Wafer Scale Fabrication of Dense and High Aspect Ratio Sub-50 nm

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Wafer scale fabrication of dense and high aspect ratio sub-50 nm nanopillars from phase separation of cross-linkable polysiloxane/polystyrene blend Yang Li, Yuli Hao, Chunyu Huang, Xingyao Chen, Xinyu Chen, YuShuang Cui, Changsheng Yuan, Kai Qiu, Haixiong Ge, and Yanfeng Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00106 • Publication Date (Web): 31 Mar 2017 Downloaded from http://pubs.acs.org on April 4, 2017

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ACS Applied Materials & Interfaces

Wafer scale fabrication of dense and high aspect ratio sub-50 nm nanopillars

from

phase

separation

of

cross-linkable

polysiloxane/polystyrene blend Yang Li,1,3 Yuli Hao,1,3 Chunyu Huang,2,3 Xingyao Chen,1,3 Xinyu Chen, 1,3

Yushuang Cui,1,3 Changsheng Yuan,1,3 Kai Qiu,2,3 Haixiong Ge,2,3,* &

Yanfeng Chen2,3 1

Department of Materials Science and Engineering, College of

Engineering and Applied Sciences, National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China 2

National Laboratory of Solid State Microstructures, School of Physics,

Nanjing University, Nanjing 210093, China 3

Collaborative Innovation Center of Advanced Microstructures, Nanjing,

210093, China

Corresponding Author *E-mail: [email protected]

Abstract: In this article, we demonstrated a simple and effective approach to fabricate dense and high aspect-ratio sub-50 nm pillars based on phase separation of a polymer blend composed of a cross-linkable polysiloxane and polystyrene (PS). In order to obtain the phase-separated domains with nanoscale size, a liquid prepolymer of cross-linkable 1

ACS Paragon Plus Environment


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polysiloxane was employed as one moiety for increasing the miscibility of the polymer blend. After phase separation via spin-coating, the dispersed domains of liquid polysiloxane with sub-50 nm size could be solidified by a UV-exposure. The solidified polysiloxane domains took the role of etching mask for formation of high aspect ratio nanopillars by O2 reactive ion etching (RIE). The aspect ratio of the nanopillars could be further amplified by introduction of a polymer transfer layer underneath the polymer blend film. The effects of spin speeds, the weight ratio of the polysiloxane/PS blend and the concentration of polysiloxane/PS blend in toluene on the characters of the nanopillars were investigated. The gold coated nanopillar arrays exhibited a high Raman scattering enhancement factor in the range of 108~109 with high uniformity across over the wafer scale sample. A superhydrophobic surface could be realized by coating a self-assembled monolayers (SAM) of fluoroalkyltrichlorosilane on the nanopillar arrays. Sub-50 nm silicon nanowires (SiNWs) with high aspect-ratio about 1000 were achieved by using the nanopillars as etching mask through a metal-assisted chemical etching process. They showed an ultra-low reflectance of approximately 0.1% for wavelengths ranging from 200 to 800 nm.

Keywords: phase separation, dense and high aspect ratio nanopillars, superhydrophobic

surface,

surface

enhanced

2


Raman

scattering,

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antireflective film

1. Introduction Dense and high aspect ratio nanopillar arrays are of great interest for numerous potential applications such as electronics,1-4 optics,5,

6

energy harvest and storage,7-9 catalysis,10 and chemical and biological sensing11-14 due to their high surface-to-volume ratio. Nanopillar arrays have been successfully fabricated by a variety of top-down lithographic

approaches

based

on

e-beam,15

interference lithography,16 and nanoimprint.17 They usually involve expensive instruments, which probably require a time-consuming fabrication process, to obtain nanostructures with desired shapes and characteristics. Hence, top-down approaches offer high fidelity and high controllability, but need high cost. Alternative to top-down lithographic techniques, self-assembly methods such as phase separation of diblock copolymers (DBC) and anodic aluminum oxide template are widely used.18-21 Particularly, well-ordered nanodots array with 3 nm in diameter over large areas was successfully achieved by the self-assembly of block copolymers with a suitable choice of immiscible segments and their chain lengths.22 However, besides the complicated synthetic routes of block copolymers, annealing for phase separation is a time consuming process with rigorous requirements. In 3



