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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 7 — Apr. 8, 2013
  • pp: 8450–8459
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A nanoscale conical polymethyl methacrylate (PMMA) sub-wavelength structure with a high aspect ratio realized by a stamping method

Dae-Seon Kim, Dong-Hyun Kim, and Jae-Hyung Jang  »View Author Affiliations


Optics Express, Vol. 21, Issue 7, pp. 8450-8459 (2013)
http://dx.doi.org/10.1364/OE.21.008450


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Abstract

A high aspect ratio conical sub-wavelength structure (SWS) was designed by using rigorous coupled-wave analysis (RCWA) method and was realized on polymethyl methacrylate (PMMA) film using a stamping technique. The silicon template containing a hexagonal array of conical holes with a period of 350 nm and an aspect ratio of 2.8 was fabricated by electron-beam (e-beam) lithography followed by a two-step etching process. The SWS with a high aspect ratio was easily transferred from the fabricated silicon template to PMMA film using the stamping method. The replicated PMMA SWS has an array of cones with nanoscale tips and an aspect ratio higher than 2.8. The average reflectance and transmittance of the PMMA film with the conical SWS in the wavelength ranging from 500 and 1500 nm was improved from 7.1 and 91.1% to 4.3 and 94.2%, respectively, as compared to flat PMMA film.

© 2013 OSA

1. Introduction

The reflection loss at the surface of a material is indispensable phenomena due to the mismatch of the refractive index between the material and the air. In order to reduce the surface reflection loss, sub-wavelength structures (SWSs) having a periodic nanostructure with a smaller period than the wavelength of the incident light have recently attracted research interest for their applications in optical devices [1

1. Y. F. Huang, S. Chattopadhyay, Y. J. Jen, C. Y. Peng, T. A. Liu, Y. K. Hsu, C. L. Pan, H. C. Lo, C. H. Hsu, Y. H. Chang, C. S. Lee, K. H. Chen, and L. C. Chen, “Improved broadband and quasi-omnidirectional anti-reflection properties with biomimetic silicon nanostructures,” Nat. Nanotechnol. 2(12), 770–774 (2007). [CrossRef] [PubMed]

4

4. K. C. Sahoo, Y. Li, and E. Y. Chang, “Shape effect of silicon nitride subwavelength structure on reflectance for silicon solar cells,” IEEE Trans. Electron. Dev. 57(10), 2427–2433 (2010). [CrossRef]

]. Especially for solar cells, a broadband low reflection property is required because the solar power spectrum is distributed throughout the UV-Vis-NIR wavelength range [3

3. M. Y. Chiu, C. H. Chang, M. A. Tsai, F. Y. Chang, and P. C. Yu, “Improved optical transmission and current matching of a triple-junction solar cell utilizing sub-wavelength structures,” Opt. Express 18(S3Suppl 3), A308–A313 (2010). [CrossRef] [PubMed]

,4

4. K. C. Sahoo, Y. Li, and E. Y. Chang, “Shape effect of silicon nitride subwavelength structure on reflectance for silicon solar cells,” IEEE Trans. Electron. Dev. 57(10), 2427–2433 (2010). [CrossRef]

]. SWSs with a periodic surface structures operating in the visible wavelength range require large-area nanoscale patterning technology. The initial formation of a nanoscale pattern can be conventionally achieved by various lithographic methods, such as electron-beam (e-beam) lithography [5

5. O. Dial, C. C. Cheng, and A. Scherer, “Fabrication of high-density nanostructures by electron beam lithography,” J. Vac. Sci. Technol. B 16(6), 3887–3890 (1998). [CrossRef]

], holographic lithography [6

6. Y. M. Song, S. Y. Bae, J. S. Yu, and Y. T. Lee, “Closely packed and aspect-ratio-controlled antireflection subwavelength gratings on GaAs using a lenslike shape transfer,” Opt. Lett. 34(11), 1702–1704 (2009). [CrossRef] [PubMed]

], nanoimprint lithography [7

7. Q. Chen, G. Hubbard, A. Shields, C. Liu, D. W. E. Allsopp, W. N. Wang, and S. Abbott, “Broadband moth-eye antireflection coatings fabricated by low-cost nanoimprinting,” Appl. Phys. Lett. 94(26), 263118 (2009). [CrossRef]

9

9. N. Koo, U. Plachetka, M. Otto, J. Bolten, J. H. Jeong, E. S. Lee, and H. Kurz, “The fabrication of a flexible mold for high resolution soft ultraviolet nanoimprint lithography,” Nanotechnology 19(22), 225304 (2008). [CrossRef] [PubMed]

], and colloidal lithography [10

10. D. S. Kim, M. S. Park, and J. H. Jang, “Fabrication of a cone-shaped subwavelength structures by utilizing a confined convective self-assembly technique and inductively-coupled-plasma reactive ion etching,” J. Vac. Sci. Technol. B 29(2), 020602 (2011). [CrossRef]

]. In spite of the excellent performance in the realization of nanoscale patterns, large-area SWS fabrication is still challenging because advanced etching processes as well as nanoscale lithography are required for the realization of an individual SWS with a desired sidewall profile.

