- Email: [email protected]
Focused ion beam patterning of diamondlike carbon films
Diamond and Related Materials 8 (1999) 1246–1250
Focused ion beam patterning of diamondlike carbon films A. Stanishevsky * Institute for Plasma Research, University of Maryland, College Park, MD 20742, USA Received 8 September 1998; accepted 5 February 1999
Abstract For applications of diamondlike carbon films in optics, microelectronics and other fields, it is in some cases necessary to form submicron size patterns. A finely focused beam of 50 keV Ga+ ions was used to mill various patterns in amorphous carbon films prepared by a pulsed cathodic arc discharge method. The trenches with width down to 30 nm and depth-to-width ratios up to 25 have been milled. The minimum trench width down to 20 nm has been achieved. The nanotips with a radius down to 35–40 nm were fabricated. The influence of the focused ion beam parameters on the film’s surface modification as well as on the shape of fabricated pattern elements is discussed. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Amorphous carbon film; Focused ion beam; Ion milling; Patterning
1. Introduction Diamond and diamondlike carbon (DLC ) films are promising materials for a number of optical, electronic and mechanical applications. Recent examples which require that a specific pattern be formed in these films include field emitters [1], AFM tips [2], sensors [3] and lithographic masks [4]. While a selective nucleation of diamond films is widely used to grow patterned structures [5], laser cutting and dry etching are applied for both diamond and DLC films [6–8]. Selective deposition and laser cutting are rough to form patterns of a submicron size. Nanometric scale patterning of carbon films produced by scanning force microscopy has been reported [9]. Among the variety of the patterning techniques available, focused ion beams ( FIB) [10] provide high precision and flexibility of the pattern layouts. There are a few publications which mention FIB processing of diamond [11–15], though this method is common in making transmission electron microscopy samples of diamond films [12]. Mori and Yoshikawa [13] have shown that 25 keV Ga+ FIB allows fabrication of trenches with a width of 2–3 mm in diamond films. FIB was used by Taniguchi et al. [14] and Russell et al. [15] to fabricate a field emitter tip with a radius of <100 nm in a diamond crystal. The goal of this work is to study the influence of the * Fax: +1-301-314-9437. E-mail address: [email protected] (A. Stanishevsky)
beam parameters and the films’ properties on the capabilities of FIB fabrication of submicron patterns in hard amorphous carbon films.
2. Experimental In this work, a focused beam of Ga+ ions was used to mill submicron patterns in hydrogen-free amorphous carbon films. The main material used in the experiments was the high sp3/sp2 ratio hydrogen-free amorphous carbon films prepared by the filtered pulsed cathodic arc discharge (PCAD) method [16 ]. The discharge parameters were: pulse duration 300 ms; pulses repetition time 1 s; discharge current 4 kA; starting discharge voltage 400 V. The substrate temperature during the deposition process was 313 K. Carbon films with a thickness of 0.3–1.0 mm were deposited under these conditions. Earlier analysis of these films showed sp3-sites fraction of up to 80%, low concentration of impurities, a residual stress level of ca 8–10 GPa, and electric resistance of (4–8)108 Vcm [16 ]. These films will be referred to as tetrahedral amorphous carbon (ta-C ) films. One sample of PCAD carbon film was prepared at the substrate temperature of 723 K. This film had <10% sp3-bonded fraction, and will be referred to as graphitelike carbon (g-C ) film. Other samples for comparison of FIB milling results were bulk type IIa diamond crystals and microwave chemical vapor deposition
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A. Stanishevsky / Diamond and Related Materials 8 (1999) 1246–1250
(CVD) polycrystalline diamond film. The latter sample was prepared by the microwave CVD method using the mix of 2% CH in hydrogen at a gas pressure of 80 Torr 4 and a substrate temperature of 1223 K. The resulting film was polycrystalline diamond with a crystallite size of 1–2 mm. A Micrion FIB-2500 machine was used to pattern diamond and ta-C films. This machine is equipped with a liquid gallium ion source and operates at an accelerating voltage of up to 50 kV. The details of the FIB milling process can be found elsewhere [10]. The milling was performed by precise pixel-by-pixel replacement (digital scan) of the beam along the rectangular region or line. The beam dwell time in one particular pixel was set in the range of 1–50 ms whereas the distance between pixels was set constant at 10 nm to provide sufficient beam overlap. The total dose (D ) of ions impinging the i surface is expressed in nC mm−2, and 1 nC mm−2 corresponds to 6.24×1017 ions cm−2. The ion current range of 1.4–1500 pA with the corresponding beam diameter in the range of 6–100 nm was used in these experiments. FIB milled patterns were observed in situ in the FIB column using the secondary electron/secondary ion (SE/SI ) imaging capabilities of the equipment. The films’ thickness, depth and shape of milled patterns, and the surface roughness were estimated both from SE/SI
1247
images and atomic force microscopy ( TMX-2000 Explorer) data. The sp3/sp2 ratio and the specific mass density of ta-C films were known a priori from EELS and XPS data [16 ].
