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Fabrication of optical grayscale masks for tapered microfluidic devices
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Microelectronic Engineering 85 (2008) 1077–1082 www.elsevier.com/locate/mee
Fabrication of optical grayscale masks for tapered microfluidic devices Volker Nock *, Richard J. Blaikie MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Electrical and Computer Engineering, University of Canterbury, Private Bag 4800, Christchurch, New Zealand Received 21 September 2007; received in revised form 17 January 2008; accepted 18 January 2008 Available online 8 February 2008
Abstract In this paper we report a process using electron beam lithography (EBL) to provide high-resolution prototyping of sub-wavelength patterned grayscale masks. The fabrication of such masks and the characterisation of grayscale patterning in thick optical resist are described. In addition, the application of these masks for the creation of three-dimensional tapered microstructures for use as masters in soft lithography for microfluidic devices is demonstrated. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Electron beam lithography; Grayscale lithography; Thick photoresist; Tapered microchannels
1. Introduction Tapering in microfluidic flow devices can be used to induce and study various effects like mixing [1] and shear in cell-culture bioreactors [2]. Furthermore, the wall shear-stress characteristics of vertically tapered microchannels allow to control species transport to cells cultured in such a channel [3]. Optical lithography using grayscale masks is the main high-throughput process that can be used to create complex tapered microstructures for optical and fluidic applications [4–8]. However, pattern detail on the mask has to be below the resolution limit of the exposure tool used for the optical lithography step. This makes grayscale mask fabrication the most critical and expensive step in the process. Since the tapered features often make up only a small area of the device, it can be uneconomical to fabricate a full grayscale mask. Hence we have developed a novel hybrid mask making process based on the combined use of fast, lower resolution optical lithography and slower, high-resolution electron beam lithography (EBL) during mask fabri*
Corresponding author. Tel.: +64 33642987x7123; fax: +64 33642761. E-mail addresses: [email protected] (V. Nock), [email protected] (R.J. Blaikie). 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.01.088
cation. By integrating locally confined grayscale features with larger binary channels on a single glass mask for optical lithography, a flexible tool with faster turnaround for prototyping of low-cost microfluidic devices is created. In the following we describe the hybrid mask fabrication process, calibration of optical grayscale lithography in standard photoresist and mask application to create microchannel molds with integrated tapered features. 2. Mask fabrication Grayscale masks were fabricated by EBL and reactive ion etching (RIE) in SF6 plasma. Fig. 1 shows a schematic of the two step main process. Standard microscope slides and a 150 lm thick sputter-deposited absorber layer of tungsten are used as the mask substrate (a). An electron beam resist poly(methyl-methacrylate) (PMMA) is spunon at 4000 rpm for 1 min and soft baked for 90 s at 180 °C (b). Desired grayscale patterns are written into the PMMA using a Raith 150 electron beam lithography system with an acceleration voltage of 10 kV and an areal dose of 60 lC/cm2 (c). After development in 3:1 IPA:MIBK and a 70 s hardbake at 100 °C the pattern is transferred into the tungsten layer via reactive ion etching in an Oxford Plasmalab 80Plus RIE system (d). Typical etching
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fabricated. Fig. 3 shows an optical micrograph of this mask after PMMA development. The overlay indicates the fundamental building blocks of the grayscale mask and their combination to achieve different transmission levels. In the example shown, a side length of 1 lm for the UC and 0.1 lm for a single PP were defined, allowing for the creation of 100 individual grayscale levels or height steps in photoresist exposed through this mask. 3. Calibration of grayscale lithography
Fig. 1. Schematic of the grayscale mask fabrication process.
