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Influence of metal co-deposition on silicon nanodot patterning dynamics during ion-beam sputtering
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Nanotechnology Nanotechnology 25 (2014) 415301 (13pp)
doi:10.1088/0957-4484/25/41/415301
Influence of metal co-deposition on silicon nanodot patterning dynamics during ionbeam sputtering R Gago1, A Redondo-Cubero2, F J Palomares1 and L Vázquez1 1
Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, E-28049 Madrid, Spain 2 Departamento de Física Aplicada, Universidad Autónoma de Madrid, E-28049 Madrid, Spain E-mail: [email protected] Received 16 April 2014, revised 31 July 2014 Accepted for publication 6 August 2014 Published 24 September 2014 Abstract
We address the impact of metal co-deposition in the nanodot patterning dynamics of Si(100) surfaces under normal-incidence 1 keV Ar+ ion-beam sputtering (IBS). In particular, the effect of both the metal nature (Fe or Mo) and flux has been studied. Morphological and compositional evolution were followed by atomic force microscopy (AFM) and Rutherford backscattering spectrometry, respectively. For the same type of impurity, the dynamics is faster for a higher codeposition flux, which also drives to larger asymptotic roughness and wavelength. Mo codeposition yields rougher surfaces for a lower metal coverage than Fe and, remarkably, higher ordered patterns. X-ray photoelectron spectroscopy reveals the formation of silicide bonds even before pattern onset, stressing the relevant role of the affinity of the co-deposited metals for silicon. Further, current-sensing AFM performed at the initial and asymptotic stages indicates that the nanodot structures are metal-rich, resulting in coupled compositional and morphological patterns. These results are discussed in terms of phase segregation, morphology-driven local flux variations of impurities and silicide formation. This analysis reveals that the underlying (concurrent) mechanisms of pattern formation are complex since many processes can come into play with a different relative weight depending on the specific patterning conditions. From a practical point of view, it is shown that, by proper selection of the process parameters, IBS with metal co-deposition can be used to tune the dynamics and pattern properties and, interestingly, to produce highly ordered arrays. Keywords: pattern formation, silicon, ion beam sputtering, co-deposition (Some figures may appear in colour only in the online journal) 1. Introduction
The understanding of nanopattern formation by IBS on amorphous or amorphizable materials has experienced a significant turning point due to the elucidation of the critical role played by (metal) impurities [2, 3]. Henceforth, two main fields can be roughly distinguished: (i) IBS patterning under low or impurity-free conditions and (ii) IBS patterning with intentional impurity co-deposition. For the former case, it has been clearly established that, in the case of monoelemental targets, (ripple) patterns only emerge above a critical ion incidence angle [4, 5]. On this evidence, pattern formation below this threshold [6] (e.g., near-normal ion incidence) has
Ion beam sputtering (IBS) has emerged in recent decades as a promising bottom-up approach for surface nanostructuring, capable of inducing nanopatterns in a wide range of targets (including semiconductors, metals and insulators) with, typically, ripple, dot, or hole nanostructures [1]. This versatile nanofabrication method comprises a single-step process with high throughput (it can pattern relatively wide surface areas of up to several tens of cm2 in short irradiation times, typically a few minutes). 0957-4484/14/415301+13$33.00
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to be attributed to the (inadvertent) presence of metal impurities during the irradiation [7]. Compositional issues have even broader implications in the field of IBS nanopatterning since they may also be relevant in alloys or compound targets [8–11] as, indeed, observed experimentally for GaSb surfaces [12, 13]. Despite the complexity added by the (a priori, undesirable) presence of impurities, IBS with concurrent metal deposition is emerging as a strategy to both tune the pattern properties and to improve the understanding of the fundamentals of nanopatterning of multi-component systems. For example, this approach has inspired a novel method, the socalled ‘surfactant sputtering’ [14], capable of modifying the surface morphology from enhanced smoothing to nanostructuring. From a practical point of view, the addition of codeposition can be used to yield large corrugations at moderate ion fluences [15], tune the pattern nanostructures [16], design the pattern symmetry [17] or improve the pattern ordering [18]. Further, the fundamental interest in this topic has led to the development of specific theoretical models on IBS nanopatterning with concurrent metal deposition [19–21]. Nevertheless, the underlying mechanisms that originate the pattern still remain under debate. The phenomenology of the simultaneous supply of Fe impurities has been systematically studied in recent years for the paradigmatic case of silicon targets [15, 22–24]. Typically, the experimental set-ups employed are rather similar, with a co-sputtered metal plate located adjacent to the target acting as the impurity source. This configuration allows the study of the pattern morphological dependence on the metal content as the latter decreases with distance from the plate edge. Here, the flux of co-deposited (metal) species has a rather defined direction, yielding mostly ripple morphologies or even an initial transient nanodot pattern. It has also been revealed that iron silicide forms on the surface [16, 22, 23, 25], which seems to be a key step for surface patterning. This finding has been extended to other metal impurities [26, 27]. However, this necessary condition is not sufficient since, despite the formation of silicides, large metal coverages do not yield any pattern [27]. This scenario suggests that patterns emerge due to lateral modulations of the sputtering rate driven by compositional changes (silicide formation). In this respect, a large angle between the ion and impurity fluxes seems to be more efficient for pattern formation [15], which highlights the relevance of height fluctuations and local flux variations (locations with the higher metal deposition rate receive a lower ion flux) in order to induce such lateral chemical inhomogeneities. The role of (ion-induced) phase-segregation effects has also been proposed in order to induce a compositional pattern [26]. Although several observations [15] do not reconcile with the previous phenomenology, the lack of patterns for large metal coverages could be explained by the fact that a homogeneous silicide layer would not tend to decompose [27]. In any case, a more general picture considers the amplifying feedback between compositional and height fluctuations [27]. As mentioned above, systematic studies on the incorporation of impurities during IBS mostly refer to ripple
patterning. This situation partially relies on the common use of a rather directional metal flux (by locating an adjacent metal plate to the silicon target) that promotes such morphology. In contrast, so far similar studies for stable nanodot morphologies (i.e., without a dot to ripple pattern transition) addressing the impact of different metals or the co-deposition flux during the irradiation have not been performed. One practical reason for that could be the difficulty in performing systematic experiments since for each (metal content) condition a new experiment is required (in contrast to the ripple patterns, in which a continuous range of metal contents is achieved in a single shot by varying the distance to the adjacent metal plate). As an example of this complexity, Feassisted nanodot patterns with a variety of designed pattern symmetries and wavelengths can be produced by adjusting the size and arrangement of different metal pieces surrounding the silicon targets [17]. From the theoretical point of view, the main specific work dealing with nanodot pattern formation by IBS with metal co-deposition is that by Bradley [21], which covers the initial patterning stages (the so-called linear regime). As indeed claimed by Bradley, there is a need for more systematic experimental studies to test the different theories and understand the mechanisms of metal-assisted nanodot patterning. In this work, we aim to contribute to filling this gap by studying the effect of the metal’s nature and its incorporation rate on the nanodot pattern formation dynamics. We have selected Fe and Mo as co-deposition species since they yield pronounced patterns [26]. Further, the morphological and compositional dynamics on the irradiated surfaces are both studied in parallel. The results allow us to discuss the eventual mechanism(s) of pattern formation and evidence IBS with metal co-deposition as an effective way to tune the nanodot pattern dynamics and characteristics in terms of amplitude (roughness), wavelength and, remarkably, ordering.
