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Abstract
In this study, a method is proposed for the rapid fabrication of a hexagonal compound eye microlens array (HCE-MLA) using maskless lithography technique based on digital micromirror device (DMD), in which a hexahedron array is lithographed, and subsequently, a microlens structure is lithographed layer by layer upon the hexahedron. Owing to the high filling rate of the hexagon and the aspheric surface of the paraboloid profile of the microlens, after the distribution of the required exposure dose of HCE-MLA was calculated based on dose modulation, a series of circular pattern with different radius could be obtained by equal-arc-mean slicing strategy that can adapt to the variable curvature of the target contour and improve its reconstruction precision. Then, after a hexagonal photoresist island was fabricated on the substrate, and the dose accumulated over multiple exposures, the required exposure dose profiles were reconstructed on the hexagonal photoresist island. Finally, a high-quality HCE-MLA with a smooth surface was fabricated via thermal reflow. The fabrication of the designed HCE-MLA using this method was demonstrated in experiments. The results indicate that the actual profile of HCE-MLA shows good agreement with the design profile and that simultaneously increasing the number of the slicing layers can improve the quality of the profile. The research indicated that this method is feasible and convenient for the fabrication of high-quality HCE-MLA.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The compound eye has been widely used in optoelectronics, medical treatment, digital displays [1–3] and other fields, owing to its advantages of small size, low weight, large field of view, and high time resolution [4,5], etc. Especially for hexagonal compound eye microlens array (HCE-MLA), it has a significantly high fill factor [6]. The microlens array with a high fill factor will capture most incident light to increase the signal-noise-ratio and optical performance [7]. However, how to obtain such a high-quality of HCE-MLA with a simple and low-cost method is a challenging issue that needs to be resolved. Furthermore, the metasurface has been regarded as a promising technique to modulate the phase, amplitude, polarization of the light at the subwavelength scale. Compared with the conventional optical devices which manipulate light via the accumulation effect along the curved optical path, the two-dimensional metasurface could realize the manipulation along flat medium surface. Matesurface-based microlens arrays are increasingly being researched and fabricated such as planar and broadband microlens arrays [8,9].
At present, some methods to fabricate the compound eye microlens arrays exist, including three-dimensional (3D) electron beam lithography [10], ultra-precision machining technology [11], femtosecond laser induced two-photon polymerization technique [12], droplet method [13] and gray-scale mask lithography [14], etc. However, these technologies have the disadvantages of complicated process flow, low efficiency, high cost, and long operational time. In recent years, a novel photolithographic method based on digital micromirror device is introduced, which is convenient for performing 3D microstructure fabrication [15]. Thus, the computer converts the mask image into binary data to control the digital micromirror devices (DMDs) in order to generate a virtual mask. Therefore, the mask patterns can be generated from time to time during production, thereby avoiding the high cost and long fabrication cycles associated with the use of physical masks. Compared to the conventional lithography methods, it can avoid misalignment errors, and thus, has a better repeatability in the fabrication of 3D microstructures [16].
To improve the accuracy of manufacturing 3D microstructures in DMD-based maskless lithography, a single scan method was developed recently, in which the fabrication process of 3D microstructure was finished by one-time scan [17]. In general, the method to fabricate a 3D microstructure in maskless lithography involves the use of multilayer slicing in order to reconstruct the design model. The reported slicing strategy includes an equal-height scan strategy, equal-exposure-dose scan strategy [18,19], and equal-arc-mean slicing strategy [20]. An equal-height scan strategy can produce any complex microstructure, but the precision of the microstructure is affected by the number of slicing layers and movement accuracy of the z-axis platform. An equal-exposure-dose scan strategy cannot adapt to the design contours with variant curvatures, as it will cause large profile errors in model reconstruction when the design profile is a curve with variant curvature. We adopt the equal-arc-mean slicing strategy to fabricate HCE-MLA. By quantizing the design profile into many layers according to the principle of equal-arc, a series of suitable patterns is generated. This method can better adapt to the curvature change of the design profile and ensure the profile accuracy.
In order to fabricate a practical microlens, typically, the microlens in photoresist is first transferred to Polydimethylsiloxane (PDMS) and then cured by heating. After peeling off the photoresist, a concave PDMS pattern is formed, and then the UV-curing resin is injected into the concave template to obtain a microlens structure with optical transmission [21].
