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New insights into subsurface imaging of carbon nanotubes in polymer composites via scanning electron microscopy
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 085703 (http://iopscience.iop.org/0957-4484/26/8/085703) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 26 (2015) 085703 (12pp)
doi:10.1088/0957-4484/26/8/085703
New insights into subsurface imaging of carbon nanotubes in polymer composites via scanning electron microscopy Minhua Zhao1,7, Bin Ming2, Jae-Woo Kim4, Luke J Gibbons6, Xiaohong Gu3, Tinh Nguyen3, Cheol Park4,5, Peter T Lillehei4, J S Villarrubia2, András E Vladár2 and J Alexander Liddle1 1
Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, USA 2 Physical Measurement Lab, National Institute of Standards and Technology, Gaithersburg, USA 3 Engineering Lab, National Institute of Standards and Technology, Gaithersburg, USA 4 NASA Langley Research Center, Hampton, USA 5 Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, USA 6 Department of Materials Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, USA 7 University of Maryland, Department of Materials Science and Engineering, College Park, USA E-mail: [email protected] and [email protected] Received 8 November 2014, revised 30 December 2014 Accepted for publication 13 January 2015 Published 4 February 2015 Abstract
Despite many studies of subsurface imaging of carbon nanotube (CNT)-polymer composites via scanning electron microscopy (SEM), significant controversy exists concerning the imaging depth and contrast mechanisms. We studied CNT-polyimide composites and, by threedimensional reconstructions of captured stereo-pair images, determined that the maximum SEM imaging depth was typically hundreds of nanometers. The contrast mechanisms were investigated over a broad range of beam accelerating voltages from 0.3 to 30 kV, and ascribed to modulation by embedded CNTs of the effective secondary electron (SE) emission yield at the polymer surface. This modulation of the SE yield is due to non-uniform surface potential distribution resulting from current flows due to leakage and electron beam induced current. The importance of an external electric field on SEM subsurface imaging was also demonstrated. The insights gained from this study can be generally applied to SEM nondestructive subsurface imaging of conducting nanostructures embedded in dielectric matrices such as graphene-polymer composites, silicon-based single electron transistors, high resolution SEM overlay metrology or e-beam lithography, and have significant implications in nanotechnology. S Online supplementary data available from stacks.iop.org/NANO/26/085703/mmedia Keywords: scanning electron microscopy, subsurface imaging, carbon nanotube polymer composites (Some figures may appear in colour only in the online journal) 1. Introduction
resolution subsurface imaging methods that can capture the three-dimensional (3D) material morphology. For example, understanding the dispersion of carbon nanotubes (CNTs) in a polymer matrix is critical for constructing the process-structure-properties relationships that enable the rational
With the increasing deployment of nanocomposite materials, whose properties depend on the state of dispersion of the reinforcing nanomaterial, there is a growing need for high0957-4484/15/085703+12$33.00
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subsurface CNTs originate from SEs emitted by the CNTs themselves or from the near-surface polymer matrix? (ii) Does the electron beam have to reach a given CNT for subsurface imaging? (iii) What is the maximum SEM imaging depth for CNT polymer composites? (iv) What is the role of SE recapture process for SEM subsurface imaging? The main purpose of the present work is to clarify these issues and help establish SEM subsurface imaging as a useful and facile method to provide quantitative information on nanocomposite microstructure. We present the results obtained by SEM subsurface imaging of CNTs embedded in polyimide composites using beam accelerating voltages in the range of 0.3–30 kV. While the CNT contrast is not only dependent on beam accelerating voltage, but also varies with beam current and dwell time per pixel, as defined by scanning speed and scanning area [12]. An exhaustive discussion of the effects of all these parameters is beyond the scope of this work. Here we will focus on the effect of beam accelerating voltage on the CNT contrast. The SEM subsurface imaging depth is determined by 3D reconstruction of captured stereo-pair images. The influence on subsurface imaging of a polymer-rich layer near the composite surface, revealed by cross-sectional SEM imaging, is discussed. The effect of an external electric field on the SEM contrast of CNT polymer composites is also studied.
improvement of CNT-nanocomposite performance. Electric force microscopy (EFM), a special type of scanning probe microscopy (SPM) based on long-range electrostatic interactions between a probe and a sample, has been used for subsurface imaging of CNTs dispersed in polymer composites [1–3]. However, the analysis of the results from this technique requires considerable care if quantitative information is to be extracted. While typically considered to be a surface characterization technique, scanning electron microscopy (SEM) has also been applied to provide a quantitative assessment of CNT dispersion in a polymer matrix [4]. Interestingly, an imaging depth up to 1000 nm was calculated using geometric relations under different stage tilting angles [4]. While it was generally agreed that the contrast of subsurface CNTs originated from variations in secondary electron (SE) yield due to charge contrast [4–17], direct SE emission from nanotubes much beyond a few 10 nm depth should not be detectable due to the shallow escape depth of SEs [18]. In general, the depth at which CNTs can be imaged by SEM in polymer composites is still a controversial topic, with estimates ranging from less than 50 nm [5], to hundreds of nanometers [6] or several micrometers [7]. In addition, the reason the contrast of CNTs embedded in polymer composites varies with the primary electron beam accelerating voltage is not well understood. SEM contrast here refers to the intensity difference of collected SE imaging signals, which is determined by both SE generation and collection conditions. The contrast formed by CNTs on insulator surfaces has been ascribed to electron beam induced current (EBIC) [19, 20] or local potential differences [21]. There are only a few studies of SEM subsurface imaging of CNTs in a polymer matrix based on charge-contrast imaging [4–7, 15–17]. Loos et al [7] tried to relate the contrast of CNTs with the depth they were located in composites: bright CNTs were closer to the surface than those appeared dark, while no quantitative measurement of CNT depth was attempted. Recently, Li et al [6, 17] did an extensive study on the imaging mechanism, examining the effect of depth and accelerating beam voltages on the visibility of CNTs in an epoxy matrix by SEM. They proposed that more electrons were trapped at the CNT–epoxy interface at high beam voltage resulting in enhanced SE emission at the CNT/epoxy interface which was responsible for the bright contrast of embedded CNTs, while the dark contrast of CNTs at low beam accelerating voltage was ascribed to lower SE emission at the interface than that of epoxy. On the other hand, Zhang et al reported SEM subsurface imaging of conductors embedded in an insulating film by a non-penetrating low-energy electron beam [13]. The contrast of embedded structures was attributed to the different SE recapture rate under the surface local electric field, which returns more SE to the surface when the local potential is high than when it is low. Hence, the effect of the SE collection process, such as the SE recapture process during SEM scanning, could be important for the interpretation of CNTs contrast, while it was not taken into consideration by Li et al [6, 17]. In summary, some open questions remain for SEM subsurface imaging of CNTs, including: (i) Does the signal of
2. Experiments8 Single-wall carbon nanotube (SWCNT)-polyimide nanocomposite films were prepared by in situ polymerization as described previously [22]. Two types of SWCNTs were incorporated into the composites: purified laser-ablated (LA) and high-pressure carbon monoxide decomposition (HiPco) SWCNTs. The polymer matrix was an electroactive polyimide, 2,6-bis(3-aminophenoxy) benzonitrile/4,4-oxydiphthalic anhydride ((β-CN)APB/ODPA). As-received anhydrous dimethyl formamide was used as a solvent. A series of SWCNT-polyimide nanocomposite films with SWCNT concentrations at mass fractions of (0, 0.05, 0.2, 0.5, 2 and 10)% were prepared as follows. The SWCNT-poly (amic acid) solution was cast onto a glass plate and dehydrated in a dry air-flowing chamber. Subsequently, the dried tack-free film was thermally imidized in a nitrogen-circulating oven to obtain a solvent-free SWCNT-polyimide film having a thickness between 25 and 65 μm. After removing the films from the glass plates, the smoother interface side (glass–film interface) was used for EFM and SEM imaging. This side of the film usually displayed a thin surface layer containing few CNTs due to the wall depletion effect [23]. The details of the EFM setup for subsurface imaging of CNTs in polymer films has been described previously [1]. 8
Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology or the National Aeronautics and Space Administration, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. 2
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Figure 1. EFM and SEM subsurface imaging at the same location of 0.5% SWCNT(LA)-PI composite. All the fields of view are 10 μm. (a) SPM height image, 50 nm full height scale. (b) EFM phase image, −3 V bias, 20° full phase scale. (c) SEM image at 15 kV beam accelerating voltage (d) SEM image at 1 kV beam accelerating voltage.
