Anti-adhesion treatment for nanoimprint stamps using atmospheric pressure plasma CVD (APPCVD)

Applied Surface Science 261 (2012) 441–446

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Anti-adhesion treatment for nanoimprint stamps using atmospheric pressure plasma CVD (APPCVD) Chien-Li Wu a , Cho-Yun Yang a , Tai-Pang An a , Je-Wei Lin b , Cheng-Kuo Sung a,∗ a b

Department of Power Mechanical Engineering, National Tsing Hua University, Hsinchu, Taiwan Plasma Science and Application Department, Mechanical and Systems Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan

a r t i c l e

i n f o

Article history: Received 25 June 2012 Received in revised form 3 August 2012 Accepted 7 August 2012 Available online 16 August 2012 Keywords: Anti-adhesion Atmospheric pressure plasma CVD (APPCVD) Nanoimprint Stamp

a b s t r a c t Nanoimprint lithography (NIL) provides a practical method for producing high-precision nanostructures efficiently and at low cost. Interfacial interactions between the imprint stamp and the forming material are crucial for high-quality pattern transfer, with critical dimensions and large aspect ratios. This study conducted an in-depth investigation into anti-adhesion treatments for imprint stamps using atmospheric pressure plasma chemical vapor deposition (APPCVD). Quantitative analyses of adhesion force, surface roughness and contact angle were applied to verify the enhanced imprint capability and release performance of the stamps. Experimental results confirmed that APPCVD-prepared anti-adhesion coatings provided improvements in quality and produced few defects in imprinted replicas. The proposed technique can simultaneously provide surface modification and thin-film deposition, depending on the precursors used. Additionally, the use of oxygen-containing plasma resulted in greater durability of the anti-adhesion coatings. The major advantages of the technique include low-temperature treatments, large-format scalability with good uniformity, short process times, and a non-vacuum process environment. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Nanoimprint processes used in the preparation of resist patterns require good adhesion asymmetry; thus, bonding strength between the forming material (imprint resist) and the substrate must be sufficiently strong. By contrast, releasing forces, including friction and sheer forces in the sidewall region, and the adhesion and normal forces present at the top and bottom regions of the stamp must be low to avoid damaging imprinted structures [1,2]. Adhesion asymmetry grows in importance as the stamp aspectratio increases because frictional forces that arise from sidewall sliding motions become the dominant releasing force [1]. Researchers have used various approaches to reduce stamp surface energy and enhance the release performance of nanoimprint stamps, including preparing anti-adhesion layers by dip coating [3–5], using vapor-phase deposition [5–9], and applying low surface-energy materials such as ETFE (ethylene tetrafluoroethylene) and PTFE (polytetrafluoroethene) [10,11]. However, these techniques are generally time consuming and have limited process windows; thus, it would be difficult to make improvements in imprint quality. It is difficult to provide uniform anti-adhesion

∗ Corresponding author. Tel.: +886 3 574 2918; fax: +886 3 571 5314. E-mail address: [email protected] (C.-K. Sung). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.029

properties over large areas. For nickel and nickel–phosphorous alloy stamp materials, a surface pretreatment, such as providing a SiOx coating, is required to enhance durability and bonding strength of the anti-adhesion layer [12,13]. Fluorinated polymer sheets usually have poor formability, and are unsuited to highprecision nanoimprint applications, especially when considering surface roughness. The use of high-temperature thermal forming processes for fabricating a polymer replica can reduce the service life of the original stamp. APPCVD provides high-quality surface coatings using low surface-energy materials such as fluoroalkylsilane (FAS). The method does not require vacuum conditions, and thus, can be used in continuous roll-to-roll processes. APPCVD can be used to simultaneously provide a hydrophilic surface treatment, and a hydrophobic anti-adhesion coating, dependent on the choice of precursor. This study focuses on the APPCVD application for antiadhesion layer deposition onto the surface of nanoimprint stamps, to provide enhanced release performance and improved imprint quality.

2. Experimental details Unlike conventional anti-adhesion processes, the APPCVD system requires numerous adjustments to optimize surface energy, roughness, and uniformity including the distance between the plasma jet nozzle and the substrate (Dn ) and the number

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Fig. 1. Schematic of atmospheric pressure plasma CVD.

Table 1 Experimental setup of APPCVD process. Stamp

Material

Plasma treatment No precursor

Plain surface

Nanoscale gratings

Si Ni Ni–P

Si Ni–P

FAS precursor

FAS precursor

Analysis Contact angle Contact angle Surface roughness Adhesion force Element analysis Imprint quality

