Combined Ammonia and Electron Processing of a Carbon-Rich Ruthenium Nanomaterial Fabricated by Electron-Induced Deposition

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Article Combined Ammonia and Electron Processing of a Carbon-Rich Ruthenium Nanomaterial Fabricated by Electron-Induced Deposition

Markus Rohdenburg 1,* , Johannes E. Fröch 2 , Petra Martinovi´c 1 , Charlene J. Lobo 2 and Petra Swiderek 1,* 1 Institute for Applied and Physical Chemistry (IAPC), Fachbereich 2 (Chemie/Biologie), University of Bremen, Leobener Str. 5 (NW2), 28359 Bremen, Germany; [email protected] 2 School of Mathematical and Physical Sciences, University of Technology Sydney, Ultimo, NSW 2007, Australia; [email protected] (J.E.F.); [email protected] (C.J.L.) * Correspondence: [email protected] (M.R.); [email protected] (P.S.); Tel.: +49-421-218-63203 (M.R.); +49-421-218-63200 (P.S.) Received: 13 July 2020; Accepted: 10 August 2020; Published: 12 August 2020

Abstract: Ammonia (NH3)-assisted puriﬁcation of deposits fabricated by focused electron beam-induced deposition (FEBID) has recently been proven successful for the removal of halide contaminations. Herein, we demonstrate the impact of combined NH3 and electron processing on FEBID deposits containing hydrocarbon contaminations that stem from anionic cyclopentadienyl-type ligands. For this purpose, we performed FEBID using bis(ethylcyclopentadienyl)ruthenium(II) as the precursor and subjected the resulting deposits to NH3 and electron processing, both in an environmental scanning electron microscope (ESEM) and in a surface science study under ultrahigh vacuum (UHV) conditions. The results provide evidence that nitrogen from NH3 is incorporated into the carbon content of the deposits which results in a covalent nitride material. This approach opens a perspective to combine the promising properties of carbon nitrides with respect to photocatalysis or nanosensing with the unique 3D nanoprinting capabilities of FEBID, enabling access to a novel class of tailored nanodevices.

Keywords: focused electron beam-induced deposition; deposit post-processing; ammonia; Ru deposition; carbon nitride

1. Introduction Nanoscale materials fabricated by focused electron beam-induced deposition (FEBID) drive the development of novel devices for a variety of applications [1–3]. In FEBID, volatile precursor molecules that contain an element of the desired material are adsorbed on a surface and fragmented under a tightly focused electron beam. In this process, a solid deposit is formed, while volatile fragments desorb and are pumped away. However, the electron-induced fragmentation of organometallic precursors used to produce metallic materials is often incomplete and less volatile ligands remain behind, leading to strongly contaminated deposits [1,4,5]. Therefore, purification processes have been devised, in which unwanted elements are removed by thermal treatment, continued electron irradiation, reactions with process gases, or a combination thereof [4]. As a particularly successful and mild post-deposition purification process, electron irradiation in the presence of H2O vapor was recently brought forward. This process was applied to deposits produced from the precursors trimethyl(methylcyclopentadienyl)-platinum(IV) (MeCpPtMe3)[6], tetrakis(trifluorophosphine)platinum(0) (Pt(PF3)4)[7], dimethyl(acetylacetonate)

Micromachines 2020, 11, 769; doi:10.3390/mi11080769 www.mdpi.com/journal/micromachines Micromachines 2020, 11, 769 2 of 17

gold(III) (Me2Au(acac)) [8], and, more recently, from bis(ethylcyclopentadienyl)ruthenium(II) ((EtCp)2Ru) [9]. In all cases, the carbon residue was efficiently removed and it was demonstrated that even the shape of complex 3D nanostructures can be preserved during such treatment [8]. In a related process, dimethyl(trifluoroacetylacetonate)gold(III) and H2O vapor were injected simultaneously during the FEBID process, again yielding a high-purity Au deposit [10]. In addition, H2O was used under electron irradiation to etch away hexagonal boron nitride (hBN) for fabrication of silver nanowire–nitride heterostructures on surfaces [11]. While FEBID generally strives to produce highly pure metals, impure deposits may also have interesting properties. Such deposits are often nanogranular materials consisting of metal nanoparticles embedded in an amorphous carbonaceous matrix [6,9,12]. For instance, Pt in a carbonaceous matrix deposited from MeCpPtMe3 can serve as humidity [13] or strain sensing material [14]. This function relies on the electric conductivity of the material which is governed by tunneling of the charge between the metal grains [1]. The height of the barrier that determines the efficiency of this transport process depends on the distance between the metallic grains which is altered by mechanical strain [1], but can also be influenced by contact of H2O with the material, which is exploited to measure humidity [13]. It is obvious that the efficiency of the latter type of sensing by such nanogranular materials depends on the uptake of the analyte by the carbonaceous matrix. This property can be tuned by providing stronger binding sites which, for instance, can be achieved by incorporating nitrogen in the carbon network. Following this strategy, amorphous carbon nitride has been suggested as a low-cost and environmentally friendly material for gas sensing [15]. While research on carbon nitride materials has predominantly focused on crystalline forms and their applications as emerging materials for semiconducting devices, photocatalysis, energy storage devices, catalyst supports or catalysts [16–18], amorphous carbon nitrides are particularly easy to produce [15]. The useful properties of carbon nitrides may open new perspectives when transferred to nanoscale devices in a direct write approach. In analogy to the diverse synthetic strategies for bulk carbon nitride materials by polymerization of N-containing heterocycles [16], this was previously attempted by electron-induced crosslinking of 1,2-diaminopropane (1,2-DAP) [19]. However, amines tend to be difficult to handle in vacuum systems. Here, we demonstrate that electron irradiation, in the presence of ammonia (NH3), performed on carbon-rich deposits produced from the organometallic precursor (EtCp)2Ru, converts the carbonaceous matrix to a material with the typical composition of carbon nitride (C3N4). This study builds on our previous work regarding the electron-induced chemistry of NH3. With respect to FEBID applications, we demonstrated that chlorine-contaminated ruthenium 3 3 deposits produced from η -allyl ruthenium tricarbonyl chloride (η -C3H5)Ru(CO)3Cl can be purified by electron exposure in the presence of NH3 [20]. Additionally, NH3 enhances the rate of metal reduction when applied as a precursor ligand [21,22]. However, NH3 also adds to unsaturated hydrocarbon structures in an electron-induced reaction [23–25]. This latter aspect, in particular, is exploited here and opens a novel pathway to the nanoscale fabrication of carbon nitride devices by FEBID. For this contribution, we performed processing of FEBID deposits produced from (EtCp)2Ru via high-energy electron irradiation in the presence of NH3 in an electron microscope and analyzed the resulting material in terms of chemical composition and deposit morphology, employing energy-dispersive X-ray spectroscopy (EDX) and atomic force microscopy (AFM). In addition, we performed a surface science study on model deposits fabricated under cryogenic conditions and investigated the impact of simultaneous electron beam and NH3 processing using a combination of reflection–absorption infrared spectroscopy (RAIRS), Auger electron spectroscopy (AES), and methods relying on mass spectrometry (MS), namely thermal desorption spectrometry (TDS) and experiments on electron-stimulated desorption (ESD) monitoring volatile irradiation products.