using conductive carbon tape. No additional sample treatment, such as coating with a conductive layer, was used. The beam accelerating voltage was varied between 0.3 and 30 kV, while the beam current was maintained at 43 pA. 3D SEM images were generated by taking stereo pairs at stage tilting angles of −5° and +5° using 3D reconstruction software Scandium (Soft Imaging System GmbH, Version 5.0, Build 1120). The sample was kept at the eucentric tilting position for a 4 mm working distance. A through-lens detector, situated inside the electron optical column, was used to collect the SE with high efficiency (supplemental figure S1 for proof of SE dominated CNT signal), while excluding most of the backscattered electrons signals. The SEM images had 1024 pixels and 768 pixels in the horizontal and vertical directions, respectively. Unless otherwise noted, a beam dwell time of 40 μs was used for each image pixel, while the sample current during scanning was recorded simultaneously using a picoammeter. The average values obtained for full-frame scanning were reported. Rectangular cross-sections on the CNT-polyimide nanocomposite were produced using FIB by the staircase method at a gallium (Ga) beam current of 97 pA.
Briefly, it is based on a two-pass scanning technique. During the first pass, the topographic image is acquired in normal tapping mode using an atomic force microscope (AFM). The second pass takes place with the conductive AFM probe raised above the sample to a fixed distance. The sample is rescanned with a bias voltage applied between the probe and AFM stage, following the previously-recorded topography to maintain a constant tip–sample separation. The dc bias voltage applied to the AFM probes ranged from −12 to +12 V and the sample surface-to-tip distance was maintained at 20 nm. Simultaneously, an ac bias voltage was applied to the AFM stage at the same frequency used to drive the AFM cantilever with a peak-to-peak amplitude of 2 V. The EFM imaging was carried out at low humidity (6 kV): bright CNT contrast. Both dark and bright CNT contrast was observed for beam voltage between 4 and 6 kV, which was ascribed to a mixed condition of Regions II and III. Interestingly, three
3. Results and discussions 3.1. Demonstration of SEM subsurface imaging of CNTs dispersed in a polymer composite
Figure 1 shows EFM and SEM subsurface images of CNTpolyimide composites taken at the same location, relative to the ‘X’ shaped scratch on the surface as a location mark. While the conventional SPM height image (figure 1(a)) did not show subsurface CNTs, both EFM (figure 1(b)) and SEM (figures 1(c)–(d)) revealed CNTs under the surface. Increasing the SEM acceleration voltage from 1 (figure 1(d)) to 15 kV (figure 1(c)), made more subsurface CNTs visible. In addition, the contrast of the CNTs inverted, going from dark to bright. Furthermore, it is important to note that the ‘X’ mark is still visible in the EFM image (figure 1(b)), while it is not visible (figure 1(c)) or hardly visible (figure 1(d)) in the SEM images. This indicates that the contrast of the SEM images is mainly dominated by subsurface features rather than surface topography. Furthermore, the features of subsurface CNTs revealed by EFM (figure 1(b)) and 1 kV SEM (figure 1(d)) is so similar that there could be a common 4
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Figure 3. (a) SEM contrast and imaging depth changes at different location of 0.5% SWCNT(LA)-PI film revealed by various beam voltages. Scale bar 5 μm. (b) Corresponding curve of sample current versus beam voltage, which is divided into three regions: Region I: E1 < E < E2; Region II: E2 < E < E3; Region III: E > E3. Error bars are one standard deviation of current measurement. (c) Schematic curve of total electron yield (σ) versus electron landing energy (E). Inset: schematic of a SEM beam interaction with a dielectric thin film attached to a conductive substrate. ρ+ is positive charge density very close to the top surface. ρ− is negative charge density in the electron beam interaction volume. t+ is the thickness of positive charge layer and t is the thickness of a dielectric thin film. R is the maximum depth of electron beam interaction. E3 is defined as the critical energy when the dielectric thin film charges positively (beginning when R(E3) is of the order of t).
corresponding regions can also be identified on the sample current versus accelerating voltage curves in figure 3(b), which were recorded simultaneously while capturing figure 3(a): (i) positive sample current for Region I, (ii) rapidly increasing negative sample current with beam accelerating voltage for Region II, and (iii) almost saturated sample current in Region III. Two critical energies E2 and E3 are marked in figure 3(b). According to the well-known total electron yield (σ) versus primary electron landing energy (E) curve as shown in figure 3(c), E2 is the secondary cross-over energy [26] when the surface charging changes from positive to negative, which is between 0.5 and 0.6 keV for polyimide [27]. For a thin dielectric thin film on a conducting substrate, a third critical energy [28], E3, occurs when the electron beam range R is comparable to or greater than the film thickness, t, resulting in positive charging when E > E3. A schematic of this concept is shown in the inset of figure 3(c). The surface potential (Vs) of a dielectric thin film attached to a grounded
substrate can be estimated by [29]
(
)
Vs ≈ ρ+ t+ + ρ− t /2 t / ε ,
(1)
where ρ+ is positive charge density due to SE escaping from the region very close to the top surface. It is approximated here as constant throughout the SE escape depth, t+. The negative charge density, ρ−, is likewise approximated as uniform through the entire dielectric film thickness, t. The dielectric constant of the film is ε. The approximation assumes t ≫ t+ and R(EPE) > t with R(E) the range at energy E and EPE the landing energy of the primary electrons. The thickness of the negative charge layer is ordinarily determined by the range, R(EPE), of the energetic primary electrons, which is much larger than the escape depth of the much less energetic SE [29]. However, when R(EPE) is greater than the film thickness, t, (as assumed in equation (1)) much of the beam penetrates into the conductive substrate, resulting in a lower total negative charge per unit area, ρ−t, while the 5
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However, its thickness is only 200–300 nm for the former (figure 5(a1)), but 2–3 μm for the latter (figure 5(b1)). Hence much higher beam energy is needed for the electrons to penetrate the thicker polymer-rich layer and be dissipated by the underlying dense, percolated CNT layer. In fact, the maximum electron beam range in polyimide derived from the Monte Carlo simulation is about 300 nm at 4 kV and 2 μm at 10 kV respectively, which is a good match to both the thickness of polymer-rich layer and E3 value for the two samples. 3.3. Estimation of imaging depth for SEM subsurface imaging
The maximum depth at which embedded CNTs can be distinguished from the surrounding polymer matrix, referred to as the imaging depth, is specific to the sample properties and imaging conditions. The previously reported imaging depth was derived indirectly by coating a known depth of polymer onto the surface of a CNT polymer composite [6]. Estimation of the imaging depth for a specific CNT nanocomposite based on geometric relations has also been previously reported [4]. In this study, the SEM imaging depth of SWCNTs embedded in a polyimide matrix was determined by 3D graphs reconstructed from SEM images captured at different tilt angles. Figures 6 and 7 show 3D images of LA and HiPCO SWCNTs embedded in a polyimide matrix respectively. The red arrows point to features that had a very intense contrast and were used as references for the film surfaces. The depth of LA– CNT marked by the blue arrow was measured to be 258 nm ± 28 nm in figure 6, while it was 728 nm ± 56 nm for the HiPco CNT in figure 7 (the uncertainty number was estimated from the pixel size and tilting angle from the stereopair images). Therefore, the subsurface imaging depth of SWCNTs embedded in a polyimide matrix is confirmed to be at least hundreds of nanometers, which is consistent with our previous results based on geometrical relations [4]. It was also revealed that the bundle size of LA–SWCNTs was much smaller than that of HiPco–SWCNTs, indicating a better dispersion of LA–SWCNTs in the polyimide matrix. The reason why a poorly dispersed sample will have a deeper imaging depth than a better dispersed sample can be accounted for by referring to CNT dispersions in figure 5. As shown in figure 5(a1), there are dense and uniform CNT networks located near the surface of the film for a better dispersed sample, this will result in a more uniform surface potential, which prevents detection of the more deeply embedded CNTs.
