We demonstrate the cost-effective and facile method of fabricating close-packed microlens arrays using photoinduced two-dimensional (2-D) surface relief structures as original templates. 2-D surface relief structures are produced by successive inscription of two beams interference patterns with different grating vectors on azopolymer films. The employed exposure dose of 1st inscription stage and 2nd inscription stage are optimized to obtain symmetrical modulation heights. These photoinduced 2-D surface relief structures on azopolymer films are used directly to mold PDMS, and PDMS molds were then transferred onto photopolymer to imprint microlens arrays. Using this method, tetragonally and hexagonally close-packed microlens arrays are successfully fabricated in rapid and cost-effective way.
© 2007 Optical Society of America
Microlens arrays are practically important to the optoelectronic applications, for example, charge-coupled devices (CCD) cameras, light-emitting diodes [1–3], confocal microscopes , optical waveguide devices , and optical connectors . Over the years, various strategies were adapted to fabricate microlens arrays. These strategies make use of ink-jet printing , thermal-reflow [8,9], gray-scale photolithography , and plasma etching . Although these methods can produce uniform microlens arrays collectively, there are limitations that require expensive equipments or elaborate multi-step processes. As alternatives, self-assembly methods were used to address these limitations [12, 13]. Unfortunately, most of self-assembled structures have undesirable defects inherently.
Recently, the soft-lithographic-enabled fabrication methods have attracted considerable attentions [14, 15]. In this case, once PDMS molds are developed, microlens arrays can be generated reproducibly by simple replication of molds. However, the fabrication methods of original templates are usually based on the conventional photolithography that also suffers from cost-high, meticulous processes and limited ability to provide close-packed microlens arrays [16,17]. Accordingly, facile, low cost, and versatile method of fabricating closed-packed templates structures is required for further development of soft-lithographic-enabled fabrication.
In this paper, we explore a novel method of fabricating close-packed microlens arrays over the large area using soft imprint lithography. In particular, we employed photoinduced 2-D surface relief structure on azopolymer films as template for microlens arrays. The power of this method lies in the fact that templates for microlens arrays with close-packed structures are fabricated simply by single-step holographic inscription. Contrary to conventional photolithography and holographic lithography, this process does not require etching or photoresists. Furthermore, the geometry of microlens arrays can be well defined and controlled by simply adjusting wavevectors of writing beams. Using this approach, we are able to fabricate the tetragonally and hexagonally close-packed microlens arrays in rapid and cost-effective way.
2. Photoinduced 2-D surface relief structures
The formation of surface relief structures on azopolymer by holographic inscription is well known phenomena. When intensity or polarization interference pattern is imaged onto azopolymer film, either amorphous or liquid crystalline, surface relief structures can be formed spontaneously at temperatures well below Tg [18–21]. This photoinduced mass transportation is closely associated with light-forced transition between trans- and cis-molecular states that differ in polarity and length of molecules [22, 23].
Two ways of holographic inscription can be used to fabricate the 2-D surface relief structure on azopolymer. The first is that three beams interference pattern is imaged onto azopolymer films . By using this method, 2-D periodic surface relief structures could be produced directly. To date, various 2-D periodic structures based on three beams interference pattern are well established experimentally and theoretically [25, 26]. The second is that the two beams interference patterns with different grating vectors are overwritten successively by rotating azopolymer films. For instance, successive inscription of two beams interference patterns with orthogonal grating vectors and 60 ° rotated grating vectors on the same region can produce the tetragonal and hexagonal structures, respectively [24, 27]. Among the above two methods, we selected the latter one, because different geometry of microlens arrays can be fabricated by simply rotating azopolymer films, while very precise and elaborate adjustments of wavevectors and polarization of each beams are required for three beams interference.
