Thin Solid Films 519 (2010) 1705–1711

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O− density measurements in the pulsed-DC reactive magnetron sputteringof titanium

Robert Dodd, ShaoDong You, James W. Bradley ⁎Department of Electrical Engineering and Electronics, University of Liverpool, Brownlow Hill, Liverpool, L69 3GJ, UK

⁎ Corresponding author.E-mail address: j.w.bradley@liv.ac.uk (J.W. Bradley).

0040-6090/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.tsf.2010.06.056

a b s t r a c t

a r t i c l e i n f o

Article history:Received 26 February 2010Received in revised form 11 May 2010Accepted 24 June 2010Available online 1 July 2010

Keywords:OxygenNegative ionsPulsed-DC magnetronReactive sputteringPhoto-detachmentTime resolvedLangmuir probe

Using eclipse laser photo-detachment in conjunction with Langmuir probing, the density of O− ions in areactive pulsed magnetron (100 kHz, 55% duty) plasma has been determined at different times during thepulse period at a set of positions along the centre line axis of the discharge. The magnetron was operated at afixed average power of 400 W with an oxygen partial pressure of 10% of the total pressure 1.33 Pa. Theresults show the plasma is weakly electro-negative, with a negative ion-to-electron density ratio α up to amaximum of 0.63.During the plasma on-phase (at all chosen measurement positions) the O− density was found to reach amaximum directly after initiation of the voltage pulse decreasing weakly during the rest of the on-phase. Onthe transition from on-to-off phases of the pulse the negative ion density was found to fall (by 60% both closeand far from the target but only 10% near the discharge centre), with the O− density remaining almostconstant during the rest of the afterglow.The spatial structure of the O− density reveals a distinct peak 75 mm from the target close to but not at theposition of the null in the magnetic field, falling by a factor of eight for increasing distances up to 30 mm bothtowards and away from the target. The highest O− density recorded at this position was 1×1016 m−3, at atime of 2.12 μs into the pulse. From a comparison between on- and off-phase densities and using an intuitivemodel of the plasma, the results indicate that most negative ions are created in the bulk plasma. The densityof target-borne O− ions is estimated to be about 1×1015 m−3 varying little with position, possibly forming abeam-like structure.

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1. Introduction

Pulsed-DC magnetron sputtering is a valuable physical vapourdeposition technique used for the reactive deposition of functional thinfilms including engineering quality oxides and nitrides [1,2]. Whenoperated in argon–oxygen atmospheres copious amounts of negativeions can be produced, particularly the O− ion. Mass spectroscopicstudies [3] over the last few years have shown that many of these ionsare highly energetic with energies up to the full cathode fall potential.

Investigations have revealed, 1) the dominant negative ionbombarding the substrate is O− [4], 2) these ions originate from thecathode surface, 3) the maximum bombarding energy is given by thecathode fall potential [5,6], e.g. in excess of 450 eV, 4) the energydistribution functionsof themost abundant ionO−, are broadwith threedistinct peaks, 5) the production of O− at the target increases as thesecondary electron emission coefficient of the target material increases[7–9], and 6) the energetic negative ions can take the form of a beam

with their spatial distribution dependent on the magnetic fieldgeometry at the cathode [8].

Recently, using eclipse laser photo-detachment it has been shownthat in the DC sputtering of titania, the O− particle densities can be ofthe same order or greater than the electron density [9]. These resultsagree to within an order of magnitude with those from the Langmuirprobe study of Straňák [10], who found O− densities up to 10% of theelectron density. Even in small quantities energetic negative ions(beams) in the plasma can deliver energy fluxes at an insulating thinfilm many times higher than delivered by the positive ions.