2. Materials and Methods Electron beam irradiation was performed in an environmental scanning electron microscope (ESEM): the FEI Nova NanoSEM (FEI Company, Eindhoven, The Netherlands) with a background Micromachines 2020, 11, 769 3 of 17 pressure of 3 10 6 mbar. Unless otherwise stated, FEBID was performed using a 5 keV beam at a × − beam current of 500 pA on Si substrates with a native SiO2 layer (SiO2/Si). A focused Gaussian electron beam was rastered over the region of interest and the irradiation process was recorded in real time. Compositional analysis was conducted by EDX (Oxford Instruments INCA x-sight, Oxford Instruments, Abingdon, UK) spectroscopy after processing by exposing an area of 500 500 nm2 to a 5 keV electron × beam at a beam current of 300 pA while collecting ejected X-rays for 1 min. Characterization of deposit morphology was conducted using tapping mode AFM (Digital Instruments Dimension 3100, Digital Instruments, Bresso, Italy) at intervals during the FEBID process. The surface science experiments were performed in an ultrahigh vacuum (UHV) chamber with 10 a base pressure of about 10− mbar [26]. It contains a polycrystalline Ta sheet used as substrate held at 110 K by liquid N2 cooling. The sample temperature is controlled by resistive heating of two thin Ta ribbons spot-welded to the thicker Ta sheet and is measured using a type E thermocouple press-fitted to the Ta substrate. The setup is equipped with a quadrupole mass spectrometer (QMS) residual gas analyzer (300 amu, Stanford Research Systems, Sunnyvale, CA, USA) with electron impact ionization at 70 eV, a commercial flood gun (SPECS FG 15/40, SPECS Surface Nano Analysis GmbH, Berlin, Germany) for electron irradiation in the range of E0 = 1–500 eV, an Auger electron spectrometer (STAIB DESA 100, STAIB INSTRUMENTS GmbH, Langenbach, Germany), and a sputter gun operated with Ar+ ions at a kinetic energy of 3 keV. All AE spectra were recorded using an electron energy of 5 keV in derivative acquisition mode. The infrared beam of a commercial RAIR spectrometer (IFS 66v/S, Bruker Optics GmbH, Ettlingen, Germany) is guided through a KBr window into the UHV chamber, allowing for vibrational spectroscopy of adsorbates on the Ta sheet under grazing incidence conditions. The RAIR spectrometer is equipped with a liquid nitrogen-cooled MCT detector with a sensitivity limit 1 1 1 down to 750 cm− . RAIR spectra were collected from 4000 to 750 cm− with a resolution of 4 cm− by averaging 400 single scans. The optics system was continuously purged with N2 to eliminate contributions of residual vapors to the RAIR spectra. Model deposits were fabricated from (EtCp)2Ru using a slightly modified protocol, as described in reference [9]. Briefly, precursor vapor was leaked into the UHV chamber via a gas handling manifold, where the pressure was measured with a Baratron pressure transducer with mTorr reading. An amount of precursor vapor corresponding to a pressure drop of 2 mTorr in the manifold was condensed onto the cooled Ta sheet, resulting in a coverage of 7–10 monolayers (ML) [9]. The film was then irradiated 2 2 at E0 = 31 eV with a total dose in the range 40–60 mC/cm . For a dose of 40 mC/cm , ESD of desorbing products had ceased according to our recent study [9]. Material retained under cryogenic conditions was subsequently desorbed by a TDS run and a bakeout (450 K for 30 s). Products that are observed during these ESD and TDS procedures are described in detail in [9]. After the Ta sheet with the remaining material had cooled back to 110 K, a film of NH3 corresponding to a pressure drop of 4 mTorr in the gas handling manifold was condensed on top of the model deposit. The NH3 film thickness was estimated to be at least in the range of a monolayer in accordance with [20]. The resulting layer 2 was then again irradiated at E0 = 31 eV with a total dose of 40 mC/cm while monitoring RAIRS and ESD. After irradiation was terminated, a subsequent TDS run followed by a bakeout (450 K, 30 s) removed from the surface the remaining material that was retained at 110 K. This cycle of NH3 dosing, irradiation at E0 = 31 eV, and final TDS with bakeout was repeated several times. After some of these NH3 processing cycles, AES was performed to monitor the elemental composition of the remaining material on the Ta sheet.

3. Results

3.1. Deposit Formation, Combined Electron and NH3 Processing, and Post-Processing Analysis

FEBID was used to fabricate square-shaped deposits from the precursor (EtCp)2Ru on a SiO2/Si substrate. The deposits had a height of 270 nm as observed by both SEM and AFM imaging (see Figure1, left side, and Supplementary Materials, Figure S1). The chemical composition of the deposits was Micromachines 2020, 11, 769 4 of 17

Micromachines 2020, 11, x 4 of 17 determined by EDX as C 87.1%, Ru 9.7%, and O 3.2%. The latter signal can be ascribed to O from the the SiO2 native layer of the substrate. However, the C:Ru ratio of ~9:1 agrees with former EDX studies SiO2 native layer of the substrate. However, the C:Ru ratio of ~9:1 agrees with former EDX studies of of the composition of FEBID deposits fabricated from (EtCp)2Ru [27]. the composition of FEBID deposits fabricated from (EtCp)2Ru [27].

Figure 1. Scanning electron microscope (SEM) (top) and atomic force microscope (AFM) (bottom) Figureimages 1. of Scanning a 1 1 electronµm2 deposit microscope fabricated (SEM) by ( focusedtop) and electron atomic beam-inducedforce microscope deposition (AFM) (bottom (FEBID)) × 2 imagesfrom (EtCp) of a 12 ×Ru 1 µm on a deposit SiO2/Si fabricated substrate (byleft focused), and of electron the deposit beam-induced after post-processing deposition (FEBID) under 30 from min (EtCp)electron2Ru irradiation on a SiO2/Si at substrate 5keV in an (left atmosphere), and of the of 0.11deposit mbar after NH post3 (right-processing). The scale under bar 30 applies min electron to both irradiationSEM images. at 5keV in an atmosphere of 0.11 mbar NH3 (right). The scale bar applies to both SEM images. Combined electron irradiation and NH3 processing was carried out by scanning the 5 keV electron 2 beamCombined in a 1.2 electron1.2 µm regionirradiation around and the NH deposits3 processing in a background was carried chamber out by pressurescanning of the 0.11 5 mbarkeV × electronNH3. According beam in a to1.2 SEM × 1.2 andµm2 AFM,region the around deposit the heightdeposits significantly in a background increased chamber from pressure initially of 270 0.11 to mbar480 nm NH (+3.~80%) According after to 30 SEM min ofand processing. AFM, the deposit This result height is in significantly contrast toanalogous increased from postprocessing initially 270 of to(EtCp) 480 nm2Ru (+ deposits ~80%) after with 30 H min2O instead of processing. of NH3 .This In this result previous is in contrast study, to the analogous deposit height postprocessing decreased 2 ofstrongly (EtCp)2 afterRu deposits short purification with H2O instead (13 s at 5of keV NH and3. In 5.6this nA previous for a 500 study,500 the nm depositdeposit) height in the decreased presence × stronglyof ~0.5mbar after short H2O andpurification 5 keV electron (13 s at irradiation5 keV and 5.6 [9]. nA Rather for a than500 × removing500 nm2 deposit) material, in processingthe presence in ofNH ~0.53 leads mbar to H incorporation2O and 5 keV of electron the process irradiation gas into [9]. the Rather deposit than and removing consequent material, swelling, processing as is obvious in NHfrom3 leads the occurrence to incorporation of large of bubble-typethe process gas structures into the in deposit the halo and regions consequent of the swelling, deposits (seeas is SEMobvious and fromAFM the images occurrence in Figure of1 large, right bubble-type side). Electron-induced structures in fragmentation the halo regions of NHof the3 that deposits has di (seeffused SEM into and the AFMdeposit images yields in N Figure2 (see also1, right ESD side). experiments Electron-induced in Section 3.2.2fragmentation) and releases of NH atomic3 that hydrogen has diffused (AH) into that thecan, deposit among yields other N reactions,2 (see also recombine ESD experiments to form Hin2 Section[22,25,28 3.].2.2.) These and gases releases can atomic be held hydrogen responsible (AH) for thatthe observedcan, among swelling other reactions, and bubble recombine formation. to form H2 [22,25,28]. These gases can be held responsible for theTo observed evaluate swelling changes and in thebubble chemical formation. composition of the deposited material under combined electronTo evaluate beam and changes NH3 processing, in the chemical we performed composit EDXion of after the several deposited stages material of purification. under combined This was electrondone by beam exposing and a NH set of3 processing, FEBID deposits we performed with comparable EDX after size several and thickness stages (Supplementaryof purification. This Materials, was doneFigure by S1) exposing to varying a NHset 3ofprocessing FEBID deposits durations with in thecomparable presence ofsize the and 5 keV thickness electron beam.(Supplementary The results Materials,are depicted Figure as circles S1) to invarying Figure NH2a.3 NH processing3 post-processing durations in for the less presence than 10 of min the had 5 keV no electron effect on beam. either Thematerial results composition are depicted or as deposit circles heightin Figure as obvious2a. NH3 post-processing from AFM images for (seeless Supplementarythan 10 min hadMaterials, no effect onFigure either S2). material After 10composition min of post-processing, or deposit height however, as obvious a decrease from in AFM the relative images C (see ( 17% Supplementary compared to − Materials,the as-deposited Figure composition)S2). After 10 min and Ruof post-pro ( 3.2%)cessing, signals washowever, observed, a decrease accompanied in the relative by an increase C (−17% in − comparedthe N signal to (the+18.5%) as-deposit and aed minor composition) increase inand O (Ru+1.1%). (−3.2%) As signals the height was gain observed, after 10 accompanied min is negligible by an(see increase Supplementary in the N Materials,signal (+18.5%) Figure and S2), a the minor chemical increase changes in O only(+1.1%). relate As to the the height deposit gain volume after and10 minnot tois anegligible variation (see in the Supplementary contribution of Materials, the underlying Figure substrate S2), the to chemical the EDX changes spectra. Theonly rapid relate decrease to the depositin the C volume signal and and incorporation not to a variation of N continued in the cont untilribution 30 min of of the processing underlying and subsequently,substrate to the declined. EDX spectra.Finally, theThe longest rapid processingdecrease in time the of C 60 signal min resulted and incorporation in a deposit withof N a maximumcontinued heightuntil 30 of ~600min nmof processing(see Supplementary and subsequently, Materials, declined. Figure S2) Finally, consisting the of longest 37.5% Cprocessing ( 49.6%), 49.0%time of N, 60 8.3% min Ru, resulted and 5.2% in Oa − deposit with a maximum height of ~600 nm (see Supplementary Materials, Figure S2) consisting of 37.5% C (−49.6%), 49.0% N, 8.3% Ru, and 5.2% O (circles in Figure 2a). The Ru and O content was, thus, nearly unaffected by the combined electron beam and NH3 processing. However, the decreased

