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Fabrication of anti-reflective structures using hot embossing with a stainless steel template irradiated by femtosecond laser
Microelectronic Engineering 88 (2011) 2908–2912
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Fabrication of anti-reflective structures using hot embossing with a stainless steel template irradiated by femtosecond laser Tsung-Fu Yao a, Ping-Han Wu b, Tzong-Ming Wu b, Chung-Wei Cheng b, Sen-Yeu Yang a,⇑ a b
Department of Mechanical Engineering, National Taiwan University, Taipei 10617, Taiwan, ROC ITRI South, Industrial Technology Research Institute, No. 8, Gongyan Rd., Liujia Distriction, Tainan 734, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 25 January 2011 Accepted 21 March 2011 Available online 3 April 2011 Keywords: Nanostructure Femtosecond laser Polycarbonate Gas-assisted Hot embossing Anti-reflective
a b s t r a c t Periodic nanostructures have played critical roles in key components for optical applications. Femtosecond laser-induced periodic surface structures (FLIPSS) in nano scale on many material surfaces have drawn much attention in recent years. However, the relatively low throughput of the direct inducing process limits its potential in regard to high volume manufacturing. In this study, fast replication of FLIPSS on Polycarbonate (PC) films by gas-assisted a hot embossing process is demonstrated. A selected surface area of a stainless steel thin plate is irradiated by femtosecond laser pulses to form a kind of periodic anti-reflective nanostructure with a period measuring 600–700 nm. The stainless plate was then used as a template for the hot embossing process. PC films with periodic nanostructures are then replicated quickly. Compared to the original PC film, at least up to 5% of the reflectivity of the fabricated PC films with nanostructures was reduced. It proves that the gas-assisted hot embossing could procure uniform transcription for antireflection PC film with stainless steel template that is irradiated by femtosecond laser pulses. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Periodic nanostructures have played critical roles in key components for application in display devices, solar cell, light-emitting diode, and other optical devices. In recent years, many researchers have tried to increase the light transmission in optical systems by reducing reflections through the use of periodic nanostructures. Anti-reflection can be achieved by coating multi-layers and by forming sub-wavelength structures on the surface of optical components. Both designs gradually increase the effective refractive indices along the light path. Due to the demand for a low cost and easy process, anti-reflection by incorporating nanostructures on optical components is drawing great attention [1,2]. Anti-reflective nanostructures have been fabricated using various methods, including: interference lithography process [3], multi-axis ultraprecision machine [4], gray scale photolithography [5], photoresist reflow [6], excimer laser ablation [7], modified LIGA process [8] etc. However, most of the processes for fabricating periodic nanostructures are either complicated or costly, requiring expensive facilities. Moreover, contamination generated by photoresist, complicated preparation in LIGA process, or excimer laser ablation have resulted in the abovementioned manufacturing processes. Recently, the femtosecond laser has been shown to be effective for surface micro/nano modification due to its minimal thermal ⇑ Corresponding author. E-mail addresses: [email protected] (C.-W. Cheng), [email protected] (S.-Y. Yang). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.03.023
and mechanical damage to metal and other materials. When irradiating metal with single-beam femtosecond laser pulses near the ablation threshold, the formation of surface structures, such as ripples (periodic nanostructures) or self-organized micro structures, are observed [9–14]. This technique is referred to as femtosecond laser-induced periodic surface structure (FLIPSS). The formation of periodic nanostructures may be attributed to the actions of the optical interference of incident femtosecond laser irradiation, with a surface scattered wave [13] and two plasmon decay models [14]. Regarding the application of the FLIPSS technique, the authors [15] prepared anti-reflection structures for photovoltaic cells using FLIPSS; the results show a photocurrent increase of about 30% in the laser-textured zones. In [16], the super hydrophobic surfaces on polypropylene (PP) materials used injection molding and a stainless steel mold prepared by