Modification of Aromatic Self-Assembled Monolayers by Electron

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Modification of Aromatic Self-Assembled Monolayers by Electron Irradiation: Basic Processes and Related Applications Can Yildirim, Matthias Füser, Andreas Terfort, and Michael Zharnikov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11269 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 22, 2016

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The Journal of Physical Chemistry

Modification of Aromatic Self-Assembled Monolayers by Electron Irradiation: Basic Processes and Related Applications

Can Yildirim,1 Matthias Füser,2 Andreas Terfort,2 and Michael Zharnikov1* 1

Applied Physical Chemistry, Heidelberg University, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany

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Institut für Anorganische und Analytische Chemie, Universität Frankfurt, Max-von-LaueStraße 7, 60438 Frankfurt, Germany

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ACS Paragon Plus Environment


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Abstract

The effect of electron irradiation on aromatic thiolate self-assembled monolayers (SAMs) with oligophenyl, acene, and oligo(phenylene ethylene) (OPE) backbones, containing from one to three phenyl rings, was studied, with emphasis on the basic irradiation-induced processes and performance of these films as negative resists in electron lithography. All films exhibited similar behavior upon the irradiation, with clear dominance of cross-linking, taking hold of the systems at already very early stages of the treatment. The cross-sections for the modification of the SAM matrix and the damage of the SAM-substrate interface were determined for the primary electron energy of 50 eV, frequently used for the fabrication of carbon nanomembranes (CNM). They show only slight dependence on the backbone character as demonstrated by the example of the three-ring films. The two-ring systems exhibited the best performance as lithographic resists, with an optimal dose of 10-20 mC/cm2 at 0.5-1 keV. The performance of the one-ring and three-ring systems was limited by a poor ability to form an extensive cross-linking network and by high resistivity of the pristine films to the etching agents, respectively. Another process, associated with the poor lithographic performance of the three-ring systems but occurring at high doses for the two-ring systems as well, was a spontaneous release of the cross-linked films within the irradiated areas, in form of CNM pieces. From the lithographic data, cross-sections of the irradiation-induced crosslinking were derived and discussed in context of backscattering and secondary electron yield. For the three-ring systems, fabrication of CNMs was demonstrated, for the first time in the OPE case.

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1. Introduction Self-assembled monolayers (SAMs) represent an indispensible element of modern nanotechnology.1-3 Their numerous applications rely mostly on flexibility of their design, allowing a combination of different functional groups within the general architecture of the SAM constituents, comprised of a suitable anchoring (head) group, rod-like spacer, and a terminal tail group. Along with the flexibility of the chemical design, additional possibilities are provided by modification of SAMs by physical means such as ultraviolet light and electron irradiation. The respective options include, among others, preparation of sophisticated mixed SAMs,4,5 SAM metallization,6,7 SAM-based lithography and derived nanofabrication,8-19 and fabrication of SAM-based carbon nanomembranes (CNMs)20-26. In context of lithography and CNM fabrication, aromatic SAMs are of particular importance. In contrast to aliphatic monolayers, which become mostly desintegrated upon electron irradiation,27 the reaction of aromatic SAMs to this treatment is dominated by extensive crosslinking, following the cleavage of C−H bonds in the SAM matrix.8,27-30 Such a cross-linking transforms the molecular assembly into a stable and homogeneous 2D film, preventing, at the same time, desorption of molecular fragments and complete damage of the SAM-substrate interface. Accordingly, aromatic SAMs can potentially serve as a negative resist for electron lithography8,10,27 and, after a separation from the substrate, represent CNMs,20-26 which can also be transformed into graphene-like sheets by subsequent pyrolysis.25,26,31 So far, most of the experiments related to electron lithography and CNM fabrication, relying on the aromatic SAMs, were performed with biphenyl-based monolayers.8-10,26,29 However, as was shown recently, CNMs and the derived graphene-like sheets can be prepared from other aromatic precursors as well.25,32,33 This was demonstrated for a variety of molecules, including those with oligophenyl and acene backbones of variable length.25 The emphasis of these studies was put on the properties of the resulting CNMs, including their structural,25 mechanical,32 and electric transport33 characteristics, while the exact behavior of the different CNM precursors under electron irradiation was addressed only marginally. In this context, in the given study, we perform detailed spectroscopic characterization of the changes occurring in the SAMs with oligophenyl (non-fused) and acene (fused) backbones (see Figure 1) upon electron irradiation. To test a broader variety of rod-like aromatic SAM precursors, we also study a series of SAMs with oligo(phenylene ethylene) (OPE) backbone of variable length (see Figure 1). Further, in addition to the spectroscopic experiments, which give only indirect information about irradiation-induced cross-linking, crucial in the given case, dedicated lithographic studies were performed, to monitor the cross-linking behavior and to test the 3



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suitability of different aromatic SAMs as a negative resist for electron lithography. Finally, fabrication of CNMs was demonstrated, which was of particular importance in the case of the OPE-based SAMs that had not been utilized so far for this purpose.

