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In-situ observation of structural and chemical transitions in B4C based layered systems
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Full Length Article
In-situ observation of structural and chemical transitions in B4C based layered systems ⁎
Christian Morawea, , Phakkhananan Pakawanitb, Ratchadaporn Supruangnetb, Narong Chanlekb, Dechmongkhon Kaewsuwanc, Jean-Christophe Peffena, Sylvain Labouréa a
ESRF - The European Synchrotron, 71 Avenue des Martyrs, 38043 Grenoble, France Synchrotron Light Research Institute (Public Organization), 111 University Avenue, Nakhon Ratchasima 30000, Thailand c Research Network NANOTEC-SUT on Advanced Nanomaterials and Characterization, School of Physics, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand b
A R T I C LE I N FO
A B S T R A C T
Keywords: X-ray multilayers Multilayer degradation In-situ X-ray reflectivity Thermal annealing
In the context of multilayer based X-ray optics developments, thin layered systems of C/[Pt/B4C] multilayers and C/Pt/B4C/Cr stacks with variable cap layer thicknesses were deposited on Si wafers using DC magnetron sputtering. The samples were studied with in-situ X-ray reflectivity techniques during annealing in air up to temperatures of 300 °C. Simulated X-ray spectra of B4C/Cr reveal a considerable thickness loss of the B4C layer at elevated temperatures. The effect is amplified and accelerated when a thin Pt top layer is added but attenuated and slowed down by an additional C cap layer. Energy dispersive X-ray spectroscopy data indicate the overall depletion of B in samples after the annealing process. In-depth studies using X-ray photoelectron spectroscopy techniques show clear evidence of chemical modifications in the original B4C layer and confirm the structural modifications derived from the X-ray reflectivity data. This study demonstrates the catalyzing role of Pt in the degradation of B4C based layered structures in air and the potential protective function of C cap layers.
1. Introduction Historically, the first applications of multilayer (ML) structures as optical elements were developed for the extreme ultraviolet (EUV) [1,2], driven by the need for efficient optics in this energy range where strong absorption in matter dominates. Following significant improvements in mirror polishing and deposition technology such as plasma assisted processes, their use was extended to higher photon energies. Today many modern X-ray synchrotron light sources use ML coatings as optical elements such as monochromators and focusing devices [3,4]. They are fabricated by depositing alternating layers of low and high electron density materials on polished mirror substrates [5,6]. An incoming wave is reflected at each interface of the ML stack. When all reflected amplitudes interfere in phase, a characteristic Bragg peak can be observed, similar to diffraction in single crystals. Smooth and uniform layers with sharp and stable interfaces need to be coated to obtain decent reflectance values. Due to their bandwidth of typically 1% MLs provide about 100 times more intensity than perfect single crystal monochromators and are therefore of particular interest for flux limited applications such as spectroscopy, tomography, or nano-imaging [7]. In many cases Metal/B4C MLs are very attractive due to their convenient ⁎
optical properties in the hard x-ray range and various metal and compound based systems have been investigated in the past [8–11]. Recent investigations [12,13] showed that low d-spacing Pd/B4C MLs, in contrast to many other B4C based systems, degrade in air over periods of days by depletion and oxidation of B. Their d-spacing drops and the layered structure collapses. Their life time can be extended to months or years by appropriate cap layers. C appears to be an efficient protective layer material. In addition, due to low absorption at high photon energies, thin C cap layers have no significant impact on the ML performance in the hard X-ray range. Due to the chemical similarities between Pd and Pt, it seemed useful to extend the study to Pt/B4C MLs. In addition, the Pt/B4C system is of interest for optics on high energy 3rd generation synchrotron beamlines, because it performs very efficiently below the Pt Kα absorption edge at photon energies up to 78.40 keV. While long term stability remains an important property, dedicated studies of the underlying dynamics are relatively cumbersome on the involved time scales. To rescale the reaction speed to manageable periods, temperature dependent in-situ X-ray experiments were carried out that provide information as a function of both temperature and time. Additional insight into the chemistry of the thin films was gained
Corresponding author. E-mail address: [email protected] (C. Morawe).
https://doi.org/10.1016/j.apsusc.2019.144920 Received 7 August 2019; Received in revised form 10 October 2019; Accepted 1 December 2019 Available online 04 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Christian Morawe, et al., Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.144920
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Laboratory using a LEO 1530 scanning electron microscope. The 20 keV electron beam probes a volume of about 1 μm3 starting from the sample surface, penetrating into the thin films and, in most cases, even into the Si substrate. The EDX detection accuracy for light elements such as B and C is rather limited and not suitable for a full quantitative data analysis of the spectra. In addition, the generation of C surface contamination during the sample exposure has to be taken into account for the interpretation of the spectra.
Table 1 Applied power and growth rates of the layer materials. Material
Power P [W]
Rate R [nm/s]
B4C C Cr Pt
500 500 100 50
0.18 0.24 0.31 0.19
2.4. Scanning transmission electron microscopy (STEM)
by studying selected samples with energy dispersive X-ray spectroscopy (EDX) and with in-depth X-ray photoelectron spectroscopy (XPS) techniques performed at the Synchrotron Light Research Institute (SLRI, Thailand).
Sample cross-sections were prepared by focused ion beam using an FEI STRATA 400 tool. The STEM observations were carried out on these cross-sections using an FEI Tecnai OSIRIS microscope.
2. Experimental techniques
2.5. X-ray photoelectron spectroscopy (XPS)
2.1. Magnetron sputter deposition
The XPS measurements were performed on a PHI 5000 VersProbe II instrument at the SUT-NANOTEC-SLRI joint research facility within the Synchrotron Light Research Institute (SLRI), Thailand. A monochromatic Al Kα X-ray beam (1486.6 eV) was used as an excitation source. During each experimental step, a full spectrum from E = 0 eV to 1400 eV was taken in steps of 1.0 eV. In addition, regions of interest (ROI) were measured with a step size of 0.1 eV. These ROI cover the vicinity of the following emission lines: B1s, C1s, N1s, O1s, Si2p, Cr2p, and Pt4f. The data analysis was done using reference tables of standard spectra [16]. The experiment took place in two different sections of the machine: in the reaction chamber the sample was heated, exposed to air, and cooled down close to RT; in the analysis chamber it was etched with an Ar+ ion gun, exposed to the X-ray beam, and XPS spectra were taken. Exposure to air was done at a pressure of 100 kPa (1 bar). Ion etching took place at an acceleration voltage of 1 kV and a current of 7 mA.
All coatings were made at the ESRF multilayer deposition facility [4] using DC magnetron sputtering on Si(1 0 0) wafers. The deposition process took place in an Ar atmosphere at a working pressure of 0.1 Pa. All samples were deposited in dynamic mode where the substrate moves in front of the sputter sources. Table 1 summarizes the applied power and the respective growth rates of all involved materials. Samples that were subject to long term observations only were simply stored in air. For those coatings dedicated to annealing experiments, three identical samples were coated during each deposition run. One of them was immediately transferred into a vacuum storage chamber and maintained in a rough vacuum below 100 Pa. A second sample was kept at room temperature in air. The third sample was used to carry out the in-situ annealing experiments. All samples that were measured with XPS at SLRI were sealed under protective N2 atmosphere after removal from the vacuum storage.
3. Experimental results
2.2. X-ray reflectivity (XRR) and in-situ thermal annealing
3.1. Ex-situ XRR and STEM of Pt/B4C MLs
All scans were carried out on a laboratory X-ray reflectometer, operating with a microfocus Cu tube at 8048 eV, a Montel type ML collimator, and a Si(1 1 1) double crystal monochromator [14]. To provide a reasonable time resolution during the in-situ annealing steps, repeated fast specular reflectivity scans taking no longer than 10 min were performed. Simulation software based on the Parratt formalism [15] allows for the precise determination of thicknesses, mass densities, and interface widths. The sample annealing was done using a furnace (Anton Paar DHS 900) that was mounted horizontally on the sample stage of the reflectometer. The Si substrates were attached to the heating plate using a heat sink compound paste providing sufficient thermal contact. Before starting the experiments the temperature reading of the furnace was calibrated using a Si wafer and a thermocouple attached to its upper surface. The absolute accuracy was better than 1 °C, the initial overshoot was below 10 °C, and the setup stabilized after 10–20 min. A first scan was done at room temperature (RT). Then the temperature was ramped up in steps of typically 25 °C from RT up to a maximum of 300 °C. Ideally, isothermal annealing would be carried out to separate the effects of temperature and time. In practice, the sample was maintained at each temperature no longer than 1 h. In the absence of visible changes in the spectra and to limit the total time of the experiment the temperature was then ramped up to the next level. At the end of each annealing cycle a control scan was added after the sample had cooled down to RT.
