We present one-step method of fabricating hierarchical multiscale grating patterns by using holographic lithography on azobenzene thin films. In this study, we investigate the growth behavior of surface relief gratings in terms of surface morphology change regarding various optical conditions of different fringe visibility, exposure dose and polarization modes of the light interference pattern. The results reveal that different-sized diffractive gratings could be fabricated orthogonally at the same time. We also explain that these orthogonal gratings were developed through the different light-induced deformation mechanism.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Light-driven structuring has a great deal of advantages of its simple and accurate fabrication of micro/nano patterns with different shapes, pitch and aspect ratio. Recently, the photofluidization of azopolymers has been extensively studied because anisotropic movements of azo materials along the light polarization direction provided a new route to give unusual nano-sized patterns without an expensive photomask [1–7]. From fundamental studies for understanding the photofluidization phenomena of azopolymers, the most critical key factors influencing photofluidization were found to be as follows: (1) optical experimental conditions (light wavelength, polarization direction of light, beam intensity, fringe visibility, light incoming angle, and exposure dose), (2) chemical structures and physicochemical properties of azopolymers (molecular structure and doping ratio of azo moiety, glass transition temperature, molecular architecture and molecular weight of polymer backbone), (3) molecular interaction of azopolymers (hydrogen bonding between adjacent azopolymers). As a rule of thumb, the photofluidization is explained that a repeated change in chemical structure of azobenzene group, known as cis-trans isomerization, upon light exposure leads to continuous volumetric change (0.5 Å of cis- and 1.5 Å of trans-form), which results in the photomechanical effect [8–10]. Unlike conventional polymers, azopolymers exhibit photoinduced softening under its glass transition temperature (Tg) that enables fluidized azopolymers to flow along the light polarization direction, and eventually, azo-moieties are aggregated and form linear or dome-like structures [11, 12].
A complete explanation regarding the fundamental mechanism of the aforementioned photofluidization of azopolymers has not been fully discovered yet. However, several significant light-induced fluidic properties of azopolymers had been exploited through a series of well-designed experiments. For instance, it was directly observed that azopolymers exhibited transition of mechanical property upon light exposure from elastic solid to viscous fluidic behavior with nano-indentation study, which was represented by the characteristic hysteresis of viscoelastic materials in load-displacement curves [13, 14]. In addition, anisotropic nanomovement of azopolymers, which is the most important feature that different from the conventional thermal-induced fluidization, was demonstrated by irradiating the polarized light on scratched azobenzene thin film in a manner of two perpendicular canals and observing the change in surface morphology as a function of irradiation time with atomic force microscopy (AFM). This simple but logical experiment offered the inevitable evidence about the directional fluidity of azopolymers, showing that the selective filling of scratched canal occurs along the direction of light polarization . Furthermore, from the previous studies, the photofluidized-azopolymers spontaneously form nanodomes by creeping and aggregation, and then these aggregated domes linked with adjacent ones, structuring quasiperiodic willow-like nanostructures and randomly arrayed hexagons when exposed to linear and circular polarized light, respectively [15–20]. In addition, it revealed that the average periodicity, usually a few hundreds of nanometer, of willow-like structure strongly depends on both wavelength of incident light and molecular weight of azopolymers [15, 21].
