Rapid and Highly Compact Purification for Focused Electron Beam

Jun 4, 2014 - Recently, Plank et al. demonstrated an alternative postgrowth approach (for the same precursor) using low temperature oxygen fluxes at 5...
0 downloads 0 Views 4MB Size
Download PDF
Article pubs.acs.org/JPCC

Rapid and Highly Compact Purification for Focused Electron Beam Induced Deposits: A Low Temperature Approach Using Electron Stimulated H2O Reactions Barbara Geier,† Christian Gspan,† Robert Winkler,† Roland Schmied,† Jason D. Fowlkes,‡ Harald Fitzek,† Sebastian Rauch,∥ Johannes Rattenberger,† Philip D. Rack,‡,§ and Harald Plank*,†,∥ †

Graz Centre for Electron Microscopy, Steyrergasse 17, 8010 Graz, Austria Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States ∥ Institute for Electron Microscopy and Nanoanalysis, Graz University of Technology, Steyrergasse 17, 8010 Graz, Austria ‡

S Supporting Information *

ABSTRACT: Focused electron beam induced deposition (FEBID) is an important synthesis method as it is an extremely flexible tool for fabricating functional (3D) structures with nanometer spatial resolution. However, FEBID has historically suffered from carbon impurities up to 90 at %, which significantly limits the intended functionalities. In this study we demonstrate that MeCpPtIVMe3 deposits can be fully purified by an electron-beam assisted approach using H2O vapor at room temperature, which eliminates sample and/or gas heating and complicated gas delivery systems, respectively. We demonstrate that local pressures of 10 Pa results in an electron-limited regime, thus enabling high purification rates of better than 5 min·nA−1·μm−2 (30 C·cm−2) for initially 150 nm thick deposits. Furthermore, TEM measurements suggest the purification process for the highly compact deposits occurs via a bottom-up process.

■

INTRODUCTION Focused electron beam induced deposition (FEBID) has attracted increasing attention as a direct-write synthesis method due to its capability to fabricate 3-dimensional structures with sub-10 nm spatial resolution.1−4 The method is based on the electron-induced, local decomposition of gaseous precursor molecules, which are injected into a scanning electron or dual ion/electron beam system via a localized gas injection system.1,5 On the basis of the precursor chemistry a variety of different functionalities can be achieved ranging from conductive, insulating, semiconducting and magnetic.6 During the past decade a diverse range of applications have been demonstrated such as (nano)lithography,7−11 magnetic storage or sensing,12−15 stress−strain nanosensors,16,17 nanooptics,18,19 nanoscale gas sensors,20 and others.21−23 However, for higher performance FEBID based concepts, the chemistry/purity has to be improved. This liability has been a main issue during the last several decades as FEBID and ion beam assisted deposition structures typically suffer from severe carbon content up to 90 at %, often reducing or even masking the intended functionalities.1,6 Although a few examples of pure materials after fabrication have been demonstrated,24,25 most efforts have focused on in situ and/or postgrowth purification processes including fabrication on hot substrates,26,27 synchronized laser assisted FEBID,28,29 coflow with reactive gases,30 or other in situ/ex situ processes.31−36 Recently, Mehendale et al. © 2014 American Chemical Society

introduced a combined postgrowth approach for (MeCpPtIVMe3) deposits by an e-beam assisted purification with oxygen gases at elevated temperatures of 120 °C, which led to nominally pure Pt deposits with purification rates of about 6 min·μm−2 for purification currents of 24 nA at 5 keV.37 Although the carbon was entirely removed, the deposit morphology suffered from severe porosity, limiting the approach for many nanoscale structures. Recently, Plank et al. demonstrated an alternative postgrowth approach (for the same precursor) using low temperature oxygen fluxes at 50 °C for the fast purification toward pure Pt at rates better than 6 min·μm−2 for purification currents of 1.6 nA at 5 keV.38 The major advantage of this method, however, was the high fidelity morphologies realized after purification as the deposits were shown to be compact and retained the original lateral dimensions without the evolution of severe porosity. In this study we demonstrate an even simpler postgrowth purification approach by using room temperature H2O vapor together with a scanning e-beam. As we will show a local H2O pressure of 10 Pa allows complete carbon removal of (MeCpPtIVMe3) based deposits at rates of better than 5 min· nA−1·μm−2. It is also found that local pressures of 10 Pa are Received: April 8, 2014 Revised: May 22, 2014 Published: June 4, 2014 14009

dx.doi.org/10.1021/jp503442b | J. Phys. Chem. C 2014, 118, 14009−14016

The Journal of Physical Chemistry C

■

Article

RESULTS Efficiency. As absolute quantification of light elements and thin layers via EDXS is very complicated, we followed a previously used, semiquantitative approach by calculating the C/Pt peak ratio after background-subtracting the substrate spectra.37,38 This approach gives a C/Pt reference ratio of 0.08 for pure Pt as the Pt−N peak overlaps the C−K peak (please note the reference value varies slightly for each EDXS system, which explains the different reference value compared to previous studies). While in situ EDXS only provides qualitative saturation behavior, ex situ measurements are used to estimate the final carbon content by comparison to the target value of 0.08 for pure Pt. The initial comparison focuses on the influence of the H2O chamber pressure, using a purification beam current of 2.5 nA, 4 nm PoP, and 1 μs DT. As summarized in Figure 1a by in situ EDXS measurements the

sufficient to reach electron-limited conditions during purification making the selection of beam current, pixel dwell time, and pixel point pitch less critical. Finally, this approach allows for very high morphological stability with low lateral shrinkage of less than 4%, while 3-dimensional surface features are unaffected. Furthermore, transmission electron microscopy investigations revealed the purification as a bottom-up process, leading to very compact Pt films with grain growth from originally 2−3 nm (as-deposited) to 6−9 nm (fully purified).

