modeling the current waveform in reactive high power...

Modeling the current waveform in reactive high power impulse magnetron sputtering

Modeling the current waveform in reactive highpower impulse magnetron sputtering

Jón Tómas Guðmundsson1,2,3, Daniel Lundin3,Michael A. Raadu1, C. Huo1, Nils Brenning1

and Tiberu M. Minea3

1Department of Space and Plasma Physics, School of Electrical Engineering,KTH – Royal Institute of Technology, Stockholm, Sweden

2 Science Institute, University of Iceland, Reykjavik, Iceland3 Laboratoire de Physique des Gaz et Plasmas - LPGP, UMR 8578 CNRS,

Université Paris-Sud, 91405 Orsay Cedex, [email protected]

Ruhr-Universität Bochum,Bochum, Germany

September 29., 2016


Introduction

Reactive sputtering, where metal targetsare sputtered in a reactive gasatmosphere to deposit compoundmaterials is of utmost importance invarious technologiesIn reactive sputtering processes areactive gas O2, N2, or CH4 etc. is mixedto the noble working gas for oxide, nitride,or carbide depositionHiPIMS deposition generally givesdenser, smoother films og highercrystallinity than dcMS grown films

Helmersson et al. (2006) Thin Solid Films 513 1

Magnus et al. (2012) IEEE EDL 33 1045


HiPIMS - Voltage - Current - Timecharacteristics


HiPIMS - Voltage - Current - time

In non-reactive discharge thecurrent waveform shows aninitial pressure dependent peakthat is followed by a secondphase that is power and materialdependentThe initial phase is dominated bygas ions, whereas the laterphase has a strong contributionfrom self-sputteringThe non-reactive case is wellunderstood

From Gudmundsson et al. (2012), JVSTA 30 030801



Ar discharge with Ti targetThe initial peak in current resultslarge flux of atoms from the targetCollisions of the sputtered atomswith the working gas result inheating and expansion of theworking gas – rarefactionA significant fraction of thesputtered atoms experienceelectron impact ionization (theionization mean free path ∼ 1 cm)and are attracted back to the targetto participate in the sputteringprocess – self-sputtering

From Magnus et al. (2011) JAP 110 083306



During reactive sputtering, areactive gas is added to the inertworking gasThe current waveform in thereactive Ar/N2 HiPIMS dischargewith Ti target is highly dependenton the pulse repetition frequencyN2 addition changes the plasmacomposition and the targetcondition can also change due tothe formation of a compound on itssurface

0 100 200 300 400

0

5

10

15

20

25

30

t [µs]

Id [A]

80 Hz

60 Hz

30 Hz

10 Hz

(b)

After Magnus et al. (2011) JAP 110 083306



Similarly for the Ar/O2discharge, the currentwaveform is highly dependenton the repetition frequency andapplied voltage which is linkedto oxide formation on the targetThe current is found toincrease significantly as thefrequency is lowered

0 100 200 300 400−20

−10

0

10

20

30

40

50

60

70

80

t [µs]

Id [A]

50 Hz

30 Hz

20 Hz

18 Hz

15 Hz (a)

After Magnus et al. (2012), JVSTA 30 050601



As the oxygen flow isincreased a transitionto oxide mode isobserved

0 100 200 300 400

0

5

10

15

20

25

30

t [µs]

Id [A]

0 sccm O

2

2 sccm O2

5.6 sccm O2

The current waveforms for an Ar/O2 discharge with a Ti target where the

oxygen flow rate is varied – 600 V, 50 Hz and 0.6 Pa

From Gudmundsson et al. (2013), ISSP 2013, p. 192

Gudmundsson (2016) Plasma Phys. Contr. Fusion 58 014002



Similar behaviour has been reported for various target andreactive gas combinations

The current increases with decreased repetition frequencyThe current waveform maintains its shape for Ar/O2discharge with Nb targetFrom Hála et al. (2012), JPD 45 055204

The current waveform becomes distinctly triangular forAr/N2 discharge with Hf targetFrom Shimizu et al. (2016), JPD 49 065202



The current increases withincreased partial pressure of thereactive gas

The current waveformbecomes distinctly triangularfor Ar/N2 discharge with AltargetFrom Moreira et al. (2015), JVSTA 33 021518

