palmucci 2013 journal of physics d applied physics

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Mass spectrometry diagnostics of short-pulsed HiPIMS discharges

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2013 J. Phys. D: Appl. Phys. 46 215201

(http://iopscience.iop.org/0022-3727/46/21/215201)

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IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 46 (2013) 215201 (11pp) doi:10.1088/0022-3727/46/21/215201

Mass spectrometry diagnostics ofshort-pulsed HiPIMS dischargesMaria Palmucci1 Nikolay Britun1 Tiago Silva1 Rony Snyders1,2and Stephanos Konstantinidis1

1 Chimie des Interactions Plasma-Surface, CIRMAP, UMONS Research Institute for Materials Scienceand Engineering, Universite de Mons, Place du Parc 23, B-7000 Mons, Belgium2 Materia Nova Research Center, Parc Initialis, Avenue N. Copernic 1, B-7000 Mons, Belgium

E-mail: [email protected]

Received 7 December 2012, in final form 2 April 2013Published 9 May 2013Online at stacks.iop.org/JPhysD/46/215201

AbstractTime-resolved mass spectrometry (MS) study of a high-power impulse magnetron sputteringdischarge (HiPIMS) operating in the short-pulse regime (5 s) at 1 kHz of the repetitionfrequency is undertaken. Several time-resolved effects related to both Ti+ and Ar+ ion energydistribution functions (IEDF) are found. In particular, the dynamics of both the low- (05 eV)and high- (530 eV) energy regions presented in Ti+ IEDFs is clarified. According to ourresults the sputtered and ionized Ti arrive at the virtual substrate position in the form of twowaves, with the first one representing the high-energy Ti+, and the second one responsible forthe low-energy Ti+. An essential decrease in the population of the energetic Ti+ group isobserved at the moment of the arrival of the low-energy group, which is explained by thecharge exchange processes, as well as by the refilling process afterwards. The role of Armetastables presumably generated at the end of the plasma pulse for further Ti ionization isstressed as well. The time-averaged IEDFs for Ti+ and Ar+ are additionally analysed. Theeffective ion temperatures are calculated for these species for the above-mentioned energyranges. A considerable increase in the effective ion temperature for the high-energy Ti+ isfound, which is directly related to the elevation of the high-energy tail in the time-averagedIEDFs with increasing discharge voltage. Possible mechanisms for such an elevation arediscussed.

(Some figures may appear in colour only in the online journal)

1. Introduction

High-power impulse magnetron sputtering (HiPIMS) is aphysical vapour deposition (PVD) process which has attractedconsiderable interest over the last few decades since itsinvention by Mozgrins group [1], and its further improvementsby Kouznetsov et al in 1999 [24]. HiPIMS plasmas arecharacterized by a high electron densityne (up to 1019 m3 [5]),which in turn leads to a higher ionization degree of the sputteredparticles (typically above 50%) compared with that observedin conventional direct current magnetron sputtering (DCMS)discharges (about 1% [6]) where ne is typically up to 1016 m3[5]. Different discharge parameters such as pulse width, peakcurrent, target voltage to name but a few allow controlling theion-to-neutral ratio, the metal-to-argon ion ratio, as wellas the energy of the ions bombarding a growing film. Theseparameters are of key importance since they strongly influence

the physico-chemical characteristics of the deposited materialssuch as their density, crystallinity, microstructure, etc [7, 8].

In HiPIMS, which is an example of a fast time-changingplasma discharge, the use of time-resolved diagnostics iscrucial in order to gain information about dynamics of thedischarge. Following this purpose, several groups haveemployed time-resolved Langmuir probes for characterizationof HiPIMS discharges. The corresponding results generallydemonstrate the existence of several groups of electrons withdifferent kinetic energies, i.e. cold and hot electrons whichare generated at different parts of the HiPIMS period [9, 10].In fact, at the beginning of the on-time, the ions impingingthe cathode result in secondary electron emission from itssurface. These hot electrons gain their kinetic energy upto the level equivalent to that of the cathode potential andgive rise to a rather high electron temperature (Te) duringthe discharge ignition. As the discharge current increases,

0022-3727/13/215201+11$33.00 1 2013 IOP Publishing Ltd Printed in the UK & the USA

J. Phys. D: Appl. Phys. 46 (2013) 215201 M Palmucci et al

Te decreases and exhibits a bimodal distribution suggestingthe dominance of argon species at the early stages of thedischarge, which is followed by the dominance of metallicspecies due to gas rarefaction in the cathode vicinity [10, 11].Similar observations have also been obtained by means ofoptical emission spectroscopy (OES) when a decrease in thesignal related to argon species correlates with an increase inthat related to the metallic species [6, 12]. It is to be noted thatsuch a variation of Te throughout the period induces a variationof the plasma potential (Vp) from a few to about ten volts.

