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Concomitant effects of the substrate temperature and the plasmachemistry on the chemical properties of propanethiol plasma polymerprepared by ICP discharges☆
Damien Thiry a,⁎, Francisco J. Aparicio a, Nikolay Britun a, Rony Snyders a,b
a Chimie des Interactions Plasma-Surface (ChIPS), CIRMAP, Université de Mons, 23 Place du Parc, B-7000 Mons, Belgiumb Materia Nova Research Center, Parc Initialis, B-7000 Mons, Belgium
☆ This manuscript is based on work presented at the SAnnual Technical Conference in Providence, Rhode Island⁎ Corresponding author. Tel.: +32 65 55 49 45; fax: +
In this work, the plasma polymerization of propanethiol was investigated aiming to give new insights into thegrowth mechanism of such material. The plasma polymers films (PPF) were synthesized using the two plasmamode production of ICP discharges, namely the capacitive (E) and the inductive (H) mode. Using the E mode,the atomic sulfur content in the PPF (at.%S) was found to be higher (~40%) than in the precursor (25%) whichwas explained by the trapping of molecules presenting a high S/C ratio in the PPF network. This explanation isvalidated by aging experiments revealing a strong decrease of at.%S likely due to the release of the trapped species.In contrast, using the Hmode, at.%S is significantly lower (17%–25%) and stable under aging. This different behaviorregarding chemical properties of thePPF as a function of the dischargemodeused for their synthesiswasunderstoodby considering the concomitant effect of the substrate temperature (Ts) and the plasma chemistry for both ICPmodes. It is shown that in the H mode, Ts ranges from 60 to 90 °C compared to 30–35 °C in E mode. This inducesa decrease of the residence time of the sulfur-based molecules at the growing film interface and, ultimately a de-crease of at.%S. On the other hand, experiments carried out for similar Ts but usingbothmodes reveal the importanceof the plasma chemistry on the chemical composition of the films. Indeed, in these conditions, at.%S was correlatedto the amount of H2S in the discharge which is therefore identified as the trapped sulfur-basedmolecules. Our dataallow highlighting the concomitant effect of both substrate temperature and plasma chemistry in order to under-stand the evolution of the chemical properties of propanethiol PPF prepared in the E andHmode of an ICP discharge.
During last decades, deposition of organic films by plasma polymeri-zation has become a well-established technique for the synthesis offunctional thin films with application in different fields such as photonics[1], biomaterials [2] and protective coatings [3]. In a few words, thissynthetic procedure is based on the plasma activation of organic vaporinducing the dissociation of an organic precursor and the reorganizationof the resulting charged and neutral species on the surfaces exposedto the discharge. This process involves both gas phase and surfacereactions [4]. The activation in the gas phase occurs mainly throughcollisions between the organic precursor and energetic electronsgenerated in the discharge. In cold plasmas, the mean electron energyis typically in the same range than the chemical bond energies of organicmolecules (2–4 eV) and much lower than the ionization potential(N9 eV). Therefore, it is assumed that the neutral moieties (particularlyfree radicals) are the main species contributing to the growth of the
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layer [5]. On the other hand, at the plasma/growing-film interface thedrop of potential across the plasma sheath accelerates the generatedpositive ions which impinge upon the film with a kinetic energy that,in case of a floating substrate, ranges from 10 to 15 eV [6]. During theprocess, the film is also continuously irradiated by UV photon comingfrom the radiative deexcitation of the excited molecules. The energybrought to the growing film by both phenomena can induce severalkinds of processes such as chemical bond breaking, ion-induced etchingand eventually film heating [4,7–9]. Thus, both gas phase and surfacereactions determine the functionality and the cross-linking degree ofthe deposited layers.
