Note: Buoyant-force assisted liquid membrane electrochemical etching for nano-tip preparation Yongbin Zeng, Yufeng Wang, Xiujuan Wu, Kun Xu, and Ningsong Qu Citation: Review of Scientific Instruments 85, 126105 (2014); doi: 10.1063/1.4903870 View online: http://dx.doi.org/10.1063/1.4903870 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Note: Advancement in tip etching for preparation of tunable size scanning tunneling microscopy tips Rev. Sci. Instrum. 86, 026104 (2015); 10.1063/1.4907706 Uniform nano-ripples on the sidewall of silicon carbide micro-hole fabricated by femtosecond laser irradiation and acid etching Appl. Phys. Lett. 104, 241907 (2014); 10.1063/1.4883880 Electrochemical etching technique: Conical-long-sharp tungsten tips for nanoapplications J. Vac. Sci. Technol. B 32, 031806 (2014); 10.1116/1.4873700 Note: Axially pull-up electrochemical etching method for fabricating tungsten nanoprobes with controllable aspect ratio Rev. Sci. Instrum. 83, 106109 (2012); 10.1063/1.4764580 The art of electrochemical etching for preparing tungsten probes with controllable tip profile and characteristic parameters Rev. Sci. Instrum. 82, 013707 (2011); 10.1063/1.3529880
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 126105 (2014)
Note: Buoyant-force assisted liquid membrane electrochemical etching for nano-tip preparation Yongbin Zeng, Yufeng Wang, Xiujuan Wu, Kun Xu, and Ningsong Qua) Jiangsu Key Laboratory of Precision and Micro-Manufacturing Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, People’s Republic of China
(Received 16 July 2014; accepted 29 November 2014; published online 15 December 2014) A liquid membrane electrochemical etching process for preparing nano-tips is proposed by the introduction of buoyant force to the lower tip, in which the lower portion of the anodic wire is immersed into a floating layer. A mathematical model of this method is derived. Both calculation and experimental results demonstrate that the introduction of buoyant force can significantly decrease the tip radius. The lubricating oil and deionized water floating layers were tested for the processing of nano-tips. Further, high-aspect-ratio nano-electrodes were prepared by applying a relative vertical movement to the anodic wire. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4903870] Nano-tips are critical components in diverse fields and applications such as scanning probe microscopy (SPM),1, 2 field emitters,3 nanomanipulators,4 nano-lithography,5 micromachining,6 and cellular studies.7 Many investigations focusing on the preparation of very sharp tips have been conducted recently, including the use of micro-grinding, cutting, chemical or electrochemical etching, focused ionbeam machining, and semiconductor micromachining-based methods.8 Electrochemical etching is more widely preferred for the processing of nano-tips because of its ease of use, reproducibility, and ability to provide high-quality nano-tips at low cost. Microelectrodes have been fabricated by an electrochemical etching in which the anode wire is immersed in the electrolyte.9–13 However, the so called immersion method is affected by many factors and is difficult to prepare nanotips. Another reliable approach to the processing of nano-tips, called drop-off method, is more reliable and easier to control. With the drop-off method, etching occurs at the interface between air and the electrolyte, causing the portion of the anode immersed in the solution to drop off when its weight exceeds the tensile strength of the etched neck.4 Ibe et al. examined the influences of meniscus shape, length of wire immersed in the solution and etching potential on the shape and sharpness of the wire above the air and electrolyte interface and were able to process nano-tips with a curvature of 20 nm.14 Cavallini et al.15 and Kim et al.16 employed a ring as the cathode and a thin film of solution as the electrolyte to etch nickel nanotips. It can thus be seen that the drop-off method is widely accepted and has been extensively studied for the fabrication of nano-tips. However, consideration needs to be given to the fact that the very thin film of electrolyte could easily be affected by the bubbles generated during the process and by the shape of the meniscus.14 In addition, the drop-off method still requires further investigation with regard to the fabrication of nano-tips with smaller scales, and, so far, there has been little examination of the exact details of the forming process. a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0034-6748/2014/85(12)/126105/3/$30.00