Among the other plasma diagnostics techniques, massspectrometry (MS) is proven to be a powerful tool to monitorthe ion dynamics in HiPIMS discharges by examining theion energy distribution function (IEDF). However, if thebasics of the IEDF for DCMS and bipolar-pulsed magnetronsputtering (BPMS) discharges are rather well known [1316],our knowledge is sparse in the domain of HiPIMS discharges,which remain unclear in many aspects [3, 17]. For example, atypical IEDF obtained in HiPIMS energy-wise reveals severalgroups of particles corresponding to different ionized specieswith low-, mid- and high-energies, which in addition areproduced at different stages of the discharge period [5]. Itis accepted that the low-energy peaks corresponding to argonand metallic ions, which generally dominate intensity-wise,are attributed to the thermalized ions reaching the groundedmass spectrometers orifice with kinetic energies close to theplasma potential [3, 1720]. The peak corresponding to thehigh-energy sputtered particles is generally attributed to (i)back-reflected ions from the target surface and/or (ii) metallicatoms ejected from the target with the so-called SigmundThompson (ST) distribution [21], and further ionized (e.g. byelectron impact) in the dense plasma region [20, 22].

Several hypotheses have been presented in the literatureto explain the origin of the peak of the IEDF appearing atintermediate energies (around 38 eV) [20, 23]. In fact, thecorresponding ion energy group has not always been detectedfor both argon and metallic ions. According to Hecimovicet al [23], the second peak of the Cr+ IEDF (i) might beattributed to Cr2+ ions which undergo a charge exchange withargon atoms, or (ii) might be the result of a wave of sputteredparticles due to the sharp drop in both Vp and Te observedduring the discharge ignition. In the other works, the mid-energy peak of Ar+ IEDF has been attributed to the partiallythermalized high-energy particles, as well as to the presenceof gas rarefaction [17, 24]. On the other hand, Greczynski andHultman [20] suggested that the energy of the mid-energy ionsin the Ar+ and Ti+ IEDFs (which is roughly equal to 34 eV)is close to that of the main ion peak (0.51.3 eV). This peakin turn might be assigned to the thermalized ions acceleratedin the plasma-spectrometer sheath during the variation of Vpthroughout the period, as discussed in [20]. Such a mid-energyion peak has also been reported before where its existence forC+ IEDFs has been explained by the positive values of Vp,which increase additionally during the on-time [25].

It is especially important to note that most of the worksdevoted to the ion dynamics in HiPIMS discharges deal withthe pulse duration ( ) ranged from 70 to 200 s or higher. Insuch long-pulse discharges, gas rarefaction, which favours

Figure 1. Schematic diagram of the experimental setup.

self-sputtering, has been pointed out to be partially responsiblefor generally low deposition rates in HiPIMS compared withDCMS [26]. It has also been reported that some of HiPIMSpower supplies are not capable of maintaining a constant targetvoltage during the entire pulse resulting sometimes in twodischarge regimes (dc-like and HiPIMS-like), and furthercomplicating the understanding of HiPIMS discharges [20].

Summarizing the above-mentioned argumentation, inorder to skilfully monitor the ignition of a HiPIMS discharge,this work deals with the short-pulse ( = 5 s) HiPIMS case,when the target current does not reach a plateau, and the plasmais still in a current rise phase, as described in [27]. Havingthe goal to enhance the level of understanding of the plasmadynamics in HiPIMS discharges, both energy- and time-resolved MS measurements of an ArTi HiPIMS discharge areconsidered and discussed in detail in the following sections.

2. Experimental

All the experiments were carried out in a cylindrical stainlesssteel chamber, 450 mm long, with 250 mm diameter, pumpedto a residual pressure of about 106 Torr by a turbomolecularpump combined with a membrane pump. The pumpingspeed was adjusted through a closed-loop throttle valve tokeep the Ar pressure constant (5 mTorr or 0.67 Pa). Argon(99.999% purity) was introduced into the chamber at a constantflow rate of 40 sccm by means of a mass flow controller(Brooks Instrument 5850 TR). Figure 1 represents a diagramof the experimental setup where the position of the massspectrometer is shown relatively to the magnetron cathode(target). The magnetron system itself consists of a planarbalanced magnetron source with a circular titanium target (Ti,99.99% purity), 100 mm in diameter. The pulsed discharges inthis study were sustained using a high-power pulse generatordescribed elsewhere [28]. The used power supply ensured ashort rise time of the discharge current, as well as generally

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Figure 2. Typical magnetron voltage and current waveforms in aHiPIMS discharge operated at 5 mTorr of Ar pressure. The pulseduration is 5 s, the repetition frequency is 1 kHz.

Table 1. ToF calculated according to the m/z ratio and the kineticenergy of ions.

Kinetic energyIon m/z range (eV) ToF (s) ToF average (s)Ar+ 40 125 84.4384.10 84.3Ti+ 48 130 92.8392.03 92.4

stable and reproducible performance of the HiPIMS discharge.The power supply without a pre-ionization option is utilizedin this work. The pulse frequency () was varied in the range0.510 kHz in order to keep the average power (P ) constantat 300 W, which is defined according to the following relation:

P = 1

0

I (t)U (t) dt (1)

where I (t) and U(t) are the discharge current and targetvoltage, respectively. I (t) was measured by a high-currentprobe (Tektronix AM503B) and U(t) by a high-voltage probe(Tektronix P5100). The typical I (t) and U(t) waveformsrecorded by an oscilloscope (Tektronix TDS2024B) arepresented in figure 2. In the experimental results shown in thefollowing sections the target voltage during the pulse was keptat the level of about 1.2 kV, unless mentioned otherwise. Basedon the measured waveforms, U(t) remains nearly constantthroughout the entire pulse duration.

