nanomaterials Article

Fabrication of Silicon Nanobelts and Nanopillars by Soft Lithography for Hydrophobic and Hydrophilic Photonic Surfaces Estela Baquedano 1 , Ramses V. Martinez 2,3 , José M. Llorens 1 and Pablo A. Postigo 1, * 1 2 3

*

Instituto de Microelectrónica de Madrid, CSIC, Tres Cantos, 28760 Madrid, Spain; [email protected] (E.B.); [email protected] (J.M.L.) School of Industrial Engineering, Purdue University, 315 N. Grant Street, West Lafayette, IN 47907, USA; [email protected] Weldon School of Biomedical Engineering, Purdue University, 206 S. Martin Jischke Drive, West Lafayette, IN 47907, USA Correspondence: [email protected]; Tel.: +34-91-806-0700

Academic Editors: Krasimir Vasilev and Melanie Ramiasa Received: 18 January 2017; Accepted: 8 May 2017; Published: 11 May 2017

Abstract: Soft lithography allows for the simple and low-cost fabrication of nanopatterns with different shapes and sizes over large areas. However, the resolution and the aspect ratio of the nanostructures fabricated by soft lithography are limited by the depth and the physical properties of the stamp. In this work, silicon nanobelts and nanostructures were achieved by combining soft nanolithography patterning with optimized reactive ion etching (RIE) in silicon. Using polymethylmethacrylate (PMMA) nanopatterned layers with thicknesses ranging between 14 and 50 nm, we obtained silicon nanobelts in areas of square centimeters with aspect ratios up to ~1.6 and linewidths of 225 nm. The soft lithographic process was assisted by a thin film of SiOx (less than 15 nm) used as a hard mask and RIE. This simple patterning method was also used to fabricate 2D nanostructures (nanopillars) with aspect ratios of ~2.7 and diameters of ~200 nm. We demonstrate that large areas patterned with silicon nanobelts exhibit a high reflectivity peak in the ultraviolet C (UVC) spectral region (280 nm) where some aminoacids and peptides have a strong absorption. We also demonstrated how to tailor the aspect ratio and the wettability of these photonic surfaces (contact angles ranging from 8.1 to 96.2◦ ) by changing the RIE power applied during the fabrication process. Keywords: soft lithography; photonic surface; hydrophobic; hydrophilic; silicon nanobelts

1. Introduction Vertical nanostructures with aspect ratios >1 are essential in a variety of fields such as photonics and nanophotonics [1,2], microfluidics [3,4], microelectromechanical systems (MEMs) [5] and even in lab-on-a-chip bio-applications such as DNA separation [6]. Soft lithography was introduced as a low-cost alternative to conventional lithography [7] and has been shown to be a powerful method of generating reproducible nanopatterns and nanostructures with features sizes ranging from 30 to 100 µm [8] using elastomeric stamps made of polydimethylsiloxane (PDMS). Unfortunately, the low Young’s modulus of elastomeric formulations (such as PDMS, h-PDMS, and PTFE) limits the aspect ratio of the nanostructures that can be achieved using soft lithography [9]. When the aspect ratio is too high or too low, the nanostructures on the PDMS stamp tend to deform and produce defects in the patterned areas due to the pressure exerted on the stamp during the patterning process [8–10]. Nanoimprint lithography (NIL) is also affected by this limitation, requiring complex post-processing steps to achieve deep nanostructures [11].