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many applications, apart from the minimum feature size, cost and throughput are becoming more and more important factors. The shrinkage of size of nanostructures in an inexpensive and high throughput way is continuously very attractive. Authors have reported a new method to fabricate nanopillar and nanodot arrays by using phase separation of polymer blends.23, 24 This approach offers several advantages such as low-cost, high-throughput, wafer-scale and facile processing. In our previous report,25 silicon containing polymer was introduced into the polymer blend for achieving high aspect ratio nanopillar arrays through a selective etching process. So far, seldom studies have been reported to employ the phase separation of polymer blends to form sub-100 nm patterns because the phase separation of most polymer blends generally forms the structures larger than 100 nm. Even though the phase separation of polymer blends with optimized parameters can form sub-100 nm domains,26, 27 the low aspect ratio of the phase-separated morphologies becomes a challenge for further pattern transfer. In this research, a liquid cross-linkable prepolymer of acrylated polysiloxane was utilized as one of the two components of the polymer blend for formation of high resolution and high aspect ratio nanostructures based on phase separation approach. Compared with other nanofabrication techniques, especially the self-assembly of 4


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block copolymers, the components of the polymer blend are easily obtained

and

commercially

available.

The

formation

of

phase-separated nanostructures only involves a single spin-coating step, which is a very simple, wafer-scale, low-cost and time saving process. By optimizing the phase separation parameters, we can obtain dense nanopatterns with a sub-50nm resolution. After phase separation of the polymer blend through a spin-coating process, the sub-50 nm dispersed domains of liquid polysiloxane were solidified by a UV-light exposure. Dense and high aspect ratio nanopillar arrays with diameter of sub-50 nm can be obtained by using the solidified polysiloxane domains as etching mask for an O2 RIE process. These nanopillar arrays are compatible with existing nanofabrication techniques for further transferring them to various nanodevices. To illustrate the potential of this approach, the nanopillar arrays coated with Au provides a surface enhanced Raman scattering (SERS) enhancement factor up to 109 with high uniformity. Furthermore, the dense and high aspect ratio nanopillars can be converted to a superhydrophobic surface

by coating

a

fluoroalkyltrichlorosilane.

self-assembled

monolayer

(SAM)

of

High

vertically

aligned

Si

density

nanowires with a high aspect ratio about 1000 were achieved through the obtained nanopillars in combination with metal-assisted chemical etching (MaCE) method. They exhibited an ultra-low reflectance of 5



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approximately 0.1% for wavelengths ranging from 200 nm to 800 nm.

2. Results and discussion 2.1 Phase separation of cross-linkable polysiloxane/polystyrene polymer blend It is well known that morphologies of phase separated polymer blends are generally controlled by many factors including polymer molecular structures, composition, molecular weights, and nature of substrate surfaces. The size of the dispersed phase has been found to depend strongly on the compatibility of the two polymer constituents. Generally, the smaller size of the dispersed phase corresponds to the better compatibility of the polymer blend. In order to increase the compatibility, the components of the polymer blends may be selected based on the principle of similar polarity. In addition, one of the most important factors affecting the miscibility of the blends composed of low molecular weight materials is the combinatorial entropy of mixing which is much larger than that of high molecular weight polymers.28 The entropy contribution is the reason that liquid-liquid mixtures offer a much better miscibility than polymer-liquid combinations and polymer-polymer mixtures has even much lower miscibility. With this knowledge in mind, in order to obtain an ultra-small dispersed phase during phase separation of polymer blends, we employed a liquid prepolymer of cross-linkable acrylated 6


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polysiloxane with a low viscosity of about 100 cP as one constituent. Polystyrene (PS) with Mn=100,000 was used as the other component for all the experiment described here. The static water contact angle measurement was used to qualitatively characterize the polarity of the polymers. The water contact angles of acrylated polysiloxane and PS are 83⁰ and 89⁰, respectively. The surface of PS and polysiloxane exhibited similar water contact angles, which indicated the similar polarity of the two materials. Solutions of polysiloxane/PS blends were prepared by dissolving polysiloxane and PS into toluene with various weight ratios of polysiloxane/PS. In addition, a photoinitiator, Irgacure 184 (2 wt% vs. the acrylated polysiloxane), was added into the solution. Figure 1a schematically presents the fabrication procedure of dense and high aspect nanopillars based on the phase separation of the acrylated polysiloxan/PS blends. The films of the polymer blends were formed on substrates by spin-coating, and then an UV-exposure was used to solidify the liquid polysiloxane domains in a nitrogen atmosphere by crosslinking the acrylate groups through a free radical polymerization. The PS moiety in the film was removed and the polysiloxane was oxidized into silica-like species by the O2 RIE. The aspect ratio of the etched structure can be further amplified by introduction of an organic transfer layer, such as SU-8 photoresist layer, underneath the blend film. The nanopillars of silica-like composition can take on the role as an O2 etching mask for 7