The current waveformmaintains its shape for Ar/O2discharge with Nb targetFrom Hála et al. (2012), JPD 45 055204



At high frequencies, nitride or oxideis not able to form between pulses,and self-sputtering by Ti+-ions(singly and multiply charged) from aTi target is the dominant processγsee is practically zero for singlycharged metal ions impacting atarget of the same metalAt low frequency, the long off-timeresults in a nitride or oxide layerbeing formed on the target surfaceand self-sputtering by Ti+- andN+-ions or O+-ions from TiN or TiO2takes place

0 100 200 300 400−20

−10

0

10

20

30

40

50

60

70

80

t [µs]

Id [A]

50 Hz

30 Hz

20 Hz

18 Hz

15 Hz (a)

0 100 200 300 400

0

5

10

15

20

25

30

t [µs]

Id [A]

80 Hz

60 Hz

30 Hz

10 Hz

(b)

From Magnus et al. (2011), JAP 110 083306

and Magnus et al. (2012), JVSTA 30 050601

Gudmundsson (2016) PPCF 58 014002


Ionization region model studies of reactiveHiPIMS


Ionization region model studies of reactive HiPIMS

The ionization region model(IRM) was developed to improvethe understanding of the plasmabehaviour during a HiPIMSpulse and the afterglowThe main feature of the model isthat an ionization region (IR) isdefined next to the race trackThe IR is defined as an annularcylinder with outer radii rc2,inner radii rc1 and lengthL = z2 − z1, extends from z1 toz2 axially away from the target

The definition of the volume covered by the IRM

From Raadu et al. (2011), PSST 20 065007



The species assumed in the IRM are

electronsargon atoms Ar(3s23p6), warm argon atoms in the groundstate ArW, hot argon atoms in the ground state ArH, Arm

(1s5 and 1s3) (11.6 eV), argon ions Ar+ (15.76 eV)titanium atoms Ti(a 3F), titanium ions Ti+ (6.83 eV), doublyionized titanium ions Ti2+ (13.58 eV)oxygen molecule in the ground state O2(X3Σ−

g ), themetastable oxygen molecules O2(a1∆g) (0.98 eV) andO2(b1Σg) (1.627 eV), the oxygen atom in the ground stateO(3P), the metastable oxygen atom O(1D) (1.96 eV), thepositive ions O+

2 (12.61 eV) and O+ (13.62 eV), and thenegative ion O−

Toneli et al. (2015) J. Phys. D. 48 325202



The particle balance equation for a particular species isgiven by

dnspecies

dt=∑

Rspecies,process

The reaction rate Rj,volume for a given volume reaction of aspecies j is

Rj,volume = k ×∏

i

nr,i [m−3s−1]

The rate at which species are sputtered off the target (Tiand O) is

Rn,sputt =

∑i ΓRT

i SRTYi(E)

VIR

and i stands for the ion involved in the process, here i =Ti+, Ti2+, Ar+, O+, and O+

2 , ΓRTi is the flux of ion i towards

the target



For each ion there is a loss rate given as

Ri,loss = −ΓBP

i SBP + ΓRTi SRT

VIR

where i stands for the particular ion and SBP is the area ofthe annular cylinder facing the lower density plasmaoutside the IR (bulk plasma), and ΓBP

i is the flux of ion iacross the boundary towards the lower density plasmaThus the flux out of the IR towards the lower densityplasma is reduced as required to obtain the assumed ionback-attraction probability β

ΓBPi =

(1β− 1)

SIR

SBPΓRT

i



Gas rarefaction lowers the density of the process gasinside the IR below the value in the surrounding gasreservoir, ng,0

This gives a back-diffusion (gain) term

Rg,refill =12

vran(ng,0 − ng)SRT

VIR

where the subscript g stands for the atoms and moleculesof the working gas Ar and O2



The return flux of recombined positive argon ions Ar+ fromthe target is treated as two groups of atoms with differenttemperatures

a hot component ArH that returns from the target with atypical sputter energy of a few electron voltsa warm component ArW that is assumed to be embedded inthe target at the ion impact, and then return to the surfaceand finally leave with the target temperature, at most 0.1 eV