We have recently investigated the plasma polymer growth frompropanethiol discharge [10]. It was demonstrated that the atomic sulfurcontent in the layers can be adjusted in a large range (from 18% to 40%).Such a high sulfur concentration opens up, for example, the possibilityfor the development of high refractive index plasma polymer thinfilms of interest for optical applications [11–13]. In addition, it wasshown that the stability of the coatings in terms of sulfur content wasalso strongly influenced by the plasma parameters. For the synthesisof the films, an RF inductively coupled plasma (ICP) source, using an in-ternal coil as electrode, was employed. An important feature of this kindof plasma reactor is that the applied power can be capacitive or
inductively coupled into the discharge. Thus the process can operate intwo modes with outstanding differences in terms of electrical and plas-ma properties [14–17]. At low power, the discharge is characterized byrather low electron density and faint light emission. In this regime,known as capacitive (E) mode, the discharge is maintained by theelectrostatic field developed between the coil extremities [17]. Increasingthe power up to a threshold value results in a shift to the so-called induc-tive (H) mode. In this regime, the plasma is mainly sustained by theelectric field induced by the oscillating magnetic field associated to theRF current flowing through the coil [18]. In comparison to the E mode,such inductively excited plasma has about one order ofmagnitude higherplasma density and hence much stronger light emission. The E–H transi-tion phenomena and the different characteristics of the E and H modeshave been subject of both theoretical [19] and experimental [16,20–22]studies on Ar, N2, Ar/N2, O2, Ar/O2, H2 and Ar/H2 plasmas mainly usedfor material etching in semiconductor processing or in sterilizationapplications. However, up to now, it has received rather little attentionin the plasma polymer domain.
In our previouswork, itwas shown that the E–H transition strongly af-fects both the functionality of the propanethiol plasma polymer films(PPPF) in terms of sulfur content and chemical stability of the films [10].However, some aspects about the impact of the E–H transition on (i)the chemical composition of the plasma, (ii) the energetic conditions atthe surface (particularly on the substrate temperature) and theirinfluence on the films properties remain unclear. The plasma chemistryand the nature of the produced neutral species have an important effecton the chemical composition of the films, as it has been extensivelyreported [23,24]. Although often omitted for the interpretation of thedata in the plasma polymerization field, the substrate temperature alsoaffects several surface processes such as adsorption, desorption,chemical sputtering,… and consequently the plasma polymer film(PPF) properties [25–29]. Moreover, this also represents an importantparameter reflecting the energy flux to the substrate through ionicbombardment, photon irradiation and surface reactions (exothermicphysi- and chemi-sorption processes) [25,26]. Therefore, the changein the chemical composition of the layers might also be related tosubstrate temperature and its correlation with the physicochemicalprocesses occurring at the growing-film/plasma interface during thedeposition.
The main aim of this work is to provide additional insights on thelayer growth mechanism of PPPF deposited by ICP discharges. In thiscontext, both plasma chemistry and thermal conditions of the substratewere correlated to the chemical composition and stability of the samplesdeposited by using capacitively and inductively coupled plasmas.
2. Materials and methods
Propanethiol (Sigma Aldrich, 99% Purity) was plasma polymerized on1 × 1 cm2 Si wafers (110). Before their introduction into the chamber,the substrates were ultrasonically washed in hexane during 15 min andrinsed with methanol.
The depositions were carried out in a metallic vacuum chamber:65 cm in length and 35 cm in diameter schematically represented at 3Din the Fig. 1. The reactorwas pumpedby a combinationof turbomolecularand primary pumps allowing to reach a residual pressure lower than2 × 10−6 Torr. More details about the description of the depositionchamber can be found elsewhere [30]. During the synthesis, the workingpressure fixed at 40 mTorr, was controlled by a throttle valve connectedto a capacitive gauge. The plasmawas generated using a one-turn induc-tive water-cooled copper coil (10 cm in diameter) located inside thechamber at the distance 10 cm in front of the substrate. The coil was con-nected to an Advanced Energy RF (13.56 MHz) power supply via amatching network. The precursor flow rate was fixed for all the experi-ments at 10 sccm. During the film depositions, the substrate was at thefloating potential, and the applied power (PRF) varied from 20 W to100 W.
The emission spectra were acquired from 230 to 805 nm using anAndor Shamrock-750 spectrometer equipped with an iStar DH740ICCD camera. The E–H transition was detected by measuring the meanintensity of the acquired emission spectra [10,30]. Assuming a coronamodel in the discharge, the mean intensity mainly reflects the changesin the electron excitation level in plasma, which could result in anabrupt variation of the plasma emission at the transition point [31].