3. Results and discussion It is obvious that the interaction of energetic ion beams will modify the film’s surface, and will influence the properties of the fabricated structures. The material is not removed at an initial stage of milling due to the well-known effect of the swelling. The ion dose at which the swelling was observed is in the range of 6.25× 1015–3.12×1017 ions cm−2 (or 0.01–0.5 nC mm−2), as shown in Fig. 1. Even assuming that all the gallium ions were implanted, this could not produce such an expansion of the material. TRIM simulation showed that for the ion beam energy used, the most radiation damage occurs within the surface layer of 30 nm depth. The volume of the swelled material corresponds fairy well to the mass density change from 3.1 to 2.3 g cm−3 in this surface region. The first value is the mass density of the original film, while the second value is close to the density of g-C films [16 ]. The same effect was observed for CVD diamond film [17], but it was negligi-
Fig. 1. AFM image of swelling in FIB milled ta-C film on an initial stage of milling. The heights of the swelled regions with corresponding ion doses (in nC mm−2) are shown in inserts.
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A. Stanishevsky / Diamond and Related Materials 8 (1999) 1246–1250
ble for a g-C film. It should be noted that Wasson et al. [4] found that ion bombardment of amorphous graphitic film by 20 keV He+ ions destroys graphitic features, while the low density of the original film is maintained. McCulloch et al. [18] have shown that 200 keV Xe+ ion irradiation results in the sp2-bonded fraction of ~0.6 in ta-C films. Preliminary results of a Raman study show that a peak at 1550 cm−1 for FIB irradiated ta-C films is broadened and shifted to the lower wavenumbers with increasing ion dose. This indicates phase transformation, most probably, a lower degree of amorphization. The D-line appears at 1340 cm−1, and the absorption coefficient is increased. It is assumed that mostly sp2bonding between carbon atoms occurs. The results of FIB milling yield measurements for different carbon materials are given in Table 1. There is a slightly higher milling yield for CVD diamond film in comparison with bulk diamond. This is because the ion milling goes faster on the sloped crystalline facets in the polycrystalline film. The mean value of the milling yield for ta-C and g-C films follows the material mass density. The results given in Table 1 are for a beam current >30 pA. The milling yield was lower by a factor of 2.3 at a beam current of 3 pA, probably because of smaller beam overlap and redeposition effect. The milling yield is also significantly reduced when very narrow trenches are milled. The dwell time influences little the milling yield. However, it becomes an important factor when very narrow trenches are milled. Short dwell times (<1 ms) reduce the milling yield and may even totally hinder the trench formation for a high sp3-bonded carbon material [17]. The difference in the surface morphology of milled areas was negligible in the range of beam currents used. This means that in all cases the beam overlap was large enough to avoid nonuniform milling. AFM measurements taken after FIB milling of the 200 nm layer showed some increase of the surface roughness up to 0.35–0.45 nm, which originally was ca 0.2–0.3 nm. This result is consistent with tendencies of the surface morphology changes for other materials under ion bombard-
Fig. 2. Profiles of FIB milled trenches in ta-C film using beam current (numbered from the left) of 32, 3, 3, 32, 7, 7, 32 and 32 pA. Resulting trench sizes are shown in Table 2.