Recently, stamping techniques have been proposed to fabricate SWSs because such techniques have the potential advantage of a low-cost manufacturing process. The templates containing the nanoscale array of holes were created on a silicon or glass substrate by a conventional lithographic method. Polymers such as polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), and polydimethylsiloxane (PDMS) were utilized as the injection materials due to their high transparency in the wavelengths of the visible and near-infrared range. After transferring the nanoscale patterns onto the polymer, the template can be reused many times. The replicated SWSs on polymer substrates can be mounted on the surfaces of optical devices for various applications [11

11. Y. Kanamori and K. Hane, “Broadband antireflection subwavelength gratings for polymethyl methacrylate fabricated with molding technique,” Opt. Rev. 9(5), 183–185 (2002). [CrossRef]

17

17. Y. Li, S. Minoru, and H. Kazuhiro, “Micro-optical components based on silicon mold technology,” Opt. Lasers Eng. 41(3), 545–552 (2004). [CrossRef]

]. A PMMA SWS with a period of 200 nm and an aspect ratio of 1 and a PDMS SWS with a period of 450 nm and an aspect ratio of 0.2 were fabricated by Kanamori et al. [11

11. Y. Kanamori and K. Hane, “Broadband antireflection subwavelength gratings for polymethyl methacrylate fabricated with molding technique,” Opt. Rev. 9(5), 183–185 (2002). [CrossRef]

] and Nam et al. [12

12. H. J. Nam, J. H. Kim, D. Y. Jung, J. B. Park, and H. S. Lee, “Two-dimensional nanopatterning by PDMS relief structures of polymeric colloidal crystals,” Appl. Surf. Sci. 254(16), 5134–5140 (2008). [CrossRef]

]. Chen et al. and Han et al. realized a solar cell integrated with a PMMA SWS (period: 700 nm and aspect ratio: 0.7) [13

13. J. Y. Chen and K. W. Sun, “Enhancement of the light conversion efficiency of silicon solar cells by using nanoimprint anti-reflection layer,” Sol. Energy Mater. Sol. Cells 94(3), 629–633 (2010). [CrossRef]

] and a PVC SWS (period: 270 nm, aspect ratio: 0.6) [14

14. K. S. Han, H. Lee, D. Kim, and H. Lee, “Fabrication of anti-reflection structure on protective layer of solar cells by hot-embossing method,” Sol. Energy Mater. Sol. Cells 93(8), 1214–1217 (2009). [CrossRef]

]. However, the performance enhancement of the optical devices was quite limited due to the small aspect ratio or the large period of the SWS. With small aspect ratio of nanostructures in SWS, the anti-reflecting properties will be limited to visible wavelength range only. For the successful application of optical devices operating in infrared as well as visible wavelength range, SWS with a period smaller than 400 nm and an aspect ratio higher than 2 are generally required. Ting et al. realized a high density disordered Ni-Co nanostructures and utilized it as a mother mold for hot-embossing method requiring pressured condition [15

15. C. J. Ting, M. C. Huang, H. Y. Tsai, C. P. Chou, and C. C. Fu, “Low cost fabrication of the large-area anti-reflection films from polymer by nanoimprint/hot-embossing technology,” Nanotechnology 19(20), 205301 (2008). [CrossRef] [PubMed]

,16

16. H. Y. Tsai and C. J. Ting, “Optical characteristics of moth-eye structures on poly(methyl methacrylate) and polycarbonate sheets fabricated by thermal nanoimprinting processes,” Jpn. J. Appl. Phys. 48(6), 06FH19 (2009). [CrossRef]

]. The replicated PMMA SWS exhibited disordered nanotips with diameters of 100~150 nm and height of 480~600 nm. It exhibited excellent anti-glaring characteristics [15

15. C. J. Ting, M. C. Huang, H. Y. Tsai, C. P. Chou, and C. C. Fu, “Low cost fabrication of the large-area anti-reflection films from polymer by nanoimprint/hot-embossing technology,” Nanotechnology 19(20), 205301 (2008). [CrossRef] [PubMed]

]. To achieve high quality SWS by using stamping method, whose performance is comparable with other SWS fabricated directly on top of semiconductors and glass substrates, it is imperative to develop fabrication technique of negative patterns of conical SWS on top of templates, and high fidelity stamping technique.

In this study, we report the successful fabrication of a silicon template consisting of conical holes with a high aspect ratio, which is a negative pattern of a conical SWS, by means of e-beam lithography and a subsequent two-step etching process. Complementary conical SWS were transferred onto PMMA film using a stamping technique in a vacuum. In the two-step etching process, the e-beam writing time could be dramatically reduced and the high etching selectivity of SiO2 against the silicon substrate could enable deep etching, which led to a high aspect ratio nanostructure. The optical properties of the PMMA film with a conical SWS as replicated by the stamping technique were measured and discussed.

