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Surface tracks by MeV C60 impacts on mica and PMMA
Nuclear Instruments and Methods in Physics Research B 143 (1998) 503±512
Surface tracks by MeV C60 impacts on mica and PMMA M. D obeli
a,*
, F. Ames b, C.R. Musil c, L. Scandella c, M. Suter b, H.A. Synal
a
a
b
Paul Scherrer Institute, ETH-H onggerberg, CH-8093 Z urich, Switzerland Institute of Particle Physics, ETH-H onggerberg, CH-8093 Z urich, Switzerland c Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland Received 4 June 1998; received in revised form 14 July 1998
Abstract Muscovite mica has been bombarded by C60 ions with energies between 2 and 15 MeV. The produced surface tracks have been analyzed by atomic force microscopy (AFM). The particle impacts formed hillocks with a height which increases with energy from 2 to 6 nm. For impacts of I, Ge3 , C8 , C10 , Cu9 , and Cu10 MeV ions, hillocks smaller than 0.6 nm have been found. Irradiation with GeV monoatomic particles at similar energy loss performed by other research groups has only changed the frictional properties of the mica surface but did not produce any hillocks. The results can be understood qualitatively by taking into account the large dierence in the velocity of the projectiles and the secondary particles along the track. PMMA irradiated with MeV Cn , Gen , and I ions exhibited holes with diameters between 18 and 45 nm after development. The measured track area as a function of energy loss shows an approximately linear behavior, pointing towards a diusive spreading of energy. Ó 1998 Elsevier Science B.V. All rights reserved. PACS: 61.46 (Clusters); 61.48 (Fullerenes) Keywords: Cluster ions; Fullerene; Particle tracks; Atomic force microscopy
1. Introduction In the last few years, MeV cluster ion beams have become available at several accelerator facilities [1±6]. The beams have mainly been used to study nonlinear eects in secondary particle emission, stopping power and point defect production
* Corresponding author. Tel.: +41 1 633 2045; fax: +41 1 633 1067; e-mail: [email protected]
[7±9]. MeV cluster beams oer the unique possibility to deposit large amounts of energy into particle tracks of very small diameter. Since nonlinear eects in the energy loss of clusters are relatively small [8], the dE/dx of a cluster can be well approximated by the sum of the dE/dx of its constituents. The energy loss of e.g. a C60 fullerene particle at 10 MeV is about the same as the one of a uranium projectile at 1 GeV (see Fig. 1). The maximum energy loss that can be reached with a C60 molecule (at a few hundred MeV) is approximately twice as high as the maximum that can be
0168-583X/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 5 1 1 - 4
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methacrylate) resist and studied the diameters of the resulting tracks after development.
2. Experimental setup
Fig. 1. Electronic energy loss versus nuclear energy loss as a function of energy for a selection of monoatomic ions and for carbon clusters of dierent sizes at a constant energy per C atom. Data are produced by TRIM [10].
obtained with any monoatomic particle and may be up to 200 keV per nm [10]. In addition, the transport of energy away from the ion track is smaller for the heavy and relatively slow clusters, since the secondary particles (electrons and recoiling nuclei) have a smaller range. This leads to an extremely high energy input per volume. The manifestations of the very dense energy deposition have proven to be spectacular in some metals irradiated with MeV C60 ions [11]. Large columnar defects with amorphous zones much bigger than those for any other monoatomic projectile have been found by TEM. In this investigation, we studied surface tracks induced by MeV cluster ions on muscovite mica. Mica has been chosen because it is known to be radiation sensitive and its surface is very well suited for imaging by atomic force microscopy (AFM). Because of its relatively complicated composition and crystal structure, mica is not the ideal candidate for a detailed understanding of damage formation. However, the dependence of surface track features on projectile parameters like velocity, energy, and energy loss can give valuable insight into the basic processes behind track formation [12,17,18,26]. With regard to possible applications of single ion tracks to lithography, we also irradiated thin ®lms of PMMA (polymethyl
Irradiations have been performed at the PSI/ ETH Tandem accelerator laboratory at ETHH onggerberg. The essential part of the setup is shown in Fig. 2. Negative cluster ions were produced with a high current cesium sputter ion source [13,14]. Massive sputter cathodes of graphite, copper and crystalline germanium have been used for production of Cn (n 6 10), Cun and Gen ions, respectively. Fullerene cathodes have been prepared by hammering C60 powder (Hoechst, ``lab grade'' > 96% C60 ) into copper targets with a hole 1.5 mm in diameter and 2 mm in depth. The cesium sputter energy was lowered to 5 keV, which optimizes the ratio between sputtered Cÿ 60 ions and destroyed material at the surface. With a Cs current of approximately 1 mA, typical negative ion currents of the order of 100 pA could be obtained for C60 . The negative ions were analyzed by a mass spectrometer consisting of an electrostatic de¯ector and a dipole magnet and injected into the 6 MV EN tandem accelerator. By lowering the injection energy to 15 keV, the mass range of the system could be extended to nearly 800 amu. Fig. 3 shows mass spectra obtained from copper and C60 cathodes. Charge exchange to positive charge states at the high voltage terminal of the accelerator was performed with an Ar gas stripper at very low density. To prevent the clusters from breaking up by collisions with residual gas molecules in the acceleration tubes, a titanium sublimation pump
Fig. 2. Experimental setup for cluster acceleration.
