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Occurrence of particle debris field during focused Ga ion beam milling of glassy carbon
Applied Surface Science 256 (2010) 5952–5956
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Occurrence of particle debris field during focused Ga ion beam milling of glassy carbon Qin Hu, William O’Neill ∗ Centre for Industrial Photonics, Institute for Manufacturing, Department of Engineering, University of Cambridge, Alan Reece Building, 17 Charles Babbage Road, Cambridge, CB3 0FS, UK
a r t i c l e
i n f o
Article history: Received 9 February 2010 Accepted 18 March 2010 Available online 25 March 2010 Keywords: Focused ion beam (FIB) Glassy carbon Milling Nano-particles Ultrasonic Cleaning
a b s t r a c t To explore the machining characteristics of glassy carbon by focused ion beam (FIB), particles induced by FIB milling on glassy carbon have been studied in the current work. Nano-sized particles in the range of tens of nanometers up to 400 nm can often be found around the area subject to FIB milling. Two ion beam scanning modes – slow single scan and fast repetitive scan – have been tested. Fewer particles are found in single patterns milled in fast repetitive scan mode. For a group of test patterns milled in a sequence, it was found that a greater number of particles were deposited around sites machined early in the sequence. In situ EDX analysis of the particles showed that they were composed of C and Ga. The formation of particles is related to the debris generated at the surrounding areas, the low melting point of gallium used as FIB ion source and the high contact angle of gallium on glassy carbon induces de-wetting of Ga and the subsequent formation of Ga particles. Ultrasonic cleaning can remove over 98% of visible particles. The surface roughness (Ra ) of FIB milled areas after cleaning is less than 2 nm. © Published by Elsevier B.V.
1. Introduction Glassy carbon (GC) is an advanced non-graphitizing carbon material with combined properties of a glass and a ceramic. Its significant properties include high service temperature (up to 3000 ◦ C), low thermal expansion (<5 × 10−6 /K), extreme resistance to thermal shock and chemical attack [1–4]. So far GC is mainly used as material for electrodes and crucibles [5–10]. The application of GC as tool material for hot-embossing based glass imprinting has been reported [11,12]. Now we are extending this application in laser-based micro- and nano-lithography, and using GC as a template material in high temperature. This work involves exploring the machining characteristics of GC using a Ga source FIB machining system. In this study, a crossbeam focused ion beam (FIB)/scanning electron microscope (SEM) was used to mill micro-features on GC and take surface images in situ. Nano-sized particles were found at or around the FIB processed area. The particle phenomenon induced by FIB milling on glassy carbon has not been reported previously. In this paper, two different ion beam scanning modes were employed to machine micro-features on the surface of GC substrates. The distribution of particles was determined for a range of milling sequences. Ultrasonic cleaning was employed to remove particles from the body of the machined zones and the outlying surface.
∗ Corresponding author. Tel.: +44 01223 748272; fax: +44 01223 338076. E-mail address: [email protected] (W. O’Neill). 0169-4332/$ – see front matter © Published by Elsevier B.V. doi:10.1016/j.apsusc.2010.03.085
Atomic force microscopy (AFM) was used to characterize the newly formed structures. A mechanism for particle formation is proposed. 2. Experimental details Polished SIGRADUR G GC (Ra < 50 nm) supplied by HTW in Germany was used in this study. The GC has the following properties: service temperature up to 3000 ◦ C, no open porosity, Vickers hardness of 230 HV, flexural strength of 260 MPa, Young’s modulus of 35 GPa, compressive strength of 480 MPa, specific electrical resistance of 45 m (at 30 ◦ C), and thermal conductivity of 6.3 W/(K m) (at 30 ◦ C) [13]. FIB milling was conducted using a Crossbeam Zeiss 1540 FIB/SEM system with a gallium liquid metal ion source. Gallium ions were accelerated to 30 keV and the ion current was kept at 100 pA during the milling process. The FIB system has an analogue scan generator and two scanning modes were used in the current work, a slow single scan and a fast repetitive scan (see Fig. 1). In slow single scan mode each line is scanned for a certain time, sequentially one after another. The sectional line scan is moved across the interaction zone to produce the machined pocket. In this mode the scan speed is controlled by the system software. In fast repetitive scan mode, the ion beam scans over the patterned area in a fast raster scan. The scanning is controlled by scan frequency in x and y directions (fx , fy ). In current study fx = 20000 Hz and fy = 0.01 Hz. Surface morphology was imaged in situ by SEM via Zeiss 1540 FIB/SEM system, and ex situ by AFM in tapping mode via Veeco Dimension 3100 AFM/SPM system. Elemental analysis of particles
Q. Hu, W. O’Neill / Applied Surface Science 256 (2010) 5952–5956
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Fig. 1. Ga ion beam scanning modes: (a) slow single scan and (b) fast repetitive scan. In slow single scan mode each line is scanned for a certain time one after another one. In fast repetitive scan mode the ion beam scans repeatedly over the whole area until the desired depth is reached. The arrow indicates the scan direction.