2. Experimental details

The conical PMMA SWS was fabricated according to the process flow presented in Fig. 1
Fig. 1 The fabrication process sequence of the silicon template and conical PMMA SWS
. A 100-nm-thick SiO2 layer was deposited on a silicon substrate via plasma-enhanced chemical vapor deposition (PECVD). The SiO2-covered-silicon substrate was then coated with an e-beam resist and a 40-nm-diameter hole with a 350-nm-period hexagonal pattern with an area of 0.5 cm2 was defined on the surface of the e-beam resist using e-beam lithography. The exposed area was reduced to minimize the e-beam writing time. After the development stage, the defined sample was chemically etched in a buffered-HF solution for 125 seconds. The hexagonal pattern on the e-beam resist was transferred to a 100-nm-thick SiO2 layer. The resulting SiO2 layer with enlarged circular patterns was used as an etch mask in the subsequent dry etching processing of the silicon. The dry etching of silicon to realize conical holes was carried out by an inductively coupled-plasma reactive-ion etching (ICP-RIE) system operated under a gas mixture of SiCl4 (10 SCCM, cubic centimeter per minute at STP) and Ar (2.5 SCCM) at a chamber pressure of 10 mTorr. Finally, the silicon template with conical holes was realized by an ICP-RIE process which lasted 7 minutes. This two-step etching process involving the wet etching of SiO2 and the ICP-RIE etching of silicon was exploited to fabricate a conical hole with a high aspect ratio.

The aforementioned stamping technique was then used to transfer the conical SWS to the PMMA film. Before the stamping process, SiNx with a thickness of 10 nm was deposited onto a silicon template to make the surface hydrophobic. The modification of the surface state was important to separate the PMMA easily during the separating process [18

18. J. H. Shin, K. S. Han, and H. Lee, “Anti-reflection and hydrophobic characteristics of M-PDMS based moth-eye nano-patterns on protection glass of photovoltaic systems,” Prog. Photovolt. Res. Appl. 19(3), 339–344 (2011). [CrossRef]

]. A PMMA solution was applied to the SiNx-coated silicon template and the sample was then treated under a vacuum for 30 min to remove any air bubbles. Next, the PMMA was heated to a temperature of 80°C for 3 hours to remove any solvents. After cooling down on a hot plate to 40°C, the PMMA film was separated from the silicon template. The total reflectance and transmittance values of the PMMA film with and without the conical SWS were measured at wavelengths ranging from 300 to 1500 nm using a UV-Vis-NIR spectrophotometer (Cary 5000 Varian) and an integrating sphere.

3. Results and discussion

3.1 Design of SWS

The optical characteristics of SWSs are affected by the shape of the nanoscale periodic structures. To investigate the effects of the shape, such as the tip size and aspect ratio of the PMMA SWS on the reflectance characteristics, a three-dimensional rigorous coupled-wave analysis (RCWA) was carried out. In this simulation, cone-shaped nanostructures were arranged in hexagonal configuration where the nanoscale cones can be most closely packed, which is suitable for gradual change in the effective refractive index from the top to the bottom of the structures.

Figure 2
Fig. 2 Calculated average reflectance of the PMMA film with the conical and truncated conical SWS as a function of the height of the SWS in wavelengths ranging from 300 to 1500 nm
shows the theoretically calculated average reflectance values in the wavelength range of 300-1500 nm as a function of the height of the PMMA SWS for different tip sizes of 300, 200, 100, and 0 nm. The models of PMMA SWS were constructed through schematic diagrams, as presented in the inset of Fig. 2. The period, width, and space between neighboring structures were fixed at 350, 300, and 50 nm, respectively. On the other hand, the aspect ratio of the PMMA SWS was controlled by adjusting the height of the structure. The average reflectance of flat PMMA film is 3.84% and those of PMMA films with conical and truncated conical SWSs are lower compared to the flat PMMA film. As shown in Fig. 2, for sample A, the effective refractive index from the top to the bottom of the SWS is constant because the effective medium is determined by the volume of its layer. The SWS in sample A acts as a thin film with a specific refractive index. For the concise conical SWS in sample D, the average reflectance was reduced dramatically with increasing the height of the structure because the conical SWS has a continuously changed effective refractive index from the top to the bottom of the structure [19

19. E. B. Grann, M. G. Moharam, and D. A. Pommet, “Artificial uniaxial and biaxial dielectrics with use of two-dimensional subwavelength binary gratings,” J. Opt. Soc. Am. A 11(10), 2695–2703 (1994). [CrossRef]

,20

20. D. H. Raguin and G. M. Morris, “Antireflection structured surfaces for the infrared spectral region,” Appl. Opt. 32(7), 1154–1167 (1993). [CrossRef] [PubMed]

]. This graded transformation of the effective refractive index can suppress the surface reflection. The average reflectance of the conical SWS with a height exceeding 500 nm reaches the minimum value. The truncated conical SWSs with tip sizes of 200 and 100 nm exhibited higher average reflectance levels than the conical SWS due to the mismatch of the refractive index from the air to the tip of the structure. For a low height of the structure, on the other hand, lower average reflectance than that of the conical SWS is exhibited. The rapid change of the graded effective refractive index through the short length is rather less efficient than a discontinuity of the refractive index between the air and the tip of the structure in creating an effective refractive index. The minimum height of the conical SWS to obtain better performance than the truncated conical SWS is approximately 500 nm. The shape of the nanostructure is also a key parameter for decreasing the surface reflection. If the conical SWS does not have a sufficient height, optimization of the tip size is very important.

