- Email: [email protected]
Electron-beam-induced deposition of carbonaceous microstructures using scanning electron microscopy
applied
surface science ELSEXIER
Applied Surface Science 113/l 14 (1997) 269-273
Electron-beam-induced deposition of carbonaceous microstructures using scanning electron microscopy Naruhisa Miura a3*, Hideaki Ishii a, Jun-ichi Shirakashi Makoto Konagai a
b2’,Akira Yamada a,
a Deportment qf Electricul and Electronic Engineering, Tokyo Institute ofTechnology2-12-l 0-oka.vama, Meguro-ku, Tobo 152, Japan ’ Electrotechnical Laboratop. 1-l-4 CJmezono, Tsukuba-shi. Ibaraki 305, Japan
Abstract Scanning electron microscopy (SEM) was utilized for deposition of carbonaceous microstructures. Conic-, wire- and square-shaped structures were fabricated with various scanning modes. A narrow carbonaceous wire with a width of 30 nm was obtained at an electron-beam current of 15 pA. The conic-structure was successfully applied to a cantilever of an atomic force microscope (AFM) and a planar resolution was extremely improved. Furthermore, the square-shaped structure was also employed as the insulator in a metal-insulator-metal (MIM) diode and a clear non-linear I-V characteristic was observed at
room temperature. Kewords:
Electron-beam-induced
deposition;
Carbonaceous
microstructures:
1. Introduction Sub-micrometer scale processing is necessary for future ultra large scale integration (ULSI) devices, quantum effect devices and single electron transistors (SET). To date, electron-beam lithography has satisfied the requirements for such processing. However, other technology such as STM/AFM nanooxidation [l-4] and focused ion beam (FIB) processing [.5-71 have recently become available. Electronbeam-induced deposition (EBID) is also one of the candidates for the above technology. For example, Broers et al. employed this technique for fabricating
* Corresponding author. Tel.: + 8 l-3-57342554: 57342897: e-mail: [email protected]. ’ Tel.: + 81-298-585 194: fax: + 81-298-585523. 0169-4332/97/$17.00 Copyright PII SOl69-4332(96)00767-2
fax:
+ 8 I-5
Atomic force microscopy;
MIM diode
a metallic nano-wire and demonstrated its advantages for ultra-high resolution processing [8]. When scanning electron microscopy (SEMI is used to monitor microscopic structures, the surface being observed is inevitably contaminated. This is due to the fact that residual hydrocarbon molecules remain in the SEM chamber even at a low base pressure (- lop6 Pa) and are decomposed by irradiation due to the electron-beam. This results in the deposition of the carbonaceous material on the area that is exposed. Since this process occurs only on the areas that are exposed to the electron-beam, carbonaceous structures with sub-micrometer dimensions can easily be fabricated. Hence the use of SEM is another method for the fabrication of nanostructures by EBID without any modification of the SEM system. In our study, we successfully prepared various
0 1997 Elsevier Science B.V. All rights reserved
270
N. Miura et al. /Applied Surface Science I13 / I I4 C1997) 269-273
carbonaceous microstructures using different SEM scanning modes. Furthermore, the carbonaceous materials were used as a cantilever of an atomic force microscope (AFM) and as an insulator in a metal-insulator-metal (MIMI diode.
60
I
,
I
,
I
[
I
,
I,
I
,
I
,
I
,
,
(
Acc.Volt 30kV Msg. 50k r5 3
60
-. w
heigM width
,
_ _
ccl E z .E
2. Experimental The deposition of the carbonaceous material was performed in an SEM (Hitachi, S-800) system. The base pressure of the processing chamber was 5 X lop6 Pa. The oil from the diffusion pump is the source of the hydrocarbon molecules which we consider as being the precursors of the carbonaceous material. It was possible to fabricate conic-, wireand square-shaped structures by changing the scanning modes of the SEM. The working distance was fixed at 10 mm and the dependence of CaJbOII wire height and width on the beam-current was investigated. The detailed processing procedure is described elsewhere [q].
