- Email: [email protected]
Fabricating bi-layered metallic wire-grid polarizers by nanoimprint and O2 plasma etching
Microelectronic Engineering 102 (2013) 53–59
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Fabricating bi-layered metallic wire-grid polarizers by nanoimprint and O2 plasma etching Chia-Meng Chen, Pei-Lun Niu, Cheng-Kuo Sung ⇑, Cheng-Huan Chen Department of Power Mechanical Engineering, National Tsing Hua University, 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan, ROC
a r t i c l e
i n f o
Article history: Available online 26 June 2012 Keywords: Insertion structure Protrusion structure Bi-layered structure Metallic wire-grid polarizer (WGP) Nanoimprint O2 plasma etching
a b s t r a c t This paper proposes an efficient process for fabricating bi-layered metallic wire-grid polarizers (WGP), which consist of a metallic (aluminum) layer on the top and a dielectric (PMMA) layer at the bottom. The proposed architecture was fabricated from the insertion structure, which was accomplished through only three steps, i.e., nanoimprint, aluminum deposition and chemical mechanical polishing (CMP), to embed the Al wire grating into PMMA substrate. By taking the advantage of the characteristic of the insertion structure, this technique fabricated PMMA wires with O2 plasma etching by employing the nano-scale Al wire gratings as a mask to achieve bi-layered structures. The proposed bi-layered structures of metallic WGP can achieve superior optical performance, such as the extinction ratio of 497 and brightness gain of 1.18, from 0° to 40° of incident angles at a wavelength of 650 nm of incident light. In this paper, FE-SEM and FIB images show that the bi-layered wire-grid structure with wire gratings 100 nm in linewidth, 240 nm in pitch, and 300 nm in total height, i.e., 150 nm each for Al and PMMA wire gratings, was successfully replicated on a PMMA substrate of 1 cm2. Various O2 plasma etching periods were employed to accomplish the desirable bi-layered structures as well as its optical performance. In addition, we demonstrate that the deeper PMMA wire gratings of bi-layered structures cannot acquire the higher extinction ratio because of the increases of the P-polarization as well as the S-polarization. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Nanoimprint lithography (NIL) proposed by Chou et al. in 1995 has been considered as one of the most promising techniques in fabricating nano-scale patterns with high throughput, low cost and good fidelity [1,2]. NIL has also been applied in a broad range of optical systems, such as WGPs [3–5], transparent metal electrodes for organic solar cells [6] and organic light-emitting diodes [7]. Most of the WGPs possess protrusion structures [3–5], which feature the metallic wire gratings on the substrate and are achieved by lift-off or etching process. The prominent way to improve their optical performance, such as extinction ratio, is to reduce the linewidth and pitch of Al wire gratings. However, it will make fabrication process more difficult and costly. For example, one can hardly achieve a mold with a pitch less than 200 nm but sufficient area and life. Both parallel and cross-stacking of the bilayered nanostructure have been demonstrated to possess higher polarization efficiency and extinction ratio without a further reduction of pitch and linewidth of Al wire gratings [8,9]. However, the proposed bi-layered structures were fabricated via e-beam
lithography and lift-off process, which makes large-area devices unattainable. Herein, we provide an efficient process to construct bi-layered structures by merely O2 plasma etching with the Al wire gratings as a mask. In this process, the bi-layered structure is realized from insertion structure [10], which can be efficiently accomplished by only three steps, i.e., nanoimprint, aluminum deposition and CMP technique. In addition, the bi-layered structure can achieve higher optical performance, such as extinction ratio, than the protrusion structure with the same linewidth, pitch and height of Al wire gratings by 2.2 times between 0° and 40° of incident angle at a wavelength of 650 nm of incident light in optical measurement. Because this architecture features an Al layer on the top of a dielectric one, it is intended to be applied for fabricating metallic wire gratings of bi-layered structure on the PMMA or PET as a polarizer of flexible display devices. Accordingly, the fabricated device with bi-layered structures has potential to be utilized in polarizers, polarization beam splitters and optical isolators [11,12].
2. Experiments ⇑ Corresponding author. E-mail address: [email protected] (C.-K. Sung). 0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.05.025
In this study, Si mold is fabricated by laser interference lithography with nano-scale wire gratings of 100 nm in linewidth,
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C.-M. Chen et al. / Microelectronic Engineering 102 (2013) 53–59
Fig. 1. Process flow for fabricating a Si mold.
nano oimp printting & demo olding S mo Si old
Al thin film m de epos sition CMP P pro oces ss Al resid dual l ayerr
P MA PMM Fig. 2. Process flow for fabricating insertion structures.
Fig. 3. The process conditions during nanoimprint.
