Laser micromilling of convex microfluidic channels onto glassy carbon for glass molding dies

Optics and Lasers in Engineering 57 (2014) 58–63

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Laser micromilling of convex microfluidic channels onto glassy carbon for glass molding dies Shih-Feng Tseng a, Ming-Fei Chen b,n, Wen-Tse Hsiao a, Chien-Yao Huang a, Chung-Heng Yang b, Yu-Sheng Chen b a b

Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu 30076, Taiwan Department of Mechatronics Engineering, National Changhua University of Education, Changhua 50007, Taiwan

art ic l e i nf o

a b s t r a c t

Article history: Received 15 March 2013 Received in revised form 12 November 2013 Accepted 22 November 2013 Available online 1 February 2014

This study reports the fabrication of convex microfluidic channels on glassy carbon using an ultraviolet laser processing system to produce glass molding dies. The laser processing parameters, including various laser fluences and scanning speeds of galvanometers, were adjusted to mill a convex microchannel on a glassy carbon substrate to identify the effects of material removal. The machined glassy carbon substrate was then applied as a glass molding die to fabricate a glass-based microfluidic biochip. The surface morphology, milled width and depth, and surface roughness of the microchannel die after laser micromilling were examined using a three-dimensional confocal laser scanning microscope. This study also investigates the transcription rate of microchannels after the glass molding process. To produce a 180 μm high microchannel on the GC substrate, the optimal number of milled cycles, laser fluence, and scanning speed were 25, 4.9 J/cm2, and 200 mm/s, respectively. The width, height, and surface roughness of milled convex microchannels were 119.6 7 0.217 μm, 180.26 70.01 μm, and 0.672 7 0.08 μm, respectively. These measured values were close to the predicted values and suitable for a glass molding die. After the glass molding process, a typical glass-based microchannel chip was formed at a molding temperature of 660 1C and the molding force of 0.45 kN. The transcription rates of the microchannel width and depth were 100% and 99.6%, respectively. Thus, the proposed approach is suitable for performing in chemical, biochemical, or medical reactions. & 2014 Published by Elsevier Ltd.

Keywords: Microfluidic channel Glassy carbon Ultraviolet laser processing system Glass molding die Laser micromilling

1. Introduction With the recent rapid development of the biomedical industry, the requirements for complex designs, mass-production processes, and low-cost products for the fabrication of glass-based microfluidic devices have increased significantly. Glass materials have excellent optical properties, such as a high refractive index and low ultraviolet (UV) absorption, and high chemical and thermal stability. Consequently, they are considerably more suitable than plastic and silicon materials for applications involving high temperature, humid, or harsh environments [1]. Because of these advantages, glass materials are primarily used in biomedical and biophotonic devices. Researchers have developed micro/nano hotembossing and imprinting techniques for glass materials and applied them to microfluidic-filter parts, fluidic-channel parts, biochips, artificial organs, deoxyribonucleic acid (DNA) sequencers, and other applications [2–5].

n

Corresponding author. Tel.: þ 886 4 7232105x7294; fax: þ 886 4 7211149. E-mail address: [email protected] (M.-F. Chen).

0143-8166/$ - see front matter & 2014 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.optlaseng.2013.11.011

The laser direct writing technique can create designed patterns and remove unnecessary materials. This processing method produces surface texturing, scribing, engraving, grooving, milling, cutting, drilling, and patterning through varying laser-material interactions. However, the laser direct writing on normal transparent materials, such as glass, sapphire, and fused silica substrates, requires a high laser fluence to generate nonlinear absorption through multi-photon ionization. This method also has a low machining efficiency [6]. Some studies have discussed glass-based microfluidic structures fabricated using different laser sources and processing techniques. Chung et al. [7] adopted a water-assisted CO2 laser process to ablate Pyrex glass and medical-grade glass substrates to produce crackless fluidic channels and holds. Ablated fluidic glass chips have hydrophilic properties that are suitable for low-cost fluidic chip applications. Hanada et al. [8] demonstrated the fabrication of three-dimensional (3D) hollow microstructures for the dynamic observation of living cells and microorganisms in fresh water using a femtosecond laser direct writing. An embedded microchannel structure can be used to analyze the continuous motion of Euglena gracilis and elucidate the information transmission process in Pleurosira laevis. Chen and Darling [9] investigated the ablation rates and laser micromachining precision of sapphire, silicon, and Pyrex glass samples micromachined by near