ment [19]. The slope of sidewalls of milled trenches strongly depends on the beam current. An example of FIB milled pattern at the different beam currents is shown in Fig. 2. The slope angle was calculated to change from 70 to 88° while the beam current dropped from 700 to 3 pA. The slope occurs because the beam tail rounds the edges of milled features and limits the smallest size of patterns. A lower beam current produces better quality patterns, but the time required to mill a pattern is much longer. Very fine patterns were formed at a beam current of 1.8–3 pA, as shown in Figs. 3 and 4. The width of the trench in Fig. 3 is estimated to be as small as 20 nm. Such a size can be partially explained by swelling of the sidewalls. The traces indicating the area of redeposition of sputtered material are visible at the open ends of the trench. The ion dose used in this case was 10 nC mm−2 and a trench depth of 300–350 nm was estimated. The size of trenches in the pattern shown in Fig. 4 is down to a 30 nm width while a depth-towidth ratio of up to 25 has been achieved. Some rounding of the edges and slightly sloped sidewalls in milled trenches are observed even at such a low current. The size of milled elements was measured at half of the depth at a 60° tilt. It should be noted that a different
Table 1 FIB milling and atomic yields of different carbon materials, the mean value of the milling yield for ta-C films follows the material mass density and the atomic yield was calculated as: Y (atom ion−1)=M rN e/A, where A is the atomic weight of the material, M is the volume milling yield, r 0 r r is the mass density, N is Avogadro’s number and e is the specific electron charge, the atomic yield for diamond may depend on the crystalline 0 orientation (Ref. [14]) Sample
Diamond
CVD diamond
ta-C, 0.4 mm thick
ta-C, 0.9 mm thick
g-C, 0.17 mm thick
Milling yield (m3 nC−1) Atomic yield (atoms ion−1) sp3-bonding (%) Specific mass (g cm−3) Film thickness (mm)
0.08 2.3 100 3.515 Bulk
0.1 2.8 100 3.5 2
0.1 2.5 80 3.25 0.35
0.12 2.9 45 2.9 0.92
0.14 2.8 <10 2.4 0.17
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A. Stanishevsky / Diamond and Related Materials 8 (1999) 1246–1250
Fig. 5. Process induced deformation of FIB milled stripes with the width of <80 nm.
Fig. 3. FIB milled 20 nm trench in ta-C film. Two areas of 200 nm depth were originally milled, and the trench connects these areas. The ion dose used for the trench formation was a priori found to provide the depth of ca 400 nm. The spread of the redeposited material is visible at the both ends of the trench.
trenches is larger than it was set. The beam scan procedure, as well as the beam tail, can be a reason. It was expected to have even wider trenches, taking into consideration the ion dose used. It should be remembered that the estimated beam diameter is <12 nm at the FIB parameters used. A combination of self-focusing effect [20,21], redeposition and swelling of the sidewall layer can also contribute to the observed result of milling. However, the detailed mechanism leading to such width and high aspect ratio of FIB milled trenches in ta-C is unclear. Reducing the width of the wall between the milled trenches results in its deformation, as shown in Fig. 5. The ion beam causes a modification to some surface region of the sidewalls. It may lead to the stress relaxation in this layer. Thus, the distribution of stress in the milled strip becomes nonuniform that results in the observed effect. Beam induced modification of the trench sidewall can influence the final shape and quality of the milled pattern. However, the deformation of the walls is reduced when the milling of trenches is performed simultaneously. An example of a FIB milled high aspect ratio structure in ta-C film on silicon at an optimized beam strategy and parameters is shown in Fig. 6. Minimum radius of the ta-C tip is 35–40 nm, while the total ta-C/Si tip height is ca 2 mm. Note that ta-C film thickness on the
Fig. 4. FIB milled trenches with the widths of 30–40 nm, and depthto-width ratios of up to 25. The details are shown in Table 2.
width was initially set for the trenches shown in Fig. 4. Table 2 gives results of the trench measurements in comparison to preset parameters. The width of milled
Table 2 The widths of FIB milled trenches in ta-C film in comparison with preset widths Trench No.
Preset trench size (nm) Milled trench size (nm) Ion dose (nC mm−2) Approx. trench depth (nm)
1
2
3
4
5
6
7
10 30 30 660
10 30 20 540
20 35 20 680
30 40 20 760
6 30 50 570
10 30 40 800
20 40 30 870
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A. Stanishevsky / Diamond and Related Materials 8 (1999) 1246–1250
Acknowledgements The author would like to thank Professor J. Melngailis for useful discussions on this work. Thanks are also due to Professor A. Badzian for an opportunity to use CVD equipment to prepare the diamond films, and Dr L.Khriachtchev for the Raman experiments.