240 nm in pitch and 150 nm in height, as shown in Fig. 1. Fig. 2 illustrates the manufacturing process of the insertion structure, which consists of nanoimprint, Al thin film deposition and CMP process. First, a self-developed thermal nanoimprint machine was used to implement the nanoimprint process under the designed imprint conditions for pattern transfer. Fig. 3 explains process conditions of temperature and pressure during nanoimprint. First, the imprint temperature and pressure were set to be 120 °C and 12.5 MPa, respectively, and the imprint period was about 360 s. Secondly, the imprinted devices were cooled down to room temperature and the imprint pressure was held for 60 s in order to obtain better pattern transfer of nano-scale wire gratings on PMMA substrate. Thirdly, DC sputtering was utilized to deposit Al thin film on patterned PMMA substrate with 300 nm in height, which is designed higher than that of the pattern
on PMMA for polishing process. During Al thin-film deposition process, the Al particle must be as small as possible to increase the cavity-filling of high aspect-ratio patterned structure on PMMA substrate for better optical performance. In order to facilitate filling of Al grains into the mold cavities of 100 nm, we decreased the power of DC sputtering machine to 100 W, which led to slow down the speed of Al deposition so as to reduce the Al grain sizes. Other recipes of Al DC sputtering were Ar2 flow rate at 10 sccm and chamber pressure at 3 10 4 Torr. The residual layer of Al thin film was removed by CMP process for fabricating the insertion structure with Al wire gratings. The precision lapping and polishing machine (manufactured by Mabuchi S&T Inc.) was modified as the CMP equipment. The schematic setup of CMP process is illustrated in Fig. 4. The specimen with insertion structures was polished on a flannelette polishing plate.
C.-M. Chen et al. / Microelectronic Engineering 102 (2013) 53–59
55
Fig. 4. Schematic diagram of CMP processing.
Table 1 Processing parameters of CMP.
eters including power, O2 flow rate and Ar2 flow rate were fixed for all the tests.
Parameters
Levels
Units
Pressure Velocity Polishing time
0.05 50 10
kg/cm2 rpm s
The slurry was composed of alumina as abrasives and H2O2 as an oxidizer. The average of abrasive grits of CMP slurry was around 30–50 nm. Table 1 shows the CMP process parameters used in this experiment, which was completed within 30 s. The dimensions of the finished insertion structure after CMP process with Al wire gratings are 100 nm in linewidth, 240 nm in pitch and 150 nm in height. Fig. 5, respectively, illustrates the top and cross-section views of the insertion structures with Al wire gratings after CMP. We utilized focused ion beam (FIB) to observe the depth and fill of Al wire gratings of the insertion structures. The Pt layer was deposited on the substrate for protecting the cross-section structure. No cavities and defects were found in Al wire gratings because the Al particles were tiny enough during Al deposition process. In addition, it demonstrates that the polishing process can not only remove Al residual layer on PMMA substrate, but also smooth the surface of insertion structure. The finished surface area of insertion structures after nanoimprint and CMP process was about 1 cm2. We randomly chose four areas, 10.0 10.0 lm2 each, for AFM analysis. Fig. 6 shows good uniformity of area-1 surface. The roughness of four areas ranges from 5.721 to 7.836 nm, which are close to the bare PMMA surface as shown in Table 2. This measured data shows that the process can fabricate the embedded structure with flat surfaces. Then, we utilized O2 plasma etching treatment to remove the PMMA in the insertion structures to construct bi-layered structures by using Al wire gratings as a mask. The process flow is shown in Fig. 7. Table 3 summarizes the O2 plasma etching process parameters used in this experiment. The etching time with three levels were selected at 3.5, 4.5 and 5.5 min. Other process param-
3. Results and discussions Figs. 8–10 illustrate the cross-section views of the bi-layered structures with Al wire gratings on the top of PMMA ones. The process parameters were the same with power 150 W, O2 flow rate 30 sccm and Ar2 flow rate 10 sccm, except that only the etching time were varied, i.e., 3.5, 4.5 and 5.5 min, respectively. The finished surface areas of the bi-layered structures after O2 plasma etching process were about 1 cm2. We utilized FIB to observe the depths of Al wire gratings of the bi-layered structures. The Pt layer was deposited on the substrate for protecting the cross-sections of the structures. Fig. 8 indicates that the bi-layered structures with the Al wire gratings on the top of PMMA ones are successfully constructed by using O2 plasma etching under aforementioned process parameters. In O2 plasma etching process, the free radical will cause the chemical etching and reflect from the bottom of the etched trench to form the undercutting [13]. Fig. 9 indicates that the Al wire gratings are separated from PMMA because of the undercut effect, which was happened on PMMA wire gratings, with the longer O2 plasma etching time, i.e., 4.5 min. Moreover, Fig. 10 illustrates that the Al wire gratings are separated from PMMA and the surfaces of the PMMA wire gratings are damaged because of the undercut effect and the collision of the positive ions [14], with the long-term O2 plasma etching of 5.5 min. Therefore, it is crucial to control the O2 plasma etching time in order to obtain satisfactory architectures. As the pitch of metallic wire gratings reduces to less than a half of the wavelength of incident light, it offers polarizing function [15]. Fig. 11 illustrates that the incident light with polarization parallel to the Al gratings (S-polarization) is reflected, while the perpendicular polarization (P-polarization) is transmitted. The optical characteristics such as the extinction ratio (ER) can be adjusted by controlling the pitch, the duty ratio and the height of Al wire gratings [11]. Herein, the ER is defined as the ratio of transmittance of P-polarization to that of S-polarization. In this study,
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C.-M. Chen et al. / Microelectronic Engineering 102 (2013) 53–59 Table 2 Roughness of four selected finished areas and bare PMMA surface.
