- Email: [email protected]
Modification of thin gold films with the scanning tunneling microscope
Surface Science 471 (2001) L129±L133
www.elsevier.nl/locate/susc
Surface Science Letters
Modi®cation of thin gold ®lms with the scanning tunneling microscope U. Weber *, R. Houbertz 1, U. Hartmann Institute of Experimental Physics, University of Saarbr ucken, P.O. Box 151150, D-66041 Saarbr ucken, Germany Received 17 January 2000; accepted for publication 10 November 2000
Abstract Thin gold ®lms, which were deposited by sputter deposition onto highly oriented graphite surfaces, were investigated and modi®ed by means of a scanning tunneling microscope. By applying short voltage pulses to the vertical piezoelectric element or to the tunneling tip, hole patterns were generated. Physical mechanisms underlying the modi®cations are discussed for the two methods. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Diusion and migration; Gold; Graphite
The dimensions of device interconnections used in integrated circuits have continuously decreased as the level of integration has increased. Electroand thermomigration-induced as well as (thermal) stress-induced failures of thin ®lm interconnects are the most prominent physical processes which can limit the reliability of integrated circuits and thus create a barrier for further miniaturization [1]. Electromigration has to be considered as a major cause of interconnect failure in integrated circuits [2] when a suciently high electrical current density up to 106 A/cm2 ¯ows through a
* Corresponding author. Present address: Department of Physics, University of Kaiserslautern, P.O. Box 3049, D-67653 Kaiserslautern, Germany. Tel.: +49-631-205-4053; fax: +49631-205-2834. E-mail address: [email protected] (U. Weber). 1 Present address: Sandia National Laboratories, MS 9161, Livermore, CA 94550, USA, and Fraunhofer Institut f ur silicatforschung, ORMOCER Dept., Micro Systems Technology, Neunerplatz 2, D-97082 w urzburg, Germany.
material. Thermomigration, on the other hand, can occur if a temperature gradient (minimum value of 1273 K/cm) is present along a sample [3]. Among all materials used in microelectronics, Au is one of the most inert materials used, for example, in Au/Ge/Ni or Au/Ni interconnects [4,5] or as passivation layers in integrated circuits with chip on board technology [6]. The latter particularly increases the interest in the behavior of thin gold ®lms upon applying stress like, for example high electrical ®elds. Scanning tunneling microscopy (STM) has proven to be not only suitable for high-resolution topographic and spectroscopic imaging at a subnm and at a meV scale [7], but also as a tool for nm-scale structuring of conducting or semiconducting surfaces down to atomic scale, some of which are reviewed in the literature [8±10]. Beside its ability of electronically mapping surfaces, STM allows to locally apply current densities in the order of 107 ±109 A/cm2 , which can account for electromigration processes [11].
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 0 ) 0 0 9 4 9 - 3
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U. Weber et al. / Surface Science 471 (2001) L129±L133
In this letter, we report on STM modi®cations of thin granular Au ®lms deposited onto highly oriented pyrolitic graphite (HOPG) surfaces, in order to investigate the behavior of thin Au ®lms under high electrical and mechanical stress. The data are discussed with respect to electro- and thermomigration as well as to mechanical indentation processes. The experiments were performed in a standard UHV chamber (base pressure 2 10ÿ10 mbar) by a commercial AFM/STM system (Omicron). As substrates, HOPG samples were used, which were prepared by peeling o several HOPG layers with an adhesive tape. This procedure is known to produce lm-size, atomically ¯at terraces, most frequently separated by monoatomic steps. Thin gold ®lms were deposited by ion sputtering EAr 1:2 keV at low sputtering rates to generate granular ®lms with thicknesses of