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δ-A15 and bcc phases coexist in sputtered chromium coatings with moderate oxygen contents
Thin Solid Films 693 (2020) 137676
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δ-A15 and bcc phases coexist in sputtered chromium coatings with moderate oxygen contents
T
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J. Peralta , J. Esteve, A. Lousa Departament de Física Aplicada, Universitat de Barcelona, Martí i Franquès 1, E-08028 Barcelona, Spain
A R T I C LE I N FO
A B S T R A C T
Keywords: Chromium thin films Direct-current magnetron sputtering Phase coexistence δ-A15 crystalline phase X-ray reflectivity X-ray diffraction
Thin films of nanocrystalline Cr with grain sizes of around 20 nm were grown on glass substrates by D.C. magnetron sputtering in an argon atmosphere containing oxygen. The thickness of the films ranged between 30 and 135 nm, and the oxygen content between 11% and 13%. We studied the chemical composition and crystalline structure by means of X-ray spectroscopy, X-ray reflectivity and X-ray diffraction in both Bragg-Brentano and grazing incidence geometries. Our results show the coexistence of metastable δ-A15 and bcc phases in all the Cr films. The combination of the two X-ray diffraction modes allowed us to observe that growth of the δ-A15 phase in the [200] direction is favored when a certain critical film thickness is exceeded for equivalent values of oxygen content. Our results show the importance of the coexistence of both crystalline Cr phases and being aware of their textures, in order to avoid possible misinterpretations of phase transformations associated with critical stress, thickness or oxygen content.
1. Introduction Thin films of Cr are widely used as adhesion layers [1–3], as photomasks in integrated circuits [4,5], and also protective coatings against wear and corrosion [6,7]. For some of applications, control of the formation of the different crystalline phases may be important or useful as the same base material can exhibit different physical properties between one phase and another [8–11]. For example, it has been observed that Cr with a non-bcc δ-A15 crystalline structure is not anti-ferromagnetically ordered and gives rise to a surface ferromagnetic phase [12]. This δ-A15 Cr is a metastable phase with a primitive cubic crystalline structure that transforms into bcc Cr around 430 °C [13]. Better understanding of the correlation between deposition parameters and the formation of the bcc and δ-A15 crystalline phases increase the possibility of structurally engineering thin Cr films. Coexistence of the bcc and δ-A15 crystalline phases has previously been reported by several authors [14–19]. Our group has previously reported a mixture of the δ-A15 and bcc Cr phases in pure nanocrystalline Cr films with grain sizes of between 13 nm and 20 nm, when deposited under non-equilibrium conditions favored by low deposition rates and temperatures [19]. Many other studies have focused on the formation of the δ-A15 phase or its transformation into bcc. The former observations have been controversial [20] and the conclusions of some studies may appear contradictory. For example, it has been observed
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that the δ-A15 structure is a common feature of metals with a native bcc structure when impurities such as N, O and C are incorporated during the growing process [15,20–22]. Consequently, O'Keefe et al. suggested that the δ-A15 phase is an impurity-locked structure [15], pointing out that its formation has been misinterpreted as a phase transformation induced by stress and critical thickness [22,23]. Those results, however, are not supported by observations made by Chu et al., who found that a 20%-35% impurity content within the films seemed unlikely to play an important role in the formation of the δ-A15 phase [13]. Additionally, Shaginyan et al. observed that the δ-A15 phase can be obtained in film form only under particular deposition conditions of relatively high pressure (5 Pa), concluding that the principal conditions for its formation are film growth from low-energy atomic flux onto substrates at a temperature below 400 °C [24]. Interestingly, while most of the studies have addressed the effect of temperature on the formation of the crystalline δ-A15 and bcc phases in thin Cr films, information regarding their formation in processes at room temperature is rather scarce. Thus, the formation of the crystalline phases of Cr is a significant issue that remains open and consequently is worthy of further study for technological and scientific purposes. The formation of the δ-A15 and bcc crystalline phases of Cr has most commonly been addressed using X-ray diffraction in Bragg-Brentano geometry (XRD). It has been established that in nanocrystalline Cr films, both phases are either moderately or strongly textured.
Corresponding author. E-mail address: [email protected] (J. Peralta).
https://doi.org/10.1016/j.tsf.2019.137676 Received 11 June 2019; Received in revised form 25 October 2019; Accepted 27 October 2019 Available online 30 October 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.
Thin Solid Films 693 (2020) 137676
J. Peralta, et al.
Consequently, few peaks are observed in the XRD data, thus making identification of the two crystalline Cr phases more difficult. Surprisingly, no studies of the δ -A15 crystalline structure of sputtered thin Cr films using grazing incidence X-ray diffraction (GIXRD) have previously been published. Such a study may allow us to observe crystalline planes inclined with respect to the surface that are not seen in XRD, thereby providing valuable complementary information on the structural characterization of the films. Therefore, in the present work, thin Cr films of between 30 and 135 nm were deposited on glass substrates at room temperature by D.C. magnetron sputtering in an argon atmosphere containing oxygen, at two different deposition rates. We used a combination of X-ray reflectometry (XRR), XRD, GIXRD and Xray photoelectron spectroscopy (XPS) measurements to evaluate the effect of deposition rate and thickness on the formation of the δ-A15 and bcc crystalline structures of the Cr films.
reflectivity curves. The crystallographic phases and structural properties of the films were studied by XRD and GIXRD using the X'Pert PRO MPD system working with CuKα X-ray radiation. The XRD measurements were obtained across a range of 2θ between 5° and 120° with 0.02° steps. In order to enhance the signal quality, the data were collected at 201 s per point using a detector array of 255 × 255 pixels in such a way that the total measurement time was about 80 min. The GIXRD measurements were obtained across a range of 5° and 105° with 0.05° steps at 8 s per point. The GIXRD measurements were taken with an incidence angle of around 0.38° to minimize the background signal from the glass substrate.
