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TEM observation of Ag microcones
Surface Science 180 (1987) L103-L108 North-Holland, Amsterdam
SURFACE
SCIENCE
TEM OBSERVATION M. TANEMURA,
L103
LETTERS OF Ag MICROCONES
S. MORISHITA,
Department of Systems Engineering, Applied Gokiso-cho, Showa-ku, Nagoya 466, Japan Received
20 October
1986; accepted
Y. FUJIMOTO Physics Laboratoty,
for publication
23 October
and F. OKUYAMA Nagoya
Institute
*
of Technology,
1986
Transmission electron microscopy of ion-induced microcones of Ag revealed that their tip and shank regions were entirely different in their crystalline state. The tip region exhibited a less developed crystallinity, being composed of crystallites and platelets of Ag, whereas the shank region was usually monocrystalline. It is supposed that the tips of the evolving cones were at too high a temperature and pressure to maintain the original bulk structure.
Ion bombardment of metallic targets generally modifies the topographical structure of the target surface [l]. The most typical topographical modification due to ion bombardment is the formation of conical structures, which has been interpreted in terms of the so-called left-standing theory [2]. When a polycrystalline thin film of Ag is bombarded with Ar+ ions accelerated to a few keV, the film surface becomes covered with densely spa9ed conical projections having microscopic dimensions [3]. Disagreeing with the left-standing model, these microcones were not impurity-seeded, so that a new model must be found to explain their formation. As stated elsewhere [3], the determination of their crystalline state by means of transmission electron microscopy (TEM) will lead to the elucidation of their formation mechanism. A 700 nm thick Ag film, deposited on an air-cleaved KC1 plate by a DC sputtering technique, was removed from the substrate in water, and then put on a vertically held slit mesh with its upper end cut by scissors (fig. 1). The sample thus prepared was mounted in a JAMP-10s scanning Auger microprobe, and the folded portion was bombarded for 20 min with normally incident 3 keV Ar+ ions, at a current density of 150 pA/cm2. Thereafter, the sample was transferred to a JEM-2000FX electron microscope, in order to determine the crystalline state of cones formed at the bombarded area. Fig. 2a shows the bright-field image of a typical Ag projection formed in the above-mentioned manner. Clearly, the projection consisted of two structures: a shank shaped in a truncated cone and a tip placed on the shank. The * To whom correspondence
should be addressed.
0039-6028/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
L104
M. Tanemura et al. / TEM ohseruution
ofAg
microcones
Ion Beam
I
Fig. 1. Scheme of ion bombardment,
with target configuration
drawn
schematically.
ED pattern. image of a typical Ag cone, and (b) corresponding Fig. 2. (a) Bright-field Dark .-field image formed by the encircled spots in (b). The arrow head in (a) indicates a crystallite grown on the tip region.
Cc) Ag
M. Tanemura et al. / TEM observation of Ag microcones
L105
tip region was more transparent to the electron beam, indicating that the tip and shank regions were quite different in crystallinity. For convenience, the crystalline states at the tip and shank regions are referred to as phase A and phase B, respectively, hereinafter. Shown in fig. 2b is the electron diffraction (ED) pattern from the entire projection. All the diffraction spots came from the fee lattice of Ag, but they were not arrayed periodically. The dark-field image in fig. 2c, produced by 111 and 002 spots, discloses Ag microcrystallites distributed on and in the tip region. Another feature of the tip region is that
Fig. 3. (a) Bright-field image and (b) ED pattern of a cone with a less sharp phase boundary. (c) Sketch of the ED pattern, showing the (310) reflection from the Ag lattice. (d) Dark-field image formed by the (131) spot. Indicated by the arrow head in (a) is the phase boundary.
L106
M. Tanemura
et al. / TEM observation of Ag MK~OCO~~S
its apex was not pointed, but rounded. As is well known, an ion beam tends to sharpen the cone apex through its etching effect. The above fact may therefore indicate that a surface tension existed on the tip region during ion bombardment. In the second example, presented in fig. 3a, the phase boundary was less sharply defined, and the ED pattern from the whole structure comprised periodically arranged spots, as shown in fig. 3b. By a careful analysis, the network formed by the diffraction spots was determined to be the (310) reciprocal lattice plane of Ag (fig. 3~). As revealed by the dark-field image in fig. 3d, the phase B region was monocrystalline and Ag microcrystallites in the phase A region possessed the same orientation as the phase B region. These may suggest that the phase A and B regions constituted one single crystal before ion bombardment, and that the upper region of the crystal lost the original bulk structure during ion bombardment.
Fig. 4. Ag cone with a defect-rich tip region. (a) Bright-field image and (b) ED pattern of the cone. (c), (d) Dark-field images produced by the spots C and D in (b), respectively.
