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Sacrificial film-assisted nanoimprint lithography
Microelectronic Engineering 83 (2006) 542–546 www.elsevier.com/locate/mee
Sacrificial film-assisted nanoimprint lithography Yongan Xu, Wei Zhao, Hong Y. Low
*
Institute of Materials Research and Engineering, Molecular and Performance Materials Cluster, 3 Research Link, Singapore 117602, Singapore Received 16 August 2005; received in revised form 25 November 2005; accepted 10 December 2005 Available online 6 January 2006
Abstract We have demonstrated a nanopattterning technique that combines the use of sacrificial film and nanoimprint lithography. The sacrificial film serves as a ‘transient substrate’ during the nanoimprinting steps. The use of a sacrificial film improves the patterning yield significantly because the de-molding is achieved by etching off the sacrificial film, instead of a mechanical de-molding as in conventional nanoimprint lithography. This patterning technique is an easy method to build up multilayer structure from a single type of polymer. The method is also highly versatile; both substrate supported and freestanding nanostructures can be easily achieved by this technique. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Nanoimprinting lithography; Multilayer structure; Sacrificial film
1. Introduction Multilayer or 3D structures are useful for a number of potential applications, such as micro and nanoelectricalmechanical systems (MEMS and NEMS), e.g., fluidics channel, actuators, lenses, resonators, sensors, integrated circuit (IC) devices, photonic band gap structures (waveguides) [1] and as template for biological applications (e.g., scaffold for cell growth and tissue engineering) [2]. Existing fabrication methods, such as X-ray, ion-beam, ebeam lithography are slow and expensive [3]. Nanoimprinting lithography (NIL) offers a very attractive alternative to the fabrication of sub-micron and nanometer-scale features, largely due to simpler, faster, and inexpensive process, making it a potential technique for replacing photolithography in mass production [4]. A still important issue in NIL is the patterning yield especially for 3D nanostructures. In conventional NIL, the patterning yield depends on an effective anti-adhesion treatment of the
*
Corresponding author. Tel.: +65 68748133; fax: +65 68727528. E-mail address: [email protected] (H.Y. Low).
0167-9317/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2005.12.004
mold, a uniform temperature and pressure control and a particle-free environment. Non-uniformity in the antiadhesion treatment often results in poor yield [5,6]. There have been some effort to improve the patterning yield, for example, a combination of soft mold and UV curable resin was reported to yield wafer level patterning [7]. We have been primary interested in fabrication of 3D structure, for example multilayer scaffold. It is almost impossible to use conventional NIL to fabricate the multilayer structures because the basis for NIL is to deform the polymer by heat and mechanical force, this would undoubtedly cause deformation of the underlying structure. Using reversal imprinting, multilayer structure consisting of polymer with different glass transition temperature was reported [8–10]. However, in reversal imprinting, there is a requirement that the multilayer is made up of polymers with subsequently lower Tg. In another example, a cross-linked polymer is required to be the underlying layer [11]; these techniques limit the choice of materials and application of the technique. In this work, we show that by using a sacrificial film as a ‘transient’ substrate, a bi-layer nanostructure made up of a single type of polymer can be easily obtained. The technique overcomes many of the above-described limitations.