■

EXPERIMENTAL SECTION Fabrication. 2 × 2 μm2 Pt−C structures were deposited on Si substrates (with 3 nm native SiO2) using the MeCpPtIVMe3 precursor. In previous studies of Pt−C FEBID deposits we found evidence for two different types of carbon matrixes depending on the synthesis parameters (and thus the ratelimiting regime) during fabrication.36,39 For electron-limited conditions an increased content of incompletely dissociated precursor molecules was realized, while precursor-limited conditions result in polymerized carbon due to precursor fragments and chamber residue. In order to investigate how different carbon matrixes affect subsequent purification rates/ mechanisms we have used two differently synthesized Pt−C materials in two different working regimes: (1) high beam currents of 1600 nA (5 keV) with dwell times (DT) and point pitches (PoP) of 100 μs and 26 nm, respectively, to establish precursor-limited conditions; and (2) low beam currents of 25 pA (5 keV) with 1 μs DT and 10 nm PoP for more electronlimited conditions. The deposit thickness after preparation was consistently chosen in the range of 60−75 nm as previous studies suggested efficiency saturation around 80 nm using oxygen as the purification gas.38 To investigate the thickness dependence, a 30−300 nm thickness series was prepared using intermediate conditions of 5 keV primary energy, 98 pA beam current, 10 μs DT, and 15 nm PoP. All deposits were characterized via atomic force microscopy (AFM) in both asprepared and fully purified conditions. Purification. In order to establish defined and spatially homogeneous pressure conditions during purification all postgrowth experiments were performed in an environmental scanning electron microscope (ESEM). As the electron beam can be strongly scattered in the gas phase for high chamber pressures initial current measurements were performed to estimate effective, local beam currents.40 After sample transfer to the ESEM, the chamber was initially evacuated to 3 × 10−6 mbar. Subsequently, energy dispersive X-ray spectroscopy (EDXS) was performed to obtain reference spectra of the Pt−C deposits in the as-deposited state. Next, the e-beam was blanked and the chamber pressure was set to a defined H2O pressure at room temperature. The purification process was then initiated by unblanking the e-beam using a primary energy of 5 keV. Simultaneously, in situ EDXS measurements were performed using integration times of 180 s followed by a 60 s pause, which defined the time resolution. To investigate the influence of the beam parameters during purification, different beam currents, POPs, and DTs were used, while the scan area was kept constant at 4.0 × 3.5 μm2. After purification the microscope chamber was again brought into high-vacuum mode (3 × 10−6 mbar), and the final EDXS spectra of fully purified deposits were measured, which are denoted as ex situ measurements.

Figure 1. (a) In situ time evolution of the C/Pt peak area ratios for low (10 Pa, black squares) and high (100 Pa, red circles) H2O chamber pressures during purification together with the applied doses on the top axis; (b) summary of uncorrected ex situ EDXS spectra after purification at different beam currents (see legend) together with as-deposited (at 1600 pA) and substrate spectra (purification settings were always 5 keV, 4 nm PoP, and 1 μs DT).

progressive carbon removal is reflected by the decreasing C/Pt ratios, which approach a constant value after purification times of 2 min·μm−2, independent of the chamber pressure of 10 or 100 Pa. Hence, this pressure insensitivity is suggestive of a process that is limited by the electron-stimulation rather than the mass transport. Figure 1b summarizes uncorrected EDXS spectra of as-prepared (red) and 60 min purified deposits performed at different purification currents (see legend) together with the substrate reference (black). As described, the carbon type in the Pt−C deposits depends on the FEBID deposition regime;36,39 thus we compared the influence of the original FEBID conditions on purification rates and achievable purities. The e-beam assisted purification procedures used constant PoPs and DTs of 4 nm and 1 μs and varied the current from 0.7 to 5.1 nA at constant H2O pressure of 10 Pa. 14010

dx.doi.org/10.1021/jp503442b | J. Phys. Chem. C 2014, 118, 14009−14016

The Journal of Physical Chemistry C

Article

Figure 2. (a) In situ evolution of the C/Pt ratios for 75 nm (FEBID current 1600 pA, upper panel) and 68 nm (FEBID current 25 pA, lower panel) thick Pt−C deposits, fabricated in more precursor-limited (upper) and electron-limited (lower) conditions (note also the different original C/Pt ratio). While purification point pitches (4 nm), dwell times (1 μs), and H2O pressures (10 Pa) were kept constant, different purification currents have been used (see legend); (b) purification efficiency normalized to the used beam current carried out for the upper panel (1600 pA deposits) in (a).