FLIPSS. In this study, we present an effective fabrication method for anti-reflective nanostructure using hot-embossing of PC films with stainless steel template. The anti-reflective nanostructures on a stainless steel surface are first prepared by FLIPSS with femtosecond laser (wavelength 800 nm), as shown in Fig. 1a. The period of the fabricated nanostructures is approximately 600–700 nm. The fabrication of nanostructures from the template onto the polycarbonate (PC) film is then performed. As shown in Fig. 1b, with the gas-assisted hot-embossing, an array of nanostructures is formed by partial protrusion of the softened film into the template under the effects of the capillary force and surface tension. Using gas as the pressing media, pressure can be imposed uniformly upon the substrates, resulting in the homogeneous fabrication of
T.-F. Yao et al. / Microelectronic Engineering 88 (2011) 2908–2912
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plate, and was then incident upon a polarizing beam splitter. The reflected component was routed to a power detector in order to measure the laser energy, while the transmitted component was passed through a mechanical shutter. The laser beam was passed through a reflective mirror system in such a manner that it entered the objective lens (numerical aperture 0.26, M Plan Apo NIR, Mitutoyo), and was incident in the normal direction on the surface of a specimen mounted on an X–Y axial micro-positioning stage, with a precision greater than 1 lm. In order to increase the throughput and reduce the diffraction effect, the laser beam was focused at a position of 860 lm below the stainless steel template surface. 2.2. Measurement of fabricating the anti-reflective structure template
Fig. 1. Description of the fabricating processes of periodic nanostructures on PC film: (a) femtosecond laser modification on stainless steel template; (b) hot embossing to PC film; (c) PC film with periodic nanostructures.
nanostructures on the whole substrate, as shown in Fig. 1c. The morphologies of the stainless steel template and PC films are observed by scanning electron microscopy (SEM, JEOL, JSM-6500F) and the heights are measured by atomic force microscope (AFM, Bruker, D3100) Furthermore, the anti-reflection characteristics of the PC film are evaluated by a spectrophotometer (Hitachi, U3501). 2. Fabrication of the anti-reflective structure template using a femtosecond laser 2.1. Femtosecond laser fabrication of the anti-reflective structure template Fig. 2 shows the schematic layout of the experimental setup; the polished stainless steel template was machined using a regenerative amplified mode-locked Ti:Sapphire laser (SPIT FIRE, Spectra-Physics) with a central wavelength of 800 nm, a repetition rate of 1 kHz, and pulse duration of 120 fs. The polarized Gaussian laser beam was initially attenuated via a rotatable half-wave
Fig. 3a shows the photograph of the stainless steel template after femtosecond laser irradiation of an area 20 20 mm2 by laser power 130 mW and scanning speed of 2 mm/s, by linearly polarized femtosecond laser beam. It can be seen that the structured area exhibits various colors. In order to confirm and compare the duplicated situation between the template and the replicates, we printed five notes (see Fig. 3b) on the stainless steel template, with the notes to be transcribed to the PC films. Fig. 3c presents the magnification SEM images of the template near the five notes. It can be found that the periodic-like nanostructures with periods of 600–700 nm formed. Before employing hot embossing to duplicate the anti-reflected structures on PC films, the nanostructure topography on the stainless steel template was first measured. The nanostructure heights in the areas of the five notes were measured by AFM, with the results summarized in Table 1. Note that we derived nine height data of AFM results to obtain the average height for each note. The average height of the whole stainless steel template, i.e. 202.90 nm, was obtained from the average height of the five notes. 3. Fabrication of the anti-reflective nanostructure on pc films using gas-assisted hot embossing 3.1. Gas-assisted hot embossing for replication of nanostructures onto PC films The experimental setup for performing the gas-assisted hot embossing is shown in Fig. 4. The process consists of four steps:
Fig. 2. Schematic layout of the experimental setup.
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Fig. 3. (a) Photograph of the stainless steel template after femtosecond laser irradiation; (b) the five notes diagram on the template; (c) SEM images near the five notes, respectively.
