Topography Measurements Using High Mass Resolution Time-of

Jun 19, 2019 - An unconventional approach using the time-of-flight secondary ion mass spectrometry (TOF-SIMS) technique to determine the height ...
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Article Cite This: Anal. Chem. 2019, 91, 8864−8872

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Topography Measurements Using High Mass Resolution Time-ofFlight Secondary Ion Mass Spectrometry: Application to Banknotes Alice Bejjani,*,† Manale Noun,† Serge Della-Negra,‡ Raymond Tannous,§ Georges Chalhoub,§ Mazen Hamdan,§ and Bilal Nsouli† †

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Research and Development Department, Lebanese Atomic Energy Commission-CNRS, P.O. Box 11-8281, Beirut 1107 2260, Lebanon ‡ Institut de Physique Nucléaire, CNRS-IN2P3, 91406 Orsay, France § Cash Operations Department, Banque du Liban, Masraf Lubnan Street, P.O. Box 11-5544, Beirut 1111 2034, Lebanon S Supporting Information *

ABSTRACT: An unconventional approach using the time-of-flight secondary ion mass spectrometry (TOF-SIMS) technique to determine the height topography at the microscale is detailed in this work with an application to cotton paper banknotes. The study was conducted by determining the effect of all related factors and parameters on the height measurement by taking the simplest model made from two Post-it sheets. For each sample, the difference in the TOF of the same secondary ion coming from two different heights was successfully attributed to the step height of the studied areas’ topography, which was measured using classic methods. The measurement was independent of the orientation of the topography with regard to the primary ion beam and the electron beam azimuth. Moreover, the adjustment of the extraction gap with different layers has no effect on such measurements. However, a range of the analyzer acceptance energy values could be considered to achieve the expected outcomes only if the different analyzers’ component energies are also changing accordingly. Heights between 20 and 180 μm were successfully measured using this new method. An added benefit to this method over other height measurement methods is the ability to discern areas with different chemical compositions, which eventually may help aid understanding of the sample in question.

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shaped sample holder.6−8 Later, J.C. Lee et al. also demonstrated the topography and field effects on the inner side and the walls of a microvia hole and proposed a solution to reduce these effects by tilting the sample using a special sample holder.9 Moreover, many successful attempts were made to rectify the topography artifacts at the nanoscale level in 2D and 3D TOF-SIMS images by combining them with the corresponding scanning or atomic force microscopy (SFM/ AFM) images using various processing methods.10−12 In this work, we took advantage of the high mass resolution of TOF-SIMS, one of the benefits of this technique, and applied it to a representative limitation of quantitative analysis, which is microscale topography. Using this technique, we established a new methodology to measure the height distribution of a sample surface. This new way of measuring height/topography consists of selecting one secondary ion and then monitoring its spectral signature and its image distribution throughout the area of interest. The selected ion

he time-of-flight secondary ion mass spectrometry (TOFSIMS) technique has undergone a dramatic improvement in its mass resolution (M/ΔM up to 18 000), spatial resolution (down to 60 nm), sensitivity (ppm/ppb range), and detection efficiency.1,2 TOF-SIMS is one of the most powerful and versatile techniques providing surface information for a wide range of organic and inorganic materials. The challenge is to apply this method to materials with surface topography on the microscale, which is considered a barrier to the quantitative and spatial characterization. Examples of these materials include fibers, pharmaceuticals, electronics, and paints. Hagenhoff was one of the first to present the mechanisms resulting from microscale topography that affect contrast in TOF-SIMS images.3 Later, Rangarajan and Tyler demonstrated the influence of topographical features on the total ion yields and interpretation of TOF-SIMS images by examining spherical or cylindrical samples.4 McDonnell et al. showed that a semiquantitative topographical map generated by matrixenhanced SIMS can determine whether the peaks’ maxima are due to the topography or to the actual features.5 J.L.S. Lee et al. provided solutions to reduce the impact of the topography and field effects with the assistance of multivariate image analysis by utilizing delayed extraction or simply by using a V© 2019 American Chemical Society

