microfluidic channels has been demonstrated inside fused silica [16-18]. However, fused silica presents much less efficiency and lower through-put in manufacturing compared with photosensitive glasses such as Forturan glass [19]. Photosensitive glasses also possess added advantage over fused silica in that heat treatment may be carried out to smoothen surfaces of microstructures through transformation to a glass ceramic [20]. The ability to directly form 3D microstructures in Foturan glass using lasers, together with its resistance to high temperature and corrosion, as well as high optical transparency, have made Foturan glass particularly attractive as a platform for the bioanalysis microdevices [11,19, 21-22].

Abstract – A laser direct writing technique was used successfully to carry out subsurface micromachining of three-dimensional microfluidic structures. It involves simple steps of femtosecond (fs) laser irradition to project a latent image of channels or chambers of various dimensions into a photosensitive Foturan glass, thermal annealing to produce crystallites of lithium metasilicates in the laser-irradiated regions, and use of a diluted hydrofluoric acid solution to remove the crystallised structures through selective chemical etching. The etched surfaces may be smoothened significantly through a secondary thermal annealing process. A microfluidic reagent mixer and reactor consisting of four cubic chambers and multiple channels was produced inside a single piece of glass to demonstrate that the technique can be used for rapid device fabrication without recourse to the cumbersome and expensive processes of alignment, stacking, bonding or assembly of the individual microcomponents. Keywords: Microfluidic channels, Microfluidic chambers, Femtosecond laser, Photosensitive glass

1

BACKGROUND

Researches on chip-based microfluidic systems or micro-total analysis system (µ-ATS) have increased tremendously since Manz et al. [1] published the landmark paper on miniaturised total analysis system in 1990, much like the revolution in the electronic industry after the invention of transistor and integrated circuit [2]. The recent trend towards the use of polymeric substrates has been primarily driven by the fact that these materials are less expensive and more adapted to mass-production techniques than silica-based substrates. Polymers have assumed the leading role as substrate materials for microfluidic devices in the recent years [3,4]. However, there still exists an issue of material incompatibility which needs to be addressed. Polymeric channels, valves, and pumps tend to react with organic solvent samples and cause contamination and device degradation [5]. Silica-based microfluidic circuits are still considered to be more suitable for applications involving use of strong chemicals. Currently, microfluidic devices are mainly fabricated by photolithography and other replicating techniques by use of master moulds. They are

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Z. K. Wang, H. Y. Zheng, and W. Zhou

However, many issues related to fs laser subsurface micromachining are poorly understood, including effects of fs laser irradiation on size of microfluidic channel and achievable structural aspect ratio. Of particular interest is the study of fs laser induced crystallisation in Foturan glass. As HF preferential etching process depends on the transformation of the glass from amorphous phase to crystalline structure, the major factors affecting the glass crystallisation process were investigated with regards to the fs laser process and the post thermal annealing process in this study. Special effort was made to produce different configurations of micro-fluidic chambers and channels to demonstrate that fs laser can be successfully used for rapid fabrication of three-dimensional microfluidic structures inside the photosensitive glass without the need to assemble the individual components. 2 • •

•

605°C at 3°C/min and then held for another hour); and (3) Etching at room temperature in a solution of 10% hydrofluoric (HF) acid diluted with water in an ultrasonic bath. An example is given later in Fig. 10 to illustrate the processes of fabrication.

Fs laser

Objective lens

Foturan glass 600 µm

OBJECTIVE Micromachining of 3D structures of microdevice components inside the transparent materials. The completion of fabrication and integration of micro-components in one single procedure glass without the need of assembling individual components. The increasing level of complexity of microfluidic-based systems in a single chip.