2. Experimental part All solvents and chemicals were purchased from Sigma-Aldrich. The SAM precursors used in this study are shown in Figure 1, along with their abbreviations. They were either purchased from Sigma-Aldrich (PT, BPT, TPT, NphT, and OPE3) or custom-synthesized (OPE2 and AnthT). The recipe for the syntheses of AnthT can be found in the literature.34 OPE2 was synthesized according to the following procedure, using S-(4-(phenylethynyl)phenyl, synthesized according to a literature protocol35. A mixture of CuI (0.09 g, 0.5 mmol, 0.06 eq) and Pd(dppf)Cl2 (0.16 g, 0.21 mmol, 0.03 eq) was dissolved in N,N-diisopropylamine (25 mL) and degassed by purging with argon for 30 minutes. To this solution, S-(4iodophenyl) ethanethioate (2.3 g, 8.1 mmol, 1.0 eq) and phenylacetylene (0.97 mL, 0.90 g, 8.8 mmol, 1.1 eq) were added and stirred at room temperature overnight. The solvent was removed in vacuum and the residue was redissolved in DCM and filtered through a celite pad. Further purification by column chromatography (10:1 hexanes/ethyl acetate) yielded the desired product as a yellow solid. Yield: 1.70 g (83%).1H NMR (250 MHz, CDCl3) δ = 7.61 – 7.49 (m, 4H), 7.45 – 7.32 (m, 5H), 2.44 (s, 3H). The gold substrates were prepared by thermal evaporation of 70-75 nm of gold (99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense) that had been precoated with a 5 nm titanium adhesion layer. For the CNM fabrication, analogous substrates without the titanium layer were used. The films were polycrystalline, exposing preferably (111) orientated surfaces of individual crystallites. The SAMs were prepared by immersion of the fresh substrates into 0.5-1 mmol solutions of the respective precursors in absolute ethanol (NphT and AnthT) or DMF (PT, BPT, TPT, OPE2, and OPE3) for 24 h at room temperature. For deprotection of the OPE2 and OPE3 precursors, triethylamine was added. After immersion, the films were rinsed with ethanol or DMF and blown dry with argon. Extensive characterization showed no evidence of impurities or oxidative degradation products. In addition, reference SAMs of hexadecanethiolate (HDT) were prepared on the same substrates using a standard procedure.36 The effect of electron irradiation was monitored by laboratory X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and secondary electron microscopy 4


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(SEM). Both irradiation and characterization were performed at room temperature. The spectroscopic experiments were only conducted for the TPT, AnthT, and OPE SAMs, representative of the respective series (see Figure 1). The samples for the spectroscopic experiments were homogeneously irradiated with a flood gun (FG20, Specs, Germany), at a base pressure better than 1×10-8 mbar. The electron energy was set to 50 eV and the dose was calibrated by an array of Faraday cups. The XPS characterization was performed in situ, immediately after the irradiation and without exposure of the samples to ambient. The measurements were carried out with a MAX200 (Leybold-Heraeus) spectrometer equipped with an Mg Kα X-ray source (200 W) and a hemispherical analyzer. Normal emission geometry was used. The recorded spectra were corrected for the spectrometer transmission and the binding energy (BE) scale was referenced to the Au 4f7/2 emission at 84.0 eV.37 The spectra were fitted by symmetric Voigt functions and a Shirley-type background. To fit the S 2p3/2,1/2 doublets we used two peaks with the same full width at half-maximum (fwhm), the standard37 spin-orbit splitting of ~1.2 eV (verified by fit), and a branching ratio of 2 (S 2p3/2/S 2p1/2). The fits were performed self-consistently: the same fit parameters were used for identical spectral regions. The effective thickness of the monolayers was calculated using a standard procedure,38 based on the C 1s/Au 4f intensity ratio. A standard expression for the attenuation of the photoemission signal was assumed;39 attenuation lengths reported in ref 40 were used. To determine the spectrometer-specific coefficients, we took molecular films of known thickness as direct reference; the respective samples were measured at the same conditions as the aromatic SAMs. As a reference film we used a HDT monolayer on Au(111). To monitor the cross-linking effects and the performance of the SAMs as resists, lithographic patterns on their basis were fabricated by e-beam lithography (EBL) and proximity printing lithography (PPL) which involved irradiation of the sample by a homogeneous and broad electron flux through a mask. The EBL experiments were performed for all SAMs studied (Figure 1), while the PPL experiments were conducted with the AnthT monolayers only. The EBL patterns were written by a LEO 1530 scanning electron microscope equipped with a Raith Elphy Plus Pattern Generator System. The e-beam energy was varied from 0.5 to 5 keV; the dose was calibrated by a Faraday cup; and the base pressure was ~5×10-6 mbar. The PPL patterns were written by homogeneous electron irradiation through a mask (Quantifoil, R 1/4, Plano, representing holey (1 µm) carbon film over Cu 400 mesh) placed on the SAM surface. The same flood gun as for the spectroscopic experiments was used. The e-beam energy was 5