A first series of [Pt/B4C]N MLs with d-spacings Λ varying from 1.0 nm to 10.0 nm was deposited. The filling factor was set close to Γ = 0.5 and the number of periods N was selected such that the total thickness remained constant at about 60 nm. The MLs were stored in air at RT and repeatedly measured as a function of time. MLs with Λ < 2 nm started to degrade within hours after exposure to air. Those with Λ > 4 nm remained stable over months showing only minor modifications in the spectra. Two of the most interesting cases with Λ = 2.5 nm and 3.0 nm are presented in Fig. 1. Three scans from each ML are compared, taken immediately after the deposition and about 5 and 9 months later. Fig. 1(a) shows data from a 2.5 nm period sample. A clear shift of the first Bragg peak from θ = 1.8° to 2.3° is observed after 9 months, corresponding to a d-spacing contraction of more than 25%. The intermediate spectrum indicates the coexistence of both periodicities with the reduced d-spacing stack on top of the original, as confirmed by simulations. Similar observations were made on the 3.0 nm period ML (Fig. 1(b)), only that the evolution of the ML appears to slow after 5 months. The 3.0 nm ML shown in Fig. 1 was characterized, 11 months after the deposition, using cross sectional STEM imaging. Fig. 2 shows an STEM image of this sample covering the full stack of 20 bi-layers. It clearly demonstrates the coexistence of two periodic structures. A thinner, less regular section, including pinholes, sits on top of the thicker, preserved stack, which confirms the X-ray data analysis. It appears that the degradation front starts at the ML surface and penetrates vertically into the stack while maintaining the periodic structure. This is a remarkable difference to the degradation of Pd/B4C MLs, where the order is heavily disturbed after the transition [12]. X-ray
2.3. Energy dispersive X-ray spectroscopy (EDX) All EDX investigations were made at the ESRF Microimaging 2
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Fig. 1. Evolution of X-ray reflectivity spectra with time of two Pt/B4C MLs. For better visibility the scans are vertically offset.
Fig. 2. STEM cross section image of a 3 nm period [Pt/B4C]20 ML after 11 months in air at RT. Dark grey indicates Pt, lighter grey B4C or C. The degraded section covers the upper, the preserved part of the ML the lower half of the total stack.
diffraction experiments on the same series of Pt/B4C MLs show that Pt forms weakly (1 1 1)-oriented crystals while B4C layers are amorphous.
3.2. In-situ XRR and thermal annealing of Pt/B4C MLs Based on the results of Section 3.1, a [Pt(1.42 nm)/B4C(1.64 nm)]20 ML was selected to test the annealing setup. An X-ray reflectivity spectrum measured up to θ = 5° before the annealing process is shown as the upper black curve in Fig. 3 measured in the pristine, as-deposited, state. It contains three Bragg peaks, the first being at 1.5°, and distinct Kiessig fringes caused by the total thickness of the stack. Simulations to the data return a clean periodic structure and interface widths of about 0.3 nm RMS. Subsequently, a sequence of reflectivity scans was performed while the temperature was ramped up to 250 °C in steps of 25 °C. An overview plot is given in Fig. 3. To limit the total number of data it shows only the last scan taken at each temperature. One observes an initial evolution up to 100 °C where a broad maximum appears at about 2.2°. Between 125 °C and 150 °C a marked transition occurs where the main peak at 1.5° disappears and is replaced by a new peak at 2.6° that shifts to 2.8° as the temperature is further increased to 200 °C and later to 250 °C. The new peak remains after the ML has cooled down to RT (lower black curve). The critical angle of total reflection shifts to higher values indicating an increase of the average ML density. A simulation of
Fig. 3. Sequence of reflectivity scans versus temperature of a [Pt(1.42 nm)/ B4C(1.64 nm)]20 ML ramped up to 250 °C in steps of 25 °C. For better visibility the scans are vertically offset.
the last scan at RT shows that the d-spacing has shrunk by almost 50% to Λ = 1.576 nm. The thickness loss can be explained by the almost entire depletion of B from B4C, while the Pt thickness remains nearly 3
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Table 2 Transition temperature and time of Pt/B4C MLs with different C cap layer thicknesses. C cap layer thickness
Ttrans
ttrans
0.0 0.5 1.0 2.0
(150 °C) 125 °C 225 °C 250 °C
(< 50 min) 400 min 30 min 500 min
nm nm nm nm
constant. Pure thermal expansion effects are negligible compared to the observed transformation. Similar annealing cycles were performed on identical Pt/B4C MLs but capped with protective C layers of thicknesses between 0.5 nm and 2.0 nm. The overall outcome of these experiments is comparable to the observations of Fig. 3. The essential differences can be found in the temperature level Ttrans where the transition occurs and in the time ttrans it takes to complete it. The results are summarized in Table 2. It appears obvious that the transition temperature increases with growing C cap layer thickness. Note that for the sample without C cap layer the experiment was performed faster than for the following MLs and might therefore overestimate the actual transition temperature. It appears likely that the equivalent of 0.5 nm of C is insufficient to form a uniform cap layer and that the protective effect is not efficient. A more detailed analysis of ttrans would require very long isothermal annealing cycles for each sample. Here it is important to note that all samples transform on comparable and, for these experiments, accessible time scales.
3.3. In-situ XRR and thermal annealing of C/Pt/B4C/Cr layered systems In-situ annealing studies of MLs have a clear advantage: they provide a strong spectral response and basic structural properties such as their d-spacing can be derived in a straight forward way. However, this strength becomes an inconvenience when the well-ordered periodic structure is modified or disturbed. In this case, a clear interpretation of X-ray reflectivity data becomes difficult or even impossible. It was therefore decided to coat simple C/Pt/B4C/Cr stacks on Si with variable C and Pt cap layer thicknesses and to perform similar annealing experiments as carried out on periodic MLs. All stacks start with a thin Cr bottom layer with the only purpose to produce sufficient X-ray optical contrast. A B4C film deposited directly on a Si substrate would be hardly visible due to the similar optical densities of the two materials for hard X-rays. The first example of this series is a B4C(19.5 nm)/Cr(2.1 nm)/Si stack. An X-ray reflectivity scan up to θ = 3° taken at RT before the annealing process is shown as the upper black curve in Fig. 4. It clearly indicates Kiessig interference fringes corresponding to the thickness of the B4C film on the Cr bottom layer, the latter causing the broad intensity bump up to θ = 2°. As in the ML case, a sequence of reflectivity scans was performed while the temperature was ramped up to 300 °C in steps of 50 °C. An overview plot is given in Fig. 4. For the sake of clarity only the last scan of each temperature level is shown. One observes stable behaviour until reaching 300 °C, where the layered structure changes dramatically. To better understand the process, Fig. 5 shows a chronological sequence of scans taken at a constant temperature of 300 °C. Until t ≈ 400 min, no major alteration occurs. After that, the signal contrast drops and the total thickness seems to decrease. At t ≈ 700 min the contrast improves again and a different layer structure appears that stabilizes towards the end of the cycle after t > 1000 min. The underlying vertical density profile based on a 3-layer model was optimized to fit the experimental data. It consists of a Cr bottom layer followed by a B4C film and a low density C-rich top layer (Fig. 6). The latter already forms during temperature ramping and attains a thickness of about 1.5 nm. Once the temperature has reached 300 °C it slowly
Fig. 4. Sequence of reflectivity scans of a B4C/Cr/Si sample with the temperature ramped up to 300 °C in steps of 50 °C. For better visibility the scans are vertically offset.
grows while the B4C thickness decreases slightly from its initial 18.5 nm down to about 18 nm at t ≈ 250 min. After this point, the B4C thickness drops linearly with time to 3 nm after about 700 min. At the same time the top layer thickness increases from 1.5 nm to about 10 nm. Further annealing causes no major modifications in the layers. Similar annealing experiments were performed using a Pt(1.0 nm)/ B4C(18.6 nm)/Cr(2.1 nm)/Si stack and a C(1.0 nm)/Pt(1.0 nm)/ B4C(18.6 nm)/Cr(2.1 nm)/Si sample. For the two respective RT data sets, simulations based on a 3-layer and a 4-layer model agree well with the experimental data. All individual layers and their properties can be clearly identified. As in the previous case both samples were annealed and measured with in-situ X-ray reflectivity. In the following, only the scans taken at constant annealing temperature and during the principal transformation of the layered structure are shown. Fig. 7 summarizes the situation for the Pt/B4C/Cr/Si stack, where significant changes appear at 150 °C and after about 400 min. At this point, a 4th low density layer had to be added to the model to obtain agreement with the measured data. Fig. 8 indicates the transition of the C/Pt/B4C/Cr/Si sample that occurs at 175 °C and after about 250 min. Beyond this point a 5th low density layer had to be added to the model to fit the data correctly. The respective vertical density profiles of the two samples and their evolution with time are given in Figs. 9 and 10. In the Pt/B4C/Cr stack (Fig. 9) both metallic layers are well defined and can be identified quite easily during the whole process. The evolution of low density layers, due to their relatively weak X-ray scattering power, is more difficult to model. The rapid transition zone after 600 min is particularly difficult to simulate. During this period it cannot be excluded that the layered 4
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Fig. 5. Sequence of reflectivity scans of a B4C/Cr/Si sample versus time at constant temperature of 300 °C. For better visibility the scans are vertically offset.
