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Wafer focusing measurement of optical lithography system based on Hartmann–Shack wavefront testing
Optics and Lasers in Engineering 66 (2015) 128–131
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Wafer focusing measurement of optical lithography system based on Hartmann–Shack wavefront testing Xianchang Zhu n, Song Hu, Lixin Zhao State Key Laboratory of Optical Technologies for Microfabrication, Institute of Optics and Electronics, The Chinese Academy of Science, Chengdu, Sichuan 610209, China
art ic l e i nf o
a b s t r a c t
Article history: Received 4 September 2014 Accepted 4 September 2014
To improve the focusing measurement precision of wafer in optical lithography instrument (OLI), a method based on Hartmann–Shack (HS) testing principle is introduced. Defocus of wafer is immediately detected by measuring the image change between plane and spherical wavefront. As defocus is measured by every sub-lens of microlens array (MLA), serials of defocus position are calculated at single shot of CCD sensor. Choose the average in this measurement the outstanding advantage of this technology is the high accuracy and efficiency. With an experiment to validate the feasibility, the accuracy of focusing measurement is indicated as 20 nm. & 2014 Elsevier Ltd. All rights reserved.
keywords: Focusing Microlens array Lithography
1. Introduction
2. Methodology
In order to adjust wafer at the focal plane of projection lens, wafer focusing technique (WFT) [1,2] is a key technology in OLI [3]. Focusing precision of wafer affect the finally exposure effect [4]. Based on triangle method, defocus of wafer is detected by measuring the image remove of gap in CCD sensor [5] or position sensitive detector (PSD) [6] with micron precision in traditional OLI. In recent times, resolution and exposure field of view (FOV) of OLI is increased [7]. While on one hand, the improvement of resolution reduces the depth of focus of OLI; on the other hand, large exposure FOV needed higher focusing measurement precision, WFT have been of interest and investigated widely [8,9]. Diffractive grating [10] is applied for focusing testing based on Talbot-moiré effect [11,12] and interferometry-spatial-phase imaging [13], probe beam scanner [14,15] are also introduced. Although these WFT have nanometer precision, the measurement system is complex and the measurement efficiency for large FOV need be improved. In this paper, a simple WFT based on HS wavefront testing principle [16] is proposed. With a simple measurement system composed by MLA and CCD sensor, wafer position is immediately detected by measuring the image change between plane and spherical wavefront.
The focusing measurement system introduced in this paper is composed by light source, beam expander and collimate system, 4f-optical system, MLA and CCD sensor, shown in Fig. 1 Plane wavefront from the beam expander and collimator system was transformed by 4f optical system and imaged by MLA in CCD sensor. When wafer position at the co-focal plane of 4f optical system, the wavefront measured by MLA is exactly plane wavefront and diffractive light intensity distribute as Bessel function: 3 2 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 =2f 2J kd ð x nd Þ þ ð y md Þ 1 2 N M 6 7 2J 1 ðZ Þ 7 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi I ¼ ∑I 0 ¼ ∑ ∑ I0 6 4 5 Z 2 2 n¼1m¼1 kd ðx ndÞ þ ðy mdÞ =2f 2 ð1Þ In this equation f2 and d are the focal length and diameter of sub-lens in MLA, M and N are the row and column number of these sub-lenses. When wafer placed at a defocus position of projection lens, the wavefront measured by MLA is spherical wavefront and the defocus is calculated in Fig. 1 (
s1 ¼ 2h sin α
s2 ¼ 2h cos α
n
Corresponding author. E-mail address: [email protected] (X. Zhu).
http://dx.doi.org/10.1016/j.optlaseng.2014.09.001 0143-8166/& 2014 Elsevier Ltd. All rights reserved.
ð2Þ
Based on the triangle method, defocus of wafer h result in axisdefocus s1 and vertical axis-defocus s2. To analyze defocus of wafer, MLA and CCD position shown in Fig. 2
X. Zhu et al. / Optics and Lasers in Engineering 66 (2015) 128–131
129
Fig. 1. Illustration of focusing measurement system.
Fig. 2. Illumination of component position in measurement system.