Received: January 8, 2019 Accepted: June 19, 2019 Published: June 19, 2019 8864

DOI: 10.1021/acs.analchem.9b00114 Anal. Chem. 2019, 91, 8864−8872

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the sample (conductor or insulator) and its topography direction relative to the primary ion beam azimuth, (iii) the adjustment of the extraction gap at the topmost or at the lowest layer of the sample, and (iv) the use of low-energy electron flow for charge compensation and the choice of the optimum analyzer acceptance energy (reflector) value for a multilayer sample, in the case of insulators. To determine the effect of the factors mentioned above on topography measurements using the TOF-SIMS technique, we began first with the simplest model that we can create and control, e.g., two sheets of Post-it notes. The two sheets form a sample with a micrometric step. The step height value obtained, in TOF (ns), is correlated to the value, in micrometers, found using a conventional instrument. Additionally, we demonstrate an important asset of this new topography measurement technique when it is combined with chemical findings: the ease to choose from one area, consisting of multiple heights, and different subareas (so-called regions of interest (ROIs)) to compare their surface chemical compositions.15

is conditioned by its abundance in the studied area and by different factors that will be discussed later. Given that this secondary ion is collected by the analyzer from different heights, i.e., different times of flight (TOFs), its spectral signature will appear as either multiple peaks or a broadened peak (which is usually considered a peak with low mass resolution). This property depends on the sensitivity of the analyzer for differentiating between the same ions emitted from areas having different kinetic energies. We applied the methodology to measure the topography of cotton paper banknotes. They are basically composed of different printing layers superimposed onto a cotton sheet and preserved by a transparent protection layer. These banknotes are distinguished from any other printed papers by the exclusivity of the printing procedures and ink used to form the so-called security features. These features, which guarantee protection of banknotes against counterfeiting, are verified visually, by touch, or by an appropriate instrument. From the list of the security features that rely on the senses of sight and touch of the banknotes’ user, we mention the intaglio printing (which produces a raised tactile relief) and the embossed and raised ink printing techniques. These printmaking techniques have dissimilar processes of preparation, but their end-products share a common quality, i.e., microscale height that defines the characteristics of each feature. Measuring the height of these features and controlling its changes with time (during its circulation in the market) is fundamental for the quality control of banknotes. Indeed, the surface topography of such samples could be measured by a large range of instrumentation with stylus and optical profilers at one end of the range and AFM at the other.10,11,13,14 Each measurement technique has its strong points and limitations relative to the others while providing the same outcome. Although stylus profilers have a number of disadvantages compared to optical profilers, they are still the most common instruments for measuring microscale surface topography. They do physically contact the surface, i.e., present a risk of damaging it, and the measurement of an area in scanning mode takes time (a few hours). Nevertheless, interpretation of stylus profiler data is simpler than interpretation of the outputs of optical instruments.8,12 The main downside of optical profilers is that low-reflectivity surfaces (e.g., a banknote’s protection layer) cannot be measured. By contrast, AFM instruments provide threedimensional topographical information with lateral resolution ranging between the atomic scale and tens of micrometers. However, a single-scan AFM image is relatively small and timeconsuming (a few hours) to obtain. In the case of the TOFSIMS technique, measuring the two-dimensional topography of a large surface in static mode takes a matter of a few minutes. Even though TOF-SIMS is an expensive technology compared to its rivals (which are considered physical analysis tools), it possesses a major advantage. In fact, this technique simultaneously provides both elemental and molecular data regarding the chemical composition of the analyzed surface.9 In the case of banknotes, this kind of versatile analytical method facilitates the understanding of the micro/macroapproach of banknote aging by simultaneously defining and controlling the thickness of the printing and protection layers, their chemical changes, and their cross-interactions. However, such measurement might be affected by the following factors, which are discussed in detail in this paper: (i) the choice of the secondary ion, (ii) the electrical properties of

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EXPERIMENTAL SECTION TOF-SIMS Description. The experiments were performed using the TOF-SIMS V mass spectrometer (ION-TOF GmbH, Mü n ster, Germany) at the Lebanese Atomic Energy Commission (LAEC−CNRSL). We used bismuth clusters (Bi3+) at an energy of 25 keV with a low intensity of 0.09 pA to keep all the ions under the saturation limits. The pulsed beam hits the sample surface with an angle of incidence of 45° with respect to the sample normal. Positive secondary ions were extracted at an energy of −2 keV passing through a single-stage reflector before hitting a single microchannel plate detector followed by a scintillator and a photomultiplier (see Figure S1). The correct setting of the reflector voltage, through the adjustment of a virtual control parameter called the “surface potential”, is described for each sample individually in the Results and Discussion. Charge compensation was provided by 21 eV electrons delivered by a flood gun mounted at an angle of 57° to the sample normal. The Z adjustment (extraction gap of 1.5 mm) was performed manually by following the acceptance circle. Unless otherwise stated, no internal mass calibration was performed to determine the TOF difference of the same ion from different spectra. The mass resolution at m/ z 23 was between 2700 and 4500 depending on the chosen parameters. Additional information is detailed in the Supporting Information. Materials and Sample Preparation. To simulate the simplest case of topography/height in a sample, we simply adhered a piece of a Post-it sheet onto another one using its own strip glue in a way that the edge of the interface between the two sheets is in the studied area (see Figure S-2.a). The thickness of the used Post-it sheets was 99 ± 1 μm and 103 ± 1 μm with and without the adhesive, respectively. The step height between the lower layer (LW; see Figure S-2.d) and the upper layer (UP) was ∼ 93 μm measured using a KEYENCE 3D laser scanning confocal microscope VK-X 200. This same sample was analyzed via TOF-SIMS while oriented in four different directions. Experiments were performed while the UP, taken as a reference, was placed at the right, left, top, and bottom of the image (see Figure S-4). These four orientations place the edge of the interface parallel or perpendicular to the primary ion and electron ion beam azimuth direction. Samples from specimen and real banknotes were also analyzed; namely, an area presenting different heights from the 8865