XPS measurements were performed using a PHI 5000 VersaProbeapparatus connected under vacuum to the deposition chamber. Amonochromatized Al Kα line (1486.6 eV) was used as a photon source.The atomic relative concentration of each element was calculated frompeak areas taking into account the respective photoionisation cross-sections, the electron inelastic mean free path, and the transmissionfunction of the spectrometer [32].
The substrate temperature (Ts) measurements were performedusing a thermocouple in contact with the substrate holder surfacethrough a screw.
The plasma composition was investigated by a quadrupole HAL EQP1000mass spectrometer supplied byHiden Analytical. The spectrometerwas connected to the chamber by a 100 μm extraction orifice. To allowneutrals species detection, they were ionized by electron impact (EI)with an electron kinetic energy fixed at 20 eV in order to avoid excessivefragmentation of the precursor in the ionization chamber. For clarity, dueto the high production of atomic and molecular hydrogen in thedischarge as usually encountered in plasma polymerization, the spectraare presented from m/z = 10 to 100 [33]. In order to take into accountthe fragmentation of the precursor in the spectrometer itself, each signalrecorded in the plasma was treated following Eq. (1) [24]:
Ic mð Þ ¼ Im Plasma ONð Þ‐Im Plasma OFFð Þ: IProp: Plasma ONð ÞIProp: Plasma OFFð Þ 1
where Ic (m) is the calculated peak intensity for m/z = m, Im (PlasmaON) and Im (Plasma OFF) represent the experimental peak intensity formass m when the plasma is switched ON and OFF, respectively. IProp. isthe intensity corresponding to the precursor signal.
This data treatment allowed to subtract the contribution of thefragmentation in the spectrometer itself and hence to differentiate thespecies originated from the plasma and those produced in the instrument[24]. Atfirst sight, IProp. might be identified to peak intensity atm/z = 76.However, the possible production of CS2molecules having the samemassthan the precursor may lead to a wrong interpretation of the data.Nevertheless, due to the relative abundance of the isotope 34S (~4%),both molecules can be discriminated by examining their isotropicdistribution [34]. A previous study revealed that the signal at m/z =76in the E mode is exclusively due to propanethiol and thereforeIProp. = I76. Whereas for the H plasmas, the isotopic studies showed
Fig. 3. Evolution of the S/C ratio measured by XPS for (i) as-deposited (in situ) and (ii)aged (20 h in air) PPF. The thickness of the coatings ranges from 200 to 250 nm in the Emode while for the H mode from 270 to 370 nm.
4 D. Thiry et al. / Surface & Coatings Technology 241 (2014) 2–7
that the m/z = 76 peak is attributed to CS2 and the amount ofinteger precursor reaching the spectrometer is negligible [30].Consequently, in this case, IProp. = 0 in Eq. (1).
3. Results and discussion
As recently shown, the propanethiol discharge generated in our ICPreactor exhibits an abrupt E–H transition mode [10]. This is apparent asa sharp increase in the mean emission intensity (Fig. 2a) of thedischarge due to the plasma density increase occurring at the transition.This enables to establish awell-defined transition power (Ptr.) forwhichthe discharge shifts from the E to the H mode.
On the other hand, as a general trend, the atomic sulfur content(at.%S) decreases as a function of PRF (Fig. 2b). For PRF = Ptr, a largedecrease in the at.%S is observed and reveals the importance of thedischarge mode on the chemical composition of the layers. An impor-tant observation concerns the E-mode prepared films for which theat.%S is much higher than in precursor itself (40% vs 25%). This peculiarobservation could be explained by the presence of unbounded mole-cules physically trapped into the plasma polymer network during thelayer growth as it was also claimed in the literature for other plasmapolymer families [35–38]. In our case, we could expect that thesemolecules present a high S/C ratio explaining the extra sulfur content.