Figure 3(a)
Fig. 3 Calculated reflectance of the (a) conical PMMA SWSs with heights of 0, 250, 500, and 750 nm (b) truncated conical PMMA SWSs with tip sizes of 0, 50, 100, and 150 nm
shows the calculated reflectance of PMMA film without and with the conical SWS and truncated conical SWS, showing heights of 250, 500, and 750 nm with respect to the incident wavelength, which ranges from 300 to 1500 nm. In this calculation, a PMMA SWS with a period of 350 nm was employed. The flat PMMA film exhibited an average reflectance of 3.8%, as described above. As shown in Fig. 3(a), the average reflectance levels of the conical PMMA SWSs with heights of 250, 500, and 750 nm are respectively 1.4, 0.6, and 0.5%. The reflectance of the conical PMMA SWS with a height of 750 nm is lower than 0.6% throughout the entire wavelength range investigated in this study. Compared with the conical PMMA SWS with height of 750 nm, the reflectance levels of the conical PMMA SWSs with heights of 250 and 500 nm were lower only at a short wavelength range but tended to increase the reflectance from 650 and 1250 nm.

To investigate the effect of the tip size of the nanoscale cones, the reflectance of the truncated conical PMMA SWS was calculated, as shown in Fig. 3(b). The period and height of the truncated conical SWS were fixed respectively to 350 and 750 nm, but the tip size varied from 0 to 150 nm. The average reflectance of the conical PMMA SWS with tip size of 0 nm is 0.5% and those levels of truncated conical PMMA SWSs with tip sizes of 50, 100, and 150 nm are respectively 0.5, 0.5, and 0.6%. Although a low average reflectance was obtained with the truncated conical SWSs, a ripple pattern in the reflectance spectrum was observed due to the mismatch in the effective refractive index between the air and the tip of the truncated conical structure. To reduce the surface reflection with the broadband wavelength, the truncated conical SWS is not an adequate shape due to the oscillation in the reflection spectrum. At tip size below least 50 nm is needed for broadband low reflection which is obtained from the conical SWS with a tip size of 0 nm.

From the calculations here, it can be concluded that the PMMA SWS can achieve the best antireflective properties for broadband wavelengths ranging from 300 to 1500 nm only when the tip diameter smaller than 50 nm and the height of the cones exceeds 750 nm, which corresponds to an aspect ratio of 2.5.

In order to realize a concise conical hole in the silicon substrate, the ICP-RIE process was performed for 7 minutes. As the etching time was decreased, however, the hole exhibited different sidewall profiles, showing a truncated conical hole with a low height. To determine the optimized appearance of the silicon template for the process proposed in this study, a RCWA simulation was carried out using the replicated PMMA SWS models. Figure 4
Fig. 4 Calculated reflectance of the PMMA SWS from different silicon templates fabricated by the ICP-RIE process with times of 5, 6, and 7 minutes
shows the calculated reflectance of the replicated PMMA SWS according to the silicon template, which is shown in the inset. The average reflectance levels of the replicated PMMA SWS from different silicon templates fabricated by ICP-RIE process times of 5, 6, and 7 minutes are 1.2, 0.8 and 0.6%. The high aspect ratio and narrow tip of the conical PMMA SWS contribute to the low reflectance in a wide wavelength range.

3.2 Fabrication of SWS

As demonstrated by the RCWA calculation, SWSs with a high aspect ratio and a narrow tip are required to achieve low reflection in broadband wavelengths. The realization of nanoscale periodic structures with aspect ratio higher than 2.5 for the template needs dry-etching process which requires thick etch mask resistant to ion bombardment. Considering that the conventional lithography techniques such as e-beam lithography, nano-imprint, and holographic lithography rely on thin resists for the creation of a nanoscale pattern, the thin resists cannot endure the long-term exposure to the dry etching process. Thereby, the two-step etching process was utilized in this study to create a thick SiO2 etch mask with a nanoscale periodic structures and high selectivity to the silicon. The SiO2 layer can realize the high aspect ratio conical holes via a dry etching process. Even though there are many lithographic techniques with high throughput, e-beam lithography technique was utilized in this study for the fast prototyping.

During the e-beam lithography process for the fabrication of the silicon template, the exposed area of the e-beam on the surface was minimized to reduce the process time and cost. However, a primary pattern on the e-beam resist with a thickness of 15 nm and a hole diameter of 40 nm was not suitable for an etch mask in the dry etching process, as the thin e-beam resist could not endure long-term dry etching. In addition, the duty ratio of the structure, 0.11, as calculated as the ratio of the structure diameter of 40 nm to the structure period of 350 nm, is not effective considering the optical properties. After patterning by e-beam lithography, the pattern transferring process from the e-beam resist to the SiO2 layer was performed by the wet etching process. The hexagonal pattern on the e-beam resist was transferred to a 100-nm-thick SiO2 layer, which is a sufficient thickness to tolerate dry etching. For the wet etching process, the diameter of the hole on the SiO2 layer was also increased from 40 to 311 nm owing to the non-directional isotropic etching. The high duty ratio of the etch mask induced from the wet etching process achieved a high duty ratio of the structure in the final conical SWS, creating greater continuity of the effective refractive index between the bottom of the SWS and the substrate. To fabricate the structure with a high aspect ratio and a duty ratio for excellent optical properties, the two-step etching was used. The hole diameter on the SiO2 layer was controlled by the wet etching time.