In this paper, we first calculated the distribution of the exposure dose on the design profile based on dose modulation and obtained a series of circular patterns with different radius by equal-arc-mean slicing strategy. Then, we designed multilayer masks pattern by L-Edit software and fabricated a hexagonal photoresist island on the substrate by using digital lithography, in order to reconstruct the design profile by the dose accumulated over multiple exposures on the hexagonal photoresist island. Finally, a high-quality HCE-MLA with a smooth surface was fabricated via thermal reflow. We can easily control the shape and size of multilayer slicing and generate dynamic masks of corresponding parameters by DMD-based maskless lithography in order to fabricate HCE-MLA with a matching design profile and a smooth surface. By comparing the resulting profile obtained from the different number of slicing layers, we found that the larger the number of slicing layers is, the closer the resulting profile is to the design profile.
2. Microstructure fabrication process and method
A schematic of DMD-based real-time maskless lithography system is shown in Fig. 1. The basic components are exposure light source, the DMD chip, and the projection system. The near-ultraviolet beam with a wavelength of 405 nm emitted by an ultraviolet light-emitting diode (LED) was homogenized and collimated, which is thus converted to an exposure light with a desired uniform illumination and a high degree of collimation. The exposure light was reflected by the mirror and casting on a DMD chip, which is a spatial light modulator manufactured by Texas Instruments. The DMD chip in our system, which has a full field width of 2048 × 1024 pixels and each micromirror has the dimension of 10.8 × 10.8 μm2, can switch between two states, with each micromirror switching according to the pixel information of the binary image and representing one pixel of the pattern controlled by the computer, which replaces the physical binary mask. When one pixel of the pattern is white, the corresponding micromirror causes deflection of + 12°, and the incident light is reflected into the objective lens to form an aerial image and focused on the photoresist surface. When the binary image is black, the corresponding micromirror causes a deflection of −12°, and the incident light is reflected and cannot enter the projection lens. Our photolithography system can fabricate a line with a minimum width of 0.6 μm, which entirely complies with the requirements of the design work.
To fabricate the HCE-MLA, we proposed the process as schematically illustrated in Fig. 2. The process flow was as follow. First, a clean substrate was spin-coated at 2000 rpm with the positive photoresist AZ 4620, and then, the substrate with photoresist was prebaked at 100 °C for 5 min, the Tg temperature (glass transition temperature) of the photoresist AZ 4620 is 125 °C, thus, a cured photoresist layer with a thickness of ~9 μm was obtained, as illustrated in Fig. 2(a). The substrate with photoresist was ultraviolet (UV)-exposed on the DMD-based maskless lithography, with the power of ultraviolet light source being 5.5 W, and the virtual mask pattern is a hexagonal array. Then, the substrate was developed in AZ 400 K solution for 4 min, and the hexagonal cylinder microstructures were obtained, as illustrated in Fig. 2(b). Next, the hexagonal cylinder microstructure was put into the experimental system with multiple exposures, the circles of different radius were used as the virtual mask pattern, the design profile was reconstructed on the hexagonal cylinder array with dose modulation, and the hexagonal microlens arrays were obtained after development in AZ 400 K solution, as shown in Fig. 2(c). The magnified image of a single microlens of existing steps was shown in Fig. 2(f). The hexagonal microlens array were placed on a hotplate of 125 °C for 10 min, and the HCE-MLA can be obtained through the contact thermal reflow process, which can clearly smooth the profile surface of HCE-MLA, as shown in Fig. 2(d). Finally, a high-quality HCE-MLA with smooth surface was fabricated, as shown in Fig. 2(e).
Fig. 2 Schematic of HCE-MLA fabrication process. (a) One-layer thick photoresist via spin-coating on a substrate. (b) Hexagonal cylinder microstructures fabricated after exposure and development in the DMD-based maskless lithography. (c) Fabrication of hexagonal micro-lens array via dose-modulation with equal-arc-mean slicing strategy. (d) Smooth of the counter surface via thermal reflow for high quality profile. (e) The fabricated HCE-MLA. (f) Magnified image of single hexagonal microlens with steps after development.