Figure 4. (a) Selected SEM images at different location of 0.5%
SWCNT(Hipco)-PI film revealed by various beam voltages. Scale bar 5 μm. (b) Corresponding curve of sample current versus beam voltage, which is divided into three regions: Region I: E1 < E < E2; Region II: E2 < E < E3; Region III: E > E3. Error bars are one standard deviation of current measurement.
positive charge per unit area, ρ+t+, is little changed. According to equation (1), when ρ−t gets small enough, either because the range gets large enough (making ρ− small) or the film thickness (t) gets small, there is a point at which Vs becomes positive. In fact, such results have been reported by both experiment [30] and simulation [31]. In the meantime, we observed a different E3 value for another type of SWCNT (Hipco)-PI film as shown in figure 4. The SEM contrast became mixed at 10 kV and changed to fully bright contrast at around 15 kV (only selected SEM images are shown in figure 4(a). The full sets of SEM images are included in supplemental figure S3). The overall progression of image contrast variation, with the similar Regions (I, II and III), can be identified in the sample current versus beam voltage curve shown in figure 4(b). So why is the E3 different for these two samples with the same polymer matrix and CNT loading? We can find the clues from the crosssectional SEM images and Monte Carlo simulations. There is a polymer-rich layer very close to the surface for both 0.5% SWCNT(LA)-PI film and 0.5% SWCNT(Hipco)-PI film.
3.4. Insight of SEM subsurface imaging from FIB crosssectional measurements
Cross-sectional measurements made on FIB-milled samples generated surprising results. The two interesting points shown in figure 8 were that (i) there were no embedded CNTs observable at the cross-section cut by the Ga-ion FIB; even at the side wall; (ii) there was an oval-shaped ring structure with bright contrast surrounding the rectangular FIB cut, under which few embedded CNTs were revealed. Both findings can 6
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Figure 5. Thickness of polymer-rich layer for 0.5% SWCNT(LA)-PI films and 0.5% SWCNT(Hipco) film revealed by SEM cross-sectional imaging of cryo-fractured surface (a1) 0.5% SWCNT (LA)-PI film. (b1) 0.5% SWCNT (Hipco)-PI film. Monte Carlo simulations of electron beam interaction depths in polyimide and compared to the thickness of the polymer-rich layer in (a1) and (b1) respectively. The scale bar is 100 nm for figures (a1)–(a4) and 1 μm for figures (b1)–(b4).
be accounted for if we hypothesize that Ga implantation leads to locally uniform surface potentials. It is well known that Ga ions are implanted into the surface during FIB processing [32]. Some of the Ga ions will also be scattered into the surrounding areas. That is why an oval-shaped ring structure was observed at the rectangular FIB cut. During FIB crosssection cutting, so many Ga ions were implanted that a conductive layer could form at the cross-section surface or nearby area, which would result in a uniform surface potential and thus prevent the observation of subsurface CNTs (see proof of Ga implantation in supplemental figure S4). We will show in the next section that the observed subsurface imaging and contrast reversals can be explained by a model in which the contrast is due to modulation of the surface potential by the presence or absence of subsurface CNTs. The disappearance of contrast FIB cross-section results can also be explained using the same model.
CNT contrast change with respect to the SEM accelerating voltage. In Region I (E1 < E < E2), the surface of the sample is positively charged. Although the primary electrons may not reach the top layer of CNTs, the positively charged surface could attract electrons from the top layer CNTs or underlying high-density CNT networks to the film surface through current leakage. By current leakage, we refer to the flow of charge in the insulator due to the very high (MV m−1) local electric fields created by charge injection and build up in the material. Such leakage is consistent with the observed positive sample current in figure 3(b). In particular, at the location just above the embedded CNTs (position B), the leakage current effect is much stronger than in the polymer matrix (position A) due to the proximity of CNTs to the surface. These provide a low-resistance path for electrons. Hence, the positive surface potential is lower at position B than at position A, as sketched in figure 9. Since some of the SEs will be attracted back to the surface by the positive surface potential as indicated by the recapture current (IRE) in figure 9, this results in a lower recapture current at position B than at position A. In the meantime, the detected SE signal
3.5. Contrast mechanism of SEM subsurface imaging
Based on these observations, a simplified 1D model, schematically illustrated in figure 9, is used to account for the 7
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Figure 6. (Top) Stereo image pairs of 0.5% SWCNT(LA)-PI film captured by 28 kV SEM at tilting angles of −5° and +5° respectively. The horizontal fields of view are 10.6 μm (the size of horizontal and vertical grid is 500 and 100 nm respectively). (Bottom) 3D SEM image of 0.5% SWCNT(LA)-PI film by reconstruction of stereo image pairs. The red arrow points to a CNT very close to the surface, while the blue one points to a deeply embedded CNT.
(IS) is given by IS = ISE − IRE = IP δ (1 − α ),
In Region II (E2 < E < E3), the electron beam is nonpenetrating since primary electrons do not have enough energy to reach the embedded conductive CNT networks. The electron-injected regions are negatively-charged in general, reversing the situation in Region I. Now electrons are the excess charge, and it is these that will be conducted away from the surface by the EBIC effect and current leakage as illustrated in figure 9, in agreement with the observed negative sample current shown in figures 3(b) and 4(b). Negative charges transporting between the electron-injected region and film substrate will be readily dissipated by near-surface CNTs (position B), causing a less negative surface potential at position B than at position A (polymer matrix without near-
(2)
where ISE is the emitted SE signal at the polyimide surface, IP is the primary beam current, δ is the SE yield and α is the SE recapture rate. Since the SE yield δ will be little changed from position A to B due to their slight potential difference, the detected SE signal IS will be stronger at position B because αB < αA. That is why embedded CNTs show positive contrast in Region I. The imaging contrast of subsurface CNTs arises from the different SE recapture rate at the polyimide surface due to non-uniform positive surface potential at positions with and without subsurface CNTs. 8
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Figure 7. (Top) Stereo image pairs of 0.5% SWCNT (Hipco)-PI film captured by 28 kV SEM at tilting angles of −5° and +5° respectively. The horizontal fields of view are 15 μm (the size of horizontal and vertical grid is 1000 and 500 nm respectively). (Bottom) 3D SEM image by reconstruction of stereo image pairs. The red arrow points to a feature at the surface, while the blue one points to a deeply embedded CNT.