In the work, the buoyant force provided by a floating layer is introduced to the liquid membrane electrochemical etching process in which the lower end of the anodic wire is immersed into a floating layer so that it is subjected to an upthrust. In the proposed method, as illustrated in Figure 1(b), compared with the conventional electrochemical etching drop-off method, a drop of potassium hydroxide solution rather than a thin film of aqueous solution is used as the electrolyte, to avoid the variations in meniscus shape and reduce the vibration of the anodic wire caused by bubbles generated during the etching process. The lower portion of the anodic wire is immersed in a floating layer contained in a cup-shaped electrode receiver (in the conventional drop-off method, the lower end is suspended in air). A polycrystalline tungsten wire with a diameter of 0.2 mm is used as the anode and is threaded at the center of a tungsten ring of about 8 mm diameter that acts as the cathode. During the liquid membrane electrochemical etching, the thickness of the liquid membrane is set as 5 mm. Upon the application of a DC power supply, electrochemical reactions occur and the anodic material is removed in ionic form. As shown in Figure 2, a liquid meniscus is formed around the wire surface at the air/electrolyte interface owing to the liquid surface tension and this meniscus determines the shape of the upper tip.14 In the etching process, a region of necking with a conical angle forms owing to the variation in diffusion layer thickness along the anode axis.10 During electrochemical etching, the diameter of the necking region decreases. Finally, the lower part drops off owing to tensile failure. The electrochemical etching of the lower tip stops immediately when it drops off. Meanwhile, the upper tip is still immersed in electrolyte and thus becomes blunted. Henceforth, this study will mainly focus on the lower tip. The maximum current density along the anodic wire surface occurs at the electrolyte/air interface.17 Thus, the anodic wire divides into two parts, namely, the upper tip above the interface and the lower tip below the interface. It is obvious that the radius of the etched anodic wire at the electrolyte/air interface decreases and thus the tensile stress increases with etching time. The anodic wire is divided into two parts
85, 126105-1
© 2014 AIP Publishing LLC
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126105-2
Zeng et al.
Rev. Sci. Instrum. 85, 126105 (2014)
FIG. 3. Variation of tip radius with floating layer thickness. FIG. 1. Experimental principle for nano-tip preparation using (a) the conventional and (b) proposed electrochemical etching drop-off method.
owing to tensile failure, and the minimum divided radius of the tungsten is G − FR − FB rmin = , (1) π σW where σ W is the ultimate stress of the tungsten wire. The forces exerted on the lower tip include the gravity (G), the surface tension of the liquid drop (FR ), and the buoyancy generated by the floating layer (FB ), which can be expressed as 2 d r0 − rmin 2 z + rmin dz + r0 (Ll − d) , G = ρW π d 0 (2) FR =
2π γ r02 , R0
FB = πρf gr02 Hi ,
(3) (4)
where ρ f is the density of the floating layer, γ is the coefficient of liquid surface tension, R0 is the radius of the liquid drop, d is the liquid membrane thickness, Ll is the length of the lower tip, and Hi is the height of the lower tip that is immersed in the floating layer.
FIG. 2. Principle of liquid membrane electrochemical etching assisted with buoyant force.
The buoyancy provided by the floating layer is proportional to the length of the lower tip that immersed in the floating layer. Experiments have been conducted with immersed lengths ranging from 5 to 20 mm. Figure 3 shows a comparison between the mathematical and experimental results. The specific parameters are as follows: the surface tension coefficient of water and oil γ ≈ 0.1 N/m, initial radius of tungsten wire r0 = 0.2 mm, density of W wire ρ w = 19.3 g/m3 , thickness of liquid membrane d = 4 mm, radius of liquid drop R0 = 4 mm, and the ultimate stress of the tungsten wire σ W = 2.069 × 103 MPa. The differences between these results are probably caused by variations in the immersed length due to measurement error. It is verified that the tip radius decreases with increasing immersed length under the experimental conditions of a lower part length of 25 mm, an electrolyte concentration of 1 M, and an applied voltage of 4 V. The maximum depth of the floating layer could not exceed 20 mm in case that it would touch the liquid drop. As shown in Figure 4, the tip radius increases with increasing length of the lower end, while the length immersed in the floating layer is held constant. As the weight of the lower tip increases with increasing length, the lower tip drops off at a larger radius. As shown by Eq. (4), the buoyancy due to the floating layer is proportional to the density of the floating layer. In addition to lubricating oil (density 0.8 g/cm3 , viscosity 1 × 10−3 Pa s), water (density 1 g/cm3 , viscosity 1.5 × 10−3 Pa s) was also used in the experiments. As shown in Figure 5, the tips that were fabricated using a deionized water floating layer
FIG. 4. Variation of tip radius with lower end length.