Fig. 5. Top-view and cross-section view of insertion structures with Al wire gratings inside PMMA after CMP process.
we measured the optical characteristics of the bi-layered structures with wire gratings 100 nm in linewidth, 240 nm in pitch, and 300 nm in total height, i.e., 150 nm each for Al and PMMA wire gratings, by using an incident light with a wavelength of 650 nm. Fig. 12 shows the optical measurement setup of the insertion structures based on Malus’s law [16]. We rotated the insertion structure at 0° and 90°, respectively, for optical detection. The detector measured the intensity of transmitted light at both
Four selected areas
Roughness (Rmax, nm)
1 2 3 4 Bare PMMA surface
5.721 5.978 7.836 6.172 5.294
degrees with P/S polarization. To divide the measured values of P-polarization by S-polarization attains the value of extinction ratio. Fig. 13 exhibits the total transmission efficiency of incident light through the bi-layered structure. The average transmittance of P-polarization is 0.751. Fig. 14 shows the measurement results for two different types of nanopatterns of Al wire gratings. The ER of bi-layered structures, with 100 nm in linewidth, 240 nm in pitch and 300 nm in total height, i.e., 150 nm each for Al and PMMA wire gratings, is conspicuously higher than the companion protrusion structures with the same linewidth, pitch and height of Al wire gratings. The result shows that the largest ER of the proposed architecture indicated with blue curve is over 1750.3 at 40° of an incident angle and the average ER of 497 from 0° to 40° of incident angles in this optical experiment. When the incident angle surpasses 40°, the light cannot pass through the polarizer because of total internal reflection (TIR) [17]. The curves in the figure should be the higher the better for the ER from 0° to 40° of incident angles. In practice, the higher ER will raise the contrast of LCD [18]. In this paper, we have demonstrated that bi-layered structures can achieve higher optical performance, such as ER, than the protrusion structures with the same linewidth, pitch and height of Al wire gratings. However, the deeper PMMA wire gratings of bilayered structures cannot acquire the higher extinction ratio, because the P-polarization increases as well as the S-polarization. Fig. 15 indicates the optimal depth of PMMA wire gratings after optical measurement. The result shows the most appropriate depth for achieving the best extinction ratio of the bi-layered structures, with 100 nm in linewidth, 240 nm in pitch and 150 nm in height of Al wire gratings, is 150 nm. Thus, it is very crucial to harness the etched depth of PMMA wire gratings in O2 plasma etching process. Besides, the brightness gain of the proposed bi-layered structures is 1.18, which is better than the protrusion structures of 1.12, in a wavelength of 650 nm of incident light of back-light unit. The definition of brightness gain is given as the ratio of transmittance of P-polarization and recycled transmittance of P-polarization of back-light unit to the transmittance of P-polarization.
Fig. 6. AFM topographic image of area-1 of the finished surface of the insertion structure.
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C.-M. Chen et al. / Microelectronic Engineering 102 (2013) 53–59
O2 plasma etching
Remained O2 plasma etching
Al PMMA Insertion structures
Bi-layered structures
Protrusion structures Fig. 7. Process flow for fabricating bi-layered structures.
4. Conclusions
Table 3 Processing parameters of O2 plasma etching treatment. Parameters
Levels
Units
Power O2 flow Ar2 flow Etching time
150 30 10 3.5, 4.5, 5.5
W sccm sccm min
In this paper, we have demonstrated an efficient process to fabricate a stable bi-layered structure with high throughput by nanoimprint and O2 plasma etching process for improving optical performance of metallic WGPs. This technique is able to fabricate PMMA wire gratings simply by O2 plasma etching with the employment of the nano-scale Al wire gratings as a mask to achieve bi-layered structures. We can accomplish the desirable bi-layered structures with power at 150 W, O2 flow rate at 30 sccm,
Pt Al 240 nm 100 nm
PMMA
150 nm
Al
150 nm
PMMA Schematic diagram of bi-layered structures Fig. 8. Cross-sectional view of the bi-layered structures with 3.5 min etching time.
Pt Al 240 nm 100 nm
Al
150 nm
PMMA 150 nm
PMMA Schematic diagram of bi-layered structures Fig. 9. Cross-sectional view of the bi-layered structures with 4.5 min etching time.
