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Smooth and high-rate reactive ion etching of diamond
Diamond and Related Materials 11 (2002) 824–827
Smooth and high-rate reactive ion etching of diamond Yutaka Andoa,*, Yoshiki Nishibayashia, Koji Kobashia, Takashi Hiraob, Kenjiro Ourab a
FCT ProjectyJFCC, Center for Advanced Research Projects, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan b Faculty of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan
Abstract Diamond surfaces with patterned Al masks were etched by a reactive ion etching (RIE) system under conditions that the RF power was 100–280 W, the CF4yO2 ratio was 0–12.5% and the gas pressure 2–40 Pa. It was found that the roughness of the etched diamond surface decreased with an increase in the CF4 yO2 ratio, although this reduced the selective etching ratio of diamond against Al. The gas pressure also affected the surface roughness and the etching anisotropy. The etching rate of diamond considerably increased upon a small addition of CF4 in O2 . Based on these results, we were successful in an anisotropic etching of diamond at a very high rate (;9.5 mmyh) with a smooth etched surface (Ra -0.4 nm), a high selective etching ratio of diamond vs. Al, and a high aspect ratio (the heightydiameter was ;8 for array structures and ;25 for exceptional cases) by choosing appropriate etching conditions. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Reactive ion etching; Anisotropic etching; Field emission; Columnar structure
1. Introduction Diamond is receiving considerable attention as a promising material for high power semiconductor devices, cold cathodes, and micromechanical devices because of its unique properties such as negative electron affinity, high thermal conductivity, high hardness, and chemical inertness. Microfabrication of diamond is an inevitable technique to realize such applications. Considerable efforts have already been made to fabricate various structures on diamond for applications w1–6x. For instance, an etching rate of 12 mmyh with a selective etching ratio of 20 between diamond and an Al mask was achieved by ion-beam assisted etching using a 2keV Xeq beam with a reactive gas flux of nitrogen dioxide w1x. In w3x, a line and space pattern with a 1mm width was fabricated by RIE using a gas mixture of Ar and O2. In w4x, controlled surface morphologies from porous to smooth were achieved using a CF4 and O2 gas mixture in a RIE system. In the present work, we have developed a microfabri*Corresponding author. Tel.: q81-6-6879-4146; fax: q81-6-68794147. E-mail address: [email protected] (Y. Ando).
cation technique of diamond by investigating the etching rate and the etching profiles as functions of the RF power, the CF4 yO2 ratio, and the gas pressure to make sharp columnar structures. 2. Experimental A reactive ion etching system (Anelva L-201D-L) with a maximum 300 W RF (13.56 MHz) power was used for etching diamond. The base pressure was less than 9.8=10y5 Pa. The diamonds used were HP-HT single crystal Ib diamonds with (100) surface and Bdoped diamond with (100) and (111) surfaces, and Al masks were formed on diamonds by conventional photolithography. The flow rate of the process gases, CF4 and O2 was controlled by mass flow controllers so that the CF4 yO2 ratios were below 25%. Total gas pressure was changed from 2 to 40 Pa. The RF power was changed from 100 to 280 W. The sample holder was water-cooled, but the diamond temperature, measured by an optical pyrometer, increased to 470–570 K during the experiments.
0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 6 1 7 - 3
Y. Ando et al. / Diamond and Related Materials 11 (2002) 824–827
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3. Results and discussion
Fig. 1. SEM micrographs of diamond surface etched under different CF4yO2 flow ratios.
An optical microscopy, a scanning electron microscopy (SEM), and an atomic force microscopy (AFM) were used to observe the diamond surface morphology, the etching depth, and the surface roughness. X-Ray photoelectron spectroscopy (XPS) was used to investigate atomic compositions at etched diamond surfaces.
Fig. 2. XPS spectra observed for etched diamond surface.