conditions used were 80 sccm SF6 flow rate, 0.15 Torr pressure and 200 W RF power for 10 s. In the second step larger channel patterns were integrated with the grayscale pattern on the mask by standard optical lithography in AZ1518 photoresist. For pattern transfer into the tungsten layer the same RIE process as before was used. High aspect-ratio optical grayscale lithography (e and f) in AZ4562 positive and SU-8 negative binary photoresist (Microchemicals Inc.) was investigated using the fabricated masks. For use with negative resist exposure has to be performed in substrate penetration [9], as shown on the right in Fig. 1(e and f). The grayscale formation with a binary optical lithography mask is by transmission through arrays of opaque pixels close to or below the resolution of the available lithography system (Pcritical). Patterns on the mask are constructed of unit cells (UC), within which primary pixels (PP, opaque or clear) are written with pitch P < Pcritical; the ratio of opaque to clear PPs determines the net transmission through each unit cell, which then creates a modulated exposure dose across the surface of the photoresist. Fig. 2 shows a SEM micrograph of a mask with a test pattern after RIE. The pattern features both different unit cell pitches and filling factors used to determine mask aligner resolution (Karl Su¨ss MA6). The insets show close-ups of the unit cell pitch. Unit cell filling is obtained by subdivision into primary pixels of 100 nm-by-100 nm during EBL. To calibrate grayscale lithography in standard photoresist a mask with different optical transmission levels was
Optical masks fabricated with the process described above were used to calibrate grayscale lithography in positive tone photoresist. AZ4562 resist was chosen because it can be spin-coated up to a thickness of 60 lm in one single application. This makes it ideal as direct master for replicamolding of microfluidic devices in elastomers such as polydimethylsiloxane (PDMS). Fig. 4 shows the individual components of the calibration process. An optical micrograph of the grayscale mask used for the exposure is shown in Fig. 4a after the final RIE step. This mask was used to expose 60 lm thick AZ4562 on a 22 22 mm2 Si substrate in a Karl Su¨ss MA6 mask aligner. Fig. 4b shows a SEM micrograph of the resist structure after exposure and development in a dilute KOH solution. The top two rows of pillars correspond to relative transmission values ranging from 100% to 10% in 10% steps. The same transmission ranges are realized in the two ramp structures with step widths of 50 lm (top) and 25 lm (bottom), respectively. To obtain the resist thickness as a function of mask transmission the rows of pillars were scanned using a DEKTAK profilometer. A plot of the surface scans and the resulting calibration graph is shown in Fig. 4c. The dashed line corresponds to a linear fit of the thickness data. Surface roughness on this sample was found to be relatively high, which however, could be significantly reduced by optimisation of the development process and a final hardbake with controlled reflow. 4. Mask application Use of the grayscale masks was investigated with both AZ4562 positive resist and SU-8 negative resist as potential mold materials. Fig. 5 shows four examples of tapered structures fabricated in 200 lm thick SU-8 negative resist (a and b) and AZ4562 positive resist of 60 lm and 20 lm thickness in (c) and (d), respectively. SU-8 was applied to 120–150 lm thick glass coverslips (ESCO) and exposed by placing the grayscale mask in contact with the substrate backside. As shown in Fig. 5a, diffraction effects at the mask/substrate stack lead to partial cross-linking of SU-8 for a certain exposure dose range. These areas could not be removed during development. Furthermore, the maskresist separation induced by exposure through the substrate leads to a tapering of structure sidewalls (Fig. 5a and b),
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Fig. 2. SEM micrograph of a grayscale mask after RIE. Different filling factors and unit cell pitches are realized to test mask aligner resolution.
Fig. 3. Optical micrograph of the mask used for calibration of grayscale lithography in standard photoresist. The overlay indicates the principle of mask layout.
making SU-8 not ideal for channel fabrication in combination with grayscale features. For structures in AZ4562 a consistent replication of the grayscale patterns and individual height levels, as realized on the mask, was found. An additional controlled hardbake step above the resist reflow temperature was used to remove the initial surface roughness typical for diffraction-based grayscale lithography. Fig. 6 shows a SEM micrograph of a vertically-tapered microchannel in AZ4562 resist (a) and the corresponding surface scans (b). To fabricate this structure the binary pattern of the channel was combined with the grayscale part
on the optical mask. The mask was then used to fabricate the structure by single exposure of the resist in the MA6 optical mask aligner. The vertical arrow (across channel) in Fig. 6a corresponds to the scan of the initial channel depth depicted on the left in Fig. 6b. Plotted on the right in Fig. 6b is the scan along the channel length (horizontal arrow in Fig. 6a). The channel tapers vertically in six steps from the initial depth of 22 lm to a minimum of 7 lm over a length of 1 mm. After hardbake the structure can be directly used as microchannel by sealing it with an appropriate lid. Alternatively, the structure can be used as
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Fig. 4. Calibration of grayscale lithography in thick positive photoresist AZ4562: an optical micrograph of the calibration mask (a), the corresponding resist structure (b) and a plot of resist thickness vs. relative mask transmission (c) are shown.
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Fig. 5. SEM micrographs of photoresist structures fabricated using grayscale mask. Both SU-8 negative (a and b) and AZ4562 positive resist (c and d) were investigated.