2. Experimental details 2.1. Sample preparation
Si(100) targets were sputtered with 1 keV Ar+ ions at normal incidence. The ions were extracted from a commercial broadbeam (3 cm beam diameter) CSC Kaufman-type ion gun (Veeco©) located 25 cm away from the target. The current density was measured prior to each irradiation with a Faraday cup and set to ∼150 μA cm−2. This relatively low current was set to attain a slow patterning dynamics [28]. Metal codeposition was carried out by placing on the sample a 1 mm thick Fe or Mo metal plate with a countersunk hole (to enhance co-deposition under normal incidence). This set-up is similar to that depicted by Zhou et al in [29]. Here, the uncovered part of the Si(100) surface is sputtered with concurrent incorporation of metal impurities from the co-sputtered plate. Plates with different hole diameters, d, were fabricated in order to tune the metal incorporation rate since this work is based on the premise that the metal flux can be increased by reducing d. The patterning dynamics has been 2
Fe content (at.%)
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geometry of the mask results in an effective isotropic metal flux at the target centre that induces the formation of stable nanodot patterns. However, as we move towards the mask edge the metal flux becomes more intense and directional, and ripple patterns appear. This situation is depicted schematically in figure 1(b) where typical atomic force microscopy (AFM) images at different locations at the sample surface with respect to the centre (C) are shown (note that the sample contour is only shown as a reference and it is not at the same scale). Remarkably, the ripple morphologies have different orientations at right (R), left (L), up (U) or down (D) positions, being aligned parallel to the closest mask perimeter. The definition of the ripple orientation by the metal flux direction has indeed been observed by other groups [7, 15] and predicted theoretically [30]. It should be noted that all the morphological and compositional analysis shown hereafter has been done systematically at the centre position.
Mask edge
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2.2. Sample characterization
The surface morphology after irradiation was imaged by AFM with a Nanoscope IIIa equipment (Veeco©) and an Agilent PicoPlus 5500, the latter being able to operate in conductive mode (C-AFM) [31]. For the topographical measurements, silicon cantilevers were employed with a nominal radius, r, of 8 nm and opening angle, θ, smaller than 52°. These measurements were carried out in air and tapping mode. Diamond coated tips were employed for the C-AFM measurements (models CDT-FMR from NanoWorld, r ∼ 83 ± 17 nm and θ < 47°, and DCP11 from NT-MDT, r ∼ 60 ± 10 nm and θ < 44°). In the C-AFM mode, imaging biases in the 3–9 V range at both polarities were employed. This analysis was complemented with Kelvin mode measurements (KFM) using silicon tips coated with gold nanoparticles in the 2–3 nm range (from NEXT TIP, Spain) [32]. In both electrical modes, the measurements were performed under nitrogen ambient. For quantification of the surface morphology, the surface roughness (w) and the characteristic pattern wavelength (ℓ ) were evaluated from the AFM images. The latter was extracted from the power spectral density (PSD) function defined as PSD(k) = 〈H(k)H(−k)〉. Here, the angular brackets denote spatial averaging and H(k) stands for the Fourier transform function of the surface height profile at vector position r, h(r)–〈h〉, where 〈h〉 is the mean surface height and k represents the wavevector with modulus k = |k|. Compositional analysis of the irradiated targets was performed by Rutherford backscattering spectrometry (RBS). The measurements were carried out with a 2 MeV He+ probing beam (∼1 mm2) and the backscattered particles were detected with a Si detector (energy resolution of 15 keV) located at a scattering angle of 170°. Additional compositional and chemical analysis was extracted from high-resolution XPS acquired with a SPECS Phoibos 150 hemispherical analyser using monochromatic Al Kα radiation. Fe2p (Mo3d) and Si2p core-level XPS spectra were recorded from as-processed samples at normal take-off angle using an energy step of 0.025 eV and a pass-energy of 7 eV, which provides an overall instrumental peak broadening of ∼0.4 eV. Data analysis was
Figure 1. (a) Axial XPS compositional profile from a Si(100) sample
irradiated for 30 min (∼2 × 1018 ions cm−2) under Fe co-deposition with a Fe12 mask. The data have been extracted by scanning at different positions the relative intensity of the Fe2p and Si2p corelevels (normalized to the sensitivity factor). The error in the values is ∼1 at.%. (b) Illustration of the morphological changes (AFM 1 × 1 μm2 scans) in the irradiated targets (Si) as we move away from the centre (C) towards the sample edge. The dashed line represents the border between irradiated (outer area) and non-irradiated (inner area) zones.
studied for Fe masks with d = 12 and 14 mm (labelled as Fe12 and Fe14, respectively) and Mo with d = 12 mm (labelled as Mo12). In this way, we can assess the effect of both the metal incorporation rate (Fe12 vs Fe14) and the metal nature (Fe12 vs Mo12). A critical issue of the present masking set-up is that the metal flux and direction are not uniform along the sample surface. That is, closer to the mask edge the metal impurities should impinge at a higher pace and with a preferential direction. This would result in an inhomogeneous metal coverage on the surface. Figure 1(a) shows a typical surface profile of the metal content measured by x-ray photoelectron spectroscopy (XPS) along one target axis for a sample irradiated for 30 min (∼2 × 1018 ions cm−2) with a Fe12 mask. Clearly, there is a peak in the metal concentration closer to the mask edge. This profile is akin to that observed when using a metal plate aside of the sample (asymmetric set-up) [7, 15, 29] but, here, the circular (symmetric) configuration of the metal mask imposes a quasi-specular profile with a broad plateau (several millimetres wide) at the sample centre. The metal flux directionality also has strong implications for the pattern homogeneity along the sample surface. Due to the reduced plate thickness in comparison with d, metal atoms impinge on the silicon target at a large angle with respect to the surface normal. As commented above, the circular 3
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Figure 2. AFM 2 × 2 μm2 images showing the evolution of Si(100) surfaces during IBS with metal co-deposition for different mask
diameters, d, and materials (Fe or Mo). The vertical range (Δz) for each image is also indicated.
done using the Casa XPS processing software (Casa Software Ltd, Cheshire, UK). The relative peak area after background subtraction and the corresponding elemental sensitivity factors were used to calculate the atomic concentrations.
coarsens with time. The nanopatterns are less defined for Fe14 than for Fe12 in the sense that smaller and more isolated structures are produced. In addition, for all systems a typical long-wavelength corrugation develops under prolonged irradiation, which corresponds to the development of surface kinetic roughening [33]. This regime seems to depend, to some extent, on the metal nature and content. Thus, for Fe incorporation, long-wavelength corrugation starts earlier and results in a more defined morphology for the smallest mask (larger Fe content, see below). In addition, the long-range wavelengths are larger for Mo (right column) than for Fe (middle column) for equivalent mask sizes. Remarkably, Mo
3. Results 3.1. Surface morphology
Figure 2 shows selected AFM images of the morphological evolution for Fe14, Fe12 and Mo12 masks. In the three cases, the surface morphology evolves toward a nanodot pattern that 4
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Figure 3. PSD curves extracted from the AFM images at the initial
patterning stages for co-deposition with Fe14, Fe12 and Mo12 masks.