The etching rates of diamond by pure O2 plasma under different RF powers was measured as reference experiments. The etching rate increased from 1.3 to 2.5 mmyh with an increase of the RF power from 100 to 200 W, and then saturated for higher RF power. Since the RIE system we used is designed to use a lower power for longer time, the RF power was fixed at 200 W for ordinary experiments. Fig. 1 show the surface morphology of the etched diamond under different CF4 y O2 flow ratios. The RF power and the gas pressure were 200 W and 5 Pa, respectively. The cylindrical or Lshaped projections in Fig. 1 are the Al masked areas. The diamond surface etched by pure O2 plasma is extremely rough as seen in Fig. 1a. Upon a small addition of CF4 to the O2 plasma, however, the morphology of etched surface changed from a porous to a needle-like shape (see Fig. 1a,b,c). A further addition of CF4 led to a smooth etched surface, although the Al mask also etched and removed (see Fig. 1d,e). Fig. 2 shows XPS spectra for the diamond surfaces etched with O2 or O2qCF4 plasma. The C1s, O1s, and OKLL bands were clearly seen for both surfaces. Note that the bands of Si and Fe are impurities from the substrate holder, sputtered by O2 plasma. It seemed that the sputtered materials from the substrate acted as micromasks, because their etching rates are lower than that of diamond w7x. Thus, the change in surface roughness observed in Fig. 1 is ascribed to such micromasks by sputtering the substrate holder. It is known in case of Si etching that a polymer film is deposited during etching using halogen-containing gases, which leads to a smooth etching surface w8x. A similar effect is expected for diamond etching using CF4 containing gases. Fig. 3 shows the morphology of the diamond surface etched under different gas pressures. The RF power and the CF4 yO2 flow ratios were 200 W and 5%, respectively. It was found that the lowering of the gas pressure led to a smooth surface. The shallow hollows, observed at the etched surface shown in Fig. 3a, originated from the mechanical polishing of the as-received diamond (see Fig. 4). We found that the surface roughness of asreceived diamond strongly affects the etching results of
Fig. 3. SEM micrographs of diamond surface etched under different gas pressures.
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Y. Ando et al. / Diamond and Related Materials 11 (2002) 824–827
Fig. 4. Diamond surface before (a) and after etching (b).
Fig. 5. SEM micrographs of diamond microcylindrical array fabricated by RIE.
diamond. Thus, we fabricated a microcylindrical array of diamond using a diamond with a very smooth etched surface, as shown in Fig. 5. The mean roughness Ra for the etched diamond surface was less than 0.4 nm by AFM measurements (see Fig. 6). This value, RaF0.4 nm, is smaller than the value for the diamond substrate surface before etching. Fig. 7 shows a typical example
Fig. 7. Typical examples of diamond microcylinders fabricated by RIE.
of diamond microcylinder fabricated by RIE. The maximum etching depth of the diamond was approximately 22.5 mm, as shown in Fig. 7a. Since the microcylinder with the 22.5-mm height shown in Fig. 7a required 140 min, the average etching rate of diamond is approxi-
Fig. 6. Diamond surface before (a) and after etching (b) observed by AFM.
Y. Ando et al. / Diamond and Related Materials 11 (2002) 824–827
Fig. 8. SEM micrographs of B-doped diamond microcylindrical array fabricated by RIE.
