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Microsyst TechnolDOI 10.1007/s00542-014-2094-y

TechnIcal PaPer

Fabrication of various nano‑structured nickel stamps using anodic aluminum oxide

Jang Min Park

received: 19 June 2013 / accepted: 18 January 2014 © Springer-Verlag Berlin heidelberg 2014

such as using block copolymer and anodic aluminum oxide (aaO) (hamley 2003; Masuda 2005). Particularly for a mass-production of the nano-structured surface, polymer molding processes such as a nano imprint lithography, UV embossing, hot embossing and injection molding have been widely investigated (Schift et al. 2005; lee et al. 2007; Worgull et al. 2006; Pranov et al. 2006). Beside those men-tioned above, one can find many kinds of fabrication and replication techniques for the nano-structured surfaces in the literatures (Park et al. 2002; Gale 1997; Michel et al. 2001; lee et al. 2004a, 2005; Masuda 2005).

amongst the various fabrication methods of the nano-structured surfaces, it might be noted that aaO shows vari-ety of applications recently. For example, two-dimensional photonic crystal can be realized directly from two-step anodization, and by introducing additional processes one can replicate the nano-structure array with various materi-als (Masuda 2005; lee et al. 2004b, 2005). aaO itself can be also employed in the nano imprint lithography or UV embossing as a master stamp for a replication of the pol-ymeric nano-structured surfaces (lee et al. 2004a, 2007). Furthermore, aaO process can be utilized in combination with micro fabrication technologies to realize a more com-plicated multi-scale structure, so-called ‘micro/nano com-bined structure’ (Jee et al. 2005; Zhang et al. 2006; Kim et al. 2007; Yin et al. 2007; Park et al. 2008).

In the present work, four kinds of nano-structured nickel stamps are fabricated using aaO: positive and nega-tive dimple structured stamps; positive and negative pore structured stamps. For this purpose, two main processes of aaO and nickel electroforming (ne) are employed. aaO process enables fabrication of master templates having closely-packed nano dimple or pore structures with various sizes (Masuda 2005). and by ne process one can achieve nano-structured nickel stamps from the master templates.

Abstract This paper presents a fabrication method of various nickel stamps based on anodic aluminum oxide (aaO) and nickel electroforming (ne) processes. By aaO process, master templates which have closely-packed nano dimple or pore structure array are fabricated. Then nickel stamps having negative surface topology of the master tem-plates are fabricated by ne process. also positive nickel stamps are fabricated from the negative stamps by ne process with the help of nickel passivation technique. So achieved nickel stamps are employed in injection molding and hot embossing processes for replication of nano-struc-tured surfaces.

1 Introduction

Surface topologies due to a periodic arrangement or a closely-packed distribution of the nano-scale structures can affect macroscopic properties of the surface such as a reflectivity, wettability, adhesiveness and so on (Wilson and hutley 1982; Kim et al. 2006; Yan et al. 2007). In this regard, so-called ‘nano-structured surfaces’ have a broad range of applications from macro-scale products like an anti-reflective film (David et al. 2002) to micro-scale units for miniaturized systems like a detection unit of a microflu-idic system (choi and cunningham 2006).

Primary fabrication processes of the nano-structured surface include lithography methods like e-beam lithogra-phy, extreme UV lithography and X-ray lithography (Bro-ers 1988; Bjorkholm 1998), and self-assembly methods

J. M. Park (*) School of Mechanical engineering, Yeungnam University, Gyeongsan 712-749, South Koreae-mail: jangmin.park@gmail.com

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It should be noted that the nickel stamps fabricated in this work can be readily employed in polymer molding pro-cesses such as hot embossing and injection molding which requires high strength of the stamp. Some of the polymer replication results by hot embossing and injection molding are also presented in the later part of this paper.

2 Experimental

2.1 Master template fabrication

Figure 1 shows schematic diagram of the fabrication pro-cess for aaO master templates (Masuda 2005). Bulk aluminum plate (99.999 %, Good Fellow, Fig. 1a) is first electropolished in a mixture of perchloric acid and etha-nol (1:4 volume ratio) at 7 °c for 5 min to remove sur-face irregularities as shown in Fig. 1b. a platinum plate is used as a cathode and a constant voltage of 20 V is applied between anode (aluminum plate) and cathode during the