Figure 5(a)
Fig. 5 (a) SEM images of the SiO2 layer patterned by different wet etching times of 75, 100, 125, and 150 seconds, and (b) dependence of the hole diameter on the wet etching time
shows scanning electron microscopy (SEM) images of a SiO2 layer patterned by wet etching. The samples fabricated by wet etching of 75, 100, 125, and 150 seconds have hole diameters of 205, 263, 311, and 350 nm. The dependence of the hole diameter on the wet etching time is shown in Fig. 5(b). As the etching time is increased, the diameter of the hole tends to increase.

Figure 6
Fig. 6 SEM images of silicon templates fabricated using the ICP-RIE etching process after a wet etching process with different etching times. The detailed etching parameters are shown in the insets of each figure. The insets of Figs. 6(a) and 6(d) are high-magnification images of the etch mask before the ICP-RIE etching process.
shows SEM images of the fabricated silicon templates with different sidewall profiles. The silicon templates were fabricated by the ICP-RIE etching process for (a) 7, (b) 8, and (c) 9 minutes after the wet etching of SiO2 for 125 seconds. For the samples shown in Figs. 6(d), 6(e), and 6(f), etching times of 7, 8, and 9 minutes in the ICP-RIE chamber were used after the wet etching of SiO2 for 150 seconds. The shape of the SiO2 etch mask was controlled by adjusting the wet etching time. As shown in the inset of Figs. 6(a) and 6(d), the outlines of the hole on the SiO2 surface have different shapes, which led to the unique appearance of the final structure. The flat surface of the etch mask as shown in the inset of Fig. 6(a) provided a smooth surface of the structure after the dry etching process. Comparably, the etch mask consisting of a triangular array on top of the SiO2 surface, as shown in the inset of Fig. 6(d), created a sharper point at the top of the fabricated structure. The etch mask controlled by wet etching is also an important factor for the determination of the shape of the structure in the silicon template. As shown in Fig. 6, the two-step etching process was also exploited to create various shapes of the structure.

Figure 7
Fig. 7 (a) top and (b) cross-sectional SEM images of a silicon template consisting of a hexagonal pattern with a period of 350 nm.
shows top and cross-sectional SEM images of the fabricated silicon template with a conical hole. The conical hole exhibited a period of 350 nm, a duty ratio of 0.8, and an aspect ratio of 2.8. The hexagonal pattern realized on the e-beam resist by e-beam lithography was suitably transferred to the final silicon template. The duty ratio of the conical hole in the silicon template was controlled by the wet etching time. As the wet etching time was increased, the hole diameter on the SiO2 layer was also increased, which lead to an increase in the high duty ratio via the dry etching process. On the other hand, the profile and aspect ratio were determined by the ICP-RIE etching condition.

After the curing process, the PMMA film was peeled off from the silicon template on the hot plate at a constant temperature of 40°C. The PMMA film when cooled down to room temperature became hard enough so that it was difficult to separate it from the silicon template. Figure 8
Fig. 8 SEM images of replicated PMMA film with a conical SWS. A separating process was performed at temperatures of (a) 40 and (b) 60°C, respectively.
shows SEM images of the fabricated PMMA film after separating it from the silicon template on a hot plate at both 40 and 60°C. As shown in Fig. 8(a), the replicated PMMA SWS is precisely conical. In comparison, the sample in Fig. 8(b) exhibits an extended conical SWS when the separating process was performed at a temperature of 60°C. The structural parameters such as the period of 350 nm and the aspect ratio of 2.8 in Fig. 8(a) are in good agreement with those from the original structure of the silicon template. These findings mean that the conical SWS on the PMMA film can be perfectly transferred from the silicon template. For comparison, the materials, periods, and the aspect ratios of various SWSs in other studies [11

11. Y. Kanamori and K. Hane, “Broadband antireflection subwavelength gratings for polymethyl methacrylate fabricated with molding technique,” Opt. Rev. 9(5), 183–185 (2002). [CrossRef]

15

15. C. J. Ting, M. C. Huang, H. Y. Tsai, C. P. Chou, and C. C. Fu, “Low cost fabrication of the large-area anti-reflection films from polymer by nanoimprint/hot-embossing technology,” Nanotechnology 19(20), 205301 (2008). [CrossRef] [PubMed]

] and the SWS fabricated in this study were tabulated, as shown in Table 1

Table 1. Comparison of the Materials, Shapes, Periods, Heights and Aspect Ratio of the Various SWSs

table-icon
View This Table
. The PMMA SWS fabricated by the two-step etched silicon template can achieve well ordered SWS with high aspect ratio.