Fig. 7. Sequence of reflectivity scans of a Pt/B4C/Cr/Si sample versus time at constant temperature of 150 °C. For better visibility the scans are vertically offset.
thickness after t ≈ 700 min. Apart from the Pt top layer, the final state of the sample is similar to what has been observed on the B4C/Cr stack. In the C/Pt/B4C/Cr case (Fig. 10), similar comments apply as far as the simulations are concerned, except for the presence of the additional C top layer. Here, no visible changes are observed during ramping at T < 175 °C. The principal transitions take place at 175 °C with an initially slow, then rapidly accelerating transformation of the original B4C layer into a low density C-rich layer. The process ends after about 300 min with the loss of almost 50% of the original total thickness and with virtually no B4C left in the stack. Apart from the C top layer, the final state of the sample is similar to what has been observed on B4C/Cr and Pt/B4C/Cr. The above experiments show that the critical transition temperature Ttrans to oxidize B4C, deplete B, and form a C-rich layer drops from 300 °C for a pure B4C layer to 150 °C after adding a 1 nm thin Pt top layer. With an additional 1 nm thin C cap layer, Ttrans increases again to 175 °C. The characteristic reaction times are of the same order of magnitude. Table 3 summarizes the principal results of the in-situ annealing experiments of the three samples discussed in Section 3.3. The transition time ttrans is defined as the time between the beginning and the end of the B4C thickness loss. This outcome is comparable to what was found for Pt/B4C MLs in Section 3.2.
Fig. 6. Layer thickness evolution with time of a B4C/Cr stack based on a 3-layer model.
structure is disrupted and temporarily replaced by areas with variable thickness that cannot be simulated using the conventional Parratt formalism. The initial ramping steps before reaching the main transition at 150 °C already cause a gradual modification from the original B4C into a C-rich layer. This process continues and ends with the virtual disappearance of B4C and the loss of nearly 50% of the total sample
3.4. EDX studies Four of the samples investigated in Section 3.3 were measured with EDX. Both the B4C(19.5 nm)/Cr(2.1 nm)/Si and the Pt(1.0 nm)/ B4C(18.6 nm)/Cr(2.1 nm)/Si stack were analyzed before and after the 5
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Fig. 10. Layer thickness evolution with time of a C/Pt/B4C/Cr stack based on a 4-to-5-layer model.
Table 3 Principal results of the in-situ annealing experiments on the three samples discussed in Section 3.3. Sample structure [nm]
Ttrans
ttrans
B4C19.5/Cr2.1 Pt1.0/B4C18.6/Cr2.1 C1.0/Pt1.0/B4C18.6/Cr2.1
300 °C 150 °C 175 °C
450 min 700 min 300 min
Fig. 8. Sequence of reflectivity scans of a C/Pt/B4C/Cr/Si sample versus time at constant temperature of 175 °C. For better visibility the scans are vertically offset.
Fig. 11. EDX spectra of samples B4C/Cr/Si (red curves) and Pt/B4C/Cr/Si (blue curves) before (solid lines) and after the annealing cycle (broken lines). (For interpretation of references to colour the reader is referred to the web version of this article.)
enhanced presence of O, which could not be deduced from X-ray reflectivity data only.
3.5. XPS experiments
Fig. 9. Layer thickness evolution with time of a Pt/B4C/Cr stack based on a 3to-4-layer model. The dashed arrow indicates the expected ion etching trajectory during the XPS experiment, the solid arrow the so-detected transition zone (see Section 3.5).
The use of low d-spacing MLs for depth profiled XPS studies may suffer from the sample complexity and from the risk to wash out the signal contrast when the erosion profile and the probing depth exceed the single layer thicknesses [13]. Consequently, a pristine but otherwise identical sample to the Pt/B4C/Cr stack used for in-situ annealing in Section 3.3 was studied with XPS on BL 5.3 at SLRI. After insertion into the vacuum chamber the sample surface was cleaned by a short exposure (30 s) to the Ar+ ion beam. During the initial annealing step at
annealing process. The respective spectra are shown in Fig. 11. In both cases the B lines were visible before the annealing cycles (solid lines) but disappeared after (broken lines). Instead, the O signals grew significantly. These spectra confirm the B depletion and indicate the 6
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Table 4 Ar+ ion etching cycles applied to sample Pt(1.0 nm)/B4C(18.6 nm)/Cr (2.1 nm)/Si. Both the incremental and the cumulated etch times, tetch(inc) and tetch(tot) are given in columns 2 and 3. The estimated total etch depth d is given in column 4, and the removed material in column 5.
Fig. 12. Overview XPS scans versus binding energy of a Pt/B4C/Cr stack before (blue curve) and after annealing (red curve) in air up to 175 °C. For better visibility the scans are vertically offset. (For interpretation of references to colour the reader is referred to the web version of this article.)
Scan No
tetch(inc) [min]
tetch(tot) [min]
d [nm]
#29 #31 #33 #35 #37 #39 #41 #43 #45 #47 #49 #51 #53 #55 #57
0.0 0.2 0.2 0.6 1.0 1.0 2.0 2.0 2.0 2.0 5.0 10.0 10.0 10.0 10.0
0.0 0.2 0.4 1.0 2.0 3.0 5.0 7.0 9.0 11.0 16.0 26.0 36.0 46.0 56.0
0.00 0.31 0.61 1.53 0.53 1.06 2.12 3.18 4.24 5.30 7.95 13.3 18.6
Material
Pt Pt Pt C-rich C-rich C-rich B4 C B4 C B4 C B4 C B4 C B4 C Cr Si
Basic Application code and its Solver function. The spectral shapes of interest were modelled with Gaussian functions and their background simulated using the Shirley algorithm [17]. The results given in Fig. 13 reveal that at this point B occurs both as carbide and oxide (Fig. 13(a)) while most of the C appears to be in its elemental state with contributions from various oxide or nitride peaks (Fig. 13(b)). The line assignments are similar to those reported by other authors [18–21]. Subsequently, a series of ion etching cycles as summarized in Table 4 was carried out on the annealed stack. Based on the technical parameters of the ion gun the etch rates of Pt and B4C were estimated as R(Pt) = 2.10 nm/min and R(B4C) = 0.72 nm/min. The expected material removal trajectory is indicated by the dashed arrow in Fig. 9. A selection of high resolution XPS spectra taken after subsequent etching steps is summarized in Fig. 14 (B1s, C1s, N1s, and O1s lines) and Fig. 15 (Pt4f, Cr2p, and Si2p lines). The sequence runs from the top to the bottom with increasing depth. As expected, the Pt signal drops quickly during the first etch cycles. Some residual material remains in oxidized states. After an etching time of about 5 min both the B and the C peaks shift from their initial positions assigned mainly to B2O3 and CC bonds as shown in Fig. 13(a) and (b) to a constellation corresponding
100 °C the sample was first maintained under vacuum to detect potential modifications that may occur before being exposed to air. Then it was annealed in air at 100 °C, 125 °C, and 150 °C during 10 min each, and finally heated up to 175 °C and maintained there for a total time of 60 min. The maximum temperature was set 25 °C above the level used during the in-situ X-ray reflectivity study because the limited available time on the XPS instrument imposed an accelerated protocol. The annealing process was interrupted for each XPS spectrum, since the sample had to cool down and needed to be transferred from the reaction stage to the analysis chamber and back. After the final annealing step the sample was kept under vacuum and cooled down close to room temperature. The upper curve (blue) in Fig. 12 shows a survey scan taken after the initial short etch cycle. It indicates the presence of B, C, and Pt, as one would expect from a clean surface. The lower scan (red) was recorded after the end of all annealing cycles in air. The B and Pt signals have dropped. A strong O and a faint N peak have appeared. They indicate that within the shallow XPS probing depth of about 2 nm the annealing process has altered the layer properties. A detailed peak analysis was carried out under Microsoft Excel 2007 based on the Visual
Fig. 13. High resolution data and simulations of the B1s (a) and C1s lines (b) after annealing but before Ar+ ion etching. 7
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Fig. 14. Local XPS spectra versus Ar+ etch time near the B1s (a), C1s (b), N1s (c), and O1s lines (d). For better visibility the scans are vertically offset.