Figure 2a shows the time evolution of all the experiments, revealing very similar rates for both deposition regimes. This suggests that the type of incorporated carbon (incompletely dissociated vs polymerized for 25 pA and 1600 pA, respectively) does not affect the purification rates within the margin of the experiment. It can also be noted that, as expected, higher currents result in faster purification. In order to estimate the purification efficiency, the time axes of the in situ EDXS spectra (Figure 2a) were normalized by the beam currents. As can be seen in Figure 2b (representatively shown for the 1600 pA deposits), all the curves basically converge except the two highest currents (dashed and dotted curves), which simply lack in situ time resolution as the process is very fast (see also Figure 2a). Similar analysis of the 25 pA deposits revealed identical behavior (see Supporting Information Figure S1). From these experiments we can estimate the requisite areal dose to purify the 75 nm/68 nm thick films to less than 5 min·nA−1·.μm−2 equal to a dose of 30 C·cm−2. These results also strongly corroborate an electron-limited purification regime as no efficiency decay is found for increasing currents. To investigate the latter in more detail the DTs were also varied by 2 orders of magnitude revealing no influence on the overall efficiency (see Supporting Information Figure S2). These findings further support electron-limited conditions during purification at H2O pressures of 10 Pa. To estimate possible influences on the achievable purities, ex situ EDXS measurements were performed for a variety of DTs, PoPs, and e-beam currents as summarized in Figure 3. As can be seen, the achievable C/Pt ratios (the error bars of ±0.005 are not shown for clarity) are very close to the reference target value of 0.08 and virtually independent of the dwell times, point pitches, beam currents, or H2O chamber pressures. Again these results strongly suggest an electron-limited purification regime, which essentially simplifies purification toward a simple straightforward setup. Morphology and Shape Fidelity. To determine the morphological quality/fidelity of the deposits, AFM measurements were conducted prior to and after purification. Figure 4a compares an initially 60 nm thick Pt−C deposit (left) followed by 5 min purification (right) using 1 nA beam current, 10 Pa chamber pressure, 4 nm PoP, and 1 μs DT (AFM images are shown with the same height scale). As shown in the upper

Figure 3. Ex situ C/Pt ratios (±0.05) of fully purified Pt deposits (initially between 60 and 75 nm high) for different DTs (top), PoPs (bottom), beam currents, and H2O chamber pressures (see legends). The green bars represent the target value of 0.080 (±0.005) for pure Pt, experimentally determined for the used EDXS system.

panel of Figure 4b by normalized AFM height cross sections (indicated by dashed lines in Figure 4a) the deposits experience minimal lateral shrinking of less than 4% (Δx = 70 nm at full width at half-maximum), while the overall shape is completely unaffected. Roughness analyses at the central areas (1 × 1 μm2) reveal practically identical root-mean-square (rms) values of around (0.5 ± 0.1) nm. The associated volume loss of Pt−C structures (deposited at 1600 pA) during purification was determined to be 69 ± 3 vol % for all deposits shown in Figure 3 (full parameter variation), which is very close to the predicted value around 70 vol % discussed previously.38 Additional AFM measurements on partially purified deposits reveal homogeneous vertical shrinking without any pore formation during purification (an absolute height comparison is given in Supporting Information Figure S3). In contrast, Pt−C structures deposited at low currents of 25 pA gave a higher volume loss of 77 ± 3 vol %. This is consistent with the higher initial carbon content of low current deposit as reflected by the associated C/Pt ratios in Figure 2a (2.08 vs 1.32 for 25 pA and 1600 pA deposition current, respectively). On the basis of these results the FEBID fabrication was changed to achieve a 3D concave architecture as shown in Figure 4c by AFM height images (different vertical scales). As can be seen by this 14011

dx.doi.org/10.1021/jp503442b | J. Phys. Chem. C 2014, 118, 14009−14016

The Journal of Physical Chemistry C

Article

Figure 5. (a) TEM bright field image of a partially purified Pt−C layer initially 56 nm high, revealing the purification as a bottom-up process; (b) STEM EELS line scan for carbon (red squares) across a fully purified Pt layer (initially 105 nm high) together with a TEM bright field image on top. The blue STEM EELS line scan shows the oxygen signal of SiO2 (substrate) and SiOx (TEOS protection layer), which acts as an interface indicator revealing the Pt layer as entirely carbon free.

Figure 4. (a) AFM height images of an as-prepared (left) and fully purified deposit (right) in the same Z scale revealing the highly compact character after purification; (b, upper) normalized AFM cross sections (see dotted lines in panel a) confirming that the relative morphology is maintained after full purification (beside a small lateral shrink of less than 4%); (c) as-deposited and fully purified 3D-shaped deposit together with relative AFM cross section (b, lower) revealing the capability to maintain even 3D surface features.

Chemical analyses via scanning transmission electron microscopy (STEM) based electron energy loss spectroscopy (EELS) confirmed the absence of carbon in fully purified layers (doses > 30 C·cm−2). Figure 5b shows an EELS line scan of carbon (red squares) across an initially 105 nm thick Pt−C layer, which was fully purified (∼180 C·cm−2) together with the STEM EELS oxygen signal (blue circles) acting as interface marker for the underlying SiO2 layer (substrate) and the SiOx protection layer (TEOS). This correlated measurement (see also the TEM bright field image on top) reveals the entire carbon removal across the FEBID deposit (see also Supporting Information Figure S4). Closer examination via high-resolution TEM reveals furthermore slight growth of Pt grains from 2−3 nm (as-prepared) to 6−9 nm after full purification, as representatively shown in Figure 6 for a fully purified 20 nm Pt layer, initially 58 nm thick. The bottom right inset gives the internal structure of the as-prepared deposit in the same lateral scale for a direct before−after comparison, which clearly reveals the Pt grain growth. To investigate the penetration depth of the purification process, an initially 300 nm thick Pt−C deposit was subjected to the postgrowth treatment at 10 Pa and 5 nA beam current using DTs and PoPs of 1 μs and 4 nm, respectively. Figure 7 shows a TEM bright field image (top) of the final deposit after a 40 min ESEM treatment (∼600 C·cm−2). As can be seen there is a 45 nm thick, highly dense top layer followed by a gradually less dense area. Correlated STEM EELS line