3.3 Optical characteristics

The simulated and measured reflectance spectra for the PMMA films with and without conical SWS were compared in Fig. 9(a)
Fig. 9 (a) Comparison of the simulated and measured reflectance of the PMMA films and (b) the measured transmittance for PMMA film with and without conical SWS
under the incident wavelength ranging from 300 to 1500 nm. For this simulation, the reflection from the flat backside of PMMA film was also considered. Compared with the calculated results presented in section 3.1, the reflectance values in Fig. 9(a) is ~3.5% higher, which is attributed to the multiple reflections on the interface between the air/front side of the PMMA and the back side of the PMMA/air. Although the conical SWS can perfectly decrease the reflection on the front side, the light transmitted from the front side was reflected to the back side, thus contributing to the total reflection.

The calculated reflectance which includes the effect of the backside reflection matches very well with the measured reflectance. The average measured reflectance of the PMMA films with and without SWS in the wavelengths between 500 and 1500 nm were 7.1 and 4.3%, respectively. If the backside reflection is neglected, the reflection from the front side of SWS can be calculated to be as low as 0.6% from the measured total reflectance and transmittance values according to [21

21. H. A. Macleod, Thin-Film Optical Filter, 3rd ed. (Institute of Physics Publishing, 2001).

]. The refection from the surface of the SWS can be estimated to be reduced from 3.8% to 0.6% by the effect of SWS in this study.

The measured transmittance spectra of the PMMA films with and without SWS were compared in Fig. 9(b). The two transmission dips observed at wavelength around 1150 and 1380 nm are due to the absorption characteristics of the PMMA. The transmittance of the PMMA film with SWS was higher than that of the flat PMMA film at wavelengths longer than 470 nm. The average transmittance in wavelengths between 500 and 1500 nm was increased from 91.1 to 94.2% by introducing conical SWS at the film surface. The drop point at a specific wavelength in the transmission spectra is dependent on the period of the SWS [22

22. Y. M. Song, H. J. Choi, J. S. Yu, and Y. T. Lee, “Design of highly transparent glasses with broadband antireflective subwavelength structures,” Opt. Express 18(12), 13063–13071 (2010). [CrossRef] [PubMed]

]. By reducing the period of the SWS, the transmission spectra can be pushed down to shorter wavelength.

4. Conclusion

The shape of a nanoscale periodic structure was designed by means of a RCWA calculation. According to a simulation study, a conical SWS with an aspect ratio higher than 2.5 and a tip diameter smaller than 50 nm is required to achieve broadband antireflective properties from a PMMA-based SWS. In order to meet the above design requirements, a two-step etching process for a silicon template and a stamping method was carried out. E-beam lithography and the wet etching of SiO2 were utilized to fabricate a hexagonal patterned SiO2 layer, which was used as an etch mask during the ICP-RIE etching process. In the ICP-RIE etching process, the hexagonal pattern on the wet-etched SiO2 mask was transferred to a silicon substrate, creating final silicon template with high aspect ratio conical holes. Next, the PMMA SWS was replicated using a silicon template and the stamping technique. The fabricated PMMA film containing conical SWS exhibited a significant improvement of its optical properties in the UV-Vis-NIR wavelength range. The fabrication of PMMA SWS using the two-step etching and stamping technique is an excellent method for the formation of SWSs with a simple and low-cost process as well as good optical performance.

Acknowledgment

This work was supported by a NRF grant (No. 20110017603), the New & Renewable Energy (No. 20123010010110) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy, The World Class University (WCU) program at GIST through a grant provided by the Ministry of Education, Science and Technology (MEST) of Korea (No. R3110026) and by the Core Technology Development Program for Next-Generation Solar Cells of the Research Institute for Solar and Sustainable Energies (RISE), GIST.

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O. Dial, C. C. Cheng, and A. Scherer, “Fabrication of high-density nanostructures by electron beam lithography,” J. Vac. Sci. Technol. B 16(6), 3887–3890 (1998). [CrossRef]

6.

Y. M. Song, S. Y. Bae, J. S. Yu, and Y. T. Lee, “Closely packed and aspect-ratio-controlled antireflection subwavelength gratings on GaAs using a lenslike shape transfer,” Opt. Lett. 34(11), 1702–1704 (2009). [CrossRef] [PubMed]

7.

Q. Chen, G. Hubbard, A. Shields, C. Liu, D. W. E. Allsopp, W. N. Wang, and S. Abbott, “Broadband moth-eye antireflection coatings fabricated by low-cost nanoimprinting,” Appl. Phys. Lett. 94(26), 263118 (2009). [CrossRef]

8.

Z. Yu, H. Gao, W. Wu, H. Ge, and S. Y. Chou, “Fabrication of large area subwavelength antireflection structures on Si using trilayer resist nanoimprint lithography and liftoff,” J. Vac. Sci. Technol. B 21(6), 2874–2877 (2003). [CrossRef]

9.

N. Koo, U. Plachetka, M. Otto, J. Bolten, J. H. Jeong, E. S. Lee, and H. Kurz, “The fabrication of a flexible mold for high resolution soft ultraviolet nanoimprint lithography,” Nanotechnology 19(22), 225304 (2008). [CrossRef] [PubMed]

10.