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Fig. 4. Accelerator transmission of C60 ions as a function of the stripper gas target density.
Fig. 3. Mass spectra of negative secondary cluster ions obtained with a Cs sputter ion source from a copper (top) and a C60 cathode (bottom).
has been used at the stripper housing. Fig. 4 displays the accelerator transmission as a function of the stripper gas density. At the exit of the accelerator an analysis of the energy per charge ratio of the high energy ions was done by a 15° electrostatic de¯ector. At the focal point of the de¯ector the ions can be detected by introducing a channel-plate assembly into the beam. For additional identi®cation of the ions, a silicon detector can be introduced instead of the channel-plate. Upon fragmentation of a cluster at the accelerator terminal the energy is distributed among the fragments proportional to their size. Therefore, a mass spectrum of the beam can be obtained by scanning the voltage of the electrostatic de¯ector. Fig. 5 shows the distribution of
Fig. 5. Mass spectrum obtained after tandem acceleration. Injected particle is C60 .
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fragment masses in charge states 1 and 2 upon injection of C60 . During irradiations, the beam is raster scanned across a square aperture of variable size in front of the sample. Typicallly, the size of the irradiated area was 5 ´ 5 mm2 . Fluences were chosen between 108 and 1010 cmÿ2 corresponding to 1 and 100 tracks per lm2 . At this areal density, tracks can easily be found by AFM and there is no signi®cant overlap between neighboring tracks. Muscovite mica was cut into pieces of approximately 1 cm2 and freshly cleaved before irradiation. PMMA layers of 85 nm thickness were spun onto the gold covered surface of oxydized silicon wafers. After irradiation, the PMMA was developed for 60 s in 1:3 MiBK:IPA and heated on a hot plate for 10 min at 100o C in order to remove solvents and absorbed water from the PMMA surface. The topography of the surface tracks was analyzed by AFM. Measurements were carried out with a commercial microscope (Autoprobe CP, Park Scienti®c Instruments) operating in air. Silicon cantilevers (Ultralevers) with a typical spring constant of 0.16 N/m were chosen. The AFM was operated in the contact mode at a contact force of a few nN. Topographic images were obtained by using a scanner with a maximum operational area of 5 ´ 5 lm2 . Cross talk between lateral force and topography could be excluded by taking forward and backward scan images. 3. Results 3.1. Mica For all investigated particles (except C4 at 4 MeV), hillocks of dierent sizes have been identi®ed on the mica surface at an areal density corresponding well to the irradiation ¯uence (see Fig. 6). It was carefully veri®ed that the hillocks were not artifacts due to altered frictional forces in the vicinity of the track. Such frictional forces near ion tracks have been observed by several authors before [17,18]. The height of the hillocks varied between 0.2 and 6 nm, depending on projectile and energy. The measured hillock diameters were be-
Fig. 6. AFM topographic images of irradiated mica. Top: 77 MeV iodine. Bottom: 7.5 MeV C60 .
tween 10 and 35 nm. In contrast to the height of the hillocks, their diameter was not well reproducible because the measurement depends on the AFM tip used. During the numerous measurements, a great number of dierent tips were used and a thorough unfolding of the measured diameters with the tip shape has not been performed. We therefore present data only on the hillock height, which is reliable within the given experimental errors. In Table 1, the experimental data are compiled for all projectiles. In addition, the energy loss, as calculated by TRIM [10], is given. It has been assumed that the energy loss of a cluster is the sum of the energy loss of its constituents. In Fig. 7 the measured hillock heights are plotted as a function of the total energy loss. 3.2. PMMA After development, holes were found in the PMMA ®lm for all particles and energies. In the case of PMMA, all samples have been measured under identical conditions with one single AFM tip. The diameters (FWHM) of the tracks varied between 15 and 45 nm depending on the projectile used (see Table 2). A few examples of AFM
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Table 1 Measured hillock height on mica. The lower part of the table gives a collection of similar results obtained by other research groups Particle
Mass (amu)
Energy (MeV)
Energy/ atom (MeV)
dE/dx (el.) (keV/nm)
dE/dx (nucl.) (keV/nm)
dE/dx (tot) (keV/nm)
Hillock height (nm)
Variance (nm)
I Ge3 Cu9 Cu11 C4 C8 C10 C60 C60 C60 C60 C60 C60
127 218 572 699 12 96 120 720 720 720 720 720 720
77 8.0 7.5 7.5 4.0 8.0 7.3 15.0 11.3 7.5 5.0 4.0 2.0
77.0 2.7 0.83 0.68 1.0 1.0 0.73 0.25 0.19 0.13 0.08 0.07 0.03
12.5 5.1 8.7 9.7 4.9 9.8 10.6 34.9 29.4 23.7 19.8 15.6 11.4
0.1 1.2 5.5 7.3 0.04 0.08 0.13 1.7 2.0 2.7 3.5 4.0 5.6
12.6 6.3 14.2 16.9 5.0 9.9 10.7 36.6 31.4 26.4 23.3 19.6 17.0
0.6 0.2 0.3 0.6 0 0.4 0.4 5.5 4.8 4.25 3.3 2.7 1.8
0.2 0.1 0.1 0.2 0.2 0.1 0.1 0.5 0.5 0.5 0.5 0.7 0.5
Pba Xea Ib Krc Krc Krc Krc Krc C60 d
207 131 127 84 84 84 84 84 720
2400 1500 78.0 2900 2700 2100 1300 640 0.38
24.0 14.2 12.5 4.2 4.4 5.2 6.8 9.2 45.0
0.02 0.01 0.12 0.002 0.002 0.002 0.0036 0.007 1.2
24.0 14.2 12.6 4.2 4.4 5.2 6.8 9.2 46.2
0 0 0.5 0 0 0 0 0 3.3
± ± 0.2 ± ± ± ± ± 0.1
2400 1500 78 2900 2700 2100 1300 640 23
a
From Ref. [17]. From Ref. [15]. c From Ref. [18]. d From Ref. [16]. b
topographic images of the developed PMMA surfaces are given in Fig. 8. 4. Discussion 4.1. Mica
Fig. 7. Measured hillock heights on mica versus total energy loss of the ions. The symbols are marked with ion species and energy in MeV (in parentheses).
