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Fabrication of tapered waveguide by ashed photoresist
Microelectronic Engineering 88 (2011) 2721–2724
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Fabrication of tapered waveguide by ashed photoresist Han-Hyoung Kim, Jang-Hwan Han, Da-Hyeok Lee, Beom-Hoan O, Seung-Gol Lee, El-Hang Lee, Se-Geun Park ⇑ Micro-photonics Advanced Research Center, Inha University, Incheon, South Korea
a r t i c l e
i n f o
Article history: Available online 26 January 2011 Keywords: Tapered waveguide Ashing Soft lithography
a b s t r a c t It is necessary to interconnect various optical components with different input or output sizes in hybrid integrated optics. Integration of optical devices on an optical printed circuit board sometimes requires a tapered waveguide to match a backbone waveguide to a smaller optical device in order to minimize optical loss. In this work a backbone waveguide of 20 25 lm2 cross-section is tapered to a 3 5 lm2 crosssection. The tapered waveguide is fabricated by soft lithography, which imprints an optical polymer using a polydimethlysiloxane mold. A master of the tapered waveguide structure used to make the mold is prepared by plasma ashing of patterned photoresist. The ratio of the stripping rate (vertical etching) to the trimming rate (horizontal etching) can be changed as a result of the line-width-dependent stripping rate. Thus narrower portions of a line become thinner (vertically) than the wider portions of the line during a single ashing step. Fabricated tapered waveguide shows higher optical transmission than that of an abruptly matched waveguide. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Integrated optics is a very promising solution for high data rate transmission with wide bandwidth by integrating many optical devices such as sources, modulators, detectors and other passive devices onto one substrate. Planar waveguides fabricated on the substrate are commonly used to interconnect those devices. However there is often a size mismatch between devices and components, for example, optical fiber and waveguide, and passive component and waveguide. In order to make effective connection among the devices, prism coupling, tapered optical fiber and grating coupling have been proposed. Because connecting points of both sides are different from each other in height and width, the coupling should be matched in both horizontal and vertical dimensions. Therefore it would be ideal to have a tapered waveguide structure for coupling with little loss [1,2]. Soft lithography has been known as a next generation patterning technology because of its possibility of low cost and mass production. General steps in soft lithography are preparation of a master, preparation of mold templates from the master and fabrication of final structure by imprinting materials of interest with the mold. Therefore, it is most important to design and prepare the master in the first step. In this work, we propose a new method to make tapered waveguide structures by plasma ashing of photo⇑ Corresponding author. Address: 253 Yonghyun-dong, Incheon 402-751, South Korea E-mail address: [email protected] (S.-G. Park). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.01.052
resist. A tapered structure has a thicker, wider part, and a gradually shrinking transition to a thinner, narrower part. The isotropic and anisotropic nature of plasma etching is utilized to achieve different final thickness of photoresist for the same initial thickness, depending on the patterned line width. In the conventional photolithography, the final thickness of photoresist layer after UV exposure and development is same for all patterns. In order to get different photoresist thickness after development, graded exposure methods have been proposed but it is very expensive [3]. Photoresist has successfully been removed by isotropic plasma ashing by oxygen radicals [4], and this isotropic nature has been applied to reduce pattern width of photoresist further [5]. In this work, ashing rates in the vertical and horizontal directions of photoresist patterns are investigated in a mixture of oxygen and nitrogen gases. Differences in the initial widths of the patterns of the same thickness results in different final thicknesses and widths because of width dependent ashing rate in our system. These results are applicable to prepare a master of tapered waveguide for soft lithography. The soft lithography is known to be a key technology to explore the optical print circuit board and integrated photonics studies [6,7]. 