Micromachines 2020, 11, 769 5 of 17 Micromachines 2020, 11, x 5 of 17

(circlesC:Ru ratio in Figure indicates2a). The that Ru C andwas Olost content from was,the deposit thus, nearly and unareplacedﬀected by by N. the Note combined that the electron C:N ratio beam of andthe obtained NH3 processing. material However,is that of a thetypical decreased carbon C:Ru nitride, ratio i.e., indicates C3N4 [16–18]. that C We was note lost that from the the density deposit of andamorphous replaced C by3N N.4 (1.3–1.4 Note that g/cm the3 [29]) C:N as ratio compared of the obtained to that of material a RuC9 deposit is that of(3.2 a typicalg/cm3 [9]) carbon can account nitride, 3 i.e.,for a C volume3N4 [16 increase–18]. We of note roughly that 230–250%. the density A ofheight amorphous increase C of3N ~220%4 (1.3–1.4 (see Supplementary g/cm [29]) as compared Materials, 3 toFigure that ofS2) a compares RuC9 deposit well (3.2with g this/cm estimate.[9]) can accountThe lateral for growth a volume after increase processing of roughly as clearly 230–250%. seen in AFigure height 1 increase(top), however, of ~220% shows (see Supplementary that additional Materials,swelling due Figure to evolution S2) compares of gas well must with be this involved estimate. in Thethe overall lateral growthvolume afterincrease. processing We additionally as clearly note seen that in Figure electron-induced1 (top), however, reactions shows of thatNH3 additional with the Si swellingsubstrate due are to unlikely evolution to ofsignificantly gas must be contribute involved in to the the overall increase volume in the increase. N EDX We signal. additionally In a control note thatexperiment electron-induced in absence reactions of a (EtCp) of NH2Ru3 withdeposit, the Sionly substrate a small are N unlikely signal was to signiﬁcantly detected after contribute 30 min toof theprocessing increase (see in the Supplementary N EDX signal. Materials, In a control Figure experiment S3). in absence of a (EtCp)2Ru deposit, only a small N signal was detected after 30 min of processing (see Supplementary Materials, Figure S3).

Figure 2. Results of NH3-assisted processing experiments in an environmental scanning electron microscopeFigure 2. Results (ESEM): of (NHa) Evolution3-assisted ofprocessing the atomic experiments composition in withan environmental time during combined scanning electronelectron microscope (ESEM): (a) Evolution of the atomic composition with time during combined electron beam and NH3 processing of deposits produced by FEBID from (EtCp)2Ru on a SiO2/Si substrate. The beam and NH3 processing of deposits produced by FEBID from (EtCp)2Ru on a SiO2/Si substrate. The deposits were either used as is (circles) or pre-purified using H2O-assisted electron beam purification (triangles)deposits were in accordance either used with as is reference (circles) or [8 ].pre-purified (b) SEM and using EDX H mapping2O-assisted images electron of abeam deposit purification that has (triangles) in accordance with reference [8]. (b) SEM and EDX mapping images of a deposit that has been subject to the combined electron beam and NH3-processing for 60 min. (c) SEM and EDX mapping been subject to the combined electron beam and NH3-processing for 60 min. (c) SEM and EDX images of a deposit that has been subject to the same processing as in (b) but after H2O-assisted purificationmapping images for 30 of min. a deposit that has been subject to the same processing as in (b) but after H2O- assisted purification for 30 min.

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EDX mapping (Figure2b) of a deposit that was post-processed for 30 min reveals that the distribution of C, Ru, and N reflects the egg-shaped thick central area. SEM imaging shows that processing has generated a hole in the middle of the deposit that is also visible in the EDX map. In contrast, O is seen mainly in the outer regions where the Si substrate contributes to EDX intensities. The effect of the combined electron beam and NH3 processing was also studied on (EtCp)2Ru deposits that had been pre-purified by electron irradiation in the presence of H2O[9]. In this case, the carbon content was significantly reduced before NH3 processing. After H2O-assisted purification (5 keV, 1 nA, 0.13 mbar H2O, 30 min), the deposits typically contained 47.9% Ru, 43.8% C, and 8.3% O (triangles in Figure2a). NH 3 post-processing of such pre-purified deposits yielded comparable results to unpurified deposits: 60 min of processing resulted in a composition of 12.2% C ( 31.6%), − 20.4% N, 51.0% Ru, and 16.3% O. The carbon content was, thus, again reduced at the expense of N intake into the deposit. Note that the carbon content after an equal processing time of 60 min was lower when the deposits were pre-purified by electrons in the presence of H2O (12.2% C) as compared to those that were not pre-purified (37.5% C). This also results in a different C:N ratio of the processed deposits (with pre-purification: 0.60, without pre-purification: 0.77). Carbon content and C:N ratio in the deposit can, thus, be tuned by adjusting the processing conditions. Similar to deposits that were not pre-purified in the presence of H2O, a height increase from initially ~100 nm (after H2O-assisted purification) to a maximum of ~600 nm was observed by AFM (see Supplementary Materials, Figure S4). In terms of EDX elemental mapping (Figure2c), the highest Ru concentration was found in the central region where the purification yielded a densified Ru deposit with reduced C content. C is also predominantly observed in this central region. The N signal, however, is spread across the whole processing area, pointing to incorporation of N both in the central deposit area and in the thinner halo region. In line with the mild character of the H2O-assisted purification [9], O was not very pronounced in the central region of the deposit but was concentrated in the halo and the surrounding area where the Si substrate contributes to the EDX intensities.

