- Email: [email protected]
Latent (sub-surface) tracks in mica studied by tapping mode scanning force microscopy
Nuclear Instruments and Methods in Physics Research B 111 (1996) 87-90
Beam Interactions with Materials CbAtoms
ELSEWIER
Latent ( sub-surface) tracks in mica studied by tapping mode scanning force microscopy D.D.N. Barlo Daya a,* , C.T. Reimann
b, A. Hall& b, B.U.R. Sundqvist
b, P. Hakansson
b
a Deparhent of Physics, University of Colombo, Colombo 3, Sri Lanka b Division of Ion Physics, Department of Radiation Sciences, Uppsala University, Box 535, S-751 21 Uppsala, Sweden
Received 30 September 1995 Abstract This short communication presents information on the structure of latent (i.e subsurface) tracks in muscovite mica due to single 78.2 MeV lz71 ions incident at an angle of 67” with respect to the surface normal. ‘Ihe latent tracks have been studied for the first time using tapping mode scanning force microscopy. Images of an underlying mica surface, exposed by cleaving after ion bombardment, displayed hillocks accompanied by raised tails. These features were significantly lower in height compared to similar features observed on the originally outermost mica surface. The inner back surface of the mica sheet cleaved away also displayed shallow hillocks with raised tails, but on that surface, the tails stretched in the opposite direction to those observed on the originally outermost mica surface. On both surfaces exposed by cleaving, the tail is located in close proximity to the latent ion track, showing a symmetry of expansion along the track. The present results provide more evidence for a permanent expansion occurring around each ion track, as described qualitatively by theoretical models.
1. Introduction The registration of bulk ion tracks as a result of the passage of energetic particles through insulating materials is a well known phenomenon which has been studied extensively for more than three decades [l]. The registration of ion tracks is considered to be a consequence of several secondary processes, including propagation of shock waves, heat conduction, bond breaking, atomic displacements, and phase changes, which occur upon relaxation of electronically deposited energy into the nuclear system of the material. A clear understanding of the structure and dimensions of ion tracks is vital, not only for identification of the incident ions but also for achieving an understanding of the mechanisms behind the track formation. The property of enhanced chemical reactivity in the regions in proximity to the ion tracks has made it possible to enlarge the tracks by chemical means (etching) so that they can be visualized using either optical [l] or scanning electron microscopy [2,31 techniques. The etching approach does not provide information about the initial structure and nature of the ion tracks since a large amount of material is removed in the etching process. After the invention of near-field scanning
’ Corresponding author. Fax + 94 1 583810, tel. + 94 1 584777, e-mail [email protected]. 0168-583X/96/$15.00
probe techniques such as scanning tunnelling microscopy [4] and scanning force microscopy @FM) [5], it has become possible to investigate surfaces and surface features with a resolution of a few A without employing any pre-treatment of the surfaces. These new scanning techniques are convenient tools to study unetched tracks and can therefore provide new, useful information about the damage mechanisms. Sputtering, or ejection of material from a solid surface, is one of the well known consequences of the ion-solid interaction 16-81 which provide indirect clues about defect formation by incident ions. Crater formation in sputtering of organic solids has recently been observed in SFM studies [9-131. The formation of topological hillocks on the surfaces of insulating materials like mica [ 14-161 as a result of ion impacts is one of the observations recently made with SFM. The formation of surface topological hillocks [ 141 accompanied by raised tails 114,171 located over the penetrating ion track provides experimental evidence for a theoretical model of material rearrangement based on a material response to large radial forces associated with the transient energy density of the track [18]. This model is related closely to the pressure pulse [6,19] and shock wave [20] models of sputtering of organic targets. The observation of surface tracks in the form of topological hillocks on mica leads to speculation about possible
0 1996 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)01285-0
88
D.D.N. Barlo Daya et al./Nucl.
Instr. and Meth. in Phys. Res. B 11 I (19961 U-90
crater-like defects located under the hillocks [ 15,21,22]. The objective of the present work was to apply SFM in order to probe the bulk damage in mica caused by incident 78.2 MeV 1271ions. In an earlier work [23], the observation of crater-like defects on swift-heavy-ion-irradiated mica examined after cleaving away the originally irradiated surface was interpreted as an artefact of the continuous repulsive contact mode,SFM due to greater material softness in the damaged track zones. In a similar work, tip-scanning-direction-dependent image contrast was taken to imply that the latent tracks were flat regions of increased friction [24,25]. In the present study, tapping mode SFM (TM-SFM) has been employed to study the latent tracks in mica. TM-SFM involves only intermittent and gentle tip-surface contact, practically eliminating lateral (frictive) forces and substantially reducing normal forces [26], and thus may offer an improved means for examining latent tracks.