was conducted by energy dispersive X-ray spectroscopy (EDS) from EDAX, which was configured within the Zeiss 1540 FIB/SEM system. 3. Results Directly after FIB milling, nano-sized particles can often be found in and around the milled areas. Fig. 2 shows a SEM image of two letters written on GC by the FIB system. The letters OO were part of a collection of 15 letters written on the surface. The whole collection were milled at 100 pA for 100 s. Slow single scan mode was used during the process. The gallium ion beam was scanned from top to bottom line by line. The production of these two letters occurred at the same time, and it can be seen that the particles are mainly distributed towards the upper part of the letters, the area machined first. Fig. 3(a) shows a group of square patterns milled on GC with a volume of 5 m × 5 m × 0.82 m at 100 pA for 15 min (corresponding to an average dose of 3.6 nC/m2 ) in fast repetitive scan mode. The numbers in Fig. 3(a) represent the milling sequence. The square pattern at the top-left was milled first, and the pattern at the bottom-right was milled last. The distance between two adjacent squares is 5 m. Many particles can be seen on the floor of the milled patterns, with exception of the last one to be milled. Fig. 3(b) shows the surface morphology of a corner of a milled pattern, with the sample tilted at 54◦ when the SEM image was taken. The size of the particles ranges from tens of nanometers up to
Fig. 2. SEM image of two letters written on glassy carbon by a FIB system at 100 pA. Ga ion beam scans line by line from top to bottom, and the two letters were milled at the same time. Nano-sized particles are found at the milled area, especially the upper part of letters. Sample was tilted 54◦ when the SEM image was taken.
100 nm. Very few particles adhere to the sidewalls of milled pockets. One example is illustrated in Fig. 3(c). Particles found on the sidewalls are normally larger than those found on the floor of the machined feature. The contact angle of particles adhering to the sidewalls is over 170◦ . Plan view images of the 2nd, 8th and 9th square in the milling sequence are shown in Fig. 3(d–f). The number of particles in the last two squares is much reduced, especially for the last square, where only one large particle was observed. Various milling sequences have been tested, with the result that the pattern milled last always has the minimum number of particles attached. In order to determine the nature of the particles, elemental analysis of the particles was conducted by EDS. The result for a particle adhering to the sidewall is shown in Fig. 4. A focused electron beam of energy 20 keV was incident on the centre of the particle, with a take-off angle of 29.90◦ . The intense electron irradiation did not cause any change in the shape or size of the particle. Only gallium and carbon signals were detected. This is not surprising since the GC is composed of carbon and the ion source is gallium. During the FIB milling process gallium ions were accelerated to 30 keV. The main ion–target interactions include milling, implantation and deposition. The processed area is left with some gallium ions due to deposition and implantation. This is confirmed by EDS analysis. The ion milling process can locally increase the surface temperature [14,15]. The temperature rise for Si, GaAs and SiO2 samples could be 2, 8 and 230 ◦ C, respectively, after 30 keV Ga+ bombardment at normal incidence [14]. The temperature of thin glass sample could reach 300 ◦ C in less than 2 s after 6 keV Ar+ bombardment at an incident angle of 80◦ [15]. FIB induced temperature rise for GC is not clear. Because the melting point of gallium is 29.8 ◦ C [16], several degrees above the room temperature, any gallium staying on the surface may be in liquid state during prolonged FIB processing. Because gallium does not wet on glassy carbon (the contact angle is ∼138◦ [17]), liquid gallium could combine with carbon debris and form particles. There are also some gallium ions in FIB unprocessed regions, due to ion scattering, although no particles are found in those regions and the amount of gallium may not be enough to initiate particle formation through de-wetting. FIB milling on silicon and ITO glass has been tested under the same parameter settings, similar particles were not found around the milled features. This may be attributed to the different surface energies for Si and ITO substrates. Comparing the single pattern milled by slow single scans (Fig. 2) and fast repetitive scans (Fig. 3(f)), it was found that fewer particles are left around the pattern milled by fast repetitive scans, although the pattern shown in Fig. 3(f) has been processed for a much longer time (more material has been sputtered away). In slow single scan mode, the ion beam scan line moves from the top of the pattern to the bottom until the desired pattern size is reached. The debris that
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Q. Hu, W. O’Neill / Applied Surface Science 256 (2010) 5952–5956
Fig. 3. (a) SEM image of a group of 5 m × 5 m square patterns on glassy carbon. Each square pattern was milled at 100 pA for 15 min (corresponding to a dose of 3.6 nC/m2 ) in fast repetitive mode. The number under each pattern represents the milling sequence. (b) Corner of a milled pattern. (c) Particle sticking on the sidewall. (d)–(f) The 2nd, 8th and 9th pattern in the group. Sample was tilted 54◦ when SEM images (b) and (c) were taken.