2. Experimental There are two experimental parts in this work; the first part is to study ashing process to optimize photoresist stripping (vertical direction etching) and trimming (horizontal direction etching)
H.-H. Kim et al. / Microelectronic Engineering 88 (2011) 2721–2724
rates for different pattern widths. The second is to fabricate tapered waveguide by the soft lithography. An inductively coupled plasma (ICP) was used to generate plasma, where a 13.56 MHz RF power is connected to planar spiral ICP antenna (ICP power) and another to substrate holder. In our experiment RF power to the substrate holder, which causes dc self-bias, is not used because any small RF power has caused too much anisotropic etching and too much vertical stripping of photoresist. In the conventional ashing down stream oxygen radicals are used for isotropic etching of photoresist [4]. In this experiment both ions and radicals are used to obtain line-width-dependent stripping. The substrate holder is water-cooled and its temperature is kept below 40 °C during ashing. For ashing condition optimization studies, positive photoresist, AZ9260, was spin-coated to 9 lm thick on silicon wafers and was exposed and developed to have line patterns with several different widths. Ashing is done by changing ICP RF power, chamber pressure and gas mixture ratio of oxygen and nitrogen. For fabrication of a tapered waveguide, 30 lm thick photoresist is prepared by multiple spin-coating of AZ9260 [8]. Photoresist pattern with wider input and narrower output terminals is patterned using a mask aligner, Karlsuss MJB4, as shown in Fig. 1a. The photoresist pattern is ashed by oxygen/nitrogen plasma as shown in Fig. 1b, where the wider part is thicker (vertically) and the narrower part is thinner. This pattern is the master for the soft lithography, and this master is used to make mold made of polydimethlysiloxane (PDMS) as shown Fig. 1c. PDMS layer was cured in an oven at 80 °C for 30 min. Final step of the soft lithography is to make replica into UV curing optical polymer material by imprinting as shown in Fig. 1d. The final structure of a tapered waveguide is shown schematically in Fig. 1e.
3. Results and discussions Photoresist ashing behavior can be changed by changing RF power to the coupling antenna of the inductively coupled plasma, chamber process pressure and gas mixture ratio of O2/N2. Ash rate was measured as the vertical ash rate (stripping rate) and horizontal ash rate (trimming rate). Purpose of this study is to investigate the stripping to trimming rate ratio (S/T ratio) for two different pattern widths, 5 and 10 lm, respectively. When ICP power was varied from 800 to 1200 W, the S/T ratio of 5 lm line was abruptly increased to 2.5 and then decreased to 1.3, but that of 10 lm line was not changed much as shown in Fig. 2a. When process pressure was increased as shown in Fig. 2b, the S/T ratio of both lines
4.0
Stripping/Trimming Ratio
2722
5 um pattern 10 um pattern
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 600
800
1000
1200
1400
Source power [W]
(a)
Stripping/Trimming Ratio
4.0 3.5
5 um pattern 10 um pattern
3.0 2.5 2.0 1.5 1.0 0.5 0.0
2.0
2.5
3.0
3.5
4.0
Chamber pressure [mTorr]
(b) Stripping/Trimming Ratio
4.0 3.5
5 um pattern 10 um pattern
3.0 2.5 2.0 1.5 1.0 0.5 0.0
0
20
40
60
80
100
O2 /(N2 +O2 ) [%]
(c) Fig. 1. Soft lithography process step: (a) patterning of photoresist by the conventional photolithography, (b) master preparation by plasma ashing, (c) PDMS mold fabrication, (d) imprinting with PDMS mold, and (e) final replica structure.
Fig. 2. Stripping to trimming rate ratio in terms of; (a) RF source power, (b) chamber pressure, and (c) O2/N2 mixture ratio.
