Angular Distribution of Molecules Sputtered by Gas Cluster Ion Beams

Article pubs.acs.org/JPCC

Angular Distribution of Molecules Sputtered by Gas Cluster Ion Beams and Implications for Secondary Neutral Mass Spectrometry Matthias Lorenz,*,† Alexander G. Shard,† Jonathan D. P. Counsell,‡ Simon Hutton,‡ and Ian S. Gilmore† †

National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom Kratos Analytical Ltd., Trafford Wharf Rd., Manchester M17 1GP, United Kingdom

‡

S Supporting Information *

ABSTRACT: We present experimental data on the angular distribution of Irganox 1010 organic molecules sputtered by large argon gas cluster projectiles (E/n = 5 eV, 10 keV Ar2000). Ejection probability distributions as derived from deposit patterns on planar collector surfaces were recorded at various angles of incidence of the primary cluster ion beam. The sputtered material is ejected at polar angles, on average, greater than 45° from the surface normal. At normal incidence there is no azimuthal dependence in the ejecta distribution, but the ejecta are forward directed even at incidence angles as low as 15°. After this initial rapid change, the ejecta distribution shows a rather weak dependence on the incidence angle of the primary ion beam and the polar, and azimuthal angles of preferred ejection remain relatively constant. Ejecta distributions agree with previously published results from molecular dynamics simulations for organic molecules sputtered with large argon gas cluster projectiles and are consistent with the picture derived from experimental data for metal target species. The close chemical resemblance of collector and target materials as identified by secondary ion mass spectrometry, and the large total volume of deposits accounting for over 75% of the sputtered material as inferred from symmetry considerations, indicate that a large fraction of the sputtered material is intact molecules. Findings are discussed with respect to the utilization of large cluster projectiles as primary ion beams in secondary neutral mass spectrometry.

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flux of ejecta has to be maximized. This also applies to the timing of sputtering and photoionization steps for schemes utilizing pulsed light sources. The angular distribution of ejectathe differential sputter yieldis thus a critical parameter for the design of an SNMS experiment. Primary ion beams for SNMS need to provide high sputtering yields for intact molecules. It is important for the advancement of high spatial resolution MSI to maximize the number of ejected molecules and atoms for a fixed surface area under primary ion bombardment, i.e., to improve the access to sample material in its depth.4 Low damage sputtering of organic material has become possible with the advent of cluster ion beams.12−14 Over the past few years, gas cluster ion beams (GCIB) have become the dominant ion beam for sputtering of organic materials as they exhibit low damage for organic molecules.15 Importantly, this has enabled depth profiling and 3D imaging of almost all organics. Experiments with a depth resolution of the order of 5 nm have been reported.16 More recently, these beams have become available as analysis primary ion beams.17 Most of the more recent implementations of SNMS instruments utilize smaller cluster projectiles as analysis beams, such as C60 or metal clusters. Molecular dynamics

INTRODUCTION Secondary ion mass spectrometry (SIMS) is a surface analytical technique that utilizes charged species formed during the bombardment of analyte surfaces with primary ion projectiles. For organic molecules, only a small fraction of the sputtered material is accessible for analysis as intact, desolvated, ionized molecules (secondary ion yields for organic molecules are assumed to be on the order of 10−6−10−3).1,2 The fraction of ionized molecules that are accessible for SIMS analysis further depends on the chemical environment at the sputter site, a phenomenon termed the “matrix effect”.3 This dependence significantly limits the direct quantitative application of SIMS. The low secondary ion yield for organic molecules sets limits for the achievable spatial resolution in mass spectrometry imaging (MSI), simply due to the extremely limited number of ions in the ejecta plume that are formed from the finite number of sputtered surface molecules and atoms.4 Secondary neutral mass spectrometry (SNMS) techniques aim to access a part of the large fraction of neutral species in the cloud of sputtered material by postionization in a dedicated ionization step subsequent to the sputtering event.5−8 Many postionization techniques utilize photoionization schemes that provide rather small ionization volumeswhich can be of dimensions as small as 10 μm,9,10 e.g., for ionization in the “strong field regime”.11 In order for SNMS techniques to access a large number of neutral species available in the ejecta plume, the overlap of the confined photoionization volume with the © 2016 American Chemical Society

Received: July 7, 2016 Revised: September 14, 2016 Published: September 22, 2016 25317

DOI: 10.1021/acs.jpcc.6b06821 J. Phys. Chem. C 2016, 120, 25317−25327

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The Journal of Physical Chemistry C

Figure 1. (a, c, e) Polar distribution of ejecta for various types of primary ion projectiles. Plotted is the probability of ejection (as inferred from the distribution of deposits on collector surfaces) vs the polar angle from published experimental work for sputtered metals. All plots are normalized and cos θ plots (blue line) are included for reference. (a) Atomic projectiles with E/n = 20 keV at normal (solid line) and 60° (dashed line) angle of incidence. Data for atomic projectiles with E/n = 5 keV at 60° (dash-dot-dot line) are included for comparison. Data are from refs 15 and 22. (c) Small cluster projectiles with E/n = 333 eV at normal (solid line) and 70° (dashed line) angle of incidence. Data are from ref 26. (e) Large cluster projectiles with E/n = 6.7 eV at normal (solid line) and 60° (dashed line) angle of incidence. The 10° (dash-dot line) case is included to illustrate the strong angular dependence for small deviation from normal impact. Data are from ref 15. (b, d, f) Illustrations of models commonly utilized to describe the different mechanisms of sputtering with atomic, small, and large cluster projectiles (see text). Plots are adapted from refs 25 and 18.

