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UV-nanoimprinting using non-transparent molds and non-transparent substrates
Microelectronic Engineering 88 (2011) 2004–2008
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
UV-nanoimprinting using non-transparent molds and non-transparent substrates R. Kirchner a,⇑, A. Finn a, L. Teng a, M. Ploetner a, A. Jahn a, L. Nueske b, W.-J. Fischer a a b
Institute of Semiconductor and Microsystems Technology, Technische Universitaet Dresden, 01062 Dresden, Germany Fraunhofer-Institute for Photonic Microsystems, Maria-Reiche-Strasse 2, 01109 Dresden, Germany
a r t i c l e
i n f o
Article history: Available online 31 January 2011 Keywords: UV nanoimprint Non-transparent mold Opaque mold NT-UV-NIL Antisticking layer
a b s t r a c t A new ultraviolet assisted nanoimprint lithography technique with an exposure through non-transparent molds and a nm-resolution capability is reported. The UV imprint material was not cured by direct irradiation, but substantially exposed to indirect and diffuse irradiation. The nanoimprint molds consisted of a transparent support and a non-transparent, patterned element. Successful imprints were conducted on transparent glass and polymer foils placed on non-transparent substrate holders as well as on SiO2 on Si. The reported technique enables the application of non-transparent mold materials like Si, new mold materials and alternative antisticking layers like metals in UV-NIL. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Nanoimprint lithography (NIL) is today an alternative to current state-of-the-art, diffraction limited optical lithography. NIL enables the top-down patterning of polymers as binary lithography masks with sub-25 nm features [1]. It is further able to directly replicate completely new polymer patterns with high aspect ratios and 3D-shapes. The patterned polymer serves for example as microlens [2], microfluidic channel [3], micro-electro-mechanical [4] or photonic structure [5]. Direct patterning is straight forward in terms of technological complexity. Respective applications require only a single layer imprint step. UV-assisted nanoimprint lithography (UV-NIL) [6] is very promising for both, imprinting binary polymeric masks and directly patterning functional polymer structures. UV-NIL offers an improved microelectronics process compatibility due to reduced process temperatures and pressures compared to thermal nanoimprint lithography (T-NIL). For direct patterning, UV-NIL offers the integration of a wide spread spectrum of functionalities by a dedicated resist synthesis [7,8]. Moreover, negative imprint molds, i.e., molds with a small proportion of recessed cavities, are difficult to replicate with T-NIL [9]. The imprint material has to travel large distances under the mold to fill the cavities. This requires very low viscous imprint materials to reduce the process time and pressure. UV-NIL resists are mainly UV-curing, low viscous monomers and are highly suited for negative molds. Additionally, UV-NIL enables a hybrid technique, which avoids the imprint inherent residual layer for UV-curing materials [10] and thus an additional process step for certain applications. So far, the required UV-transparent quartz molds are a major cost ⇑ Corresponding author. Tel.: +49 35146332653; fax: +49 35146337021. E-mail address: [email protected] (R. Kirchner). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.01.066
driver for state-of-the-art UV-NIL. The etching of SiO2 is more challenging and thus more expensive than that of Si. Therefore, costefficient Si molds are desirable for UV-NIL. The degradation of the predominantly used fluorinated organosilane antisticking layers [11] is a challenge for UV-NIL mold durability. Metals are a possible solution as demonstrated for TNIL [9]. However, UV-NIL requires a transparent antisticking layer. One solution for using non-transparent molds and antisticking layers is the usage of transparent substrates [12]. Thereby, a transparent chuck and a UV-source below this chuck are required (see Fig. 1b). Another solution is an inverted imprint setup with the substrate being placed on top of the mold (see Fig. 1c). Unfortunately, this is not very convenient for stamp-and-repeat imprinting. In usual UV-NIL systems (through-mold-exposure), the UV source is located above the mold and the substrate is located below on a non-transparent chuck (see Fig. 1a). This work presents a new technique, termed non-transparent UV-NIL (NT-UV-NIL), which uses non-transparent molds (see Fig. 1d and e) in a through-mold-exposure system. 2. Materials and methods 2.1. Imprint material As imprint material, a customized resist formulation with 67.5% w/w tert-butyl acrylate, 30% w/w ethylene glycol diacrylate and 2.5% w/w 2-hydroxy-2-methylpropiophenone as photoinitiator was used. All chemicals were purchased from Aldrich. The free-radical polymerization is sensitive to oxygen. Thin layers as they occur in UV-NIL were successfully cured with doses of about 6000 mJ/cm2 with fully transparent molds. This is because the dissolved oxygen is rapidly used up by the polymerization process
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Fig. 1. (a) Standard UV-NIL imprint situation. A setup for non-transparent molds with (b) a bottom UV-source and (c) a top UV-source (transparent substrate holder required). (d) NT-UV-NIL setup requiring no change compared to the standard setup. (e) NT-UV-NIL mold and the exposure situation.