surface CNT) as shown in figure 6. Since a negative surface potential will enhance SE emission according to voltage contrast SEM imaging [33, 34], the embedded CNTs have dark contrast. As to Region III (E > E3), the electron-injected regions are positively charged because the primary electron range is now sufficiently greater than the distance from the surface to the percolated CNT network as shown in figure 9. In the meantime, there is an electron leakage current flowing from the substrate to the surface due to the slightly positive surface potential. This current reduces the positive surface potential. Specifically, at position B when the
embedded CNT is within the electron beam, EBIC permits a larger neutralizing current. So the surface potential at position B will be less than that at position A as shown in figure 6. Similar to the case of Region I, this generates a bright CNT contrast in Region III during SEM subsurface imaging. On the other hand, the embedded CNTs within the percolated CNT networks might not be individually resolved if large numbers of CNTs are located at a depth similar to that of the electron-injected region. Furthermore, the high-density CNT networks will also prevent the electron beam from resolving other CNTs located below this layer. This is consistent with the observed different 9
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share of the image. This could be explained by considering the energy barrier between the sample and the detector. At 0 V bias, the barrier varied, with electrons that escaped from locations with low potential energy (high surface potential) facing a higher barrier than those that escaped from regions with higher potential energy (low surface potential). As discussed above, this was what gave rise to the observed contrast. When the substrate potential was raised by Vb, all electrons must overcome an additional barrier, eVb. When the total barrier (due to the charging-induced surface variation plus the imposed sample bias) became large enough, none of the electrons would have enough energy to overcome the barrier. Of course, those that originated from low potential energy regions (i.e., the darker parts of the image) reached this point first. Consequently, one expected the dark regions to spread as observed in the bottom row of figure 10. In this row the landing energy was such that the sample charged positively. Escaping electrons already had a barrier to overcome, even before the additional barrier imposed by the sample bias. In the upper row, the landing energy was such that the surface potential was negative when there was no additional imposed bias. Electrons that escaped the immediate vicinity of the surface accelerated away from the surface, arriving at the detector with excess energy. Addition of a small positive bias initially reduced this excess energy, but had no effect on electron recapture, because any excess was enough to avoid recapture. This is why there is little or no change in the upper row of figure 10. Changing the bias on sample holder is one way to change the extraction field. Of course, the opposite change on the other electrode is equivalent (since only differences in potential are important). The important role of the external electric field on the SEM contrast opens an opportunity to optimize charge-based SEM subsurface imaging by tuning the external electric field.
Figure 8. SEM cross-sectional observation of 0.5% LA–SWCNT/PI film after Ga FIB cutting. No subsurface CNTs are observed by SEM at FIB cut region observed from 52° tilting angle.
imaging depths for the two CNT-polymer composites in figures 6 and 7. Furthermore, the length and connectivity of CNTs may also play an important role in the above contrast mechanism. An isolated short CNT or a small group of connected CNTs with low capacitance would reach its steady-state charge on a time scale (i.e. charging time constant τ = RC) short compared to the measurement (i.e. pixel dwell time). It would cease to be a source or sink of charge capable of maintaining a surface potential differential during continuing e-beam exposure, so it would not be visible. CNTs that are part of a large connected network (e.g., connected to ground) can continuously conduct the charge required to maintain surface potential differences. This is partly manifested in figure 4(a): almost all the detectable embedded CNTs are curved and look like parts of round-shaped networks, which have larger capacitance and thus a longer time constant for charging. That is to say, not all CNTs located within the electron beam-sample interaction volume may be visible due to their limited charging-time constant. Similar observations were reported by Nagase et al, [35] where it was found that only an embedded nanoscale Si pattern connected to a large area Si plate could be detected by SEM. Compared to the un-connected nanoscale Si pattern, the connected one has much larger capacitance and thus a longer time constant for charging.
4. Conclusions Subsurface SEM imaging methods have been successfully applied to non-destructively characterize the dispersion of SWCNTs embedded in polyimide composites. Contrast was investigated over a broad range of beam accelerating voltages from 0.3 to 30 kV and was ascribed to modulation of the effective SE emission yield (taking into account SE recapture) at the polymer surface because of non-uniform surface potential. The non-uniform potential was due to varying proximity to buried conducting CNTs, to which charge flows through leakage or EBIC. Furthermore, the electron beam might only reveal part of the CNT networks within the beam range. The maximum SEM subsurface imaging depth for CNT polymer composites is dependent on beam voltages and the dispersion of CNTs in polymer, with greater imaging depths for a given CNT loading in samples with poorer dispersion. SEM subsurface imaging can also be tuned by applying an external electric field. Finally, the application of these techniques is not limited to the subsurface imaging of CNT-polymer composites. They are generally applicable to conducting nanostructures embedded in a dielectric matrix,
3.6. Effect of external electric field on SEM subsurface imaging of CNT-polymer composites
The effect of an external sample bias on the SEM contrast of CNT-polymer composites is illustrated in figure 10. A dc bias from 2 to 6 V was applied to the SEM sample stage during scanning. The results for a 2 kV accelerating voltage indicated that there was little change in the contrast or brightness with the variation of bias, as shown in the top row of figure 10. On the other hand, image changes were more pronounced with increasing sample bias at 8 kV accelerating voltage, as shown in the bottom row of figure 10. The average brightness decreased as the bias increased, but rather than uniformly decreasing, the darker areas growed, engulfing an increasing 10
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Figure 9. Schematic of contrast mechanisms for CNTs embedded in polymer composites at various beam accelerating voltages. IP: primary beam current, IS: detected SE signal, and IRE is the recapture current caused by the local electric field. Point A is the position without embedded CNTs in the polymer rich layer. Point B is the position with embedded CNTs. EBIC current is represented by solid lines, while leakage current is depicted by dotted lines.
Figure 10. Stage bias effect on SEM contrast of 0.5% SWCNT(LA)-PI film at different beam accelerating voltages. Top row (a)–(c): 2 kV
beam accelerating voltage. Bottom row (d)–(f): 8 kV beam accelerating voltage.
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such as graphene polymer composites [36] or integrated circuit conductors covered by a dielectric layer. This may have significant implications in non-destructive imaging of silicon nanostructures embedded in silicon oxide for single electron transistors [35], high resolution SEM overlay metrology [37] or e-beam lithography [30], opening up a broad range of applications in nanotechnology. Further optimization of these techniques through experiment and Monte Carlo simulation is underway.
[14] Jbara O, Fakhfakh S, Belhaj M and Rondot S 2004 Microsc. Microanal. 10 697–710 [15] Jesse S, Guillorn M A, Ivanov I N, Puretzky A A, Howe J Y, Britt P F and Geohegan D B 2006 Appl. Phys. Lett. 89 013114 [16] Misiego C R and Pipes R B 2013 Compos. Sci. Technol. 85 43–9 [17] Li W J and Bauhofer W 2011 Carbon 49 3891–8 [18] Goldstein J, Newbury D, Joy D, Lyman C, Echlin P, Lifshin Sawyer L and Michael J 2003 Scanning Electron Microscopy and X-Ray Microanalysis 3rd edn (New York: Springer) pp 91–3 [19] Homma Y, Suzuki S, Kobayashi Y and Nagase M 2004 Appl. Phys. Lett. 84 1750–2 [20] Zhou Y S, Yi K J, Mahjouri-Samani M, Xiong W, Lu Y F and Liou S H 2009 Appl. Surf. Sci. 255 4341–6 [21] Brintlinger T, Chen Y F, Durkop T, Cobas E and Fuhrer M S 2002 Appl. Phys. Lett. 81 2454–6 [22] Park C, Ounaies Z, Watson K A, Crooks R E, Smith J, Lowther S E, Connell J W, Siochi E J, Harrison J S and Clair T L S 2002 Chem. Phys. Lett. 364 303 [23] Park C, Kang J H, Harrison J S, Costen R C and Lowther S E 2008 Adv. Mater. 20 2074 [24] Martin J W, Embree E and VanLandingham M R 2002 US Patent, US 6.490.913 B1 [25] Drouin D, Couture A R, Joly D, Tastet X, Aimez V and Gauvin R 2007 Scanning 29 92–101 [26] Joy D C and Joy C S 1999 Micros. Microanal. 4 475–80 [27] Nitta K, Hagiwara Y, Takeda Y, Takahashi M and Miyazaki E 2010 24th Int. Symp. on Discharges and Electrical Insulation in Vacuum (Braunschweig) pp 552–5 [28] Reimer L, Golla U, Bongeler R, Kassens M, Schindler B and Senkel R 1992 Optik 92 14–22 [29] Cazaux J 1986 J. Appl. Phys. 59 1418–30 Our equation (1) corresponds to this reference’s equation (28), albeit with correction of a factor of 2 typographical error on the negative charge density term [30] Liu W, Ingino J and Pease R F 1995 J. Vac. Sci. Technol. B 13 1979–83 [31] Li W Q and Zhang H B 2010 Micron 41 416–22 [32] Cooper D, Ailliot C, Barnes J P, Hartmann J M, Salles P, Benassayag G and Dunin-Borkowski R E 2010 Ultramicroscopy 110 383–9 [33] Rosenkranz R 2011 J. Mater. Sci., Mater. Electron. 22 1523–35 [34] Vijayaraghavan A, Marquardt C W, Dehm S, Hennrich F and Krupke R 2010 Carbon 48 494–500 [35] Nagase M, Fujiwara A, Kurihara K and Namatsu H 2003 Japan. J. Appl. Phys. 42 318–25 [36] Bernard C, Nguyen T, Pellegrin B, Holbrook R D, Zhao M H and Chin J 2011 J. Phys.: Conf. Ser. 304 012063 [37] Koike T, Ikeda T, Abe H and Komatsu F 1999 Japan. J. Appl. Phys. 38 7159–63
Acknowledgments The work of Dr Minhua Zhao was supported by NIST-ARRA senior fellowship award in measurement science and technology and Cooperative Research Program in Nanoscience and Technology between the University of Maryland and NIST. Luke Gibbons and Cheol Park acknowledge support by National Science Foundation CMMI-0928839 in part.