Fig. 6. Application of grayscale masks to fabricate a vertically-tapered microchannel. A SEM micrograph of the channel in AZ4562 photoresist (a) indicates the direction of the surface scans (b), obtained by profilometry.
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inverse molds for replication in PDMS or a similar elastomer. 5. Conclusion By combining electron beam lithography and standard optical lithography during mask fabrication, we have shown the application of optical grayscale masks to the replication of vertically-tapered microchannels. A novel process for the fabrication of grayscale masks was developed, combining the advantages of optical lithography and EBL during mask writing. Using such masks, grayscale lithography was investigated in thick SU-8 negative and AZ4562 positive resist. AZ4562 was identified as more suited for mold fabrication and calibrated for grayscale lithography. Applicability of the mask fabrication process was demonstrated by fabrication of a vertically-tapering microchannel in thick photoresist. The process provides simple integration of tapered features into low-cost microfluidic devices, and thereby the potential to study threedimensional flow and species transport phenomena.
Acknowledgements The authors would like to thank David Melville for fruitful discussions and Helen Devereux and Gary Turner for technical assistance. References [1] N.-T. Nguyen, Z. Wu, J. Micromech. Microeng. 15 (2005) R1–R16. [2] H. Lu, L.Y. Koo, W.M. Wang, D.A. Lauffenburger, L.G. Griffith, K.F. Jensen, Anal. Chem. 76 (2004) 5257–5264. [3] V. Nock, R.J. Blaikie, T. David, New Zealand Med. J. 120 (1252) (2007) 2–3. [4] C.M. Waits, A. Modafe, R. Ghodssi, J. Micromech. Microeng. 13 (2003) 170–177. [5] J.C. Galas, B. Belier, A. Aassime, J. Palomo, D. Bouville, J. Aubert, J. Vac. Sci. Technol. B 22 (3) (2004) 1160–1162. [6] K.-Y. Hung, F.-G. Tseng, H.-P. Chou, Microsyst. Technol. 11 (4) (2005) 365–369. [7] M.A. Afromowitz, US Pat. 7045089 (2006). [8] R. Mori, K. Hanai, Y. Matsumoto, Proc. uTAS (2004) 333–335. [9] M.C. Peterman, P. Huie, D.M. Bloom, H.A. Fishman, J. Micromech. Microeng. 13 (2003) 380–382.
Microelectronic Engineering 85 (2008) 1077–1082 www.elsevier.com/locate/mee
Fabrication of optical grayscale masks for tapered microfluidic devices Volker Nock *, Richard J. Blaikie MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Electrical and Computer Engineering, University of Canterbury, Private Bag 4800, Christchurch, New Zealand Received 21 September 2007; received in revised form 17 January 2008; accepted 18 January 2008 Available online 8 February 2008
Abstract In this paper we report a process using electron beam lithography (EBL) to provide high-resolution prototyping of sub-wavelength patterned grayscale masks. The fabrication of such masks and the characterisation of grayscale patterning in thick optical resist are described. In addition, the application of these masks for the creation of three-dimensional tapered microstructures for use as masters in soft lithography for microfluidic devices is demonstrated. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Electron beam lithography; Grayscale lithography; Thick photoresist; Tapered microchannels
1. Introduction Tapering in microfluidic flow devices can be used to induce and study various effects like mixing [1] and shear in cell-culture bioreactors [2]. Furthermore, the wall shear-stress characteristics of vertically tapered microchannels allow to control species transport to cells cultured in such a channel [3]. Optical lithography using grayscale masks is the main high-throughput process that can be used to create complex tapered microstructures for optical and fluidic applications [4–8]. However, pattern detail on the mask has to be below the resolution limit of the exposure tool used for the optical lithography step. This makes grayscale mask fabrication the most critical and expensive step in the process. Since the tapered features often make up only a small area of the device, it can be uneconomical to fabricate a full grayscale mask. Hence we have developed a novel hybrid mask making process based on the combined use of fast, lower resolution optical lithography and slower, high-resolution electron beam lithography (EBL) during mask fabri*
Corresponding author. Tel.: +64 33642987x7123; fax: +64 33642761. E-mail addresses: [email protected] (V. Nock), [email protected] (R.J. Blaikie). 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.01.088
cation. By integrating locally confined grayscale features with larger binary channels on a single glass mask for optical lithography, a flexible tool with faster turnaround for prototyping of low-cost microfluidic devices is created. In the following we describe the hybrid mask fabrication process, calibration of optical grayscale lithography in standard photoresist and mask application to create microchannel molds with integrated tapered features. 2. Mask fabrication Grayscale masks were fabricated by EBL and reactive ion etching (RIE) in SF6 plasma. Fig. 1 shows a schematic of the two step main process. Standard microscope slides and a 150 lm thick sputter-deposited absorber layer of tungsten are used as the mask substrate (a). An electron beam resist poly(methyl-methacrylate) (PMMA) is spunon at 4000 rpm for 1 min and soft baked for 90 s at 180 °C (b). Desired grayscale patterns are written into the PMMA using a Raith 150 electron beam lithography system with an acceleration voltage of 10 kV and an areal dose of 60 lC/cm2 (c). After development in 3:1 IPA:MIBK and a 70 s hardbake at 100 °C the pattern is transferred into the tungsten layer via reactive ion etching in an Oxford Plasmalab 80Plus RIE system (d). Typical etching
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fabricated. Fig. 3 shows an optical micrograph of this mask after PMMA development. The overlay indicates the fundamental building blocks of the grayscale mask and their combination to achieve different transmission levels. In the example shown, a side length of 1 lm for the UC and 0.1 lm for a single PP were defined, allowing for the creation of 100 individual grayscale levels or height steps in photoresist exposed through this mask. 3. Calibration of grayscale lithography
Fig. 1. Schematic of the grayscale mask fabrication process.