References
Fig. 6. FIB milled ~2 mm tall tips in 0. 8 mm thick ta-C film on Si. The size of 200 nm was set for two left tips, and 300 nm for the third one (shown by arrows). Minimum radius of the tip is 35–40 nm. Note the position of ta-C/Si interface on the milled structures.
tip is reduced due to the overmilling, but it is still ~500 nm.
4. Conclusion In summary, it was demonstrated that it is possible to form high aspect ratio submicron patterns in hydrogen-free DLC films by FIB milling. A minimum size of the milled trench as small as 20 nm has been obtained. A number of structures with sizes below 100 nm and high aspect ratios have been fabricated. The results of FIB milling of ta-C film are affected partially by the properties and the structure of the particular carbon material used. Swelling and modification of the surface structure of the milled material were observed and it should be taken into account for specific pattern design.
[1] A. Wisitsora-at, W.P. Kang, J.L. Davidson, D.V. Kerns, Appl. Phys. Lett. 71 (1997) 3394. [2] W. Kulisch, A. Malave, G. Lippold, W. Scholz, C. Mihalcea, E. Oesterschulze, Diamond Relat. Mater. 6 (1997) 906. [3] P.R. Chalker, C. Johnston, Phys. Status Solidi A 154 (1996) 455. [4] J.R. Wasson, J.L. Torres, H.R. Rampersad, J.C. Wolfe, P. Ruchhoeft, M. Herbordt, H. Loschner, J. Vac. Sci. Technol. B 15 (1997) 2214. [5] P.G. Roberts, D.K. Milne, P. John, M.G. Jubber, J.I.B. Wilson, J. Mater. Res. 11 (1996) 3128. [6 ] R.B. Jackman, S.S.M. Chan, F. Raybould, G. Arthur, F. Goodall, Diamond Relat. Mater. 5 (1996) 317. [7] N.N. Efremov, M.W. Geis, D.C. Flanders, G.A. Lincoln, N.P. Economou, J. Vac. Sci. Technol. B 3 (1985) 416. [8] H. Shiomi, Jpn. J. Appl. Phys. 36 (1) (1997) 7745. [9] T. Muhl, H. Bruckl, G. Weise, G. Reiss, J. Appl. Phys. 82 (1997) 5255. [10] L.R. Harriott, Nucl. Instrum. Meth. Phys. Res. B 55 (1991) 802. [11] I. Miyamoto, Bull. Japan Soc. Precision Eng. 29 (1995) 295. [12] M. Tarutani, Jpn. J. Appl. Phys. Lett. 31 (1992) L1305. [13] Y. Mori, M. Yoshikawa, Diamond Film Technol. 6 (1996) 1. [14] J. Taniguchi, N. Ohno, S. Takeda, I. Miyamoto, M. Komuro, J. Vac. Sci. Technol. B 16 (1998) 2506. [15] P.E. Russell, T.J. Stark, D.P. Griffis, J.R. Phillips, K.F. Jarausch, J. Vac. Sci. Technol. B 16 (1998) 2494. [16 ] A. Stanishevsky, E. Tochitsky, J. CVD 4 (1996) 297. [17] A. Stanishevsky, J. Superhard Mater. 20 (6) (1998) 4. [18] D.G. McCulloch, E.G. Gerstner, D.R. McKenzie, S. Prawer, R. Kalish, Phys. Rev. B 52 (1995) 850. [19] Z. Csahok, Z. Farkas, M. Menyhard, G. Gergely, Cs.S. Daroczi, Surf. Sci. 364 (1996) L600. [20] J. Melngailis, J. Vac. Sci. Technol. B 5 (1987) 465. [21] W. Driesel, Phys. Stat. Solidi A 146 (1994) 523.