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different regions: the UP and the LW of the Post-it model. The Na ions extracted from the UP, which is closer to the analyzer, arrived earlier (red peak) than the Na ions extracted from the LW (blue peak). As a result, the spectral signature of the Na ions is divided into two peaks. Otherwise, as shown in Figure 1.b, analysis of each layer could be performed separately by repositioning the area of analysis from one layer to the other. Subsequently, the values of the maximum points of the Na peaks are considered without any prior mass calibration. The difference in the TOF in nanoseconds (ΔH) between the maximum points of the Na peaks (7.7 and 6.7 ns in the case of Figures 1.a and 1.b, respectively) could be correlated with the distance difference between the two layers of the Post-it model. Although what we suggest is as simple as it sounds, we want first to elucidate the role or the effect of several parameters. The parameter list includes the following: (1) The changes in the extraction field if the analysis is performed next to or far from the border of the interface between layers. (2) The presence or absence of flood gun electrons. While our samples are considered insulators, given the relatively short analysis time, one may consider that the surface charge will not have an influence on our results. (3) The adjustment of Z (extraction gap) relative to the UP or the LW, which establishes from what step height an ion could still be extracted. (4) Adjustment of the analyzer deflection unit and acceptance energy (reflector) with respect to each layer (UP or LW), i.e., to different surface potentials. In fact, changing the value of the so-called surface potential affects different power supplies of the TOF analyzer that are related to the kinetic energy of the secondary ions (see Table S-1).

Franz Schubert-Fitaglio specimen was studied without any prior treatment. The specimen is a printed document (see Figure S-3.a) made and provided by Giesecke+Devrient, GmbH. The analyzed area, enlarged in Figure S-3.b, includes two walls with a 90° corner made by the intaglio printing technique and some embossed symbols (“G+D”). Additionally, as shown in the black squares of Figure S-3.b,c, two areas from a circulated 1000 Lebanese Lira (LL) banknote (provided by the Central Bank of Lebanon) were studied: (1) an intaglio printing area in the form of blue and green stripes and (2) one of the braille dots made by the raised ink technique. To remove the effect of the entire banknote’s roughness on the analysis and to ensure good contact with the instrument, a 2 cm2 sample surface was cut from each of the above chosen areas and was well-sandwiched between a grounded sample holder with a 1 cm2 aperture for every sample and an aluminum grounded backplane block.

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RESULTS AND DISCUSSION

We chose a sodium peak (m/z = 23 amu) as the secondary ion for the subsequent topography measurements considering the following aspects: (1) it is from the low-mass region where the background is low and the mass separation is at its maximum; (2) in the positive mode spectrum, organic ions with m/z = 23 are absent, so interference from peaks of different natures is excluded; and (3) regarding the studied samples, Na is considered a surface contaminant and hence is present almost everywhere. Post-it. A part of the mass spectrum between 22.95 and 23.09 of the interface region of the two adhered Post-it sheets is presented in Figure 1.a., showing two peaks for the Na ions. The UP was oriented to the right. Each and every ion extracted from this sample seems to have double peaks. The comparison between the images of the Na double peaks, as also shown in Figure 1.a, and the graphical representation of the analyzed area (see Figure S-2.a) reveals that Na is extracted from two