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set to 50 eV; the dose was calibrated by a Faraday cup; the base pressure in the chamber during the irradiation was better than 1×10-8 mbar. The patterned SAMs on the Au/Si substrates were immersed into a thiosulfate based etching solution for 30-40 minutes at room temperature.41 The etching bath consisted of a 1 M KOH solution containing 0.1 M K2S2O3, 10 mM K3Fe(CN)6, and 1 mM K4Fe(CN)6. Subsequently the samples were washed copiously with Millipore water and dried with nitrogen. Fabricated gold patterns were imaged by AFM and SEM using a Solver Next device (NTMDT) and a LEO 1530 scanning electron microscope (Zeiss), respectively. The AFM measurements were carried out in tapping mode under ambient conditions. In addition to the lithographic experiments, CNMs were fabricated. Gold substrates without the adhesive titanium interlayer were used. The procedure mostly followed to the wellestablished protocols of refs 21 and 22. In brief, the SAMs were homogeneously irradiated by electrons (50 eV) with a dose of 40 mC/cm2. The resulting, cross-linked monolayers were spin-coated with a polymethylmethacrylat (PMMA) layer (950 kDa; 5 % in chlorobenzene) at 4000 rpm. The PMMA/CNM/Gold/Silicon samples were then carefully immersed into water under a grazing angle to peel the PMMA/CNM/Gold layer from the silicon support, following a recipe of ref 24. The gold was subsequently dissolved in Lugol’s solution (aqueous KI/I2, 2 %). Finally, the SAM/PMMA sheets were transferred onto supporting metal grids (1500 mesh; Plano) and baked on hot plate at 50 °C for 2 min. The dissolution of the PMMA in acetone and the drying of the CNMs were carried out in a critical point dryer (Automated Critical Point Dryer, Leica EM CPD300). The resulting CNMs were imaged by SEM using a LEO 1530 device; the e-beam energy was set to 1 keV; in-lens detector was used.

3. Results 3.1 Spectroscopic Characterization For detailed monitoring of irradiation-induced processes by XPS, the TPT, AnthT, and OPE3 SAMs were selected, representative of the oligiphenyl, acene, and OPE series. These films exhibited similar behavior upon electron irradiation. Au 4f, C 1s, and S 2p XPS spectra of the pristine and irradiated OPE3 SAMs are presented in Figure 2, representative of the TPT and AnthT monolayers as well. The irradiation was performed homogeneously over the sample, the electron energy was set to 50 eV, and the dose was varied. The Au 4f spectra in Figure 2a exhibit the Au 4f7/2,5/2 doublet from the gold substrate. The intensity of the Au 4f7/2,5/2 components does not change noticeably with increasing dose, 6


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suggesting a very low extent (if any at all) of irradiation-induced material loss. The C 1s spectrum of the pristine OPE3 SAM in Figure 2b exhibits a strong peak corresponding to the OPE backbone42 and a minor shoulder at higher BE, alternatively assigned to the carbon atom bonded to the sulfur headgroup or to shake-up processes in the aromatic matrix (see discussion in ref 43). No features corresponding to possible oxygen contamination or to the remaining protection groups were observed, which was also supported by the O 1s spectrum (not shown). The intensity of the C 1s peaks as well as the character and shape of the C 1s spectrum of the OPE3 SAM do not exhibit any perceptible changes upon irradiation. This suggests, in agreement with the Au 4f data, a very low extent (if any at all) of irradiationinduced material loss, which is also supported by the behavior of the effective thickness. The respective dependence of this parameter for the TPT, AnthT, and OPE3 SAMs on irradiation dose is shown in Figure 3a. Accordingly, the effective thickness did not decrease in the course of the irradiation but even increased slightly. The latter is presumably related to the irradiation-induced adsorption of residual gas molecules on the SAM surface, well-known in the electron-beam induced deposition community.44,45 The doses applied (1 mC/cm2 corresponds to ~13 primary electrons pro SAM constituent) were obviously sufficiently high to induce such an adsorption even at the UHV conditions in the electron irradiation chamber. The lack of any perceptible changes in the C 1s spectrum upon the irradiation can lead to a wrong conclusion that no chemical changes in the SAM matrix occur. This is, however, not true as follows from a detailed analysis of these spectra. The derived parameters, viz. the BE position and FWHM of the main C 1s peak for the TPT, AnthT, and OPE3 SAMs are shown in Figures 3b and 3c, respectively, as functions of the irradiation dose. As seen in these figures, these values exhibited small but well-traceable changes upon the irradiation, with a saturation-like behavior achieved already at ~10 mC/cm2. These changes can be associated with progressive cross-linking of the SAM constituents, following the extensive cleavage of C−H bonds in the SAM matrix. The threshold for such processes lies at ~7 eV,46,47 far below the electron energy in our experiments. Both BE positions and FWHMs of the main C 1s peak for the SAMs studied exhibit an exponential-like variation with the increasing dose, with the respective, leveling off behavior at high doses. Such a behavior is typical for SAMs27 and can be described by a function I = Isat + (Ipris − Isat ) × exp(-σQ/eSirrad ),

(1)

where I is the value of a characteristic film parameter in a course of irradiation, Ipris and Isat are the parameter values for the pristine and strongly irradiated films (a leveling off behavior), 7