is deposited on previous scanned area (upper part) may combine with local gallium and form particles. While the debris deposited on the forward scan area (lower part) may be removed by scanning. Similar to the case of fast repetitive scan, debris on the patterned area may be removed by successive scans. So directly after the milling process, fewer particles are found for the pattern milled in fast repetitive scan mode. If only one single pattern is to be milled on glassy carbon, fast repetitive scan mode is recommended. This scan mode is also suggested for features that need flat bottoms, vertical sidewalls and high-aspect-ratios. Ultrasonic cleaning in de-ionized water was used to remove the particles after FIB milling. Results show that over 98% of the particles can be removed. Fig. 5(a) shows a square pattern after ultrasonic cleaning for 10 min. The morphology of the pattern before cleaning is shown in Fig. 3(d). At the site of each particle there is a small surface feature left on surface after ultrasonic cleaning. The AFM image taken at the base of the pocket is shown in Fig. 5(b). The surface roughness Ra (arithmetic average) of measured area is 1.60 nm, and Rq (root mean square) is 2.19 nm. Although the formation of particles increases surface roughness, after simple
cleaning, a high quality surface with Ra less than 2 nm can still be achieved. Fig. 6(a) shows the last square in the milling sequence after ultrasonic cleaning. The image of this pattern before cleaning is
Fig. 4. EDS analysis of a particle sitting on the sidewall of a machined square. Focused electron beam with energy of 20 kV was incident on the centre of the particle. The take-off angle was 29.90◦ .
Q. Hu, W. O’Neill / Applied Surface Science 256 (2010) 5952–5956
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Fig. 5. (a) SEM image of a pattern after ultrasonic cleaning in de-ionized water for 10 min. The image of the pattern before cleaning is shown in Fig. 3(d). All visible particles are removed and (b) AFM image of the base of the pattern.
shown in Fig. 3(f). The AFM image of the bottom of the pattern is illustrated in Fig. 6(b). The surface roughness Ra of the measured area is 0.347 nm, and Rq is 0.471 nm. As this region has no visible particles before cleaning, the measured surface roughness repre-
sents the roughness directly induced by FIB milling without particle adhesion. Compared with the region that has experienced a number of particle formation sites, e.g., the area shown in Fig. 5(b), particles can increase the final surface roughness by as much as 5 times. As a rule of thumb, regions that require better surface finish should be produced later in the milling sequence. 4. Conclusions Particles induced by FIB milling on glassy carbon have been studied in the current work. Nano-sized particles can often be found on the surface of milled patterns. If only one single pattern needs to be milled on glassy carbon, a fast repetitive scan mode is preferred over a slow single scan mode, as few particles are found. For a group of patterns milled within a sequence, most particles are found at neighbouring patterns milled early in the sequence. X-ray diffraction analysis of the particles have shown them to be composed of carbon and gallium. Ultrasonic cleaning can remove over 98% particles. After cleaning, regions that suffered particle formation were up to 5 times rougher than regions originally free of particles. Surfaces with a roughness value of Ra less than 2 nm were achieved for GC samples after FIB milling and ultrasonic cleaning. It is suggested that regions that require a good surface finish should be milled later in the milling sequence. Acknowledgment The work is part of 3D Mintegration Grand Challenge project (http://www.3d-mintegration.com/index.php) sponsored by UK’s Engineering and Physical Sciences Research Council (EPSRC Reference: EP/C534212/1). References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Fig. 6. (a) SEM image of a square after ultrasonic cleaning in de-ionized water for 10 min. The image of the pattern before cleaning is shown in Fig. 3(f) and (b) AFM image of the base of the pattern.
