Combined optical emission and resonant absorption

Spectrochimica Acta Part B 103–104 (2015) 99–105

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Combined optical emission and resonant absorption diagnostics of an Ar-O2-Ce-reactive magnetron sputtering discharge A.A. El Mel a,b, S. Ershov a, N. Britun a,⁎, A. Ricard c, S. Konstantinidis a, R. Snyders a,d a

Chimie des Interactions Plasma-Surface (ChIPS), Research Institute for Materials Science and Engineering, Université de Mons, Place du Parc 23, Mons B-7000, Belgium Institut des Matériaux Jean Rouxel, Université de Nantes, CNRS, 2 rue de la Houssinière B.P. 32229, Nantes Cedex 3 44322, France Université de Toulouse, UPS, INPT, LAPLACE (Laboratoire Plasma et Conversion d'Energie), 118 route de Narbonne, Toulouse Cedex 9 F-31062, France d Materia Nova Research Center, Parc Initialis, Avenue Copernic 1, Mons B-7000, Belgium b c

a r t i c l e

i n f o

Article history: Received 23 March 2014 Accepted 23 November 2014 Available online 4 December 2014 Keywords: Optical emission spectroscopy Optical absorption spectroscopy Cerium Argon metastables Magnetron sputtering

a b s t r a c t We report the results on combined optical characterization of Ar-O2-Ce magnetron sputtering discharges by optical emission and resonant absorption spectroscopy. In this study, a DC magnetron sputtering system equipped with a movable planar magnetron source with a Ce target is used. The intensities of Ar, O, and Ce emission lines, as well as the absolute densities of Ar metastable and Ce ground state atoms are analyzed as a function of the distance from the magnetron target, applied DC power, O2 content, etc. The absolute number density of the Arm is found to decrease exponentially as a function of the target-to-substrate distance. The rate of this decrease is dependent on the sputtering regime, which should be due to the different collisional quenching rates of Arm by O2 molecules at different oxygen contents. Quantitatively, the absolute number density of Arm is found to be equal to ≈3 × 108 cm−3 in the metallic, and ≈5 × 107 cm−3 in the oxidized regime of sputtering, whereas Ce ground state densities at the similar conditions are found to be few times lower. The absolute densities of species are consistent with the corresponding deposition rates, which decrease sharply during the transition from metallic to poisoned sputtering regime. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Metal oxide thin films are the focus of many researches studies due to the wide variety of physical and chemical properties that can be tuned, e.g., by inducing small changes in their band structures [1–3]. Among the studied oxides, cerium oxide is of particular interest owing to its potential application in gas sensors [4], anti-corrosion coatings [5], CMOS transistors [6], and sensors [7]. Among the various approaches that can be used to synthesize CeO thin films stands magnetron [8] sputtering, which is a powerful technique usually implemented in industry for deposition of high-quality coatings of various materials at low temperature and over large areas. The strength of this technique arises from the possibility of understanding and controlling the various mechanisms occurring within the plasma discharges. Thanks to this control, the characteristics of the deposited coatings can be tailored accurately according to the particular application [9]. The plasma needs to be rigorously characterized, however, in order to implement the above-mentioned control. Several tools can be used to characterize the plasma discharges, including optical emission spectroscopy (OES) [10–16], resonant optical

⁎ Corresponding author. E-mail address: [email protected] (N. Britun).

http://dx.doi.org/10.1016/j.sab.2014.11.009 0584-8547/© 2014 Elsevier B.V. All rights reserved.