atomic (e.g., Ar+), medium-size cluster (e.g., C60), and large cluster (e.g., Ar3000) projectiles. The angular distribution of material ejected in the impact of atomic projectiles varies qualitatively with projectile energy.21 It resembles a cosine distribution in the keV region (cf. Figure 1a for 20 keV Ar+ at normal incidence15 and 19 keV Kr+ at 60° oblique incidence22) and becomes “under-cosine” (“heartshaped”) for lower projectile energies and “over-cosine” for the higher energy regime.23 In the keV region under oblique incidence, the preferred polar angle of ejection shifts slightly from normal in the forward direction relative to the incoming primary ion beam. The cosine character of the distribution is retained for oblique incidence angles for sputtering conditions that can be described by the random collision cascade theory24 (i.e., for primary ions of moderate mass in the keV regime). Information about the impact angle and direction of the primary ion is lost in the linear cascade sputtering process (cf. Figure 1b, red collision cascade path, with primary ions in blue and secondary ions in green). This causes the ejection to be

(MD) simulations suggest significant differences for the angular distribution of organic molecules sputtered with large Ar clusters as compared to C60 projectiles,18 which would have an impact on the instrumental design of an SNMS platform that utilizes large gas cluster projectiles. Differences have also been observed in experimental studies for elemental species but no experimental data is available for organic material. Sputtering processes have broad application and have been subject to fundamental studies in various fields such as surface analysis, surface etching, cleaning, and thin film deposition.19 Historically, sputtering processes were limited to atomic projectiles sputtering metals and inorganic compounds.20 Significant differences exist for cluster ion projectiles regarding the angular distribution of ejecta. But also within the group of cluster ion projectiles, significant changes with the size, energy, and impact angle of the projectiles have been reported. Figure 1 compiles published experimental data for the angular distribution of ejecta from inorganic materials sputtered with 25318


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surface normal cause a strongly directed emission in the specular direction.29 The angular dependence has been observed to be extremely sensitive close to normal incidence in the range from 0° to 10° but then becomes less pronounced above 10° (cf. Figure 1e for 20 keV Ar3000 at 10° and 60° oblique angle of incidence).15,29 MD simulations predict a similar behavior for organic molecules and explain this phenomenon by an intense flux of redirected Ar atoms (which would stay in the crater for the normal incidence case) over the rim of the crater, blocking the direct ejection from the bottom of the crater and “washing out” intact molecules as well as organic fragments (cf. Figure 1f, lower panel). This “washing out” effect causes a strong azimuthal anisotropy.18 The angular distribution of ejecta has been studied by direct detection of the emitted molecules or particles. Techniques utilized include the angular scanning analysis/detection of ejecta with quartz crystal microbalance (QCM),35−38 mass spectrometry,39−43 laser-induced fluorescence (LIF),44 and cavity ring-down spectroscopy (CRDS).45 Nonscanning approaches include the imaging of the ejecta plume on a position-sensitive MCP detector.27,46 The direct detection of ejecta is usually quite elaborate and requires dedicated instruments. An experimentally simple (and very common) approach to assess the angular distribution of ejection of sputtered material is to capture the ejecta on surfaces and utilize thickness mapping techniques to measure the distribution of deposits in a subsequent step. Usually, curved collector surfaces are used with semicircular profile (i.e., semicylindrical or hemispherical) that are equipped with at least one opening for the incoming primary ion beam. A hemispherical surface enables simultaneous access to azimuthal and polar angles, but many studies report only the change of the polar angle of ejection, assuming azimuthal symmetry. Transparent hemispherical or semicylindrical collector surfaces enable photometric evaluation of deposits.47 Flexible surfaces such as foils32,48−52 or composite surfaces of planar elements23,29,48,53,54 enable easy access by common surface analysis techniques. Among the techniques successfully utilized for mapping the thickness of deposited material are photometry,48 elastic recoil detection analysis (ERDA),55 electron probe microanalysis (EPMA),23,52 Rutherford backscattering spectroscopy (RBS),29,32 Auger spectrometry,56 profilometry,53,54 SIMS depth profiling,51 and radioactive tracer measurements.57 Planar collector surfaces can be utilized,56 drastically simplifying the experiment in both the sputtering/collection step and the surface analysis. Thickness maps of the collected organic material have to be corrected for the sputtering geometry based on simple geometric considerations. However, this approach faces interpretational challenges including the angular and kinetic energy dependence of the sticking coefficient.58 Data for the angular dependence of sticking coefficients for large organic molecules are sparse, and the kinetic energy of the ejecta is currently not known. The deposits on planar collectors critically depend on the angular distribution of emission itself, characterized by the half-width of the emission.59 The correction of thickness values based on the sputtering geometry becomes an essential part for interpreting features and their position in the maps recorded on planar collectors. In this paper we present experimental data on the angular distribution of Irganox 1010 sputtered with large Ar gas cluster