S.-F. Tseng et al. / Optics and Lasers in Engineering 57 (2014) 58–63

UV (355 nm) and mid-UV (266 nm) nanosecond pulsed Nd:YAG lasers for microfluidic applications. Their experimental results indicate that mid-UV lasers could efficiently micromachine the transparent materials, especially in microfluidic systems. Yen et al. [10] used a widely available CO2 laser to develop an ablation system for the rapid manufacture of crack-free glass microfluidic chips and cell-array chips. The laser-ablated glass samples had a minimum trench width and maximum trench depth of 95 μm and 225 μm, respectively. Cheng et al. [11] developed a rapid prototyping platform for fabricating a debris-free and crack-free glass microfluidic chip using a Q-switched DPSS Nd:YAG laser with a wavelength of 266 nm. When the minimal laser power used for glass etching was 20 mW, the channel width was approximately 6 μm and the aspect ratio was 0.5. This microfluidic system, which includes a microreactor and a microconcentrator, was fabricated to demonstrate the flexibility of the laser direct writing method. To enhance the machining efficiency of microfluidic channels, this study focuses on the laser micromilling of convex microchannels on glassy carbon (GC) substrates using a UV laser processing system to prepare glass molding dies. The laser processing parameters, including various laser fluences and scanning speeds of galvanometers, were adjusted to mill the convex microchannel on the GC surface and determine the effects of material removal. The machined GC was then used as a glass molding die to fabricate a glass-based microfluidic device. The surface morphology, machined depth, and surface roughness of the microchannel die after laser micromilling were examined using a 3D confocal laser scanning microscope. This study also investigates the transcription rate of microchannels after the glass molding process.

2. Experimental details 2.1. Nanosecond pulsed UV laser processing system Fig. 1 shows a schematic diagram of the UV laser processing system. A nanosecond pulsed Nd:YVO4 UV laser processing system (Coherent, Inc. model AVIA 355-14) with a wavelength of 355 nm was used for milling microfluidic structures on GC surfaces. The laser beam passed through three reflective mirrors, a beam expander with 2  magnification, and a galvanometer system (Raylase AG model SS-15) with a focus shifter that could adjust the focus range in the Z-direction from þ15 mm to  15 mm. The telecentric lens used in this system had a focal length of 110 mm and a scanning area of 60 mm  60 mm. The specifications of the UV laser in this system included a maximal power of 16.8 W, transverse mode of TEM00, and maximal pulse repetition frequency of 300 kHz. The pulsed energy and pulsed width were 170 μJ and 28 ns, respectively, at a 100 kHz pulsed repetition frequency. The nominal values of the laser beam diameter at the exit port and minimum spot size were approximately 3.5 mm and 50 μm, respectively. The average output power, pulse repetition frequency, the scanning speed of galvanometers, and the number of milled cycles on the GC substrate were adjusted by the system's human machine interface (HMI), enabling automatic control during the laser machining process.

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Fig. 1. Schematic diagram of the UV laser processing system.

Table 1 Physical properties of the glassy carbon (GC) material for glass molding dies. Physical properties

SPI-GlasTM 22 GC

Bulk density (g/cm3) Maximum service temperature (1C) Thermal conductivity (30 1C), (W/mK) Coefficient of thermal expansion (20–200 1C), (  10  6/K) Young's modulus (kN/mm2) Compressive strength (kN/mm2) Flexural strength (N/mm2) Vickers hardness (HV)

1.42 3000 6.3 2.6 35 480 260 230

presents a summary of the physical properties of the GC material. The experimental specimens were commercial GC substrates with dimensions of 25 mm  25 mm  4 mm (SPI Supplies, USA). Before and after laser micromilling, the specimens were cleaned by ultrasonic cleaning equipment to remove motes and subsequently dried by a pressured gas jet. A spectrophotometer (LAMBDA 950 UV/Vis/NIR) was used to determine the light reflectance (R) of the GC substrate. Fig. 2 shows the spectra measurement data. Because the GC substrate was opaque at the Nd:YVO4 laser wavelength, the reflectance value in the Nd:YVO4 laser ultraviolet spectrum (wavelength@355 nm) was approximately 20%. Consequently, the absorptance (A) was calculated to be 80% by substituting reflectance values into A ¼1  R. Because the GC substrate had high absorption in the UV light spectrum, a higher laser pulse repetition rate and scanning speed of galvanometers were preferable in the laser micromilling process, thereby increasing manufacturing efficiency and capacity.