MS measurements were performed with an energy-resolving mass spectrometer HAL 7 EQP 1000 (HidenAnalytical, UK). The grounded orifice of the instrument waspositioned at 80 mm in front of the target surface, i.e. in thevirtual substrate position (see figure 1). Taking into account theorifice geometry, the solid angle was estimated to be equal toaround 102 srad. For time-resolved measurements, the massspectrometer circuit was triggered by a transistor-transistorlogic (TTL) pulse sequence generated at the output of theutilized power supply. The data were recorded during theentire period which implies that the measurement time wasmuch longer in the time-resolved mode compared with thetime-averaged one. A 2 s gate width was used during thepulse. The gate width was set at 20 s, 120 s after the pulse

Figure 3. Ti+ (a) and Ar+ (b) IEDFs measured as a function of timein the ArTi HiPIMS discharge during its entire period.

and at 50 s after 300 s. The dwell time was set to collectall the data during 1000 consecutive pulses. In order to beindependent from varying the pathway of ions in the massspectrometer, their time-of-flight (ToF) from the orifice to thedetector was additionally calculated considering the equipmentparameters according to [3]. The ToF values for each sort ofions are given in table 1 for lowest and highest values of thecorresponding kinetic energy range. As the variations in ToFdo not exceed 1%, all the data for a given sort of ion werecorrected by its average ToF value.

3. Results and discussion

3.1. Time-resolved mass spectrometry results

A general image of the short-pulse HiPIMS dischargewhich is a focus of this study can be obtained from the 3Drepresentation of the time-resolved IEDFs shown in figure 3.As one can observe, the studied ions are represented by twogroup of particles, one at energies of about 1.5 eV (referredto as Ar+low, Ti

+low below), whereas the second representing

the range of about 1030 eV (Ti+high) (figure 3(a)). The high-energy group of Ti+ appears abruptly in the MS spectra aftera definite time delay (t) and quickly disappears afterwards.At the same time, a similar high-energy group of Ar+ is muchless numerous compared with the one of Ti+, as presentedin figure 3(b). The presence of high-energy particles maypoint out the influence of the energetic ionized particles (Ti+and Ar+), which might be accelerated and elastically reflected

3


Figure 4. Energy mass spectra of Ti+ measured in the HiPIMSdischarge at different t during its period.

from the target surface [29], differently for Ar+ and Ti+. Moreremarks on this effect are given below.

More particularities of the described time-resolvedevolution become available after the comparison of theTi+ IEDFs taken at different t , as shown in figure 4,which confirms the previous observations. As we can seeadditionally, the position of the second broad maximum maydrift by several eV sometimes demonstrating the splitting ofthe maximum (referred to as peak 2 and peak 3 in this work).

A detailed analysis of the time evolution of both peakpositions, as well as the particles density associated with them,etc, is given below. The corresponding data are presented infigure 5. The maxima found experimentally were fitted by theGaussian curves and further analysed in a time-resolved way.

Positions of the energy maxima. As shown in figure 5(a),the energy corresponding to the low-energy group of particlesevolves similarly in time for both Ar+ and Ti+. The visiblediscrepancies at the beginning of the plasma period arerelated to the uncertainties of the peaks fitting at low timedelays. The evolution of the Ar+ low-energy peak positionis often associated with the plasma potential (Vp). It isobserved that the position of the intensity maximum of Ar+lowchanges throughout the period, which is confirmed by thetime-resolved Langmuir probe measurements [10, 11]. Theseresults, however, have to be interpreted properly taking intoaccount the particularities of each plasma diagnostics tool. Infact, Langmuir probe measurements give the instantaneousVp values which match the pulse shape, whereas in MS, the Vpvalues are shifted and somewhat extended in time out of theplasma pulse. The discrepancy between both techniques mayalso come from the difference between the electrons and ionsmobility [30].

The data in figure 5(a) corresponding to the 1224 eVenergy range represent the second and third (when visible)energy maxima found in our measurements. These maximaappear in figure 3(a) at t 27 s, in the form of a singlepeak, whereas the third maximum becomes distinguishableafter t 50 s. The positions of these energy maxima havea trend to be gradually separated as a function of time, having

Figure 5. Time evolution of the position (a), value (b) and the areaunder the curve (c) corresponding to three energy maxima visible infigure 4, representing low- and high-energy Ti+. Two waves of thesputtered Ti+ particles coming to the MS detector after a certaindelay time are visible. Ar+ time evolution is given for comparison.

final energies of about 12 eV and 23 eV at the end. However,due to the large width of these peaks, as well as their proximity,the described maxima are considered together in the frameworkof this study, especially taking into account their similar timeevolution (shown in figures 5(b) and (c)).

Values of the energy maxima. The time evolution of the peakmaxima found based on the corresponding fit is representedin figure 5(b). This quantity, however, does not representthe number of particles that can be associated with the foundenergy peaks and is given for an illustrative purpose. The areasunder the curves corresponding to the particle population fora chosen energy range are more suitable parameters for thedischarge characterization, as described below.