largely independent of the impact angle for a wide range of incidence angles, except for glancing angles. Ejecta show no azimuthal preference and their distribution is rotationally symmetric with respect to the surface normal. Reduction of the primary ion energy renders the ejecta distribution under-cosine and dependent on the incident angle (cf. Figure 1a for 5 keV Kr+ at 60° oblique angle of incidence22). This effect can be understood by an increasing importance of shorter (hence shallower) cascades due to a reduced amount of energy deposited onto target atoms (cf. Figure 1b, yellow path). Momentum randomization is not completely achieved in a shallow cascade. For normal incidence, primary recoil processes intensify the ejection at high polar angles, causing the distribution to become undercosine.25 Ejecta of metals sputtered with medium-size cluster projectiles, such as C60 (at 20 keV), show an under-cosine distribution with little dependence on the impact angle of the primary ion beam (cf. Figure 1c for 20 keV C60 at normal vs 70° oblique angle of incidence26). Also for organic material sputtered with C60 projectiles at normal impact (20 keV projectile energy), ejection primarily at polar angles smaller than 45° has been observed, with a preferred ejection around 30° polar angle.27 MD simulations for the sputtering of organic molecules with medium-size cluster projectiles (C60, Ar366) predict a similar independence of the ejecta distribution from the projectile impact angle, even at polar impact angles as large as 45°.28 Ejecta in these simulations are initially emitted in the specular direction but dominated by the large number of lowenergy molecules ejected close to the surface normal (cf. Figure 1d). For metals sputtered with large cluster projectiles (e.g., Ar3000), significant deviation from the cosine distribution has been observed for the normal impact (cf. Figure 1e for 20 keV Ar3000 at normal impact).15,29 The significantly under-cosine contribution as observed for metals (e.g., Cu) has been termed “lateral sputtering” and was supported by MD simulations.30,31 The effect of lateral sputtering has gained importance as technique for surface smoothing.15 MD simulations for organic molecules predict a similar behavior (for benzene sputtered with Ar2953 cluster projectiles) and explain the phenomenon by the formation of a layer of projectile atoms in the crater after normal impact that effectively blocks the emission of ejecta from the bottom of the crater (“blocking effect”; cf. Figure 1f, upper sketch).18 Molecules are mainly ejected from the rim of the crater and leave the crater at large polar angles. Recent work on the normal incidence sputtering of metals (W, Mo) with E/ n = 5 eV 10 keV Ar2000 projectiles, however, revealed a significant contribution of atoms ejected close to the surface normal.32 A proposed mechanism is capable of explaining the difference to the previously reported lateral sputtering for Cu on the basis of different elastic (Young’s) moduli of the target materials.33 This mechanism has minor significance for elements with a relatively low elastic moduli (Cd, Cu, In, Au) which consequently show a predominantly lateral emission,29,32,33 while metals with higher elastic moduli (W, Mo) are ejected at small polar angles close to the surface normal.32 Organic molecules have generally low elastic moduli (6.1 N/m2 for solid benzene34 vs 11.6 N/m2 for Cd32) and would therefore be expected to demonstrate lateral emission. Ejecta of metals sputtered with large cluster projectiles (e.g., Ar3000) show a pronounced dependence on the impact angle of the primary ion beam. Polar angles as low as 10° relative to the 25319


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Irganox 1010 on silicon (about 1.7 μm thick) were prepared by physical vapor deposition (PVD), using a Mantis QBox Deposition System (Mantis Deposition Ltd., Thame, UK). The thickness of the organic layers was measured by ellipsometry, using the instrument described above. Software. Data were processed using software developed inhouse based on the Matlab R2014a (MathWorks, Natick, MA) software package. Sputtering Experiments. Custom holders were designed and built in-house from aluminum to mount and align three 10 × 10 mm2 silicon wafers pairwise perpendicularlytwo in a vertical orientation as collector plates and one in horizontal orientation with the sputter target organic layer (cf. Figures 2a and 2b). The vertical silicon wafers were screw-mounted in slots of 3 mm depth; the sputter target was held in place by a metal spring clip.

projectiles at variable angle of incidence. Irganox 1010 is chosen because a great deal is understood about its sputtering yield under GCIB irradiation.60−62 Maps of the absolute thickness of collected organic deposits enable quantification of the amount of material. SIMS analysis of the deposits provides information on the chemical identity of the material ejected when sputtered by large Ar gas cluster projectiles.