In this study, we used epoxy-based azopolymer (PDO 3). The detail synthesis procedures are demonstrated in previous report . Azopolymer films are prepared by spin-coating onto pre-cleaned slide glass, and thickness was controlled to 800 nm. For holographic inscription, 488 nm Ar+ laser was used as writing beams. In particular, employed polarization of two writing beams was orthogonal circular, since mixing of two orthogonal circular polarized beams leads to the most effective formation of surface relief gratings . The polarizations of each beam were controlled by adjusting half- and quarter-wave plates. Figure 1 shows the optical setup for holographic inscription.
Especially, exposure to a different dose between 1st and 2nd inscription stages is significantly important in the process of successive overwriting with different grating vectors. This is because the modulation heights of sinusoidal grating, formed at 1st inscription stage, could influence on the formation of second sinusoidal grating at 2nd inscription stage. After 1st inscription, the thickness of azopolymer films in the regions of peaks is increased with respect to the thickness of raw azopolymer film, due to the increased mass of azopolymer resulting from the mass transportation. Therefore, to obtain the symmetric modulation height, 2nd inscription should induce the larger mass transportation of azopolymer than 1st inscription in the regions of peaks resulting from surface relief structures. The mass of azopolymer to be diffused can be controlled by the adjustment of exposure dose: generally, the diffusion of azopolymer is increased with the exposure dose. Consequently, larger exposure does of 2nd inscription is essential for the symmetric modulation height than that of 1st inscription. To verify the effect of exposure dose ratio (ratio of 2nd exposure dose to 1st exposure dose) on the symmetry of modulation heights (ratio of 1st grating modulation height to 2nd grating modulation height), templates for microlens arrays on azopolymer films were generated by successive overwriting with various exposure dose ratios. A plot of the symmetry of modulation heights versus exposure dose ratio is described in Fig. 2(c). It is observed that exposure dose ratios in the range of 0.88 from 0.81 give relatively symmetrical modulation heights. Upon this results, we wrote 1st and 2nd gratings for 3900 sec and 4800 sec, respectively (intensities of the two beams were equally 2.5 mW/cm2).
Firstly, we fabricated pristine tetragonal microlens arrays by successive overwriting of two beams interference patterns with orthogonal grating vectors. As shown in Fig. 3(a), atomic force microscope (AFM) investigation reveals that tetragonally close-packed microlens arrays over large area were fabricated successfully on azopolymer film. The average modulation heights of tetragonal microlens arrays in 1st grating vector and 2nd grating vector were 300 nm and 340 nm, respectively. In this case, employed incident angles of two beams were 7°, and the observed diameter of microlens was 2 μm. This is in accordance with Bragg conditions.
To manufacture the pristine hexagonal microlens arrays, two beams interference patterns with 60 ° between their grating vectors were sequentially overwritten by rotating azopolymer films. As shown in Fig. 3(b), the pristine microlens arrays showed regular triangular structures. In this case, each microlens shows the elliptical shape, not spherical because rotation angle of the azopolymer films was 60 °. The average modulation heights of the hexagonal microlens arrays in 1st grating vectors and 2nd grating vectors were also 300 nm and 340 nm, respectively. Both pristine microlens arrays on azopolymer films show close-packed arrangement with good homogeneity over the large area, while conventional photolithography only gave non-close-packed arrangement. These patterned azopolymer films could be used directly as templates for microlens arrays to mold PDMS without additional surface treatment.
3. Fabrication of microlens arrays by soft imprint lithography
Figure 4 depicts the process of soft imprint lithography for fabrication of microlens arrays using photoinduced 2-D surface relief structures as pristine templates. After holographic inscription of azopolymer films, PDMS prepolymer was poured onto the patterned azopolymer films. Since less viscous PDMS prepolymer can improve the penetration into the interstices of microlens arrays, we prepared the prepolymer having higher content of 20 wt% curing agent than conventional content of 10 wt%, and then de-aerated in vacuum to remove the remaining bubbles in prepolymer. The cast prepolymer was cured at 70 °C oven for three hours. The PDMS molds, released from patterned azopolymer films, were studied by AFM. Figure 5 shows the topography of the obtained PDMS molds having tetragonal and hexagonal holes arrays. Although there is little degradation of holes compared to the pristine microlens arrays on azopolymer films (less than 5 nm), PDMS molds with excellent fidelity were fabricated. This means that PDMS prepolymer efficiently penetrates into interstices of the patterned azopolymer films as we expected. It is also important to point out that these PDMS molds can be used, repeatedly.
