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Miniature electrostatic column for a compact scanning electron microscope
Microelectronic Engineering 61–62 (2002) 317–321 www.elsevier.com / locate / mee
Miniature electrostatic column for a compact scanning electron microscope T. Ambe*, H. Tanaka, H. Teguri, I. Honjo 1 , A. Ito Fujitsu Laboratories Ltd., 10 -1 Morinosato-Wakamiya, Atsugi 243 -0197, Japan
Abstract We designed and fabricated a low-aberration miniature column with electrostatic lenses. It includes two octupole aligners for optical adjustment, and two objective lenses, one for main focusing and the other for low-magnification observation. The optical characteristics of the objective lenses were calculated with Munro’s program. We obtained values of 2.9 mm for the chromatic aberration coefficient and 14 mm for the spherical aberration coefficient, at a working distance of 2 mm. We also developed a compact concentric sputter ion pump with a TFE e-gun. The pumping speed is 6 l / sec, and the end pressure is 10 27 Pa. By using this miniature column and the TFE e-gun assembly, we developed a compact scanning electron microscope. We achieved 10-nm resolution at 1.3-keV landing energy. 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrostatic; Miniature column; Low aberration; Concentric sputter ion pump
1. Introduction Electron beam systems are powerful tools for semiconductor manufacture. Low-energy electrons are especially attractive for a variety of applications, such as inspection, testing, and lithography, because of the reduced extent of penetration into the sample, the reduced damage and heating of the sample, and the ability to irradiate insulating samples without surface charging [1]. The scaled aberration coefficients of miniature columns are particularly suitable for generating higher resolution beams of low-energy electrons [2]. We designed and fabricated a low-aberration miniature column to achieve higher resolution beams with low-energy electrons. 2. Low-aberration miniature column Fig. 1 shows a photograph and a diagram of the miniature column we designed. It is about 74 mm * Corresponding author. Tel.: 1 81-46-250-8226; fax: 1 81-46-250-8842. E-mail address: [email protected] (T. Ambe). 1 Present address: KLA-Tencor Japan Ltd. 0167-9317 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0167-9317( 02 )00477-X
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Fig. 1. Photograph and diagram of the miniature electron beam column. The column includes two octupole aligners for optical adjustment and two objective lenses, one for main focusing (Main OL) and the other for low-magnification observation (Sub OL).
in length and 27 mm in diameter. It includes aligners for optical adjustment, a limiting aperture 40 mm in diameter, a deflector with a 10-mm bore diameter, and two objective lenses, one for main focusing (Main OL), and the other for low-magnification observation (Sub OL). All the optical components are electrostatic. For the purpose of surface inspection, the column must have a conical shape for tilted sample observation. Therefore, we simulated two types of main OL shapes, the Einzel-type and pseudo bi-potential-type (Fig. 2), while considering physical limitations. We used Munro’s program to calculate the optical properties. Our design goal was 10-nm resolution at 1-keV landing energy. This required a chromatic aberration coefficient of 3 mm or less. We found that, at a 2-mm working distance, the chromatic aberration coefficient of the Einzel lens could not be reduced to 3 mm or less.
Fig. 2. Simulated lens shapes.
T. Ambe et al. / Microelectronic Engineering 61 – 62 (2002) 317 – 321
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In addition, it was difficult to place the three electrodes of the Einzel lens in a narrow space. On the other hand, we found that the chromatic aberration coefficient of the pseudo bi-potential lens could be reduced to 3 mm or less. This was simply by removing the upper electrode of the Einzel lens. We then simulated and improved this lens shape, and finally we obtained a chromatic aberration coefficient of 2.9 mm and a spherical aberration of 14 mm at a 2-mm working distance. The final semi-convergence angle with respect to the optical axis is 6.6 mrad The observation area with the main OL is about 200 mm square. To achieve a wider observation area, we placed an additional objective lens (sub OL) between the beam aperture and the deflector. In this case, the observation area without distortion was 1 mm in diameter. Fig. 3 shows the low-aberration objective lens in detail. We improved the design several times to achieve a concentricity of 5 mm or better. All the electron optical components of this miniature column were fabricated by precision machining and stacked in a cylindrical sleeve to achieve this concentricity. All the electrodes were made of gold-plated phosphorus bronze, the sleeves were made of permalloy, and the insulation devices were made with MACOR —a machinable ceramic. Power was supplied to each electrode with pins that were simply inserted into the electrodes.
3. TFE e-gun assembly Fig. 4 shows the TFE e-gun assembly. We developed our own original concentric-type of sputtering ion pump (SIP) to make the gun assembly as small as possible. The electron gun sits inside the pump. The axial magnetic field produced by the ion pump has a focusing effect on the emitted electrons, so it reduces the aberration coefficients by almost a half. The pumping speed is about 6 l / sec, and the end pressure is 10 27 Pa.
Fig. 3. Low-aberration objective lens.