electropolishing. First-anodization is then carried out using 0.1 M phosphoric acid (85 %, aldrich) at −5 °c for 16 h with a constant voltage of 195 V, which results in a porous alumina layer as shown in Fig. 1c. The low tem-perature of −5 °c is adopted to reduce a serious burning of the aluminum plate during the anodization, and ethanol is additionally introduced in the phosphoric acid solution to prevent a freezing of the solution (li et al. 2006). The alumina layer is then completely etched out using alumina etchant (1.8 wt% chromic acid and 6 wt% phosphoric acid) at 65 °c for 12 h to realize a close-packed nano dimple array (Fig. 1d). The dimple structures have a hexagonal arrangement and their center-to-center distance is approxi-mately 500 nm (Masuda et al. 1998). If second-anodization is further carried out with the same processing condition as the first-anodization but only for several minutes, one can obtain a porous alumina layer as shown in Fig. 1e. The alumina layer is constructed with close-packed funnel geometry having wide circular opening at the top and nar-row hole at the bottom of which diameter is approximately 100 nm (lee et al. 2005). In the present study, both dimple (Fig. 1d) and pore (Fig. 1e) structured master templates are fabricated and used for a subsequent ne process.

2.2 nickel electroforming

an experimental setup for the electroforming process is shown schematically in Fig. 2. nickel sulfamate compo-sition is used as an electrolyte solution, and the composi-tion is the most popular one due to a low internal stress of the deposits and high rate of deposition (Schlesinger and Paunovic 2000). The bath temperature is kept constant at 55 °c. Since ph of the electroforming solution usually rises in a normal operating condition, it is carefully con-trolled to be around four using sulfamic acid. a current density of 1–8 ma/cm2 is applied between anode and cath-ode using Dc power supplier (agilent, e3649). The cur-rent density has a linear relationship with the growth rate of

Fig. 1 Fabrication process of aaO master template

Fig. 2 ne setup

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nickel deposit and the relationship can be represented as a following equation (Schlesinger and Paunovic 2000).

where s, t, a, I and A are deposit thickness, time, current efficiency ratio, current and surface area to be electro-formed, respectively (I/A would be the current density). according to this relationship, the growth rate of nickel is about 1.17–9.39 μm/h in the present case when current efficiency is assumed to be 95.5 %. at the early stage of the electroforming, a low current density of 1 ma/cm2 is applied for 1 h and in this stage the nano dimple or pore structures would be mostly covered by the nickel layer. Then the current density is gradually increased to 8 ma/cm2 and is maintained for about 6 days to achieve a final thickness around 1.3 mm.

Shown in Fig. 3 are schematic representations of the ne processes where the left and right column correspond to the case with dimple and pore structure, respectively. The mas-ter templates are first cleaned with ethanol and de-ionized water for 20 min, respectively (Fig. 3a). Particularly in the pore structure case (right part in Fig. 3a), a seed-layer with a thickness of 20 nm is coated by using e-beam evaporator prior to the electroforming process since porous alumina layer is electrically non-conducting material. nickel is then electroformed on the master template using the electro-forming setup (Fig. 3b). after electroforming is completed, the nickel layer is separated from the master templates (Fig. 3c). Second electroforming can be further carried out using the negative type of the nickel stamps shown in Fig. 3c to fabricate the positive type of the stamps. For that purpose, the surfaces of negative type stamps are passivated using potassium dichromate solution (Fig. 3d). after the passivation, the nickel surface is still electrically conduct-ing enough for a subsequent electroforming process. Sec-ond electroforming is then carried out in the same manner mentioned before (Fig. 3e). after the second electroform-ing is completed, two pieces are finally separated with the help of the passivation (Fig. 3f).

2.3 Polymer molding

The nickel stamps can be readily employed for the hot embossing and injection molding for a replication of nano-structured polymer surfaces. Figure 4 shows sche-matic diagrams of the hot embossing process which has three major steps of pre-heating both substrate and stamp (Fig. 4a), embossing (Fig. 4b) and demolding after cool-ing (Fig. 4c) (lee 2002). a polymethylmethacrylate plate with a thickness of 1 mm is used as a polymer substrate and pre-heating temperature of 130 °c is applied (Fig. 4a). The substrate is then embossed by the nickel stamp with a

s

t=

12.294aI

A

pressure of 10 MPa for 5 min at the pre-heated temperature (Fig. 4b). after the embossing step, both stamp and poly-mer substrate is cooled down to a room temperature while holding the embossing pressure. after the cooling is fin-ished, the polymer substrate is finally demolded from the stamp (Fig. 4c).

