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Micro and nano patterning by focused ion beam enhanced adhesion
Nuclear Instruments and Methods in Physics Research B 267 (2009) 1376–1380
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Micro and nano patterning by focused ion beam enhanced adhesion Neeraj Shukla, Sarvesh K. Tripathi, Mihir Sarkar, Nitul S. Rajput, Vishwas N. Kulkarni * Department of Physics, Indian Institute of Technology Kanpur, Kanpur 208 016, India
a r t i c l e
i n f o
Article history: Available online 29 January 2009 PACS: 68.35.Np 82.80.Ej 68.37.Hk Keywords: Thin film adhesion Focused ion beam EDS SEM Nano pattern generation
a b s t r a c t We report a new method of generating nano and micro patterns using focused ion beam (FIB) induced adhesion. The method utilizes selective irradiation of thin metallic films grown on substrates by focused ion beam followed by peel off. After peel off of the irradiated thin film it is observed that the ion beam scanned portions are retained on the substrate, creating nano and micro patterns. The method is suitable for materials of which the adhesion to the substrate can be improved by ion bombardment. The phenomenon has been demonstrated by creating gold nano patterns of different shapes and sizes ranging from 500 nm to 5 lm on SiO2–Si substrate using 10–30 keV Ga FIB at beam currents up to 10 pA. The mechanism involved in the process has been discussed. The technique could be utilized to prepare micro and nano patterns of thin films deposited on an appropriate substrate for optical, plasmonic and sensor related applications. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Nano patterns are becoming backbone of modern and futuristic technology for the fabrication of electronic, photonic, plasmonic and memory devices [1–3]. There are few ways of making nano patterns of thin films. Among them are electron beam lithography [4] and electron beam induced deposition (EBID) [5]. Conventional electron beam lithography is also popular for fabricating nanostructures which is a multi step process. EBID is versatile in making nano patterns of different shapes and sizes, but the process is limited to the few elemental species for which organometallic precursor gases are available. Also the patterns made by EBID invariably contain carbon and hydrogen as impurities. In the recent years, focused ion beam has emerged as a powerful tool for fabrication of structures of low dimensions. The technique utilizes cracking of precursor gas molecules (organometallic) adsorbed over a substrate under focused ion beam impact. The volatile component escapes the substrate and the residue remains on the surface as the grown material [6]. There are several ion matter interaction phenomena e.g. secondary ion/electron emission, ion beam induced adhesion etc. which can be gainfully exploited in the fabrication of nanostructures. Recently, we have used the effect of secondary ion emission to deposit Pt and W on the side walls of a nano hole [7].
In this paper we explore the phenomenon of focused ion beam induced adhesion for making nano and micro pattern of a metallic film on SiO2–Si substrate. The term adhesion implies that the thin film on the substrate does not deteriorate at the normal working temperature. The formation of a well attached thin film coating on a substrate is termed as good adhesion which depends mainly on the nature of substrate–film interface and their surface energies. Ion beams provide a variety of well established ways of improving adhesion of thin films on the substrates [8–11]. Enhancement of adhesion of thin films has been well established by several researchers using energetic ion bombardment of different substrate–film combinations [12,13]. Therefore, it is worth to explore the efficacy of the focused ion beams to achieve adhesion of thin films over a micro and nano dimensions. We have observed that irradiation of focused Ga ions in the energy range of 10– 30 keV enhances adhesion of thin gold film on SiO2 substrate. This phenomenon has been utilized, by combining it with the peel off technique [14], to fabricate micro and nano size patterns of Au film of different shapes viz, circle, square, star etc. on SiO2. The results of these investigations, comprising ion energy and fluence dependence, on the quality of the patterns i.e. the relative percentage retention of gold and the mechanism involved, are presented in this paper. 2. Experiment details
* Corresponding author. Tel.: +91 512 2597985/2597986; fax: +91 512 2590914/ 2590103. E-mail address: [email protected] (V.N. Kulkarni). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.01.048
Experiments have been performed using a dual beam FIB system equipped with a field emission Ga ion source, high resolution
N. Shukla et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1376–1380
1377
Fig. 1. Schematic diagram, depicting the peel off scheme.
