Generation of Clean Iron Structures by Electron-Beam-Induced

Even though the underlying physical and chemical processes are not yet fully understood,(11, 12) EBID is now established as a repair tool for new gene...
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Generation of Clean Iron Structures by Electron-Beam-Induced Deposition and Selective Catalytic Decomposition of Iron Pentacarbonyl on Rh(110) Thomas Lukasczyk, Michael Schirmer, Hans-Peter Steinr€uck, and Hubertus Marbach* Lehrstuhl f€ ur Physikalische Chemie II and Interdisciplinary Center for Molecular Materials (ICMM), Universit€ at Erlangen-N€ urnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany Received May 6, 2009. Revised Manuscript Received June 18, 2009 We explore the electron-beam-induced deposition (EBID) of iron pentacarbonyl, Fe(CO)5, in ultrahigh vacuum (UHV) on clean and modified Rh(110) surfaces by scanning electron microscopy (SEM), scanning Auger microscopy (SAM), and local Auger electron spectroscopy (AES). In EBID a highly focused electron beam is used to locally decompose the iron pentacarbonyl precursor molecules with the goal to generate pure iron nanostructures. It is demonstrated that the selectivity of the process strongly depends on the surface properties. On a perfect, clean Rh(110) surface almost no selectivity is observed; i.e., deposition of Fe is found on irradiated and nonirradiated surface regions due to catalytic decomposition of the Fe(CO)5. However, on a structurally nonperfect Rh(110) surface and on a Tiprecovered Rh(110) surface high selectivity is found; i.e., Fe deposits are primarily formed in irradiated regions. The role of catalytic and autocatalytic growth of iron on clean Rh respective iron deposits is discussed. The purity of the Fe deposits was always very high (>88%). It is demonstrated that the deposited Fe structures can be selectively oxidized to iron oxide by exposure to oxygen. Furthermore, attempts to write Fe line deposits were also successful, and line diameters smaller than 25 nm could be achieved.

Introduction The realization of structures in the nanometer regime is a central task in the still evolving field of nanotechnology. Even though the production of certain nanostructures is a well-established process in industry or science (e.g., carbon nanotubes1-6 or other nanoparticles7-10), the generation of arbitrarily shaped nanoscaled objects with defined chemical composition is still a challenge. A technique that has the potential to achieve these goals is the electron-beam-induced deposition (EBID).11,12 In this direct-write method a highly focused electron beam from a scanning electron microscope (SEM) or transmission electron microscope (TEM) is utilized to locally induce the dissociation of precursor molecules adsorbed on a surface. In Figure 1 a scheme of the ideal EBID process with iron pentacarbonyl, Fe(CO)5, is sketched. The adsorbed precursor molecules are locally irradiated by electrons from a highly focused electron beam. These primary electrons (PEs) can either directly dissociate the precursor molecule or interact with the substrate. Thereby, elastic collisions result in backscattered electrons (BSEs) and inelastic collisions yield secondary electrons (SEs) with lower exit energies. The backscattered electrons are concentrated around the impact of the PEs (∼800 nm diameter on rhodium, as calculated from a Monte Carlo simulation; see Figure S1 in the Supporting Information) *Corresponding author: Fax +49 (0)9131/85-28867; Ph +49 (0)9131/8527316; e-mail [email protected]. (1) Ando, Y.; Zhao, X.; Sugai, T.; Kumar, M. Mater. Today 2004, 7, 22. (2) Baddour, C. E.; Briens, C. Int. J. Chem. React. Eng. 2005, 3, 1. (3) See, C. H.; Harris, A. T. Ind. Eng. Chem. Res. 2007, 46, 997. (4) Baugham, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (5) Terrones, M. Annu. Rev. Mater. Res. 2003, 33, 419. (6) Terranova, M. L.; Sessa, V.; Rossi, M. Chem. Vap. Deposition 2006, 12, 315. (7) Kinge, S.; Crego-Calama, M.; Reinhoudt, D. N. ChemPhysChem 2008, 9, 20. (8) Pileni, M. P. Langmuir 1997, 13, 3266. (9) Tao, A. R.; Huang, J.; Yang, P. Acc. Chem. Res. 2008, 41, 1662. (10) White, R. J.; Luque, R.; Budarin, V. L.; Clark, J. H.; Macquarrie, D. J. Chem. Soc. Rev. 2008, 38, 481. (11) Utke, I.; Hoffman, P.; Melngailis, J. J. Vac. Sci. Technol., B 2008, 24, 1197. (12) van Dorp, W. F.; Hagen, C. W. J. Appl. Phys. 2008, 104, 081301/1.