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C.-M. Chen et al. / Microelectronic Engineering 102 (2013) 53–59
Pt
Al 240 nm 100 nm
Al
150 nm
PMMA 150 nm
PMMA Schematic diagram of bi-layered structures Fig. 10. Cross-sectional view of the bi-layered structures with 5.5 min etching time.
Incident light
Reflected light
Incident angle
Al wire gratings
Total transmission efficiency
1
0.9 0.8 0.7 0.6
0.5 0.4
Tp (P-polarization)
0.3 0.2
0.1 0
0
4
8 12 16 20 24 28 32 36 40
Incident angles Fig. 13. Total transmission efficiency of incident light through the bi-layered structure.
PMMA
Bi-layered structures
P-polarization
Transmitted light Fig. 11. Schematic view of bi-layered structures employed as a WGP.
Ar2 flow rate at 10 sccm and the etching time at 3.5 min in O2 plasma etching treatment. The bi-layered structures can achieve higher optical performance, such as extinction ratio, than the companion
protrusion structures with the same linewidth, pitch and height of Al wire gratings by 2.2 times between 0° and 40° of incident angle at a wavelength of 650 nm of incident light in optical measurement. Also, in a wavelength of 650 nm of incident light of backlight unit, the brightness gain of the proposed bi-layered structures is 1.18, which is better than the protrusion structures of 1.12. Furthermore, we have observed that the deeper PMMA wire gratings of bi-layered structures cannot acquire the higher extinction ratio, because of the increase of the P-polarization as well as the S-polarization. Thus, it is very crucial to harness the etched depth of PMMA wire gratings in O2 plasma etching process.
Fig. 12. Schematic diagram of optical measurement setup.
C.-M. Chen et al. / Microelectronic Engineering 102 (2013) 53–59
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Acknowledgement
2000 1800
The authors gratefully thank for the financial support by the National Science Council of ROC under Contract Nos. NSC 97-2221-E007-045-MY3 and NSC 97-2221-E-007-048-MY3.
Extinction Ratio
1600 1400 1200 1000 800
Bi-layered structures
600
Protrusion structures [1] S.Y. Chou, P.R. Krauss, P.J. Renstrom, Applied Physics Letters 67 (1995) 3114– 3116. [2] Z. Yu, P. Deshpande, W. Wu, J. Wang, S.Y. Chou, Applied Physics Letters 77 (2000) 927–929. [3] K.D. Lee, S.H. Kim, J.D. Park, J.Y. Kim, S.J. Park, Proceeding of SPIE 6462 (2007) 4620. [4] L. Chen, J.J. Wang, F. Walters, X. Deng, M. Buonanno, S. Tai, X. Liu, Journal of Vacuum Science & Technology B 25 (2007) 2654–2657. [5] F. Meng, J. Chu, Z. Han, K. Zhao, in: Nanotechnology, IEEE-NANO, 2007, pp. 942–946. [6] M.G. Kang, M.S. Kim, J. Kim, L.J. Guo, Advanced Materials 20 (2008) 4408–4413. [7] C.Y. Cheng, C.N. Hong, Japanese Journal of Applied Physics 45 (2006) 8915– 8919. [8] C.C. Chen, C.H. Chen, Optical Review 16 (4) (2009) 416–421. [9] K.W. Chien, H.P.D. Shieh, Applied Optics 43 (9) (2004) 1830–1834. [10] C.M. Chen, C.K. Sung, Microelectronic Engineering 87 (2010) 872–875. [11] J.B. Young, H.A. Graham, E.W. Peterson, Applied Optics 4 (1965) 1023–1026. [12] S.W. Ahn, K.D. Lee, J.S. Kim, S.H. Kim, J.D. Park, S.H. Lee, P.W. Yoon, Nanotechnology 16 (2005) 1874–1877. [13] Hong Xiao, Introduction to Semiconductor Manufacturing Technology, Pearson. [14] Wen-an Loong, Nanometer Technologies for Semiconductors, Wunan. [15] C.H. Chen, P.C. Chen, C.C. Chen, Microelectronic Engineering 86 (2009) 1107– 1110. [16] D. Amrani, P. Paradis, Journal Physics Education 3 (2) (2009) 229–231. [17] I. Moreno, J.J. Araiza, M. Avendano-Alejo, Optics Letters 30 (8) (2005) 914–916. [18] S.H. Kim, J.D. Park, K.D. Lee, Nanotechnology 17 (2006) 4436–4438.
400 200 0 0
4
8 12 16 20 24 28 32 36 40
Incident angles Fig. 14. ER of Al wire gratings with various structures.
Optimal depth
600
Extinction Ratio
500 400 300
Tp ↑, Ts ↑
200 100 0 20
60
90
110
130
150
170
190
220
Depth of etched PMMA wire gratings (nm) Fig. 15. ER of etched PMMA wire gratings with various depths.
