Direct laser writing (DLW) via two-photon absorption (TPA) has attracted much attention as a new microfabrication technique because it can be applied to fabricate complex, three-dimensional (3D) microstructures. In this study, 3D microstructures and micro-optical devices of micro-lens array on the micrometer scale are fabricated using the negative photoresist SU-8 through TPA with a femtosecond laser pulse under a microscope. The effects of the irradiation conditions on linewidths, such as laser power, writing speed, and writing cycles (a number of times a line is overwritten), are investigated before the fabrication of the 3D microstructures. Various microstructures such as woodpiles, hemisphere and microstructures, 3D micro-lens and micro-lens array for micro-optical devices are fabricated. The shape of the micro-lens is evaluated using the shape analysis mode of a laser microscope to calculate the working distance of the fabricated micro-lenses. The calculated working distance corresponds well to the experimentally measured value. The focusing performance of the fabricated micro-lens is confirmed by the TPA fluorescence of an isopropyl thioxanthone (ITX) ethanol solution excited by a Ti:sapphire femtosecond laser at 800 nm. Micro-lens array (assembled 9 micro-lenses) are fabricated. Nine independent woodpile structures are simultaneously manufactured by DLW via TPA to confirm the multi-focusing ability using the fabricated micro-lens array.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Photolithography has been widely used in the field of semiconducting manufacturing and microscale fabrication, such as for micro- and nanoelectromechanical systems (MEMS and NEMS, respectively)  and polymer waveguides . Surface modifications with micro- and nanostructured topographies, such as array of asymmetric two-dimensional (2D) and three-dimensional (3D) surface features, have been extensively investigated to enhance the interface performance in polymers, metals and ceramics .
3D micro/nano scale structures can be fabricated using a micro 3D printing technique based on direct laser polymerization of a photoresist. Specifically, direct-laser writing (DLW) via two-photon polymerization (TPP) and two-photon absorption (TPA) is widely used to fabricate photonic crystals [4–6], photonic metamaterials [7–14], and scaffolding for cell cultures [15–22].
A recent review article  summarizes the state-of-the-art and emerging applications for ultrafast laser processing techniques, including the DLW method, due to TPP and TPA. The processing (writing) speed of conventional fabrication techniques is ~100 μm s−1 and can be extended to 5 cm s−1 , which makes the DLW method suitable for industrial applications.
Conventional methods to fabricate glass lenses and optical components in the dimension of millimeter and larger are well-established. However, for the optical components in the dimension of sub-millimeter and smaller, the fabrication methods are still challenging, because of the limit of the classical methods.
DLW via TPP and TPA is a useful technique to fabricate micro optical tips and components using highly transparent photoresists. A micro-lens array with a different curvature (MLADC) unit lens was fabricated with DLW using a femtosecond laser . The MLADC is useful for curvature correction and real-time 3D imaging . Ultracompact multi-lens objectives were produced using a two-photon DLW method . Ink jet printing method has been reported to fabricate the convex and concave micro-lenses . Either method provides the novel ways to fabricate the micro-optics in the dimensions of sub-millimeter or smaller scale. DLW method has the flexibility to fabricate spherical and aspherical micro-lenses with various sizes.
In this paper, we investigate the 3D lithography of a commercially available negative photoresist SU-8 to manufacture micrometer-scale 3D structures, micro-lens and the micro-lens array consisting of 9 single micro-lenses for the optical elements. We used DLW via TPA for the micro-fabrications of the 3D microstructures. Working-distance of micro-lens was evaluated from the radius of the curvature of micro-lens, which was compared with the measured value. Using the micro-lens array (assembly of 9 single micro-lenses) we simultaneously fabricate 9 woodpile microstructures using a negative photoresist with DLW via TPA. To our knowledge, this is the first demonstration that the simultaneous fabrication of woodpiles microstructures by DLW via TPA with multi-beams using the micro-lens array fabricated by the same DLW via TPA. This can be compared with the previous impressive results reported with multiple beams using a spatial light modulator .
The negative photoresist SU-8 (Microchem, USA) was used for fabrication. SU-8 consists of an epoxy monomer, photo acid emitter, and solvent. Isopropyl thioxanthone (ITX) was used as a fluorescent dye to monitor the two-photon excitation through a fabricated micro-lens. ITX has strong absorption at 400 nm and emits blue fluorescence (maximum peak at 450 nm).
SU-8 was spin-coated on a glass substrate at 1500 (3000) rpm for 60 s. The obtained spun-coated SU-8 thin film was soft baked at 65 °C for 10 min followed by at 90 °C for 50 min. The resulting sample film had a thickness of 30 μm.