For hillocks on mica, some additional data are available from other research groups. Most of the data refer to impacts of GeV heavy ions [17,18] but two data points also exist for MeV iodine and C60 [15,16]. We compiled all the existing data in Table 1. The data points for iodine at 77 MeV (this work) and 78 MeV [15] are in complete agreement. For all C60 ions up to 15 MeV investigated in this work, the hillock size monotonically increases with energy. However, the 23 MeV C60 data point of
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Table 2 Measured track diameters in PMMA Particle
Energy (MeV)
dE/dx (el.) (keV/nm)
Track diameter (nm)
Variance (nm)
I I Ge Ge3 C4 C8 C60
77.0 30.0 2.7 8.0 4.0 8.0 7.5
6.5 4.6 0.7 2.1 2.8 5.6 14.5
31.6 28.0 19.1 28.0 29.2 32.4 45.0
3.1 3.2 2.6 4.1 3.6 3.9 2.7
Fig. 8. AFM topographic images of irradiated and developed PMMA. Irradiation ¯uence was between 109 and 1010 cmÿ2 .
Ref. [16] is slightly lower than expected. It cannot be decided if this is due to dierences in e.g. sample preparation or analysis procedure or if there is a deeper physical reason related to the process of hillock formation.
Hillock and dip formation has been reported for GeV heavy ions in earlier publications. Thorough AFM investigations, however, proved that this was only an artifact due to a change in friction at the point of impact of the fast heavy ions. For
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all GeV particles the vertical size of the impact feature turned out to be zero within the experimental error (a fraction of a nm). As shown in Table 1, most of the particles under investigation are in a regime where electronic energy loss dominates. Only for the C60 particles below 7.5 MeV and for the Ge and Cu clusters does the nuclear energy loss exceed 10% of the total energy loss. Several mechanisms for surface track formation by electronic stopping have been proposed in the past. Among the most important are the Ôthermal spikeÕ [19], the Ôpressure pulseÕ [20], the Ôshock waveÕ [21] or the ÔCoulomb explosionÕ model [22]. Every model predicts a speci®c monotonic increase of the feature size with stopping power of the particles. We therefore plotted all the available data versus particle energy and velocity and versus electronic, nuclear and total energy loss in order to see if any obvious systematic behavior exists. As an example, all the available experimental data on hillock heights on mica are plotted as a function of electronic and nuclear energy loss in Fig. 9. However, it is not possible to attribute the shape of any of the curves obtained to any of the speci®c models. In every case the GeV particles produce discontinuous features in the curves. The 3-dimensional plot of hillock height versus electronic energy loss (Se ) and particle velocity (vp ), however, reveals the existence of two distinct regions (Fig. 10) in the vp ±Se plane. Only particles with high energy loss and low velocity produce signi®cant hillocks, while fast particles never form hillocks, no matter how high their energy loss is. This behavior qualitatively corresponds to the ÔultratrackÕ model [23] that has been re®ned in the past years by several authors [24±26]. This model assumes that a part of the deposited energy is carried away from the central track by the recoiling secondary particles (especially electrons). For fast particles the range of the most energetic secondary electrons (Re ) increases approximately with the square of the velocity of the primary ion. Therefore, the energy deposition is signi®cantly `diluted' for the GeV monoatomic ions while it stays concentrated in a very narrow cylinder for the slow clusters. This is depicted in Fig. 11 for a 7 MeV C60 and a 1 GeV Au particle at an equal total energy
509
Fig. 9. Measured hillock height on mica versus electronic (top), and nuclear (bottom) energy loss. Filled circles: data from this work. GSI: from Ref. [17], GANIL: from Ref. [18], Orsay: from Ref. [16], Uppsala: from Ref. [15].
loss of 22 keV/nm. Of course, this picture only gives a qualitative understanding of the mechanism. For example, the fact has been neglected that the secondary particles with the highest energies are those emitted along the direction of the primary ion. While it is possible to estimate the track diameter with this kind of model [27], a prediction of the hillock height is not as straightforward. However, the argument that the hillock height increases with increasing Se and decreasing ve is further supported by Fig. 12 where the hillock ÿ1=2 . With the exheight is plotted versus Se vp ception of the 23 MeV C60 from Ref. [16], most of
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Fig. 11. Qualitative schematic picture of particle tracks for Au and C60 projectiles. Silicon is assumed as target material. Re is the range of the most energetic secondary electrons, Rr is the maximum range of recoiling target nuclei. The drawing is not to scale.