3.2. Surface Science Study

3.2.1. Formation of Model Deposits A surface science study was performed to elucidate the chemical form of N incorporated into the (EtCp)2Ru deposits during the combined electron beam and NH3 processing. As a model system for FEBID deposits, thin layers (7–10 ML corresponding to an average thickness of 7–10 nm [9]) of (EtCp)2Ru were condensed on a cryogenic surface (110 K), extensively irradiated (E0 = 31 eV, 40–60 mC/cm2), and subsequently, annealed (450 K) [9]. The products that desorb during both electron exposure and annealing have been described in detail elsewhere [9]. Briefly, methane (CH4), ethene (C2H4), and ethane (C2H6) are lost during ESD, along with a small fraction of Cp-containing species (see Supplementary Materials, Figure S5a–c for a thicker 13–20 ML precursor film). This accounts for the high carbon content of as-deposited material from (EtCp)2Ru. It is clear that most of the observed products relate to fragmentation of the Et side chains of the EtCp ligands. All ESD signals leveled off after an exposure of 40 mC/cm2 [9]. An exposure in the range 40–60 mC/cm2 in combination with an even thinner precursor film is, thus, sufficient to convert the molecular precursor film into a model deposit. The persistent Cp moieties of the precursor become embedded into the deposit upon electron exposure. This is also obvious from the reduced intensity of the precursor desorption signal at ~210 K in post-exposure TDS as compared to a non-irradiated film (see Supplementary Materials, Figure S5d) [8]. In addition, some further C2 fragments desorb between 100 and 150 K. Note that this desorption temperature is higher than observed previously for condensed mixtures of C2H4 (about 65 K), C2H6 (about 70 K) and NH3 (around 100 K) [25]. The delayed desorption of the C2 products in post-exposure TDS of (EtCp)2Ru must, thus, be ascribed to trapping of the small hydrocarbons in the model deposit. Micromachines 2020, 11, 769 7 of 17

Micromachines 2020, 11, x 7 of 17 In the present study, the formation of the deposit was also monitored by RAIRS, which shows 1 meanthe CH free stretching path of 31 bands eV electrons of the saturatedin condensed Et sidematerial chains is below between 1 nm 2800 [30], and some 3000 precursor cm− as in the deeper most layersintense is signalsleft intact (Figure after 3an). exposure In line with of 40 the mC/cm layer thickness2, as obvious of 7–10 from nm the [ remaining9] and the intensity fact that theof the typical CH stretchingmean free pathbands. of After 31 eV electronsTDS, the inremaining condensed inte materialnsity was is below strongly 1 nm reduced, [30], some and precursor the band in deepershape 2 altered,layers is pointing left intact to afterremoval an exposure of the intact of 40 precur mC/cmsor, asmolecules obvious that from were the remainingnot decomposed intensity by of electron the CH irradiation.stretching bands. After TDS, the remaining intensity was strongly reduced, and the band shape altered, pointing to removal of the intact precursor molecules that were not decomposed by electron irradiation.

Figure 3. Reﬂection–absorption infrared (RAIR) spectra of a pristine 7–10 ML (EtCp)2Ru ﬁlm on a Figure 3. Reflection–absorption infrared (RAIR) spectra of a pristine 7–10 ML (EtCp)2Ru film on a Ta Ta substrate held at 110 K directly after precursor dosing (0 mC/cm2), after electron irradiation at 2 substrate held at 110 K 2directly after precursor dosing (0 mC/cm ), after electron irradiation at E0 = 31 E0 = 31 eV (40 mC/cm ), and after a subsequent TDS run to 450 K. A RAIR spectrum acquired from eV (40 mC/cm2), and after a subsequent TDS run to 450 K. A RAIR spectrum acquired from the clean the clean Ta sheet just before (EtCp)2Ru dosing served as a background for all spectra shown here. Ta sheet just before (EtCp)2Ru dosing served as a background for all spectra shown here. The infrared The infrared spectrum of the precursor shows two major regions of signals (marked in red): Aliphatic spectrum of the precursor shows two major regions of signals (marked1 in red): Aliphatic and aromatic and aromatic C–H stretches (ν(CH)) between 2800 and 3100 cm− and aliphatic and aromatic C–C C–H stretches (ν(CH)) between 2800 and 3100 cm−1 and aliphatic and aromatic C–C stretches (ν(CC)) stretches (ν(CC)) as well as bending vibrations of the terminal –CH3 groups (δ(CH3)) between 1200 −1 as well as bending1 vibrations of the terminal –CH3 groups (δ(CH3)) between 1200 and 1500 cm [31]. and 1500 cm− [31].

3.2.2.3.2.2. First First Cycle Cycle of of Electron Electron Exposure Exposure in in Presence Presence of of NH NH33

WhenWhen NH3 isis adsorbedadsorbed on on the the deposit deposit and and electron electron exposure exposure at E0 =at31 E0 eV = is31 resumed, eV is resumed, MS acquired MS acquiredduring ESD during (Figure ESD4 )(Figure shows 4) dominant shows dominant signals of signals NH 3 ( mof/ zNH17)3 and(m/z N17)2 ( mand/z 28),N2 ( them/z latter28), the pointing latter pointingto the electron-induced to the electron-induced release of hydrogenrelease of [20hy,drogen22]. Note [20,22]. that the Note intensity that the at m intensity/z 16 (intensity at m/z ratio 16 (intensitym/z 16:m/ zratio17 = m~0.8)/z 16: ism not/z 17 enhanced = ~0.8) is as not compared enhanced to theas compared reference MSto the of NHreference3 [32]. Therefore,MS of NH and3 [32]. in Therefore,contrast to and our in previous contrast study to our on previous ESD from study cisplatin, on ESD there from is no cisplatin, evidence there for desorptionis no evidence of NH for2• desorptionradicals during of NH processing.2• radicalsAdditional during processing. low-intensity Additional signals atlow-intensitym/z 26–30 are signals present, at m indicating/z 26–30 thatare present,more of indicating a C2 product that ismore formed. of a C2 Here, product the presence is formed. of Here,m/z 30 the points presence to C 2ofH m6 [/32z 30] in points line withto C2 theH6 [32]reducing in line action with the of NHreducing3. Intensity action at ofm NH/z 303. Intensity could also atbe m/ relatedz 30 could todiazene also be related (N2H2) to desorption, diazene (N but2H2 it) desorption, but it would be surprising to detect such an unstable species but not the more stable NH3 irradiation product hydrazine (N2H4) at m/z 32 [33].

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would be surprising to detect such an unstable species but not the more stable NH3 irradiation product Micromachines 2020, 11, x 8 of 17 hydrazine (N2H4) at m/z 32 [33].

Figure 4. Mass spectrum (m/z 10 50) of the volatile species produced upon ESD (E0 = 31 eV) of an NH3 Figure 4. Mass spectrum (m/z 10−−50) of the volatile species produced upon ESD (E0 = 31 eV) of 2an NH3 ﬁlm condensed on top of a model deposit fabricated by electron irradiation (E0 = 31 eV, 40 mC/cm ) from film condensed on top of a model deposit fabricated by electron irradiation (E0 = 31 eV, 40 mC/cm2) 7 to 0 ML (EtCp)2Ru on a Ta substrate held at 110 K and subsequent annealing (450 K). The spectrum from 7 to 0 ML (EtCp)2Ru on a Ta substrate held at 110 K and subsequent annealing (450 K). The represents an average of three mass scans obtained at the start of irradiation with the ultrahigh vacuum spectrum represents an average of three mass scans obtained at the start of irradiation with the (UHV) chamber background signals subtracted. ultrahigh vacuum (UHV) chamber background signals subtracted.

In Figure5, RAIR spectra are shown for a NH 3 ﬁlm that has been condensed on top of an (EtCp)2Ru In Figure 5, RAIR spectra are shown for a NH3 film that has been condensed on top of an model deposit at T = 110 K in the pristine state (top), after several stages of electron exposure at (EtCp)2Ru model deposit at T = 110 K in the pristine state (top), after several stages of electron E0 = 31 eV (middle), and after a ﬁnal thermal annealing step to T = 450 K in the course of a TDS exposure at E0 = 31 eV (middle), and after a final thermal annealing step to T = 4501 K in the course of run (bottom). NH3 vibrations are marked with dashed black lines: At 3382 cm− , the asymmetric a TDS run (bottom). NH3 vibrations are marked with dashed black1 lines: At 3382 cm−1, the asymmetric NH3 stretch is observed, along with an asymmetric (1622 cm− ) and two symmetric deformations NH3 stretch is observed,1 along with an asymmetric (1622 cm−1) and two symmetric deformations (1121 and 1072 cm− ). The band locations are in good agreement with assignments published for NH3 (1121 and 1072 cm−1). The band locations are in good agreement with assignments published for NH3 condensed on a highly ordered pyrolytic graphite (HOPG) surface [34]. The symmetric NH3 stretch at condensed1 on a highly ordered pyrolytic graphite (HOPG) surface [34]. The symmetric NH3 stretch 3299 cm− [34] is overlaid with O–H stretches stemming from water ice frozen onto the entry window at 3299 cm−1 [34] is overlaid with O–H stretches stemming from water ice frozen onto the entry of the cooled infrared detector (all H2O-related vibrations [35] are marked with dashed blue lines in window of the cooled infrared detector (all H2O-related vibrations [35] are marked with dashed blue Figure5). lines in Figure 5).