ion optics, high resolution milling and gas chemistry functionalities for deposition of C, Pt and W along with e-beam patterning, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) facility. The schematic of the process adapted for making nano dimensional patterns is shown in Fig. 1(a) shows an as deposited Au film on SiO2–Si substrate. The cross sectional view of the film after Ga irradiation forming a pattern on the film is shown in Fig. 1(b)–(d) illustrate removal of the unirradiated portion of the film by peel off technique leaving the irradiated portions on the substrate. Thin films of gold in the thickness range of 70–300 nm were thermally evaporated under clean and high vacuum on a 300 nm layer of SiO2 grown on Si substrate by wet oxidation. These films were irradiated by Ga ion beam at different energies ranging from 10 keV to 30 keV at currents in the range of 3–10 pA to enhance adhesion in the irradiated area. After peel off, patterns of dimensions ranging from 5 lm 5 lm down to 500 nm 500 nm over an area of 10 lm 10 lm are achieved. The bitmaps of typical patterns used to create the structures are shown in Fig. 2(a). The unirradiated portions were removed by peel off method using adhesive scotch tape as illustrated in Fig. 1(c) and (d). Array of several shapes like circles, squares, rectangles, stars etc. have been made by the above method.
The shape distortion (sharpness of the edges), fidelity of the structures and amount of material retained has been studied using in situ SEM and EDS as a function of ion energy, fluence and size of the pattern. Cross sectional SEM and map scan EDS of the virgin sample and of the patterns has been done in order to infer the interfacial changes occurring after focused ion beam irradiation. 3. Results and discussion Fig. 2(b) and (c) show the patterns of Au e.g. stars and discs of dimensions 500 nm, created on the SiO2–Si substrate using irradiation of ion beam of energy 20 keV at current 4 pA and peel off. Fig. 3(a) manifests the SEM images of the patterns made by successively increasing the ion beam fluence at a constant ion beam energy of 20 keV at a current of 4 pA, over areas of 5 lm 5 lm followed by peel off. EDS analysis of the patterns confirms that there is a threshold fluence of 1.8 1016 ions/cm2 at which retention of the film starts (Fig. 3(b)). It may be noted that percentage retention of gold reduces as the fluence is further increased because of sputtering. SEM image of the patterns of gold of dimensions 5 lm 5 lm made by successive increment of Ga ion beam energy in the range of 10–30 keV at a constant fluence of 1.8 1016 ions/cm2 is shown in Fig. 4(a). In this case percentage
Fig. 2. (a) Bit map images of the patterns Acquired by FIB, (b) and (c) array of patterns of different shapes showing fidelity of shapes.
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Fig. 3. Variation of percentage retention of gold as a function of fluence of Ga FIB at energy of 20 keV and current of 4 pA, (a) SEM image of the patterns formed after peel off at various fluence (ions/cm2), (b) shows that here is a threshold fluence of 1.8 1016 ions/cm2 at which retention initiates to occur and then retention decreases as a function of fluence owing to sputtering.
Fig. 4. Dependence of percentage retention of gold on varying the energy of Ga FIB in the range of 10–30 keV at constant fluence using currents in the range of 3–10 pA, (a) SEM image of the pattern formed after peel off and (b) shows that adhesion improves as ion beam energy is increased keeping fluence constant.
N. Shukla et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1376–1380
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Fig. 5. (a) The trajectory profile and range of 30 keV Ga ions in gold simulated by SRIM, (b) the 30 keV Ga ion distribution in Au. The projected range is about 9.3 nm and straggling is 5.6 nm.