Neglected the thickness of MLA Δf, the difference of image spot between plane and spherical wavefront is calculated:
Calculated by Newton equation in geometrical optics: 2
L¼
f1 δ 2f 2 s1
ð3Þ
In this equation, f1 and f2 is the focal length of 4f system; s1 is the axis-defocus; and δ is the distance between 4f system and MLA. And for the spherical wavefront measurement, every subwavefront subdivided by MLA is calculated as plane wavefront, which means: the curvature center of wavefront under testing O, center of sub-lens P in MLA and image spot center of this sub-lens in CCD sensor M are on the same line, showed in Fig. 3
ds d d ¼ ¼ L zþL f 2 þL
ð4Þ
While ds is the distance between image spot of spherical wavefront and light axis. Composited all these analysis, defocus of wafer is detected actual time: 2
h¼
N 1 M f1 ∑ ∑ MN m ¼ 1 n ¼ 1 2 ðdsmn f 2 =dmn dsmn Þ þ δ þ 2f 2 sin α
ð5Þ
130
X. Zhu et al. / Optics and Lasers in Engineering 66 (2015) 128–131
In this measurement system, every sub-lens has its own measurement result of wafer defocus, so serials of date are calculated at single shot of CCD sensor. Choose the average in this measurement, the testing precision and efficiency is improved.
Fig. 3. Plane and spherical wavefront measurement in HS sensor.
3. Experiment To validate the feasibility and indicate the measurement precision of this technology, an experiment is proceeding. In this system, a reflex lens is applied instead of the wafer, which is shown in Fig. 4. Reflex lens is controlled by piezo driver with the resolution is 10 nm and capacitive sensor is applied for reference of this measurement system. A MLA with focal length is 10 mm and diameter of sublens is 0.1 mm is also applied. Defocus of reflex lens make the wavefront under testing in MLA changed from plane to spherical wavefront and detected by CCD sensor, shown in Fig. 5. Be different from other focusing measurement, the key function of this system is measure the wafer position. After times of technical exposure experiment, the actual focal plane of projection lens is ascertained and focal plane may be changed by environment, mask position and so on, so wafer position need be measured and adjusted at every exposure process of OLI. To indicate the measurement precision of wafer position, reflex lens drive by piezo diver from nearby focus position in Fig. 5(a) to defocus position in Fig. 5(b) as 1 μm detected by capacitive sensor and the measurement result based on HS wavefront testing principle is showed in Table 1. Image of 20 sub-lenses (4 rows, 5 columns) are detected by CCD sensor at single shot. Analyzing these measurement results and
Fig. 4. Measurement system for WFT.
Fig. 5. Distribution of image spot detected by CCD sensor: (a) approximate plane wavefront nearby the focus position; (b) spherical wavefront at defocus position; and (c) defocus calculation by analyzing difference between plane and spherical wavefront.
Table 1 Measurement result contrasted with capacitive sensor. MLA (4 rows and 5 columns sub-lenses) Focusing measurement by MLA (nm)
995.5 992.6 996.7 1003.8
Capacitive sensor 1000.3 991.7 996.4 997.8
Statistics: average 998.6 nm; PV 16.7 nm; RMS: 4.3 nm
989.1 996.0 1000.4 1001.8
1003.3 1001.7 1003.4 1005.8
999.0 999.0 998.6 998.3
Reflex lens controlled by Piezo driver as 1 μm 1000 nm
X. Zhu et al. / Optics and Lasers in Engineering 66 (2015) 128–131
contrasting with capacitive sensor, the final measurement result of this technology is 998.6 712.9 nm and measurement precision precedes 20 nm.
4. Conclusion Totally, a technology based on HS wavefront testing principle is detailed introduced. With a 4f-optcial system and MLA, the measurement system is simply setup. In the experiment, the measurement precision and feasibility is demonstrated. The outstanding advantage of this technology is high accuracy and efficiency as serials of sub-lenses are imaged and defocus are calculated at single shot of CCD sensor.
References [1] Singer Jonathan P, Lee Jae-Hwang, Kooi Steven E, Thomas Edwin L. Rapid fabrication of 3D terahertz split ring resonator arrays by novel single-shot direct write focused proximity field nanopatterning. Opt Express 2012;20 (10):11097–108. [2] Chen X, Yao H, Chen X. Classical simulation of atomic beam focusing and deposition for atom lithography. Chin Opt Lett 2004;2:187–9. [3] Keskinbora Keskinbora, Corinne Grévent, Michael Bechtel, Markus Weigand, Eberhard Goering, Achim Nadzeyka, et al. Ion beam lithography for Fresnel zone plates in X-ray microscopy. Opt Express 2013;21(10):11747–56.
