High Temporal Resolution Ion Energy Distribution Functions in HIPIMS Discharges

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High Temporal Resolution Ion EnergyDistribution Functions in HIPIMS Discharges

Anurag Mishra, Gregory Clarke, Peter Kelly, James W. Bradley*

A technique for obtaining high time resolution ion energy distribution functions (IEDFs) at thesubstrate in depositing plasma has been demonstrated, and applied to a high power impulsemagnetron sputtering (HIPIMS) discharge. Key to this technique is the electrostatic gating ofions inside the instrument end cap. To demonstrate the performance of this technique, IEDFmeasurements with a 2 ms time-resolution have been made with the following HIPIMSoperating conditions: a repetition rate of 100 Hz, a pulse width of 100 ms, a pressure of0.26 Pa and a peak power density of 2.5 kW cm�2. The orifice of the mass spectrometer waspositioned facing the racetrack region of the circular magnetron cathode. The Arþ ions weredetected 8ms after initiation of the discharge voltage pulse, exhibiting a narrow distribution ofenergies, while Tiþ ions were detected 14 ms after the initiation, showing a high-energy tailextending up to 100 eV. The time-evolution of Tiþ ions show that the metal flux starts to bebuilt up at the substrate position at times 20 ms after the pulse initiation.

Introduction

High power impulse magnetron sputtering (HIPIMS) is a

newly developed IPVD (ionized physical vapour deposi-

tion) technique,[1] characterized by very high peak power

density (typically in the range of 500–3 000 W � cm�2),

pulse frequencies from 1 to 1 000 Hz, pulse widths from 50

to 500 ms, and low duty factors (<5%). Such operational

window prevents target heating[2] and yields very high

fractions of ionized metal flux.[1–3] It utilizes a short

intense impulse of very high peak power to produce high

plasma density in the vicinity of the target. The presence of

this dense plasma enhances the degree of ionization of the

sputtered metal flux up to 99%, depending on the target

material.[4]

A. Mishra, G. Clarke, James W. BradleyDepartment of Electrical Engineering and Electronics, BrownlowHill, University of Liverpool, Liverpool, UKE-mail: [email protected]. KellySurface Engineering Group, Dalton Research Institute,Manchester Metropolitan University, Manchester, UK

Plasma Process. Polym. 2009, 6, S610–S614

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The achievement of those high fractions of ionized

metal flux allows the charged particles constituting metal

flux in HiPIMS discharges to be controlled by externally

applied electric and magnetic fields, making the technique

therefore useful for high-aspect-ratio trench filling in the

semiconductor industry.[1] Surface pre-treatment in

HIPIMS environments provides also ultra clean sub-

strate–coating interfaces due to the dense and droplet-

free ionized metal flux, improving by this way the

adhesion of the coatings.[5] Another application includes

the pre-treatment of the substrates before the deposition

of CrN films, which results in higher corrosion and wear

resistance of the films.[6]

HIPIMS discharges pass through several distinct phases

during the cathode voltage pulse evolution. Time-resolved

optical emission spectroscopic studies (OES) reveal a

transition from a gas dominated phase to metal phase

plasma.[7] Due to this transient nature of the discharge it is

therefore necessary to characterize the plasma in time-

resolved mode, if a detailed study of physical processes is

desired.

A major drawback of HiPIMS is the lower deposition rate

observed in comparison to conventional dc magnetron

sputtering at the same nominal average power. Christie[8]

DOI: 10.1002/ppap.200931601

High Temporal Resolution Ion Energy Distribution Functions . . .

Figure 1. A schematic diagram of the experimental setup with themagnetron and the mass spectrometer.

developed a phenomenal model for the HiPIMS discharge

and suggested that attraction of metal ions back to the

target might be a possible cause of the lower deposition

rates.[8] Konstantinidis et al.[9] demonstrated the impor-

tance of plasma conductivity and Bohlmark et al.[3]

showed that the magnetic field arrangement may be

responsible for the lower deposition rate. Recently,

Emmerlich et al.[10] pointed out that the nonlinear

sputtering yield may also contribute to the low deposition

rate.