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METHODOLOGY

3.1

Experiments

Foturan glass was used in the study. It is a photosensitive glass composed of lithium aluminosilicate doped with trace amounts of silver, cerium, sodium and antimony [10]. A Ti:sapphire regenerative amplifier fs laser system (CPA2001 ClarkMXR) emitting pulses at repetition rate of 1 kHz with a wavelength of 775 nm was used in the direct writing experiments. Pulse energy was adjusted over a range of 0.1 to 4.0 µJ using polariser and neutral density (ND) filters. The focusing system was a 50x objective lens with a numerical aperture (N.A.) of 0.55. A 3 mm aperture was placed before the objective lens to block the outer part of the original output beam so as to improve the beam quality, as shown in Fig. 1. The laser beam was kept stationary, but the sample was moved at speeds of 0.1 to 1.5 mm/s on a PC-controlled motorised XYZ stage. The fabrication procedure of microfluidic channels includes three steps: (1) Drawing latent images of microfluidic structures inside the glass by scanning a tightly focused fs laser beam; (2) Thermal annealing (the temperature was first ramped up to 500°C at 5°C/min, held at this temperature for 1 h, raised to

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Aperture

XYZ - stage Fig. 1. Illustration of experimental setup used for fs laser subsurface micromachining.

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RESULTS & DISCUSSION

4.1

Microstructure Crystallisation

The ability to fabricate microfluidic structures inside the glass relies upon the formation of microhollow structures by selectively etching away the crystallised parts in HF solution. Therefore, the major factors affecting the glass crystallisation process have to be studied with regards to the fs laser writing process and the subsequent annealing process. Figure 2 shows the micro-Raman spectra of samples obtained under various laser writing parameters and thermal treatments. It can be seen from Fig. 2(a) that structure of the original Foturan glass is amorphous and the structure remained amorphous when either only laser direct writing process or only thermal annealing (either once or twice) was applied to the sample. It is very interesting to observe from Fig. 2(b) that thermal annealing after fs laser exposure resulted in crystallisation, although neither laser exposure alone or anealling alone could crystallise the amorphous stucture. The peaks in Raman spectra in Fig. 2(b) were identified to correspond to crystallites of lithium metasilicates (Li2SiO3). The study revealed a laser pulse energy threshold for crystallisation. As shown in Fig. 2(b), when the energy was less than 0.125 µJ (corresponding to a measured laser fluence of 0.64 J/cm2), no glass crystallisation was observed even after annealing.

Fabrication of three-dimensional microfluidic structures inside glass using femtosecond laser

Origin al glass Laser-exposed on ly Annealed only Annealed (twice)

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b) 300.0 μJ

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W avenu mb er (cm )

(a) Amorphous structures after laser-exposure alone or thermal annealing alone

(b) Crystallisation in samples annealed after laser-exposure (laser scanning speed 0.5 mm/s)

Fig. 2. Micro-Raman spectra samples obtained under various laser writing parameters and thermal treatments.

4.2

Geometric Control of Microchannels

Glass pieces with a size of 10 mm by 10 mm and thickness of 2 mm were used in the laser experiment. Fs laser direct writing was carried out from one end surface to the other to draw microfuidic channels at 600 µm below the top surface of the Foturan glass. Figure 3 shows some examples of channels successfully produced using laser energy of 1.1 µJ at speed of 1.5 mm/s. It is noted that the energy used is well above the threshold energy for crystallisation, which is 0.125 µJ as shown in Fig. 2(b).

a)

b) 50 µm

50 µm

c) 200 µm Fig. 3. Optical micrographs showing: a) top view of microfluidic channel directly written with fs laser at 600 µm below surface, b) top view of the channel after thermal annealing and HF etching for 10 min, and c) cross-sectional view of a microchannel entrance after thermal annealing and etching for 20 min.