Figure 4. Temporal evolution of the pattern characteristics for metal co-deposition with Fe14, Fe12 and Mo12 masks. Dashed lines are a guide to the eye only.
co-deposition drives to a more pronounced pattern and, interestingly, with a higher ordering than in the Fe case. The AFM images have been analysed in order to address and quantify the eventual differences in the temporal evolution. First, figure 3 shows the PSD functions extracted from the AFM images at the shortest irradiation times in order to study the pattern onset. The fingerprint of this particular stage is the appearance of a peak in the PSD curve that indicates the selection of a characteristic wavelength given by ℓ ∼ 1/k. For Fe14, surface roughening starts earlier (note the overall higher intensity of the PSD at 2 min) although, finally, the pattern onset is retarded with respect to the other cases (see PSD at 5 min). Under this condition, the surface develops a relatively disordered pattern as derived from the broad and low intensity of the PSD peak at longer times. Further, ℓ is lower (larger k) with respect to Fe12. The evolution for Fe12 and Mo12 is similar but the higher intensity of the PSD peak indicates a more defined pattern in the latter case. The promotion of ordering by Mo co-deposition is evident by the presence of secondary peaks (harmonics) in the PSD function. These results reveal the potential of IBS with metal co-deposition to improve the pattern quality as suggested in [34]. A detailed quantitative evolution of the surface morphology is depicted in figure 4. We can see a typical behaviour where w (figure 4(a)) and ℓ (figure 4(b)) initially
increase sharply with irradiation time until a saturation value is attained asymptotically. This pattern evolution is an example of interrupted coarsening [28]. Further, the asymptotic features (w and ℓ ) are larger for Fe12 than for Fe14. In the case of Mo12, patterns with higher w but similar ℓ are obtained with respect to Fe12. Figure 4(c) shows the evolution of the pattern ordering (ξ), as extracted from the inverse of the full width at half maximum of the PSD peak. This value has been normalized to ℓ in order to estimate the correlation length or mean typical size of the ordered domains. In line with previous data [35], the analysis indicates a shortrange ordering that increases with sputtering time until reaching a saturation. Whereas in the case of Fe the saturation in ξ seems to be unaffected by the mask size, considerably higher ξ (nearly two-fold values) are obtained in the case of Mo with respect to Fe. As mentioned above, this fact is very interesting from a practical point of view and shows the potential of the technique as a nanofabrication tool for highly ordered patterns. 3.2. Compositional analysis
In order to understand the morphological evolution shown above, the residual metal content after irradiation has been 5
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ratio for pure elemental materials (∼1.3 as tabulated in [36]). Since the mask and beam geometry are equivalent for the Fe12 and Mo12 masks, the SY ratio can be taken as an estimate of the relative flux attained with the two metals. Hence, under this assumption and considering that the ratio is preserved in the steady-state situation, we can also expect that the removal (SY) of both co-deposited metals on the silicon target will be quite similar. This statement is further supported below. 3.3. Chemical state of co-deposited metals
As commented above, a relevant issue for the compositional and morphological evolution concerns silicide formation and lateral chemical inhomogeneities. In particular, it has been suggested that pattern formation could initiate as a result of ion-induced phase separation from a saturated Fe-Si mixture [26]. Within this framework, it is interesting to assess whether the chemical state of the impurities changes upon pattern onset due to compound or cluster formation. Thus, the Fe2p3/2 and Mo3d5/2 XPS core-levels have been recorded in as-processed samples with Fe12 and Mo12 masks, respectively, before (2.5 min) and after (10 min) the pattern onset. The results are shown in figure 6 together with the reference data from pure metal foils (these spectra have been acquired after removal of surface oxidation by in situ ion beam cleaning). First, note that the XPS compositional values differ from RBS as higher metal contents are obtained for Mo12 than in Fe12. However, since metal atoms present an in-depth distribution along the ion-induced amorphous layer [25], this can be understood from the larger escape depth (probed depth) for photoelectrons with energies around the Mo3d core-level than at the Fe2p counterpart. In agreement with previous results [16], the core-level spectra of nanopatterned samples show a clear chemical shift (∼0.4 eV) with respect to pure metals as an indication of silicide bonds. As reported in [37, 38], the chemical shift for iron silicides at the Fe2p3/2 core-level increases in binding energy with the Si content and, although the difference between the different phases is small, it has a maximum value of 0.3–0.4 eV for FeSi2. Moreover, the spectral lineshape of silicides becomes narrower and more symmetric as the Si content increases [38]. In our nanopatterned samples (figure 6), the chemical shift and the symmetrical lineshape in the Fe2p3/2 core-levels suggest the dominance of Si-rich silicides. This is somewhat foreseeable due to the low Fe content (15° and 20° for FeSi2 and MoSi2, respectively). It should be noted that the results with 2 keV Ar+ are similar to those of 2 keV Kr+, so the energy seems to be a more important factor than the ion itself in the behaviour depicted in figure 9. In any case, this result suggests that the SR difference may be a relevant parameter for oblique incidence IBS with impurities. Indeed, under such conditions and with a wide angle between the ion and the Fe fluxes, the change of the local ion incidence angle would result in local changes in the SY that may be relevant for the nanopatterning process [22]. Figure 9 also indicates that the angular behaviour of the relative SR slightly differs for each metal silicide. In order to check whether this effect could be relevant for the pattern formation or explain the differences observed between Mo- and Fe-assisted patterns, we have analysed the surface slope distributions (not shown) in the AFM images for both systems. The analysis reveals that there is a widening of the slope distribution with irradiation time. In the steady-state regime, the maximum surface angles, with a very low relative weight, are close to 17° and 27° for Fe12 and Mo12 patterns, respectively. Therefore, a strong influence of local changes on the sputtering yield due to the surface morphology (i.e., variations of the incidence angle of the ion beam with respect to the local surface normal) does not seem likely. Based on the TRIDYN data, a composition-dependent SR does not seem plausible in order to explain the patterning 10
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this stress could be attributed to the volume shrinkage upon silicide formation [39, 49]. In our case, the comparison of different metals and levels of impurities allows us to further discuss this issue since the stress upon silicide formation may differ from metal to metal and with the metal to silicon ratio. Therefore, the amount of stress caused by the level of impurities (Fe12 vs Fe14) or the different silicides (Fe12 vs Mo12) could be related with the results observed here. In fact, the volume shrinkage upon silicide formation from metal on silicon reactions is more pronounced for MoSi2 (∼27%) than for FeSi2 (∼22%) [39]. Moreover, the fact that at the initial patterning stage the morphological instabilities correspond to silicide-rich spots (see figure 7) is in agreement with the prediction of the model by Zhou et al [29] when tensile stress is operating. Another consideration, probably also related to the stress field, could be associated with the structure of the ion-induced amorphous layer in analogy to the impurity-free nanopatterning process [5]. That is, silicide formation also results in the attenuation of the ion range with respect to Si [16, 23], yielding a shallower ion-induced amorphous layer the higher the metal content. Under these considerations, the modulation of the amorphous layer thickness driven by the (lateral distribution of) silicide regions could also influence the roughening process. Interestingly, Zhang et al [17] have also hinted at a trend between the pattern characteristics under metal co-deposition and the thickness of the ion-induced amorphous layer, although their calculations should be corrected with the particular metal content (they assume a pure silicon target). In any case, if we consider the simulation results in table 1, the magnitude of the stress should be strong enough to overcome the effect of SR differences in order to yield, as indeed observed, metal-rich nanostructures. Our study shows that the role of silicide formation is complex and strongly dependent on the particular experimental configuration. Therefore, further systematic investigations are still needed to fully understand this issue. Due to the results from BCA models, it would also be desirable to have experimental measurements of the SR for different silicides to provide more solid conclusions about its role in surface roughening. In parallel, it would be interesting to test different working conditions based on clear tendencies given by the simulations. Further, although stress caused by silicide formation could explain the larger structures in the case of Mo co-deposition, a question that still needs to be addressed is the role of the impurity in the pattern ordering. The formation of larger aspect ratio nanostructures and the ordering process may be correlated. However, further insight into this relevant phenomenon is still needed. Therefore, pattern formation and the resulting morphological characteristics should be systematically compared under the assistance of different metals, with special attention to the corresponding silicide properties.
