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A simple and novel method is proposed for the fabrication of aspherical SU-8 microlens arrays with a wide range of tunable focal lengths utilizing a soft SU-8 stamping process and an electro-static pulling method. In the proposed approach, an SU-8 stamp incorporating a micro-nozzle array and a reservoir containing unexposed SU-8 is fabricated on a glass substrate using a dose-controlled exposure process. Microlens arrays with diameters ranging from 20 to 500 μm and various radii of curvature are successfully fabricated using the proposed method. The low surface roughness (Ra = 3.84 nm) and high dimensional uniformity of the SU-8 microlens arrays (variation < 5% designed diameter) confirm both the optical quality of the individual microlenses and the general feasibility of the fabrication method. The innovative fabrication method proposed in this study provides a simple and efficient means of producing high quality aspherical microlens arrays with tunable focal lengths.
©2010 Optical Society of America
1. Introduction
Microlens arrays have many applications in the optical field, ranging from optical communications [1], to flat panel displays [2], optical storage devices [3], and scanning micro-optical systems [4]. As a result, the literature contains many proposals for the fabrication of microlens arrays utilizing a wide variety of techniques, including photoresist (PR) or sol-gel glass reflowing [5,6], hot embossing, replica molding [7,8], ink-jet printing [9,10], gray scale photolithography [11], and excimer laser ablation [12].
Due to the natural laws of physics, liquids tend to conjugate in a spherical form. The simplest lens structure is that of a liquid droplet, and thus droplet arrays fabricated using PR reflow [5], ink-jet printing [10], dielectrophoretic [13] or surface modification-based liquid incision techniques [14] have attracted particular attention in recent years. Of these various techniques, the PR reflow method is amongst the most simple and controllable, and is therefore one of the most commonly applied. However, the reflow process has a number of drawbacks for microlens array fabrication, including a limited lens shape (i.e. semispherical only), the need for a high processing temperature and a long processing time. Micro-droplet jet printing is also commonly used for the fabrication of commercial microlens arrays. However, as in the reflow method described above, the possible lens geometries are also limited by the physical properties of the lens material. Furthermore, the lens size is constrained by the jet head design.
The external force extrusion method provides a straightforward means of effecting a change in the volume of fabricated droplet lenses through the application of a mechanical or hydraulic force. Various researchers have demonstrated the feasibility of realizing aspherical lenses using electrostatic force modulation methods [15,16]. Nevertheless, only large-scale lenses of millimeters in size were demonstrated. Asphirical microlens array fabricated utilizing electrostatic pulling approach is still lacking as of today.
This paper proposes a novel method for the fabrication of microlens arrays with a high surface quality and a tunable focal length using SU-8 negative photoresist (PR). SU-8 PR has a high optical transmittance in the visible to near-IR wavelength range and a high refractive index (~1.6). Furthermore, it has better mechanical strength and chemical resistance than other common polymers such as polycarbonate (PC) [17] or PMMA [7]. As a result SU-8 is ideally suited to microlens array fabrication [9]. In the proposed fabrication method, an SU-8 stamp and cartridge structure is fabricated using a novel dose-controlled exposure process. The stamp / cartridge structure is then pressed into contact with an ITO glass substrate maintained at a temperature higher than the glass transition temperature (Tg) of the unexposed SU-8 within the stamp. The stamp is then removed from the substrate, leaving a constant volume of SU-8 droplets on the substrate in the form of a microlens array. Significantly, the shape and radii of curvature of the individual microlenses within the array can be easily controlled through an appropriate design of the micro-nozzle openings in the stamp and the stamping temperature, respectively. Having formed the basic microlens structures on the ITO substrate, the surface profiles of the microlenses are modified via the developed electro-static pulling process in order to tune their optical properties (i.e. their focal lengths).