Any changes in these parameters also have a direct impact on the mass resolution, on the secondary ion emission and path, and on the shape of the peaks. This impact may cause inaccurate measurement of the TOF difference between the existing layers. Figure 2 gives a preview of the various outputs due to the different combinations of the above parameters. The first comparison is between the results of the interface area analysis of the Post-it model after adjusting Z on the UP (Figure 2.a) and on the LW (Figure 2.b). In both cases, the flood gun was not applied. This test reveals that wherever the adjustment of the sample distance from the detector is done, the extraction of the secondary ions from the LW is low and inhomogeneous. Thus, the right-side peak of Na does not reflect the actual distribution of this layer topography. The total ion images of these two situations also show the lack of emission from the borders of the UP. However, when the flood gun is applied without even adjusting the surface potential (Figure 2.c), the extraction from both layers is sufficient and consistent. The second test analyzes the interface of the Post-it model in the presence of flood gun electrons and, with the software, automatically adjusts the surface potential of the UP (Figure 2.d) and then the LW (Figure 2.e). The software considers −100 and −60 V as the surface potentials of the UP and LW, respectively. In fact, when −100 V is applied as the surface potential of the sample (−80 V for the reflector), most of the ions extracted from the LW have splashed onto the reflector bottom plate, and only a few of them were transmitted toward

Figure 1. (a) Part of the Post-it model’s interface edge region spectrum between m/z = 22.95 and 23.09, showing two Na peaks with their images separated and overlaid. (b) Overlay of spectral parts between 22.95 and 23.09 of spectra collected separately from the UP and LW layers of the Post-it model. *AnaTGT = one analysis for the two layers together at the interface. AnaSEP = Two separate analyses, one for each layer. 8866

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which we varied between ±60 V with steps of 20 V. Figures 3.a and 3.b present the impact of this variation on the Na peak TOF values (in ns). The layers of the Post-it model, oriented to the right, are analyzed separately with a corresponding adjustment of Z on the UP and LW. The trends in the values of the Na peaks doubly extracted (AnaTGT) from the edge of the interface are presented in Figure S-5. Moreover, to obtain a good understanding of these variations, a conducting model (fine silver sample) was also analyzed with similar conditions (see Figure 3.c). In all cases, the variation in Na peak TOF has a parabolic shape as a function of the surface potential values independent of the changes in the other parameters (ZUP, ZLW, AnaLW, AnaUP, conductor, or insulator). Whereas these curves have the same leading coefficient, their vertices vary in the x direction with the variation in the sample’s properties (conductor, insulator, thickness of the insulator, etc.) This behavior can indicate the real surface potential value of each sample. However, the variation in the vertex of each layer in the y direction designates the changes in the acceleration time when the distance between the sample and the analyzer is modified. Because the reflector is a single-stage reflector, the variation in the time-of-flight is affected by the variation in the surface potential based on the first-order partial differential equation. The reflector does not compensate for the variation in TOF in the extraction, acceleration, and postacceleration areas. However, the curves in Figure 3 reveal that the relation between the variation in the TOF and the surface potential is constant. This outcome is explained by observing the trend in modifications that occurred to the components of the analyzer (presented in Table S-1) when the surface potential was changing. In fact, the lens, extractor, and postacceleration voltages change by the same amount but in an opposite polarity compared to that of the reflector and the surface potential value. This approach attempts to maintain the same TOF in the reflector and in the free flight area. We must note that regardless of the orientation of the sample’s topography relative to the primary beam azimuth direction (top, bottom, left, or right), similar behavior and variations in the Na ions’ TOF values are observed according to the surface potential. Nevertheless, any topography measurement is determined by the difference in heights between layers. Therefore, in Figure 3.d, we present the variation in the step height (ΔH) in nanoseconds between the two layers with respect to the chosen surface potential value, to the Z adjustment, and to the location of analysis, as explained in Figure 1 (AnaTGT or AnaSEP). The UP of the Post-it is oriented to the right side of the image. Moreover, in Figure 4, we group the results of the variation in the step height (ΔH) with the same Z adjustment and type of analysis with regard to the orientation of the sample and chosen surface potential values. The linear correlations (R2 > 0.99) between the step height and the surface potential establish several facts. The most conspicuous fact is that the orientation of the topography’s step with regard to the primary ion and the electron flood gun beams does not influence its determination (Figure 4). The same ΔH value is found for the four orientations regardless of the other parameters. However, as shown in Figure 3.d, the ΔH values calculated from the separate analysis of the two layers (with Z adjusted at the UP) are equal to those calculated from the analysis of the interface edge (with Z adjusted at the LW). In contrast to the ΔH values found when the parameters are reversed, the lowest values of

Figure 2. Na+ spectral signature extracted from the interface area of the Post-it model and the total ion image obtained using different parameters: (a) adjustment of Z on the UP without applying the flood gun, (b) adjustment of Z on the LW without applying the flood gun, (c) adjustment of Z on the LW while applying the flood gun and a surface potential of 0 V, (d) adjustment of Z on the UP while applying the flood gun and a surface potential of −100 V, and (e) adjustment of Z on the LW while applying the flood gun and a surface potential of −60 V. For presentation purposes only, spectra are mass calibrated and normalized by the total ion dose. The y-axes are individually scaled. The UP is on the right side of the total ion image.