Fig. 15. Local XPS spectra versus Ar+ etch time near the Pt4f (a), Cr2p (b), and Si2p lines (c). For better visibility the scans are vertically offset.
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Declaration of Competing Interest
to B4C like compounds. For B the process continues until about 16 min when the peak splits into B-N and B-C bonds. The C peak position remains constant with further etching. With the shift of the B and C lines an N peak appears and grows until the Cr bottom layer is reached. The presence of N in the deeper zones of the sample coincides with the observation of the B-N line. The initial O peak seems to shift from Pt-O to B-O and later to Cr-O and SiO2, although the latter positions are difficult to distinguish from the data. The first detection of Cr after 36 min and Si after 46 min as well as the gradual loss of the B and C signals is clear evidence that the whole stack down to the bottom layer and the substrate was removed. The appearance of the Cr signal allows for a re-calibration of the ion etch rates. Based on the original Pt and B4C layer thicknesses and on the integrated etch time the experimental etch rates would be R(Pt) = 1.53 nm/min and R(B4C) = 0.52 nm/min, that is about 27% less than estimated from the ion gun performance. Note that ion etching of a 20 nm thick film stack is not a clean layer-bylayer removal process, but subject to re-deposition and mixing of materials, and the resolution of XPS is limited by the probing depth. The so-calculated etch depth d per material is given in the 4th column, the concerned material in the 5th column of Table 4. Using the measured removal rates the observed chemical transition zone from the upper C- and O-rich layer into the lower B4C-like film would begin at a depth of about 2 nm below the Pt cap layer and saturate 6 nm deeper. This is close to the zone expected from the in-situ Xray reflectivity data (Fig. 9), accounting for differences in the annealing process such as temperature, exposure and ramping times. Surprisingly, N is mainly detected in the deeper pristine B4C layer while O is found closer to the surface.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors would like to acknowledge the team of the CRG beamline BM32 for lending the in-situ furnace, I. Snigireva for measuring the EDX spectra, and R. Barrett for proof-reading the manuscript. The STEM studies were carried out by the company SERMA TECHNILOGIES (France). This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References [1] J.B. Dinklage, X-ray diffraction by multi-layered thin-film structures and their diffusion, J. Appl. Phys. 38 (1967) 3781–3785, https://doi.org/10.1063/1.1710211. [2] E. Spiller, Low-loss reflection coatings using absorbing materials, Appl. Phys. Lett. 20 (1972) 365–367, https://doi.org/10.1063/1.1654189. [3] A. Kazimirov, D.-M. Smilgies, Q. Shen, X. Xiao, Q. Hao, E. Fontes, et al., Multilayer X-ray optics at CHESS, J. Synchrotron Radiat. 13 (2006) 204–210, https://doi.org/ 10.1107/S0909049506002846. [4] C. Morawe, Graded multilayers for synchrotron optics, in: AIP Conf. Proc., AIP, 2007, pp. 764–769, https://doi.org/10.1063/1.2436173. [5] J.H. Underwood, T.W. Barbee Jr., Layered synthetic microstructures as Bragg diffractors for X rays and extreme ultraviolet: theory and predicted performance, Appl. Opt. 20 (1981) 3027, https://doi.org/10.1364/AO.20.003027. [6] T.W. Barbee Jr., Multilayers for X-ray optics, Opt. Eng. 25 (1986) 258898, , https:// doi.org/10.1117/12.7973929. [7] E. Ziegler, Multilayers for high heat load synchrotron applications, Opt. Eng. 34 (1995) 445, https://doi.org/10.1117/12.194837. [8] C. Morawe, J.-C. Peffen, G.O. Hignette, E. Ziegler, Design and performance of graded multilayers, in: A.M. Khounsary, A.K. Freund, T. Ishikawa, G. Srajer, J.C. Lang (Eds.), SPIE’s Int. Symp. Opt. Sci. Eng. Instrum., International Society for Optics and Photonics, 1999, pp. 90–99, https://doi.org/10.1117/12.370083. [9] S. Braun, P. Gawlitza, M. Menzel, A. Leson, M. Mertin, F. Schäfers, Reflectance and resolution of multilayer monochromators for photon energies from 400–6000 eV, in: AIP Conf. Proc., AIP, 2007, pp. 493–496, https://doi.org/10.1063/1.2436106. [10] P. Oberta, Y. Platonov, U. Flechsig, Investigation of multilayer X-ray optics for the 6 keV to 20 keV energy range, J. Synchrotron Rad. 19 (2012) 675–681, https://doi. org/10.1107/S0909049512032153. [11] H. Jiang, Z. Wang, J. Zhu, Interface characterization of B4C-based multilayers by Xray grazing-incidence reflectivity and diffuse scattering, J. Synchrotron Rad. 20 (2013) 449–454, https://doi.org/10.1107/S0909049513004329. [12] C. Morawe, R. Supruangnet, J.-C. Peffen, Structural modifications in Pd/B4C multilayers for X-ray optical applications, Thin Solid Films 588 (2015) 1–10, https:// doi.org/10.1016/j.tsf.2015.04.037. [13] R. Supruangnet, C. Morawe, J.-C. Peffen, H. Nakajima, S. Rattanasuporna, P. Photongkama, N. Jearanaikoon, W. Busayaporn, Chemical modification of B4C cap layers on Pd/B4C multilayers, Appl. Surface Sci. 367 (2016) 347–353 https:// doi:10.1016/j.apsusc.2016.1.180. [14] Ch. Morawe, J.-Ch. Peffen, P. Pakawanit, The new ESRF thin-film X-ray reflectometer, Proc. SPIE 1076005, 2018, https://doi.org/10.1117/12.2319833. [15] L.G. Parratt, Surface studies of solids by total reflection of X-rays, Phys. Rev. 95 (1954) 359–369, https://doi.org/10.1103/PhysRev. 95.359. [16] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray photoelectron spectroscopy, 1992, https://doi.org/10.1002/sia.740030412. [17] D. Shirley, High-resolution X-ray photoemission spectrum of the valence bands of gold, Phys. Rev. B 5 (1972) 4709–4714, https://doi.org/10.1103/PhysRevB.5. 4709. [18] L.G. Jacobsohn, R.K. Schulze, M.E.H. Maia da Costa, M. Nastasi, X-ray photoelectron spectroscopy investigation of boron carbide films deposited by sputtering, Surf. Sci. 572 (2004) 418–424, https://doi.org/10.1016/j.susc.2004.09.020. [19] H. Moreno Fernàndez, D. Rogler, G. Sauthier, M. Thomasset, R. Dietsch, V. Carlino, E. Pellegrin, Characterization of carbon-contaminated B4C-coated optics after chemically selective cleaning with low-pressure RF plasma, Sci. Reports 8 (2018) 1293, https://doi.org/10.1038/s41598-018-19273-6. [20] Y. Sun, Q. Meng, M. Qian, B. Liu, K. Gao, Y. Ma, et al., Enhancement of oxidation resistance via a self-healing boron carbide coating on diamond particles, Sci. Rep. 6 (2016) 1–6, https://doi.org/10.1038/srep20198. [21] P.N. Rao, U.K. Goutam, P. Kumar, M. Gupta, T. Ganguli, S.K. Rai, Depth-resolved compositional analysis of W/B4C multilayers using resonant soft X-ray reflectivity, J. Synchrotron Rad. 26 (2019) 793–800, https://doi.org/10.1107/ S1600577519002339. [22] D.D. Wagman, W.H. Evans, V.B. Parker, R.H. Schumm, I. Halow, S.M. Bailey, K.L. Churney, R.L. Nuttall, The NBS tables of chemical thermodynamic properties, J. Phys. Chem. Ref. Data. 11 (Suppl) (1982) 2.