40 !,k; 20 0
20
40
60
Beam current
80
100
[ pA]
Fig. 1. Dependence of the shape of the carbonaceous electron-beam current.
wire on the
scattering at the surface, resulting in a lateral growth of carbon. Therefore, we obtained a narrower wire with a high aspect ratio for a low beam current, which is suitable for the fabrication of nanostructures. 3.2. Application tilever
of a conic structure
to AFM can-
3. Results and discussion
3.1. Growth of a carbonaceous
wire
In our previous study, it was found that a high acceleration voltage and high magnification are optimal for making wires with a high aspect ratio. In this study, we further optimized the electron-beam conditions to fabricate a narrower carbonaceous wire. Fig. 1 shows the dependence of the carbon wire height and width on the beam current. The acceleration voltage was set to 30 kV. The electron-beam was scanned once in each procedure during the in-lineanalysis mode and a wire of about 2 pm long was obtained. Carbonaceous wires with a width of 30 nm were obtained at a low beam current of 15 pA. The height of the carbon wire decreased with increasing beam current, whereas the width increased, which suggests that a high aspect ratio and narrow wire can be obtained by using a low beam current. The increase of the beam current causes both local heating and re-evaporation of the carbon and consequently the vertical growth is suppressed. Furthermore, the increase of the beam current also causes electron
To demonstrate the EBID process, we fabricated conic-shaped structures at the tip of an atomic force microscope (AFM) cantilever. Some reports have studied this technique [lo-121 and observed trench structures. In our study, we tried to resolve the wire structures with a small line width and spacing. A sample for the AFM observation consisted of four wires with a width of 50 nm and spacing of 200 nm. We could not obtain a clear image with a normal cantilever, because the line width and spacing of the wires was less than the resolution. Therefore, we used the conic structure of the tip of the AFM cantilever. An AFM image of the wires taken by a conicshaped carbon needle is shown in Fig. 2(a). The SEM image of the new cantilever is shown in Fig. 2(b). The base cantilever consisted of insulating Si,N,. Therefore, before the growth of the carbon needle, a small amount of Pt-Pd alloy was deposited on it by rf sputtering to make the surface electrically conducting. The carbon growth was performed by just focusing the electron-beam onto a spot for a few minutes. The carbon needle had a height of 1 Frn
N. Miura
Fig. 2. (a) AFM image of the carbonaceous needle on top of the cantilever.
et al. /Applied
211
Surface Science 113 / I14 (1997) 269-273
wires (50 nm width,
and a diameter of 60 nm. The needle was also coated with Pt-Pd alloy to strengthen its adhesion to the cantilever. By using this cantilever, the clear picture of the carbonaceous wires was obtained as shown in Fig. 2(a). The left wall of the wires is seen to be blurred, while the right is sharp. This is attributed to the slight curvature of the needle as shown in Fig. 2(b). In this experiment, it was demonstrated that this carbonaceous needle is very effective especially
100
nm height, 200 nm spacing). (b) SEM image of the carbonaceous
for the observation aspect ratios. 3.3. Application
of nano-scale
of a square structure to MIM diode
Carbonaceous films phous carbon has been using chemical vapor [ 141 techniques, while
Fig. 3. AFM image of the surface of the carbonaceous
structures with high
such as diamond-like amordeposited by several authors deposition [ 131 or sputtering square-shaped carbonaceous
tilm (7 X 7 pm, 100 nm height).
212
N. Miura et al/Applied
Sur$ace Science 113/114
films can be grown at an arbitrary position by twodimensional electron-beam scanning. This is a great advantage for fabricating nanometer-size devices. To demonstrate this advantage, we fabricated a MIM diode. The carbon deposition was carried out in a surface analysis mode, and the SEM system parameters were set to 30 kV and 34 pA. The film was grown on a Si substrate with a size of 6 pm square. Fig. 3 shows an AFM image of the surface of the carbonaceous film. The surface is smooth with the roughness being less than about 2 nm. Although the surface morphology shows some stripes due to the scanning beam deposition, no pinholes were observed. Therefore, MIM diodes were fabricated and the electrical properties were characterized. The fabrication of the MIM diode was carried out as follows: thermally oxidized Si was used as a substrate. A Au base electrode was fabricated using the lift-off technique. Then the carbonaceous film was deposited on the base electrode in the junction region. Finally, a counter (top) electrode was evaporated. A schematic and an AFM image of the device are shown in Fig. 4(a) and (b), respectively. The
(4
counter electrode
(Au&
(1997) 269-273
Voltage Fig. 5. Current-voltage
characteristics
M of the MIM diode.