retention of gold increases as we increase the ion beam energy for a constant fluence which reflects that adhesion improves as we increase the ion beam energy (Fig. 4(b)). One thing can be noticed that the edges of the patters are not very sharp in Fig. 3 but are sharp in Figs. 2 and 4. Actually heat conduction is spherically symmetric and is not linear. Hence edges can not be very sharp in the patterns made by this technique. Therefore, this sets a limit to the minimum achievable size which is 500 nm for Au–SiO2 system. In addition the sharpness and the minimum achievable size by this method would depend on the thermal conductivity and the thickness of the film. Detailed investigations in this direction are in progress. In general the adhesion between a metallic film and an oxide substrate happens to be poor because when interfacial energies are larger, the work required to separate unit area of two phases forming an interface is reduced resulting in poor adhesion [15]. The ion beam enhances the adhesion of a film on a substrate by one of the several processes, which occur as the ion traverses the interface, namely reconfiguring interface structure, tailoring elemental abundance, adding reactive species, roughening or dispersing the interface [10]. In order for any of these effects to operate, it is necessary that the ion penetrates or reaches the interface. The range of 30 keV Ga ions in gold in the present context has been calculated utilizing ‘‘Stopping and Range of Ions in Matter – SRIM 2006” simulation [16]. Fig. 5 enunciates the depth profile and range of 30 keV Ga ions in the Au–SiO2 system as obtained from the SRIM simulation. This shows that the projected range of 30 keV Ga ions in gold film is 9.3 nm and the longitudinal straggling is 5.6 nm while the thickness of the gold film employed for
the fabrication of patterns is 70 nm. Therefore, in the present case Ga ions do not reach to the film–substrate interface and the processes mentioned above are not expected to play role in the improvement of adhesion. In view of this, an alternate mechanism has to be sought to explain low energy focused ion beam induced adhesion in the present investigations. Thermal annealing at elevated temperatures is known to enhance adhesion of variety of films–substrate combination [17]. It may be noted that ion bombardment of a surface also increases the substrate temperature [18,19]. In case of focused ion beam irradiation, local temperature rise is expected to occur at the place where the beam hits the substrates. In the following, we estimate this local temperature rise which occurs during FIB scanning. One of the major features of FIB is the high current density of few A/cm2 it employs, which in present case is in the range of 1–4 A/cm2 corresponding to power density of the order of 105 Watt/cm2. In the literature it has been shown that a proton beam having current density of 3.2 10 6 A/cm2 and power density of 8 Watt/cm2 gives a temperature rise of about 573 K on thermally insulating sample [20], also a proton beam having power density of 38 Watt/cm2 raises temperature of materials with low thermal conductivity to 603 K [21]. In the case of 30 keV Ga ions beam irradiation at a beam current of 0.3 nA, the energy deposition calculations performed for a carbon pillar of 4 lm height and 90 nm radius, show a temperature rise of about 1400 K [22]. For these ion beam parameters, the current density and power density turns out to be 8.8 A/cm2 and 2.64 105 Watt/cm2, respectively. In view of these reported data, the large current density used in the present case is expected to locally increase the temperature in the ion beam irradiated zones.
Fig. 6. Surface morphology of the Gold film on SiO2, (a) before and (b) after irradiation.
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Fig. 7. (a) and (c) are cross sectional SEM images of unirradiated and irradiated samples in EDS mode, respectively, and (b) and (d) represents Au map scan EDS of unirradiated and irradiated samples, respectively (here gold can be differentiated by purple dots). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