Although, up till now there has been little quantitative data on thenumbers of negative ions present in the magnetron plasma it is knownthat energetic negative oxygen ions in sufficient numbers can damagethe film at the substrate. For instance, in the RF sputter deposition ofTa2Zn3O8 [11], the sputter deposition of aluminum doped zinc oxide(AZO) [12] and indium tin oxidefilm (ITO) [13] negative ions have beenshown to cause energetic re-sputtering of a film. These can cause aslowing or complete suppression of the film growth, leading to surfacenon-uniformities, ripples, and the development of etch pits. Thisphenomenon was seen in the sputtering of BaTiO3 films [14]. Also,compressive stresses can be introduced this way through atomicpeening as in the deposition of ITO and AZO films [13]. Other problems

1706 R. Dodd et al. / Thin Solid Films 519 (2010) 1705–1711

arising from energetic bombardment include the reduction in therefractive index of Al2O3 films, the relative reduction in crystallinity ofY2O3 films and the reduction in resistivity of sputtering AZO films [15].The degradation of crystallinity and superconductivity, for hightemperature superconductor YBaCuO films has also been attributed tothis effect [16].

Although mass spectrometry had been used to good effect onreactive magnetron plasmas to detect and measure the energy ofnegation ions, only those ions energetic enough to overcome the sheathpotential adjacent to the instrument orifice can actually be detected.This potential is typically a few volts, and therefore negative ions withlow energies or those thermalized in the plasma cannot be detected atall by this technique. In some cases, to overcome this problem, theplasma potential can be artificially lowered using a pulsed bias [17].Also,mass spectrometry does not allowabsolute densitymeasurementswithout complicated calibration.

However, amore effectiveway to detect negative ions and find theirabsolute density n− (of any energy) is to use laser photo-detachment[18,19]. Laser photo-detachment in conjunction with Langmuir probedetection of the photo-ionised electrons has been done in RF capacitiveand inductive plasmas [20], bucket source plasmas for neutral beamheating in fusion [21], magnetised plasma [22], and hydrogen divertorsimulator [23] for negative ion species such as O2

− and O− and H−, F−

and D− [24].In our previous work [9], we have used the eclipse photo-

detachment technique to determine the negative ion density on thecentre line of the DC magnetron operating in reactive conditions. Theeclipse method [25–28], where the probe is protected from the laserbeam by forming a shadow is important in depositing plasmas, sincespurious results for the photo-detached current and derived negativeion density (n−) can be obtained due to extra electron generation fromthe laser beam incident on the depositing material on the probe. Thephotoelectronsbecomethermalized in~1 μs [9], so, the collection rate ofthe background and photoelectrons to the probe, across the magneticfield (0–100 Galong themeasurement line),will not differ substantially.Therefore, we argue that the current ratio is insensitive to themagneticfield in this regime and this ratio provides a good estimate of negativeions. InRef. [9], itwas found that theO− ion dominates overO2

−with thelatter less than 2% of the total observed. Also, the maximum n− valuewas 7×1015 m−3 at 25 mTorr total pressure and 50% oxygen by partialpressure. At typical operating conditions n− was found to be about4×1015 m−3 or 0.4 of the electron density.

Here we extend the reactive DC magnetron study to the pulsedenvironment using a commercial pulsed-DC power supply anddetermine the temporal variation of the negative ion density as afunction of distance along the discharge centre line. This provides notonly the total negative ion density (largely O−) but also an estimate ofthe relative numbers of negative ions created at the cathode surfacecompared to the number created in the bulk, the latter of which can notbe easily detected by mass spectrometry.

2. Experimental method and set-up

2.1. The magnetron sputtering and photo-detachment system

The experimental set-up is shown in Fig. 1. A detailed description ofthe experimental set-up can be found in Ref. [9]. The magnetron, fittedwith a 150 mm titanium target, was pulsed at 100 kHz, 55% duty cycleusing anAdvanced Energy Inc. Pinnacle Plus power supply at an averageapplied power of 400W. This supply works in the bipolar mode with apositive potential at the target during the off-time of ~+30 V. After aninitial peak of 1 kW, the powerwas reduced to 600W then increased to800W during the on-phase of the pulse. The working pressure wasregulated to be 1.33 Pa with an oxygen partial pressure fixed at 10% ofthe overall total pressure. A delay pulse generator, Stanford DG535, was

used to control the timing and synchronisation for both the Langmuirprobe and laser system.