Y. Xu et al. / Microelectronic Engineering 83 (2006) 542–546
2. Experimental The silicon (Si) molds used consist of sub-micro or nanometer sized pattern with 1:1 duty cycle grating feature with pattern size varies from 75 to 500 nm, and the aspect ratios are all 1:1. All the Si molds were cleaned with acetone, isopropanol, and oxygen plasma cleaning (O2 flow rate: 50 sccm, 100 W, 250 mTorr for 5 min) prior to a silane treatment. For the 500 nm mold, the silane treatments were carried out in a dry N2 glove box where the relative humidity (RH) was kept at 18–20%. The Si molds were treated with a variety of 20 mM silane solutions in anhydrous heptane for 10 min at room temperature. The silane used was 1H,1H,2H,2H-perfluorodecyl-trichloro silane (FDTS, 96%) obtained from Alfa Aesar, which was used directly without any further purification. After the silane treatment, the molds were rinsed with anhydrous heptane immediately and then dried at 100 °C before use. For the 75 nm mold, the silane treatments were carried out in a chamber by vapor phase deposition method [12]. The polymer solution used was poly(methyl methacrylate) (PMMA, average Mw 120 k, glass transition temperature, Tg = 114 °C) in toluene. The solution concentration was 7% in weight percentage, giving film thickness in 260 and 800 nm for 75 and 500 nm mold, respectively, when spin cast at 3000 rpm for 30 s and baked at 100 °C for 5 min. The imprinting processes were carried out using a NIL400 from Obducat, Sweden. The substrates were treated by oxygen plasma for 1–5 min to obtain a high surface energy before the imprinting process. The PMMA spin coated mold and substrate were then brought together and pressed under 4 MPa at a temperature above the Tg of the PMMA (e.g., 150 °C) for 5 min. The mold and the substrate were then cooled down to 70 °C and separated, resulting in grating patterns transferred to the substrate. The aluminum film used in this work is from Diamond Company, and its polished side was treated by O2 plasma before pressed onto the spin-coated polymer film. To remove the aluminum substrate after imprinting process, the sample was immersed in a dilute HCl solution (0.5– 1 M) for about 30 min by wet etching. An oxygen plasma etching (O2 flow rate: 10 sccm, 100 W, 250 mTorr, 30– 90 s) may be carried out to remove the residual layer of the polymer film. Scanning electron micrograph (SEM, JOEL, FESEM JSM6700F) was used to inspect the topography of the imprinted structures at an accelerating voltage of 5 kV. Atomic force microscope (AFM) scanning was conducted on a Multimode Digital Instruments in tapping mode.
543
num film is used at various stages during the imprinting process and in various modes. The variations are described below. 3.1. Single-layer nanostructure 3.1.1. By reversal imprint lithography Fig. 1 schematically illustrates the fabrication process by combining the use of sacrificial film with reversal imprint lithography. Briefly, a polymer film is coated onto a mold (Si mold with 75 nm grating, duty cycle 1:1), and an oxygen plasma treated Al film (sacrificial film) is pressed against the polymer coated mold. The sandwich structure is then placed inside the imprinting machine and imprinting was carried out at 150 °C and 4 MPa. Upon de-molding, the polymer film with the desired structure is transferred to the high surface energy Al film, resulted in an Al film supported polymer structure. The Al film can then be etched away by immersing in a dilute HCl solution, resulting in a freestanding single-layer structure (Step (e)). Fig. 2 shows the SEM and AFM images of a single-layer PMMA grating structure on aluminum foil imprinted from a silicon mold with 75 nm gratings. In this case once the Al foil is etched, the 75 nm gratings will be a freestanding film. 3.1.2. By conventional NIL To incorporate the sacrificial film approach in a conventional NIL, an additional hard substrate is used. In Fig. 3, an adhesive layer is placed between the aluminum film and a Si substrate. A polymer solution is then spin-coated onto the Al film, as shown in Step (a). In Step (b), FDTS treated mold, for instance a mold with 500 nm gratings, is pressed Sacrificial substrate with high surface energy Polymer coated on silane treated mold
Imprint above Tg of polymer
Separate below Tg of polymer
Sacrificial substrate supported pattern
Step a
Step b
Step c
Step d
3. Results and discussion The sacrificial film used in this study need to meet two requirements: mechanically conformal (i.e., a ductile film) and chemically etchable. A commercially available aluminum film is found to meet the above criteria. The alumi-
Free standing single layer
Step e
Fig. 1. Schematic illustration of the fabrication of a substrate supported or freestanding single-layer structure by the combination of sacrificial film and NIL.
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Y. Xu et al. / Microelectronic Engineering 83 (2006) 542–546
Fig. 2. (a) SEM image of a single-layer, 75 nm PMMA grating structure that was transferred onto an Al film. (b) AFM image showing the cross-section view of the 75 nm PMMA grating on Al film.