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Several top-down methods have been developed to enable the fabrication of high-aspect-ratio nanopatterns. Some methods use a double layer of resist and a thin film like hard mask among the layers to increase the high aspect ratio in the bottom resist [10,12]. However, for transferring the pattern into Si, it is always necessary to continue the process with a subsequent etching [7]. In this work, a process without a second layer resist is developed through the optimization of the reactive ion etching (RIE) process to transfer the pattern into Si. The RIE process uses only SF6 , CHF3 , and a SiOx mask in a standard RIE chamber, avoiding any cryogenic or other deep-RIE procedures. Using this patterning-etching procedure, we obtained gratings made out of nanobelts with aspect ratios >1 using conventional DVD and Blu-ray discs as soft lithographic masters in a simple and low-cost approach [13–15]. RIE is a reliable method to achieve high-aspect-ratio silicon nanostructures [16] that can use a mixture of diverse gases such as O2 :Ar or Cl2 [10,17]. SF6 plasma is commonly used as the main reactive agent to etch silicon due to its capacity to generate atomic fluorine (F), which has a high affinity for Si atoms to form SiFx (x = 14 , 2, 4) volatile species [18,19]. The addition of small concentrations of O2 in the SF6 plasma leads to the generation of F due to the following oxidation reaction: O + SFx → SOFx −1 + F (x ≤ 5) [18]. To our knowledge, the best aspect ratios (as high as 107) have been achieved using deep RIE (DRIE) [20], and values of 50 have been reported using Inductively Coupled Plasma (ICP-RIE) [21]. Unfortunately, the DRIE and ICP-RIE systems are not as extended as conventional RIE systems due to their higher price. In general, using conventional RIE systems, it is difficult to achieve aspect ratios larger than 10 and lateral dimensions smaller than 200 nm when using Ni-masks and electron-beam lithography on thicker PMMA layers (around 300 nm) [22]. This manuscript describes the cost-effective fabrication of nanostructures 535-nm-tall and 197-nm-diameter (aspect ratio 2.71) over large areas using a conventional RIE system and starting from PMMA layers as thin as 14 nm. This simple fabrication process has two steps: (i) soft lithography is used to pattern large silicon surfaces with PMMA nanostructures; (ii) the PMMA nanostructures are used as a mask during the RIE etching of a sacrificial SiOx layer that is 14 nm thick. We varied the RIE power applied to SF6 /O2 plasma to find the etching conditions that optimized the aspect ratio of the resulting nanostructures and to control the wetting properties of the final photonic surfaces. 2. Experiment Figure 1a shows the fabrication process sequence. First, a thin layer of SiOx ~14 nm thick was deposited on a Si wafer to serve as a hard mask by a plasma-enhanced chemical vapor deposition (PECVD) system (Surface Technology Systems 310PC-DF) at 300 ◦ C. We fabricated elastomeric stamps using the following masters: (i) digital versatile discs (DVDs) with a lineal grating of 775 nm period, a 400 nm linewidth, and a 150 nm depth. DVDs were replicated by casting a 5 mm layer of Sylgard 184 PDMS (Dow Corning Corporation, Seneffe, Belgium); (ii) Blu-ray (BR) discs with a lineal grating of 325 nm, a 200 nm linewidth, and a 25 nm depth. We replicated BR discs using a composite formed by a stiff layer of h-PDMS (30–40 µm thick) supported by a flexible layer (5 mm thick) of Sylgard 184 PDMS [23]. The patterning process over the Si/SiOx substrate required 5 µL of resist (5% PMMA 996k in gamma-butyrolactone (GBL)) added with a micropipette. The resist solution was covered with the PDMS stamp and pressed between two glass slides. The resist was cured under vacuum for 3 h. Figure 1b,c shows the atomic force microscope (AFM) images of the nanopatterns obtained in the PMMA. The depth of the nanopattern was 50 nm using the DVD stamp (DVD-PDMS sample). For the sample fabricated with a BR stamp (BR-hPDMS sample), the depth was 15 nm. The limiting factor for a higher aspect ratio is the thickness of SiOx , which can be etched using the PMMA. Using our etching recipe, this thickness is between 14 and 20 nm. After that, two RIE etchings were carried out (Oxford Plasmalab 80). A cleaning procedure of the RIE chamber with Ar and O2 was run before each etching to ensure reproducibility. The first etching (CHF3 25.0 sccm, chamber pressure 20.0 mTorr, power 50 W, wafer temperature 30 ◦ C) was used to transfer the nanopattern to the SiOx hard mask. The second etching (SF6 /O2 12.0/3.0 sccm, chamber pressure 90.0 mTorr, wafer temperature 30 ◦ C, 1 min) was

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Nanomaterials 2017, 7, 109 of 11W to used to transfer the nanopattern to the Si [24]. We varied the RIE power between 10 and 3100 Nanomaterials 2017, 109 aspect ratio, while keeping the rest of the parameters fixed. Each experiment 3 of 11 explore its effect in7,the power between 10 and 100 W to explore its effect in the aspect ratio, while keeping the rest of the was carried out three times in order to verify its reproducibility.

parameters fixed.10 Each was carried out three in ratio, order while to verify its reproducibility. power between andexperiment 100 W to explore its effect in thetimes aspect keeping the rest of the parameters fixed. Each experiment was carried out three times in order to verify its reproducibility.

Figure 1. (a) Schematicillustration illustration of of the fabrication AFM image of the nanopattern Figure 1. (a) Schematic fabricationprocess; process;(b)(b) AFM image of the nanopattern obtained by soft lithography using DVD stamp; (c) a nanopattern obtained by soft lithography using obtained by1.soft using DVD (c) a nanopattern obtained by soft using a Figure (a) lithography Schematic illustration of stamp; the fabrication process; (b) AFM image of lithography the nanopattern a Blu-ray (BR) stamp. obtained soft lithography using DVD stamp; (c) a nanopattern obtained by soft lithography using Blu-ray (BR) by stamp. a Blu-ray (BR) stamp.