(a) the stack of mold (periodic nanostructure template) and PC substrate was placed in the hot plate; (b) A poly(ethylene terephthalate) (PET) film (thickness 188 lm) used as the sealing film was placed upon the stack, and the stack was then heated by a heating coil to plasticize the PC substrate after the chamber was closed; (c) nitrogen gas was poured into the chamber to produce a uniform pressing pressure over the sealing film; and (d) the mold and substrate were cooled, and then the nitrogen gas was expelled when the temperature was below the glass transition temperature of the PC material. After the four steps, the PC films of 125 lm thickness with replicated nanostructures from mold were obtained. The most important parameters in the hot-embossing process are temperature and gas pressure. When the thermoplastic film is heated above the glass transition temperature, the film is forced to conform to the features in the surface of the mold by the pressure. Pressing pressure lends an impetus force to the polymer
Table 1 The nanostructure heights in the areas of the five notes upon the stainless steel template.
Height (nm)
Center
Downleft
Downright
Upleft
Upright
Average
215.15
200.13
196.13
204.09
198.98
202.90
material to fill in the cavities; as with the purpose of heat energy, the pressure imposed by the nitrogen gas has a positive correlation with the nanostructure heights on PC films. In this study, the temperature of the embossing process was set to three levels: 150, 160, and 170 °C, which are all higher than the glass transition temperature of thermoplastic film. The pressure was also set to three levels:20, 30, and 40 kgf/cm2. With the abovementioned parameter setting, the experiment will fabricate nine hot-embossing PC films from each parameter combination. 3.2. Result and discussion of the PC films fabricated by gas-assisted hot embossing Fig. 5 presents the SEM images of the fabricated periodic structures on the PC films at temperatures ranging from 150 to 170 °C and pressure ranging from 20 to 40 kgf/cm2, respectively. It was found that when embossing periodic structures from stainless steel template to PC films, it more obviously complied with increased temperature and pressure. The average heights (measured by AFM) of PC film nanostructures fabricated using a constant temperature of 170 °C and pressure 20, 30, 40 kgf/cm2 were found to be 171.65 nm (20 kgf/cm2), 211.15 nm (30 kgf/cm2), and 218.64 nm (40 kgf/cm2), respectively. It was also found that the heights of the two PC films fabricated by 30 and 40 kgf/cm2 exhibit no remarkable difference and was approximately the average heights of the stainless steel template nanostructures (202.90 nm, see Table 1). Therefore, pressure of 30 kgf/cm2 was
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Table 2 The nanostructure heights in the areas of the five notes upon the PC film fabricated with temperature 170 °C and pressure 30 kgf/cm2.
Height (nm)
Center
Downleft
Downright
Upleft
Upright
Average
217.57
219.41
193.03
218.87
191.01
207.98
Fig. 6. Reflectivity of the PC films with/without nanostructures.
Fig. 4. Process of the gas-assisted hot embossing.
enough to duplicate the structure from stainless steel template onto PC films. In addition, the average heights (measured by AFM) of PC film nanostructures fabricated using a constant pressure of 30 kgf/cm2 and temperatures of 150, 160, 170 °C were found to be 185.90 nm (150 °C), 202.97 nm (160 °C), and 211.15 nm (170 °C), respectively. It was clear that the height of the PC film nanostruc-
Fig. 5. SEM images of the PC films fabricated by gas-assisted hot embossing with pressure of 20 kgf/cm2 (left column), 30 kgf/cm2 (middle column), 40 kgf/cm2 (right column) and temperatures as follows: (a–c) 150 °C; (d–f) 160 °C; (h–j) 170 °C.
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films with structures was reduced from 9% to below 5% at wavelengths of 400–800 nm, and the reflectivity increased gradually from 0.5–1% to 3–5% when the irradiated wavelength increased from 400 to 800 nm. Fig. 7 shows the photograph of the PC film fabricated at 170 °C and pressure of 30 kgf/cm2. It can be seen that the structured area exhibits transparent color. 4. Conclusions
Fig. 7. Photograph of the fabricated PC film.