For a specific type of positive photoresist, there exist the exposure thresholds, Eth and Ec, Eth is defined as the minimum exposure dose required for the initial reaction of the photoresist, and Ec is defined as the required exposure dose for maximum curing depth, with the total height of design profile being H. There is a value of contrast γ, which is not only related to the thickness of the photoresist, but also to the distribution of the design profile, which was defined as the linear slope of the contrast curve, as shown in Eq. (1) [22],
where 0 ≤ h (x, z) ≤ H, Eth ≤ E (x, z) ≤ Ec.
The Eq. (2) can be obtained from Eq. (1).
It shows the relationship of the calculated exposure dose, E (x, z), with the design profile, h (x, z).
According to the fabrication process and method proposed in this paper, the hexagonal photoresist island was first fabricated, with its bottom side length being 15 μm and the height being 9 μm, as shown in Figs. 3(a) and 3(b).
Fig. 3 (a) Hexagon with a length of 15 μm. (b) Hexagonal cylinder with a height of 9 μm. (c) Designed parabolic profile. (d) Paraboloid. (e) Model of single hexagonal compound eye microlens.
The designed cross-sectional profile h (x, z) of the HCE-MLA is a parabola, as shown in Fig. 3(c), its bottom radius, R, is 15 μm, and maximum curing depth, H, is 9 μm, the profile can be expressed as,
The paraboloid is shown in Fig. 3(d) based on Eq. (4). Then, a model of single hexagonal compound eye microlens can be obtained by the Boolean sum on the hexagonal cylinder and paraboloid, as shown in Fig. 3(e).
The photoresist model used in this fabrication process is AZ 4620 (a thick film positive photoresist from Clariant Corporation), which can be identified from the relationship between the exposure dose and the development depth used, where the exposure threshold Eth and the contrast γ of the photoresist were 35.88 mJ/cm2 and 0.4334, respectively, when the maximum depth of development, H, was 9 μm, and the required exposure dose was 360.5 mJ/cm2. Then, the exposure dose distribution for the parabolic profile can be calculated according to Eqs. (2) and (3), as shown in Fig. 4.
On the basis of the distribution of the calculated exposure dose across a parabolic profile, we need to investigate the slicing strategy in order to improve the profile quality and fabrication efficiency. Herein, we propose and use a new slicing strategy called equal-arc-mean slicing strategy, which quantifies the design profile into several layers, according to the principle of equal arc. Contrary to the equal-height scan and equal-exposure-dose scan, this method can accommodate the curvature change of the design profile and improve the profile accuracy, as shown in Fig. 5.
Assuming that the design profile with height H and bottom radius R, represented by the red curve, we divide H into n layers according to the principle of equal arc, with the length of each arc being ∆S and the height of the ith step layer being hi, represented by a blue line, as described in Eq. (5),
Thus, the radius of the ith slice is ri, and the width of the ith step is ∆ri = ri-ri-1, (∆r1 = r1, i = 2, …, n.). In order to compensate the precision error caused by photoresist deformation during the thermal reflow process, the corresponding radius of the ith layer can be expressed as Eq. (6).
3. Results and discussions
To evaluate the validity of the fabrication process and method proposed in Section 2, the HCE-MLAs with a parabolic profile were experimentally fabricated. First, the multilayer masks must be drawn for multiple exposures on the fabrication process. Then, the hexagonal side length is set to 15 μm, and a series of circles with different radius data can be obtained by the equal-arc-mean slicing strategy. Next, the designed mask pattern is drawn in the L-Edit, which is a software for design integrated circuit layout developed by Tanner Research.
A schematic of the multilayer mask pattern is shown in Fig. 6(a), in which different colors represent different mask layers. In the digital lithography system, the DMD chip converts these masks into a binary image in order to replace the traditional physical mask. As shown from the expanded view of the multilayer mask pattern in Fig. 6(b), the underlying layer is composed of regular hexagons, and each layer from top to the bottom is the circle of radius from small to large.
Fig. 6 Mask pattern of the HCE-MLA. (a) Multilayer mask pattern. (b) Expansion view of the multilayer mask pattern.
After the mask pattern was drawn, the substrate coated with the photoresist was exposed to the first layer mask in the lithography machine, and then was developed using the developing solution. The images were measured by the scanning electron microscope (S-3400N, Hitachi Limited). Figure 7 showed scanning electron microscope (SEM) image of the hexagonal photoresist island array. These arrays are arranged in a symmetric order with uniform spacing, and the lines of each hexagonal photoresist island are clearly visible.