conditions used were 80 sccm SF6 flow rate, 0.15 Torr pressure and 200 W RF power for 10 s. In the second step larger channel patterns were integrated with the grayscale pattern on the mask by standard optical lithography in AZ1518 photoresist. For pattern transfer into the tungsten layer the same RIE process as before was used. High aspect-ratio optical grayscale lithography (e and f) in AZ4562 positive and SU-8 negative binary photoresist (Microchemicals Inc.) was investigated using the fabricated masks. For use with negative resist exposure has to be performed in substrate penetration [9], as shown on the right in Fig. 1(e and f). The grayscale formation with a binary optical lithography mask is by transmission through arrays of opaque pixels close to or below the resolution of the available lithography system (Pcritical). Patterns on the mask are constructed of unit cells (UC), within which primary pixels (PP, opaque or clear) are written with pitch P < Pcritical; the ratio of opaque to clear PPs determines the net transmission through each unit cell, which then creates a modulated exposure dose across the surface of the photoresist. Fig. 2 shows a SEM micrograph of a mask with a test pattern after RIE. The pattern features both different unit cell pitches and filling factors used to determine mask aligner resolution (Karl Su¨ss MA6). The insets show close-ups of the unit cell pitch. Unit cell filling is obtained by subdivision into primary pixels of 100 nm-by-100 nm during EBL. To calibrate grayscale lithography in standard photoresist a mask with different optical transmission levels was
Optical masks fabricated with the process described above were used to calibrate grayscale lithography in positive tone photoresist. AZ4562 resist was chosen because it can be spin-coated up to a thickness of 60 lm in one single application. This makes it ideal as direct master for replicamolding of microfluidic devices in elastomers such as polydimethylsiloxane (PDMS). Fig. 4 shows the individual components of the calibration process. An optical micrograph of the grayscale mask used for the exposure is shown in Fig. 4a after the final RIE step. This mask was used to expose 60 lm thick AZ4562 on a 22 22 mm2 Si substrate in a Karl Su¨ss MA6 mask aligner. Fig. 4b shows a SEM micrograph of the resist structure after exposure and development in a dilute KOH solution. The top two rows of pillars correspond to relative transmission values ranging from 100% to 10% in 10% steps. The same transmission ranges are realized in the two ramp structures with step widths of 50 lm (top) and 25 lm (bottom), respectively. To obtain the resist thickness as a function of mask transmission the rows of pillars were scanned using a DEKTAK profilometer. A plot of the surface scans and the resulting calibration graph is shown in Fig. 4c. The dashed line corresponds to a linear fit of the thickness data. Surface roughness on this sample was found to be relatively high, which however, could be significantly reduced by optimisation of the development process and a final hardbake with controlled reflow. 4. Mask application Use of the grayscale masks was investigated with both AZ4562 positive resist and SU-8 negative resist as potential mold materials. Fig. 5 shows four examples of tapered structures fabricated in 200 lm thick SU-8 negative resist (a and b) and AZ4562 positive resist of 60 lm and 20 lm thickness in (c) and (d), respectively. SU-8 was applied to 120–150 lm thick glass coverslips (ESCO) and exposed by placing the grayscale mask in contact with the substrate backside. As shown in Fig. 5a, diffraction effects at the mask/substrate stack lead to partial cross-linking of SU-8 for a certain exposure dose range. These areas could not be removed during development. Furthermore, the maskresist separation induced by exposure through the substrate leads to a tapering of structure sidewalls (Fig. 5a and b),
V. Nock, R.J. Blaikie / Microelectronic Engineering 85 (2008) 1077–1082
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Fig. 2. SEM micrograph of a grayscale mask after RIE. Different filling factors and unit cell pitches are realized to test mask aligner resolution.