4. Discussion and conclusions Both periodic C/[Pt/B4C]N MLs and specific C/Pt/B4C/Cr stacks exhibit B oxidation and depletion upon annealing in air. These findings are similar to those obtained during previous studies of Pd/B4C MLs [12,13]. The respective free energies of the formation of oxides of both metals ΔfG0(PdO) = -85 kJ/mol and ΔfG0(Pt3O4) = -163 kJ/mol [22] are indeed among the lowest (least favorable) of all metals employed in short period ML coatings for X-ray optics. This weak reactivity prevents Pd and Pt from forming protective barriers with B4C and accelerates the reaction channels that lead to B oxidation and depletion. The considerable drop of the observed transition temperature from 300 °C to 150 °C underpins the catalytic action of the thin Pt layer on top of the B4C film. Additional C cap layers clearly slow down B depletion and increase the transition temperature. The deposition of protective cap layers has become a common practice to improve the life time of ML based X-ray optics. The detection of N during the XPS experiments and the possible formation of BN was not observed in air-annealed Pt/B4C samples without exposure to the Ar+ beam, as can be seen from the EDX spectra shown in Fig. 11. A possible explanation might be N contamination during the XPS experiment. Further tests would be required to clarify this issue. The newly applied technique of in-situ X-ray reflectivity scans with simultaneous sample annealing in air, complemented by in-depth XPS investigations, returns a detailed and quantitative description of the degradation process in thin Pt/B4C layered systems. The interplay between the reactive B4C, the catalytic Pt top layer, and the protective C cap can be observed and anticipated as a function of thickness, temperature, and time. In principle, these experiments can be applied to any given thin film system. They provide a convenient tool to explore stability issues and help optimizing ML fabrication strategies. They might also trigger theoretical investigations and modeling efforts and stimulate further studies in the field.
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Full Length Article
In-situ observation of structural and chemical transitions in B4C based layered systems ⁎
Christian Morawea, , Phakkhananan Pakawanitb, Ratchadaporn Supruangnetb, Narong Chanlekb, Dechmongkhon Kaewsuwanc, Jean-Christophe Peffena, Sylvain Labouréa a
ESRF - The European Synchrotron, 71 Avenue des Martyrs, 38043 Grenoble, France Synchrotron Light Research Institute (Public Organization), 111 University Avenue, Nakhon Ratchasima 30000, Thailand c Research Network NANOTEC-SUT on Advanced Nanomaterials and Characterization, School of Physics, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand b
A R T I C LE I N FO
A B S T R A C T
Keywords: X-ray multilayers Multilayer degradation In-situ X-ray reflectivity Thermal annealing
In the context of multilayer based X-ray optics developments, thin layered systems of C/[Pt/B4C] multilayers and C/Pt/B4C/Cr stacks with variable cap layer thicknesses were deposited on Si wafers using DC magnetron sputtering. The samples were studied with in-situ X-ray reflectivity techniques during annealing in air up to temperatures of 300 °C. Simulated X-ray spectra of B4C/Cr reveal a considerable thickness loss of the B4C layer at elevated temperatures. The effect is amplified and accelerated when a thin Pt top layer is added but attenuated and slowed down by an additional C cap layer. Energy dispersive X-ray spectroscopy data indicate the overall depletion of B in samples after the annealing process. In-depth studies using X-ray photoelectron spectroscopy techniques show clear evidence of chemical modifications in the original B4C layer and confirm the structural modifications derived from the X-ray reflectivity data. This study demonstrates the catalyzing role of Pt in the degradation of B4C based layered structures in air and the potential protective function of C cap layers.
1. Introduction Historically, the first applications of multilayer (ML) structures as optical elements were developed for the extreme ultraviolet (EUV) [1,2], driven by the need for efficient optics in this energy range where strong absorption in matter dominates. Following significant improvements in mirror polishing and deposition technology such as plasma assisted processes, their use was extended to higher photon energies. Today many modern X-ray synchrotron light sources use ML coatings as optical elements such as monochromators and focusing devices [3,4]. They are fabricated by depositing alternating layers of low and high electron density materials on polished mirror substrates [5,6]. An incoming wave is reflected at each interface of the ML stack. When all reflected amplitudes interfere in phase, a characteristic Bragg peak can be observed, similar to diffraction in single crystals. Smooth and uniform layers with sharp and stable interfaces need to be coated to obtain decent reflectance values. Due to their bandwidth of typically 1% MLs provide about 100 times more intensity than perfect single crystal monochromators and are therefore of particular interest for flux limited applications such as spectroscopy, tomography, or nano-imaging [7]. In many cases Metal/B4C MLs are very attractive due to their convenient ⁎
optical properties in the hard x-ray range and various metal and compound based systems have been investigated in the past [8–11]. Recent investigations [12,13] showed that low d-spacing Pd/B4C MLs, in contrast to many other B4C based systems, degrade in air over periods of days by depletion and oxidation of B. Their d-spacing drops and the layered structure collapses. Their life time can be extended to months or years by appropriate cap layers. C appears to be an efficient protective layer material. In addition, due to low absorption at high photon energies, thin C cap layers have no significant impact on the ML performance in the hard X-ray range. Due to the chemical similarities between Pd and Pt, it seemed useful to extend the study to Pt/B4C MLs. In addition, the Pt/B4C system is of interest for optics on high energy 3rd generation synchrotron beamlines, because it performs very efficiently below the Pt Kα absorption edge at photon energies up to 78.40 keV. While long term stability remains an important property, dedicated studies of the underlying dynamics are relatively cumbersome on the involved time scales. To rescale the reaction speed to manageable periods, temperature dependent in-situ X-ray experiments were carried out that provide information as a function of both temperature and time. Additional insight into the chemistry of the thin films was gained
Corresponding author. E-mail address: [email protected] (C. Morawe).
https://doi.org/10.1016/j.apsusc.2019.144920 Received 7 August 2019; Received in revised form 10 October 2019; Accepted 1 December 2019 Available online 04 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Christian Morawe, et al., Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.144920
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Laboratory using a LEO 1530 scanning electron microscope. The 20 keV electron beam probes a volume of about 1 μm3 starting from the sample surface, penetrating into the thin films and, in most cases, even into the Si substrate. The EDX detection accuracy for light elements such as B and C is rather limited and not suitable for a full quantitative data analysis of the spectra. In addition, the generation of C surface contamination during the sample exposure has to be taken into account for the interpretation of the spectra.
Table 1 Applied power and growth rates of the layer materials. Material
Power P [W]
Rate R [nm/s]
B4C C Cr Pt
500 500 100 50
0.18 0.24 0.31 0.19
2.4. Scanning transmission electron microscopy (STEM)
by studying selected samples with energy dispersive X-ray spectroscopy (EDX) and with in-depth X-ray photoelectron spectroscopy (XPS) techniques performed at the Synchrotron Light Research Institute (SLRI, Thailand).
Sample cross-sections were prepared by focused ion beam using an FEI STRATA 400 tool. The STEM observations were carried out on these cross-sections using an FEI Tecnai OSIRIS microscope.
2. Experimental techniques
2.5. X-ray photoelectron spectroscopy (XPS)
2.1. Magnetron sputter deposition
The XPS measurements were performed on a PHI 5000 VersProbe II instrument at the SUT-NANOTEC-SLRI joint research facility within the Synchrotron Light Research Institute (SLRI), Thailand. A monochromatic Al Kα X-ray beam (1486.6 eV) was used as an excitation source. During each experimental step, a full spectrum from E = 0 eV to 1400 eV was taken in steps of 1.0 eV. In addition, regions of interest (ROI) were measured with a step size of 0.1 eV. These ROI cover the vicinity of the following emission lines: B1s, C1s, N1s, O1s, Si2p, Cr2p, and Pt4f. The data analysis was done using reference tables of standard spectra [16]. The experiment took place in two different sections of the machine: in the reaction chamber the sample was heated, exposed to air, and cooled down close to RT; in the analysis chamber it was etched with an Ar+ ion gun, exposed to the X-ray beam, and XPS spectra were taken. Exposure to air was done at a pressure of 100 kPa (1 bar). Ion etching took place at an acceleration voltage of 1 kV and a current of 7 mA.
All coatings were made at the ESRF multilayer deposition facility [4] using DC magnetron sputtering on Si(1 0 0) wafers. The deposition process took place in an Ar atmosphere at a working pressure of 0.1 Pa. All samples were deposited in dynamic mode where the substrate moves in front of the sputter sources. Table 1 summarizes the applied power and the respective growth rates of all involved materials. Samples that were subject to long term observations only were simply stored in air. For those coatings dedicated to annealing experiments, three identical samples were coated during each deposition run. One of them was immediately transferred into a vacuum storage chamber and maintained in a rough vacuum below 100 Pa. A second sample was kept at room temperature in air. The third sample was used to carry out the in-situ annealing experiments. All samples that were measured with XPS at SLRI were sealed under protective N2 atmosphere after removal from the vacuum storage.