Thus there is a loss out of the IR

RArZ,loss = −vrannArZSRT

VIR

where Z stands for H for hot or W for warm argon atoms



In addition there is a change in the neutral density due tokick-out by collisions with fast sputtered atoms comingfrom the targetFor each of the neutrals Y of the working gas, includingeach of the metastable states, the particle balanceincludes a loss term

Rn,kick−out = −12

vran,X

LFcoll

mX

mY

∑nX ,i∑

i nY ,inY

The sum is taken over all the states of that sputteredspecies and

Fcoll = 1 − exp(−L/λX ,gas

)is the probability of a collision inside the IR, where λX ,gas isthe mean free path for an atom X sputtered off the target



The sputter yield for the variousbombarding ions was calculatedby TRIDYN for

Metal mode – Ti targetPoisoned mode – TiO2 target

The yields correspond to theextreme cases of either clean Tisurface and a surface completelyoxidized (TiO2 surface)The sputter yield is much lowerfor poisoned target

The sputter yield data is from Tomas Kubart, Uppsala University

0 500 1000 1500 20000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Sput

ter y

ield

[ato

ms/

ion]

Energy [eV]

Ar+ / Ti yield O+ / Ti yield Ti+ / Ti yield

(a)

0 500 1000 1500 20000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

(b)

Sput

ter y

ield

[ato

ms/

ion]

Energy [eV]

Ar+ / Ti yield Ar+ / O yield O+ / Ti yield O+ / O yield Ti+ / Ti yield Ti+ / O yield



The model is applied to exploreAr/O2 discharge with Ti target inboth metal mode and oxide(poisoned) modeThe IRM is a semi-empiricalmodel in the sense that it uses ameasured discharge currentwaveform as a main inputparameterFor this study we use themeasured curve for Ar/O2 with Titarget at 50 Hz for metal modeand at 15 Hz for poisoned mode

0 100 200 300 400−20

−10

0

10

20

30

40

50

60

70

80

t [µs]

Id [A]

50 Hz

30 Hz

20 Hz

18 Hz

15 Hz (a)

After Magnus et al. (2012), JVSTA 30 050601



The gas rarefaction isobserved for the argonatoms but is moresignificant for the O2moleculeThe density of Ti atoms ishigher than the O2 densityThe atomic oxygendensity of is over oneorder of magnitude lowerthan the molecular oxygendensity – the dissociationfraction is low

0 100 200 300 400 5001014

1015

1016

1017

1018

1019

1020

1021

Ar ArW

ArH

Ti O2

O

Parti

cle

dens

ity [m

-3]

t [ s]

(a)

The temporal evolution of the neutral species with 5 %

oxygen partial flow rate for Ar/O2 discharge with Ti

target in metal mode.

Gudmundsson et al. (2016), PSST, accepted 2016



Gas rarefaction isobserved for both argonatoms and O2 moleculesThe density of Ti atoms islower than both the O2density and atomicoxygen densityThe atomic oxygendensity is higher than theO2 density towards theend of the pulse

0 100 200 300 400 5001014

1015

1016

1017

1018

1019

1020

1021

Ar ArW

ArH

Ti O2

O

Parti

cle

dens

ity [m

-3]

t [ s]

(b)



target in poisoned mode.




Ar+ and Ti+-ionsdominate the dischargeTi2+-ions follow by roughlyan order of magnitudelower densityThe O+

2 and O+-iondensity is much lower 0 100 200 300 400 500

1014

1015

1016

1017

1018

1019

1020

1021

(a)

Ar+ Ti+ Ti2+

O2+ O+ O-

Parti

cle

dens

ity [m

-3]

t [ s]







Ar+-ions dominate thedischargeTi+, O+, have very similardensity, but the temporalvariation is different, andthe O+

2 density is slightlylowerThe Ti2+-ion densityincreases fast with timeand overcomes the Ti+

density towards the end ofthe pulse

0 100 200 300 400 5001014

1015

1016

1017

1018

1019

1020

1021

(b)

Ar+ Ti+ Ti2+

O2+ O+ O-

Parti

cle

dens

ity [m

-3]

t [ s]







Ar+ and Ti+-ionscontribute mostsignificantly to thedischarge current

0 100 200 300 400 500

0

10

20

30

40

50

60

70

80

(a)