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but eventually also generate additional SEs on their way (range typically a few nanometers11), which results in the so-called secondary electron cone. The interaction of secondary electrons from this cone with the adsorbed precursor molecules causes a broadening of the EBID deposit as compared to the size of the primary beam. The spatial distribution of the BSEs and thus the broadening of the deposit is determined, e.g., by the substrate material, which has therefore a direct influence on the EBID process.11-13 Ideally, the volatile products of the electron-induced dissociation process, e.g., CO in case of Fe(CO)5, desorb from the surface and are pumped out of the chamber, leaving a deposit of pure iron. By controlling the position of the electron beam on the surface, e.g., by a lithographic attachment, the shape and position of the deposits can be controlled and even three-dimensional structures can be generated.14 Depending on the nature of the precursor molecule, e.g., metal-containing (Pt,15-20 W,21-26 (13) Amman, M.; Sleight, J. W.; Lombardi, D. R.; Welser, R. E.; Deshpande, M. R.; Reed, M. A.; Guido, L. J. J. Vac. Sci. Technol., B 1996, 14, 54. (14) Koops, H. W. P.; Kretz, J.; Rudolph, M.; Weber, M. J. Vac. Sci. Technol., B 1993, 11, 2386. (15) Weber, M.; Koops, H. W. P.; Rudolph, M.; Kretz, J.; Schmidt, G. J. Vac. Sci. Technol., B 1995, 13, 1364. (16) Koops, H. W. P.; Sch€ossler, C.; Kaya, A.; Weber, M. J. Vac. Sci. Technol., B 1996, 14, 4105. (17) Koops, H. W. P.; Kaya, A.; Weber, M. J. Vac. Sci. Technol., B 1995, 13, 2400. (18) Gazzadi, G. C.; Frabboni, S. J. Vac. Sci. Technol., B 2005, 23, L1. (19) Yavas, O.; Ochiai, C.; Takai, M.; Hosono, A.; Okuda, S. Appl. Phys. Lett. 2000, 76, 3319. (20) Barry, J. D.; Ervin, M.; Molstad, J.; Wickenden, A.; Brintlinger, T.; Hoffman, P.; Meingailis, J. J. Vac. Sci. Technol., B 2006, 24, 3165. (21) van Dorp, W. F.; van Someren, B.; Hagen, C. W.; Kruit, P. Nano Lett. 2005, 5, 1303. (22) Koops, H. W. P.; Weiel, R.; Kern, D. P.; Baum, T. H. J. Vac. Sci. Technol., B 1988, 6, 477. (23) Hoyle, P. C.; Ogasawara, M.; Cleaver, J. R. A.; Ahmed, H. Appl. Phys. Lett. 1993, 62, 3043. (24) Shimojo, M.; Mitsuishi, K.; Tameike, A.; Furuya, K. J. Vac. Sci. Technol., B 2004, 22, 742. (25) Matsui, S.; Mori, K. J. Vac. Sci. Technol., B 1986, 4, 299. (26) Matsui, S.; Ichihashi, T.; Mito, M. J. Vac. Sci. Technol., B 1989, 7, 1182.

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Figure 1. Scheme of the ideal EBID process with the precursor molecule Fe(CO)5. (a) Dosage and adsorption of Fe(CO)5 molecules. (b) Local exposure of the adsorbed molecules to the electron beam; the dashed line encloses the backscattered electron (BSE) interaction volume, and red lines indicate trajectories that reach the surface. (c) Decomposition of Fe(CO)5 molecules exposed to the electron beam and desorption of the volatile fragments (i.e., CO) and the unexposed Fe(CO)5. (d) Pure iron deposit.