2. Experiment Uniform irradiation [27] was performed on freshlycleaved muscovite mica pieces (KAl,(OH),Si,AlO,,) by charge-equilibrated 78.2 MeV ‘*‘I ions from the Uppsala EN tandem Van de Graaff accelerator at ion angle of incidence of 67” with respect to the surface normal and to a fluence of - lo9 ions/cm*. III, Digital InTappingMode TM SFM (NanoScopeB struments, CA, USA) was employed to scan mica surfaces under ambient conditions with NanoProbeTM silicon tips with nominal radius of curvature 10 nm and cone angle 36”. The cantilevers were oscillated near resonance ( - 275 kHz) with a drive amplitude corresponding to a detector signal of 4 V. When probing a sample, imaging feedback was based on damping of the free oscillations of the cantilever by a few percent by intermittent tip-surface contact. A freshly-cleaved and then ion-bombarded mica surface was first scanned by TM-SFM. Next, the irradiated mica surface was cleaved again and both newly-exposed surfaces were scanned by TM-SFM. This procedure allowed us to compare surface track dimensions with the dimensions of exposed bulk latent tracks.
3. Results and discussion On the originally outermost mica surface (denoted a-b in Fig. l), conical shaped hillocks accompanied by raised tails oriented over the penetrating ion tracks were observed. Hillock heights (HH) were - 1.2 nm and the hillocks displayed basal widths (HW) - 37 nm. The lengths of the tails (TL) ranged up to - 99 nm. Measured heights were in excellent agreement with our previously
a
C
(i)
e
r
Cleavage Plane
Hill\ock
(ii) a
b
d f Ion Fig. 1. (i) Top views of tapping mode scanning force microscopy images of mica surfaces. a-b: the mica surface irradiated by 78.2-MeV lz71 ions at an ion angle of incidence of 67” with respect to the surface normal. e-f: the buried surface layer of mica freshly exposed by cleaving off a layer of the mica. c-d: the inner backside surface of the mica layer cleaved away. (ii) A schematic illustration of the symmetry of the expansion in the ion track constrained by the layered nature of mica and as viewed from the side. The images labelled a-b, c-d, and e-f in (i) correspond to the surfaces labelled the same in (ii) but with au obvious rotation. Ions proceed in the direction from the hillocks to the tails.
published measurements [14]; hillock widths and tail lengths were in fair agreement with our previous measurements [14]. We note that deduction of the tail lengths is quite subjective. Furthermore, in the present set of measurements, we measured relatively few surface tracks: since feature sizes are broadly distributed [15], generally fair but not excellent agreement with previous extensive measurements is not surprising. After cleaving the irradiated mica, TM-SFM revealed
D.D.N. Barlo Daya et al./Nucl.
Instr. and Meth. in Phys. Res. B III (1996) 87-90
hillocks on the exposed substrate surface (denoted e-f in Fig. 1) accompanied by raised tails again located over the ion track penetrating into the substrate. These features (HH = 0.29 nm, HW = 32 nm, TL = 92 nm) were more shallow than, though otherwise comparable with, the features observed on the originally outermost surface. The mica layer peeled away was probed on its exposed underside surface (denoted c-d in Fig. 1) and shallow hillocks with raised tails were also found (HH = 0.22 nm, HW = 26 nm, TL = 67 nm). These tails stretched away from the hillock in the opposite direction with respect to the tails observed on either the uncleaved irradiated mica (a-b in Fig. 1) or the upward-oriented surface exposed by cleaving (e-f in Fig. 1). That is, the tails on the exposed underside surface were located “under” the ion track passing through the peeled mica layer. Since the range of 78.2-MeV lz71 ions (0.6 MeV/u) in mica (_ 10 km> is of the same order of magnitude as the layer thickness of mica usually cleaved away (on average 5-10 pm as estimated from weight measurements), cleaving out a relatively thinner mica layer in order to observe latent tracks was challenging. It was not guaranteed that one could observe the latent tracks after cleaving each and every mica sample irradiated in our studies. (Of course, for studies involving faster ions with much larger range [23251, such difficulties are not encountered). When peeling away mica layers, it is likely that these layers are nonuniform in thickness, as visual inspection of the surfaces under an optical microscope typically revealed tall step edges. Considering consequent variations in the amount of energy lost by the 127I ions in traversing various thicknesses of mica, and the resulting variations in electronic