2. Experimental
3.1. Chemical composition
2.1. Sample preparation
All the films presented a high Cr content of around 83%, with slight C and Ar contamination of 3% and 2% respectively. Films deposited at 4 Å/s presented an oxygen content of 11%, while this value increased to 13% for films deposited at 2 Å/s. Comparison of the Cr 2p core level spectra of the films deposited at 2 and 4 Å/s obtained after 10 min of Ar etching with a spectrum of a pure Cr film obtained in situ [25] is presented in Fig. 1. The Cr 2p spectra of the films presented a spin orbit splitting of 9.3 eV, which is typical of metallic Cr [25]. In the high-energy region, we note the absence of the broad satellite band around 597 eV, which is characteristic of Cr2O3 [26]. Additionally, a slight broadening of the Cr 2p3/2 and Cr 2p1/2 peaks is observed with respect to the pure Cr reference spectrum; this can be attributed to the presence of a certain density of Cr–O and Cr–OH bonds in the film [25]. Collectively, these data suggest that the films are metallic and no forms of Cr sub-oxides are present. An example of a normalized O 1 s spectrum obtained for films
3. Results and discussion
The thin Cr films were deposited on glass using a commercial D.C. magnetron sputtering system from Hartec Anlagenbau GmbH, Germany. The system was equipped with a 658.5 × 272.5 mm2 Cr target of 99.99% purity. The substrates were located 10 cm from the target. Prior to deposition, the substrates were cleaned in boiling deionized water for 1 min, then subjected to ultrasonic agitation for 1 min and finally dried with N2. The system was evacuated to a base pressure of 8 × 10−3 Pa. This base pressure was selected to simulate industrial conditions where a certain degree of residual O-containing species is allowed. Although under these conditions the residual gas composition was not controlled, the relative humidity (RH) was recorded prior to each deposition. The plasma was generated in an atmosphere of Ar at a constant pressure of 1.3 × 10−1 Pa. The glass substrates were biased at −65 V and were not intentionally heated. Two series of samples, L and H, were obtained at 8 and 16A values of the cathode current, respectively. Under these conditions, deposition rates of 2 and 4 Å/s for L and H series, were deduced from profilometry measurements. The deposition time was adjusted to yield films between 30 and 135 nm thick. 2.2. Film characterization The thickness of the films was measured by mechanical profilometry with a Dektak 150 surface profiler (Veeco instruments, USA). To do so, lines were painted using permanent markers on the surface of the substrates before the deposition processes. The tint and the sputtered material deposited onto it, were removed using isopropanol prior to the thickness measurements. This process produced sharp steps between the substrate and film surfaces. The chemical composition was studied by means of XPS in a PHI 5500 Multitechnique System (Physical Electronics, USA) with a monochromatic X-ray source, Aluminum Kα line of 1486.6 eV and 350 W. In order to remove contamination, prior to data acquisition the surface of the samples was sputtered for ten minutes with a 4 eV Ar+ ion source. The area analyzed was a circle of 0.8 mm diameter. General spectra were acquired with steps of 0.8 eV for all the samples. Highresolution Cr 2p and O 1 s spectra were acquired with a resolution of 0.1 eV and corrected to C 1 s at 284.8 eV. All measurements were taken in an ultra-high vacuum chamber at a pressure around 1 × 10−6 Pa. XRR measurements were performed using an X'Pert PRO MRD (PANalytical, The Netherlands), with a CuKα X-ray beam with wavelength λ = 1.54 Å of 100 µm cross section. Specular XRR intensities were obtained across an angular range of 2θ between 0° and 6° The thickness, roughness and density of the films were varied using the PANalytical X'Pert Reflectivity software, in order to minimize the differences between the experimental data and the calculated X-ray
Fig. 1. XPS spectra of Cr 2p and O 1 s of the thin films of Cr. Comparison of the Cr 2p spectra of films deposited at 2 and 4 Å/s with a spectrum obtained in situ by Rogojanu [25] (up); Effect of RH in the O 1 s spectra of Cr thin films (down). For clarity, only the thicker samples of the H series are shown. 2
Thin Solid Films 693 (2020) 137676
J. Peralta, et al.
The results obtained from the fitting of the XRR scans are presented in Table 1. The thickness values calculated from the XRR fitting are congruent with those acquired by profilometry. For films above 60 nm thick, the fitting process was impossible because the decrease in the reflected X-ray intensity attenuated the interference fringes. However, the diminution of the reflected X-ray intensity in those films can be attributed to a general trend of roughness increasing with thickness, as revealed by the XRR results when d < 60 nm. In our range of study, the density of the films did not show an apparent dependency on thickness, but decreased from 6.2 g/cm3 to 5.9 g/cm3 when the deposition rate decreased from 4 to 2 Å/s. These density values are significantly lower than that of bulk Cr (7.19 g/cm3) [32] and slightly lower than those reported by Matyi et al. that reported Cr films deposited by electron beam evaporation with 4% hydrogen content (between 6.9 g/cm3 and 6.5 g/cm3). The reduced density of our Cr films can be related to the presence of oxygen, as detected in the XPS analysis [31]. The XRD diffractograms of the Cr films of both series of samples are presented in Fig. 3. The patterns present relatively broad peaks and a considerably low signal-to-noise ratio, typical of nanocrystalline thin films. These patterns suggest a mixture of the δ-A15 and bcc Cr phases. The most intense peak in all the patterns is located around 44° This peak is broad and asymmetric, suggesting that it may have two contributions corresponding to the bcc (110) and δ-A15 (210) planes. Moreover, the presence of the two crystalline phases is evident in the thicker films in both series, which exhibit the δ-A15 (200) and bcc (211) peaks. A clear effect of the thickness of the crystalline structure of the Cr films can be observed. As shown in Fig. 3, in those films of 65–40 nm thickness, only the peak at 44° was observed, accompanied by small traces of the δ-A15 (200) and bcc (211) peaks. The 44° peak became wider, less intense and more asymmetric as thickness decreased, in such a way that the 30 nm thick films were practically amorphous for both series. The results reveal an evolution from an ordered polycrystalline to a nearly amorphous structure, with decreasing thickness. This pattern is probably due to increased substrate temperature, provoked by ion bombardment during film growth. The previous discussion can be qualitatively applied to both series of samples. However, some effects of deposition rate on the structure of the Cr films were observed. The XRD peaks presented small shifts to lower values of 2θ when the deposition rate was decreased from 4 to 2 Å/s. These shifts indicate a larger distance between crystal planes and may be caused by the combination of two factors: residual stress and incorporation of oxygen into the crystal lattice [33]. A description of residual stress in the Cr films is given in what follows, as part of the GIXRD discussion. Fig. 4 shows the GIXRD patterns of both series of Cr films. As in the measurements obtained in θ−2θ configuration, only peaks correspondent to the δ-A15 and bcc Cr phases were observed, with more peaks of such structures present: bcc (200) and (220), and δ-A15 (211) peaks. Consequently, the coexistence of the δ-A15 and bcc phases is confirmed.