of treatment cycles (Nt ). A schematic of the APPCVD system is shown in Fig. 1. The plasma jet was generated at 576 W using compressed dry air (CDA) at a flow rate of 42 slm. The FAS precursor (1H,1H,2H,2H-perfluorooctyltriethoxysilane, CF3 (CF2 )5 CH2 CH2 Si(OCH2 CH3 )3 ) was vaporized at 70 ◦ C and carried into the plasma chamber near the nozzle exit by a stream of argon gas at a flow rate of 300 sccm. FAS molecules, containing CFx and CHx, adhered well to the stamp surface. The layout of the APPCVD system provides only a small effective plasma jet; thus, the X-Y scanning stage allows a travel distance of 60 cm to provide large-area deposition and surface treatment. Deposition uniformity is obtained by controlling the number of treatment cycles (Nt ), which can be considered as the number of deposition layers. Therefore, the scanning mechanism also provides a uniform surface treatment/coating, even though the plasma jet only contributes to a small effective area. To determine the effectiveness of the APPCVD process, we compared three stamp materials: silicon, nickel, and nickel–phosphorous (Ni–P) alloy. An 800 nm nickel film was deposited onto silicon wafers using an E-gun thermal evaporator. A 632-nm-thick Ni–P alloy film was formed on silicon wafers by electroless deposition. Nickel and Ni–P alloy are commonly used for fabricating durable imprint stamps; the amorphous characteristics of Ni–P alloy offer good resistance to chemical corrosion and better surface roughness than nickel alone [14]. We used two types of stamp samples, one with a plain surface and the other with structural gratings. The use of these two surface types simplified the assessment of surface properties; for example, the contact angle obtained after APPCVD treatment could be isolated from variation in other parameters such as surface roughness, linewidth, duty ratio, and structural height [15]. Nanoscale gratings with a pitch of 240 nm, height of 300 nm, and linewidth of 96 nm (Fig. 2a) were formed in the silicon stamps using photolithography and etching. The Ni–P stamps were replicated from silicon stamps by electroless deposition (Fig. 2b). Table 1 shows the experimental setup for

Fig. 2. SEM images of the (a) silicon stamp and (b) Ni–P stamp.

Table 2 Contact angle of three stamp materials before and after APPCVD treatment. Substrate

Treatment

Si

Ni

Ni–P

Contact angle

Before After

51.65 14.52

76.86 14.52

86.14 49.62

testing APPCVD performance for each stamp material and surface texture. 3. Results and discussion To confirm the stability of the APPCVD system and to optimize bonding strength between the anti-adhesion coatings and stamp surface [16], we compared plain surface untreated samples with plain surface samples treated by APPCVD but without using the FAS precursor. The APPCVD-treated samples developed hydrophilic surfaces, with a lower water contact angle than that of the samples without treatment (Table 2) because of the oxygen (21% of the CDA gas) plasma effect [9]. All three stamp materials (silicon, nickel, and Ni–P alloy) underwent oxidation and OH groups production of the sample surfaces by the oxygen plasma [9,16–18], and thus, APPCVD treatment with CDA plasma was effective in each case. We applied APPCVD without FAS as pretreatment for plain RCAcleaned silicon wafer samples. The FAS precursor was then applied sequentially. The nozzle distance (Dn ) and the number of treatment cycles (Nt ) were varied to optimize the APPCVD process because the target samples were affected by both the CDA plasma and the vaporized FAS precursor. Nt was initially set to 10 cycles.

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Fig. 3. The nozzle distances with respect to (a) contact angle and (b) surface roughness when Nt = 10. Table 3 Chemical composition of the deposited anti-adhesion coatings. Element (at.%)

F 1s

O 1s

C 1s

Si 2p

Dn = 20 mm Dn = 35 mm Dn = 60 mm

8.17 29.34 18.6

49.62 24.15 27.8

10.62 19.7 15.94

31.59 27.33 37.6

The experimental results (Fig. 3) show that for maximal contact angle and adequate surface roughness, the optimal nozzle distance Dn is 30 mm, the same as the APPCVD system working distance. Fig. 3(a) shows that small contact angles for plasma-treated substrates were obtained at Dn distances less than 25 mm because the oxygen plasma dominates the process and modifies surface properties to hydrophilic behavior. In Fig. 3(b), the surface roughness of plasma-treated samples was analyzed by atomic force microscopy (AFM). Dn distances of less than 30 mm resulted in rougher surfaces because of FAS self-polymerization; by contrast, greater nozzle distances also produced excessive surface roughness because the FAS precursor is more affected by ambient conditions as the plasma jet distance increases. The AFM images shown in Fig. 4 indicate that particles appearing on the silicon sample surface were polymerized FAS, and not contaminants or protrusions of the substrate caused by plasma etching. X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical composition of APPCVD-treated samples prepared with 10 treatment cycles (Nt = 10) for three different working distances. The results displayed in Table 3 show that the atomic composition ratios of the elements C, O, F, and Si calculated from XPS spectra indicated a strong oxygen plasma effect for Dn = 20 mm [20]. The greatest fluorine composition was obtained for Dn = 35 mm. This

Fig. 4. Surface roughness of plain silicon wafers (a) before and (b) after APPCVD processing with Nt = 10 and Dn = 30 mm.

distance provided a balance between the oxygen plasma effect and FAS polymerization. Thus, the APPCVD system simultaneously provided FAS anti-adhesion coating and modification of the surface by oxygen plasma, resulting in improved bond strength between the substrate and the anti-adhesion layer [9,16]. This study demonstrated that the plasma jet nozzle distance affects surface roughness; this study then investigated how the number of treatment cycles, i.e., the number of deposition layers,