Fabrication of three-dimensional structures
The laser source is a femtosecond Ti:sapphire pulse laser (Spectra Physics, Mai Tai, wavelength: 800 nm, pulse width: 100 fs, repetition rate: 80 MHz). The femtosecond laser beam is introduced to an Olympus microscope BX611WI equipped with an oil-immersed objective lens (Olympus Uplan FLN, × 100, NA = 1.30). Three-dimensional microstructures were fabricated inside the SU-8 film on Newport VP-25XA-XYZ stages controlled by an ALPS 3861. Travel range of each stage is 25 mm with 100 nm resolution. Maximum speed of stage is 25 mm s−1. The laser intensity is attenuated by an attenuator (ATT), and the writing speed is 100 μm s−1, 200, 300, 400, and 450 μm s−1. A schematic of the DLW apparatus is shown in Fig. 1.
After laser irradiation, the sample film was baked at 65 °C for 10 min and at 95 °C for 50 min to complete the cross-linking reaction. The un-irradiated part was removed using a developer (SU-8 Developer, Microchem, USA) for 10 min. After development, the sample was rinsed with 2-propanol, followed by drying. A UV lamp (wavelength: 365 nm) illuminates the entire sample for 10 min followed by baking at 65 °C for 10 min and further baking at 95 °C for 30 min to complete the cross-linking reaction inside the micro-lens.
The fabricated structures were observed using a scanning electron microscope (Hitachi SEM S-3000, Japan). Before SEM observation, platinum was sputtered on the sample film to give a conductivity using ion-sputtering (Hitachi E-1010, Japan).
The three-dimensional structure of the fabricated materials was measured using a 3D laser scanning confocal microscope with a 408 nm wavelength (Keyence, VK-X200, Japan).
The roughness of the fabricated materials was measured using an atomic force microscope in close contact mode (AFM, Pacific Nanotechnology Nano-R, USA).
3. Results and Discussion
Vertical and horizontal linewidths
Linewidth and line-depth are important parameters for microstructure fabrication using the DLW method. The laser power I, writing speed v, and writing cycle x, a number of times a line is overwritten, significantly affect the linewidth of the fabricated structures. For the laser power dependence of the linewidth, the laser power is increased from 6.0 to 10 mW in 0.5-mW increments for single-pass writing with a writing speed of 20 μm s−1. The measured linewidth and line-depth are summarized in Table 1. For the laser writing speed dependence of the linewidth and line-depth, the writing speed is increased from 10 to 50 μm s−1 in 5-μm s−1 increments with a fixed laser power of 10 mW and single-pass writing. The measured linewidth and line-depth are summarized in Table 2. For the linewidth and line-depth dependence on the writing cycle, the writing cycle is increased from 1 to 10 cycles when the laser power is changed from 2.5 to 5.0 mW. The laser writing speed is fixed at 20 μm s−1. The measured linewidth and line-depth are summarized in Table 3.
The dependence of the linewidth and line-depth of the positive photoresist of Novolak/diazonaphtoquinone (DNQ) on the laser power and writing speed has been analyzed using an exposure kinetics model for DNQ using two-photon absorption with a femtosecond laser . In that model, the linewidth is proportional to . Analogous to Novolak/DNQ , the present linewidths are analyzed in the same manner. The present obtained linewidth and line-depth are sensitive to the laser power, writing speed, and writing cycle. The linewidth and line-depth are plotted in Fig. 2 as a function of where is the dose of the laser illumination. For multi-pass writing (writing cycle is higher than 2), the plots of the linewidth and line-depth at different writing cycles and different laser powers form a universal relationship, which is shown by the solid line in Fig. 2. For single-pass writing, the plots of the linewidth and line-depth at different laser powers and writing speeds form another universal relationship, which is shown by the dashed line in Fig. 2. The minimum linewidth is measured to be 0.7 μm, as shown in Fig. 2, which is smaller than the diameter of the diffraction limited airy-disc of 0.75 μm. This is due to the narrowing of the focal area using the two-photon excitation. The diameter of the airy-disc, 2r, is defined by the equation of , where λ is the wavelength of laser beam, 800 nm, and NA = 1.3. On the other hand, the vertical width extends along the direction of beam propagation. The aspect ratio between the line-depth and linewidth is in the range of 3 to 4. The detailed aspect ratios are summarized in Tables 1, 2 and 3.