the detector, causing a huge decrease in the intensity of their signature. However, when −60 V is considered the surface potential of the sample, secondary ions are properly and almost equally extracted from the two layers. In the case of fibers, this phenomenon is elucidated elsewhere with a recommendation to set the reflector voltage +20 V higher than the average value of the surface potential of the uppermost and lowest layers.6 However, for our Post-it model and for complex insulating samples such as banknotes, when the reflector value is chosen by means of the surface potential adjustment using this recommended method or the ION-TOF routine, a spectrum with the best mass resolution and peak shapes is not always reached. Thus, an unexpected value of the surface potential/reflector could be the optimum value. Accordingly, all the parameters mentioned earlier were studied with reference to the value of the surface potential, 8867

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Figure 3. Variation in the TOF values of the maximum point of the Na peaks extracted separately from (a) the UP and (b) the LW of the Post-it sample, with right side orientation, with regard to the area analyzed and to the position of the Z adjustment versus the surface potential value. (c) Variation in the TOF values of the maximum point of the Na peaks extracted from the surface of a fine silver sample with two different Z adjustments versus the surface potential value. The vertical dotted lines indicate the location of the curve vertices. (d) Variation in the step height between the two layers (ΔH) of the Post-it, with right side orientation, versus the surface potential value and the Z adjustment of upper (ZUP) or lower (ZLW) layer. AnaUP = analysis of the upper layer, AnaLW = analysis of the lower layer, AnaTGT = analysis of the two layers together at the interface, and AnaSEP = separate analysis of each layer.

Figure 4. Variation in the step height between the two layers of the Post-it model versus the surface potential value, the four orientations of the sample’s UP (with regard to the primary ion beam azimuth direction), the Z adjustment (ZLW or ZUP), and the type of analysis (AnaTGT or AnaSEP).

ΔH are those obtained when the analyses of the two layers are separately performed (with Z adjusted at the LW). For the analysis of a larger area, using scan mode, the results show the same trend as that presented in Figure S-6. The negative slope of these straight lines is due to the reflector’s characteristics, which affect the transmission and the mass resolution in different directions. As the surface potential (i.e.,

the reflector) voltage increases, the secondary ions fail to follow the ideal pathway through the reflector and do not reach the detector. This effect causes a decrease in their intensity and in the mass separation. These lines also confirm the repeatability of how we chose the maximum peak value. In fact, the software provides the peak centroid value with respect to its area calculation and 8868

DOI: 10.1021/acs.analchem.9b00114 Anal. Chem. 2019, 91, 8864−8872

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Analytical Chemistry borders. When the peak has a significant tail (on the right side), the centroid axis differs from the maximum one.16 Thus, we decided to manually choose the full width at half-maximum part of the peak and take the value that corresponds to its center. Another test using three Post-it sheets was performed, and it is detailed in Figure S-7. In real samples with multiple topography layers and different directions of the interface with respect to the beam direction, the samples can be considered more complicated cases than the Post-it samples. However, based on the above study, the chosen areas from different banknotes were all studied while applying flood gun electrons and a surface potential between −60 and 0 V. In that surface potential range, the sensitivity of this topography measurement method is at its maximum. The 1000 Lebanese Lira Banknote. Two areas from the 1000 LL banknote were studied: (i) blue and green stripes (width ∼200 μm) with two different heights and (ii) a braille dot, which is an embossed 1 mm2 cylinder (see Figure S-3). Stripes. The stripes are formed using the intaglio printing technique in such a way that the blue and green stripes have two definite heights. This approach is intended to create a certain optical illusion that the stripes’ colors are changing when the banknote is tilted in different directions. This color change is one of the most commonly used security features in Lebanese banknotes. Moreover, the height between the stripes and the height variation due to the circulation of the banknote in the market directly affect the quality of this security feature. Thus, to obtain an average value of the height difference between the blue and green stripes, different areas of a 500 × 500 μm2 raster size were analyzed. The surface potential was set to −50 V, and the extraction gap was adjusted on the blue stripes. Figure 5.a shows the spectral signature of the two Na peaks extracted from the blue stripes (peak in blue color) and from the green stripes (peak in green color). Additionally, the optical and total ion images of the analyzed area are presented. The average difference in TOF between the maxima of the two peaks from two different areas of similar dimensions was found to be approximately 3 ns. Braille Dot. Braille dots were created on all Lebanese banknotes using the raised ink printing technique with the intention to help blind people to distinguish, with their sense of touch, different denominations. Therefore, the quality control of the height of these braille dots before and during circulation is a must. The scan stage option was used on an area of 2 × 2 mm2 covering the dot and its surroundings for two reasons: (1) the diameter of the dot is higher than the maximum raster size of the beam (500 μm), and (2) the objective is to obtain an average of the height of the round borders of the dot relative to the surface of the banknote. The beam raster size was 500 × 500 μm2, and the adjustment of Z and the charge compensation (surface potential of −65 V) was performed relative to the surface of the banknote (from the right side of the dot). As shown in Figure 5.b, the spectral signature of Na seemed to be composed of multiple peaks interfering each other. This outcome indicates that the Na peak is seemingly not extracted from only two regions: (1) the braille dot as the uppermost layer in this area and (2) the surface of the banknote as the lowest layer. In fact, from the overlay image of the Na peaks from the selected areas, we can easily conclude that the surroundings of the braille dot have three different heights. Its highest point (right corner) is comparable to the surface of the braille dot.