The resulting PDMS molds are used to imprint close-packed microlens arrays onto liquid precursor (NOA 65, Norland Inc.) films that can be photocured by ultraviolet (UV), according to Fig. 4. The liquid precursor films were prepared by spin-coating onto pre-cleaned glass slides, and PDMS molds were subsequently placed against these films. In this case, downward pressing of the molds is necessary to facilitate the penetration of liquid precursors and displace the air bubbles. The films were then exposed to UV for 30 min, and PDMS molds were released from photocured microlens arrays. Fabricated microlens arrays are investigated by AFM and scanning electron microscope (SEM) study, confirming that highly-ordered and dense microlens arrays were successfully fabricated with small roughness, as shown in Fig. 6 and Fig. 7, respectively.
The average modulation heights of the two types of microlens arrays in 1st grating vectors and 2nd grating vectors were 290 nm and 325 nm, respectively. The contraction ratio was almost maintained at the level for the pristine microlens arrays (less than 4.4 %). This implies that both microlens arrays were replicated with excellent resolution and fidelity, because PDMS prepolymer and photopolymer (NOA 65) efficiently filled the azopolymer template and PDMS molds, respectively. This soft imprint process was repeated several times using fabricated PDMS molds, and same modulation heights of both microlens arrays could be generated reproducibly. However, it should be considered that the lifetime of PDMS molds is limited, because PDMS molds become brittle with repeated exposure to UV. Figure 8 shows the section of scan data of both microlens arrays along the vertical direction of Fig. 6.
Finally, we characterized the optical properties of the microlens arrays. The focal lengths for tetragonal and hexagonal microlens arrays were measured to be 13.9 μm and 14.8 μm, respectively. We also checked the focal properties by using brightfield optical microscopy (OM). The microlens arrays were positioned on the sample stage of an OM, and white light was projected on them. Figure 9 displays the OM images of both microlens arrays that were taken at different focal plane. These OM images also confirm that close-packed microlens arrays with very few defects are successfully fabricated over the large area. Figure 9(a) is OM images of tetragonal and hexagonal microlens arrays at the focal plane of microlens arrays. These clearly show that the regions of microlens are bright color, because light is focused by close-packed microlens arrays. Figure 9(b) shows OM images of tetragonal and hexagonal microlens arrays at out of focal plane. In this case, light was scattered by the microlens arrays, and thereby regions of microlens turn to darker than in the focal region. The focal spot distribution was also clearly revealed, as shown in Fig. 10.
In summary, we have proposed and investigated the new method by which pristine microlens arrays, to be replicated, were developed easily by successive holographic inscription on azopolymer films. This method provides advantages such as easy and rapid fabrication without organic solvent, precise control of the geometry of microlens arrays, and ability to form close-packed structures with very few defects. To obtain symmetrical structure, exposure dose difference between 1st inscription stage and 2nd inscription stage is found to be a critical parameter. By using patterned azopolymer film as pristine template, highly ordered and dense microlens arrays with tetragonal and hexagonal structures are transferred to the photopolymer (NOA 65) successfully. Additionally, it is expected that the fabrication of close-packed submicron-lens arrays will be possible by simply adjusting incident angle of writing beams. We believe that this approach makes the fabrication of close-packed microlens arrays easier and opens new path towards the fabrication of more sophisticated type of microlens arrays.
This research was supported by a grant (code#: M105KO010027-07K1501-02710) from ‘Center for Nanostructured Materials Technology’ under ‘21st Century Frontier R&D Programs’ of the Ministry of Science and Technology, Korea.
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