Focused ion beam patterning of diamondlike carbon films A. Stanishevsky * Institute for Plasma Research, University of Maryland, College Park, MD 20742, USA Received 8 September 1998; accepted 5 February 1999
Abstract For applications of diamondlike carbon films in optics, microelectronics and other fields, it is in some cases necessary to form submicron size patterns. A finely focused beam of 50 keV Ga+ ions was used to mill various patterns in amorphous carbon films prepared by a pulsed cathodic arc discharge method. The trenches with width down to 30 nm and depth-to-width ratios up to 25 have been milled. The minimum trench width down to 20 nm has been achieved. The nanotips with a radius down to 35–40 nm were fabricated. The influence of the focused ion beam parameters on the film’s surface modification as well as on the shape of fabricated pattern elements is discussed. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Amorphous carbon film; Focused ion beam; Ion milling; Patterning
1. Introduction Diamond and diamondlike carbon (DLC ) films are promising materials for a number of optical, electronic and mechanical applications. Recent examples which require that a specific pattern be formed in these films include field emitters [1], AFM tips [2], sensors [3] and lithographic masks [4]. While a selective nucleation of diamond films is widely used to grow patterned structures [5], laser cutting and dry etching are applied for both diamond and DLC films [6–8]. Selective deposition and laser cutting are rough to form patterns of a submicron size. Nanometric scale patterning of carbon films produced by scanning force microscopy has been reported [9]. Among the variety of the patterning techniques available, focused ion beams ( FIB) [10] provide high precision and flexibility of the pattern layouts. There are a few publications which mention FIB processing of diamond [11–15], though this method is common in making transmission electron microscopy samples of diamond films [12]. Mori and Yoshikawa [13] have shown that 25 keV Ga+ FIB allows fabrication of trenches with a width of 2–3 mm in diamond films. FIB was used by Taniguchi et al. [14] and Russell et al. [15] to fabricate a field emitter tip with a radius of <100 nm in a diamond crystal. The goal of this work is to study the influence of the * Fax: +1-301-314-9437. E-mail address: [email protected] (A. Stanishevsky)
beam parameters and the films’ properties on the capabilities of FIB fabrication of submicron patterns in hard amorphous carbon films.
2. Experimental In this work, a focused beam of Ga+ ions was used to mill submicron patterns in hydrogen-free amorphous carbon films. The main material used in the experiments was the high sp3/sp2 ratio hydrogen-free amorphous carbon films prepared by the filtered pulsed cathodic arc discharge (PCAD) method [16 ]. The discharge parameters were: pulse duration 300 ms; pulses repetition time 1 s; discharge current 4 kA; starting discharge voltage 400 V. The substrate temperature during the deposition process was 313 K. Carbon films with a thickness of 0.3–1.0 mm were deposited under these conditions. Earlier analysis of these films showed sp3-sites fraction of up to 80%, low concentration of impurities, a residual stress level of ca 8–10 GPa, and electric resistance of (4–8)108 Vcm [16 ]. These films will be referred to as tetrahedral amorphous carbon (ta-C ) films. One sample of PCAD carbon film was prepared at the substrate temperature of 723 K. This film had <10% sp3-bonded fraction, and will be referred to as graphitelike carbon (g-C ) film. Other samples for comparison of FIB milling results were bulk type IIa diamond crystals and microwave chemical vapor deposition
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 9 ) 0 0 11 0 - 7
A. Stanishevsky / Diamond and Related Materials 8 (1999) 1246–1250
(CVD) polycrystalline diamond film. The latter sample was prepared by the microwave CVD method using the mix of 2% CH in hydrogen at a gas pressure of 80 Torr 4 and a substrate temperature of 1223 K. The resulting film was polycrystalline diamond with a crystallite size of 1–2 mm. A Micrion FIB-2500 machine was used to pattern diamond and ta-C films. This machine is equipped with a liquid gallium ion source and operates at an accelerating voltage of up to 50 kV. The details of the FIB milling process can be found elsewhere [10]. The milling was performed by precise pixel-by-pixel replacement (digital scan) of the beam along the rectangular region or line. The beam dwell time in one particular pixel was set in the range of 1–50 ms whereas the distance between pixels was set constant at 10 nm to provide sufficient beam overlap. The total dose (D ) of ions impinging the i surface is expressed in nC mm−2, and 1 nC mm−2 corresponds to 6.24×1017 ions cm−2. The ion current range of 1.4–1500 pA with the corresponding beam diameter in the range of 6–100 nm was used in these experiments. FIB milled patterns were observed in situ in the FIB column using the secondary electron/secondary ion (SE/SI ) imaging capabilities of the equipment. The films’ thickness, depth and shape of milled patterns, and the surface roughness were estimated both from SE/SI
1247
images and atomic force microscopy ( TMX-2000 Explorer) data. The sp3/sp2 ratio and the specific mass density of ta-C films were known a priori from EELS and XPS data [16 ].