To measure the negative ion density it is necessary to photo-detachthe extra electron residing on all the negative ions in the chosen volumeof theplasmaunder investigation andcollect thembyanelectrical probeat a chosenmeasurement point. In this study, we have chosen to do thisusing a pulsed Q-switched Nd:YAG laser (Quantel Brilliant B) at awavelength of 532 nm with an energy density of 240 mJ cm−2. Ablocking wire was used outside the chamber to shadow the probe soeliminating the ablation of insulating material caused by the incidenceof the laser on the probe [9]. During the photo-detachment experimentsa variable DC power supply in series with a resistor was used toelectrically bias the probe.

The collecting (Langmuir) probe was situated on the dischargecentre line. The target (cathode) was moved vertically, with respect totheprobe, over a distanceof 90 mm. Theposition of the probe relative tothe target encompasses the position of the magnetic null (Bz=0) andeither side of it. The probe consisted of 250 μm diameter tungsten wirewith a re-entrant ceramic sleeve located outside of the laser beam. The7 mm long extension of the ‘L’ shaped probe tip (3 mmshort extension)was aligned co-axially with the laser beam. This was positionedcentrally by having the stem offset by 3.5 mm. To obtain conventionaltime-resolved Langmuir probe current–voltage characteristics anESPion probe acquisition system (from HIDEN Analytical Ltd) wassynchronisedwith themagnetron voltage supply. The time resolutionofthe Langmuir probe characteristics was 1 μs.

Photo-detached electron currents were measurements using theLangmuir probe described above placed at different positions alongthe centre of the discharge, from 35 to 125 mm from the target. Sincethe probe is oriented parallel to the target surface, the measurementscan only be spatially averaged over this distance, indicated by theshaded region in Fig. 2. The magnetic field configuration, also shownin Fig. 2 (B-field strengths and directions) was determined using aHall probe (GM04 from Hirst Magnetic Instruments Ltd). As we cansee the photo-detachment measurement position includes themagnetic null (Bz, Br=0, at z=60 mm). Due to restrictions of theprobe and magnetron positioning no photo-detachment measure-ments closer than 35 mm to the target were possible.

Optimum settings, of Langmuir probe bias and laser energy densitywere used, the laser beam radius was 4 mm and the laser energydensity was 240 mJ cm−2 [9].

3. Results and discussion

During the experiments the following operating conditions wereused namely 400W average discharge powers, 1.33 Pa total pressure,and a gasmixture ration of 90% argon to 10% oxygen by partial pressure.The cathode voltage (Vd), current (Id) and instantaneous power(product of Id (t) and Vd (t)) waveforms are shown in Fig. 3a and brespectively.

From Fig. 3a, we can see a number of distinct features; between 0and 0.5 μs there is a rapid downward swing of Vd from +30 V to−450 V. This is continued until 5.5 μs when there is a rapid swingpositive to +175 V during the 0.1 μs of the on-to-off transient phase.During the discharge on-phase Id rises gradually over 5 μs from 0 to2.0 A. This is followed by a rapid current reversal during the on-to-offtransition down to −3 A. A large fraction of this is displacementcurrent [29]. Langmuir probe measurements reveal that in the off-phase the plasma potential (Vp) is uniformly elevated to all positionsin the plasma to about +30 V. In the quiescent on-time Vp attains avalue a few volts above ground. During the two voltage transients, on–off and off–on, the plasma potential is highly dynamic, particularlyduring the on–off phases where it can swing up to values greater than+200 V for a period of 0.5 μs. Because of this we have not attemptedto make any Langmuir probe or photo-detachment measurements attimes from 0 to 1 and 5.5 to 7.0 μs.

Fig. 1. A schematic diagram of the experimental set-up. M1 and M2 — turning mirrors, VA — variable aperture, BW — blocking wire, BS — beam splitter, PM — power meter, PD —

photodiode, W1 andW2—windows, LBA— laser beam analyser, MFC1 and MFC2—mass flow controllers, C=1 μF, Cbp=1000 μF R1=47 Ω, R2=4.7 kΩ and LP— Langmuir probe.