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T. Ambe et al. / Microelectronic Engineering 61 – 62 (2002) 317 – 321
Fig. 4. Diagram and photograph of the TFE e-gun assembly. The pumping speed is 6 l / sec. The end pressure is 10 27 Pa. The e-gun is of conventional type, and the energy spread is assumed to be 0.8 eV.
4. SEM operation We developed a compact SEM by using the miniature column and the TFE e-gun assembly. Fig. 5 shows an image taken with this compact SEM. The sample was gold deposited on a carbon substrate. The gap indicated on the image is about 10 nm. The electron energy was 1.3 keV, and the working distance was 2 mm.
5. Discussion The final bore diameter was 1 mm, as shown in Fig. 3. To reduce the aberration, calculation showed that the bore diameter should be large. For a bore diameter of 2 mm, however, the electric field
Fig. 5. SEM image obtained with the miniature column of gold deposited on a carbon substrate. The gap is about 10 nm.
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strength would be 100 V/ mm at a 2-mm working distance from the sample surface. In this case, almost all of the secondary electrons would escape upstream through the final bore. To reduce the electric field strength at the sample surface, we reduced the final bore diameter to 1 mm, but this caused the aberrations of the lens to increase. We used traditional secondary electron detectors such as scintillators and photo-multipliers, and placed them inside the vacuum chamber. Therefore, the detection efficiency of the secondary electrons was not good. To improve the detection efficiency, we are considering using an in-lens detector. We expect this to improve the aberration and the detection efficiency.
6. Conclusion We have designed and developed a low-aberration electrostatic miniature column. It is 74 mm in length and 27 mm in diameter. The chromatic and spherical aberration coefficients of the main OL are 2.9 and 14 mm, respectively at a 2-mm working distance. We also developed a concentric-type SIP with a TFE gun to make the gun assembly as small as possible. The pumping speed is 6 l / sec, and the end pressure is 10 27 Pa. We developed a compact SEM system by using the miniature column and achieved 10-nm resolution at 1.3 keV. For further studies, we are considering using an in-lens detector to improve the detection efficiency of the secondary electrons, thereby improving the SEM image resolution.
Acknowledgements We wish to acknowledge Y. Endo, M. Asai, and T. Ishizuka for their helpful discussions and contributions to this paper. We also thank S. Wakana for his valuable discussion and support.
References [1] Y.-H. Lee, R. Browning, N. Maluf, G. Owen, R.F.W. Pease, J. Vac. Sci. Technol. B (1992) 3094–3098. [2] T.H.P. Chang, D.P. Kern, L.P. Murray, J. Vac. Sci. Technol. B (1990) 1698–1705.
Miniature electrostatic column for a compact scanning electron microscope T. Ambe*, H. Tanaka, H. Teguri, I. Honjo 1 , A. Ito Fujitsu Laboratories Ltd., 10 -1 Morinosato-Wakamiya, Atsugi 243 -0197, Japan
Abstract We designed and fabricated a low-aberration miniature column with electrostatic lenses. It includes two octupole aligners for optical adjustment, and two objective lenses, one for main focusing and the other for low-magnification observation. The optical characteristics of the objective lenses were calculated with Munro’s program. We obtained values of 2.9 mm for the chromatic aberration coefficient and 14 mm for the spherical aberration coefficient, at a working distance of 2 mm. We also developed a compact concentric sputter ion pump with a TFE e-gun. The pumping speed is 6 l / sec, and the end pressure is 10 27 Pa. By using this miniature column and the TFE e-gun assembly, we developed a compact scanning electron microscope. We achieved 10-nm resolution at 1.3-keV landing energy. 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrostatic; Miniature column; Low aberration; Concentric sputter ion pump
1. Introduction Electron beam systems are powerful tools for semiconductor manufacture. Low-energy electrons are especially attractive for a variety of applications, such as inspection, testing, and lithography, because of the reduced extent of penetration into the sample, the reduced damage and heating of the sample, and the ability to irradiate insulating samples without surface charging [1]. The scaled aberration coefficients of miniature columns are particularly suitable for generating higher resolution beams of low-energy electrons [2]. We designed and fabricated a low-aberration miniature column to achieve higher resolution beams with low-energy electrons. 2. Low-aberration miniature column Fig. 1 shows a photograph and a diagram of the miniature column we designed. It is about 74 mm * Corresponding author. Tel.: 1 81-46-250-8226; fax: 1 81-46-250-8842. E-mail address: [email protected] (T. Ambe). 1 Present address: KLA-Tencor Japan Ltd. 0167-9317 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0167-9317( 02 )00477-X
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T. Ambe et al. / Microelectronic Engineering 61 – 62 (2002) 317 – 321
Fig. 1. Photograph and diagram of the miniature electron beam column. The column includes two octupole aligners for optical adjustment and two objective lenses, one for main focusing (Main OL) and the other for low-magnification observation (Sub OL).
in length and 27 mm in diameter. It includes aligners for optical adjustment, a limiting aperture 40 mm in diameter, a deflector with a 10-mm bore diameter, and two objective lenses, one for main focusing (Main OL), and the other for low-magnification observation (Sub OL). All the optical components are electrostatic. For the purpose of surface inspection, the column must have a conical shape for tilted sample observation. Therefore, we simulated two types of main OL shapes, the Einzel-type and pseudo bi-potential-type (Fig. 2), while considering physical limitations. We used Munro’s program to calculate the optical properties. Our design goal was 10-nm resolution at 1-keV landing energy. This required a chromatic aberration coefficient of 3 mm or less. We found that, at a 2-mm working distance, the chromatic aberration coefficient of the Einzel lens could not be reduced to 3 mm or less.