Populations of different energy groups of species. The totalamount of species (in the arbitrary units, though) presentedin the HiPIMS discharge at a certain moment of time andcorresponding to a certain energy range can be representedby the area under the curve in a measured IEDF. The timeevolution of this parameter for the species considered in thisstudy is given in figure 5(c). It should be noted that sometime corrections should be made in order to understand the

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real timing based on this figure, namely the position of thedetector (80 mm from the target) should be taken into account,as well as the fact that the increase in the electron densityand the density of the sputtered species normally happens atthe end of the on- or at the right beginning of the off-time inHiPIMS [9, 23]. Several important observations can be madebased on the time-resolved results presented in figure 5(c).(a) There are two (high- and low-) energy groups of Ti+

participating in the resulting time-resolved shape of theionized sputtered species coming to the detector.

(b) These two groups have different arrival times, as well asdifferent growth times at the moment of their detection,representing two waves of Ti+, as indicated in figure 5(c).

(c) The arrival time for the first Ti+ wave is about t =t1 27 s, whereas for the second one it is aboutt = t2 38 s.

(d) The first Ti+ wave increases abruptly at the beginningshowing a total growth time of about t2t1 10 s,afterwards it starts to decay. The growth time for thesecond wave is equal to t3t2 32 s.

(e) The decay for the high-energy group of Ti+ (high-energygroup) is nearly exponential. Its overall measurabledensity drop is more than three orders of magnitude.

(f) At the same time, the low-energy group of Ti+ reveals yetanother broad maximum at t 400500 s. Finally, itspopulation returns back to the background value detectedat the on-time beginning (106 counts in figure 5(c)).

(g) The argon ions are detected much earlier (at tAr 12 s)revealing much longer growth time (about 80100 s).

(h) The total growth of the Ar+ population accounts for aboutthree orders of magnitude.

(i) The background values (e.g. at t = t0) of the Ar+density are about one order of magnitude lower than thatdetected for Ti+.

3.2. Analysis of the time-resolved resultsIn this section, the time-resolved data shown above areanalysed. This is first of all related to the time behaviour ofthe Ti and Ar ions during the HiPIMS discharge period, assummarized in the observations (a)(i) made previously.

Relation to the ST energy distribution. The presence of twogroups of the sputtered species in the discharge with verydifferent mean energies (1.5 eV versus 1223 eV) pointsout the differences between a classical sputtering process,when the energy distribution of the sputtering species canbe described by a ST distribution, and the HiPIMS process.The ST distribution mainly characterizes the DCMS-likesputtering, and can be presented in the form [21]

dNdE

E(E + Eb)

3 , (2)

where E is the energy of a sputtered particle, and Eb is thesurface binding energy of the sputtered cathode.

A comparison between the ST distribution calculated forTi (Eb is taken to be 4.9 eV [21]) and the Ti+ IEDFs measured

Figure 6. Comparison between the normalized ST distributioncalculated for Ti sputtered atoms (Eb = 4.9 eV), and the Ti+ IEDFsmeasured in HiPIMS in this work at t = 30, 60 and 100 s(mutually normalized in the low-energy range).

in HiPIMS at differentt is presented in figure 6. In spite of thefact that the ST distribution describes neutral atoms, whereasthe presented IEDFs are related to Ti+, the IEDFs determinedin this work coincide well with the ST distribution in the energyrange of about 02 eV. This fact confirms that the Ti+low grouporiginates from the low-energy sputtered Ti (


Figure 7. Ti+ IEDFs measured at two different target voltages Uapplied during the HiPIMS pulse. The mean ion energies Emeancalculated by equation (3) are given for comparison; t = 44 s.

Ti+. Figure 7 shows that Emean corresponding to the high-energy Ti+ changes roughly proportionally toU , supporting thehypothesis of the back-reflected ions. These ions, after beingaccelerated in the sheath, should have an energy proportionalto the target voltage before transmitting it to the otherrelatively slow discharge species by collisions. According tothe additional calculation based on the time-resolved IEDFsgiven in figure 3, the total population of the high-energy Ar+appears to be about 30 times less than that of Ti+ at themoment of their maximum appearance in the discharge (att 45 s). This value is comparable to the differencein the background ion populations for these species beforethe plasma pulse (observation (i), section 3.1). Assumingthe presence of rarefaction in the discharge, the amount ofback-reflected Ar+ (during the whole pulse duration) mightfurther decrease compared with Ti+, confirming our suggestionfor the high-energy tails of the observed IEDFs. Followingthese considerations, the third energy peak sometimes visiblein the measured IEDFs (see also [31]) might be due to theback-reflection of the double-charged Ti ions (also presentin HiPIMS), since the energy corresponding to the thirdmaximum (when it is visible) is always approximately twice ashigh as the energy of the second one (24 eV versus 13 eVsee figure 5(a)). These arguments, however, require additionalverifications.