Fig. 10. Three-dimensional plot of measured hillock height versus particle velocity vp and electronic energy loss Se . Bottom: projection on vp ±Se plane.
the data points lie on a continously increasing, smooth curve. This representation was found in a completely heuristic manner, and we cannot oer a quantitative interpretation of this ®nding. Experimentally, a very similar behavior of damage as a function of Se and vp was discovered some years ago for heavy ion irradiation of magnetic insulators [28]. 4.2. PMMA For focused ion beam irradiation with a large variety of ions with energies between 30 and 200 keV, PMMA is known to become soluble in the
Fig. 12. Measured hillock height on mica as a function of electronic energy loss divided by the square root of the particle velocity Se vp ÿ1=2 .
developer above a threshold energy deposition density of 6.7 eV/nm3 [29]. If the area of developed MeV ion tracks is plotted versus electronic energy loss, an approximately linear dependence is found (Fig. 13). A straight line ®t to the data yields a ratio between energy loss (per track length) and track area of 8.2 eV/nm3 , which compares quite well with the threshold energy density found for
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particle tracks in PMMA over a wide range. This has already led to a ®rst technical application of cluster beams to single ion lithography [30].
References
Fig. 13. Diameter and area of developed holes in PMMA as a function of electronic energy loss. The straight lines correspond to a constant volume density of deposited energy. Solid line: best ®t to data. Dashed line: threshold energy density found for keV ions.
keV ions. This points towards a diusive behavior of energy transport. Unfortunately, there is no data available on GeV ion tracks in PMMA. Therefore, it is dicult to check if there is a similar velocity dependence as in the case of hillock formation on mica. 5. Conclusions The vertical size of hillocks produced by MeV ions on a mica surface is not a simple function of the energy loss of the ions. It is necessary to take into account the projectile velocity in order to bring all the available experimental data in qualitative agreement. This is probably due to the fact that some of the deposited energy is immediately transported away from the track core by secondary particles, the range of which increases with the projectile velocity. This means that Ôultra-trackÕ models are at least an essential addition to other theories of track formation. In the case of PMMA, no such in¯uence of the particle velocity on the track size could be demonstrated by the existing data. Experiments with GeV heavy ions would be necessary in order to produce a clearly visible eect. Nevertheless, irradiation with cluster ions of dierent size and energy oers a means of tailoring the diameter of
[1] E. Arai, H. Funaki, Y. Ishiguro, M. Katayama, Y. Oguri, K. Shinizu, M. Shimizu, Particle Accelerators 37/38 (1992) 39. [2] V.S. Ramamurthy, D.P. Mahapatra, R.T. Yadav, G.V. Ravi Prasad, A. Gopalakrishna, Pramana J. Phys. 39 (1992) 583. [3] S. Della-Negra, A. Brunelle, Y. Le Beyec, J.M. Curaudeau, J.P. Mouron, B. Waast, P. H akansson, B.U.R. Sundqvist, E. Parilis, Nucl. Instr. and Meth. B 74 (1993) 453. [4] K. Bousso®ane-Baudin, A. Brunelle, P. Chaurand, J. Depauw, S. Della-Negra, P. H akansson, Y. Le Beyec, Int. J. Mass Spectrom. Ion Processes 130 (1994) 73. [5] Ch. Tomaschko, Ch. Schoppmann, D. Brandl, H. Voit, Nucl. Instr. and Meth. B 88 (1994) 6. [6] M. D obeli, R.M. Ender, U.S. Fischer, M. Suter, H.A. Synal, D. Vetterli, Nucl. Instr. and Meth. B 94 (1994) 388. [7] K. Bousso®ane-Baudin, A. Brunelle, P. Chaurand, J. Depauw, S. Della-Negra, P. H akansson, Y. Le Beyec, Nucl. Instr. and Meth. B 88 (1994) 61. [8] K. Baudin, A. Brunelle, M. Chabot, S. Della-Negra, J. Depauw, D. Gardes, P. H akansson, Y. Le Beyec, A. Billebaud, M. Fallavier, J. Remillieux, J.C. Poizat, J.P. Thomas, Nucl. Instr. and Meth. B 94 (1994) 341. [9] M. D obeli, F. Ames, R.M. Ender, M. Suter, H.A. Synal, D. Vetterli, Nucl. Instr. and Meth. B 106 (1995) 43. [10] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [11] H. Dammak, A. Dunlop, D. Lesueur, A. Brunelle, S. Della-Negra, Y. Le Beyec, Phys. Rev. Lett. 74 (1995) 1135. [12] D.D.N. Barlo Daya, C.T. Reimann, A. Hallen, B.U.R. Sundqvist, P. H akansson, Nucl. Instr. and Meth. B 111 (1996) 87. [13] Th.R. Niklaus, F. Ames, G. Bonani, M. Suter, H.-A. Synal, Nucl. Instr. and Meth B 92 (1994) 96. [14] F. Ames, M. D obeli, C.R. Musil, P.W. Nebiker, L. Scandella, M. Suter, H.A. Synal, Nucl. Instr. and Meth. B 112 (1996) 64. [15] D.D.N. Barlo Daya, A. Hallen, P. H akansson, B.U.R. Sundqvist, C.T. Reimann, Nucl. Instr. and Meth. B 103 (1995) 454. [16] D.D.N. Barlo Daya, A. Hallen, J. Eriksson, J. Kopniczky, R. Papaleo, C.T. Reimann, P. H akansson, B.U.R. Sundqvist, A. Brunelle, S. Della-Negra, Y. Le Beyec, Nucl. Instr. and Meth. B 106 (1995) 38. [17] T. Hagen, S. Grafstr om, J. Ackermann, R. Neumann, C. Trautmann, J. Vetter, N. Angert, J. Vac. Sci. Technol. B 12 (1994) 1555.