Micromachines 2020, 11, 769x 99 of of 17

Figure 5. RAIR spectra collected for a pristine NH multilayer (0 mC/cm2) condensed on top of a 3 2 Figure 5. RAIR spectra collected for a pristine NH3 multilayer2 (0 mC/cm ) condensed on top of a deposit fabricated by electron irradiation (E0 = 31 eV, 40 mC/cm ) from 7 to 10 ML (EtCp)2Ru on a Ta deposit fabricated by electron irradiation (E0 = 31 eV, 40 mC/cm2) from 7 to 10 ML (EtCp)2Ru on a Ta substrate held at 110 K and subsequent annealing (450 K), for the same layer with ongoing electron substrate held at 110 K and subsequent annealing (450 K), for the same layer with ongoing electron exposure at E0 = 31 eV, and after post-irradiation thermal annealing to 450 K in the course of a TDS exposure at E0 = 31 eV, and after post-irradiation thermal annealing to 450 K in the course of a TDS run. The dashed blue vertical lines correspond to H2O vibrations due to water ice frozen onto the run. The dashed blue vertical lines correspond to H2O vibrations due to water ice frozen onto the detector window [35], the dashed black lines correspond to NH3 vibrations, and the red areas to detector window [35], the dashed black lines correspond to NH3 vibrations, and the red areas to changes correlating to the model deposit. A RAIR spectrum acquired just before NH3 dosing served changes correlating to the model deposit. A RAIR spectrum acquired just before NH3 dosing served as a background for all spectra shown here. Inset: Enlarged view of the RAIR spectra at 0 mC/cm2 as a background for all spectra shown here. Inset: Enlarged view of the RAIR spectra at 0 mC/cm2 and and after TDS for the spectral range 1000–1200 cm2. The dashed red line marks the position of the CN after TDS for the spectral range 1000–1200 cm1 2. The dashed red line marks the position of the CN stretch of gaseous methylamine at 1044 cm− [36]. The CN stretch of methylamine in Kr solution is stretch of gaseous methylamine1 at 1044 cm−1 [36]. The CN stretch of methylamine in Kr solution is blueshifted by only 3 cm− [37]. blueshifted by only 3 cm−1 [37].

Upon electron exposure at E0 = 31 eV, all NH3-related bands decreased in intensity in line with Upon electron exposure at E0 = 31 eV, all NH3-related bands decreased in intensity in line with eﬃcient ESD of NH3, as observed previously at even lower E0 than applied here [25]. Additionally, efficient ESD of NH3, as observed previously at even lower E0 than1 applied here [25]. Additionally, negative signals in the range of aliphatic CH stretches (2800–3000 cm− ), and CC stretches/CH3 bending negative signals in the range1 of aliphatic CH stretches (2800–3000 cm−1), and CC stretches/CH3 vibrations (1200–1500 cm− ) evolved as the electron dose was increased. Since the state after thermal bending vibrations (1200–1500 cm−1) evolved as the electron dose was increased. Since the state after annealing of the model deposit and just before NH3 dosing is reﬂected in the background that was thermal annealing of the model deposit and just before NH3 dosing is reflected in the background employed for all spectra, these negative signals indicate loss of CH and CC stretches while NH3 is that was employed for all spectra, these negative signals indicate loss of CH and CC stretches while consumed. Furthermore, post-NH3-processing TDS (see Supplementary Materials, Figure S6) shows NHagain3 is some consumed. desorption Furthermore, of a product post-NH with3 a-processingm/z 28 fragment TDS (see between Supplementary 100 and 150 Materials, K. This isFigure possibly S6) shows again some desorption of a product with a m/z 28 fragment between 100 and 150 K. This is also a C2 product, but some contribution of N2 trapped in the model deposit cannot be ruled out. possibly also a C2 product, but some contribution of N2 trapped in 1the model deposit cannot be ruled2 In addition, we observed a weak vibrational feature at ~1044 cm− after processing with 10 mC/cm out.for all In higher addition, electron we dosesobserved and a after weak TDS vibrational (see inset infeature Figure at5). ~1044 The position cm−1 after of this processing feature coincides with 10 mC/cmwith the2 for position all higher of the electron CN stretching doses and in methylamine after TDS (see [36 in,37set]. in The Figure CN stretching 5). The position vibration of ofthis saturated feature coincides with the position of the CN stretching in methylamine1 [36,37]. The CN stretching vibration amines is typically located in the range 1020–1250 cm− [38]. Combined NH3 and electron processing of saturated amines is typically located in the range 1020–1250 cm−1 [38]. Combined NH3 and electron of deposits fabricated from (EtCp)2Ru, thus, not only leads to further removal of hydrocarbons but processingalso to the formationof deposits of C–Nfabricated bonds. from (EtCp)2Ru, thus, not only leads to further removal of hydrocarbons but also to the formation of C–N bonds.

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3.2.3. Effect of Repeated Processing Cycles of Electron Exposure in Presence of NH3 3.2.3. Effect of Repeated Processing Cycles of Electron Exposure in Presence of NH3 Beyond the first processing cycle, the chemical composition of the (EtCp)2Ru model deposits was Beyond the first processing cycle, the chemical composition of the (EtCp) Ru model deposits was monitored by AES. The spectra were acquired after different numbers of proces2 sing cycles consisting monitored by AES. The spectra were acquired after different numbers of processing cycles consisting of NH3 dosing and electron irradiation at E0 = 31 eV (40 mC/cm2) at 110 K and a subsequent TDS run of NH dosing and electron irradiation at E = 31 eV (40 mC/cm2) at 110 K and a subsequent TDS run (Figure3 6a). 0 (Figure6a).

FigureFigure 6.6.( a(a)) Derivative Derivative Auger Auger electron electron (AE) (AE) spectra spectra collected collected for a modelfor a model deposit deposit fabricated fabricated by electron by irradiation (E = 31 eV, 60 mC/cm2) from 7 to 10 ML (EtCp) Ru on a Ta substrate held at 110 K and electron irradiation0 (E0 = 31 eV, 60 mC/cm2) from 7 to 10 ML2 (EtCp)2Ru on a Ta substrate held at 110 subsequentK and subsequent annealing annealing (450 K), (450 in theK), as-depositedin the as-deposit stateed and state after and di afterfferent different cycles cycles of processing. of processing. Each cycle consisted of dosing of a multilayer NH film (4 mTorr) onto the deposit held at 110 K, electron Each cycle consisted of dosing of a multilayer3 NH3 film (4 mTorr) onto the deposit held at 110 K, irradiation at 110 K (E = 31 eV, 40 mC/cm2), and annealing (450 K). (b) Enlarged view of the N KLL electron irradiation at 0110 K (E0 = 31 eV, 40 mC/cm2), and annealing (450 K). (b) Enlarged view of the 2 regionN KLL of region the model of the deposit model afterdeposit 320 after mC/ cm320 processingmC/cm2 processing (black curve) (black and curve) of the and bare ofTa the substrate bare Ta after exposure to the exact same 320 mC/cm2 processing (red curve). substrate after exposure to the exact same 320 mC/cm2 processing (red curve). Unfortunately, the most intense Ru signal in AES (MNN at 277 eV [39]) and the C KLL signal Unfortunately, the most intense Ru signal in AES (MNN at 277 eV [39]) and the C KLL signal (275 eV [39]) are located too close to each other to be resolved by our instrument. Therefore, a broad (275 eV [39]) are located too close to each other to be resolved by our instrument. Therefore, a broad signal accounting for both species is the only feature observed for the as-deposited model deposit. signal accounting for both species is the only feature observed for the as-deposited model deposit. With ongoing processing, this signal did not significantly decrease in intensity but a new feature at With ongoing processing, this signal did not significantly decrease in intensity but a new feature at 386 eV appeared, indicative of N [39]. The N KLL signal in AES is a sensitive probe for the chemical 386 eV appeared, indicative of N [39]. The N KLL signal in AES is a sensitive probe for the chemical state of N bound to another element. An earlier study has compared the N KVV signals in AES state of N bound to another element. An earlier study has compared the N KVV signals in AES measured in differential mode from diverse N-containing elemental layers [40]. While the exact measured in differential mode from diverse N-containing elemental layers [40]. While the exact position of the N KVV bands shifted slightly with increasing N content, as exemplified for Ru-N position of the N KVV bands shifted slightly with increasing N content, as exemplified for Ru-N samples, it was shown that the fine structure of the bands is characteristic of the element in which samples, it was shown that the fine structure of the bands is characteristic of the element in which N N was incorporated [40]. In particular, both Ta–N and Ru–N revealed narrow distances between the was incorporated [40]. In particular, both Ta–N and Ru–N revealed narrow distances between the minima and maxima, while covalent nitrides such as C–N exhibited wide peak structures [40]. For the minima and maxima, while covalent nitrides such as C–N exhibited wide peak structures [40]. For present AES data (Figure6b), the distances between the main maxima are on average 13 eV in the case the present AES data (Figure 6b), the distances between the main maxima are on average 13 eV in the of Ta reacted with NH3, in excellent agreement with the spectrum reported in reference [40], where a case of Ta reacted with NH3, in excellent agreement with the spectrum reported in reference [40], where a similar line shape was also observed for Ru–N. In contrast, the AES data obtained after