Figure 5. (a) Optical image of a 500 × 500 μm2 area from the stripes of the 1000 LL banknote with the spectral signature of the Na peaks extracted from the upper layer (green stripes) and lower layer (blue stripes) and with their ion images separated and overlaid. (b) Total ion image of a 2 × 2 mm2 scanned region of the 1000 LL banknote that includes a braille dot with the spectral signature of Na peaks and with their ion images separated and overlaid. (c) Total ion image of a 2 × 2 mm2 scanned region of the 1000 LL banknote that includes a braille dot with the Na spectral signature of a 400 × 400 μm2 ROI, presented as squares with different colors (scan mode with a minimum raster size of 500 × 500 μm2).

After examination of the total ion image presented in Figure 5.b, the difference in the surface properties between the braille dot and its environment are evident. If the protocol used in the stripe and Post-it cases is applied to measure the topography height from the Na spectrum, as presented in Figure 5.b, by taking the difference between the maximum of the Na peaks, the outcome will not be accurate or even successful. In fact, the height difference between the various areas is smaller than the mass separation. The alternative ways to achieve this measurement are to either analyze each area separately or simply choose different ROIs from the total area studied and then overlay their spectra without any mass calibration (Figure 5.c). The difference in TOF between the peaks’ center indicates the distance between these ROIs. Thus, the difference in height between the dot and the upper left and the lower left sides of the surface of the banknote are 4 and 7.4 ns, respectively. The position of the red and blue peaks in Figure 5.c confirms the initial observation mentioned above; i.e., the upper right side of the surface of the banknote has the same height as the braille dot. This braille dot was taken from a used 1000 LL that had been circulating in the market for 4−5 months. The 1000 LL, which is the lowest Lebanese denomination, has the highest 8869

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ammonium cations that are extracted from only the stripes and the areas surrounding the braille dot (Figures 6.a and 6.b, black stars).17 However, the other series (black circle, Figure 6.c) is most likely related to ethylated Surfynol 104 with the basic molecular formula C14H26O2. These last fragments have the same image distribution as the green pigment of the braille dot (Figure 6.d). In fact, Surfynol is usually used as a surfactant in the pigment industry. Finally, the peaks around m/z 576 are related to the phthalocyanine blue pigment (black triangle, Figure 6.c) with the molecular formula C32H16Cu1N8, which also seems to be concentrated in the same areas as the green pigment (Figure 6.e).This type of study opens the discussion about further investigating whether this green pigment or its surfactant is not compatible with the varnish mixture, which would cause inhomogeneous deposition of the varnish layer. Franz Schubert-Fitaglio Specimen. A 1.0 × 1.0 mm2 area from the Franz Schubert-Fitaglio specimen was chosen to study the variation in its height. This zone is composed of an L-shaped dark blue wall and an embossed G symbol. The area is analyzed using the stage scan with a 500 × 500 μm2 raster size, a surface potential of −60 V, and an extraction gap adjusted relative to the lower layer. As expected for an orientation perpendicular to the primary beam azimuth direction, the total ion image of the top of the wall, as presented in Figure 7.a, is shifted to the left side of the image, leaving behind (on the right side) a black shadow area.