Fig. 2. Simulated lens shapes.
T. Ambe et al. / Microelectronic Engineering 61 – 62 (2002) 317 – 321
319
In addition, it was difficult to place the three electrodes of the Einzel lens in a narrow space. On the other hand, we found that the chromatic aberration coefficient of the pseudo bi-potential lens could be reduced to 3 mm or less. This was simply by removing the upper electrode of the Einzel lens. We then simulated and improved this lens shape, and finally we obtained a chromatic aberration coefficient of 2.9 mm and a spherical aberration of 14 mm at a 2-mm working distance. The final semi-convergence angle with respect to the optical axis is 6.6 mrad The observation area with the main OL is about 200 mm square. To achieve a wider observation area, we placed an additional objective lens (sub OL) between the beam aperture and the deflector. In this case, the observation area without distortion was 1 mm in diameter. Fig. 3 shows the low-aberration objective lens in detail. We improved the design several times to achieve a concentricity of 5 mm or better. All the electron optical components of this miniature column were fabricated by precision machining and stacked in a cylindrical sleeve to achieve this concentricity. All the electrodes were made of gold-plated phosphorus bronze, the sleeves were made of permalloy, and the insulation devices were made with MACOR —a machinable ceramic. Power was supplied to each electrode with pins that were simply inserted into the electrodes.
3. TFE e-gun assembly Fig. 4 shows the TFE e-gun assembly. We developed our own original concentric-type of sputtering ion pump (SIP) to make the gun assembly as small as possible. The electron gun sits inside the pump. The axial magnetic field produced by the ion pump has a focusing effect on the emitted electrons, so it reduces the aberration coefficients by almost a half. The pumping speed is about 6 l / sec, and the end pressure is 10 27 Pa.
Fig. 3. Low-aberration objective lens.
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T. Ambe et al. / Microelectronic Engineering 61 – 62 (2002) 317 – 321
Fig. 4. Diagram and photograph of the TFE e-gun assembly. The pumping speed is 6 l / sec. The end pressure is 10 27 Pa. The e-gun is of conventional type, and the energy spread is assumed to be 0.8 eV.
4. SEM operation We developed a compact SEM by using the miniature column and the TFE e-gun assembly. Fig. 5 shows an image taken with this compact SEM. The sample was gold deposited on a carbon substrate. The gap indicated on the image is about 10 nm. The electron energy was 1.3 keV, and the working distance was 2 mm.
5. Discussion The final bore diameter was 1 mm, as shown in Fig. 3. To reduce the aberration, calculation showed that the bore diameter should be large. For a bore diameter of 2 mm, however, the electric field
Fig. 5. SEM image obtained with the miniature column of gold deposited on a carbon substrate. The gap is about 10 nm.
T. Ambe et al. / Microelectronic Engineering 61 – 62 (2002) 317 – 321
321
strength would be 100 V/ mm at a 2-mm working distance from the sample surface. In this case, almost all of the secondary electrons would escape upstream through the final bore. To reduce the electric field strength at the sample surface, we reduced the final bore diameter to 1 mm, but this caused the aberrations of the lens to increase. We used traditional secondary electron detectors such as scintillators and photo-multipliers, and placed them inside the vacuum chamber. Therefore, the detection efficiency of the secondary electrons was not good. To improve the detection efficiency, we are considering using an in-lens detector. We expect this to improve the aberration and the detection efficiency.
6. Conclusion We have designed and developed a low-aberration electrostatic miniature column. It is 74 mm in length and 27 mm in diameter. The chromatic and spherical aberration coefficients of the main OL are 2.9 and 14 mm, respectively at a 2-mm working distance. We also developed a concentric-type SIP with a TFE gun to make the gun assembly as small as possible. The pumping speed is 6 l / sec, and the end pressure is 10 27 Pa. We developed a compact SEM system by using the miniature column and achieved 10-nm resolution at 1.3 keV. For further studies, we are considering using an in-lens detector to improve the detection efficiency of the secondary electrons, thereby improving the SEM image resolution.
Acknowledgements We wish to acknowledge Y. Endo, M. Asai, and T. Ishizuka for their helpful discussions and contributions to this paper. We also thank S. Wakana for his valuable discussion and support.
References [1] Y.-H. Lee, R. Browning, N. Maluf, G. Owen, R.F.W. Pease, J. Vac. Sci. Technol. B (1992) 3094–3098. [2] T.H.P. Chang, D.P. Kern, L.P. Murray, J. Vac. Sci. Technol. B (1990) 1698–1705.