smooth edge profile of the base electrode indicates that no electrical short path exists between the base and counter electrodes, whereas the edge of the counter electrode is a little rough. From this figure, the thickness of the carbonaceous film was measured to be 30 nm and the junction size 3 X 3 pm. The current-voltage characteristics of this diode at 300 and 175 K are shown in Fig. 5. The intermediate carbonaceous film was found to be working as an insulator and an energy barrier to the electrons since the current is suppressed in the low bias region. At higher biases, the current begins to flow through the film due to thermionic emission and/or tunneling. The current is suppressed and the threshold voltage shifts towards higher voltages at lower temperatures. Therefore, it is found that the tunneling current dominates in this temperature region. The resistivity of the film was estimated from the lower bias region to be on the order of 10” LR cm.
4. Conclusion
Fig. 4. (a) Schematic of the MIM @i-layer diode. (b) AFM image of the MIM diode (7 X 7 pm, 700 nm height).
The fabrication of carbonaceous microstructures was studied by electron-beam-induced deposition using SEM. It was found that a low beam current is optimal for fabrication of narrow wires. A wire with a width of 30 nm was obtained at a beam current of 15 pA. A conic-shaped structure was successfully employed as a cantilever of an AFM and a drastic improvement of the planar resolution was demonstrated. Furthermore, a square-shaped structure was
N. Miura
et al./Applied
Sur$ace SL?ence 113/114
employed as an intermediate layer of a MIM diode and it was found that the carbonaceous films acted as an insulator.
References [l] K. Matsumoto, S. Takahashi, M. Ishii, M. Hoshi. A. Kurokawa, S. Ochimura and A. Ando, Jpn. J. Appl. Phys. 34 (1995) 1387. [2] K. Matsumoto, M. Ishii, J. Shirakashi, K. Segawa. Y. Oka, B.J. Vartanian and J.S. Harris, Tech. Dig. Int. Electron Device Meet. (1995) 363. [3] J. Shirakashi. M. Ishii, K. Matsumoto, N. Miura and M. Konagai, Abstr. 54th Device Research Conf. (DRC) (1996). [4] N. Kramer, H. Birk, J. Jorritsma and C. Schonenberger, Appl. Phys. Lett. 66 (1995) 1325.
(1997) 269-273
273
[51 M.J. Vasile. D. Grigg, J.E. Griffith, E. Fitzgerald and P.E. Russell, J. Vat. Sci. Technol. B 9 (1991) 3569. [61 J.S. Hub, M.I. Shepard and J. Melngailis, J. Vat. Sci. Technol. B 9 (1991) 173. [71Y. Gotoh. T. Ohtake. N. Fujita, K. moue. H. Tsuji and J. Ishikawa. J. Vat. Sci. Technol. B 13 (19951 465. 181A.N. Broers. W.W. Molzen, J.J. Cuomo and N.D. Wittels, Appl. Phys. Lett. 29 t 1976) 596. [91N. Miura. H. Ishii, A. Yamada and M. Konagai. Jpn. J. Appl. Phys., submitted. [lOI K.I. Schiffmann, Nanotechnology 4 (19931 163. [Ill Y. Akama, E. Nishimura. A. Sakai and H. Murakami. J. Vat. Sci. Technol. A 8 (1990) 429. 1121K.L. Lee, D.W. Abraham. F. Secord and L. Landstein. J. Vat. Sci. Technol. B 9 (1991) 3562. [I31M. Ramsteiner, J. Wagner, Ch. Wild and P. Koidl. J. Appl. Phys. 62 (1987) 729. [141M. Yoshikawa. G. Katagiri, H. Ishida. A. Ishitani and T. Akamatsu, J. Appl. Phys. 64 (1988) 6464.