approximately 2±5 nm. For the data presented here, 5 nm thick Au ®lms were used. After preparation, the samples were immediately transferred into UHV without any further preparation like, e.g., thermal annealing, and subsequently investigated and modi®ed by STM. Several methods for generating small indentations in thin ®lms are reported in the literature (see, e.g. Refs. [12±14]). For example, the modi®cation of thin Au ®lms in Ref. [14] was carried out by enlarging the interaction force between an atomic force microscope probe and a sample, which was shown to allow modi®cations in the range between 25 and 100 nm. Similar modi®cations were carried out using voltage pulses between tip and sample, which can be compared with earlier ®ndings elsewhere [15]. These observations suggest that the mechanism underlying the modi®cations is a thermomigration process combined with an electromigration process due to the high mobility of metal atoms on the graphite surface [16]. Accordingly, our ®ndings (cf. Fig. 1), obtained upon applying a 5 5 pattern of fairly low voltage pulses to the tip (10 ls, 2.5 V), show ± irrespective of the polarity ± holes in the ®lms with diameters of about 50 nm. The occurrence of a hole as well as its lateral extent seem to strongly depend on the atomic shape of the tip. This does not allow a re-
liable generation of a hole pattern involving holes of a well-de®ned shape and size (cf. Fig. 1). The typical diameters which were achieved in one pattern varied from 20 to 60 nm, where the number of produced holes is around 10% of the number of manipulation processes. We have been not able to produce hillocks as reported in Ref. [15], probably due to the ®lm thickness which in our case was much lower (30 nm in Ref. [15]). Usually, from 25 pulses, only two large holes were generated, which show signi®cant accumulation of material around their edges. Sometimes, a pattern could be generated, although it is very dicult to distinguish between the individual holes due to the large amount of material piled up around them (cf. Fig. 1(b)). Evaluating the total volumes of the holes in Fig. 1(a) and comparing it to the total volume of the accumulated material, con®rms that both volumes are identical. From this observation, it can be concluded, that no material from the tip is involved in the modi®cation (e.g. by ®eld emission as in Ref. [13]). Additionally to the modi®cation of the Au ®lm, an in¯uence of the modi®cation procedure on the underlying HOPG substrate has to be discussed. As mentioned earlier, the samples were transferred into UHV without further processing like, e.g., annealing. Thus, it is expected that water is present at the surface, which might have an in¯uence on the modi®cation process. It could be shown, however, that the HOPG surface itself is signi®cantly modi®ed by the sputtering process leading to near-surface defects intercalation of the gold in the graphite lattice [18]. Both possibilities will be discussed now in more detail. For the high voltages it is very likely that the upper HOPG plane is in¯uenced by the modi®cation process. It is well known that HOPG can be modi®ed in the presence of water adsorbed at the surface, which already was observed by several groups (e.g. Ref. [17]). A chemical reaction between the carbon and the water takes place, resulting in the modi®cation/a removal of the upper exposed HOPG layer. Besides this observation, which was achieved on clean HOPG surfaces, in our case also a further promotion of the intercalation of the gold in the graphite lattice has to be assumed. For the work presented here, neither of
CE N IE SC RS CE E A TT RF LE SU
U. Weber et al. / Surface Science 471 (2001) L129±L133
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Fig. 1. Sputter-deposited Au ®lms after applying a 5 5 pattern of voltage pulses of 2.5 V for 10 ls between tip and sample. (a) represents only two out of 25 intended holes and (b) represents a complete hole pattern. The image size is (a) 375 375 nm2 and (b) 730 730 nm2 .