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[18] F. Thibaudau, J. Cousty, E. Balanzat, S. Bouard, Phys. Rev. Lett. 67 (1991) 1582. [19] R.E. Johnson, R. Evatt, Radiat. E. 52 (1980) 187. [20] R.E. Johnson, B.U.R. Sundqvist, Phys. Today (1992) 28. [21] S. Bitensky, E.S. Parilis, Nucl. Instr. and Meth. B 21 (1987) 26. [22] W.L. Brown, W.M. Augustyniak, E. Brody, B. Cooper, L.J. Lanzerotti, A. Ramirez, R. Evatt, R.E. Johnson, Nucl. Instr. and Meth. 170 (1980) 321. [23] R. Katz, Nuclear Track Detection 2 (1978) 1. [24] A. Hedin, P. H akansson, B.U.R. Sundqvist, R.E. Johnson, Phys. Rev. B 31 (1985) 1780.
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Surface tracks by MeV C60 impacts on mica and PMMA M. D obeli
a,*
, F. Ames b, C.R. Musil c, L. Scandella c, M. Suter b, H.A. Synal
a
a
b
Paul Scherrer Institute, ETH-H onggerberg, CH-8093 Z urich, Switzerland Institute of Particle Physics, ETH-H onggerberg, CH-8093 Z urich, Switzerland c Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland Received 4 June 1998; received in revised form 14 July 1998
Abstract Muscovite mica has been bombarded by C60 ions with energies between 2 and 15 MeV. The produced surface tracks have been analyzed by atomic force microscopy (AFM). The particle impacts formed hillocks with a height which increases with energy from 2 to 6 nm. For impacts of I, Ge3 , C8 , C10 , Cu9 , and Cu10 MeV ions, hillocks smaller than 0.6 nm have been found. Irradiation with GeV monoatomic particles at similar energy loss performed by other research groups has only changed the frictional properties of the mica surface but did not produce any hillocks. The results can be understood qualitatively by taking into account the large dierence in the velocity of the projectiles and the secondary particles along the track. PMMA irradiated with MeV Cn , Gen , and I ions exhibited holes with diameters between 18 and 45 nm after development. The measured track area as a function of energy loss shows an approximately linear behavior, pointing towards a diusive spreading of energy. Ó 1998 Elsevier Science B.V. All rights reserved. PACS: 61.46 (Clusters); 61.48 (Fullerenes) Keywords: Cluster ions; Fullerene; Particle tracks; Atomic force microscopy
1. Introduction In the last few years, MeV cluster ion beams have become available at several accelerator facilities [1±6]. The beams have mainly been used to study nonlinear eects in secondary particle emission, stopping power and point defect production
* Corresponding author. Tel.: +41 1 633 2045; fax: +41 1 633 1067; e-mail: [email protected]
[7±9]. MeV cluster beams oer the unique possibility to deposit large amounts of energy into particle tracks of very small diameter. Since nonlinear eects in the energy loss of clusters are relatively small [8], the dE/dx of a cluster can be well approximated by the sum of the dE/dx of its constituents. The energy loss of e.g. a C60 fullerene particle at 10 MeV is about the same as the one of a uranium projectile at 1 GeV (see Fig. 1). The maximum energy loss that can be reached with a C60 molecule (at a few hundred MeV) is approximately twice as high as the maximum that can be
0168-583X/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 5 1 1 - 4
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M. D obeli et al. / Nucl. Instr. and Meth. in Phys. Res. B 143 (1998) 503±512
methacrylate) resist and studied the diameters of the resulting tracks after development.