surface science ELSEXIER
Applied Surface Science 113/l 14 (1997) 269-273
Electron-beam-induced deposition of carbonaceous microstructures using scanning electron microscopy Naruhisa Miura a3*, Hideaki Ishii a, Jun-ichi Shirakashi Makoto Konagai a
b2’,Akira Yamada a,
a Deportment qf Electricul and Electronic Engineering, Tokyo Institute ofTechnology2-12-l 0-oka.vama, Meguro-ku, Tobo 152, Japan ’ Electrotechnical Laboratop. 1-l-4 CJmezono, Tsukuba-shi. Ibaraki 305, Japan
Abstract Scanning electron microscopy (SEM) was utilized for deposition of carbonaceous microstructures. Conic-, wire- and square-shaped structures were fabricated with various scanning modes. A narrow carbonaceous wire with a width of 30 nm was obtained at an electron-beam current of 15 pA. The conic-structure was successfully applied to a cantilever of an atomic force microscope (AFM) and a planar resolution was extremely improved. Furthermore, the square-shaped structure was also employed as the insulator in a metal-insulator-metal (MIM) diode and a clear non-linear I-V characteristic was observed at
room temperature. Kewords:
Electron-beam-induced
deposition;
Carbonaceous
microstructures:
1. Introduction Sub-micrometer scale processing is necessary for future ultra large scale integration (ULSI) devices, quantum effect devices and single electron transistors (SET). To date, electron-beam lithography has satisfied the requirements for such processing. However, other technology such as STM/AFM nanooxidation [l-4] and focused ion beam (FIB) processing [.5-71 have recently become available. Electronbeam-induced deposition (EBID) is also one of the candidates for the above technology. For example, Broers et al. employed this technique for fabricating
* Corresponding author. Tel.: + 8 l-3-57342554: 57342897: e-mail: [email protected]. ’ Tel.: + 81-298-585 194: fax: + 81-298-585523. 0169-4332/97/$17.00 Copyright PII SOl69-4332(96)00767-2
fax:
+ 8 I-5
Atomic force microscopy;
MIM diode
a metallic nano-wire and demonstrated its advantages for ultra-high resolution processing [8]. When scanning electron microscopy (SEMI is used to monitor microscopic structures, the surface being observed is inevitably contaminated. This is due to the fact that residual hydrocarbon molecules remain in the SEM chamber even at a low base pressure (- lop6 Pa) and are decomposed by irradiation due to the electron-beam. This results in the deposition of the carbonaceous material on the area that is exposed. Since this process occurs only on the areas that are exposed to the electron-beam, carbonaceous structures with sub-micrometer dimensions can easily be fabricated. Hence the use of SEM is another method for the fabrication of nanostructures by EBID without any modification of the SEM system. In our study, we successfully prepared various
0 1997 Elsevier Science B.V. All rights reserved
270
N. Miura et al. /Applied Surface Science I13 / I I4 C1997) 269-273
carbonaceous microstructures using different SEM scanning modes. Furthermore, the carbonaceous materials were used as a cantilever of an atomic force microscope (AFM) and as an insulator in a metal-insulator-metal (MIMI diode.
60
I
,
I
,
I
[
I
,
I,
I
,
I
,
I
,
,
(
Acc.Volt 30kV Msg. 50k r5 3
60
-. w
heigM width
,
_ _
ccl E z .E
2. Experimental The deposition of the carbonaceous material was performed in an SEM (Hitachi, S-800) system. The base pressure of the processing chamber was 5 X lop6 Pa. The oil from the diffusion pump is the source of the hydrocarbon molecules which we consider as being the precursors of the carbonaceous material. It was possible to fabricate conic-, wireand square-shaped structures by changing the scanning modes of the SEM. The working distance was fixed at 10 mm and the dependence of CaJbOII wire height and width on the beam-current was investigated. The detailed processing procedure is described elsewhere [q].
