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Hot roll embossing in thermoplastic foils using dry-etched silicon stamp and multiple passes
Microelectronic Engineering 88 (2011) 2679–2682
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Hot roll embossing in thermoplastic foils using dry-etched silicon stamp and multiple passes Khaled Metwally ⇑,1, Samuel Queste, Laurent Robert, Roland Salut, Chantal Khan-malek FEMTO-ST Institute, UMR CNRS 6174, 32 Avenue de l’Observatoire, 25044 Besançon cedex, France
a r t i c l e
i n f o
Article history: Available online 16 February 2011 Keywords: Roll-to-roll Hot embossing Silicon stamp Dry-etching Flexible microfluidic devices COC PMMA
a b s t r a c t Hot roll embossing is a promising technique for manufacturing and patterning of micron and sub-micron features. It attracted attention due to its high volume production and large area processing. In this work, we describe a hot-roll-embossing process for manufacturing flexible devices in different commercially available thermoplastic polymer foils using a microstructured silicon wafer as a flat stamping tool. Larger features of a 100 lm width were defined by photolithography and dry etching. A process combining deep reactive ion etching and reactive ion etching of silicon in fluorinated plasma was developed to achieve patterns in silicon with slightly positively tapered sidewalls to allow for easy demoulding. 100 lm features were successfully replicated in cyclic-olefin-copolymer (COC) and poly-methyl-methacrylate (PMMA), using relatively low temperature and multiple passes. Smaller features with 1 lm width were patterned by electron beam lithography and transferred by DRIE in silicon stamps, then replicated in COC foils. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years the necessity of having disposable devices has lead to the usage of polymers in fabrication as they are low cost and offer capability for high volume production. In particular, foil-based solutions are emerging, particularly in applications requiring higher mechanical flexibility, lighter weight, faster heat transfer efficiency and lower material consumption [1,2]. Several techniques like micro-thermoforming [3], hot or UV embossing [4] or lamination techniques [5,6] can be used for producing microdevices on polymer thin films. Roll embossing, where rollers are used instead of the flat plates of planar embossing, is emerging as a viable fabrication technology which provides advantages such as reduced cycle time (no cooling of rollers) and large processing area (no fixed format and advancement of film between the rollers). It also allows continuous or semi-continuous manufacturing [7–9], hence producing a very large number of devices with the same conditions in one run. One of the challenges in large area roll embossing is how to fabricate an embossing master with low cost and high resolution. The solution chosen most of the times is to use a thin and flexible patterned stamp (generally electroplated nickel [10], but also thinned silicon [11]) which is wrapped around the roll cylinder. In this ⇑ Corresponding author. Tel.: +33 381853978; fax: +33 381853998. E-mail address: [email protected] (K. Metwally). Erasmus Mundus student – European Union Master for Mechatronics and MicroMechatronics (EU4M). 1
0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.02.009
work we used a silicon wafer structured using clean-room technologies as a stamping tool and a configuration similar to a ‘‘roll on flat’’ configuration. Performing hot embossing in roller-based or planar configuration results in different ways of applying temperature and pressure. In a configuration involving roller(s), heating occurs only during a short time, which depends on the roller speed whereas a planar configuration involves a long thermal cycle resulting from heating, holding temperature and cooling of the tool and supporting platens. The applied pressure in the contact area of the polymer with the roller is parabolic as a function of time. In contrast, in planar embossing, a constant pressure is uniformly applied on the whole polymer surface which is simultaneously patterned [12]. Influence of parameters such as roller temperature, applied pressure, roller speed and preheating of polymer substrate prior to embossing was reviewed by various groups [7–9]. We introduce here as well the number of passes through the roller as additional embossing parameter. It offers several advantages as it allows embossing at lower temperatures and decreases the internal residual stress in the embossed polymer film. Multiple passes can be applied in multi-roller embossing system that will allow full control of thermal cycle without affecting the production rate. In this work, we report on our experiments of hot roll embossing using dry-etched silicon stamp, starting by the manufacture of Si-moulds, followed by replication in COC and PMMA thermoplastic foils. Discussion of results will be performed in comparison with literature.
