- Email: [email protected]
A nanoimprint lithography for fabricating SU-8 gratings for near-infrared to deep-UV application
Available online at www.sciencedirect.com
Microelectronic Engineering 85 (2008) 914–917 www.elsevier.com/locate/mee
A nanoimprint lithography for fabricating SU-8 gratings for near-infrared to deep-UV application Shen-Qi Xie a, Jing Wan a, Bing-Rui Lu a, Yan Sun b, Yifang Chen c, Xin-Ping Qu a, Ran Liu a,* a
State key lab of Asic and system, Department of Microelectronics, Fudan University, Shanghai 200433, China b Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China c Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, UK Received 5 October 2007; received in revised form 17 December 2007; accepted 18 January 2008 Available online 2 February 2008
Abstract We demonstrate the nanofabrication of the transmission SU-8 gratings with periods from 200 nm (5000 lines/mm) to 1 lm (1000 lines/mm) with different trench depths for applications from near-infrared to deep-UV wavelength. The imprint property of SU-8 under various pressures and temperatures was systematically studied and agreed well with the simulation results. The effects of trench depth on diffraction intensity profiles were simulated and results show periodic diffraction orders with varying intensity distributions, which is in good accordance with the results from optical measurements. The high optical transmittance of SU-8 in the visible range and its low volume shrinkage coefficient make the developed process an ideal candidate for high-volume manufacturing of various gratings at low cost. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Nanoimprint lithography (NIL); SU-8; Phase gratings; Reactive ion etch (RIE); Diffraction
1. Introduction With their unique properties, diffraction and sub-wavelength gratings have taken an indispensable place in optical instrumentation and applications in optics, opto-electronics, communications, nanophotonic and nanobio-science. Modern optical systems often require high reliability, robustness and functional integration of gratings [1]. However, optic components with nanometer features are usually expensive due to the complex in manufacture. Recently, nanoimprint lithography has been applied for the fabrication of metallic and dielectric gratings [2]. Based on our earlier work on imprint into SU-8 resist [3], in this paper we further extend this NIL technique to the fabrication of transmission SU-8 gratings applicable in a broader wavelength range from 1 lm (1000 lines/mm) down to 200 nm (5000 lines/mm) with various trench depths. The *
Corresponding author. Tel./fax: +86 21 55664548. E-mail address: [email protected] (R. Liu).
0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.01.072
imprint property of SU-8 under various pressures and temperatures was systematically studied. Theoretical simulation of imprint mechanism into SU-8 described by a modified equation of squeezed flow [4] was undertaken. Both characterizations and theoretical analysis show the unique characteristics of fabricated SU-8 phase gratings, which offers potential applications in multiple imaging devices or beam splitters [5,6]. It is expected that the developed technique will hopefully be adapted in industry for high volume and high quality manufacturing of various gratings for optical wavelengths from near-infrared to deep-UV range. 2. Experimental details To carry out nanoimprint, high aspect ratio silicon grating templates with different periods were first fabricated [7]. A 2.5 lm thick SU-8 was first spin-coated onto the Pyrex substrates and then soft baked at 95 °C on a hotplate for 10 min to evaporate all the solvents in the SU-8 resist.