Based on our measurements, the high-energy Ti+ havethe mean energy Emean equal to about 15 eV. In the firstapproximation, the net velocity of these species is about8 km s1, giving the flight time to the detector (80 mm formthe target) equal to about 10 s. However, the actual arrivaltime measured in this work is about 27 1 s (see figure 5).Assuming that most of the sputtered species are produced at theend of the 5 s pulse, the final flying time is reduced to 22 s.This value is still about twice as large as that calculated solelybased on the particle energy. Since the mean energy valueof these species measured at 80 mm from the target alreadyaccounts for a possible thermalization during the flight, wefinally arrive at the conclusion that this group of ions originates

at t 22 s (arrival) 10 s (flight time) = 12 s, i.e. afterthe end of the HiPIMS pulse3.

It should also be noted that the growth time of the first Ti+wave at the detector position is about 10 s (see figure 5(c)),i.e. twice longer than the pulse duration. This fact additionallypoint out certain energy dissipation during the flight for thisgroup of species. This is also true for the second Ti+ wave.Since the initial time profile for the sputtered species (and itsduration) is unknown, and it may not coincide with the HiPIMSpulse, the role of the energy dissipation during the flight timecannot be determined precisely.

If the low-energy component in the IEDFs taken atdifferent t follows the ST distribution, the intensity of thehigh-energy one changes essentially, as shown in figure 6.According to figure 5(c), the population of the high-energyTi+ surpasses the low-energy population between time t1 andt2, nearly approaching the level of Ar+ (at t = 30 s), anddecreases afterwards. The same rate of decrease for peaks 2and 3 implies the same dissipation kinetics for these high-energy species.

Low-energy Ti+. The low-energy Ti ions have an averageenergy of 1.5 eV, which decays gradually down to about0.7 eV at the end of the plasma off-time, where these speciescan be considered as thermalized [23]. At the beginning ofthe off-time the measured energy of 1.5 eV corresponds to thenet velocity of 2.5 km s1. This value is in good agreementwith the previous observations in HiPIMS made using opticalspectroscopy [12], and Langmuir probe measurements [32].

The measured flight time for this group of species isabout 32 s, which is in very good agreement with theexperimental results (flight time t25 s 38 s5 s =33 s). This conclusion is also consistent with both STdistribution predicting the velocity of the sputtered Ti particlesto be about 3 km s1, and with the numerous measurements ofthis velocity presented elsewhere [3337].

Low-energy Ar+. Based on our measurements, the Ar ions aremainly distributed within the low-energy peak (see figure 3(b)),having the average energy close to that of Ti+low. The timeevolution of the population density for these species have tobe described as well.

First, the measured background level of Ar+ is about 10times lower than that of Ti+ low, as shown in figure 5(c). Thisis presumably due to the presence of relatively cold electrons(Te < 1 eV) in the discharge till the end of the HiPIMS off-time [27], which are still capable of ionizing the sputtered Ti(ionization threshold is 6.8 eV), whereas their energy might betoo low for Ar ionization (ionization threshold is 15.76 eV).The low Ar+ population might also be partially related to therarefaction in this time interval.

Second, as mentioned above, the growth of Ar+ densityis much slower than that of both groups of Ti+ and takes3 This energy, as well as the estimated net velocity are given here forillustrative purpose. Theoretically, having very different energies, the speciesfrom the whole high-energy region in the found IEDF should arrive to thedetector at different time delays, which does not happen in our case, however(figure 3(a)). The observed contradiction happens likely due to the net velocityof the detected species may be different from that determined by their kineticenergy.

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Figure 8. Ratio between the Ar and Ti electron impact ionizationrates calculated for different Te based on the Maxwellian EEDF.Zone I corresponds to the Te range typically reached in HiPIMSduring the off-time, whereas zone II represents possible Te values atthe end of the pulse. Inset: three normalized EEDFs calculated atdifferent Te along with the electron impact ionization cross sectionsfor Ar and Ti [42]. The overlapping interval (grey filling) isproportional to the ionization rate Qion.

a much longer time. The moment of time when the Ar+population starts to increase should be assigned to the arrival ofthe electrons non-confined in the targets vicinity resulting inthe Ar ionization wave observable by the mass spectrometer.This suggestion is supported by the growth of the electrondensity at the end of the on-time reported in [38, 39], as wellas by the velocity of the bulk gas excitation wave recentlyfound by OES, which stays in the range 724 km s1 dependingon the bulk gas [12]. Giving the Ar ionization arriving timeequal to 7 s, this velocity is estimated to be equal to80 mm/7 s = 11 km s1 in our case.