comparison as well as by normalized cross sectional profiles across the diagonals in the lower panel of Figure 4c, even pseudo-3D structured deposits widely preserve their relative morphology, which is attributed to the homogeneous vertical shrink widely maintaining the lateral footprint shape and dimension. This behavior makes this purification route very promising for more complex geometries, which are currently under investigation. Internal Structure. Transmission electron microscopy (TEM) investigations were performed to provide insight in the purification mechanism, penetration depths, and structural and chemical details. Figure 5a shows TEM bright field image of a partially purified Pt−C deposit (∼15 C·cm−2), initially 56 nm thick and subsequently covered with a SiOx protection layer (via FEBID of TEOS precursor). There are three different zones clearly recognizable: (1) a highly dense layer at the bottom (∼5 nm) and (2) partially densified layer in the center (∼18 nm), followed by (3) virtually unaffected areas with characteristic 2−3 nm Pt nanoclusters dispersed in a carbon matrix.34,36,37,39,41 This image clearly reveals the purification as a bottom-up process previously suggested by the authors.38 14012

dx.doi.org/10.1021/jp503442b | J. Phys. Chem. C 2014, 118, 14009−14016

The Journal of Physical Chemistry C

Article

the local pressure to be ∼3 × 10−2 Pa and a current density of ∼500 A·cm−2. This results in an oxygen flux of ∼1 × 1016 O2 s−1·cm−2 and an electron flux ∼3 × 1021 e−·s−1·cm−2. For the present 10 Pa H2O study, the H2O flux (ΓH2O = P/(2πmkT)1/2 where P is pressure, m is the molecule mass, k is Boltzmann’s constant, and T is temperature) is ∼4 × 1019 H2O s−1·cm−2, and the electron flux ranges from ∼8 × 1019 to 4 × 1020 e−·s−1· cm−2. Thus, it is clear that an entirely different regime emerges as the electron and purification gas flux are comparable. Interestingly, if one estimates the total number of electrons per cm2 required to purify a 70 nm thin film (i.e., 30 C·cm−2) is ∼2 × 1020 e·cm−2. The total number of carbon atoms in a 70 nm thick PtC4 that is 1 cm2 is ∼2 × 1017. Thus, the cumulative efficiency for the electron-limited regime is ∼0.1%. Hence, the experimental evidence suggesting an electron-limited regime is reasonable because both the equilibrium coverage and the diffusive permeation into the Pt−C deposit will both be enhanced relative to our previous investigation. For comparison, the average diffusion coefficient for water in several polymer materials is D = 1 × 10−8 cm2·s−1.42 Thus, a diffusion distance of 70 nm ((4Dt)0.5) only takes ∼1 ms. These findings allow us furthermore to conclude that the observed efficiencies represent an intrinsic limitation based on the surface coverage and/or solubility of water in the Pt−C deposit and the electron-stimulated cross section of the water on the surface and in the matrix. Concerning the details of the purification process itself we have to start with the fact that it is a bottom-up process observed via TEM of the partially purified Pt−C deposits (Figure 5). On the basis of this finding we have to consider two possibilities: (1) formation of reactive components on the deposit surface, which diffuse inside along the carbon network, followed by electron stimulated reactions at areas of highest electron densities (according to the interaction volume) forming, e.g., CO, CO2, or CHx, which leave the deposit; or (2) diffusion of water molecules inside the deposit followed by electron stimulated dissociation (again according to the interaction volume) and subsequent formation of the abovementioned volatile species. For both assumptions, however, the interaction volume in the original Pt−C material plays an essential role. Hence, we conducted electron trajectory simulations via the software package CASINO43 assuming a wide range of chemical composition of 10−20 at % Pt in a carbon matrix. Figure 8a shows the depth distribution of 5 keV electrons assuming PtC10 (red bars) and PtC4 (blue bars). As can be seen, even for the highest carbon content material (PtC10, red), primary electrons do not reach much more than 200 nm into the deposit. The associated electron densities are schematically indicated by the same colors at the right assuming a 300 nm thick deposit. As evident, for 5 keV the deepest areas of 80− 120 nm (for PtC10 and PtC4, respectively) are not reached by the electrons (white areas) and should therefore be unaffected from the purification process. However, this is in conflict with the STEM EELS results in Figure 7 clearly revealing deeper areas as partially purified. To explain this discrepancy the cumulative volume loss has to be taken into account as well. On the basis of TEM/STEM EELS results (Figure 7), we modeled therefore a layer sequence consisting of 45 nm pure Pt followed by a 50 nm thick region in which the carbon content is gradually increasing from PtC to PtC5 (equivalent to 17 at % Pt as suggested by EDXS based absolute quantification). The associated depth distribution of 5 keV electrons is shown in

Figure 6. High-resolution TEM image of an initially 58 nm thick Pt layer after full purification. Comparison to the as-deposited structure in the bottom right inset reveal the Pt grain growth from 2−3 nm to about 6−9 nm (representatively indicated by red arrows).

Figure 7. Combined TEM bright field image (top) and STEM EELS line scan data (bottom) of an initially 300 nm thick Pt−C deposit after long purification (∼600 C·cm−2) to reveal the penetration depth of the purification process. While the topmost 45 nm are fully purified (see EELS carbon signal), the underlying areas (∼50 nm) are only partly purified. The STEM EELS oxygen signal again acts as an indicator for the SiO2 (substrate) and SiOx (TEOS protection layer) interface.

scans across this deposit are shown in Figure 7 (bottom) for carbon (red squares) and oxygen (blue circles), which confirms that the top ∼45 nm is fully purified. However, there is a 50 nm thick region underneath, which shows increasing carbon content for deeper regions. The SiO2 interface again can be identified by the increasing oxygen signal shown by blue circles in Figure 7. Experiments with decreasing Pt−C layer thicknesses reveal full purification for initial layer thicknesses up to 150 nm resulting in 49 nm thick layers, which is consistent with the fully purified region in Figure 7 (see Supporting Information Figure S5).