In order to validate these considerations, the aging properties of thelayers are evaluated by comparing the S/C ratio measured in situ for theas-deposited PPF and after 20 h aging in the air (Fig. 3). The data indi-cate, for the lowest PRF used (PRF = 20 W), a significant decrease inthe S/C ratio after aging suggesting the release of the sulfur-based mol-ecules. One should note that, close to the transition but still in the Emode, at.%S is stable. This could be attributed to an increase in thecross-linking density with PRF as reported for allylamine plasma poly-mers which could prevent the diffusion of the trapped sulfur-basedmolecules [32,35–37,39]. This phenomenon can be explained as follows.In addition to the increase in the electron density and its influenceon the flow of ions toward the substrate, it has also been reported thatespecially in the E mode, an increase in the electron temperature hasalso to be considered when increasing the applied power [22,40]. Thelatter directly affects the value of the floating potential and hence the
Fig. 2. Evolution of (a) mean intensity and (b) at.%S as a function of PRF for a fixed deposi-tion time of 30 min. At this condition, the thickness of the coating varies from 60 to180 nm in the E mode while for the H mode from 600 to 800 nm.
energy of the bombarding ions. For example, in their work, Haddowet al have measured an increase of the ion energy from 5 to 25 eV fora corresponding power range of 2–15 W [41]. At the same time, theintensity of UV irradiation is also proportional to PRF [32]. The combina-tion of these effects could inducemore chemical bond breaking at the in-terface and hence a higher cross-linking degree for the E sampledeposited at higher power. Such aging behaviour in the E mode hasalready been observed for other pressure conditions [30].
For the H-mode prepared PPF, at.%S is lower and stable throughaging. The stability of the PPF suggests that, in this case, (i) a higherpercentage of sulfur groups are covalently bonded to the film and/or(ii) the crosslinking degree of the PPF is higher. It should be emphasizedthat the E–H transition is generally accompanied by a drop in thefloatingpotential which in turn reduces the energy of the bombarding ions[17,22]. In the case of theHmode, this effect is probably counterbalancedby a significant increase in the ion flow and UV photon irradiation at thegrowing film interface likely contributing to ensure a high cross-linkingdegree.
In Fig. 2b and 3, it is also important to stress, the lower at.%S foundfor the PPF deposited in the H mode. This likely results from importantvariation in the thermal conditions of the substrate. This could be ratio-nalized by considering the influence of Ts on the residence time (τ) ofphysisorbedmolecules on a surface given by the Frenkel's equation [28]:
τ ¼ τ0 expEactkTs
� �2
Here τ0 describes the smallest possible residence time and correspondsto the inverse of the vibrational frequency of the surface bond which isof the order of 10−12–10−13 s [28]. Eact represents the activation energyfor the thermal desorption, k the Boltzmann constant and Ts the sub-strate temperature.
Eq. (2) reveals that the variation of Ts strongly affects τ and thereforethe trapping probability of particles. Therefore, in the context of thetrapping scenario, Ts measurements are performed as a function of PRFand the deposition time (Fig. 4). It can be clearly observed how Ts grad-ually increaseswith the deposition time. This temporal evolution is con-sistent with the literature [25,42–44]. As it was reported, a certainperiod of time is needed to reach an equilibrium at which the loss andthe gain of energy from the surface are equal [26]. This leads to a station-ary state and consequently to a constant Ts which, as clearly observed inthe Fig. 4, depends on the experimental conditions. In order to evaluatethe impact of the process parameters on the substrate thermal condi-tions, the final Ts reached after a deposition time of 30 min is depictedin Fig. 5 as a function of PRF. For a given mode, Ts increases (from30 °C to 35 °C in the E mode and from 65 °C to 90 °C in the H mode)whereas, as other experimental parameters (emission intensity, at.%S),it strongly increases at the transition power (from ~35 °C to ~65 °C).The latter effect is attributed to the variation of the energy flux towards
the growing film. Considering a PECVD process, the main factorscontributing to the substrate heating are the ionic bombardment andthe exothermic surface reactions (e.g. through radical chemisorption).Both these phenomena being directly proportional to the electron den-sity, it explains that an important load of energy occurs when transitingfrom E to H mode as depicted in Fig. 5 [25,26].
As the samples analysed in Fig. 2b correspond to a deposition time of30 min, the data in Fig. 5 also provide information about the finaltemperature of these experiments. Thus, according to the proposedtrapping hypothesis and Eq. (2), the Ts reached in the H mode shouldbe high enough to significantly reduce the residence time of the stablesulfur-based particles at the growing film interface and hencepreventing their trapping in the polymer network.