stopping power at the various surface/interfaces imaged, some differences might well be expected between hillock/tail dimensions on the two exposed mica surfaces (c-d and e-f). Based on previous studies [14-161, it appears reasonable to conclude that all hillocks and tails observed in the present work are topological, although TM-SFM has in some cases displayed substantial height measurement errors [28]. In studies involving much swifter ions [23-251 than those employed in the present studies, no hillocks were observed. However, in those studies [23-251, the deposited energy density was likely to have been much lower than in the present work owing to a much larger secondary electron ultratrack radius for the swifter atomic ions. We probably observe hillocks as a more dramatic response to a very high deposited energy density. Each mica layer is about 1 nm thick and can be characterized qualitatively as having a “layer-gap” structure in which the “gaps” have a somewhat lower density than the “layers” [29]. It may thus be supposed that, even in the bulk, there should be room for a hillock to develop. Hillock and tail formation can be seen as a response to a high deposited energy density. A thermal spike mechanism in which a localized melt is created and rapidly
89
solidifies to an amorphous phase has been suggested [30]. If the amorphous phase is characterized by a lower material density than the surrounding unmelted material, stress is generated in the material around the ion track. At any interface, the stress would be manifested by a hillock-tail “signature”. When mica is bombarded by swift heavy atomic ions, the resulting latent tracks are indeed characterized by lower density [31]. When the deposited energy density in an ion track is high, there is also a transient high energy density gradient which can imply a significant radial force acting on surrounding material. This idea has been used to describe the sputtering of organic solids [6,19,20,32] in response to swift-heavy-atomic-ion bombardment, but it may also account for radial plastic deformation of bulk material [18]. Why were the hillocks observed on interior surfaces exposed by cleaving much lower than the hillocks observed on the originally outer mica layer? One reason for this could be that the ion electronic stopping power is much reduced by the time the ions have penetrated a distance into the mica comparable to their total range. When the ions are incident on the mica, their velocity is below the peak in the velocity dependence of the ion electronic stopping power, so as the ions slow down, their stopping power will be reduced. However, hillock widths and tail lengths were not so drastically reduced in the bulk. We therefore suppose that there is also a “containment effect”. At the originally outermost surface of the mica, there is no mechanical constraint on hillock formation. In the bulk, however, the expansion of material in adjacent layers mechanically interferes, as shown schematically (Fig. 1 (ii)). A certain amount of stress energy may even be stored in the lattice to be released upon cleaving.
4. Conclusions Tapping mode scanning force microscopy shows that the intersection with a cleavage plane of a latent (i.e. sub-surface) track in mica due to surface-grazing 78.2 MeV lz71 ions is a topological hillock with a raised tail located in the material proximate to the nearby sub-interface ion track. The hillocks observed on both exposed surfaces after cleaving the irradiated mica are shallow compared with the hillocks on the originally irradiated mica surface but are otherwise similar in structure. The observation of hillocks with tails oriented in opposite directions on the two surfaces exposed by cleaving the mica shows the symmetry of the damage around the bulk ion track. The observed symmetry of the latent tracks in mica is further evidence for a radial plastic deformation around each MeV ion track due to the action of a shock wave or a pressure pulse. Alternatively, the region around each ion track may undergo a phase transition to a less dense amorphous phase which imparts a permanent local stress to the material.
90
D.D.N. Bar10 Daya et al./Nucl.
Instr. and Meth. in Phys. Res. B 111 (1996) 87-90
Acknowledgements The authors thank the Swedish Research Council for Engineering Sciences (TFR), the Swedish National Board for Industrial and Technical Development (NUTEK), and the Knut and Alice Wallenberg Foundation. D.D.N.B.D. thanks the International Science Programs (ISP) of the Uppsala University for a fellowship. The authors thank T.R. Ariyaratne for helpful discussions.