S.-Q. Xie et al. / Microelectronic Engineering 85 (2008) 914–917
The SU-8 was then heated at a certain temperature above the glass transmission temperature (Tg) in oven for 10 min. After that, a pressure was applied on the template for another 10 min with the heat on. Before raising the template, a UV exposure by a 365 nm UV light was undertaken for 5 min to cure the SU-8 and turn it into a rubbery state under 200 °C[8]. Templates were finally separated from the substrates to form the fully cross-linked SU-8 gratings. Fig. 1 shows the imprinted gratings with periods of 1 lm, 300 nm and 200 nm. 3. Results and discussions 3.1. Simulation of the imprint property In the SU-8 based NIL, the imprint process can be described by the equation of squeezed flow which bases on the assumption of an incompressible Newtonian fluid of certain viscosity. The quasi stationary solution of this problem under constant external force is described analyt-
915
ically by the so-called Stefan-equation [9] which was already applied to simulate specific characteristics of the imprint process [10,11]. However, these basic simulations are unable to cover a realistic imprint process in which not only one stamp feature of given size s0 is imprinted but rather a number of these features, separated by cavities of width w and of height H for a plane geometry, just like the grating structures. Eq. (1) of squeezed flow modified by Bogdanski et al. [4] to enlarge the simulations to realistic imprint conditions describes the change of polymer thickness h(t) below the elevated stamp patterns. 1 1 2pðtÞt ¼ þ h2 ðt; T Þ h20 gðT ÞS
ð1Þ
where g(T) is the viscosity of SU-8, which is determined by an experimental function regarding imprint temperature T; p is the imprinting pressure; h0 is the initial thickness of the spin-coated SU-8 layer and t is the imprint time. A field of width for N periodic lines within the field is described by S ¼ N ðs0 þ wÞ, where s0 is the width of grating and w is the width of trench. The trench depth can be given as d ¼ ½h0 hðt; T Þðs0 þ wÞ=w
ð2Þ
d will reach the maximum when it is equal to the height of trench depth H. In our imprint experiment, 500 nm period silicon grating templates (1 mm 1 mm) with 1120 nm in height were used. The imprint properties of SU-8 were studied by optimization of the imprint pressure and imprint temperatures to achieve required imprint depth for different usage. Fig. 2 illustrates the trench depth variation with imprint pressures under different temperatures. The experimental results, shown in the monochrome data points are compared to the simulation results drawn in chromatic curves acquired from the Eqs. (1) and (2). As a UV curing process was introduced to further cure the SU-8 after imprint, vertical
Trench depth (nm)
1200 1000 800 600 400 200 20
40
60
E100oC E140oC
E120oC E200oC
S100oC S140oC
S120oC S200oC
80
100
Pressure (bar) Fig. 1. SEM photos showing the imprinted SU-8 gratings. Cross sectional views of (a) 1 lm period SU-8 gratings with 730 nm in height and (b) 300 nm period SU-8 gratings with 420 nm in height. (c) 200 nm period SU8 gratings with 260 nm in height.
Fig. 2. Relationship of the imprint depth against the pressure with the temperature of 100 °C, 120 °C, 140 °C and 200 °C. The monochrome data points are experimental results and the chromatic curves are simulation results.
916
S.-Q. Xie et al. / Microelectronic Engineering 85 (2008) 914–917
profile with least damage from the template releasing process was obtained, which made the measurement results reliable. It is found that at a fixed temperature, the imprint depth increases with the increasing imprint pressure until it reaches the height of the templates, which is around 1120 nm. Due to the variation of viscosity under different temperature, the pressure used to achieve such a depth also gradually shifts from 47 bar at 200 °C to 75 bar at 120 °C. For trench depth lower than the template’s height, in order to achieve the same depth, obviously higher temperature requires lower pressure. As a result, two extremes of the process window were obtained: one corresponds to the lowest pressure applicable for the highest temperature used (200 °C in this work), and the other is the lowest temperature applicable under the highest pressure (85 bar in this work). These two extremes are practically useful in some cases when low pressure is demanded while the temperature is not an important issue and vice versa. From Fig. 2 it can be seen that the experimental results agreed well with the simulation results, indicating reliable description of the experimental data using the modified. It can be conclude that the imprint trench depth can be predicted by using this simulation model. 3.2. Optical properties of the phase grating The fabricated SU-8 gratings were actually one kind of transmission phase grating which can be described by composing of two different transparent materials. The refraction indexes of the two kinds of material are noted by n1 = 1.55 (for SU-8) and n2 = 1 (for air), respectively. If the wavelength of the incident light is k, the transmission function of the phase grating, E(x), can be defined as the addition of two separate parts from n1 and n2. x ð3Þ EðxÞ ¼ ½E1 ðxÞ þ E2 ðxÞrectð Þ L
Fig. 3. The intensity distribution of the diffraction field of the SU-8 phase grating with trench depth from 0 to 2 lm. The grating’s period is 1 lm and the wavelength of incident light is 632.8 nm.