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Fig. 5. The influence of number of treatment cycles, Nt , on (a) contact angle and (b) surface roughness.

affects surface roughness. Surface energy generally decreases with increasing thickness of the anti-adhesion layer because the coating layer becomes denser and more uniform. Our results show that for nozzle distances of 30 and 35 mm, surface roughness was significantly affected by increasing the number of treatment cycles (Fig. 5b). The contact angles of plain Si, Ni, and Ni–P substrates were at their maximum values: roughly 120◦ for Nt values greater than 30 (Fig. 5a). This maximum value was caused by the crosslinked FAS network, which had insufficient space to accommodate substrate silane molecules [19,21], resulting in the surface roughness profiles shown in Fig. 5(b). All four materials showed similar trends of increasing roughness with increasing Nt . However, nickel

Fig. 6. Adhesive force measured by AFM with respect to different number of treatment cycles and stamp materials.

Fig. 7. SEM images of thermally imprinted resist structures on PET substrate using the silicon stamp with FAS coating deposited by APPCVD. (a) SEM cross-sectional view and (b) SEM plane view.

exhibited significantly greater surface roughness compared with the other materials because of its native crystalline structure (RMS roughness = 5.6 nm without APPCVD treatment). For Nt values less than 15, the Ni–P material exhibited comparable surface roughness to the silicon samples because of its amorphous structure (RMS roughness = 0.9 nm without APPCVD treatment). In addition to acceptable surface roughness, good release performance is essential. Contact mode AFM force plots were used in the study to measure adhesion forces of APPCVD treated samples. Fig. 6 shows that optimal values for Dn and Nt are 35 mm and 30 cycles, respectively. Si had a surface energy of 13.44 mJ/m2 for Nt = 25 and Dn = 35 mm. Silicon treatment using Dn = 35 mm resulted in lower adhesion forces and slightly greater surface roughness than that formed using Dn = 30 mm, which shows good correlation with the contact angles shown in Fig. 5(a). To provide excellent imprint quality and stamp durability, antiadhesion treatments must allow for low surface energy deposition, low surface roughness, and good uniformity. As the stamp’s dimensions become smaller, it is more important to exercise control over the anti-adhesion coating thickness. This study successfully treated imprint stamps using APPCVD parameters of Nt = 30 and Dn = 35 mm to provide an anti-adhesion layer thickness of less than

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gratings imprinted by APPCVD-treated Ni–P stamps also showed good quality (Fig. 8). Furthermore, the proposed anti-adhesion process demonstrated excellent release performance by imprinting more than 100 copies of high aspect-ratio resist structures with narrow linewidths on silicon oxide substrates (Fig. 9a and b) while providing good imprint uniformity over a large area of 5 cm × 5 cm. Only ±5 nm dimensional variation was found in the imprint experiments. For even larger stamps, the X–Y scanning stage allows a travel distance of 60 cm to provide uniform deposition and surface treatment. A typical APPCVD process for 6-inch-wide sample can be accomplished within 20 min using only one plasma jet nozzle. The more nozzles equipped, the less process time will be needed. Ultimately, by applying the combination of scanning stage and multiple nozzles, efficiency and uniformity can be obtained in an ideal way. 4. Conclusion Fig. 8. SEM image of thermally imprinted PMMA gratings by the Ni–P stamp.

5 nm. Thermal NIL was performed to assess release performance and service life of silicon and Ni–P stamps. Fig. 7(a) shows imprint for the APPCVD-treated silicon stamp; the thermoplastic imprint resists (mr-I 8020R, Micro Resist Technology) were imprinted at 150 ◦ C at 3 MPa for 180 s on PET (polyethylene terephthalate) substrates. After making 80 thermal imprints, the release performance remained almost pristine, without any obvious defects on the imprinted replica, indicating that stamp life met the practical needs of imprint applications. PMMA (polymethyl methacrylate)

In this study, the APPCVD system was used to modify the surface properties of Si, Ni, and Ni–P stamp materials, and to deposit uniform anti-adhesion coatings with short process times and low working temperatures. The process parameters nozzle distance, Dn , and the number of treatment cycles, Nt , were optimized to minimize surface energy and surface roughness. The influences of the FAS precursors and the oxygen plasma on bond strength, surface roughness, and uniformity were investigated and controlled by varying the process conditions. The APPCVD system provided enhanced imprints with contact angles greater than 120◦ . The imprinting stamps prepared using the proposed antiadhesion techniques produced high-quality replicas and exhibited good stamp life. APPCVD offers good scalability for large-volume nanoimprint applications. Acknowledgments The authors express their thanks to the National Science Council, R.O.C., for financially supporting this study under contract numbers NSC 99-2622-E-007-005-CC1 and NSC 98-2622-E-007-017-CC1. References

Fig. 9. SEM cross-sectional image shows high aspect ratio resist gratings with small linewidth were achieved. (a) the 1st imprint replica and (b) the 100th replica.

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