Figure 9. TRYDIN calculations of the relative SR ratio for Si with
respect to metal disilicide targets, SRSi/SRMeSi2, for different incidence angles, ions and energies. Data above (below) unity correspond to faster erosion of Si (FeSi2). The error bars include both statistical deviations in the simulation results and an uncertainty in the SBE of 0.1 eV for the silicide compounds.
(at least) under our working conditions. However, there could be additional considerations in this respect. First, Hofsäss et al [26] interestingly suggests the eventual contribution of SY amplification to increase the removal rate of silicon atoms. This mechanism has been observed by co-deposition of heavy elements, which drives to an enhanced sputtering of the lighter matrix element by reflection of projectiles from forward-implanted impurities [47]. However, assuming a phase segregation scheme, this situation would take place preferably in the silicide regions and not in the pure silicon regions (or with low Fe content) and, further, it would result in Fe-rich silicides that are not observed in our XPS analysis. In a recent work [27] where a similar controversy has been found, it has been argued that, due to the continuous supply of co-sputtered impurities, the relevant parameter governing the SR in silicide regions is, in fact, the partial SR for Si atoms. However, we find two unclear points in this argument. First, they calculate the partial SR assuming the total atomic density of the silicide instead of just that for Si atoms. Second, this scenario applies only for the steady-state situation since it assumes that metal restoration occurs only on the silicide patches by lateral (thermal or ion-induced) diffusion or geometrical effects [27] (i.e., it does not explain the initial regime from a flat surface). All these arguments show that the faster erosion of silicon compared to silicide regions is not as straightforward as originally thought. In view of the above, other mechanisms rather than the SR difference may be plausible in order to develop the pattern at near-normal incidence. Based on our C-AFM data, these mechanisms could probably occur along with phase segregation. For instance, Zhou et al [29] proposed that the stress generated upon silicide formation may also be relevant for ion-induced patterning under metal co-deposition. In fact, the presence of tensile stress has indeed been detected in nanodot patterns produced with Mo co-deposition [48]. The origin of
5. Conclusion In summary, nanodot patterning dynamics on Si(100) by normal-incidence low-energy IBS under co-deposition of Fe or Mo impurities has been studied. The pattern evolution 11
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depends both on the metal’s nature and its incorporation rate. On the one hand, a higher flux (for the same type of impurity) yields a faster dynamics and larger asymptotic ℓ . By comparing Fe and Mo co-deposition, it is found that higher w is obtained in the latter case for a relatively lower metal coverage and, remarkably, the ordering within the patterns is considerably enhanced. Further, XPS has revealed the formation of silicide bonds from the early irradiation stages, stressing the relevant role of the affinity of co-deposited metals for silicon. C-AFM analysis performed at the initial and asymptotic stages also indicates that nanodot structures are metal-rich, resulting in coupled compositional and morphological patterns. These results were discussed in terms of phase segregation, influence of morphology on local flux variations of impurities and silicide formation. It is concluded that the underlying (concurrent) mechanisms of pattern formation are complex since many processes, with a different relative weight depending on the specific experimental conditions, can come into play. In any case, from a practical point of view, it is shown that, by proper selection of the process parameters, IBS with metal co-deposition can be used as an effective way of tuning the pattern characteristics and, interestingly, of achieving highly ordered arrays.
[12] Le Roy S, Barthel E, Brun N, Lelarge A and Søndergård E 2009 J. Appl. Phys. 106 094308 [13] El-Atwani O, Allain J P, Cimaroli A and Ortoleva S 2011 J. Appl. Phys. 110 074301 [14] Hofsäss H and Zhang K 2008 Appl. Phys. A 92 517 [15] Macko S, Frost F, Engler M, Hirsch D, Höche T, Grenzer J and Michely T 2011 New J. Phys. 13 073017 [16] Sánchez-García J A, Vázquez L, Gago R, Redondo-Cubero A, Albella J M and Czigány Zs 2008 Nanotechnology 19 355306 Sánchez-García J A, Gago R, Caillard R, Redondo-Cubero A, Martin-Gago J A, Palomares F J, Fernández M and Vázquez L 2009 J. Phys.: Condens. Matter 21 224009 [17] Zhang K, Bobes O and Hofsäss H 2014 Nanotechnology 25 085301 [18] Norris S A 2013 J. Appl. Phys. 114 204303 [19] Kree R, Yasseri T and Hartmann A K 2009 Nucl. Instrum. Methods Phys. Res. B 267 1403 [20] Zhou J and Lu M 2010 Phys. Rev. B 82 125404 [21] Bradley R M 2011 Phys. Rev. B 83 195410 [22] Zhang K, Brötzmann M and Hofsäss H 2011 New J. Phys. 13 013033 [23] Redondo-Cubero A, Gago R, Palomares F J, Mücklich A, Vinnichenko M and Vázquez L 2012 Phys. Rev. B 86 085436 [24] Zhang K, Brötzmann M and Hofsäss H 2012 AIP Adv. 2 032123 [25] Khanbabaee B, Arezki B, Biermanns A, Cornejo M, Hirsch D, Lützenkirchen-Hecht D, Frost F and Pietsch U 2013 Thin Solid Films 527 349 [26] Hofsäss H, Zhang K, Pape A, Bobes O and Brötzmann M 2013 Appl. Phys. A 111 653 [27] Engler M, Frost F, Müller S, Macko S, Will M, Feder R, Spemann D, Hübner R, Facsko S and Michely T 2014 Nanotechnology 25 115303 [28] Muñoz-García J, Gago R, Vázquez L, Sánchez-García J A and Cuerno R 2010 Phys. Rev. Lett. 105 026101 [29] Zhou J, Facsko S, Lu M and Möller W 2011 J. Appl. Phys. 109 104315 [30] Bradley R M 2012 Phys. Rev. B 85 115419 [31] Teichert C and Beinik I 2011 Scanning Probe Microscopy in Nanoscience and Nanotechnology ed B Bhushan vol 2 chapter 23 (Berlin: Springer) [32] Martínez L, Tello M, Díaz M, Román E, Garcia R and Huttel Y 2011 Rev. Sci. Instrum. 82 023710 Hormeño S, Penedo M, Manzano C V and Luna M 2013 Nanotechnology 24 395701 [33] Castro M, Cuerno R, Vázquez L and Gago R 2005 Phys. Rev. Lett. 94 016102 [34] Bradley R M 2013 Phys. Rev. B 87 205408 [35] Gago R, Vázquez L, Plantevin O, Metzger T H, Muñoz-García J, Cuerno R and Castro M 2006 Appl. Phys. Lett. 89 233101 [36] Behrisch R and Eckstein W 2007 Sputtering by Particle Bombardment (Berlin: Springer) [37] Egert B and Panzner G 1984 Phys. Rev. B 4 2091 [38] Kinsinger V, Dezsi I, Steiner P and Langouche G 1990 J. Phys.: Condens. Matter 2 4955 [39] Murarka S P 1983 Silicides for VLSI Applications (New York: Academic Press Inc.) [40] Slaughter J M, Shapiro A, Kearney P A and Falco C M 1991 Phys. Rev. B 44 3854 [41] Ozaydin-Ince G and Ludwig Jr K F 2009 J. Phys.: Condens. Matter 21 224008 [42] Ziegler J F, Biersack J P and Littmark U 1985 The Stopping and Range of Ions in Solids (New York: Pergamon) www. srim.org [43] Möller W, Eckstein W and Biersack J P 1988 Comput. Phys. Commun. 51 355
Acknowledgments We are indebted to R Cuerno, M Castro and J Muñoz-García for fruitful suggestions and discussions. This work has been supported by grants FIS2012-38866-C05-05 and MAT201018432 (MECO, Spain), and CSD2008-00023 (MICINN, Spain). ARC acknowledges financial support under the ‘Juan de la Cierva Program’ (MECO, Spain) through grant JCI2012-14509.