Fig. 2. XRR profiles of the thinnest samples (around 30 nm thick) of each series. For clarity, the XRR data are shifted along the intensity scale. The solid symbols correspond to experimental data while the continuous lines correspond to the computed curve.
deposited at RH of 38%, black line, and 53%, red line, registered prior to the deposition process is presented in Fig. 1. A broadening and shift towards higher binding energy values of the O 1 s peak is observed when the RH is 53%. These changes may be explained in terms of hydroxylation or hydration of the Cr films. Introduction of OH– and water ligands may result from the high residual water content in the vacuum chamber, and so we expect that at 53% RH, the contribution of such species provokes those features in the O 1 s spectra. Similar results have been observed before in Cr oxide films exposed to water vapor [27]. 3.2. Film density and structural characterization A comparison of the XRR data fitting for the thinnest samples in each series is presented in Fig. 2. For the fitting procedure we have used a bilayer structural model, consisting of (a) a slightly oxidized Cr layer and (b) a low-density native top layer of Cr(OH)3. The fitted curves obtained via the proposed model result in good agreement with the experimental scans for θ ≤ 1.1°, allowing to determine with enough precision the density, roughness and thickness of the films of Cr. For θ ≥ 1.1° the quality of the data fitting slightly decreases, suggesting that a more sophisticated structure including an extra interfacial layer between substrate and layer (a) may be required [28–30]. Nevertheless, the bilayer model resulted to be a suitable compromise to reduce the number of parameters during the fitting procedure. The fitted curves allowed us to determine that a native hydroxide layer of Cr(OH)3 around 3 nm thick is formed on the surface of our Cr films. The presence of such a native layer in sputtered thin Cr films has previously been suggested [14,31], and is probably not detected via the XPS spectra due to the Ar etching performed before data acquisition.
Table 1 Some parameters and physical properties of the Cr thin films deposited: relative humidity (RH), deposition rate (DRp) and thickness all measured by profilometry (dp); and thickness (dXRR), density (ρXRR) and roughness (Ra) measured by XRR. Sample
RH (%)
DRp (Å/s)
dp (nm)
dXRR (nm)
ρXRR (gr/cm3)
Ra (nm)
L022 L030 L040 L050 L100a H022 H030 H040 H050 H100
40 40 40 52 34 40 40 39 53 38
2.1 2.2 2.2 2.0 2.1 3.3 4.1 4.0 3.9 4.1
29 ± 1 39 ± 1 53 ± 1 61 ± 1 123 ± 2 26 ± 3 39 ± 1 55 ± 1 65 ± 2 135 ± 1
29.5 37.5 48.7 – – 24.7 37.8 50.8 – –
6.0 5.9 5.9 – – 6.3 6.2 6.2 – –
2.3 3.6 3.9 – – 2.0 3.1 3.8 – –
± ± ± ± ± ± ± ± ± ±
0.1 0.1 0.0 0.1 0.0 0.4 0.1 0.1 0.1 0.0
3
± 0.1 ± 0.1 ± 0.2
± 0.1 ± 0.2 ± 0.1
± 0.1 ± 0.1 ± 0.1
± 0.1 ± 0.1 ± 0.1
± 0.1 ± 0.1 ± 0.1
± 0.1 ± 0.1 ± 0.1
Thin Solid Films 693 (2020) 137676
J. Peralta, et al.
Fig. 3. θ−2θ XRD scans of L series (a) and H series (b) Cr films of different thicknesses. For clarity, the diffractograms are shifted along the intensity scale. The vertical dotted lines represent the position of the peaks in the standard powder patterns of the bcc (black) and δ-A15 (blue) phases.
shown in Fig. 5. The fit obtained with two Lorentzian profiles presents quite a good correlation coefficient (0.9993) with small and uniformly distributed fluctuations of the residual values. This fit is clearly better than that using only one Lorentzian profile, which presents a poorer correlation coefficient (0.997) and wide fluctuations of the residual values. These results agree with a mixture of the crystalline phases, indicating that the asymmetry of the peak is the convolution of the bcc (110) and δ-A15 (210) peaks. The deconvolution of the most intense peak of the GIXRD patterns is shown in Fig. 6. The grain size was estimated for both phases using
Moreover, no Cr oxide phases were detected in θ−2θ or GIXRD configurations, indicating that the oxygen detected by XPS was not enough to promote the formation of Cr oxide crystallites, but it was incorporated into the crystalline Cr lattice. The coexistence of the δ-A15 and bcc phases reinforces the possibility of two contributions to the peak located at 44°. In order to clarify this point, a detailed analysis of the asymmetry of the peak located around 2θ–44° in the GIXRD patterns was performed via two approaches: curve-fitting processes using one and two Lorentzian profiles. A typical comparison of the fits with their respective residual plots is
Fig. 4. GIXRD patterns of the Cr films of L (a) and H (b) series. For clarity, the diffractograms are shifted along the intensity scale. 4
Thin Solid Films 693 (2020) 137676
J. Peralta, et al.
Fig. 7. Residual stress of the bcc phase for both series of the Cr films as a function of thickness. A dashed line is drawn to divide the positive values assigned to tensile stress and negative values assigned to compressive stress.
the tilt angle, ψ, equal to the Bragg angle, θ, minus the angle of incidence, γ. The residual stress corresponding to the bcc phase is displayed as a function of thickness for both series of films in Fig. 7. The Cr films exhibit intrinsic tensile residual stress, which tends to relax as thickness increases. This behavior has been observed before by other authors for Cr films deposited by D.C. sputtering at low temperatures and Ar working pressures [35,36]. A shift to the region of more negative values in the residual stress curve is observed when the deposition rate was decreased from 4 to 2 Å/s. This behavior can be attributed to two different effects: ion bombardment and incorporation of oxygen [37,38]. The residual stress is highly dependent on the ratio of bombarding species to depositing species. The flux of bombarding Ar+ is equal for both series, but the average energy per atom is increased in the L series due to the diminished flux of sputtered Cr. This results in increased mobility of Cr adatoms and thus a buildup of a compressive component in the residual stress of those films [35]. It has also been observed that the incorporation of oxygen gives rise to a compressive contribution to the residual stress of sputtered metallic films [37]. A combination of both contributions results in an overall relaxation of the tensile residual stress, which can even turn into compressive stress in our range of study. The zero-stress lattice parameter, a0, can be calculated from Eq. (2) [34]:
Fig. 5. Typical comparison of the two fitting models used to analyze the asymmetry of the most intense peak in the GIXRD diffractograms: fit using one Lorentzian profile (red) and two Lorentzian profiles (blue). The open triangles represent the residuals for each fitting model.
