Fabrication of Large Area, Ordered Nanoporous Structures on Various Substrates for Potential Electro-Optic Applications

Abstract

Nanoporous structures have attracted great attention in electronics, sensor and storage devices, and photonics because of their large surface area, large volume to surface ratio, and potential for high-sensitivity sensor applications. Normally, electron or ion beam patterning can be used for nanopores fabrication by direct writing. However, direct writing is a rather expensive and time-consuming method due to its serial nature. Therefore, it may not translate to a preferred manufacturing process. In this research, a perfectly ordered large-area periodic pattern in an area of approximately 1 cm² has been successfully fabricated on various substrates including glass, silicon, and polydimethylsiloxane, using a two-step process comprising visible light-based multibeam interference lithography and subsequent pattern transfer processes of reactive ion etching and nanomolding. Additionally, the multibeam interference lithography templated anodized aluminum oxide process has been described. Since the fabrication area in multibeam interference lithography can be extended by using a larger beam size, it is highly cost effective and manufacturable. Furthermore, although not described here, an electrodeposition process can be utilized as a pattern transfer process. This large-area perfectly ordered nanopore array will be very useful for high-density electronic memory and photonic bandgap and metamaterial applications.

1. Introduction

Because of their large surface area, large volume to surface ratio, and potential for high sensitivity sensor applications, nanoporous structures have drawn great attention in electronics, sensor and storage devices, and photonics [1,2,3,4,5,6]. Electron or ion beam patterning can be used for nanopore fabrication by direct writing [7,8,9,10]. However, direct writing is a rather expensive and time-consuming method due to its serial nature. Therefore, it may not translate to a preferred manufacturing process. Meantime, metal anodization processes, i.e., anodized aluminum oxide (AAO) and anodized titanium oxide (ATO) processes, are used for large-area nanopore templates [11,12,13,14], but their array patterns tend to contain locally disordered portions, which would limit their usage for electronic memory devices or photonic devices requiring perfectly ordered arrays over a large area. Multibeam interference lithography (MIL), however, could be a good candidate for the patterning of perfectly ordered large-area photonic crystals and can be applied for photonic, electronic applications [15,16,17,18,19]. MIL can form interference patterns on photosensitive polymer. In our previous work, MIL and two-photon lithography were combined by two-step exposures and a single development process. This new approach, adding patterns on the repeated nanoporous structures, suggested a new path from restricted usages of nanoporous structures to various applications but essentially, the newly fabricated structures were still made of the same material, polymer, and there was a limit to their usage for diverse applications [20]. To overcome its limitations and construct a functional electronic or photonic device, pattern transfer to the underneath layers or substrates is required.

In this work, three-beam interference lithography with a visible light source is used to form hexagonal symmetric interference patterns on a large photoresist (PR) layer. Subsequently, various pattern transfer processes to different substrates such as reactive ion etching (RIE) to glass and Si substrates, nanomolding to a polydimethylsiloxane (PDMS) substrate, and anodization to an aluminum-coated substrate were performed. By combining MIL and RIE/nanomolding, nanoporous polymer structures could be transferred to various substrates and this can provide the opportunity to be used for a variety of applications for electronic and photonic devices.

2. Materials and Methods

To fabricate nanoporous structures with a large area, multibeam interference lithography was performed using a cw Nd:YVO4 laser (Verdi V6, Coherent, Palo Alto, CA, USA) with a wavelength of 532 nm. The whole covered area by MIL was approximately 1 cm² and the covered area can be adjusted by varying the aperture size of the beam. As PR, Epon SU-8 (Miller-Stephenson Chemical, Inc., Sylmar, CA, USA) was selected due to its good solubility and transparency in visible and UV light. The SU-8 was doped with a photosensitizer, Rubrene (Sigma-Aldrich, Inc., Burlington, MA, USA) and a photoacid generator, diaryliodonium hexafluoroantimonate (PC-2506, Polyset, Mechanicville, NY, USA) to respond to the green laser. Figure 1 shows the experimental setup and a schematic of three-beam interference lithography where the incident angle of each beam was 45° from the vertical line, and the phase difference was 120° between beams in the horizontal plane. Depending on the phase angle of each incident beam and the PR type, either a hexagonal symmetric nanopore array or a nanopillar array was formed.

Figure 2 shows the nanoporous structures on the substrate fabricated by MIL. As is shown, well-ordered interference patterns were fabricated, which covered 1 cm². Depending on the optical property of the substrate, a buffer layer was used between the substrate and the photopatterning layer to minimize the reflection.