bution is also a function of the height of the grating, while not only determined by the period but also the trench width for a certain wavelength of incident light. The trench depth can be well controlled by the imprint condition. Fig. 4 shows the diffraction patterns of the 500 nm period SU-8 gratings with different trench depths, which demonstrates the effect of trench depth on the periodic variation of the intensity distribution. Apparently, the intensity of the first diffraction orders decreased with the reduction in trench depth, which closely agrees with the simulation results. However, the variations of the zero order cannot be observed, which may be caused by the transparent area outside the imprinted one (1 mm2).
where 1 E1 ðxÞ ¼ rectðx=dÞcombðx=aÞ expðj2pHn1 =kÞ a 1 x a=2 Þcombðx=aÞ expðj2pHn2 =kÞ E2 ðxÞ ¼ rectð a ad
ð4Þ ð5Þ
In which, d denotes the line width of the grating; H is the thickness of the grating. w represents the width of trench depth and a is the period of the grating. The total width of the grating is L. As known that the field distribution of Fraunhofer diffraction of an aperture is the Fourier transform of the amplitude distribution across the aperture. Making Fourier transformation of Eq. (3), we can have the intensity distribution of the Fraunhofer diffraction of the phase grating [12]. Fig. 3 shows the intensity distribution of the diffraction field of the 1 lm period SU-8 grating with different trench depths when using a 632.8 nm incident light. Periodic diffraction orders with varying intensity distribution are observed, indicating that the diffraction intensity distri-
Fig. 4. Diffraction patterns of the 500 nm period SU-8 phase gratings with different trench depth. (a) 540 nm; (b) 320 nm; (c) 220 nm; (d) 140 nm. The incident light is a 632.8 nm laser.
S.-Q. Xie et al. / Microelectronic Engineering 85 (2008) 914–917
917
4. Conclusions
References
A nanofabrication technique of the transmission SU-8 gratings with various periods and different trench depth for application from near-infrared to deep-UV wavelength has been successfully developed. The imprint properties have been studied and the experimental results agree well with the simulation results described by a modified equation of squeezed flow. The SU-8 phase grating was demonstrated both experimentally and theoretically. The developed process is applicable not only for plastic gratings, but also for other nanostructures such as nanofluidics and nanophotonics.
[1] Jian Wang, Hubert Kostal, Laser Focus World 41 (12) (2005) 76. [2] Xudi Wang, Yifang Chen, Shaojun Fu, Zheng Cui, Microelectron. Eng. 84 (2007) 872–876. [3] Bing-Rui Lu, Shen-Qi Xie, Yan Sun, Yifang Chen, Xin-Ping Qu, Ran Liu, Abstract No. 1P-529, in: International Conference on Nanoscience & Technology, Beijing, China, 2007. [4] N. Bogdanski, M. Wissen, A. Ziegler, H.-C. Scheer, Microelectron. Eng. 78–79 (2005) 598–604. [5] W.-H. Lee, Appl. Opt. 18 (13) (1979) 2152. [6] C. David, B. No¨hammer, H.H. Solak, Appl. Phys. Lett. 81 (17) (2002). [7] Shen-Qi Xie, Bing-Rui Lu, Yan Sun, Yifang Chen, Xin-Ping Qu, Ran Liu, Abstract No. 1O-068, in: International Conference on Nanoscience & Technology, Beijing, China, 2007. [8] R. Feng, R.J. Farris, J. Micromech. Microeng. 13 (2003) 80. [9] J. Stefan, Akad. Wiss. Math. Natur. 69 (1874) 713. [10] H. Schift et al., in: C.M. Sotomayor Torres (Ed.), Alternative Lithography, Plenum, 2003. [11] L.G. Baraldi, PhD Thesis, ETH Zu¨ rich, 1994. [12] Hua Gao, Min Ouyang, Yu Wang, Yan Shen, Jing Zhou, Dahe Liu, Optik 118 (2007) 452–456.
