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Single-step 3D nanolithography using plasma polymerized hexane films
Microelectronic Engineering 98 (2012) 167–170
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Single-step 3D nanolithography using plasma polymerized hexane films Rasmus H. Pedersen ⇑, Kiryl Kustanovich, Nikolaj Gadegaard University of Glasgow, School of Engineering, Rankine Building, Oakfield Avenue, Glasgow G12 8LT, United Kingdom
a r t i c l e
i n f o
Article history: Available online 20 July 2012 Keywords: Plasma polymerization E-beam lithography 3D nanopatterning Conformal resist coating
a b s t r a c t The fabrication of nanostructured textures on top of a microstructured topography is a topic of considerable interest, with a number of different approaches having been proposed, most of which are either cumbersome, or represent a radical departure from standard fabrication processes. In this work, we present a simple fabrication scheme for obtaining nanopatterned structures in 3D. The scheme relies on the use of plasma polymerized hexane as an electron beam resist. We utilize the unique property of plasma polymerization where the films are deposited with conformal coverage across pre-structured surfaces. This enables high resolution electron beam lithography to be performed on a pre-patterned/etched substrate. As a demonstrator, we show that nanometric line gratings can be produced at both the top and bottom surfaces of preetcheded, 15–20 lm deep, box arrays. The final structure can be replicated in PDMS, to provide a soft working stamp for subsequent 3D patterning. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction In recent years, interest has accelerated in the development of methods for fabrication of 3D structures on the micro- and nanoscale, e.g. for applications in optical/photonic control [1,2], support structures for biological studies [3,4], or complex stamps for nanoimprint of e.g. electronic or microfluidic components [5,6]. Two different classes of 3D structures can be identified. ‘‘True’’ 3D are structures including e.g. overhanging structures or porous materials. Such structures can be fabricated on the micro and nanoscale using e.g. 2-photon polymerization [7], sequential reverse nanoimprint [8], advanced etching techniques [9], direct micro/nanomanipulation [10], or self-assembly [11]. Many structures do not, however, require such undercuts, i.e. they can be unambiguously projected onto a 2D surface. Such structures are sometimes denoted 2.5-dimensional, the term used in machining (2.5D structures can be created by a simple 3-axis milling machine). Examples are surfaces with multiple structured levels or structures with a controlled, smooth, variation in height. Obtaining this subset of 3D structures on the micro and nanoscale is generally much simpler than full 3D, and can be performed exploiting tools and processes already in use for 2D planar processing. Examples include tuning of dry etch parameters [12], greyscale lithography (photon or beam-based) [13–15], thermal reflow of polymer layers [4] or a series of sequential 2D lithography and etch steps [6]. In this work, we present a simple process to allow nanotopography to be created on a previously defined structure, obtaining a complex 2.5D ⇑ Corresponding author. Tel.: +44 1413306691. E-mail addresses: [email protected] (R.H. Pedersen), Nikolaj. [email protected] (N. Gadegaard).
structure using only a single high resolution lithography step. The process uses a plasma polymerized thin film as resist, exploiting the fact that plasma polymerized layers are deposited conformally over an existing topography [16]. Plasma polymers have been previously shown to be capable of use as DUV or electron beam resists [17,18]. As a demonstrator of the process, we show the fabrication of nanoscale line grating on the top and bottom surfaces of a deep (>15 lm) microscale structure. This process involves the coating of resist over a predefined structure. As shown in Fig. 1(a) attempting to spin coat resist on such a sample leads to a very low quality film coverage, with large variations in thickness around the predefined structure, and tails from the obstruction towards the edge of the sample. Conversely, for plasma polymerized film, as shown in Fig. 1(b), the deposition is not affected by the predefined structure, and the obtained resist film is defect-free and highly uniform. It should be noted that the typical alternative to spin coating for these types of processes, spray coating [19], will avoid the defects seen in Fig. 1(a), but it cannot reproduce the high uniformity in thickness obtained by plasma polymerization. Another alternative to providing this type of structure is thermoforming of a polymer film [20,21]. Such processes, however, are limited to application in plastic materials, and will result in some deformation of the defined structure. 2. Experimental Silicon substrate pieces, approximately 25 25 mm2 in size, were used. Firstly, a number of gold registration markers were produced on the substrate by electron beam lithography and lift-off. Both the following lithographic processes were aligned using these
0167-9317/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.07.054
168
R.H. Pedersen et al. / Microelectronic Engineering 98 (2012) 167–170
Fig. 1. Illustration of the coating of resist on a pre-patterned surface (in this case, 20 lm box arrays). (a) Spin coating of PMMA, the resist uniformity is very low and the defect density is high. (b) Plasma polymerization of hexane. The preexisting topography is covered without defects, and film uniformity is excellent.
