- Email: [email protected]
Microstructures of GaAs fabricated by finely focused ion beam lithography
Microelectronic North-Holland
Engineering
277
9 (1989) 277-279
MICROSTRUCTURES
OF GaAs
FABRICATED
BY FINELY
Takao SHIOKAWA, Pi1 Hyon KIM, 2anabu Koichi TOYODA and Susumu NAMBA
FOCUSED
HAMAGAKI,
ION BEAM
LITHOGRAPHY
Tamio HARA, Yoshlnobu
AOYAGI,
$IKEN, The Institute of Physical and Chemical Research, Frontier Research Programs, Wako, Saltama 351-01 JAPAN RIKEN, ++ Microstructures of fine line and dot patterns on GaAs are fabricated by Be fine focused ion beam lithography and chlorine reactive ion etching using electron beam excited plasma. Microstructures with a vertical side wall of fine line and dot in fabricated GAS have 40 nm width with a high aspect ratio of about 17 and 80 nm dot diameter with a high aspect ratio of about 8. It is found that the process has a high potential for microstructure fabrication in several tenth nanometer regions.
1. INTRODUCTION Maskless processing using focused ion beam (FIB) is potentially important in microfabrications with a dimension less than 100 nm. This is because the FIB has a small beam diameter with less than 100 nm, a high ion current density of about 1 A/cm2 and many ion species [1,21. The diameter of the FIB is one of important factors to fabrIcate the fine pattern. The beam diameter with several tenth ++ nanometers is obtained by+Be as an n-type dopant for GaAs and by Ga as a sputter etching ion in liquid metal ion sources [3,41. The FIB using those ion species can be expected to fabricate the fine pattern with several tenth nanometer regions. In maskless implantaion using Be ++ FIB, the line width of implanted region is larger than the diameter of the ion beam, because the incident ion is scattered in a solid [51. In maskless sputter etching and maskiess assisted etching of a solid using Ga FIB, the width of the etched line at the surface is lager than the diameter of the ion beam [6,71. The cross-sectional profile of the etched sample is not a rectangle shape. The spreading of fabricated width compared with the beam diameter is due to the large tail of the current density profile in FIB [81. Therefore, ion implantation and ion etching using those FIB are difficult to fabricate the fine pattern with a high aspect ratio and a rectangle shape of the same size as the beam diameter with several tenth nanometers. On the other hand, maskless lithography using Be++ and Gaf FIB has reported on fabrication of several tenth nanometer patterns in PMMA resist, CMS resist and a negative-acting bilevel resist [9,10,111. We reported that a microstructure of GaAs with a high aspect ratio could be fabricated by Be++ fine FIB lithography and Cl2 reactive ion etching [121. In this paper, we report that microstructures of fine line and dot with high
0167-9317/89/$3.50
0 1989, Elsevier Science Publishers
B.V. (North-Holland)
aspect ratio and size less than 100 nm in GaAs can+te performed by maskless lithography using Be fine FIB and pattern transfer with reactive ion etching using electron beam excited plasma (EBEP) [131.
Z.EXPERIMENTAL
PROCEDURES
Maskless ion beam lithography was done by a 100 kV FIB system with Au-Si-Be ternary alloy ion source [141. A CMS (Toyo Soda Manufacturing Co. CxM-CMS (R)) resist layer with the film thickness of 300 nm was formed by spin coating on n-GaAs (100) substrate. Prebaking was performed at 120 OC for 30 min. The resist was exposed by a 100 kV Be++ fine FIB which has a diameter with several tenth nanometers [3,101. Fine patterning of resist was exposed by a single line scanning for the fine line pattern and a single pass irradiation for the dot pattern. The beam current of Be++ was about 1 PA. In the fabrication of the several tenth nanometer pattern, the precise focusing is one of important factors. The silver conductlng paste on resist was used as a focusing marker. After the ion beam exposure, the conducting paste was removed. The CMS resist on the GaAs substrate was developed by 200 set dip in a solution of isoamly acetate (iAAc) and ethylcellosolve (EC) with a 30:70 ratio. The rinse was ca,rried out by two step procSSStZS. The 1st rinse was 30 set dip in a solution of iAAc and EC with a lo:90 ratio. The 2nd rinse was a 30 set dip in isopropyl alcohol. Post baking was performed at 130 OC for 30 min. The transfer of the fine resist pattern on n-GaAs substrate was performed by Cl2 reactive ion etching using EBEP. This 1s because the low energy reactive ion etching has an advantage of the damage less [151. The acceleration voltage was 130 V. The etchlnq ion beam current density was about 2.1 mA/cm', measured on the stage without the suppressing secondary electron. The etching time was 50
T. Shiokawa et al. / Microstructures
278
sec. The etching rate ratio of virgin resist to GaAs was about 2.5. After the dry etching, the resist as a mask was removed by J-100 resist strip for 30 min at 100 OC. Microstructures of GaAs fabricated were observed by a scanning electron microscope (SEM).