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(b) Fig. 2. Setup of the NT-UV-NIL imprint experiments with different imprint systems.
and the diffusion of new oxygen into thin layers is slower than the polymerization reaction. Kinetic curing models and rate coefficients required for them are very system specific and difficult to obtain for a UV-NIL material [13]. The lowest exposure dose used to cure the imprint material was 3000 mJ/cm2, which is not the minimum achievable. Minimal exposure doses required for curing found in the literature for comparable monomer systems were 150 mJ/cm2 [14] with another photoinitiator and 477 mJ/cm2 with the same photoinitiator [13]. For other monomer systems with the same photoinitiator, minimal doses of 250 mJ/cm2 [15] and 1134 mJ/cm2 [16] were reported for good monomer conversion. For other UV-NIL resists, minimum curing doses of 300 mJ/cm2 were reported [14]. In summary, a reasonable minimum curing dose of about 500 mJ/cm2 could be extracted from the literature. A thermally initiated curing of the monomer system in this work was ruled out. Additionally, the commercial resist mr-UVCur21SF (micro resist technology GmbH, Germany) was used. 2.2. Imprint setups The large scale potential of the NT-UV-NIL mechanism was investigated with a contact mask exposure tool (AL6-2, EV Group GmbH) and a stamp-and-repeat imprint system (NPS300, SET S.A.S) (see Fig. 2a and b). The laboratory potential of NT-UV-NIL was investigated with a customized imprint press (GeSiM mbH) (see Fig. 2c). 2.2.1. Al6-2 In the AL6-2 tool, a stack of a NT-UV-NIL mold and the substrate was put between a transparent lower chuck (normally the
place of the wafer) and a transparent upper chuck (normally the place of the photomask) (see Fig. 2a). After dispensing the resist between the mold and the substrate, the complete stack was pressed with 1 bar. A UV flood exposure was done by a mercury arc lamp with a power density of 18 mW/cm2 for the 300– 400 nm spectral range. After releasing the pressure, the mold was peeled off. Diffuse reflections from AL6-2 machine parts and the substrate-chuck interface might have been possible but they were expected to be small compared to the primary exposure due to the transparent chuck and a blackened machine interior. However, they could not be ruled out completely. 2.2.2. NPS300 In contrast to the AL6-2, the NPS300 has a non-transparent SiC chuck and a tunable, 250 mW/cm2, 365 ± 15 nm LED UV-source in a 65 mm 65 mm transparent upper chuck (compare Fig. 2b). It is obvious that for the NPS300 the support of a NT-UV-NIL mold has to be transparent and the structured element must be at least smaller than the upper chuck so that the light is able to reach the polymer. In this setup, the UV-light could only propagate laterally into the imprint material, either through the thin film of imprint material, the transparent substrate (compare Fig. 1e) or through the transparent film on a non-transparent substrate. 2.2.3. Laboratory press The universal practicability of NT-UV-NIL was investigated with a simple laboratory press (see Fig. 2c). The substrate was placed on a metal support. The UV-source had an F-spectrum (iron doped mercury arc lamp) with a power density of 68 mW/cm2 in the 300–400 nm spectral range and a broad angular intensity
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Fig. 3. NT-UV-NIL imprints of (a) dense resist lines with a period of 3.5 lm with the AL6-2 on a 2.3 mm glass substrate and (b) of a test structure with the NPS300 on a transparent 1 mm thick glass slide.