Silane treated stamp Spin coated polymer Sacrificial film HEL or NIL
Step a
Adhesion layer Hard substrate
Press at proper temperature and pressure
Step b
Separate at low temperature
Step c
Remove from hard substrate
Sacrificial film supported structure Step d1
Dissolve sacrificial film
Free standing structure Step d2
Fig. 3. Schematic of the application of sacrificial film approach in conventional NIL technique.
onto the polymer film at 150 °C and 4 MPa. After de-molding, the Si substrate is removed by peeling off the adhesive layer to yield an aluminum film supported polymer structure (Step (d1)), or the Al film can be further removed to yield a freestanding structure (Step (d2)). This method is contrasted to a recent report on the bi-layer scaffold where a spin coated polyvinyl alcohol (PVA) is used as a sacrificial layer [13]. Fig. 4 is the example of aluminum film supported 500 nm PMMA gratings imprinted by conventional NIL in combination with a sacrificial film. If the aluminum film is etched by a dilute HCl solution, this PMMA grating will be a freestanding structure. 3.2. Bi-layer nanostructures Fig. 5 shows a schematic of the steps for the fabrication of bi-layer scaffold structure. Following the imprinting pro-
cess in Fig. 1, PMMA gratings on a glass substrate (Fig. 5(a1)) and on the Al film (Fig. 5(a2) and (a3)) are obtained. With the polymer pattern on the Al film, it is possible to carry out the two following processes: (1) fabrication of substrate supported structure: the polymer structure on the aluminum film is pressed at 2 MPa and 80 °C for 5 min onto another polymer structure on a glass substrate for 5 min (Fig. 5(b1)). After the imprinting process, the Al film is etched away to yield a glass substrate supported bi-layer structure (Fig. 5(c1)). This imprinting process was carried out at a mild temperature with near 100% pattern transfer yield. Note that the pattern transfer yield refers to the pattern transferred from the Al film to another polymer structure, and because the Al film is chemically etched away, nearly the entire pattern on the Al film is transferred; (2) fabrication of freestanding structure: the polymer structure on the aluminum film is pressed
Y. Xu et al. / Microelectronic Engineering 83 (2006) 542–546
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Fig. 4. (a) SEM images of 500 nm PMMA gratings on Al film, imprinted by NIL process in combination with a sacrificial film. (b) AFM image showing the cross-section view of the 500 nm PMMA gratings on Al film.
Fig. 5. Schematic of the imprinting process of substrate supported or free-standing bi-layer structure assisted by using sacrificial films.
onto another polymer structure also on Al film at 80 °C and 2 MPa for 5 min (Fig. 5(b2)). A freestanding bi-layer structure (Fig. 5(c2)) is obtained by etching away both Al films. Again, the entire polymer structure from the Al film remained after the Al film etching, yielding 100% pattern transfer. In NIL processes, the imprinting yield is greatly affected by the effectiveness of an anti-adhesion treatment in order to ensure a clean de-molding. In this technique, since the ‘transient substrate’ is sacrificed after the imprinting process, the yield is independent of the surface energy difference among the mold, substrate and the polymer. It is principally different from the mechanical de-molding step involved in current imprinting techniques. Because of the good conformability of thin film, the Al foil has a better and more uniform contact with the polymer layer compare to a rigid glass or silicon wafer substrate. Hence, a low imprinting temperature is sufficient to transfer the pattern from the mold to the Al film. The de-molding method by chemical etching of the Al film results in a 100% pattern
transfer yield with minimum pattern defects as commonly seen in mechanical de-molding. Fig. 6 is a SEM image of a bi-layer PMMA scaffold structure supported on a glass substrate comprising 500 nm-wide gratings on both the bottom and the top.
Fig. 6. SEM image of a bi-layer PMMA scaffold structure supported on a glass substrate comprising 500 nm gratings on both the bottom and the top.
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Fig. 7. SEM images of a bi-layer PMMA scaffold structure supported on glass substrate: (a) enlarged view and (b) zoomed out view, imprinted from a silicon mold with 75 nm gratings.