M. Tanemura et al. / TEM observation of Ag microcones
L107
Clearer evidence that phase A also had the fee structure of Ag is presented in fig. 4. In the ED pattern shown in fig. 4b, the spots C and D corresponded to 002 and 111 reflections, respectively, and the dark-field images produced by these spots, given in figs. 4c and 4d, prove that the spots arose from electrons scattered at lattices in the tip region. What is surprising is that the dark-field image formed by the (111) spot exhibited parallel and concentric moire fringes [4] (see also fig. 5). This implies that the tip region involved platelets, or plate-like crystallites, with normals parallel to the incident electron beam. During the course of its evolution, the cone was oriented to the Ar+-ion beam [3], and the electron beam was nearly perpendicular to the cone’s central axis. Hence, the developed surfaces of the platelets may have been parallel to the ion beam. In view of the fact that the electron beam transmission was higher at the tip region than at the shank, the stacking of the platelets must have been fault-rich. At the present time, we have no clue as to the exact cause of the phase A formation. However, we are convinced that it was not impurity-triggered, because Auger signals originating from impurities were never detected from the ion-bombarded area. Our tentative interpretation is that the formation of phase A is ascribable to the heating and compressive effects of impinging Ar+ ions. Perhaps, the tips of developing cones were at so high a temperature and pressure, owing to a head-on impact with Arf ions, that they could not maintain the original bulk structure. We are now continuing TEM observations of Cu and MO microcones, and a more detailed discussion will soon become possible regarding this problem.
Fig. 5. Enlarged photograph
of fig. 4d, revealing parallel and concentric moire fringes. The arrows indicate rotation fringes.
L108
M. Tanemura et al. / TEM observation
ofAg
microcones
Unfortunately, the TEM data cited above provided no clear information on the cone formation mechanism. If phase A was sputter-protecting, the cone formation was due solely to the etching effect of the Ar+-ion beam. The growth of Ag microcrystallites on the surface of the phase A region (see fig. 2a), however, strongly indicates that a particle supply to the tip region took place during ion bombardment. We thus envisage a real growth process took place in combination with the ion-etching process forming the cones. Sincere thanks discussions.
are expressed
to Professor
H. Morikawa
for stimulating
References [l] [2] [3] [4]
G.K. Wehner, J. Vacuum Sci. Technol. A3 (1985) 1821, and papers cited therein. A.D.G. Stewart and M.W. Thompson, J. Mater. Sci. 4 (1969) 56. M. Tanemura and F. Okuyama, J. Vacuum Sci. Technol. A4 (1986) 2369. P.B. Hirsch, A. Howie, R.B. Nicholson, D.W. Pashley and M.J. Whelan, Electron of Thin Crystals (Butterworths, London, 1967) ch. 7.
Microscopy
SURFACE
SCIENCE
TEM OBSERVATION M. TANEMURA,
L103
LETTERS OF Ag MICROCONES
S. MORISHITA,
Department of Systems Engineering, Applied Gokiso-cho, Showa-ku, Nagoya 466, Japan Received
20 October
1986; accepted
Y. FUJIMOTO Physics Laboratoty,
for publication
23 October
and F. OKUYAMA Nagoya
Institute
*
of Technology,
1986
Transmission electron microscopy of ion-induced microcones of Ag revealed that their tip and shank regions were entirely different in their crystalline state. The tip region exhibited a less developed crystallinity, being composed of crystallites and platelets of Ag, whereas the shank region was usually monocrystalline. It is supposed that the tips of the evolving cones were at too high a temperature and pressure to maintain the original bulk structure.
Ion bombardment of metallic targets generally modifies the topographical structure of the target surface [l]. The most typical topographical modification due to ion bombardment is the formation of conical structures, which has been interpreted in terms of the so-called left-standing theory [2]. When a polycrystalline thin film of Ag is bombarded with Ar+ ions accelerated to a few keV, the film surface becomes covered with densely spa9ed conical projections having microscopic dimensions [3]. Disagreeing with the left-standing model, these microcones were not impurity-seeded, so that a new model must be found to explain their formation. As stated elsewhere [3], the determination of their crystalline state by means of transmission electron microscopy (TEM) will lead to the elucidation of their formation mechanism. A 700 nm thick Ag film, deposited on an air-cleaved KC1 plate by a DC sputtering technique, was removed from the substrate in water, and then put on a vertically held slit mesh with its upper end cut by scissors (fig. 1). The sample thus prepared was mounted in a JAMP-10s scanning Auger microprobe, and the folded portion was bombarded for 20 min with normally incident 3 keV Ar+ ions, at a current density of 150 pA/cm2. Thereafter, the sample was transferred to a JEM-2000FX electron microscope, in order to determine the crystalline state of cones formed at the bombarded area. Fig. 2a shows the bright-field image of a typical Ag projection formed in the above-mentioned manner. Clearly, the projection consisted of two structures: a shank shaped in a truncated cone and a tip placed on the shank. The * To whom correspondence
should be addressed.
0039-6028/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
L104
M. Tanemura et al. / TEM ohseruution
ofAg
microcones
Ion Beam
I
Fig. 1. Scheme of ion bombardment,
with target configuration
drawn
schematically.