3.
RESULTS AND DISCUSSION
TO fabricate the more fine pattern using FIB, it is necessary to use the beam at the lower beam current because the beam diameter increases with the beam current [141. The microfabrication using ion beam at lower beam current has several problems such as fine focusing and the stability of the FIB for long fabrication time. To obtain the fine focusing condition at low beam current, scanning ion microscope (SIM) photographs of the same regions of the conducting paste at two different ion beam current are shown in Figure 1 (a) and (b). The SIM photograph (a) at beam current of 0.7 pA shows the high resolution of the sever-al tenth nanometers at the suitable contrast.
of GaAs
current was determined by the contrast and the signal to noise ratio in SIM image. The beam current in the experiment system was about 0.5-1.0 pA. The beam diameter was estimated to be several tenth nanometers (40-60 nm) from the resolution in SIM image of latex [161. The shift of the beam position was estimated to be less than 100 nm for 5 min from the image shift between SIM photographs in a high magnification. The time of 5 min in position stability is much longer than the scanning time less than 1 set for fine line and the beam dwell time less than 10m3 set for dot.
Figure
(a)
-1
Urn
2
Figure 2 shows a SEM photograph of the microstructure of GaAs with the minimum width fabricated by 100 kV Be++ fine FIB lithography The using EBEP. and Cl2 reactive ion etching line dose of the FIB was 3.4 x lo7 ions/cm. The SEM photograph shows the cleaved surface of fabricated GaAs. The fabricated GaAs structure is continuous in length of 0.3 mm. The width and height are about 40 nm and 680 nm, respecThe minimum width is nearly equal to tively. The microstructure of GaAs the beam diameter. has a high aspect ratio of about 17 and the vertical side wall.
Figure 1 The SIM photograph (b) at beam current of 0.08 pA do not show clearly the microstructure of several tenth nanometers because of the low signal to noise ratio. Therefore, the fine FIB at low beam current can not be used in the fabrication of the microstructure with several The practical beam tenth nanometer regions.
-
100 nm Figure
3
Figure 3 shows 70 degree tilt angle SEM photograph of fabricated GaAs dot with the minimum diameter. The dose of the FIB is about
T. Shiokuwa et al. / Microstructures
240 ions (beam dwell time is 0.14 Ins). The diameter and the height of the GaAs dot are about 80 "m and 660 "m, respectively. The dot has the vertical side wall and a high aspect ratio of about 8. The fabricated GaAs has characteristic microstructures of a high aspect ratio with a height of about 700 "m. The height of fabricated GaAs is larger than the projected range (576 nm) for 100 kV Be++ in GaAs. In this case, the projected range in GaAs is reduced by the deceleration of ion through the resist film. The influence of the damage in GaAs during ion beam lithography may be reduced by the use of bottom region in fabricated GaAs.
4. CONCLUSION In conclusion, high aspect ratio microstructures of a 40 "m width fine line and an 80 "m diameter dot were fabricated on GaAs by 100 kV Be++ fine FIB lithography and Cl2 reactive ion etching using EBEP. The processes are attractive for fabrication of microstructures with several tenth nanometer regions because of maskless and dry ones.