The surface of the structure is rough. This is due to the rough surface of the Al film, dissolving the Al film results in the PMMA surface with a similar surface roughness to that of the Al film. However, this rough protrusion surface can be optimized and smoothed by using a well-polished Al film or other sacrificial films, and will not be a limitation of this technique. In another example, a silicon mold with 75 nm grating is used for the fabrication of a PMMA bi-layer nanostructure. Fig. 7(a) shows the close up image of the imprinted scaffold while Fig. 7(b) shows a large area of the bi-layer nanostructure indicating a large size scaffold can be obtained by using this technique and the continuous bilayer nanostructure covers an area of a few millimeters. A larger area with continuous nanostructures can be obtained by using an Al film with smoother surface, since the rough surface will cause defects on the multilayer structures, such as particles, breaks in the scaffold and disconnected structures. Finally, it should be mentioned that although in our experiment, we use Al film as the sacrificial substrate; the process and principle described in this paper can also be applied by using other commercially available materials, such as water-soluble polymer sheets, and thin film of ceramics, Al, Zn, Fe or Cu. 4. Conclusions We have presented in this paper, a technique for imprinting single and bi-layer structure with near 100% pattern transfer yield. As noted throughout, a key feature of this technique is the use of a mechanically conformable and chemically etchable film as a sacrificial component in the nanoimprinting lithographic processes. The pattern transfer yield during the de-molding process is significantly improved. The entire pattern, supported on a sacrificial film, can be transferred to the target substrate by chemically etching off the sacrificial film. This provides a 100% pattern transfer yield. The pattern transfer can be carried out at milder temperatures and pressures, assisted by the
conformal nature of the sacrificial film. This advantage enables the imprinting of a multilayer structure comprising a single type of material, thereby broadening the choice of materials for multilayer structures and allowing for easy preparation of freestanding 3D structures. Lastly, this method is applicable to most of the current imprinting lithography techniques such as, conventional NIL or hotembossing lithography (HEL) [14], reversal imprinting lithography, duo-mold imprinting [15], and mold-assisted lithography (MAL) or step-and-flash imprint lithography (SFIL) [16]. The sacrificial film is typically inexpensive, thus adding to the benefit of a low cost nanofabrication process for mass production. References [1] G. Kiriakidis, N. Katsarakis, Mater. Phys. Mech. 1 (2000) 20. [2] I. Zein, D.W. Hutmacher, K.C. Tan, S.H. Teoh, Biomaterials 23 (2002) 1169. [3] M. Geissler, Y. Xia, Adv. Mater. 16 (2004) 1249. [4] S.Y. Chou, P.R. Krauss, P.J. Renstrom, Science 272 (1996) 85. [5] R.W. Jaszewski, H. Schift, B. Schnyder, A. Schneuwly, P. Groning, Appl. Sur. Sci. 143 (1999) 301. [6] S. Park, C. Padeste, H. Schift, J. Gobrecht, Microelectron. Eng. 67 (2003) 252. [7] U. Plachetka, M. Bender, A. Fuchs, B. Vratzov, T. Glinsner, F. Lindner, H. Kurz, Microelectron. Eng. 167 (2004) 73–74. [8] L.R. Bao, X. Cheng, X.D. Huang, L.J. Guo, S.W. Pang, A.F. Yee, J. Vac. Sci. Technol. B 20 (2002) 2881. [9] X.D. Huang, L.R. Bao, X. Cheng, L.J. Guo, S.W. Pang, A.F. Yee, J. Vac. Sci. Technol. B 20 (2002) 2872. [10] X. Sun, L. Zhuang, W. Zhang, S.Y. Chou, J. Vac. Sci. Technol. B 16 (1998) 3922. [11] Z. Yu, H. Gao, W. Wu, H. Ge, S.Y. Chou, J. Vac. Sci. Technol. B 21 (2003) 2874. [12] M. beck, M. Graczyk, I. Maximov, E.L. Sarwe, T.G.I. Ling, M. Keil, L. Montelius, Microelectron. Eng. 61 (2002) 441. [13] Y. Yang, S. Basu, D.L. Tomasko, L.J. Lee, S. Yang, Biomaterials 26 (2005) 2585. [14] S.Y. Chou, P.R. Krauss, P.J. Renstrom, J. Vac. Sci. Technol. B 14 (1996) 4129. [15] Y.P. Kong, H.Y. Low, S.W. Wang, A.F. Yee, J. Vac. Sci. Technol. B 22 (2004) 3251. [16] M. Colburn, S. Johnson, M. Stewart, S. Damle, T. Bailey, B. Choi, M. Wdlake, T. Michaelson, Proc. SPIE. 379 (1999) 3676.