One of the signatures of the local temperature rise is seen in terms of the increase in the grain size of the film at the irradiated region as compared to the unirradiated one, which has been shown in Fig. 6. This local rise in temperature would tend to increase the adhesion of the film due to diffusion of the species across the interface. To confirm this possibility of diffusion across the interface, cross sectional SEM and gold map scan EDS of gold film as deposited and ion beam irradiated, has been performed, the results of which are shown in Fig. 7. In order to save the interface from ion induced damage during sample preparation for cross sectional SEM using ion beam milling, a layer of tungsten of thickness 500 nm was deposited over the film. Fig. 7(a) and (b) shows the cross sectional SEM of the Au–SiO2 interface in the case of as deposited film and its gold map scan EDS (here gold can be differentiated by purple dots), respectively. In this case interface is found out to be very sharp and clear. Fig. 7(c) and (d) shows the similar cross sectional SEM image of ion beam irradiated film followed by its gold map scan EDS, respectively. The gold map scan EDS in this case, shows dispersion of Au across the interface with presence of Au atoms in the SiO2 matrix. This reveals the diffusion of Au across the interface after ion beam irradiation, which enhances the adhesion of the film. At higher ion fluence, sputtering of the surface is dominant which results in the thinning of the film as exhibited in Fig. 3(b). Highly energetic ions induce more adhesion as seen in Fig. 4(b). This can be understood from the fact that ions having higher energy deposit more thermal energy thereby improving the adhesion as a function of energy. Ion beam irradiation also induces Ga contamination in the irradiated film. These effects can be minimized by optimizing the incident ion fluence. Under optimized conditions of current, energy and fluence obtained from Figs. 3 and 4, nano patterns of Au film of dimensions down to 500 nm 500 nm have been achieved with good fidelity and reproducibility. 4. Conclusion A novel method involving focused ion beam enhanced adhesion combined with peel off technique has been devised to make patterns of Au film of various shapes and sizes ranging from 500 nm to 5 lm on a SiO2–Si substrate. The adhesion improves because of local temperature rise in the irradiated regions and diffusion
of Au atoms across the interface. The method can be used to prepare templates for making plasmonic and fluorescent nano materials and devices. Acknowledgements We thank, Prof. G.K. Mehta, Dr. S. Dhamodharan for many fruitful discussions, Mr. Sudhanshu Srivastava for his help in the experiments and the staff of the ion beam laboratory at IIT Kanpur for their assistance. We acknowledge financial support from NSTI, Department of Science and Technology India, and Indian Institute of Technology Kanpur for procurement of the FIB system. References [1] H.-J. Maas, J. Heimel, H. Fuchs, U.C. Fischer, J.C. Weeber, A. Dereux, J. Microsc. 209 (2003) 241. [2] T. Matsuyama, Y. Kawata, Jpn. J. Appl. Phys. 45 (2006) 1438. [3] E. Ozbay, Science 311 (2006) 189. [4] M. Trinker, S. Groth, S. Haslinger, S. Manz, T. Betz, I.B. Joseph, T. Schumm, J. Schmiedmayer, Appl. Phys. Lett. 92 (2008) 254102. [5] S. Graells, R. Alcubilla, G. Badenes, R. Quidant, Appl. Phys. Lett. 91 (2007) 21112. [6] S.K. Tripathi, N. Shukla, V.N. Kulkarni, Nucl. Instr. and Meth. B 266 (2008) 1468. [7] S.K. Tripathi, N. Shukla, V.N. Kulkarni, Nanotechnology 20 (2009) 075304. [8] J.E. Griffith, Y. Qiu, T.A. Tombrello, Nucl. Instr. and Meth. 198 (1982) 607. [9] H. Fladry, N. Tengen, G.K. Wolf, Nucl. Instr. and Meth. B 91 (1994) 575. [10] J.E.E. Baglin, IBM J. Res. Develop. 38 (1994) 413. [11] J.E.E. Baglin, in: Y. Pauleau (Ed.), Materials and Processes for Surface and Interface Engineering, Kluwer, Dordrecht, 1995, p. 111. [12] C.R. Wie, C.R. Shi, M.H. Mendenhall, R.P. Livi, T. Vreeland Jr., T.A. Tombrello, Nucl. Instr. and Meth. B 9 (1985) 20. [13] D.K. Sood, J.E.E. Baglin, Nucl. Instr. and Meth. B 19/20 (1987) 954. [14] Robert Lacombe, Adhesion Measurements Methods, Taylor and Francis – CRC Press, 2006 (Chapter 1). [15] M. Ohring, Material Science of Thin Films, Academic Press, San Diego, 2002 (Chapter 12). [16] J.F. Ziegler, Nucl. Instr. and Meth. B 219–220 (2004) 1027. [17] T.P. Nguyen, J. Ip, P. Le Rendu, A. Lahmar, Surf. Coat. Technol. 141 (2001) 108. [18] G. Dearnaley, J.H. Freeman, R.S. Nelson, J. Stephen, Ion Implantation, NorthHolland, Amesterdam, 1973 (Chapter 4). [19] N. Bohr, The penetration of atomic particles through matter (Det Kgl. Danske Vid. Sels. Mat-fys. Med. XVIII. 8, 1948). [20] Y.M. Parka, D.S. Koa, K.W. Yia, I. Petrovb, Y.W. Kim, Ultramicroscopy 107 (2007) 663. [21] Donald F. Peach, David W. Lane, Mike J. Sellwood, Nucl. Instr. and Meth. B 249 (2006) 677. [22] S.K. Tripathi, N. Shukla, S. Dhamodharan, V.N. Kulkarni, Nanotechnology 19 (2008) 205302.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Micro and nano patterning by focused ion beam enhanced adhesion Neeraj Shukla, Sarvesh K. Tripathi, Mihir Sarkar, Nitul S. Rajput, Vishwas N. Kulkarni * Department of Physics, Indian Institute of Technology Kanpur, Kanpur 208 016, India