3. Results and discussion It is obvious that the interaction of energetic ion beams will modify the film’s surface, and will influence the properties of the fabricated structures. The material is not removed at an initial stage of milling due to the well-known effect of the swelling. The ion dose at which the swelling was observed is in the range of 6.25× 1015–3.12×1017 ions cm−2 (or 0.01–0.5 nC mm−2), as shown in Fig. 1. Even assuming that all the gallium ions were implanted, this could not produce such an expansion of the material. TRIM simulation showed that for the ion beam energy used, the most radiation damage occurs within the surface layer of 30 nm depth. The volume of the swelled material corresponds fairy well to the mass density change from 3.1 to 2.3 g cm−3 in this surface region. The first value is the mass density of the original film, while the second value is close to the density of g-C films [16 ]. The same effect was observed for CVD diamond film [17], but it was negligi-
Fig. 1. AFM image of swelling in FIB milled ta-C film on an initial stage of milling. The heights of the swelled regions with corresponding ion doses (in nC mm−2) are shown in inserts.
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A. Stanishevsky / Diamond and Related Materials 8 (1999) 1246–1250
ble for a g-C film. It should be noted that Wasson et al. [4] found that ion bombardment of amorphous graphitic film by 20 keV He+ ions destroys graphitic features, while the low density of the original film is maintained. McCulloch et al. [18] have shown that 200 keV Xe+ ion irradiation results in the sp2-bonded fraction of ~0.6 in ta-C films. Preliminary results of a Raman study show that a peak at 1550 cm−1 for FIB irradiated ta-C films is broadened and shifted to the lower wavenumbers with increasing ion dose. This indicates phase transformation, most probably, a lower degree of amorphization. The D-line appears at 1340 cm−1, and the absorption coefficient is increased. It is assumed that mostly sp2bonding between carbon atoms occurs. The results of FIB milling yield measurements for different carbon materials are given in Table 1. There is a slightly higher milling yield for CVD diamond film in comparison with bulk diamond. This is because the ion milling goes faster on the sloped crystalline facets in the polycrystalline film. The mean value of the milling yield for ta-C and g-C films follows the material mass density. The results given in Table 1 are for a beam current >30 pA. The milling yield was lower by a factor of 2.3 at a beam current of 3 pA, probably because of smaller beam overlap and redeposition effect. The milling yield is also significantly reduced when very narrow trenches are milled. The dwell time influences little the milling yield. However, it becomes an important factor when very narrow trenches are milled. Short dwell times (<1 ms) reduce the milling yield and may even totally hinder the trench formation for a high sp3-bonded carbon material [17]. The difference in the surface morphology of milled areas was negligible in the range of beam currents used. This means that in all cases the beam overlap was large enough to avoid nonuniform milling. AFM measurements taken after FIB milling of the 200 nm layer showed some increase of the surface roughness up to 0.35–0.45 nm, which originally was ca 0.2–0.3 nm. This result is consistent with tendencies of the surface morphology changes for other materials under ion bombard-
Fig. 2. Profiles of FIB milled trenches in ta-C film using beam current (numbered from the left) of 32, 3, 3, 32, 7, 7, 32 and 32 pA. Resulting trench sizes are shown in Table 2.
ment [19]. The slope of sidewalls of milled trenches strongly depends on the beam current. An example of FIB milled pattern at the different beam currents is shown in Fig. 2. The slope angle was calculated to change from 70 to 88° while the beam current dropped from 700 to 3 pA. The slope occurs because the beam tail rounds the edges of milled features and limits the smallest size of patterns. A lower beam current produces better quality patterns, but the time required to mill a pattern is much longer. Very fine patterns were formed at a beam current of 1.8–3 pA, as shown in Figs. 3 and 4. The width of the trench in Fig. 3 is estimated to be as small as 20 nm. Such a size can be partially explained by swelling of the sidewalls. The traces indicating the area of redeposition of sputtered material are visible at the open ends of the trench. The ion dose used in this case was 10 nC mm−2 and a trench depth of 300–350 nm was estimated. The size of trenches in the pattern shown in Fig. 4 is down to a 30 nm width while a depth-towidth ratio of up to 25 has been achieved. Some rounding of the edges and slightly sloped sidewalls in milled trenches are observed even at such a low current. The size of milled elements was measured at half of the depth at a 60° tilt. It should be noted that a different
Table 1 FIB milling and atomic yields of different carbon materials, the mean value of the milling yield for ta-C films follows the material mass density and the atomic yield was calculated as: Y (atom ion−1)=M rN e/A, where A is the atomic weight of the material, M is the volume milling yield, r 0 r r is the mass density, N is Avogadro’s number and e is the specific electron charge, the atomic yield for diamond may depend on the crystalline 0 orientation (Ref. [14]) Sample
Diamond
CVD diamond
ta-C, 0.4 mm thick
ta-C, 0.9 mm thick
g-C, 0.17 mm thick
Milling yield (m3 nC−1) Atomic yield (atoms ion−1) sp3-bonding (%) Specific mass (g cm−3) Film thickness (mm)
0.08 2.3 100 3.515 Bulk
0.1 2.8 100 3.5 2
0.1 2.5 80 3.25 0.35
0.12 2.9 45 2.9 0.92
0.14 2.8 <10 2.4 0.17
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A. Stanishevsky / Diamond and Related Materials 8 (1999) 1246–1250
Fig. 5. Process induced deformation of FIB milled stripes with the width of <80 nm.