References
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Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Fabricating bi-layered metallic wire-grid polarizers by nanoimprint and O2 plasma etching Chia-Meng Chen, Pei-Lun Niu, Cheng-Kuo Sung ⇑, Cheng-Huan Chen Department of Power Mechanical Engineering, National Tsing Hua University, 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan, ROC
a r t i c l e
i n f o
Article history: Available online 26 June 2012 Keywords: Insertion structure Protrusion structure Bi-layered structure Metallic wire-grid polarizer (WGP) Nanoimprint O2 plasma etching
a b s t r a c t This paper proposes an efficient process for fabricating bi-layered metallic wire-grid polarizers (WGP), which consist of a metallic (aluminum) layer on the top and a dielectric (PMMA) layer at the bottom. The proposed architecture was fabricated from the insertion structure, which was accomplished through only three steps, i.e., nanoimprint, aluminum deposition and chemical mechanical polishing (CMP), to embed the Al wire grating into PMMA substrate. By taking the advantage of the characteristic of the insertion structure, this technique fabricated PMMA wires with O2 plasma etching by employing the nano-scale Al wire gratings as a mask to achieve bi-layered structures. The proposed bi-layered structures of metallic WGP can achieve superior optical performance, such as the extinction ratio of 497 and brightness gain of 1.18, from 0° to 40° of incident angles at a wavelength of 650 nm of incident light. In this paper, FE-SEM and FIB images show that the bi-layered wire-grid structure with wire gratings 100 nm in linewidth, 240 nm in pitch, and 300 nm in total height, i.e., 150 nm each for Al and PMMA wire gratings, was successfully replicated on a PMMA substrate of 1 cm2. Various O2 plasma etching periods were employed to accomplish the desirable bi-layered structures as well as its optical performance. In addition, we demonstrate that the deeper PMMA wire gratings of bi-layered structures cannot acquire the higher extinction ratio because of the increases of the P-polarization as well as the S-polarization. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Nanoimprint lithography (NIL) proposed by Chou et al. in 1995 has been considered as one of the most promising techniques in fabricating nano-scale patterns with high throughput, low cost and good fidelity [1,2]. NIL has also been applied in a broad range of optical systems, such as WGPs [3–5], transparent metal electrodes for organic solar cells [6] and organic light-emitting diodes [7]. Most of the WGPs possess protrusion structures [3–5], which feature the metallic wire gratings on the substrate and are achieved by lift-off or etching process. The prominent way to improve their optical performance, such as extinction ratio, is to reduce the linewidth and pitch of Al wire gratings. However, it will make fabrication process more difficult and costly. For example, one can hardly achieve a mold with a pitch less than 200 nm but sufficient area and life. Both parallel and cross-stacking of the bilayered nanostructure have been demonstrated to possess higher polarization efficiency and extinction ratio without a further reduction of pitch and linewidth of Al wire gratings [8,9]. However, the proposed bi-layered structures were fabricated via e-beam
lithography and lift-off process, which makes large-area devices unattainable. Herein, we provide an efficient process to construct bi-layered structures by merely O2 plasma etching with the Al wire gratings as a mask. In this process, the bi-layered structure is realized from insertion structure [10], which can be efficiently accomplished by only three steps, i.e., nanoimprint, aluminum deposition and CMP technique. In addition, the bi-layered structure can achieve higher optical performance, such as extinction ratio, than the protrusion structure with the same linewidth, pitch and height of Al wire gratings by 2.2 times between 0° and 40° of incident angle at a wavelength of 650 nm of incident light in optical measurement. Because this architecture features an Al layer on the top of a dielectric one, it is intended to be applied for fabricating metallic wire gratings of bi-layered structure on the PMMA or PET as a polarizer of flexible display devices. Accordingly, the fabricated device with bi-layered structures has potential to be utilized in polarizers, polarization beam splitters and optical isolators [11,12].
2. Experiments ⇑ Corresponding author. E-mail address: [email protected] (C.-K. Sung). 0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.05.025
In this study, Si mold is fabricated by laser interference lithography with nano-scale wire gratings of 100 nm in linewidth,
54
C.-M. Chen et al. / Microelectronic Engineering 102 (2013) 53–59
Fig. 1. Process flow for fabricating a Si mold.
nano oimp printting & demo olding S mo Si old
Al thin film m de epos sition CMP P pro oces ss Al resid dual l ayerr
P MA PMM Fig. 2. Process flow for fabricating insertion structures.
Fig. 3. The process conditions during nanoimprint.