3. Experimental results
2.2. X-ray reflectivity (XRR) and in-situ thermal annealing
3.1. Ex-situ XRR and STEM of Pt/B4C MLs
All scans were carried out on a laboratory X-ray reflectometer, operating with a microfocus Cu tube at 8048 eV, a Montel type ML collimator, and a Si(1 1 1) double crystal monochromator [14]. To provide a reasonable time resolution during the in-situ annealing steps, repeated fast specular reflectivity scans taking no longer than 10 min were performed. Simulation software based on the Parratt formalism [15] allows for the precise determination of thicknesses, mass densities, and interface widths. The sample annealing was done using a furnace (Anton Paar DHS 900) that was mounted horizontally on the sample stage of the reflectometer. The Si substrates were attached to the heating plate using a heat sink compound paste providing sufficient thermal contact. Before starting the experiments the temperature reading of the furnace was calibrated using a Si wafer and a thermocouple attached to its upper surface. The absolute accuracy was better than 1 °C, the initial overshoot was below 10 °C, and the setup stabilized after 10–20 min. A first scan was done at room temperature (RT). Then the temperature was ramped up in steps of typically 25 °C from RT up to a maximum of 300 °C. Ideally, isothermal annealing would be carried out to separate the effects of temperature and time. In practice, the sample was maintained at each temperature no longer than 1 h. In the absence of visible changes in the spectra and to limit the total time of the experiment the temperature was then ramped up to the next level. At the end of each annealing cycle a control scan was added after the sample had cooled down to RT.
A first series of [Pt/B4C]N MLs with d-spacings Λ varying from 1.0 nm to 10.0 nm was deposited. The filling factor was set close to Γ = 0.5 and the number of periods N was selected such that the total thickness remained constant at about 60 nm. The MLs were stored in air at RT and repeatedly measured as a function of time. MLs with Λ < 2 nm started to degrade within hours after exposure to air. Those with Λ > 4 nm remained stable over months showing only minor modifications in the spectra. Two of the most interesting cases with Λ = 2.5 nm and 3.0 nm are presented in Fig. 1. Three scans from each ML are compared, taken immediately after the deposition and about 5 and 9 months later. Fig. 1(a) shows data from a 2.5 nm period sample. A clear shift of the first Bragg peak from θ = 1.8° to 2.3° is observed after 9 months, corresponding to a d-spacing contraction of more than 25%. The intermediate spectrum indicates the coexistence of both periodicities with the reduced d-spacing stack on top of the original, as confirmed by simulations. Similar observations were made on the 3.0 nm period ML (Fig. 1(b)), only that the evolution of the ML appears to slow after 5 months. The 3.0 nm ML shown in Fig. 1 was characterized, 11 months after the deposition, using cross sectional STEM imaging. Fig. 2 shows an STEM image of this sample covering the full stack of 20 bi-layers. It clearly demonstrates the coexistence of two periodic structures. A thinner, less regular section, including pinholes, sits on top of the thicker, preserved stack, which confirms the X-ray data analysis. It appears that the degradation front starts at the ML surface and penetrates vertically into the stack while maintaining the periodic structure. This is a remarkable difference to the degradation of Pd/B4C MLs, where the order is heavily disturbed after the transition [12]. X-ray
2.3. Energy dispersive X-ray spectroscopy (EDX) All EDX investigations were made at the ESRF Microimaging 2
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Fig. 1. Evolution of X-ray reflectivity spectra with time of two Pt/B4C MLs. For better visibility the scans are vertically offset.
Fig. 2. STEM cross section image of a 3 nm period [Pt/B4C]20 ML after 11 months in air at RT. Dark grey indicates Pt, lighter grey B4C or C. The degraded section covers the upper, the preserved part of the ML the lower half of the total stack.
diffraction experiments on the same series of Pt/B4C MLs show that Pt forms weakly (1 1 1)-oriented crystals while B4C layers are amorphous.
3.2. In-situ XRR and thermal annealing of Pt/B4C MLs Based on the results of Section 3.1, a [Pt(1.42 nm)/B4C(1.64 nm)]20 ML was selected to test the annealing setup. An X-ray reflectivity spectrum measured up to θ = 5° before the annealing process is shown as the upper black curve in Fig. 3 measured in the pristine, as-deposited, state. It contains three Bragg peaks, the first being at 1.5°, and distinct Kiessig fringes caused by the total thickness of the stack. Simulations to the data return a clean periodic structure and interface widths of about 0.3 nm RMS. Subsequently, a sequence of reflectivity scans was performed while the temperature was ramped up to 250 °C in steps of 25 °C. An overview plot is given in Fig. 3. To limit the total number of data it shows only the last scan taken at each temperature. One observes an initial evolution up to 100 °C where a broad maximum appears at about 2.2°. Between 125 °C and 150 °C a marked transition occurs where the main peak at 1.5° disappears and is replaced by a new peak at 2.6° that shifts to 2.8° as the temperature is further increased to 200 °C and later to 250 °C. The new peak remains after the ML has cooled down to RT (lower black curve). The critical angle of total reflection shifts to higher values indicating an increase of the average ML density. A simulation of
Fig. 3. Sequence of reflectivity scans versus temperature of a [Pt(1.42 nm)/ B4C(1.64 nm)]20 ML ramped up to 250 °C in steps of 25 °C. For better visibility the scans are vertically offset.
the last scan at RT shows that the d-spacing has shrunk by almost 50% to Λ = 1.576 nm. The thickness loss can be explained by the almost entire depletion of B from B4C, while the Pt thickness remains nearly 3
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Table 2 Transition temperature and time of Pt/B4C MLs with different C cap layer thicknesses. C cap layer thickness
Ttrans
ttrans
0.0 0.5 1.0 2.0
(150 °C) 125 °C 225 °C 250 °C
(< 50 min) 400 min 30 min 500 min
nm nm nm nm
constant. Pure thermal expansion effects are negligible compared to the observed transformation. Similar annealing cycles were performed on identical Pt/B4C MLs but capped with protective C layers of thicknesses between 0.5 nm and 2.0 nm. The overall outcome of these experiments is comparable to the observations of Fig. 3. The essential differences can be found in the temperature level Ttrans where the transition occurs and in the time ttrans it takes to complete it. The results are summarized in Table 2. It appears obvious that the transition temperature increases with growing C cap layer thickness. Note that for the sample without C cap layer the experiment was performed faster than for the following MLs and might therefore overestimate the actual transition temperature. It appears likely that the equivalent of 0.5 nm of C is insufficient to form a uniform cap layer and that the protective effect is not efficient. A more detailed analysis of ttrans would require very long isothermal annealing cycles for each sample. Here it is important to note that all samples transform on comparable and, for these experiments, accessible time scales.
3.3. In-situ XRR and thermal annealing of C/Pt/B4C/Cr layered systems In-situ annealing studies of MLs have a clear advantage: they provide a strong spectral response and basic structural properties such as their d-spacing can be derived in a straight forward way. However, this strength becomes an inconvenience when the well-ordered periodic structure is modified or disturbed. In this case, a clear interpretation of X-ray reflectivity data becomes difficult or even impossible. It was therefore decided to coat simple C/Pt/B4C/Cr stacks on Si with variable C and Pt cap layer thicknesses and to perform similar annealing experiments as carried out on periodic MLs. All stacks start with a thin Cr bottom layer with the only purpose to produce sufficient X-ray optical contrast. A B4C film deposited directly on a Si substrate would be hardly visible due to the similar optical densities of the two materials for hard X-rays. The first example of this series is a B4C(19.5 nm)/Cr(2.1 nm)/Si stack. An X-ray reflectivity scan up to θ = 3° taken at RT before the annealing process is shown as the upper black curve in Fig. 4. It clearly indicates Kiessig interference fringes corresponding to the thickness of the B4C film on the Cr bottom layer, the latter causing the broad intensity bump up to θ = 2°. As in the ML case, a sequence of reflectivity scans was performed while the temperature was ramped up to 300 °C in steps of 50 °C. An overview plot is given in Fig. 4. For the sake of clarity only the last scan of each temperature level is shown. One observes stable behaviour until reaching 300 °C, where the layered structure changes dramatically. To better understand the process, Fig. 5 shows a chronological sequence of scans taken at a constant temperature of 300 °C. Until t ≈ 400 min, no major alteration occurs. After that, the signal contrast drops and the total thickness seems to decrease. At t ≈ 700 min the contrast improves again and a different layer structure appears that stabilizes towards the end of the cycle after t > 1000 min. The underlying vertical density profile based on a 3-layer model was optimized to fit the experimental data. It consists of a Cr bottom layer followed by a B4C film and a low density C-rich top layer (Fig. 6). The latter already forms during temperature ramping and attains a thickness of about 1.5 nm. Once the temperature has reached 300 °C it slowly
Fig. 4. Sequence of reflectivity scans of a B4C/Cr/Si sample with the temperature ramped up to 300 °C in steps of 50 °C. For better visibility the scans are vertically offset.