Scherrer's well-known formula and the full-width-at-half-maximum values obtained from the deconvolutions. The grain size was slightly dependent on thickness, tending to increase roughly 50% when the thickness was increased across the studied range. The grain size showed dependence on the deposition rate, decreasing from 20.0 ± 0.5 nm to 15.9 ± 0.2 nm (average calculated for both bcc (110) and δ-A15 (210) peaks) when the deposition rate decreased from 4 to 2 Å/s. The residual stress of the films, σ, is calculated using the GIXRD method as follows [34]:
σ=
αE β (1 + ν ) + 2αν
(1)
where E = 279 GPa and ν = 0.21 are the Young's modulus and Poisson coefficient of Cr, and α and β are the slope and vertical intercept of the linear regression of the lattice parameter aψ as a function of sin2ψ, with
Fig. 6. Deconvolution of the most intense GIXRD peak in the L (left) and H (right) series. The experimental data and fitted Lorentzian curves are presented as open dots and solid lines respectively. 5
Thin Solid Films 693 (2020) 137676
J. Peralta, et al.
oxygen impurities dissolved in the crystal lattice, with a native hydroxide layer of Cr(OH)3 around 3 nm thick on the surface. Our combined XRD and GIXRD analysis has proved to be an effective method to study the coexistence of the δ-A15 and bcc Cr phases, where a thickness-related change in crystalline orientation of the δ-A15 phase permits us to rule out a previously postulated thickness-induced phase transformation from bcc to δ-A15 in Cr films. The deposition rate plays a major role in determining the structural and mechanical properties of Cr films deposited in an Ar atmosphere containing oxygen, where a decrease in the deposition rate from 4 to 2 Å/s results in a decrease from 6.2 g/cm3 to 5.9 g/cm3 in the density of the films, and induces a reduction of grain size from 20 nm to 16 nm. The residual stress of our Cr films is found to be strongly dependent on the deposition rate, changing from highly tensile to compressive when the rate decreases from 4 to 2 Å/s. The behavior of the residual stress is explained in terms of a buildup of compressive stress provoked by the incorporation of oxygen and an increased Ar+ ion bombardment contribution when the ratio of incoming Cr/Ar atoms decreases.
Fig. 8. Zero-stress lattice parameter, a0, as a function of thickness. The dashed line marks the value of a0 in bulk Cr.
a0 =
β (1 + ν ) + 2αν 1+ν
(2)
The zero-stress lattice parameter a0 is presented as a function of thickness for the bcc phase in Fig. 8. The results suggest that a0 is slightly dependent on deposition rate. The value of the error is approximately 0.013° for most points, suggesting an uncertainty in the value of a0, but not a change in its tendency independently of the overlapping of the error bars. From the results is reasonable to assume that a0 increased in our films in comparison with the reported value for bulk Cr [9]. This result can be attributed to a distortion of the lattice caused by the incorporation of atoms of a different size from the Cr atoms in the film [39]. The increase in a0 may indicate that oxygen atoms are interstitially located in the crystalline lattice of our Cr films. The influence of thickness on the texture of the δ-A15 phase in the Cr films was studied by comparison of the XRD diffraction patterns obtained in the θ−2θ and grazing-incidence configurations. In the initial growth regime (30 to 65 nm), the films preferentially grew in the [210] direction. Then, for the thicker films in both series (120 and 135 nm), the δ-A15 (200) peak intensity increased significantly. As the intensity of the (210) diffraction is ~6 times more intense than that of (200) in the standard powder pattern of the δ-A15 Cr phase, a thickness-related change from a preferred growing orientation in the δ-A15 [210] direction to a weak δ-A15 [200] direction may be inferred. This analysis is consistent with the progression of the δ-A15 (200) peak with thickness, as shown in Fig. 6, where a diminution of the intensity as thickness increased indicates a narrowing of the angular distribution, i.e., the crystallites tend to grow in the δ-A15 [200] direction more and more perpendicular to the surface with thickness. As an induced transformation from bcc to δ-A15 is highly unlikely at deposition temperatures below 450 °C, we conclude that the observed trends are provoked by a thickness-related effect on the crystalline orientation of the δ-A15 phase. This change in crystalline orientation exhibited in the δ-A15 phase may be controlled by strain energies related to residual stress buildup as thickness increases. Therefore, ignoring the progress of residual stress of the Cr films may lead to misinterpretations of thickness-induced phase transformations from bcc to δ-A15. In spite of the fact that the details of the formation of the δ-A15 crystalline phase are not yet completely clear, our results show that the crystalline nanostructure of the Cr films evolves in a competitive fashion due to contributions from oxygen impurities and ion bombardment. Consequently, a complete description of the formation of the δ-A15 and bcc crystalline phases is not possible from only considering the incorporation of impurity atoms.
Declaration of Competing Interest 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. Acknowledgments The authors gratefully acknowledge the Scientific and Technological Centers of the University of Barcelona. Particular thanks are due to Josep Bassas and Lorenzo Calvo for their assistance in the XRR, XRD, GIXRD and XPS measurements, as well to our colleague Mr. Marc Torres for his valuable contribution in the edition of this document. This work was supported by the “Secretaria d'Universitats i Recerca del Departament d'Economia i Coneixement” from the “Generalitat de Catalunya” of Spain under contract 2013 DI 070. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.tsf.2019.137676. References [1] D. Park, S.J. Shin, T.S. Oh, Stretchable characteristics of thin au film on polydimethylsiloxane substrate with parylene intermediate layer for stretchable electronic packaging, J. Electron. Mater. 47 (2018) 9–17, https://doi.org/10.1007/ s11664-017-5722-3. [2] Y. Wang, Z. Li, J. Xiao, Stretchable thin film materials: fabrication, application, and mechanics, J. Electron. Packag. Trans. ASME. 138 (2016) 1–22, https://doi.org/10. 1115/1.4032984. [3] R.A. Guerrero, J.T. Barretto, J.L V Uy, I.B. Culaba, B.O. Chan, Effects of spontaneous surface buckling on the diffraction performance of an Au-coated elastomeric grating, Opt. Commun. 270 (2007) 1–7, https://doi.org/10.1016/j.optcom.2006. 08.024. [4] C. Acikgoz, M.A. Hempenius, J. Huskens, G.J. Vancso, Polymers in conventional and alternative lithography for the fabrication of nanostructures, Eur. Polym. J. 47 (2011) 2033–2052, https://doi.org/10.1016/j.eurpolymj.2011.07.025. [5] D.M. Mattox, Handbook of Physical Vapor Deposition (PVD) Processing, second ed., William Andrew, 2010. [6] P. Pearson, The history and future of aircraft turbine engine bearing steels, in: J. Hoo, W. Green (Eds.), Bear. Steels Into 21st Century, first ed., ASTM International, 1998, , https://doi.org/10.1520/STP12138S. [7] Z. Zeng, L. Wang, L. Chen, J. Zhang, The correlation between the hardness and tribological behaviour of electroplated chromium coatings sliding against ceramic and steel counterparts, Surf. Coat. Technol. 201 (2006) 2282–2288, https://doi. org/10.1016/j.surfcoat.2006.03.038. [8] L. Saraf, C. Wang, M.H. Engelhard, D.R. Baer, Temperature-induced phase separation in chromium films, Appl. Phys. Lett. 82 (2003) 2230–2232, https://doi.org/10. 1063/1.1565686. [9] W. Feng, D.D. Dung, S. Cho, Ferromagnetism in tetragonally distorted chromium, Phys. Rev. B 82 (2010) 132401, , https://doi.org/10.1103/PhysRevB.82.132401. [10] M.R. Fitzsimmons, J.A. Eastman, R.A. Robinson, J.W. Lynn, On the possibility of a two-state magnetic structure for nanocrystalline chromium, NanoStructured Mater.