As another intriguing light-induced structuring of azopolymers, there has been much effort to control the grating formation, which enables to precisely transfer the light interference pattern (LIP) on azobenzene thin film. Based on the continuous volumetric change of azopolymers by photo-isomerization, mass transport of azopolymers occurs from bright region toward the dark side of light intensity gradient [13, 22]. Therefore, two-beam coupled holographic lithography could generate linearly developed one-dimensional surface relief gratings (SRGs) on the azopolymer film in the form of sinusoidal shape with the same periodicity and k-vector of LIP [2, 23]. Furthermore, aspect ratio and duty cycle of SRG patterns could be manipulated easily by varying intensity, polarization of light exposure, and thickness of azopolymer film. Recently, multiplexing the LIP exposure on azobenzene thin film has been reported to construct various diffraction gratings with different morphologies including linear, cubic, hexagonal and quasiperiodic patterns [24–26]. Meanwhile, unusual morphologies of SRGs had been observed where unexpected nanostructures were expressed on the aforementioned micro-sized linear gratings [21, 27]. It seems that the nanostructures were regularly formed with averaged periodicity of hundreds of nanometers. Furthermore, the nanostructures were oriented orthogonal to SRG when the polarization direction of LIP is perpendicular to k-vector (i.e. LIP of s-polarized two beams). These hierarchically structured multiscale patterns were also observed with various polarization modes of ± 45° and orthogonal circular beams . As an attempt to utilize these unusual nanogratings, two-step holographic inscription of azopolymers with different light exposure conditions was studied as follows . Firstly, single beam irradiation with circular polarization constructs hexagonal arrays of nanodomes in a manner of random distribution. Secondly, a subsequent exposure of LIP inscribes micro-sized one-dimensional gratings on the nanodomes-expressed surface of azopolymers, resulting in complex-periodic gratings with considerable roughness along vertical as well as horizontal direction.
Here, we study holographic lithography for high-throughput and scalable optical grating with hierarchical micro/nano structures through a one-step process. This single exposure lithography technique is based on the in situ construction of SRGs and spontaneously formed nanostructures, induced by different mass transport phenomena of azopolymers. First, micro-sized SRGs are developed with respect to LIP. Second, nanodome structures are spontaneously generated by uniform light intensity along the perpendicular direction of k-vector. As a result, various morphologies of hierarchical structures with controlled duty cycle and shapes could be fabricated in large scale through one-step process.
2. Experimental details
2.1 Preparation of azobenzene thin film
Poly (Disperse Red 1 methacrylate) (PDR1) was purchased from Sigma-Aldrich and used as photosensitive azopolymer. The chemical structure and UV-Visible absorption spectra (measured by UV-Vis spectrometer, UV-1800 from SHIMADZU) of PDR1 were reported in our previous study . The PDR1 powder was dissolved in chloroform by 3 wt% with vigorous stirring, and then the PDR1 solution was filtered by a PTFE filter with a 0.45 μm pore size to remove undissolved particles. The glass substrates for spin coating was prepared by sonication in acetone and IPA for 20 and 10 minutes, respectively. Ultraviolet/Ozone treatment was applied on pre-cleaned glass substrates for good wettability before spin coating. The droplet of the PDR1 solution was cast on the glass substrate, and it was spin coated at 1000 rpm for 30 seconds. The residual solvents in the PDR1 thin film were removed by thermal annealing at 95 °C for 24 hours. The smooth surface of azopolymer coating was achieved without any defects. The thickness of the azobenzene thin film was 320 nm (measured by alpha-step, alpha-step IQ from KLA Tencor).
2.2 Optical apparatus for two-beam coupled holographic lithography
Construction of the surface relief grating was performed through a LIP assisted photo-fabrication process by two-beam coupled holographic lithography. A diode-pumped solid-state laser with a wavelength of 532 nm was used as a light source and split into two beams with equal intensity of 33 mW/cm2 (measured by photodetector, OP-2 VIS from COHERENT) as shown in Fig. 1. The incident angle of the laser beam was 12.2°, forming a grating period of 1260 nm as calculated from Bragg’s law as shown in Fig. 1(b). By rotating the first HW after the light source, the light intensity of transmitted and reflected beam through PBS was precisely controlled, and therefore fringe visibility (see section3.1) of LIP could be controlled from 0 to 1.