40 !,k; 20 0
20
40
60
Beam current
80
100
[ pA]
Fig. 1. Dependence of the shape of the carbonaceous electron-beam current.
wire on the
scattering at the surface, resulting in a lateral growth of carbon. Therefore, we obtained a narrower wire with a high aspect ratio for a low beam current, which is suitable for the fabrication of nanostructures. 3.2. Application tilever
of a conic structure
to AFM can-
3. Results and discussion
3.1. Growth of a carbonaceous
wire
In our previous study, it was found that a high acceleration voltage and high magnification are optimal for making wires with a high aspect ratio. In this study, we further optimized the electron-beam conditions to fabricate a narrower carbonaceous wire. Fig. 1 shows the dependence of the carbon wire height and width on the beam current. The acceleration voltage was set to 30 kV. The electron-beam was scanned once in each procedure during the in-lineanalysis mode and a wire of about 2 pm long was obtained. Carbonaceous wires with a width of 30 nm were obtained at a low beam current of 15 pA. The height of the carbon wire decreased with increasing beam current, whereas the width increased, which suggests that a high aspect ratio and narrow wire can be obtained by using a low beam current. The increase of the beam current causes both local heating and re-evaporation of the carbon and consequently the vertical growth is suppressed. Furthermore, the increase of the beam current also causes electron
To demonstrate the EBID process, we fabricated conic-shaped structures at the tip of an atomic force microscope (AFM) cantilever. Some reports have studied this technique [lo-121 and observed trench structures. In our study, we tried to resolve the wire structures with a small line width and spacing. A sample for the AFM observation consisted of four wires with a width of 50 nm and spacing of 200 nm. We could not obtain a clear image with a normal cantilever, because the line width and spacing of the wires was less than the resolution. Therefore, we used the conic structure of the tip of the AFM cantilever. An AFM image of the wires taken by a conicshaped carbon needle is shown in Fig. 2(a). The SEM image of the new cantilever is shown in Fig. 2(b). The base cantilever consisted of insulating Si,N,. Therefore, before the growth of the carbon needle, a small amount of Pt-Pd alloy was deposited on it by rf sputtering to make the surface electrically conducting. The carbon growth was performed by just focusing the electron-beam onto a spot for a few minutes. The carbon needle had a height of 1 Frn
N. Miura
Fig. 2. (a) AFM image of the carbonaceous needle on top of the cantilever.
et al. /Applied
211
Surface Science 113 / I14 (1997) 269-273
wires (50 nm width,
and a diameter of 60 nm. The needle was also coated with Pt-Pd alloy to strengthen its adhesion to the cantilever. By using this cantilever, the clear picture of the carbonaceous wires was obtained as shown in Fig. 2(a). The left wall of the wires is seen to be blurred, while the right is sharp. This is attributed to the slight curvature of the needle as shown in Fig. 2(b). In this experiment, it was demonstrated that this carbonaceous needle is very effective especially
100
nm height, 200 nm spacing). (b) SEM image of the carbonaceous
for the observation aspect ratios. 3.3. Application
of nano-scale
of a square structure to MIM diode
Carbonaceous films phous carbon has been using chemical vapor [ 141 techniques, while
Fig. 3. AFM image of the surface of the carbonaceous
structures with high
such as diamond-like amordeposited by several authors deposition [ 131 or sputtering square-shaped carbonaceous
tilm (7 X 7 pm, 100 nm height).
212
N. Miura et al/Applied
Sur$ace Science 113/114
films can be grown at an arbitrary position by twodimensional electron-beam scanning. This is a great advantage for fabricating nanometer-size devices. To demonstrate this advantage, we fabricated a MIM diode. The carbon deposition was carried out in a surface analysis mode, and the SEM system parameters were set to 30 kV and 34 pA. The film was grown on a Si substrate with a size of 6 pm square. Fig. 3 shows an AFM image of the surface of the carbonaceous film. The surface is smooth with the roughness being less than about 2 nm. Although the surface morphology shows some stripes due to the scanning beam deposition, no pinholes were observed. Therefore, MIM diodes were fabricated and the electrical properties were characterized. The fabrication of the MIM diode was carried out as follows: thermally oxidized Si was used as a substrate. A Au base electrode was fabricated using the lift-off technique. Then the carbonaceous film was deposited on the base electrode in the junction region. Finally, a counter (top) electrode was evaporated. A schematic and an AFM image of the device are shown in Fig. 4(a) and (b), respectively. The
(4
counter electrode
(Au&
(1997) 269-273
Voltage Fig. 5. Current-voltage
characteristics
M of the MIM diode.