H.-H. Kim et al. / Microelectronic Engineering 88 (2011) 2721–2724 Table 1 Ashing rates in terms of ICP RF source power. Source power (W)
Trimming rate (nm/min)
Stripping rate of 5 lm line (nm/min)
Stripping rate of 10 lm line (nm/min)
800 1000 1200
540 760 980
480 1850 1260
460 560 860
increased. In all cases of the S/T ratio of narrower line of 5 lm is higher than that of 10 lm line. Fig. 2c shows the dependence of the S/T ratios on the gas mixture ratio. For 5 lm lines, the highest S/T ratio can be found at 60% of oxygen, and for 10 lm lines the maximum is found at 20% of oxygen. It is desirable to have high S/T ratio in order to prepare as close as a square shape from
2723
5 lm lines. Table 1 lists the stripping rate and trimming rate of two lines of different width in terms of ICP power. Trimming rate is the same for both lines, but stripping rates are different from each other, and this is why the S/T ratios are different for different line width. We call this as line-width-dependent stripping rate in ashing reaction. Fig. 3 shows that after ashing different aspect ratio of photoresist lines can be obtained. Fig. 3a shows the definition of aspect ratio and Fig. 3b the cross section of original pattern with aspect ratio of 1.7. By changing ashing conditions, the aspect ratios of 1.2 and 3.0 can be obtained as shown Fig. 3c and d, respectively. Considering the different S/T ratio and aspect ratio resulted by ashing reaction, a tapered waveguide with 20(W) 25(H) lm2 input and 3(W) 5(H) lm2 output was designed and fabricated as shown in Fig. 4. Fig. 4a shows the master structure made of photoresist and Fig. 4b shows the final replica made of optical polymer
Fig. 3. Cross sectional views of photoresist lines: (a) definition of aspect ratio, (b) aspect ratio 1.7 of original pattern, (c) aspect ratio 1.2 of ashed line, and (d) aspect ratio 3.0 of ashed line.
Fig. 4. SEM images; (a) master structure of a tapered waveguide made of photoresist and (b) final replica made by the soft lithography.
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to the input port of the tapered waveguide through 50 lm diameter multi-mode optical fiber as shown in Fig. 5a. Light (bright spot in Fig. 5b) comes out from the output port of the 2 cm long tapered waveguide. 4. Conclusions Line-width-dependent stripping rate is applied to prepare a tapered waveguide master structure in photoresist. Plasma conditions are investigated to increase the ratio of stripping to trimming rates while trimming rate is independent on line width. From this master a PDMS mold is made and soft lithography is employed successfully to fabricate a tapered optical waveguide with 20 25 lm2 input to 3 5 lm2 output terminals. Acknowledgment This work has been supported by the Inha University through its Basic Research Promotion Program. References Fig. 5. (a) Top view of the fabricated tapered waveguide along with optical fiber, and (b) picture of a bright spot which is the light coming out from the waveguide output port.
resin on cladding layer. From this figures it is found that the replica has thinner lines than that of the master. This is due to shrinkage of the polymer resin and possible incomplete filling into smaller hollow pattern [9,10]. In order to measure the optical performance of the tapered waveguide, the top cladding layer was coated. The index of reflection of cladding layer is 1.45 and that of core part is 1.48. The tapered waveguide serves as the core and is sandwiched by top and bottom cladding layers. An 850 nm laser source is connected
[1] E.J. Teo, A.A. Bettiol, M.B.H. Breese, P. Yang, G.Z. Mashanovich, W.R. Headley, G.T. Reed, D.J. Blackwood, Opt. Express 16 (2008) 573–578. [2] I. Hardy, P. Grosso, D. Bosc, IEEE Photonics Technol. Lett. 17 (2005) 1028. [3] H. Spelthahn, A. Poghossian, M.J. Schöninga, Electrochim. Acta 54 (2009) 6010– 6014. [4] K. Inomata, H. Koinuma, Appl. Phys. Lett. 66 (1995) 2188–2190. [5] S.G. Park, S.K. Yang, J.K. Yang, J. Korean Phys. Soc. 49 (2006) S732–S735. [6] E.H. Lee, I.J. Chin, Y.K. Kwon, S.G. Lee, B.H. O, S.G. Park, K.H. Kim, Solid State Phenom. 124–126 (2007) 459–462. [7] H.-H. Kim, B.-H. O, S.-G. Lee, S.-G. Park, Microelectron. Eng. 87 (2010) 1033– 1036. [8] P. Dixit, C.W. Tan, L. Xu, N. Lin, J. Miao, J.H.L. Pang, P. Backus, R. Preisser, J. Micromech. Microeng. 17 (2007) 1078–1086. [9] M.H. Lee, M.D. Huntington, W. Zhou, J.C. Yang, T.W. Odom, Nano Lett., in press. [10] Y. Xia, G.M. Whitesides, Annu. Rev. Mater. Sci. 28 (1998) 153–184.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Fabrication of tapered waveguide by ashed photoresist Han-Hyoung Kim, Jang-Hwan Han, Da-Hyeok Lee, Beom-Hoan O, Seung-Gol Lee, El-Hang Lee, Se-Geun Park ⇑ Micro-photonics Advanced Research Center, Inha University, Incheon, South Korea