The injection molding is a more complicated process than the hot embossing since molten polymer experiences high rate of deformation inside the mold cavity with a non-isothermal condition during the process cycle (Greener and Wimberger-Friedl 2006). The mold cavity geometry is shown in Fig. 5, and polycarbonate (lexan 141r, Ge

Fig. 3 Schematic diagrams of the electroforming procedure using dimple (left) and pore (right) structure template

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Plastics) is used as a thermoplastic resin. a conventional processing condition shown in Table 1 is applied for this experiment and particularly the nano-pore structured stamp

is employed as a mold insert. Of course, the effect of pro-cessing condition on the transcription property is an issue of a great importance to be investigated further in the future.

Fig. 4 Schematic diagram of hot embossing process, a pre-heating stamp and polymer substrate, b embossing and c cooling and demold-ing

Fig. 5 Mold cavity geometry of injection molding

Table 1 Injection molding processing condition

Mold temperature 90 °c

Melt temperature 310 °c

Packing pressure 150 MPa

Packing time 2 s

cooling time 15 s

Filling time 0.12 s

Fig. 6 SeM images of four nickel stamps having a negative dimple, b positive dimple, c negative pore and d positive pore structures

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3 Results and discussion

Shown in Fig. 6 are scanning electron microscope (SeM) images of four different kinds of the nickel stamps. all

the SeM images in this paper are taken with tilting angle of 20° to clarify the shapes of the nano structures. Fig-ure 6a and b corresponds to the upper and lower parts of Fig. 2f in the left column, respectively. Dimple struc-tures in these figures clearly match with each other, and even sharp valleys between the dimples are found to have a fine transcription without serious damages. Similarly Fig. 6c and d shows upper and lower parts of Fig. 2f in the right column, respectively. Pillar structures in Fig. 6c and pore structures in Fig. 6d also confirm a highly accu-rate transcription of the nano-structures by the ne pro-cess. These nano-structured nickel stamps are useful par-ticularly for polymer molding processes because of their high mechanical properties. Young’s modulus of nickel is of order of 1011 Pa in a room temperature while that of common thermoplastics is of order of 109 Pa. This is quite important for a molding process of the nano-structured surface since a deformation of the stamp can result in a significant defect and inhomogeneous transcription of the nano-structures.

Shown in Fig. 7 are SeM images of replicas by the hot embossing. each figure in Fig. 7 corresponds to the repli-cation result using the nickel stamp shown in Fig. 6 in the same order. The closely-packed nano-structure arrays can be clearly observed for all of the cases and their shapes are consistent with those of nickel stamps. Shown in Fig. 8 are a SeM image and a photograph (inset) of injection molded replica using the nickel stamp shown in Fig. 6d. The nano-structures are supposed to have a protuberance-like shape as shown in Fig. 7d but have a rather lens-like shape which is because of incomplete filling of molten polymers into the pore cavity. This is mainly due to the rapid cooling of molten polymers at the interface with the nickel stamp, thus the mold temperature is one of the most important

Fig. 7 SeM images of the hot embossed replicas

Fig. 8 Picture and SeM image of injection molded replica

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parameters for the nano-structure injection molding (Yoshii and Kuramoto 1994). In this regard, there are several ongo-ing researches to control the mold temperature actively to achieve a high accuracy of replication while maintaining the total process cycle time in a similar level, recently (rha et al. 2008). In the photograph of Fig. 8, one can observe structural colors which reflect the presence of the closely-packed nano-structures over the replica surface. however, one should note that the replication property of the nano-structure will depend on the location since the pressure and temperature history will differ depending on its location (Xu et al. 2005), and investigating the effect of process-ing condition is of a great importance for further studies of nano-structure replications.

4 Concluding remarks

In the present study, a simple and flexible fabrication method for various nano-structured nickel stamps is pre-sented based on aaO and ne processes. Two kinds of closely-packed nano-structure arrays are fabricated by aaO process and they are transcribed to the nickel stamps in a positive and negative fashion with a high precision by ne process. Due to the high mechanical strength of the nickel stamp, they can be directly employed in the thermo-plastic molding processes like a hot embossing and injec-tion molding for a mass-production of the nano-structured surfaces. Finally it might be mentioned that by utilizing micro fabrication processes in combination with the pre-sent fabrication process one can realize more complicated structures such as a micro/nano combined structure for var-ious applications (Kim et al. 2007; Puukilainen et al. 2007; Jeong et al. 2006).

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