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higher aspect ratio ones by a selective etching process. After the O2 RIE, a residual layer remained at the bottom among the nanopillars was eliminated by a fluorine-based RIE. Then, the nanopillars can be transferred into the SU-8 layer by second O2 RIE due to their resistance to O2 RIE. Figure 1b shows top-view and cross-sectional scanning electron microscopy (SEM) images of the cross-linkable polysiloxane/PS blend film on a silicon wafer with a weight ratio of 9:1 and 5 wt% toluene solution at a spin speed of 3000 r·min-1 after UV-exposure and O2 RIE. Densely packed nanopillars with a diameter of sub-50 nm and aspect ratio above 4 were obtained. It can be concluded that the continuous phase of the polymer blend film was composed of PS and the dispersed phase was composed of polysiloxane domains because the PS could be selectively depleted by the O2 RIE while the polysiloxane could only be converted to silica-like species and resistant to O2 RIE. After O2 RIE, a residual layer was seen at the bottom among the nanopillars in the cross-sectional SEM image shown in the inset of Figure 1b. After phase separation, there must be a small fraction of polysiloxane still distributed in the PS-rich phase. The organic components in the PS-rich phase were depleted by O2 RIE, whereas the polysiloxane was transformed into a silica-like residual layer and remained on the bottom among the nanopillars. Figure 1c shows the cross-sectional SEM image of nanopillars formed on a SU-8 film under the same other experimental conditions as the ones in 8


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Figure 1b. Before transferring the nanopillars into the underlying SU-8 layer by O2 RIE, another step of fluorine based RIE was required to remove the residual layer because the silica-like species are resistant to O2 RIE. Since the thickness of the residual layer was sub-scale to the height of the O2 etched nanopillars, a limited fluorine-based RIE could only remove the residual layer and have little influence on the nanopillars. After the selective etching process of a fluorine-based RIE and O2 RIE, the nanopillars were successfully transferred into the SU-8 layer as shown in figure 1d. The aspect ratio of the obtained nanopillars was close to 10:1. The closing and warping (deformation) of the nanopillars may be driven by the electron beam during the SEM observation especially for high-aspect-ratio polymer nanostructures. Figure 2a and b show the atomic force microscope (AFM) height and phase images of the same sample in figure 1b without O2 etching, respectively. The AFM height image exhibits that a great number of isolated sub-50 nm small islands are randomly distributed in a continuous matrix and slightly protruded about 1.6 nm from the continuous background. The corresponding phase image reveals that these protruded domains are the dispersed darker regions in the phase image because they have almost identical shapes and sizes at the same relative positions in both images. It can be confirmed that the dispersed phase was composed of acrylated polysiloxane in combination with the aforementioned SEM 9



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observation. In this study, a selective etching experiment was used to gain a better insight into the phase morphology of the spin-coated polysiloxane/PS blend film. Figure 2c shows the curves of film thickness of pure PS and crosslinked polysiloxane versus etching time under O2 and fluorine based RIE, respectively. The film thicknesses before and after etching were measured by interferometric measurements. The film thickness of the pure PS and polysiloxane was reduced linearly under fluorine based RIE at a similar rate of 0.2 nm/s. The O2 RIE rate of PS remained almost unchanged at 1.0 nm/s during whole O2 RIE process while the O2 RIE rate of the polysiloxane dropped rapidly and reached a very low stable value of approximately zero with the extension of the etching time, which meant that the polysiloxane component had a high resistance to O2 RIE. In this experiment, the blend films were first etched by a fluorine based RIE in different etching times, followed by an over-etching of O2 RIE. Figure 2d and e show the cross-sectional SEM images of the blend films etched by the fluorine based RIE for 600 and 1200 s, respectively. The insets are top-view SEM images of the same samples followed with an over-etching of O2 RIE. The thickness of the blend film decreased from 230 nm to 163 nm with 600 s fluorine based RIE. Both components were depleted equally because the PS and polysiloxane had the similar etching rate under fluorine based RIE. Followed with an over O2 RIE, smaller isolated nanodots with a 10