a r t i c l e
i n f o
Article history: Available online 26 January 2011 Keywords: Tapered waveguide Ashing Soft lithography
a b s t r a c t It is necessary to interconnect various optical components with different input or output sizes in hybrid integrated optics. Integration of optical devices on an optical printed circuit board sometimes requires a tapered waveguide to match a backbone waveguide to a smaller optical device in order to minimize optical loss. In this work a backbone waveguide of 20 25 lm2 cross-section is tapered to a 3 5 lm2 crosssection. The tapered waveguide is fabricated by soft lithography, which imprints an optical polymer using a polydimethlysiloxane mold. A master of the tapered waveguide structure used to make the mold is prepared by plasma ashing of patterned photoresist. The ratio of the stripping rate (vertical etching) to the trimming rate (horizontal etching) can be changed as a result of the line-width-dependent stripping rate. Thus narrower portions of a line become thinner (vertically) than the wider portions of the line during a single ashing step. Fabricated tapered waveguide shows higher optical transmission than that of an abruptly matched waveguide. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Integrated optics is a very promising solution for high data rate transmission with wide bandwidth by integrating many optical devices such as sources, modulators, detectors and other passive devices onto one substrate. Planar waveguides fabricated on the substrate are commonly used to interconnect those devices. However there is often a size mismatch between devices and components, for example, optical fiber and waveguide, and passive component and waveguide. In order to make effective connection among the devices, prism coupling, tapered optical fiber and grating coupling have been proposed. Because connecting points of both sides are different from each other in height and width, the coupling should be matched in both horizontal and vertical dimensions. Therefore it would be ideal to have a tapered waveguide structure for coupling with little loss [1,2]. Soft lithography has been known as a next generation patterning technology because of its possibility of low cost and mass production. General steps in soft lithography are preparation of a master, preparation of mold templates from the master and fabrication of final structure by imprinting materials of interest with the mold. Therefore, it is most important to design and prepare the master in the first step. In this work, we propose a new method to make tapered waveguide structures by plasma ashing of photo⇑ Corresponding author. Address: 253 Yonghyun-dong, Incheon 402-751, South Korea E-mail address: [email protected] (S.-G. Park). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.01.052
resist. A tapered structure has a thicker, wider part, and a gradually shrinking transition to a thinner, narrower part. The isotropic and anisotropic nature of plasma etching is utilized to achieve different final thickness of photoresist for the same initial thickness, depending on the patterned line width. In the conventional photolithography, the final thickness of photoresist layer after UV exposure and development is same for all patterns. In order to get different photoresist thickness after development, graded exposure methods have been proposed but it is very expensive [3]. Photoresist has successfully been removed by isotropic plasma ashing by oxygen radicals [4], and this isotropic nature has been applied to reduce pattern width of photoresist further [5]. In this work, ashing rates in the vertical and horizontal directions of photoresist patterns are investigated in a mixture of oxygen and nitrogen gases. Differences in the initial widths of the patterns of the same thickness results in different final thicknesses and widths because of width dependent ashing rate in our system. These results are applicable to prepare a master of tapered waveguide for soft lithography. The soft lithography is known to be a key technology to explore the optical print circuit board and integrated photonics studies [6,7]. 