The ion energy distribution function (IEDF) at the

substrate is a crucial discharge parameter that determines

the properties of the deposited thin films. Bohlmark

et al.[11] carried out time-resolved IEDFs measurements

and found that in the initial phase of the discharge, Tiþ

ions featured a high-energy tail of the form of the

Sigmund–Thomson sputter distribution. A number of

works have reported that two waves of plasma arrive at

the substrate,[2,9] namely a noble gas rich plasma followed

by metal-rich plasma. Further time-resolved IEDFs mea-

surements have been reported,[7] demonstrating similar

time evolution of IEDFs in reactive and non-reactive

ambient during HIPIMS operation. Vlcek et al.[6] performed

time-average measurements and found that metal ions are

strongly dominant (up to 92%) in the total ion flux onto the

substrate and the energy distribution is pressure depen-

dent with a broadened low energy peak at low pressure.[6]

Ehiasarian et al.[7] demonstrated that metal ion flux in

HiPIMS was five times larger than in conventional

sputtering techniques. Hecimovic et al.[12] realized time-

averaged IEDF measurements at various discharge cur-

rents and pressures and demonstrated that the high

energy tail of metal ions increases with discharge current.

They also demonstrated that at high pressure the IEDFs of

the sputtering gas show a single peak while at lower

pressure it shows a bi-Maxwellian distribution. Burcalova

et al.[13] showed that the efficiency of magnetron

sputtering process and transport of the sputtered particle

to the substrate decreases with increasing average target

power loading. Recently, Andersson et al.[14] carried out

experiments with a titanium target at peak current

density 3.3 A � cm�2, repetition rate 100 Hz, and pulse

width 150 ms. They found Ti4þ ions in HIPIMS discharge. It

was concluded that these multi-charged ions are formed

under a certain condition, i.e. sufficiently long discharge

pulses to allow a runaway process to be developed. Most of

these measurements are time-averaged and thus provide a

static picture of the discharges used.

Few publications on time-resolved measurements of

IEDFs, carried out up to date, have very low resolution (two

or three different times during the discharge pulse) and,

consequently, do not provide a full picture of the IEDF

evolution. The present work is original in the following

sense: it demonstrates a newly developed gating techni-



que in order to achieve IEDF measurements in HIPIMS

discharges with time resolution better than 2 ms over the

entire cathode voltage pulse period.

Experimental Part

All the experiments were carried out in a purpose built stainless

steel vacuum chamber (provided by Gencoa Ltd, UK) of 60 cm in

length and 40 cm in diameter, pumped by rotary and turbo

molecular pump to give a base pressure of 2� 10�4 Pa. It was

equipped with a V-TECH 150 planar circular magnetron having

titanium cathode target (99.995% purity and diameter 150 mm)

and was driven by a SINEX 3.0 (Chemfilt IonSputtering AB2)

HIPIMS power supply at an average power of 680 W. The

repetition frequency was 100 Hz with pulse width of 100 ms. The

experiments were performed in non-reactive mode at argon gas

pressure of 0.26 Pa and flow rate of 30 sccm, regulated by a

Baratron feedback (MKS 627) and a mass flow controller (MKS

247), respectively.

Voltage and current waveforms were recorded using a 100:1

voltage probe (P5100 Tektronix) and a 20:1 current probe (TCP04

Tektronix) in conjunction with a 10:1 current probe (TCP202

Tektronix), respectively, all attached between the power supply

and the magnetron.

An electrostatic ion shuttering technique in conjunction with

HIDEN EQP 300 [Electrostatic Quadrupole Plasma (EQP) analyser]

energy-resolved mass spectrometer[6] was used for time-resolved

IEDFs measurements. The spectrometer was equipped with 458energy sector and SEM (secondary electron multiplier) to count the

ions. It provides energy resolution up to 0.1 eV but in the present

experiments a 0.5 eV energy resolution was chosen due to the long

acquisition times (�3 min) in a single scan. Before choosing this

energy resolution setting, the IEDFs were scanned at 0.1 and 0.5 eV

energy resolutions and no significant changes, in counts or in the

shape of IEDFs, were observed. The mass spectrometer barrel was

located opposite the racetrack at a distance of 10 cm (see Figure 1).

This orientation provides direct line-of-sight between the instru-

ment and surface of the cathode, increasing the probability for the

sputtered target material to enter the instrument without

undergoing any collisions. The plasma ions were sampled through

www.plasma-polymers.org S611

A. Mishra, G. Clarke, P. Kelly, J. W. Bradley

Figure 2. The schematic diagram of the electrostatic ion shutter-ing technique.