[10] [11]
R.E. Franklin, Proc. Roy. Soc. A 209 (1951) 196. P.J.F. Harris, Int. Mater. Rev. 42 (1997) 206. P.J.F. Harris, Philos. Mag. 84 (2004) 3159. G. M. Jenkins, K. Kawamura, Polymeric carbons—carbon fibre, glass and char, Cambridge, 1976. B. Brunetti, E. Desimoni, Electroanalysis 21 (2009) 772. M.E. Ghica, C.M.A. Brett, J. Electroanal. Chem. 629 (2009) 35. M.A. Montero, M.R.G. de Chialvo, A.C. Chialvo, J. Mater. Chem. 19 (2009) 3276. L. Gomez, R. Rodriguez-Amaro, Langmuir 25 (2009) 4799. M. Kuhnke, T. Lippert, E. Ortelli, G.G. Scherer, A. Wokaun, Thin Solid Films 453–454 (2004) 36. C.E.M. de Oliveira, M.M.G. de Carvalho, C.A.C. Mendonc¸a, C.R. Miskys, G.M. Guadalupi, M. Battagliarin, M.I.M.S. Bueno, J. Crystal Growth 186 (1998) 487. S.W. Youn, M. Takahashi, H. Goto, R. Maeda, Microelectron. Eng. 83 (2006) 2482.
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Q. Hu, W. O’Neill / Applied Surface Science 256 (2010) 5952–5956
[12] M. Takahashi, K. Sugimoto, R. Maeda, Jpn. J. Appl. Phys. 44 (2005) 5600. [13] Hochtemperatur Werkstoffe GmbH, http://www.htw-germany.com/ technology.php5?lang=en&nav0=2. [14] T. Ishitani, H. Kaga, J. Electron Microsc. 44 (1995) 331.
[15] Y.M. Park, D.-S. Ko, K.-W. Yi, I. Petrov, Y.-W. Kim, Ultramicroscopy 107 (2007) 663. [16] D.R. Lide, Handbook of Chemistry and Physics, 89th ed., CRC, 2008. [17] J.V. Naidich, J.N. Chuvashov, J. Mater. Sci. 18 (1983) 2071.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Occurrence of particle debris field during focused Ga ion beam milling of glassy carbon Qin Hu, William O’Neill ∗ Centre for Industrial Photonics, Institute for Manufacturing, Department of Engineering, University of Cambridge, Alan Reece Building, 17 Charles Babbage Road, Cambridge, CB3 0FS, UK
a r t i c l e
i n f o
Article history: Received 9 February 2010 Accepted 18 March 2010 Available online 25 March 2010 Keywords: Focused ion beam (FIB) Glassy carbon Milling Nano-particles Ultrasonic Cleaning
a b s t r a c t To explore the machining characteristics of glassy carbon by focused ion beam (FIB), particles induced by FIB milling on glassy carbon have been studied in the current work. Nano-sized particles in the range of tens of nanometers up to 400 nm can often be found around the area subject to FIB milling. Two ion beam scanning modes – slow single scan and fast repetitive scan – have been tested. Fewer particles are found in single patterns milled in fast repetitive scan mode. For a group of test patterns milled in a sequence, it was found that a greater number of particles were deposited around sites machined early in the sequence. In situ EDX analysis of the particles showed that they were composed of C and Ga. The formation of particles is related to the debris generated at the surrounding areas, the low melting point of gallium used as FIB ion source and the high contact angle of gallium on glassy carbon induces de-wetting of Ga and the subsequent formation of Ga particles. Ultrasonic cleaning can remove over 98% of visible particles. The surface roughness (Ra ) of FIB milled areas after cleaning is less than 2 nm. © Published by Elsevier B.V.