these processes can be either excluded or con®rmed, since the bottom of the holes could not be imaged with the STM tip. Holes of smaller extent can be produced more reliably with respect to their size as well as their position by applying short voltage pulses to the z piezo of the STM (henceforth referred to as z pulses), thus causing the tip to move towards the sample and back again at a time scale of 100 ls. The z pulses applied would correspond to equilibrium displacements of 9.3±11.3 nm. These values, however, do not yield the actual displacement of the tip due to the inertia of the piezoelectric element. For a pulse duration of 100 ls, the z piezo only moves approximately 1/3 of its equilibrium value towards the sample, which results in displacements between 3.1 and 3.8 nm. During this procedure, the tip bias was kept at the tunneling voltage of 0.1 V. Fig. 2 shows four STM images, which were recorded before (Fig. 2(a)) and after applying short z pulses (Fig. 2(b)±(d)). The pulses were arranged such that 5 5 patterns were generated. This modi®cation procedure results in holes within the Au ®lm. Compared to the holes generated by applying voltage pulses onto the tunneling tip, the ones depicted in Fig. 2 show nearly no accumulation of material piled up around their edges. There are several possibilities for where the removed material could be located. One is that the material is picked up by the STM tip. This then
would result in a variation of the tunneling distance, thus causing the tip to retract from the sample surface after every z pulse. That was never observed during the present experiments. On the other hand, an analysis of the inter-grain distances in the unmodi®ed Au ®lm and in close vicinity to the generated holes show signi®cant dierences. The averaged inter-grain distances within the unmodi®ed ®lm (cf. Fig. 2(a)) are in the range between 3 and 4 nm, which is the same value as for the modi®ed ®lm far away from the indentations (cf. Fig. 2(b)±(d)). In close vicinity to the generated holes, however, the distances between the Au grains are much smaller or even close to zero. Furthermore, nearly no additional material is visible at the edges of the holes, as usually observed for varying the gap voltage (cf. Fig. 1). This points towards the fact that the Au grains were mainly moved in-plane during the modi®cation procedure, thus causing a compression of the Au ®lm in close vicinity to the holes. However, in some cases, a low amount of material is piled up around the hole, whereas the inter-grain distances are still visible. This is similar to experiments using an AFM to move gold clusters on HOPG surfaces [19]. Due to the poor sticking of Au clusters, large clusters (100 nm) could be easily moved in-plane without modifying the clusters themselves. For sputtered Au ®lms, however, sticking is assumed to be higher due to the presence of sputterinduced surface defects in the HOPG lattice [18].
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U. Weber et al. / Surface Science 471 (2001) L129±L133
Fig. 2. Sputter-deposited Au ®lm on HOPG: (a) before and after applying a 5 5 pattern of z pulses of (b) 9.3 nm, (c) 10.7 nm and (d) 11.3 nm. The image sizes are 375 375 nm2 .
Nevertheless, the grains can still be shifted, so that sticking is not prominent enough compared with the tip±sample (Au±Au) interactions. The data show, that holes can be reproducibly generated within an uncertainty of around 5 nm. The modi®cations show lateral extents of 6±14 nm in Fig. 2(b) and 11±27 nm in Fig. 2(d). A comparison of the four topographies shows that the indentations become larger after applying higher pulses which results from a larger penetration depth of the tip into the Au ®lm, thus enhancing the interaction forces between the tip and the ®lm surface due to the much larger interaction radius. After a subsequent z pulse of 12 nm (i.e. Dz 4 nm), the tip has lost its imaging properties, which was reproducibly found in the experiments. This modi®cation technique which allows mechanical modi®cations of metal ®lms on a length scale of 5 nm at room
temperature ± keeping high quality imaging capability of the tunneling tip ± yields smaller structures and a much better control compared to the literature (e.g. Ref. [10] and references therein). Several mechanisms can be taken into account for the formation of the indentations as achieved after applying z pulses. In the following, these mechanisms will be discussed carefully with respect to accessible experimental parameters. In accounting for the size of the z pulses it becomes clear that the tip penetrates the Au ®lm, as in a typical nanoindentation experiment (see, e.g., Refs. [20,21]). Apart from nanoindentation, electromigration and thermomigration should be taken into account. Electromigration seems to play a minor role, since the modi®cations show nearly no dependence on the polarity of the tunneling voltage. In com-
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U. Weber et al. / Surface Science 471 (2001) L129±L133
parison to Ref. [15], where an obvious polarity dependence was observed, the ®lms in our case are only 5 nm thick which may reduce the lateral electronic transport due to an enhanced resistivity arising from the grain interconnections, when compared to bulk Au. Even if the current is assumed to only ¯ow in radial direction within the Au ®lm, the current for a modi®cation of 5 nm in diameter would have to be as high as 1.6 lA (for larger modi®cations even much higher) to give rise to electromigration. Nanoindentation and thermomigration cannot easily be distinguished. The arising temperatures do not seem to exceed the melting point of gold. In the case of melting, one would expect the gold grains to form bigger entities, like it was observed for the modi®cation of thin silver ®lms in Ref. [22]. Therefore, a combination of the latter two mechanisms has to be assumed here. For this type of modi®cation, adhesion of ®lm material to the tip cannot be excluded. Comparing the volumes of individual grains before and after the modi®cation, we found indeed, that some material disappeared during the modi®cation. However, adhesion could not be clearly identi®ed either, since the apparent surface structures in the STM topographies are strongly in¯uenced by the shape of the imaging tip. For small tip penetration depths, no signi®cant in¯uence on the modi®cation process should be expected since Au is not very reactive, particularly with water. It is thus not very likely that OHÿ anions are involved in a chemical process with the Au surface, initiated by the applied z pulses. In conclusion, we have presented a reliable modi®cation technique for thin Au ®lms on HOPG using STM. This yields holes with diameters in the 5±30 nm range. The data clearly show that electromigration is not the underlying process for the observed modi®cation behavior, while nanoindentation and thermomigration are assumed to account for the generated structures.