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The microchannel in Fig. 3(b) is observed to be tapered. The tapered shape was found to be related to the chemical etching process. Etching always started from the part of channel in direct contact with HF and then moved inside gradually, resulting in progressively shorter etching time and thus smaller channel szie for the part of channel exposed to HF at later stage. Cross-sections of the microchannels were studied and found to be elongated along the direction of laser beam propagation, as shown in Fig. 3(c). This elongated shape is mainly derived from the longitudinal distribution of the spatial intensity of the focused beam. When the material is exposed to fs laser, the refractive index is locally modified within an ellipsoidal shape whose size is defined by the N.A. of the optical system and by the laser parameters such as beam waist and energy [23]. The elongated shape may be a big advantage for some applications, but it may have to be tuned for some other applications. If the aspect ratio of cross-section is defined as the ratio of the depth to the width, then its value typically ranges from 1.7 to 4.3 under the conditions of laser scanning speed of 0.1 to 1.5 mm/s, laser energy of 0.4 to 2.1 µJ, and HF etching time of 10 to 50 min. Figure 4 shows that the aspect ratio can be tuned by adjusting scanning speed and etching time. Figure 5 shows some typical images of the embedded microfluidic channels fabricated using different writing parameters and etched in HF solution for the same time of 10 min. The channel size becomes smaller with decreasing laser energy and increasing scanning speed. The width of the channel was measured from the smallest section and is shown in Fig. 6 as a function of laser energy and writing speed.


manual handling and in order to combine multi-functions on the same component, the chips are generally not smaller than a few square centimeters [24]. The width of channels presented in Fig. 6 ranges from 13 to 27 µm. It is quite narrow for typical µ-TAS applications where the channel width is usually larger than 50 µm. Figure 8 shows the controlling of the channel width by multiscanning the laser beam horizontally at a pitch of 5 µm. The channel width can be easily adjusted by laser beam multi-writing. Meanwhile, the crosssectional profile of the microchannel is easily tunable from elliptical to rectangular so as to meet the different requirements of microlfuidic channels.

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Laser writing speed, mm/s Fig. 4. Effect of laser writing speed and HF etching time on aspect ratio of the channel entrance.

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Fig. 7. Effect of HF etching time on microfluidic channel length at various writing speeds.

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Fig. 5. Optical micrographs showing embedded microfluidic channels produced using different laser writing parameters. Width control by laser multi-scanning with a shift of 5 µm in between.

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Fig. 8. Increasing channel width through multi-scanning of the laser beam with a pitch of 5 µm (the white numbers indicate number of laser scan).

Laser writing speed, mm/s

Fig. 6. Effect of laser energy and speed on microfluidic channel width.

4.3

Microchannel length was also measured and is shown in Fig. 7 to decrease with either increasing laser scanning speed or increasing etching time. When laser scanning speed increases, laser irradition time reduces, making microstructure crystallisation more difficult during the subsequent annealing treatment. As etching time increases, the channel becomes longer, resulting in slower material removal. In general, microfluidic components, although often called microchips, are not so small as cell phone or computer chips. For reasons of easy

Surface Smoothening Through Secondary Annealing

Since the crystallites of the lithium metasilicate must be developed by the heat treatment to form a connected etchable network [25], the removal of the crystallites by etching leaves a coral-like surface morphology behind. This surface may adversely affect the reagent flow in the microchannel. Therefore, effort was made to smoothen the surface through secondary thermal annealing after chemical etching.

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The current study focuses on subsurface micromachining of microfluidic structures. It is noted that fs laser direct writing can also be used to produce optical waveguide [23] and microlens [26] inside Foturan glass. Therefore, the direct writing technique makes it possible to integrate microoptical and microfluidic components into a single chip to highly enhance the functionality and performance of the microchip, e.g., for photonic biosensing, as shown in Fig. 12.