Due to the temporal increase in Ts, one can wonder if the depositiontime influences at.%S for films produced in the H mode. Hence, thetemporal variation of Ts was compared with the evolution of at.%S fordifferent deposition times considering the highest power studied inthis work, namely PRF = 100 W (Fig. 6). At low deposition time, thefilms exhibit rather high at.%S that decreases with the deposition time(i.e. as Ts increases). At temperature higher than 80 °C, the at.%S reachesa practically constant value (within the confidence intervals) of ~17%that likely corresponds to the sulfur atoms forming the plasma
Fig. 5. Evolution of Ts as a function of PRF for a fixed deposition time of 30 min.
polymeric network. As these experiments are carried out under an iden-tical set of plasma conditions, these data additionally confirm the influ-ence of the Ts on the chemical composition of the PPPF. According to ourtrapping hypothesis, the trend observed in Fig. 6 might tentatively berationalized as follows. At beginning of the deposition (i.e low Ts), phys-ical trapping of stable molecules with high sulfur concentration occurs,but as the Ts increases with the deposition time, this phenomenon islimited by shorter residence time of these sulfur-based molecules. Thisleads to a decrease in at.%S down to a stable value of ~17% that mightcorrespond to the sulfur atoms chemically bonded in the plasma poly-meric matrix.
It is also important to note that owing to the evolution of thetemperature during the deposition process, gradient layers in terms ofat.%S as a function of the thickness are produced. In addition to thevariation of at.%S, a decrease in the deposition rate with Ts has also tobe considered (data not shown) as extensively reported in plasmapolymerization [28,43,45–47].
Having a closer look to these data, one should notice that consideringTs is not enough to explain the differences in the chemical composition ofthe PPF prepared in E- and H- modes. Indeed, by comparing the PPFdeposited in both modes for similar Ts (combining the data recorded inthe Figs. 2b, 4 and 6) as depicted in the Fig. 7, we observe that at.%S islower for the H-mode deposited PPF (~30% vs ~40%). In order to explainthis behaviour, it is therefore necessary to take into account themodification of the plasma composition when transiting from the E- tothe H-mode. This has been done by RGA mass spectrometry.
Fig. 8a shows theRGAmass spectrummeasured for the propanethiolvapour (Plasma OFF). Except for the peak at m/z = 76 (correspondingto the precursor), all peaks are due to the fragmentation of the latter
Fig. 7. Correlation between at.%S and Irel.(H2S) as a function of the discharge mode forTs = 30 °C which corresponds to a deposition time of 30 min for PRF = 20 W and1 min for PRF = 100 W (see Figs. 4 and 6).
Fig. 8.Mass spectrum of the (a) propanethiol vapour (Plasma OFF). Mass spectra of (b) E(PRF = 20 W) and (c) H (PRF = 100 W) modes discharges treated according to Eq. (1).
6 D. Thiry et al. / Surface & Coatings Technology 241 (2014) 2–7
in the spectrometer ionization source and are listed in Table 1. Correctedmass spectra (according to Eq. (1), see experimental section) of plasmassustained in the E (PRF = 20 W) and H mode (PRF = 100 W) areshown in Fig. 8b and c. In both cases, a great variety of species are de-tected. Globally, carbon-based fragments ([CyHx]+), hydrogenated sul-fur ([HxS]+) and carbon/sulfur-based species ([CyHxS]+) are identified(see Table 1 for the peak labeling). The numerous peaks recorded inthe mass spectra suggest complicated fragmentation patterns whichhave been partly identified using theoretical calculations in a previouswork [30].
Considering the E mode plasma, an important observation concernsthe production of a large amount of H2S molecules (most intense peakin Fig. 8b). Stable, this molecule does not take part in the growth ofthe film but could be physisorbed at the growing film interface and itis proposed to explain the high sulfur concentration of these E depositedfilms by trapping within the nascent plasma polymeric matrix.
In comparison, in the H mode, the major change concerns the com-plete disappearance of the precursor signal correlated with an importantproduction of CS2 molecules (Fig. 8c). Again, both species appear for asimilarm/z ratio but, as alreadymentioned, using the isotopic distributionof the elements, we were able to discriminate the latter [30].