■

DISCUSSION All of the experimental evidence corroborates that an H2O based chamber pressure of 10 Pa is sufficient to achieve electron limited conditions during purification. First of all, initial experiments at 10 and 100 Pa yielded comparable purification rates. Furthermore, even significant variation of dwell time or beam current does not show an influence on purification efficiencies (Figure 3). As a comparison to our previous O2 purification in a high vacuum SEM,38 we estimated 14013

dx.doi.org/10.1021/jp503442b | J. Phys. Chem. C 2014, 118, 14009−14016

The Journal of Physical Chemistry C

Article

used a scanning electron beam at constant energies of 5 keV in combination with room temperature H2O vapor as the reactive gas in an ESEM to provide spatially homogeneous pressure conditions. We demonstrated that a partial pressure of 10 Pa is sufficient to reach electron limited conditions during purification for a wide range of purification currents and patterning parameters making the postgrowth process very simple as no special parameter window emerges and the total purification time is simply related to the beam current. TEM investigations confirmed the entire and spatially homogeneous carbon removal up to an initial thickness of 150 nm and revealed a slight Pt grain growth from 2−3 to 6−9 nm. The purification process itself was confirmed to be a bottom-up process and suggests a correlation with the electron interaction volume. Detailed analysis of thick Pt−C deposits demonstrated furthermore the importance of the electron energy during purification as areas beneath the electron interaction volume in the pure Pt are not fully purified. Concerning the process itself we therefore suggest electron stimulated reactions of either water molecules or reactive fragments (e.g., dissociated at the surface) inside the deposit via formation of volatile CO, CO2, or CHx groups. In addition to high purification efficiencies of better than 5 min·nA−1·μm−2 (30 C·cm−2), this approach confirmed the ability to maintain even pseudo 3-dimensional surface morphologies with minimal lateral shrinking and without any porosity, which demonstrates the high potential of this electron assisted H2O purification at room temperature.

Figure 8. CASINO based electron trajectory simulations (5 keV primary energy) together with layer schemes for (a) 300 nm PtC4 (blue bars) and PtC10 (red bars); and (b) a layer sequence of 45 nm pure Pt followed by a 50 nm region with a gradient from Pt to PtC5 based on TEM based findings (see right side for TEM bright field image).

Figure 8b by the green bars together with a layer scheme and the resultant TEM bright field image. As can be seen, the strong carbon volume loss lead to the situation that electrons can penetrate the 45 nm thick, fully purified top region leading to electron penetration to the underlying region, which can explain the gradual carbon increase for deeper areas shown in Figure 7. A systematic variation of the chemistry for the deepest 50 nm revealed that even a gradient from PtC to PtC2 leads to the presence of electrons in the deepest areas (see Supporting Information Figure S6), which explains the partial purification suggested by STEM EELS measurements. However, because of the low number of electrons that penetrate into the underlying layer in the deposits (as the Pt top layer is highly dense) the final stages of purification for the deepest layers is very slow. This regime shift is consistent with the observation that we could not observe any significant change in C/Pt ratios after 40 min of purification. This is also consistent with the fact that initially 150 nm thick deposits can be fully purified (see Supporting Information Figure S5) resulting in a 49 nm thick Pt layer. These combined findings also demonstrate that for full purification of thicker deposits the electron energy during purification has to be adapted, which is the subject of ongoing research. It is anticipated that new regimes will emerge for thicker deposits and higher energy as electron stimulated reactions will change (likely decrease) with increasing energy and eventually diffusive transport of products and reactants through the thickening Pt layer may also alter the process. Finally, it should be mentioned that AFM results of pore- and crack-free structures after purification are consistent with the close packing of Pt grains observed via TEM (Figure 6) and the observed volume loss around 70 vol %, which is very close to theoretically predicted values.38 Together with the observation of homogeneous vertical shrink while maintaining their lateral footprint shape and dimension, we attribute the pore- and crack-free structure to the bottom-up type purification process (see Figure 5) where CO, CO2, or CHx byproducts can apparently diffuse out of the Pt−C matrix and possibly via grain boundaries allowing the Pt grains to rearrange in an ideal manner.

■

METHODOLOGY FEBID deposits were fabricated with a FEI Nova 200 dual beam system (FEI, The Netherlands) equipped with FEI gas injection systems (GIS) using MeCpPtIVMe3 precursor. The GIS nozzle was placed in 108 μm distance to the substrate at an angle of 52°, while the long GIS axis was exactly aligned with the area of deposition. 10 × 10 mm2 Si substrates with a 3 nm SiO2 top layer have been precleaned via (1) acetone, (2) supersonic assisted isopropanol cleaning, and (3) CO2 spraying followed by immediate transfer to the NOVA 200 dual beam instrument. While establishing a chamber background pressure between 5−6 × 10−6 mbar, the GIS was heated to 45 °C for at least 30 min prior to any deposition. After positioning the sample with a blanked e-beam, the GIS valve was opened for at least 2 min to establish a thermodynamic equilibrium between precursor adsorption and desorption providing a constant surface coverage. All deposits have then been fabricated using the internal pattering engine with the parameters specified in the main text. Different layer thicknesses have been achieved via adapted number of patterning loops. After successful fabrication, the samples have immediately been transferred to a QUANTA 200 ESEM (FEI, The Netherlands) for postgrowth purification experiments. The instrument was equipped with an EDAX XL-30 EDXS system (EDAX, USA), which was used for all EDXS analyses. The pressure inside the specimen chamber is monitored by a capacitance gauge (Pfeiffer CMR262, accuracy 0.2% of the measured value) and controlled by the user interface. Caused by the relatively large chamber pressure in comparison to the gas inlet region, pressure and temperature conditions (room temperature) are in equilibrium and stable. For further analysis, all spectra were corrected by the reference spectra taken from surrounding areas of the samples. To follow temporal evolution of carbon removal and final purities, corrected intensities have been integrated from 120 to 330 eV (C−K and Pt−N) and 1950−2220 eV (Pt−M)