Fig. 2. Fabrication procedure. (a) Box arrays are defined and etched using UV lithography. (b) Plasma polymerized hexane is deposited and exposed to an electron beam. If the focus is not controlled, features on the top and bottom will not be written with the same spot size (front). If the focus is controlled depending on location, high resolution lithography can be obtained on both surfaces (rear). (c) Lines defined after development of the resist.
marks. Using standard UV lithography and lift-off of 150 nm of aluminum, an array of 10 lm boxes with 10 lm spacing is defined and subsequently transferred by deep reactive ion etching to a depth of 15–25 lm (STS ASE), after which the aluminum mask was removed. The box array was aligned to the registration markers. Thus the structure shown in Fig. 2(a) was obtained. Prior to plasma polymer deposition, the sample was given an O2 plasma treatment at 100 W for 1 min, to improve adhesion. A 60 nm thick layer of plasma polymerized hexane (ppHex) was deposited in a borosilicate chamber at 50 W RF power and a pressure of 0.4 mbar. Further details of the ppHex deposition conditions have been published previously [22]. Electron beam lithography was carried out in a 100 kV Vistec VB6 UHR EWF tool. Full details of the performance of ppHex as a resist for e-beam lithography is published previously [18]. Due to the nature of the crosslinking process in the material [23], a relatively large dose is required. The dose requirement also increases heavily as the linewidth is decreased. Lines of nominally 100 nm width and either 500 nm or 1 lm pitch were defined across the pre-patterned area, at an exposure dose of 25 mCcm 2. During exposure, the lithography tool automatically focuses the electron beam on the surface using a laser height meter reading. However, the resolution of the height meter means the microstructured surface is not resolved. Therefore, exposed patterns on the top and bottom surface will be exposed at different beam spot sizes, result-
Fig. 3. Measurement of linewidth of nominally 100 nm wide lines defined on a preetched substrate (etch depth: 16 lm). With no control of the focus, a 40 nm difference in linewidth is observed. By tuning the electron beam focus, lines on the top surface can be defined with the optimal linewidth. Equal linewidth can also be obtained with a smaller defocus of approximately 5 lm on both levels, but the linewidth is increased from the design specification.
ing in pattern distortion (Fig. 2(b) front). However, by use of the registration markers, the electron beam focus can be intentionally moved depending on the location of the pattern to be written,
R.H. Pedersen et al. / Microelectronic Engineering 98 (2012) 167–170
169
Fig. 4. Scanning Electron Microscope images of line arrays defined at the top and bottom surface of a pre-etched 20 lm tall feature. (a) Using alignment and control of the beam focus high resolution features are obtained on both surfaces. (b) If beam focus is not controlled, linewidth control on the top surface is poor.
allowing for optimal focus on both top and bottom level in a single exposure (Fig. 2(b) rear). After exposure, the ppHex was developed in a 1:1 mixture of cyclopentanone and o-xylene for 5 min using ultrasonic agitation, Fig. 2(c). Finally, the produced pattern was transferred into the silicon substrate to a depth of 100 nm by reactive ion etching, and the ppHex removed in an O2 plasma.