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Rev. Sci. Instrum. 85, 126105 (2014)
FIG. 5. Variation of tip radius with applied voltage.
were smaller than those fabricated using oil. This is because the upthrust generated by deionized water is larger owing to its greater density. For instance, the tip radius of nano-tips fabricated with a water floating layer was 19 nm, compared with 24 nm for an oil floating layer, under an applied voltage of 3 V, a 1 M electrolyte solution, and an immersed depth of 8 mm. With the length of the lower tip of 20 mm, immersed depth of 8 mm in the lubricating oil, applied voltage of 4 V and electrolyte concentration of 1 M, a tip radius of 15 nm was obtained with buoyant force assistance, compared with 39 nm without buoyant force assistance (a decrease in tip radius by 160%), as shown in Figure 6. The results show that
FIG. 7. Nano-electrodes with an average radius of about 55 nm, an aspect ratio greater than 40, and a tip radius of 29 nm.
the introduction of buoyant force can significantly decrease the tip radius compared with etching under the same experimental conditions but in the absence of buoyant force. To fabricate high-aspect-ratio nano-electrodes, a relative vertical movement (a straight reciprocating motion) was also applied to the anodic wire during liquid membrane electrochemical etching. It was thus possible to obtain a nanoelectrode with a high aspect ratio. As illustrated by the example in Figure 7, nano-electrodes with an average radius of about 55 nm, an aspect ratio greater than 40, and a tip radius of 29 nm could be fabricated, with an applied voltage of 3 V, an electrolyte concentration of 1 M, and relative movement of amplitude 0.4 mm and velocity 1 μm/s. This work was conducted under the sponsorship of the National Natural Science Foundation of China (51375238), the Jiang Su Natural Science Foundation (BK20131361), and Jiangsu Innovation Program for Graduate Education (CXZZ13_0153). 1 C.
FIG. 6. SEM images of lower tips: (a) without buoyant force assistance and (b) with buoyant force assistance.
Albonetti, I. Bergenti, M. Cavallini, V. Dediu, M. Massi, J. F. Moulin, and F. Biscarini, Rev. Sci. Instrum. 73, 4254 (2002). 2 C. W. Lee, H. J. Kang, C. C. Chung, and W. Jin, Microsyst. Technol. 15, 1663 (2009). 3 M. Pichaud, A. Muller, and M. Drechsler, Surf. Sci. 26, 14 (1971). 4 R. Hobara, N. Nagamura, S. Hasegawa, I. Matsuda, Y. Yamamoto, M. Miyatake, and T. Nagamura, Rev. Sci. Instrum. 78, 053705 (2007). 5 Y. F. Lu, Z. H. Mai, G. Qiu, and W. K. Chim, Appl. Phys. Lett. 75, 2359 (1999). 6 M. Kock, V. Kirchner, and R. Schuster, Electrochem. Acta 48, 3213 (2003). 7 A. Hermans and R. M. Wightman, Langmuir 22, 10348 (2006). 8 R. Hobara, S. Yoshimoto, and S. Hasegawa, J. Surf. Sci. Nanotechnol. 5, 94 (2007). 9 Y. M. Lim, H. J. Lim, J. R. Liu, and S. H. Kim, J. Mater. Process. Technol. 141, 251 (2003). 10 Y. M. Lim and S. H. Kim, Int. J. Mach. Tools Manuf. 41, 2287 (2001). 11 Z. W. Fan, L. W. Hourng, and C. Y. Wang, Precis. Eng. 34, 489 (2010). 12 S. H. Choi, S. H. Ryu, D. K. Choi, and C. N. Chu, Int. J. Adv. Manuf. Technol. 31, 682 (2007). 13 B. Ghoshal and B. Bhattacharyya, Int. J. Mach. Tools Manuf. 64, 49 (2013). 14 J. P. Ibe, P. P. Bey, Jr., S. L. Brandow, R. A. Brizzolara, N. A. Burnham, D. P. DiLella, K. P. Lee, C. R. K. Marrian, and R. J. Colton, J. Vac. Sci. Technol. A8, 3570 (1990). 15 M. Cavallini and F. Biscarini, Rev. Sci. Instrum. 71, 4457 (2000). 16 K. B. Kim and J. Choi, J. Korean Phys. Soc. 55, 2427 (2009). 17 X. J. Wu, N. S. Qu, Y. B. Zeng, and D. Zhu, Int. J. Adv. Manuf. Technol. 69, 723 (2013).
