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NEA photocathode for SEM application
Microelectronic Engineering 67–68 (2003) 951–954 www.elsevier.com / locate / mee
NEA photocathode for SEM application T. Ohshima* Central Research Laboratory, Hitachi Ltd., 1 -280, Higashi-koigakubo, Kokubunji, Tokyo 185 -8601, Japan
Abstract Negative electron affinity (NEA) photocathodes were examined as monochromatic electron sources for achieving low acceleration voltage in SEMs. An electron gun structure was developed that makes use of transmission type NEA photocathodes for SEM application, and measurements showed it had electron beam divergence with narrow half angle of 4.2 mrad in FWHM at 3-kV acceleration. The low value of the angle is expected to make up for the large source size to obtain brightness as high as that of a Zr-O / W thermal field emitter (TFE) source. Obtained resolution was 50 nm at 1-kV acceleration voltage. 2003 Elsevier Science B.V. All rights reserved. Keywords: Negative electron affinity (NEA); Photocathode; Electron beam source; Scanning electron microscope (SEM)
1. Introduction Low acceleration voltage ( , 1 kV) with high resolution for SEM measurements is attractive in the LSI and biotechnology fields as a means of minimizing damage to samples. However, chromatic aberration (Cc) increases greatly with acceleration voltage of 1 kV or less since it is proportional to DV/V, where DV is energy spread of electron beam and V is acceleration voltage. To achieve high resolutional, low acceleration SEM, monochromatic electron sources are desired in which DV is smaller than the 0.4 eV of conventional Zr-O / W thermal field emitters (TFEs). As such electron sources, negative electron affinity (NEA) photocathodes have been reported whose normal energy distribution is less than 0.1 eV [1,2], and whose precise energy of total energy distribution coincides with thermionic emission near room temperatures [3]. These features make them attractive for many researchers. One attempt was made to apply NEA photocathodes for SEM, resulting in a low resolutional image of several micrometers [4]. In this work, the author presents a new electron gun structure suitable for SEM application and measured results of SEM images at a low acceleration voltage (1 kV).
* Tel.: 181-42-323-1111; fax: 181-42-327-7713. E-mail address: [email protected] (T. Ohshima). 0167-9317 / 03 / $ – see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00159-X
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T. Ohshima / Microelectronic Engineering 67–68 (2003) 951–954
Fig. 1. Laser optics of the gun: (a) schematic view, (b) light focus measured by optical microscope, and (c) one dimensional intensity plot of the focus.
2. Experiments Making the electron beam source small is important to obtain a high-resolution image. The laser optics of the gun to obtain fine focus on the photocathode are shown in Fig. 1a. There is an aspherical focusing lens above the photocathode in vacuum, the position of which can be adjusted to focus on GaAs film through the glass substrate. A transmission photocathode that is a p-type GaAs thin film attached on a glass substrate is suitable for this application because it enables laser beam and electron optics to be separated. The lens corrects spherical aberration of the vacuum / glass system, enabling small focus size to be obtained. A laser diode (LD) along with a collimater lens and prisms generate a parallel light beam with a round cross-section. This parallel beam minimizes aberration when the light beam passes through the glass plate of the viewing port. A light focus pattern on the photocathode measured by the optical microscope from the opposite side is shown in Fig. 1b. The one-dimensional plot of light intensity shown in Fig. 1c revealed that the spot diameter is 1 mm in FWHM. This is the smallest size near the diffraction limit. The focus position of the lens is determined by magnified focus image that is projected on a CCD using reflected light from the cathode. A schematic illustration of the gun structure is shown in Fig. 2. Since NEA surfaces are known to be quite sensitive to contamination, surface preparation devices are included in the gun chamber. These include a Cs source (SAES Getters Cs dispenser which consists of cesium cromate wraped in a foil heater, evaporates Cs by heating), an oxygen source (variable leak valve with Ag pipe reservoir), and a substrate heater for surface cleaning. Also, an atomic hydrogen source that consists of a W heater, alumina cover and hydrogen gas source was used to remove contaminants from the atmosphere [5]. The photocathode moves between the gun center and the surface preparation position. The entire gun structure except for the linear motion drive and feedthroughs were built within a 152-mm diameter cylinder, which was small enough to mount on SEM (Hitachi S-6000) optics.
T. Ohshima / Microelectronic Engineering 67–68 (2003) 951–954
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Fig. 2. Schematic cross-section of the electron gun mounted on a SEM system.