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2. Experimental 2.1. Si-mould fabrication The mould was fabricated on a single-side polished 1 mm thick 300 silicon wafer by means of photolithography and dry etching techniques. Firstly, the wafer was cleaned in piranha for 3 min followed by de-ionized water rinse for 3 min and dehydration on a hotplate at 100 °C for 10 min. Positive photoresist SPR220-3.0 was spin coated to define the 2.4 lm thick layer, then soft baked on hotplate from 25 to 115 °C in 15 min on hotplate from 25 to 115 °C in 15 min, hold at 115°C for 5 min, then cooled down to room temperature. The resist was exposed using a EVG620 double side alignment exposure with a power 300 mJ/cm2. The post-exposure bake on hotplate was done for 5 min at 115 °C. The unexposed SPR220-3.0 was developed away with MF-26A. After the photolithographic step, a sequential combination of deep reactive ion etching using a modified Bosch process and reactive ion etching in sulphur hexafluoride (SF6) plasma was done to achieve slightly positively tapered sidewalls to ease the demoulding. Deep anisotropic dry etching in silicon was performed in an Adixen (Alcatel Vacuum Technology) A 601 E etcher based on high-density inductively coupled plasma. The modified Bosch process derived out of the standard commercially available Bosch process consisted in the alternating steps of passivation and etching using octo-fluoro-butane (C4F8) gas and SF6 gas, respectively. An under-passivation cycle was done by reducing the passivation flow gas and increasing the flow of etching gas. It consisted of a 2-s step with 40 sccm of C4F8 followed by a 7-s step with 500 sccm of SF6. During the process, the pressure was regulated by a threshold valve. In particular the pressure during the SF6 cycle was maintained between 3.2 102 and 3.6 102 mbar to prevent the micromasking effect due to the presence of small particles which block the silicon etching and create the ‘‘grass.’’ Another important parameter to control during the etching process is the temperature of the substrate, which was maintained at 20 °C. After removing the photoresist mask a RIE etching process was performed in a Fluorine Plassys RIE etcher under SF6 plasma for 12 min using a power of 75 W and a gas flow of 20 sccm. Finally, an anti-sticking layer to further facilitate mould release was deposited on the patterned wafers under a flow of 500 sccm of C4F8 passivation gas for 5 min. Smaller features from 1 lm to sub-micronic pattern size were produced by electron beam lithography using a Raith E_line pattern generator operated at 10 keV in 460 nm thick ZEP520A electron beam sensitive resist from ZEON Corp. This resist was chosen because it has a good etch resistance to fluorinated plasma and can be used as a sole etching mask for deep reactive ion etching for high aspect ratio sub-micronic features with an aspect ratio up to 30. Pattern transfer for 1 lm wide ridges was performed using a slightly modified Bosch process with low bias power (40 W) and shorter cycle time (3 s on SF6-250 sccm steps and 1 s on C4F8-265 sccm step) to reduce the scalloping of the sidewalls.
2.2. Roll embossing process Roll embossing was conducted using a Rohm & Haas 350HR laminator. It consists of two rollers with metallic cylinders covered with a rubbery material. The rotating motor is attached to the lower roller, the speed of rotation being an adjustable parameter. The upper roller has a degree of freedom in vertical translation allowing two positions, separation or contact (with the lower roller). The contact pressure is adjustable via a relief valve pressurized by air supply. The upper roller is rotated by transmission during contact with the lower roller. A temperature range from room temperature to 130 °C is
allowed for both rollers and the temperature of each roll can be regulated independently. The pressure can be adjusted from 0 to 8 bars and the feed rate can be increased continuously up to 3 m/min. Thin films of two different thermoplastic materials were fed as flexible polymer substrates, cyclic-olefin-copolymer (COC) (Topas 8007 from Ticona GmbH) and poly-methyl-methacrylate (PMMA) (Goodfellow) of respective nominal thickness of 130 and 125 lm. Their glass transition temperature (Tg) is equal to 78 and 105 °C, respectively. A structured silicon wafer used as a stamping tool was placed with its features upwards on a supporting metallic plate. The polymer/silicon stamp assembly was then forced to pass between both embossing rollers under given embossing pressure, temperatures, and rolling speed. Several experiments were performed to optimize the filling of microcavities of the stamp. Pattern transfer in polymer foils was investigated as a function of roller temperature, applied pressure, feeding rate, and number of passes. Both mould and replicas were characterized using optical microscopy (leica), profilometry (Tencor Alpha-Step 200) and scanning electron microscopy (SEM) (Leica Cambridge Stereoscan S-440). 