a r t i c l e
i n f o
Article history: Available online 29 January 2009 PACS: 68.35.Np 82.80.Ej 68.37.Hk Keywords: Thin film adhesion Focused ion beam EDS SEM Nano pattern generation
a b s t r a c t We report a new method of generating nano and micro patterns using focused ion beam (FIB) induced adhesion. The method utilizes selective irradiation of thin metallic films grown on substrates by focused ion beam followed by peel off. After peel off of the irradiated thin film it is observed that the ion beam scanned portions are retained on the substrate, creating nano and micro patterns. The method is suitable for materials of which the adhesion to the substrate can be improved by ion bombardment. The phenomenon has been demonstrated by creating gold nano patterns of different shapes and sizes ranging from 500 nm to 5 lm on SiO2–Si substrate using 10–30 keV Ga FIB at beam currents up to 10 pA. The mechanism involved in the process has been discussed. The technique could be utilized to prepare micro and nano patterns of thin films deposited on an appropriate substrate for optical, plasmonic and sensor related applications. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Nano patterns are becoming backbone of modern and futuristic technology for the fabrication of electronic, photonic, plasmonic and memory devices [1–3]. There are few ways of making nano patterns of thin films. Among them are electron beam lithography [4] and electron beam induced deposition (EBID) [5]. Conventional electron beam lithography is also popular for fabricating nanostructures which is a multi step process. EBID is versatile in making nano patterns of different shapes and sizes, but the process is limited to the few elemental species for which organometallic precursor gases are available. Also the patterns made by EBID invariably contain carbon and hydrogen as impurities. In the recent years, focused ion beam has emerged as a powerful tool for fabrication of structures of low dimensions. The technique utilizes cracking of precursor gas molecules (organometallic) adsorbed over a substrate under focused ion beam impact. The volatile component escapes the substrate and the residue remains on the surface as the grown material [6]. There are several ion matter interaction phenomena e.g. secondary ion/electron emission, ion beam induced adhesion etc. which can be gainfully exploited in the fabrication of nanostructures. Recently, we have used the effect of secondary ion emission to deposit Pt and W on the side walls of a nano hole [7].
In this paper we explore the phenomenon of focused ion beam induced adhesion for making nano and micro pattern of a metallic film on SiO2–Si substrate. The term adhesion implies that the thin film on the substrate does not deteriorate at the normal working temperature. The formation of a well attached thin film coating on a substrate is termed as good adhesion which depends mainly on the nature of substrate–film interface and their surface energies. Ion beams provide a variety of well established ways of improving adhesion of thin films on the substrates [8–11]. Enhancement of adhesion of thin films has been well established by several researchers using energetic ion bombardment of different substrate–film combinations [12,13]. Therefore, it is worth to explore the efficacy of the focused ion beams to achieve adhesion of thin films over a micro and nano dimensions. We have observed that irradiation of focused Ga ions in the energy range of 10– 30 keV enhances adhesion of thin gold film on SiO2 substrate. This phenomenon has been utilized, by combining it with the peel off technique [14], to fabricate micro and nano size patterns of Au film of different shapes viz, circle, square, star etc. on SiO2. The results of these investigations, comprising ion energy and fluence dependence, on the quality of the patterns i.e. the relative percentage retention of gold and the mechanism involved, are presented in this paper. 2. Experiment details
* Corresponding author. Tel.: +91 512 2597985/2597986; fax: +91 512 2590914/ 2590103. E-mail address: [email protected] (V.N. Kulkarni). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.01.048
Experiments have been performed using a dual beam FIB system equipped with a field emission Ga ion source, high resolution
N. Shukla et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1376–1380
1377
Fig. 1. Schematic diagram, depicting the peel off scheme.