Acknowledgements The work was supported by National high technology program (2006AA03Z352) and the ‘‘985” Micro/nanoelectronics science and Technology Innovation Platform.
Microelectronic Engineering 85 (2008) 914–917 www.elsevier.com/locate/mee
A nanoimprint lithography for fabricating SU-8 gratings for near-infrared to deep-UV application Shen-Qi Xie a, Jing Wan a, Bing-Rui Lu a, Yan Sun b, Yifang Chen c, Xin-Ping Qu a, Ran Liu a,* a
State key lab of Asic and system, Department of Microelectronics, Fudan University, Shanghai 200433, China b Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China c Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, UK Received 5 October 2007; received in revised form 17 December 2007; accepted 18 January 2008 Available online 2 February 2008
Abstract We demonstrate the nanofabrication of the transmission SU-8 gratings with periods from 200 nm (5000 lines/mm) to 1 lm (1000 lines/mm) with different trench depths for applications from near-infrared to deep-UV wavelength. The imprint property of SU-8 under various pressures and temperatures was systematically studied and agreed well with the simulation results. The effects of trench depth on diffraction intensity profiles were simulated and results show periodic diffraction orders with varying intensity distributions, which is in good accordance with the results from optical measurements. The high optical transmittance of SU-8 in the visible range and its low volume shrinkage coefficient make the developed process an ideal candidate for high-volume manufacturing of various gratings at low cost. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Nanoimprint lithography (NIL); SU-8; Phase gratings; Reactive ion etch (RIE); Diffraction
1. Introduction With their unique properties, diffraction and sub-wavelength gratings have taken an indispensable place in optical instrumentation and applications in optics, opto-electronics, communications, nanophotonic and nanobio-science. Modern optical systems often require high reliability, robustness and functional integration of gratings [1]. However, optic components with nanometer features are usually expensive due to the complex in manufacture. Recently, nanoimprint lithography has been applied for the fabrication of metallic and dielectric gratings [2]. Based on our earlier work on imprint into SU-8 resist [3], in this paper we further extend this NIL technique to the fabrication of transmission SU-8 gratings applicable in a broader wavelength range from 1 lm (1000 lines/mm) down to 200 nm (5000 lines/mm) with various trench depths. The *
Corresponding author. Tel./fax: +86 21 55664548. E-mail address: [email protected] (R. Liu).
0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.01.072
imprint property of SU-8 under various pressures and temperatures was systematically studied. Theoretical simulation of imprint mechanism into SU-8 described by a modified equation of squeezed flow [4] was undertaken. Both characterizations and theoretical analysis show the unique characteristics of fabricated SU-8 phase gratings, which offers potential applications in multiple imaging devices or beam splitters [5,6]. It is expected that the developed technique will hopefully be adapted in industry for high volume and high quality manufacturing of various gratings for optical wavelengths from near-infrared to deep-UV range. 2. Experimental details To carry out nanoimprint, high aspect ratio silicon grating templates with different periods were first fabricated [7]. A 2.5 lm thick SU-8 was first spin-coated onto the Pyrex substrates and then soft baked at 95 °C on a hotplate for 10 min to evaporate all the solvents in the SU-8 resist.