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100 mm in diameter orifice drilled on the grounded end cap of the

instrument.

The electrostatic shutter[15] was made from a stainless steel

mesh having 57 wires/inch and a 45% geometrical transparency. It

was situated 0.5 mm behind the orifice (see Figure 2). An

electrostatic pulse, produced by an external pulse generator, was

biasing the mesh via a feedthrough. Both the grid and the detector

were gated by an external delay generator at a prescribed time

delay. The time t¼0 position was defined as the zero cutting edge

of the target voltage pulse at the off-to-on transition.

During ion counting, the grid was biased atþ150 V to repel all the

positive ions. During the ion acquisition, the grid voltage was

ramped and maintained at �22 V for a duration �tg (the time

resolution window), allowing the ions to reach the detector via

spectrometer’s ion optics (see Figure 3). It was experimentally found

that the best setting of grid and extractor biases were �22 and �25

V, respectively, to achieve the highest count rates. The fall and rise

times of the gate pulse were 40 and 70 ns, respectively, and these

were considerably shorter than the shuttering window width used

in these experiments (i.e. 2 ms).

At a time of 2 ms after the end of the mesh gating pulse, the

detector enabling pulse was sent by the EQP signal board allowing

the digital ion counting pulses to be counted within this pulse, as

Figure 3 shows. The detector gating pulse was chosen to be 250 ms

so that the heaviest (300 amu) and the least energetic (1 eV) ions,

Figure 3. The electrostatic ion shuttering and detector pulse waveforms.



which need 200 ms to reach the detector (SEM) starting from the

orifice of the instrument, could readily be detected during the

pulse period. The time lag (2 ms) between the end of the mesh

gating and the beginning of the detector gating pulse prevents any

false ion detection at the SEM due to electromagnetic interference

generated by large rapid change of the grid biasing (from �22 to

150 V in about 20 ns).

Experimental Results and Discussion

As mentioned in Experimental Set-Up Section, all time-

resolved measurements of the IEDFs were performed at an

argon pressure of 0.26 Pa, a repetition rate of 100 Hz, a

pulse width of 100 ms and an average power of 680 W. The

peak current and power density were 2.6 A � cm�2 and 2.5

kW � cm�2, respectively.

The cathode voltage pulse Vd and current pulse Id are

shown in Figure 4. Two phases of the voltage pulse can be

clearly identified. These are a transient on-phase and a

stable off-phase. Average power was calculated as follows:

Pav ¼ ð1=tÞZ

IdðtÞVdðtÞdtð1Þ ð1Þ

where I(t) is target current and V(t) target voltage as

functions of time and t is the pulse period.

Before performing the detailed experiments, a pre-

liminary experiment was carried out to assess the

performance of the electrostatic shuttering technique.

For this, a summation of time-resolved IEDFs at different

times during the pulse was compared to a time-averaged

IEDF. The plot is shown in Figure 5. Both of these IEDFs

have been recorded in different experiments and with

different tuning parameters of the mass spectrometer. The

time-averaged IEDF was recorded without mesh in the

instrument. As can be seen from plot, the IEDFs match well

except that the sum of time-resolved IEDFs has a hump at

105 eV and lower number of counts. This hump is artefact

produced by the biased mesh when there is fast transition

from �22 to 150 V, at the end of gating time window. To

confirm that this hump is indeed an artefact produced by

the mesh, more experiments were carried out with

Figure 4. The discharge voltage and current waveforms.

DOI: 10.1002/ppap.200931601

High Temporal Resolution Ion Energy Distribution Functions . . .

Figure 5. The plot showing the comparison of summation of time-resolved IEDFs and time-averaged IEDFs of Arþ.

Figure 7. The plot of time resolved IEDFs of Tiþ with 2 ms timeresolution. The hump at þ105 eV is an artefact of the bias meshshutter technique.

different shuttering voltages on mesh and it was found

that the position of hump follows the mesh biasing

voltage. The lower number of counts in the peak in the

time-resolved IEDF and the slight mismatch in both IEDFs

can be attributed to different tuning parameters of mass

spectrometer as both of these scans were recorded during

different experiments. This test gave us the confidence

that the used gating technique was performing well.

