1. Introduction

Recently, there has been much effort on the soft lithography to fabricate and transfer micro- and nano-patterns for a variety of applications such as optoelectronics, fluidic mechanics, and biologics because it provides simple, fast, and cost-effective processes as well as high throughputs [1,2]. In most of reports on the soft lithography, conformable and elastomeric stamps using polydimethylsiloxane (PDMS) which has various advantages including low free surface energy, flexibility, transparency, and hardness [1–5] are required to transfer patterns precisely on secondary substrates. The nanopatterns of silicon (Si) templates prepared by conventional fabrication processes with photo- or e-beam lithography and dry etching are negatively transferred to the PDMS stamps. The replica molding, one of soft lithographic techniques, is more generally used for the nanopattern transferring due to its good fidelity [4,5]. The replicas can be turned out with various materials such as ultraviolet (UV) and thermally curable polymers or epoxies [3].

Meanwhile, sapphire has been widely used in optical and optoelectronic applications such as high-durability windows, insulating substrates, and optical lenses due to its good thermal/chemical stability and mechanical durability [6,7]. However, the transmission of sapphire (T~85%) is relatively lower compared to other transparent substrates such as glass and quartz (T~92%) because of its higher refractive index of ~1.76 at visible wavelengths [8]. Over the past years, the subwavelength gratings (SWGs) have been exploited as an alternative of conventional thin-film antireflection coatings (ARCs) [9,10]. The SWGs effectively suppress the surface Fresnel reflection in wide ranges of wavelengths and incident angles due to a linearly and continuously gradient effective refractive index profile between air and the material, so called “moth eye effect” [9]. And thus, for the transparent materials, the transmission can be enhanced by employing antireflective SWGs. Furthermore, the water contact angle can be varied with roughness and a transition between hydrophilicity and hydrophobicity is observed [11]. Therefore, it is very meaningful to analyze the optical properties and wettability by applying the SWGs onto SU8 film/sapphires. In this work, we investigated the total and diffuse transmittance properties and wetting behavior of sapphires with SU8 polymer surface structures consisting of conical SWGs by the soft lithography method. For comparison with the experimentally measured optical results, the theoretical calculations were also performed using the rigorous coupled-wave analysis (RCWA) method.

2. Experimental and simulation modeling details

Figure 1 shows the schematic diagram for the fabrication procedure of SWGs on SU8 film/sapphire substrate using the soft lithography method. The two-dimensional (2D) hexagonal patterned SWGs with a conical shape on Si templates were prepared by laser interference lithography and dry etching process as reported in our previous work [10]. In order to transfer subwavelength patterns without deformation and distortion, a hard PMDS (h-PDMS) solution, which has higher compression modulus (9.0 N/mm²) than that (2.0 N/mm²) of a soft PDMS (s-PDMS, Sylgard 184) [4,5], was spin-coated onto the Si templates, and then cured at a temperature of 75° for 25 min. After that, the s-PDMS solution which can reduce pressure and enable conformal contact with the substrate was poured on the h-PDMS/Si template and subsequently cured at a temperature of 75° for 2 h. The PDMS stamp consisted of h-PDMS/s-PDMS was peeled off from the Si templates, thus creating the negative patterns of SWGs. For the double side-polished sapphire substrates with a size of 1.5 × 1.5 cm², the samples were ultrasonically cleaned in acetone, methanol, and de-ionized water for 10 min, respectively. The SU8 (Micro Chem Corp.) negative photoresist, one of UV curable polymers, was spin-coated on the sapphire substrates and cured provisionally with pressing (3.0 kgf) the PDMS stamp at a temperature of 85 ºC. After the UV exposure for 20 min, the SWGs on SU8 film/sapphire substrates at one-side surface were formed by peeling off the PDMS stamp. By repeating the above process at the other surface of sapphire substrates, the SWGs on the both-side surfaces of SU8 film/sapphires were finally fabricated.

 

Fig. 1 Schematic diagram for the fabrication procedure of SWGs on SU8 film/sapphire substrate using the soft lithography method.

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The structural and morphological properties of the fabricated SWGs on SU8 film/sapphires were observed by using a scanning electron microscope (SEM; LEO SUPRA 55, Carl Zeiss). The total and diffuse transmittances were measured by using a UV-vis-NIR spectrophotometer (Cary 5000, Varian) with an integrating sphere at normal incidence. A contact angle measurement system (Phoenix-300, SEO Co., Ltd.) was used to analyze the surface wettability depending on surface morphology. For the theoretical optical analysis of SWGs on SU8 film/sapphires, the modeling and simulation were performed based on the RCWA method using a commercial software (DiffractMOD 3.1, Rsoft Design Group). It was assumed that the light enters the SWGs from air at normal incident angle (θ_i = 0°) and the thicknesses of sapphire substrate and SU8 film are set to be 400 and 5 μm, respectively. The shape of SWGs was represented by 3D equations with the Cartesian coordinate system (i.e., scalar-valued function of three variables, f(x,y,z)) as below [12]:

3. Results and discussion

Figure 2(a) shows the top-view and cross-sectional SEM images of the transferred SWGs on SU8 film/sapphires for different periods of 350, 500, and 600 nm. It can be observed that the cone-shaped SWGs with closely-packed periodic 2D hexagonal patterns were well formed on the SU8 polymer films without distinct deformation and distortion in appearance from the Si templates by the soft lithography method. From the measured SEM images, the average bottom diameter and height of the fabricated conical SWGs were estimated to be about 291 ± 14 and 193 ± 8 nm, 415 ± 22 and 231 ± 16 nm, and 491 ± 27 and 351 ± 18 nm for periods of 350, 500, and 600 nm, respectively. For all the samples, the ratio of the bottom diameter of SWGs to the period between SWGs (R_bdp) was approximately 0.82. The thickness of SU8 film coated on the sapphire substrate was about 5 μm. The photographs of a water droplet on the bare sapphire, flat SU8 film/sapphire, and transferred SWGs with periods of 350, 500, and 600 nm on SU8 film/sapphires are shown in Fig. 2(b). The water contact angles were measured and averaged at three different positions on the surface of samples. The bare sapphire and flat SU8 film/sapphire exhibited a hydrophilic surface, indicating the water contact angles (θ_c) of ~54 and 74°, respectively. On the contrary, for the samples with SWGs on SU8 films, higher θ_c values of ~101, 99, and 93° for periods of 350, 500, and 600 nm, respectively, were obtained compared to the flat SU8 film due to the enhanced surface roughness [9].

 

Fig. 2 (a) Top-view and cross-sectional SEM images of the transferred SWGs on SU8 film/sapphires for different periods of 350, 500, and 600 nm and (b) photographs of a water droplet on the bare sapphire, flat SU8 film/sapphire, and transferred SWGs with periods of 350, 500, and 600 nm on SU8 film/sapphires.

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Figure 3(a) shows the measured total transmittance spectra of bare sapphire, flat SU8 film/sapphire, and transferred SWGs on SU8 film/sapphires for different periods of 350, 500, and 600 nm at one-side and both-side surfaces. For the flat SU8 film coated on the sapphire substrate, the increased total transmittance was obtained compared to the bare sapphire over a wide wavelength region of 420-1000 nm, exhibiting an average total transmittance value (T_avg) of ~87.7% (T_avg~85.6% for the bare sapphire). This is attributed to the step gradient refractive index profile between air (n = 1) and the sapphire (n~1.76) via the SU8 film (n~1.59). For the samples with SWGs, the transmittance was further enhanced due to the more linearly graded effective refractive index distribution from air to the SU8 film via the SWGs. Particularly, for the period of 350 nm, the transmittance spectrum of the sample with SWGs at one-side surface was higher than those of the other samples at wavelengths of 450-1000 nm, indicating the T_avg value of ~89.1% in the wavelength region of 400-1000 nm (i.e., T_avg~86.7 and 84.9% for the samples with periods of 500 and 600 nm, respectively). In addition, the transmittance strongly depended on the period of SWGs on SU8 film/sapphires. The low transmittance region of < 85% was shifted towards the longer wavelengths and became broader with increasing the period of SWGs. This may be caused by the diffraction losses. When the light with a normal incident angle (θ_i = 0°) enters the grating structure with a period of Λ, the angle of the reflected diffraction waves, θ_r,m, in the m-th diffraction order are given by a well-known grating equation [9]: sinθ_r,m = mλ/Λn, where n is the refractive index of incident medium and λ is the incident wavelength of light. From this equation, the zeroth order diffractive wave is only allowed and the others are disappeared when a period of SWGs becomes much less than the wavelength of incident light. For this reason, as the period of SWGs is decreased, the high transmittance region of > 91% at one-side surface is extended to the shorter wavelengths, that is, over a wide wavelength range. The weak oscillations in the spectra of the SU8 film coated samples including flat and SWGs were observed due to the constructive and destructive interferences of incident light owing to the difference of refractive indices for constituent media (i.e., air, SU8 film, and sapphire). It is clear that the SWGs considerably reduced the reflection losses at the both-side surfaces of SU8 film/sapphire. Especially, for the SWGs with the period of 350 nm at both-side surfaces, a higher T_avg value of ~94.3% was obtained over a wide wavelength region of 400-1000 nm. This value is much higher than that (i.e., T_avg~85.6%) of the bare sapphire.

 

Fig. 3 Measured (a) total and (b) diffuse transmittance spectra of bare sapphire, flat SU8 film/sapphire, and transferred SWGs on SU8 film/sapphires for different periods of 350, 500, and 600 nm at one-side and both-side surfaces. The haze ratio values at λ = 525 and 635 nm of the corresponding samples and photographs of the diffracted light patterns obtained from the light penetration through the sample with the SWGs on the SU8 film/sapphire at the period of 600 nm for λ = 525 and 635 nm are shown in the left and right insets of (b), respectively.