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decreased contrast were left as shown in the inset, compared with the sample without fluorine etching in Figure 1. This result indicated that the polysiloxane phase still remained in the blend film after 600 s fluorine based RIE. As the fluorine etching time extended to 1200 s, the film thickness decreased to 99 nm, and after an over O2 etching, the whole blend film was etched off as shown in Figure 2e. It can be inferred that the polysiloxane phase had been depleted in this fluorine etching stage and the left film was removed by O2 etching without the protection of polysiloxane as the etching mask. These results allowed us to draw a schematic (Figure 2f) of the phase-separated morphology that the dispersed liquid polysiloxane domains floated on the surface of the continuous PS phase. The phase separated morphology may be formed by a balance of surface tensions. On the other hand, since the liquid polysiloxane was mutually miscible with the toluene, PS was more quickly precipitated from the solution and solidified first to form a continuous film with the toluene evaporation. The polysiloxane tended to stay longer in the toluene-rich phase. Since the relative densities of the PS, polysiloxane and toluene were 1.05, 1.1 and 0.87, respectively, the polysiloxane phase containing more toluene may lead to a lower density. The phase with a lower density could be driven toward the surface by a buoyant force. To control the parameters of the nanopillar arrays formed from the phase 11



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separation, we investigated the effects of spin speed, the weight ratio of the polysiloxane/PS blend and concentration of polysiloxane/PS blend in toluene on the phase separated morphologies. Figure 3 exhibits top-view SEM images of the O2 etched polysiloxane/PS blend films with four different weight ratios, 1:3, 1:4, 1:5 and 1:6. The spin speed was fixed at 3000 rpm, and the concentration of the polysiloxane was set at 1%. With the increase of the PS to polysiloxane weight ratio, the morphologies of the O2 etched samples were transformed from the interconnected domains into isolated islands. Since the concentration of polysiloxane was fixed, the increase of the PS to polysiloxane weight ratio would increase the PS concentration, which led to the increase of the viscosity of the blend solution. The higher viscosity could suppress the coalescence of the polysiloxane droplets dispersed in the PS matrix. Hence, the increase of the PS fraction in the blends resulted in the formation of the isolated polysiloxane islands and the decrease of the characteristic sizes of the islands. In this study, it was found that the spin speed and the solution concentration also had impacts on the phase separation structures. Figure 4 shows the top-view SEM images of O2 etched samples prepared with different spin speeds at 3000, 4000, 5000 and 6000 rpm in a constant polysiloxane/PS weight ratio of 1:5 and fixed solution concentration of 6 wt %. As the spin speed increased, the morphologies of the O2 etched 12


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samples changed from the interconnected domains to isolated islands as shown in Figure 4. Furthermore, with the increase of spin speeds, the average feature size of the isolated islands and their standard deviation decreased, and the number density of the islands increased. The evaporation of the toluene was accelerated with the higher spin speed and the coalescence of droplets is time dependent. There was not enough time for the dispersive polysiloxane droplets to coalesce into larger droplets before the faster solidification of PS continuous at higher spin-speed, which led to the diameter of the island smaller. Figure 5 shows the phase separation morphologies of the polysiloxane/PS blends after O2 RIE process with the concentration of 2 wt%, 3 wt%, 4 wt% and 5 wt% while the initial polysiloxane/PS ratio in the solution was set at 1:9. It was observed that the lower solution concentration of the polysiloxane/PS blends resulted in the smaller feature size of the isolated polysiloxane domains. It has been found that the phase-separated domain sizes of polymer blends were increased linearly by increasing solution concentration.29-31 The thickness of the spin-coated film is proportional to solution concentration. During the spin-coating process, it took less time for the thinner film to evaporate the solvent and solidify, which provides less chance for the phase-separated domain coarsening into larger ones, resulting in smaller feature size and better uniformity. The parameters of the nanopillars prepared under different experimental conditions were 13



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summarized in Table I.