REFERENCES [ll
Seliger, R.L., Ward, J.W., Wang, V., and Kubena, R.l., Appl.Phys.Lett. 34 (1979) 310. [2] Melngailis, J., J.Vac.Sci.Technol. B5 (1987) 469. [31 Arimoto, H., Takamori, A., Miyauchi, E. and Hashimoto, H., Jpn.J.Appl.Phys. 22 (1983) L280.
of GaAs
R.L.,Joyce, R.J., Ward, J.W., H.L., Stratton, F.P. and Brault, J.Vac.Sic.Technol. B6 (1988) 353. R.G., [5] Miyauchi, E., Arimoto, H., Bamba. Y., Takamori, A., Hashimoto, H., and UtsUmi. T., Jp".J.Appl.PhyS. 22 (1983) L423. [6] Kubena, R.L., seliger, R.L., and Stevens, E.H., Thin Solid Films 92 (1982) 165. [7] Ochiai, Y., Shihoyama, K., Shiokawa, T., Toyoda, K., Masuyama, A., Game, K., and B4 (1986) Namba, S., J.Vac.Sci.Technol. 333. [8] Kubena, R.L., and Ward, J.W., Appl.Phys. Lett. 51 (1987) 1960. [9] Shiokawa, T., Aoyagi, Y., Kim,P.H., Toyoda, K., and Namba, S., Jp".J.Appl.Phys. 23 (1984) L232. [lo] Matsui, S., Mori, K., Shiokawa. T., Toyoda, K., and Namba, S., J.Vac.Sci. Technol. B6 (1987) 853. [ll] Kubena, R.L., Joyce, R.J., Ward, J.W., Gravin,H.L., Stratton, F.P., and Brault, 50 (1987) 1589. R.G., Appl.Phys.Lett. [12] Shiokawa, T., Kim,P.H., Hamagaki, M., Hara, K., Aoyagi, Y., Toyoda, K., and Namba, S., Jpn.J.app1.Phy.s. 27 (1988) L1160. [13] Hara, T., Hamagaki,M., Sanda,A., Aoyagi, Y., and Namba,S., J.Vac.Sci.Technol. B5 (1987) 366. [14] Shiokawa, T., Kim, P.H., Toyoda, K., Namba, S., Matsui, T., and Game, K., J.Vac.Sci.Technol. Bl (1983) 1117. [151 Yu, J.-Z., Hara,T., Hamagaki, M., Yoshinaga, Y, Aoyagi, Y., and Namba, S., J.Vac.Sci.Technol. B6 (1988) to be published. [16] Ishitani, T., Tamura, H., Todokoro. H., J.Vac.Sci.Technol. 20 (1982) 80.
[4] Kubena, Garvin,
279
Engineering
277
9 (1989) 277-279
MICROSTRUCTURES
OF GaAs
FABRICATED
BY FINELY
Takao SHIOKAWA, Pi1 Hyon KIM, 2anabu Koichi TOYODA and Susumu NAMBA
FOCUSED
HAMAGAKI,
ION BEAM
LITHOGRAPHY
Tamio HARA, Yoshlnobu
AOYAGI,
$IKEN, The Institute of Physical and Chemical Research, Frontier Research Programs, Wako, Saltama 351-01 JAPAN RIKEN, ++ Microstructures of fine line and dot patterns on GaAs are fabricated by Be fine focused ion beam lithography and chlorine reactive ion etching using electron beam excited plasma. Microstructures with a vertical side wall of fine line and dot in fabricated GAS have 40 nm width with a high aspect ratio of about 17 and 80 nm dot diameter with a high aspect ratio of about 8. It is found that the process has a high potential for microstructure fabrication in several tenth nanometer regions.
1. INTRODUCTION Maskless processing using focused ion beam (FIB) is potentially important in microfabrications with a dimension less than 100 nm. This is because the FIB has a small beam diameter with less than 100 nm, a high ion current density of about 1 A/cm2 and many ion species [1,21. The diameter of the FIB is one of important factors to fabrIcate the fine pattern. The beam diameter with several tenth ++ nanometers is obtained by+Be as an n-type dopant for GaAs and by Ga as a sputter etching ion in liquid metal ion sources [3,41. The FIB using those ion species can be expected to fabricate the fine pattern with several tenth nanometer regions. In maskless implantaion using Be ++ FIB, the line width of implanted region is larger than the diameter of the ion beam, because the incident ion is scattered in a solid [51. In maskless sputter etching and maskiess assisted etching of a solid using Ga FIB, the width of the etched line at the surface is lager than the diameter of the ion beam [6,71. The cross-sectional profile of the etched sample is not a rectangle shape. The spreading of fabricated width compared with the beam diameter is due to the large tail of the current density profile in FIB [81. Therefore, ion implantation and ion etching using those FIB are difficult to fabricate the fine pattern with a high aspect ratio and a rectangle shape of the same size as the beam diameter with several tenth nanometers. On the other hand, maskless lithography using Be++ and Gaf FIB has reported on fabrication of several tenth nanometer patterns in PMMA resist, CMS resist and a negative-acting bilevel resist [9,10,111. We reported that a microstructure of GaAs with a high aspect ratio could be fabricated by Be++ fine FIB lithography and Cl2 reactive ion etching [121. In this paper, we report that microstructures of fine line and dot with high
0167-9317/89/$3.50
0 1989, Elsevier Science Publishers
B.V. (North-Holland)
aspect ratio and size less than 100 nm in GaAs can+te performed by maskless lithography using Be fine FIB and pattern transfer with reactive ion etching using electron beam excited plasma (EBEP) [131.