grows while the B4C thickness decreases slightly from its initial 18.5 nm down to about 18 nm at t ≈ 250 min. After this point, the B4C thickness drops linearly with time to 3 nm after about 700 min. At the same time the top layer thickness increases from 1.5 nm to about 10 nm. Further annealing causes no major modifications in the layers. Similar annealing experiments were performed using a Pt(1.0 nm)/ B4C(18.6 nm)/Cr(2.1 nm)/Si stack and a C(1.0 nm)/Pt(1.0 nm)/ B4C(18.6 nm)/Cr(2.1 nm)/Si sample. For the two respective RT data sets, simulations based on a 3-layer and a 4-layer model agree well with the experimental data. All individual layers and their properties can be clearly identified. As in the previous case both samples were annealed and measured with in-situ X-ray reflectivity. In the following, only the scans taken at constant annealing temperature and during the principal transformation of the layered structure are shown. Fig. 7 summarizes the situation for the Pt/B4C/Cr/Si stack, where significant changes appear at 150 °C and after about 400 min. At this point, a 4th low density layer had to be added to the model to obtain agreement with the measured data. Fig. 8 indicates the transition of the C/Pt/B4C/Cr/Si sample that occurs at 175 °C and after about 250 min. Beyond this point a 5th low density layer had to be added to the model to fit the data correctly. The respective vertical density profiles of the two samples and their evolution with time are given in Figs. 9 and 10. In the Pt/B4C/Cr stack (Fig. 9) both metallic layers are well defined and can be identified quite easily during the whole process. The evolution of low density layers, due to their relatively weak X-ray scattering power, is more difficult to model. The rapid transition zone after 600 min is particularly difficult to simulate. During this period it cannot be excluded that the layered 4
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Fig. 5. Sequence of reflectivity scans of a B4C/Cr/Si sample versus time at constant temperature of 300 °C. For better visibility the scans are vertically offset.
Fig. 7. Sequence of reflectivity scans of a Pt/B4C/Cr/Si sample versus time at constant temperature of 150 °C. For better visibility the scans are vertically offset.
thickness after t ≈ 700 min. Apart from the Pt top layer, the final state of the sample is similar to what has been observed on the B4C/Cr stack. In the C/Pt/B4C/Cr case (Fig. 10), similar comments apply as far as the simulations are concerned, except for the presence of the additional C top layer. Here, no visible changes are observed during ramping at T < 175 °C. The principal transitions take place at 175 °C with an initially slow, then rapidly accelerating transformation of the original B4C layer into a low density C-rich layer. The process ends after about 300 min with the loss of almost 50% of the original total thickness and with virtually no B4C left in the stack. Apart from the C top layer, the final state of the sample is similar to what has been observed on B4C/Cr and Pt/B4C/Cr. The above experiments show that the critical transition temperature Ttrans to oxidize B4C, deplete B, and form a C-rich layer drops from 300 °C for a pure B4C layer to 150 °C after adding a 1 nm thin Pt top layer. With an additional 1 nm thin C cap layer, Ttrans increases again to 175 °C. The characteristic reaction times are of the same order of magnitude. Table 3 summarizes the principal results of the in-situ annealing experiments of the three samples discussed in Section 3.3. The transition time ttrans is defined as the time between the beginning and the end of the B4C thickness loss. This outcome is comparable to what was found for Pt/B4C MLs in Section 3.2.
Fig. 6. Layer thickness evolution with time of a B4C/Cr stack based on a 3-layer model.
structure is disrupted and temporarily replaced by areas with variable thickness that cannot be simulated using the conventional Parratt formalism. The initial ramping steps before reaching the main transition at 150 °C already cause a gradual modification from the original B4C into a C-rich layer. This process continues and ends with the virtual disappearance of B4C and the loss of nearly 50% of the total sample
3.4. EDX studies Four of the samples investigated in Section 3.3 were measured with EDX. Both the B4C(19.5 nm)/Cr(2.1 nm)/Si and the Pt(1.0 nm)/ B4C(18.6 nm)/Cr(2.1 nm)/Si stack were analyzed before and after the 5
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Fig. 10. Layer thickness evolution with time of a C/Pt/B4C/Cr stack based on a 4-to-5-layer model.
Table 3 Principal results of the in-situ annealing experiments on the three samples discussed in Section 3.3. Sample structure [nm]
Ttrans
ttrans
B4C19.5/Cr2.1 Pt1.0/B4C18.6/Cr2.1 C1.0/Pt1.0/B4C18.6/Cr2.1
300 °C 150 °C 175 °C
450 min 700 min 300 min
Fig. 8. Sequence of reflectivity scans of a C/Pt/B4C/Cr/Si sample versus time at constant temperature of 175 °C. For better visibility the scans are vertically offset.
Fig. 11. EDX spectra of samples B4C/Cr/Si (red curves) and Pt/B4C/Cr/Si (blue curves) before (solid lines) and after the annealing cycle (broken lines). (For interpretation of references to colour the reader is referred to the web version of this article.)
enhanced presence of O, which could not be deduced from X-ray reflectivity data only.
3.5. XPS experiments
Fig. 9. Layer thickness evolution with time of a Pt/B4C/Cr stack based on a 3to-4-layer model. The dashed arrow indicates the expected ion etching trajectory during the XPS experiment, the solid arrow the so-detected transition zone (see Section 3.5).
The use of low d-spacing MLs for depth profiled XPS studies may suffer from the sample complexity and from the risk to wash out the signal contrast when the erosion profile and the probing depth exceed the single layer thicknesses [13]. Consequently, a pristine but otherwise identical sample to the Pt/B4C/Cr stack used for in-situ annealing in Section 3.3 was studied with XPS on BL 5.3 at SLRI. After insertion into the vacuum chamber the sample surface was cleaned by a short exposure (30 s) to the Ar+ ion beam. During the initial annealing step at
annealing process. The respective spectra are shown in Fig. 11. In both cases the B lines were visible before the annealing cycles (solid lines) but disappeared after (broken lines). Instead, the O signals grew significantly. These spectra confirm the B depletion and indicate the 6
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Table 4 Ar+ ion etching cycles applied to sample Pt(1.0 nm)/B4C(18.6 nm)/Cr (2.1 nm)/Si. Both the incremental and the cumulated etch times, tetch(inc) and tetch(tot) are given in columns 2 and 3. The estimated total etch depth d is given in column 4, and the removed material in column 5.
Fig. 12. Overview XPS scans versus binding energy of a Pt/B4C/Cr stack before (blue curve) and after annealing (red curve) in air up to 175 °C. For better visibility the scans are vertically offset. (For interpretation of references to colour the reader is referred to the web version of this article.)
