Nanoscale Structural Damage due to Focused Ion Beam Milling of Silicon with Ga ions E. Salvatia* , L. R. Brandta, C. Papadakia, H. Zhanga, S.M. Mousavib D. Wermeillec, A.M. Korsunskya
a
University of Oxford, Engineering Science Department, Parks Road, Oxford, UK b
c
Karlstad University, Universitetsgatan 2, 651 88 Karlstad, Sweden
ESRF European Synchrotron Radiation Facility - XMaS Beamline - 71, avenue des Martyrs, CS 40220, 38043 Grenoble Cedex 9, France
Keywords Ga-ion amorphisation, XRR, eigenstrain
Abstract The exposure of sample to Focused Ion Beam leads to Ga-ion implantation, damage, material amorphisation, and the introduction of sources of residual stress; namely eigenstrain. In this study we employ synchrotron X-ray Reflectivity technique to characterise the amorphous layer generated in a single crystal Silicon sample by exposure to Ga-ion beam. The thickness, density and interface roughness of the amorphous layer were extracted from the analysis of the reflectivity curve. The outcome is compared with the eigenstrain profile evaluated from residual stress analysis by Molecular Dynamics and TEM imaging reported in the literature.
Introduction One of the most common techniques in bespoke nanoscale fabrication is undoubtedly Focused Ion Beam (FIB) milling. This technique is able to perform material removal by means of ion beam exposure. Ga-ion exposure not only leads to material removal, but is also the cause of material damage within a shallow surface layer. The interaction of irradiating ions with the target material causes dislocation nucleation, amorphisation, density and elastic property change [1-3]. Evidently, these effects combine to alter the overall mechanical response of fabricated objects. Although only recently the inelastic deformation that gives rise to the residual stress was characterised by means of the eigenstrain theory [4], still many questions remain open about the origin of such inelastic deformation. Starting from residual stress profile from literature of Si single crystal sample irradiated with Ga+ ion at low grazing angle, we extracted the underlying eigenstrain distribution using FEM modelling. The eigenstrain approach provides a powerful tool for the prediction of residual stress distributions in complex geometries [5]. Following, synchrotron X-ray Reflectivity (XRR) was then employed to interrogate the amorphous layer that arises under the surface of the material and compared with the results extracted from literature of an experiment using similar Ga-ion exposure specifications. Attention was principally paid to the density change experienced by the material, upon Ga irradiation, with the purpose of searching correlation with the underlying eigenstrain distribution and the density change profile extracted from literature Transmission Electron Microscopy (TEM) imaging analysis.
*Corresponding Author email:
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Methods and Material XRR technique is particularly suitable for the study of very thin layers (from few to tens nm), therefore suitable for this case-study [6-9]. In the present study the software GenX© was used to fit XRR profiles [10]. FIB milling was performed at the grazing angle of 10˚. This low angle is chosen to reflect the conditions experienced by the sample material during surface polishing milling [11]. A single crystal Silicon sample was exposed to the FIB at the energy of 30keV, current of 0.3 nA and ion dose of 2.0x109 ions/µm2 . The XRR experiment was conducted on the BM28 - XMaS beamline at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). X-ray beam energy was chosen to be 12.4keV. The beam size was set to 0.5mm in width (parallel to sample surface) and 0.2mm in height (in the direction close to the sample normal). The sample was mounted on a diffractometer and data collection of the reflection was performed using an APD KF16. In order to achieve adequate agreement between the fit and experimental data, the amorphous layer was thought to be composed from two layers distinct in terms of density (and hence refractive index).. The thickness of each sub-layer was considered to be a fitting parameter sought by the software as part of the refinement. Moreover, it was found that an additional surface layer had to be introduced to achieve the match that represented the SiOx phase arising from the exposure of Si amorphous layer to air.
Results and Discussion The fitting process led to the results illustrated in Figure 1(a). In order to show the details of the fitting, a close-up of the plot in Figure 1(a) is reported in Figure 1(b).
Figure 1. XRR profiles. (a) Experimental intensity profiles for Ga-ion treated samples (open symbols), and fitting using GenX (continuous line). (b) Magnified plot at low momentum transfer Q. The relevant parameters that produced the best fit are summarised in Table 1. For the sake of convenience, material density is expressed as % change from the pristine Si condition. Layer SiOx 1 st Si amorphous 2 nd Si amorphous
Thickness [nm] Density change ∆𝜌 2.8 19.5 -10.0% 3.7 1.7% Table 1. The abstracted layer properties.