Figure 3 shows the microstructures fabricated by linear line drawings with single-pass writing. The laser power is 5.0 mW, and the writing speed is 20 μm s−1. Woodpiles, assemblies of cubic structures, and pyramid structures on the micrometer scale are fabricated. As shown in Fig. 3(b), each line has a 0.7 μm horizontal width and a 2.3 μm vertical width. Figure 4 shows the curved hemispherand structures that were fabricated with two types of drawings patterns. One pattern is shown in Fig. 4(a), and the SEM images of the resulting structures are shown in Fig. 4(b) and Fig. 4(c) (enlarged scale). The other pattern is shown in Fig. 4(d), and the SEM image of the resulting structures are shown in Fig. 4(e) and Fig. 4(f) (enlarged scale). It is clearly seen that the writing pattern of Fig. 4(d) and SEM images of Fig. 4(e) and Fig. 4(f) give a smoother surface on the hemispherand.
As shown in Fig. 4, the assembly of circular patterns gives a smooth surface for the curved structures. Figure 5 shows the writing procedure of the micro-lens from the top and side views. The circular lines were drawn with a spacing of 0.5 μm. The radius and height of micro-lens are 168 and 24 μm, respectively. The resulting micro-lens has a radius of curvature of 600 μm. The laser intensity is 7.0 mW, and the writing speed is 100 μm s−1. The micro-lens was fabricated using the optical system shown in the Experimental section.
The structure of the micro-lens is observed using a laser microscope and an SEM. Figure 6 shows the structural images of the micro-lens fabricated by DLW: laser microscope image in Fig. 6(a), 3D image using the shape analysis mode in Fig. 6(b), and in Fig. 6(c) curvature profile measured along the red line in Fig. 6(a), Fig. 6(d) shows the SEM images, and Fig. 6(e) is the enlarged SEM image of white rectangular area in Fig. 6(d). From the 3D image in Fig. 6(b), the micro-lens is formed by assembling the concentric circles. The radius of the fabricated micro-lens is 169.5 μm, and the height is 32.6 μm, which is slightly larger than the thickness of SU-8 film, but this is within an experimental error. As seen in Fig. 6(a) and Fig. 6(d), micro-lens consists of three parts. This is because thatthe multiple programming is employed: The outer rings are fabricated using one program, then next inner rings fabricated using next program. Thus the change of the programming caused the small gap on the surface. The gap is within 0.4 μm.
To complete the cross-linking inside of the fabricated micro-lens, the following cross-linking reaction was performed. After the development, the sample was illuminated for 10 min using a UV lamp. After the UV illumination and to complete the cross-linking reaction, the sample was baked at 65 °C for 30 min, and at 95 °C for 30 min. To confirm the structure of the fabricated micro-lens, the sample was observed using a laser microscope again. The resultant images are shown in Fig. 7: laser microscope image in Fig. 7(a), 3D image of shape analysis in Fig. 7(b), in Fig. 7(c) curvature profile measured along the red line in Fig. 7(a), curvature fitting by a polynomial function in Fig. 7(d), SEM image in Fig. 7(e), and in Fig. 7(f) the enlarged SEM image of white rectangular area in Fig. 7(e). The radius of the fabricated micro-lens is 170.3 μm, and the height is 27.9 μm. As seen by comparing the curvature in Fig. 7(e) with Fig. 6(d), the cross-linking the inside the sample gives a smooth surface. The gap shown in Fig. 6 are disappeared because of cross-linking reaction inside. The surface roughness measured by AFM is within 0.2 μm.
The focusing performance of the micro-lens is measured using the apparatus shown in Fig. 8. A laser beam from a Ti:sapphire laser is focused using the fabricated micro-lens, and the focused beam is passed through a counter objective lens with a working distance of 0.6 mm to obtain a collimated beam. The distance between the micro-lens and an objective lens is 1.52mm, and the working distance of the micro-lens is determined to be 0.92 mm.
To evaluate the working distance, the curvature of micro-lens is calculated using Eq. (1);Eq. (2) using the method of curve fitting. The value of a is defined by x = 158 where the first derivative of f(x) is zero. The radius of curvature R is calculated to be 499 μm.
Since the micro-lens is a plano-convex type, the working distance (WD) is defined using the Eq. (3);
The WD value of 0.84 mm is determined, which is nearly the same as the value measured.
Two-photon excitation measurements using the fabricated micro-lens
To confirm the focusing ability of the fabricated micro-lens, a fluorescence of isopropyl thioxanthone (ITX) through two-photon excitation using a Ti:sapphire laser at 800 nm was monitored. Figure 9(a) shows the schematic of the apparatus for the two-photon excitation measurement through the fabricated micro-lens. The laser beam that passed through the micro-lens was focused on the ITX ethanol solution (5 wt% solution) in a silicon rubber cell sandwiched by two glass substrates. The fluorescence from the focusing point was monitored with a CCD camera, and the laser power was 880 mW. Photographs of the two-photon exited fluorescence from the ITX ethanol solution are shown in Fig. 9(b). Clear blue fluorescence is observed from the two-photon excitation through the fabricated micro-lens.