The time resolved IEDFs of Arþ and Tiþ, obtained at

different parts of pulse cycle are shown in Figure 6 and 7,

respectively. These measurements were recorded after a

prescribed time delay from the initiation of the cathode

voltage pulse, with respect to ground potential (end cap of

mass spectrometer was on ground potential). The time

resolution was 2 ms and the measurements were recorded

between 0 and 30 ms. The first appearance of Arþ was

observed 8 ms after the initiation of the pulse.

As can be seen in Figure 6, Arþ ions have a distribution

extending up to 20 eV. Further into the pulse, the energy

Figure 6. The plot of time resolved IEDFs of Arþ with 2 ms timeresolution. The hump at þ105 eV is an artefact of the bias meshshutter technique.



distribution of Arþ ions spreads up to 35 eV, due to possibly

the transfer of energy and momentum to them by Tiþ via

collisions. Similar energy spreads in the Arþ IEDF have

been reported by Bohlmark et al.[11] for similar pressures.

The Tiþ ions in Figure 7 appear 14 ms after the initiation

of the cathode voltage pulse, some 6 ms after the arrival of

Arþ at the substrate. It shows clearly that there are two

distinct phases of plasma—gas rich plasma followed by

metal rich plasma in agreement with other publica-

tions.[2,9,16] The highest counts in the energy distribution

of Arþ are seen at 3 eV and the energies spread up to 20 eV.

Similarly the maximum in the Tiþ ions counts in the

energy distribution at 22 eV however these ions extend up

to 100 eV. Bohlmark et al.[11] have also reported the similar

results. The observed energy of Tiþ ions can be considered

to come from two components, firstly due to plasma

potential Vp at which they were created and secondly from

the sputtering kinetic energy of Ti atoms before ionization.

The origin of higher energy peak of Tiþ is attributed to

sputtering energy of metal atoms and can be explained as

follows: Metal ions are sputtered from the target by the

impact of argon gas ions and then they diffuse away in

the plasma and get ionized in dense plasma region due to

the collisions with electrons, trapped in strong magnetic

field of magnetron. These ions therefore have typically

the energy same as the sputtered atoms, given by the

Sigmund–Thompson distribution

FTð"Þ �Eb

ð"þ EbÞ3 ð2Þ

where Eb is the binding energy of metal atoms of target

and " is the sputtered atom energy.

Conclusion

A technique for obtaining high time-resolution energy

distribution function measurements in a HiPIMS discharge

using the commercial energy resolved mass spectrometer

www.plasma-polymers.org S613

A. Mishra, G. Clarke, P. Kelly, J. W. Bradley

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(HIDEN Ltd, EQP 300) has been demonstrated successfully.

The key to this technique is the electrostatic gating of

the ions inside the instrument’s end cap. To demonstrate

the performance of this technique, the time-resolved

IEDFs measurement with 2 ms time resolution were

carried in the HIPIMS discharge pulsed at frequency

of 100 Hz, with a pulse width of 100 ms and average power

of 680 W.

The excellent agreement between the summation of all

the time-resolved IEDFs measurements at different times

during the pulse and a time-averaged measurement

recorded by the mass spectrometer demonstrated that

this technique is quite suitable for very high time-resolved

measurements of IEDFs in the HiPIMS discharge.

It was found that Arþ energy distribution has its

maximum energy at 3 eV (corresponding to plasma

potential) and extends up to 20 eV at 2 ms after the

initiation of the discharge pulse. As the pulse develops, this

energy spread extends to 35 eV possibly due to the energy

transfer from Tiþ via binary collisions.

The IEDFs of Tiþ ions has its maximum counts at 22 eV,

due to the fact that energy of Tiþ ions is largely determined

by the energy of sputtered atoms as described the

Sigmund–Thomson energy distribution function. The

IEDFs of Tiþ spread up to 50 eV at 2 ms after the initiation

of cathode voltage pulse and further into the pulse, ions are

detected with energies extending up to 100 eV.

Acknowledgements: Authors are gratefully acknowledged EPSRCfor their funding support.

Received: September 12, 2008; Accepted: February 11, 2009; DOI:10.1002/ppap.200931601



Keywords: HIPIMS; ion energy distribution function; massspectrometry; pulsed-magnetron; time resolved

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High Temporal Resolution Ion Energy Distribution Functions in HIPIMS Discharges