Population of Ti+ compared with Ar+. In terms of the arrivaltime for different groups of particles, as well as their relativepopulations, the above-mentioned results can be illustrated bycalculations of the relative ionization rate for Ar and Ti. Takingthe expression for ionization rateQion in the form (e.g. [40, 41])

Qion =

Eion

ion(E)

2Eme

f (E) dE, (4)

where Eion is the electron ionization threshold, ion(E) is theelection impact ionization cross section, E is the electronenergy, f (E) is the EEDF, a curve representing the ratiobetween the Ar and Ti ionization rates can be obtained. Sucha curve is shown in figure 8 where a Maxwellian EEDF isassumed in the discharge [27], and the values of ion(E) for Arand Ti are taken from [42]. At the end of the HiPIMS pulse, Temay increase up to 26 eV due to a wave of energetic electrons[12, 27] which are especially favourable in the presence ofrarefaction [4]. Afterwards the electrons cool down to typically0.51 eV, as a result of numerous collisions with the sputteringmaterial [38, 39, 43]. This fact is represented by two Te zones

in figure 8, with zone I representing the cold, and zone II hotelectrons in the discharge. As we can see, the cold electronscannot ionize Ar efficiently, resulting in an Ar/Ti ionizationrate ratio of about (15) 105. Provided the Ar density inthe reactor is about 34 orders of magnitude higher than thatof Ti [44], the Ar+/Ti+ ion density ratio is 0.1, which is veryclose to the result shown in figure 5(c).

Furthermore, the Ti+ high population in its maximumnearly reaches the one of Ar+ (at t 30 s). Sucha proximity implies that the ratio between the Ar and Tiionization rates is nearly equal to the Ti/Ar density ratio inthe discharge at that moment of time. Assuming the Ti/Ardensity ratio is about 103 (based on the time-resolved opticalabsorption measurements performed in [44]), we obtain thatTe is not surpassing the limit of about 2 eV corresponding tothe beginning of zone II in our case (see figure 8). In spite ofsuch a small Te change, this conclusion is in good agreementwith several observations performed in this pressure range inHiPIMS [9, 38, 39].

Timing for different groups of particles. In this section,we will try to give the explanation for the time-resolvedphenomena summarized in figure 5. As we can see, the waveof ionization protruding with a speed of about 11 km s1 isfaster than the velocity of both low- and high-energy Ti afterthe sputtering. In addition, it should be taken into account thatin the used balanced magnetron source a dense electron cloudconfined in the target vicinity (at 3 cm [27]) is formed, aswas verified additionally by OES for our magnetron source(not shown). As a result, the sputtered Ti undergoes ionizationwhen flying through this region. Based on the estimatedarrival and flight times for the measured species, as wellas on the change of their population in time, a simplifiedchart reproducing the main processes taking place in the areaadjacent to the detector can be built. Presented in figure 9, thischart shows mainly the wave of electrons, both high- and low-energy groups of Ti+, as well as Ar+ coming to the detectoralong with possible plasma kinetic reactions which might beresponsible for a certain process.

The most critical reactions for the found kinetics of thestudied discharge representing the ionization, recombination,charge exchange processes and so on are summarized below:

Ti + e Ti+ + 2e (R1)

Ar + e Ar+ + 2e (R2)Ar + e Armet + e (R3)

Ti+high + Tilow Tihigh + Ti+low (R4)Ti+high + Ar Ti+low + Armet (R5)Ti+high + Ar Tihigh + Ar+ (R6)

Tilow + Armet Ti+low + Ar + e (R7)X+ + e X, (X = Ar, Ti) (R8)

These reactions tend to illustrate only the main processesobserved in this study by MS measurements in HiPIMS. Fora better understanding of the contribution of each reaction

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Figure 9. Schematic chart illustrating the population change for themain groups of particles at the position of the detector (80 mm awayfrom the cathode) as a function of t . The population behaviourcorresponds to the log-vertical scale, representing about three ordersof magnitude of the total change in the case of Ar+ and Ti+. Time t4is 400500 s. The reactions with higher importance are markedin bold.

(as well as the other possible reactions), correspondingcalculations should be undertaken accounting for the reactionrates as well as the other quantitative data, which is outside thescope of this paper.

A few additional remarks should be made based onfigure 9. First, since the electron confinement zone locatedclose to the target is supposed to be the main ionization regionin the HiPIMS discharges with a balanced magnetron source,the evolution of the electron density sketched in figure 9 shouldbe associated with the non-confined electrons only. In addition,since the high-energy groups of sputtered Ti cross the electronconfinement zone before the low-energy ones, the high-energyTi gets ionized first, which in turn results in the essentialcooling of the electrons in the confinement zone [12, 45]. Suchan electron cooling (i.e. the Te decrease) should result in aweaker ionization of the following groups of the sputteredatoms passing the confinement zone, which have lower energyand longer arrival time.

Second, the production of the Ar metastables (Armet)in the discharge volume should be mentioned, since it isthe essential factor for Penning ionization of the low-energygroup of Ti at the position of the detector. This process issupposed to be essential after refilling starts (roughly after50100 s according to [23]). In fact, Armet can particularlybe produced as a result of collisions with the energetic Ti+(reaction (R5)) and be used for further Ti ionization (reaction(R7)). According to our recent study involving resonant opticalabsorption [44], performed under somewhat different HiPIMSconditions, Armet are mainly produced right after the dischargepulse, resulting in further generation of Ti metastables (seefigure 10), and in the slow increase of the Ti+ population due

Figure 10. Time evolution of the absolute density of Ti+, as well asTi and Ar metastables during a HiPIMS pulse measured by opticalabsorption spectroscopy. The Ar pressure is 20 mTorr, the pulseduration is 20 s, distance from target is 50 mm; adapted from [44].

to the Penning ionization (reaction (R7)). This is also observedin this study in the time interval between t3 and t4 (see figure 5and figure 9).