1. Introduction Glassy carbon (GC) is an advanced non-graphitizing carbon material with combined properties of a glass and a ceramic. Its significant properties include high service temperature (up to 3000 ◦ C), low thermal expansion (<5 × 10−6 /K), extreme resistance to thermal shock and chemical attack [1–4]. So far GC is mainly used as material for electrodes and crucibles [5–10]. The application of GC as tool material for hot-embossing based glass imprinting has been reported [11,12]. Now we are extending this application in laser-based micro- and nano-lithography, and using GC as a template material in high temperature. This work involves exploring the machining characteristics of GC using a Ga source FIB machining system. In this study, a crossbeam focused ion beam (FIB)/scanning electron microscope (SEM) was used to mill micro-features on GC and take surface images in situ. Nano-sized particles were found at or around the FIB processed area. The particle phenomenon induced by FIB milling on glassy carbon has not been reported previously. In this paper, two different ion beam scanning modes were employed to machine micro-features on the surface of GC substrates. The distribution of particles was determined for a range of milling sequences. Ultrasonic cleaning was employed to remove particles from the body of the machined zones and the outlying surface.
∗ Corresponding author. Tel.: +44 01223 748272; fax: +44 01223 338076. E-mail address: [email protected] (W. O’Neill). 0169-4332/$ – see front matter © Published by Elsevier B.V. doi:10.1016/j.apsusc.2010.03.085
Atomic force microscopy (AFM) was used to characterize the newly formed structures. A mechanism for particle formation is proposed. 2. Experimental details Polished SIGRADUR G GC (Ra < 50 nm) supplied by HTW in Germany was used in this study. The GC has the following properties: service temperature up to 3000 ◦ C, no open porosity, Vickers hardness of 230 HV, flexural strength of 260 MPa, Young’s modulus of 35 GPa, compressive strength of 480 MPa, specific electrical resistance of 45 m (at 30 ◦ C), and thermal conductivity of 6.3 W/(K m) (at 30 ◦ C) [13]. FIB milling was conducted using a Crossbeam Zeiss 1540 FIB/SEM system with a gallium liquid metal ion source. Gallium ions were accelerated to 30 keV and the ion current was kept at 100 pA during the milling process. The FIB system has an analogue scan generator and two scanning modes were used in the current work, a slow single scan and a fast repetitive scan (see Fig. 1). In slow single scan mode each line is scanned for a certain time, sequentially one after another. The sectional line scan is moved across the interaction zone to produce the machined pocket. In this mode the scan speed is controlled by the system software. In fast repetitive scan mode, the ion beam scans over the patterned area in a fast raster scan. The scanning is controlled by scan frequency in x and y directions (fx , fy ). In current study fx = 20000 Hz and fy = 0.01 Hz. Surface morphology was imaged in situ by SEM via Zeiss 1540 FIB/SEM system, and ex situ by AFM in tapping mode via Veeco Dimension 3100 AFM/SPM system. Elemental analysis of particles
Q. Hu, W. O’Neill / Applied Surface Science 256 (2010) 5952–5956
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Fig. 1. Ga ion beam scanning modes: (a) slow single scan and (b) fast repetitive scan. In slow single scan mode each line is scanned for a certain time one after another one. In fast repetitive scan mode the ion beam scans repeatedly over the whole area until the desired depth is reached. The arrow indicates the scan direction.
was conducted by energy dispersive X-ray spectroscopy (EDS) from EDAX, which was configured within the Zeiss 1540 FIB/SEM system. 3. Results Directly after FIB milling, nano-sized particles can often be found in and around the milled areas. Fig. 2 shows a SEM image of two letters written on GC by the FIB system. The letters OO were part of a collection of 15 letters written on the surface. The whole collection were milled at 100 pA for 100 s. Slow single scan mode was used during the process. The gallium ion beam was scanned from top to bottom line by line. The production of these two letters occurred at the same time, and it can be seen that the particles are mainly distributed towards the upper part of the letters, the area machined first. Fig. 3(a) shows a group of square patterns milled on GC with a volume of 5 m × 5 m × 0.82 m at 100 pA for 15 min (corresponding to an average dose of 3.6 nC/m2 ) in fast repetitive scan mode. The numbers in Fig. 3(a) represent the milling sequence. The square pattern at the top-left was milled first, and the pattern at the bottom-right was milled last. The distance between two adjacent squares is 5 m. Many particles can be seen on the floor of the milled patterns, with exception of the last one to be milled. Fig. 3(b) shows the surface morphology of a corner of a milled pattern, with the sample tilted at 54◦ when the SEM image was taken. The size of the particles ranges from tens of nanometers up to
Fig. 2. SEM image of two letters written on glassy carbon by a FIB system at 100 pA. Ga ion beam scans line by line from top to bottom, and the two letters were milled at the same time. Nano-sized particles are found at the milled area, especially the upper part of letters. Sample was tilted 54◦ when the SEM image was taken.