2. Experimental setup
Fig. 1. Electronic energy loss versus nuclear energy loss as a function of energy for a selection of monoatomic ions and for carbon clusters of dierent sizes at a constant energy per C atom. Data are produced by TRIM [10].
obtained with any monoatomic particle and may be up to 200 keV per nm [10]. In addition, the transport of energy away from the ion track is smaller for the heavy and relatively slow clusters, since the secondary particles (electrons and recoiling nuclei) have a smaller range. This leads to an extremely high energy input per volume. The manifestations of the very dense energy deposition have proven to be spectacular in some metals irradiated with MeV C60 ions [11]. Large columnar defects with amorphous zones much bigger than those for any other monoatomic projectile have been found by TEM. In this investigation, we studied surface tracks induced by MeV cluster ions on muscovite mica. Mica has been chosen because it is known to be radiation sensitive and its surface is very well suited for imaging by atomic force microscopy (AFM). Because of its relatively complicated composition and crystal structure, mica is not the ideal candidate for a detailed understanding of damage formation. However, the dependence of surface track features on projectile parameters like velocity, energy, and energy loss can give valuable insight into the basic processes behind track formation [12,17,18,26]. With regard to possible applications of single ion tracks to lithography, we also irradiated thin ®lms of PMMA (polymethyl
Irradiations have been performed at the PSI/ ETH Tandem accelerator laboratory at ETHH onggerberg. The essential part of the setup is shown in Fig. 2. Negative cluster ions were produced with a high current cesium sputter ion source [13,14]. Massive sputter cathodes of graphite, copper and crystalline germanium have been used for production of Cn (n 6 10), Cun and Gen ions, respectively. Fullerene cathodes have been prepared by hammering C60 powder (Hoechst, ``lab grade'' > 96% C60 ) into copper targets with a hole 1.5 mm in diameter and 2 mm in depth. The cesium sputter energy was lowered to 5 keV, which optimizes the ratio between sputtered Cÿ 60 ions and destroyed material at the surface. With a Cs current of approximately 1 mA, typical negative ion currents of the order of 100 pA could be obtained for C60 . The negative ions were analyzed by a mass spectrometer consisting of an electrostatic de¯ector and a dipole magnet and injected into the 6 MV EN tandem accelerator. By lowering the injection energy to 15 keV, the mass range of the system could be extended to nearly 800 amu. Fig. 3 shows mass spectra obtained from copper and C60 cathodes. Charge exchange to positive charge states at the high voltage terminal of the accelerator was performed with an Ar gas stripper at very low density. To prevent the clusters from breaking up by collisions with residual gas molecules in the acceleration tubes, a titanium sublimation pump
Fig. 2. Experimental setup for cluster acceleration.
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Fig. 4. Accelerator transmission of C60 ions as a function of the stripper gas target density.
Fig. 3. Mass spectra of negative secondary cluster ions obtained with a Cs sputter ion source from a copper (top) and a C60 cathode (bottom).
has been used at the stripper housing. Fig. 4 displays the accelerator transmission as a function of the stripper gas density. At the exit of the accelerator an analysis of the energy per charge ratio of the high energy ions was done by a 15° electrostatic de¯ector. At the focal point of the de¯ector the ions can be detected by introducing a channel-plate assembly into the beam. For additional identi®cation of the ions, a silicon detector can be introduced instead of the channel-plate. Upon fragmentation of a cluster at the accelerator terminal the energy is distributed among the fragments proportional to their size. Therefore, a mass spectrum of the beam can be obtained by scanning the voltage of the electrostatic de¯ector. Fig. 5 shows the distribution of
Fig. 5. Mass spectrum obtained after tandem acceleration. Injected particle is C60 .
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fragment masses in charge states 1 and 2 upon injection of C60 . During irradiations, the beam is raster scanned across a square aperture of variable size in front of the sample. Typicallly, the size of the irradiated area was 5 ´ 5 mm2 . Fluences were chosen between 108 and 1010 cmÿ2 corresponding to 1 and 100 tracks per lm2 . At this areal density, tracks can easily be found by AFM and there is no signi®cant overlap between neighboring tracks. Muscovite mica was cut into pieces of approximately 1 cm2 and freshly cleaved before irradiation. PMMA layers of 85 nm thickness were spun onto the gold covered surface of oxydized silicon wafers. After irradiation, the PMMA was developed for 60 s in 1:3 MiBK:IPA and heated on a hot plate for 10 min at 100o C in order to remove solvents and absorbed water from the PMMA surface. The topography of the surface tracks was analyzed by AFM. Measurements were carried out with a commercial microscope (Autoprobe CP, Park Scienti®c Instruments) operating in air. Silicon cantilevers (Ultralevers) with a typical spring constant of 0.16 N/m were chosen. The AFM was operated in the contact mode at a contact force of a few nN. Topographic images were obtained by using a scanner with a maximum operational area of 5 ´ 5 lm2 . Cross talk between lateral force and topography could be excluded by taking forward and backward scan images. 3. Results 3.1. Mica For all investigated particles (except C4 at 4 MeV), hillocks of dierent sizes have been identi®ed on the mica surface at an areal density corresponding well to the irradiation ¯uence (see Fig. 6). It was carefully veri®ed that the hillocks were not artifacts due to altered frictional forces in the vicinity of the track. Such frictional forces near ion tracks have been observed by several authors before [17,18]. The height of the hillocks varied between 0.2 and 6 nm, depending on projectile and energy. The measured hillock diameters were be-
Fig. 6. AFM topographic images of irradiated mica. Top: 77 MeV iodine. Bottom: 7.5 MeV C60 .