References [1] Muñoz-García J, Vázquez L, Cuerno R, Sánchez-García J A, Castro M and Gago R 2009 Toward Functional Nanomaterials ed Z Wang (New York: Springer) pp 323–98 [2] Ozaydin G, Özcan A S, Wang Y, Ludwig K F, Zhou H, Headrick R L and Siddons D P 2005 Appl. Phys. Lett. 87 163104 [3] Teichert C, Hofer C and Hlawacek G 2006 Adv. Eng. Mater. 8 1057 [4] Madi C S, George H B and Aziz M J 2009 J. Phys.: Condens. Matter 21 224010 [5] Castro M, Gago R, Vázquez L, Muñoz-García J and Cuerno R 2012 Phys. Rev. B 86 214107 [6] Gago R, Vázquez L, Cuerno R, Varela M, Ballesteros C and Albella J M 2001 Appl. Phys. Lett. 78 3316 [7] Macko S, Frost F, Ziberi B, Förster D F and Michely T 2010 Nanotechnology 21 085301 [8] Shenoy V B, Chan W L and Chason E 2007 Phys. Rev. Lett. 98 256101 [9] Bradley R M and Shipman P D 2010 Phys. Rev. Lett. 105 145501 [10] Shipman P D and Bradley R M 2011 Phys. Rev. B 84 085420 [11] Motta F C, Shipman P D and Bradley R M 2012 J. Phys. D 45 122001 12
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[47] Berg S and Katardjiev I 1996 Surf. Coat. Technol. 84 353 [48] Ozaydin G, Ludwig K F, Zhou H and Headrick R L 2008 J. Vac. Sci. Technol. B 26 551 [49] Sander D, Enders A and Kirschner J 1995 Appl. Phys. Lett. 67 1833
[44] Zaporozchenko V I, Stepanova M G and Vojtusik S S 1996 Vacuum 47 421 [45] Moroni E G, Wolf W, Hafner J and Podloucky R 1999 Phys. Rev. B 59 12860 [46] McMahan A K, Klepeis J E, van Schilfgaarde M and Methfessel M 1994 Phys. Rev. B 50 10742
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Influence of metal co-deposition on silicon nanodot patterning dynamics during ion-beam sputtering
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 415301 (http://iopscience.iop.org/0957-4484/25/41/415301) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 25 (2014) 415301 (13pp)
doi:10.1088/0957-4484/25/41/415301
Influence of metal co-deposition on silicon nanodot patterning dynamics during ionbeam sputtering R Gago1, A Redondo-Cubero2, F J Palomares1 and L Vázquez1 1
Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, E-28049 Madrid, Spain 2 Departamento de Física Aplicada, Universidad Autónoma de Madrid, E-28049 Madrid, Spain E-mail: [email protected] Received 16 April 2014, revised 31 July 2014 Accepted for publication 6 August 2014 Published 24 September 2014 Abstract
We address the impact of metal co-deposition in the nanodot patterning dynamics of Si(100) surfaces under normal-incidence 1 keV Ar+ ion-beam sputtering (IBS). In particular, the effect of both the metal nature (Fe or Mo) and flux has been studied. Morphological and compositional evolution were followed by atomic force microscopy (AFM) and Rutherford backscattering spectrometry, respectively. For the same type of impurity, the dynamics is faster for a higher codeposition flux, which also drives to larger asymptotic roughness and wavelength. Mo codeposition yields rougher surfaces for a lower metal coverage than Fe and, remarkably, higher ordered patterns. X-ray photoelectron spectroscopy reveals the formation of silicide bonds even before pattern onset, stressing the relevant role of the affinity of the co-deposited metals for silicon. Further, current-sensing AFM performed at the initial and asymptotic stages indicates that the nanodot structures are metal-rich, resulting in coupled compositional and morphological patterns. These results are discussed in terms of phase segregation, morphology-driven local flux variations of impurities and silicide formation. This analysis reveals that the underlying (concurrent) mechanisms of pattern formation are complex since many processes can come into play with a different relative weight depending on the specific patterning conditions. From a practical point of view, it is shown that, by proper selection of the process parameters, IBS with metal co-deposition can be used to tune the dynamics and pattern properties and, interestingly, to produce highly ordered arrays. Keywords: pattern formation, silicon, ion beam sputtering, co-deposition (Some figures may appear in colour only in the online journal) 1. Introduction
The understanding of nanopattern formation by IBS on amorphous or amorphizable materials has experienced a significant turning point due to the elucidation of the critical role played by (metal) impurities [2, 3]. Henceforth, two main fields can be roughly distinguished: (i) IBS patterning under low or impurity-free conditions and (ii) IBS patterning with intentional impurity co-deposition. For the former case, it has been clearly established that, in the case of monoelemental targets, (ripple) patterns only emerge above a critical ion incidence angle [4, 5]. On this evidence, pattern formation below this threshold [6] (e.g., near-normal ion incidence) has
Ion beam sputtering (IBS) has emerged in recent decades as a promising bottom-up approach for surface nanostructuring, capable of inducing nanopatterns in a wide range of targets (including semiconductors, metals and insulators) with, typically, ripple, dot, or hole nanostructures [1]. This versatile nanofabrication method comprises a single-step process with high throughput (it can pattern relatively wide surface areas of up to several tens of cm2 in short irradiation times, typically a few minutes). 0957-4484/14/415301+13$33.00
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to be attributed to the (inadvertent) presence of metal impurities during the irradiation [7]. Compositional issues have even broader implications in the field of IBS nanopatterning since they may also be relevant in alloys or compound targets [8–11] as, indeed, observed experimentally for GaSb surfaces [12, 13]. Despite the complexity added by the (a priori, undesirable) presence of impurities, IBS with concurrent metal deposition is emerging as a strategy to both tune the pattern properties and to improve the understanding of the fundamentals of nanopatterning of multi-component systems. For example, this approach has inspired a novel method, the socalled ‘surfactant sputtering’ [14], capable of modifying the surface morphology from enhanced smoothing to nanostructuring. From a practical point of view, the addition of codeposition can be used to yield large corrugations at moderate ion fluences [15], tune the pattern nanostructures [16], design the pattern symmetry [17] or improve the pattern ordering [18]. Further, the fundamental interest in this topic has led to the development of specific theoretical models on IBS nanopatterning with concurrent metal deposition [19–21]. Nevertheless, the underlying mechanisms that originate the pattern still remain under debate. The phenomenology of the simultaneous supply of Fe impurities has been systematically studied in recent years for the paradigmatic case of silicon targets [15, 22–24]. Typically, the experimental set-ups employed are rather similar, with a co-sputtered metal plate located adjacent to the target acting as the impurity source. This configuration allows the study of the pattern morphological dependence on the metal content as the latter decreases with distance from the plate edge. Here, the flux of co-deposited (metal) species has a rather defined direction, yielding mostly ripple morphologies or even an initial transient nanodot pattern. It has also been revealed that iron silicide forms on the surface [16, 22, 23, 25], which seems to be a key step for surface patterning. This finding has been extended to other metal impurities [26, 27]. However, this necessary condition is not sufficient since, despite the formation of silicides, large metal coverages do not yield any pattern [27]. This scenario suggests that patterns emerge due to lateral modulations of the sputtering rate driven by compositional changes (silicide formation). In this respect, a large angle between the ion and impurity fluxes seems to be more efficient for pattern formation [15], which highlights the relevance of height fluctuations and local flux variations (locations with the higher metal deposition rate receive a lower ion flux) in order to induce such lateral chemical inhomogeneities. The role of (ion-induced) phase-segregation effects has also been proposed in order to induce a compositional pattern [26]. Although several observations [15] do not reconcile with the previous phenomenology, the lack of patterns for large metal coverages could be explained by the fact that a homogeneous silicide layer would not tend to decompose [27]. In any case, a more general picture considers the amplifying feedback between compositional and height fluctuations [27]. As mentioned above, systematic studies on the incorporation of impurities during IBS mostly refer to ripple