240 nm in pitch and 150 nm in height, as shown in Fig. 1. Fig. 2 illustrates the manufacturing process of the insertion structure, which consists of nanoimprint, Al thin film deposition and CMP process. First, a self-developed thermal nanoimprint machine was used to implement the nanoimprint process under the designed imprint conditions for pattern transfer. Fig. 3 explains process conditions of temperature and pressure during nanoimprint. First, the imprint temperature and pressure were set to be 120 °C and 12.5 MPa, respectively, and the imprint period was about 360 s. Secondly, the imprinted devices were cooled down to room temperature and the imprint pressure was held for 60 s in order to obtain better pattern transfer of nano-scale wire gratings on PMMA substrate. Thirdly, DC sputtering was utilized to deposit Al thin film on patterned PMMA substrate with 300 nm in height, which is designed higher than that of the pattern
on PMMA for polishing process. During Al thin-film deposition process, the Al particle must be as small as possible to increase the cavity-filling of high aspect-ratio patterned structure on PMMA substrate for better optical performance. In order to facilitate filling of Al grains into the mold cavities of 100 nm, we decreased the power of DC sputtering machine to 100 W, which led to slow down the speed of Al deposition so as to reduce the Al grain sizes. Other recipes of Al DC sputtering were Ar2 flow rate at 10 sccm and chamber pressure at 3 10 4 Torr. The residual layer of Al thin film was removed by CMP process for fabricating the insertion structure with Al wire gratings. The precision lapping and polishing machine (manufactured by Mabuchi S&T Inc.) was modified as the CMP equipment. The schematic setup of CMP process is illustrated in Fig. 4. The specimen with insertion structures was polished on a flannelette polishing plate.
C.-M. Chen et al. / Microelectronic Engineering 102 (2013) 53–59
55
Fig. 4. Schematic diagram of CMP processing.
Table 1 Processing parameters of CMP.
eters including power, O2 flow rate and Ar2 flow rate were fixed for all the tests.
Parameters
Levels
Units
Pressure Velocity Polishing time
0.05 50 10
kg/cm2 rpm s
The slurry was composed of alumina as abrasives and H2O2 as an oxidizer. The average of abrasive grits of CMP slurry was around 30–50 nm. Table 1 shows the CMP process parameters used in this experiment, which was completed within 30 s. The dimensions of the finished insertion structure after CMP process with Al wire gratings are 100 nm in linewidth, 240 nm in pitch and 150 nm in height. Fig. 5, respectively, illustrates the top and cross-section views of the insertion structures with Al wire gratings after CMP. We utilized focused ion beam (FIB) to observe the depth and fill of Al wire gratings of the insertion structures. The Pt layer was deposited on the substrate for protecting the cross-section structure. No cavities and defects were found in Al wire gratings because the Al particles were tiny enough during Al deposition process. In addition, it demonstrates that the polishing process can not only remove Al residual layer on PMMA substrate, but also smooth the surface of insertion structure. The finished surface area of insertion structures after nanoimprint and CMP process was about 1 cm2. We randomly chose four areas, 10.0 10.0 lm2 each, for AFM analysis. Fig. 6 shows good uniformity of area-1 surface. The roughness of four areas ranges from 5.721 to 7.836 nm, which are close to the bare PMMA surface as shown in Table 2. This measured data shows that the process can fabricate the embedded structure with flat surfaces. Then, we utilized O2 plasma etching treatment to remove the PMMA in the insertion structures to construct bi-layered structures by using Al wire gratings as a mask. The process flow is shown in Fig. 7. Table 3 summarizes the O2 plasma etching process parameters used in this experiment. The etching time with three levels were selected at 3.5, 4.5 and 5.5 min. Other process param-
3. Results and discussions Figs. 8–10 illustrate the cross-section views of the bi-layered structures with Al wire gratings on the top of PMMA ones. The process parameters were the same with power 150 W, O2 flow rate 30 sccm and Ar2 flow rate 10 sccm, except that only the etching time were varied, i.e., 3.5, 4.5 and 5.5 min, respectively. The finished surface areas of the bi-layered structures after O2 plasma etching process were about 1 cm2. We utilized FIB to observe the depths of Al wire gratings of the bi-layered structures. The Pt layer was deposited on the substrate for protecting the cross-sections of the structures. Fig. 8 indicates that the bi-layered structures with the Al wire gratings on the top of PMMA ones are successfully constructed by using O2 plasma etching under aforementioned process parameters. In O2 plasma etching process, the free radical will cause the chemical etching and reflect from the bottom of the etched trench to form the undercutting [13]. Fig. 9 indicates that the Al wire gratings are separated from PMMA because of the undercut effect, which was happened on PMMA wire gratings, with the longer O2 plasma etching time, i.e., 4.5 min. Moreover, Fig. 10 illustrates that the Al wire gratings are separated from PMMA and the surfaces of the PMMA wire gratings are damaged because of the undercut effect and the collision of the positive ions [14], with the long-term O2 plasma etching of 5.5 min. Therefore, it is crucial to control the O2 plasma etching time in order to obtain satisfactory architectures. As the pitch of metallic wire gratings reduces to less than a half of the wavelength of incident light, it offers polarizing function [15]. Fig. 11 illustrates that the incident light with polarization parallel to the Al gratings (S-polarization) is reflected, while the perpendicular polarization (P-polarization) is transmitted. The optical characteristics such as the extinction ratio (ER) can be adjusted by controlling the pitch, the duty ratio and the height of Al wire gratings [11]. Herein, the ER is defined as the ratio of transmittance of P-polarization to that of S-polarization. In this study,
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C.-M. Chen et al. / Microelectronic Engineering 102 (2013) 53–59 Table 2 Roughness of four selected finished areas and bare PMMA surface.