2.3 Characterization of azopolymer surface
The resultant surface topology of azopolymer was characterized by atomic force microscope (AFM) (XE-7 from Park Systems) with silicon tip (PPP-NCHR, non-contact mode). To measure water contact angle (CA) and optical property of photo-inscribed azopolymer surface, surface patterns of azopolymer were transferred to the environmentally robust material by using the nano-imprinting technique. To do this, polydimethylsiloxane (PDMS) elastomer resin (Silgard 184, Dow Corning) was prepared by mixing resin and curing agent with a weight ratio of 10:1. Well-mixed PDMS resin then poured onto azopolymer surface. Undesired air bubbles in the resin were deaerated by using vacuum oven. The clear resin was thermally cured at 70 °C for 6 hours. Cured PDMS replica was exfoliated carefully from azopolymers and fully crosslinked at 100 °C for 30 minutes. The patterns of PDMS replica was transferred to the optically transparent UV-curable resin (NOA 65, Norland) as follows. NOA was dropped on the pre-cleaned glass substrate and then spin-coated at 1500 rpm for 15 seconds. PDMS replica put on the NOA coated glass with the vertical pressure of 1kgf, and residual air bubbles between replica and NOA were removed by using vacuum oven. The UV light was irradiated to NOA for 12 minutes to be fully cured by the photo-crosslinking reaction, followed by carefully peeling off the PDMS replica molds. Contact angle analyzer (SDS-TEZD from FEMTOFAB) was used for water contact angle analysis, and 10 μl of water droplet was formed for each measurement. Transmittance and reflectance of azopolymer patterns were also measured by using UV-Vis spectrometer (UV-1800 from SHIMADZU). The broadband light source (300-2600 nm, SLS201/M from THORLABS) and photodetector (OP-2 VIS, COHERENT) were used for measurement of viewing angle. By rotating either the light source or photodetector, the viewing angle was estimated with respect to light extraction and light acceptance.
3. Results and Discussion
3.1 Various optical condition for fabricating hierarchical micro/nano structures
Conventional SRG texturing process is performed through two-beam coupling optical apparatus of equal light intensity to achieve a high contrast of LIP. In general, the contrast of LIP is defined as fringe visibility (υ), shown in Eq. (1).Eq. (2).
Meanwhile, there are several works that are reporting unusual SRG structures with not only micro-sized SRG but also unexpected nano-sized structures, simultaneously. These orthogonally fabricated nanogratings, however, were rarely studied. We speculate that these unexpected orthogonal gratings are originated from anisotropic photofluidized mass flow according to following reason: First, the period of nanogratings has similar periodicity with spontaneously formed nanodomes which constructed by single beam irradiation. Second, single beam induced nanodomes oriented perpendicular to the polarization direction of incident light through anisotropic creeping and aggregation. In the case of S:S polarization mode, for example, SRG structures with unexpected orthogonal nanogratings could be explained by aforementioned anisotropic photofluidization of azopolymers with uniform light intensity along the perpendicular direction of k-vector (i.e. y-axis of Fig. 1(b)).
To demonstrate our hypothesis, we designed two different experiments as follows: First, various holographic inscriptions with different fringe visibilities were conducted on PDR1 film to investigate the light contrast effect of LIP on both micro-sized SRG and nanograting patterns along the x- and y-axis, respectively. Second, growth behaviors of both gratings were monitored as a function of exposure dose with a fixed fringe visibility. More in detail, as shown in Table 1, we designed different light contrast conditions of fv1-fv5 with fringe visibility from 0.999 to 0, respectively. It is expected that the driving force of fabricating micro-sized SRG decrease whereas the polarization-directed mass flow of forming orthogonal nanograting increases as fringe visibility changes from fv1 to fv5.