4. Conclusions We report the deposition of nanocrystalline thin films of Cr with oxygen contents ranging between 11% and 13%, at room temperature, by D.C. magnetron sputtering at two different deposition rates in an argon atmosphere containing oxygen. A combination of XRR, XPS and XRD analysis permitted us to establish the structure of the Cr films, which consisted of a Cr layer with 6
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δ-A15 and bcc phases coexist in sputtered chromium coatings with moderate oxygen contents
T
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J. Peralta , J. Esteve, A. Lousa Departament de Física Aplicada, Universitat de Barcelona, Martí i Franquès 1, E-08028 Barcelona, Spain
A R T I C LE I N FO
A B S T R A C T
Keywords: Chromium thin films Direct-current magnetron sputtering Phase coexistence δ-A15 crystalline phase X-ray reflectivity X-ray diffraction
Thin films of nanocrystalline Cr with grain sizes of around 20 nm were grown on glass substrates by D.C. magnetron sputtering in an argon atmosphere containing oxygen. The thickness of the films ranged between 30 and 135 nm, and the oxygen content between 11% and 13%. We studied the chemical composition and crystalline structure by means of X-ray spectroscopy, X-ray reflectivity and X-ray diffraction in both Bragg-Brentano and grazing incidence geometries. Our results show the coexistence of metastable δ-A15 and bcc phases in all the Cr films. The combination of the two X-ray diffraction modes allowed us to observe that growth of the δ-A15 phase in the [200] direction is favored when a certain critical film thickness is exceeded for equivalent values of oxygen content. Our results show the importance of the coexistence of both crystalline Cr phases and being aware of their textures, in order to avoid possible misinterpretations of phase transformations associated with critical stress, thickness or oxygen content.
1. Introduction Thin films of Cr are widely used as adhesion layers [1–3], as photomasks in integrated circuits [4,5], and also protective coatings against wear and corrosion [6,7]. For some of applications, control of the formation of the different crystalline phases may be important or useful as the same base material can exhibit different physical properties between one phase and another [8–11]. For example, it has been observed that Cr with a non-bcc δ-A15 crystalline structure is not anti-ferromagnetically ordered and gives rise to a surface ferromagnetic phase [12]. This δ-A15 Cr is a metastable phase with a primitive cubic crystalline structure that transforms into bcc Cr around 430 °C [13]. Better understanding of the correlation between deposition parameters and the formation of the bcc and δ-A15 crystalline phases increase the possibility of structurally engineering thin Cr films. Coexistence of the bcc and δ-A15 crystalline phases has previously been reported by several authors [14–19]. Our group has previously reported a mixture of the δ-A15 and bcc Cr phases in pure nanocrystalline Cr films with grain sizes of between 13 nm and 20 nm, when deposited under non-equilibrium conditions favored by low deposition rates and temperatures [19]. Many other studies have focused on the formation of the δ-A15 phase or its transformation into bcc. The former observations have been controversial [20] and the conclusions of some studies may appear contradictory. For example, it has been observed
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that the δ-A15 structure is a common feature of metals with a native bcc structure when impurities such as N, O and C are incorporated during the growing process [15,20–22]. Consequently, O'Keefe et al. suggested that the δ-A15 phase is an impurity-locked structure [15], pointing out that its formation has been misinterpreted as a phase transformation induced by stress and critical thickness [22,23]. Those results, however, are not supported by observations made by Chu et al., who found that a 20%-35% impurity content within the films seemed unlikely to play an important role in the formation of the δ-A15 phase [13]. Additionally, Shaginyan et al. observed that the δ-A15 phase can be obtained in film form only under particular deposition conditions of relatively high pressure (5 Pa), concluding that the principal conditions for its formation are film growth from low-energy atomic flux onto substrates at a temperature below 400 °C [24]. Interestingly, while most of the studies have addressed the effect of temperature on the formation of the crystalline δ-A15 and bcc phases in thin Cr films, information regarding their formation in processes at room temperature is rather scarce. Thus, the formation of the crystalline phases of Cr is a significant issue that remains open and consequently is worthy of further study for technological and scientific purposes. The formation of the δ-A15 and bcc crystalline phases of Cr has most commonly been addressed using X-ray diffraction in Bragg-Brentano geometry (XRD). It has been established that in nanocrystalline Cr films, both phases are either moderately or strongly textured.
Corresponding author. E-mail address: [email protected] (J. Peralta).
https://doi.org/10.1016/j.tsf.2019.137676 Received 11 June 2019; Received in revised form 25 October 2019; Accepted 27 October 2019 Available online 30 October 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.
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Consequently, few peaks are observed in the XRD data, thus making identification of the two crystalline Cr phases more difficult. Surprisingly, no studies of the δ -A15 crystalline structure of sputtered thin Cr films using grazing incidence X-ray diffraction (GIXRD) have previously been published. Such a study may allow us to observe crystalline planes inclined with respect to the surface that are not seen in XRD, thereby providing valuable complementary information on the structural characterization of the films. Therefore, in the present work, thin Cr films of between 30 and 135 nm were deposited on glass substrates at room temperature by D.C. magnetron sputtering in an argon atmosphere containing oxygen, at two different deposition rates. We used a combination of X-ray reflectometry (XRR), XRD, GIXRD and Xray photoelectron spectroscopy (XPS) measurements to evaluate the effect of deposition rate and thickness on the formation of the δ-A15 and bcc crystalline structures of the Cr films.
reflectivity curves. The crystallographic phases and structural properties of the films were studied by XRD and GIXRD using the X'Pert PRO MPD system working with CuKα X-ray radiation. The XRD measurements were obtained across a range of 2θ between 5° and 120° with 0.02° steps. In order to enhance the signal quality, the data were collected at 201 s per point using a detector array of 255 × 255 pixels in such a way that the total measurement time was about 80 min. The GIXRD measurements were obtained across a range of 5° and 105° with 0.05° steps at 8 s per point. The GIXRD measurements were taken with an incidence angle of around 0.38° to minimize the background signal from the glass substrate.