absorption spectroscopy (ROAS) [10–12], laser-induced fluorescence [17–19], glow discharge mass spectrometry [11,20], and Fabry–Perot interferometry [21]. Among these techniques, in situ OES and ROAS allow non-intrusive determination of several important plasma parameters [10,11,22–24]. For example, OES is often employed to evaluate the gas temperature [10,11,15,22–25]. ROAS can be used to determine the absolute number density of the plasma species [10,22,24–26]. In the context of sputtering, while OES is widely employed since the beginnings of magnetron-based technology to characterize sputtered metal atoms [27,28], the interpretation of OES data in terms of population density is however not straightforward as, by definition, only the excited energy levels are probed. Therefore, OES can only provide qualitative information on the plasma chemistry or requires otherwise the knowledge of the excitation pathways at the location where OES data are obtained [29,30]. Electron energy distribution function and careful Langmuir probe measurements are required to achieve this goal [31]. To the contrary, ROAS can provide the number density of atoms in a relatively straightforward way. Indeed, determining the excitation pathways of the probed atoms is not mandatory as the ground state energy (sub)levels are detected, and/or eventually the long living metastable states. Most of the time, the use of ROAS was limited on metal atoms (Al [32], Ti [10,21,22,24,33], Cu [34], and Ni [35]) sputtered in pure argon plasma. Much less data are reported for ROAS in so-called reactive sputtering discharges, when a reactive gas such as oxygen [36] or

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A.A. El Mel et al. / Spectrochimica Acta Part B 103–104 (2015) 99–105

Table 1 Spectral lines of Ar, O, and Ce used in this study.

Emission

Element

Wavelength (nm)

Transition (lower–upper)

Nature of lower level

Absorption oscillator strength (fji)

Ar

763.51⁎ 750.39 777.19 777.42 777.54 520.17⁎⁎ 794.82 811.53 461.05⁎⁎

3p54s(1s5)–3p54p(2p6) 3p54s(1s2)–3p54p(2p1) 2p3(4S°)3s 5S02–2p3(4S°)3p 5P3 2p3(4S°)3s 5S02–2p3(4S°)3p 5P2 2p3(4S°)3s 5S02–2p3(4S°)3p 5P1 4f 5d 6s2 3H°4–4f2(3 F)5d (4 F)6s 0A5 3p54s(1s3)–3p54p(2p4) 3p54s(1s5)–3p54p(2p9) 4f 5d 6s2 1G°4–4f2(1G)5d (2I)6s 0A5

Metastable Radiative Metastable Metastable Metastable Excited (0.16 eV) Metastable Metastable Ground

– – – – – – 0.56 0.51 ~11

O

Absorption

Ce Ar Ce

⁎ Spectral information for Ar (Paschen representation) is taken from ref. [48]. ⁎⁎ Spectral information for Ce is taken from ref. [49].

nitrogen [11] is admixed to the argon gas in order to produce metal oxide or metal nitride compounds, respectively. We have recently reported on the characterization of Ce-Ar-O2 plasma discharge using ROAS [37]. However, our study was limited only on the evaluation of the Ce ground state density (CeGS). In order to broaden the comprehension of the Ce-Ar-O2 plasma discharge and understand its impact on the characteristics of the deposited thin films, a further characterization the rest of the species constituting the plasma is necessary. Argon metastables (Arm) are among the neutral species which have an essential role in sputtering discharges due to their interaction with various plasma species or the growing thin film on the substrate surface. In case of Penning ionization for example, the reaction of the Arm with the neutral metal atoms in high-pressure (N 0.1 Torr) sputtering discharges results in the formation of ionized metal species in the plasma [13]. The collisional process, which may occur between Arm and O2 molecules in an Ar-O2 plasma discharge resulting in fragmentation of the O2 molecules, is another example of the possible reactions taking place inside the plasma bulk. The production of chemically reactive O atoms is obviously of great relevance for plasma-based surface modification processes. It is also important to mention that the quenching (i.e., drop in number density) of the Arm is among the main consequences of the reactions occurring between the Arm and the other species of the plasma. This effect is expected to vary spatially in respect to the cathode position, resulting in a non homogenous distribution of the neutral species in the magnetron sputtering discharge [12, 22,26,38] in which the magnetic field, generated by the permanent magnets located under the cathode, confines the plasma in the target vicinity. As a consequence such spatial variation, the deposition rate and the characteristics of the synthesized films may evolve when varying the substrate position relatively to the target surface. In this context, the aim of this study is to investigate the spatial distribution of the neutral species in Ar-O2-Ce-reactive magnetron discharge. More precisely, the spatial distribution of absolute number densities of Arm states are determined by probing different zones of the plasma extending between the target and the substrate using OES and ROAS techniques. The obtained values are compared to the absolute number density of CeGS. Furthermore, the impact of the spatial evolution of the absolute number densities of Arm and CeGS on the deposition rate of cerium oxide thin films is also addressed.