patterning. This situation partially relies on the common use of a rather directional metal flux (by locating an adjacent metal plate to the silicon target) that promotes such morphology. In contrast, so far similar studies for stable nanodot morphologies (i.e., without a dot to ripple pattern transition) addressing the impact of different metals or the co-deposition flux during the irradiation have not been performed. One practical reason for that could be the difficulty in performing systematic experiments since for each (metal content) condition a new experiment is required (in contrast to the ripple patterns, in which a continuous range of metal contents is achieved in a single shot by varying the distance to the adjacent metal plate). As an example of this complexity, Feassisted nanodot patterns with a variety of designed pattern symmetries and wavelengths can be produced by adjusting the size and arrangement of different metal pieces surrounding the silicon targets [17]. From the theoretical point of view, the main specific work dealing with nanodot pattern formation by IBS with metal co-deposition is that by Bradley [21], which covers the initial patterning stages (the so-called linear regime). As indeed claimed by Bradley, there is a need for more systematic experimental studies to test the different theories and understand the mechanisms of metal-assisted nanodot patterning. In this work, we aim to contribute to filling this gap by studying the effect of the metal’s nature and its incorporation rate on the nanodot pattern formation dynamics. We have selected Fe and Mo as co-deposition species since they yield pronounced patterns [26]. Further, the morphological and compositional dynamics on the irradiated surfaces are both studied in parallel. The results allow us to discuss the eventual mechanism(s) of pattern formation and evidence IBS with metal co-deposition as an effective way to tune the nanodot pattern dynamics and characteristics in terms of amplitude (roughness), wavelength and, remarkably, ordering.
2. Experimental details 2.1. Sample preparation
Si(100) targets were sputtered with 1 keV Ar+ ions at normal incidence. The ions were extracted from a commercial broadbeam (3 cm beam diameter) CSC Kaufman-type ion gun (Veeco©) located 25 cm away from the target. The current density was measured prior to each irradiation with a Faraday cup and set to ∼150 μA cm−2. This relatively low current was set to attain a slow patterning dynamics [28]. Metal codeposition was carried out by placing on the sample a 1 mm thick Fe or Mo metal plate with a countersunk hole (to enhance co-deposition under normal incidence). This set-up is similar to that depicted by Zhou et al in [29]. Here, the uncovered part of the Si(100) surface is sputtered with concurrent incorporation of metal impurities from the co-sputtered plate. Plates with different hole diameters, d, were fabricated in order to tune the metal incorporation rate since this work is based on the premise that the metal flux can be increased by reducing d. The patterning dynamics has been 2
Fe content (at.%)
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geometry of the mask results in an effective isotropic metal flux at the target centre that induces the formation of stable nanodot patterns. However, as we move towards the mask edge the metal flux becomes more intense and directional, and ripple patterns appear. This situation is depicted schematically in figure 1(b) where typical atomic force microscopy (AFM) images at different locations at the sample surface with respect to the centre (C) are shown (note that the sample contour is only shown as a reference and it is not at the same scale). Remarkably, the ripple morphologies have different orientations at right (R), left (L), up (U) or down (D) positions, being aligned parallel to the closest mask perimeter. The definition of the ripple orientation by the metal flux direction has indeed been observed by other groups [7, 15] and predicted theoretically [30]. It should be noted that all the morphological and compositional analysis shown hereafter has been done systematically at the centre position.
Mask edge
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2.2. Sample characterization
The surface morphology after irradiation was imaged by AFM with a Nanoscope IIIa equipment (Veeco©) and an Agilent PicoPlus 5500, the latter being able to operate in conductive mode (C-AFM) [31]. For the topographical measurements, silicon cantilevers were employed with a nominal radius, r, of 8 nm and opening angle, θ, smaller than 52°. These measurements were carried out in air and tapping mode. Diamond coated tips were employed for the C-AFM measurements (models CDT-FMR from NanoWorld, r ∼ 83 ± 17 nm and θ < 47°, and DCP11 from NT-MDT, r ∼ 60 ± 10 nm and θ < 44°). In the C-AFM mode, imaging biases in the 3–9 V range at both polarities were employed. This analysis was complemented with Kelvin mode measurements (KFM) using silicon tips coated with gold nanoparticles in the 2–3 nm range (from NEXT TIP, Spain) [32]. In both electrical modes, the measurements were performed under nitrogen ambient. For quantification of the surface morphology, the surface roughness (w) and the characteristic pattern wavelength (ℓ ) were evaluated from the AFM images. The latter was extracted from the power spectral density (PSD) function defined as PSD(k) = 〈H(k)H(−k)〉. Here, the angular brackets denote spatial averaging and H(k) stands for the Fourier transform function of the surface height profile at vector position r, h(r)–〈h〉, where 〈h〉 is the mean surface height and k represents the wavevector with modulus k = |k|. Compositional analysis of the irradiated targets was performed by Rutherford backscattering spectrometry (RBS). The measurements were carried out with a 2 MeV He+ probing beam (∼1 mm2) and the backscattered particles were detected with a Si detector (energy resolution of 15 keV) located at a scattering angle of 170°. Additional compositional and chemical analysis was extracted from high-resolution XPS acquired with a SPECS Phoibos 150 hemispherical analyser using monochromatic Al Kα radiation. Fe2p (Mo3d) and Si2p core-level XPS spectra were recorded from as-processed samples at normal take-off angle using an energy step of 0.025 eV and a pass-energy of 7 eV, which provides an overall instrumental peak broadening of ∼0.4 eV. Data analysis was
Figure 1. (a) Axial XPS compositional profile from a Si(100) sample
irradiated for 30 min (∼2 × 1018 ions cm−2) under Fe co-deposition with a Fe12 mask. The data have been extracted by scanning at different positions the relative intensity of the Fe2p and Si2p corelevels (normalized to the sensitivity factor). The error in the values is ∼1 at.%. (b) Illustration of the morphological changes (AFM 1 × 1 μm2 scans) in the irradiated targets (Si) as we move away from the centre (C) towards the sample edge. The dashed line represents the border between irradiated (outer area) and non-irradiated (inner area) zones.