3. Results Fig. 3 shows measurements of the obtained linewidth of nominally 100 nm wide lines, as a function of the deliberate defocus. As expected, the data shows a parabola-like shape, corresponding to the ‘‘focus cone’’ of the electron beam. The automatic focus of the electron beam is on the bottom surface of the predefined structure, so the minimum linewidth is obtained at zero defocus. There is a slight self-induced linewidth increase, as previously observed in ppHex [18]. For patterns on the top surface, a considerable linewidth increase of approximately 40 nm is observed at zero defocus. It is observed that equal linewidth can be obtained by the intentional defocus of the structures on the top surface, in the appropriate direction. Fig. 4(a) shows SEM images of dense lines obtained on both surfaces of the structure, in a situation where the focus has been accurately controlled (for this sample, the etch depth and defocus were both approximately 20 lm). Fig. 4(b) shows a situation where the defocus was not accurately controlled. As observed, the linewidth on the top surface is considerably deformed. As discussed above, ppHex requires a very high electron beam dose as compared to traditional resists. Thus, the process as it stands is not well suited for high volume fabrication. To alleviate this issue, we demonstrate here the compatibility of the process with a replication technique. Alternatively, one might consider developing a different or enhanced plasma polymer with an improved response to electron beam irradiation. Soft lithography, i.e. the casting of a silicone replica from a (typically hard) master, is a commonly used tool e.g. for further replication of a complex structure using UV-NIL, or for direct use. We have used this method on our demonstrator structure to show compatibility. Polydimethylsiloxane (PDMS) was mixed in a 10:1 base/curing agent ratio and desiccated to remove bubbles. It was
Fig. 5. Scanning electron microscope image of replication of the multilevel structure in polydimethylsiloxane (PDMS).
then cast over the silicon sample in a custom-made holder and cured in an oven at 80 °C for 2 h. As shown in Fig. 5, the microand nano-structures have both been transferred successfully to the replica. 4. Conclusion We have described a simple fabrication scheme for producing nanostructures in 3D conformally coated plasma polymerized hexane as an electron beam resist, enabling high resolution patterning on a predefined microstructured surface. As a demonstrator of the concept, we show the definition of nanometric line gratings on both the top and bottom surface of a micropattern etched to a depth of 20 lm. The obtained structures can be replicated using any standard process – as an example, we show the casting of a PDMS replica. Acknowledgements The work has been partially funded by the Engineering and Physical Sciences Research Council (EPSRC), Grant no. EP/ F047851/1, and the Glasgow Research Partnership in engineering (GRPe). The work was supported by the EC-funded project NAPANIL (Contract no. FP7-CP-IP214249-2). References [1] J.S. Yoo et al., Sol. Energy Mat. Sol. C. 90 (18–19) (2006) 3085–3093. [2] S.Y. Lin et al., Nature 24 (2008) 1997–1999.
170 [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
R.H. Pedersen et al. / Microelectronic Engineering 98 (2012) 167–170 F. Klein et al., Adv. Mater. 23 (2011) 1341–1345. Y. Ding et al., Adv. Mater. 23 (2011) 421–425. M.D. Stewart et al., Proc. SPIE 5751 (2005) 210–218. L.H. Thamdrup et al., Nanotechnology 19 (12) (2008) 125301. A. Radke et al., Adv. Mater. 23 (2011) 3018–3021. N. Kehagias et al., Nanotechnology 18 (17) (2007) 175303. M.P. Stewart, J.M. Buriak, Adv. Mater. 12 (2000) 859–869. K. Aoki et al., Nat. Mater. 2 (2003) 117–121. S. Ndoni et al., J. Am. Chem. Soc. 125 (44) (2003) 13366–13367. W. Wu et al., Nanotechnology 21 (25) (2010) 255502. A. Rammohan et al., Sensor. Actuat. B Chem. 153 (1) (2011) 125–134.
[14] [15] [16] [17] [18] [19] [20]
S. Balslev et al., Proc. SPIE 6110 (2006) 61100C–61100C-8. M.D. Henry et al., Nanotechnology 21 (24) (2010) 245303. J. Yang et al., Adv. Mater. 21 (2009) 300–304. C.R. Hurley et al., Langmuir 26 (12) (2010) 10203–10209. R.H. Pedersen et al., Microelectron. Eng. 87 (5–8) (2010) 1112–1114. J. Linden et al., Microelectron. Eng. 88 (8) (2011) 2030–2032. S. Giselbrecht et al., Adv. Mater. (2011), http://dx.doi.org/10.1002/ adma.201102759. [21] T. Senn et al., Microelectron. Eng. 88 (9) (2011) 3043–3048. [22] M. Zelzer et al., J. Phys. Chem. B 113 (2009) 8487–8494. [23] R.H. Pedersen et al., Plasma Process. Polym. 9 (2012) 22–27.