100 nm. Very few particles adhere to the sidewalls of milled pockets. One example is illustrated in Fig. 3(c). Particles found on the sidewalls are normally larger than those found on the floor of the machined feature. The contact angle of particles adhering to the sidewalls is over 170◦ . Plan view images of the 2nd, 8th and 9th square in the milling sequence are shown in Fig. 3(d–f). The number of particles in the last two squares is much reduced, especially for the last square, where only one large particle was observed. Various milling sequences have been tested, with the result that the pattern milled last always has the minimum number of particles attached. In order to determine the nature of the particles, elemental analysis of the particles was conducted by EDS. The result for a particle adhering to the sidewall is shown in Fig. 4. A focused electron beam of energy 20 keV was incident on the centre of the particle, with a take-off angle of 29.90◦ . The intense electron irradiation did not cause any change in the shape or size of the particle. Only gallium and carbon signals were detected. This is not surprising since the GC is composed of carbon and the ion source is gallium. During the FIB milling process gallium ions were accelerated to 30 keV. The main ion–target interactions include milling, implantation and deposition. The processed area is left with some gallium ions due to deposition and implantation. This is confirmed by EDS analysis. The ion milling process can locally increase the surface temperature [14,15]. The temperature rise for Si, GaAs and SiO2 samples could be 2, 8 and 230 ◦ C, respectively, after 30 keV Ga+ bombardment at normal incidence [14]. The temperature of thin glass sample could reach 300 ◦ C in less than 2 s after 6 keV Ar+ bombardment at an incident angle of 80◦ [15]. FIB induced temperature rise for GC is not clear. Because the melting point of gallium is 29.8 ◦ C [16], several degrees above the room temperature, any gallium staying on the surface may be in liquid state during prolonged FIB processing. Because gallium does not wet on glassy carbon (the contact angle is ∼138◦ [17]), liquid gallium could combine with carbon debris and form particles. There are also some gallium ions in FIB unprocessed regions, due to ion scattering, although no particles are found in those regions and the amount of gallium may not be enough to initiate particle formation through de-wetting. FIB milling on silicon and ITO glass has been tested under the same parameter settings, similar particles were not found around the milled features. This may be attributed to the different surface energies for Si and ITO substrates. Comparing the single pattern milled by slow single scans (Fig. 2) and fast repetitive scans (Fig. 3(f)), it was found that fewer particles are left around the pattern milled by fast repetitive scans, although the pattern shown in Fig. 3(f) has been processed for a much longer time (more material has been sputtered away). In slow single scan mode, the ion beam scan line moves from the top of the pattern to the bottom until the desired pattern size is reached. The debris that
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Q. Hu, W. O’Neill / Applied Surface Science 256 (2010) 5952–5956
Fig. 3. (a) SEM image of a group of 5 m × 5 m square patterns on glassy carbon. Each square pattern was milled at 100 pA for 15 min (corresponding to a dose of 3.6 nC/m2 ) in fast repetitive mode. The number under each pattern represents the milling sequence. (b) Corner of a milled pattern. (c) Particle sticking on the sidewall. (d)–(f) The 2nd, 8th and 9th pattern in the group. Sample was tilted 54◦ when SEM images (b) and (c) were taken.
is deposited on previous scanned area (upper part) may combine with local gallium and form particles. While the debris deposited on the forward scan area (lower part) may be removed by scanning. Similar to the case of fast repetitive scan, debris on the patterned area may be removed by successive scans. So directly after the milling process, fewer particles are found for the pattern milled in fast repetitive scan mode. If only one single pattern is to be milled on glassy carbon, fast repetitive scan mode is recommended. This scan mode is also suggested for features that need flat bottoms, vertical sidewalls and high-aspect-ratios. Ultrasonic cleaning in de-ionized water was used to remove the particles after FIB milling. Results show that over 98% of the particles can be removed. Fig. 5(a) shows a square pattern after ultrasonic cleaning for 10 min. The morphology of the pattern before cleaning is shown in Fig. 3(d). At the site of each particle there is a small surface feature left on surface after ultrasonic cleaning. The AFM image taken at the base of the pocket is shown in Fig. 5(b). The surface roughness Ra (arithmetic average) of measured area is 1.60 nm, and Rq (root mean square) is 2.19 nm. Although the formation of particles increases surface roughness, after simple
cleaning, a high quality surface with Ra less than 2 nm can still be achieved. Fig. 6(a) shows the last square in the milling sequence after ultrasonic cleaning. The image of this pattern before cleaning is
Fig. 4. EDS analysis of a particle sitting on the sidewall of a machined square. Focused electron beam with energy of 20 kV was incident on the centre of the particle. The take-off angle was 29.90◦ .