The total flow of the injected gas was maintained at about 60 sccm (standard cubic centimeter per minute). The oxygen fraction, F, calculated from following relation F = f(O2) / (f(O2) + f(Ar)) (where f is the mass flow of gas), and injected into the vacuum chamber was varied between 0% and 80%. For 0% and 3% of O2, the discharge operates in the metallic mode whereas for 20%, 50%, and 80% of O2 the so-called poisoned mode takes over. The base pressure before each experiment was in the order of 10−6 Torr, whereas two different pressures were selected, namely 5 and 20 mTorr. 2.2. Plasma diagnostic setup For OES and ROAS characterization, two quartz windows facing each other were used as vacuum viewports for spectra acquisition (Fig. 1A). A Ce “Perkin Elmer” hollow cathode lamp (HCL), supplied by a 15 mA bias DC current, was used as a reference source for the ROAS measurements. The HCL stabilization time before each set of measurements was about 30 min. The collimated HCL beam (~ 1.5 cm in diameter) crossed the magnetron axis of symmetry and reached the second quartz window.

2. Experimental 2.1. Magnetron sputtering system The magnetron sputtering system used in this study is presented in Fig. 1A. A movable balanced magnetron source (Fig. 2) was used to sputter a Ce target (3 inch in diameter, and 99.99% in purity) in an Ar-O2 direct current (DC) magnetron discharge. The discharge is run in the constant current mode. Three different current intensities were selected for this study: 0.5 A, 1 A, and 1.5 A, corresponding to an electrical power of 125 W, 275 W and 400 W, respectively. The magnetron source was located in a stainless steel chamber, 63 cm long and 37 cm in diameter.

Fig. 1. Schematic illustration of the magnetron sputtering system used for the diagnostic of Ar-O-Ce magnetron plasma.


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the light emitted by the HCL), and IP represents the light intensity of the plasma with the source switched off. On the other hand, assuming a situation where the plasma line broadening is only induced by the Doppler effect, the mathematical relation between AL, L, and the absorption coefficient k0 can be written as follows: 2 AL ¼ pffiffiffi π

Z∞

2 2 2 Exp −x 1−Exp −k0 L Exp −α x dx

ð2Þ

0

where α represents the ratio of the optical source (δσS) and the plasma (δσP) line widths. Using the values of AL determined experimentally from Eq. (1), the absorption coefficient k0 can be evaluated using Eq. (2), which allows in turn calculating the absolute number density na (cm−3) of the absorbing atoms as follows: 12

na ¼ 1:21 10 ðk0 LÞ

δσ P f L

ð3Þ

where f (unitless) is the oscillator strength of a chosen spectral transition. In case of low-pressure glow discharges, the Doppler broadening is the dominant broadening mechanism of the spectral lines [39]. Thus, a Gaussian line profile can be used to fit the data and then to calculate the plasma line-width according to the following equation: −7

P

δσ ¼ 7:16 10

Fig. 2. (A) Map of the magnetic field corresponding to the balanced magnetron used in this work. (B) Evolution of the magnetic field strength measured along the magnetron axis starting from the target surface.