Micromachines 2020, 11, 769 11 of 17 similar line shape was also observed for Ru–N. In contrast, the AES data obtained after processing of the carbonaceous Ru model deposits (Figure6a,b) revealed a spacing near 20 eV between the two main maxima, this time in excellent agreement with the previous data for C–N samples [40]. This clearly shows that N from the adsorbed NH3 is incorporated in the C residue during electron exposure and does not react with Ru.

4. Discussion We now propose reaction mechanisms that can rationalize the chemical composition of FEBID deposits produced from (EtCp)2Ru, as observed after electron beam processing in the presence of NH3. Based on the observation that N2 is released in ESD experiments (Figure4) and in line with the fact that NH3 acts as a reducing agent under electron irradiation [20,22], NH3 must be decomposed under electron impact yielding AH. In addition, ionization of NH3 can lead to a proton transfer + reaction as predicted by theory [41] and observed in experiments [42] for the ionized (NH3)2• dimer (Equation (1)). + + (NH ) • NH + NH • (1) 3 2 → 4 2 Electron exposure of (EtCp)2Ru mainly leads to cleavage of the Cp-Et bonds, resulting in loss of C2 hydrocarbons from the precursor (see Section 3.2.1 and reference [9]). As a simpliﬁed model that reﬂects the composition of deposits fabricated from (EtCp)2Ru, we therefore employ ruthenocene (Cp2Ru) in + all following discussions. When a proton from NH3• is transferred to a Cp2Ru molecule instead of a neighboring NH3, the reaction would lead to cleavage of the metal–ligand bond, since cyclopentadienyl ligand-carrying precursors tend to lose their Cp ligands when they become protonated [43,44]. Equation (2) represents this reaction in the case of Cp2Ru.

+ + NH • + Cp Ru CpH + CpRu + NH • (2) 3 2 → 2 In the following, we explain how the products of Equation (2), namely cyclopentadiene (CpH, 1), + cyclopentadienyl ruthenium(II) cations (CpRu ), and amino radicals (NH2•) possibly contribute to the formation of carbon nitrides as observed after processing of the (EtCp)2Ru deposits. Combined ESD and TDS results of the surface study (see Section 3.2.1 and reference [9]) as well as EDX analysis in electron microscopes (see Section 3.1 and reference [27]) suggest that most of the Cp moieties of the EtCp ligands become embedded in the deposits upon electron exposure. The majority of 1 as the most volatile, small hydrocarbon product of Equation (2) is, thus, retained in the deposit. The cycloaddition of dienes via single electron oxidation and subsequent formation of the Diels–Alder dimer has been described in the literature [45]. Due to the presence of C=C double bonds even after dimerization, the resulting Diels–Alder adduct is prone to further crosslinking upon prolonged electron exposure, producing a larger, non-volatile hydrocarbon residue. Aside from crosslinking, the C=C double bonds can be subject to an electron-induced hydroamination reaction in the presence of NH3. This reaction has been previously observed in mixtures of various alkenes and NH3 [23–25]. Electron-induced hydroamination is initiated by electron impact ionization (EI) of either the alkene or NH3, leading to attractive forces between the resulting radical cation and its electron-rich reaction partner [23]. This produces a radical cation adduct in the deposit that can subsequently be quenched by a thermalized electron. Scheme1 shows this electron-induced hydroamination of 1 as our simpliﬁed model for chemical motifs in the hydrocarbon residue. The reaction product shown in Scheme1 is cyclopent-2-en-1-amine ( 2). Note that electron-induced hydroamination reactions are hardly regioselective [25] so that cyclopent-3-en-1-amine (3) would be another conceivable reaction product. Micromachines 2020, 11, x 12 of 17

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Scheme 1. Electron-induced hydroamination of 1.

The NH2• radicals producedScheme according 1. Electron-induced to Equations hydroamination (1) and (2) of may1. also add to C=C double bonds present in the hydrocarbonScheme 1.residue. Electron-induced This results hydroamination in direct amination of 1. of these moieties. The resultingThe NHamine2• radicals radical producedcan be saturated according by to addition Equations of (1)AH. and Note (2) maythat radical also add addition to C=C doublecan initiate bonds a presentchainThe reaction in NH the2• hydrocarboninvolving radicals produced radical residue. site according propagation This results to Equations inan directd, thus, amination (1) extends and (2) the of may thesedegree also moieties. of add crosslinking to The C=C resulting double in the aminedeposit.bonds radicalpresent Furthermore, canin the be saturatedhydrocarbonin alkenes by with additionresidue. allylic This of H AH. atreoms,sults Note substitutionin that direct radical amination of addition one ofof these canthese initiate Hmoieties. atoms a chain byThe a reactionradicalresulting under involving amine release radical radical of AHcan site isbe propagationanother saturated conceivable by and, addition thus, reaction. extendsof AH. In Note the the degree case that of radical of 1, crosslinking this addition reaction in canwould the initiate deposit. result a Furthermore,inchain formation reaction of in involving cyclopenta-2,4-dien-1-amine alkenes withradical allylic site Hpropagation atoms, (4 substitution). Scheme and, thus, 2 visualizes of extends one of these the reactiondegree H atoms of pathways crosslinking by a radical involving underin the releaseNHdeposit.2• radicals. of Furthermore, AH is another in conceivable alkenes with reaction. allylic In H the at caseoms, of substitution1, this reaction of wouldone of resultthese inH formationatoms by ofa cyclopenta-2,4-dien-1-amineradical under release of AH is ( 4another). Scheme conceivable2 visualizes reaction. the reaction In the pathways case of 1, this involving reaction NH would2• radicals. result in formation of cyclopenta-2,4-dien-1-amine (4). Scheme 2 visualizes the reaction pathways involving NH2• radicals.