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respectively, Q is the cumulative charge delivered to the surface in Coulombs, e is the electron charge, Sirrad is the area irradiated by the electron beam, and the cross-section σ is a measure of a rate at which the saturation behavior is achieved. The respective fitting curves are shown in Figures 3b and 3c, and the derived cross-sections, averaged over the FWHM and C 1s behavior, are compiled in Table 1. The values for the TPT and OPE3 SAMs are similar while that for the AnthT monolayer is noticeably lower. The latter is probably related to the conjugated character of the AnthT backbone; consequently, irradiation-induced cross-linking is less pronounced in the C 1s XPS spectrum than in the TPT and OPE3 case. Along with the SAM matrix, the SAM-substrate interface was modified upon the electron irradiation as follows from the S 2p XPS spectra of the OPE3 SAMs shown in Figure 2c, representative of the TPT and AnthT monolayers as well (similar behavior). The spectrum of the pristine OPE3 SAM exhibits a characteristic S 2p3/2,1/2 doublet at a BE position of ~162.0 eV (S 2p3/2) corresponding to the thiolate species bound to noble metal surfaces,48,49 with no traces of atomic sulfur, disulfide, unbound sulfur or oxidized species. Upon the irradiation, another component doublet at a BE position of ~163.4 eV (S 2p3/2) appears and progressively increases in intensity with increasing dose on the expense of the doublet stemming from the thiolate group. This behavior corresponds to the irradiation-induced cleavage of the original S−Au bonds and appearance of new species which might be disulfides but most likely have C−S−C character.29,50 The intensities of both S 2p component doublets as well as the total intensity of the S 2p signal for the TPT, AnthT, and OPE3 SAMs are presented in Figure 4 as functions of the irradiation dose. The total S 2p intensity changed hardly in the course of the irradiation which, along with the Au 4f and C 1s XPS data, suggests a very low extent of the irradiation-induced material loss in the SAM studied (apart from the loss of H, following the cleavage of C−H bonds). At the same time, the intensities of the doublets corresponding to the thiolate and irradiation-induced species exhibit an exponential-like dependence on the irradiation dose, with the corresponding leveling at high doses. Using similar approach as in the case of the C 1s spectra, fitting according to Eq. 1 could be performed. The respective fitting curves are shown in Figure 4, and the derived cross-sections are compiled in Table 1, averaged over the curves describing the intensities of S 2p component doublets corresponding to the thiolate and irradiation-induced species. These cross-sections are quite similar for the SAM studied, with the highest value for the TPT film and the lowest value for the OPE3 monolayer. At the same time, these parameters are by almost two orders of magnitude lower than the analogous value for the reference HDT SAM (5.5±0.4×10-16 cm2) where the irradiation-induced damage dominates over cross-linking. In contrast, in the aromatic SAMs, 8


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including those of the present study, extensive cross-linking, occurring quite fast in the course of irradiation, prevents and quenches any displacement of the SAM constituents, necessary for the cleavage of S−Au bonds, hindering the respective processes. Even if a bond can be broken, it can recombine if the respective "fragments" stay in place.51 This is certainly true for such a "fragment" as the substrate, but is also the case for the assembled molecules as far as they do not only interact weakly with each other but are covalently linked to the matrix.

3.2 Lithographic Experiments Cross-linking behavior in the aromatic matrix can only be monitored indirectly by spectroscopic experiments because of the lack of characteristic features for this process, even though the desorption of H, following the cleavage of C−H bonds and representing a prerequisite for the cross-linking, can be directly monitored based on the characteristic vibration modes8,30,52,53 or by mass-spectrometry47,53. Therefore, complementary lithographic experiments were performed. The idea of these experiments was to monitor the resist performance of the SAMs studied as function of the irradiation dose and electron energy, providing also a test for the suitability of these films for electron lithography. Two sets of the experiments were performed. Within the first set, a pattern of square-like features was "written" by EBL using a scanning electron microscope with a pattern generator system (see Section 2 for details). The irradiation dose was varied from one square-like feature to another at a fixed electron energy, which, in its turn, was varied from pattern to pattern (0.5, 1, 3, and 5 keV). Within the second set, a pattern of dot-like features was "written" by PPL using the same electron source (a flood gun) and the same electron energy (50 eV) as for the spectroscopy experiments (see Section 2 for details). This allowed to correlate the results of the spectroscopic and lithographic experiments. All patterns were etched using the same procedure and imaged by SEM (not shown) and AFM afterwards (see Section 2 for details). Aromatic SAMs perform generally as negative resists for electron lithography.8,11,27 The general idea is that the cross-linked SAM within the irradiated areas protects the underlying substrate against the etching while the non-irradiated areas, protected by the pristine SAM consisting of individual molecules, can be etched much more easily. Consequently, a pattern of well-defined, square-like or dot-like Au features was initially expected by us, with the height of the features reflecting the resist ability of the SAM, given by the extent of the crosslinking. The observations were, however, somewhat different, as illustrated by Figure 5 where the AFM images of representative Au/Si(100) patterns prepared by EBL with the NphT and 9