circulation in the market, i.e., the fastest aging among all the denominations. This rapid aging is considered the main cause for the observed irregularity in the heights of the banknote surface around the braille dot. In fact, by analyzing another braille dot taken from a brand new 1000 LL, we verified that the origin of this irregularity is not the manufacturing process (Figure S-8). In another aspect, one can perceive that the Na peak extracted from the surface of the braille dot has 1.3 times lower mass resolution than the peaks extracted from the other ROIs, the mass resolutions of which are similar. This observation not only confirms the above-mentioned suggestion about the irregularity of the dot’s surface but also gives this type of analysis another advantage. In fact, this proposed topography measurement, in which the Na peak is monitored, not only allows us to differentiate between areas with different roughness but also makes it possible to discern areas with different chemical compositions. For instance, if we compare spectra taken from ROIs from the braille dot and its surroundings and from the stripes of a brand new 1000 LL, we can uncover the following features of this denomination: (1) The spectra of each type of stripe, green and blue, are identical with no pigment signature. Hence, one can say that the protection varnish layer is deposited uniformly onto the stripe area. (2) From the high-mass region spectra presented in Figure 6.c (black rectangle), we can identify the presence of

Figure 7. (a) Total ion image of a 1 × 1 mm2 scanned region of the Franz Schubert-Fitaglio specimen with the Na peak spectral signatures and with their images separated and overlaid. (b) Optical and total ion images of a 2.5 × 2.5 mm2 scanned area from the Franz SchubertFitaglio specimen and the distribution of the m/z = 576 peak (phthalocyanine blue pigment) (scan mode with a minimum raster size of 500 × 500 μm2).

Figure 6. Two parts between 300 and 620 amu and between 860 and 1220 amu (×10 for visual purposes) from the spectra of different areas from the brand new 1000 LL: (a) the lower left side next to the braille dot, (b) the stripes, and (c) the center of the braille dot with the distribution of (d) the Surfynol peaks (black square), (e) the blue pigment (black triangle), and (f) the green pigment (black rectangle) (quaternary ammonium cation peaks are marked with black stars).

The spectral signature of Na from the scanned 1 × 1 mm2 area, as presented in Figure 7.a, reveals three different heights, which are well-highlighted in the overlay image in the same figure (green, yellow and blue). However, the tail on the right side of the Na peaks (red color) fills the shadow area. This phenomenon does not appear in the stripe case where the stripes are tilted with respect to the x and y axes. This measurement artifact is seen in the Post-it case (Figure S-4) and is related to the height of the wall and its 90° position with respect to the angle of the primary ions. The difference in height among the three highlighted areas is higher than the mass separation and can be calculated from the differences in

phthalocyanine green (Ceres green 3b with the chemical formula C32Cl16Cu1N8) within only some regions of the surface of the braille dot (Figure 6.f). This result indicates that the varnish is not homogeneously deposited on top of the braille dot. (3) In the mass region between 400 and 600, three different series are identified. The two peaks at m/z 522 [C36H76N]+ and m/z 550 [C38H80N]+ are attributed to quaternary 8870

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Article

Analytical Chemistry

in the range between 20 and 180 μm. The standard error of the TOF-SIMS values is on the order of 10 ps (for counts higher than 250 000). The asymmetric horizontal errors in the height measurement of the stripe, L wall, and braille dot samples are due to the variance between the maximum (square points) and the mean height values given by the optical profiler. Both curves include an offset resulting from the application of a surface potential. In fact, even in the case of measuring the TOF of Na peaks at a so-called “0 V surface potential”, the surface is charged by electrons delivered by the flood gun. This surface charge causes a shift ranging between 0.7 and 2 ns depending on the chosen parameters. The limit of detection of this method is comparable to the roughness of these cotton samples (1−2 μm). It was calculated from the time separation between the ROI peaks in Figure S.8. A difference of 0.6 ns between peaks was discerned. However, from the “three Post-it sheets” experiments, we concluded that the upper limit of height step determination is approximately 200 μm only if Z is adjusted halfway. We expect that the operating conditions of a surface potential between −60 and 0 V could work only for insulators similar to banknotes and cotton paper. However, a calibration curve at one surface potential value is sufficient to achieve quantitative analysis. Nevertheless, the calibration curve must be determined (or rechecked) in the case of different types of insulators (thickness, dielectric constant, etc.) Additionally, the bottom surface of the sample should be kept as flat as possible. Therefore, the electric field is affected by only the surface topography.