Fig. 3. Optical micrograph of the mask used for calibration of grayscale lithography in standard photoresist. The overlay indicates the principle of mask layout.
making SU-8 not ideal for channel fabrication in combination with grayscale features. For structures in AZ4562 a consistent replication of the grayscale patterns and individual height levels, as realized on the mask, was found. An additional controlled hardbake step above the resist reflow temperature was used to remove the initial surface roughness typical for diffraction-based grayscale lithography. Fig. 6 shows a SEM micrograph of a vertically-tapered microchannel in AZ4562 resist (a) and the corresponding surface scans (b). To fabricate this structure the binary pattern of the channel was combined with the grayscale part
on the optical mask. The mask was then used to fabricate the structure by single exposure of the resist in the MA6 optical mask aligner. The vertical arrow (across channel) in Fig. 6a corresponds to the scan of the initial channel depth depicted on the left in Fig. 6b. Plotted on the right in Fig. 6b is the scan along the channel length (horizontal arrow in Fig. 6a). The channel tapers vertically in six steps from the initial depth of 22 lm to a minimum of 7 lm over a length of 1 mm. After hardbake the structure can be directly used as microchannel by sealing it with an appropriate lid. Alternatively, the structure can be used as
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Fig. 4. Calibration of grayscale lithography in thick positive photoresist AZ4562: an optical micrograph of the calibration mask (a), the corresponding resist structure (b) and a plot of resist thickness vs. relative mask transmission (c) are shown.
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Fig. 5. SEM micrographs of photoresist structures fabricated using grayscale mask. Both SU-8 negative (a and b) and AZ4562 positive resist (c and d) were investigated.
Fig. 6. Application of grayscale masks to fabricate a vertically-tapered microchannel. A SEM micrograph of the channel in AZ4562 photoresist (a) indicates the direction of the surface scans (b), obtained by profilometry.
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inverse molds for replication in PDMS or a similar elastomer. 5. Conclusion By combining electron beam lithography and standard optical lithography during mask fabrication, we have shown the application of optical grayscale masks to the replication of vertically-tapered microchannels. A novel process for the fabrication of grayscale masks was developed, combining the advantages of optical lithography and EBL during mask writing. Using such masks, grayscale lithography was investigated in thick SU-8 negative and AZ4562 positive resist. AZ4562 was identified as more suited for mold fabrication and calibrated for grayscale lithography. Applicability of the mask fabrication process was demonstrated by fabrication of a vertically-tapering microchannel in thick photoresist. The process provides simple integration of tapered features into low-cost microfluidic devices, and thereby the potential to study threedimensional flow and species transport phenomena.
Acknowledgements The authors would like to thank David Melville for fruitful discussions and Helen Devereux and Gary Turner for technical assistance. References [1] N.-T. Nguyen, Z. Wu, J. Micromech. Microeng. 15 (2005) R1–R16. [2] H. Lu, L.Y. Koo, W.M. Wang, D.A. Lauffenburger, L.G. Griffith, K.F. Jensen, Anal. Chem. 76 (2004) 5257–5264. [3] V. Nock, R.J. Blaikie, T. David, New Zealand Med. J. 120 (1252) (2007) 2–3. [4] C.M. Waits, A. Modafe, R. Ghodssi, J. Micromech. Microeng. 13 (2003) 170–177. [5] J.C. Galas, B. Belier, A. Aassime, J. Palomo, D. Bouville, J. Aubert, J. Vac. Sci. Technol. B 22 (3) (2004) 1160–1162. [6] K.-Y. Hung, F.-G. Tseng, H.-P. Chou, Microsyst. Technol. 11 (4) (2005) 365–369. [7] M.A. Afromowitz, US Pat. 7045089 (2006). [8] R. Mori, K. Hanai, Y. Matsumoto, Proc. uTAS (2004) 333–335. [9] M.C. Peterman, P. Huie, D.M. Bloom, H.A. Fishman, J. Micromech. Microeng. 13 (2003) 380–382.