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AnthT SAM resists are presented, along with the corresponding height profiles across the Au features. The expected, square-like Au features are only observed at low doses, while the features prepared at higher doses retrace the perimeters of the written squares. The only explanation for this behavior is a release of the entire cross-linked SAM within the irradiated area upon the etching, presumably in the form of CNM pieces. Indeed, with increasing doses, cross-linking becomes more extensive but the anchoring to the substrate gets impaired progressively (see Figures 2c and 4). Accordingly, the cross-linked SAM pieces within the irradiated areas can be easily released into the etching solution as soon as the irradiation dose is sufficiently high, leaving the underlying Au substrate unprotected against the etchant. The perimeter-like Au structures observed instead of the initially written square-like features can then be only related to the joint effect of the backscattered primary electrons and the secondary electrons from the substrate.54 These electrons are reflected and emitted not only within the irradiated areas but also along their perimeter, providing a lower irradiation dose than that by the combined effect of the primary, backscattering, and secondary electrons within the predefined spots. Accordingly, the SAM along the perimeter is still well-anchored to the substrate and somewhat cross-linked, providing a protection against the etching. Similar behavior was also observed in the PPL experiments, which were only performed with the AnthT SAM resist, representative of all other monolayers of this study. AFM images of two characteristic gold patterns, prepared at irradiation doses of 6 mC/cm2 and 40 mC/cm2, are shown in Figures 6a and 6b, respectively. Whereas the predefined, dot-like features are observed at the low dose (Figures 6a), a circle-like pattern was fabricated at the high dose, following the release of the cross-linked SAM within the irradiated areas. Interestingly, there are clear effects of the SAM thickness and the electron energy, as was especially obvious in the EBL experiments. The transition from the well-defined pattern to the etching-mediated CNM release occurs at lower doses for the thicker SAMs (compare Figures 5b and 5c) and at a lower electron energy for the same SAM (compare Figures 5a and 5b). This means that the cross-linking is more efficient for a thicker monolayer, which is understandable considering that the irradiation-induced cleavage of C−H (and C−C) bonds, which is a prerequisite for the cross-linking, can be quenched by energy-transfer to the substrate.46,47 The efficiency of such a quenching decreases with increasing distance to the substrate and is, accordingly, less efficient, for longer molecules, resulting in a higher extent of cross-linking. An additional effect in this context is a stronger cross-linking in the vicinity of the SAM-ambient interface as compared to the SAM-substrate one for any particular film.55 10


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The effects of the SAM thickness and electron energy were observed for all three series studied (oligophenyls, acenes, and OPEs). Note that the first member of all three series, PT/Au, exhibited a negative resist behavior as well, even though with inferior performance (lower contrast of the lithographic patterns) as compared to the monolayers comprised of the longer molecules. This is understandable since most of the molecular models of cross-linking involve simultaneous cross-linking of several rings of a particular molecular backbone, which is necessary for the formation of extended, 2D polymer-like network.28-30 One ring is obviously not enough to form such a network of comparable quality. In contrast, all two-ring systems performed quite well as negative lithographic resists, until the onset of the spontaneous CNM release at high irradiation doses. The AnthT film performed similar as well (but only at low doses), in contrast to the TPT and OPE3 monolayers which did not provided reasonable lithographic patterns under the conditions of our experiments. To quantify the results of the lithographic experiments we plotted the height of the square-like gold features in the patterns prepared with the SAM resists as function of irradiation dose. An example of such a plot, for the NphT resist (representative of all SAMs of this study), is presented in Figure 7a. The parts of the plots at low doses represent straight lines at the given scaling and can therefore be described by exponential functions, similar to Eq. 1. The respective cross-sections for all SAMs studied except for OPE3 (the data could not be derived, see above) are compiled in Figure 7b, as functions of the primary electron energy in the range of 0.5-5 keV (first set of the experiments). For the AntT SAM, the value corresponding to an electron energy of 50 eV is also given; it was determined in the second set of the lithographic experiments. This value is additionally presented in Table 1.

3.3 CNM Fabrication The ability of the SAM studied to serve as precursors for CNMs, important in general but also in context of the given lithographic experiments (spontaneous release of the CNM pieces upon the wet chemical etching), was tested by the example of the TPT, AnthT, and OPE3 monolayers. SEM images of the respective CNMs are presented in Figure 8. Note that whereas the fabrication of the TPT- and AnthT-derived CNMs has already been reported,25 their preparation from the OPE SAM is demonstrated for the first time, extending the family of potential CNM precursors. Note also that no specific characterization of the CNMs beyond their imaging was performed since the focus of this study was put on the spectroscopic and lithographic experiments. The CNMs could be reproducibly prepared on different supports, including the metal grids used for the membranes shown in Figure 8. 11



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4. Discussion The results of the spectroscopic, lithographic, and CNM fabrication experiments agree well to each other, suggesting that the dominant effect of electron irradiation in the oligophenyl-, acene-, and OPE-based SAMs is extensive cross-linking in the aromatic matrix. The driving force of the cross-linking is the cleavage of C−H bonds in the matrix, with the subsequent desorption of released H atoms and derived H2 molecules.53 The primary electron-molecule interactions are electronic attachment, impact ionization, and impact electronic excitation, with electronic attachment being a resonant, energy-specific effect, observed usually at low electron energies (several eV),46,30 and impact electronic excitation considered to be marginally contributing only, at least at an energy of 50 eV.53 These processes result in the formation of dissociative complexes or radicals which evolve with the release of hydrogen. Such a release can occur at several different positions in a molecular chain followed by multiple and, most likely, differently directed cross-linking bonds between the neighboring molecular backbones. Some reasonable suggestions for specific structural motifs within the electron-induced cross-linking network in aromatic SAMs can be found in literature.28-30 Obviously, as shown is this study, a single phenyl ring (PT) is not entirely sufficient to form a well-defined 2D cross-linking network. Significantly, the cross-linking occurs quite rapidly in the course of irradiation. Otherwise, the damage of the SAM-substrate interface could not be hindered. Such a damage takes place very rapidly in the systems where cross-linking is a minor process, such as alkanethiolate SAMs, along with extensive desorption of molecular fragments.27,47,56 Analogous desorption is mostly blocked in the aromatic SAMs, both because of the cross-linking and the cyclic character of the aromatic rings.57 Therefore, the effective thickness of these SAMs does not change noticeably in the course of the irradiation (see Figure 3a). The dynamics of the irradiation-induced processes in the aromatic matrix and at the SAMsubstrate interface is characterized by the respective cross-sections, compiled in Table 1. We believe that these values are reliable, except probably for the SAM matrix value for the AnthT SAMs, which appears to be somewhat too low. We think that the reason for a possible underestimate is the strongly conjugated character of the anthracene backbone, so that the effect of the cross-linking, resulting in the formation of conjugated structures as well,28-30 is not that pronounced in the C 1s XPS spectrum. An additional factor is the comparably small length of the AnthT backbone, so that the primary excitation, leading to the cleavage of a C−H bond, can be quenched efficiently by dipolar damping to the gold substrate, analogous to 12