3.2 Effect of fringe visibility on micro-sized SRGs and orthogonal nanogratings
We investigated the effect of fringe visibility on constructing hierarchical micro/nano multiscale structures. More in detail, the polarization direction of two beams were controlled to be perpendicular to k-vector of LIP (i.e. S:S polarization modes denoted in Fig. 1(b)). Meanwhile, the ratio of light intensities of two beams was controlled by rotating half-wave plate with the same total beam power and thereby, fringe visibility of LIP was changed as shown in Table 1. All the samples were exposed for 45 minutes. Figures 2(a)-2(e) and Figs. 2(f)-2(j) present 2D and 3D AFM images of azopolymer samples with different fringe visibility conditions from fv1 to fv5, respectively. Figure 2(a) shows regularly oriented dome- and line-like nanogratings with a periodicity of 430 nm along the vertical direction, while SRGs were observed across the y-axis with a grating period of 1.26 μm. According to growth nature of SRG through LIP-triggered mass transfer, the position of highly modulated nanodomes is supposed to be super-trough of LIP as shown in Fig. 2(k). On the other hand, the nanogratings along the vertical direction are considered to be originated from photofluidization-induced self-assembled dome structures, which could be formed spontaneously without artificially controlled light intensity profile.
It is interesting that oblate-shaped nanogratings are developed at super-trough of LIP in the case of fv2 as shown in Fig. 2(b). Comparing optical conditions of fv1 and fv2, the light intensity of fv2 at super-trough of LIP (i.e. Imin) increased 100 times higher than that of fv1. It means that aforementioned photofluidization-induced self-assembling is also highly intensified. Meanwhile, both fv1 and fv2 has the almost similar fringe visibility that influences driving force of forming the SRG structures. Furthermore, increased photo-softening effect at super-trough of LIP provides easier mass transfer of azobenzene across the x-axis. As a result, hierarchical grating patterns were regularly fabricated across the x- as well as y-axis with different grating periods.
Meanwhile, it is noted that nanogratings were developed further in the case of fv3 whereas SRG patterns were deteriorated with significantly decreased modulation depth as shown in Figs. 2(c) and 2(m), respectively. More in detail, dome-like nanogratings formed a linkage with adjacent nanodomes. It is interpreted that photo-assembling effect by increased Imin was more dominant behavior than light-contrast based SRG construction. Furthermore, aforementioned partially connected nanogratings were fully linked each other with further decrease in fringe visibility (fv4) and formed continuous linear gratings, while vague traces of SRG patterns were only observed as shown in Fig. 2(d). In Fig. 2(e), in the case of using single beam irradiation (fv5), spontaneously formed linear nanogratings were only found without any SRG patterns. We also evaluated modulation of grating depth quantitatively across the x- and y-axis, respectively, as schematically shown in Fig. 2(l). We defined three different grating types of (1) surface relief grating, (2, 3) nanogratings at super-trough and crest of LIP, respectively. The results of evaluated grating depth are presented in Fig. 2(m). It shows the highest modulation depth at fv2 with the help of synergistic driving forces of forming SRG and nanogratings.
3.3 Growth behavior of hierarchical surface relief gratings
To investigate the growth behavior of the hierarchical SRG, we examined morphological changes of the azo polymer surface as a function of irradiation time with fixed fringe visibility of fv1. Figures 3(a)-3(e) and Figs. 3(f)-3(j) show 2D and 3D AFM images of the sample for 30- to 360-minute exposure time, respectively. The fast Fourier transform (FFT) results of the corresponding AFM images are also presented in Figs. 3(k)-3(o). It is interesting to note that the development of SRG and nanogratings in super-crest and super-trough is observed as a different way of morphological change with respect to lattice growth rate and irradiation time. At the initial stage of the 30 minutes irradiation period, semi-oblate-like nanogratings were arranged along the y-axis, but the conventional one-dimensional linear SRG pattern was not clearly observed. The semi-oblate nanogratings are due to the optical anisotropic agglomerates of azobenzene as it was already described that the nanogratings developed spontaneously with uniform regularity along the polarization direction. As can be seen in Figs. 3(r), the grating periods of the SRG and nanograting patterns were 1.26 μm and 0.43 μm, respectively.