131
[4] Kazuo Tawarayama, Hajime Aoyama, Shunko Magoshi, Yuusuke Tanaka, Seiichiro Shirai, Hiroyuki Tanaka. Recent progress of EUV full-field exposure tool in Selete. Proc SPIE 2009;7271:727118. [5] Huang T, Liu S, Yi P, Shi T. Proc SPIE 2009;7160:71602X. [6] Dohoon Kim, Won-Ick Jang, Boo-Yeon Choi, Youngjik I. Lee, Jong-Hyun Lee, Hyung Joun Yoo, et al. Focusing and leveling system using position-sensitive detectors for the wafer steppers. Proc SPIE 1994;2197:997. [7] Rhee Hyug-Gyo, Lee Yun-Woo. Enhancement of height resolution in direct laser lithography. Opt Express 2012;20(1):291–8. [8] Williams W, Saffman M. Two-dimensional atomic lithography by submicrometer focusing of atomic beams. J Opt Soc Am B 2006;23:1161–9. [9] Unno Y. Modeling of a slit-scan-type aerial image measurement sensor used for optical lithography. Appl Opt 2011;50:271–81. [10] Hori Y, Sogawa F, Asakure H, Kato M, Serizawa H. Fabrication of a focusing grating mirror by electron beam lithography. Appl Opt 1990;29:2522–6. [11] Yan Wei, Yang Yong, Chen Wangfu, Hu Song, Zhou Shaolin. Moiré-based focusing and leveling scheme for optical projection lithography [J]. Appl Opt 2010;49(32):5959–63. [12] Jangping Zhu, Song Hu, Junsheng Yu, Yan Tang, Feng Xu, Yu He, et al. Influence of tilt moiré fringe on alignment accuracy in proximity lithography. Opt Laser Eng 2013;51(4):371–81. [13] Wataabe Masahiro, Oshida Yoshitada, Nakayama Yasuhiko, Yoshida Minoru, Funatsu Ryuichi, Fujii Akira, et al. Focusing and leveling based on wafer surface profile detection with interferometry for optical lithography. In: Proceeding of SPIE, vol. 2197, Optical/Laser Microlithography VII; May 17, 1994. p. 980. [14] Dong B, Zhang G, Yang G, Gu B, Zheng S, Li D, et al. Design and fabrication of a diffractive phase element for wavelength demultiplexing and spatial focusing simultaneously. Appl Opt 1996;35:6859–64. [15] Malacara Daniel. Optical shop testing. New York: A John Wiley & Sons, Inc., Publication; 2007 (Chapter 8). [16] Wu J-j, Chen Jia-bi, Xu An-cheng, Gao Xoao-yan, Zhuang Song-lin. Focal length measurement based on Hartmann-Shack principle. Opt Int J Light Electron 2011.
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Wafer focusing measurement of optical lithography system based on Hartmann–Shack wavefront testing Xianchang Zhu n, Song Hu, Lixin Zhao State Key Laboratory of Optical Technologies for Microfabrication, Institute of Optics and Electronics, The Chinese Academy of Science, Chengdu, Sichuan 610209, China
art ic l e i nf o
a b s t r a c t
Article history: Received 4 September 2014 Accepted 4 September 2014
To improve the focusing measurement precision of wafer in optical lithography instrument (OLI), a method based on Hartmann–Shack (HS) testing principle is introduced. Defocus of wafer is immediately detected by measuring the image change between plane and spherical wavefront. As defocus is measured by every sub-lens of microlens array (MLA), serials of defocus position are calculated at single shot of CCD sensor. Choose the average in this measurement the outstanding advantage of this technology is the high accuracy and efficiency. With an experiment to validate the feasibility, the accuracy of focusing measurement is indicated as 20 nm. & 2014 Elsevier Ltd. All rights reserved.
keywords: Focusing Microlens array Lithography
1. Introduction
2. Methodology
In order to adjust wafer at the focal plane of projection lens, wafer focusing technique (WFT) [1,2] is a key technology in OLI [3]. Focusing precision of wafer affect the finally exposure effect [4]. Based on triangle method, defocus of wafer is detected by measuring the image remove of gap in CCD sensor [5] or position sensitive detector (PSD) [6] with micron precision in traditional OLI. In recent times, resolution and exposure field of view (FOV) of OLI is increased [7]. While on one hand, the improvement of resolution reduces the depth of focus of OLI; on the other hand, large exposure FOV needed higher focusing measurement precision, WFT have been of interest and investigated widely [8,9]. Diffractive grating [10] is applied for focusing testing based on Talbot-moiré effect [11,12] and interferometry-spatial-phase imaging [13], probe beam scanner [14,15] are also introduced. Although these WFT have nanometer precision, the measurement system is complex and the measurement efficiency for large FOV need be improved. In this paper, a simple WFT based on HS wavefront testing principle [16] is proposed. With a simple measurement system composed by MLA and CCD sensor, wafer position is immediately detected by measuring the image change between plane and spherical wavefront.