2.2. Sample preparation

2.3. Laser processing parameters and structures of the microfluidic channel

Glassy carbon was served as the mold material in the glassmolding process. This material has good physical properties in glass molding dies, including a high working temperature of 3000 1C in a non-oxidizing environment, a low coefficient of thermal expansion, high corrosion resistance, high thermal conductivity, and high compressive strength and hardness. Table 1

Researchers have proposed using the laser direct writing technique for hard and brittle materials because it combines a high laser fluence Nd:YVO4 laser and a high speed scanning system. When laser beams are focused on the GC surface, they produce a shallow groove along the laser-scan path. Fig. 3 schematically shows the laser processing path, laser spot overlaps, and line-scan

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Table 2 Summary of the laser micromilling parameters used for fabricating convex microchannels on the GC substrate. Laser micromilling parameters 2

Laser fluence (J/cm ) Laser spot diameter (μm) Pulse repetition frequency (kHz) Scanning speed (mm/s) Overlapping rate (%)

Values 3.1

200 80

400 60

4.9 50 20 600 40

8.1

800 20

1000 0

Fig. 2. Light reflectance and absorptance versus wavelength for the glassy carbon substrate.

Fig. 4. Schematic diagram of the microfluidic channel structures on the glassy carbon substrate after the UV laser micromilling of the glass-molding die.

Fig. 3. Schematic diagram of the laser processing path, laser spot overlaps, and line-scan spacing.

spacing. The cross lines of the laser processing path in the X and Y directions are equal in line-scan spacing. This figure shows that the dimensions of scanning area and line-scan spacing are 25 mm  25 mm and 1 μm, respectively. To enhance the machining efficiency of GC materials, high laser fluences of 3.1 J/cm2, 4.9 J/cm2, and 8.1 J/cm2 were used to mill convex microfluidic channels on the glassy carbon substrate. The laser pulse repetition rate was fixed at 20 kHz, and the scanning speeds of galvanometers were adjusted from 200 mm/s to 1000 mm/s in 200 mm/s intervals. The following equation defines the overlapping rates (OR) of the laser spot [12,13]. OR ¼

D  Bs  100% D

ð1Þ

where D and Bs are the laser spot diameter and the bite size between laser spots, respectively. The Bs values can be represented as Bs ¼

V F

ð2Þ

where V and F are the scanning speed of galvanometers and the laser pulse repetition frequency, respectively. The measured D value was approximately 50 μm. The calculated OR values ranged from 0% to 80%

in 20% intervals because the scanning speeds were adjusted from 200 mm/s to 1000 mm/s in 200 mm/s intervals, respectively. Table 2 presents a summary of the laser processing parameters used to fabricate convex microchannels on the GC substrate. Fig. 4 shows a schematic diagram of the microfluidic channel structures fabricated by the UV laser processing system for the glass-molding die. Each convex microfluidic channel on the GC substrate consisted of two inlet ports, microchannels, and an outlet port. The designed microchannel was used as a mixing area to enhance the mixing effect of reagents. Each microchannel was 120 μm wide, 180 μm high, and 10 mm long with a distance of 2 mm between neighboring microchannels. The diameters of each inlet port and outlet port were 2 mm and 7 mm, respectively.

3. Results and discussion 3.1. Laser milled depth, surface roughness, and surface morphology of glass molding dies The scanning speed of galvanometers, pulse repetition rate, and material absorption were major factors of the laser milling parameters. These parameters have a significant effect on the machined depth and surface roughness of the glass molding dies. In addition, milling microfluidic channels on GC materials depends on the laser fluences to control the machined depth. The convex channels were fabricated on the GC substrate using laser fluences of 3.1 J/cm2, 4.9 J/cm2, and 8.1 J/cm2 and the pulse repetition rate of 20 kHz, and the laser milled depth and surface roughness were measured by a 3D confocal laser scanning microscope (Fig. 5). A fitting curve with a power function was adopted to describe the relationship between the laser milled depth and the scanning speed of galvanometers in a one-cycle milling process (Fig. 5(a)). The laser milled depth increased rapidly as the laser fluence increased and the scanning speed of galvanometers decreased.