The channel width in the range of several tens of µm is too small for the purpose of measuring surface roughness using surface profilometer. Therefore, the multi-scanning technique as illustrated in Fig. 8 was used to produce a much larger channel with a cross-section of 1.5 mm by 3 mm. Secondary annealing was carried out by ramping the temperature slowly at a rate of 5°C/min to 570°C, keeping the sample at this temperature for 5 h, and then cooling it in furnace. Afterwards, the channel was sectioned along its axis to make its wall surface accessible to a Stylus Profilometer (Taylor-Hobson Form Talysurf Series 2) for carrying out the roughness measurement. Surface roughness Ra was found to be reduced significantly by the secondary annealing from 272 nm to 7 nm. The effectiveness of secondary annealing was also confirmed through observation under the optical microscope, as shown in Fig. 9.

a)

Inlet

Outlet Microchamber II

Microchamber I Micro-channel Fs laser beam

Latent images by fs laser writing

Objective lens

b) Visible images by thermal annealing

(a) Rough surface before secondary annealing

Microfluidic structures by HF etching

(b) much smoother surface after secondary annealing

Fig. 9. Optical micrographs showing the effectiveness of secondary annealing.

4.4

Fig. 10. Illustration of 3D microfabrication process inside Foturan glass.

3D Integration of Microcomponents

The same procedure of hollow structure formation can be applied to fabricate microfluidic chambers buried inside Foturan glass. The use of such an fs laser direct subsurface writing enables the true 3D fabrication, as illustrated in Fig. 10. By arranging four cubic chambers serially in the glass, we built a microfluidic reagent mixer and reactor, as shown in Fig. 11. The inlet of each chamber was opened at the top surface of the glass, and one outlet was opened at one side facet which was connected to the bigger chamber (mixer or reactor). By computer controlled movement of the glass sample on the XYZ stage, the 3D images of the mirofluidic structures of chambers and channels were continuously written inside the glass. After thermal annealing and HF etching, the integrated microfulidic structures were rapidly prototyped inside a single glass chip, as shown in Figs. 11(b)- (c). Of interest to note is that the device fabricated contains a channel with a tilted angle, as shown in Fig. 11(a). It is difficult to produce such tilted channel using other techniques except fs laser direct writing.

Reagents

a) A

B C View from right side 0.5 mm Exist

Infusion

300 μm

c)

20 μm

b)

500 μm

Fig. 11. 3D integration of different chambers and channels for storage and infusion of reagents: a) schematic of the designed microfluidic structures; b) after laser subsurface writing and annealing; and c) after HF etching for 100 min.

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Lens

Microfluidics

[2]

Mirror

[3] Pump laser beam

waveguide

[4] Detector

[5] Fig. 12. Sophisticated microdevice integrating microoptical components with microfulidic components in a single piece of glass for on-site biosensing application.

5

[6]

CONCLUSIONS [7]

Amorphous structure of the Foturan glass cannot be crystallised by fs laser irradition alone or heat treatment alone. However, crystallites of lithium metasilicates (Li2SiO3) were found to be formed by thermal annealing of the glass after fs laser exposure above the threshold fluence of 0.64 J/cm2. The crystallised structure can be removed selectively by chemical etching in HF solution. The findings led to a method of subsurface micromachining of microfluidic channels inside Foturan glass using fs laser. The channel size can be increased by increasing the laser energy or decreasing the laser scanning speed. Channels of width ranging from 13 to 27 µm can be produced easily using single laser scan, and a multi-scanning technique was developed to produce channels or chambers of much larger dimensions. Effort was made to smoothen the surface of channels produced, and evidence was shown that secondary thermal annealing after chemical etching could reduce the surface roughness Ra significantly from 272 nm to as low as 7 nm. A microfluidic reagent mixer and reactor consisting of four cubic chambers and multiple channels were produced in a single piece of glass to demonstrate the success in rapid 3D fabrication using the fs laser subsurface writing technique. 6

[8] [9]

[10]

[11]

[12]

[13]

[14] [15]

INDUSTRIAL SIGNIFICANCE [16]

This technique is potentially applied to the manufacturing of microfluidics and other micro devices. The completion of fabrication and integration of micro-components is in one single procedure glass without the need of assembling individual components. It could bring about the innovative design of microfluidics because of the features of flexible, direct fabrication process.

[17]

[18]

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