The focusing measurement system introduced in this paper is composed by light source, beam expander and collimate system, 4f-optical system, MLA and CCD sensor, shown in Fig. 1 Plane wavefront from the beam expander and collimator system was transformed by 4f optical system and imaged by MLA in CCD sensor. When wafer position at the co-focal plane of 4f optical system, the wavefront measured by MLA is exactly plane wavefront and diffractive light intensity distribute as Bessel function: 3 2 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 =2f 2J kd ð x nd Þ þ ð y md Þ 1 2 N M 6 7 2J 1 ðZ Þ 7 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi I ¼ ∑I 0 ¼ ∑ ∑ I0 6 4 5 Z 2 2 n¼1m¼1 kd ðx ndÞ þ ðy mdÞ =2f 2 ð1Þ In this equation f2 and d are the focal length and diameter of sub-lens in MLA, M and N are the row and column number of these sub-lenses. When wafer placed at a defocus position of projection lens, the wavefront measured by MLA is spherical wavefront and the defocus is calculated in Fig. 1 (
s1 ¼ 2h sin α
s2 ¼ 2h cos α
n
Corresponding author. E-mail address: [email protected] (X. Zhu).
http://dx.doi.org/10.1016/j.optlaseng.2014.09.001 0143-8166/& 2014 Elsevier Ltd. All rights reserved.
ð2Þ
Based on the triangle method, defocus of wafer h result in axisdefocus s1 and vertical axis-defocus s2. To analyze defocus of wafer, MLA and CCD position shown in Fig. 2
X. Zhu et al. / Optics and Lasers in Engineering 66 (2015) 128–131
129
Fig. 1. Illustration of focusing measurement system.
Fig. 2. Illumination of component position in measurement system.
Neglected the thickness of MLA Δf, the difference of image spot between plane and spherical wavefront is calculated:
Calculated by Newton equation in geometrical optics: 2
L¼
f1 δ 2f 2 s1
ð3Þ
In this equation, f1 and f2 is the focal length of 4f system; s1 is the axis-defocus; and δ is the distance between 4f system and MLA. And for the spherical wavefront measurement, every subwavefront subdivided by MLA is calculated as plane wavefront, which means: the curvature center of wavefront under testing O, center of sub-lens P in MLA and image spot center of this sub-lens in CCD sensor M are on the same line, showed in Fig. 3
ds d d ¼ ¼ L zþL f 2 þL
ð4Þ
While ds is the distance between image spot of spherical wavefront and light axis. Composited all these analysis, defocus of wafer is detected actual time: 2
h¼
N 1 M f1 ∑ ∑ MN m ¼ 1 n ¼ 1 2 ðdsmn f 2 =dmn dsmn Þ þ δ þ 2f 2 sin α
ð5Þ
130
X. Zhu et al. / Optics and Lasers in Engineering 66 (2015) 128–131
In this measurement system, every sub-lens has its own measurement result of wafer defocus, so serials of date are calculated at single shot of CCD sensor. Choose the average in this measurement, the testing precision and efficiency is improved.
Fig. 3. Plane and spherical wavefront measurement in HS sensor.
3. Experiment To validate the feasibility and indicate the measurement precision of this technology, an experiment is proceeding. In this system, a reflex lens is applied instead of the wafer, which is shown in Fig. 4. Reflex lens is controlled by piezo driver with the resolution is 10 nm and capacitive sensor is applied for reference of this measurement system. A MLA with focal length is 10 mm and diameter of sublens is 0.1 mm is also applied. Defocus of reflex lens make the wavefront under testing in MLA changed from plane to spherical wavefront and detected by CCD sensor, shown in Fig. 5. Be different from other focusing measurement, the key function of this system is measure the wafer position. After times of technical exposure experiment, the actual focal plane of projection lens is ascertained and focal plane may be changed by environment, mask position and so on, so wafer position need be measured and adjusted at every exposure process of OLI. To indicate the measurement precision of wafer position, reflex lens drive by piezo diver from nearby focus position in Fig. 5(a) to defocus position in Fig. 5(b) as 1 μm detected by capacitive sensor and the measurement result based on HS wavefront testing principle is showed in Table 1. Image of 20 sub-lenses (4 rows, 5 columns) are detected by CCD sensor at single shot. Analyzing these measurement results and
Fig. 4. Measurement system for WFT.
Fig. 5. Distribution of image spot detected by CCD sensor: (a) approximate plane wavefront nearby the focus position; (b) spherical wavefront at defocus position; and (c) defocus calculation by analyzing difference between plane and spherical wavefront.