■

CONCLUSIONS In conclusion we have demonstrated a rapid approach for the complete removal of carbon in MeCpPtIVMe3 based FEBID deposits, while the deposit morphology is barely affected. We 14014

dx.doi.org/10.1021/jp503442b | J. Phys. Chem. C 2014, 118, 14009−14016

The Journal of Physical Chemistry C

Article

following previous approaches.37,38 The obtained values have then been used to calculate a C/Pt ratio as an indicator for the purity. The target value for pure Pt has been experimentally determined by applying the same procedures to a 200 nm thick Pt layer in high vacuum suggesting a value of 0.08. While in situ (high pressure mode) EDXS has only qualitative character, high-vacuum EDXS measurement (ex situ) have been used to estimate final purities. AFM measurements were performed with a Dimension 3100 microscope (Digital Instruments, Bruker), equipped with a Hybrid XYZ scan head operated with a Nanoscope IVa controller and Olympus OMCL TS-160 cantilever in tapping mode. TEM and STEM images were acquired on a Tecnai F20 (FEI, The Netherlands) with a Schottky gun operating at 200 kV. EELS spectra were measured with a postcolumn energy filter from Gatan (GATAN, USA) and a 2k CCD. For EELS measurements the STEM mode was used for exact positioning and correlation of the electron beam with the sample and its composition. An energy dispersion of 0.2 eV/channel was chosen to see the carbon K-edge and the oxygen K-edge in the same EEL spectrum. To extract the signals for carbon and oxygen a power law model was used for subtracting the background. The window size for the background window was about 20 eV. Also the window size for the signal (carbon and oxygen) was 20 eV. With the extracted signal, an elemental distribution map was generated and, from this the signal, profiles for carbon and oxygen.

■

Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

■

(1) Utke, I.; Russell, P. E. Nanofabrication Using Focused Ion and Electron Beams: Principles and Applications. Oxford University Press: New York, 2012. (2) Randolph, S. J.; Fowlkes, J. D.; Rack, P. D. Focused, Nanoscale Electron-Beam-Induced Deposition and Etching. Crit. Rev. Solid State Mater. Sci. 2006, 31, 55−89. (3) van Dorp, W. F.; Hagen, C. W. A Critical Literature Review Of Focused Electron Beam Induced Deposition. J. Appl. Phys. 2008, 104 (081301), 1−42. (4) van Dorp, W.; Hansen, T. W.; Wagner, J. B.; De Hosson, J. T. B. The Role Of Electron-Stimulated Desorption in Focused Electron Beam Induced Deposition. Beilstein J. Nanotechnol. 2013, 4, 474−480. (5) Winkler, R.; Fowlkes, J. D.; Szkudlarek, A.; Utke, I.; Rack, P. D.; Plank, H. The Nanoscale Implications of a Molecular Gas Beam during Electron Beam Induced Deposition. ACS Appl. Mater. Interfaces 2014, 6, 2987−2995. (6) Botman, A.; Mulders, J. J. L.; Hagen, C. W. Creating Pure Nanostructures From Electron-Beam-Induced Deposition Using Purification Techniques: A Technology Perspective. Nanotechnology 2009, 20 (372001), 1−17. (7) Heerkens, C. T. H.; Kamerbeek, M. J.; van Dorp, W.; Hagen, C. W.; Hoekstra, J. Electron Beam Induced Deposited Etch Masks. Microelectron. Eng. 2009, 86, 961−964. (8) Guan, Y.; Fowlkes, J. D.; Retterer, S. T.; Simpson, M. L.; Rack, P. D. Pulsed Laser Dewetting of Nickel Catalyst for Carbon Nanofiber Growth. Nanotechnology 2008, 19 (505302), 1−4. (9) Lassiter, M. G.; Liang, T.; Rack, P. D. Inhibiting Spontaneous Etching Of Nanoscale Electron Beam Induced Etching Features: Solutions for Nanoscale Repair of Extreme Ultraviolet Lithography. J. Vac. Sci. Technol., B 2008, 26, 963−967. (10) Liang, T.; Frendberg, E.; Lieberman, B.; Stivers, A. Advanced Photolithographic Mask Repair Using Electron Beams. J. Vac. Sci. Technol., B 2005, 23, 3101−3105. (11) Edinger, K.; Becht, H.; Bihr, J.; Boegli, V.; Budach, M.; Hofmann, T.; Koops, H. W. P.; Kuschnerus, P.; Oster, J.; Spies, P.; Weyrauch, B. Electron-Beam-Based Photomask Repair. J. Vac. Sci. Technol., B 2004, 22, 2902−2906. (12) Gavagnin, M.; Wanzenboeck, H. D.; Belić, D.; Bertagnolli, E. Synthesis of Individually Tuned Nanomagnets for Nanomagnet Logic by Direct Write Focused Electron Beam Induced Deposition. ACS Nano 2013, 7, 777−784. (13) Gabureac, M.; Bernau, L.; Boero, G.; Utke, I. Single Superparamagnetic Bead Detection and Direct Tracing of Bead Position Using Novel Nanocomposite Nano-Hall Sensors. IEEE Trans. Nanotechnol. 2013, 12, 668−673. (14) Serrano-Ramon, L.; Cordoba, R.; Rodriguez, L. A.; Magen, C.; Snoeck, E.; Gatel, C.; Serrano, I.; Ibarra, M. R.; De Teresa, J. M. Ultrasmall Functional Ferromagnetic Nanostructures Grown By Focused Electron-Beam-Induced Deposition. ACS Nano 2011, 5, 7781−7787. (15) Gabureac, M.; Bernau, L.; Utke, I.; Boero, G. Granular Co−C Nano-Hall Sensors By Focused-Beam-Induced Deposition. Nanotechnology 2010, 21 (115503), 1−5. (16) Porrati, F.; Sachser, R.; Schwalb, C. H.; Frangakis, A. S.; Huth, M. Tuning The Electrical Conductivity Of Pt-Containing Granular Metals by Postgrowth Electron Irradiation. J. Appl. Phys. 2011, 109 (0637151), 1−7. (17) Huth, M.; Porrati, F.; Schwalb, C.; Winhold, M.; Sachser, R.; Dukic, M.; Adams, J.; Fantner, G. Focused Electron Beam Induced Deposition: A Perspective. Beilstein. J. Nanotechnol. 2012, 3, 597−619. (18) Perentes, A.; Bachmann, A.; Leutenegger, M.; Utke, I.; Sandu, C.; Hoffmann, P. Focused Electron Beam Induced Deposition of a Periodic Transparent Nano-Optic Pattern. Microelectron. Eng. 2004, 73, 412−416.