ture was close to the average height of the stainless steel template after the hot embossing process when the temperature and pressure were set to 170 °C and 30 kgf/cm2. The best parameter combination of gas-assisted hot embossing, i.e. 170 °C and pressure of 30 kgf/cm2, are used to duplicate antireflected structures on PC film. The nanostructure heights in the areas of the five notes upon PC film were measured by AFM, and the results summarized in Table 2. The average height of the whole PC film, i.e. 207.98 nm, was obtained from the average height of the five notes. Note that the heights of the stainless steel template and PC film exhibit no remarkable difference. It means that the gasassisted hot embossing has the ability to fabricate an article which has almost the same surface structure as the original mold. However, it has been found that the mold average height (202.90 nm) was less than the PC film (207.98 nm). This result is inevitable because of the height distribution upon the stainless steel template; also, ensuring the same-point in every measurement is impossible. We could confirm that the differences between the mold and the product were less than 3% and could be omitted. Fig. 6 shows the measured optical reflectivity spectrum of the PC films with structures prepared at temperatures ranging from 150 to 170 °C and pressure ranging from 20 to 40 kgf/cm2, respectively. Compared with the bare PC film, the reflectivity of the PC
This paper reports a novel and low cost fabrication process of periodic nanostructures using gas-assisted hot embossing with template made from femtosecond laser. The stainless-steel template is fabricated by the FLIPSS technique. Molds used in this paper were prepared in short-cycle-time and did not need any chemical process. By this proposed technique, an array of concave nanostructure was successfully formed by partial protrusion of the PC film with nanostructures into the nano-grooves of the mold under the effects of the capillary force and surface tension during the hot embossing process. The duplicated structures on PC film were compared with the structures on the mold by means of an AFM. It was proven that the gas-assisted hot embossing could procure a uniform transcription of PC film with stainless steel. The optical properties of the fabricated periodic structure have been proven. Compared with the bare PC film, the reflectivity of the PC films with structures was reduced from 9% to below 5% at wavelengths of 400–800 nm. References [1] C.J. Ting, M.C. Huang, H.Y. Tsai, C.P. Chou, C.C. Fu, Nanotechnology 19 (2008) 205301. [2] T.L. Chang, K.Y. Cheng, T.H. Chou, C.C. Su, H.P. Yang, S.W. Luo, Microelectron. Eng. 86 (2009) 874–877. [3] B. Paivanranta, P.Y. Baroni, T. Scharf, W. Nakagawa, M. Kuittinen, H.P. Herzig, Microelectron. Eng. 85 (2008) 1089–1091. [4] A.Y. Yi, L. Li, Opt. Lett. 30 (2005) 1707–1709. [5] K. Totsu, M. Esashi, J. Vac. Sci. Technol., B 23 (2005) 1487–1490. [6] D. Daly, R.F. Stevens, M.C. Hutley, N. Davles, Sci. Technol. 1 (1990) 759–766. [7] K. Naessens, H. Ottevaere, P. Van Daele, R. Baets, Appl. Surf. Sci. 208–9 (2003) 159–164. [8] S.K. Lee, K.C. Lee, S.S. Lee, J. Micromech. Microeng. 12 (2002) 334–340. [9] A.Y. Vorobyev, V.S. Makin, Guo Chunlei, J. Appl. Phys. 101 (2007) 034903. [10] A.M. Kietzig, S.G. Hatzikiriakos, P. Englezos, Langmuir 25 (2009) 4821–4827. [11] L. Qi, K. Nishii, Y. Namba, Opt. Lett. 34 (2009) 1846–1848. [12] B.K. Nayak, M.C. Gupta, Opt. Lasers Eng. 48 (2010) 940–949. [13] S. Sakabe, M. Hashida, S. Tokita, S. Namba, K. Okamuro, Phys. Rev. B 79 (2009) 033409. [14] M. Hashida, S. Namba, K. Okamuro, S. Tokita, S. Sakabe, Phys. Rev. B 81 (2010) 115442. [15] M. Halbwax, T. Sarnet, P. Delaporte, M. Sentis, H. Etienne, F. Torregrosa, V. Vervisch, I. Perichaud, S. Martinuzzi, Thin Solid Films 516 (2008) 6791–6795. [16] M. Groenendijk, Laser Tech. J. 5 (2008) 44–47.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Fabrication of anti-reflective structures using hot embossing with a stainless steel template irradiated by femtosecond laser Tsung-Fu Yao a, Ping-Han Wu b, Tzong-Ming Wu b, Chung-Wei Cheng b, Sen-Yeu Yang a,⇑ a b