2.2 Applications of phase-separation lithography To illustrate the potential of this approach, we formed and investigated SERS substrate, superhydrophobic surface and high antireflective surface based on the dense and high aspect ratio sub-50 nm nanopillars. It has been reported that noble metal nanoparticles with enough small gaps usually provide dramatically enhanced Raman enhancement factors (EF) up to 1011 for a new localized electromagnetic distribution with strong field intensity.32 However, the sparse hot-spots and lack of the uniformity and reproducibility in EF for the random nanostructures may limit their practical applications. Thus, the challenge to SERS for the real applications is how to fabricate large-area and uniform SERS-active substrate with dense hot-spots through a cost-efficient and reproducible way. The gold coated polymer nanopillars have been used as high efficiency surface enhanced Raman spectroscopy (SERS) substrates.33, 34 The polymer nanopillars can bend and close to trap detected molecules by the microcapillary force under exposure to liquid as well as provide ultra-small gaps between nanopillar tips as hot spots for an ultrahigh field enhancement. In this study, we fabricated the SERS substrates based on the above prepared dense nanopillar arrays. Figure 6a is the schematic diagram of the simple fabrication process of the SERS substrates. The 14


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nanopillars were prepared by the polysiloxane/PS blend with the weight ratio of 1:5 and concentration of 6 wt% at a spin speed of 5000 rpm and followed by an O2 RIE process. Finally, a 30 nm thick Au layer was deposited on nanopillar arrays by the e-beam evaporation coating with rotating the sample stage. As shown in Figure 6b, the average diameter of the gold coated pillar was 54 nm, the height was approximately 180 nm and the narrow gaps between pillars could be smaller than 10 nm. Besides the narrow distance between the top of the pillars, since the sidewall of the pillars was coated with Au, the narrow gaps between the sidewalls of the pillars could also contribute to the signal enhancement. In addition, the gold coated nanopillars can also bend and close together to form nanogaps. Figure 6c exhibits the 10-fold amplified Raman spectra of trans-1,2-bis(4-pyridyl)ethylene

(BPE)

on

bare

silicon

at

the

concentration of 10-1 M and the Raman spectra of using BPE on the pillar arrays coated with Au layer at the concentration of 10-3 M, 10-6 M and 10-9 M, respectively. The excitation wavelength was 633 nm and the exposure time was 1 s. The characteristic peaks of BPE molecule can be clearly identified at the wavelength of 1200, 1605 and 1641 cm-1. Raman scattering enhancement factor of the gold coated nanopillar arrays was estimated in the range of 108~109 by EF = (ISERS × Nbulk)/(Ibulk × NSERS) ,35 where ISERS is the Raman intensity of the 1200 cm−1 peak resulting from BPE molecules on the gold coated nanopillar arrays substrate and Ibulk is 15



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the Raman signal of the same band on the bare Si substrate. NSERS and Nbulk are the numbers of molecules on the illuminated area, which are proportional to the effective surface area of the pattern. Such a remarkable SERS performance is owing to the existence of the highly-dense nanogaps created from the highly dense gold coated nanopillars. The uniformity of the fabricated SERS substrate was also investigated. As shown in Figure 6d, we measured the Raman spectra of BPE at five different points randomly chosen on the 3×3 cm2 fabricated substrate. The variable coefficient of the intensity of Raman signal at 1200 cm−1 was measured to be less than 5%, which indicated the uniformity of the nanopillar arrays in macroscopic scale. It has been found that the parameters of the similar structures, such as the diameter, height, and especially the gold layer thickness have significant effects on the SERS performance.36,37, 38 Compared with other fabrication methods, it is quite simple to obtain nanopillars with different diameters, heights and densities by controlling the phase separation and etching conditions. In addition, the nanogap size between the gold coated nanopillars can also be tuned by the deposition thickness of the gold layer. Since the densely packed high-aspect-ratio nanopillar arrays were composed of silica-like species after O2 RIE, the surface could be converted into superhydrophobic one by coating a conventional antiadhesion layer of trichloroﬂuoroalkylsilane SAM. The dewettability 16