2. Materials and method
Un-exposed SU-8 has a glass temperature (Tg) of around 55°C and is highly sticky in the liquid phase [18]. However, following exposure, Tg increases to 225°C and the SU-8 has a low surface energy. Consequently, the surface is no longer sticky. In the present study, these temperature-dependent properties of SU-8 PR are exploited to fabricate an aspherical microlens array Fig. 1 presents the concept of the developed method for fabricating aspherical microlens array. In the proposed approach, an SU-8 stamp composed of a hard shell incorporating a micro-array of via-holes is fabricated using a dose-controlled exposure process. As shown in Fig. 1(A), the exposure conditions are carefully controlled such that the outer SU-8 PR is fully exposed, creating a hardened shell, while the inner SU-8 PR remains unexposed, thereby creating an “ink” cartridge within the stamp. In other words, the exposed SU-8 structure serves both as a stamp to define the pattern of the microlens array and a reservoir to store the SU-8 ink [19]. Following the exposure process, the stamp is impressed onto an ITO glass substrate maintained at a temperature higher than the glass temperature of the unexposed SU-8 PR. As a result, the liquid SU-8 is squeezed from the cartridge and forms an array of semispherical droplets on the surface of the ITO substrate under the combined effects of surface tension and the gravity force. The polarity of SU-8 PR molecules enables SU-8 structures to be readily deformed via the application of an external electrical field [20]. Thus, following the stamping process, the semispherical droplets on the ITO substrate are transformed into aspherical droplets by applying an electrical field with an appropriate intensity between the lower patterned substrate and an upper blank substrate (see Fig. 1(B)). In practice, the surface profiles of the microlenses formed using the stamping process described above can be adjusted via two different mechanisms, namely a post-stamping thermal reflow process or an electro-static pulling method. The formed lenses were finally UV exposed to fix the shapes.
Fig. 1 Schematic illustration of aspherical SU-8 microlens array fabrication concept: (A) SU-8 lens molding, (B) electrostaic pulling.
3. Results and discussion
Existing microlens fabrication methods such as ink-jet printing are capable only of producing ball-shape lens structures. By contrast, the SU-8 stamping/electro-static pulling method proposed in this study is capable of producing microlens arrays with a variety of geometrical shapes. For example, Fig. 2(A) and Fig. 2(B) presents SEM images of semispherical microlens array with diameter of 300 μm and hexagonal microlens array with diameter of 100 μm. In practice, the lens geometry is constrained only by the mask design used to pattern the openings in the stamp structure. Thus, a single stamp can either be patterned with multiple openings of a single design, resulting in an array of identical microlenses, or can be patterned with multiple openings with different designs in order to create an array of microlenses with different geometries for light coupling applications. Furthermore, the filling ratio of the microlens array can be easily adjusted by varying the size and pitch of the openings in the stamp structure. The proposed fabrication method enables a maximum filling ratio of 92.5% to be obtained given a suitable mask design. Accordingly, the proposed stamping / electro-static pulling method provides a highly versatile approach for the fabrication of microlens arrays for a wide variety of optical applications. Figure 2(C) and 2(D) present SEM images of a microlens before and after the electro-static pulling process. The images show that the interaction between the surface tension of the SU-8 PR, the gravity force, and the electro-static pulling force causes the profile of the microlens to change from a semispherical shape to a cone-like shape. In other words, the electro-static pulling process results in a significant reduction in the radius of curvature of the original microlens. In practice, the radii of curvature of the stamped microlens can be effectively controlled through an appropriate specification of the electro-static pulling voltage. Figures 2(E), 2(F) present OM images showing the optical behavior of microlens arrays fabricated using electro-static pulling voltages of 0 and 2500 V, respectively. The images clearly show that the focal length reduces as the pulling voltage increases.
Fig. 2 SEM images of fabricated microlens array with excellent surface flatness: (A) semispherical microlens array with diameter of 300 μm (molding temperature = 65°C), (B) hexagonal microlens array with diameter of 100 μm (molding temperature = 70°C). (C) microlens without electrostatic pulling and (D) after electro-static pulling at 2500 V. OM image showing the focus behavior of microlens arrays fabricated under different electro-static pulling voltages. (E) 0 V and (F) 2500 V.
The surface tension of SU-8 PR reduces as the environmental temperature increases. As a result, the stamping temperature provides a convenient means of adjusting the initial height of the microlenses formed on the ITO substrate. Figure 3 illustrates the variation of the fabricated lens height with the stamping temperature for microlenses with designed diameters ranging from 20 ~500 μm. The standard deviation of the height measurements was found to be less than 5% for all values of the stamping temperature, and thus the dimensional uniformity of the microlenses is confirmed. Furthermore, Fig. 3 shows that the lens height decreases approximately linearly as the stamping temperature is increased. Finally, it is observed that the maximum lens height attainable in the proposed stamping method is around 40% of the designed diameter.