Scheme 2. Possible radical substitution and addition reactions between NH2• and 1. Scheme 2. Possible radical substitution and addition reactions between NH2• and 1. The RAIRS investigation of the first processing cycle (Figure5) was carried out in situ, i.e., byThe keeping RAIRS investigation the sample continuously of the first processing under UHV cycle conditions. (Figure 5) was In our carried previous out in study situ, oni.e., theby electron-inducedkeeping the Schemesample decomposition 2.continuously Possible radical of under cisplatin,substitution UHV we andcondit identified additionions. NHInreactions our2• radicals previous between as NHstudy intermediates2• and on 1 .the electron- in an in situinduced X-ray decomposition photoelectron of spectroscopy cisplatin, we (XPS) identified study, butNH not2• radicals in a complementary as intermediates RAIRS in an study in situ that X-ray was carriedphotoelectronThe out RAIRS ex spectroscopy situ investigation [22]. The (XPS) absence of the study, first of vibrational butprocessing not in a features cycomplementarycle (Figure of the 5) NH was RAIRS2• moietiescarried study out inthat exin wassitu, situ carried RAIRSi.e., by wasoutkeeping ex explained situ the [22].sample by The consumption continuously absence of of vibrational theunder reactive UHV features radicalscondit ions.of during the In NH our handling2• previousmoieties at ambient studyin ex situon conditions the RAIRS electron- [was22]. • 1 1 Theexplainedinduced absence decomposition by of consumption NH2• vibrational of ciofsplatin, the features reactive we identified like radicalsδw(NH NHduring2)2 at radicals 1392 handling cm as− intermediates orat δambients(HNH) conditions at in 1555 an in cm situ −[22].[ 46X-ray The] in inabsencephotoelectron situ RAIRS of NH in spectroscopy2• thevibrational present study,features(XPS) thus,study, like points δbutw(NH not to2) a inat rapid a1392 complementary consumption cm−1 or δs(HNH) byRAIRS the at described 1555 study cm that−1 reactions [46] was in carried in with situ theRAIRSout hydrocarbonex insitu the [22]. present residue.The study,absence The thus, weak of pointsvibrational C–N stretchingto a rapifeaturesd featureconsumption of the present NH by2• after themoieties depositdescribed in processingex reactions situ RAIRS (Figure with was the5) togetherhydrocarbonexplained with by residue. theconsumption absence The of weak of NH the2 •C–N reactivedesorption stretching radicals in ESDfeat duringure (Figure present handling4) and after the at deposit AESambient data processing conditions (Figure6b) (Figure indicate[22]. The 5) • −1 −1 thattogetherabsence the fate ofwith NH of the2 the vibrational absence NH2• radicals of featuresNH2• must desorption like indeed δw(NH in be 2ESD) conversion at 1392(Figure cm 4) into or and δ carbon-bounds(HNH) the AES at data 1555 moieties. (Figure cm [46] 6b) in indicate in situ thatRAIRS theAfter in fate the EI, of cyclicpresent the NH C5 study,2 amines,• radicals thus, as must canpoints be indeed formedto a rapi be viaconversiond consumption the reactions into proposedcarbon-boundby the described above, moieties. typicallyreactions fragment with the viahydrocarbon ring-openingAfter EI, cyclicresidue.α cleavage C5 Theamines, weak followed as C–Ncan by be stretching H formed rearrangement via feat theure reactions [present47]. The proposedafter EI-MS deposit of 2above,, 3, processing or 4typically, however, (Figure fragment has not 5) beenviatogether ring-opening reported with tothe the αabsence cleavage best of of our followedNH knowledge.2• desorption by H We, rearrangement in therefore, ESD (Figure briefly [47]. 4) anddiscussThe theEI-MS theAES majorof data 2, 3 dissociation,(Figure or 4, however, 6b) indicate channel has • + followingnotthat been the fate reported EI of of the cyclopentanamine toNH the2 radicalsbest of our must (C knowledge.5H indeed11N, M be• We conversionat m, therefore,/z 85) as into a briefly simple carbon-bound discuss example the moieties. for major fragmentation dissociation of suchchannelAfter cyclic following EI, C5 cyclic amines. C5EI Theamines,of cyclopentanamine EI-MS as can of cyclopentanamine be formed (C via5H 11theN, exhibitsreactionsM•+ at the mproposed/z most 85) intenseas above, a simple signal typically atexamplem fragment/z 56 [32for]. + Thisfragmentationvia ring-opening fragment resultsof αsuch cleavage from cyclic the followed C5 loss amines. of by a C2 HThe radicalrearrangement EI-M fromS of cyclopentanamine M •[47].[32 The]. Scheme EI-MS 3 exhibitsof shows 2, 3, or howthe 4, however,most this kindintense has of fragmentationsignalnot been at mreported/z 56 following [32]. to This the EI bestfragment would of our proceed results knowledge. from in 2. the The We loss loss, therefore, of of a C2 C2 radicals radical briefly from yieldsdiscuss M new•+ the [32]. CC major Scheme double dissociation 3 or shows triple bondshowchannel this that followingkind may of either fragmentation EI crosslink of cyclopentanamine orfollowing be subject EI to would (C hydroamination5H11 proceedN, M•+ inat under 2m. The/z 85)prolonged loss as of aC2 simple electronradicals example exposureyields new for in theCCfragmentation presencedouble or of triple of NH such 3bonds. cyclic that C5 may amines. either Thecrosslin EI-Mk Sor of be cyclopentanamine subject to hydroamination exhibits underthe most prolonged intense •+ electronsignal at exposurem/z 56 [32]. in Thisthe presence fragment of results NH3. from the loss of a C2 radical from M [32]. Scheme 3 shows how this kind of fragmentation following EI would proceed in 2. The loss of C2 radicals yields new CC double or triple bonds that may either crosslink or be subject to hydroamination under prolonged electron exposure in the presence of NH3.

Micromachines 2020, 11, x 13 of 17 Micromachines 2020, 11, 769 13 of 17 Micromachines 2020, 11, x 13 of 17

Scheme 3. Fragmentation of 2 following EI. Scheme 3. Fragmentation of 2 following EI. Scheme 3. Fragmentation of 2 following EI. The C2 radicals released during fragmentation, i.e., vinyl (C2H3•) and ethyl (C2H5•) radicals in The C2 radicals released during fragmentation, i.e., vinyl (C2H3•) and ethyl (C2H5•) radicals in 3 2 4 2 6 the case of 2, may react with AH produced through NH decomposition• to yield C• H and C H , the caseThe ofC22 ,radicals may react released with AHduring produced fragmentation, through i.e., NH 3vinyldecomposition (C2H3 ) and to ethyl yield (C C2H2H5 4) radicalsand C2H in6, contributing to ESD intensities in the range m/z 26–30 (Figure 4). Recombination of C2H3• with NH2• contributingthe case of 2,to may ESD react intensities with AH in produced the range throughm/z 26–30 NH (Figure3 decomposition4). Recombination to yield C of2H C4 andH Cwith2H6, radicals would yield ethanimine (7) or its tautomer vinylamine (8), respectively (Scheme2 3• 4). It is NHcontributingradicals to wouldESD intensities yield ethanimine in the range (7) orm/z its 26–30 tautomer (Figure vinylamine 4). Recombination (8), respectively of C2H3• (Scheme with NH4).2• known2• that 7 trimerizes at temperatures above 230 K to yield the acetaldehyde ammonia trimer (9) Itradicals is known would that yield7 trimerizes ethanimine at temperatures(7) or its tautomer above vinylamine 230 K to yield(8), respectively the acetaldehyde (Scheme ammonia 4). It is [48]. This reaction could take place during room temperature processing in the electron microscope trimerknown (that9)[48 7]. trimerizes This reaction at temperatures could take placeabove during 230 K to room yield temperature the acetaldehyde processing ammonia in the trimer electron (9) as well as during annealing of the model deposits after processing and would yield again larger, microscope[48]. This reaction as well could as during take annealingplace during of the room model temperature depositsafter processing processing in the and electron would microscope yield again nitrogen-containing hydrocarbon species that are very likely retained in the processed deposit. larger,as well nitrogen-containing as during annealing hydrocarbon of the model species deposits that areafter very processing likely retained and would in the yield processed again deposit. larger, Recombination of C2H5• with NH2• would accordingly yield ethylamine (C2H5NH2). In previous Recombinationnitrogen-containing of C hydrocarbonH with NH specieswould that accordingly are very yieldlikely ethylamine retained in (C theH processedNH ). In previousdeposit. studies, it was argued2 5• that radical2• densities in molecular films during electron2 5 exposure2 with the studies,Recombination it was argued of C2H that5• with radical NH densities2• would in accordingly molecular ﬁlms yield during ethylamine electron (C exposure2H5NH2). with In previous the same same electron gun as used herein are low and radical recombination reactions are, thus, unlikely to electronstudies, gunit was as usedargued herein that are radical low and densities radical in recombination molecular films reactions during are, electron thus, unlikely exposure to occur with [49the]. occur [49]. The observation of a hydrocarbon residue with high nitrogen content along with the Thesame observation electron gun of as ahydrocarbon used herein are residue low and with radical high nitrogen recombination content reactions along with are, the thus, presence unlikely of C2 to presence of C2 hydrocarbons in ESD (Figure 4) during processing does not allow us to fully rule out hydrocarbonsoccur [49]. The in observation ESD (Figure of4) a during hydrocarbon processing resi doesdue with not allowhigh usnitrogen to fully content rule out along such with radical the such radical recombination reactions. The initiation of chain reactions by C2H3• and C2H5• is, however, recombinationpresence of C2 reactions.hydrocarbons The in initiation ESD (Figure of chain 4) duri reactionsng processing by C H doesand not C alHlowis, us however, to fully rule a more out a more likely scenario. 2 3• 2 5• likelysuch radical scenario. recombination reactions. The initiation of chain reactions by C2H3• and C2H5• is, however, a more likely scenario.