Department of Mechanical Engineering, National Taiwan University, Taipei 10617, Taiwan, ROC ITRI South, Industrial Technology Research Institute, No. 8, Gongyan Rd., Liujia Distriction, Tainan 734, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 25 January 2011 Accepted 21 March 2011 Available online 3 April 2011 Keywords: Nanostructure Femtosecond laser Polycarbonate Gas-assisted Hot embossing Anti-reflective
a b s t r a c t Periodic nanostructures have played critical roles in key components for optical applications. Femtosecond laser-induced periodic surface structures (FLIPSS) in nano scale on many material surfaces have drawn much attention in recent years. However, the relatively low throughput of the direct inducing process limits its potential in regard to high volume manufacturing. In this study, fast replication of FLIPSS on Polycarbonate (PC) films by gas-assisted a hot embossing process is demonstrated. A selected surface area of a stainless steel thin plate is irradiated by femtosecond laser pulses to form a kind of periodic anti-reflective nanostructure with a period measuring 600–700 nm. The stainless plate was then used as a template for the hot embossing process. PC films with periodic nanostructures are then replicated quickly. Compared to the original PC film, at least up to 5% of the reflectivity of the fabricated PC films with nanostructures was reduced. It proves that the gas-assisted hot embossing could procure uniform transcription for antireflection PC film with stainless steel template that is irradiated by femtosecond laser pulses. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Periodic nanostructures have played critical roles in key components for application in display devices, solar cell, light-emitting diode, and other optical devices. In recent years, many researchers have tried to increase the light transmission in optical systems by reducing reflections through the use of periodic nanostructures. Anti-reflection can be achieved by coating multi-layers and by forming sub-wavelength structures on the surface of optical components. Both designs gradually increase the effective refractive indices along the light path. Due to the demand for a low cost and easy process, anti-reflection by incorporating nanostructures on optical components is drawing great attention [1,2]. Anti-reflective nanostructures have been fabricated using various methods, including: interference lithography process [3], multi-axis ultraprecision machine [4], gray scale photolithography [5], photoresist reflow [6], excimer laser ablation [7], modified LIGA process [8] etc. However, most of the processes for fabricating periodic nanostructures are either complicated or costly, requiring expensive facilities. Moreover, contamination generated by photoresist, complicated preparation in LIGA process, or excimer laser ablation have resulted in the abovementioned manufacturing processes. Recently, the femtosecond laser has been shown to be effective for surface micro/nano modification due to its minimal thermal ⇑ Corresponding author. E-mail addresses: [email protected] (C.-W. Cheng), [email protected] (S.-Y. Yang). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.03.023
and mechanical damage to metal and other materials. When irradiating metal with single-beam femtosecond laser pulses near the ablation threshold, the formation of surface structures, such as ripples (periodic nanostructures) or self-organized micro structures, are observed [9–14]. This technique is referred to as femtosecond laser-induced periodic surface structure (FLIPSS). The formation of periodic nanostructures may be attributed to the actions of the optical interference of incident femtosecond laser irradiation, with a surface scattered wave [13] and two plasmon decay models [14]. Regarding the application of the FLIPSS technique, the authors [15] prepared anti-reflection structures for photovoltaic cells using FLIPSS; the results show a photocurrent increase of about 30% in the laser-textured zones. In [16], the super hydrophobic surfaces on polypropylene (PP) materials used injection molding and a stainless steel mold prepared by FLIPSS. In this study, we present an effective fabrication method for anti-reflective nanostructure using hot-embossing of PC films with stainless steel template. The anti-reflective nanostructures on a stainless steel surface are first prepared by FLIPSS with femtosecond laser (wavelength 800 nm), as shown in Fig. 1a. The period of the fabricated nanostructures is approximately 600–700 nm. The fabrication of nanostructures from the template onto the polycarbonate (PC) film is then performed. As shown in Fig. 1b, with the gas-assisted hot-embossing, an array of nanostructures is formed by partial protrusion of the softened film into the template under the effects of the capillary force and surface tension. Using gas as the pressing media, pressure can be imposed uniformly upon the substrates, resulting in the homogeneous fabrication of