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wasestimated by measuring the contact angles of water droplets on the surface. Figure 7 exhibits the static water contact angle as high as 150.5⁰±0.5⁰, indicating a superhydrophobic surface. Silicon nanowires (SiNWs) have been attracting much attention in recent years for their potential applications in optoelectronic devices,39-42 chemical and biological sensors,43-46 field emitters,47 etc. Extensive efforts have been devoted to fabrication of SiNWs. Metal-assisted chemical etching (MaCE)48-50 approach is a promising way for the fabrication of high-aspect-ratio silicon nanostructures in combination with lithographic techniques due to its simplicity, versatility and low cost. In a typical MaCE process, Si substrate partly covered by a noble metal is etched in a solution composed of HF and an oxidative agent. Generally, the etching rate of Si underneath the noble metal is much higher than the Si without noble metal coverage so that silicon nanostructures can be formed after this procedure. MaCE has been applied to fabricate arrays of well-aligned and uniformly distributed SiNWs with controlled optoelectronic properties. In this part, the SiNWs were fabricated from the aforementioned nanopillar arrays by the MaCE method. The fabrication procedure is quite simple as schematically depicted in Figure 8a. A uniform film of the polysiloxane/PS blend was firstly spin-coated on a 4’’ silicon wafer, as shown in Figure 8b. After the phase-separated film was treated with O2 and fluorine-based RIE to remove the PS component and 17



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eliminate the silica-like residual layer, respectively, the produced polymer nanopillars on silicon wafer were coated with a 15 nm thick gold film as catalyst by resistance heating vacuum evaporation. Then, MaCE step was conducted in a mixture of deionized water, H2O2 and HF for 8 min. During the MaCE process, etching occurred only in the regions of the silicon

wafer covered

by gold,

leading

to

vertically aligned

high-aspect-ratio SiNWs. After etching, we removed the polymer by immersing the substrate in toluene for 5 min and the Au film by immersing it in boiling aqua regia (3:1 (v/v) HCl/HNO3) for 30 min. Finally, the wafer-scale and high density vertically aligned Si nanowires with the aspect ratio near 1000:1 were obtained as shown in Figure 8c and d. Figure 8e exhibits the reflectance spectra of the fabricated SiNWs in Figure 8d and the polished bare silicon substrate. The SiNWs gave an ultra-low reflectance of approximately 0.1% over the entire spectral range from 200 to 800 nm. This was probably attributed to the high light trapping capability of the dense high-aspect-ratio SiNWs with the small diameter. The ultra-low reflectance in a broad band ranging from 200 to 800 nm indicated that the fabricated SiNWs could have great potential in antireflective surfaces and photovoltaics. 3. Conclusion In summary, we proposed a simple method to fabricate dense and high aspect-ratio sub-50 nm pillars based on phase separation of a polymer 18


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blend composed of a cross-linkable polysiloxane and polystyrene (PS). In order to achieve phase-separated domains with sub-50 nm size, the liquid prepolymer of cross-linkable polysiloxane was used as one component for increasing the miscibility of the polymer blend. After phase separation, the dispersed phase of cross-linkable liquid polysiloxane were solidified by a UV-exposure, which could be taken as an etching mask to form dense and high aspect-ratio nanopillars by an O2 RIE. The aspect ratio of the nanopillars was further amplified by introduction of a polymer transfer layer underneath the blend film. Compared with other nanofabrication techniques, especially the self-assembly of block copolymers, the components of the polymer blend are easily obtained and commercially available. The formation of phase-separated nanostructures only involves a single spin-coating step, which is a very simple, wafer-scale, low-cost and time saving process. The gold coated nanopillars arrays exhibited a SERS enhancement factor in the range of 108~109 with high uniformity across over the wafer scale sample. The nanopillar arrays could be converted to a superhydrophobic surface by coating a SAM of fluoroalkyltrichlorosilane. SiNWs with diameter of sub-50 nm and aspect-ratio about 1000 were obtained by using the nanopillars as etching mask through MaCE process. The SiNWs showed an ultra-low reflectance of approximately 0.1% for wavelengths ranging from 200 to 800 nm. 19