Fig. 5. Top-view and cross-section view of insertion structures with Al wire gratings inside PMMA after CMP process.
we measured the optical characteristics of the bi-layered structures with wire gratings 100 nm in linewidth, 240 nm in pitch, and 300 nm in total height, i.e., 150 nm each for Al and PMMA wire gratings, by using an incident light with a wavelength of 650 nm. Fig. 12 shows the optical measurement setup of the insertion structures based on Malus’s law [16]. We rotated the insertion structure at 0° and 90°, respectively, for optical detection. The detector measured the intensity of transmitted light at both
Four selected areas
Roughness (Rmax, nm)
1 2 3 4 Bare PMMA surface
5.721 5.978 7.836 6.172 5.294
degrees with P/S polarization. To divide the measured values of P-polarization by S-polarization attains the value of extinction ratio. Fig. 13 exhibits the total transmission efficiency of incident light through the bi-layered structure. The average transmittance of P-polarization is 0.751. Fig. 14 shows the measurement results for two different types of nanopatterns of Al wire gratings. The ER of bi-layered structures, with 100 nm in linewidth, 240 nm in pitch and 300 nm in total height, i.e., 150 nm each for Al and PMMA wire gratings, is conspicuously higher than the companion protrusion structures with the same linewidth, pitch and height of Al wire gratings. The result shows that the largest ER of the proposed architecture indicated with blue curve is over 1750.3 at 40° of an incident angle and the average ER of 497 from 0° to 40° of incident angles in this optical experiment. When the incident angle surpasses 40°, the light cannot pass through the polarizer because of total internal reflection (TIR) [17]. The curves in the figure should be the higher the better for the ER from 0° to 40° of incident angles. In practice, the higher ER will raise the contrast of LCD [18]. In this paper, we have demonstrated that bi-layered structures can achieve higher optical performance, such as ER, than the protrusion structures with the same linewidth, pitch and height of Al wire gratings. However, the deeper PMMA wire gratings of bilayered structures cannot acquire the higher extinction ratio, because the P-polarization increases as well as the S-polarization. Fig. 15 indicates the optimal depth of PMMA wire gratings after optical measurement. The result shows the most appropriate depth for achieving the best extinction ratio of the bi-layered structures, with 100 nm in linewidth, 240 nm in pitch and 150 nm in height of Al wire gratings, is 150 nm. Thus, it is very crucial to harness the etched depth of PMMA wire gratings in O2 plasma etching process. Besides, the brightness gain of the proposed bi-layered structures is 1.18, which is better than the protrusion structures of 1.12, in a wavelength of 650 nm of incident light of back-light unit. The definition of brightness gain is given as the ratio of transmittance of P-polarization and recycled transmittance of P-polarization of back-light unit to the transmittance of P-polarization.
Fig. 6. AFM topographic image of area-1 of the finished surface of the insertion structure.
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C.-M. Chen et al. / Microelectronic Engineering 102 (2013) 53–59
O2 plasma etching
Remained O2 plasma etching
Al PMMA Insertion structures
Bi-layered structures
Protrusion structures Fig. 7. Process flow for fabricating bi-layered structures.
4. Conclusions
Table 3 Processing parameters of O2 plasma etching treatment. Parameters
Levels
Units
Power O2 flow Ar2 flow Etching time
150 30 10 3.5, 4.5, 5.5
W sccm sccm min
In this paper, we have demonstrated an efficient process to fabricate a stable bi-layered structure with high throughput by nanoimprint and O2 plasma etching process for improving optical performance of metallic WGPs. This technique is able to fabricate PMMA wire gratings simply by O2 plasma etching with the employment of the nano-scale Al wire gratings as a mask to achieve bi-layered structures. We can accomplish the desirable bi-layered structures with power at 150 W, O2 flow rate at 30 sccm,
Pt Al 240 nm 100 nm
PMMA
150 nm
Al
150 nm
PMMA Schematic diagram of bi-layered structures Fig. 8. Cross-sectional view of the bi-layered structures with 3.5 min etching time.
Pt Al 240 nm 100 nm
Al
150 nm
PMMA 150 nm
PMMA Schematic diagram of bi-layered structures Fig. 9. Cross-sectional view of the bi-layered structures with 4.5 min etching time.