References 111 R.L. Fleischer, P.B. Price and R.M. Walker, Nuclear Tracks in Solids: Principles and Applications (University of California Press, 1975). La H.A. Khan, N.A. Khan and R. Spohr, Nucl. Instr. and Meth. 189 (1981) 577. [31 H.A. Khan, N.A. Khan and R. Spohr, Nucl. In&. and Meth. 189 (1981) 583. [41 H. Rohrer, Surf. Sci. 299/300 (1994) 956. [51 C.F. Quate, Surf. Sci. 299/300 (1994) 980. bl R.E. Johnson and B.U.R. Sundqvist, Phys. Today, March (1992) 28. K. Dan. Vidensk. Selsk. Mat. Fys. Medd. 43 [71 P. Hikansson, (1993) 593. [81 C.T. Reimann, K. Dan. Vidensk. Selsk. Mat. Fys. Medd. 43 (1993) 351. [PI J. Kopniczky, CT. Reimrum, A. Hallen, B.U.R. Sundqvist, P. Tengvall and R. Erlandsson, Phys. Rev. B 49 (1994) 625. 1101J. Kopniczky, A. Hall&r, N. Keskitalo, CT. Reimann and B.U.R. Sundqvist, Radiat. Meas. 25 (1995) 47. [Ill J. Eriksson, J. Kopniczky, G. Brinkmalm, R.M. Papaleo, P. Demirev, C.T. Reimann, P. H&ansson and B.U.R. Sundqvist, Nucl. Instr. and Meth. B 101 (1995) 142. [121 Ch. Tomaschko, M. Schutr, R. Berger, G. Saemamr-Ischenko, H. Voit, A. Brunelle, S. Della-Negra and Y. Le Beyec, Rap. Commun. Mass Spectrum. 9 (1995) 924. E. Wistus, J. Eriksson, P. [I31 C.T. Reimann, J. Kopniczky, HBkansson and B.U.R. Sundqvist, Int. J. Mass Spectrom. Ion Froc. 151 (1995) 147.
[14] D.D.N. Bar10 Daya, A. Hall&, J. Eriksson, J. Kopniczky, R. Papaleo, C.T. Reimann, P. HIUransson, B.U.R. Sundqvist, A. Bnmelle, S. Della-Negra and Y. Le Beyec, Nucl. Instr. and Meth. B 106 (1995) 38. [15] D.D.N. Bar10 Daya, A. Hall&, P.. H&ansson, B.U.R. Sundqvist and C.T. Reimann, Nucl. Instr. and Meth. B 103 ( 1995) 454. 1161 F. Ames, M. Dabeli, C.R. Musil, P.W. Nebiker, L. Scandella, M. Suter and H.A. Synal, Nucl. Instr. and Meth. B, to be published. 1171 I.V. Vorob’eva and E.A. Ter-Ovanes’yan, Sov. Phys. Sol. State 34 (1992) 222. [181 I.V. Vorobyova, Nucl. Tracks Radiat. Meas. 21 (1993) 449. [l 91 R.E. Johnson, B.U.R. Sundqvist, A. Hedin and D. Fenyii, Phys. Rev. B 40 (1989) 49. 1201 IS. Bitensky, A.M. Goldenberg and E.S. Parilis, in: K.G. Standing and W. Ens, eds., Methods and Mechanisms for Producing Ions from Large Molecules (Plenum, New York, 1991) p. 83. [21] R.E. Mans, P. Beiersdorfer and D. Schneider, Phys. Today, October (1994) 27. [22] D. Schneider, M.A. Briere, M.W. Clark, J. McDonald, J. Biersack and W. Siekhaus, Surf. Sci. 294 (1993) 403. 1231 F. Thibaudau, J. Cousty, E. Balanzat and S. Bouffard, Phys. Rev. Lett. 67 (1991) 1582. [241 T. Hagen, S. Grafstrijm, J. Ackermann, R. Neumann, C. Trautmann, J. Vetter and N. Angert, J. Vat. Sci. Technol. B 12 (1994) 1555. 1251 J. Ackermann, N. Angert, R. Neumann, C. Trammann, M. Dischner, T. Hagen and M. Sedlacek, Nucl. Instr. and Meth. B 107 (1996) 181. [26] Q. Zhong, D. Inniss, K. Kjoller and V.B. Elings, Surf. Sci. Lett. 290 (1993) L688. [27] A. Hall&, P.A. Ingermarsson, P. Hfikansson and B.U.R. Sundqvist, Nucl. Ins& and Meth. B 36 (1989) 345. [28] A.P. Quist, L.P. Bjiirck, C.T. Reimann, S.O. Oscarsson and B.U.R. Sundqvist, Surf. Sci. 325 (1995) L406. 1291 L.T. Chadderton, J.P. Biersack and S.L. Koul, Nucl. Track Radiat. Meas. 15 (1988) 31. [30] G. Szenes, Phys. Rev. B 51 (1995) 8026. [3 11 D. Albrecht, P. Armbruster, R. Spohr, M. Roth, K. Schaupert and H. Stuhrmann, Appl. Phys. A 37 (1985) 37. [32] C.T. Reimann, Nucl. Instr. and Meth. B 95 (1995) 181.