2. Experimental
3.1. Chemical composition
2.1. Sample preparation
All the films presented a high Cr content of around 83%, with slight C and Ar contamination of 3% and 2% respectively. Films deposited at 4 Å/s presented an oxygen content of 11%, while this value increased to 13% for films deposited at 2 Å/s. Comparison of the Cr 2p core level spectra of the films deposited at 2 and 4 Å/s obtained after 10 min of Ar etching with a spectrum of a pure Cr film obtained in situ [25] is presented in Fig. 1. The Cr 2p spectra of the films presented a spin orbit splitting of 9.3 eV, which is typical of metallic Cr [25]. In the high-energy region, we note the absence of the broad satellite band around 597 eV, which is characteristic of Cr2O3 [26]. Additionally, a slight broadening of the Cr 2p3/2 and Cr 2p1/2 peaks is observed with respect to the pure Cr reference spectrum; this can be attributed to the presence of a certain density of Cr–O and Cr–OH bonds in the film [25]. Collectively, these data suggest that the films are metallic and no forms of Cr sub-oxides are present. An example of a normalized O 1 s spectrum obtained for films
3. Results and discussion
The thin Cr films were deposited on glass using a commercial D.C. magnetron sputtering system from Hartec Anlagenbau GmbH, Germany. The system was equipped with a 658.5 × 272.5 mm2 Cr target of 99.99% purity. The substrates were located 10 cm from the target. Prior to deposition, the substrates were cleaned in boiling deionized water for 1 min, then subjected to ultrasonic agitation for 1 min and finally dried with N2. The system was evacuated to a base pressure of 8 × 10−3 Pa. This base pressure was selected to simulate industrial conditions where a certain degree of residual O-containing species is allowed. Although under these conditions the residual gas composition was not controlled, the relative humidity (RH) was recorded prior to each deposition. The plasma was generated in an atmosphere of Ar at a constant pressure of 1.3 × 10−1 Pa. The glass substrates were biased at −65 V and were not intentionally heated. Two series of samples, L and H, were obtained at 8 and 16A values of the cathode current, respectively. Under these conditions, deposition rates of 2 and 4 Å/s for L and H series, were deduced from profilometry measurements. The deposition time was adjusted to yield films between 30 and 135 nm thick. 2.2. Film characterization The thickness of the films was measured by mechanical profilometry with a Dektak 150 surface profiler (Veeco instruments, USA). To do so, lines were painted using permanent markers on the surface of the substrates before the deposition processes. The tint and the sputtered material deposited onto it, were removed using isopropanol prior to the thickness measurements. This process produced sharp steps between the substrate and film surfaces. The chemical composition was studied by means of XPS in a PHI 5500 Multitechnique System (Physical Electronics, USA) with a monochromatic X-ray source, Aluminum Kα line of 1486.6 eV and 350 W. In order to remove contamination, prior to data acquisition the surface of the samples was sputtered for ten minutes with a 4 eV Ar+ ion source. The area analyzed was a circle of 0.8 mm diameter. General spectra were acquired with steps of 0.8 eV for all the samples. Highresolution Cr 2p and O 1 s spectra were acquired with a resolution of 0.1 eV and corrected to C 1 s at 284.8 eV. All measurements were taken in an ultra-high vacuum chamber at a pressure around 1 × 10−6 Pa. XRR measurements were performed using an X'Pert PRO MRD (PANalytical, The Netherlands), with a CuKα X-ray beam with wavelength λ = 1.54 Å of 100 µm cross section. Specular XRR intensities were obtained across an angular range of 2θ between 0° and 6° The thickness, roughness and density of the films were varied using the PANalytical X'Pert Reflectivity software, in order to minimize the differences between the experimental data and the calculated X-ray
Fig. 1. XPS spectra of Cr 2p and O 1 s of the thin films of Cr. Comparison of the Cr 2p spectra of films deposited at 2 and 4 Å/s with a spectrum obtained in situ by Rogojanu [25] (up); Effect of RH in the O 1 s spectra of Cr thin films (down). For clarity, only the thicker samples of the H series are shown. 2
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The results obtained from the fitting of the XRR scans are presented in Table 1. The thickness values calculated from the XRR fitting are congruent with those acquired by profilometry. For films above 60 nm thick, the fitting process was impossible because the decrease in the reflected X-ray intensity attenuated the interference fringes. However, the diminution of the reflected X-ray intensity in those films can be attributed to a general trend of roughness increasing with thickness, as revealed by the XRR results when d < 60 nm. In our range of study, the density of the films did not show an apparent dependency on thickness, but decreased from 6.2 g/cm3 to 5.9 g/cm3 when the deposition rate decreased from 4 to 2 Å/s. These density values are significantly lower than that of bulk Cr (7.19 g/cm3) [32] and slightly lower than those reported by Matyi et al. that reported Cr films deposited by electron beam evaporation with 4% hydrogen content (between 6.9 g/cm3 and 6.5 g/cm3). The reduced density of our Cr films can be related to the presence of oxygen, as detected in the XPS analysis [31]. The XRD diffractograms of the Cr films of both series of samples are presented in Fig. 3. The patterns present relatively broad peaks and a considerably low signal-to-noise ratio, typical of nanocrystalline thin films. These patterns suggest a mixture of the δ-A15 and bcc Cr phases. The most intense peak in all the patterns is located around 44° This peak is broad and asymmetric, suggesting that it may have two contributions corresponding to the bcc (110) and δ-A15 (210) planes. Moreover, the presence of the two crystalline phases is evident in the thicker films in both series, which exhibit the δ-A15 (200) and bcc (211) peaks. A clear effect of the thickness of the crystalline structure of the Cr films can be observed. As shown in Fig. 3, in those films of 65–40 nm thickness, only the peak at 44° was observed, accompanied by small traces of the δ-A15 (200) and bcc (211) peaks. The 44° peak became wider, less intense and more asymmetric as thickness decreased, in such a way that the 30 nm thick films were practically amorphous for both series. The results reveal an evolution from an ordered polycrystalline to a nearly amorphous structure, with decreasing thickness. This pattern is probably due to increased substrate temperature, provoked by ion bombardment during film growth. The previous discussion can be qualitatively applied to both series of samples. However, some effects of deposition rate on the structure of the Cr films were observed. The XRD peaks presented small shifts to lower values of 2θ when the deposition rate was decreased from 4 to 2 Å/s. These shifts indicate a larger distance between crystal planes and may be caused by the combination of two factors: residual stress and incorporation of oxygen into the crystal lattice [33]. A description of residual stress in the Cr films is given in what follows, as part of the GIXRD discussion. Fig. 4 shows the GIXRD patterns of both series of Cr films. As in the measurements obtained in θ−2θ configuration, only peaks correspondent to the δ-A15 and bcc Cr phases were observed, with more peaks of such structures present: bcc (200) and (220), and δ-A15 (211) peaks. Consequently, the coexistence of the δ-A15 and bcc phases is confirmed.