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EXPERIMENTAL METHODS Analytical Instrumentation. Sputtering experiments were performed on a Kratos Axis Nova (Kratos Analytical Ltd., Manchester, UK) X-ray photoelectron spectrometer (XPS), equipped with a 20 keV maximum energy gas cluster ion source (GCIS). The XPS was set to acquire depth profiles on a rectangular surface area of 2 × 2 mm2 with the GCIS operated in dc mode (10 keV Ar2000, 44 nA measured on sample) and charge neutralization enabled. The acquisition was set to 100 cycles of 30 s each but was manually stopped after reaching the substrate interface as identified by the Si 2p signal (signals of O 1s, C 1s, and Si 2p were monitored after each sputtering cycle). A Woollam M-2000 DI ellipsometer (J.A. Woollam Co., Lincoln, NE) was used for mapping the thickness of collected organic layers on the two collection plates. It was equipped with focusing probes, with a minimum spot size of about 300 μm on the short axis (the actual spot size was not determined). Thickness maps were recorded for a rectangular area of 10 × 10 mm2 and a grid of 0.25 × 0.25 mm2 steps. Ellipsometric data were acquired at 70° angle from the surface normal, fitted in the wavelength range 300−900 nm using a Cauchy layer model for the organic layers, and corrected for the SiO2 oxide layer which was measured prior to the experiment. The ellipsometric measurement of thin films of Irganox 1010 is accurate to within 2%.63 SIMS spectra were recorded with an ION-TOF TOF-SIMS IV (ION-TOF GmbH, Münster, Germany) equipped with a bismuth cluster liquid metal ion gun (LMIG) for analysis and a 20 keV maximum energy GCIB gun for analysis or erosion in 3D experiments. Both beams have an angle of incidence of 45° and are rotationally offset by 90° around the sample stage normal. SIMS spectra of captured organic material were recorded in negative ion mode using either a GCIB or an LMIG analysis beam. GCIB SIMS data were acquired with E/n = 4 eV, 20 keV Ar5000 cluster projectiles with 0.14 pA beam current, and spectra were extracted as an average for square surface areas of 200 μm. LMIG SIMS spectra were recorded in a large field experiment (7 × 4 mm2, 29 × 17 patches) using a 25 keV Bi3+ analysis beam with 0.08 pA beam current. For the studies on the chemical identity of collected material, an Irganox 1010 layer (∼1.7 μm thick) was eroded with the TOFSIMS IV GCIB with E/n = 5 eV, 10 keV Ar2000 cluster projectiles at 0.89 nA beam current. Ejecta were collected on a Si wafer mounted in perpendicular orientation to the target surface using the TOF-SIMS IV equipped sample holder. Chemicals and Materials. Silicon wafers (100 mm diameter, 500 ± 25 μm thick, 15−20 μm thick SiO2 layer) were obtained from UniversityWafer (Boston, MA). Irganox 1010 was obtained from BASF Schweiz (formerly Ciba Specialty Chemicals, Basel, Switzerland), and 2-propanol (HPLC grade) was obtained from Fisher Scientific UK (Loughborough, UK). Silicon wafers of about 10 × 10 mm2 (0.5 mm thick) were cut by hand, manually cleaned with compressed air and 2propanol, and visually checked by optical microscopy. Layers of

Figure 2. (a) Schematic of the sputtering experiment showing three pairwise perpendicular 10 × 10 mm2 Si wafers, one of which is coated with an Irganox 1010 layer used as sputter target. An E/n = 5 eV, 10 keV Ar2000 beam is operated in dc mode and directed at the sputter target surface at variable angle of incidence. Ejecta emitted from the sputter crater are partially collected at the other two Si wafers and form patterns of organic deposits. (b) Photograph of the Si wafers as mounted in a custom holder and supported by a 45° wedge for tilting the sample relative to the fixed GCIB. (c) Photograph of a collector plate showing the organic deposit on a Si wafer after eroding a 1 × 1 mm2 × 1.7 μm volume of Irganox 1010 and collection at short distance of only about 1 mm. The rectangular sputter crater in the target layer is clearly visible.

The Axis Nova GCIB gun is oriented at a fixed 45° polar angle relative to the sample holder plane, and it was necessary to align the sputter target relative to the primary ion beam in azimuthal and polar orientation. Metal wedges with a defined slope were used to tilt the sample (mounted with double-sided copper tape) and adjust the polar orientation of the target layer and the collection plates (cf. Figure 2b). Deviations from the rectangular shape and crater dimensions were expected on the tilted sample surface. The actual crater dimensions varied between experiments and depended on the angle of incidence adjusted by tilting the sample surface. The 25320


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The Journal of Physical Chemistry C side length of the rectangular craters was in the range 0.7−1.6 mm. The interface was reached after about 6 min sputtering in the 15° incidence experiments (1.65 × 1.24 mm2 crater size). Multiple sets comprising two collection wafers and one target organic layer each were mounted onto individual holders to enable quick exchange between experiments (cf. Figure 2b). Several sets were prepared for the sputtering experiments and analyzed by ellipsometry within a few days after collection. Samples were kept in dust-tight containers for transport, and high temperatures were avoided. The dimensions of the rectangular sputter craters (0.7−1.6 mm side length) were chosen to be small compared to the distance of the collector plates from the sputter site (several millimeters). This minimized the effect of blurring of features in the thickness maps on the collector plates due to the spatial extent of the sputter crater compared to a point source. For the analysis of the data, a point source was assumed. Data Analysis. Thickness values for the collected organic deposits were determined at discrete positions on a square grid. For each of the positions, the exact (within experimental errors) location relative to the sputter site was determined. Polar coordinates were defined relative to an origin located at the center of the rectangular sputter crater, with a polar angle θ to the sample surface normal and an azimuthal angle φ to the “forward” direction of the primary ion beam. This definition considers the sputter site a point source located at the center of the actual crater. The correction of thickness values based on the sputtering geometry is an essential part of interpreting the maps. The geometric considerations used to derive the correction terms and details on how the angular distribution of ejection probabilities were derived from the patterns of organic deposits is included in the Supporting Information. No correction has been applied for angular and velocity dependence of the sticking coefficient for the organic material on silicon. The craters eroded were of dimensions readily visible under the microscope. The size and location of the craters could reliably be determined from calibrated optical images.