Beam Interactions with Materials CbAtoms
ELSEWIER
Latent ( sub-surface) tracks in mica studied by tapping mode scanning force microscopy D.D.N. Barlo Daya a,* , C.T. Reimann
b, A. Hall& b, B.U.R. Sundqvist
b, P. Hakansson
b
a Deparhent of Physics, University of Colombo, Colombo 3, Sri Lanka b Division of Ion Physics, Department of Radiation Sciences, Uppsala University, Box 535, S-751 21 Uppsala, Sweden
Received 30 September 1995 Abstract This short communication presents information on the structure of latent (i.e subsurface) tracks in muscovite mica due to single 78.2 MeV lz71 ions incident at an angle of 67” with respect to the surface normal. ‘Ihe latent tracks have been studied for the first time using tapping mode scanning force microscopy. Images of an underlying mica surface, exposed by cleaving after ion bombardment, displayed hillocks accompanied by raised tails. These features were significantly lower in height compared to similar features observed on the originally outermost mica surface. The inner back surface of the mica sheet cleaved away also displayed shallow hillocks with raised tails, but on that surface, the tails stretched in the opposite direction to those observed on the originally outermost mica surface. On both surfaces exposed by cleaving, the tail is located in close proximity to the latent ion track, showing a symmetry of expansion along the track. The present results provide more evidence for a permanent expansion occurring around each ion track, as described qualitatively by theoretical models.
1. Introduction The registration of bulk ion tracks as a result of the passage of energetic particles through insulating materials is a well known phenomenon which has been studied extensively for more than three decades [l]. The registration of ion tracks is considered to be a consequence of several secondary processes, including propagation of shock waves, heat conduction, bond breaking, atomic displacements, and phase changes, which occur upon relaxation of electronically deposited energy into the nuclear system of the material. A clear understanding of the structure and dimensions of ion tracks is vital, not only for identification of the incident ions but also for achieving an understanding of the mechanisms behind the track formation. The property of enhanced chemical reactivity in the regions in proximity to the ion tracks has made it possible to enlarge the tracks by chemical means (etching) so that they can be visualized using either optical [l] or scanning electron microscopy [2,31 techniques. The etching approach does not provide information about the initial structure and nature of the ion tracks since a large amount of material is removed in the etching process. After the invention of near-field scanning
’ Corresponding author. Fax + 94 1 583810, tel. + 94 1 584777, e-mail [email protected]. 0168-583X/96/$15.00
probe techniques such as scanning tunnelling microscopy [4] and scanning force microscopy @FM) [5], it has become possible to investigate surfaces and surface features with a resolution of a few A without employing any pre-treatment of the surfaces. These new scanning techniques are convenient tools to study unetched tracks and can therefore provide new, useful information about the damage mechanisms. Sputtering, or ejection of material from a solid surface, is one of the well known consequences of the ion-solid interaction 16-81 which provide indirect clues about defect formation by incident ions. Crater formation in sputtering of organic solids has recently been observed in SFM studies [9-131. The formation of topological hillocks on the surfaces of insulating materials like mica [ 14-161 as a result of ion impacts is one of the observations recently made with SFM. The formation of surface topological hillocks [ 141 accompanied by raised tails 114,171 located over the penetrating ion track provides experimental evidence for a theoretical model of material rearrangement based on a material response to large radial forces associated with the transient energy density of the track [18]. This model is related closely to the pressure pulse [6,19] and shock wave [20] models of sputtering of organic targets. The observation of surface tracks in the form of topological hillocks on mica leads to speculation about possible
0 1996 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)01285-0
88
D.D.N. Barlo Daya et al./Nucl.
Instr. and Meth. in Phys. Res. B 11 I (19961 U-90
crater-like defects located under the hillocks [ 15,21,22]. The objective of the present work was to apply SFM in order to probe the bulk damage in mica caused by incident 78.2 MeV 1271ions. In an earlier work [23], the observation of crater-like defects on swift-heavy-ion-irradiated mica examined after cleaving away the originally irradiated surface was interpreted as an artefact of the continuous repulsive contact mode,SFM due to greater material softness in the damaged track zones. In a similar work, tip-scanning-direction-dependent image contrast was taken to imply that the latent tracks were flat regions of increased friction [24,25]. In the present study, tapping mode SFM (TM-SFM) has been employed to study the latent tracks in mica. TM-SFM involves only intermittent and gentle tip-surface contact, practically eliminating lateral (frictive) forces and substantially reducing normal forces [26], and thus may offer an improved means for examining latent tracks.