Au,14-16,22,27-30 Cr25,31) or carbonaceous32 deposits can be generated using organometallic compounds or hydrocarbons, respectively. A prerequisite for the local generation of nanostructures by EBID is that deposits should only be generated at and in close vicinity of the region directly irradiated by the electron beam, i.e., in the region where electron-precursor interactions occur, which varies in size with the electron-beam parameters (e.g., beam energy) and the applied material. Therefore, the utilized sample material can directly affect the shape and size of the deposits.11-13 At this point it is important to define the term selectivity of the (27) Utke, I.; Hoffmann, P.; Dwir, B.; Leifer, K.; Kapon, E.; Doppelt, P. J. Vac. Sci. Technol., B 2000, 18, 3168. (28) Lee, K. L.; Abraham, D. W.; Secord, F.; Landstein, L. J. Vac. Sci. Technol., B 1991, 9, 3562. (29) Utke, I.; Dwir, B.; Leifer, K.; Cicoira, F.; Doppelt, P.; Hoffman, P.; Kapon, E. Appl. Phys. Lett. 2000, 53, 261. (30) Lee, K. L.; Hatzakis, M. J. Vac. Sci. Technol., B 1989, 7, 1941. (31) Kunz, R. R.; Mayer, T. M. J. Vac. Sci. Technol., B 1988, 6, 1557. (32) Guise, O.; Marbach, H.; Levy, J.; Ahner, J.; Yates, J. T. Surf. Sci. 2004, 571, 128.

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EBID process in the context of this work, since the term is often used differently (e.g., as the selectivity of electrons to break specific chemical bonds): here we understand selectivity in a more general picture; e.g., catalytic activity of the substrate is regarded as an intrinsic property of the specific system and fully accounts for the selectivity of the EBID process. In this interpretation the selectivity is simply judged by the result obtained after EBID. Even though the underlying physical and chemical processes are not yet fully understood,11,12 EBID is now established as a repair tool for new generation lithographic UV masks for semiconductor industry (45 nm node technology).33-35 One of the major challenges here is the chemical composition/purity of metallic deposits. Most EBID experiments were performed in high-vacuum (HV) systems resulting in typical metal contents of 10-60 at. %,15,16,19,22,25,29,30,36-42 with a maximum value of 85 at. % achieved for W with the precursor WF6.26 Low metal contents are mainly attributed to the simultaneous deposition of carbon from hydrocarbons (mainly residual gases from pump oil) in the HV chamber. By performing EBID in a UHV system, we have recently demonstrated that with Fe(CO)5 as precursor one can produce Fe nanodeposits on Si(100) at room temperature with at least 95% purity. In the same study we also found indications that the purity and morphology of the EBID deposits strongly depend on the condition of the Si surface.43 To further investigate the influence of the substrate on the EBID process, we now investigate the catalytically active Rh(110) surface, using the same precursor, i.e., Fe(CO)5. This combination was chosen for several reasons: (1) the catalytic activity of Rh(110) is well studied, in particular also by the authors;44-48 (2) for sample temperatures above 430 K dissociative adsorption of oxygen occurs on Rh(110),46 which opens the possibility to oxidize unintended carbon contaminations with subsequent desorption as CO or CO2; (3) chemisorbed oxygen on Rh(110) can be removed by dosing hydrogen,44-46 which might enable the selective oxidation of iron deposits and thus the generation of oxide nanostructures on a metallic substrate; (4) the choice of Fe(CO)5 allows to directly compare to the results on Si(100).43 Generally, adsorption and reaction properties of Fe(CO)5 with metal substrates like Ag,49 Ni(100),50 and Pt(111)51,52 vary strongly with the substrate. To the best of (33) Edinger, K.; Becht, H.; Bihr, J.; Boegli, V.; Budach, M.; Hofmann, T.; Koops, H. W. P.; Kuschnerus, P.; Oster, J.; Spies, P.; Weyrauch, B. J. Vac. Sci. Technol., B 2004, 2, 2902. (34) Koops, H. W. P. Innovation 2005, 16, 46. (35) Liang, T.; Frendberg, E.; Lieberman, B.; Stivers, A. J. Vac. Sci. Technol., B 2005, 23, 3101. (36) Lau, Y. M.; Chee, P. C.; Thong, J. T. L.; Ng, V. J. Vac. Sci. Technol., A 2002, 20, 1295. (37) Utke, I.; Hoffmann, P.; Berger, R.; Scandella, L. Appl. Phys. Lett. 2002, 80, 4792. (38) Cicoira, F.; Hoffmann, P.; Olsson, C. O. A.; Xanthopoulos, N.; Mathieu, H. J.; Doppelt, P. Appl. Surf. Sci. 2005, 242, 107. (39) Perentes, A.; Sinicco, G.; Boero, G.; Dwir, B.; Hoffmann, P. J. Vac. Sci. Technol., B 2007, 25, 2228. (40) Bret, T.; Utke, I.; Bachmann, A.; Hoffman, P. Appl. Phys. Lett. 2003, 83, 4005. (41) Ochiai, Y.; Fujita, J.-i.; Matsui, S. J. Vac. Sci. Technol., B 1996, 14, 3887. (42) Perentes, A.; Bret, T.; Utke, I.; Hoffmann, P.; Vaupel, M. J. Vac. Sci. Technol., B 2006, 24, 587. (43) Lukasczyk, T.; Schirmer, M.; Steinr€uck, H.-P.; Marbach, H. Small 2008, 4, 841. (44) Kiskinova, M. Chem. Rev. 1996, 96, 1431. (45) Mertens, F.; Imbihl, R. Chem. Phys. Lett. 1995, 242, 221. (46) Schaak, A.; Shaikhutdinov, S.; Imbihl, R. Surf. Sci. 1999, 421, 191. (47) G€unther, S.; Hoyer, R.; Marbach, H.; Imbihl, R.; Esch, F.; Africh, C.; Comelli, G.; Kiskinova, M. J. Chem. Phys. 2006, 124, 014706/1. (48) Schmidt, T.; Schaak, A.; G€unther, S.; Ressel, B.; Bauer, E.; Imbihl, R. Chem. Phys. Lett. 2000, 318, 549. (49) Sato, S.; Ukisu, Y.; Ogawa, H.; Takasu, Y. J. Chem. Soc., Faraday Trans. 1993, 89, 4387. (50) Xu, M.; Zaera, F. J. Vac. Sci. Technol., A 1996, 14, 415. (51) Zaera, F. Surf. Sci. 1991, 255, 280. (52) Zaera, F. Langmuir 1991, 7, 1188.