ion optics, high resolution milling and gas chemistry functionalities for deposition of C, Pt and W along with e-beam patterning, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) facility. The schematic of the process adapted for making nano dimensional patterns is shown in Fig. 1(a) shows an as deposited Au film on SiO2–Si substrate. The cross sectional view of the film after Ga irradiation forming a pattern on the film is shown in Fig. 1(b)–(d) illustrate removal of the unirradiated portion of the film by peel off technique leaving the irradiated portions on the substrate. Thin films of gold in the thickness range of 70–300 nm were thermally evaporated under clean and high vacuum on a 300 nm layer of SiO2 grown on Si substrate by wet oxidation. These films were irradiated by Ga ion beam at different energies ranging from 10 keV to 30 keV at currents in the range of 3–10 pA to enhance adhesion in the irradiated area. After peel off, patterns of dimensions ranging from 5 lm 5 lm down to 500 nm 500 nm over an area of 10 lm 10 lm are achieved. The bitmaps of typical patterns used to create the structures are shown in Fig. 2(a). The unirradiated portions were removed by peel off method using adhesive scotch tape as illustrated in Fig. 1(c) and (d). Array of several shapes like circles, squares, rectangles, stars etc. have been made by the above method.
The shape distortion (sharpness of the edges), fidelity of the structures and amount of material retained has been studied using in situ SEM and EDS as a function of ion energy, fluence and size of the pattern. Cross sectional SEM and map scan EDS of the virgin sample and of the patterns has been done in order to infer the interfacial changes occurring after focused ion beam irradiation. 3. Results and discussion Fig. 2(b) and (c) show the patterns of Au e.g. stars and discs of dimensions 500 nm, created on the SiO2–Si substrate using irradiation of ion beam of energy 20 keV at current 4 pA and peel off. Fig. 3(a) manifests the SEM images of the patterns made by successively increasing the ion beam fluence at a constant ion beam energy of 20 keV at a current of 4 pA, over areas of 5 lm 5 lm followed by peel off. EDS analysis of the patterns confirms that there is a threshold fluence of 1.8 1016 ions/cm2 at which retention of the film starts (Fig. 3(b)). It may be noted that percentage retention of gold reduces as the fluence is further increased because of sputtering. SEM image of the patterns of gold of dimensions 5 lm 5 lm made by successive increment of Ga ion beam energy in the range of 10–30 keV at a constant fluence of 1.8 1016 ions/cm2 is shown in Fig. 4(a). In this case percentage
Fig. 2. (a) Bit map images of the patterns Acquired by FIB, (b) and (c) array of patterns of different shapes showing fidelity of shapes.
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Fig. 3. Variation of percentage retention of gold as a function of fluence of Ga FIB at energy of 20 keV and current of 4 pA, (a) SEM image of the patterns formed after peel off at various fluence (ions/cm2), (b) shows that here is a threshold fluence of 1.8 1016 ions/cm2 at which retention initiates to occur and then retention decreases as a function of fluence owing to sputtering.
Fig. 4. Dependence of percentage retention of gold on varying the energy of Ga FIB in the range of 10–30 keV at constant fluence using currents in the range of 3–10 pA, (a) SEM image of the pattern formed after peel off and (b) shows that adhesion improves as ion beam energy is increased keeping fluence constant.
N. Shukla et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1376–1380
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Fig. 5. (a) The trajectory profile and range of 30 keV Ga ions in gold simulated by SRIM, (b) the 30 keV Ga ion distribution in Au. The projected range is about 9.3 nm and straggling is 5.6 nm.