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.179.16.201 On: Mon, 16 Feb 2015 18:06:24
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.179.16.201 On: Mon, 16 Feb 2015 18:06:24
REVIEW OF SCIENTIFIC INSTRUMENTS 85, 126105 (2014)
Note: Buoyant-force assisted liquid membrane electrochemical etching for nano-tip preparation Yongbin Zeng, Yufeng Wang, Xiujuan Wu, Kun Xu, and Ningsong Qua) Jiangsu Key Laboratory of Precision and Micro-Manufacturing Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, People’s Republic of China
(Received 16 July 2014; accepted 29 November 2014; published online 15 December 2014) A liquid membrane electrochemical etching process for preparing nano-tips is proposed by the introduction of buoyant force to the lower tip, in which the lower portion of the anodic wire is immersed into a floating layer. A mathematical model of this method is derived. Both calculation and experimental results demonstrate that the introduction of buoyant force can significantly decrease the tip radius. The lubricating oil and deionized water floating layers were tested for the processing of nano-tips. Further, high-aspect-ratio nano-electrodes were prepared by applying a relative vertical movement to the anodic wire. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4903870] Nano-tips are critical components in diverse fields and applications such as scanning probe microscopy (SPM),1, 2 field emitters,3 nanomanipulators,4 nano-lithography,5 micromachining,6 and cellular studies.7 Many investigations focusing on the preparation of very sharp tips have been conducted recently, including the use of micro-grinding, cutting, chemical or electrochemical etching, focused ionbeam machining, and semiconductor micromachining-based methods.8 Electrochemical etching is more widely preferred for the processing of nano-tips because of its ease of use, reproducibility, and ability to provide high-quality nano-tips at low cost. Microelectrodes have been fabricated by an electrochemical etching in which the anode wire is immersed in the electrolyte.9–13 However, the so called immersion method is affected by many factors and is difficult to prepare nanotips. Another reliable approach to the processing of nano-tips, called drop-off method, is more reliable and easier to control. With the drop-off method, etching occurs at the interface between air and the electrolyte, causing the portion of the anode immersed in the solution to drop off when its weight exceeds the tensile strength of the etched neck.4 Ibe et al. examined the influences of meniscus shape, length of wire immersed in the solution and etching potential on the shape and sharpness of the wire above the air and electrolyte interface and were able to process nano-tips with a curvature of 20 nm.14 Cavallini et al.15 and Kim et al.16 employed a ring as the cathode and a thin film of solution as the electrolyte to etch nickel nanotips. It can thus be seen that the drop-off method is widely accepted and has been extensively studied for the fabrication of nano-tips. However, consideration needs to be given to the fact that the very thin film of electrolyte could easily be affected by the bubbles generated during the process and by the shape of the meniscus.14 In addition, the drop-off method still requires further investigation with regard to the fabrication of nano-tips with smaller scales, and, so far, there has been little examination of the exact details of the forming process. a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0034-6748/2014/85(12)/126105/3/$30.00
In the work, the buoyant force provided by a floating layer is introduced to the liquid membrane electrochemical etching process in which the lower end of the anodic wire is immersed into a floating layer so that it is subjected to an upthrust. In the proposed method, as illustrated in Figure 1(b), compared with the conventional electrochemical etching drop-off method, a drop of potassium hydroxide solution rather than a thin film of aqueous solution is used as the electrolyte, to avoid the variations in meniscus shape and reduce the vibration of the anodic wire caused by bubbles generated during the etching process. The lower portion of the anodic wire is immersed in a floating layer contained in a cup-shaped electrode receiver (in the conventional drop-off method, the lower end is suspended in air). A polycrystalline tungsten wire with a diameter of 0.2 