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our knowledge, the adsorption of Fe(CO)5 on Rh has not been studied so far. The major goals of this study are (i) the generation of Fe deposits with a high purity, (ii) to investigate the selectivity of the process, (iii) to explore the influence of specific parameters on the shape and size of the deposits, and (iv) to evaluate the potential to selectively oxidize Fe deposits.

Experimental Section The experiments were performed in a two-chamber UHV system (Omicron Multiscan Lab) with a base pressure of 1.5 nm; iron content >88%), with only small contaminations of carbon and oxygen, due to the unintended dissociation of carbon monoxide ligands. In the unexposed region a significantly smaller amount of iron was found (estimated thickness: 0.2 nm). The fact that a higher amount of iron is deposited in the exposed region implies that the EBID process on sample B, i.e., the lowquality Rh(110) surface, is moderately selective. The selectivity is, however, lower than observed for EBID with Fe(CO)5 on Si(100), where no iron was detected in the unexposed area.43 DOI: 10.1021/la901612u

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Figure 7. EBID deposit with Fe(CO)5 on a Ti-covered area on sample C. (a) SEM image of an iron structure, which was generated by the irradiation of a quadratic area (white dotted line) with an electron dose of 53.1 As/cm2. The markers correspond to the positions where the Auger spectra in (b) were taken. (b) Local AE spectra taken at the positions marked in (a).

Summarizing the observations for samples A and B, it is evident that the surface quality has a high influence on the selectivity of the EBID process. While independent of surface quality the deposits contain almost pure iron, the unintended deposition of material in surface regions not exposed to the electron beam apparently depends on the actual condition of the Rh surface. With this finding one might envision local surface modifications, which inhibit the catalytic growth of iron upon dosage of Fe(CO)5, as a tool to produce a negative pattern for the generation of catalytically grown iron structures. EBID on a Ti-Precovered Rh(110) Surface (Sample C). As mentioned above, sample C is a certain region of sample A, which is partly covered with a thin layer of TiOx. Since our experimental setup allows to specifically address these regions on the surface, this substrate will now be exploited to investigate the selectivity of EBID on a TiOx-precovered Rh(110) surface as compared to clean Rh(110). For that purpose, EBID experiments were performed on both surface regions, i.e., clean and titanium covered. First, EBID was performed on clean Rh(110) areas. Figure 6a shows an SEM image of the surface before EBID with Fe(CO)5, with the clean Rh(110) region in the center brighter than the titanium covered region at the top and at the bottom. In Figure 6b the SEM image after EBID of four squares (template squares: 600 nm  600 nm) onto the clean Rh(110) region using Fe(CO)5 is shown. The former clean Rh(110) region appears significantly darkened, and four square-shaped structures can be identified, which are even darker. The local AE spectra taken on (red) and beside (blue) the irradiated area are very similar (see Figure 6c); they are dominated by the Fe signals with no Rh substrate signals visible, confirming the lack of selectivity of the EBID process on clean and well-prepared Rh(110); see also results for sample A. Since no Rh signals are observed, the deposited iron layer must be again thicker than 1.5 nm. 11936 DOI: 10.1021/la901612u