ED pattern. image of a typical Ag cone, and (b) corresponding Fig. 2. (a) Bright-field Dark .-field image formed by the encircled spots in (b). The arrow head in (a) indicates a crystallite grown on the tip region.
Cc) Ag
M. Tanemura et al. / TEM observation of Ag microcones
L105
tip region was more transparent to the electron beam, indicating that the tip and shank regions were quite different in crystallinity. For convenience, the crystalline states at the tip and shank regions are referred to as phase A and phase B, respectively, hereinafter. Shown in fig. 2b is the electron diffraction (ED) pattern from the entire projection. All the diffraction spots came from the fee lattice of Ag, but they were not arrayed periodically. The dark-field image in fig. 2c, produced by 111 and 002 spots, discloses Ag microcrystallites distributed on and in the tip region. Another feature of the tip region is that
Fig. 3. (a) Bright-field image and (b) ED pattern of a cone with a less sharp phase boundary. (c) Sketch of the ED pattern, showing the (310) reflection from the Ag lattice. (d) Dark-field image formed by the (131) spot. Indicated by the arrow head in (a) is the phase boundary.
L106
M. Tanemura
et al. / TEM observation of Ag MK~OCO~~S
its apex was not pointed, but rounded. As is well known, an ion beam tends to sharpen the cone apex through its etching effect. The above fact may therefore indicate that a surface tension existed on the tip region during ion bombardment. In the second example, presented in fig. 3a, the phase boundary was less sharply defined, and the ED pattern from the whole structure comprised periodically arranged spots, as shown in fig. 3b. By a careful analysis, the network formed by the diffraction spots was determined to be the (310) reciprocal lattice plane of Ag (fig. 3~). As revealed by the dark-field image in fig. 3d, the phase B region was monocrystalline and Ag microcrystallites in the phase A region possessed the same orientation as the phase B region. These may suggest that the phase A and B regions constituted one single crystal before ion bombardment, and that the upper region of the crystal lost the original bulk structure during ion bombardment.
Fig. 4. Ag cone with a defect-rich tip region. (a) Bright-field image and (b) ED pattern of the cone. (c), (d) Dark-field images produced by the spots C and D in (b), respectively.
M. Tanemura et al. / TEM observation of Ag microcones
L107
Clearer evidence that phase A also had the fee structure of Ag is presented in fig. 4. In the ED pattern shown in fig. 4b, the spots C and D corresponded to 002 and 111 reflections, respectively, and the dark-field images produced by these spots, given in figs. 4c and 4d, prove that the spots arose from electrons scattered at lattices in the tip region. What is surprising is that the dark-field image formed by the (111) spot exhibited parallel and concentric moire fringes [4] (see also fig. 5). This implies that the tip region involved platelets, or plate-like crystallites, with normals parallel to the incident electron beam. During the course of its evolution, the cone was oriented to the Ar+-ion beam [3], and the electron beam was nearly perpendicular to the cone’s central axis. Hence, the developed surfaces of the platelets may have been parallel to the ion beam. In view of the fact that the electron beam transmission was higher at the tip region than at the shank, the stacking of the platelets must have been fault-rich. At the present time, we have no clue as to the exact cause of the phase A formation. However, we are convinced that it was not impurity-triggered, because Auger signals originating from impurities were never detected from the ion-bombarded area. Our tentative interpretation is that the formation of phase A is ascribable to the heating and compressive effects of impinging Ar+ ions. Perhaps, the tips of developing cones were at so high a temperature and pressure, owing to a head-on impact with Arf ions, that they could not maintain the original bulk structure. We are now continuing TEM observations of Cu and MO microcones, and a more detailed discussion will soon become possible regarding this problem.
Fig. 5. Enlarged photograph
of fig. 4d, revealing parallel and concentric moire fringes. The arrows indicate rotation fringes.
L108
M. Tanemura et al. / TEM observation
ofAg
microcones
Unfortunately, the TEM data cited above provided no clear information on the cone formation mechanism. If phase A was sputter-protecting, the cone formation was due solely to the etching effect of the Ar+-ion beam. The growth of Ag microcrystallites on the surface of the phase A region (see fig. 2a), however, strongly indicates that a particle supply to the tip region took place during ion bombardment. We thus envisage a real growth process took place in combination with the ion-etching process forming the cones. Sincere thanks discussions.
are expressed
to Professor
H. Morikawa
for stimulating
References [l] [2] [3] [4]
G.K. Wehner, J. Vacuum Sci. Technol. A3 (1985) 1821, and papers cited therein. A.D.G. Stewart and M.W. Thompson, J. Mater. Sci. 4 (1969) 56. M. Tanemura and F. Okuyama, J. Vacuum Sci. Technol. A4 (1986) 2369. P.B. Hirsch, A. Howie, R.B. Nicholson, D.W. Pashley and M.J. Whelan, Electron of Thin Crystals (Butterworths, London, 1967) ch. 7.
Microscopy