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Fig. 3b shows the typical power waveform. In this case theintegrated average power is 400 W, however, at the beginning of thepulse on-phase the instantaneous power can reach a value in excess of1000 W after which it settles down to 800 W. During the period 0 to5.5 μs there will be significant sputtering of the target and depositionof oxide films on the walls of the chamber and the Langmuir probe.The build up of insulating material, at the walls, hinders the electronreturn path and can lead to significant changes in the plasma potentialand parameters.

To characterise the plasma and determine the plasma potential(Vp), electron density (ne) and electron temperature (Te) at each ofour measurement positions the Langmuir probe was used. The

Fig. 2. The magnetic field geometry and field strengths together with the ind

electron density ne was determined from the measured electronsaturation current Ie-sat (itself obtained at the plasma potential) with

Ie�sat = IeðVpÞ = eneA

ffiffiffiffiffiffiffiffiffiffiffiffikTe

2πme

sð1Þ

where Ie (Vb) is the collected electron current at probe bias Vb, A is theprobe surface area, e the electronic charge, me the electron mass andTe the electron temperature. The latter was obtained from the inverseslope of the logarithm of the electron current Ie against the retardingprobe bias Vb, knowing that Ie (Vb)= Ie (Vp) exp {−e (Vb−Vp)/kTe}.

icated location of detection Langmuir probe on the discharge centre line.

Fig. 3. a) The discharge voltage and current waveforms and b) the calculatedinstantaneous discharge power for frequency of 100 kHz and a duty factor of 55%.

Fig. 4. A plot of the measured electron current as a function of time in a) the pulse on-time and b) the pulse off-time. The chosen probe biases are +10 and +35 V and +35and +55 V respectively. The photo-detached electron current at the different biases isindicated by ΔIpd.

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The plasma potential Vp was obtained from the second derivative zerocrossing point of the electron current against Vb. As the applied biasvoltage is increased, the sheath around the probe increases in size. Asit approaches the diameter of the laser beam, the ratio of Ie and ΔIpd(photo-detached electron current) will saturate.

To successfully detach the outer electron on the negative ion thephoton energy must be above the electron binding energy. Thewavelength therefore determines which negative ions can sufferdetachment and the laser energy density determines the detachmentfraction, e.g. a 532 nm pulsed Nd:YAG with an energy densityN240 mJ cm−2, is capable of photo-detaching O2

− and O−, as thephoton energy exceeds both the binding energies and will give adetachment fraction of 1 [24]. The detachment fraction (Δn− /n−) ofthe bulk negative ions of density (n−) is also dependent on the photo-detachment cross-section, σpd. For single species photo-detachment,this is given by [24]:

Δn−n−

= 1− exp − ESσpd

hv

� �ð2Þ

where E is the incident laser energy and S is the beam cross-sectionalarea. Provided that saturation is reached namely Δn−/n−→1 in thebeam, then the current of the photo-detached electrons collected atthe probe, for a single species (as in our case), can be used to calculaten− directly using

ΔIpdIe en−

ne= α ð3Þ

where ΔIpd is the photo-detached electron current produced after thelaser energy is applied, Ie is the electron current due to thebackground electrons observed in the saturation region of the probecurrent characteristic and α is the negative ion-to-electron densityratio. As we will discuss a proportion of the negative ions in this studyhave significant energies. However, since the time it takes these fastnegative ions to cross the laser beam is much longer than the laser

pulse duration, the photo-detachment cross-section (σpd) is un-changed, compared with slow negative ions.

Typical photo-detached currents ΔIpd obtained in both the on andoff-phases of the pulse can be seen in Fig. 4a andb respectively. In Fig. 4aand b, two different probe biases are shown to illustrate the effect ofbias. We see that after the application of the laser (duration 6 ns) theelectron current is perturbed for times up to 2.5 μs. Previous steady-state DC measurements [9] have shown that the recovery time of thepositive ions displaced from the illuminated region can be up to 4 μs.However, Fig. 4 indicates that this recovery time is much less here at2.5 μs. The recovery time is characteristic of the time it takes negativeions to re-populate in the region inside the laser beam through diffusionfrom the remote plasma [30], and this time may vary with differentplasma parameters when the discharge is pulsed.