mm is used as the anode and is threaded at the center of a tungsten ring of about 8 mm diameter that acts as the cathode. During the liquid membrane electrochemical etching, the thickness of the liquid membrane is set as 5 mm. Upon the application of a DC power supply, electrochemical reactions occur and the anodic material is removed in ionic form. As shown in Figure 2, a liquid meniscus is formed around the wire surface at the air/electrolyte interface owing to the liquid surface tension and this meniscus determines the shape of the upper tip.14 In the etching process, a region of necking with a conical angle forms owing to the variation in diffusion layer thickness along the anode axis.10 During electrochemical etching, the diameter of the necking region decreases. Finally, the lower part drops off owing to tensile failure. The electrochemical etching of the lower tip stops immediately when it drops off. Meanwhile, the upper tip is still immersed in electrolyte and thus becomes blunted. Henceforth, this study will mainly focus on the lower tip. The maximum current density along the anodic wire surface occurs at the electrolyte/air interface.17 Thus, the anodic wire divides into two parts, namely, the upper tip above the interface and the lower tip below the interface. It is obvious that the radius of the etched anodic wire at the electrolyte/air interface decreases and thus the tensile stress increases with etching time. The anodic wire is divided into two parts
85, 126105-1
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126105-2
Zeng et al.
Rev. Sci. Instrum. 85, 126105 (2014)
FIG. 3. Variation of tip radius with floating layer thickness. FIG. 1. Experimental principle for nano-tip preparation using (a) the conventional and (b) proposed electrochemical etching drop-off method.
owing to tensile failure, and the minimum divided radius of the tungsten is G − FR − FB rmin = , (1) π σW where σ W is the ultimate stress of the tungsten wire. The forces exerted on the lower tip include the gravity (G), the surface tension of the liquid drop (FR ), and the buoyancy generated by the floating layer (FB ), which can be expressed as 2 d r0 − rmin 2 z + rmin dz + r0 (Ll − d) , G = ρW π d 0 (2) FR =
2π γ r02 , R0
FB = πρf gr02 Hi ,
(3) (4)
where ρ f is the density of the floating layer, γ is the coefficient of liquid surface tension, R0 is the radius of the liquid drop, d is the liquid membrane thickness, Ll is the length of the lower tip, and Hi is the height of the lower tip that is immersed in the floating layer.
FIG. 2. Principle of liquid membrane electrochemical etching assisted with buoyant force.
The buoyancy provided by the floating layer is proportional to the length of the lower tip that immersed in the floating layer. Experiments have been conducted with immersed lengths ranging from 5 to 20 mm. Figure 3 shows a comparison between the mathematical and experimental results. The specific parameters are as follows: the surface tension coefficient of water and oil γ ≈ 0.1 N/m, initial radius of tungsten wire r0 = 0.2 mm, density of W wire ρ w = 19.3 g/m3 , thickness of liquid membrane d = 4 mm, radius of liquid drop R0 = 4 mm, and the ultimate stress of the tungsten wire σ W = 2.069 × 103 MPa. The differences between these results are probably caused by variations in the immersed length due to measurement error. It is verified that the tip radius decreases with increasing immersed length under the experimental conditions of a lower part length of 25 mm, an electrolyte concentration of 1 M, and an applied voltage of 4 V. The maximum depth of the floating layer could not exceed 20 mm in case that it would touch the liquid drop. As shown in Figure 4, the tip radius increases with increasing length of the lower end, while the length immersed in the floating layer is held constant. As the weight of the lower tip increases with increasing length, the lower tip drops off at a larger radius. As shown by Eq. (4), the buoyancy due to the floating layer is proportional to the density of the floating layer. In addition to lubricating oil (density 0.8 g/cm3 , viscosity 1 × 10−3 Pa s), water (density 1 g/cm3 , viscosity 1.5 × 10−3 Pa s) was also used in the experiments. As shown in Figure 5, the tips that were fabricated using a deionized water floating layer
FIG. 4. Variation of tip radius with lower end length.
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126105-3
Zeng et al.