T.-F. Yao et al. / Microelectronic Engineering 88 (2011) 2908–2912
2909
plate, and was then incident upon a polarizing beam splitter. The reflected component was routed to a power detector in order to measure the laser energy, while the transmitted component was passed through a mechanical shutter. The laser beam was passed through a reflective mirror system in such a manner that it entered the objective lens (numerical aperture 0.26, M Plan Apo NIR, Mitutoyo), and was incident in the normal direction on the surface of a specimen mounted on an X–Y axial micro-positioning stage, with a precision greater than 1 lm. In order to increase the throughput and reduce the diffraction effect, the laser beam was focused at a position of 860 lm below the stainless steel template surface. 2.2. Measurement of fabricating the anti-reflective structure template
Fig. 1. Description of the fabricating processes of periodic nanostructures on PC film: (a) femtosecond laser modification on stainless steel template; (b) hot embossing to PC film; (c) PC film with periodic nanostructures.
nanostructures on the whole substrate, as shown in Fig. 1c. The morphologies of the stainless steel template and PC films are observed by scanning electron microscopy (SEM, JEOL, JSM-6500F) and the heights are measured by atomic force microscope (AFM, Bruker, D3100) Furthermore, the anti-reflection characteristics of the PC film are evaluated by a spectrophotometer (Hitachi, U3501). 2. Fabrication of the anti-reflective structure template using a femtosecond laser 2.1. Femtosecond laser fabrication of the anti-reflective structure template Fig. 2 shows the schematic layout of the experimental setup; the polished stainless steel template was machined using a regenerative amplified mode-locked Ti:Sapphire laser (SPIT FIRE, Spectra-Physics) with a central wavelength of 800 nm, a repetition rate of 1 kHz, and pulse duration of 120 fs. The polarized Gaussian laser beam was initially attenuated via a rotatable half-wave
Fig. 3a shows the photograph of the stainless steel template after femtosecond laser irradiation of an area 20 20 mm2 by laser power 130 mW and scanning speed of 2 mm/s, by linearly polarized femtosecond laser beam. It can be seen that the structured area exhibits various colors. In order to confirm and compare the duplicated situation between the template and the replicates, we printed five notes (see Fig. 3b) on the stainless steel template, with the notes to be transcribed to the PC films. Fig. 3c presents the magnification SEM images of the template near the five notes. It can be found that the periodic-like nanostructures with periods of 600–700 nm formed. Before employing hot embossing to duplicate the anti-reflected structures on PC films, the nanostructure topography on the stainless steel template was first measured. The nanostructure heights in the areas of the five notes were measured by AFM, with the results summarized in Table 1. Note that we derived nine height data of AFM results to obtain the average height for each note. The average height of the whole stainless steel template, i.e. 202.90 nm, was obtained from the average height of the five notes. 3. Fabrication of the anti-reflective nanostructure on pc films using gas-assisted hot embossing 3.1. Gas-assisted hot embossing for replication of nanostructures onto PC films The experimental setup for performing the gas-assisted hot embossing is shown in Fig. 4. The process consists of four steps:
Fig. 2. Schematic layout of the experimental setup.
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Fig. 3. (a) Photograph of the stainless steel template after femtosecond laser irradiation; (b) the five notes diagram on the template; (c) SEM images near the five notes, respectively.