Table 1 Measurement result contrasted with capacitive sensor. MLA (4 rows and 5 columns sub-lenses) Focusing measurement by MLA (nm)
995.5 992.6 996.7 1003.8
Capacitive sensor 1000.3 991.7 996.4 997.8
Statistics: average 998.6 nm; PV 16.7 nm; RMS: 4.3 nm
989.1 996.0 1000.4 1001.8
1003.3 1001.7 1003.4 1005.8
999.0 999.0 998.6 998.3
Reflex lens controlled by Piezo driver as 1 μm 1000 nm
X. Zhu et al. / Optics and Lasers in Engineering 66 (2015) 128–131
contrasting with capacitive sensor, the final measurement result of this technology is 998.6 712.9 nm and measurement precision precedes 20 nm.
4. Conclusion Totally, a technology based on HS wavefront testing principle is detailed introduced. With a 4f-optcial system and MLA, the measurement system is simply setup. In the experiment, the measurement precision and feasibility is demonstrated. The outstanding advantage of this technology is high accuracy and efficiency as serials of sub-lenses are imaged and defocus are calculated at single shot of CCD sensor.
References [1] Singer Jonathan P, Lee Jae-Hwang, Kooi Steven E, Thomas Edwin L. Rapid fabrication of 3D terahertz split ring resonator arrays by novel single-shot direct write focused proximity field nanopatterning. Opt Express 2012;20 (10):11097–108. [2] Chen X, Yao H, Chen X. Classical simulation of atomic beam focusing and deposition for atom lithography. Chin Opt Lett 2004;2:187–9. [3] Keskinbora Keskinbora, Corinne Grévent, Michael Bechtel, Markus Weigand, Eberhard Goering, Achim Nadzeyka, et al. Ion beam lithography for Fresnel zone plates in X-ray microscopy. Opt Express 2013;21(10):11747–56.
131
[4] Kazuo Tawarayama, Hajime Aoyama, Shunko Magoshi, Yuusuke Tanaka, Seiichiro Shirai, Hiroyuki Tanaka. Recent progress of EUV full-field exposure tool in Selete. Proc SPIE 2009;7271:727118. [5] Huang T, Liu S, Yi P, Shi T. Proc SPIE 2009;7160:71602X. [6] Dohoon Kim, Won-Ick Jang, Boo-Yeon Choi, Youngjik I. Lee, Jong-Hyun Lee, Hyung Joun Yoo, et al. Focusing and leveling system using position-sensitive detectors for the wafer steppers. Proc SPIE 1994;2197:997. [7] Rhee Hyug-Gyo, Lee Yun-Woo. Enhancement of height resolution in direct laser lithography. Opt Express 2012;20(1):291–8. [8] Williams W, Saffman M. Two-dimensional atomic lithography by submicrometer focusing of atomic beams. J Opt Soc Am B 2006;23:1161–9. [9] Unno Y. Modeling of a slit-scan-type aerial image measurement sensor used for optical lithography. Appl Opt 2011;50:271–81. [10] Hori Y, Sogawa F, Asakure H, Kato M, Serizawa H. Fabrication of a focusing grating mirror by electron beam lithography. Appl Opt 1990;29:2522–6. [11] Yan Wei, Yang Yong, Chen Wangfu, Hu Song, Zhou Shaolin. Moiré-based focusing and leveling scheme for optical projection lithography [J]. Appl Opt 2010;49(32):5959–63. [12] Jangping Zhu, Song Hu, Junsheng Yu, Yan Tang, Feng Xu, Yu He, et al. Influence of tilt moiré fringe on alignment accuracy in proximity lithography. Opt Laser Eng 2013;51(4):371–81. [13] Wataabe Masahiro, Oshida Yoshitada, Nakayama Yasuhiko, Yoshida Minoru, Funatsu Ryuichi, Fujii Akira, et al. Focusing and leveling based on wafer surface profile detection with interferometry for optical lithography. In: Proceeding of SPIE, vol. 2197, Optical/Laser Microlithography VII; May 17, 1994. p. 980. [14] Dong B, Zhang G, Yang G, Gu B, Zheng S, Li D, et al. Design and fabrication of a diffractive phase element for wavelength demultiplexing and spatial focusing simultaneously. Appl Opt 1996;35:6859–64. [15] Malacara Daniel. Optical shop testing. New York: A John Wiley & Sons, Inc., Publication; 2007 (Chapter 8). [16] Wu J-j, Chen Jia-bi, Xu An-cheng, Gao Xoao-yan, Zhuang Song-lin. Focal length measurement based on Hartmann-Shack principle. Opt Int J Light Electron 2011.