Scheme 4. Possible reaction pathways of vinyl radicals produced during fragmentation of 2. Scheme 4. Possible reaction pathways of vinyl radicals produced during fragmentation of 2. The iminium cations 5 and 6, which are the fragmentation products in Scheme3, could protonate Scheme 4. Possible reaction pathways of vinyl radicals produced during fragmentation of 2. further CpThe moieties iminium in cations the deposit. 5 and 6 Note, which again are that the 2fragmentationis just a simpliﬁed products model in Scheme compound 3, could representing protonate further Cp moieties in the deposit. Note again that 2 is just a simplified model compound likelyThe structural iminium elements cations 5 inand the 6, which crosslinked are the carbon fragmentation residue ofproducts the deposit in Scheme after 3, electron-induced could protonate representing likely structural elements in the crosslinked carbon residue of the deposit after electron- hydroamination.further Cp moieties Therefore, in the deposit. various similarNote again small that CxN 2y isfragments just a simplified may be the model outcome compound of the induced hydroamination. Therefore, various similar small CxNy fragments may be the outcome of the proposedrepresenting reaction likely sequence. structural Overall, elements electron-induced in the crosslinked crosslinking carbon residue and further of the NHdeposit3 addition after electron- to these proposed reaction sequence. Overall, electron-induced crosslinking and further NH3 addition to unsaturatedinduced hydroamination. species can explain Theref theore, increasingvarious similar N content small ofCx theNy fragments non-volatile may residue be the remaining outcome of after the these unsaturated species can explain the increasing N content of the non-volatile residue remaining processingproposed reaction of (EtCp) sequence.2Ru deposits. Overall, electron-induced crosslinking and further NH3 addition to after processing of (EtCp)2Ru deposits. theseAside unsaturated from fragmentation species can explain after EI, the amines increasing can also N content undergo of crosslinking the non-volatile initiated residue by dissociative remaining Aside from fragmentation after EI, amines can also undergo crosslinking initiated by electronafter processing attachment of (EtCp) (DEA).2Ru For deposits. instance, it was previously proposed that electron attachment to dissociative electron attachment (DEA). For instance, it was previously proposed that electron 1,2-DAPAside leads from to an fragmentation electronically excitedafter EI, anion, amines which can relaxes also via undergo dehydrogenation crosslinking leaving initiated behind by C- dissociative electron attachment (DEA). For instance, it was previously proposed that electron

Micromachines 2020, 11, 769 14 of 17 and N-centered radicals that can crosslink the molecular ﬁlm [19]. Such reactions can also explain why volatile amines like ethylamine are not observed in ESD during processing because similar DEA resonances associated with H− loss were found for aliphatic amines like n-propylamine [50,51]. Since primary electrons with energies above the ionization threshold such as those used in the present study generate low-energy secondary electrons (LESEs) upon interaction with solid matter, initiation of DEA to amine species in the deposit may likely contribute to the observed crosslinking after processing. Finally, CpRu+ as a further reaction product in Equation (2) is an example of a cationic species that exposes a metal center where additional reactions can occur. For instance, it has been reported that C–N bond formation between ethene and an NH2 moiety is facilitated in a cationic Ni(II) amide complex [52]. Such amide complexes may form by addition of NH2− anions to an accessible Ru(II) + center in CpRu . Note that NH2− anions are a major product of DEA to cyclic amines like reported for cyclopropylamine [53] as well as for NH3 [54–56]. Reactions occurring at accessible Ru(II) centers in the deposit and triggered by LESEs, thus, likely enable formation of new C–N bonds and also rearrangement of amines already present in the residue according to the reactions proposed in Schemes1 and2. Such reactions are of special relevance, since it was reported that NH 3 can be used as an etching agent for amorphous carbon under electron exposure [57], which would additionally expose further accessible Ru sites that can contribute to C–N bond formation. To conclude, the described reactions contribute to a complex chemical environment in the deposits during processing that involves multiple sites prone to crosslinking or incorporation of N from NH3. The observed C:N ratio of ~3:4 after processing, thus, cannot be assigned to a single chemical mechanism yielding a deﬁned carbon nitride material with known stoichiometry. Rather, the multitude of possible reaction pathways needs to be held responsible for the composition of the obtained material after combined electron and NH3 processing. Future studies may aim at a thorough understanding of the dominant chemical features in dependence of the processing conditions. For instance, XPS can yield valuable information on the bonding motifs of C, N, and Ru in the processed deposits. Especially,the role of accessible Ru centers and intermediate species attached to them could be further investigated by this approach.

5. Conclusions

Our present study provides evidence that combined NH3 and electron processing of deposits fabricated from the FEBID precursor (EtCp)2Ru yields deposits with Ru embedded in a material with the chemical composition of a carbon nitride (C3N4). NH3, thus, becomes incorporated in the deposit via the formation of covalent C–N bonds. In the case of pre-purification of the deposits using electron exposure in the presence of H2O vapor, the carbon content of the matrix can be reduced before N is incorporated, resulting also in an overall lower C:N ratio. By a combination of H2O purification and NH3 processing, the stoichiometry of the obtained material can, thus, be tuned. The results of a surface science study on combined NH3 and electron processing of (EtCp)2Ru model deposits suggest that the electron-induced reactions of NH3 with the cyclopentadienyl-type ligands are responsible for the formation of amine species that degrade and crosslink the EtCp ligands under prolonged electron exposure. The process described in this contribution may allow access to the fabrication of a new class of carbon nitride functional nanodevices by FEBID. The affinity of nitrogen as a binding site for e.g., H2O is higher than that of pure carbon compounds. Therefore, carbon–nitride devices could in further studies be tested for their ability to outperform existing sensing concepts relying on carbonaceous FEBID deposits.

Supplementary Materials: The following are available online at http://www.mdpi.com/2072-666X/11/8/769/s1, Figure S1: AFM Data of As-Deposited Pads Before NH3 Treatment, Figure S2: AFM Data for Various Stages of NH3 Treatment, Figure S3: Electron-Induced Reactions of NH3 with the Si Substrate, Figure S4: AFM Data for Various Stages of NH3 Treatment of H2O-Puriﬁed Deposits, Figure S5: ESD and TDS Data on Model Deposit Formation, Figure S6: TDS After NH3 Treatment. Micromachines 2020, 11, 769 15 of 17

Author Contributions: Conceptualization, M.R. and P.S.; formal analysis, M.R., J.E.F. and P.S.; investigation, M.R., J.E.F. and P.M.; resources, C.J.L. and P.S.; data curation, M.R., J.E.F. and P.M.; writing—original draft preparation, M.R. and P.S.; writing—review and editing, M.R., J.E.F., C.J.L. and P.S.; visualization, M.R.; supervision, M.R., C.J.L. and P.S.; project administration, C.J.L. and P.S.; funding acquisition, C.J.L. and P.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by DFG, grant number Sw26/13-2 and DAAD, PPP project grant number 57317041. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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