D. S. Kim, M. S. Park, and J. H. Jang, “Fabrication of a cone-shaped subwavelength structures by utilizing a confined convective self-assembly technique and inductively-coupled-plasma reactive ion etching,” J. Vac. Sci. Technol. B 29(2), 020602 (2011). [CrossRef]

11.

Y. Kanamori and K. Hane, “Broadband antireflection subwavelength gratings for polymethyl methacrylate fabricated with molding technique,” Opt. Rev. 9(5), 183–185 (2002). [CrossRef]

12.

H. J. Nam, J. H. Kim, D. Y. Jung, J. B. Park, and H. S. Lee, “Two-dimensional nanopatterning by PDMS relief structures of polymeric colloidal crystals,” Appl. Surf. Sci. 254(16), 5134–5140 (2008). [CrossRef]

13.

J. Y. Chen and K. W. Sun, “Enhancement of the light conversion efficiency of silicon solar cells by using nanoimprint anti-reflection layer,” Sol. Energy Mater. Sol. Cells 94(3), 629–633 (2010). [CrossRef]

14.

K. S. Han, H. Lee, D. Kim, and H. Lee, “Fabrication of anti-reflection structure on protective layer of solar cells by hot-embossing method,” Sol. Energy Mater. Sol. Cells 93(8), 1214–1217 (2009). [CrossRef]

15.

C. J. Ting, M. C. Huang, H. Y. Tsai, C. P. Chou, and C. C. Fu, “Low cost fabrication of the large-area anti-reflection films from polymer by nanoimprint/hot-embossing technology,” Nanotechnology 19(20), 205301 (2008). [CrossRef] [PubMed]

16.

H. Y. Tsai and C. J. Ting, “Optical characteristics of moth-eye structures on poly(methyl methacrylate) and polycarbonate sheets fabricated by thermal nanoimprinting processes,” Jpn. J. Appl. Phys. 48(6), 06FH19 (2009). [CrossRef]

17.

Y. Li, S. Minoru, and H. Kazuhiro, “Micro-optical components based on silicon mold technology,” Opt. Lasers Eng. 41(3), 545–552 (2004). [CrossRef]

18.

J. H. Shin, K. S. Han, and H. Lee, “Anti-reflection and hydrophobic characteristics of M-PDMS based moth-eye nano-patterns on protection glass of photovoltaic systems,” Prog. Photovolt. Res. Appl. 19(3), 339–344 (2011). [CrossRef]

19.

E. B. Grann, M. G. Moharam, and D. A. Pommet, “Artificial uniaxial and biaxial dielectrics with use of two-dimensional subwavelength binary gratings,” J. Opt. Soc. Am. A 11(10), 2695–2703 (1994). [CrossRef]

20.

D. H. Raguin and G. M. Morris, “Antireflection structured surfaces for the infrared spectral region,” Appl. Opt. 32(7), 1154–1167 (1993). [CrossRef] [PubMed]

21.

H. A. Macleod, Thin-Film Optical Filter, 3rd ed. (Institute of Physics Publishing, 2001).

22.

Y. M. Song, H. J. Choi, J. S. Yu, and Y. T. Lee, “Design of highly transparent glasses with broadband antireflective subwavelength structures,” Opt. Express 18(12), 13063–13071 (2010). [CrossRef] [PubMed]

OCIS Codes
(220.4241) Optical design and fabrication : Nanostructure fabrication
(050.6624) Diffraction and gratings : Subwavelength structures

ToC Category:
Optical Design and Fabrication

History
Original Manuscript: February 7, 2013
Revised Manuscript: March 10, 2013
Manuscript Accepted: March 15, 2013
Published: March 29, 2013

Citation
Dae-Seon Kim, Dong-Hyun Kim, and Jae-Hyung Jang, "A nanoscale conical polymethyl methacrylate (PMMA) sub-wavelength structure with a high aspect ratio realized by a stamping method," Opt. Express 21, 8450-8459 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-7-8450