S.-Q. Xie et al. / Microelectronic Engineering 85 (2008) 914–917
The SU-8 was then heated at a certain temperature above the glass transmission temperature (Tg) in oven for 10 min. After that, a pressure was applied on the template for another 10 min with the heat on. Before raising the template, a UV exposure by a 365 nm UV light was undertaken for 5 min to cure the SU-8 and turn it into a rubbery state under 200 °C[8]. Templates were finally separated from the substrates to form the fully cross-linked SU-8 gratings. Fig. 1 shows the imprinted gratings with periods of 1 lm, 300 nm and 200 nm. 3. Results and discussions 3.1. Simulation of the imprint property In the SU-8 based NIL, the imprint process can be described by the equation of squeezed flow which bases on the assumption of an incompressible Newtonian fluid of certain viscosity. The quasi stationary solution of this problem under constant external force is described analyt-
915
ically by the so-called Stefan-equation [9] which was already applied to simulate specific characteristics of the imprint process [10,11]. However, these basic simulations are unable to cover a realistic imprint process in which not only one stamp feature of given size s0 is imprinted but rather a number of these features, separated by cavities of width w and of height H for a plane geometry, just like the grating structures. Eq. (1) of squeezed flow modified by Bogdanski et al. [4] to enlarge the simulations to realistic imprint conditions describes the change of polymer thickness h(t) below the elevated stamp patterns. 1 1 2pðtÞt ¼ þ h2 ðt; T Þ h20 gðT ÞS
ð1Þ
where g(T) is the viscosity of SU-8, which is determined by an experimental function regarding imprint temperature T; p is the imprinting pressure; h0 is the initial thickness of the spin-coated SU-8 layer and t is the imprint time. A field of width for N periodic lines within the field is described by S ¼ N ðs0 þ wÞ, where s0 is the width of grating and w is the width of trench. The trench depth can be given as d ¼ ½h0 hðt; T Þðs0 þ wÞ=w
ð2Þ
d will reach the maximum when it is equal to the height of trench depth H. In our imprint experiment, 500 nm period silicon grating templates (1 mm 1 mm) with 1120 nm in height were used. The imprint properties of SU-8 were studied by optimization of the imprint pressure and imprint temperatures to achieve required imprint depth for different usage. Fig. 2 illustrates the trench depth variation with imprint pressures under different temperatures. The experimental results, shown in the monochrome data points are compared to the simulation results drawn in chromatic curves acquired from the Eqs. (1) and (2). As a UV curing process was introduced to further cure the SU-8 after imprint, vertical
Trench depth (nm)
1200 1000 800 600 400 200 20
40
60
E100oC E140oC
E120oC E200oC
S100oC S140oC
S120oC S200oC
80
100
Pressure (bar) Fig. 1. SEM photos showing the imprinted SU-8 gratings. Cross sectional views of (a) 1 lm period SU-8 gratings with 730 nm in height and (b) 300 nm period SU-8 gratings with 420 nm in height. (c) 200 nm period SU8 gratings with 260 nm in height.
Fig. 2. Relationship of the imprint depth against the pressure with the temperature of 100 °C, 120 °C, 140 °C and 200 °C. The monochrome data points are experimental results and the chromatic curves are simulation results.
916
S.-Q. Xie et al. / Microelectronic Engineering 85 (2008) 914–917
profile with least damage from the template releasing process was obtained, which made the measurement results reliable. It is found that at a fixed temperature, the imprint depth increases with the increasing imprint pressure until it reaches the height of the templates, which is around 1120 nm. Due to the variation of viscosity under different temperature, the pressure used to achieve such a depth also gradually shifts from 47 bar at 200 °C to 75 bar at 120 °C. For trench depth lower than the template’s height, in order to achieve the same depth, obviously higher temperature requires lower pressure. As a result, two extremes of the process window were obtained: one corresponds to the lowest pressure applicable for the highest temperature used (200 °C in this work), and the other is the lowest temperature applicable under the highest pressure (85 bar in this work). These two extremes are practically useful in some cases when low pressure is demanded while the temperature is not an important issue and vice versa. From Fig. 2 it can be seen that the experimental results agreed well with the simulation results, indicating reliable description of the experimental data using the modified. It can be conclude that the imprint trench depth can be predicted by using this simulation model. 3.2. Optical properties of the phase grating The fabricated SU-8 gratings were actually one kind of transmission phase grating which can be described by composing of two different transparent materials. The refraction indexes of the two kinds of material are noted by n1 = 1.55 (for SU-8) and n2 = 1 (for air), respectively. If the wavelength of the incident light is k, the transmission function of the phase grating, E(x), can be defined as the addition of two separate parts from n1 and n2. x ð3Þ EðxÞ ¼ ½E1 ðxÞ þ E2 ðxÞrectð Þ L
Fig. 3. The intensity distribution of the diffraction field of the SU-8 phase grating with trench depth from 0 to 2 lm. The grating’s period is 1 lm and the wavelength of incident light is 632.8 nm.