2. Experimental There are two experimental parts in this work; the first part is to study ashing process to optimize photoresist stripping (vertical direction etching) and trimming (horizontal direction etching)
H.-H. Kim et al. / Microelectronic Engineering 88 (2011) 2721–2724
rates for different pattern widths. The second is to fabricate tapered waveguide by the soft lithography. An inductively coupled plasma (ICP) was used to generate plasma, where a 13.56 MHz RF power is connected to planar spiral ICP antenna (ICP power) and another to substrate holder. In our experiment RF power to the substrate holder, which causes dc self-bias, is not used because any small RF power has caused too much anisotropic etching and too much vertical stripping of photoresist. In the conventional ashing down stream oxygen radicals are used for isotropic etching of photoresist [4]. In this experiment both ions and radicals are used to obtain line-width-dependent stripping. The substrate holder is water-cooled and its temperature is kept below 40 °C during ashing. For ashing condition optimization studies, positive photoresist, AZ9260, was spin-coated to 9 lm thick on silicon wafers and was exposed and developed to have line patterns with several different widths. Ashing is done by changing ICP RF power, chamber pressure and gas mixture ratio of oxygen and nitrogen. For fabrication of a tapered waveguide, 30 lm thick photoresist is prepared by multiple spin-coating of AZ9260 [8]. Photoresist pattern with wider input and narrower output terminals is patterned using a mask aligner, Karlsuss MJB4, as shown in Fig. 1a. The photoresist pattern is ashed by oxygen/nitrogen plasma as shown in Fig. 1b, where the wider part is thicker (vertically) and the narrower part is thinner. This pattern is the master for the soft lithography, and this master is used to make mold made of polydimethlysiloxane (PDMS) as shown Fig. 1c. PDMS layer was cured in an oven at 80 °C for 30 min. Final step of the soft lithography is to make replica into UV curing optical polymer material by imprinting as shown in Fig. 1d. The final structure of a tapered waveguide is shown schematically in Fig. 1e.
3. Results and discussions Photoresist ashing behavior can be changed by changing RF power to the coupling antenna of the inductively coupled plasma, chamber process pressure and gas mixture ratio of O2/N2. Ash rate was measured as the vertical ash rate (stripping rate) and horizontal ash rate (trimming rate). Purpose of this study is to investigate the stripping to trimming rate ratio (S/T ratio) for two different pattern widths, 5 and 10 lm, respectively. When ICP power was varied from 800 to 1200 W, the S/T ratio of 5 lm line was abruptly increased to 2.5 and then decreased to 1.3, but that of 10 lm line was not changed much as shown in Fig. 2a. When process pressure was increased as shown in Fig. 2b, the S/T ratio of both lines
4.0
Stripping/Trimming Ratio
2722
5 um pattern 10 um pattern
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 600
800
1000
1200
1400
Source power [W]
(a)
Stripping/Trimming Ratio
4.0 3.5
5 um pattern 10 um pattern
3.0 2.5 2.0 1.5 1.0 0.5 0.0
2.0
2.5
3.0
3.5
4.0
Chamber pressure [mTorr]
(b) Stripping/Trimming Ratio
4.0 3.5
5 um pattern 10 um pattern
3.0 2.5 2.0 1.5 1.0 0.5 0.0
0
20
40
60
80
100
O2 /(N2 +O2 ) [%]
(c) Fig. 1. Soft lithography process step: (a) patterning of photoresist by the conventional photolithography, (b) master preparation by plasma ashing, (c) PDMS mold fabrication, (d) imprinting with PDMS mold, and (e) final replica structure.
Fig. 2. Stripping to trimming rate ratio in terms of; (a) RF source power, (b) chamber pressure, and (c) O2/N2 mixture ratio.