Rev. Sci. Instrum. 85, 126105 (2014)
FIG. 5. Variation of tip radius with applied voltage.
were smaller than those fabricated using oil. This is because the upthrust generated by deionized water is larger owing to its greater density. For instance, the tip radius of nano-tips fabricated with a water floating layer was 19 nm, compared with 24 nm for an oil floating layer, under an applied voltage of 3 V, a 1 M electrolyte solution, and an immersed depth of 8 mm. With the length of the lower tip of 20 mm, immersed depth of 8 mm in the lubricating oil, applied voltage of 4 V and electrolyte concentration of 1 M, a tip radius of 15 nm was obtained with buoyant force assistance, compared with 39 nm without buoyant force assistance (a decrease in tip radius by 160%), as shown in Figure 6. The results show that
FIG. 7. Nano-electrodes with an average radius of about 55 nm, an aspect ratio greater than 40, and a tip radius of 29 nm.
the introduction of buoyant force can significantly decrease the tip radius compared with etching under the same experimental conditions but in the absence of buoyant force. To fabricate high-aspect-ratio nano-electrodes, a relative vertical movement (a straight reciprocating motion) was also applied to the anodic wire during liquid membrane electrochemical etching. It was thus possible to obtain a nanoelectrode with a high aspect ratio. As illustrated by the example in Figure 7, nano-electrodes with an average radius of about 55 nm, an aspect ratio greater than 40, and a tip radius of 29 nm could be fabricated, with an applied voltage of 3 V, an electrolyte concentration of 1 M, and relative movement of amplitude 0.4 mm and velocity 1 μm/s. This work was conducted under the sponsorship of the National Natural Science Foundation of China (51375238), the Jiang Su Natural Science Foundation (BK20131361), and Jiangsu Innovation Program for Graduate Education (CXZZ13_0153). 1 C.
FIG. 6. SEM images of lower tips: (a) without buoyant force assistance and (b) with buoyant force assistance.
Albonetti, I. Bergenti, M. Cavallini, V. Dediu, M. Massi, J. F. Moulin, and F. Biscarini, Rev. Sci. Instrum. 73, 4254 (2002). 2 C. W. Lee, H. J. Kang, C. C. Chung, and W. Jin, Microsyst. Technol. 15, 1663 (2009). 3 M. Pichaud, A. Muller, and M. Drechsler, Surf. Sci. 26, 14 (1971). 4 R. Hobara, N. Nagamura, S. Hasegawa, I. Matsuda, Y. Yamamoto, M. Miyatake, and T. Nagamura, Rev. Sci. Instrum. 78, 053705 (2007). 5 Y. F. Lu, Z. H. Mai, G. Qiu, and W. K. Chim, Appl. Phys. Lett. 75, 2359 (1999). 6 M. Kock, V. Kirchner, and R. Schuster, Electrochem. Acta 48, 3213 (2003). 7 A. Hermans and R. M. Wightman, Langmuir 22, 10348 (2006). 8 R. Hobara, S. Yoshimoto, and S. Hasegawa, J. Surf. Sci. Nanotechnol. 5, 94 (2007). 9 Y. M. Lim, H. J. Lim, J. R. Liu, and S. H. Kim, J. Mater. Process. Technol. 141, 251 (2003). 10 Y. M. Lim and S. H. Kim, Int. J. Mach. Tools Manuf. 41, 2287 (2001). 11 Z. W. Fan, L. W. Hourng, and C. Y. Wang, Precis. Eng. 34, 489 (2010). 12 S. H. Choi, S. H. Ryu, D. K. Choi, and C. N. Chu, Int. J. Adv. Manuf. Technol. 31, 682 (2007). 13 B. Ghoshal and B. Bhattacharyya, Int. J. Mach. Tools Manuf. 64, 49 (2013). 14 J. P. Ibe, P. P. Bey, Jr., S. L. Brandow, R. A. Brizzolara, N. A. Burnham, D. P. DiLella, K. P. Lee, C. R. K. Marrian, and R. J. Colton, J. Vac. Sci. Technol. A8, 3570 (1990). 15 M. Cavallini and F. Biscarini, Rev. Sci. Instrum. 71, 4457 (2000). 16 K. B. Kim and J. Choi, J. Korean Phys. Soc. 55, 2427 (2009). 17 X. J. Wu, N. S. Qu, Y. B. Zeng, and D. Zhu, Int. J. Adv. Manuf. Technol. 69, 723 (2013).
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