retention of gold increases as we increase the ion beam energy for a constant fluence which reflects that adhesion improves as we increase the ion beam energy (Fig. 4(b)). One thing can be noticed that the edges of the patters are not very sharp in Fig. 3 but are sharp in Figs. 2 and 4. Actually heat conduction is spherically symmetric and is not linear. Hence edges can not be very sharp in the patterns made by this technique. Therefore, this sets a limit to the minimum achievable size which is 500 nm for Au–SiO2 system. In addition the sharpness and the minimum achievable size by this method would depend on the thermal conductivity and the thickness of the film. Detailed investigations in this direction are in progress. In general the adhesion between a metallic film and an oxide substrate happens to be poor because when interfacial energies are larger, the work required to separate unit area of two phases forming an interface is reduced resulting in poor adhesion [15]. The ion beam enhances the adhesion of a film on a substrate by one of the several processes, which occur as the ion traverses the interface, namely reconfiguring interface structure, tailoring elemental abundance, adding reactive species, roughening or dispersing the interface [10]. In order for any of these effects to operate, it is necessary that the ion penetrates or reaches the interface. The range of 30 keV Ga ions in gold in the present context has been calculated utilizing ‘‘Stopping and Range of Ions in Matter – SRIM 2006” simulation [16]. Fig. 5 enunciates the depth profile and range of 30 keV Ga ions in the Au–SiO2 system as obtained from the SRIM simulation. This shows that the projected range of 30 keV Ga ions in gold film is 9.3 nm and the longitudinal straggling is 5.6 nm while the thickness of the gold film employed for
the fabrication of patterns is 70 nm. Therefore, in the present case Ga ions do not reach to the film–substrate interface and the processes mentioned above are not expected to play role in the improvement of adhesion. In view of this, an alternate mechanism has to be sought to explain low energy focused ion beam induced adhesion in the present investigations. Thermal annealing at elevated temperatures is known to enhance adhesion of variety of films–substrate combination [17]. It may be noted that ion bombardment of a surface also increases the substrate temperature [18,19]. In case of focused ion beam irradiation, local temperature rise is expected to occur at the place where the beam hits the substrates. In the following, we estimate this local temperature rise which occurs during FIB scanning. One of the major features of FIB is the high current density of few A/cm2 it employs, which in present case is in the range of 1–4 A/cm2 corresponding to power density of the order of 105 Watt/cm2. In the literature it has been shown that a proton beam having current density of 3.2 10 6 A/cm2 and power density of 8 Watt/cm2 gives a temperature rise of about 573 K on thermally insulating sample [20], also a proton beam having power density of 38 Watt/cm2 raises temperature of materials with low thermal conductivity to 603 K [21]. In the case of 30 keV Ga ions beam irradiation at a beam current of 0.3 nA, the energy deposition calculations performed for a carbon pillar of 4 lm height and 90 nm radius, show a temperature rise of about 1400 K [22]. For these ion beam parameters, the current density and power density turns out to be 8.8 A/cm2 and 2.64 105 Watt/cm2, respectively. In view of these reported data, the large current density used in the present case is expected to locally increase the temperature in the ion beam irradiated zones.
Fig. 6. Surface morphology of the Gold film on SiO2, (a) before and (b) after irradiation.
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Fig. 7. (a) and (c) are cross sectional SEM images of unirradiated and irradiated samples in EDS mode, respectively, and (b) and (d) represents Au map scan EDS of unirradiated and irradiated samples, respectively (here gold can be differentiated by purple dots). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