Fig. 4. Homogeneous NT-UV-NIL imprint of an electrode array on 18 lm USG with (a) macroscopic overview, (b) center of the imprint field and (c) resolved 150 nm line. (d) NT-UV-NIL imprint with insufficient UV-dose on 4.5 lm USG.
distribution. Again, the light could not pass directly through the non-transparent Si chip during the flood exposure. 2.2.4. Light diffusor To enhance diffuse light reflections, the imprint press was put in an aluminum chamber. This chamber provided additionally a N2 atmosphere to eliminate the oxygen curing inhibition. 2.3. Substrates To reveal the NT-UV-NIL limits, further experiments were conducted with the laboratory imprint press on 70 lm glass slides and 30 lm polyethylene terephtalat (PET) foils. These transparent substrates were placed on a Si substrate with 1 lm SiO2. To achieve thinner transparent films, 18 lm, 9 lm, 4.5 lm undoped silicate glass (USG) was deposited by plasma enhanced chemical vapor deposition. The thinnest films were 1 lm and 140 nm thermal SiO2 on Si. Further substrates were a 175 lm PET foil, 625 lm quartz wafers as well as 1 and 2.3 mm glass slides.
2.4. NT-UV-NIL molds The molds were unstructured or structured Si chips sized 10 mm 10 mm. Additionally, a 15 mm 15 mm quartz chip with a non-transparent, antireflective Cr surface was used with the NPS300. During the imprint process, the non-structured Cr surface faced the imprint material and the substrate. A 1H,1H,2H,2H-perfluorodecyltrichlorosilane (ABCR GmbH, Germany) antisticking layer (ASL) was deployed for all molds. 3. Results and discussion 3.1. Successful NT-UV-NIL imprints 3.1.1. Al6-2 A Si mold with a line-space pattern and a period of 3.5 lm was successfully replicated into the imprint material. The complete resist was cured and the mold structure was perfectly transferred (see Fig. 3a).
R. Kirchner et al. / Microelectronic Engineering 88 (2011) 2004–2008
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Fig. 5. NT-UV-NIL imprint of (a) resist lines with a period of 3.5 lm with a laboratory press on a 175 lm thin PET foil and (b) dense, 25 nm thick Al lines after imprint with the same mold, 25 nm Al sputtering and lift off on a 2.3 mm thick glass substrate.
3.1.2. NPS300 The ability for stamp-and-repeat imprinting of NT-UV-NIL was demonstrated with a structured Si chip. The chip was directly placed without a support under the transparent upper chuck and pressed with about 0.6 bar into a before dispensed resist depot on a 1 mm glass slide. The structure on the silicon mold was successfully replicated into the imprint material (see Fig. 3b) by a 60 s exposure with an intensity of 200 mW/cm2. To increase the non-transparent area, the larger, unstructured Cr mold was imprinted on a 1 mm glass slide. The resist was completely cured within 30 s at 100 mW/cm2 demonstrating the potential of the NT-UV-NIL for large non-transparent molds. Further detailed studies on the mold size were not conducted. NT-UV-NIL imprints were homogeneous and completely cured over a 10 mm 10 mm imprint field (see Fig. 4a and b). Isolated 150 nm features were resolved (see Fig. 4c). The resist curing proceeded from the periphery of the mold to its center. Hence, insufficient doses lead to a partial pattern transfer (see Fig. 4d). As a consequence, completely cured imprints received in peripheral areas higher UV-intensities than in central ones. The distribution of the received intensities in NT-UV-NIL is, thus, not as homogeneous as in UV-NIL. 3.1.3. Laboratory press Successful imprints were achieved on a 175 lm thin PET foil and on a 2.3 mm glass substrate which was further used to realize dense Al lines with an acetone lift off process (see Fig. 5). 3.2. Influence of the imprint setup NT-UV-NIL imprints were successfully achieved independent of particular imprint setups. However, the required curing time for the imprint of the same unstructured Si mold on a 675 lm quartz substrate could be reduced from 30 to 15 min by using the aluminum chamber as light diffusor. 3.3. Influence of the substrate With the Cr-mold, an imprint was conducted with the NPS300 on a Si wafer having 90 nm SiO2 and a polymethylmethacrylate (PMMA) thinner than 1 lm on top. The resist was not cured, indicating a lower limit for the usable thickness of the transparent substrate layer. Further experiments with the laboratory press revealed that on USG-films thinner than 4.5 lm no curing was possible in reasonable processing times (see Fig. 6). After an unreasonable 2 h (489.6 J/cm2) exposure, only a partial curing with uncured resist in the mold center was possible on 4.5 lm USG. On 9 lm USG almost all imprint material was cured within 1h (244.8 J/cm2). On 18 lm USG complete curing was achieved within 1 h with shorter
Fig. 6. Relation between the thickness of the transparent layer and the curing dose ((a) = without Al-chamber).