3. Results and discussion Angular distribution of the electron beam from the gun as monitored by a phosphor plate is shown in Fig. 3a. Current distribution measured by a Faraday cup under the aperture on the phosphor plate is shown in Fig. 3b. Beam opening half angle was found to be 4.2 mrad, and the value of angular distribution is narrower than that of conventional needle shaped point source such as Zr-O / W TFEs. Brightness, one of the important parameters for high resolutional SEM, is determined by the emission current density in solid angle / size of the electron source. The source size of an NEA photocathode cannot be reduced to less than 1 mm due to electron diffusion in GaAs and diffraction limit of light spot size, whereas the virtual source size of a TFE is as small as several tens of nanometers. On the other hand, the current density in solid angle of NEA photocathode will be 0.23 A / sr when the emission current is 1.3 mA, whereas that of a TFE is several tens of mA / sr. As a result, NEA
Fig. 3. Electron beam from NEA gun: (a) spot by electron beam on a phosphor plate; and (b) current distribution of electron beam measured under a hole on the plate.
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T. Ohshima / Microelectronic Engineering 67–68 (2003) 951–954
Fig. 4. SEM image. Sample, Cu mesh; acceleration voltage 1 kV, averaging frame number 32, cathode emission current 0.3–1 mA.
photocathodes are expected to provide brightness as high as 3310 6 A / sr cm 2 at 3 kV, a value equivalent to that provided by TFEs. Observed SEM images are shown in Fig. 4. They are taken in TV scan mode with an average of 32 frames, and the total time for measurement was 0.64 s. A resolution as high as 50 nm was observed, this demonstrating the feasibility of NEA photocathodes for SEM application. Advances in electron optics are expected to improve resolution by more than one order of magnitude, since the estimated chromatic aberration is far below this resolution level. For example, there still remains the problem of surface contamination, which results in rather steep emission current decay, e.g. from 2 to 1 mA within 40 min, during 1-kV SEM measurement. Another problem which remains to be solved is that of magnetic leakage flux near the photocathode. Studies to improve the resolution are underway.
Acknowledgements The author gratefully acknowledges Dr Hideo Todokoro and Satoru Fukuhara of Hitachi High Technology Company, and Hiroyuki Shinada of Central Research Laboratory, Hitachi Ltd. for their helpful discussions and suggestions.
References [1] J.X. Zhou et al., Nucl. Instrum. Meth. B56 / 57 (1991) 1171. [2] A.W. Baum et al., Proc. SPIE 2522 (1995) 208. [3] T. Ohshima, in: Extended Abstracts (60th Autumn Meeting, Kobe, Japan, 1999), The Japanese Society of Applied Physics, 1999, p. 635. [4] C. Sanford, N. MacDonald, J. Vac. Sci. Technol. B6 (1988) 2005. [5] K.A. Elamrawi, H.E. Elsayed-Ali, J. Vac. Sci. Technol. A 17 (3) (1999) 823.
NEA photocathode for SEM application T. Ohshima* Central Research Laboratory, Hitachi Ltd., 1 -280, Higashi-koigakubo, Kokubunji, Tokyo 185 -8601, Japan
Abstract Negative electron affinity (NEA) photocathodes were examined as monochromatic electron sources for achieving low acceleration voltage in SEMs. An electron gun structure was developed that makes use of transmission type NEA photocathodes for SEM application, and measurements showed it had electron beam divergence with narrow half angle of 4.2 mrad in FWHM at 3-kV acceleration. The low value of the angle is expected to make up for the large source size to obtain brightness as high as that of a Zr-O / W thermal field emitter (TFE) source. Obtained resolution was 50 nm at 1-kV acceleration voltage. 2003 Elsevier Science B.V. All rights reserved. Keywords: Negative electron affinity (NEA); Photocathode; Electron beam source; Scanning electron microscope (SEM)
1. Introduction Low acceleration voltage ( , 1 kV) with high resolution for SEM measurements is attractive in the LSI and biotechnology fields as a means of minimizing damage to samples. However, chromatic aberration (Cc) increases greatly with acceleration voltage of 1 kV or less since it is proportional to DV/V, where DV is energy spread of electron beam and V is acceleration voltage. To achieve high resolutional, low acceleration SEM, monochromatic electron sources are desired in which DV is smaller than the 0.4 eV of conventional Zr-O / W thermal field emitters (TFEs). As such electron sources, negative electron affinity (NEA) photocathodes have been reported whose normal energy distribution is less than 0.1 eV [1,2], and whose precise energy of total energy distribution coincides with thermionic emission near room temperatures [3]. These features make them attractive for many researchers. One attempt was made to apply NEA photocathodes for SEM, resulting in a low resolutional image of several micrometers [4]. In this work, the author presents a new electron gun structure suitable for SEM application and measured results of SEM images at a low acceleration voltage (1 kV).
* Tel.: 181-42-323-1111; fax: 181-42-327-7713. E-mail address: [email protected] (T. Ohshima). 0167-9317 / 03 / $ – see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00159-X
952
T. Ohshima / Microelectronic Engineering 67–68 (2003) 951–954
Fig. 1. Laser optics of the gun: (a) schematic view, (b) light focus measured by optical microscope, and (c) one dimensional intensity plot of the focus.