Figure 8. Measurements on an unexposed area of sample C after Fe(CO)5 dosage. (a) SEM image of the corresponding region, displaying areas of the Ti-free (dark) and Ti-covered (bright) surface. (b) SAM image for the element iron; red areas correspond to a high iron coverage. (c) SAM image for the element titanium; blue areas correspond to a high titanium coverage. Yellow areas in (b) and (c) correspond to a low amount of the respective element.

The AE spectrum (green) measured on the titanium-containing region shows the expected Rh and Ti peaks, but only minor Fe signals, indicating that Fe(CO)5 is not decomposed catalytically in this region. In other words, the catalytic effect of the clean Rh(110) surface can be effectively inhibited via a surface modification, i.e., an ultrathin TiOx layer. The Auger spectra also explain the contrast inversion in the SEM images after the Fe(CO)5 dosage: the titanium-covered regions (upper and lower part of the image) in Figure 6b appear brighter than the central region, since iron, now completely covering the previously clean Rh areas, has a lower electron yield than rhodium covered with a thin Ti layer (again associated with the higher atomic number of Rh compared to Fe).59 Small portions of the titanium-covered region close to the exposed areas remain dark after the dosage, which is attributed to EBID via scattered primary electrons. To further investigate selectivity, EBID was also performed on titanium-covered regions. The SEM image in Figure 7a shows a section of the titanium-covered region, with a dark area on the left. This feature was generated via EBID of a square template, as Langmuir 2009, 25(19), 11930–11939

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Figure 9. Scheme summarizing the behavior of Fe(CO)5 on clean and Ti-covered Rh(110). (a) Adsorption of Fe(CO)5 molecules on the clean and TiOx-covered surface. (b) Catalytic dissociation of Fe(CO)5 on the clean Rh surface: Fe atoms stick to the surface while the CO ligands are mainly removed from the system. On the Ti-covered region Fe(CO)5 desorbs molecularly without decomposition. (c) Additional Fe(CO)5 molecules adsorb on the Fecovered Rh surface and are dissociated autocatalytically. On the Ti-covered surface the molecules again desorb molecularly. (d) After the Fe(CO)5 dosage the free Rh surface is covered with a thick Fe layer, while no Fe was deposited on the Ti-covered area.

indicated by the dotted white line, with the same electron dose as for the top right square in Figure 6b. Peculiar features in Figure 7a are the bright clusters that appear in and around the exposed area. Their origin will be discussed in detail below. Local AES was performed on the EBID deposit (red marker in Figure 7a). The corresponding red spectrum in Figure 7b is dominated by the Fe signals with a minor O peak, indicating that the EBID deposit mainly consists of iron, similar to the results obtained on the clean Rh surface. No Rh substrate signal is observed, indicating that the deposit is again thicker than 1.5 nm. The green spectrum in Figure 7b was measured on the nearby unexposed area (green marker in Figure 7a). Apart from the Rh substrate peaks and signals from the TiOx layer, no other signals are observed. We therefore conclude that the EBID process on the titanium-covered surface is strictly selective, in contrast to the observations on the clean Rh surface. Langmuir 2009, 25(19), 11930–11939