curing times being expected. Thus, the limit for the transparent layer thickness was about 18 lm for reasonable process times well below 1 h with the used setup. The curing time is expected to be reducible even further by improved light guiding measures, which was beyond the scope of this work. Notable was, that the 5 min minimal curing time for the 35 lm PET foil was significantly shorter than the 15 min for the slightly thinner 30 lm adhesive tape (see Fig. 6). Transparency cannot account for this effect. The adhesive tape was even more transparent at 365 nm than the PET foil. But the PET foil (118 nm RMS) was much rougher than the tape (7 nm RMS). This gives evidences that the roughness of the substrate and thus diffuse scattering plays an important role for NT-UV-NIL. Those effects are aspects for further research. NT-UV-NIL with another material, mr-UVCur21SF, required 2 min (30 J/cm2) on 18 lm USG and 10 min (150 J/cm2) on 9 lm for complete curing with the NPS-300. This was not possible on 4.5 lm USG up to 10 min. 4. Conclusion and outlook A new technique, NT-UV-NIL, was used to imprint UV-curable resists on transparent substrates, foils and thin films by indirect exposure. Special measures against oxygen inhibition were not mandatory. Enough UV-intensity reached the resist indirectly. NT-UV-NIL is well suited for direct imprinting and lift-off. It enables the use of cost-efficient Si molds and new non-transparent materials, which were so far not usable for ‘‘through-mold-exposure’’
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systems. It is a route towards non-transparent ASLs, e.g., metals, to avoid organosilane ASL degradation in UV-NIL. NT-UV-NIL worked with a typical stamp-and-repeat UV-NIL mold size. We speculate that the achieved resolution was only limited by the available imprint molds. This is currently under further investigation. Acknowledgements This work was partly funded by the DFG as Research Training Group ‘‘Nano- and Biotechnologies for Packaging of Electronic Systems’’ (DFG 1401/1), the Fraunhofer-Society and the Saechsische AufbauBank (SAB). References [1] S.Y. Chou, P.R. Krauss, P.J. Renstrom, Appl. Phys. Lett. 67 (1995) 3114–3116. [2] H. Schmitt, M. Rommel, A.J. Bauer, L. Frey, A. Bich, M. Eisner, R. Voelkel, M. Hornung, Microelectron. Eng. 87 (2010) 1074–1076.