Q. Hu, W. O’Neill / Applied Surface Science 256 (2010) 5952–5956
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Fig. 5. (a) SEM image of a pattern after ultrasonic cleaning in de-ionized water for 10 min. The image of the pattern before cleaning is shown in Fig. 3(d). All visible particles are removed and (b) AFM image of the base of the pattern.
shown in Fig. 3(f). The AFM image of the bottom of the pattern is illustrated in Fig. 6(b). The surface roughness Ra of the measured area is 0.347 nm, and Rq is 0.471 nm. As this region has no visible particles before cleaning, the measured surface roughness repre-
sents the roughness directly induced by FIB milling without particle adhesion. Compared with the region that has experienced a number of particle formation sites, e.g., the area shown in Fig. 5(b), particles can increase the final surface roughness by as much as 5 times. As a rule of thumb, regions that require better surface finish should be produced later in the milling sequence. 4. Conclusions Particles induced by FIB milling on glassy carbon have been studied in the current work. Nano-sized particles can often be found on the surface of milled patterns. If only one single pattern needs to be milled on glassy carbon, a fast repetitive scan mode is preferred over a slow single scan mode, as few particles are found. For a group of patterns milled within a sequence, most particles are found at neighbouring patterns milled early in the sequence. X-ray diffraction analysis of the particles have shown them to be composed of carbon and gallium. Ultrasonic cleaning can remove over 98% particles. After cleaning, regions that suffered particle formation were up to 5 times rougher than regions originally free of particles. Surfaces with a roughness value of Ra less than 2 nm were achieved for GC samples after FIB milling and ultrasonic cleaning. It is suggested that regions that require a good surface finish should be milled later in the milling sequence. Acknowledgment The work is part of 3D Mintegration Grand Challenge project (http://www.3d-mintegration.com/index.php) sponsored by UK’s Engineering and Physical Sciences Research Council (EPSRC Reference: EP/C534212/1). References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Fig. 6. (a) SEM image of a square after ultrasonic cleaning in de-ionized water for 10 min. The image of the pattern before cleaning is shown in Fig. 3(f) and (b) AFM image of the base of the pattern.
[10] [11]
R.E. Franklin, Proc. Roy. Soc. A 209 (1951) 196. P.J.F. Harris, Int. Mater. Rev. 42 (1997) 206. P.J.F. Harris, Philos. Mag. 84 (2004) 3159. G. M. Jenkins, K. Kawamura, Polymeric carbons—carbon fibre, glass and char, Cambridge, 1976. B. Brunetti, E. Desimoni, Electroanalysis 21 (2009) 772. M.E. Ghica, C.M.A. Brett, J. Electroanal. Chem. 629 (2009) 35. M.A. Montero, M.R.G. de Chialvo, A.C. Chialvo, J. Mater. Chem. 19 (2009) 3276. L. Gomez, R. Rodriguez-Amaro, Langmuir 25 (2009) 4799. M. Kuhnke, T. Lippert, E. Ortelli, G.G. Scherer, A. Wokaun, Thin Solid Films 453–454 (2004) 36. C.E.M. de Oliveira, M.M.G. de Carvalho, C.A.C. Mendonc¸a, C.R. Miskys, G.M. Guadalupi, M. Battagliarin, M.I.M.S. Bueno, J. Crystal Growth 186 (1998) 487. S.W. Youn, M. Takahashi, H. Goto, R. Maeda, Microelectron. Eng. 83 (2006) 2482.
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Q. Hu, W. O’Neill / Applied Surface Science 256 (2010) 5952–5956
[12] M. Takahashi, K. Sugimoto, R. Maeda, Jpn. J. Appl. Phys. 44 (2005) 5600. [13] Hochtemperatur Werkstoffe GmbH, http://www.htw-germany.com/ technology.php5?lang=en&nav0=2. [14] T. Ishitani, H. Kaga, J. Electron Microsc. 44 (1995) 331.
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