Aiming to correlate the evolution of the at.%S in the films with theplasma chemistry, a particular attention is paid to the evolution of thecorrected intensity of H2S with respect to the sum of signals associatedto carbon-based molecules (m/z = 15, 26–29, 39–43, see Table 1 forthe peak attribution) which could potentially take part in the growth
Table 1Labeling of the mass spectra peaks (Fig. 8).
m/z Ions
15–16 [CHx]+ x = 3–426–30 [C2Hx]+ x = 2–632–34 [HxS]+ x = 0–239–44 [C3Hx]+ x = 3–844–48 [CHxS]+ x = 0–456–62 [C2HxS]+ x = 0–664 [S2]+
68–76 [C3HxS]+ x = 0–8; [CS2]+
of the layer. The chosen carbon-based species correspond to radicalsor to molecules containing an unsaturation which are species suscepti-ble to be grafted at the growing film interface. The evolution of this ratio,Irel.(H2S), as a function of PRF, calculated following the Eq. (3) is shownin Fig. 9:
Irel: H2Sð Þ ¼ IC H2Sð Þ
IC m=z ¼ 15ð Þ þXm=z¼29
m=z¼26
IC m=zð Þ þXm=z¼43
m=z¼39
IC m=zð Þ3
where IC (H2S) is the corrected intensity calculated following Eq. (1) form/z = 34. Ic (m/z) is the corrected intensity calculated followingEq. (1)for the ratio m/z.
Irel. (H2S) significantly decreases for PRF = Ptr. illustrating the impactof the discharge mode on the chemical composition of the plasma(Fig. 9). From these data, it is clear that, in addition to the influence ofTs, the evolution of the plasma chemistry also contributes to reducetheproportion of trappedH2Smoieties in thenetwork. This is illustratedin Fig. 7 revealing that the lower is Irel. (H2S), the lower is the at.%S in thefilm for an equivalent Ts. The combination of our results reveals thatat.%S is controlled by both Ts and the plasma chemistry.
It should be noticed that despite the production of CS2 molecules inthe H mode plasma (which could also represent potential trappedsulfur-based species), their trapping in the network is limited. This islikely related to the chemical structure of the CS2 which are linearand do not possess a permanent dipole moment in contrast to H2S.This feature could significantly reduce the physisorption energy andconsequently the residence time of the particles at the surface [48].
4. Conclusion
In this work, the impact of the plasma mode production using ICPsdischarges on propanethiol plasma polymer films properties was investi-gated. It was shown that the discharge mode strongly influences thechemical composition of the layer, especially the sulfur content. At lowpower, in the E mode, the films exhibit high sulfur content of ~40%which strongly decreases after aging in the air. The latter behaviour isassociated with the release of molecules presenting a high S/C ratiosuch as H2S trapped during the PPF growth and identified in importantquantity in the plasma by mass spectrometry. In H mode, the sulfurcontent is significantly lower (ranging 17% to 25%) and stable likelyresulting from a lower proportion of trapped species in the PPF networkand/or a higher cross-linking degree.
The significant difference of composition and of ageing behaviourdepending on the discharge mode is correlated with important variationof (i) the substrate temperature and (ii) the plasma composition. Indeed,transiting from E to H mode results in an increase of substrate tempera-ture likely limiting the trapping phenomenon by reducing the residence
Fig. 9. Evolution of Irel. (H2S) as a function of PRF.
time of the adsorbed H2S molecules. In addition, by comparing samplesprepared at equivalent surface temperature but using the E and Hmodes, we have shown that the plasma composition, especially the H2Scontent, is also a key parameter.
It is therefore clear that, both plasma chemistry and substratetemperature which are strongly influenced by the discharge modeplay a key role in determining the chemical composition and stabilityof the propanethiol plasma polymers.
Acknowledgments
The authors thank F.R.I.A grant of the Communauté de Française deBelgique and the Belgian Government through the “Pôle d'AttractionInteruniversitaire” (PAI, P7/34, “Plasma-Surface Interaction”, Ψ) forfinancial support. N.B is post-doc researcher of the FNRS Belgium.
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