References [1] Zhao M H, Gu X H, Lowther S E, Park C, Jean Y C and Nguyen T 2010 Nanotechnology 21 225702 [2] Cadena M J, Misiego R, Smith K C, Avila A, Pipes B, Reifenberger R and Raman A 2013 Nanotechnology 24 135706 [3] Thompson H T, Barroso-Bujans F, Herrero J G, Reifenberger R and Raman A 2013 Nanotechnology 23 135701 [4] Lillehei P T, Kim J W, Gibbons L J and Park C 2009 Nanotechnology 20 325708 [5] Kovacs J Z, Andresen K, Pauls J R, Garcia C P, Schossig M, Schulte K and Bauhofer W 2007 Carbon 45 1279–88 [6] Li W J, Buschhorn S T, Schulte K and Bauhofer W 2011 Carbon 49 1955–64 [7] Loos J, Alexeev A, Grossiord N, Koning C E and Regev O 2005 Ultramicroscopy 104 160–7 [8] Chung K T, Reisner J H and Campbell E R 1983 J. Appl. Phys. 54 6099–112 [9] Zhang R Y, Wei Y, Nagahara L A, Amlani I and Tsui R K Nanotechnology 2006 17 272–6 [10] Ura K 1998 J. Electron Microsc. 47 143–7 [11] Ura K and Aoyagi S 2000 J. Electron Microsc. 49 157–62 [12] Feng R J, Zhang H B and Ura K 2003 J. Electron Microsc. 52 455–8 [13] Zhang H B, Feng R J and Ura K 2004 Sci. Prog. 87 249–68
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New insights into subsurface imaging of carbon nanotubes in polymer composites via scanning electron microscopy
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 085703 (http://iopscience.iop.org/0957-4484/26/8/085703) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 26 (2015) 085703 (12pp)
doi:10.1088/0957-4484/26/8/085703
New insights into subsurface imaging of carbon nanotubes in polymer composites via scanning electron microscopy Minhua Zhao1,7, Bin Ming2, Jae-Woo Kim4, Luke J Gibbons6, Xiaohong Gu3, Tinh Nguyen3, Cheol Park4,5, Peter T Lillehei4, J S Villarrubia2, András E Vladár2 and J Alexander Liddle1 1
Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, USA 2 Physical Measurement Lab, National Institute of Standards and Technology, Gaithersburg, USA 3 Engineering Lab, National Institute of Standards and Technology, Gaithersburg, USA 4 NASA Langley Research Center, Hampton, USA 5 Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, USA 6 Department of Materials Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, USA 7 University of Maryland, Department of Materials Science and Engineering, College Park, USA E-mail: [email protected] and [email protected] Received 8 November 2014, revised 30 December 2014 Accepted for publication 13 January 2015 Published 4 February 2015 Abstract
Despite many studies of subsurface imaging of carbon nanotube (CNT)-polymer composites via scanning electron microscopy (SEM), significant controversy exists concerning the imaging depth and contrast mechanisms. We studied CNT-polyimide composites and, by threedimensional reconstructions of captured stereo-pair images, determined that the maximum SEM imaging depth was typically hundreds of nanometers. The contrast mechanisms were investigated over a broad range of beam accelerating voltages from 0.3 to 30 kV, and ascribed to modulation by embedded CNTs of the effective secondary electron (SE) emission yield at the polymer surface. This modulation of the SE yield is due to non-uniform surface potential distribution resulting from current flows due to leakage and electron beam induced current. The importance of an external electric field on SEM subsurface imaging was also demonstrated. The insights gained from this study can be generally applied to SEM nondestructive subsurface imaging of conducting nanostructures embedded in dielectric matrices such as graphene-polymer composites, silicon-based single electron transistors, high resolution SEM overlay metrology or e-beam lithography, and have significant implications in nanotechnology. S Online supplementary data available from stacks.iop.org/NANO/26/085703/mmedia Keywords: scanning electron microscopy, subsurface imaging, carbon nanotube polymer composites (Some figures may appear in colour only in the online journal) 1. Introduction
resolution subsurface imaging methods that can capture the three-dimensional (3D) material morphology. For example, understanding the dispersion of carbon nanotubes (CNTs) in a polymer matrix is critical for constructing the process-structure-properties relationships that enable the rational
With the increasing deployment of nanocomposite materials, whose properties depend on the state of dispersion of the reinforcing nanomaterial, there is a growing need for high0957-4484/15/085703+12$33.00
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subsurface CNTs originate from SEs emitted by the CNTs themselves or from the near-surface polymer matrix? (ii) Does the electron beam have to reach a given CNT for subsurface imaging? (iii) What is the maximum SEM imaging depth for CNT polymer composites? (iv) What is the role of SE recapture process for SEM subsurface imaging? The main purpose of the present work is to clarify these issues and help establish SEM subsurface imaging as a useful and facile method to provide quantitative information on nanocomposite microstructure. We present the results obtained by SEM subsurface imaging of CNTs embedded in polyimide composites using beam accelerating voltages in the range of 0.3–30 kV. While the CNT contrast is not only dependent on beam accelerating voltage, but also varies with beam current and dwell time per pixel, as defined by scanning speed and scanning area [12]. An exhaustive discussion of the effects of all these parameters is beyond the scope of this work. Here we will focus on the effect of beam accelerating voltage on the CNT contrast. The SEM subsurface imaging depth is determined by 3D reconstruction of captured stereo-pair images. The influence on subsurface imaging of a polymer-rich layer near the composite surface, revealed by cross-sectional SEM imaging, is discussed. The effect of an external electric field on the SEM contrast of CNT polymer composites is also studied.