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the mechanism suggested in ref 46. Significantly, the cross- sections in Table 1 correlate well with the literature values. In particular, the cross section for the electron beam (50 eV) damage of the SAM-substrate interface in the BPT SAMs on Au was estimated at 1.75×10-17 cm2 (average value for the damage of the pristine headgroups and formation of new, sulfurderived species) in a series of independent experiments,56 which is very close to the value of (1.7±0.3)×10-17 cm2 obtained for the TPT monolayers in the present work. Also, the effective cross-section of the hydrogen content loss in the TPT SAMs on Au was estimated at (2.74.7)×10-17 cm2 for electron processing at 50 eV by high-resolution electron energy loss spectroscopy and electron-stimulated desorption experiments,53 in excellent agreement with our SAM matrix value of (3.7±0.9)×10-17 cm2 for the same system. Significantly, the latter correlation suggests indirectly that our cross-section values for the modification of the SAM matrix, obtained on the basis of the C 1s XPS spectra, reflect predominantly the hydrogen content loss in the matrix. The cross-linking in the SAM matrix should basically mimic the dynamics of the hydrogen content loss, occurring by creation of C−C bonds between two neighboring molecules following dissociation of C−H bonds. In addition, a reaction of electronically excited molecules or radicals accompanied by hydrogen (H or H2) abstraction is possible. It looks, however, that the effect of the cross-linking has a faster dynamics than the hydrogen content loss. Otherwise, extensive damage of the SAM/substrate interface, occurring at the very low irradiation doses in the films where the cross-linking is a minor process only, could not be prevented. The respective cross-section of this process in such systems, AT SAMs on Au(111), was estimated at 5.5±0.4×10-16 cm2 in this work and at 2.5×10-16 cm2 in ref 58 for 50 eV electron processing, which is by one order of the magnitude higher than all the crosssections determined from the spectroscopy data for the aromatic SAMs in the given work (Table 1). A further argument lies in the comparably high values of the cross-sections for the cross-linking processes determined within the lithographic experiment. A direct comparison to the spectroscopy-derived values can be done for the AnthT SAM only (see Table 1 and Figure 7b) since the same primary energy (50 eV) was applied in both spectroscopic and dedicated (the second set) lithographic experiments. The values for the other SAMs, obtained at the higher electron energies only, can be tentatively scaled following the behavior of the curve for the AnthT film (Figure 7b); accordingly, the cross-sections at 50 eV are similar to those at 1 keV.

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The energy dependence of the cross-linking cross-section for the AnthT SAM in Figure 7b represents a bell-like curve, mimicked, starting from 500 eV, by the analogous curves for all other SAMs of this study with the exception of PT/Au which seems to be a special case (see above). Significantly, the modification of the SAMs is mediated not only by the primary electron beam but also by the backscattering (elastic) and (true) secondary electrons. The electron backscattering factor depends on the atomic number of the substrate material and electron energy, being quite large for Au (close to 1.0 at 500 eV).59 Significantly, it decreases with increasing electron energy,59 similar to the cross-section curves in Figure 7b (starting from 500 eV). The secondary electron yield exhibits a bell-like behavior with increasing electron energy, with a maximum at ~500 eV for gold,60 mimicking the curve for the AnthT SAM in Figure 7b. Accordingly, a significant effect of the secondary and backscattering electrons within the entire impact of the electron irradiation can be assumed, at least at high kinetic energies, above 500 eV. In particular, this effect is responsible for the perimeter-like "decoration" of the predefined spots in the lithographic patterns written with a high dose (Figures 5 and 6). For low kinetic energies, such as e.g. 50 eV, the contribution of the secondary electrons is believed to be small, referring, however, to specific, resonant dissociative electronic attachment processes,53 whereas non-resonant processes, leading also to the loss of H, can be mediated by the secondary electrons as well. Note that the yield of the true secondary electrons is generally higher than that of the backscattering, elastic electrons, except for very low primary energies (less than 30 eV), but the secondary electrons mostly have low kinetic energies, with a maximum of the distribution centered at 6-7 eV.54 Consequently, some of the secondary electrons are well capable of cleaving C−H bonds, since the threshold for the respective process was estimated at ~7 eV.46,47 The final issue of the discussion is the performance of the SAMs studied as negative resists for electron lithography. According to our experiments, the two-ring systems (BPT, NphT, and OPE2) are most suitable for this purpose, but the dose should be carefully selected to be high enough to initiate sufficient cross-linking but low enough to avoid a spontaneous release of CNM pieces in the case of wet etching procedure. A dose of 10-20 mC/cm2 appeared to be most suitable at 0.5-1 keV (see Figure 5) and is somewhat higher, up to 30-40 mC/cm2, at higher primary energies. In contrast, the PT SAMs exhibited a poor performance as resist, which is presumably related to a limited extent of cross-linking, associated with one-ring system (see above). A special case are the three-ring systems, with only the AnthT monolayer exhibiting a reasonable resist performance (see Figure 5), even though at low doses only, inferior to that of 14