2. Experiment Uniform irradiation [27] was performed on freshlycleaved muscovite mica pieces (KAl,(OH),Si,AlO,,) by charge-equilibrated 78.2 MeV ‘*‘I ions from the Uppsala EN tandem Van de Graaff accelerator at ion angle of incidence of 67” with respect to the surface normal and to a fluence of - lo9 ions/cm*. III, Digital InTappingMode TM SFM (NanoScopeB struments, CA, USA) was employed to scan mica surfaces under ambient conditions with NanoProbeTM silicon tips with nominal radius of curvature 10 nm and cone angle 36”. The cantilevers were oscillated near resonance ( - 275 kHz) with a drive amplitude corresponding to a detector signal of 4 V. When probing a sample, imaging feedback was based on damping of the free oscillations of the cantilever by a few percent by intermittent tip-surface contact. A freshly-cleaved and then ion-bombarded mica surface was first scanned by TM-SFM. Next, the irradiated mica surface was cleaved again and both newly-exposed surfaces were scanned by TM-SFM. This procedure allowed us to compare surface track dimensions with the dimensions of exposed bulk latent tracks.
3. Results and discussion On the originally outermost mica surface (denoted a-b in Fig. l), conical shaped hillocks accompanied by raised tails oriented over the penetrating ion tracks were observed. Hillock heights (HH) were - 1.2 nm and the hillocks displayed basal widths (HW) - 37 nm. The lengths of the tails (TL) ranged up to - 99 nm. Measured heights were in excellent agreement with our previously
a
C
(i)
e
r
Cleavage Plane
Hill\ock
(ii) a
b
d f Ion Fig. 1. (i) Top views of tapping mode scanning force microscopy images of mica surfaces. a-b: the mica surface irradiated by 78.2-MeV lz71 ions at an ion angle of incidence of 67” with respect to the surface normal. e-f: the buried surface layer of mica freshly exposed by cleaving off a layer of the mica. c-d: the inner backside surface of the mica layer cleaved away. (ii) A schematic illustration of the symmetry of the expansion in the ion track constrained by the layered nature of mica and as viewed from the side. The images labelled a-b, c-d, and e-f in (i) correspond to the surfaces labelled the same in (ii) but with au obvious rotation. Ions proceed in the direction from the hillocks to the tails.
published measurements [14]; hillock widths and tail lengths were in fair agreement with our previous measurements [14]. We note that deduction of the tail lengths is quite subjective. Furthermore, in the present set of measurements, we measured relatively few surface tracks: since feature sizes are broadly distributed [15], generally fair but not excellent agreement with previous extensive measurements is not surprising. After cleaving the irradiated mica, TM-SFM revealed
D.D.N. Barlo Daya et al./Nucl.
Instr. and Meth. in Phys. Res. B III (1996) 87-90
hillocks on the exposed substrate surface (denoted e-f in Fig. 1) accompanied by raised tails again located over the ion track penetrating into the substrate. These features (HH = 0.29 nm, HW = 32 nm, TL = 92 nm) were more shallow than, though otherwise comparable with, the features observed on the originally outermost surface. The mica layer peeled away was probed on its exposed underside surface (denoted c-d in Fig. 1) and shallow hillocks with raised tails were also found (HH = 0.22 nm, HW = 26 nm, TL = 67 nm). These tails stretched away from the hillock in the opposite direction with respect to the tails observed on either the uncleaved irradiated mica (a-b in Fig. 1) or the upward-oriented surface exposed by cleaving (e-f in Fig. 1). That is, the tails on the exposed underside surface were located “under” the ion track passing through the peeled mica layer. Since the range of 78.2-MeV lz71 ions (0.6 MeV/u) in mica (_ 10 km> is of the same order of magnitude as the layer thickness of mica usually cleaved away (on average 5-10 pm as estimated from weight measurements), cleaving out a relatively thinner mica layer in order to observe latent tracks was challenging. It was not guaranteed that one could observe the latent tracks after cleaving each and every mica sample irradiated in our studies. (Of course, for studies involving faster ions with much larger range [23251, such difficulties are not encountered). When peeling away mica layers, it is likely that these layers are nonuniform in thickness, as visual inspection of the surfaces under an optical microscope typically revealed tall step edges. Considering consequent variations in the amount of energy lost by the 127I ions in traversing various thicknesses of mica, and the resulting variations in electronic