[3] C.A. Mills, E. Martinez, F. Bessueille, G. Villanueva, J. Bausells, J. Samitier, A. Errachid, Microelectron. Eng. 78–79 (2005) 695–700. [4] Y. Zhao, T. Cui, J. Micromech. Microeng. 13 (2003) 430–435. [5] C.-Y. Chao, W. Fung, L.J. Guo, IEEE J. Quantum. Elect. 12 (2006) 134–142. [6] J. Haisma, M. Verheijen, K. van den Heuvel, J. van den Berg, J. Vac. Sci. Technol. B 14 (1996) 4124–4128. [7] J. Kettle, P. Coppo, G. Lalev, C. Tattershall, S. Dimov, M. Turner, Microelectron. Eng. 85 (2008) 850–852. [8] Y. Ofir, I.W. Moran, C. Subramani, K.R. Carter, V.M. Rotello, Adv. Mater. 22 (2010) 3608–3614. [9] H.-C. Scheer, H. Schulz, T. Hoffmann, C.M. Sotomayor-Torres, Handbook of Thin Film Materials, Nanomaterials and Magnetic Thin Films, vol. 5, Academic Press, 2002, pp. 49. [10] Y. Chen, F. Carcenac, C. Ecoffet, D.J. Lougnot, H. Launois, Microelectron. Eng. 46 (1999) 69–72. [11] D. Truffier-Boutry, M. Zelsmann, J.D. Girolamo, J. Boussey, C. Lombard, B. Pépin-Donat, Appl. Phys. Lett. 94 (2009) 044110. [12] H.S. Lee, D.S. Kim, T.H. Kwon, Microsyst. Technol. 13 (2007) 593–599. [13] C.G.W. Michael, D. Dickey, AIChE J. 52 (2006) 777784. [14] H. Schmitt, L. Frey, H. Ryssel, J. Vac. Sci. Technol. B 25 (2007) 785–790. [15] C. Decker, D. Decker, Polymer 38 (1997) 2229–2237. [16] L. Lecamp, B. Youssef, C. Bunel, Polymer 38 (1997) 6089–6096.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
UV-nanoimprinting using non-transparent molds and non-transparent substrates R. Kirchner a,⇑, A. Finn a, L. Teng a, M. Ploetner a, A. Jahn a, L. Nueske b, W.-J. Fischer a a b
Institute of Semiconductor and Microsystems Technology, Technische Universitaet Dresden, 01062 Dresden, Germany Fraunhofer-Institute for Photonic Microsystems, Maria-Reiche-Strasse 2, 01109 Dresden, Germany
a r t i c l e
i n f o
Article history: Available online 31 January 2011 Keywords: UV nanoimprint Non-transparent mold Opaque mold NT-UV-NIL Antisticking layer
a b s t r a c t A new ultraviolet assisted nanoimprint lithography technique with an exposure through non-transparent molds and a nm-resolution capability is reported. The UV imprint material was not cured by direct irradiation, but substantially exposed to indirect and diffuse irradiation. The nanoimprint molds consisted of a transparent support and a non-transparent, patterned element. Successful imprints were conducted on transparent glass and polymer foils placed on non-transparent substrate holders as well as on SiO2 on Si. The reported technique enables the application of non-transparent mold materials like Si, new mold materials and alternative antisticking layers like metals in UV-NIL. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Nanoimprint lithography (NIL) is today an alternative to current state-of-the-art, diffraction limited optical lithography. NIL enables the top-down patterning of polymers as binary lithography masks with sub-25 nm features [1]. It is further able to directly replicate completely new polymer patterns with high aspect ratios and 3D-shapes. The patterned polymer serves for example as microlens [2], microfluidic channel [3], micro-electro-mechanical [4] or photonic structure [5]. Direct patterning is straight forward in terms of technological complexity. Respective applications require only a single layer imprint step. UV-assisted nanoimprint lithography (UV-NIL) [6] is very promising for both, imprinting binary polymeric masks and directly patterning functional polymer structures. UV-NIL offers an improved microelectronics process compatibility due to reduced process temperatures and pressures compared to thermal nanoimprint lithography (T-NIL). For direct patterning, UV-NIL offers the integration of a wide spread spectrum of functionalities by a dedicated resist synthesis [7,8]. Moreover, negative imprint molds, i.e., molds with a small proportion of recessed cavities, are difficult to replicate with T-NIL [9]. The imprint material has to travel large distances under the mold to fill the cavities. This requires very low viscous imprint materials to reduce the process time and pressure. UV-NIL resists are mainly UV-curing, low viscous monomers and are highly suited for negative molds. Additionally, UV-NIL enables a hybrid technique, which avoids the imprint inherent residual layer for UV-curing materials [10] and thus an additional process step for certain applications. So far, the required UV-transparent quartz molds are a major cost ⇑ Corresponding author. Tel.: +49 35146332653; fax: +49 35146337021. E-mail address: [email protected] (R. Kirchner). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.01.066