3. Results and discussion 3.1. Si mould Usually, a demoulding force is required in thermal embossing in order to release the mould from replicas. Different techniques have been used to decrease this force, hence ease the demoulding of the embossed part from the mould. They include fabrication of tapered side walls by control of etching conditions, for example involving a wet anisotropic etching step in the fabrication of the mould in silicon crystal [13,14]. In particular, etching in potassium hydroxide solution results in a high surface smoothness at the atomic scale; however it produces structures with a fixed angle (54.7° along the (1 1 1) surface for (1 0 0) oriented silicon wafers). Additionally, an anti-sticking layer can be deposited to ease demoulding [15]. For instance, a release layer such as Teflon-like film may decrease the ripple effect of DRIE scalloping; however, the anti-sticking layer needs to be renewed after a number of replications. A microfluidic design was etched in the 1 mm thick silicon wafer with microfeatures of 100 lm nominal width and 30 lm depth. The resultant ridge (inverse relief pattern of channel to be obtained on polymer replica) got an upper width of approx. 90 lm and lower width of 100 lm. The modified DRIE process used in this work enabled a sloped sidewall profile favourable for demoulding as shown in Fig. 1. The inward taper angle was 78° with respect to the silicon wafer plane. The RIE step was used to suppress the Si undercut created by etching with SF6 gas and smoothen the roughness of the sidewalls caused by the lower passivation time used during the DRIE step. In addition, a 900 nm thick release layer was deposited in C4F8 plasma at the end of the DRIE etching process. The modified Bosch process was then extended to achieve submicronic features. However, in this case the patterning on top was performed by e-beam lithography. Arrays of lines were patterned with a 1 lm width and transferred into a depth of 5 lm. This process led to vertical walls with very light scalloping of the sidewalls as shown in Fig. 2 inset. Silicon ridges with features size of 300 nm width and 6 lm depth were also fabricated (Fig. 3). The darker part on top of the Si columns is the resin which remains after etching. 3.2. Replication in thermoplastic foils Microchannels of 100 lm width and 30 lm depth were successfully replicated by roll embossing in Topas 8007 COC, and PMMA
K. Metwally et al. / Microelectronic Engineering 88 (2011) 2679–2682
Fig. 1. SEM picture of dry-etched silicon stamp cross-section.
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Fig. 4. SEM picture of dry-etched silicon stamp with 100 lm width and 30 lm depth. Insets: replication in PMMA and COC foil.
Fig. 2. SEM picture of silicon stamp with ridges of 1 lm width, 5 lm height and 2 lm pitch produced by DRIE using a modified Bosch process. Inset: Magnified view of profile (6.5K) of the 1 lm wide ridges.
Fig. 5. SEM picture of channels with 1 lm width and 2 lm pitch roll embossed on COC foil. Transferred depth is in the order of 1 lm.
Fig. 3. SEM picture of sub-micronic features with 300 nm width and 6 lm depth (cleaved wafer with 80° tilt angle).
(Fig. 4) using a dry-etched flat Si-mould. The COC replica was produced in a single pass at a temperature of 70–110 °C for upper and lower rolls, respectively, using a pressure of 5.5 bar and a feed rate of 0.5 m/min. The PMMA replica was produced in four passes at a temperature of 130 °C for both upper and lower rollers while applying a pressure of 5.5 bar and a feed rate 0.1 m/min. First set of trials for roll embossing with smaller features was conducted in Topas 8007 COC. Embossed fine features of 1 lm width and 5 lm depth are shown in Fig. 5; in 10 passes at a temperature of 70–110 °C for upper and lower rolls, using a pressure of 6 bar and a feed rate of 0.1 m/min. However, the transferred depth was smaller than the height of the feature on the silicon mould, about 1 vs. 5 lm. We did not conduct experiments in PMMA for fine features however it is predicted to require higher temperature and higher pressure to be replicated. It is well known that the filling of mould microcavities can be improved by: increasing the embossing temperature which decreases the polymer viscosity, applying a higher pressure, reducing the feed rate, and increasing the number of passes. Experiments are in progress.