mately 9.5 mmyh. The etching rate of the diamond increased from 2.5 to 9.5 mmyh with a small addition of CF4 in O2. It is not known at the present stage how the etching rate increased so much by only a small addition of CF4. The fact that the density of atomic oxygen in the plasma increases by addition of CF4 w9x can be one of the reasons for increasing the etching rate. Fig. 7b shows the diamond microcylinder with a maximum aspect ratio (heightydiameter) of approximately 25 that we have achieved. The maximum aspect ratio of diamond microcylinders with an array structure was approximately 8, as shown in Fig. 7c. From the result of Fig. 7b, we believe that the maximum aspect ratio can be increased by optimizing the Al lithography technique. Similar etching experiments were applied for B-doped (100) and (111) diamond. As a result, microcylindrical arrays with high aspect ratios and a smooth etched surface were fabricated, irrespective of doping and crystal orientation (as shown in Fig. 8). 4. Conclusion Diamond surfaces with patterned Al masks were etched by reactive ion etching using a gas mixture of
827
CF4 and O2. It was found that the surface roughness of the etched diamond decreased with an increase of CF4 y O2 ratio, although the increase of CF4 yO2 ratio reduced the selective etching ratio of diamond against Al. The gas pressure also affected the surface roughness and the anisotropy of etching. The etching rate of diamond considerably increased from 2.5 to 9.5 mmyh by a small addition of CF4 in O2. The etched diamond surface was very smooth, RaF0.4 nm. The diamond microcylinders with very high aspect ratios (the heightydiameter was approx. 8 for array structures and approx. 25 for exceptional cases) were successfully fabricated. Acknowledgments This work was supported by the FCT Project, which was consigned to JFCC by NEDO. References w1x N.N. Efremow, M.W. Geis, D.C. Flanders, G.A. Lincoln, N.P. Economou, J. Vac. Sci. Technol. B 3 (1) (1985) 416. w2x G.S. Sandhu, W.K. Chu, Appl. Phys. Lett. 55 (5) (1989) 437. w3x S. Shikata, Y. Nishibayashi, T. Tomikawa, N. Toda, N. Fujimori, Proceedings of the Second International Conference on the Applications of Diamond Films and Related Materials, MYU, Tokyo, 1993. w4x H. Shiomi, Jpn. J. Appl. Phys. 36 (1997) 7745. w5x S. Okuyama, S.I. Matsushita, A. Fujishima, Chem. Lett. (2000) 534. w6x H. Masuda, M. Watanabe, K. Yasui, D. Tryk, T. Rao, A. Fujishima, Adv. Mater. 12 (6) (2000) 444. w7x E.S. Baik, Y.J. Baik, J. Mater. Res. 15 (4) (2000) 923. w8x K. Hirobe, K. Kawamura, K. Nojiri, J. Vac. Sci. Technol. B 5 (2) (1987) 594. w9x F.D. Egitto, F. Emmi, R.S. Horwath, V. Vukanovic, J. Vac. Sci. Technol. B 3 (3) (1985) 893.
Smooth and high-rate reactive ion etching of diamond Yutaka Andoa,*, Yoshiki Nishibayashia, Koji Kobashia, Takashi Hiraob, Kenjiro Ourab a
FCT ProjectyJFCC, Center for Advanced Research Projects, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan b Faculty of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan
Abstract Diamond surfaces with patterned Al masks were etched by a reactive ion etching (RIE) system under conditions that the RF power was 100–280 W, the CF4yO2 ratio was 0–12.5% and the gas pressure 2–40 Pa. It was found that the roughness of the etched diamond surface decreased with an increase in the CF4 yO2 ratio, although this reduced the selective etching ratio of diamond against Al. The gas pressure also affected the surface roughness and the etching anisotropy. The etching rate of diamond considerably increased upon a small addition of CF4 in O2 . Based on these results, we were successful in an anisotropic etching of diamond at a very high rate (;9.5 mmyh) with a smooth etched surface (Ra -0.4 nm), a high selective etching ratio of diamond vs. Al, and a high aspect ratio (the heightydiameter was ;8 for array structures and ;25 for exceptional cases) by choosing appropriate etching conditions. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Reactive ion etching; Anisotropic etching; Field emission; Columnar structure
1. Introduction Diamond is receiving considerable attention as a promising material for high power semiconductor devices, cold cathodes, and micromechanical devices because of its unique properties such as negative electron affinity, high thermal conductivity, high hardness, and chemical inertness. Microfabrication of diamond is an inevitable technique to realize such applications. Considerable efforts have already been made to fabricate various structures on diamond for applications w1–6x. For instance, an etching rate of 12 mmyh with a selective etching ratio of 20 between diamond and an Al mask was achieved by ion-beam assisted etching using a 2keV Xeq beam with a reactive gas flux of nitrogen dioxide w1x. In w3x, a line and space pattern with a 1mm width was fabricated by RIE using a gas mixture of Ar and O2. In w4x, controlled surface morphologies from porous to smooth were achieved using a CF4 and O2 gas mixture in a RIE system. In the present work, we have developed a microfabri*Corresponding author. Tel.: q81-6-6879-4146; fax: q81-6-68794147. E-mail address: [email protected] (Y. Ando).