Roughness [nm] 0.6 2.3 0.4
The overall thickness of the amorphous layer was found to be ~23 nm. To test the correctness of this conclusion, more complex analysis was carried out assuming a three-layer structure; however,
the results came out very similar, and a similar overall thickness of ~23nm provided the best fit; this consistent with the literature reports of the results obtained using TEM [12] and shown in Figure 2(b). Furthermore, there is agreement between this depth of amorphisation and the predictions of molecular dynamics simulations reported previously [4]. As seen in the previously reported experimental evidence shown in Figure 2(b) [12] from through-thickness TEM imaging, no sharp interfaces are present within the affected layer. This is a further indication that the interfaces are not atomically flat, but rather correspond to diffuse transitions. In fact, the 2nd modelled Si layer can be thought as the diffuse interface itself between amorphised material and the unaffected material lying beyond this depth. Further insight into this “fuzzy interface” transition region can be obtained by noting that the interface roughness between the 1st and the 2nd layer is comparable to the thickness of the 2nd layer itself. The resulting density variation obtained from the analysis suggests that the layer modified by the ion beam interaction is itself inhomogeneous, characterised by significant changes with depth within the ~23nm range from the surface. Figure 2(a) illustrates the reconstructed density change ∆𝜌 with respect to the density of the parent (pristine) material. In order to validate the results of the XRR experiment, we analysed a TEM image from the relevant literature [12]. In this study, a Silicon pillar of 200nm diameter was machined using FIB at low grazing angle with processing conditions similar to those of our experiment. Pixel intensity in TEM imaging depends on the local material thickness, and absorption that is related to material density. Therefore, quantitative information regarding the material density change can be extracted from TEM images. Using the image shown in Figure 2(b) [12], we performed the analysis of intensity variation along the x direction in the region delimited by a square box region shown in white. By quantifying the sample absorption, the plot shown in Figure 2(c) was obtained. Since the pillar sample had cylindrical shape, the absorption profile is affected by the geometric effect. Nonetheless, a peak of absorption was found in the region close to the interface between the amorphised layer and the unaffected parent material. This provides confirmation for the conclusion we drew from XRR analysis that material densification takes place in this region.
Figure 2. Density change analysis within the layer affected by the ion beam. (a) Relative density change within the layer affected by the ion beam obtained from XRR analysis. (b) The image of FIBmilled Si pillar obtained using TEM [12]. (c) Normalised absorption plot extracted by intensity integration over the y direction from the TEM image performed in the region indicated by the white square as a function of horizontal position x. It can be surmised that that process of ion interaction with the sample surface can be viewed in two stages. In Stage I, Ga ions travel through the material while losing some energy in the layer confined between 5-20nm; the decrease in density is observed (by up to 10%), corresponding to material swelling. Finally, in Stage II, at depths of ~20-25nm, Ga ions come to stop, and a mild increase in density is observed. In order to shed some light onto the correlation between eigenstrain and density change within the affected layer, we extracted the eigenstrain distribution from the residual elastic strain distribution provided by MD simulations [4]. As shown in this work, FEM modelling allows prescribing an eigenstrain profile based on the residual elastic strain obtained by MD simulation data. The curves for the residual elastic strain from MD, eigenstrain, and eigenstrain-based residual elastic strain prediction are shown in Figure 3(a). In Figure 3(b), the two trends (density change and eigenstrain) are compared with the purpose of establish a correlation.
Figure 3 (a) Simulated and fitted residual elastic strain compared with the obtained inverse eigenstrain profile. Here xx and yy strain components refer to the in-plane coordinates with respect to the sample surface. (b) Comparison of the density change (solid lin e) and eigenstrain profile (dashed curve). It is evident that the trend in the evaluated XRR density profile is correlated with the eigenstrain distribution provided by the MD simulation. This observation suggests that a relationship may be established between eigenstrain and the density change. However, it is important to note that this correlation cannot be simply one-to-one, as some aspects of material damage and deformation may be accommodated through other mechanisms that are not correlated with eigenstrain, e.g. (nano)voiding, fissuring, decohesion, etc. The swelling (density decrease) or densification (density increase) caused by FIB-induced damage is likely to be anisotropic, causing differential deformation in the in-plane and out-of-plane directions. Furthermore, volumetric change is not translated into stress-inducing eigenstrain in its entirety: some elements of inelastic deformation do not lead to residual stress generation. Also, the conversion of structural modification induced by radiation into eigenstrain is related to the changes in the material modulus.
Conclusions The experimental evidence collected and interpreted in this work are of considerable relevance to the task of advancing the understanding of the characteristics of the damaged layer produced by Gaion irradiation. The superficial layer of modified density in Si subjected to normal incidence Ga-ion beam milling was studied using XRR and found to be ~23nm in thickness, in good agreement with previous measurements and observations reported in the literature. The layer of modified properties could be further sub-divided into two layers, separated by diffuse interfaces, with different densities. An evident proportional correlation between the density change and the eigenstrain distribution was found. This results indicates that the arising of eigenstrain cannot be imputed mainly to the volume transformation occurring in the amorphous layer. Rather, other material processes are of more relevant importance (e.g. nano voiding, defect population and elastic properties modifications). Moreover, by analysing the TEM image from the literature, confirmatory evidence for material densification was found within the interfacial layer that was referred to as the “2nd layer” in our analysis. The result is therefore consistent with the experimentally based quantification of density change by XRR analysis.
Acknowledgements AMK acknowledges funding received for the MBLEM laboratory at Oxford through EU FP7 project iSTRESS (604646) and the EPSRC funded UK Materials Science National Facility (XMaS).
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