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When the scanning speed was set to 1000 mm/s, all laser milled depths were lower than 2.5 μm. The maximum laser milled depth on GC substrates was approximately 10 μm when the scanning speed and the laser fluence were set to 200 mm/s and 8.1 J/cm2, respectively. The laser milled depths in the one-cycle milling process ranged from a few micrometers to tens of micrometers. Fig. 5(b) shows the average surface roughness (Ra) versus various

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scanning speeds of galvanometers in the one-cycle milling process. These results show that the surface roughness decreased rapidly as the laser fluence decreased and the scanning speed of galvanometers increased. When the scanning speed was set to 200 mm/s at laser fluences of 3.1 J/cm2, 4.9 J/cm2, and 8.1 J/cm2, the Ra values were 1.28 μm, 2.5 μm, and 3.94 μm, respectively. All of the Ra values were lower than 1 μm when the scanning speed was greater than 400 mm/s. The dependence of surface roughness at the higher scanning speed and lower laser fluence created a smooth channel surface. This phenomenon was attributed to the relationship between the heat accumulation and overlap of laser spots. By increasing the scanning speed or decreasing the laser fluence, the accumulated temperature on GC surface decreased, reducing debris buildup and surface roughness [14–17].

3.2. Laser micromilling of the microfluidic channel for glass molding dies

Fig. 5. Relationship between the laser milled depth (a) and surface roughness (b) at various scanning speeds of galvanometers in the one-cycle milling process.

The previous one-cycle milling process adopted a scanning speed of 200 mm/s and a pulse repetition rate of 20 kHz to obtain the maximum milled depth under different laser fluences. To achieve a 180 μm high microchannel on the GC substrates, the optimal number of milled cycles, laser fluence, and scanning speed were 25, 4.9 J/cm2, and 200 mm/s, respectively. Fig. 6(a) and (b) respectively shows a photo and surface morphology of the microfluidic channel for glass molding dies as taken by a digital camera and 3D laser confocal microscope, respectively, after the laser micromilling of convex microchannel structures. The dark area represents the laser milled region on the GC substrate and the light area was the laser un-milled region (Fig. 6(a)). The boundary between the dark and light areas is clearly distinguishable, implying that the laser-milled process was conducted smoothly and uniformly. The surface morphology of the partial microchannel was measured using image assembly techniques and Fig. 6(b) was constructed by 25 pieces of assembly blocks. The dimension of each assembly block was 200 μm  283 μm with a 1000  magnification. These results clearly show that the milled edges of the convex microchannels on the GC substrate had straight lines and vertical walls around the patterned boundaries. The width, height, and surface roughness of milled convex microchannels, as estimated using a 3D laser confocal microscope, were 119.6 70.217 μm, 180.26 70.01 μm, and 0.672 70.08 μm, respectively. These values are close to the designed values and good for glass molding dies. To avoid breaking down of the convex microchannel edge during the glass molding process, the convex microchannel edge on the GC die was machined using the onecycle laser milling process to obtain dull edges and stepped shapes (Fig. 6(b)). However, the milled surface had micro-scale roughness. Because the GC die was a brittle material, laser direct writing process with the high laser fluence produced no significant recast

Fig. 6. Photographs of (a) the glass molding die after laser micromilling onto the glassy carbon substrate with convex microchannels and (b) enlarged partial convex microchannels as measured by the 3D laser confocal microscope.

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layer near the milled edges, and no milled debris remained or piled up on the surface of the GC substrates [18]. 3.3. Glass molding microfluidic channels High precision molding machines are widely used to fabricate glass lenses with low cost, high precision or special requirements, such as aspherical lenses, freeform lenses, f–θ lenses, meniscus lenses, micro lens arrays, fiber arrays, and so on. In the glass molding process adopted in this study, a preform glass was pressed in a pumped atmosphere of nitrogen (N2) gas. The applied temperatures are typically 20–40 1C higher than the glass yield point. Polished soda-lime glass substrates measuring 76 mm  26 mm  1.2 mm were used as the preform glass. Moreover, the transformation temperature of the soda-lime glass was approximately 564 1C. Fig. 7 shows the thermal and force loading data when using a soda-lime glass molding cycle to fabricate a microchannel chip. The red and blue lines in Fig. 7 represent the glass molding temperature and force, respectively. During one molding cycle for one-sample test, the temperature of the soda-lime glass surface was ramped up to 660 1C from 25 1C over a 300 s time interval. The molding force was then raised from 0 kN to 0.45 kN immediately. After keeping the temperature at 660 1C for 300 s, the glass molding temperature dropped to 25 1C with one stage cooling over 600 s and the molding force was dropped to 0 kN with a two-stage releasing force. To investigate the transcription depth and width of the microchannels, the molding temperature and force were adjusted from 620 1C to 660 1C in 10 1C intervals and from 0.25 kN to 0.45 kN in 0.05 kN intervals. The samples were maintained for 350 s in N2 atmosphere at a 50 lit/min flow rate using a glass molding machine (GMP-207HV, Toshiba Machine Co. Ltd., Japan). The transcription depth and width and surface morphology of molded glass-based microchannels were examined using a 3D laser confocal microscope. Fig. 8 shows the relationship between the transcription depth and width at various glass molding temperatures at a fixed molding force of 0.45 kN. These results show that the maximal depth values of molded microchannels increased nonlinearly from 46.17 μm to 179.5 μm as the glass molding temperature increased from 620 1C to 660 1C, respectively. When the glass molding temperatures were adjusted from 630 1C to 650 1C,