Third, the population exchange between the high- andlow-energy groups of Ti+ during the time interval t2t3, visiblein figure 5(c) and figure 9, should be stressed. Such a processis likely to happen as a result of the efficient charge exchangebetween these two groups of Ti+, after the low-energy (andrather weakly ionized) Ti+ group arrives at the detector vicinity.Such an exchange results in about one order of magnitudedecrease in the high-energy Ti+ and in roughly a similarincrease in the Ti+low population. The further decrease (withthe same rate) of the Ti+high group after time t3 is likely due tothe combination of several factors, in particular represented byreactions (R4) and (R5).

Finally, the presence of the described charged particlesthroughout the entire HiPIMS period is very important asthey play a role of pre-ionization for the next pulse, sothe additional pre-ionization is not required for most of theHiPIMS discharges with comparable repetition rates [46, 47].Furthermore, the metal ions are transported in a medium whichis still filled with electrons and ions. This situation might,therefore, enhance the ion transport as suggested previouslyin [6].

3.3. Time-averaged mass spectrometry results

Having rather high-repetition rate as well as short-pulseduration, the considered HiPIMS discharge cannot becharacterized rigorously by the time-resolved temperaturecorresponding to the heavy particles presented in the gasmixture (e.g. ions or neutrals) at each moment of time.The reason for this is that the thermalization time constant(defining the thermalization rate of heavy particles in thedischarge, and thus the characteristic time of thermalization)for the translationaltranslational energy relaxation, being inthe range of several hundred s at 5 mTorr, is roughly one

8


Figure 11. Results of the time-averaged MS measurements of Ti+(a), (b), and Ar+ (c), (d) in the HiPIMS discharge. The changesinduced by the different applied voltages U result in changing theeffective Ti+ temperatures shown for Ti+ (b). Inset: the effective Ti+temperatures calculated based on the Maxwellian fit of the datashown in (b).

Table 2. Effective ion temperatures calculated as a result ofMaxwellian distribution fits based on the actual dischargeparameters in the corresponding energy ranges.

Teffi (eV)Ar+low Ti

+low Ti

+high

U (V) Ip (A) Jp (A cm2) 59 eV 59 eV 1230 eV800 48 0.15 0.89 1.61 4.661000 122 0.39 0.76 1.27 5.801100 166 0.53 0.80 1.31 6.231200 192 0.61 0.81 1.28 6.67

order of magnitude higher than the HiPIMS pulse durationconsidered in this work [48, 49]. This difference might beeven larger taking into account the rarefaction further reducingthe Ar pressure. Since the gas refilling itself may takeabout 100 s [50], which is also much longer than the pulseduration, the only time-averaged effective ion temperature(T effi ) perceptible during a relatively long time in HiPIMScan be considered.

The results of the time-averaged argon and titanium IEDFmeasurements are given in figure 11. The ion temperatures aredetermined for two energy ranges, as shown in figure 11(a).Since the total measured IEDF has a bi-Maxwellian nature,the Maxwellian fits are applied for each energy range,corresponding to the low- and high- energy group of Ar+or Ti+ (see figure 11), similarly to the considerations describedin the previous sections. The obtained T effi for the low-energyspecies are close for Ar+ and Ti+, whereas T effi found for thehigh-energy Ti+ are much larger. Moreover, in the last caseT effi reveals the dependence on the applied voltage (see table 2and the inset in figure 11(a) and (d)). This fact can be observedin figure 11(b) where the high-energy tail of Ti+ extends as thetarget voltage increases from 800 to 1200 V.

Considering the last result, a suggestion about the increasein the total amount of the sputtered species in the whole energyrange [51] with increase in the voltage is barely acceptablein our case, since only the tail of the IEDF is affected. Atthe same time, considering the DCMS discharges, a simulatedenergy distribution function for the back-reflected Ar+ showsthe appearance of a high-energy component increasing from50 to 200 eV while the incident ion energy increases from200 to 800 eV [29]. In the HiPIMS discharges, since thegas rarefaction can be rather strong due to the high targetcurrent values compared with DCMS [4, 52], the sputteredmetal density may exceed several times the gas density in thetarget vicinity. In such a situation, the sputtering probabilityof titanium target by Ti+ (self-sputtering) is enhanced ascompared with DCMS processes [26, 53], which may furtherpromote the effect of back-reflected ions. On figure 11(a),the time-averaged contribution of Ti+ with energy higher than30 eV is 23 orders of magnitude lower compared with the low-energy peak intensity meaning roughly that during the wholeHiPIMS period less than 1% of the particles have energy above30 eV. This result is in good agreement with the probability ofthe so-called back-attracted Ti+ () estimated as 0 0.3in [53], and with the SRIM simulations showing that only3% of atoms are back-reflected from the target [54]. Itis noteworthy that the SRIM simulations do not take intoaccount the post-ionization of these reflected neutrals in theplasma. Moreover, the mass spectrometer orifice only acceptsthe ions within a rather small solid angle (102 srad) whichmay explain the discrepancy between the experimental andtheoretical data.