driver for state-of-the-art UV-NIL. The etching of SiO2 is more challenging and thus more expensive than that of Si. Therefore, costefficient Si molds are desirable for UV-NIL. The degradation of the predominantly used fluorinated organosilane antisticking layers [11] is a challenge for UV-NIL mold durability. Metals are a possible solution as demonstrated for TNIL [9]. However, UV-NIL requires a transparent antisticking layer. One solution for using non-transparent molds and antisticking layers is the usage of transparent substrates [12]. Thereby, a transparent chuck and a UV-source below this chuck are required (see Fig. 1b). Another solution is an inverted imprint setup with the substrate being placed on top of the mold (see Fig. 1c). Unfortunately, this is not very convenient for stamp-and-repeat imprinting. In usual UV-NIL systems (through-mold-exposure), the UV source is located above the mold and the substrate is located below on a non-transparent chuck (see Fig. 1a). This work presents a new technique, termed non-transparent UV-NIL (NT-UV-NIL), which uses non-transparent molds (see Fig. 1d and e) in a through-mold-exposure system. 2. Materials and methods 2.1. Imprint material As imprint material, a customized resist formulation with 67.5% w/w tert-butyl acrylate, 30% w/w ethylene glycol diacrylate and 2.5% w/w 2-hydroxy-2-methylpropiophenone as photoinitiator was used. All chemicals were purchased from Aldrich. The free-radical polymerization is sensitive to oxygen. Thin layers as they occur in UV-NIL were successfully cured with doses of about 6000 mJ/cm2 with fully transparent molds. This is because the dissolved oxygen is rapidly used up by the polymerization process
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Fig. 1. (a) Standard UV-NIL imprint situation. A setup for non-transparent molds with (b) a bottom UV-source and (c) a top UV-source (transparent substrate holder required). (d) NT-UV-NIL setup requiring no change compared to the standard setup. (e) NT-UV-NIL mold and the exposure situation.
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(b) Fig. 2. Setup of the NT-UV-NIL imprint experiments with different imprint systems.
and the diffusion of new oxygen into thin layers is slower than the polymerization reaction. Kinetic curing models and rate coefficients required for them are very system specific and difficult to obtain for a UV-NIL material [13]. The lowest exposure dose used to cure the imprint material was 3000 mJ/cm2, which is not the minimum achievable. Minimal exposure doses required for curing found in the literature for comparable monomer systems were 150 mJ/cm2 [14] with another photoinitiator and 477 mJ/cm2 with the same photoinitiator [13]. For other monomer systems with the same photoinitiator, minimal doses of 250 mJ/cm2 [15] and 1134 mJ/cm2 [16] were reported for good monomer conversion. For other UV-NIL resists, minimum curing doses of 300 mJ/cm2 were reported [14]. In summary, a reasonable minimum curing dose of about 500 mJ/cm2 could be extracted from the literature. A thermally initiated curing of the monomer system in this work was ruled out. Additionally, the commercial resist mr-UVCur21SF (micro resist technology GmbH, Germany) was used. 2.2. Imprint setups The large scale potential of the NT-UV-NIL mechanism was investigated with a contact mask exposure tool (AL6-2, EV Group GmbH) and a stamp-and-repeat imprint system (NPS300, SET S.A.S) (see Fig. 2a and b). The laboratory potential of NT-UV-NIL was investigated with a customized imprint press (GeSiM mbH) (see Fig. 2c). 2.2.1. Al6-2 In the AL6-2 tool, a stack of a NT-UV-NIL mold and the substrate was put between a transparent lower chuck (normally the
place of the wafer) and a transparent upper chuck (normally the place of the photomask) (see Fig. 2a). After dispensing the resist between the mold and the substrate, the complete stack was pressed with 1 bar. A UV flood exposure was done by a mercury arc lamp with a power density of 18 mW/cm2 for the 300– 400 nm spectral range. After releasing the pressure, the mold was peeled off. Diffuse reflections from AL6-2 machine parts and the substrate-chuck interface might have been possible but they were expected to be small compared to the primary exposure due to the transparent chuck and a blackened machine interior. However, they could not be ruled out completely. 2.2.2. NPS300 In contrast to the AL6-2, the NPS300 has a non-transparent SiC chuck and a tunable, 250 mW/cm2, 365 ± 15 nm LED UV-source in a 65 mm 65 mm transparent upper chuck (compare Fig. 2b). It is obvious that for the NPS300 the support of a NT-UV-NIL mold has to be transparent and the structured element must be at least smaller than the upper chuck so that the light is able to reach the polymer. In this setup, the UV-light could only propagate laterally into the imprint material, either through the thin film of imprint material, the transparent substrate (compare Fig. 1e) or through the transparent film on a non-transparent substrate. 2.2.3. Laboratory press The universal practicability of NT-UV-NIL was investigated with a simple laboratory press (see Fig. 2c). The substrate was placed on a metal support. The UV-source had an F-spectrum (iron doped mercury arc lamp) with a power density of 68 mW/cm2 in the 300–400 nm spectral range and a broad angular intensity
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Fig. 3. NT-UV-NIL imprints of (a) dense resist lines with a period of 3.5 lm with the AL6-2 on a 2.3 mm glass substrate and (b) of a test structure with the NPS300 on a transparent 1 mm thick glass slide.