2. Experiments Making the electron beam source small is important to obtain a high-resolution image. The laser optics of the gun to obtain fine focus on the photocathode are shown in Fig. 1a. There is an aspherical focusing lens above the photocathode in vacuum, the position of which can be adjusted to focus on GaAs film through the glass substrate. A transmission photocathode that is a p-type GaAs thin film attached on a glass substrate is suitable for this application because it enables laser beam and electron optics to be separated. The lens corrects spherical aberration of the vacuum / glass system, enabling small focus size to be obtained. A laser diode (LD) along with a collimater lens and prisms generate a parallel light beam with a round cross-section. This parallel beam minimizes aberration when the light beam passes through the glass plate of the viewing port. A light focus pattern on the photocathode measured by the optical microscope from the opposite side is shown in Fig. 1b. The one-dimensional plot of light intensity shown in Fig. 1c revealed that the spot diameter is 1 mm in FWHM. This is the smallest size near the diffraction limit. The focus position of the lens is determined by magnified focus image that is projected on a CCD using reflected light from the cathode. A schematic illustration of the gun structure is shown in Fig. 2. Since NEA surfaces are known to be quite sensitive to contamination, surface preparation devices are included in the gun chamber. These include a Cs source (SAES Getters Cs dispenser which consists of cesium cromate wraped in a foil heater, evaporates Cs by heating), an oxygen source (variable leak valve with Ag pipe reservoir), and a substrate heater for surface cleaning. Also, an atomic hydrogen source that consists of a W heater, alumina cover and hydrogen gas source was used to remove contaminants from the atmosphere [5]. The photocathode moves between the gun center and the surface preparation position. The entire gun structure except for the linear motion drive and feedthroughs were built within a 152-mm diameter cylinder, which was small enough to mount on SEM (Hitachi S-6000) optics.
T. Ohshima / Microelectronic Engineering 67–68 (2003) 951–954
953
Fig. 2. Schematic cross-section of the electron gun mounted on a SEM system.
3. Results and discussion Angular distribution of the electron beam from the gun as monitored by a phosphor plate is shown in Fig. 3a. Current distribution measured by a Faraday cup under the aperture on the phosphor plate is shown in Fig. 3b. Beam opening half angle was found to be 4.2 mrad, and the value of angular distribution is narrower than that of conventional needle shaped point source such as Zr-O / W TFEs. Brightness, one of the important parameters for high resolutional SEM, is determined by the emission current density in solid angle / size of the electron source. The source size of an NEA photocathode cannot be reduced to less than 1 mm due to electron diffusion in GaAs and diffraction limit of light spot size, whereas the virtual source size of a TFE is as small as several tens of nanometers. On the other hand, the current density in solid angle of NEA photocathode will be 0.23 A / sr when the emission current is 1.3 mA, whereas that of a TFE is several tens of mA / sr. As a result, NEA
Fig. 3. Electron beam from NEA gun: (a) spot by electron beam on a phosphor plate; and (b) current distribution of electron beam measured under a hole on the plate.
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T. Ohshima / Microelectronic Engineering 67–68 (2003) 951–954
Fig. 4. SEM image. Sample, Cu mesh; acceleration voltage 1 kV, averaging frame number 32, cathode emission current 0.3–1 mA.
photocathodes are expected to provide brightness as high as 3310 6 A / sr cm 2 at 3 kV, a value equivalent to that provided by TFEs. Observed SEM images are shown in Fig. 4. They are taken in TV scan mode with an average of 32 frames, and the total time for measurement was 0.64 s. A resolution as high as 50 nm was observed, this demonstrating the feasibility of NEA photocathodes for SEM application. Advances in electron optics are expected to improve resolution by more than one order of magnitude, since the estimated chromatic aberration is far below this resolution level. For example, there still remains the problem of surface contamination, which results in rather steep emission current decay, e.g. from 2 to 1 mA within 40 min, during 1-kV SEM measurement. Another problem which remains to be solved is that of magnetic leakage flux near the photocathode. Studies to improve the resolution are underway.
Acknowledgements The author gratefully acknowledges Dr Hideo Todokoro and Satoru Fukuhara of Hitachi High Technology Company, and Hiroyuki Shinada of Central Research Laboratory, Hitachi Ltd. for their helpful discussions and suggestions.
References [1] J.X. Zhou et al., Nucl. Instrum. Meth. B56 / 57 (1991) 1171. [2] A.W. Baum et al., Proc. SPIE 2522 (1995) 208. [3] T. Ohshima, in: Extended Abstracts (60th Autumn Meeting, Kobe, Japan, 1999), The Japanese Society of Applied Physics, 1999, p. 635. [4] C. Sanford, N. MacDonald, J. Vac. Sci. Technol. B6 (1988) 2005. [5] K.A. Elamrawi, H.E. Elsayed-Ali, J. Vac. Sci. Technol. A 17 (3) (1999) 823.