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the NphT film. On first glance, this is hardly explainable, since the respective monolayers become extensively cross-linked in the course of electron irradiation. However, effective etching of the substrate relies on the difference in the protection ability of the monomolecular resist within and beyond the irradiated areas. The protection ability of the cross-linked threering films is certainly high enough, but, presumably, it is also the case for the pristine areas, relying on the well-known higher structural quality of the longer backbone SAMs as compared to the shorter ones.42,48,49,61 Thus, no pronounced contrast etching is observed, until, at high doses, a spontaneous release of the CNM pieces occurs while the non-irradiated areas still show a persistent protection behavior. As a result, not the non-irradiated but the irradiated areas become preferably etched, resulting in an inverse (positive) contrast of the lithographic patterns. Nevertheless such a contrast can be much more easily achieved with the aliphatic SAMs, which by themselves represent a positive resist for lithographic applications.27

5. Conclusions The effect of electron irradiation on aromatic thiolate SAMs was studied, with a particular emphasis on the evolution of the basic irradiation-induced processes and performance of these films as negative resists in electron lithography. The most basic aromatic systems, with rodlike oligophenyl, acene, and OPE backbones, were addressed, with the length of the backbone being varied and with the OPE-based films being investigated for the first time in the given context. All SAMs studied exhibited similar behavior upon the electron treatment, with clear dominance of cross-linking, following the cleavage of C−H bonds in the SAM matrix and slowing down the damage of the SAM-substrate interface and material loss from the film. The cross-sections for the modification of the SAM matrix and the damage of the SAM-substrate interface were determined for the primary electron energy of 50 eV, frequently used for the CNM fabrication.25,29 The derived values were found to be similar for a particular process, showing only slight difference for the different backbones. The effect of cross-linking took hold of the systems at already very early stages of the treatment, affecting the other, irradiation-induced processes and ensuring performance of the films as a negative lithographic resist. The two-ring systems (BPT, NphT, and OPE2) exhibited the best performance as lithographic resists, with an optimal dose of 10-20 mC/cm2 at 0.5-1 keV. The one-ring system (PT) showed a poor performance, which was explained by a limited ability to form an extensive, 2D cross-linking network. The three-ring systems (TPT, AnthT, and OPE3) exhibited a poor lithographic performance, because of the high resistivity of the pristine films to the etching agents, resulting in a low etching contrast between the 15



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irradiated and non-irradiated areas. Another process associated with the poor lithographic performance of the three-ring systems, but occurring at high doses for the two-ring systems as well, is a spontaneous release of the cross-linked SAMs within the irradiated areas, in the form of CNM pieces. This process resulted either in spot-perimeter-decoration patterns or even in a reversal of the lithographic contrast. From the lithographic data, cross-sections of the irradiation-induced cross-linking were derived for all SAMs studied. These cross-sections decreased with increasing electron energy in the energy range of the experiments (0.5 - 5 keV). For the AnthT film, where the data for 50 eV were obtained as well, the dependence of the cross-section on the electron energy exhibited a bell-like curve with a maximum at 500 eV, mimicking the behavior of the secondary electron yield from the gold substrate. For the three-ring systems (TPT, AnthT, and OPE3), fabrication of CNMs was demonstrated, reproducing the literature data25 for the TPT and AnthT films and getting this result for the first time for the OPE3 monolayer.

Acknowledgement MZ appreciates a financial support of the German Research Society (Deutsche Forschungsgemeinschaft; DFG). We thank Prof. A . Turchanin for useful advices regarding the preparation of the CNMs.

Author information Corresponding Author *Phone: +49 6221 54 4921. Fax: +49 6221 54 6199. E-mail: [email protected]

Notes The authors declare no competing financial interest.