L133
Acknowledgements This work was supported by BMBF (grant: 03N1023B9). References [1] D.G. Pierce, P.G. Brusius, Microelectron. Reliab. 37 (1997) 1053. [2] S.J. Krumbein, IEEE Trans. Reliab. 44 (1995) 539. [3] R.E. Hummel, Internat. Mat. Rev. 39 (1994) 97. [4] N. Braslau, J.B. Gunn, J.L. Staples, Solid-State Electron. 10 (1967) 381. [5] T.S. Kuan, P.E. Batson, T.N. Jackson, H. Rupprecht, E.L. Wilkie, J. Appl. Phys. 54 (1983) 6952. [6] M. Paul, H. Berek, G. Kaden, W. Schneider, Mater. Corros. 48 (1997) 171. [7] I.-W. Lyo, Ph. Avouris, Science 245 (1989) 1369. [8] C.F. Quate, in: L. Esaki (Ed.), Highlights of the Eighties and Future Prospects in Condensed Matter Physics, NATO ASI, Plenum Press, New York 1991. [9] E.J. Heller, M.F. Crommie, C.P. Lutz, D.M. Eigler, Nature 369 (1994) 464. [10] R.M. Nyenegger, R.M. Penner, Chem. Rev. 97 (1997) 1195. [11] U. Stauer, in: R. Wiesendanger, H.J. G untherodt (Eds.), Scanning Tunneling Microscopy II, Springer, Berlin, 1992, p. 273. [12] Y.Z. Li, R.P. Andres, R. Reifenberger, Appl. Phys. Lett. 55 (1989) 2366. [13] C.S. Chang, W.B. Su, T.T. Tsong, Phys. Rev. Lett. 72 (1994) 574. [14] H.W. Schumacher, B. Kracke, B. Damaschke, Thin Solid Films 264 (1995) 268. [15] M. Ohto, S. Yamaguchi, K. Tanaka, Jpn. J. Appl. Phys. 34 (1995) L694. [16] E. Ganz, K. Sattler, J. Clarke, Phys. Rev. Lett. 60 (1988) 1856. [17] H. Chang, A.J. Bard, J. Am. Chem. Soc. 112 (1990) 4598. [18] R. Houbertz, U. Weber, U. Hartmann, Appl. Phys. A 66 (1998) S149. [19] D.M. Schaefer, R. Reifenberger, A. Patil, R.P. Andres, Appl. Phys. Lett. 66 (1995) 1012. [20] U. Landman, W.D. Luedtke, N.A. Burnham, R.J. Colton, Science 248 (1990) 454. [21] S.P. Baker, Thin Solid Films 308 (1997) 289. [22] U. Hodel, U. Memmert, U. Hartmann, Phys. Rev. B 54 (1996) 17888.