On the basis of the hexagonal photoresist island array, the exposure process was continued in the lithography system. When the last layer of masks was used for fabricate the HCE-MLAs with the method of dose-modulation, the area above the photoresist island array that can be used for exposure becomes very narrow because the edge of the hexagonal cylinder is a sharp corner, and may be cleared after exposure and development. In order to verify the condition of the edge of the photoresist after exposure and development. The last layer of masks was used for exposure as a test experiment firstly. Figure 8 was showed the SEM images of a test experiment using the last layer of the design mask. Figures 8(a) and 8(b) showed the images of test effect after development. It can be seen that the photoresist island still presents a regular hexagon and are neatly arranged. Figures 8(c) and 8(d) showed an enlarged view of the edge of the cylinder. It can be obviously seen from the figure that the sharp corner of the hexagonal cylinder still exists, and the steps appearing can be clearly seen after exposure and development.
Fig. 8 The SEM image of a test experiment using the last layer of the design masks. (a) The image of photoresist island array after exposure and development. (b) The image of single photoresist island. (c) Magnifying image of the sharp corner of the hexagonal cylinder. (d) Magnifying image of the edge of the hexagonal cylinder.
According to the distribution of the required exposure dose on the parabolic profile, a dose modulation method was used to accumulate multiple exposures with the 10-layer circular masks from small to large on the hexagonal photoresist islands. Figure 9 showed the SEM image of hexagonal microlens array with steps. As can be seen from the image that the shape of the bottom edge of the hexagonal array remains intact, and the steps obtained by the multiple exposure are very clear.
Fig. 9 The SEM image of hexagonal microlens array with steps via multiple exposures for reconstruction of the contour.
The HCE-MLA was finally fabricated after development and thermal reflow, as shown in Fig. 10. A clean and smooth HCE-MLA is shown.
Here, in order to compare the agreement of design profile and the profile of the fabricated HCE-MLA, considering the effect of different slicing layers on fabrication accuracy, the number of slicing layers was set to five and ten layers. Each such set of slicing layers of HCE-MLA was fabricated, and their profiles were measured by the Profiler (PGI1240, Taylor Hobson Limited). The blue solid line represents the design profile that is a parabolic profile, the green dashed lines and the red dotted lines indicate measured values of the HCE-MLA fabricated that the number of slicing was five and ten layers, respectively. The diagram for comparing different numbers of slicing layers relative to the parabolic profile was shown in Fig. 11, the measurement profile and design profile are closer at the bottom of the microlens, which reveals that real profile and design profile have a small deviation, it means that the radius values of each microlens differ less. With the value in the height of the microlens increases, the matching value of the design curve and the real curve decreases, which mean that radius values of different microlenses have small deviation. For the error between the real profile and design profile along with the radius of micro-lens, the main reason that may be the bottom temperature is greater than the temperature value of the upper part of the microlens during the thermal reflow process. In summary, the resulting profile of HCE-MLA is highly consistent with the design profile, and the resulting profile is closer to the design profile with increasing number of slicing layers.
4. Conclusions
To fabricate HCE-MLAs, first, the hexagonal cylinder microstructures were fabricated. Then, a photoresist layer with a thickness of ~9 μm was UV-exposed during DMD-based maskless lithography. Next, the circle profile of different radius used as the virtual mask pattern was reconstructed on the hexagonal cylinder array with the dose modulation method. Finally, a HCE-MLA with smooth surface can be fabricated through a contact thermal reflow process. Simultaneously, the superimposed hexagonal cylinder and paraboloid profiles obtained by the Boolean sum were theoretically analyzed. The required exposure dose was obtained through the curve characterizing the relationship between the exposure dose and the development depth in certain practical situation of the exposure threshold, the contrast of photoresist and the maximum depth of development. On the basis of distribution of calculated exposure dose for the parabolic profile, an equal-arc-mean slicing strategy was chosen to accommodate curvature change of the design profile and improve the profile accuracy, and as the number of slicing layers is increased, the quality of profile can be further improved. The experimental results indicate that the fabricated profile of HCE-MLA shows a good agreement with the design profile. At the same time, this method has the advantages of rapid, convenient, and real-time processing over other methods.
Funding
National Natural Science Foundation of China (NSFC) (grant no.61475037) and Science and Technology Research Projects of Guangdong Province (2016B090918124).
Acknowledgments
In addition, we would like to thank Editage [www.editage.cn] for English language editing.
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