cation technique of diamond by investigating the etching rate and the etching profiles as functions of the RF power, the CF4 yO2 ratio, and the gas pressure to make sharp columnar structures. 2. Experimental A reactive ion etching system (Anelva L-201D-L) with a maximum 300 W RF (13.56 MHz) power was used for etching diamond. The base pressure was less than 9.8=10y5 Pa. The diamonds used were HP-HT single crystal Ib diamonds with (100) surface and Bdoped diamond with (100) and (111) surfaces, and Al masks were formed on diamonds by conventional photolithography. The flow rate of the process gases, CF4 and O2 was controlled by mass flow controllers so that the CF4 yO2 ratios were below 25%. Total gas pressure was changed from 2 to 40 Pa. The RF power was changed from 100 to 280 W. The sample holder was water-cooled, but the diamond temperature, measured by an optical pyrometer, increased to 470–570 K during the experiments.
0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 6 1 7 - 3
Y. Ando et al. / Diamond and Related Materials 11 (2002) 824–827
825
3. Results and discussion
Fig. 1. SEM micrographs of diamond surface etched under different CF4yO2 flow ratios.
An optical microscopy, a scanning electron microscopy (SEM), and an atomic force microscopy (AFM) were used to observe the diamond surface morphology, the etching depth, and the surface roughness. X-Ray photoelectron spectroscopy (XPS) was used to investigate atomic compositions at etched diamond surfaces.
Fig. 2. XPS spectra observed for etched diamond surface.
The etching rates of diamond by pure O2 plasma under different RF powers was measured as reference experiments. The etching rate increased from 1.3 to 2.5 mmyh with an increase of the RF power from 100 to 200 W, and then saturated for higher RF power. Since the RIE system we used is designed to use a lower power for longer time, the RF power was fixed at 200 W for ordinary experiments. Fig. 1 show the surface morphology of the etched diamond under different CF4 y O2 flow ratios. The RF power and the gas pressure were 200 W and 5 Pa, respectively. The cylindrical or Lshaped projections in Fig. 1 are the Al masked areas. The diamond surface etched by pure O2 plasma is extremely rough as seen in Fig. 1a. Upon a small addition of CF4 to the O2 plasma, however, the morphology of etched surface changed from a porous to a needle-like shape (see Fig. 1a,b,c). A further addition of CF4 led to a smooth etched surface, although the Al mask also etched and removed (see Fig. 1d,e). Fig. 2 shows XPS spectra for the diamond surfaces etched with O2 or O2qCF4 plasma. The C1s, O1s, and OKLL bands were clearly seen for both surfaces. Note that the bands of Si and Fe are impurities from the substrate holder, sputtered by O2 plasma. It seemed that the sputtered materials from the substrate acted as micromasks, because their etching rates are lower than that of diamond w7x. Thus, the change in surface roughness observed in Fig. 1 is ascribed to such micromasks by sputtering the substrate holder. It is known in case of Si etching that a polymer film is deposited during etching using halogen-containing gases, which leads to a smooth etching surface w8x. A similar effect is expected for diamond etching using CF4 containing gases. Fig. 3 shows the morphology of the diamond surface etched under different gas pressures. The RF power and the CF4 yO2 flow ratios were 200 W and 5%, respectively. It was found that the lowering of the gas pressure led to a smooth surface. The shallow hollows, observed at the etched surface shown in Fig. 3a, originated from the mechanical polishing of the as-received diamond (see Fig. 4). We found that the surface roughness of asreceived diamond strongly affects the etching results of
Fig. 3. SEM micrographs of diamond surface etched under different gas pressures.
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Y. Ando et al. / Diamond and Related Materials 11 (2002) 824–827
Fig. 4. Diamond surface before (a) and after etching (b).