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In the following the origin of the bright features on the titanium-covered surface after Fe(CO)5 dosage will be investigated. These features are visible mostly as small protrusions with a diameter of roughly 10 nm (e.g., compare Figure 7a) or for example as longish features with a granular structure in the top right corner in Figure 7a. In Figure 8 a SEM image (a) of the unexposed area is shown along with two images of the same surface acquired via scanning Auger microscopy (SAM), revealing the distribution of iron (b) and titanium (c); to obtain these images, the electron analyzer was set to the Fe LMM peak energy at 702 eV and Ti LMM peak at 382 eV, respectively. Red regions in Figure 8b indicate a high amount of iron, while blue regions in Figure 8c correspond to a high amount of titanium. A comparison between parts b and c of Figure 8 evidences that a high local amount of Ti coincides with basically no Fe and vice versa. On the basis of the SAM images, the bright features in the SEM image are identified as iron. Interestingly, these Fe features appear bright in SEM, whereas the more or less continuous iron film (e.g., in the left part and lower right corner of Figure 8a) appears darker than the rest of the surface. This observation is attributed to the wellknown tilt and edge effects,61 indicating the three-dimensional nature of these bright features: apparently they are higher than the titanium-covered surface areas, and they are therefore assigned as small iron clusters. The origin of these iron clusters on the titanium-covered regions is interpreted as due to local defects (holes) in the titanium film, locally exposing the clean Rh and thus catalytically decomposing the Fe(CO)5 precursor. The interaction between Fe(CO)5 and the surface is summarized in the scheme in Figure 9. On the clean Rh(110) surface (sample A and Ti-free regions of sample C) the precursor molecules dissociate even without any electron assistance (Figure 9a). This catalytic decomposition leads to the deposition of iron, while the CO ligands mainly desorb (Figure 9b). Additional precursor molecules impinging onto these regions are also dissociated via an autocatalytic effect of the predeposited iron (Figure 9c). Therefore, the areas of the former clean Rh(110) surface are covered with a thick Fe layer at the end of the Fe(CO)5 dosage (Figure 9d). In terms of the EBID process this catalytic behavior means a complete loss of selectivity, as the electron interaction is not necessary to induce the dissociation of the precursor molecules. The only difference is a slightly higher amount of oxygen in the irradiated areas, accompanied by a darker appearance in the SEM images, due to electron-induced dissociation of small amounts of carbon monoxide (see Figures 4a,b, 5a, and 6). For the TiOx-modified Rh(110) surface (sample C) the situation is completely different (left part of the scheme in Figure 9). Here, the catalytic activity is inhibited by the thin overlayer, and Fe(CO)5 adsorbs and desorbs intact in the area unexposed to electrons. In the irradiated areas, the precursor molecules are dissociated, resulting in the deposition of iron with minor contaminations (see Figure 7). This implies that the EBID process is selective on the titanium-covered region, as iron is only observed on areas that were irradiated by electrons. The observed selectivity is similar to that found on the nonperfect Rh(110) surface (sample B, see Figures 4c and 5b), which is characterized by only minor contaminations but limited long-range order. The origin of the selectivity for sample B, which is not as high as found for the TiOx-covered regions on sample c, is not clear at the moment and will be subject to further investigation. A significant influence of the small carbon contamination visible before the EBID process (see blue spectrum in Figure 5b) appears unlikely, since a similar amount was observed during the characterization of sample (61) Jeol, A Guide to Scanning Microscope Observation, 2006.

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Figure 11. Two ellipses deposited via EBID on a lesser prepared surface than sample B with an electron dose of 80 μAs/cm each.

Figure 10. Oxidation of iron EBID deposits into iron oxide structures. The markers in the SEM images correspond to the positions where the spectra were taken. (a) SEM: blow-up of the bottom EBID deposit from Figure 4c. (b) Auger spectra taken in and beside the circular EBID structure in (a). (c) SEM image of the circular EBID structure in (a) after the oxidation procedure. (d) Auger spectra taken in and beside the deposit in (c) after the oxidation procedure. (e) SEM image of the circular EBID structure in (c) after the subsequent reduction procedure. (f) Auger spectra acquired in and beside the deposit in (e) after the reduction procedure.

A (see blue spectrum in Figure 5a). It might be interpreted that the actual atomic arrangement determines the catalytic activity of the Rh(110) surface, which is assumed to be rather unordered for sample B compared to sample A. Selective Oxidation of Fe Deposits. To further explore the potential of our approach to generate tailored nanostructures, we performed an experiment with the goal to oxidize an iron nanostructure, which was produced by EBID. For that purpose we used the nonperfect Rh(110) surface (sample B), as we were able to selectively deposit Fe via EBID on this surface without other species being present (as would be the case for sample C). Figure 10 shows the corresponding data. The starting point was the structure, which corresponds to the bottom EBID deposit shown in Figure 4c (circle with 500 nm diameter; deposited with an electron dose of 39.9 As/cm2). Figure 10a,b shows the corresponding SEM image and AE spectra after the deposit was heated for 30 min at ∼525 K to exclude heating effects on the deposit morphology during the oxidation. The SEM micrograph (Figure 10a) displays a dark circular area, which was generated via EBID. The AE spectrum of the irradiated area (dark blue) shows intense Fe peaks, with only a minor O peak and a small but significant C signal visible. From the damping of the Rh signal the thickness of the iron layer is again estimated to be higher than 1.5 nm (note that again the Rh substrate signal is too small to determine the signal intensity properly). The spectrum on the unexposed surface area (dark green) exhibits small Fe signals, indicating that the selectivity is not perfect and only some catalytic/autocatalytic dissociation of Fe(CO)5 on the nonirradiated surface was observed. To oxidize the iron structure in Figure 10a, a sequence of alternatively dosing oxygen (three times) and hydrogen (two 11938 DOI: 10.1021/la901612u