(a) the stack of mold (periodic nanostructure template) and PC substrate was placed in the hot plate; (b) A poly(ethylene terephthalate) (PET) film (thickness 188 lm) used as the sealing film was placed upon the stack, and the stack was then heated by a heating coil to plasticize the PC substrate after the chamber was closed; (c) nitrogen gas was poured into the chamber to produce a uniform pressing pressure over the sealing film; and (d) the mold and substrate were cooled, and then the nitrogen gas was expelled when the temperature was below the glass transition temperature of the PC material. After the four steps, the PC films of 125 lm thickness with replicated nanostructures from mold were obtained. The most important parameters in the hot-embossing process are temperature and gas pressure. When the thermoplastic film is heated above the glass transition temperature, the film is forced to conform to the features in the surface of the mold by the pressure. Pressing pressure lends an impetus force to the polymer
Table 1 The nanostructure heights in the areas of the five notes upon the stainless steel template.
Height (nm)
Center
Downleft
Downright
Upleft
Upright
Average
215.15
200.13
196.13
204.09
198.98
202.90
material to fill in the cavities; as with the purpose of heat energy, the pressure imposed by the nitrogen gas has a positive correlation with the nanostructure heights on PC films. In this study, the temperature of the embossing process was set to three levels: 150, 160, and 170 °C, which are all higher than the glass transition temperature of thermoplastic film. The pressure was also set to three levels:20, 30, and 40 kgf/cm2. With the abovementioned parameter setting, the experiment will fabricate nine hot-embossing PC films from each parameter combination. 3.2. Result and discussion of the PC films fabricated by gas-assisted hot embossing Fig. 5 presents the SEM images of the fabricated periodic structures on the PC films at temperatures ranging from 150 to 170 °C and pressure ranging from 20 to 40 kgf/cm2, respectively. It was found that when embossing periodic structures from stainless steel template to PC films, it more obviously complied with increased temperature and pressure. The average heights (measured by AFM) of PC film nanostructures fabricated using a constant temperature of 170 °C and pressure 20, 30, 40 kgf/cm2 were found to be 171.65 nm (20 kgf/cm2), 211.15 nm (30 kgf/cm2), and 218.64 nm (40 kgf/cm2), respectively. It was also found that the heights of the two PC films fabricated by 30 and 40 kgf/cm2 exhibit no remarkable difference and was approximately the average heights of the stainless steel template nanostructures (202.90 nm, see Table 1). Therefore, pressure of 30 kgf/cm2 was
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Table 2 The nanostructure heights in the areas of the five notes upon the PC film fabricated with temperature 170 °C and pressure 30 kgf/cm2.
Height (nm)
Center
Downleft
Downright
Upleft
Upright
Average
217.57
219.41
193.03
218.87
191.01
207.98
Fig. 6. Reflectivity of the PC films with/without nanostructures.
Fig. 4. Process of the gas-assisted hot embossing.
enough to duplicate the structure from stainless steel template onto PC films. In addition, the average heights (measured by AFM) of PC film nanostructures fabricated using a constant pressure of 30 kgf/cm2 and temperatures of 150, 160, 170 °C were found to be 185.90 nm (150 °C), 202.97 nm (160 °C), and 211.15 nm (170 °C), respectively. It was clear that the height of the PC film nanostruc-
Fig. 5. SEM images of the PC films fabricated by gas-assisted hot embossing with pressure of 20 kgf/cm2 (left column), 30 kgf/cm2 (middle column), 40 kgf/cm2 (right column) and temperatures as follows: (a–c) 150 °C; (d–f) 160 °C; (h–j) 170 °C.
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films with structures was reduced from 9% to below 5% at wavelengths of 400–800 nm, and the reflectivity increased gradually from 0.5–1% to 3–5% when the irradiated wavelength increased from 400 to 800 nm. Fig. 7 shows the photograph of the PC film fabricated at 170 °C and pressure of 30 kgf/cm2. It can be seen that the structured area exhibits transparent color. 4. Conclusions
Fig. 7. Photograph of the fabricated PC film.