3. Results and Discussion

As pattern transfer methods, three different approaches are shown in Figure 3. Figure 3a used a (RIE) process for a pattern transfer method to a Si or glass substrate. When glass is used as a substrate, the buffer layer between the PR and the substrate may not be critical since the refractive index difference between SU-8 (n = 1.59) and glass (n = 1.50) is relatively small and no significant portion of optical dose is reflected to generate parasitic patterns. On the other hand, when Si is used, reflection between SU-8 and Si (n = 3.42) would be quite significant due to its large refractive index difference, and the buffer layer design becomes very critical. SU-8 doped with Rhodamine B (RhB) dye (Sigma-Aldrich, Inc., Burlington, MA, USA) has been used because of its strong absorption of green light and high quantum efficiency. First, RhB was dissolved in cyclopentanone and mixed with SU-8 and photoacid generator, where the weight ratio of SU-8:RhB:PC was 10:0.1:1. It was spin-coated on a polished Si substrate at 2000 rpm for 40 s. After soft baking it on a hotplate at 95 °C for a minute, it was flood-exposed under the multiband UV light for one minute and hard baked at 160 °C for 10 min. The resultant thickness was approximately 2.5 μm. Next, a SU-8 PR layer with a weight ratio of SU-8:Rubrene:PC of 100:0.2:2.5 was spin-coated at 4000 rpm for 40 s, followed by soft baking at 95 °C for a minute (Figure 3(a1)). Three-beam interference lithography was performed on a circular exposure area with a diameter of 1 cm. The cumulative intensity of the exposure energy was 0.6 W cm⁻² and exposed beams for 3 to 8 sec gave the total exposure dosage for 2 to 5 J cm⁻². When the incident beams reached the PR layer, which has very good thermal stability, the exposed areas increased in molecular weight and photochemically transformed to form insoluble products [21]. The specimen was post baked on a hot plate at 95 °C for a minute and cooled down to room temperature, followed by development in propylene glycol monomethyl ether acetate (PGMEA). The thickness was approximately 300–400 nm (Figure 3(a2)). RIE was performed in an oxygen-dominant environment for buffer layer patterning (Figure 3(a3)) and in a chlorine-dominant environment for Si layer patterning (Figure 3(a4)). Figure 4 shows a fabricated photonic crystal membrane at the stage depicted in Figure 3(a2).

The three layers are clearly seen with the top layer patterned in a hexagonal symmetric shape. Figure 5 shows a pattern transferred glass substrate, where RIE has been performed in O₂:CF₄ of 15:3.5 sccm with a power of 20 W for 30 s for buffer layer etching and in O₂:CH₄ of 5:40 sccm with 70 W for 2 min for glass etching [13]. Figure 3b shows a pattern transfer process using polydimethylsiloxane (PDMS) nanomolding. After the interference lithography and RIE process with PR on the glass substrate to make nanoporous glass structures as shown in Figure 3a, PDMS was poured on the nanoporous glass surface and cured at 60 °C for 2 h. After cooling down to room temperature, a nanopillar array, which is the negative form of the nanopore array, was formed upon separation. Figure 6a is the top view of a perfectly ordered hexagonal symmetric nanopore array, with the inset showing a magnified oblique view. Figure 6b shows a replicated PDMS structure after nanomolding, with the inset showing a magnified view of the pillar array. The scanning electron microscopy image demonstrates good fidelity. The anodized aluminum oxide (AAO) process converts pure aluminum into aluminum oxide in a specific shape, as shown in Figure 7, when it is anodized in an acidic environment under a bias voltage. The pore size and the pitch between pores can be controlled by the magnitude of the bias voltage and the type of acids. For example, the pore pitch is in the range of 50–60 nm at a bias voltage of 19–25 V in sulfuric acid (H₂SO₄), 220 nm–300 nm at 120–250 V in oxalic acid (H₂C₂O₂), and 405–500 nm at 160–195 V in phosphoric acid (H₃PO₄). If the bias voltage is lower than the optimal range, pores formed are not well ordered. If the bias voltage is higher than the optimal range, the barrier layer breaks down and substrate burning occurs. Fabricated anodized aluminum oxide layer samples are shown in Figure 8. A two-step AAO process has been used to form more uniform pore sizes. First, aluminum foil undergoes thermal pretreatment at 400 °C for one hour, which forms a thin layer of oxide layer, preventing initial breakdown upon applying bias voltage. The first anodization is performed in oxalic acid at 40 V for 6 min, which gives rise to relatively random pores, as shown in Figure 8a. The first anodized layer is etched away using a solution consisting of 1.8 wt% chromic acid and 6 wt% phosphoric acid for an hour. After etching, small bumps are formed along the barrier depicted in Figure 7. The bumps serve as guides for the second anodization. The second anodization is performed in oxalic acid at 40 V for 4 min. Optionally, pore widening step is performed using 6 wt% phosphoric acid for an hour. Figure 8b,c are the cross-section view and the top view, respectively. The average pore size and the average pitch are 60 nm and 100 nm, respectively [15]. Figure 3c shows a direct pattern transfer onto an aluminum layer using AAO immediately followed by MIL. The anodization is being performed using phosphoric acid at 160 V to have a pore pitch of 500 nm. As an alternative to the direct anodization process after MIL, template-assisted anodization can be used. However, there are some challenges, including reflection and pore size matching issues between one from MIL and one from AAO. In order to preserve the perfect pore ordering from MIL, further work is required.

4. Conclusions

A perfectly ordered large-area periodic pattern in an area of approximately 1 cm² has been successfully fabricated on various substrates including glass, Si, and PDMS, using a two-step process comprising visible light-based MIL and subsequent pattern transfer processes of RIE and nanomolding. Additionally, the MIL-templated AAO process has been described. Since the fabrication area in MIL can be extended by using a larger beam size, it is highly cost effective and manufacturable. This large-area perfectly ordered nanopore array will be very useful for high-density electronic memory and photonic bandgap and metamaterial applications, sensors, nanolens and photonics [22].