Fig. 3. FIB milled 20 nm trench in ta-C film. Two areas of 200 nm depth were originally milled, and the trench connects these areas. The ion dose used for the trench formation was a priori found to provide the depth of ca 400 nm. The spread of the redeposited material is visible at the both ends of the trench.
trenches is larger than it was set. The beam scan procedure, as well as the beam tail, can be a reason. It was expected to have even wider trenches, taking into consideration the ion dose used. It should be remembered that the estimated beam diameter is <12 nm at the FIB parameters used. A combination of self-focusing effect [20,21], redeposition and swelling of the sidewall layer can also contribute to the observed result of milling. However, the detailed mechanism leading to such width and high aspect ratio of FIB milled trenches in ta-C is unclear. Reducing the width of the wall between the milled trenches results in its deformation, as shown in Fig. 5. The ion beam causes a modification to some surface region of the sidewalls. It may lead to the stress relaxation in this layer. Thus, the distribution of stress in the milled strip becomes nonuniform that results in the observed effect. Beam induced modification of the trench sidewall can influence the final shape and quality of the milled pattern. However, the deformation of the walls is reduced when the milling of trenches is performed simultaneously. An example of a FIB milled high aspect ratio structure in ta-C film on silicon at an optimized beam strategy and parameters is shown in Fig. 6. Minimum radius of the ta-C tip is 35–40 nm, while the total ta-C/Si tip height is ca 2 mm. Note that ta-C film thickness on the
Fig. 4. FIB milled trenches with the widths of 30–40 nm, and depthto-width ratios of up to 25. The details are shown in Table 2.
width was initially set for the trenches shown in Fig. 4. Table 2 gives results of the trench measurements in comparison to preset parameters. The width of milled
Table 2 The widths of FIB milled trenches in ta-C film in comparison with preset widths Trench No.
Preset trench size (nm) Milled trench size (nm) Ion dose (nC mm−2) Approx. trench depth (nm)
1
2
3
4
5
6
7
10 30 30 660
10 30 20 540
20 35 20 680
30 40 20 760
6 30 50 570
10 30 40 800
20 40 30 870
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A. Stanishevsky / Diamond and Related Materials 8 (1999) 1246–1250
Acknowledgements The author would like to thank Professor J. Melngailis for useful discussions on this work. Thanks are also due to Professor A. Badzian for an opportunity to use CVD equipment to prepare the diamond films, and Dr L.Khriachtchev for the Raman experiments.
References
Fig. 6. FIB milled ~2 mm tall tips in 0. 8 mm thick ta-C film on Si. The size of 200 nm was set for two left tips, and 300 nm for the third one (shown by arrows). Minimum radius of the tip is 35–40 nm. Note the position of ta-C/Si interface on the milled structures.
tip is reduced due to the overmilling, but it is still ~500 nm.
4. Conclusion In summary, it was demonstrated that it is possible to form high aspect ratio submicron patterns in hydrogen-free DLC films by FIB milling. A minimum size of the milled trench as small as 20 nm has been obtained. A number of structures with sizes below 100 nm and high aspect ratios have been fabricated. The results of FIB milling of ta-C film are affected partially by the properties and the structure of the particular carbon material used. Swelling and modification of the surface structure of the milled material were observed and it should be taken into account for specific pattern design.
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