With further light irradiation, two differently shaped nanostructures were fabricated with the same grating period of SRG along the y-axis as shown in Fig. 3(b). From the nature of SRG formation, highly modulated gratings of semi-oblate shape are considered newly sprouted nanodome structures that positioned at the super-trough of LIP. On the other hand, willow-like linear nanogratings were expressed at super-crest of LIP. It is interesting that two distinguished orthogonal nanogratings arrayed alternately along the k-vector, which conjectured as the result of minimizing structural interfacial energy between adjacent nanodomes. We believe that spontaneously formed nanodomes at super-crest of LIP change to willow-like linear shape through the elongation itself by LIP-induced mass transfer across the x-axis. Vice versa, nanodomes at super-trough of LIP show oblate-like structures because of compressive force due to polarization-dependent anisotropic movements of azopolymer.
Figure 3(c) presents the most impressive morphology with several notable features as followings: First, SRG patterns developed entirely compared to disconnected nanograting textures of Fig. 3(b). Second, nanogratings at super-trough of LIP grew as the arch-like structures along the central position of one-dimensional linear SRG gratings. As a result, well-ordered hierarchical spine-like grating patterns could be fabricated with orthogonal grating vectors. Meanwhile, as shown in Fig. 3(d), the structures above changed to Chinese abacus-like shape because photofluidized anisotropic aggregation of azobenzene kept broadening nanogratings across the y-axis. Moreover, it was observed that oblate-like nanogratings formed between abacus-like grating patterns in Fig. 3(e). It is interpreted that mass transfer across the x-axis by light intensity gradient was almost saturated whereas spontaneously created nanodomes by photofluidized anisotropic movements of azobenzene kept growing.
To confirm this, we evaluated averaged modulation depth of three different gratings with respect to exposure time as shown in Fig. 3(p). It presents that SRG structures show saturated exponential growth behavior whereas the modulation of nanogratings at super-crest of LIP monotonically increase. This difference in grating growth behavior of SRG and nanogratings could explain the final morphology of Fig. 3(e). We believe that photo-soften azobenzenes at super-crest of LIP were mass transferred to super-trough side by light intensity gradient. As a result, SRG and nanogratings at super-trough grew faster than gratings in the bright field. However, nanogratings at super-crest developed further even after optical modulation of SRGs saturated.
Figure 3(q) presents the distribution of modulation depth of overall gratings as a function of exposure time, and it corresponds to the morphological transition of azobenzene surface directly. The inset graph shows slightly separated distribution peak at 30 minutes of irradiation time, indicating different nanogratings formed at super-crest and super-trough of LIP as shown in Fig. 3(a), respectively. Moreover, distribution peak of SRG was shown after 45 minutes of irradiation time. With further exposure, the average modulation depth of each grating increased continuously, and finally, all the distribution peaks were merged into one broad peak. We also show power spectral density (PSD) of azobenzene surface patterns in Fig. 3(r). It shows three different grating periods of 0.43, 0.72, and 1.26 μm. The periodicity of 0.43 μm and 1.26 μm indicates the interval length of nanogratings and SRGs patterns along the vertical and horizontal direction, respectively. The other peak at 0.72 μm is conjectured as the diagonal interval length of adjacent nanogratings which constructing the repetitive hexagonal array.
From the observation of surface morphology change of azobenzene surface, the growth nature of hierarchical gratings could be summarized with three steps: 1) spontaneously formation of nanodomes with energy minimized structural configuration along polarization direction, 2) elongation of nanodomes at super-crest of LIP by mass transfer along k-vector, 3) broadening nanodomes along the y-axis and formation of abacus-shaped nanogratings. It is interesting that a few micrometer-sized irregular waviness is observed across the y-aixs as shown in Fig. 4(a) that presenting large-scale surface morphology of 45 minutes sample. The magnified view of Fig. 4(b) presents kinks along well-developed nanogratings. We believe that structural configuration between adjacent nanodomes significantly influences the growth of hierarchical gratings. Comparing Figs. 4(c) and 4(d), nanograting patterns could be clearly inscribed when photo-assembling of nanodomes formed in hexagonal arrays. Moreover, as shown in Fig. 4(e), relatively uniform patterns of hierarchical gratings were developed with further photo-assembling effect through further light irradiation.