tween 10 and 35 nm. In contrast to the height of the hillocks, their diameter was not well reproducible because the measurement depends on the AFM tip used. During the numerous measurements, a great number of dierent tips were used and a thorough unfolding of the measured diameters with the tip shape has not been performed. We therefore present data only on the hillock height, which is reliable within the given experimental errors. In Table 1, the experimental data are compiled for all projectiles. In addition, the energy loss, as calculated by TRIM [10], is given. It has been assumed that the energy loss of a cluster is the sum of the energy loss of its constituents. In Fig. 7 the measured hillock heights are plotted as a function of the total energy loss. 3.2. PMMA After development, holes were found in the PMMA ®lm for all particles and energies. In the case of PMMA, all samples have been measured under identical conditions with one single AFM tip. The diameters (FWHM) of the tracks varied between 15 and 45 nm depending on the projectile used (see Table 2). A few examples of AFM
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Table 1 Measured hillock height on mica. The lower part of the table gives a collection of similar results obtained by other research groups Particle
Mass (amu)
Energy (MeV)
Energy/ atom (MeV)
dE/dx (el.) (keV/nm)
dE/dx (nucl.) (keV/nm)
dE/dx (tot) (keV/nm)
Hillock height (nm)
Variance (nm)
I Ge3 Cu9 Cu11 C4 C8 C10 C60 C60 C60 C60 C60 C60
127 218 572 699 12 96 120 720 720 720 720 720 720
77 8.0 7.5 7.5 4.0 8.0 7.3 15.0 11.3 7.5 5.0 4.0 2.0
77.0 2.7 0.83 0.68 1.0 1.0 0.73 0.25 0.19 0.13 0.08 0.07 0.03
12.5 5.1 8.7 9.7 4.9 9.8 10.6 34.9 29.4 23.7 19.8 15.6 11.4
0.1 1.2 5.5 7.3 0.04 0.08 0.13 1.7 2.0 2.7 3.5 4.0 5.6
12.6 6.3 14.2 16.9 5.0 9.9 10.7 36.6 31.4 26.4 23.3 19.6 17.0
0.6 0.2 0.3 0.6 0 0.4 0.4 5.5 4.8 4.25 3.3 2.7 1.8
0.2 0.1 0.1 0.2 0.2 0.1 0.1 0.5 0.5 0.5 0.5 0.7 0.5
Pba Xea Ib Krc Krc Krc Krc Krc C60 d
207 131 127 84 84 84 84 84 720
2400 1500 78.0 2900 2700 2100 1300 640 0.38
24.0 14.2 12.5 4.2 4.4 5.2 6.8 9.2 45.0
0.02 0.01 0.12 0.002 0.002 0.002 0.0036 0.007 1.2
24.0 14.2 12.6 4.2 4.4 5.2 6.8 9.2 46.2
0 0 0.5 0 0 0 0 0 3.3
± ± 0.2 ± ± ± ± ± 0.1
2400 1500 78 2900 2700 2100 1300 640 23
a
From Ref. [17]. From Ref. [15]. c From Ref. [18]. d From Ref. [16]. b
topographic images of the developed PMMA surfaces are given in Fig. 8. 4. Discussion 4.1. Mica
Fig. 7. Measured hillock heights on mica versus total energy loss of the ions. The symbols are marked with ion species and energy in MeV (in parentheses).
For hillocks on mica, some additional data are available from other research groups. Most of the data refer to impacts of GeV heavy ions [17,18] but two data points also exist for MeV iodine and C60 [15,16]. We compiled all the existing data in Table 1. The data points for iodine at 77 MeV (this work) and 78 MeV [15] are in complete agreement. For all C60 ions up to 15 MeV investigated in this work, the hillock size monotonically increases with energy. However, the 23 MeV C60 data point of
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Table 2 Measured track diameters in PMMA Particle
Energy (MeV)
dE/dx (el.) (keV/nm)
Track diameter (nm)
Variance (nm)
I I Ge Ge3 C4 C8 C60
77.0 30.0 2.7 8.0 4.0 8.0 7.5
6.5 4.6 0.7 2.1 2.8 5.6 14.5
31.6 28.0 19.1 28.0 29.2 32.4 45.0
3.1 3.2 2.6 4.1 3.6 3.9 2.7
Fig. 8. AFM topographic images of irradiated and developed PMMA. Irradiation ¯uence was between 109 and 1010 cmÿ2 .
Ref. [16] is slightly lower than expected. It cannot be decided if this is due to dierences in e.g. sample preparation or analysis procedure or if there is a deeper physical reason related to the process of hillock formation.