To ensure all the photoelectrons, produced in the laser beam, in thevicinity of the probe are collected, it is necessary to vary the probe bias(Vb) at voltagesmore positive than the plasmapotential and look for theonset of saturation in Ie. Fig. 5 shows the measured photoelectroncurrent (ΔIpd) as a function of probe bias for both the on and off-times.

We can see clearly in Fig. 5 during the on-phase that for probepotentials below the plasma potential, (varying from +1 to +2 V) inthis phase, there is no photoelectron currentΔIpd. The same can be seenin the off-phase for biases below +32 V since the plasma potentialvaries between+28 and+32 V in this phase. From a plot of the ratio ofΔIpd/Ie against the probe bias Vb (where Ie is the electron current due tobackground electrons in the absence of the laser pulse) the degree ofelectronegativityα for the plasma at the probe position can be obtained.

Fig. 6 shows this ratio for both on and off-phases. In the pulse on-phase, the current ratio (ΔIpd/Ie) increases with increasing Vb up toVb=+35 V, after which it slightly decreases. At this optimum bias the

Fig. 5. A plot of the photo-detached electron current (ΔIpd) against probe bias att=2.5 μs (on-phase) and t=7.5 μs respectively (off-phase) at 75 mm from the target.

Fig. 7. Plot of the electron densities against distance along the central axis for a numberof chosen times during a) the on-time and b) the off-time. Steady-state measurementsmade in 200 W DC magnetron conditions are also shown.

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probe sheath essentially fills the beam and all the local photoelectronsare collected. In the pulse off-phase, this optimum bias was +50 V.

3.1. Temporally and spatially-resolved photo-detachment measurements

Having obtained the optimum probe bias and with the standardoperating conditions, time-resolved photo-detachment and conven-tional Langmuir probe measurements were undertaken at differenttarget-to-probe distances.

Fig. 7a and b shows the measured background electron density neat the 10 chosen positions on the discharge axis for times during boththe on and off-phases of the pulse. For comparison we also plot theelectron and negative ion densities for a steady-state DC conditionpower of 200 W, 10% oxygen partial pressure and +35 V probe bias.Previous work has shown that a power of 200 Wwas chosen as it gavea maximum value of negative ions [9]. A high electron density (ne)combined with a high α gives an optimum negative ion density (n−).At 400 W in the DC case the displayed negative ion densities were 30%lower, but we did notmake a full spatial scan and hence the results arenot shown here. From Fig. 7a the electron density peaks at 65 mmcoinciding with the position of the magnetic null. The maximumelectron density is 2×1016 m−3. This type of spatial distribution in theplasma density has been seen elsewhere and is attributed to magneticmirroring of electrons in the null region [31]. In the off-phase theelectron becomes more spatially uniform with the density valuesdecreasing from the on-phase.

The spatial profiles of electron temperature (Te) in the differentphases of the pulse are not shown here (due to space limitations)however Te was found not to vary significantly with distance butvaries in time. However, it did reach a maximum during the on-phasewhen the probe was situated 75 mm from the target. During the off-phase it remained constant with distance. From 0 to 2 μs, Te climbsfrom about 2 to just under 3.4 eV. In the off-phase, Te wasmeasured tobe 2 eV dropping to 1.8 eV in the last 2.5 μs.

Fig. 6. A plot of the ratio of photo-detached electron current to background electroncurrent (ΔIpd/Ie) against probe bias at t=2.5 μs (on-phase) and t=7.5 μs respectively(off-phase) at 75 mm from the target. The required value α is determined at 35 V bias inthe on-phase and +50 V in the off-phase.