Fig. 8. Relationship between the transcription depth and width at various glass molding temperatures and a fixed molding force of 0.45 kN.

Fig. 9. Relationship between the transcription depth and width on various glass molding forces at the fixed molding temperature of 660 1C.

Fig. 7. History of the thermal and force loading in a soda-lime glass molding cycle to fabricate the microchannel chip for the one-sample test. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

the maximal depth values of molded microchannels on the sodalime glass substrates increased slightly from 73.56 μm to 90.83 μm. The transcription rate also increased slightly from 40.8% to 50.4%. By increasing the glass molding temperatures to 660 1C, the maximal depth of molded microchannels increased significantly to 179.5 μm and the transcription rate increased to 99.6%. Due to the use of the higher molding temperature, the molded glass had a lower viscosity that could improve the transcription depth. These results are close to the designed values of microchannel depths. The transcription width values of the molded microchannels on the soda-lime glass substrates were close at various glass molding temperatures. The average widths of the molded microchannels were approximately 120 μm and all transcription rates were 100%. This study adopts a fitting curve with power function to describe the relationship between the transcription depth and width on various molding forces at a fixed glass molding temperature of 660 1C (Fig. 9). The maximal depth value of molded microchannels significantly increased from 74.32 μm to 179.5 μm as the molding force increased from 0.25 kN to 0.45 kN. When the molding force was adjusted from 0.25 kN to 0.45 kN in 0.05 kN

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Fig. 10. A photograph of the typical glass-based microchannel chip fabricated by the glass molding process at a molding temperature of 660 1C and a molding force of 0.45 kN (a) and a 3D topography of the microchannel chip assembled from 4250 pieces of assembled blocks (b).

intervals, the transcription rates were 31%, 45%, 54.1%, 70%, and 99.6%, respectively. Using a higher molding force value of 0.45 kN and the higher glass molding temperature of 660 1C produced a higher microchannel transcription rate. The maximal width values of molded microchannels on the soda-lime glass substrates remained almost unchanged as the glass molding temperature increased. The average widths of molded microchannels were close to 120 μm, and all transcription rates were 100%. Fig. 10(a) and (b) shows a photo and surface morphology of the glass-based microchannel chip as taken by a digital camera and the 3D laser confocal microscope, respectively, after the glass molding process. The typical glass-based microchannel chip was performed using a molding temperature of 660 1C and a molding force of 0.45 kN. The surface morphology of the glass-based microchannel chip was measured using image assembly techniques, and Fig. 10(b) was constructed by 4250 pieces of assembly blocks. The dimension of each assembly block was 200 μm  283 μm measured by the 3D laser confocal microscope. These results clearly demonstrate that the molded microchannel on the soda-lime substrate had straight grooves, smooth edges and corners, flat channels, and circular inlet and outlet ports. The average width, height, and surface roughness of the molded microchannels estimated by the 3D laser confocal microscope were 119.4 μm, 179.5 μm, and 0.207 μm, respectively. The transcription rates of the microchannel width and depth were 100% and 99.6%, respectively.

4. Conclusion This study presents the fabrication of glass-based microchannel chips on soda-lime glass substrates using laser micromilling and glass molding processes. After laser micromilling, the machined edge of convex microchannels on the GC substrate had straight lines and vertical walls around the patterned boundaries. To produce a 180 μm high microchannel on the GC substrates, the optimal number of milled cycles, laser fluence, and scanning speed were 25, 4.9 J/cm2, and 200 mm/s, respectively. The width, height, and surface roughness of milled convex microchannels were 119.6 70.217 μm, 180.267 0.01 μm, and 0.672 70.08 μm, respectively. The typical glass-based microchannel chip after the glass molding process was achieved using a molding temperature of 660 1C and a molding force of 0.45 kN. The measured results clearly show that the molded microchannel on the soda-lime substrate had straight grooves, smooth edges and corners, flat channels, and circular inlet and outlet ports. The transcription rates of microchannel width and depth were 100% and 99.6%, respectively. Finally, the Ra values of glass-based microchannels were better than those of laser-milled microchannels.