ture was close to the average height of the stainless steel template after the hot embossing process when the temperature and pressure were set to 170 °C and 30 kgf/cm2. The best parameter combination of gas-assisted hot embossing, i.e. 170 °C and pressure of 30 kgf/cm2, are used to duplicate antireflected structures on PC film. The nanostructure heights in the areas of the five notes upon PC film were measured by AFM, and the results summarized in Table 2. The average height of the whole PC film, i.e. 207.98 nm, was obtained from the average height of the five notes. Note that the heights of the stainless steel template and PC film exhibit no remarkable difference. It means that the gasassisted hot embossing has the ability to fabricate an article which has almost the same surface structure as the original mold. However, it has been found that the mold average height (202.90 nm) was less than the PC film (207.98 nm). This result is inevitable because of the height distribution upon the stainless steel template; also, ensuring the same-point in every measurement is impossible. We could confirm that the differences between the mold and the product were less than 3% and could be omitted. Fig. 6 shows the measured optical reflectivity spectrum of the PC films with structures prepared at temperatures ranging from 150 to 170 °C and pressure ranging from 20 to 40 kgf/cm2, respectively. Compared with the bare PC film, the reflectivity of the PC
This paper reports a novel and low cost fabrication process of periodic nanostructures using gas-assisted hot embossing with template made from femtosecond laser. The stainless-steel template is fabricated by the FLIPSS technique. Molds used in this paper were prepared in short-cycle-time and did not need any chemical process. By this proposed technique, an array of concave nanostructure was successfully formed by partial protrusion of the PC film with nanostructures into the nano-grooves of the mold under the effects of the capillary force and surface tension during the hot embossing process. The duplicated structures on PC film were compared with the structures on the mold by means of an AFM. It was proven that the gas-assisted hot embossing could procure a uniform transcription of PC film with stainless steel. The optical properties of the fabricated periodic structure have been proven. Compared with the bare PC film, the reflectivity of the PC films with structures was reduced from 9% to below 5% at wavelengths of 400–800 nm. References [1] C.J. Ting, M.C. Huang, H.Y. Tsai, C.P. Chou, C.C. Fu, Nanotechnology 19 (2008) 205301. [2] T.L. Chang, K.Y. Cheng, T.H. Chou, C.C. Su, H.P. Yang, S.W. Luo, Microelectron. Eng. 86 (2009) 874–877. [3] B. Paivanranta, P.Y. Baroni, T. Scharf, W. Nakagawa, M. Kuittinen, H.P. Herzig, Microelectron. Eng. 85 (2008) 1089–1091. [4] A.Y. Yi, L. Li, Opt. Lett. 30 (2005) 1707–1709. [5] K. Totsu, M. Esashi, J. Vac. Sci. Technol., B 23 (2005) 1487–1490. [6] D. Daly, R.F. Stevens, M.C. Hutley, N. Davles, Sci. Technol. 1 (1990) 759–766. [7] K. Naessens, H. Ottevaere, P. Van Daele, R. Baets, Appl. Surf. Sci. 208–9 (2003) 159–164. [8] S.K. Lee, K.C. Lee, S.S. Lee, J. Micromech. Microeng. 12 (2002) 334–340. [9] A.Y. Vorobyev, V.S. Makin, Guo Chunlei, J. Appl. Phys. 101 (2007) 034903. [10] A.M. Kietzig, S.G. Hatzikiriakos, P. Englezos, Langmuir 25 (2009) 4821–4827. [11] L. Qi, K. Nishii, Y. Namba, Opt. Lett. 34 (2009) 1846–1848. [12] B.K. Nayak, M.C. Gupta, Opt. Lasers Eng. 48 (2010) 940–949. [13] S. Sakabe, M. Hashida, S. Tokita, S. Namba, K. Okamuro, Phys. Rev. B 79 (2009) 033409. [14] M. Hashida, S. Namba, K. Okamuro, S. Tokita, S. Sakabe, Phys. Rev. B 81 (2010) 115442. [15] M. Halbwax, T. Sarnet, P. Delaporte, M. Sentis, H. Etienne, F. Torregrosa, V. Vervisch, I. Perichaud, S. Martinuzzi, Thin Solid Films 516 (2008) 6791–6795. [16] M. Groenendijk, Laser Tech. J. 5 (2008) 44–47.