Sacrificial film-assisted nanoimprint lithography Yongan Xu, Wei Zhao, Hong Y. Low
*
Institute of Materials Research and Engineering, Molecular and Performance Materials Cluster, 3 Research Link, Singapore 117602, Singapore Received 16 August 2005; received in revised form 25 November 2005; accepted 10 December 2005 Available online 6 January 2006
Abstract We have demonstrated a nanopattterning technique that combines the use of sacrificial film and nanoimprint lithography. The sacrificial film serves as a ‘transient substrate’ during the nanoimprinting steps. The use of a sacrificial film improves the patterning yield significantly because the de-molding is achieved by etching off the sacrificial film, instead of a mechanical de-molding as in conventional nanoimprint lithography. This patterning technique is an easy method to build up multilayer structure from a single type of polymer. The method is also highly versatile; both substrate supported and freestanding nanostructures can be easily achieved by this technique. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Nanoimprinting lithography; Multilayer structure; Sacrificial film
1. Introduction Multilayer or 3D structures are useful for a number of potential applications, such as micro and nanoelectricalmechanical systems (MEMS and NEMS), e.g., fluidics channel, actuators, lenses, resonators, sensors, integrated circuit (IC) devices, photonic band gap structures (waveguides) [1] and as template for biological applications (e.g., scaffold for cell growth and tissue engineering) [2]. Existing fabrication methods, such as X-ray, ion-beam, ebeam lithography are slow and expensive [3]. Nanoimprinting lithography (NIL) offers a very attractive alternative to the fabrication of sub-micron and nanometer-scale features, largely due to simpler, faster, and inexpensive process, making it a potential technique for replacing photolithography in mass production [4]. A still important issue in NIL is the patterning yield especially for 3D nanostructures. In conventional NIL, the patterning yield depends on an effective anti-adhesion treatment of the
*
Corresponding author. Tel.: +65 68748133; fax: +65 68727528. E-mail address: [email protected] (H.Y. Low).
0167-9317/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2005.12.004
mold, a uniform temperature and pressure control and a particle-free environment. Non-uniformity in the antiadhesion treatment often results in poor yield [5,6]. There have been some effort to improve the patterning yield, for example, a combination of soft mold and UV curable resin was reported to yield wafer level patterning [7]. We have been primary interested in fabrication of 3D structure, for example multilayer scaffold. It is almost impossible to use conventional NIL to fabricate the multilayer structures because the basis for NIL is to deform the polymer by heat and mechanical force, this would undoubtedly cause deformation of the underlying structure. Using reversal imprinting, multilayer structure consisting of polymer with different glass transition temperature was reported [8–10]. However, in reversal imprinting, there is a requirement that the multilayer is made up of polymers with subsequently lower Tg. In another example, a cross-linked polymer is required to be the underlying layer [11]; these techniques limit the choice of materials and application of the technique. In this work, we show that by using a sacrificial film as a ‘transient’ substrate, a bi-layer nanostructure made up of a single type of polymer can be easily obtained. The technique overcomes many of the above-described limitations.
Y. Xu et al. / Microelectronic Engineering 83 (2006) 542–546
2. Experimental The silicon (Si) molds used consist of sub-micro or nanometer sized pattern with 1:1 duty cycle grating feature with pattern size varies from 75 to 500 nm, and the aspect ratios are all 1:1. All the Si molds were cleaned with acetone, isopropanol, and oxygen plasma cleaning (O2 flow rate: 50 sccm, 100 W, 250 mTorr for 5 min) prior to a silane treatment. For the 500 nm mold, the silane treatments were carried out in a dry N2 glove box where the relative humidity (RH) was kept at 18–20%. The Si molds were treated with a variety of 20 mM silane solutions in anhydrous heptane for 10 min at room temperature. The silane used was 1H,1H,2H,2H-perfluorodecyl-trichloro silane (FDTS, 96%) obtained from Alfa Aesar, which was used directly without any further purification. After the silane treatment, the molds were rinsed with anhydrous heptane immediately and then dried at 100 °C before use. For the 75 nm mold, the silane treatments were carried out in a chamber by vapor phase deposition method [12]. The polymer solution used was poly(methyl methacrylate) (PMMA, average Mw 120 k, glass transition temperature, Tg = 114 °C) in toluene. The solution concentration was 7% in weight percentage, giving film thickness in 260 and 800 nm for 75 and 500 nm mold, respectively, when spin cast at 3000 rpm for 30 s and baked at 100 °C for 5 min. The imprinting processes were carried out using a NIL400 from Obducat, Sweden. The substrates were treated by oxygen plasma for 1–5 min to obtain a high surface energy before the imprinting process. The PMMA spin coated mold and substrate were then brought together and pressed under 4 MPa at a temperature above the Tg of the PMMA (e.g., 150 °C) for 5 min. The mold and the substrate were then cooled down to 70 °C and separated, resulting in grating patterns transferred to the substrate. The aluminum film used in this work is from Diamond Company, and its polished side was treated by O2 plasma before pressed onto the spin-coated polymer film. To remove the aluminum substrate after imprinting process, the sample was immersed in a dilute HCl solution (0.5– 1 M) for about 30 min by wet etching. An oxygen plasma etching (O2 flow rate: 10 sccm, 100 W, 250 mTorr, 30– 90 s) may be carried out to remove the residual layer of the polymer film. Scanning electron micrograph (SEM, JOEL, FESEM JSM6700F) was used to inspect the topography of the imprinted structures at an accelerating voltage of 5 kV. Atomic force microscope (AFM) scanning was conducted on a Multimode Digital Instruments in tapping mode.