Acknowledgment The authors thank the National Science Council of Taiwan for financially supporting this research under Contract nos. NSC 1022622-E-492-008-CC3 and NSC 102-2221-E-492-011 and MGþ 4C Vertical Integration Enhancement Project between Industries and Academia at Science Parks under Contract no. 102MG06. References [1] Tseng SF, Hsiao WT, Huang KC, Chen MF, Lee CT, Chou CP. Characteristics of Ni–Ir and Pt–Ir hard coatings surface treated by pulsed Nd:YAG laser. Surf Coat Technol 2010;205:1979–84. [2] Huang CY, Kuo CH, Hsiao WT, Huang KC, Tseng SF, Chou CP. Glass biochip fabrication by laser micromachining and glass-molding process. J Mater Process Technol 2012;212:633–9. [3] Takahashi M, Murakoshi Y, Maeda R, Hasegawa K. Large area micro hot embossing of Pyrex glass with GC mold machined by dicing. Microsyst Technol 2007;13:379–84. [4] Chen Q, Chen Q, Milanese D, Ferraris M. Micro-structures fabrication on glasses for micro-fluidics by imprinting technique. Microsyst Technol 2009;15:1067–71. [5] Youn SW, Takahashi M, Goto H, Maeda R. A study on focused ion beam milling of glassy carbon molds for the thermal imprinting of quartz and borosilicate glasses. J Micromech Microeng 2006;16:2576–84. [6] Zhou Y, Wu B. Experimental study on infrared nanosecond laser-induced backside ablation of sapphire. J Manuf Processes 2010;12:57–61. [7] Chung CK, Chang HC, Shih TR, Lin SL, Hsiao EJ, Chen YS, et al. Water-assisted CO2 laser ablated glass and modified thermal bonding for capillary-driven biofluidic application. Biomed Microdevices 2010;12:107–14. [8] Hanada Y, Sugioka K, Kawano H, Ishikawa IS, Miyawaki A, Midorikawa K. Nano-aquarium for dynamic observation of living cells fabricated by femtosecond laser direct writing of photostructurable glass. Biomed Microdevices 2008;10:403–10. [9] Chen TC, Darling RB. Laser micromachining of the materials using in microfluidic by high precision pulsed near and mid-ultraviolet Nd:YAG lasers. J Mater Process Technol 2008;198:248–53. [10] Yen MH, Cheng JY, Wei CW, Chuang YC, Young TH. Rapid cell-patterning and microfluidic chip fabrication by crack-free CO2 laser ablation on glass. J Micromech Microeng 2006;16:1143–53. [11] Cheng JY, Yen MH, Wei CW, Chuang YC, Young TH. Crack-free direct-writing on glass using a low-power UV laser in the manufacture of a microfluidic chip. J Micromech Microeng 2005;15:1147–56. [12] Tseng SF, Hsiao WT, Huang KC, Chiang D. The effect of laser patterning parameters on fluorine-doped tin oxide films deposited on glass substrates. Appl Surf Sci 2011;257:8813–9. [13] Chen MF, Chen YP, Hsiao WT, Wu SY, Hu CW, Gu ZP. A scribing laser marking system using DSP controller. Opt Lasers Eng 2008;46:410–8. [14] Tseng SF, Hsiao WT, Chiang D, Huang KC, Chou CP. Mechanical and optoelectric properties of post-annealed fluorine-doped tin oxide films by ultraviolet laser irradiation. Appl Surf Sci 2011;257:7204–9. [15] Kam DH, Shah L, Mazumder J. Femtosecond laser machining of multi-depth microchannel networks onto silicon. J Micromech Microeng 2011;21:045027. [16] Eaton SM, Zhang H, Herman PR, Yoshino F, Shah L, Bovatsek J, et al. Heat accumulation effects in femtosecond laser-written waveguides with variable repetition rate. Opt Express 2005;13:4708–16. [17] Sakakura M, Shimizu M, Shimotsuma Y, Miura K, Hirao K. Temperature distribution and modification mechanism inside glass with heat accumulation during 250 kHz irradiation of femtosecond laser pulses. Appl Phys Lett 2008;93:231112. [18] Huang CY, Hsiao WT, Huang KC, Chang KS, Chou HY, Chou CP. Fabrication of a double-sided micro-lens array by a glass molding technique. J Micromech Microeng 2011;21:085020.