3. Results and Discussion 3. Results and Discussion 3.1. Optimization of the RIE Process 3.1. Optimization of the RIE Process 3.1. Optimization of the RIE Process We determined first the etching time necessary to transfer the PMMA nanopattern to the 14-nm-

3. Results and Discussion

We determined first the etching time necessary to transfer the PMMA nanopattern to the thick We SiOxdetermined by AFM inspection. Duringtime the RIE process, x and the PMMA were etched at the same rate. first the etching necessary toSiO transfer PMMA nanopattern to the 14-nm14-nm-thick SiO by AFM inspection. During the RIE process, SiO and PMMA were etched at x x The between PMMA and SiO is 1:1. FigureSiO 2 shows that 30were s were not enough timerate. to thickselectivity SiOx by AFM inspection. During thex RIE process, x and PMMA etched at the same the completely same rate.transfer Thebetween selectivity betweenSiO PMMA SiO2xshows is 1:1.that Figure 2 shows that stime were not the nanopatterns SiOand x surface (DVD-PDMS After a 30 1 min RIE The selectivity PMMA and onto x the is 1:1. Figure 30samples). s were not enough to enough time to completely transfer the nanopatterns onto the SiO surface (DVD-PDMS samples). x process, the transfer nanopatterns were transferred over large areas(DVD-PDMS of the SiOx surface. BR-hPDMS completely the nanopatterns onto the SiO x surface samples). After a 1samples min RIE After a 1 min process, the nanopatterns were transferred over areas of SiO transferred the nanopatterns after 30 s; however, since of etching did notthe change the x surface. process, theRIE nanopatterns were transferred over large areas1ofmin the SiO xlarge surface. BR-hPDMS samples BR-hPDMS samples transferred the 30 s; since 1 did minnot of etching nanopattern significantly, we decided to use 1 min after for since both samples. transferred the nanopatterns afternanopatterns 30 s; however, 1however, min of etching change did the not change the nanopattern significantly, wetodecided tofor useboth 1 min for both samples. nanopattern significantly, we decided use 1 min samples.

Figure 2. AFM images after reactive ion etching (RIE) etching of a 14-nm-thick SiOx layer with CHF3. 3 plasma; (b)SiO image ofwith DVD(a) AFM of DVD-PDMS sample after 30 s of(RIE) etching with CHF Figure 2.image AFM images after reactive ion etching (RIE) etching of x layer with CHFCHF 3. Figure 2. AFM images after reactive ion etching etching ofaa14-nm-thick 14-nm-thickAFM SiO x layer 3. 3 plasma; (c) AFM image of BR-hPDMS sample PDMS sample after 1 min of RIE etching with CHF plasma; (b) AFM image of DVD(a) AFM image DVD-PDMS sample s ofs etching withwith CHF3CHF (a) AFM image ofofDVD-PDMS sampleafter after3030 of etching plasma; (b) AFM image of 3 3etching plasma;with (d) AFM of BR-hPDMS sample after 1 min sample of RIE after 30 sample s of RIEafter etching with 3image plasma; (c) AFM image of BR-hPDMS PDMS 1 min ofCHF RIE CHF DVD-PDMS sample after 1 min of RIE etching with CHF 3 plasma; (c) AFM image of BR-hPDMS sample etching with CHF 3 plasma. after 30 s of RIE etching with CHF3 plasma; (d) AFM image of BR-hPDMS sample after 1 min of RIE

after 30 s of RIE etching with CHF3 plasma; (d) AFM image of BR-hPDMS sample after 1 min of RIE etching with CHF3 plasma. etching with CHF3 plasma.

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We also studied the influence of the RIE power during the etching of the Si wafer. Increasing We also studied the influence of the RIE power during the etching of the Si wafer. Increasing the the RIE power morereactive reactivespecies speciesand and improves ionic bombardment. affects RIE power produces produces more improves thethe ionic bombardment. This This affects the the etching rate rate andand both thethe width and thepatterned patternednanostructures. nanostructures. pictures etching both width androughness roughness of of the SEMSEM pictures showshow that no like like “grass” or any other high-level roughness S1). Ten Ten different different RIE thatartifacts no artifacts “grass” or any other high-level roughnessisispresent present (Figure (Figure S1). RIEwere powers were used starting to as as much W.etching The etching was fixed 1 min. powers used starting from 10from to as10much 100as W.100 The time time was fixed at 1 at min. Figure 3 Figure 3 shows the AFM images of the nanopatterns obtained after the etching (Figure S2). shows the AFM images of the nanopatterns obtained after the etching (Figure S2).