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Single-step 3D nanolithography using plasma polymerized hexane films Rasmus H. Pedersen ⇑, Kiryl Kustanovich, Nikolaj Gadegaard University of Glasgow, School of Engineering, Rankine Building, Oakfield Avenue, Glasgow G12 8LT, United Kingdom
a r t i c l e
i n f o
Article history: Available online 20 July 2012 Keywords: Plasma polymerization E-beam lithography 3D nanopatterning Conformal resist coating
a b s t r a c t The fabrication of nanostructured textures on top of a microstructured topography is a topic of considerable interest, with a number of different approaches having been proposed, most of which are either cumbersome, or represent a radical departure from standard fabrication processes. In this work, we present a simple fabrication scheme for obtaining nanopatterned structures in 3D. The scheme relies on the use of plasma polymerized hexane as an electron beam resist. We utilize the unique property of plasma polymerization where the films are deposited with conformal coverage across pre-structured surfaces. This enables high resolution electron beam lithography to be performed on a pre-patterned/etched substrate. As a demonstrator, we show that nanometric line gratings can be produced at both the top and bottom surfaces of preetcheded, 15–20 lm deep, box arrays. The final structure can be replicated in PDMS, to provide a soft working stamp for subsequent 3D patterning. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction In recent years, interest has accelerated in the development of methods for fabrication of 3D structures on the micro- and nanoscale, e.g. for applications in optical/photonic control [1,2], support structures for biological studies [3,4], or complex stamps for nanoimprint of e.g. electronic or microfluidic components [5,6]. Two different classes of 3D structures can be identified. ‘‘True’’ 3D are structures including e.g. overhanging structures or porous materials. Such structures can be fabricated on the micro and nanoscale using e.g. 2-photon polymerization [7], sequential reverse nanoimprint [8], advanced etching techniques [9], direct micro/nanomanipulation [10], or self-assembly [11]. Many structures do not, however, require such undercuts, i.e. they can be unambiguously projected onto a 2D surface. Such structures are sometimes denoted 2.5-dimensional, the term used in machining (2.5D structures can be created by a simple 3-axis milling machine). Examples are surfaces with multiple structured levels or structures with a controlled, smooth, variation in height. Obtaining this subset of 3D structures on the micro and nanoscale is generally much simpler than full 3D, and can be performed exploiting tools and processes already in use for 2D planar processing. Examples include tuning of dry etch parameters [12], greyscale lithography (photon or beam-based) [13–15], thermal reflow of polymer layers [4] or a series of sequential 2D lithography and etch steps [6]. In this work, we present a simple process to allow nanotopography to be created on a previously defined structure, obtaining a complex 2.5D ⇑ Corresponding author. Tel.: +44 1413306691. E-mail addresses: [email protected] (R.H. Pedersen), Nikolaj. [email protected] (N. Gadegaard).
structure using only a single high resolution lithography step. The process uses a plasma polymerized thin film as resist, exploiting the fact that plasma polymerized layers are deposited conformally over an existing topography [16]. Plasma polymers have been previously shown to be capable of use as DUV or electron beam resists [17,18]. As a demonstrator of the process, we show the fabrication of nanoscale line grating on the top and bottom surfaces of a deep (>15 lm) microscale structure. This process involves the coating of resist over a predefined structure. As shown in Fig. 1(a) attempting to spin coat resist on such a sample leads to a very low quality film coverage, with large variations in thickness around the predefined structure, and tails from the obstruction towards the edge of the sample. Conversely, for plasma polymerized film, as shown in Fig. 1(b), the deposition is not affected by the predefined structure, and the obtained resist film is defect-free and highly uniform. It should be noted that the typical alternative to spin coating for these types of processes, spray coating [19], will avoid the defects seen in Fig. 1(a), but it cannot reproduce the high uniformity in thickness obtained by plasma polymerization. Another alternative to providing this type of structure is thermoforming of a polymer film [20,21]. Such processes, however, are limited to application in plastic materials, and will result in some deformation of the defined structure. 2. Experimental Silicon substrate pieces, approximately 25 25 mm2 in size, were used. Firstly, a number of gold registration markers were produced on the substrate by electron beam lithography and lift-off. Both the following lithographic processes were aligned using these
0167-9317/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.07.054
168
R.H. Pedersen et al. / Microelectronic Engineering 98 (2012) 167–170
Fig. 1. Illustration of the coating of resist on a pre-patterned surface (in this case, 20 lm box arrays). (a) Spin coating of PMMA, the resist uniformity is very low and the defect density is high. (b) Plasma polymerization of hexane. The preexisting topography is covered without defects, and film uniformity is excellent.