improvement of CNT-nanocomposite performance. Electric force microscopy (EFM), a special type of scanning probe microscopy (SPM) based on long-range electrostatic interactions between a probe and a sample, has been used for subsurface imaging of CNTs dispersed in polymer composites [1–3]. However, the analysis of the results from this technique requires considerable care if quantitative information is to be extracted. While typically considered to be a surface characterization technique, scanning electron microscopy (SEM) has also been applied to provide a quantitative assessment of CNT dispersion in a polymer matrix [4]. Interestingly, an imaging depth up to 1000 nm was calculated using geometric relations under different stage tilting angles [4]. While it was generally agreed that the contrast of subsurface CNTs originated from variations in secondary electron (SE) yield due to charge contrast [4–17], direct SE emission from nanotubes much beyond a few 10 nm depth should not be detectable due to the shallow escape depth of SEs [18]. In general, the depth at which CNTs can be imaged by SEM in polymer composites is still a controversial topic, with estimates ranging from less than 50 nm [5], to hundreds of nanometers [6] or several micrometers [7]. In addition, the reason the contrast of CNTs embedded in polymer composites varies with the primary electron beam accelerating voltage is not well understood. SEM contrast here refers to the intensity difference of collected SE imaging signals, which is determined by both SE generation and collection conditions. The contrast formed by CNTs on insulator surfaces has been ascribed to electron beam induced current (EBIC) [19, 20] or local potential differences [21]. There are only a few studies of SEM subsurface imaging of CNTs in a polymer matrix based on charge-contrast imaging [4–7, 15–17]. Loos et al [7] tried to relate the contrast of CNTs with the depth they were located in composites: bright CNTs were closer to the surface than those appeared dark, while no quantitative measurement of CNT depth was attempted. Recently, Li et al [6, 17] did an extensive study on the imaging mechanism, examining the effect of depth and accelerating beam voltages on the visibility of CNTs in an epoxy matrix by SEM. They proposed that more electrons were trapped at the CNT–epoxy interface at high beam voltage resulting in enhanced SE emission at the CNT/epoxy interface which was responsible for the bright contrast of embedded CNTs, while the dark contrast of CNTs at low beam accelerating voltage was ascribed to lower SE emission at the interface than that of epoxy. On the other hand, Zhang et al reported SEM subsurface imaging of conductors embedded in an insulating film by a non-penetrating low-energy electron beam [13]. The contrast of embedded structures was attributed to the different SE recapture rate under the surface local electric field, which returns more SE to the surface when the local potential is high than when it is low. Hence, the effect of the SE collection process, such as the SE recapture process during SEM scanning, could be important for the interpretation of CNTs contrast, while it was not taken into consideration by Li et al [6, 17]. In summary, some open questions remain for SEM subsurface imaging of CNTs, including: (i) Does the signal of
2. Experiments8 Single-wall carbon nanotube (SWCNT)-polyimide nanocomposite films were prepared by in situ polymerization as described previously [22]. Two types of SWCNTs were incorporated into the composites: purified laser-ablated (LA) and high-pressure carbon monoxide decomposition (HiPco) SWCNTs. The polymer matrix was an electroactive polyimide, 2,6-bis(3-aminophenoxy) benzonitrile/4,4-oxydiphthalic anhydride ((β-CN)APB/ODPA). As-received anhydrous dimethyl formamide was used as a solvent. A series of SWCNT-polyimide nanocomposite films with SWCNT concentrations at mass fractions of (0, 0.05, 0.2, 0.5, 2 and 10)% were prepared as follows. The SWCNT-poly (amic acid) solution was cast onto a glass plate and dehydrated in a dry air-flowing chamber. Subsequently, the dried tack-free film was thermally imidized in a nitrogen-circulating oven to obtain a solvent-free SWCNT-polyimide film having a thickness between 25 and 65 μm. After removing the films from the glass plates, the smoother interface side (glass–film interface) was used for EFM and SEM imaging. This side of the film usually displayed a thin surface layer containing few CNTs due to the wall depletion effect [23]. The details of the EFM setup for subsurface imaging of CNTs in polymer films has been described previously [1]. 8
Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology or the National Aeronautics and Space Administration, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. 2
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Figure 1. EFM and SEM subsurface imaging at the same location of 0.5% SWCNT(LA)-PI composite. All the fields of view are 10 μm. (a) SPM height image, 50 nm full height scale. (b) EFM phase image, −3 V bias, 20° full phase scale. (c) SEM image at 15 kV beam accelerating voltage (d) SEM image at 1 kV beam accelerating voltage.
using conductive carbon tape. No additional sample treatment, such as coating with a conductive layer, was used. The beam accelerating voltage was varied between 0.3 and 30 kV, while the beam current was maintained at 43 pA. 3D SEM images were generated by taking stereo pairs at stage tilting angles of −5° and +5° using 3D reconstruction software Scandium (Soft Imaging System GmbH, Version 5.0, Build 1120). The sample was kept at the eucentric tilting position for a 4 mm working distance. A through-lens detector, situated inside the electron optical column, was used to collect the SE with high efficiency (supplemental figure S1 for proof of SE dominated CNT signal), while excluding most of the backscattered electrons signals. The SEM images had 1024 pixels and 768 pixels in the horizontal and vertical directions, respectively. Unless otherwise noted, a beam dwell time of 40 μs was used for each image pixel, while the sample current during scanning was recorded simultaneously using a picoammeter. The average values obtained for full-frame scanning were reported. Rectangular cross-sections on the CNT-polyimide nanocomposite were produced using FIB by the staircase method at a gallium (Ga) beam current of 97 pA.
Briefly, it is based on a two-pass scanning technique. During the first pass, the topographic image is acquired in normal tapping mode using an atomic force microscope (AFM). The second pass takes place with the conductive AFM probe raised above the sample to a fixed distance. The sample is rescanned with a bias voltage applied between the probe and AFM stage, following the previously-recorded topography to maintain a constant tip–sample separation. The dc bias voltage applied to the AFM probes ranged from −12 to +12 V and the sample surface-to-tip distance was maintained at 20 nm. Simultaneously, an ac bias voltage was applied to the AFM stage at the same frequency used to drive the AFM cantilever with a peak-to-peak amplitude of 2 V. The EFM imaging was carried out at low humidity (6 kV): bright CNT contrast. Both dark and bright CNT contrast was observed for beam voltage between 4 and 6 kV, which was ascribed to a mixed condition of Regions II and III. Interestingly, three
3. Results and discussions 3.1. Demonstration of SEM subsurface imaging of CNTs dispersed in a polymer composite
Figure 1 shows EFM and SEM subsurface images of CNTpolyimide composites taken at the same location, relative to the ‘X’ shaped scratch on the surface as a location mark. While the conventional SPM height image (figure 1(a)) did not show subsurface CNTs, both EFM (figure 1(b)) and SEM (figures 1(c)–(d)) revealed CNTs under the surface. Increasing the SEM acceleration voltage from 1 (figure 1(d)) to 15 kV (figure 1(c)), made more subsurface CNTs visible. In addition, the contrast of the CNTs inverted, going from dark to bright. Furthermore, it is important to note that the ‘X’ mark is still visible in the EFM image (figure 1(b)), while it is not visible (figure 1(c)) or hardly visible (figure 1(d)) in the SEM images. This indicates that the contrast of the SEM images is mainly dominated by subsurface features rather than surface topography. Furthermore, the features of subsurface CNTs revealed by EFM (figure 1(b)) and 1 kV SEM (figure 1(d)) is so similar that there could be a common 4
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Figure 3. (a) SEM contrast and imaging depth changes at different location of 0.5% SWCNT(LA)-PI film revealed by various beam voltages. Scale bar 5 μm. (b) Corresponding curve of sample current versus beam voltage, which is divided into three regions: Region I: E1 < E < E2; Region II: E2 < E < E3; Region III: E > E3. Error bars are one standard deviation of current measurement. (c) Schematic curve of total electron yield (σ) versus electron landing energy (E). Inset: schematic of a SEM beam interaction with a dielectric thin film attached to a conductive substrate. ρ+ is positive charge density very close to the top surface. ρ− is negative charge density in the electron beam interaction volume. t+ is the thickness of positive charge layer and t is the thickness of a dielectric thin film. R is the maximum depth of electron beam interaction. E3 is defined as the critical energy when the dielectric thin film charges positively (beginning when R(E3) is of the order of t).