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Figure 3(b) shows the measured diffuse transmittance spectra of the corresponding samples. For the bare sapphire and flat SU8 film/sapphire, there are almost no diffused lights in the wavelength region of 400-1000 nm. On the other hand, for the samples with SWGs, as the period was increased, the diffuse transmittance was increased at visible wavelengths of 400-700 nm. This is due to the higher orders of diffracted waves in the transmission of SWGs with larger periods, as mentioned above. Particularly, for the large period of 600 nm, the SWGs on both sides of the SU8 film/sapphire exhibited a strong diffused transmittance spectrum. This can be confirmed by the right inset of Fig. 3(b). The photographs show the diffracted light patterns obtained from the light penetration through the sample with the SWGs on the SU8 film/sapphire at the period of 600 nm at both wavelengths (λ) of 532 and 625 nm, respectively. The conventional green and red lasers were used as light sources. It can be observed that the transparent substrates with SWGs at larger periods than the incident light wavelengths can scatter the light due to the higher orders of diffractive waves. The haze ratio (H), which is defined by the ratio of the diffuse (T_diffuse) transmission to the total (T_total) transmission, i.e., H = T_diffuse/T_total, is often used to characterize the light scattering properties of samples. For all the samples, at λ = 525 and 635 nm, the H values were summarized in the left inset of Fig. 3(b). As expected, the bare sapphire and flat SU8 film/sapphire exhibited relatively low H values. However, for the samples with SWGs, the H values were increased with increasing the period. For the period of 600 nm, the highest H values of 22.2 and 11.3% and 47.2 and 27.6% at λ = 525 and 635 nm for one- and both-side surfaces were obtained, respectively. These values were much higher than those (i.e., H: 5.2 and 4.9% at λ = 525 and 635 nm) of the bare sapphire.

Figure 4 shows the contour plots of variations of the calculated (i) total and (ii) diffuse transmittance spectra of the SWGs on SU8 film/sapphire at one- and both-side surfaces for different (a) periods (height: 300 nm) and (b) heights (period: 500 nm) of SWGs. The 3D simulation model used in this calculation is also shown in the inset of Fig. 4(a). In these simulations, the R_bdp and O_T of SWGs were estimated to be 0.82 and 1, respectively. As shown in (i) of Fig. 4(a), the total transmittance region of < 90% is shifted towards the longer wavelengths with increasing the period of SWGs due to the diffraction losses caused by the higher order waves in transmitted lights. Clearly, the both-side SWGs have transmittance values (i.e., red part) higher than 90% in the wider wavelength region compared to one with SWGs at one-side surface. Also, from the calculated diffuse transmittance results in (ii) of Fig. 4(a), the both-side SWGs exhibit a broader scattered light region than that of SWGs at one-side surface over a wavelength range of 400-700 nm. This may be ascribed to the more enhanced scattering lights due to the other side SWGs. The transmission is also dependent on the height of SWGs. As shown in Fig. 4(b), at heights of < 200 nm, the total transmittance values of < 90% at wavelengths of 600-1000 nm, and the diffuse transmittance values of > 20% at wavelengths of 400-450 nm are observed, respectively. Thus, to achieve the highly-transparent characteristics, the SWGs with heights larger than 200 nm should be formed on the surface of transparent medium [13]. Furthermore, in the short UV wavelength region of 400-500 nm, the lower total transmittance region (i.e., blue part) of < 80% becomes slightly broader due to the increased light absorption caused by the thicker SU8 film [14]. Similarly, the sample with SWGs at both-side surfaces exhibits the improved total and diffuse transmittance for the effect of height of SWGs. Overall, these simulation values roughly show a similar trend to the measured spectra though there exists a discrepancy at some wavelengths between the measured and calculated results due to the difficulty in matching exactly the geometric simulation model to the actual fabricated samples as well as the refractive index mismatch of the sapphire and SU8 film used in this experiment and calculation. Consequently, the incorporation of antireflective subwavelength patterned SU8 polymer surface structures into sapphires significantly contributes to the enhancement of their transmittance characteristics over a broad wavelength region.

 

Fig. 4 Contour plots of variations of the calculated (i) total and (ii) diffuse transmittance spectra of the SWGs on SU8 film/sapphire at one- and both-side surfaces for different (a) periods (height: 300 nm) and (b) heights (period: 500 nm) of SWGs.

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4. Conclusion

The conical SWGs with 2D periodic hexagonal patterns were fabricated on the UV curable SU8 polymer film/sapphire substrates using the Si templates with SWGs by the soft lithography technique. Their optical transmittance and wetting behaviors were investigated, together with a theoretical optical simulation using the RCWA method. The SU8 SWGs enhanced the total and diffuse transmittance properties, which are dependent on the period and height of SWGs, of the bare sapphire over a wide wavelength region of 400-1000 nm, indicating the large haze effect at visible wavelengths. Also, the SU8 SWGs exhibited a hydrophobic surface with water contact angles of > 90°. These results can give a better insight into the antireflective nanostructures, which can be simply and easily fabricated by the cost-effective soft lithography method, with broadband high transparency on sapphires for high-performance optical elements and optoelectronic devices.