TheO−densityn− values (derivedusingEq. (3)) are shown in Fig. 8aand b and reveal a distribution that is highly dependent on position,with peak values seen at 75 mm from the target in both the on and off-phases. This is different from the electron density distribution. Thedistribution of bulk negative ions will be dependent both on the spatialdistribution of electrons (since electrons essentially create these ionsthrough dissociative electron attachment O2+e−→O−+O [32]) andthe potential structure of the plasma and a combination of these willdetermine where low energy negative ions reside in the downstreamplasma as shown in PIC-MMCmodelling of a reactivemagnetron systembyBultinck [33]. In our casewe do not know theplasma potential valuesat radial positions away from the line of measurement. If it is morepositive with increasing radial distance then this may account for thepeaks in negative and electron densities occurring at different positionson the centre line. The highest O− density was seen at 2.12 μs with avalueof 1×1016 m−3 and this valuedecreasesmarginally during theon-phases for most positions. At a position of 75 mm, the peak electron tonegative ion ratio (α) was 0.63. The O− density results in the off-phaseare consistently lower than in the on-phase by an amount that isdependent on position. For instance, for positions close to and far awayfrom the target, the negative ion density was found to fall by 60%,however, close to themagnetic null this fall was found to be 10%. Duringthe off-phase of the discharge, the negative ion density remains at analmost constant value at each measured position.

To enhance the data the background electron density (ne), O−

density (n−) and their ratio (α) are shown in contour plots, Fig. 9a–c.Plotting thedata thisway clearly reveals the peak electrondensity in theon-phase at 65 mm with a loss of this structure when the power is nolonger applied in the afterglow. The reduction in negative ion densitygoing from on- to off-phase can be seen in Fig. 9b, with retention of the

Fig. 8. A plot of the O− density against distance along the central axis for a number ofchosen times during a) the on-time and b) the off-time. Steady-state measurementsmade in 200 W DC magnetron conditions are also shown.

Fig. 9. Space–time plots of a) the background electron density (ne), b) the negative O−

ion density (n−) and c) the negative ion-to-electron density ratio (α).

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peak structure that is clearly lost in the background electron density ne(Fig. 9a). Fig. 9c shows the spatial–temporal distribution of α. Thehighest value is at 2.12 μs at 75 mm with a value of 0.63. The lowestmeasuredvalue of alphawas0.05 close to and far from the target. Due tothe voltage transients, in the phase between pulse on and pulse off-times it was not possible to acquire any electron and negative iondensity data. Therefore, we do not know if the negative ion densityincreases or decreases immediately after the pulse on-phase, and ourmethod of estimating the negative ions created at the target may beunder or over estimated. However, we expect the plasma parametersnot to change toomuch in the phase andwecan assign (froma variationin themeasureddata) an estimated error in the fast negative ion densityof ±40%. Steady-state DC measurements cannot distinguish fromwhere the negative ions originate. However, in the pulsedmodewe canget an estimate of the proportion of negative ions created in the bulkfrom those originating from the target. We do that by assuming thatonce thedischarge enters theoff-phase theproduction and sputteringofnegative ions at the cathode cease. Since these are fast (energies of theequivalent of the target voltage and fractions of that down to low valuesdue to dissociation en route [5,6]) they will exit the system b2 μs.Therefore, in themiddle of the off-phasewe assumeall the negative ionsare due to those created in thebulk. The difference then between theO−

densities at the end of the on-time and those from the end of the off-timewill give an indication of those fromthe target. These are calculatedand shown in Fig. 10. Despite the inherent error of this crude method(±40%calculated fromconsideration of thedifference in rate coefficientof dissociative electron attachment between 3.4 and 1.9 eV), we can saywith some confidence that themajority of ions are originating from theplasma. The inferred target-borne ions have a density that lies between0.6×1015 m−3 and 1×1015 m−3, weakly dependent on the positionalong the centre line. This is in marked contrast to the bulk created

negative ions and native electrons that both have a strong dependencyon position as seen in Figs. 7a,b and 8a,b. We propose that these ionsform an energetic beam along the centre line as observed and describedbyMahieu et al. [8]. This calculation is valid provided the production anddestruction rates of the bulk negative ions are the same during thesetimes. In the on-phase O− will be created largely through dissociative

Fig. 10. A plot of the negative ion densities (n−), at the end of on-phase t=5.03 μs andmiddle of off-phase t=7.64 μs and the differenceΔn− representing the inferred densityof negative ions originating from the cathode target.