It is known that a drop in the deposition rate in HiPIMScan be observed when both the ionization rate of the metallicatoms and the self-sputtering processes are favourable [26, 53].Therefore, an increase in U , and hence Ip, leads to an increasein the self-sputtering process and consequently to the loss ofthe deposited material at the substrate. In this study the dataextracted from the x-ray fluorescence analysis (XRF, BrukerS4-Pioneer spectrometer) performed on titanium films show a39% decrease of the amount of deposited material when U isincreased from 800 to 1200 V. This effect, widely observedin the literature [52, 55], provides additional proof that theobserved tail elevation effect might be attributed to the back-reflected ions in our case.

Another possible explanation for the found high-energytails in the time-averaged IEDFs can be related to theazimuthally accelerated ions, as was mentioned above anddescribed in [31] and in [56], where the high-energy tails (up to80 eV) were observed when a mass spectrometer was locatedperpendicular to the target surface. The origin of the foundenergy tails in these works is explained by the existence ofthe so-called modified two-stream instability (MTSI) drivenby the relative drifts between the fast electrons and slow ionsin the presence of a magnetic field. This effect leads to theappearance of azimuthal forces caused by the azimuthal currentand might be virtually responsible for the observed elevationof the high-energy tails, along with the back-reflected Ti+, asmentioned in the previous sections.

9


4. Summary

In this work, the plasma dynamics of a short-pulse HiPIMSdischarge, during sputtering of a Ti target in Ar gas, isinvestigated using MS diagnostics.

As a result of the time-resolved measurements thepresence of high- and low-energy groups for Ti ions isconfirmed. The time evolution of the average energy andthe population of these groups of species are analysed. Asa result of this analysis, two waves of the incoming Ti+ arefound. The first wave represents the sputtered and ionizedTi with a mean energy Emean of about 15 eV, whereas thesecond one corresponds to the low-energy Ti+ group (Emean 1.5 eV) arriving at the virtual substrate position with certainretardation.

The estimated arrival times for the various groups ofspecies correlate with the measured mean energies of thesespecies, whereas their somewhat extended growth time at themoment of detection compared with the pulse duration pointsout a definite energy dissipation of the arriving species. Lowerpopulation of the low-energy Ti+ group at the moment of arrivalmight be mainly explained by cooling of the confined electronsin the target vicinity during the on-time, as a result of collisionswith the energetic sputtered Ti passing this region previously.

The population of the energetic group of the detected Ti+decays fast (nearly exponentially) after reaching its maximum,whereas the second low-energy Ti+ reveals the additional widemaximum corresponding to the middle of the plasma off-time. Several particularities, such as the population exchangebetween two groups of Ti+ found after the arrival of the low-energy Ti+, as well as the further slow increase in the low-energy Ti+ population are clearly detected. While the firsteffect may happen due to the charge exchange mechanism(reactions (R4) and (R5)), the second might be due to theformation of Ar metastables, detected previously in HiPIMSby time-resolved absorption spectroscopy. The influence ofAr metastables, however, is supposed to be weak, taking intoaccount rather minor elevation of the Ti+low population in thetime interval between t3 and t4. At the same time, furtherfast decay of the energetic Ti+high population, until it becomesundetectable, is attributed mainly to the collisions with theslow particles, e.g. Ar, especially at the moment when refillingeffect starts (reactions (R4)(R6)).

The time-averaged IEDF measurements performed in thesame discharge for Ti+ and Ar+ indicate that the effective iontemperatures for low-energy Ti+ and Ar+ are close to each other(1.5 eV), whereas this temperature is voltage-dependent forthe high-energy group of Ti+. The last effect is directly relatedto the elevation of the high-energy Ti+ population (a tail ofIEDF) with voltage, which is explained by a combination ofseveral factors in this work.

Finally, the role of the back-reflected ions in the observedeffects needs to be mentioned. The proportionality between thecalculated mean ion energy in the Ti+ IEDF and the dischargevoltage U (figure 7) points out a possible contribution ofthese energetic particles to the energy transfer, which is alsoconfirmed by the high-energy IEDF tail elevation in the time-averaged case. On the other hand, the population of high-energy Ar+ found in this study is roughly 30 times less

than that of Ti+ at t 45 s where these populationstake their maxima. The presence of rarefaction inherent inHiPIMS, however, additionally supports the determined lowAr+ population. The exact measurement of the rarefaction (e.g.in terms of Ar or Ar+ density changes during the dischargeperiod) was not the goal of this work. Such measurementsmay be performed, e.g. by time-resolved optical absorptionspectroscopy in HiPIMS in the future.

Acknowledgments

The authors would like to acknowledge the financial supportof the Belgian Government through the IAP program (P06/08)as well as M Michiels and D Walrave (Materia Nova) forthe technical contribution to this work. N Britun andS Konstantinidis are postdoctoral researcher and researchassociate of the Fonds National de la Recherche Scientifique(FNRS), respectively.

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11

1. Introduction2. Experimental3. Results and discussion3.1. Time-resolved mass spectrometry results3.2. Analysis of the time-resolved results3.3. Time-averaged mass spectrometry results

4. Summary Acknowledgments References

palmucci 2013 journal of physics d applied physics

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