times) was performed with the background pressure set to 1  10-6 mbar and the sample at ∼525 K (∼10 min for each step). The SEM image in Figure 10c after this procedure shows the circular deposit with unchanged size and shape. The contrast of the structure is weaker than in Figure 10a (i.e., the deposit appears not as dark), which is most probably due to changes of the electron yield in the exposed and unexposed area, induced by adsorbed oxygen and the conversion of iron into iron oxide. The corresponding AE spectra are shown in Figure 10d. The spectrum of the irradiated area (light blue) exhibits intense oxygen and smaller Fe peaks as compared to before the oxygen dosage (Figure 10b). The small C and Rh signals prior to the oxidation procedure have now vanished. This indicates that the deposited iron is indeed oxidized by the given procedure, resulting in a thicker layer (FexOy vs Fe), which leads to a further damping of the Rh substrate signal. For carbon, a reaction with oxygen to CO or CO2, which subsequently desorb, is likely, leading to a purification of the EBID deposit. The spectrum on the nonirradiated area (light green) displays only a minor oxygen peak due to the oxidation of the small amounts of iron present in this region. Otherwise, the spectrum remains unchanged as compared to before the oxidation. These results clearly demonstrate that the oxidation procedure is selective, as only the iron covered areas exhibit oxygen signals. One should also note here that oxygen is sensitive to radiation damage, as AE spectra taken not at one spot but while laterally scanning a certain area of the unexposed surface show a higher oxygen peak (not shown here). In order to check the chemical stability of the oxidized iron structures, hydrogen was dosed into the chamber once more (1  10-6 mbar, ∼525 K, ∼10 min). The SEM image and the AE spectra in Figures 10e and 10f, respectively, are identical with the ones prior to the hydrogen treatment (Figures 10c and 10d, respectively). This is an indication that the deposited iron was indeed transformed into iron oxide by the applied procedure and that it is stable against molecular hydrogen. EBID of Line Structures. After having evaluated the selectivity of the EBID process on various surfaces by writing area deposits of several 100 nm diameter, we want to briefly address the potential to write line structures. This is certainly only possible for substrates, where the EBID process is highly selective, i.e., samples B and C in the present study. In the following, we will show one successful example for a surface that has been treated with a lower number of cleaning cycles than sample B but already shows a similarly clean Auger spectrum. In Figure 11 the SEM image of two spatially well-defined ellipses is depicted, each deposited with an electron line dose of 80 μAs/cm. Both structures comprise continuous lines with a diameter below 25 nm. Auger Langmuir 2009, 25(19), 11930–11939

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spectra measured on the same surface for a circular deposit of ∼500 nm diameter show again a high purity of the Fe deposit, similar to the one on sample B.

Summary and Conclusions We have investigated the electron-beam-induced deposition of Fe(CO)5 on clean and modified Rh(110) surfaces by SEM, SAM, and local Auger electron spectroscopy. Our UHV experiments yield a novel aspect for the technique of EBID, since the utilization of a well-prepared single-crystal surface with a high longrange order is not possible under the typically applied highvacuum conditions. Our study clearly shows that the selectivity of the process, i.e., the deposition only in irradiated regions, strongly depends on the status of the surface. In contrast to Si(100), the EBID process on a perfect and clean Rh(110) surface is not selective, which means that Fe deposits are formed on the whole surface by catalytic decomposition of Fe(CO)5, irrespective of electron irradiation. This process is then followed by autocatalytic decomposition to Fe on already Fe-covered regions. In contrast to that, for a structurally nonperfect Rh(110) surface and for a Rh(110) surface precovered with TiOx, EBID is selective, which means that Fe deposition mainly occurs in regions, which are simultaneously irradiated with the electron beam. The purity of the Fe deposits is always very high (g88%), which is comparable to results of our previous investigation on Si(100), but much higher than the values of