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detachment of the O2 molecule O2+e−→O−+O [32], with a ratecoefficient that increases with electron temperature Te up toTe=5.19 eV. The main loss process will be by ion–ion recombination,O−+O2

+→O+O2, O−+O2+→3O and O−+O+→2O [34], with otherprocesses such as electron induced detachment, O−+e→O+2e,usually negligible. In the off-phase as Te falls (measured from ~3.4 to~2 eV at all chosen positions) the production rate through the reactionO2+e−→O−+O becomes negligible and we may expect the O−

density to fall. However, this is not the case, as can be seen clearlyin Fig. 9b, with O− reducing only marginally. The maximum changeis at positions furthest from the peak O− density with a 60% fall.In other pulsed plasmas at similar pressures it has been seen that theO− density may in fact increase dramatically in the afterglow, by forinstance a factor between 1.5 and 5 at times less than 5 μs into off-time with 0.63 Pa pressure [35]. This production of O− (in the coolingplasma) can be explained through the electron attachment reactione+O2

M→O− +O where O2M=(A3Σu

+,C3Δu,c1Σu−) are highly excited

metastable oxygen molecules. This reaction channel has a rate coef-ficient which increases with deceasing Te peaking at a value of about4×10–8 cm3s–1 at 0.5 eV [36]. The observation that the negative iondensities do not fall significantly in the off-time may imply thepresence of metastable oxygen molecules in the magnetron holdingup the O− density through the mechanism described above. A fulldetailed study of the negative ion chemistry to explain the observa-tions is beyond the scope of this paper.

Inspection of Fig. 10 shows that close to the target and at thefurthest extremity, negative ions originating from the target make upapproximately 60% of the total density, however approaching theposition of the peakO− density (i.e. 75 mm from the target), this dropsto b10%. In a previous work [9] we postulated that the energy flux ofnegative ions to the substrate can greatly exceed that from the positiveions, however, in that study, the fraction of fast negative ions from thecathode was unknown. Here, in the on-phase, at the peak value of α(0.63), the fast negative ions contribute about 10% to the total negativeion density. Using a method of calculation (and the assumed ionicenergies) discussed in Ref. [9], the effective value α from the fastnegative ions is ~0.63×0.1=0.063, giving a negative ion to positiveion energyflux ratio at the substrate of P−/P+~20. However, the actualratio at the substrate may be lower than this due to the divergenceof the negative ions (perhaps in the form of a beam), as they areaccelerated away from the race track area of the cathode. This highfigure for the energy flux ratio clearly has implications in the ion-assisted film growth processes, to be discussed in future works.

4. Conclusions

Using a Langmuir probe and laser photo-detachment technique, thedensity (n−) of O− ions in a low-pressure (1.33 Pa) pulsed magnetron

discharge, operating in argon–oxygenmixtures, has been determined atdifferent times during the pulse period and for different positions on thedischarge axis. In addition, the density of the background electrons ne,was also obtained under the same set of conditions.

It was found that n− and ne have peak values at 65 and 75 mm fromthe target respectively, both in the on and off-phases. During the on-phase, n− attained a value of 1×1016 m−3 shortly after initiation of thevoltage pulse and this remains almost constant throughout theremainder of the on-phase, whilst during the off-phase n− decaysgraduallywith time. Themaximummeasured ratio ofn− tonewas0.63at75 mm from the target at 2.12 μs into the pulse. By considering n− valuesboth at the end of the on-phase and mid-point of the off-phases, andusing a simple intuitive model we calculate that the density of negativeions originating from the cathode is approximately 1×1015 m−3 with adensity profile that is almost constant along the axis (constituting a non-diverging beam). The results indicate that most of the negative ions arecreated in the bulk plasma (close to the position of the null in themagnetic field observed on the discharge axis) through the most likelyroute of dissociative electron attachment.

Acknowledgment

The authors would like to that the EPSRC for funding the project.

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