Figure 3. 3. AFM afterRIE RIE with SF2 6mixture /O2 mixture plasma forRIE different powers. Figure AFMimages images after with SF6/O plasma for different powers. RIE The images on The images on the left correspond to DVD-PDMS samples, while those on the right were obtained the left correspond to DVD-PDMS samples, while those on the right were obtained using BR-hPDMSusing BR-hPDMS stamps. stamps.

Tables 1 and 2 show the results for DVD-PDMS and BR-hPDMS samples, respectively. The Tables 1 and 2 show the results for DVD-PDMS and BR-hPDMS samples, respectively. The results results were the average of the three experiments for each RIE power.

were the average of the three experiments for each RIE power.

Table 1. Results obtained for the different RIE powers studied in the DVD-PDMS samples.

Table 1. Results obtained for the different RIE powers studied in the DVD-PDMS samples. 10 W Roughness (nm) 2.1 Linewidth (nm) 10 W 38020 W Depth (nm) 26.5 Roughness (nm) 2.1 8 Selectivity SiOx/Si 1.8 Linewidth (nm) 380 440 Aspect Ratio 0.07

Depth (nm) Selectivity SiOx /Si Aspect Ratio

26.5

80

1.8

5.7

0.07

0.18

20 W 8 440 30 W 80 24.5 5.7 607 0.18

275

RIE Power 30 W 40 W 50 W 60 W RIE Power 24.5 35 40 45 607 40 W615 50 W 656 60478 W 275 312 403 297 35 40 45 19.6 22.2 28.7 21.2 615 656 478 0.45 0.51 0.61 0.62

70 W 57.5 426 70 W 460 57.5 32.8 426 1.08

80 W 50 36880 W 440 50 31.4 368 1.19

90 W 87.5 41090 W 432 87.5 30.85 410 1.05

100 W 65 368100 W 391 65 27.92 368 1.06

312

403

297

460

440

432

391

19.6

22.2

28.7

21.2

32.8

31.4

30.85

27.92

0.45

0.51

0.61

0.62

1.08

1.19

1.05

1.06

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Table 2. Results obtained for the different RIE powers studied in the BR-hPDMS samples.

Table 2. Results obtained for the different RIE powers studied in the BR-hPDMS samples. RIE Power

10 W Roughness (nm) 2.5 Roughness (nm) Linewidth (nm) 175 Linewidth (nm) Depth (nm) 5.96 Depth (nm) Selectivity 0.42 SiOx/Si SiOxSelectivity /Si Aspect Ratio 0.03 Aspect Ratio

20 W 10 W 2.5 2.5 190 175 2.52 5.96 0.18 0.42 0.01 0.03

RIE Power 30 W 40 W 50 W 60 W 70 W 80 W 90 W 20 W 30 W 40 W 50 W 60 W 70 W 80 W 90 W 100 W 25.5 60 45 53 47.5 55 50 2.5299 25.5 225 60 45 53 47.5142 55 201 50 53.5 269 190 119 19040 299 62 225 269 190 142225 201 266119 127 140 236 154 2.52 40 62 140 236 225 266 154 204 2.85 4.42 10 16.85 16.07 19 11 0.18 2.85 4.42 10 16.85 16.07 19 11 16.57 0.13 0.13 0.270.27 0.52 1.24 1.581.58 1.32 1.321.29 1.29 0.01 0.52 1.24 1.61

100 W 53.5 127 204 16.57 1.61

When theRIE RIEpower powerwas wasincreased, increased, surface roughness increased (Figure forkinds both When the thethe surface roughness alsoalso increased (Figure 4) for4) both kinds of samples with a maximum value of 87.5 nm at 90 W in the case of DVD-PDMS and 60 nm at of samples with a maximum value of 87.5 nm at 90 W in the case of DVD-PDMS and 60 nm at 80 W 80 W in BR-hPMDS. in BR-hPMDS.

Dependence of the roughness of the nanostructures fabricated fabricated on the RIE power used for Figure 4. Dependence DVD-PDMS and BR-hPDMS samples.