Afterwards, it was guided by an optical fiber to a detector. A shutter, settled in front of the HCL, was used to cut the HCL signal in order to collect only the plasma signal when required. All the emission spectra were acquired using an Andor SR750 monochromator equipped with a 1800 g/ mm diffraction grating, which results in ~0.04 nm of spectral resolution. An Andor iStar 740 intensified charge coupled device (ICCD) camera was used as a detector for accumulation of the emission light from plasma and/or HCL. The studied spectral lines, as well as the corresponding transitions for studied species can be found in Table 1. Few hundreds of accumulations on the ICCD combined with typically 300 ms of exposure time were utilized for typical signal measurements. For the data analyses, several central pixels on the ICCD were used corresponding to the wavelength integration (in the peak vicinity) of about 0.01–0.05 nm roughly. 3. ROAS theory A detailed introduction to the resonant ansorption technique can be found elsewhere [39]. In brief, the ROAS method consists of measuring the fraction of the absorbed radiation AL obtained for an effective optical length L (cm). Experimentally, AL can be determined according to the following relation: AL ¼ 1−

ðIPS −IP Þ ; IS

ð1Þ

where IPS stands for the intensity of the light emitted simultaneously by the plasma and the source, Is is the source line intensity (i.e., intensity of

σ0

1=2 Tg M

ð4Þ

where σ0 (cm−1) is the wavenumber corresponding to the center of the measured peak, M (g/mol) is the atomic weight of the analyzed element (here Ar or Ce), and Tg (K) is the gas temperature. Substituting Eq. (4) in Eq. (3), the general expression of the absolute number density of the absorbing species, na, can finally be obtained as follows: 5

na ¼ 8:66 10 ðk0 LÞ

σ 0 T g 1=2 f L M

ð5Þ

4. Results and discussion 4.1. Gas temperature According to Eqs. (4) and (5), the gas temperature, Tg, is a required input parameter for the calculation of the absolute number density of the different absorbing species present in the plasma. Using optical methods, Tg can be evaluated using several ways, including the analysis of the rotational spectrum of simple diatomic molecules. In particular, molecular nitrogen can be added to the gas mixture and the rotational temperature derived from the N2 first positive system can be determined [10,11,33]. In this type of plasma, the gas composition would not affect the gas temperature significantly [37]. Following this considerations, in this work, the gas temperatures was determined using an Ar + 20% N2 mixture. Since this method implies that the temperatures of each group of species present in the plasma are the same, the thermalization of the sputtered species is a critical condition that should be taken into account. It has been reported elsewhere that a sputtered metallic atom requires typically several mean free paths to get thermalized with the surrounding environment [24,37,40,41]. For example, at 15 mTorr, the mean free path of 5 eV sputtered atoms of Cu or Ti is about 16 mm [38,40]. This indicates that, in such conditions, if the working distance d (e.g. distance between the cathode and the analyzed region) equals at least 16 mm, the sputtered metal species can be considered thermalized with the bulk gas atoms [24,42]. For this reason, in order to

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determine the gas temperature reliably, rather high working pressure of 20 mTorr is selected in this study. The smallest distance from the cathode was 25 mm in our case. It should be noted that, in spite of the fact the gas temperature is monitored at 25 mm away from the target, the gas temperature may be still far from reality since the measurements can be affected by the presence of a fraction of non-thermalized species. On the other hand, at higher distances, it can be assumed that all the Ce atoms are thermalized and are at about the same temperature as the Ar atoms, the O2 and the N2 molecules. Fig. 3 illustrates the evolution of the measured Tg at two different working distances d (i.e., 25 mm and 75 mm) as a function of the electrical power applied to the target. At both distances, a linear increase of Tg is observed when the power increases. It can be noticed that approaching the cathode (from 75 mm to 25 mm) results in a slight linear temperature shift toward higher values. Based on this result, the obtained Tg values are extrapolated for the other distances (in the 50–100 mm range), as shown in Fig. 3. 4.2. Optical emission spectroscopy In Fig. 4, the evolution of the intensities of the emission lines (logscale) of Ce, O, and Ar are presented as a function of the distance from the target for several Ar/O2 gas mixtures. For all the studied emission lines, a sharp decay in the emission intensity is followed by a much lower decay rate (at d N 50 mm). The high line intensities observed at 25 mm is related to the fact that at such distance from the target surface, the analyzed region corresponds to the glow discharge's area, where the magnetic field is intense and the plasma is very dense. This is related to the well-known electron confinement effect in magnetron sputtering in the target vicinity (where the magnetic field is strong and has a shape of a spherical segment; see Fig. 2). Out of this region (d N 50 mm in our case), the electron density is much lower [43], which results in much weaker excitation rates of the plasma species (compared to Fig. 3 in [22]). The impact of oxygen is found to vary from one element to another. In case of Ar (Fig. 4A), increasing the oxygen fraction induces a decrease of the Ar 763 nm line intensity. An opposite evolution is observed in case of the O lines (Fig. 4B), where the increase of the oxygen fraction in the plasma leads to an increase of the O 777 nm triplet intensity. It should be mentioned that the evolution of the O line intensity as a function of distance for the different considered oxygen fractions exhibits almost the same slope. This result indicates that for the three oxygen fractions, the mechanism leading to the spatial attenuation of the O line intensity is identical. For the Ce line (Fig. 4C), the emission signal