543
num film is used at various stages during the imprinting process and in various modes. The variations are described below. 3.1. Single-layer nanostructure 3.1.1. By reversal imprint lithography Fig. 1 schematically illustrates the fabrication process by combining the use of sacrificial film with reversal imprint lithography. Briefly, a polymer film is coated onto a mold (Si mold with 75 nm grating, duty cycle 1:1), and an oxygen plasma treated Al film (sacrificial film) is pressed against the polymer coated mold. The sandwich structure is then placed inside the imprinting machine and imprinting was carried out at 150 °C and 4 MPa. Upon de-molding, the polymer film with the desired structure is transferred to the high surface energy Al film, resulted in an Al film supported polymer structure. The Al film can then be etched away by immersing in a dilute HCl solution, resulting in a freestanding single-layer structure (Step (e)). Fig. 2 shows the SEM and AFM images of a single-layer PMMA grating structure on aluminum foil imprinted from a silicon mold with 75 nm gratings. In this case once the Al foil is etched, the 75 nm gratings will be a freestanding film. 3.1.2. By conventional NIL To incorporate the sacrificial film approach in a conventional NIL, an additional hard substrate is used. In Fig. 3, an adhesive layer is placed between the aluminum film and a Si substrate. A polymer solution is then spin-coated onto the Al film, as shown in Step (a). In Step (b), FDTS treated mold, for instance a mold with 500 nm gratings, is pressed Sacrificial substrate with high surface energy Polymer coated on silane treated mold
Imprint above Tg of polymer
Separate below Tg of polymer
Sacrificial substrate supported pattern
Step a
Step b
Step c
Step d
3. Results and discussion The sacrificial film used in this study need to meet two requirements: mechanically conformal (i.e., a ductile film) and chemically etchable. A commercially available aluminum film is found to meet the above criteria. The alumi-
Free standing single layer
Step e
Fig. 1. Schematic illustration of the fabrication of a substrate supported or freestanding single-layer structure by the combination of sacrificial film and NIL.
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Y. Xu et al. / Microelectronic Engineering 83 (2006) 542–546
Fig. 2. (a) SEM image of a single-layer, 75 nm PMMA grating structure that was transferred onto an Al film. (b) AFM image showing the cross-section view of the 75 nm PMMA grating on Al film.
Silane treated stamp Spin coated polymer Sacrificial film HEL or NIL
Step a
Adhesion layer Hard substrate
Press at proper temperature and pressure
Step b
Separate at low temperature
Step c
Remove from hard substrate
Sacrificial film supported structure Step d1
Dissolve sacrificial film
Free standing structure Step d2
Fig. 3. Schematic of the application of sacrificial film approach in conventional NIL technique.