studied for Fe masks with d = 12 and 14 mm (labelled as Fe12 and Fe14, respectively) and Mo with d = 12 mm (labelled as Mo12). In this way, we can assess the effect of both the metal incorporation rate (Fe12 vs Fe14) and the metal nature (Fe12 vs Mo12). A critical issue of the present masking set-up is that the metal flux and direction are not uniform along the sample surface. That is, closer to the mask edge the metal impurities should impinge at a higher pace and with a preferential direction. This would result in an inhomogeneous metal coverage on the surface. Figure 1(a) shows a typical surface profile of the metal content measured by x-ray photoelectron spectroscopy (XPS) along one target axis for a sample irradiated for 30 min (∼2 × 1018 ions cm−2) with a Fe12 mask. Clearly, there is a peak in the metal concentration closer to the mask edge. This profile is akin to that observed when using a metal plate aside of the sample (asymmetric set-up) [7, 15, 29] but, here, the circular (symmetric) configuration of the metal mask imposes a quasi-specular profile with a broad plateau (several millimetres wide) at the sample centre. The metal flux directionality also has strong implications for the pattern homogeneity along the sample surface. Due to the reduced plate thickness in comparison with d, metal atoms impinge on the silicon target at a large angle with respect to the surface normal. As commented above, the circular 3
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Figure 2. AFM 2 × 2 μm2 images showing the evolution of Si(100) surfaces during IBS with metal co-deposition for different mask
diameters, d, and materials (Fe or Mo). The vertical range (Δz) for each image is also indicated.
done using the Casa XPS processing software (Casa Software Ltd, Cheshire, UK). The relative peak area after background subtraction and the corresponding elemental sensitivity factors were used to calculate the atomic concentrations.
coarsens with time. The nanopatterns are less defined for Fe14 than for Fe12 in the sense that smaller and more isolated structures are produced. In addition, for all systems a typical long-wavelength corrugation develops under prolonged irradiation, which corresponds to the development of surface kinetic roughening [33]. This regime seems to depend, to some extent, on the metal nature and content. Thus, for Fe incorporation, long-wavelength corrugation starts earlier and results in a more defined morphology for the smallest mask (larger Fe content, see below). In addition, the long-range wavelengths are larger for Mo (right column) than for Fe (middle column) for equivalent mask sizes. Remarkably, Mo
3. Results 3.1. Surface morphology
Figure 2 shows selected AFM images of the morphological evolution for Fe14, Fe12 and Mo12 masks. In the three cases, the surface morphology evolves toward a nanodot pattern that 4
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Figure 3. PSD curves extracted from the AFM images at the initial
patterning stages for co-deposition with Fe14, Fe12 and Mo12 masks.
Figure 4. Temporal evolution of the pattern characteristics for metal co-deposition with Fe14, Fe12 and Mo12 masks. Dashed lines are a guide to the eye only.
co-deposition drives to a more pronounced pattern and, interestingly, with a higher ordering than in the Fe case. The AFM images have been analysed in order to address and quantify the eventual differences in the temporal evolution. First, figure 3 shows the PSD functions extracted from the AFM images at the shortest irradiation times in order to study the pattern onset. The fingerprint of this particular stage is the appearance of a peak in the PSD curve that indicates the selection of a characteristic wavelength given by ℓ ∼ 1/k. For Fe14, surface roughening starts earlier (note the overall higher intensity of the PSD at 2 min) although, finally, the pattern onset is retarded with respect to the other cases (see PSD at 5 min). Under this condition, the surface develops a relatively disordered pattern as derived from the broad and low intensity of the PSD peak at longer times. Further, ℓ is lower (larger k) with respect to Fe12. The evolution for Fe12 and Mo12 is similar but the higher intensity of the PSD peak indicates a more defined pattern in the latter case. The promotion of ordering by Mo co-deposition is evident by the presence of secondary peaks (harmonics) in the PSD function. These results reveal the potential of IBS with metal co-deposition to improve the pattern quality as suggested in [34]. A detailed quantitative evolution of the surface morphology is depicted in figure 4. We can see a typical behaviour where w (figure 4(a)) and ℓ (figure 4(b)) initially
increase sharply with irradiation time until a saturation value is attained asymptotically. This pattern evolution is an example of interrupted coarsening [28]. Further, the asymptotic features (w and ℓ ) are larger for Fe12 than for Fe14. In the case of Mo12, patterns with higher w but similar ℓ are obtained with respect to Fe12. Figure 4(c) shows the evolution of the pattern ordering (ξ), as extracted from the inverse of the full width at half maximum of the PSD peak. This value has been normalized to ℓ in order to estimate the correlation length or mean typical size of the ordered domains. In line with previous data [35], the analysis indicates a shortrange ordering that increases with sputtering time until reaching a saturation. Whereas in the case of Fe the saturation in ξ seems to be unaffected by the mask size, considerably higher ξ (nearly two-fold values) are obtained in the case of Mo with respect to Fe. As mentioned above, this fact is very interesting from a practical point of view and shows the potential of the technique as a nanofabrication tool for highly ordered patterns. 3.2. Compositional analysis
In order to understand the morphological evolution shown above, the residual metal content after irradiation has been 5
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ratio for pure elemental materials (∼1.3 as tabulated in [36]). Since the mask and beam geometry are equivalent for the Fe12 and Mo12 masks, the SY ratio can be taken as an estimate of the relative flux attained with the two metals. Hence, under this assumption and considering that the ratio is preserved in the steady-state situation, we can also expect that the removal (SY) of both co-deposited metals on the silicon target will be quite similar. This statement is further supported below. 3.3. Chemical state of co-deposited metals
As commented above, a relevant issue for the compositional and morphological evolution concerns silicide formation and lateral chemical inhomogeneities. In particular, it has been suggested that pattern formation could initiate as a result of ion-induced phase separation from a saturated Fe-Si mixture [26]. Within this framework, it is interesting to assess whether the chemical state of the impurities changes upon pattern onset due to compound or cluster formation. Thus, the Fe2p3/2 and Mo3d5/2 XPS core-levels have been recorded in as-processed samples with Fe12 and Mo12 masks, respectively, before (2.5 min) and after (10 min) the pattern onset. The results are shown in figure 6 together with the reference data from pure metal foils (these spectra have been acquired after removal of surface oxidation by in situ ion beam cleaning). First, note that the XPS compositional values differ from RBS as higher metal contents are obtained for Mo12 than in Fe12. However, since metal atoms present an in-depth distribution along the ion-induced amorphous layer [25], this can be understood from the larger escape depth (probed depth) for photoelectrons with energies around the Mo3d core-level than at the Fe2p counterpart. In agreement with previous results [16], the core-level spectra of nanopatterned samples show a clear chemical shift (∼0.4 eV) with respect to pure metals as an indication of silicide bonds. As reported in [37, 38], the chemical shift for iron silicides at the Fe2p3/2 core-level increases in binding energy with the Si content and, although the difference between the different phases is small, it has a maximum value of 0.3–0.4 eV for FeSi2. Moreover, the spectral lineshape of silicides becomes narrower and more symmetric as the Si content increases [38]. In our nanopatterned samples (figure 6), the chemical shift and the symmetrical lineshape in the Fe2p3/2 core-levels suggest the dominance of Si-rich silicides. This is somewhat foreseeable due to the low Fe content (15° and 20° for FeSi2 and MoSi2, respectively). It should be noted that the results with 2 keV Ar+ are similar to those of 2 keV Kr+, so the energy seems to be a more important factor than the ion itself in the behaviour depicted in figure 9. In any case, this result suggests that the SR difference may be a relevant parameter for oblique incidence IBS with impurities. Indeed, under such conditions and with a wide angle between the ion and the Fe fluxes, the change of the local ion incidence angle would result in local changes in the SY that may be relevant for the nanopatterning process [22]. Figure 9 also indicates that the angular behaviour of the relative SR slightly differs for each metal silicide. In order to check whether this effect could be relevant for the pattern formation or explain the differences observed between Mo- and Fe-assisted patterns, we have analysed the surface slope distributions (not shown) in the AFM images for both systems. The analysis reveals that there is a widening of the slope distribution with irradiation time. In the steady-state regime, the maximum surface angles, with a very low relative weight, are close to 17° and 27° for Fe12 and Mo12 patterns, respectively. Therefore, a strong influence of local changes on the sputtering yield due to the surface morphology (i.e., variations of the incidence angle of the ion beam with respect to the local surface normal) does not seem likely. Based on the TRIDYN data, a composition-dependent SR does not seem plausible in order to explain the patterning 10