Figure Figure 55 shows shows the the change change in in linewidth, linewidth, depth, depth, selectivity, selectivity, and and aspect aspect ratio ratio versus versusthe theRIE RIEpower. power. Figure 5a shows an increase in the linewidth when the RIE power is lower than 70 W for DVD-PDMS Figure 5a shows an increase in the linewidth when the RIE power is lower than 70 W for DVD-PDMS samples. Higher RIE RIE powers powersmaintain maintainthe thelinewidth linewidthwith withrespect respecttotothe theoriginal original linewidth (previous samples. Higher linewidth (previous to to RIE process). the caseofofBR-hPDMS BR-hPDMSsamples, samples,the thechange changeininthe thelinewidth linewidthis is less less abrupt. abrupt. That That thethe RIE process). InIn the case could due to to the the increased increased difficulty difficulty for for the the reactive reactive species species to to penetrate penetrate between between the the patterned patterned could be be due lines during the etching. Figure 5b shows the variation of the depth with the RIE power for lines during the etching. Figure 5b shows the variation of the depth with the RIE power for both both sets sets of of samples. samples. As As the the RIE RIE power power increases, increases, the the depth depth of of the the nanostructures nanostructures is is higher higher for for the the DVD-PDMS DVD-PDMS samples. This is is not notrelated relatedtotothe theinitial initialthickness thickness PMMA resist rather the geometry samples. This of of thethe PMMA resist butbut rather withwith the geometry and and dimensions of the stamp used. The maximum depth achieved is 460 nm at 70 W. the BRdimensions of the stamp used. The maximum depth achieved is 460 nm at 70 W. For the For BR-hPDMS hPDMS the maximum depth achieved is at 266 80higher W. ForRIE higher RIE powers, SiOisx samples,samples, the maximum depth achieved is 266 nm 80nm W. at For powers, the SiOx the mask mask is overetched. This produces a drop in the Si depth that can be etched. Figure 5c shows the overetched. This produces a drop in the Si depth that can be etched. Figure 5c shows the selectivity selectivity (SiOx/Si) for the SF6/O2 etching at different RIE powers. In the DVD-PDMS samples, the (SiO x /Si) for the SF6 /O2 etching at different RIE powers. In the DVD-PDMS samples, the highest highest selectivity is achieved W. For BR-hPDMS a high selectivity of 1:19 is selectivity of 1:32.8of is 1:32.8 achieved at 70 W. at For70BR-hPDMS samples, samples, a high selectivity of 1:19 is achieved achieved at 80 W. The aspect ratio (depth/linewidth) is shown in Figure 5d. In the DVD-PDMS at 80 W. The aspect ratio (depth/linewidth) is shown in Figure 5d. In the DVD-PDMS samples, the samples, the highest ratio is 1.19 at 80 W; in1.58 BR-hPDMS, highest aspect ratio isaspect 1.19 obtained at 80obtained W; in BR-hPDMS, at 70 W. 1.58 at 70 W. To fabricate 2D arrays of nanostructures (nanopillars), we performed similar optimization optimization of To fabricate 2D arrays of nanostructures (nanopillars), we performed aa similar of the etching by varying the RIE power. The rest of the conditions were the same than the the etching by varying the RIE power. The rest of the conditions were the same than used used for for the fabrication of the the Si Si nanobelts. nanobelts. A A master master with with nanoholes nanoholes of of diameter diameter ~150 ~150 nm nm and and aa period period ~400 ~400 nm fabrication of nm was used to obtain the stamp. Figure 6 shows a SEM image of the obtained nanopattern. was used to obtain the stamp. Figure 6 shows a SEM image of the obtained nanopattern. Table showsthe thedimensions dimensionsand andthe theselectivity selectivityvalues values obtained nanostructures. Figure Table 33 shows obtained forfor 2D2D nanostructures. Figure 7a 7a shows the relationship between the depth and the RIE power. We obtained a maximum depth of shows the relationship between the depth and the RIE power. We obtained a maximum depth of 535 nm at 70 W. Figure 7b shows an increase in the diameter of the nanopillars for RIE powers from 535 nm at 70 W. Figure 7b shows an increase in the diameter of the nanopillars for RIE powers from 70 70 to W. 80 We W. found We found the highest selectivity SiOto x/Si to be 1:38.2 at 70 W (see Figure 7c). Figure 7d to 80 the highest selectivity of SiOofx /Si be 1:38.2 at 70 W (see Figure 7c). Figure 7d shows shows the aspect ratio (depth/diameter) of the nanopillars with a maximum at RIE 70 W of RIE the aspect ratio (depth/diameter) of the nanopillars with a maximum of 2.71 atof702.71 W of power. power.