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4 Experimental details: Materials: All the polymer materials and chemical reagents are commercially available and used without further purification. PS (Mw = 100,000, Mw/Mn= 1.06) was obtained from Sigma-Aldrich. Acrylated polysiloxane and trichloro(1H,1H,2H,2H-perfluorooctyl) silane were purchased from Gelest. SU-8 2150 photoresist was purchased from MicroChem. The p-type silicon wafers with an orientation of (100) were cleaned by a standard RCA cleaning process before use. Preparation of nanopillar arrays: Polymer blend solutions with various polysiloxane/PS weight ratios and concentrations were prepared by dissolving each mixture of PS and acrylated polysiloxane prepolymer in toluene (concentration expressed as % w/w). The blend film was formed by spin-coating the solutions onto a substrate (the temperature of 25℃ and the humidity of 30% RH) and subsequently treated with the UV exposure for 1 min to cure the polysiloxane prepolymer in a nitrogen atmosphere. PS and SU-8 were removed by RIE with O2 gas flow rate of 10 sccm, process pressure of 2 Pa and RF power of 40 W (RIE 100, Oxford) and the residual polysiloxane layer was etched by CHF3 RIE with flow rate of 20 sccm, process pressure of 2 Pa and RF power of 50 W. Preparation of gold coated nanopillars: The nanopillar structures were firstly fabricated with the polysiloxane/PS blend (weigh ratio=1:5, 6 wt%) at the spin speed of 5000 rpm. An Au layer with a thickness of 30 nm was deposited on the nanopillar arrays by vacuum e-beam evaporation coating. Preparation of superhydrophobic surface: The nanopillar arrays were coated with trichloro(1H,1H,2H,2H-perfluorooctyl) silane by a vapor phase deposition. Preparation of high-aspect-ratio SiNWs: The nanopillar arrays were firstly fabricated on a silicon wafer with the polysiloxane/PS blends (weigh ratio=1:9, 5 wt%) at the spin speed of 3000 rpm. The nanopillar arrays were treated with the fluorine-based RIE to eliminate the residual layer covered the silicon wafer. An Au layer with a thickness of 15 nm was deposited on the nanopillar arrays by vacuum e-beam evaporation coating. The sample was submerged in a HF/H2O2 aqueous solution for 20


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MaCE and the molar proportion of HF/H2O2/H2O is 17:3:100. During the MaCE process, only the areas of the silicon wafer contacted with Au were etched, leading to vertically aligned high-aspect-ratio SiNWs. Characterization: All the SEM images were detected using field-emission scanning electron microscope (ZEISS ULTRA-55). The distribution of pillar diameter and density of pillars were obtained from the SEM image by the software Nano Measurer. The SERS signals were recorded using an upright confocal Raman microscope (Labram Aramis Raman Spectrometer, Horiba Scientific) equipped with a nitrogen-cooled multichannel CCD detector and through a 50×objective. 633 nm wavelength laser was used with exposure time of 1 second for BPE. The power at the sample was 3 mW. All the samples were soaked in the prepared BPE solutions (ethanol) with known concentrations for 20 min, and subsequently dried in air as well. When measuring the Raman signal of the bulk sample, we chose five points at different areas with a relatively homogeneous color under a microscope. The static water contact angle was measured using a contact angle meter (JC2000 CS). The films of pure PS and acrylated polysiloxane were spin coated on silicon wafers, respectively. The acrylated polysiloxane film was further cured by a UV exposure in a nitrogen atmosphere. Water contact angles of the cured and uncured film of acrylated polysiloxane were both measured. (water contact angle of cured film: 83⁰; uncured film: 82⁰) Contact angles were measured at five different locations on each sample using 5 µL liquid droplets; the average values are reported in this paper. The reflectance spectra were detected using an UV-VIS-NIR spectrophotometer (UV-3600, Shimadzu) at normal incident angle.

21



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Table 1. Summarization of feature size distribution of the nanopillars with various Polysiloxane/PS weight ratios, spin speeds and concentrations. Average Polysiloxane/PS

Concentration

Spin speed

weight ratio

(%)

(rpm)

Standard deviation

Feature density

(nm)

(features inch-2)

feature size (nm) 1:3

4

3000

--

--

--

1:4

5

3000

--

--

--

1:5

6

3000

--

--

--

1:6

7

3000

67

16

3.19 × 1010

1:5

6

4000

74

23

1.19 × 1011

1:5

6

5000

67

22

1.03 × 1011

1:5

6

6000

57

15

1.44 × 1011

1:9

2

3000

--

--

--

1:9

3

3000

17

3

6.40 × 1011

1:9

4

3000

23

5

3.40 × 1011

1:9

5

3000

40

6

8.42 × 1010

22


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Figure 1

Figure 1. (a) Schematic of the fabrication procedure of dense and high aspect nanopillars based on the phase separation of the acrylated polysiloxane/PS blends. (b) Top-view and cross-sectional SEM images of nanopillars formed on a bare silicon wafer. (c) Cross-sectional SEM image of the nanopillars formed on a 150-nm-thick SU-8 layer. (d) Cross-sectional SEM image of nanopillars with aspect ratios as high as 10:1 formed from the nanopillars in figure c by a selective etching of the SU-8 layer. The polymer blend concentration was 5 wt% with a polysiloxane to PS ratio of 1:9 (w/w) and the spin speed was 3000 rpm.