corresponding regions can also be identified on the sample current versus accelerating voltage curves in figure 3(b), which were recorded simultaneously while capturing figure 3(a): (i) positive sample current for Region I, (ii) rapidly increasing negative sample current with beam accelerating voltage for Region II, and (iii) almost saturated sample current in Region III. Two critical energies E2 and E3 are marked in figure 3(b). According to the well-known total electron yield (σ) versus primary electron landing energy (E) curve as shown in figure 3(c), E2 is the secondary cross-over energy [26] when the surface charging changes from positive to negative, which is between 0.5 and 0.6 keV for polyimide [27]. For a thin dielectric thin film on a conducting substrate, a third critical energy [28], E3, occurs when the electron beam range R is comparable to or greater than the film thickness, t, resulting in positive charging when E > E3. A schematic of this concept is shown in the inset of figure 3(c). The surface potential (Vs) of a dielectric thin film attached to a grounded
substrate can be estimated by [29]
(
)
Vs ≈ ρ+ t+ + ρ− t /2 t / ε ,
(1)
where ρ+ is positive charge density due to SE escaping from the region very close to the top surface. It is approximated here as constant throughout the SE escape depth, t+. The negative charge density, ρ−, is likewise approximated as uniform through the entire dielectric film thickness, t. The dielectric constant of the film is ε. The approximation assumes t ≫ t+ and R(EPE) > t with R(E) the range at energy E and EPE the landing energy of the primary electrons. The thickness of the negative charge layer is ordinarily determined by the range, R(EPE), of the energetic primary electrons, which is much larger than the escape depth of the much less energetic SE [29]. However, when R(EPE) is greater than the film thickness, t, (as assumed in equation (1)) much of the beam penetrates into the conductive substrate, resulting in a lower total negative charge per unit area, ρ−t, while the 5
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However, its thickness is only 200–300 nm for the former (figure 5(a1)), but 2–3 μm for the latter (figure 5(b1)). Hence much higher beam energy is needed for the electrons to penetrate the thicker polymer-rich layer and be dissipated by the underlying dense, percolated CNT layer. In fact, the maximum electron beam range in polyimide derived from the Monte Carlo simulation is about 300 nm at 4 kV and 2 μm at 10 kV respectively, which is a good match to both the thickness of polymer-rich layer and E3 value for the two samples. 3.3. Estimation of imaging depth for SEM subsurface imaging
The maximum depth at which embedded CNTs can be distinguished from the surrounding polymer matrix, referred to as the imaging depth, is specific to the sample properties and imaging conditions. The previously reported imaging depth was derived indirectly by coating a known depth of polymer onto the surface of a CNT polymer composite [6]. Estimation of the imaging depth for a specific CNT nanocomposite based on geometric relations has also been previously reported [4]. In this study, the SEM imaging depth of SWCNTs embedded in a polyimide matrix was determined by 3D graphs reconstructed from SEM images captured at different tilt angles. Figures 6 and 7 show 3D images of LA and HiPCO SWCNTs embedded in a polyimide matrix respectively. The red arrows point to features that had a very intense contrast and were used as references for the film surfaces. The depth of LA– CNT marked by the blue arrow was measured to be 258 nm ± 28 nm in figure 6, while it was 728 nm ± 56 nm for the HiPco CNT in figure 7 (the uncertainty number was estimated from the pixel size and tilting angle from the stereopair images). Therefore, the subsurface imaging depth of SWCNTs embedded in a polyimide matrix is confirmed to be at least hundreds of nanometers, which is consistent with our previous results based on geometrical relations [4]. It was also revealed that the bundle size of LA–SWCNTs was much smaller than that of HiPco–SWCNTs, indicating a better dispersion of LA–SWCNTs in the polyimide matrix. The reason why a poorly dispersed sample will have a deeper imaging depth than a better dispersed sample can be accounted for by referring to CNT dispersions in figure 5. As shown in figure 5(a1), there are dense and uniform CNT networks located near the surface of the film for a better dispersed sample, this will result in a more uniform surface potential, which prevents detection of the more deeply embedded CNTs.
Figure 4. (a) Selected SEM images at different location of 0.5%
SWCNT(Hipco)-PI film revealed by various beam voltages. Scale bar 5 μm. (b) Corresponding curve of sample current versus beam voltage, which is divided into three regions: Region I: E1 < E < E2; Region II: E2 < E < E3; Region III: E > E3. Error bars are one standard deviation of current measurement.
positive charge per unit area, ρ+t+, is little changed. According to equation (1), when ρ−t gets small enough, either because the range gets large enough (making ρ− small) or the film thickness (t) gets small, there is a point at which Vs becomes positive. In fact, such results have been reported by both experiment [30] and simulation [31]. In the meantime, we observed a different E3 value for another type of SWCNT (Hipco)-PI film as shown in figure 4. The SEM contrast became mixed at 10 kV and changed to fully bright contrast at around 15 kV (only selected SEM images are shown in figure 4(a). The full sets of SEM images are included in supplemental figure S3). The overall progression of image contrast variation, with the similar Regions (I, II and III), can be identified in the sample current versus beam voltage curve shown in figure 4(b). So why is the E3 different for these two samples with the same polymer matrix and CNT loading? We can find the clues from the crosssectional SEM images and Monte Carlo simulations. There is a polymer-rich layer very close to the surface for both 0.5% SWCNT(LA)-PI film and 0.5% SWCNT(Hipco)-PI film.
3.4. Insight of SEM subsurface imaging from FIB crosssectional measurements
Cross-sectional measurements made on FIB-milled samples generated surprising results. The two interesting points shown in figure 8 were that (i) there were no embedded CNTs observable at the cross-section cut by the Ga-ion FIB; even at the side wall; (ii) there was an oval-shaped ring structure with bright contrast surrounding the rectangular FIB cut, under which few embedded CNTs were revealed. Both findings can 6
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Figure 5. Thickness of polymer-rich layer for 0.5% SWCNT(LA)-PI films and 0.5% SWCNT(Hipco) film revealed by SEM cross-sectional imaging of cryo-fractured surface (a1) 0.5% SWCNT (LA)-PI film. (b1) 0.5% SWCNT (Hipco)-PI film. Monte Carlo simulations of electron beam interaction depths in polyimide and compared to the thickness of the polymer-rich layer in (a1) and (b1) respectively. The scale bar is 100 nm for figures (a1)–(a4) and 1 μm for figures (b1)–(b4).
be accounted for if we hypothesize that Ga implantation leads to locally uniform surface potentials. It is well known that Ga ions are implanted into the surface during FIB processing [32]. Some of the Ga ions will also be scattered into the surrounding areas. That is why an oval-shaped ring structure was observed at the rectangular FIB cut. During FIB crosssection cutting, so many Ga ions were implanted that a conductive layer could form at the cross-section surface or nearby area, which would result in a uniform surface potential and thus prevent the observation of subsurface CNTs (see proof of Ga implantation in supplemental figure S4). We will show in the next section that the observed subsurface imaging and contrast reversals can be explained by a model in which the contrast is due to modulation of the surface potential by the presence or absence of subsurface CNTs. The disappearance of contrast FIB cross-section results can also be explained using the same model.
CNT contrast change with respect to the SEM accelerating voltage. In Region I (E1 < E < E2), the surface of the sample is positively charged. Although the primary electrons may not reach the top layer of CNTs, the positively charged surface could attract electrons from the top layer CNTs or underlying high-density CNT networks to the film surface through current leakage. By current leakage, we refer to the flow of charge in the insulator due to the very high (MV m−1) local electric fields created by charge injection and build up in the material. Such leakage is consistent with the observed positive sample current in figure 3(b). In particular, at the location just above the embedded CNTs (position B), the leakage current effect is much stronger than in the polymer matrix (position A) due to the proximity of CNTs to the surface. These provide a low-resistance path for electrons. Hence, the positive surface potential is lower at position B than at position A, as sketched in figure 9. Since some of the SEs will be attracted back to the surface by the positive surface potential as indicated by the recapture current (IRE) in figure 9, this results in a lower recapture current at position B than at position A. In the meantime, the detected SE signal
3.5. Contrast mechanism of SEM subsurface imaging
Based on these observations, a simplified 1D model, schematically illustrated in figure 9, is used to account for the 7
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Figure 6. (Top) Stereo image pairs of 0.5% SWCNT(LA)-PI film captured by 28 kV SEM at tilting angles of −5° and +5° respectively. The horizontal fields of view are 10.6 μm (the size of horizontal and vertical grid is 500 and 100 nm respectively). (Bottom) 3D SEM image of 0.5% SWCNT(LA)-PI film by reconstruction of stereo image pairs. The red arrow points to a CNT very close to the surface, while the blue one points to a deeply embedded CNT.