Figure 3. (a, c, d) Angular distribution of ejection probabilities vs the azimuthal (φ) and polar (θ) angles in a spherical coordinate system with the sputter crater located at the origin. (b, e) Polar distribution of ejection probabilities vs polar angle (θ) for different azimuthal angles (φ). (a, b) Normal incidence of the GCIB on the target surface. (c, d, e) GCIB positioned at 15° to target surface normal. (c) Ejecta collected in backward and in sideways direction. (d) Ejecta collected in forward (i.e., in specular direction with respect to the GCIB) and in sideways direction. (e) Combined representation of the data for the normal (green) and 15° (black) impact case for the plane that contains the primary ion beam (φ = 0°). All plots are normalized to individual scales.

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RESULTS AND DISCUSSION The graphs in Figure 3a,c,d show the distribution of ejection probabilities for the azimuthal (φ) and polar (θ) angles of ejection. The diagrams are oriented in a way that the primary ion beam is traveling in the direction of 0° azimuthal angle φ (except for the trivial case of normal incidence where all velocity components other than normal to the target surface are essentially zero). The primary ion cluster beam impacts the surface at a variable polar angle θ, indicated by the “X” in the diagrams. With this orientation of the primary ion beam, contributions at 0° azimuthal angle are directed with the same azimuth as the incoming primary ion beam, contributions at −90° < φ < 90° azimuthal angle are directed in “forward” direction, contributions with φ < −90° or φ > 90° are ejected in “backward” direction. Panels b and e in Figure 3 show the ejection probability as plots against the polar angle θ. Panel b combines several profiles with 0° ≤ θ ≤ 90° (data for 45° ≤ θ ≤ 90°) for different azimuthal angles in one graph. The graph in panel e covers the full polar angle range −90° ≤ θ ≤ 90° (data for −90° ≤ θ ≤ −50° and 40° ≤ θ ≤ 90°) for the plane with azimuthal angle φ = 0° (and also φ = 180°); i.e., it shows the profile for the plane that contains the primary ion beam. This

plot is directly comparable with those shown in Figure 1a,c,e for other types of projectiles and analytes. The finite dimensions of the silicon wafers used for collecting the ejected material limit the angular range interrogated. Variations in the location of sputter craters relative to the collector plates between experiments are the reason for differences in the ranges for the two angles covered. Normal Incidence. Figure 3a,b shows the case of normal incidence with E/n = 5 eV, 10 keV Ar2000 projectiles. No azimuthal dependence is expected. There is a polar distribution visible in the plots, with a maximum around 55°. No data for polar angles lower than 45° have been obtained due to the limited dimensions of the collector plates. But a reduced ejection of material at polar angles lower than 45° can be extrapolated from the available data. The preferred ejection at high polar angles is to be expected from published work on the 25321


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Estimation of Transfer Efficiency. Rectangular craters were eroded, with dimensions of 1.65 × 1.24 mm2 on the surface for this particular experiment (15° experiment with front and side collector plate). The XPS signal was monitored during the sputtering run to make sure that the target layer was eroded all the way through down to the Si substrate interface. The average thickness of PVD prepared sample layers was 1765 nm as measured by ellipsometry prior to sputtering. The total captured volume of 27.4 × 1014 nm3 accounts for 76% of the total volume of eroded organic material of 36.1 × 1014 nm3. A fraction of 30% of the sputtered material could be collected in the forward quadrant alone. Table 1 summarizes the data for different azimuthal ranges.