Scan No
tetch(inc) [min]
tetch(tot) [min]
d [nm]
#29 #31 #33 #35 #37 #39 #41 #43 #45 #47 #49 #51 #53 #55 #57
0.0 0.2 0.2 0.6 1.0 1.0 2.0 2.0 2.0 2.0 5.0 10.0 10.0 10.0 10.0
0.0 0.2 0.4 1.0 2.0 3.0 5.0 7.0 9.0 11.0 16.0 26.0 36.0 46.0 56.0
0.00 0.31 0.61 1.53 0.53 1.06 2.12 3.18 4.24 5.30 7.95 13.3 18.6
Material
Pt Pt Pt C-rich C-rich C-rich B4 C B4 C B4 C B4 C B4 C B4 C Cr Si
Basic Application code and its Solver function. The spectral shapes of interest were modelled with Gaussian functions and their background simulated using the Shirley algorithm [17]. The results given in Fig. 13 reveal that at this point B occurs both as carbide and oxide (Fig. 13(a)) while most of the C appears to be in its elemental state with contributions from various oxide or nitride peaks (Fig. 13(b)). The line assignments are similar to those reported by other authors [18–21]. Subsequently, a series of ion etching cycles as summarized in Table 4 was carried out on the annealed stack. Based on the technical parameters of the ion gun the etch rates of Pt and B4C were estimated as R(Pt) = 2.10 nm/min and R(B4C) = 0.72 nm/min. The expected material removal trajectory is indicated by the dashed arrow in Fig. 9. A selection of high resolution XPS spectra taken after subsequent etching steps is summarized in Fig. 14 (B1s, C1s, N1s, and O1s lines) and Fig. 15 (Pt4f, Cr2p, and Si2p lines). The sequence runs from the top to the bottom with increasing depth. As expected, the Pt signal drops quickly during the first etch cycles. Some residual material remains in oxidized states. After an etching time of about 5 min both the B and the C peaks shift from their initial positions assigned mainly to B2O3 and CC bonds as shown in Fig. 13(a) and (b) to a constellation corresponding
100 °C the sample was first maintained under vacuum to detect potential modifications that may occur before being exposed to air. Then it was annealed in air at 100 °C, 125 °C, and 150 °C during 10 min each, and finally heated up to 175 °C and maintained there for a total time of 60 min. The maximum temperature was set 25 °C above the level used during the in-situ X-ray reflectivity study because the limited available time on the XPS instrument imposed an accelerated protocol. The annealing process was interrupted for each XPS spectrum, since the sample had to cool down and needed to be transferred from the reaction stage to the analysis chamber and back. After the final annealing step the sample was kept under vacuum and cooled down close to room temperature. The upper curve (blue) in Fig. 12 shows a survey scan taken after the initial short etch cycle. It indicates the presence of B, C, and Pt, as one would expect from a clean surface. The lower scan (red) was recorded after the end of all annealing cycles in air. The B and Pt signals have dropped. A strong O and a faint N peak have appeared. They indicate that within the shallow XPS probing depth of about 2 nm the annealing process has altered the layer properties. A detailed peak analysis was carried out under Microsoft Excel 2007 based on the Visual
Fig. 13. High resolution data and simulations of the B1s (a) and C1s lines (b) after annealing but before Ar+ ion etching. 7
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Fig. 14. Local XPS spectra versus Ar+ etch time near the B1s (a), C1s (b), N1s (c), and O1s lines (d). For better visibility the scans are vertically offset.
Fig. 15. Local XPS spectra versus Ar+ etch time near the Pt4f (a), Cr2p (b), and Si2p lines (c). For better visibility the scans are vertically offset.
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Declaration of Competing Interest
to B4C like compounds. For B the process continues until about 16 min when the peak splits into B-N and B-C bonds. The C peak position remains constant with further etching. With the shift of the B and C lines an N peak appears and grows until the Cr bottom layer is reached. The presence of N in the deeper zones of the sample coincides with the observation of the B-N line. The initial O peak seems to shift from Pt-O to B-O and later to Cr-O and SiO2, although the latter positions are difficult to distinguish from the data. The first detection of Cr after 36 min and Si after 46 min as well as the gradual loss of the B and C signals is clear evidence that the whole stack down to the bottom layer and the substrate was removed. The appearance of the Cr signal allows for a re-calibration of the ion etch rates. Based on the original Pt and B4C layer thicknesses and on the integrated etch time the experimental etch rates would be R(Pt) = 1.53 nm/min and R(B4C) = 0.52 nm/min, that is about 27% less than estimated from the ion gun performance. Note that ion etching of a 20 nm thick film stack is not a clean layer-bylayer removal process, but subject to re-deposition and mixing of materials, and the resolution of XPS is limited by the probing depth. The so-calculated etch depth d per material is given in the 4th column, the concerned material in the 5th column of Table 4. Using the measured removal rates the observed chemical transition zone from the upper C- and O-rich layer into the lower B4C-like film would begin at a depth of about 2 nm below the Pt cap layer and saturate 6 nm deeper. This is close to the zone expected from the in-situ Xray reflectivity data (Fig. 9), accounting for differences in the annealing process such as temperature, exposure and ramping times. Surprisingly, N is mainly detected in the deeper pristine B4C layer while O is found closer to the surface.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors would like to acknowledge the team of the CRG beamline BM32 for lending the in-situ furnace, I. Snigireva for measuring the EDX spectra, and R. Barrett for proof-reading the manuscript. The STEM studies were carried out by the company SERMA TECHNILOGIES (France). This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References [1] J.B. Dinklage, X-ray diffraction by multi-layered thin-film structures and their diffusion, J. Appl. Phys. 38 (1967) 3781–3785, https://doi.org/10.1063/1.1710211. [2] E. Spiller, Low-loss reflection coatings using absorbing materials, Appl. Phys. Lett. 20 (1972) 365–367, https://doi.org/10.1063/1.1654189. [3] A. Kazimirov, D.-M. Smilgies, Q. Shen, X. Xiao, Q. Hao, E. Fontes, et al., Multilayer X-ray optics at CHESS, J. Synchrotron Radiat. 13 (2006) 204–210, https://doi.org/ 10.1107/S0909049506002846. [4] C. Morawe, Graded multilayers for synchrotron optics, in: AIP Conf. Proc., AIP, 2007, pp. 764–769, https://doi.org/10.1063/1.2436173. [5] J.H. Underwood, T.W. Barbee Jr., Layered synthetic microstructures as Bragg diffractors for X rays and extreme ultraviolet: theory and predicted performance, Appl. Opt. 20 (1981) 3027, https://doi.org/10.1364/AO.20.003027. [6] T.W. Barbee Jr., Multilayers for X-ray optics, Opt. Eng. 25 (1986) 258898, , https:// doi.org/10.1117/12.7973929. [7] E. Ziegler, Multilayers for high heat load synchrotron applications, Opt. Eng. 34 (1995) 445, https://doi.org/10.1117/12.194837. [8] C. Morawe, J.-C. Peffen, G.O. Hignette, E. Ziegler, Design and performance of graded multilayers, in: A.M. Khounsary, A.K. Freund, T. Ishikawa, G. Srajer, J.C. Lang (Eds.), SPIE’s Int. Symp. Opt. Sci. Eng. 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Supruangnet, C. Morawe, J.-C. Peffen, H. Nakajima, S. Rattanasuporna, P. Photongkama, N. Jearanaikoon, W. Busayaporn, Chemical modification of B4C cap layers on Pd/B4C multilayers, Appl. Surface Sci. 367 (2016) 347–353 https:// doi:10.1016/j.apsusc.2016.1.180. [14] Ch. Morawe, J.-Ch. Peffen, P. Pakawanit, The new ESRF thin-film X-ray reflectometer, Proc. SPIE 1076005, 2018, https://doi.org/10.1117/12.2319833. [15] L.G. Parratt, Surface studies of solids by total reflection of X-rays, Phys. Rev. 95 (1954) 359–369, https://doi.org/10.1103/PhysRev. 95.359. [16] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray photoelectron spectroscopy, 1992, https://doi.org/10.1002/sia.740030412. [17] D. Shirley, High-resolution X-ray photoemission spectrum of the valence bands of gold, Phys. Rev. B 5 (1972) 4709–4714, https://doi.org/10.1103/PhysRevB.5. 4709. [18] L.G. Jacobsohn, R.K. Schulze, M.E.H. Maia da Costa, M. 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4. Discussion and conclusions Both periodic C/[Pt/B4C]N MLs and specific C/Pt/B4C/Cr stacks exhibit B oxidation and depletion upon annealing in air. These findings are similar to those obtained during previous studies of Pd/B4C MLs [12,13]. The respective free energies of the formation of oxides of both metals ΔfG0(PdO) = -85 kJ/mol and ΔfG0(Pt3O4) = -163 kJ/mol [22] are indeed among the lowest (least favorable) of all metals employed in short period ML coatings for X-ray optics. This weak reactivity prevents Pd and Pt from forming protective barriers with B4C and accelerates the reaction channels that lead to B oxidation and depletion. The considerable drop of the observed transition temperature from 300 °C to 150 °C underpins the catalytic action of the thin Pt layer on top of the B4C film. Additional C cap layers clearly slow down B depletion and increase the transition temperature. The deposition of protective cap layers has become a common practice to improve the life time of ML based X-ray optics. The detection of N during the XPS experiments and the possible formation of BN was not observed in air-annealed Pt/B4C samples without exposure to the Ar+ beam, as can be seen from the EDX spectra shown in Fig. 11. A possible explanation might be N contamination during the XPS experiment. Further tests would be required to clarify this issue. The newly applied technique of in-situ X-ray reflectivity scans with simultaneous sample annealing in air, complemented by in-depth XPS investigations, returns a detailed and quantitative description of the degradation process in thin Pt/B4C layered systems. The interplay between the reactive B4C, the catalytic Pt top layer, and the protective C cap can be observed and anticipated as a function of thickness, temperature, and time. In principle, these experiments can be applied to any given thin film system. They provide a convenient tool to explore stability issues and help optimizing ML fabrication strategies. They might also trigger theoretical investigations and modeling efforts and stimulate further studies in the field.
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