Z.EXPERIMENTAL
PROCEDURES
Maskless ion beam lithography was done by a 100 kV FIB system with Au-Si-Be ternary alloy ion source [141. A CMS (Toyo Soda Manufacturing Co. CxM-CMS (R)) resist layer with the film thickness of 300 nm was formed by spin coating on n-GaAs (100) substrate. Prebaking was performed at 120 OC for 30 min. The resist was exposed by a 100 kV Be++ fine FIB which has a diameter with several tenth nanometers [3,101. Fine patterning of resist was exposed by a single line scanning for the fine line pattern and a single pass irradiation for the dot pattern. The beam current of Be++ was about 1 PA. In the fabrication of the several tenth nanometer pattern, the precise focusing is one of important factors. The silver conductlng paste on resist was used as a focusing marker. After the ion beam exposure, the conducting paste was removed. The CMS resist on the GaAs substrate was developed by 200 set dip in a solution of isoamly acetate (iAAc) and ethylcellosolve (EC) with a 30:70 ratio. The rinse was ca,rried out by two step procSSStZS. The 1st rinse was 30 set dip in a solution of iAAc and EC with a lo:90 ratio. The 2nd rinse was a 30 set dip in isopropyl alcohol. Post baking was performed at 130 OC for 30 min. The transfer of the fine resist pattern on n-GaAs substrate was performed by Cl2 reactive ion etching using EBEP. This 1s because the low energy reactive ion etching has an advantage of the damage less [151. The acceleration voltage was 130 V. The etchlnq ion beam current density was about 2.1 mA/cm', measured on the stage without the suppressing secondary electron. The etching time was 50
T. Shiokawa et al. / Microstructures
278
sec. The etching rate ratio of virgin resist to GaAs was about 2.5. After the dry etching, the resist as a mask was removed by J-100 resist strip for 30 min at 100 OC. Microstructures of GaAs fabricated were observed by a scanning electron microscope (SEM).
3.
RESULTS AND DISCUSSION
TO fabricate the more fine pattern using FIB, it is necessary to use the beam at the lower beam current because the beam diameter increases with the beam current [141. The microfabrication using ion beam at lower beam current has several problems such as fine focusing and the stability of the FIB for long fabrication time. To obtain the fine focusing condition at low beam current, scanning ion microscope (SIM) photographs of the same regions of the conducting paste at two different ion beam current are shown in Figure 1 (a) and (b). The SIM photograph (a) at beam current of 0.7 pA shows the high resolution of the sever-al tenth nanometers at the suitable contrast.
of GaAs
current was determined by the contrast and the signal to noise ratio in SIM image. The beam current in the experiment system was about 0.5-1.0 pA. The beam diameter was estimated to be several tenth nanometers (40-60 nm) from the resolution in SIM image of latex [161. The shift of the beam position was estimated to be less than 100 nm for 5 min from the image shift between SIM photographs in a high magnification. The time of 5 min in position stability is much longer than the scanning time less than 1 set for fine line and the beam dwell time less than 10m3 set for dot.
Figure
(a)
-1
Urn
2
Figure 2 shows a SEM photograph of the microstructure of GaAs with the minimum width fabricated by 100 kV Be++ fine FIB lithography The using EBEP. and Cl2 reactive ion etching line dose of the FIB was 3.4 x lo7 ions/cm. The SEM photograph shows the cleaved surface of fabricated GaAs. The fabricated GaAs structure is continuous in length of 0.3 mm. The width and height are about 40 nm and 680 nm, respecThe minimum width is nearly equal to tively. The microstructure of GaAs the beam diameter. has a high aspect ratio of about 17 and the vertical side wall.