Figure 4(A) shows the surface profiles of microlenses fabricated at a stamping temperature of 60°C and then reflowed at temperatures ranging from 60~100°C. In the post stamping reflow process, once the ambient temperature exceeds the glass temperature of the uncured SU-8 PR, a phase change occurs. As a result, the combined effects of the surface tension and the gravity force cause a self-planarizing of the microlenses. Due to the high viscosity of the molten SU-8 PR, the original surface profile is not recovered after the reflow process. Figure 4(B) illustrates the effect of the pulling voltage on the surface profile of a semispherical lens with a designed diameter of 500 μm and fabricated at a stamping temperature of 70°C. Note that the electro-static pulling process was performed at a temperature of 80°C with a gap distance of 770 μm between the upper and lower ITO glass substrates. The results clearly show that the electrical field counteracts the effects of gravity and the thermally-induced capillary force in causing an expansion of the droplet at temperatures greater than the glass temperature of uncured SU-8. For example, the droplet profile following an electro-static pulling process performed at 19500 V/cm is very similar to that of the original droplet prior to the pulling process. In other word, the effects of surface tension and gravity at a reflow temperature of 80°C are equal and opposite to those of the electro-static pulling force at a voltage of 19500 V/cm. For applied voltages greater than 19500 V/cm, however, the pulling force in the upward direction is greater than the combined surface tension and gravity force acting in the downward direction. Hence, an elongation of the droplet occurs, causing the lens profile to change from a spherical to an aspherical shape.
Fig. 4 (A) Scanned surface profiles of 500 μm microlens fabricated at 60°C and then reflowed at temperatures between 70°C and 100°C. (B) Measured optical behavior of microlens arrays fabricated under different electro-static pulling voltages from 0 V/cm to 32500 V/cm. Note that these profiles were obtained by analyzing the lens contours of the SEM images using commercial software of Matlab®.
The microlens arrays fabricated using the proposed SU-8 stamping/electro-static pulling method have an excellent dimensional uniformity. The surface roughness properties of the fabricated lenses were evaluated using a surface profile meter (Model: SJ-400, Mitutoyo, Japan) over a scanning length of 50 μm. The results showed that the average surface roughness (Ra) (evaluated over 20 samples) was just 3.84 nm. Furthermore, the use of the soft baking technique in the stamp fabrication process reduces the solvent content in the glue mixed with the SU-8 PR. As a result, no volume shrinkage of the fabricated lenses occurs, and thus the fabricated microlens arrays are free of surface wrinkling and shape deformation effects. Overall, the low surface roughness and high dimensional uniformity of the SU-8 microlens structures confirm both the optical quality of the individual microlenses and the general feasibility of the fabrication method. The optical performance of the fabricated microlens arrays was evaluated by measuring the light intensity at the focal plane. Figure 5 shows the focused light spots and light intensity profiles of a microlens array with a lens diameter of 300 μm and a radius of curvature of 230 μm. It can be seen that the focused light spots have a large diameter and a uniform intensity (variation < 5%). In other words, the results confirm the excellent optical performance of the microlens arrays fabricated using the proposed stamping / electro-static pulling method.
Fig. 5 (A) Photographs of light emitted from a spherical microlens array with a lens diameter of 300 μm and a radius of curvature of 230 μm, (B) measured intensity profiles. Note that “D” in (A) denotes spot diameter and “I” in (B) denotes spot intensity. Note that this microlens array was fabricated with a stamping temperature of 70°C and without the electro-static pulling process.
4. Conclusion
This paper has presented a simple and rapid approach for the fabrication of aspherical SU-8 microlens arrays utilizing a novel stamping and electro-static pulling method. The proposed method enables the fabrication of microlens structures with a variety of lens shapes and optical properties through an appropriate specification of the mask design, stamping temperature and external electrical field. The experimental results have shown that the fabricated microlenses have both a high surface quality and a high optical uniformity. In other words, the proposed approach provides a viable means for the mass fabrication of precise microlens arrays for a variety of optical applications.
Acknowledgments
The financial support from National Science Council of Taiwan is greatly acknowledged (NSC 97-2221-B-110-018-MY3).
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