Fig. 2. XRR profiles of the thinnest samples (around 30 nm thick) of each series. For clarity, the XRR data are shifted along the intensity scale. The solid symbols correspond to experimental data while the continuous lines correspond to the computed curve.
deposited at RH of 38%, black line, and 53%, red line, registered prior to the deposition process is presented in Fig. 1. A broadening and shift towards higher binding energy values of the O 1 s peak is observed when the RH is 53%. These changes may be explained in terms of hydroxylation or hydration of the Cr films. Introduction of OH– and water ligands may result from the high residual water content in the vacuum chamber, and so we expect that at 53% RH, the contribution of such species provokes those features in the O 1 s spectra. Similar results have been observed before in Cr oxide films exposed to water vapor [27]. 3.2. Film density and structural characterization A comparison of the XRR data fitting for the thinnest samples in each series is presented in Fig. 2. For the fitting procedure we have used a bilayer structural model, consisting of (a) a slightly oxidized Cr layer and (b) a low-density native top layer of Cr(OH)3. The fitted curves obtained via the proposed model result in good agreement with the experimental scans for θ ≤ 1.1°, allowing to determine with enough precision the density, roughness and thickness of the films of Cr. For θ ≥ 1.1° the quality of the data fitting slightly decreases, suggesting that a more sophisticated structure including an extra interfacial layer between substrate and layer (a) may be required [28–30]. Nevertheless, the bilayer model resulted to be a suitable compromise to reduce the number of parameters during the fitting procedure. The fitted curves allowed us to determine that a native hydroxide layer of Cr(OH)3 around 3 nm thick is formed on the surface of our Cr films. The presence of such a native layer in sputtered thin Cr films has previously been suggested [14,31], and is probably not detected via the XPS spectra due to the Ar etching performed before data acquisition.
Table 1 Some parameters and physical properties of the Cr thin films deposited: relative humidity (RH), deposition rate (DRp) and thickness all measured by profilometry (dp); and thickness (dXRR), density (ρXRR) and roughness (Ra) measured by XRR. Sample
RH (%)
DRp (Å/s)
dp (nm)
dXRR (nm)
ρXRR (gr/cm3)
Ra (nm)
L022 L030 L040 L050 L100a H022 H030 H040 H050 H100
40 40 40 52 34 40 40 39 53 38
2.1 2.2 2.2 2.0 2.1 3.3 4.1 4.0 3.9 4.1
29 ± 1 39 ± 1 53 ± 1 61 ± 1 123 ± 2 26 ± 3 39 ± 1 55 ± 1 65 ± 2 135 ± 1
29.5 37.5 48.7 – – 24.7 37.8 50.8 – –
6.0 5.9 5.9 – – 6.3 6.2 6.2 – –
2.3 3.6 3.9 – – 2.0 3.1 3.8 – –
± ± ± ± ± ± ± ± ± ±
0.1 0.1 0.0 0.1 0.0 0.4 0.1 0.1 0.1 0.0
3
± 0.1 ± 0.1 ± 0.2
± 0.1 ± 0.2 ± 0.1
± 0.1 ± 0.1 ± 0.1
± 0.1 ± 0.1 ± 0.1
± 0.1 ± 0.1 ± 0.1
± 0.1 ± 0.1 ± 0.1
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Fig. 3. θ−2θ XRD scans of L series (a) and H series (b) Cr films of different thicknesses. For clarity, the diffractograms are shifted along the intensity scale. The vertical dotted lines represent the position of the peaks in the standard powder patterns of the bcc (black) and δ-A15 (blue) phases.
shown in Fig. 5. The fit obtained with two Lorentzian profiles presents quite a good correlation coefficient (0.9993) with small and uniformly distributed fluctuations of the residual values. This fit is clearly better than that using only one Lorentzian profile, which presents a poorer correlation coefficient (0.997) and wide fluctuations of the residual values. These results agree with a mixture of the crystalline phases, indicating that the asymmetry of the peak is the convolution of the bcc (110) and δ-A15 (210) peaks. The deconvolution of the most intense peak of the GIXRD patterns is shown in Fig. 6. The grain size was estimated for both phases using
Moreover, no Cr oxide phases were detected in θ−2θ or GIXRD configurations, indicating that the oxygen detected by XPS was not enough to promote the formation of Cr oxide crystallites, but it was incorporated into the crystalline Cr lattice. The coexistence of the δ-A15 and bcc phases reinforces the possibility of two contributions to the peak located at 44°. In order to clarify this point, a detailed analysis of the asymmetry of the peak located around 2θ–44° in the GIXRD patterns was performed via two approaches: curve-fitting processes using one and two Lorentzian profiles. A typical comparison of the fits with their respective residual plots is
Fig. 4. GIXRD patterns of the Cr films of L (a) and H (b) series. For clarity, the diffractograms are shifted along the intensity scale. 4
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Fig. 7. Residual stress of the bcc phase for both series of the Cr films as a function of thickness. A dashed line is drawn to divide the positive values assigned to tensile stress and negative values assigned to compressive stress.
the tilt angle, ψ, equal to the Bragg angle, θ, minus the angle of incidence, γ. The residual stress corresponding to the bcc phase is displayed as a function of thickness for both series of films in Fig. 7. The Cr films exhibit intrinsic tensile residual stress, which tends to relax as thickness increases. This behavior has been observed before by other authors for Cr films deposited by D.C. sputtering at low temperatures and Ar working pressures [35,36]. A shift to the region of more negative values in the residual stress curve is observed when the deposition rate was decreased from 4 to 2 Å/s. This behavior can be attributed to two different effects: ion bombardment and incorporation of oxygen [37,38]. The residual stress is highly dependent on the ratio of bombarding species to depositing species. The flux of bombarding Ar+ is equal for both series, but the average energy per atom is increased in the L series due to the diminished flux of sputtered Cr. This results in increased mobility of Cr adatoms and thus a buildup of a compressive component in the residual stress of those films [35]. It has also been observed that the incorporation of oxygen gives rise to a compressive contribution to the residual stress of sputtered metallic films [37]. A combination of both contributions results in an overall relaxation of the tensile residual stress, which can even turn into compressive stress in our range of study. The zero-stress lattice parameter, a0, can be calculated from Eq. (2) [34]:
Fig. 5. Typical comparison of the two fitting models used to analyze the asymmetry of the most intense peak in the GIXRD diffractograms: fit using one Lorentzian profile (red) and two Lorentzian profiles (blue). The open triangles represent the residuals for each fitting model.