ASSOCIATED CONTENT

S Supporting Information *

Complementary purification efficiencies (renormalized) for more electron-limited conditions during deposition; timeresolved in situ EDXS during purification for different dwell times and beam currents; absolute height comparison via AFM in as-deposited, partial and fully purified states; STEM-EELS raw data for as-prepared and fully purified deposits; TEM of an initially 150 nm thick Pt−C deposit after full purification; CASINO-based electron simulations for 5 keV electrons for different chemistries to estimate penetration depths. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*(H.P.) E-mail: [email protected]. Phone: +43 316 873 8821. Notes

The authors declare no competing financial interest.

■

REFERENCES

ACKNOWLEDGMENTS

The authors gratefully acknowledge Prof. Ferdinand Hofer, Prof. Werner Grogger, Ing. Hartmuth Schröttner, and Dr. JooHyon Noh for support. We also acknowledge financial support by the 1) COST action CELINA (Nr. CM1301), 2) EUROSTARS project TRIPLE-S (Nr. E! 8213), and 3) ESTEEM2 (EU FP7 program, FP7/2007-2013, Nr. 312483). P.D.R. acknowledges support from Intel Corporation (and Ted Liang as program mentor) via the direct funding program at the Semiconductor Research Corporation (SRC-2012-In-2310) and matching funds from the Center for Materials Processing. J.D.F. acknowledges support from the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National 14015

dx.doi.org/10.1021/jp503442b | J. Phys. Chem. C 2014, 118, 14009−14016

The Journal of Physical Chemistry C

Article

Curing for Focused Electron-Beam-Induced Pt Deposits. J. Vac. Sci. Technol., B 2011, 29 (051801), 1−7. (37) Mehendale, S.; Mulders, J. J. L.; Trompenaars, P. H. F. A New Sequential EBID Process for the Creation of Pure Pt Structures From MeCpPtMe3. Nanotechnology 2013, 24 (145303), 1−7. (38) Plank, H.; Noh, J.-H.; Fowlkes, J. D.; Lester, K.; Lewis, B. B.; Rack, P. D. Electron Beam Assisted Oxygen Purification At Low Temperatures for Electron Beam Induced Pt Deposits: Towards Pure and Pore-Free Structures. ACS Appl. Mater. Interfaces 2014, 6, 1018− 1024. (39) Plank, H.; Haber, T.; Gspan, C.; Kothleitner, G.; Hofer, F. Chemical Tuning of Ptc Nanostructures Fabricated via Focused Electron Beam Induced Deposition. Nanotechnology 2013, 24 (1753605), 1−8. (40) Rattenberger, J.; Wagner, J.; Schröttner, H.; Mitsche, S.; Zankel, A. A Method to Measure the Total Scattering Cross Section and Effective Beam Gas Path Length in a Low-Vacuum SEM. Scanning 2009, 31, 1−7. (41) Plank, H.; Gspan, C.; Dienstleder, M.; Kothleitner, G.; Hofer, F. The Influence of Beam Defocus on Volume Growth Rates for Electron Beam Induced Platinum Deposition. Nanotechnology 2008, 19 (485302), 1−9. (42) Metayer, M.; Labbe, M.; Marais, S.; Langevin, D.; Chappey, C.; Dreux, F.; Brainville, M.; Belliard, P. Diffusion of Water through Various Polymer Films: A New High Performance Method of Characterization. Polym. Test. 1999, 18, 533−549. (43) Drouin, D.; Couture, A. R.; Joly, D.; Tastet, C.; Aimez, V.; Gauvin, R. CASINO V2.42: A Fast and Easy-to-Use Modeling Tool for Scanning Electron Microscopy and Microanalysis Users. Scanning 2007, 29, 92−101.