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Figure 5. (a)5.Variation of linewidth versus the RIE power in the SFSF /O plasma etching. (b) Variation 22 plasma Figure (a)(a) Variation of of linewidth versus thethe RIE power in in the SF 66 /O 2 plasma etching. (b)(b) Variation Figure 5. Variation linewidth versus RIE power the 6/O etching. Variation of depth with the RIE power ininthe SF /O etching. (c) Change in the selectivity to SiO x /Si 22 plasma etching. (c)(c) Change in in thethe selectivity to to SiO x/Si with of of depth with the RIE power thethe SFSF 66/O 6/O 2 plasma plasma etching. Change selectivity SiO x/Si with depth with the RIE power in with the RIE power the SF plasma etching. (d) Aspect ratio versus the RIE power in the SF thethe RIE power inin the SFSF 6/O 2/O plasma etching. (d) Aspect ratio versus the RIE power in the SF 6 /O 2 plasma RIE power in the 66/O 2 plasma etching. (d) Aspect ratio versus the RIE power in the SF 6 /O 2 plasma 2 6 /O2 etching. etching. plasma etching.

Figure 6. 6. SEM images 2D nanostructures silicon fabricated using 7070 WW of RIE power. Figure SEM images of 2D nanostructures silicon fabricated using power. Figure 6. SEM images of of 2D nanostructures ofof silicon fabricated using 70 Wof ofRIE RIE power. Table 3. 3. Results obtained in in 2D2D nanostructures forfor different RIE powers. Table Results obtained nanostructures different RIE powers.

Table 3. Results obtained in 2D nanostructures for different RIE powers. RIE Power RIE Power Diameter (nm) Diameter (nm) Depth (nm) Depth (nm) x/Si Selectivity SiO x/Si Selectivity SiO Aspect Ratio Aspect Ratio

Diameter (nm) Depth (nm) Selectivity SiOx /Si Aspect Ratio

60 60 WW 170170 353353 25.2 25.2 60 W 2.07 2.07

170 353

70 70 WW 197197 535535 7038.2 W38.2 2.71 2.71

197 535

80 80 WW 198198 RIE Power 497497 35.5 35.5 80 W 2.51 2.51

198 497

90 90 WW 173173 360360 25.7 25.7 90 W 2.08 2.08

100

100100 WW 178178 372372 26.5 W26.5 2.09 2.09

173 360

178 372

25.2

38.2

35.5

25.7

26.5

2.07

2.71

2.51

2.08

2.09

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Figure 7. (a) Variation of depth versus the RIE power in the SF6 /O2 plasma etching of 2D nanopillars. Figure 7. (a) Variation of depth versus the RIE power in the SF6/O2 plasma etching of 2D nanopillars. (b) Variation of theofdiameter versus the the RIERIE power. (c)(c) Selectivity dependenceononthe the RIE /Si) dependence RIE (b) Variation the diameter versus power. Selectivity(SiO (SiOx x/Si) power.power. (d) Aspect ratio dependence on the RIE power. (d) Aspect ratio dependence on the RIE power.