One of the signatures of the local temperature rise is seen in terms of the increase in the grain size of the film at the irradiated region as compared to the unirradiated one, which has been shown in Fig. 6. This local rise in temperature would tend to increase the adhesion of the film due to diffusion of the species across the interface. To confirm this possibility of diffusion across the interface, cross sectional SEM and gold map scan EDS of gold film as deposited and ion beam irradiated, has been performed, the results of which are shown in Fig. 7. In order to save the interface from ion induced damage during sample preparation for cross sectional SEM using ion beam milling, a layer of tungsten of thickness 500 nm was deposited over the film. Fig. 7(a) and (b) shows the cross sectional SEM of the Au–SiO2 interface in the case of as deposited film and its gold map scan EDS (here gold can be differentiated by purple dots), respectively. In this case interface is found out to be very sharp and clear. Fig. 7(c) and (d) shows the similar cross sectional SEM image of ion beam irradiated film followed by its gold map scan EDS, respectively. The gold map scan EDS in this case, shows dispersion of Au across the interface with presence of Au atoms in the SiO2 matrix. This reveals the diffusion of Au across the interface after ion beam irradiation, which enhances the adhesion of the film. At higher ion fluence, sputtering of the surface is dominant which results in the thinning of the film as exhibited in Fig. 3(b). Highly energetic ions induce more adhesion as seen in Fig. 4(b). This can be understood from the fact that ions having higher energy deposit more thermal energy thereby improving the adhesion as a function of energy. Ion beam irradiation also induces Ga contamination in the irradiated film. These effects can be minimized by optimizing the incident ion fluence. Under optimized conditions of current, energy and fluence obtained from Figs. 3 and 4, nano patterns of Au film of dimensions down to 500 nm 500 nm have been achieved with good fidelity and reproducibility. 4. Conclusion A novel method involving focused ion beam enhanced adhesion combined with peel off technique has been devised to make patterns of Au film of various shapes and sizes ranging from 500 nm to 5 lm on a SiO2–Si substrate. The adhesion improves because of local temperature rise in the irradiated regions and diffusion
of Au atoms across the interface. The method can be used to prepare templates for making plasmonic and fluorescent nano materials and devices. Acknowledgements We thank, Prof. G.K. Mehta, Dr. S. Dhamodharan for many fruitful discussions, Mr. Sudhanshu Srivastava for his help in the experiments and the staff of the ion beam laboratory at IIT Kanpur for their assistance. We acknowledge financial support from NSTI, Department of Science and Technology India, and Indian Institute of Technology Kanpur for procurement of the FIB system. References [1] H.-J. Maas, J. Heimel, H. Fuchs, U.C. Fischer, J.C. Weeber, A. Dereux, J. Microsc. 209 (2003) 241. [2] T. Matsuyama, Y. Kawata, Jpn. J. Appl. Phys. 45 (2006) 1438. [3] E. Ozbay, Science 311 (2006) 189. [4] M. Trinker, S. Groth, S. Haslinger, S. Manz, T. Betz, I.B. Joseph, T. Schumm, J. Schmiedmayer, Appl. Phys. Lett. 92 (2008) 254102. [5] S. Graells, R. Alcubilla, G. Badenes, R. Quidant, Appl. Phys. Lett. 91 (2007) 21112. [6] S.K. Tripathi, N. Shukla, V.N. Kulkarni, Nucl. Instr. and Meth. B 266 (2008) 1468. [7] S.K. Tripathi, N. Shukla, V.N. Kulkarni, Nanotechnology 20 (2009) 075304. [8] J.E. Griffith, Y. Qiu, T.A. Tombrello, Nucl. Instr. and Meth. 198 (1982) 607. [9] H. Fladry, N. Tengen, G.K. Wolf, Nucl. Instr. and Meth. B 91 (1994) 575. [10] J.E.E. Baglin, IBM J. Res. Develop. 38 (1994) 413. [11] J.E.E. Baglin, in: Y. Pauleau (Ed.), Materials and Processes for Surface and Interface Engineering, Kluwer, Dordrecht, 1995, p. 111. [12] C.R. Wie, C.R. Shi, M.H. Mendenhall, R.P. Livi, T. Vreeland Jr., T.A. Tombrello, Nucl. Instr. and Meth. B 9 (1985) 20. [13] D.K. Sood, J.E.E. Baglin, Nucl. Instr. and Meth. B 19/20 (1987) 954. [14] Robert Lacombe, Adhesion Measurements Methods, Taylor and Francis – CRC Press, 2006 (Chapter 1). [15] M. Ohring, Material Science of Thin Films, Academic Press, San Diego, 2002 (Chapter 12). [16] J.F. Ziegler, Nucl. Instr. and Meth. B 219–220 (2004) 1027. [17] T.P. Nguyen, J. Ip, P. Le Rendu, A. Lahmar, Surf. Coat. Technol. 141 (2001) 108. [18] G. Dearnaley, J.H. Freeman, R.S. Nelson, J. Stephen, Ion Implantation, NorthHolland, Amesterdam, 1973 (Chapter 4). [19] N. Bohr, The penetration of atomic particles through matter (Det Kgl. Danske Vid. Sels. Mat-fys. Med. XVIII. 8, 1948). [20] Y.M. Parka, D.S. Koa, K.W. Yia, I. Petrovb, Y.W. Kim, Ultramicroscopy 107 (2007) 663. [21] Donald F. Peach, David W. Lane, Mike J. Sellwood, Nucl. Instr. and Meth. B 249 (2006) 677. [22] S.K. Tripathi, N. Shukla, S. Dhamodharan, V.N. Kulkarni, Nanotechnology 19 (2008) 205302.