sputtering characteristics of metals with large cluster ions (cf. Figure 1). Values of about θ = 55° are lower than for the metals, but far from the polar angles observed for C60 sputtered organic material of about θ = 30°.27 There is a slight asymmetry visible in the plot in Figure 3a with respect to the edge where the two collection plates meet. This asymmetry can be attributed to the incomplete capture on one of the collector plates and the “rebouncing” of material which is then captured on the other plate. The sticking of the organic material on silicon has its own angular dependence which convolves with the data on the angular distribution of ejection.58 A decreased capture efficiency of material toward the edges of the collector plates is to be expected. To reduce the experimental error, maps have been cropped to the inner part with a maximum impact angle of ejecta on the collector surface of 60° (corresponding to a corrective factor of χ ≤ 8; cf. Supporting Information for details). 15° Polar Angle. Figures 3c−e show the case of 15° impact, also for E/n = 5 eV, 10 keV Ar2000 projectiles, i.e., a steep angle of incidence and only a slight deviation from the normal case shown in panels a and b. The slight change in the impact angle results in a surprisingly directed ejection (cf. Figure 3e for a direct comparison of the two cases for the plane with φ = 0° (180°)). Much of the material is ejected in forward direction (to the right-hand side in the graphs). The polar angle distribution of ejecta in forward direction (0° ≤ θ ≤ 90°) is very similar to the case of normal incidence. Azimuthal Distribution of Ejected Material. The total volume of organic material captured on the individual plates and its spatial distribution can be derived from the thickness maps by numerical integration. The thickness maps for the 15° case shown in Figure 3d are included in the Supporting Information as Figure SI-2. The amount ejected in the forward quadrant (azimuthal angle interval −45° to 45°) can be estimated to account for at least 10.9 × 1014 nm3, summing up 8.6 × 1014 nm3 captured on the front collector plate and 2.3 × 1014 nm3 captured on the side collector (cf. Figure SI-2). A significant volume of the sputtered material is ejected in a sideways direction. A volume of 7.3 × 1014 nm3 is collected in one of the side quadrants (azimuthal angle interval 45° to 135°). Mirror symmetry is assumed along the φ = 0° plane and a similar deposit volume can be expected for the second side quadrant (azimuthal angle interval −45° to −135°). The larger part of this volume (64.4%) is deposited forward of ±90°, i.e., within the intervals 45° to 90° and −45° to −90°. Thickness maps (data not shown) from the sputtering experiment equipped with side and back collector plates (shown in Figure 3c) provided a volume of 1.4 × 1014 nm3 for the material captured in the backward quadrant (−135° over 180° to 135°) and 5.3 × 1014 nm3 for one of the side quadrants. The ratio of the values for side and backward quadrants shall be used to estimate a volume of 1.9 × 1014 nm3 for the backward quadrant in the first experiment. Of the estimated total captured volume of 27.4 × 1014 nm3 (100%), 74% (20.3 × 1014 nm3) was ejected in forward direction and 40% (10.9 × 1014 nm3) in an azimuthal angle range of 90° symmetrical to the plane φ = 0° (forward quadrant). Only a minor fraction of 7% (1.9 × 1014 nm3) can be estimated to be ejected in the backward quadrant. About three-quarters of the total ejecta (20.3 × 1014 nm3) is emitted downrange (−90° to 90°) and one-quarter (7.1 × 1014 nm3) uprange (−90° over 180° to 90°).

Table 1. Azimuthal Distribution of Ejecta of Irganox 1010 Sputtered with 10 keV Ar2000 Projectiles at 15° Angle of Incidence azimuthal zone

range (deg)

quadrants forward right backward left downrange uprange total

−45 to 45 45 to 135 135 to −135 −135 to −45 90 to −90 −90 to 90

volume of material collected (1014 nm3)a

fraction (%) of sputtered materialb

≥10.9 7.3 1.9 7.3 ≥20.3 7.1 27.4

30 20 5.5 20 56 20 76

a

This uses only ranges where data is useful or can be inferred by symmetry. bVolume of collected material relative to 36.1 × 1014 nm3 total volume of sputtered material.

The estimation of the amount of material transferred from the sputter target to the collector plates is based upon the assumption that the captured material has a density comparable to the target organic layer of PVD deposited Irganox 1010. This can be safely assumed, as the range of densities of organic materials with similar structures is generally rather narrow.64 It becomes obvious from the thickness maps (cf. Figure SI-2) that the finite height of the collector plates prevents a complete capture of sputtered material. Interpolation of the symmetryinferred experimental data set can be used to estimate that the loss of material due to gaps between collector plates and in the low polar angle range is small (about 5%, data not shown). A nonunity sticking probability and the possible conversion of molecules to volatile fragments would further reduce the amount available for capture. The capture of such a large fraction of sputtered material is remarkable. Trends. Figure 4 shows plots for the angular ejection for a range of angles of incidence from close-to-normal (θ = 15°) to oblique (θ = 62°). Despite the rapid change observed from normal to 15° angle of incidence, only minor trends can be seen in this compilation for a wide range of incidence angles in Figures 4a−d and 4e−h. The ejection of sputtered material becomes increasingly directed with a more oblique incidence of the primary cluster ion beam. As indicated by the dashed lines in panels a and d, most of the sputtered material appears to be ejected in forward direction within the azimuthal angle interval −90° to 90°, while for the 62° case in panel d this interval can be narrowed down to −60° to 60°. The polar angle of preferred ejection shifts only slightly toward higher values (i.e., more oblique angles) with 25322


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Figure 4. (a−d) Angular distribution of ejection probabilities vs azimuthal (φ) and polar (θ) angles in a spherical coordinate system with the sputter crater located at the origin. From (a) to (d), the GCIB is positioned at an increasing angle to the target surface normal of (a) 15°, (b) 25°, (c) 45°, and (d) 62°. (e−h) Alternative representation of the data shown in panels a−d as polar distribution of ejection probabilities vs polar angle (θ) for different azimuthal angles (φ). All plots are normalized to individual scales.