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K. Metwally et al. / Microelectronic Engineering 88 (2011) 2679–2682
4. Conclusion and outlook
References
A specific process using a combination of DRIE and RIE was developed to produce microstructured Si mould inserts for roll embossing thin flexible polymer parts. The process was extended to produce sub-micronic features. Replication was successfully demonstrated at micronic scale with foils of thermoplastic polymers using a laminator. Features of 100 lm width were transferred in 130 lm thick COC 8007 and 125 lm thick PMMA films, and microfluidic devices were produced using this process. 1 lm wide feature size was also replicated. A novelty of this work was to use relatively low temperature and multiple passes. Pattern transfer with sub-micronic features is being investigated.
[1] M. Focke, D. Kosse, C. Müller, H. Reinecke, R. Zengerle, F. von Stetten, Lab Chip 10 (2010) 1365–1386. [2] T. Velten, H. Schuck, M. Richter, G. Klink, K. Bock, C. Khan-Malek, S. Roth, H. Schoo, P. Bolt, J. Eng. Manuf. 222 (1) (2008) 107–116. [3] R. Truckenmüller, S. Giselbrecht, C. Van Blitterswijk, N. Dambrowsky, E. Gottwald, T. Mappes, A. Rolletschek, V. Saile, C. Trautmann, K.F. Weibezahn, A. Welle, Lab Chip 8 (2008) 1570–1579. [4] H. Schift, J. Vac. Sci. Technol. B 26 (2008) 1023–1071. [5] C. Khan-Malek and L. Robert, Proc. 4M2008, (2008) 96–99. [6] P. Abgrall, S. Charlot, R. Fulcrand, L. Paul, A. Boukabache, A.M. Gué, J. Microsyst. Technol. 14 (2008) 1205–1214. [7] T. Velten, H. Schuck, W. Haberer, F. Bauerfeld, Int. J. Adv. Manuf. Technol. 47 (2010) 73–80. [8] L.P. Yeo, S.H. Ng, Z.F. Wang, H.M. Xia, Z.P. Wang, V.S. Thang, Z.W. Zhong, N.F. de Rooij, J. Micromech. Microeng. 20 (1) (2010) 15017–15026. [9] D. Paul, A. Pallandre, S. Miserere, J. Weber, J.L. Viovy, Electrophoresis 28 (2007) 1115–1122. [10] N. Ishizawa, K. Idei, T. Kimura, D. Noda, T. Hattori, Microsyst. Technol. 14 (2008) 1381–1388. [11] T. Velten, F. Bauerfeld, H. Schuck, S. Scherbaum, K. Bock, C. Landesberger, Proc. Conf. DTIP (2010) 1–6. [12] S.H. Ahn, L. Guo, Am. Chem. Soc. Nano 3 (2009) 2304–2310. [13] H. Kawata, M. Matsue, K. Kubo, M. Yasuda, Y. Hirai, Microelectron. Eng. 86 (2009) 700–704. [14] M. Nordström, J. Hübner, A. Boisen, Microelectron. Eng. 83 (2006) 1269– 1272. [15] T. Hansen, D. Selmeczi, N. Larsen, J. Micromech. Microeng. 20 (2010) 015020.