Figure 1 The SIM photograph (b) at beam current of 0.08 pA do not show clearly the microstructure of several tenth nanometers because of the low signal to noise ratio. Therefore, the fine FIB at low beam current can not be used in the fabrication of the microstructure with several The practical beam tenth nanometer regions.
-
100 nm Figure
3
Figure 3 shows 70 degree tilt angle SEM photograph of fabricated GaAs dot with the minimum diameter. The dose of the FIB is about
T. Shiokuwa et al. / Microstructures
240 ions (beam dwell time is 0.14 Ins). The diameter and the height of the GaAs dot are about 80 "m and 660 "m, respectively. The dot has the vertical side wall and a high aspect ratio of about 8. The fabricated GaAs has characteristic microstructures of a high aspect ratio with a height of about 700 "m. The height of fabricated GaAs is larger than the projected range (576 nm) for 100 kV Be++ in GaAs. In this case, the projected range in GaAs is reduced by the deceleration of ion through the resist film. The influence of the damage in GaAs during ion beam lithography may be reduced by the use of bottom region in fabricated GaAs.
4. CONCLUSION In conclusion, high aspect ratio microstructures of a 40 "m width fine line and an 80 "m diameter dot were fabricated on GaAs by 100 kV Be++ fine FIB lithography and Cl2 reactive ion etching using EBEP. The processes are attractive for fabrication of microstructures with several tenth nanometer regions because of maskless and dry ones.
REFERENCES [ll
Seliger, R.L., Ward, J.W., Wang, V., and Kubena, R.l., Appl.Phys.Lett. 34 (1979) 310. [2] Melngailis, J., J.Vac.Sci.Technol. B5 (1987) 469. [31 Arimoto, H., Takamori, A., Miyauchi, E. and Hashimoto, H., Jpn.J.Appl.Phys. 22 (1983) L280.
of GaAs
R.L.,Joyce, R.J., Ward, J.W., H.L., Stratton, F.P. and Brault, J.Vac.Sic.Technol. B6 (1988) 353. R.G., [5] Miyauchi, E., Arimoto, H., Bamba. Y., Takamori, A., Hashimoto, H., and UtsUmi. T., Jp".J.Appl.PhyS. 22 (1983) L423. [6] Kubena, R.L., seliger, R.L., and Stevens, E.H., Thin Solid Films 92 (1982) 165. [7] Ochiai, Y., Shihoyama, K., Shiokawa, T., Toyoda, K., Masuyama, A., Game, K., and B4 (1986) Namba, S., J.Vac.Sci.Technol. 333. [8] Kubena, R.L., and Ward, J.W., Appl.Phys. Lett. 51 (1987) 1960. [9] Shiokawa, T., Aoyagi, Y., Kim,P.H., Toyoda, K., and Namba, S., Jp".J.Appl.Phys. 23 (1984) L232. [lo] Matsui, S., Mori, K., Shiokawa. T., Toyoda, K., and Namba, S., J.Vac.Sci. Technol. B6 (1987) 853. [ll] Kubena, R.L., Joyce, R.J., Ward, J.W., Gravin,H.L., Stratton, F.P., and Brault, 50 (1987) 1589. R.G., Appl.Phys.Lett. [12] Shiokawa, T., Kim,P.H., Hamagaki, M., Hara, K., Aoyagi, Y., Toyoda, K., and Namba, S., Jpn.J.app1.Phy.s. 27 (1988) L1160. [13] Hara, T., Hamagaki,M., Sanda,A., Aoyagi, Y., and Namba,S., J.Vac.Sci.Technol. B5 (1987) 366. [14] Shiokawa, T., Kim, P.H., Toyoda, K., Namba, S., Matsui, T., and Game, K., J.Vac.Sci.Technol. Bl (1983) 1117. [151 Yu, J.-Z., Hara,T., Hamagaki, M., Yoshinaga, Y, Aoyagi, Y., and Namba, S., J.Vac.Sci.Technol. B6 (1988) to be published. [16] Ishitani, T., Tamura, H., Todokoro. H., J.Vac.Sci.Technol. 20 (1982) 80.
[4] Kubena, Garvin,
279