90° azimuthal angular range and 68% within a 60° angular range. The 45° angle of incidence also provides the highest sputtering yields for organic molecules.62 A quick and symmetrical drop of these numbers can be observed when changing the angle of incidence by about 20° toward higher (i.e., the 62° experiment) or lower (i.e., the 25° experiment) values. About 60% are collected within the 90° azimuthal range at these impact angles, about 40% within a 60° azimuthal range. Table 2 illustrates that the ejection at 15° angle of incidence is significantly less directed than it is the case at more oblique incidence. The integrated data emphasize the extent to which ejecta of organic molecules are directed when sputtered with large cluster ions (cf. the already strongly directed emission for the 15° case in Figure 3e). The 45° case shows the most directed ejection of material for the sputtering with almost no material ejected in the backward quadrant. The sudden shift from an azimuthally uniform ejection at normal incidence to a pronounced directed ejection at only 15° deviation from the normal case with 74% of the ejecta traveling in forward direction is a very particular situation for sputtering with large gas cluster projectiles. The observations agree with previous data for the sputtering of metals with large Ar GCIB15,29 and with interpretations from MD simulations18 for the impact of cluster ion beams as discussed earlier (cf. Figure 1f). Implications for SNMS. Important implications for the instrumental design of SNMS experiments using large gas cluster ion projectiles follow from the distinct ejection characteristics of organic molecules when sputtered with these projectile species. At about 45° polar angle of ejection, a minimum applicable distance of the photoionization volume above the sample surface (as required for experimental reasons) will have a lateral offset of the most efficient photoionization region by a similar distance as consequence. The result is an off-

increasingly oblique incidence of the primary ion beam. This shift is anisotropic over the azimuthal range and most pronounced for low azimuthal deviation from the direction of the primary ion beam. Table 2 compiles data from the numerical integration of the thickness maps for the experiments presented in Figure 4. Table 2. Estimated Fraction of Total Sputtered Material in Azimuthal Angle Intervals fraction (%) of material total in φ = −150° to 150° collected within these azimuthal angle intervals angle of incidence (deg)

−90° to 90°

−60° to 60°

−45° to 45°

−30° to 30°

0 15 25 45 62

(60)a 76 89 98 94

(40)a 58 71 97 77

(30)a 40 58 94 60

(20)a 27 41 68 42

a

Fraction to expect for angle interval with material ejected homogeneously over total azimuthal range.

Shown is the volume of collected material for different azimuthal intervals as a fraction of the total volume collected in the −150° to 150° azimuthal range, i.e., 300° total range leaving out only a 60° azimuthal range in backward direction (assuming an insignificant contribution in that direction as it has been identified for the 15° case). Also included in Table 2 are the theoretical fractions for the normal incidence case where 83% of the total amount of ejected/collected material fall into the azimuthal range −150° to 150°. The integrated data confirm a trend toward a more directed ejection with increasingly oblique angles of incidence up to 45°. At 45°, the ejection is most directedwith more than 90% of the material from the full 300° azimuthal range collected in a 25323


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Figure 5. (a−d) SIMS spectra of organic deposits for Irganox 1010 sputtered with E/n = 5 eV, 10 keV Ar2000 projectiles. Spectra were recorded in negative ion mode using an E/n = 4 eV, 20 keV Ar5000 analysis beam. (a) SIMS spectrum of organic deposits on the front collector plate. (b) SIMS spectrum of Irganox 1010 used as sputter target for the experiment shown in (a). (c, d) Details of the spectra in (a) and (b).

axis geometry of sputter site, photoionization volume, and ion extraction optics. It is important to emphasize the difference to atomic and medium-size cluster projectiles frequently utilized in SNMS. Both atomic and C60 beams enable a more or less common geometry for SIMS and SNMS experiments, simply by inserting a postionization region into the path of ejecta which coincides with the path of secondary ions emerging from the sample surface. In a static extraction field, the pronounced lateral velocity component of GCIB sputtered ejecta would immediately spatially separate ionic species from neutral ejecta. Delayed extraction with a pulsed extraction field would be required in order to utilize a common geometry for SIMS and SNMS and to enable the simultaneous analysis with both techniques. The rather weak change of the polar angle of ejection with the primary ion beam angle of incidence (for off-normal angles) might make it possible to define a fixed position for the photoionization volume. On the other hand, the ejection at an almost constant polar angle relative to the sample surface normal means that ejecta trajectories are highly dependent on the local topography at the sputter site. Sample surfaces should optimally be flat and smooth. Chemical Identity of Collected Material. The total amount of collected ejecta (cf. Table 1 and discussion) does not provide any information on the degree of chemical changes (e.g., possible fragmentation) of the sputtered material during

the sputtering and collection processes. Figure 5 shows SIMS data for the deposits collected in a sputtering experiment with E/n = 5 eV 10 keV Ar2000 projectiles at 45° angle of incidence and the SIMS spectrum for the target Irganox 1010 layer for comparison. Figure 5a shows a SIMS spectrum for the collected deposits. The spectrum shows intense signals of the deprotonated analyte molecule [M−H]− (C73H107O12− at m/z 1176) and fragments that result from the loss of small hydrocarbon functions such as tert-butyl groups C(CH3)3 ([M−C4H9−H]− at m/z 1119, [M−2(C4H9)−H]− at m/z 1062). The lower mass region includes species formed in the fragmentation of the 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate substituents (the “legs” of the molecule; cf. Figure SI-3). These are m/z 205 (C14H21O−), m/z 231 (C16H23O−), and m/z 277 (C17H25O3−). Less specific but characteristic for Irganox 1010 SIMS spectra (cf. refs 61 and 65) are the ions at m/z 41 (C2HO−) and m/z 59 (C2H3O2−). Only species with sufficiently high sticking coefficients are captured on the collector plates. The chemical composition of the organic deposits thus only provides partial information about the changes occurring during the sputtering experiment (i.e., sputtering and collection steps). Smaller, more volatile species such as small fragments might not be collected. Although, our studies on the transfer efficiency show that these are a small fraction of the ejected material. It can be assumed that changes occurring during the SIMS analysis step are 25324