Acknowledgements This work was carried out within the framework of the CarnotFraunhofer PICF programme (3lP project: Multi-Reaction, MultiSample Micro-Fluidic Platform). MIMENTO technology platform is part of the RENATECH network. K.M. would like to give special thanks the Erasmus Mundus EU4M-Consormium for offering the Master scholarship, and giving him the opportunity and support for doing this project.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Hot roll embossing in thermoplastic foils using dry-etched silicon stamp and multiple passes Khaled Metwally ⇑,1, Samuel Queste, Laurent Robert, Roland Salut, Chantal Khan-malek FEMTO-ST Institute, UMR CNRS 6174, 32 Avenue de l’Observatoire, 25044 Besançon cedex, France
a r t i c l e
i n f o
Article history: Available online 16 February 2011 Keywords: Roll-to-roll Hot embossing Silicon stamp Dry-etching Flexible microfluidic devices COC PMMA
a b s t r a c t Hot roll embossing is a promising technique for manufacturing and patterning of micron and sub-micron features. It attracted attention due to its high volume production and large area processing. In this work, we describe a hot-roll-embossing process for manufacturing flexible devices in different commercially available thermoplastic polymer foils using a microstructured silicon wafer as a flat stamping tool. Larger features of a 100 lm width were defined by photolithography and dry etching. A process combining deep reactive ion etching and reactive ion etching of silicon in fluorinated plasma was developed to achieve patterns in silicon with slightly positively tapered sidewalls to allow for easy demoulding. 100 lm features were successfully replicated in cyclic-olefin-copolymer (COC) and poly-methyl-methacrylate (PMMA), using relatively low temperature and multiple passes. Smaller features with 1 lm width were patterned by electron beam lithography and transferred by DRIE in silicon stamps, then replicated in COC foils. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years the necessity of having disposable devices has lead to the usage of polymers in fabrication as they are low cost and offer capability for high volume production. In particular, foil-based solutions are emerging, particularly in applications requiring higher mechanical flexibility, lighter weight, faster heat transfer efficiency and lower material consumption [1,2]. Several techniques like micro-thermoforming [3], hot or UV embossing [4] or lamination techniques [5,6] can be used for producing microdevices on polymer thin films. Roll embossing, where rollers are used instead of the flat plates of planar embossing, is emerging as a viable fabrication technology which provides advantages such as reduced cycle time (no cooling of rollers) and large processing area (no fixed format and advancement of film between the rollers). It also allows continuous or semi-continuous manufacturing [7–9], hence producing a very large number of devices with the same conditions in one run. One of the challenges in large area roll embossing is how to fabricate an embossing master with low cost and high resolution. The solution chosen most of the times is to use a thin and flexible patterned stamp (generally electroplated nickel [10], but also thinned silicon [11]) which is wrapped around the roll cylinder. In this ⇑ Corresponding author. Tel.: +33 381853978; fax: +33 381853998. E-mail address: [email protected] (K. Metwally). Erasmus Mundus student – European Union Master for Mechatronics and MicroMechatronics (EU4M). 1
0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.02.009
work we used a silicon wafer structured using clean-room technologies as a stamping tool and a configuration similar to a ‘‘roll on flat’’ configuration. Performing hot embossing in roller-based or planar configuration results in different ways of applying temperature and pressure. In a configuration involving roller(s), heating occurs only during a short time, which depends on the roller speed whereas a planar configuration involves a long thermal cycle resulting from heating, holding temperature and cooling of the tool and supporting platens. The applied pressure in the contact area of the polymer with the roller is parabolic as a function of time. In contrast, in planar embossing, a constant pressure is uniformly applied on the whole polymer surface which is simultaneously patterned [12]. Influence of parameters such as roller temperature, applied pressure, roller speed and preheating of polymer substrate prior to embossing was reviewed by various groups [7–9]. We introduce here as well the number of passes through the roller as additional embossing parameter. It offers several advantages as it allows embossing at lower temperatures and decreases the internal residual stress in the embossed polymer film. Multiple passes can be applied in multi-roller embossing system that will allow full control of thermal cycle without affecting the production rate. In this work, we report on our experiments of hot roll embossing using dry-etched silicon stamp, starting by the manufacture of Si-moulds, followed by replication in COC and PMMA thermoplastic foils. Discussion of results will be performed in comparison with literature.