Hillock and dip formation has been reported for GeV heavy ions in earlier publications. Thorough AFM investigations, however, proved that this was only an artifact due to a change in friction at the point of impact of the fast heavy ions. For
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all GeV particles the vertical size of the impact feature turned out to be zero within the experimental error (a fraction of a nm). As shown in Table 1, most of the particles under investigation are in a regime where electronic energy loss dominates. Only for the C60 particles below 7.5 MeV and for the Ge and Cu clusters does the nuclear energy loss exceed 10% of the total energy loss. Several mechanisms for surface track formation by electronic stopping have been proposed in the past. Among the most important are the Ôthermal spikeÕ [19], the Ôpressure pulseÕ [20], the Ôshock waveÕ [21] or the ÔCoulomb explosionÕ model [22]. Every model predicts a speci®c monotonic increase of the feature size with stopping power of the particles. We therefore plotted all the available data versus particle energy and velocity and versus electronic, nuclear and total energy loss in order to see if any obvious systematic behavior exists. As an example, all the available experimental data on hillock heights on mica are plotted as a function of electronic and nuclear energy loss in Fig. 9. However, it is not possible to attribute the shape of any of the curves obtained to any of the speci®c models. In every case the GeV particles produce discontinuous features in the curves. The 3-dimensional plot of hillock height versus electronic energy loss (Se ) and particle velocity (vp ), however, reveals the existence of two distinct regions (Fig. 10) in the vp ±Se plane. Only particles with high energy loss and low velocity produce signi®cant hillocks, while fast particles never form hillocks, no matter how high their energy loss is. This behavior qualitatively corresponds to the ÔultratrackÕ model [23] that has been re®ned in the past years by several authors [24±26]. This model assumes that a part of the deposited energy is carried away from the central track by the recoiling secondary particles (especially electrons). For fast particles the range of the most energetic secondary electrons (Re ) increases approximately with the square of the velocity of the primary ion. Therefore, the energy deposition is signi®cantly `diluted' for the GeV monoatomic ions while it stays concentrated in a very narrow cylinder for the slow clusters. This is depicted in Fig. 11 for a 7 MeV C60 and a 1 GeV Au particle at an equal total energy
509
Fig. 9. Measured hillock height on mica versus electronic (top), and nuclear (bottom) energy loss. Filled circles: data from this work. GSI: from Ref. [17], GANIL: from Ref. [18], Orsay: from Ref. [16], Uppsala: from Ref. [15].
loss of 22 keV/nm. Of course, this picture only gives a qualitative understanding of the mechanism. For example, the fact has been neglected that the secondary particles with the highest energies are those emitted along the direction of the primary ion. While it is possible to estimate the track diameter with this kind of model [27], a prediction of the hillock height is not as straightforward. However, the argument that the hillock height increases with increasing Se and decreasing ve is further supported by Fig. 12 where the hillock ÿ1=2 . With the exheight is plotted versus Se vp ception of the 23 MeV C60 from Ref. [16], most of
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Fig. 11. Qualitative schematic picture of particle tracks for Au and C60 projectiles. Silicon is assumed as target material. Re is the range of the most energetic secondary electrons, Rr is the maximum range of recoiling target nuclei. The drawing is not to scale.
Fig. 10. Three-dimensional plot of measured hillock height versus particle velocity vp and electronic energy loss Se . Bottom: projection on vp ±Se plane.
the data points lie on a continously increasing, smooth curve. This representation was found in a completely heuristic manner, and we cannot oer a quantitative interpretation of this ®nding. Experimentally, a very similar behavior of damage as a function of Se and vp was discovered some years ago for heavy ion irradiation of magnetic insulators [28]. 4.2. PMMA For focused ion beam irradiation with a large variety of ions with energies between 30 and 200 keV, PMMA is known to become soluble in the
Fig. 12. Measured hillock height on mica as a function of electronic energy loss divided by the square root of the particle velocity Se vp ÿ1=2 .
developer above a threshold energy deposition density of 6.7 eV/nm3 [29]. If the area of developed MeV ion tracks is plotted versus electronic energy loss, an approximately linear dependence is found (Fig. 13). A straight line ®t to the data yields a ratio between energy loss (per track length) and track area of 8.2 eV/nm3 , which compares quite well with the threshold energy density found for
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particle tracks in PMMA over a wide range. This has already led to a ®rst technical application of cluster beams to single ion lithography [30].
References
Fig. 13. Diameter and area of developed holes in PMMA as a function of electronic energy loss. The straight lines correspond to a constant volume density of deposited energy. Solid line: best ®t to data. Dashed line: threshold energy density found for keV ions.
keV ions. This points towards a diusive behavior of energy transport. Unfortunately, there is no data available on GeV ion tracks in PMMA. Therefore, it is dicult to check if there is a similar velocity dependence as in the case of hillock formation on mica. 5. Conclusions The vertical size of hillocks produced by MeV ions on a mica surface is not a simple function of the energy loss of the ions. It is necessary to take into account the projectile velocity in order to bring all the available experimental data in qualitative agreement. This is probably due to the fact that some of the deposited energy is immediately transported away from the track core by secondary particles, the range of which increases with the projectile velocity. This means that Ôultra-trackÕ models are at least an essential addition to other theories of track formation. In the case of PMMA, no such in¯uence of the particle velocity on the track size could be demonstrated by the existing data. Experiments with GeV heavy ions would be necessary in order to produce a clearly visible eect. Nevertheless, irradiation with cluster ions of dierent size and energy oers a means of tailoring the diameter of
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