www.elsevier.nl/locate/susc
Surface Science Letters
Modi®cation of thin gold ®lms with the scanning tunneling microscope U. Weber *, R. Houbertz 1, U. Hartmann Institute of Experimental Physics, University of Saarbr ucken, P.O. Box 151150, D-66041 Saarbr ucken, Germany Received 17 January 2000; accepted for publication 10 November 2000
Abstract Thin gold ®lms, which were deposited by sputter deposition onto highly oriented graphite surfaces, were investigated and modi®ed by means of a scanning tunneling microscope. By applying short voltage pulses to the vertical piezoelectric element or to the tunneling tip, hole patterns were generated. Physical mechanisms underlying the modi®cations are discussed for the two methods. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Diusion and migration; Gold; Graphite
The dimensions of device interconnections used in integrated circuits have continuously decreased as the level of integration has increased. Electroand thermomigration-induced as well as (thermal) stress-induced failures of thin ®lm interconnects are the most prominent physical processes which can limit the reliability of integrated circuits and thus create a barrier for further miniaturization [1]. Electromigration has to be considered as a major cause of interconnect failure in integrated circuits [2] when a suciently high electrical current density up to 106 A/cm2 ¯ows through a
* Corresponding author. Present address: Department of Physics, University of Kaiserslautern, P.O. Box 3049, D-67653 Kaiserslautern, Germany. Tel.: +49-631-205-4053; fax: +49631-205-2834. E-mail address: [email protected] (U. Weber). 1 Present address: Sandia National Laboratories, MS 9161, Livermore, CA 94550, USA, and Fraunhofer Institut f ur silicatforschung, ORMOCER Dept., Micro Systems Technology, Neunerplatz 2, D-97082 w urzburg, Germany.
material. Thermomigration, on the other hand, can occur if a temperature gradient (minimum value of 1273 K/cm) is present along a sample [3]. Among all materials used in microelectronics, Au is one of the most inert materials used, for example, in Au/Ge/Ni or Au/Ni interconnects [4,5] or as passivation layers in integrated circuits with chip on board technology [6]. The latter particularly increases the interest in the behavior of thin gold ®lms upon applying stress like, for example high electrical ®elds. Scanning tunneling microscopy (STM) has proven to be not only suitable for high-resolution topographic and spectroscopic imaging at a subnm and at a meV scale [7], but also as a tool for nm-scale structuring of conducting or semiconducting surfaces down to atomic scale, some of which are reviewed in the literature [8±10]. Beside its ability of electronically mapping surfaces, STM allows to locally apply current densities in the order of 107 ±109 A/cm2 , which can account for electromigration processes [11].
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 0 ) 0 0 9 4 9 - 3
SU RF A LE CE TT S ER CIE S NC E
L130
U. Weber et al. / Surface Science 471 (2001) L129±L133
In this letter, we report on STM modi®cations of thin granular Au ®lms deposited onto highly oriented pyrolitic graphite (HOPG) surfaces, in order to investigate the behavior of thin Au ®lms under high electrical and mechanical stress. The data are discussed with respect to electro- and thermomigration as well as to mechanical indentation processes. The experiments were performed in a standard UHV chamber (base pressure 2 10ÿ10 mbar) by a commercial AFM/STM system (Omicron). As substrates, HOPG samples were used, which were prepared by peeling o several HOPG layers with an adhesive tape. This procedure is known to produce lm-size, atomically ¯at terraces, most frequently separated by monoatomic steps. Thin gold ®lms were deposited by ion sputtering EAr 1:2 keV at low sputtering rates to generate granular ®lms with thicknesses of approximately 2±5 nm. For the data presented here, 5 nm thick Au ®lms were used. After preparation, the samples were immediately transferred into UHV without any further preparation like, e.g., thermal annealing, and subsequently investigated and modi®ed by STM. Several methods for generating small indentations in thin ®lms are reported in the literature (see, e.g. Refs. [12±14]). For example, the modi®cation of thin Au ®lms in Ref. [14] was carried out by enlarging the interaction force between an atomic force microscope probe and a sample, which was shown to allow modi®cations in the range between 25 and 100 nm. Similar modi®cations were carried out using voltage pulses between tip and sample, which can be compared with earlier ®ndings elsewhere [15]. These observations suggest that the mechanism underlying the modi®cations is a thermomigration process combined with an electromigration process due to the high mobility of metal atoms on the graphite surface [16]. Accordingly, our ®ndings (cf. Fig. 1), obtained upon applying a 5 5 pattern of fairly low voltage pulses to the tip (10 ls, 2.5 V), show ± irrespective of the polarity ± holes in the ®lms with diameters of about 50 nm. The occurrence of a hole as well as its lateral extent seem to strongly depend on the atomic shape of the tip. This does not allow a re-