Scherrer's well-known formula and the full-width-at-half-maximum values obtained from the deconvolutions. The grain size was slightly dependent on thickness, tending to increase roughly 50% when the thickness was increased across the studied range. The grain size showed dependence on the deposition rate, decreasing from 20.0 ± 0.5 nm to 15.9 ± 0.2 nm (average calculated for both bcc (110) and δ-A15 (210) peaks) when the deposition rate decreased from 4 to 2 Å/s. The residual stress of the films, σ, is calculated using the GIXRD method as follows [34]:
σ=
αE β (1 + ν ) + 2αν
(1)
where E = 279 GPa and ν = 0.21 are the Young's modulus and Poisson coefficient of Cr, and α and β are the slope and vertical intercept of the linear regression of the lattice parameter aψ as a function of sin2ψ, with
Fig. 6. Deconvolution of the most intense GIXRD peak in the L (left) and H (right) series. The experimental data and fitted Lorentzian curves are presented as open dots and solid lines respectively. 5
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oxygen impurities dissolved in the crystal lattice, with a native hydroxide layer of Cr(OH)3 around 3 nm thick on the surface. Our combined XRD and GIXRD analysis has proved to be an effective method to study the coexistence of the δ-A15 and bcc Cr phases, where a thickness-related change in crystalline orientation of the δ-A15 phase permits us to rule out a previously postulated thickness-induced phase transformation from bcc to δ-A15 in Cr films. The deposition rate plays a major role in determining the structural and mechanical properties of Cr films deposited in an Ar atmosphere containing oxygen, where a decrease in the deposition rate from 4 to 2 Å/s results in a decrease from 6.2 g/cm3 to 5.9 g/cm3 in the density of the films, and induces a reduction of grain size from 20 nm to 16 nm. The residual stress of our Cr films is found to be strongly dependent on the deposition rate, changing from highly tensile to compressive when the rate decreases from 4 to 2 Å/s. The behavior of the residual stress is explained in terms of a buildup of compressive stress provoked by the incorporation of oxygen and an increased Ar+ ion bombardment contribution when the ratio of incoming Cr/Ar atoms decreases.
Fig. 8. Zero-stress lattice parameter, a0, as a function of thickness. The dashed line marks the value of a0 in bulk Cr.
a0 =
β (1 + ν ) + 2αν 1+ν
(2)
The zero-stress lattice parameter a0 is presented as a function of thickness for the bcc phase in Fig. 8. The results suggest that a0 is slightly dependent on deposition rate. The value of the error is approximately 0.013° for most points, suggesting an uncertainty in the value of a0, but not a change in its tendency independently of the overlapping of the error bars. From the results is reasonable to assume that a0 increased in our films in comparison with the reported value for bulk Cr [9]. This result can be attributed to a distortion of the lattice caused by the incorporation of atoms of a different size from the Cr atoms in the film [39]. The increase in a0 may indicate that oxygen atoms are interstitially located in the crystalline lattice of our Cr films. The influence of thickness on the texture of the δ-A15 phase in the Cr films was studied by comparison of the XRD diffraction patterns obtained in the θ−2θ and grazing-incidence configurations. In the initial growth regime (30 to 65 nm), the films preferentially grew in the [210] direction. Then, for the thicker films in both series (120 and 135 nm), the δ-A15 (200) peak intensity increased significantly. As the intensity of the (210) diffraction is ~6 times more intense than that of (200) in the standard powder pattern of the δ-A15 Cr phase, a thickness-related change from a preferred growing orientation in the δ-A15 [210] direction to a weak δ-A15 [200] direction may be inferred. This analysis is consistent with the progression of the δ-A15 (200) peak with thickness, as shown in Fig. 6, where a diminution of the intensity as thickness increased indicates a narrowing of the angular distribution, i.e., the crystallites tend to grow in the δ-A15 [200] direction more and more perpendicular to the surface with thickness. As an induced transformation from bcc to δ-A15 is highly unlikely at deposition temperatures below 450 °C, we conclude that the observed trends are provoked by a thickness-related effect on the crystalline orientation of the δ-A15 phase. This change in crystalline orientation exhibited in the δ-A15 phase may be controlled by strain energies related to residual stress buildup as thickness increases. Therefore, ignoring the progress of residual stress of the Cr films may lead to misinterpretations of thickness-induced phase transformations from bcc to δ-A15. In spite of the fact that the details of the formation of the δ-A15 crystalline phase are not yet completely clear, our results show that the crystalline nanostructure of the Cr films evolves in a competitive fashion due to contributions from oxygen impurities and ion bombardment. Consequently, a complete description of the formation of the δ-A15 and bcc crystalline phases is not possible from only considering the incorporation of impurity atoms.
Declaration of Competing Interest 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. Acknowledgments The authors gratefully acknowledge the Scientific and Technological Centers of the University of Barcelona. Particular thanks are due to Josep Bassas and Lorenzo Calvo for their assistance in the XRR, XRD, GIXRD and XPS measurements, as well to our colleague Mr. Marc Torres for his valuable contribution in the edition of this document. This work was supported by the “Secretaria d'Universitats i Recerca del Departament d'Economia i Coneixement” from the “Generalitat de Catalunya” of Spain under contract 2013 DI 070. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.tsf.2019.137676. References [1] D. Park, S.J. Shin, T.S. Oh, Stretchable characteristics of thin au film on polydimethylsiloxane substrate with parylene intermediate layer for stretchable electronic packaging, J. Electron. Mater. 47 (2018) 9–17, https://doi.org/10.1007/ s11664-017-5722-3. [2] Y. Wang, Z. Li, J. Xiao, Stretchable thin film materials: fabrication, application, and mechanics, J. Electron. Packag. Trans. ASME. 138 (2016) 1–22, https://doi.org/10. 1115/1.4032984. [3] R.A. Guerrero, J.T. Barretto, J.L V Uy, I.B. Culaba, B.O. Chan, Effects of spontaneous surface buckling on the diffraction performance of an Au-coated elastomeric grating, Opt. Commun. 270 (2007) 1–7, https://doi.org/10.1016/j.optcom.2006. 08.024. [4] C. Acikgoz, M.A. Hempenius, J. Huskens, G.J. Vancso, Polymers in conventional and alternative lithography for the fabrication of nanostructures, Eur. Polym. J. 47 (2011) 2033–2052, https://doi.org/10.1016/j.eurpolymj.2011.07.025. [5] D.M. Mattox, Handbook of Physical Vapor Deposition (PVD) Processing, second ed., William Andrew, 2010. [6] P. Pearson, The history and future of aircraft turbine engine bearing steels, in: J. Hoo, W. Green (Eds.), Bear. Steels Into 21st Century, first ed., ASTM International, 1998, , https://doi.org/10.1520/STP12138S. [7] Z. Zeng, L. Wang, L. Chen, J. Zhang, The correlation between the hardness and tribological behaviour of electroplated chromium coatings sliding against ceramic and steel counterparts, Surf. Coat. Technol. 201 (2006) 2282–2288, https://doi. org/10.1016/j.surfcoat.2006.03.038. [8] L. Saraf, C. Wang, M.H. Engelhard, D.R. Baer, Temperature-induced phase separation in chromium films, Appl. Phys. Lett. 82 (2003) 2230–2232, https://doi.org/10. 1063/1.1565686. [9] W. Feng, D.D. Dung, S. Cho, Ferromagnetism in tetragonally distorted chromium, Phys. Rev. B 82 (2010) 132401, , https://doi.org/10.1103/PhysRevB.82.132401. [10] M.R. Fitzsimmons, J.A. Eastman, R.A. Robinson, J.W. Lynn, On the possibility of a two-state magnetic structure for nanocrystalline chromium, NanoStructured Mater.
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