onto the polymer film at 150 °C and 4 MPa. After de-molding, the Si substrate is removed by peeling off the adhesive layer to yield an aluminum film supported polymer structure (Step (d1)), or the Al film can be further removed to yield a freestanding structure (Step (d2)). This method is contrasted to a recent report on the bi-layer scaffold where a spin coated polyvinyl alcohol (PVA) is used as a sacrificial layer [13]. Fig. 4 is the example of aluminum film supported 500 nm PMMA gratings imprinted by conventional NIL in combination with a sacrificial film. If the aluminum film is etched by a dilute HCl solution, this PMMA grating will be a freestanding structure. 3.2. Bi-layer nanostructures Fig. 5 shows a schematic of the steps for the fabrication of bi-layer scaffold structure. Following the imprinting pro-
cess in Fig. 1, PMMA gratings on a glass substrate (Fig. 5(a1)) and on the Al film (Fig. 5(a2) and (a3)) are obtained. With the polymer pattern on the Al film, it is possible to carry out the two following processes: (1) fabrication of substrate supported structure: the polymer structure on the aluminum film is pressed at 2 MPa and 80 °C for 5 min onto another polymer structure on a glass substrate for 5 min (Fig. 5(b1)). After the imprinting process, the Al film is etched away to yield a glass substrate supported bi-layer structure (Fig. 5(c1)). This imprinting process was carried out at a mild temperature with near 100% pattern transfer yield. Note that the pattern transfer yield refers to the pattern transferred from the Al film to another polymer structure, and because the Al film is chemically etched away, nearly the entire pattern on the Al film is transferred; (2) fabrication of freestanding structure: the polymer structure on the aluminum film is pressed
Y. Xu et al. / Microelectronic Engineering 83 (2006) 542–546
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Fig. 4. (a) SEM images of 500 nm PMMA gratings on Al film, imprinted by NIL process in combination with a sacrificial film. (b) AFM image showing the cross-section view of the 500 nm PMMA gratings on Al film.
Fig. 5. Schematic of the imprinting process of substrate supported or free-standing bi-layer structure assisted by using sacrificial films.
onto another polymer structure also on Al film at 80 °C and 2 MPa for 5 min (Fig. 5(b2)). A freestanding bi-layer structure (Fig. 5(c2)) is obtained by etching away both Al films. Again, the entire polymer structure from the Al film remained after the Al film etching, yielding 100% pattern transfer. In NIL processes, the imprinting yield is greatly affected by the effectiveness of an anti-adhesion treatment in order to ensure a clean de-molding. In this technique, since the ‘transient substrate’ is sacrificed after the imprinting process, the yield is independent of the surface energy difference among the mold, substrate and the polymer. It is principally different from the mechanical de-molding step involved in current imprinting techniques. Because of the good conformability of thin film, the Al foil has a better and more uniform contact with the polymer layer compare to a rigid glass or silicon wafer substrate. Hence, a low imprinting temperature is sufficient to transfer the pattern from the mold to the Al film. The de-molding method by chemical etching of the Al film results in a 100% pattern
transfer yield with minimum pattern defects as commonly seen in mechanical de-molding. Fig. 6 is a SEM image of a bi-layer PMMA scaffold structure supported on a glass substrate comprising 500 nm-wide gratings on both the bottom and the top.
Fig. 6. SEM image of a bi-layer PMMA scaffold structure supported on a glass substrate comprising 500 nm gratings on both the bottom and the top.
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Fig. 7. SEM images of a bi-layer PMMA scaffold structure supported on glass substrate: (a) enlarged view and (b) zoomed out view, imprinted from a silicon mold with 75 nm gratings.
The surface of the structure is rough. This is due to the rough surface of the Al film, dissolving the Al film results in the PMMA surface with a similar surface roughness to that of the Al film. However, this rough protrusion surface can be optimized and smoothed by using a well-polished Al film or other sacrificial films, and will not be a limitation of this technique. In another example, a silicon mold with 75 nm grating is used for the fabrication of a PMMA bi-layer nanostructure. Fig. 7(a) shows the close up image of the imprinted scaffold while Fig. 7(b) shows a large area of the bi-layer nanostructure indicating a large size scaffold can be obtained by using this technique and the continuous bilayer nanostructure covers an area of a few millimeters. A larger area with continuous nanostructures can be obtained by using an Al film with smoother surface, since the rough surface will cause defects on the multilayer structures, such as particles, breaks in the scaffold and disconnected structures. Finally, it should be mentioned that although in our experiment, we use Al film as the sacrificial substrate; the process and principle described in this paper can also be applied by using other commercially available materials, such as water-soluble polymer sheets, and thin film of ceramics, Al, Zn, Fe or Cu. 4. Conclusions We have presented in this paper, a technique for imprinting single and bi-layer structure with near 100% pattern transfer yield. As noted throughout, a key feature of this technique is the use of a mechanically conformable and chemically etchable film as a sacrificial component in the nanoimprinting lithographic processes. The pattern transfer yield during the de-molding process is significantly improved. The entire pattern, supported on a sacrificial film, can be transferred to the target substrate by chemically etching off the sacrificial film. This provides a 100% pattern transfer yield. The pattern transfer can be carried out at milder temperatures and pressures, assisted by the
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