liable generation of a hole pattern involving holes of a well-de®ned shape and size (cf. Fig. 1). The typical diameters which were achieved in one pattern varied from 20 to 60 nm, where the number of produced holes is around 10% of the number of manipulation processes. We have been not able to produce hillocks as reported in Ref. [15], probably due to the ®lm thickness which in our case was much lower (30 nm in Ref. [15]). Usually, from 25 pulses, only two large holes were generated, which show signi®cant accumulation of material around their edges. Sometimes, a pattern could be generated, although it is very dicult to distinguish between the individual holes due to the large amount of material piled up around them (cf. Fig. 1(b)). Evaluating the total volumes of the holes in Fig. 1(a) and comparing it to the total volume of the accumulated material, con®rms that both volumes are identical. From this observation, it can be concluded, that no material from the tip is involved in the modi®cation (e.g. by ®eld emission as in Ref. [13]). Additionally to the modi®cation of the Au ®lm, an in¯uence of the modi®cation procedure on the underlying HOPG substrate has to be discussed. As mentioned earlier, the samples were transferred into UHV without further processing like, e.g., annealing. Thus, it is expected that water is present at the surface, which might have an in¯uence on the modi®cation process. It could be shown, however, that the HOPG surface itself is signi®cantly modi®ed by the sputtering process leading to near-surface defects intercalation of the gold in the graphite lattice [18]. Both possibilities will be discussed now in more detail. For the high voltages it is very likely that the upper HOPG plane is in¯uenced by the modi®cation process. It is well known that HOPG can be modi®ed in the presence of water adsorbed at the surface, which already was observed by several groups (e.g. Ref. [17]). A chemical reaction between the carbon and the water takes place, resulting in the modi®cation/a removal of the upper exposed HOPG layer. Besides this observation, which was achieved on clean HOPG surfaces, in our case also a further promotion of the intercalation of the gold in the graphite lattice has to be assumed. For the work presented here, neither of
CE N IE SC RS CE E A TT RF LE SU
U. Weber et al. / Surface Science 471 (2001) L129±L133
L131
Fig. 1. Sputter-deposited Au ®lms after applying a 5 5 pattern of voltage pulses of 2.5 V for 10 ls between tip and sample. (a) represents only two out of 25 intended holes and (b) represents a complete hole pattern. The image size is (a) 375 375 nm2 and (b) 730 730 nm2 .
these processes can be either excluded or con®rmed, since the bottom of the holes could not be imaged with the STM tip. Holes of smaller extent can be produced more reliably with respect to their size as well as their position by applying short voltage pulses to the z piezo of the STM (henceforth referred to as z pulses), thus causing the tip to move towards the sample and back again at a time scale of 100 ls. The z pulses applied would correspond to equilibrium displacements of 9.3±11.3 nm. These values, however, do not yield the actual displacement of the tip due to the inertia of the piezoelectric element. For a pulse duration of 100 ls, the z piezo only moves approximately 1/3 of its equilibrium value towards the sample, which results in displacements between 3.1 and 3.8 nm. During this procedure, the tip bias was kept at the tunneling voltage of 0.1 V. Fig. 2 shows four STM images, which were recorded before (Fig. 2(a)) and after applying short z pulses (Fig. 2(b)±(d)). The pulses were arranged such that 5 5 patterns were generated. This modi®cation procedure results in holes within the Au ®lm. Compared to the holes generated by applying voltage pulses onto the tunneling tip, the ones depicted in Fig. 2 show nearly no accumulation of material piled up around their edges. There are several possibilities for where the removed material could be located. One is that the material is picked up by the STM tip. This then
would result in a variation of the tunneling distance, thus causing the tip to retract from the sample surface after every z pulse. That was never observed during the present experiments. On the other hand, an analysis of the inter-grain distances in the unmodi®ed Au ®lm and in close vicinity to the generated holes show signi®cant dierences. The averaged inter-grain distances within the unmodi®ed ®lm (cf. Fig. 2(a)) are in the range between 3 and 4 nm, which is the same value as for the modi®ed ®lm far away from the indentations (cf. Fig. 2(b)±(d)). In close vicinity to the generated holes, however, the distances between the Au grains are much smaller or even close to zero. Furthermore, nearly no additional material is visible at the edges of the holes, as usually observed for varying the gap voltage (cf. Fig. 1). This points towards the fact that the Au grains were mainly moved in-plane during the modi®cation procedure, thus causing a compression of the Au ®lm in close vicinity to the holes. However, in some cases, a low amount of material is piled up around the hole, whereas the inter-grain distances are still visible. This is similar to experiments using an AFM to move gold clusters on HOPG surfaces [19]. Due to the poor sticking of Au clusters, large clusters (100 nm) could be easily moved in-plane without modifying the clusters themselves. For sputtered Au ®lms, however, sticking is assumed to be higher due to the presence of sputterinduced surface defects in the HOPG lattice [18].