H.-H. Kim et al. / Microelectronic Engineering 88 (2011) 2721–2724 Table 1 Ashing rates in terms of ICP RF source power. Source power (W)
Trimming rate (nm/min)
Stripping rate of 5 lm line (nm/min)
Stripping rate of 10 lm line (nm/min)
800 1000 1200
540 760 980
480 1850 1260
460 560 860
increased. In all cases of the S/T ratio of narrower line of 5 lm is higher than that of 10 lm line. Fig. 2c shows the dependence of the S/T ratios on the gas mixture ratio. For 5 lm lines, the highest S/T ratio can be found at 60% of oxygen, and for 10 lm lines the maximum is found at 20% of oxygen. It is desirable to have high S/T ratio in order to prepare as close as a square shape from
2723
5 lm lines. Table 1 lists the stripping rate and trimming rate of two lines of different width in terms of ICP power. Trimming rate is the same for both lines, but stripping rates are different from each other, and this is why the S/T ratios are different for different line width. We call this as line-width-dependent stripping rate in ashing reaction. Fig. 3 shows that after ashing different aspect ratio of photoresist lines can be obtained. Fig. 3a shows the definition of aspect ratio and Fig. 3b the cross section of original pattern with aspect ratio of 1.7. By changing ashing conditions, the aspect ratios of 1.2 and 3.0 can be obtained as shown Fig. 3c and d, respectively. Considering the different S/T ratio and aspect ratio resulted by ashing reaction, a tapered waveguide with 20(W) 25(H) lm2 input and 3(W) 5(H) lm2 output was designed and fabricated as shown in Fig. 4. Fig. 4a shows the master structure made of photoresist and Fig. 4b shows the final replica made of optical polymer
Fig. 3. Cross sectional views of photoresist lines: (a) definition of aspect ratio, (b) aspect ratio 1.7 of original pattern, (c) aspect ratio 1.2 of ashed line, and (d) aspect ratio 3.0 of ashed line.
Fig. 4. SEM images; (a) master structure of a tapered waveguide made of photoresist and (b) final replica made by the soft lithography.
2724
H.-H. Kim et al. / Microelectronic Engineering 88 (2011) 2721–2724
to the input port of the tapered waveguide through 50 lm diameter multi-mode optical fiber as shown in Fig. 5a. Light (bright spot in Fig. 5b) comes out from the output port of the 2 cm long tapered waveguide. 4. Conclusions Line-width-dependent stripping rate is applied to prepare a tapered waveguide master structure in photoresist. Plasma conditions are investigated to increase the ratio of stripping to trimming rates while trimming rate is independent on line width. From this master a PDMS mold is made and soft lithography is employed successfully to fabricate a tapered optical waveguide with 20 25 lm2 input to 3 5 lm2 output terminals. Acknowledgment This work has been supported by the Inha University through its Basic Research Promotion Program. References Fig. 5. (a) Top view of the fabricated tapered waveguide along with optical fiber, and (b) picture of a bright spot which is the light coming out from the waveguide output port.
resin on cladding layer. From this figures it is found that the replica has thinner lines than that of the master. This is due to shrinkage of the polymer resin and possible incomplete filling into smaller hollow pattern [9,10]. In order to measure the optical performance of the tapered waveguide, the top cladding layer was coated. The index of reflection of cladding layer is 1.45 and that of core part is 1.48. The tapered waveguide serves as the core and is sandwiched by top and bottom cladding layers. An 850 nm laser source is connected
[1] E.J. Teo, A.A. Bettiol, M.B.H. Breese, P. Yang, G.Z. Mashanovich, W.R. Headley, G.T. Reed, D.J. Blackwood, Opt. Express 16 (2008) 573–578. [2] I. Hardy, P. Grosso, D. Bosc, IEEE Photonics Technol. Lett. 17 (2005) 1028. [3] H. Spelthahn, A. Poghossian, M.J. Schöninga, Electrochim. Acta 54 (2009) 6010– 6014. [4] K. Inomata, H. Koinuma, Appl. Phys. Lett. 66 (1995) 2188–2190. [5] S.G. Park, S.K. Yang, J.K. Yang, J. Korean Phys. Soc. 49 (2006) S732–S735. [6] E.H. Lee, I.J. Chin, Y.K. Kwon, S.G. Lee, B.H. O, S.G. Park, K.H. Kim, Solid State Phenom. 124–126 (2007) 459–462. [7] H.-H. Kim, B.-H. O, S.-G. Lee, S.-G. Park, Microelectron. Eng. 87 (2010) 1033– 1036. [8] P. Dixit, C.W. Tan, L. Xu, N. Lin, J. Miao, J.H.L. Pang, P. Backus, R. Preisser, J. Micromech. Microeng. 17 (2007) 1078–1086. [9] M.H. Lee, M.D. Huntington, W. Zhou, J.C. Yang, T.W. Odom, Nano Lett., in press. [10] Y. Xia, G.M. Whitesides, Annu. Rev. Mater. Sci. 28 (1998) 153–184.