3.2. Wettability 3.2. Wettability Controlling the wetting properties a varietyofofsurfaces surfaces through through the their Controlling the wetting properties of of a variety the manipulation manipulationofof their physical and/or chemical properties at the nanoscale level has attracted increased attention due to to physical and/or chemical properties at the nanoscale level has attracted increased attention due their multiple applications [25]. Hydrophilic surfaces with a water contact angle of almost 0° have ◦ their multiple applications [25]. Hydrophilic surfaces with a water contact angle of almost 0 have been successfully used as a transparent coating with antifogging and self-cleaning properties, while been successfully used as a transparent coating with antifogging and self-cleaning properties, while hydrophobic surfaces can avoid contamination, the sticking of snow, and erosion [26]. The hydrophobic surfaces can avoid contamination, the sticking of snow, and erosion [26]. The modification modification of the surface morphology and the materials used permits to adjust the wettability from of the asurface morphology and the materials used adjustand theaspect wettability froman a important hydrophilic hydrophilic material to a hydrophobic one [27].permits Specificto periods ratios play material to a hydrophobic one [27]. Specific periods and aspect ratios play an important role for role for obtaining hydrophilic or hydrophobic states [28]. The Wenzel model [29] describes sessile obtaining hydrophilic or hydrophobic states [28]. The Wenzel model [29] describes sessile droplets droplets that fully wet the surface texture. On the other hand, the Cassie–Baxter model [30] describes that fully the surface texture. On on thethe other model describes water waterwet droplets that reside partially solidhand, texturethe andCassie–Baxter partially on a raft of air[30] pockets entrapped droplets thatthe reside partially on the texture and partially onhydrophobic. a raft of airInpockets within microscopic texture thatsolid enable the surface to become additionentrapped to high ratios (>>1), the feature density, geometric length scale, and topography of withinaspect the microscopic texture that enablethe thecharacteristic surface to become hydrophobic. In addition to high the surface texture all play pivotal roles in creating hydrophobic surfaces that exhibit robust Cassie– aspect ratios (>>1), the feature density, the characteristic geometric length scale, and topography of the Baxter interfaces that can resist wetting hydrophobic under dynamic conditions have shown surface texture all playand pivotal roles in creating surfaces that[31–33]. exhibit Studies robust Cassie–Baxter that an array of high-aspect-ratio nanostructures with high densities shows hydrophobicity, with interfaces and that can resist wetting under dynamic conditions [31–33]. Studies have shown that strong resistance against transition to the Wenzel state [28,32,34]. We studied the wettability an array of high-aspect-ratio nanostructures with high densities shows hydrophobicity, with strong properties of the fabricated Si nanobelt surfaces through the measurement of the contact angle. A 0.5 resistance against transition to the Wenzel state [28,32,34]. We studied the wettability properties of the µL drop of water was placed over the nanostructured surface to measure its contact angle (Figures fabricated Si nanobelt surfaces through the measurement of the contact angle. A 0.5 µL drop of water 8a,b). Figure 8c shows that the highest value for the contact angle for DVD-PDMS samples was 96.2° was placed over measure itswas contact angle Figuresamples, 8c shows (etched withthe 30 nanostructured W of RIE power)surface and thetolowest value 8.1° (50 W).(Figure For the 8a,b). BR-hPDMS ◦ (etched with 30 W of that the highest value for the contact angle for DVD-PDMS samples was 96.2 the highest value of the contact angle was 36.1° (100 W) and the lowest value was 10.5° (50 W). We ◦ RIE power) andthat theDVD-PDMS lowest value was 8.1 W). For the BR-hPDMS samples, the highest value of the observed samples had(50 a higher hydrophobicity for low RIE powers (70 value 10.5◦ (50for W). We types observed that DVD-PDMS hPDMS samples higher RIEthe powers W).was However, both of samples, a high hydrophilic surface was obtained at W. RIE Figure 8d shows in the contact angle with the samples had a higher hydrophobicity for50low powers (70 ratio. W). In DVD-PDMS, theboth highest hydrophobicity at an surface aspect ratio 0.45; in at RIE powers However, for types of samples, awas highachieved hydrophilic was of obtained samples, 1.6. Weinfound that, forangle all surfaces, the wetting stateIncorresponds to athe Wenzel 50 W. BR-hPDMS Figure 8d shows theatchange the contact with the aspect ratio. DVD-PDMS, highest hydrophobicity was achieved at an aspect ratio of 0.45; in BR-hPDMS samples, at 1.6. We found that, for all surfaces, the wetting state corresponds to a Wenzel state and the contact angle never exceeds the critical angle needed for the transition to a Cassey–Baxter condition (Figures S3–S6 and Tables S1–S3).

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Nanomaterials 2017, 109 8 of 11 state and the7,contact angle never exceeds the critical angle needed for the transition to a Cassey–

Baxter (Figures Tables state andcondition the contact angle S3–S6 never and exceeds theS1–S3). critical angle needed for the transition to a Cassey– Baxter condition (Figures S3–S6 and Tables S1–S3).

Figure 8. (a) Image of the contact angle 0.5µL µLdroplet dropletof ofwater wateron on top top of of aa DVD-PDMS Figure 8. (a) Image of the contact angle ofof a a0.5 DVD-PDMSSiSinanobelt nanobelt (etched atImage 30 W). nanobelts etched W. (c)Contact Contact angle versus the power Figure 8. (a) theSame contact angle of aetched 0.5 µL droplet of(c) water on topangle of a DVD-PDMS Si nanobelt (etched at 30 W). (b)of(b) Same forfor nanobelts atat5050W. versus the RIE RIE powerfor for DVD-PDMS and nanobelts. (d) Contact angle versus the aspect ratiothe for DVD-PDMS (etched at 30and W). (b)BR-hPDMS Same fornanobelts. nanobelts etched at 50angle W. (c)versus Contact angle versus power forand DVD-PDMS BR-hPDMS (d) Contact the aspect ratio forRIE DVD-PDMS and BR-hPDMS nanobelts. DVD-PDMS and BR-hPDMS nanobelts. (d) Contact angle versus the aspect ratio for DVD-PDMS and BR-hPDMS nanobelts. BR-hPDMS nanobelts.