Fig. 2. Fabrication procedure. (a) Box arrays are defined and etched using UV lithography. (b) Plasma polymerized hexane is deposited and exposed to an electron beam. If the focus is not controlled, features on the top and bottom will not be written with the same spot size (front). If the focus is controlled depending on location, high resolution lithography can be obtained on both surfaces (rear). (c) Lines defined after development of the resist.
marks. Using standard UV lithography and lift-off of 150 nm of aluminum, an array of 10 lm boxes with 10 lm spacing is defined and subsequently transferred by deep reactive ion etching to a depth of 15–25 lm (STS ASE), after which the aluminum mask was removed. The box array was aligned to the registration markers. Thus the structure shown in Fig. 2(a) was obtained. Prior to plasma polymer deposition, the sample was given an O2 plasma treatment at 100 W for 1 min, to improve adhesion. A 60 nm thick layer of plasma polymerized hexane (ppHex) was deposited in a borosilicate chamber at 50 W RF power and a pressure of 0.4 mbar. Further details of the ppHex deposition conditions have been published previously [22]. Electron beam lithography was carried out in a 100 kV Vistec VB6 UHR EWF tool. Full details of the performance of ppHex as a resist for e-beam lithography is published previously [18]. Due to the nature of the crosslinking process in the material [23], a relatively large dose is required. The dose requirement also increases heavily as the linewidth is decreased. Lines of nominally 100 nm width and either 500 nm or 1 lm pitch were defined across the pre-patterned area, at an exposure dose of 25 mCcm 2. During exposure, the lithography tool automatically focuses the electron beam on the surface using a laser height meter reading. However, the resolution of the height meter means the microstructured surface is not resolved. Therefore, exposed patterns on the top and bottom surface will be exposed at different beam spot sizes, result-
Fig. 3. Measurement of linewidth of nominally 100 nm wide lines defined on a preetched substrate (etch depth: 16 lm). With no control of the focus, a 40 nm difference in linewidth is observed. By tuning the electron beam focus, lines on the top surface can be defined with the optimal linewidth. Equal linewidth can also be obtained with a smaller defocus of approximately 5 lm on both levels, but the linewidth is increased from the design specification.
ing in pattern distortion (Fig. 2(b) front). However, by use of the registration markers, the electron beam focus can be intentionally moved depending on the location of the pattern to be written,
R.H. Pedersen et al. / Microelectronic Engineering 98 (2012) 167–170
169
Fig. 4. Scanning Electron Microscope images of line arrays defined at the top and bottom surface of a pre-etched 20 lm tall feature. (a) Using alignment and control of the beam focus high resolution features are obtained on both surfaces. (b) If beam focus is not controlled, linewidth control on the top surface is poor.
allowing for optimal focus on both top and bottom level in a single exposure (Fig. 2(b) rear). After exposure, the ppHex was developed in a 1:1 mixture of cyclopentanone and o-xylene for 5 min using ultrasonic agitation, Fig. 2(c). Finally, the produced pattern was transferred into the silicon substrate to a depth of 100 nm by reactive ion etching, and the ppHex removed in an O2 plasma.