Fig. 3. Evolution of the gas temperature, Tg, as a function of the plasma parameters in the considered DC magnetron discharge.

Fig. 4. Evolution of the peak intensities of (A) Ar 763 nm, (B) O 777 nm, and (C) Ce 520 nm recorded at different oxygen fractions as a function of the target-substrate distance. The discharge current is 1 A.

was detectable only for oxygen fraction of 0% and 3%, which correspond to the metallic mode. This means that in the poisoned mode (i.e., oxygen fraction higher than 3%) a very low amount of Ce, which is probably below the detection limits of the used spectrometer system, is present in the plasma or that Ce is sputtered from the oxidized target as CeOx molecules.

4.3. Resonant optical absorption spectroscopy The spatial evolution of the absolute number density of the argon metastables, Arm, present in the discharge is given in Fig. 5. At 5 mTorr, the absolute number density decays exponentially as a function of distance at any oxygen fraction (Fig. 5A). In the metallic mode (i.e., for an oxygen fraction between 0% and 3%), the highest absolute number density of Arm, about 2 × 109 cm−3, is recorded at 25 mm from the target. In this mode, the decrease of the Arm absolute number density when d increases can be due to the fact that the plasma in the magnetron is confined near the target and is less intense as moving far away. Similar evolution is observed at 20 mTorr (Fig. 5B). Moreover, as it can be seen that the curves illustrating the evolution of the absolute number density as a function of distance exhibit different slopes when changing the oxygen fraction (Fig. 5A and B). This can be attributed to the quenching effect of Arm due to the increase of the O2 absolute number density when increasing the oxygen fraction in the gas mixture. This effect becomes stronger when increasing the O2 content in the sputtering chamber. In addition, the quenching effect is less important close to the magnetron than far away from it. This is related to the low amount of O2 molecules near the target as a consequence of the highly confined plasma, where the O2 decomposition process is more efficient. Thus, at higher working distance, the plasma becomes less dense and hence the concentration of the O2 molecules may increase. This has been demonstrated in the previous section where the emission intensity of oxygen atoms is quite high near the target and decays exponentially as a function of the distance from the target (Fig. 4B).


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Fig. 5. Evolution of the Arm absolute number density for different oxygen fractions as a function of the distance while maintaining the current discharge at 1 A at two different sputtering pressures: (A) 5 mTorr and (B) 20 mTorr. Evolution of the Arm absolute number density at 5 mTorr as a function of the distance for three different current discharges for two different oxygen fractions: (C) 0% and (D) 20%. The typical error for Arm densities is less than 10%.