3.4 Anisotropic surface property of hierarchical SRG patterns
We also investigated the surface property of optically patterned hierarchical gratings by measuring water contact angle (CA). To do this, we transferred the textures of azobenzene surface to the environmentally robust materials through nanoimprint technique as explained in the experimental section. We measured water contact angle in different observation directions of CA∥ and CA⊥, respectively, as illustrated in Fig. 5(A). Figures 5(b) and 5(c) present different models of water contact angle corresponding to CA∥ and CA⊥. We believe that micro-scale SRGs exhibit Wenzel-state water droplet where the water fully contacts with surface patterns of azobenzene thin film. Cassie-Baxter model, on the other hand, would be more suitable for nano-sized surface structures where air pocket forms at the interface between water droplet and nanostructures .
As shown in Fig. 5(d), both CAs steadily increased up to 60 minutes of irradiation time and then, CA∥ decreased slightly with further exposure whereas CA⊥ continued to increase. It explains that the surface roughness between liquid (water) and solid (azobenzene) enhanced the hydrophobicity of wetting property. Moreover, it is noteworthy that anisotropic CA was clearly observed at light irradiation of 60 minutes. It means that the hierarchical SRG patterns influenced the liquid/solid interfacial energy differently along the x- and y-axis. First, the interfacial energy across the x-axis (i.e. CA∥) is affected by micro-scale SRG patterns that act as anti-water overflow according to Wenzel model, resulting in improved hydrophobic property. Meanwhile, the water contact angle along the y-axis (i.e. CA⊥) is thought to be governed by Cassie-Baxter model where the liquid does not penetrate into the grooves. As shown in Fig. 3(p), the higher modulation depth of micro-scale grating across the x-axis than that of nanogratings along the y-axis up to irradiation time of 60 minutes explains the result of anisotropic water contact angle. The representative picture of anisotropic water contact angle is shown in Fig. 5(e). Furthermore, continuous development of nanogratings at super-crest of LIP resulted in higher CA⊥ than CA∥ at irradiation time of 360 minutes. Detailed numerical data of CA, surface roughness and surface energy of samples are summarized in Table 2.
3.5 Optical properties of hierarchical diffractive SRG patterns
To evaluate light diffraction property of fabricated grating patterns, we measured light extraction as well as acceptance performance in terms of viewing angle as shown in Fig. 6. To do this, optical transmittance and reflectance of imprinted grating patterns were measured with UV-Vis spectrometer along viewing angle as shown in Figs. 6(a) and 6(c). Figure 6(b) shows wider viewing angle of light extraction and the highly modulated optical grating sample (360 min) resulted in 4.1% increment of total light out-coupling efficiency. We believe that hierarchical multiscale gratings diffracted incident light efficiently and thus, total internal reflection between imprinted NOA and air was suppressed. Similarly, light acceptance efficiency increased 9.5% compared to the bare sample with a flat surface as shown in Fig. 6(d).
In conclusion, we studied the one-step method of fabricating hierarchical multiscale surface relief gratings. We investigated the effect of fringe visibility of LIP on the growth behavior of hierarchical gratings in terms of surface morphology, modulation depth and distribution of gratings with respect to exposure dose. Our study revealed that different-sized gratings of micro/sub-micro were developed orthogonally at the same time while minimizing structural configuration energy. The nanogratings spontaneously formed regularly along light polarization direction regardless light intensity gradient whereas micro-sized surface relief grating patterns were fabricated along the k-vector LIP. These orthogonal Bragg gratings diffracted incident light in vertical as well as horizontal direction with different diffraction angle, resulting in improved light extraction and acceptance efficiency. Furthermore, it showed anisotropic hydrophobicity that could provide a new route of guiding water flow based on different wettability.
KITECH (Korea Institute of Industrial Technology (project number EO170038)); MOTIE (Ministry of Trade, Industry & Energy (project number 10051918, 10077471, 10062356)) and KDRC (Korea Display Research Consortium) support program for the development of future devices technology for display industry.
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