smooth edge profile of the base electrode indicates that no electrical short path exists between the base and counter electrodes, whereas the edge of the counter electrode is a little rough. From this figure, the thickness of the carbonaceous film was measured to be 30 nm and the junction size 3 X 3 pm. The current-voltage characteristics of this diode at 300 and 175 K are shown in Fig. 5. The intermediate carbonaceous film was found to be working as an insulator and an energy barrier to the electrons since the current is suppressed in the low bias region. At higher biases, the current begins to flow through the film due to thermionic emission and/or tunneling. The current is suppressed and the threshold voltage shifts towards higher voltages at lower temperatures. Therefore, it is found that the tunneling current dominates in this temperature region. The resistivity of the film was estimated from the lower bias region to be on the order of 10” LR cm.
4. Conclusion
Fig. 4. (a) Schematic of the MIM @i-layer diode. (b) AFM image of the MIM diode (7 X 7 pm, 700 nm height).
The fabrication of carbonaceous microstructures was studied by electron-beam-induced deposition using SEM. It was found that a low beam current is optimal for fabrication of narrow wires. A wire with a width of 30 nm was obtained at a beam current of 15 pA. A conic-shaped structure was successfully employed as a cantilever of an AFM and a drastic improvement of the planar resolution was demonstrated. Furthermore, a square-shaped structure was
N. Miura
et al./Applied
Sur$ace SL?ence 113/114
employed as an intermediate layer of a MIM diode and it was found that the carbonaceous films acted as an insulator.
References [l] K. Matsumoto, S. Takahashi, M. Ishii, M. Hoshi. A. Kurokawa, S. Ochimura and A. Ando, Jpn. J. Appl. Phys. 34 (1995) 1387. [2] K. Matsumoto, M. Ishii, J. Shirakashi, K. Segawa. Y. Oka, B.J. Vartanian and J.S. Harris, Tech. Dig. Int. Electron Device Meet. (1995) 363. [3] J. Shirakashi. M. Ishii, K. Matsumoto, N. Miura and M. Konagai, Abstr. 54th Device Research Conf. (DRC) (1996). [4] N. Kramer, H. Birk, J. Jorritsma and C. Schonenberger, Appl. Phys. Lett. 66 (1995) 1325.
(1997) 269-273
273
[51 M.J. Vasile. D. Grigg, J.E. Griffith, E. Fitzgerald and P.E. Russell, J. Vat. Sci. Technol. B 9 (1991) 3569. [61 J.S. Hub, M.I. Shepard and J. Melngailis, J. Vat. Sci. Technol. B 9 (1991) 173. [71Y. Gotoh. T. Ohtake. N. Fujita, K. moue. H. Tsuji and J. Ishikawa. J. Vat. Sci. Technol. B 13 (19951 465. 181A.N. Broers. W.W. Molzen, J.J. Cuomo and N.D. Wittels, Appl. Phys. Lett. 29 t 1976) 596. [91N. Miura. H. Ishii, A. Yamada and M. Konagai. Jpn. J. Appl. Phys., submitted. [lOI K.I. Schiffmann, Nanotechnology 4 (19931 163. [Ill Y. Akama, E. Nishimura. A. Sakai and H. Murakami. J. Vat. Sci. Technol. A 8 (1990) 429. 1121K.L. Lee, D.W. Abraham. F. Secord and L. Landstein. J. Vat. Sci. Technol. B 9 (1991) 3562. [I31M. Ramsteiner, J. Wagner, Ch. Wild and P. Koidl. J. Appl. Phys. 62 (1987) 729. [141M. Yoshikawa. G. Katagiri, H. Ishida. A. Ishitani and T. Akamatsu, J. Appl. Phys. 64 (1988) 6464.