The same study performed previously has been repeated in the metallic (0% of O2) and in the poisoned (20% of O2) regimes of sputtering at 5 mTorr while varying the intensity of the discharge current from 0.5 A to 1.5 A (Fig. 5C and D). In both regimes, an exponential decay of the Arm absolute number density is observed as a function of distance at three discharge currents (i.e., 0.5 A, 1 A, and 1.5 A). When examining the data collected in the metallic (Fig. 5C) and the poisoned mode (Fig. 5D), one can remark that for a fixed distance the absolute number density decreases slightly as the intensity of the discharge current is increased. This decrease is, however, more pronounced in the poisoned mode (Fig. 5D) than in the metallic mode (Fig. 5C). This indicates that, in the poisoned mode, the spatial distribution of Arm within the plasma is more sensitive to the variations of the power dissipated in the discharge, as compared to the metallic mode. To explain this behavior, it is important to remind here the various reactions that Arm may undergo in the plasma. Indeed, Arm are created through electron collisions (Ar + e → Arm + e) but are also destroyed through electron collisions and either ions are generated (Arm + e → Ar+ + 2e) and/or excited states are produced (Arm + e → Ar * + e). Therefore, there is a maximum in the curve that represents the evolution of the Arm absolute number density as a function of the electron density. On the other hand, as the discharge current is increased, the ion density and the electron density are both increased. Hence, there will be an optimum in the graph that represents the evolution of the Arm absolute number density as a function of the discharge current. This is the reason why the data points representing the Arm absolute number density as obtained for a 1.5 A discharge current are located under the data points related to the 1.0 and 0.5 A working conditions, respectively (Fig. 5C and D), in both regimes (i.e., metallic and poisoned modes). Furthermore, as explained previously, Arm may also be destroyed through collisions with molecular oxygen. As the oxygen flow rate is increased, the oxygen partial pressure is increased and the argon density decreases as the collision frequency between Arm and O2 molecules is increased. This is the reason why the absolute number densities of Arm are systematically lower in Fig. 5D (20% O2) as compared to the data points presented in Fig. 5C (0% O2). Also, The destruction of argon metastable states by collisions with oxygen molecules is more pronounced as the absorption measurement is performed further away from the magnetron plasma (see Fig. 5D) because there are fewer electrons and the oxygen molecules are less dissociated at this position. In conclusion, it is expected that argon metastable atoms will be less numerous if the discharge current is large (e.g., 1.5 A) and if the density of O2 molecules is high. This is the case, i.e., for high oxygen flows

situation and far away from the target surface. This is indeed what can be understood by comparing the data points presented in Fig. 5C and D. 5. Correlation between the obtained results After characterizing the Ce-Ar-O2 sputtering discharge for different experimental conditions, in this section, we attempt to correlate these data to the deposition rate of cerium and cerium oxide thin films (Fig. 6). The deposition rate for a given O2 fraction can be defined as the ratio between the film thickness, measured by profilometry, and the deposition time selected to grow the film. For the film growth, the deposition pressure and the target-to-substrate distance were fixed at 20 mTorr and 75 mm, respectively. The deposition rate was found to increase slightly (from 110 to 140 nm/min) when increasing the oxygen fraction in the mixture from 0% to 3%, which can be attributed to the getting effect of the deposited metallic-like material [44]. When reaching the poisoned mode (at about 12% of oxygen), the deposition rate sharply drops to about 30 nm/min and stays almost constant during the further increase of the oxygen content afterwards. Such a decrease in the deposition rate was accompanied by a corresponding voltage increase on the sputtered target. The observed effects can be correlated to the sputtered Ce atoms and the Arm densities. Under our sputtering conditions, the Ce atom absolute number density in the metallic mode is around 1.5 × 108 cm−3 [37] and the one of the Arm is about 3 × 108 cm−3, whereas lower values are recorded in the poisoned mode

Fig. 6. Evolution of the discharge voltage (● and ○), the deposition rate (□), and the Arm (△) and CeGS (✰) absolute number densities as a function of the O2 fraction in gas mixture. The deposition pressure was 20 mTorr, and the current intensity was maintained at 1 A and the target-to-substrate distance was 75 mm.