3.3. Optical Measurements 3.3. Optical Measurements 3.3. Optical Measurements The interaction of materials with optical waves and photons is strongly dependent on the The interaction of materials with optical waves and photons is strongly dependent on the structure The interaction of materials withmodifications optical waves photons of is astrongly dependent the structure of their surface. Nanoscale of and the structure surface can be used on to control ofstructure their surface. Nanoscale modifications of the structure of a surface can be used to control the theirdistribution surface. Nanoscale modifications of allowing the structure of a development surface can beof used to control the lightoffield and light propagation, for the a large range of light field distribution light propagation, for the development of range of key thekey light field distribution and light propagation, allowing for the development ofaalarge large rangein ofthe components for and optical systems [35]. We allowing measured the reflectivity of the silicon nanobelts components for optical systems [35]. We measured the reflectivity of the silicon nanobelts in key componentssfor optical systems [35]. We thespectra reflectivity of polarization the silicon nanobelts in the the polarizations and p. Figure 9 shows themeasured reflectivity in the s (polarization p polarizations ssand p. 9 shows shows the reflectivity spectra polarization s (polarization polarizations and p. Figure Figure reflectivity spectra in in thethe polarization s (polarization showed lower intensity for 9both setsthe of samples) using a UV-visible elipsometer (J.A Woolamp M-p showed lower forfor both sets of of samples) using a UV-visible elipsometer (J.A Woolam M-2000FI, showed lower intensity both sets samples) using a UV-visible elipsometer (J.A Woolam M2000FI, J.A intensity Woolam Co, Lincoln, NE, USA) Measurements were performed at 45° on the DVD-PDMS ◦45° J.A Woolam Co, Lincoln, NE, USA) Measurements were performed at 45 on the DVD-PDMS and 2000FI, J.A Woolam Co, Lincoln, NE, USA) Measurements were performed at on the DVD-PDMS and BR-hPDMS samples fabricated with different RIE powers. and BR-hPDMS samples fabricated different powers. BR-hPDMS samples fabricated withwith different RIERIE powers.

Figure 9. Reflectivity spectrums for the DVD-PDMS and BR-hPDMS samples. Figure9. 9. Reflectivity Reflectivity spectrums and BR-hPDMS samples. Figure spectrumsfor forthe theDVD-PDMS DVD-PDMS and BR-hPDMS samples.

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For both cases, we obtained high oscillations in the reflectivity with maximum values reaching 50% of reflectivity. The first peak, around 280 nm, is correlated with the high reflectivity peak of silicon, although in our case this peak is much narrower due to the nanostructuration (Figure S7). This is an interesting spectral region in the UVC part of the light spectrum where most proteins show strong optical absorptions [36]. The rest of the peaks were naturally produced by the periodicity of the samples. In the DVD-PDMS samples, as well as in the BR-hPDMS samples, the spectrums showed that the highest reflectivity was achieved when the RIE power was 30 W, and decreased when the RIE power was increased. 4. Conclusions This research demonstrates a new method for the fabrication of nanogratings and nanopillars onto Si wafers with aspect ratios >1 by combining soft lithography and RIE starting from nanopatterned resist layers as thin as 14 nm. We fabricated 1D and 2D nanostructures using low-cost DVD and BR stamps, over large areas, in a simple, fast, and reproducible way. We demonstrated the control of the aspect ratio of the fabricated silicon nanostructures by varying the RIE power during the RIE process. By optimization of the RIE power, we fabricated nanobelts with aspect ratios up to 1.6 (142 nm of linewidth and 225 nm of depth, using BR-hPDMS stamps) and an aspect ratio of 2.71 for 2D nanopillars (197 nm of diameter and 535 nm of depth). We also demonstrated how the wetting properties of the photonic surfaces that are patterned using this method can be tuned. For the DVD-PDMS samples, the highest angle achieved was 96.2◦ and the lowest was 8.1◦ . For BR-hPDMS, the contact angle varied between 36.1 and 10.5◦ . Finally, the reflectivity was measured with a UV-visible elipsometer, obtaining a narrow and intense peak of intensity in the UVC spectral region (280 nm), where some aminoacids and peptides have a strong optical absorption. Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/7/5/109/s1. Acknowledgments: We acknowledge the service from the X-SEM Laboratory at IMM, and funding from MINECO under project CSIC13-4E-1794 with support from the EU (FEDER, FSE), TEC2014-54449-C3-3-R and PCIN-2013-179. Author Contributions: Estela Baquedano, Ramses V. Martinez and Pablo A. Postigo conceived and designed the experiments; Estela Baquedano performed the experiments; Estela Baquedano and Pablo A. Postigo analyzed the data; José M. Llorens performed the simulations; All authors wrote, reviewed and approved the final version of the manuscript. Conflicts of Interest: The authors declare no conflicts of interest.

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