3. Results Fig. 3 shows measurements of the obtained linewidth of nominally 100 nm wide lines, as a function of the deliberate defocus. As expected, the data shows a parabola-like shape, corresponding to the ‘‘focus cone’’ of the electron beam. The automatic focus of the electron beam is on the bottom surface of the predefined structure, so the minimum linewidth is obtained at zero defocus. There is a slight self-induced linewidth increase, as previously observed in ppHex [18]. For patterns on the top surface, a considerable linewidth increase of approximately 40 nm is observed at zero defocus. It is observed that equal linewidth can be obtained by the intentional defocus of the structures on the top surface, in the appropriate direction. Fig. 4(a) shows SEM images of dense lines obtained on both surfaces of the structure, in a situation where the focus has been accurately controlled (for this sample, the etch depth and defocus were both approximately 20 lm). Fig. 4(b) shows a situation where the defocus was not accurately controlled. As observed, the linewidth on the top surface is considerably deformed. As discussed above, ppHex requires a very high electron beam dose as compared to traditional resists. Thus, the process as it stands is not well suited for high volume fabrication. To alleviate this issue, we demonstrate here the compatibility of the process with a replication technique. Alternatively, one might consider developing a different or enhanced plasma polymer with an improved response to electron beam irradiation. Soft lithography, i.e. the casting of a silicone replica from a (typically hard) master, is a commonly used tool e.g. for further replication of a complex structure using UV-NIL, or for direct use. We have used this method on our demonstrator structure to show compatibility. Polydimethylsiloxane (PDMS) was mixed in a 10:1 base/curing agent ratio and desiccated to remove bubbles. It was
Fig. 5. Scanning electron microscope image of replication of the multilevel structure in polydimethylsiloxane (PDMS).
then cast over the silicon sample in a custom-made holder and cured in an oven at 80 °C for 2 h. As shown in Fig. 5, the microand nano-structures have both been transferred successfully to the replica. 4. Conclusion We have described a simple fabrication scheme for producing nanostructures in 3D conformally coated plasma polymerized hexane as an electron beam resist, enabling high resolution patterning on a predefined microstructured surface. As a demonstrator of the concept, we show the definition of nanometric line gratings on both the top and bottom surface of a micropattern etched to a depth of 20 lm. The obtained structures can be replicated using any standard process – as an example, we show the casting of a PDMS replica. Acknowledgements The work has been partially funded by the Engineering and Physical Sciences Research Council (EPSRC), Grant no. EP/ F047851/1, and the Glasgow Research Partnership in engineering (GRPe). The work was supported by the EC-funded project NAPANIL (Contract no. FP7-CP-IP214249-2). References [1] J.S. Yoo et al., Sol. Energy Mat. Sol. C. 90 (18–19) (2006) 3085–3093. [2] S.Y. Lin et al., Nature 24 (2008) 1997–1999.
170 [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
R.H. Pedersen et al. / Microelectronic Engineering 98 (2012) 167–170 F. Klein et al., Adv. Mater. 23 (2011) 1341–1345. Y. Ding et al., Adv. Mater. 23 (2011) 421–425. M.D. Stewart et al., Proc. SPIE 5751 (2005) 210–218. L.H. Thamdrup et al., Nanotechnology 19 (12) (2008) 125301. A. Radke et al., Adv. Mater. 23 (2011) 3018–3021. N. Kehagias et al., Nanotechnology 18 (17) (2007) 175303. M.P. Stewart, J.M. Buriak, Adv. Mater. 12 (2000) 859–869. K. Aoki et al., Nat. Mater. 2 (2003) 117–121. S. Ndoni et al., J. Am. Chem. Soc. 125 (44) (2003) 13366–13367. W. Wu et al., Nanotechnology 21 (25) (2010) 255502. A. Rammohan et al., Sensor. Actuat. B Chem. 153 (1) (2011) 125–134.
[14] [15] [16] [17] [18] [19] [20]
S. Balslev et al., Proc. SPIE 6110 (2006) 61100C–61100C-8. M.D. Henry et al., Nanotechnology 21 (24) (2010) 245303. J. Yang et al., Adv. Mater. 21 (2009) 300–304. C.R. Hurley et al., Langmuir 26 (12) (2010) 10203–10209. R.H. Pedersen et al., Microelectron. Eng. 87 (5–8) (2010) 1112–1114. J. Linden et al., Microelectron. Eng. 88 (8) (2011) 2030–2032. S. Giselbrecht et al., Adv. Mater. (2011), http://dx.doi.org/10.1002/ adma.201102759. [21] T. Senn et al., Microelectron. Eng. 88 (9) (2011) 3043–3048. [22] M. Zelzer et al., J. Phys. Chem. B 113 (2009) 8487–8494. [23] R.H. Pedersen et al., Plasma Process. Polym. 9 (2012) 22–27.