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(Ce: 1 × 107 cm−3 and Ar: 5 × 107 cm−3). The Arm absolute number density decreases when switching from the metallic to the poisoned sputtering mode results in the decrease of the sputtered CeGS, which leads to a drop in the deposition rate. Although the absolute number density of Arm is located within the range reported in literature for the sputtering of titanium in similar conditions [24], the absolute number density of the CeGS is very low compared to the one of TiGS. In addition, the recorded deposition rates of Ce in the metallic mode are very high with respect to the values of the obtained CeGS absolute number density. The following reasons may explain the contradiction in the obtained results: i- The sputtering yields of Ti and Ce are different. This possibility, however, can barely be accepted since for the used Ar ion energies used for sputtering, Ce exhibits a sputtering yield roughly two times higher than the one of Ti (see [30] and therein). ii- The porosity of the deposited films can be also the reason for obtaining a high deposition rate for such a low absolute number density of CeGS. It is known that the porosity of the sputtered films depends on the energy of the impinged particles [45]. Thus, for the same amount of sputtered Ce per unit of time, one can obtain either a thin and very dense or a thick and very porous film of Ce for impinged particles with high or low energies, respectively. In this work, the deposition rate was evaluated from the thickness of the films measured by a profilometer. Since this approach does not take into account the density of the obtained films, a possible over-estimation of the deposition rate is quite probable. To verify this issue, an evaluation of the density of the films, e.g., by Rutherford backscattering spectroscopy, would be necessary. iii- Another possible explanation can be the formation of Ce clusters (macroparticles), which contribute to the growth of the thin film, remaining undetectable by ROAS (due to the nature of the resonant absorption). Such an effect has been reported in case of titanium sputtered in pure argon plasma [46]. The extensive contribution of the Ce clusters in the formation of the resulting film may explain the high deposition rate obtained in the metallic mode, while rather low densities of both Arm and CeGS are deduced by the ROAS measurements. However, the origin of such “cluster-assisted film formation” in a magnetron sputtering discharge is still rather unclear. One of the arguments reported in a previous work can be the presence of a partial evaporation of Ce target during sputtering in the metallic mode as a result of its over-heating and surpassing the Ce melting point (about 1070 K) [37]. This effect becomes less pronounced when switching to the poisoned mode since cerium oxide exhibits a considerably higher melting point (2673 K) compared to the pure Cerium. Cormier et al. [47] reported target surface temperatures ranging from ~500 K to ~850 K for the sputtering of titanium in Ar using a balanced magnetron for DC power densities increasing from 1.3 to 10.7 W⋅cm−2.

6. Conclusions In summary, a reactive Ar-O2-Ce magnetron sputtering discharge is characterized using optical emission and resonant absorption spectroscopy. We demonstrated that the intensities of Ar, O, and Ce emission lines are the strong fuctions of the discharge parameters. Namely, they decrease as the distance from the magnetron target increases. A similar behavior is found for the absolute number density of Ar metastable atoms present in the discharge. The spatial evolution of the species absolute number density is presented at different oxygen fractions in the gas mixture and is found to be O2 admixture dependent. Namely, the collisional quenching of the Ar metastables by O2 molecules at different

oxygen contents is found to increase as a function of the oxygen admixture. When switching from the metallic to the poisoned mode, the deposition rate of the films was found to decline. This behavior is correlated with the drop in the absolute densities of the Ar metastables as well as of the ground state Ce atoms. For the measured densities of Arm, the densities of CeGS are found to be rather low, pointing out on a possible formation of Ce clusters in the magnetron discharge during the Ce target sputtering process. Acknowledgments This work is supported by the Belgian Government through the «Pôle d'Attraction Interuniversitaire» (PAI, P7/34, “Plasma-Surface Interaction,” Ψ) as well as by the DG06 through the Opti2mat program. N. Britun is a postdoc researcher, and S. Konstantinidis is a research associate of the FNRS (Fonds National de la Recherche Scientifique), Belgium. References [1] M. Buffière, S. Harel, C. Guillot-Deudon, L. Arzel, N. Barreau, J. 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