(19) Utke, I.; Jenke, M. G.; Röling, C.; Thiesen, P. H.; Iakovlev, V.; Sirbu, A.; Mereuta, A.; Caliman, A.; Kapon, E. Polarisation Stabilisation of Vertical Cavity Surface Emitting Lasers by Minimally Invasive Focused Electron Beam Triggered Chemistry. Nanoscale 2011, 3, 2718−2722. (20) Kolb, F.; Schmoltner, K.; Huth, M.; Hohenau, A.; Krenn, J.; Klug, A.; List, E. J. W.; Plank, H. Variable Tunneling Barriers in FEBID Based Ptc Metal-Matrix Nanocomposites as a Transducing Element for Humidity Sensing. Nanotechnology 2013, 24 (305501), 1−7. (21) Serrano-Ramon, L.; Cordoba, R.; Rodriguez, L. A.; Magen, C.; Snoeck, E.; Gatel, C.; Serrano, I.; Ibarra, M. R.; De Teresa, J. M. Ultrasmall Functional Ferromagnetic Nanostructures Grown By Focused Electron-Beam-Induced Deposition. ACS Nano 2011, 5, 7781−7787. (22) Fernandez-Pacheco, A.; De Teresa, J. M.; Cordoba, R.; Ibarra, M. R.; Petit, D.; Read, D. E.; O’Brien, L.; Lewis, E. R.; Zeng, H. T.; Cowburn, R. P. Domain Wall Conduit Behavior in Cobalt Nanowires Grown by Focused Electron Beam Induced Deposition. Appl. Phys. Lett. 2009, 94 (192509), 1−3. (23) Gabureac, M.; Bernau, L.; Utke, I.; Boero, G. Granular Co-C Nano-Hall Sensors By Focused-Beam-Induced Deposition. Nanotechnology 2010, 21 (115503), 1−5. (24) Fernandez-Pacheco, A.; De Teresa, J. M.; Cordoba, R.; Ibarra, M. R. Metal-Insulator Transition in Pt-C Nanowires Grown by Focused-Ion-Beam-Induced Deposition. Phys. Rev. B 2009, 79 (174204), 1−12. (25) Klein, K. L.; Randolph, S. J.; Fowlkes, J. D.; Allard, L. F.; Meyer, H. M.; Simpson, M. L.; Rack, P. D. Single-Crystal Nanowires Grown via Electron-Beam-Induced Deposition. Nanotechnology 2008, 19 (345705), 1−8. (26) Mulders, J. J. L.; Belova, L. M.; Riazanova, A. Electron Beam Induced Deposition at Elevated Temperatures: Compositional Changes and Purity Improvement. Nanotechnology 2011, 22 (055302), 1−7. (27) Cordoba, R.; Sese, J.; De Teresa, J. M.; Ibarra, M. R. High-Purity Cobalt Nanostructures Grown by Focused-Electron-Beam-Induced Deposition at Low Current. Microelectron. Eng. 2010, 87, 1550−1553. (28) Roberts, N. A.; Fowlkes, J. D.; Magel, G. A.; Rack, P. D. Enhanced Material Purity and Resolution via Synchronized Laser Assisted Electron Beam Induced Deposition of Platinum. Nanoscale 2013, 5, 408−415. (29) Roberts, N. A.; Magel, G. A.; Hartfield, C. D.; Moore, T. M.; Fowlkes, J. D.; Rack, P. D. In Situ Laser Processing in a Scanning Electron Microscope. J. Vac. Sci. Technol., A 2012, 30, 041404− 041406. (30) Langford, R. M.; Ozkaya, D.; Sheridan, J.; Chater, R. Effects of Water Vapour on Electron and Ion Beam Deposited Platinum. Microsc. Microanal. 2004, 10, 1122−1123. (31) Gopal, V.; Radilovic, V. R.; Daraio, C.; Jin, S.; Yang, P.; Stach, E. A. Rapid Prototyping of Site-Specific Nanocontacts by Electron and Ion Beam Assisted Direct-Write Nanolithography. Nano Lett. 2004, 4, 2059−2063. (32) Botman, A.; Mulders, J. J. L.; Weemaes, R.; Mentink, S. Purification of Platinum and Gold Structures after Electron-BeamInduced Deposition. Nanotechnology 2006, 17, 3779−3785. (33) Langford, R. M.; Wang, T. X.; Ozkaya, D. Reducing the Resistivity of Electron and Ion Beam Assisted Deposited Pt. Microelectron. Eng. 2007, 84, 784−788. (34) Schwalb, C. H.; Grimm, C.; Baranowski, M.; Sachser, R.; Porrati, F.; Reith, H.; Das, P.; Muller, J.; Volklein, F.; Kaya, A.; Huth, M. A Tunable Strain Sensor Using Nanogranular Metals. Sensors 2010, 10, 9847−9856. (35) Frabboni, S.; Gazzadi, G. C.; Felisari, L.; Spessot, A. Fabrication by Electron Beam Induced Deposition and Transmission Electron Microscopic Characterization of Sub-10-Nm Freestanding Pt Nanowires. Appl. Phys. Lett. 2006, 88 (213116), 1−3. (36) Plank, H.; Kothleitner, G.; Hofer, F.; Michelitsch, S. G.; Gspan, C.; Hohenau, A.; Krenn, J. Optimization of Postgrowth Electron-Beam 14016

dx.doi.org/10.1021/jp503442b | J. Phys. Chem. C 2014, 118, 14009−14016