SU RF A LE CE TT S ER CIE S NC E
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U. Weber et al. / Surface Science 471 (2001) L129±L133
Fig. 2. Sputter-deposited Au ®lm on HOPG: (a) before and after applying a 5 5 pattern of z pulses of (b) 9.3 nm, (c) 10.7 nm and (d) 11.3 nm. The image sizes are 375 375 nm2 .
Nevertheless, the grains can still be shifted, so that sticking is not prominent enough compared with the tip±sample (Au±Au) interactions. The data show, that holes can be reproducibly generated within an uncertainty of around 5 nm. The modi®cations show lateral extents of 6±14 nm in Fig. 2(b) and 11±27 nm in Fig. 2(d). A comparison of the four topographies shows that the indentations become larger after applying higher pulses which results from a larger penetration depth of the tip into the Au ®lm, thus enhancing the interaction forces between the tip and the ®lm surface due to the much larger interaction radius. After a subsequent z pulse of 12 nm (i.e. Dz 4 nm), the tip has lost its imaging properties, which was reproducibly found in the experiments. This modi®cation technique which allows mechanical modi®cations of metal ®lms on a length scale of 5 nm at room
temperature ± keeping high quality imaging capability of the tunneling tip ± yields smaller structures and a much better control compared to the literature (e.g. Ref. [10] and references therein). Several mechanisms can be taken into account for the formation of the indentations as achieved after applying z pulses. In the following, these mechanisms will be discussed carefully with respect to accessible experimental parameters. In accounting for the size of the z pulses it becomes clear that the tip penetrates the Au ®lm, as in a typical nanoindentation experiment (see, e.g., Refs. [20,21]). Apart from nanoindentation, electromigration and thermomigration should be taken into account. Electromigration seems to play a minor role, since the modi®cations show nearly no dependence on the polarity of the tunneling voltage. In com-
CE N IE SC RS CE E A TT RF LE SU
U. Weber et al. / Surface Science 471 (2001) L129±L133
parison to Ref. [15], where an obvious polarity dependence was observed, the ®lms in our case are only 5 nm thick which may reduce the lateral electronic transport due to an enhanced resistivity arising from the grain interconnections, when compared to bulk Au. Even if the current is assumed to only ¯ow in radial direction within the Au ®lm, the current for a modi®cation of 5 nm in diameter would have to be as high as 1.6 lA (for larger modi®cations even much higher) to give rise to electromigration. Nanoindentation and thermomigration cannot easily be distinguished. The arising temperatures do not seem to exceed the melting point of gold. In the case of melting, one would expect the gold grains to form bigger entities, like it was observed for the modi®cation of thin silver ®lms in Ref. [22]. Therefore, a combination of the latter two mechanisms has to be assumed here. For this type of modi®cation, adhesion of ®lm material to the tip cannot be excluded. Comparing the volumes of individual grains before and after the modi®cation, we found indeed, that some material disappeared during the modi®cation. However, adhesion could not be clearly identi®ed either, since the apparent surface structures in the STM topographies are strongly in¯uenced by the shape of the imaging tip. For small tip penetration depths, no signi®cant in¯uence on the modi®cation process should be expected since Au is not very reactive, particularly with water. It is thus not very likely that OHÿ anions are involved in a chemical process with the Au surface, initiated by the applied z pulses. In conclusion, we have presented a reliable modi®cation technique for thin Au ®lms on HOPG using STM. This yields holes with diameters in the 5±30 nm range. The data clearly show that electromigration is not the underlying process for the observed modi®cation behavior, while nanoindentation and thermomigration are assumed to account for the generated structures.
L133
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