(IS) is given by IS = ISE − IRE = IP δ (1 − α ),
In Region II (E2 < E < E3), the electron beam is nonpenetrating since primary electrons do not have enough energy to reach the embedded conductive CNT networks. The electron-injected regions are negatively-charged in general, reversing the situation in Region I. Now electrons are the excess charge, and it is these that will be conducted away from the surface by the EBIC effect and current leakage as illustrated in figure 9, in agreement with the observed negative sample current shown in figures 3(b) and 4(b). Negative charges transporting between the electron-injected region and film substrate will be readily dissipated by near-surface CNTs (position B), causing a less negative surface potential at position B than at position A (polymer matrix without near-
(2)
where ISE is the emitted SE signal at the polyimide surface, IP is the primary beam current, δ is the SE yield and α is the SE recapture rate. Since the SE yield δ will be little changed from position A to B due to their slight potential difference, the detected SE signal IS will be stronger at position B because αB < αA. That is why embedded CNTs show positive contrast in Region I. The imaging contrast of subsurface CNTs arises from the different SE recapture rate at the polyimide surface due to non-uniform positive surface potential at positions with and without subsurface CNTs. 8
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Figure 7. (Top) Stereo image pairs of 0.5% SWCNT (Hipco)-PI film captured by 28 kV SEM at tilting angles of −5° and +5° respectively. The horizontal fields of view are 15 μm (the size of horizontal and vertical grid is 1000 and 500 nm respectively). (Bottom) 3D SEM image by reconstruction of stereo image pairs. The red arrow points to a feature at the surface, while the blue one points to a deeply embedded CNT.
surface CNT) as shown in figure 6. Since a negative surface potential will enhance SE emission according to voltage contrast SEM imaging [33, 34], the embedded CNTs have dark contrast. As to Region III (E > E3), the electron-injected regions are positively charged because the primary electron range is now sufficiently greater than the distance from the surface to the percolated CNT network as shown in figure 9. In the meantime, there is an electron leakage current flowing from the substrate to the surface due to the slightly positive surface potential. This current reduces the positive surface potential. Specifically, at position B when the
embedded CNT is within the electron beam, EBIC permits a larger neutralizing current. So the surface potential at position B will be less than that at position A as shown in figure 6. Similar to the case of Region I, this generates a bright CNT contrast in Region III during SEM subsurface imaging. On the other hand, the embedded CNTs within the percolated CNT networks might not be individually resolved if large numbers of CNTs are located at a depth similar to that of the electron-injected region. Furthermore, the high-density CNT networks will also prevent the electron beam from resolving other CNTs located below this layer. This is consistent with the observed different 9
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share of the image. This could be explained by considering the energy barrier between the sample and the detector. At 0 V bias, the barrier varied, with electrons that escaped from locations with low potential energy (high surface potential) facing a higher barrier than those that escaped from regions with higher potential energy (low surface potential). As discussed above, this was what gave rise to the observed contrast. When the substrate potential was raised by Vb, all electrons must overcome an additional barrier, eVb. When the total barrier (due to the charging-induced surface variation plus the imposed sample bias) became large enough, none of the electrons would have enough energy to overcome the barrier. Of course, those that originated from low potential energy regions (i.e., the darker parts of the image) reached this point first. Consequently, one expected the dark regions to spread as observed in the bottom row of figure 10. In this row the landing energy was such that the sample charged positively. Escaping electrons already had a barrier to overcome, even before the additional barrier imposed by the sample bias. In the upper row, the landing energy was such that the surface potential was negative when there was no additional imposed bias. Electrons that escaped the immediate vicinity of the surface accelerated away from the surface, arriving at the detector with excess energy. Addition of a small positive bias initially reduced this excess energy, but had no effect on electron recapture, because any excess was enough to avoid recapture. This is why there is little or no change in the upper row of figure 10. Changing the bias on sample holder is one way to change the extraction field. Of course, the opposite change on the other electrode is equivalent (since only differences in potential are important). The important role of the external electric field on the SEM contrast opens an opportunity to optimize charge-based SEM subsurface imaging by tuning the external electric field.
Figure 8. SEM cross-sectional observation of 0.5% LA–SWCNT/PI film after Ga FIB cutting. No subsurface CNTs are observed by SEM at FIB cut region observed from 52° tilting angle.
imaging depths for the two CNT-polymer composites in figures 6 and 7. Furthermore, the length and connectivity of CNTs may also play an important role in the above contrast mechanism. An isolated short CNT or a small group of connected CNTs with low capacitance would reach its steady-state charge on a time scale (i.e. charging time constant τ = RC) short compared to the measurement (i.e. pixel dwell time). It would cease to be a source or sink of charge capable of maintaining a surface potential differential during continuing e-beam exposure, so it would not be visible. CNTs that are part of a large connected network (e.g., connected to ground) can continuously conduct the charge required to maintain surface potential differences. This is partly manifested in figure 4(a): almost all the detectable embedded CNTs are curved and look like parts of round-shaped networks, which have larger capacitance and thus a longer time constant for charging. That is to say, not all CNTs located within the electron beam-sample interaction volume may be visible due to their limited charging-time constant. Similar observations were reported by Nagase et al, [35] where it was found that only an embedded nanoscale Si pattern connected to a large area Si plate could be detected by SEM. Compared to the un-connected nanoscale Si pattern, the connected one has much larger capacitance and thus a longer time constant for charging.
4. Conclusions Subsurface SEM imaging methods have been successfully applied to non-destructively characterize the dispersion of SWCNTs embedded in polyimide composites. Contrast was investigated over a broad range of beam accelerating voltages from 0.3 to 30 kV and was ascribed to modulation of the effective SE emission yield (taking into account SE recapture) at the polymer surface because of non-uniform surface potential. The non-uniform potential was due to varying proximity to buried conducting CNTs, to which charge flows through leakage or EBIC. Furthermore, the electron beam might only reveal part of the CNT networks within the beam range. The maximum SEM subsurface imaging depth for CNT polymer composites is dependent on beam voltages and the dispersion of CNTs in polymer, with greater imaging depths for a given CNT loading in samples with poorer dispersion. SEM subsurface imaging can also be tuned by applying an external electric field. Finally, the application of these techniques is not limited to the subsurface imaging of CNT-polymer composites. They are generally applicable to conducting nanostructures embedded in a dielectric matrix,
3.6. Effect of external electric field on SEM subsurface imaging of CNT-polymer composites
The effect of an external sample bias on the SEM contrast of CNT-polymer composites is illustrated in figure 10. A dc bias from 2 to 6 V was applied to the SEM sample stage during scanning. The results for a 2 kV accelerating voltage indicated that there was little change in the contrast or brightness with the variation of bias, as shown in the top row of figure 10. On the other hand, image changes were more pronounced with increasing sample bias at 8 kV accelerating voltage, as shown in the bottom row of figure 10. The average brightness decreased as the bias increased, but rather than uniformly decreasing, the darker areas growed, engulfing an increasing 10
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Figure 9. Schematic of contrast mechanisms for CNTs embedded in polymer composites at various beam accelerating voltages. IP: primary beam current, IS: detected SE signal, and IRE is the recapture current caused by the local electric field. Point A is the position without embedded CNTs in the polymer rich layer. Point B is the position with embedded CNTs. EBIC current is represented by solid lines, while leakage current is depicted by dotted lines.
Figure 10. Stage bias effect on SEM contrast of 0.5% SWCNT(LA)-PI film at different beam accelerating voltages. Top row (a)–(c): 2 kV
beam accelerating voltage. Bottom row (d)–(f): 8 kV beam accelerating voltage.
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such as graphene polymer composites [36] or integrated circuit conductors covered by a dielectric layer. This may have significant implications in non-destructive imaging of silicon nanostructures embedded in silicon oxide for single electron transistors [35], high resolution SEM overlay metrology [37] or e-beam lithography [30], opening up a broad range of applications in nanotechnology. Further optimization of these techniques through experiment and Monte Carlo simulation is underway.
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Acknowledgments The work of Dr Minhua Zhao was supported by NIST-ARRA senior fellowship award in measurement science and technology and Cooperative Research Program in Nanoscience and Technology between the University of Maryland and NIST. Luke Gibbons and Cheol Park acknowledge support by National Science Foundation CMMI-0928839 in part.
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