Fig. 4. Homogeneous NT-UV-NIL imprint of an electrode array on 18 lm USG with (a) macroscopic overview, (b) center of the imprint field and (c) resolved 150 nm line. (d) NT-UV-NIL imprint with insufficient UV-dose on 4.5 lm USG.
distribution. Again, the light could not pass directly through the non-transparent Si chip during the flood exposure. 2.2.4. Light diffusor To enhance diffuse light reflections, the imprint press was put in an aluminum chamber. This chamber provided additionally a N2 atmosphere to eliminate the oxygen curing inhibition. 2.3. Substrates To reveal the NT-UV-NIL limits, further experiments were conducted with the laboratory imprint press on 70 lm glass slides and 30 lm polyethylene terephtalat (PET) foils. These transparent substrates were placed on a Si substrate with 1 lm SiO2. To achieve thinner transparent films, 18 lm, 9 lm, 4.5 lm undoped silicate glass (USG) was deposited by plasma enhanced chemical vapor deposition. The thinnest films were 1 lm and 140 nm thermal SiO2 on Si. Further substrates were a 175 lm PET foil, 625 lm quartz wafers as well as 1 and 2.3 mm glass slides.
2.4. NT-UV-NIL molds The molds were unstructured or structured Si chips sized 10 mm 10 mm. Additionally, a 15 mm 15 mm quartz chip with a non-transparent, antireflective Cr surface was used with the NPS300. During the imprint process, the non-structured Cr surface faced the imprint material and the substrate. A 1H,1H,2H,2H-perfluorodecyltrichlorosilane (ABCR GmbH, Germany) antisticking layer (ASL) was deployed for all molds. 3. Results and discussion 3.1. Successful NT-UV-NIL imprints 3.1.1. Al6-2 A Si mold with a line-space pattern and a period of 3.5 lm was successfully replicated into the imprint material. The complete resist was cured and the mold structure was perfectly transferred (see Fig. 3a).
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Fig. 5. NT-UV-NIL imprint of (a) resist lines with a period of 3.5 lm with a laboratory press on a 175 lm thin PET foil and (b) dense, 25 nm thick Al lines after imprint with the same mold, 25 nm Al sputtering and lift off on a 2.3 mm thick glass substrate.