stopping power at the various surface/interfaces imaged, some differences might well be expected between hillock/tail dimensions on the two exposed mica surfaces (c-d and e-f). Based on previous studies [14-161, it appears reasonable to conclude that all hillocks and tails observed in the present work are topological, although TM-SFM has in some cases displayed substantial height measurement errors [28]. In studies involving much swifter ions [23-251 than those employed in the present studies, no hillocks were observed. However, in those studies [23-251, the deposited energy density was likely to have been much lower than in the present work owing to a much larger secondary electron ultratrack radius for the swifter atomic ions. We probably observe hillocks as a more dramatic response to a very high deposited energy density. Each mica layer is about 1 nm thick and can be characterized qualitatively as having a “layer-gap” structure in which the “gaps” have a somewhat lower density than the “layers” [29]. It may thus be supposed that, even in the bulk, there should be room for a hillock to develop. Hillock and tail formation can be seen as a response to a high deposited energy density. A thermal spike mechanism in which a localized melt is created and rapidly
89
solidifies to an amorphous phase has been suggested [30]. If the amorphous phase is characterized by a lower material density than the surrounding unmelted material, stress is generated in the material around the ion track. At any interface, the stress would be manifested by a hillock-tail “signature”. When mica is bombarded by swift heavy atomic ions, the resulting latent tracks are indeed characterized by lower density [31]. When the deposited energy density in an ion track is high, there is also a transient high energy density gradient which can imply a significant radial force acting on surrounding material. This idea has been used to describe the sputtering of organic solids [6,19,20,32] in response to swift-heavy-atomic-ion bombardment, but it may also account for radial plastic deformation of bulk material [18]. Why were the hillocks observed on interior surfaces exposed by cleaving much lower than the hillocks observed on the originally outer mica layer? One reason for this could be that the ion electronic stopping power is much reduced by the time the ions have penetrated a distance into the mica comparable to their total range. When the ions are incident on the mica, their velocity is below the peak in the velocity dependence of the ion electronic stopping power, so as the ions slow down, their stopping power will be reduced. However, hillock widths and tail lengths were not so drastically reduced in the bulk. We therefore suppose that there is also a “containment effect”. At the originally outermost surface of the mica, there is no mechanical constraint on hillock formation. In the bulk, however, the expansion of material in adjacent layers mechanically interferes, as shown schematically (Fig. 1 (ii)). A certain amount of stress energy may even be stored in the lattice to be released upon cleaving.
4. Conclusions Tapping mode scanning force microscopy shows that the intersection with a cleavage plane of a latent (i.e. sub-surface) track in mica due to surface-grazing 78.2 MeV lz71 ions is a topological hillock with a raised tail located in the material proximate to the nearby sub-interface ion track. The hillocks observed on both exposed surfaces after cleaving the irradiated mica are shallow compared with the hillocks on the originally irradiated mica surface but are otherwise similar in structure. The observation of hillocks with tails oriented in opposite directions on the two surfaces exposed by cleaving the mica shows the symmetry of the damage around the bulk ion track. The observed symmetry of the latent tracks in mica is further evidence for a radial plastic deformation around each MeV ion track due to the action of a shock wave or a pressure pulse. Alternatively, the region around each ion track may undergo a phase transition to a less dense amorphous phase which imparts a permanent local stress to the material.
90
D.D.N. Bar10 Daya et al./Nucl.
Instr. and Meth. in Phys. Res. B 111 (1996) 87-90
Acknowledgements The authors thank the Swedish Research Council for Engineering Sciences (TFR), the Swedish National Board for Industrial and Technical Development (NUTEK), and the Knut and Alice Wallenberg Foundation. D.D.N.B.D. thanks the International Science Programs (ISP) of the Uppsala University for a fellowship. The authors thank T.R. Ariyaratne for helpful discussions.