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this stress could be attributed to the volume shrinkage upon silicide formation [39, 49]. In our case, the comparison of different metals and levels of impurities allows us to further discuss this issue since the stress upon silicide formation may differ from metal to metal and with the metal to silicon ratio. Therefore, the amount of stress caused by the level of impurities (Fe12 vs Fe14) or the different silicides (Fe12 vs Mo12) could be related with the results observed here. In fact, the volume shrinkage upon silicide formation from metal on silicon reactions is more pronounced for MoSi2 (∼27%) than for FeSi2 (∼22%) [39]. Moreover, the fact that at the initial patterning stage the morphological instabilities correspond to silicide-rich spots (see figure 7) is in agreement with the prediction of the model by Zhou et al [29] when tensile stress is operating. Another consideration, probably also related to the stress field, could be associated with the structure of the ion-induced amorphous layer in analogy to the impurity-free nanopatterning process [5]. That is, silicide formation also results in the attenuation of the ion range with respect to Si [16, 23], yielding a shallower ion-induced amorphous layer the higher the metal content. Under these considerations, the modulation of the amorphous layer thickness driven by the (lateral distribution of) silicide regions could also influence the roughening process. Interestingly, Zhang et al [17] have also hinted at a trend between the pattern characteristics under metal co-deposition and the thickness of the ion-induced amorphous layer, although their calculations should be corrected with the particular metal content (they assume a pure silicon target). In any case, if we consider the simulation results in table 1, the magnitude of the stress should be strong enough to overcome the effect of SR differences in order to yield, as indeed observed, metal-rich nanostructures. Our study shows that the role of silicide formation is complex and strongly dependent on the particular experimental configuration. Therefore, further systematic investigations are still needed to fully understand this issue. Due to the results from BCA models, it would also be desirable to have experimental measurements of the SR for different silicides to provide more solid conclusions about its role in surface roughening. In parallel, it would be interesting to test different working conditions based on clear tendencies given by the simulations. Further, although stress caused by silicide formation could explain the larger structures in the case of Mo co-deposition, a question that still needs to be addressed is the role of the impurity in the pattern ordering. The formation of larger aspect ratio nanostructures and the ordering process may be correlated. However, further insight into this relevant phenomenon is still needed. Therefore, pattern formation and the resulting morphological characteristics should be systematically compared under the assistance of different metals, with special attention to the corresponding silicide properties.
Figure 9. TRYDIN calculations of the relative SR ratio for Si with
respect to metal disilicide targets, SRSi/SRMeSi2, for different incidence angles, ions and energies. Data above (below) unity correspond to faster erosion of Si (FeSi2). The error bars include both statistical deviations in the simulation results and an uncertainty in the SBE of 0.1 eV for the silicide compounds.
(at least) under our working conditions. However, there could be additional considerations in this respect. First, Hofsäss et al [26] interestingly suggests the eventual contribution of SY amplification to increase the removal rate of silicon atoms. This mechanism has been observed by co-deposition of heavy elements, which drives to an enhanced sputtering of the lighter matrix element by reflection of projectiles from forward-implanted impurities [47]. However, assuming a phase segregation scheme, this situation would take place preferably in the silicide regions and not in the pure silicon regions (or with low Fe content) and, further, it would result in Fe-rich silicides that are not observed in our XPS analysis. In a recent work [27] where a similar controversy has been found, it has been argued that, due to the continuous supply of co-sputtered impurities, the relevant parameter governing the SR in silicide regions is, in fact, the partial SR for Si atoms. However, we find two unclear points in this argument. First, they calculate the partial SR assuming the total atomic density of the silicide instead of just that for Si atoms. Second, this scenario applies only for the steady-state situation since it assumes that metal restoration occurs only on the silicide patches by lateral (thermal or ion-induced) diffusion or geometrical effects [27] (i.e., it does not explain the initial regime from a flat surface). All these arguments show that the faster erosion of silicon compared to silicide regions is not as straightforward as originally thought. In view of the above, other mechanisms rather than the SR difference may be plausible in order to develop the pattern at near-normal incidence. Based on our C-AFM data, these mechanisms could probably occur along with phase segregation. For instance, Zhou et al [29] proposed that the stress generated upon silicide formation may also be relevant for ion-induced patterning under metal co-deposition. In fact, the presence of tensile stress has indeed been detected in nanodot patterns produced with Mo co-deposition [48]. The origin of
5. Conclusion In summary, nanodot patterning dynamics on Si(100) by normal-incidence low-energy IBS under co-deposition of Fe or Mo impurities has been studied. The pattern evolution 11
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depends both on the metal’s nature and its incorporation rate. On the one hand, a higher flux (for the same type of impurity) yields a faster dynamics and larger asymptotic ℓ . By comparing Fe and Mo co-deposition, it is found that higher w is obtained in the latter case for a relatively lower metal coverage and, remarkably, the ordering within the patterns is considerably enhanced. Further, XPS has revealed the formation of silicide bonds from the early irradiation stages, stressing the relevant role of the affinity of co-deposited metals for silicon. C-AFM analysis performed at the initial and asymptotic stages also indicates that nanodot structures are metal-rich, resulting in coupled compositional and morphological patterns. These results were discussed in terms of phase segregation, influence of morphology on local flux variations of impurities and silicide formation. It is concluded that the underlying (concurrent) mechanisms of pattern formation are complex since many processes, with a different relative weight depending on the specific experimental conditions, can come into play. In any case, from a practical point of view, it is shown that, by proper selection of the process parameters, IBS with metal co-deposition can be used as an effective way of tuning the pattern characteristics and, interestingly, of achieving highly ordered arrays.
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Acknowledgments We are indebted to R Cuerno, M Castro and J Muñoz-García for fruitful suggestions and discussions. This work has been supported by grants FIS2012-38866-C05-05 and MAT201018432 (MECO, Spain), and CSD2008-00023 (MICINN, Spain). ARC acknowledges financial support under the ‘Juan de la Cierva Program’ (MECO, Spain) through grant JCI2012-14509.
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