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Figure Captions Figure 1. The structures of the precursors for the SAM studied, along with their abbreviations. The precursors build three different series, with oligophenyl, acene, and OPE backbones, respectively. PT serves as the first member of all three series. Figure 2. Au 4f (a), C 1s (b), and S 2p (c) XPS spectra of the pristine and irradiated OPE3 SAMs. The doses are marked at the respective spectra. The energy of the electrons was 50 eV. The C 1s spectra are decomposed into the component peaks, viz. the main peak (green lines) and a minor shoulder (blue lines); see text for details. The S 2p spectra are decomposed into the component doublets corresponding to the pristine thiolate species (green lines) and irradiation-induced sulfur species (blue lines); the sum of these components is drawn by the red lines. Vertical solid lines in panel (c) highlight the BE positions of the doublets. Figure 3. Dependence of the effective thickness (a), BE position of the main C 1s peak (b), and FWHM of this peak (c) for the TPT (red triangles and red dashed curves), AnthT (blue squares and blue dashed curves), and OPE3 (black circles and black dashed curves) SAMs on irradiation dose. The energy of the electrons was 50 eV. The parameters in panels (b) and (c) are fitted by exponential functions according to Eq. 1 (color-coded dashed curves). The straight, color-coded dashed lines in panel (a) are guides for the eyes. Figure 4. Dependence of the total intensity of the S 2p signal (blue squares), intensity of the S 2p component doublet corresponding to the pristine thiolate species (red triangles), and intensity of the S 2p component doublet corresponding to irradiation-induced species (black circles) for the TPT (a), AnthT (b), and OPE3 (c) SAMs on irradiation dose. The intensities of the above components are fitted by exponential functions according to Eq. 1 (red and black solid lines, respectively). The gray dashed lines correspond to 1 and are meant as guides for the eyes. The energy of the electrons was 50 eV. Figure 5. AFM images (3D representation) of Au/Si(100) patterns prepared by EBL with the SAM resists as well as the height profiles along the lines shown in the images. The patterning was performed with the NphT (a,b) and AnthT (c) SAM resists at energies of 3 keV (a) and 0.5 keV (b, c). The doses corresponding to the individual square-like Au features were 5, 10, 20, 40, 80, 100, and 150 mC/cm2 (from left to the right). Figure 6. AFM images of an e-beam patterned gold film on Si(100). The patterning was performed with the AnthT SAM resists in proximity printing geometry (see text for details). The energy of the electrons was set at 50 eV. The doses were 6 mC/cm2 (a) and 40 mC/cm2 (b). 23



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Figure 7. (a) Dependence of the height of the square-like gold features in the patterns prepared with the NphT SAM resist on irradiation dose. The patterning was performed at electron energies 0.5 keV (black squares and black curve), 1 keV (red up triangles and red curve), 3 keV (blue diamonds and blue curve), and 5 keV (green down triangles and green curve). (b) Dependence of the cross-sections for irradiation-induced cross-linking on the electron energy for the SAMs of this study. The values were determined from the lithographic experiments. A legend is given. For the AntT SAM, the value corresponding to an electron energy of 50 eV is provided. This value was determined in the separate experiments (see text for details). Figure 8. SEM images of the CNMs prepared from the TPT (a), AnthT (b), and OPE3 (c) SAMs. The CNMs were supported by metal grids. The CNMs are broken in few places to provide a contrast. Note that the contrast differences between the images do not reflect the CNM quality but only slightly different imaging conditions. Also, the number of defects in the images is not representative of the general quality of a particular CNM type.

24


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Table 1. Cross sections of the irradiation-induced processes involving the SAM matrix and the SAM/substrate interface; see text for details. The values were determined from the spectroscopic experiments. For the AnthT SAMs, the effective cross-section of the crosslinking, determined from the lithographic experiments (Section 3.2), is given. The units are 10-17 cm2. Monolayer

TPT

AnthT

OPE3

SAM matrix

3.7±0.9

1.2±0.5

3.8±1.5

SAM/Au interface

1.7±0.3

1.2±0.4

1.0±0.2

cross-linking

10±3

25



SH SH

SH

NphT

AnthT

BPT

S

O SH

TPT

OPE2

PT

OPE3 O

SH

S

Figure 1

a XPS: C 1s

XPS: Au 4f

b XPS: S 2p

60mC/cm2

60mC/cm2

40mC/cm2

40mC/cm2

40mC/cm2

20mC/cm2

20mC/cm2

10mC/cm2

10mC/cm2

5mC/cm2

5mC/cm2

pristine

pristine

OPE3

60mC/cm2

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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92

c

20mC/cm2 10mC/cm2 5mC/cm2

pristine

88

84

80

76

296

292

288

284

280

170 168 166 164 162 160 158

Binding energy (eV)

Figure 2 26


284.3

a

16 14 12 10 8 6

TPT AnthT OPE3

4 2

1.60

b

1.55

C 1s FWHM (eV)

18

C 1s BE shift (eV)

20

284.2

284.1

284.0

283.9 0

c

1.50 1.45 1.40 1.35 1.30 1.25

0 10 20 30 40 50 60

1.20 0

10 20 30 40 50 60

0

10 20 30 40 50 60

2

Dose (mC/cm )

Figure 3

Sulfur content (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Effective thickness (Å)

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1.2

a

TPT

b

AnthT

c

OPE3

1.0 0.8 0.6 0.4 total

0.2

thiolate irr. induced

0.0 0

10 20 30 40 50 60

0

10 20 30 40 50 60

0

10 20 30 40 50 60

2

Dose (mC/cm )

Figure 4

27



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Figure 5

28


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Figure 6

29



60

Height (nm)

50

a

40 30 20

0.5 keV 1 keV 3 keV 5 keV

10 0 1

10

100

Irradiation dose (mC/cm2)

Cross-sections (cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.0x10-16

PT BPT TPT NphT AnthT OPE2

b

1.5x10-16

1.0x10-16

5.0x10-17

50 eV

0.0 0

1

2

3

4

5

Electron energy (keV)

Figure 7

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Figure 8

31



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TOC Graphic

32


Modification of Aromatic Self-Assembled Monolayers by Electron

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