References 111 R.L. Fleischer, P.B. Price and R.M. Walker, Nuclear Tracks in Solids: Principles and Applications (University of California Press, 1975). La H.A. Khan, N.A. Khan and R. Spohr, Nucl. Instr. and Meth. 189 (1981) 577. [31 H.A. Khan, N.A. Khan and R. Spohr, Nucl. In&. and Meth. 189 (1981) 583. [41 H. Rohrer, Surf. Sci. 299/300 (1994) 956. [51 C.F. Quate, Surf. Sci. 299/300 (1994) 980. bl R.E. Johnson and B.U.R. Sundqvist, Phys. Today, March (1992) 28. K. Dan. Vidensk. Selsk. Mat. Fys. Medd. 43 [71 P. Hikansson, (1993) 593. [81 C.T. Reimann, K. Dan. Vidensk. Selsk. Mat. Fys. Medd. 43 (1993) 351. [PI J. Kopniczky, CT. Reimrum, A. Hallen, B.U.R. Sundqvist, P. Tengvall and R. Erlandsson, Phys. Rev. B 49 (1994) 625. 1101J. Kopniczky, A. Hall&r, N. Keskitalo, CT. Reimann and B.U.R. Sundqvist, Radiat. Meas. 25 (1995) 47. [Ill J. Eriksson, J. Kopniczky, G. Brinkmalm, R.M. Papaleo, P. Demirev, C.T. Reimann, P. H&ansson and B.U.R. Sundqvist, Nucl. Instr. and Meth. B 101 (1995) 142. [121 Ch. Tomaschko, M. Schutr, R. Berger, G. Saemamr-Ischenko, H. Voit, A. Brunelle, S. Della-Negra and Y. Le Beyec, Rap. Commun. Mass Spectrum. 9 (1995) 924. E. Wistus, J. Eriksson, P. [I31 C.T. Reimann, J. Kopniczky, HBkansson and B.U.R. Sundqvist, Int. J. Mass Spectrom. Ion Froc. 151 (1995) 147.
[14] D.D.N. Bar10 Daya, A. Hall&, J. Eriksson, J. Kopniczky, R. Papaleo, C.T. Reimann, P. HIUransson, B.U.R. Sundqvist, A. Bnmelle, S. Della-Negra and Y. Le Beyec, Nucl. Instr. and Meth. B 106 (1995) 38. [15] D.D.N. Bar10 Daya, A. Hall&, P.. H&ansson, B.U.R. Sundqvist and C.T. Reimann, Nucl. Instr. and Meth. B 103 ( 1995) 454. 1161 F. Ames, M. Dabeli, C.R. Musil, P.W. Nebiker, L. Scandella, M. Suter and H.A. Synal, Nucl. Instr. and Meth. B, to be published. 1171 I.V. Vorob’eva and E.A. Ter-Ovanes’yan, Sov. Phys. Sol. State 34 (1992) 222. [181 I.V. Vorobyova, Nucl. Tracks Radiat. Meas. 21 (1993) 449. [l 91 R.E. Johnson, B.U.R. Sundqvist, A. Hedin and D. Fenyii, Phys. Rev. B 40 (1989) 49. 1201 IS. Bitensky, A.M. Goldenberg and E.S. Parilis, in: K.G. Standing and W. Ens, eds., Methods and Mechanisms for Producing Ions from Large Molecules (Plenum, New York, 1991) p. 83. [21] R.E. Mans, P. Beiersdorfer and D. Schneider, Phys. Today, October (1994) 27. [22] D. Schneider, M.A. Briere, M.W. Clark, J. McDonald, J. Biersack and W. Siekhaus, Surf. Sci. 294 (1993) 403. 1231 F. Thibaudau, J. Cousty, E. Balanzat and S. Bouffard, Phys. Rev. Lett. 67 (1991) 1582. [241 T. Hagen, S. Grafstrijm, J. Ackermann, R. Neumann, C. Trautmann, J. Vetter and N. Angert, J. Vat. Sci. Technol. B 12 (1994) 1555. 1251 J. Ackermann, N. Angert, R. Neumann, C. Trammann, M. Dischner, T. Hagen and M. Sedlacek, Nucl. Instr. and Meth. B 107 (1996) 181. [26] Q. Zhong, D. Inniss, K. Kjoller and V.B. Elings, Surf. Sci. Lett. 290 (1993) L688. [27] A. Hall&, P.A. Ingermarsson, P. Hfikansson and B.U.R. Sundqvist, Nucl. Ins& and Meth. B 36 (1989) 345. [28] A.P. Quist, L.P. Bjiirck, C.T. Reimann, S.O. Oscarsson and B.U.R. Sundqvist, Surf. Sci. 325 (1995) L406. 1291 L.T. Chadderton, J.P. Biersack and S.L. Koul, Nucl. Track Radiat. Meas. 15 (1988) 31. [30] G. Szenes, Phys. Rev. B 51 (1995) 8026. [3 11 D. Albrecht, P. Armbruster, R. Spohr, M. Roth, K. Schaupert and H. Stuhrmann, Appl. Phys. A 37 (1985) 37. [32] C.T. Reimann, Nucl. Instr. and Meth. B 95 (1995) 181.