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References

  1. Y. F. Huang, S. Chattopadhyay, Y. J. Jen, C. Y. Peng, T. A. Liu, Y. K. Hsu, C. L. Pan, H. C. Lo, C. H. Hsu, Y. H. Chang, C. S. Lee, K. H. Chen, and L. C. Chen, “Improved broadband and quasi-omnidirectional anti-reflection properties with biomimetic silicon nanostructures,” Nat. Nanotechnol.2(12), 770–774 (2007). [CrossRef] [PubMed]
  2. K. Nishioka, S. Horita, K. Ohdaira, and H. Matsumura, “Antireflection subwavelength structure of silicon surface formed by wet process using catalysis of single nano-sized gold particle,” Sol. Energy Mater. Sol. Cells92(8), 919–922 (2008). [CrossRef]
  3. M. Y. Chiu, C. H. Chang, M. A. Tsai, F. Y. Chang, and P. C. Yu, “Improved optical transmission and current matching of a triple-junction solar cell utilizing sub-wavelength structures,” Opt. Express18(S3Suppl 3), A308–A313 (2010). [CrossRef] [PubMed]
  4. K. C. Sahoo, Y. Li, and E. Y. Chang, “Shape effect of silicon nitride subwavelength structure on reflectance for silicon solar cells,” IEEE Trans. Electron. Dev.57(10), 2427–2433 (2010). [CrossRef]
  5. O. Dial, C. C. Cheng, and A. Scherer, “Fabrication of high-density nanostructures by electron beam lithography,” J. Vac. Sci. Technol. B16(6), 3887–3890 (1998). [CrossRef]
  6. Y. M. Song, S. Y. Bae, J. S. Yu, and Y. T. Lee, “Closely packed and aspect-ratio-controlled antireflection subwavelength gratings on GaAs using a lenslike shape transfer,” Opt. Lett.34(11), 1702–1704 (2009). [CrossRef] [PubMed]
  7. Q. Chen, G. Hubbard, A. Shields, C. Liu, D. W. E. Allsopp, W. N. Wang, and S. Abbott, “Broadband moth-eye antireflection coatings fabricated by low-cost nanoimprinting,” Appl. Phys. Lett.94(26), 263118 (2009). [CrossRef]
  8. Z. Yu, H. Gao, W. Wu, H. Ge, and S. Y. Chou, “Fabrication of large area subwavelength antireflection structures on Si using trilayer resist nanoimprint lithography and liftoff,” J. Vac. Sci. Technol. B21(6), 2874–2877 (2003). [CrossRef]
  9. N. Koo, U. Plachetka, M. Otto, J. Bolten, J. H. Jeong, E. S. Lee, and H. Kurz, “The fabrication of a flexible mold for high resolution soft ultraviolet nanoimprint lithography,” Nanotechnology19(22), 225304 (2008). [CrossRef] [PubMed]
  10. D. S. Kim, M. S. Park, and J. H. Jang, “Fabrication of a cone-shaped subwavelength structures by utilizing a confined convective self-assembly technique and inductively-coupled-plasma reactive ion etching,” J. Vac. Sci. Technol. B29(2), 020602 (2011). [CrossRef]
  11. Y. Kanamori and K. Hane, “Broadband antireflection subwavelength gratings for polymethyl methacrylate fabricated with molding technique,” Opt. Rev.9(5), 183–185 (2002). [CrossRef]
  12. H. J. Nam, J. H. Kim, D. Y. Jung, J. B. Park, and H. S. Lee, “Two-dimensional nanopatterning by PDMS relief structures of polymeric colloidal crystals,” Appl. Surf. Sci.254(16), 5134–5140 (2008). [CrossRef]
  13. J. Y. Chen and K. W. Sun, “Enhancement of the light conversion efficiency of silicon solar cells by using nanoimprint anti-reflection layer,” Sol. Energy Mater. Sol. Cells94(3), 629–633 (2010). [CrossRef]
  14. K. S. Han, H. Lee, D. Kim, and H. Lee, “Fabrication of anti-reflection structure on protective layer of solar cells by hot-embossing method,” Sol. Energy Mater. Sol. Cells93(8), 1214–1217 (2009). [CrossRef]
  15. C. J. Ting, M. C. Huang, H. Y. Tsai, C. P. Chou, and C. C. Fu, “Low cost fabrication of the large-area anti-reflection films from polymer by nanoimprint/hot-embossing technology,” Nanotechnology19(20), 205301 (2008). [CrossRef] [PubMed]
  16. H. Y. Tsai and C. J. Ting, “Optical characteristics of moth-eye structures on poly(methyl methacrylate) and polycarbonate sheets fabricated by thermal nanoimprinting processes,” Jpn. J. Appl. Phys.48(6), 06FH19 (2009). [CrossRef]
  17. Y. Li, S. Minoru, and H. Kazuhiro, “Micro-optical components based on silicon mold technology,” Opt. Lasers Eng.41(3), 545–552 (2004). [CrossRef]
  18. J. H. Shin, K. S. Han, and H. Lee, “Anti-reflection and hydrophobic characteristics of M-PDMS based moth-eye nano-patterns on protection glass of photovoltaic systems,” Prog. Photovolt. Res. Appl.19(3), 339–344 (2011). [CrossRef]
  19. E. B. Grann, M. G. Moharam, and D. A. Pommet, “Artificial uniaxial and biaxial dielectrics with use of two-dimensional subwavelength binary gratings,” J. Opt. Soc. Am. A11(10), 2695–2703 (1994). [CrossRef]
  20. D. H. Raguin and G. M. Morris, “Antireflection structured surfaces for the infrared spectral region,” Appl. Opt.32(7), 1154–1167 (1993). [CrossRef] [PubMed]
  21. H. A. Macleod, Thin-Film Optical Filter, 3rd ed. (Institute of Physics Publishing, 2001).
  22. Y. M. Song, H. J. Choi, J. S. Yu, and Y. T. Lee, “Design of highly transparent glasses with broadband antireflective subwavelength structures,” Opt. Express18(12), 13063–13071 (2010). [CrossRef] [PubMed]

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