Change in the writing speed to fabricate multiple micro-lenses
The next step is the fabrication of the multiple micro-lenses. To reduce the processing time for the multiple-step fabrication process, it is important to shorten the working time for the fabrication of the single micro-lens. We extend the writing speed to 200, 300, 400, and 450 μm s−1 to decrease the total time for the fabrication process. The fabrication conditions are summarized in Table 4. The increase in the writing speed is found to produce small, hollow holes in the center of the micro-lens. These holes were observed using a 3D laser scanning confocal microscope and a scanning electron microscope. We believe that the SU-8 resist solution flows away from the hole, making it hollow. We assume this occurs at faster writing speeds because the depression of the diffusion of the photo acid disturbs the thicker lines of the structure.
To enhance the diffusion of the photo acid, we used thicker sample films with a thickness of 60 μm, which is twice as large as the former one. Figure 10 shows the laser microscope image and 3D image of the micro-lens when the writing speed is varied from 200 to 450 μm s−1.
Micro-lens array fabrication
As discussed above, we can shorten the processing time for the fabrication of a single micro-lens by using a writing speed of 400 μm s−1. We then fabricate a micro-lens array (3 × 3) using this writing speed. The 3D image of the micro-lens array is shown in Fig. 11. The laser power was slightly increased to 17.5 mW to avoid hollow spots at the top of the micro-lens.
A schematic of the apparatus for multi-focusing using the micro-lens array is shown in Fig. 12. A Ti:sapphire laser beam with a diameter of 1 mmφ is illuminated onto the micro-lens array and a laser beam is transmitted through each micro-lens and focuses on the SU-8 film to fabricate the 9 separate and identical woodpile microstructures.
A total of 9 woodpile structures are simultaneously fabricated using the micro-lens array. Twenty-one beam lines with a 100 μm length are illuminated with a 5-μm interval, and the sample film was rotated 90°; at a position 1 μm higher, another set of 21 beam lines are illuminated. This process continued until a total of 800 beam lines are illuminated for the sample film. The laser power is 880 mW, and the writing speed of 10 μm s−1. Compared to the above illumination conditions, the dose is larger for the array. The SEM image of the obtained woodpile structures is shown in Fig. 13. The 9 woodpiles show a cubic microstructures and are successfully fabricated using the micro-lens array. The measured lateral length and height for each woodpile are summarized in Table 5. The 20 lines on the surface can be seen in Fig. 13(b). Unfortunately, as can be seen in Fig. 13(b), each line cannot be separated. That is due to the low NA value of the obtained micro-lens array. NA value was assumed to be between 0.2 and 0.3 with the working distance of 0.92 mm measured and the diameter of micro-lens. Thus the focusing diameter of the beam is comparable to the diffraction limit of the micro-lens between 3 and 5 μm, which is larger than the vertical distance between two lines and comparable to the lateral distance of that.
We have successfully fabricated 3D microstructures, micro-lenses, and micro-lens array for micro-optical devices using the negative photoresist SU-8 with a femtosecond DLW via TPP and TPA. Precisely designed micro-lenses can be fabricated using the DLW via TPP. The curvature of the micro-lens was evaluated from the shape analysis mode of the laser microscope. The curvature is fit by a polynomial function, from which the working distance of 0.84 mm is calculated. The calculated working distance corresponds well to the experimentally measured value of 0.92 mm. The optical performance of the fabricated micro-lens is tested using the TPA fluorescence measurements of the ITX. The micro-lens array consisting of 9 micro-lenses is fabricated. This is the first demonstration that the femtosecond laser beam with a 1 mmφ is focused on the SU-8 photoresist through the fabricated micro-lens array to simultaneously manufacture 9 independent woodpile-like microstructures using DLW via TPP. Due to the high dose, the lines are not separated. However, the present results clearly demonstrate the high potential of DLW method to fabricate the micro-lens array with any working distances and diameters in the dimensions of micron to sub-millimeter. The number of micro-lenses are basically only limited by the travel range of 3D stages, in the present case 25 mm each, and lens diameter. Multi-focusing using micro-lens array with large size area can be achieved using a collimated beam expansion of a femtosecond laser beam. This technique paves a new way to manufacture the optical components for the micro-scale optical devices.
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