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K. Metwally et al. / Microelectronic Engineering 88 (2011) 2679–2682
2. Experimental 2.1. Si-mould fabrication The mould was fabricated on a single-side polished 1 mm thick 300 silicon wafer by means of photolithography and dry etching techniques. Firstly, the wafer was cleaned in piranha for 3 min followed by de-ionized water rinse for 3 min and dehydration on a hotplate at 100 °C for 10 min. Positive photoresist SPR220-3.0 was spin coated to define the 2.4 lm thick layer, then soft baked on hotplate from 25 to 115 °C in 15 min on hotplate from 25 to 115 °C in 15 min, hold at 115°C for 5 min, then cooled down to room temperature. The resist was exposed using a EVG620 double side alignment exposure with a power 300 mJ/cm2. The post-exposure bake on hotplate was done for 5 min at 115 °C. The unexposed SPR220-3.0 was developed away with MF-26A. After the photolithographic step, a sequential combination of deep reactive ion etching using a modified Bosch process and reactive ion etching in sulphur hexafluoride (SF6) plasma was done to achieve slightly positively tapered sidewalls to ease the demoulding. Deep anisotropic dry etching in silicon was performed in an Adixen (Alcatel Vacuum Technology) A 601 E etcher based on high-density inductively coupled plasma. The modified Bosch process derived out of the standard commercially available Bosch process consisted in the alternating steps of passivation and etching using octo-fluoro-butane (C4F8) gas and SF6 gas, respectively. An under-passivation cycle was done by reducing the passivation flow gas and increasing the flow of etching gas. It consisted of a 2-s step with 40 sccm of C4F8 followed by a 7-s step with 500 sccm of SF6. During the process, the pressure was regulated by a threshold valve. In particular the pressure during the SF6 cycle was maintained between 3.2 102 and 3.6 102 mbar to prevent the micromasking effect due to the presence of small particles which block the silicon etching and create the ‘‘grass.’’ Another important parameter to control during the etching process is the temperature of the substrate, which was maintained at 20 °C. After removing the photoresist mask a RIE etching process was performed in a Fluorine Plassys RIE etcher under SF6 plasma for 12 min using a power of 75 W and a gas flow of 20 sccm. Finally, an anti-sticking layer to further facilitate mould release was deposited on the patterned wafers under a flow of 500 sccm of C4F8 passivation gas for 5 min. Smaller features from 1 lm to sub-micronic pattern size were produced by electron beam lithography using a Raith E_line pattern generator operated at 10 keV in 460 nm thick ZEP520A electron beam sensitive resist from ZEON Corp. This resist was chosen because it has a good etch resistance to fluorinated plasma and can be used as a sole etching mask for deep reactive ion etching for high aspect ratio sub-micronic features with an aspect ratio up to 30. Pattern transfer for 1 lm wide ridges was performed using a slightly modified Bosch process with low bias power (40 W) and shorter cycle time (3 s on SF6-250 sccm steps and 1 s on C4F8-265 sccm step) to reduce the scalloping of the sidewalls.
2.2. Roll embossing process Roll embossing was conducted using a Rohm & Haas 350HR laminator. It consists of two rollers with metallic cylinders covered with a rubbery material. The rotating motor is attached to the lower roller, the speed of rotation being an adjustable parameter. The upper roller has a degree of freedom in vertical translation allowing two positions, separation or contact (with the lower roller). The contact pressure is adjustable via a relief valve pressurized by air supply. The upper roller is rotated by transmission during contact with the lower roller. A temperature range from room temperature to 130 °C is
allowed for both rollers and the temperature of each roll can be regulated independently. The pressure can be adjusted from 0 to 8 bars and the feed rate can be increased continuously up to 3 m/min. Thin films of two different thermoplastic materials were fed as flexible polymer substrates, cyclic-olefin-copolymer (COC) (Topas 8007 from Ticona GmbH) and poly-methyl-methacrylate (PMMA) (Goodfellow) of respective nominal thickness of 130 and 125 lm. Their glass transition temperature (Tg) is equal to 78 and 105 °C, respectively. A structured silicon wafer used as a stamping tool was placed with its features upwards on a supporting metallic plate. The polymer/silicon stamp assembly was then forced to pass between both embossing rollers under given embossing pressure, temperatures, and rolling speed. Several experiments were performed to optimize the filling of microcavities of the stamp. Pattern transfer in polymer foils was investigated as a function of roller temperature, applied pressure, feeding rate, and number of passes. Both mould and replicas were characterized using optical microscopy (leica), profilometry (Tencor Alpha-Step 200) and scanning electron microscopy (SEM) (Leica Cambridge Stereoscan S-440). 