23



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Figure 2

Figure 2. (a) AFM height image and (b) AFM phase image of a spin-coated polysiloxane/PS blend film. (c) Curves of film thickness of pure PS and crosslinked polysiloxane versus etching time under O2 and fluorine based RIE. (d) Cross-sectional SEM images of the spin-coated polysiloxane/PS blend film with the treatment of fluorine based RIE for 600 s. The inset is the same sample followed with an over O2 RIE treatment and (e) Cross-sectional SEM images of the spin-coated polysiloxane/PS blend film with the treatment of fluorine based RIE for 1200 s. The insets is the same sample followed with an over O2 RIE treatment 24


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(f) Schematic image of the phase-separated morphologies of the polysiloxane/PS blend film. The polymer blend concentration was 5 wt% with a polysiloxane to PS ratio of 1:9 (w/w) and the spin speed was 3000 rpm.

25



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Figure 3

Figure 3. Top-view SEM images of O2 etched polysiloxane/PS blend films with different weight ratios: (a) polysiloxane/PS = 1:3; (b) polysiloxane/PS = 1:4; (c) polysiloxane/PS = 1:5; and (d) polysiloxane/PS = 1:6. The concentration of the polysiloxane was set at 1% and the spin speed was 3000 rpm.

26


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Figure 4

Figure 4. Top-view SEM images of O2 etched polysiloxane/PS blend films with various spin speeds: (a) 3000 rpm; (b) 4000 rpm; (c) 5000 rpm; and (d) 6000 rpm. The initial polymer blend concentration was set at 6 wt%, and the weight ratio was set at polysiloxane/PS = 1:5.

27



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Figure 5

Figure 5. Top-view SEM images of O2 etched polysiloxane/PS blend films formed at different polymer blend concentrations: (a) 2 wt%; (b) 3 wt%; (c) 4 wt% and (d) 5 wt%. The initial weight ratio was set at polysiloxane/PS = 1:9 and the spin speed was 3000 rpm.

28


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Figure 6

Figure 6. (a) Schematic of fabricating gold coated nanopillar arrays. (b) Top-view and cross-sectional SEM images of nanopillar arrays coated with a 30 nm thick Au layer. The polymer blend concentration was 6 wt% with a polysiloxane/PS ratio of 1:5 (w/w) and the spin speed was 5000 rpm. (c) Raman shift of BPE (10-3, 10-6 and 10-9 M) on SERS substrate of Au coated nanopillars and bare silicon wafer (10-1 M) (exposure time = 1 s). (d) Reproducibility test for SERS spectra of BPE molecules at five random different points on the Au coated nanopillars . (BPE=10-6 M; exposure time=1 s).

29



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Figure 7

Figure 7. Static water contact angle measurement of the nanopillars coated with a SAM of fluoroalkyltrichlorosilane. The polymer blend concentration was 5 wt% with a polysiloxane/PS ratio of 1:9 (w/w) and the spin speed was 3000 rpm.

30


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Figure 8

Figure 8. (a) Schematic process of the fabrication of high-aspect ratio SiNWs. The photographs of (b) spin-coated polysiloxane/PS blend film on a 4’’ silicon wafer and (c) a 4’’ anti-reflective silicon wafer of sub-50nm SiNWs. (d) SEM images of sub-50nm SiNWs with aspect ratio about 1000:1. (e) Reflectance spectra of the SiNWs and a bare Si wafer.

Acknowledgements This work was jointly supported by the National Nature Science Foundation of China (Grant No. 51473076) and the National Basic Research Program of China (973 Program) (Grant No. 2013CB632702). 31



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Table of Contents

35


Wafer Scale Fabrication of Dense and High Aspect Ratio Sub-50 nm

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