3.1.2. NPS300 The ability for stamp-and-repeat imprinting of NT-UV-NIL was demonstrated with a structured Si chip. The chip was directly placed without a support under the transparent upper chuck and pressed with about 0.6 bar into a before dispensed resist depot on a 1 mm glass slide. The structure on the silicon mold was successfully replicated into the imprint material (see Fig. 3b) by a 60 s exposure with an intensity of 200 mW/cm2. To increase the non-transparent area, the larger, unstructured Cr mold was imprinted on a 1 mm glass slide. The resist was completely cured within 30 s at 100 mW/cm2 demonstrating the potential of the NT-UV-NIL for large non-transparent molds. Further detailed studies on the mold size were not conducted. NT-UV-NIL imprints were homogeneous and completely cured over a 10 mm 10 mm imprint field (see Fig. 4a and b). Isolated 150 nm features were resolved (see Fig. 4c). The resist curing proceeded from the periphery of the mold to its center. Hence, insufficient doses lead to a partial pattern transfer (see Fig. 4d). As a consequence, completely cured imprints received in peripheral areas higher UV-intensities than in central ones. The distribution of the received intensities in NT-UV-NIL is, thus, not as homogeneous as in UV-NIL. 3.1.3. Laboratory press Successful imprints were achieved on a 175 lm thin PET foil and on a 2.3 mm glass substrate which was further used to realize dense Al lines with an acetone lift off process (see Fig. 5). 3.2. Influence of the imprint setup NT-UV-NIL imprints were successfully achieved independent of particular imprint setups. However, the required curing time for the imprint of the same unstructured Si mold on a 675 lm quartz substrate could be reduced from 30 to 15 min by using the aluminum chamber as light diffusor. 3.3. Influence of the substrate With the Cr-mold, an imprint was conducted with the NPS300 on a Si wafer having 90 nm SiO2 and a polymethylmethacrylate (PMMA) thinner than 1 lm on top. The resist was not cured, indicating a lower limit for the usable thickness of the transparent substrate layer. Further experiments with the laboratory press revealed that on USG-films thinner than 4.5 lm no curing was possible in reasonable processing times (see Fig. 6). After an unreasonable 2 h (489.6 J/cm2) exposure, only a partial curing with uncured resist in the mold center was possible on 4.5 lm USG. On 9 lm USG almost all imprint material was cured within 1h (244.8 J/cm2). On 18 lm USG complete curing was achieved within 1 h with shorter
Fig. 6. Relation between the thickness of the transparent layer and the curing dose ((a) = without Al-chamber).
curing times being expected. Thus, the limit for the transparent layer thickness was about 18 lm for reasonable process times well below 1 h with the used setup. The curing time is expected to be reducible even further by improved light guiding measures, which was beyond the scope of this work. Notable was, that the 5 min minimal curing time for the 35 lm PET foil was significantly shorter than the 15 min for the slightly thinner 30 lm adhesive tape (see Fig. 6). Transparency cannot account for this effect. The adhesive tape was even more transparent at 365 nm than the PET foil. But the PET foil (118 nm RMS) was much rougher than the tape (7 nm RMS). This gives evidences that the roughness of the substrate and thus diffuse scattering plays an important role for NT-UV-NIL. Those effects are aspects for further research. NT-UV-NIL with another material, mr-UVCur21SF, required 2 min (30 J/cm2) on 18 lm USG and 10 min (150 J/cm2) on 9 lm for complete curing with the NPS-300. This was not possible on 4.5 lm USG up to 10 min. 4. Conclusion and outlook A new technique, NT-UV-NIL, was used to imprint UV-curable resists on transparent substrates, foils and thin films by indirect exposure. Special measures against oxygen inhibition were not mandatory. Enough UV-intensity reached the resist indirectly. NT-UV-NIL is well suited for direct imprinting and lift-off. It enables the use of cost-efficient Si molds and new non-transparent materials, which were so far not usable for ‘‘through-mold-exposure’’
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systems. It is a route towards non-transparent ASLs, e.g., metals, to avoid organosilane ASL degradation in UV-NIL. NT-UV-NIL worked with a typical stamp-and-repeat UV-NIL mold size. We speculate that the achieved resolution was only limited by the available imprint molds. This is currently under further investigation. Acknowledgements This work was partly funded by the DFG as Research Training Group ‘‘Nano- and Biotechnologies for Packaging of Electronic Systems’’ (DFG 1401/1), the Fraunhofer-Society and the Saechsische AufbauBank (SAB). References [1] S.Y. Chou, P.R. Krauss, P.J. Renstrom, Appl. Phys. Lett. 67 (1995) 3114–3116. [2] H. Schmitt, M. Rommel, A.J. Bauer, L. Frey, A. Bich, M. Eisner, R. Voelkel, M. Hornung, Microelectron. Eng. 87 (2010) 1074–1076.
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