TOF among the maximum of the Na peaks. Hence, the differences between the yellow peak and the centers of the green and blue peaks are 2.7 and 7.3 ns, respectively. This specimen is coating-free. Therefore, it was interesting to compare the spectra of different areas with different heights. A 2.5 × 2.5 mm2 area was analyzed using scan mode. The images presented in Figure 7.b indicate that the phthalocyanine blue pigment was used in the standard printing technique to generate the word “Giesecke & Devrient” on the cotton substrate. By contrast, this pigment was not utilized while creating the cube area (L wall and the symbol “G+D”). This approach of establishing accurate topography as a function of the TOF difference and accordingly determining the chemical composition could be beneficial for other types of applications. In fact, any sample exhibiting microscale topography, such as animal cells, plant cells, and microelectromechanical systems (MEMs), can benefit from this method. Especially in the case of biological samples, this method can complement the label-free approach when a sample is scratched or burned by conventional profilers. Moreover, this method of topography measurement is still valid even if a secondary ion other than the Na ion is monitored. Nevertheless, some limitations could result if hydrogen ions are followed, as their TOFs and resolutions are 2 to 3 times less than those of Na ions. Furthermore, in the case of ions with greater mass than the Na ion, i.e., better time separation, the probability of these ions interfering with other ions is also larger. Correlation between the Step Heights Measured in Nanoseconds and Micrometers. Banknote samples were also scanned using a Schaefer optical profiler to obtain an average of their topographical heights (see Figure S.9). The Post-it measurements were performed using a KEYENCE 3D laser scanning confocal microscope VK-X 200. Figure 8

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CONCLUSION In this work, we demonstrated that with the high mass resolution and high mass separation of the TOF-SIMS technique, a height measurement of insulating samples with micrometer-scale topography can be performed. This measurement was carried out by monitoring the value (in nanoseconds) of the centroid of the peaks of a single positive secondary ion extracted from different heights. While using the Post-it model, we conducted a detailed study focusing on the effect of different parameters on the height measurement. We found that (i) flood gun electrons should always be present and (ii) the analysis could be performed on each layer separately, on the interface between the layers, or even on a larger area. We experimentally determined that the adjustment of the gap between the analyzer and the sample, the adjustment of the analyzer deflection unit, and the charge compensation adjustment could be performed on any layer of the sample. Importantly, the orientation of the topography with regard to the primary ion beam azimuth and to the flood gun did not have an effect on these measurements. However, to obtain the best mass separation and uniformly extract ions from the total studied region, the reflector voltage value should be kept between −60 and 0 V. These parameters are applicable to insulators similar to printed paper. The limit of detection was found to be 1−2 μm (in the range of the samples’ surface roughness). The maximum step height measured was approximately 200 μm with a condition of adjusting Z halfway. This step height calculation method works well for samples such as banknotes that have topographical variations in the applicable range. Moreover, the chemical composition results for the different layers are revealed accordingly. This method could also be useful for biological samples, provided that the measured topography is at least three times higher than the

Figure 8. Correlation between TOF measurements (ns) and different types of profiler measurements of the step heights (μm) for different studied samples (stripes and braille dot from the 1000 LL specimen, L wall from the Franz Schubert-Fitaglio specimen, and 2 and 3 Post-it sheet models).

illustrates the correlation between the differences in height measured in nanoseconds using the TOF-SIMS technique and those measured in micrometers using the above-mentioned techniques. The TOF values were calibrated using the Post-it findings at −60 and 0 V surface potentials. There is a linear agreement between this new method of finding the height using the TOF-SIMS technique and the conventional methods 8871

DOI: 10.1021/acs.analchem.9b00114 Anal. Chem. 2019, 91, 8864−8872

Article

Analytical Chemistry

(16) Liu, C.; Liu, J.; Tan, J. Confocal Axial Peak Extraction Algorithm. Confocal Microscopy; Morgan & Claypool Publishers: 2016; pp 6−10. (17) Schmidt, E. M.; Franco, M. F.; Regino, K. G.; Lehmann, E. L.; Arruda, M. A. Z.; de Carvalho Rocha, W. F.; Borges, R.; de Souza, W.; Eberlin, M. N.; Correa, D. N. Sci. Justice 2014, 54 (6), 459−464.

roughness of the sample. For quantitative analysis of new types of samples, a calibration curve is essential.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.9b00114.

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One table and nine figures detailing parts of the work and describing some choices we made while using our suggested method for measuring topography heights (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Alice Bejjani: 0000-0003-0108-6590 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is part of the project LEB0007, entitled “Strengthening Nuclear and Nuclear Related Analytical Techniques Capabilities in the Field of Forensics”, which is financially supported by the International Atomic Energy Agency (IAEA) through the Technical Cooperation Program between Lebanon and the IAEA. A sincere thank you to Dr. Michael Eller for his diligent language editing of this manuscript is given.

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DOI: 10.1021/acs.analchem.9b00114 Anal. Chem. 2019, 91, 8864−8872