3. Results and discussion 3.1. Si mould Usually, a demoulding force is required in thermal embossing in order to release the mould from replicas. Different techniques have been used to decrease this force, hence ease the demoulding of the embossed part from the mould. They include fabrication of tapered side walls by control of etching conditions, for example involving a wet anisotropic etching step in the fabrication of the mould in silicon crystal [13,14]. In particular, etching in potassium hydroxide solution results in a high surface smoothness at the atomic scale; however it produces structures with a fixed angle (54.7° along the (1 1 1) surface for (1 0 0) oriented silicon wafers). Additionally, an anti-sticking layer can be deposited to ease demoulding [15]. For instance, a release layer such as Teflon-like film may decrease the ripple effect of DRIE scalloping; however, the anti-sticking layer needs to be renewed after a number of replications. A microfluidic design was etched in the 1 mm thick silicon wafer with microfeatures of 100 lm nominal width and 30 lm depth. The resultant ridge (inverse relief pattern of channel to be obtained on polymer replica) got an upper width of approx. 90 lm and lower width of 100 lm. The modified DRIE process used in this work enabled a sloped sidewall profile favourable for demoulding as shown in Fig. 1. The inward taper angle was 78° with respect to the silicon wafer plane. The RIE step was used to suppress the Si undercut created by etching with SF6 gas and smoothen the roughness of the sidewalls caused by the lower passivation time used during the DRIE step. In addition, a 900 nm thick release layer was deposited in C4F8 plasma at the end of the DRIE etching process. The modified Bosch process was then extended to achieve submicronic features. However, in this case the patterning on top was performed by e-beam lithography. Arrays of lines were patterned with a 1 lm width and transferred into a depth of 5 lm. This process led to vertical walls with very light scalloping of the sidewalls as shown in Fig. 2 inset. Silicon ridges with features size of 300 nm width and 6 lm depth were also fabricated (Fig. 3). The darker part on top of the Si columns is the resin which remains after etching. 3.2. Replication in thermoplastic foils Microchannels of 100 lm width and 30 lm depth were successfully replicated by roll embossing in Topas 8007 COC, and PMMA
K. Metwally et al. / Microelectronic Engineering 88 (2011) 2679–2682
Fig. 1. SEM picture of dry-etched silicon stamp cross-section.
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Fig. 4. SEM picture of dry-etched silicon stamp with 100 lm width and 30 lm depth. Insets: replication in PMMA and COC foil.
Fig. 2. SEM picture of silicon stamp with ridges of 1 lm width, 5 lm height and 2 lm pitch produced by DRIE using a modified Bosch process. Inset: Magnified view of profile (6.5K) of the 1 lm wide ridges.
Fig. 5. SEM picture of channels with 1 lm width and 2 lm pitch roll embossed on COC foil. Transferred depth is in the order of 1 lm.
Fig. 3. SEM picture of sub-micronic features with 300 nm width and 6 lm depth (cleaved wafer with 80° tilt angle).
(Fig. 4) using a dry-etched flat Si-mould. The COC replica was produced in a single pass at a temperature of 70–110 °C for upper and lower rolls, respectively, using a pressure of 5.5 bar and a feed rate of 0.5 m/min. The PMMA replica was produced in four passes at a temperature of 130 °C for both upper and lower rollers while applying a pressure of 5.5 bar and a feed rate 0.1 m/min. First set of trials for roll embossing with smaller features was conducted in Topas 8007 COC. Embossed fine features of 1 lm width and 5 lm depth are shown in Fig. 5; in 10 passes at a temperature of 70–110 °C for upper and lower rolls, using a pressure of 6 bar and a feed rate of 0.1 m/min. However, the transferred depth was smaller than the height of the feature on the silicon mould, about 1 vs. 5 lm. We did not conduct experiments in PMMA for fine features however it is predicted to require higher temperature and higher pressure to be replicated. It is well known that the filling of mould microcavities can be improved by: increasing the embossing temperature which decreases the polymer viscosity, applying a higher pressure, reducing the feed rate, and increasing the number of passes. Experiments are in progress.
2682
K. Metwally et al. / Microelectronic Engineering 88 (2011) 2679–2682
4. Conclusion and outlook
References
A specific process using a combination of DRIE and RIE was developed to produce microstructured Si mould inserts for roll embossing thin flexible polymer parts. The process was extended to produce sub-micronic features. Replication was successfully demonstrated at micronic scale with foils of thermoplastic polymers using a laminator. Features of 100 lm width were transferred in 130 lm thick COC 8007 and 125 lm thick PMMA films, and microfluidic devices were produced using this process. 1 lm wide feature size was also replicated. A novelty of this work was to use relatively low temperature and multiple passes. Pattern transfer with sub-micronic features is being investigated.
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Acknowledgements This work was carried out within the framework of the CarnotFraunhofer PICF programme (3lP project: Multi-Reaction, MultiSample Micro-Fluidic Platform). MIMENTO technology platform is part of the RENATECH network. K.M. would like to give special thanks the Erasmus Mundus EU4M-Consormium for offering the Master scholarship, and giving him the opportunity and support for doing this project.