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C.-M. Chen et al. / Microelectronic Engineering 102 (2013) 53–59
Pt
Al 240 nm 100 nm
Al
150 nm
PMMA 150 nm
PMMA Schematic diagram of bi-layered structures Fig. 10. Cross-sectional view of the bi-layered structures with 5.5 min etching time.
Incident light
Reflected light
Incident angle
Al wire gratings
Total transmission efficiency
1
0.9 0.8 0.7 0.6
0.5 0.4
Tp (P-polarization)
0.3 0.2
0.1 0
0
4
8 12 16 20 24 28 32 36 40
Incident angles Fig. 13. Total transmission efficiency of incident light through the bi-layered structure.
PMMA
Bi-layered structures
P-polarization
Transmitted light Fig. 11. Schematic view of bi-layered structures employed as a WGP.
Ar2 flow rate at 10 sccm and the etching time at 3.5 min in O2 plasma etching treatment. The bi-layered structures can achieve higher optical performance, such as extinction ratio, than the companion
protrusion structures with the same linewidth, pitch and height of Al wire gratings by 2.2 times between 0° and 40° of incident angle at a wavelength of 650 nm of incident light in optical measurement. Also, in a wavelength of 650 nm of incident light of backlight unit, the brightness gain of the proposed bi-layered structures is 1.18, which is better than the protrusion structures of 1.12. Furthermore, we have observed that the deeper PMMA wire gratings of bi-layered structures cannot acquire the higher extinction ratio, because of the increase of the P-polarization as well as the S-polarization. Thus, it is very crucial to harness the etched depth of PMMA wire gratings in O2 plasma etching process.
Fig. 12. Schematic diagram of optical measurement setup.
C.-M. Chen et al. / Microelectronic Engineering 102 (2013) 53–59
59
Acknowledgement
2000 1800
The authors gratefully thank for the financial support by the National Science Council of ROC under Contract Nos. NSC 97-2221-E007-045-MY3 and NSC 97-2221-E-007-048-MY3.
Extinction Ratio
1600 1400 1200 1000 800
Bi-layered structures
600
Protrusion structures [1] S.Y. Chou, P.R. Krauss, P.J. Renstrom, Applied Physics Letters 67 (1995) 3114– 3116. [2] Z. Yu, P. Deshpande, W. Wu, J. Wang, S.Y. Chou, Applied Physics Letters 77 (2000) 927–929. [3] K.D. Lee, S.H. Kim, J.D. Park, J.Y. Kim, S.J. Park, Proceeding of SPIE 6462 (2007) 4620. [4] L. Chen, J.J. Wang, F. Walters, X. Deng, M. Buonanno, S. Tai, X. Liu, Journal of Vacuum Science & Technology B 25 (2007) 2654–2657. [5] F. Meng, J. Chu, Z. Han, K. Zhao, in: Nanotechnology, IEEE-NANO, 2007, pp. 942–946. [6] M.G. Kang, M.S. Kim, J. Kim, L.J. Guo, Advanced Materials 20 (2008) 4408–4413. [7] C.Y. Cheng, C.N. Hong, Japanese Journal of Applied Physics 45 (2006) 8915– 8919. [8] C.C. Chen, C.H. Chen, Optical Review 16 (4) (2009) 416–421. [9] K.W. Chien, H.P.D. Shieh, Applied Optics 43 (9) (2004) 1830–1834. [10] C.M. Chen, C.K. Sung, Microelectronic Engineering 87 (2010) 872–875. [11] J.B. Young, H.A. Graham, E.W. Peterson, Applied Optics 4 (1965) 1023–1026. [12] S.W. Ahn, K.D. Lee, J.S. Kim, S.H. Kim, J.D. Park, S.H. Lee, P.W. Yoon, Nanotechnology 16 (2005) 1874–1877. [13] Hong Xiao, Introduction to Semiconductor Manufacturing Technology, Pearson. [14] Wen-an Loong, Nanometer Technologies for Semiconductors, Wunan. [15] C.H. Chen, P.C. Chen, C.C. Chen, Microelectronic Engineering 86 (2009) 1107– 1110. [16] D. Amrani, P. Paradis, Journal Physics Education 3 (2) (2009) 229–231. [17] I. Moreno, J.J. Araiza, M. Avendano-Alejo, Optics Letters 30 (8) (2005) 914–916. [18] S.H. Kim, J.D. Park, K.D. Lee, Nanotechnology 17 (2006) 4436–4438.
400 200 0 0
4
8 12 16 20 24 28 32 36 40
Incident angles Fig. 14. ER of Al wire gratings with various structures.
Optimal depth
600
Extinction Ratio
500 400 300
Tp ↑, Ts ↑
200 100 0 20
60
90
110
130
150
170
190
220
Depth of etched PMMA wire gratings (nm) Fig. 15. ER of etched PMMA wire gratings with various depths.
References
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