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SNMS experiments using a fixed ionization volume. It further means that the sputtering site in SNMS experiments with large Ar cluster projectiles should be located off-axis with respect to the ion extraction optics. The observed lateral sputtering behavior with virtually constant ejection angles relative to the surface normal probably means that ejection angles strongly depend on the sample surface topography. Changes in the surface topography could easily steer the flux of emitted neutrals out of the postionization region in an SNMS experiment and cause a decrease in signal intensity. A potential velocity distribution might render the situation less straightforward for postionization with pulsed light sources such as laser. The method used here does not allow information on velocities of ejecta and angular distributions for the velocity of ejecta. Data provide no information on the physical identity of captured species (particles, clusters, or molecules). Outlook. Experiments varying the projectile cluster size could help locating the shift from a homogeneous (high E/n projectiles) to a directed (low E/n projectiles) ejection. The combination with MD simulation efforts might enable access to ejecta velocities (not easily accessible with experiments) and lead toward a generalization of experimental finding for other systems of analytes and primary ion beams.

comparable to those occurring during the sputtering experiment, since similar analysis beams were used. The collector deposits thus underwent a similar, potentially fragmenting sputtering process twice (and the potentially fragmenting collection process in addition), while the Irganox 1010 target layer only underwent the sputtering process during SIMS analysis. Figure 5b shows a SIMS spectrum of the target Irganox 1010 layer (with detailed views in panels c and d). The spectrum looks surprisingly similar to the one for the collector deposits in panel a. Only minor increase of the fragment signals at m/z 1119 and m/z 1062 relative to the deprotonated analyte molecules at m/z 1176 can be observed in the collected material. Also, the fragments at m/z 231 and m/z 205 gain relative intensity compared to the target Irganox 1010 material. The collector spectrum contains no additional negative ion signals compared to the target layer. The close similarity of the SIMS spectra of collector and target material indicates that those fragments visible in the spectra (e.g., m/z 277, m/z 231, m/z 205, m/z 59, and m/z 41) are most probably among the species with low sticking probability. They contribute to the difference of volumes of sputtered and collected material determined for the transfer from the target layer to the collector (cf. Table 1 and discussion). The fragment species are probably formed during the SIMS analysis of Irganox 1010 and thus appear in the spectra for the target layer and the collector deposits with similar intensities. The spectra in Figure 5 provide evidence for a very close chemical identity of the collector deposits and the target Irganox 1010 material. Figure SI-4 describes the change of SIMS spectra with the azimuthal and/or polar deviation from the preferred angles of ejection. Only minor intensity changes could be observed for a broad range covering an azimuthal range wider than the forward quadrant (φ = 0°−54°) and covering about the full height of the collector plate (θ = 40°−82°). The SIMS spectra look very similar, providing evidence that the material ejected over a broad range of azimuthal and polar angles has a similar composition as the target Irganox 1010 material. The chemical similarity of target and collected material illustrates that the plume of sputtered material contains a high fraction of intact molecules. This is important evidence for the utility of postionization techniques to enhance the analytical sensitivity for molecular species.

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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.jpcc.6b06821. Equations utilized to derive ejection probability distributions from thickness maps of deposits on planar collector surfaces; discussion of thickness maps recorded in 15° angle of incidence experiment; structure and fragmentation of Irganox 1010 molecule; angular dependence of chemical composition of collected material (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], [email protected]; phone +44 20 8943 6535 (M.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Martin P. Seah for helpful comments and Steve A. Smith for provision of the samples. This work forms part of the 3D nanoSIMS project in the Strategic Capability program of the National Measurement System of the UK Department of Business, Innovation and Skills.

CONCLUSIONS Implications for SNMS. Large gas clusters ions (e.g., Ar2000) are capable of sputtering organic molecules under pronounced directed ejection (in polar and azimuthal angular dimensions). A substantial fraction of the eroded organic material stays intact during the sputtering process as indicated by almost identical SIMS spectra for the fresh and captured material. Both findings indicate a high potential to access a large fraction of ejected material as intact species with suitable postionization techniques, including laser-based photoionization schemes providing only small ionization volumes. The model system Irganox 1010 as sputtered with E/n = 5 eV, 10 keV Ar2000 cluster projectiles indicates that organic material sputtered with large Ar GCIB is ejected with a strong lateral velocity component. The dependence of the ejecta distribution on the incidence angle has been identified but is rather weak (except for a sudden initial change from normal incidence). These findings indicate the potential to design

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REFERENCES

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