bution is also a function of the height of the grating, while not only determined by the period but also the trench width for a certain wavelength of incident light. The trench depth can be well controlled by the imprint condition. Fig. 4 shows the diffraction patterns of the 500 nm period SU-8 gratings with different trench depths, which demonstrates the effect of trench depth on the periodic variation of the intensity distribution. Apparently, the intensity of the first diffraction orders decreased with the reduction in trench depth, which closely agrees with the simulation results. However, the variations of the zero order cannot be observed, which may be caused by the transparent area outside the imprinted one (1 mm2).
where 1 E1 ðxÞ ¼ rectðx=dÞcombðx=aÞ expðj2pHn1 =kÞ a 1 x a=2 Þcombðx=aÞ expðj2pHn2 =kÞ E2 ðxÞ ¼ rectð a ad
ð4Þ ð5Þ
In which, d denotes the line width of the grating; H is the thickness of the grating. w represents the width of trench depth and a is the period of the grating. The total width of the grating is L. As known that the field distribution of Fraunhofer diffraction of an aperture is the Fourier transform of the amplitude distribution across the aperture. Making Fourier transformation of Eq. (3), we can have the intensity distribution of the Fraunhofer diffraction of the phase grating [12]. Fig. 3 shows the intensity distribution of the diffraction field of the 1 lm period SU-8 grating with different trench depths when using a 632.8 nm incident light. Periodic diffraction orders with varying intensity distribution are observed, indicating that the diffraction intensity distri-
Fig. 4. Diffraction patterns of the 500 nm period SU-8 phase gratings with different trench depth. (a) 540 nm; (b) 320 nm; (c) 220 nm; (d) 140 nm. The incident light is a 632.8 nm laser.
S.-Q. Xie et al. / Microelectronic Engineering 85 (2008) 914–917
917
4. Conclusions
References
A nanofabrication technique of the transmission SU-8 gratings with various periods and different trench depth for application from near-infrared to deep-UV wavelength has been successfully developed. The imprint properties have been studied and the experimental results agree well with the simulation results described by a modified equation of squeezed flow. The SU-8 phase grating was demonstrated both experimentally and theoretically. The developed process is applicable not only for plastic gratings, but also for other nanostructures such as nanofluidics and nanophotonics.
[1] Jian Wang, Hubert Kostal, Laser Focus World 41 (12) (2005) 76. [2] Xudi Wang, Yifang Chen, Shaojun Fu, Zheng Cui, Microelectron. Eng. 84 (2007) 872–876. [3] Bing-Rui Lu, Shen-Qi Xie, Yan Sun, Yifang Chen, Xin-Ping Qu, Ran Liu, Abstract No. 1P-529, in: International Conference on Nanoscience & Technology, Beijing, China, 2007. [4] N. Bogdanski, M. Wissen, A. Ziegler, H.-C. Scheer, Microelectron. Eng. 78–79 (2005) 598–604. [5] W.-H. Lee, Appl. Opt. 18 (13) (1979) 2152. [6] C. David, B. No¨hammer, H.H. Solak, Appl. Phys. Lett. 81 (17) (2002). [7] Shen-Qi Xie, Bing-Rui Lu, Yan Sun, Yifang Chen, Xin-Ping Qu, Ran Liu, Abstract No. 1O-068, in: International Conference on Nanoscience & Technology, Beijing, China, 2007. [8] R. Feng, R.J. Farris, J. Micromech. Microeng. 13 (2003) 80. [9] J. Stefan, Akad. Wiss. Math. Natur. 69 (1874) 713. [10] H. Schift et al., in: C.M. Sotomayor Torres (Ed.), Alternative Lithography, Plenum, 2003. [11] L.G. Baraldi, PhD Thesis, ETH Zu¨ rich, 1994. [12] Hua Gao, Min Ouyang, Yu Wang, Yan Shen, Jing Zhou, Dahe Liu, Optik 118 (2007) 452–456.
Acknowledgements The work was supported by National high technology program (2006AA03Z352) and the ‘‘985” Micro/nanoelectronics science and Technology Innovation Platform.