Fig. 5. SEM micrographs of diamond microcylindrical array fabricated by RIE.
diamond. Thus, we fabricated a microcylindrical array of diamond using a diamond with a very smooth etched surface, as shown in Fig. 5. The mean roughness Ra for the etched diamond surface was less than 0.4 nm by AFM measurements (see Fig. 6). This value, RaF0.4 nm, is smaller than the value for the diamond substrate surface before etching. Fig. 7 shows a typical example
Fig. 7. Typical examples of diamond microcylinders fabricated by RIE.
of diamond microcylinder fabricated by RIE. The maximum etching depth of the diamond was approximately 22.5 mm, as shown in Fig. 7a. Since the microcylinder with the 22.5-mm height shown in Fig. 7a required 140 min, the average etching rate of diamond is approxi-
Fig. 6. Diamond surface before (a) and after etching (b) observed by AFM.
Y. Ando et al. / Diamond and Related Materials 11 (2002) 824–827
Fig. 8. SEM micrographs of B-doped diamond microcylindrical array fabricated by RIE.
mately 9.5 mmyh. The etching rate of the diamond increased from 2.5 to 9.5 mmyh with a small addition of CF4 in O2. It is not known at the present stage how the etching rate increased so much by only a small addition of CF4. The fact that the density of atomic oxygen in the plasma increases by addition of CF4 w9x can be one of the reasons for increasing the etching rate. Fig. 7b shows the diamond microcylinder with a maximum aspect ratio (heightydiameter) of approximately 25 that we have achieved. The maximum aspect ratio of diamond microcylinders with an array structure was approximately 8, as shown in Fig. 7c. From the result of Fig. 7b, we believe that the maximum aspect ratio can be increased by optimizing the Al lithography technique. Similar etching experiments were applied for B-doped (100) and (111) diamond. As a result, microcylindrical arrays with high aspect ratios and a smooth etched surface were fabricated, irrespective of doping and crystal orientation (as shown in Fig. 8). 4. Conclusion Diamond surfaces with patterned Al masks were etched by reactive ion etching using a gas mixture of
827
CF4 and O2. It was found that the surface roughness of the etched diamond decreased with an increase of CF4 y O2 ratio, although the increase of CF4 yO2 ratio reduced the selective etching ratio of diamond against Al. The gas pressure also affected the surface roughness and the anisotropy of etching. The etching rate of diamond considerably increased from 2.5 to 9.5 mmyh by a small addition of CF4 in O2. The etched diamond surface was very smooth, RaF0.4 nm. The diamond microcylinders with very high aspect ratios (the heightydiameter was approx. 8 for array structures and approx. 25 for exceptional cases) were successfully fabricated. Acknowledgments This work was supported by the FCT Project, which was consigned to JFCC by NEDO. References w1x N.N. Efremow, M.W. Geis, D.C. Flanders, G.A. Lincoln, N.P. Economou, J. Vac. Sci. Technol. B 3 (1) (1985) 416. w2x G.S. Sandhu, W.K. Chu, Appl. Phys. Lett. 55 (5) (1989) 437. w3x S. Shikata, Y. Nishibayashi, T. Tomikawa, N. Toda, N. Fujimori, Proceedings of the Second International Conference on the Applications of Diamond Films and Related Materials, MYU, Tokyo, 1993. w4x H. Shiomi, Jpn. J. Appl. Phys. 36 (1997) 7745. w5x S. Okuyama, S.I. Matsushita, A. Fujishima, Chem. Lett. (2000) 534. w6x H. Masuda, M. Watanabe, K. Yasui, D. Tryk, T. Rao, A. Fujishima, Adv. Mater. 12 (6) (2000) 444. w7x E.S. Baik, Y.J. Baik, J. Mater. Res. 15 (4) (2000) 923. w8x K. Hirobe, K. Kawamura, K. Nojiri, J. Vac. Sci. Technol. B 5 (2) (1987) 594. w9x F.D. Egitto, F. Emmi, R.S. Horwath, V. Vukanovic, J. Vac. Sci. Technol. B 3 (3) (1985) 893.