Dis

char

ge c

urre

nt c

ontri

butio

ns [A

]

t [ s]

ID total

ID Ar+

ID Ti+

ID Ti2+

ID O2+

ID O+

ID se







Ar+ contribute mostsignificantly to thedischarge current – almostsolelyThe contribution ofsecondary electronemission is very small 0 100 200 300 400 500

0

10

20

30

40

50

60

70

80(b)

Dis

char

ge c

urre

nt c

ontri

butio

ns [A

]

t [ s]

ID total

ID Ar+

ID Ti+

ID Ti2+

ID O2+

ID O+

ID se







Recycling of ionizedatoms coming from thetarget are required for thecurrent generation in bothmodes of operationIn the metal modeself-sputter recyclingdominates and in thepoisoned mode workinggas recycling dominatesThe dominating type ofrecycling determines thedischarge currentwaveform

0 100 200 300 400 5001021

1022

1023

1024

1025

(a)

Arm ioniz. eC Arm ioniz. eH ArH ioniz. eC

ArH ioniz. eH ArW ioniz. eC ArW ioniz. eH

Ar ioniz. eC Ar ioniz. eH

Rea

ctio

n ra

tes

[m-3 s

-1]

t [ s]

The temporal variations of the reaction rates for electron

impact ionization of the argon atoms (ground state plus

metastable) in poisoned mode.




In the metal mode sheath energization was found to beonly 10 %

same range as the results reported earlier for an Al target

For the poisoned mode the sheath energization was 30 %,with a rising trend, at the end of the pulseThis is due to the secondary electron emission

In the poisoned mode essentially all the ions (mainly Ar+,but also O+ and Ti2+ towards the end of the pulse)contribute to the secondary electron emissionIn the metal mode only half of the ions contribute to thesecondary electron emission (Ar+ ) while the other halfdoes not contribute at all (γTi+ = 0.0)


Summary


Summary

An ionization region model was used to explore the plasmacomposition during the high power pulseComparison was made between the metal mode and thepoisoned mode

In metal mode Ar+ and Ti+-ions dominate the dischargeand are of the same order of magnitudeIn poisoned mode Ar+-ions dominate the discharge andtwo orders of magnitude lower, Ti+, O+, have very similardensity, with the O+

2 density slightly lowerIn the metal mode Ar+ and Ti+-ions contribute mostsignificantly to the discharge current while in poisonedmode Ar+ dominate

In the metal mode self-sputter recycling dominates and inthe poisoned mode working gas recycling dominates – thedominating type of recycling determines the dischargecurrent waveform


The slides can be downloaded athttp://langmuir.raunvis.hi.is/∼tumi/ranns.html

The experimental work was made in collaboration with

Dr. Fridrik Magnus, Uppsala University, Uppsala, SwedenTryggvi K. Tryggvason, University of Iceland

We got help with the sputtering yields from

Dr. Tomas Kubart, Uppsala University, Uppsala, Sweden

and the project is funded byIcelandic Research Fund Grant No. 130029-053Swedish Government Agency for Innovation Systems(VINNOVA) contract no. 2014-04876,


References

Aiempanakit, M., A. Aijaz, D. Lundin, U. Helmersson, and T. Kubart (2013). Understanding the discharge currentbehavior in reactive high power impulse magnetron sputtering of oxides. Journal of Applied Physics 113(13),133302.

Gudmundsson, J. T. (2016). On reactive high power impulse magnetron sputtering. Plasma Physics and ControlledFusion 58(1), 014002.

Gudmundsson, J. T., N. Brenning, D. Lundin, and U. Helmersson (2012). The high power impulse magnetronsputtering discharge. Journal of Vacuum Science and Technology A 30(3), 030801.

Gudmundsson, J. T., F. Magnus, T. K. Tryggvason, S. Shayestehaminzadeh, O. B. Sveinsson, and S. Olafsson(2013). Reactive high power impulse magnetron sputtering. In Proceedings of the XII International Symposiumon Sputtering and Plasma Processes (ISSP 2013), pp. 192–194.

Gudmundsson, J. T., D. Lundin, N. Brenning, M. A. Raadu, C. Huo, and T. M. Minea (accepted 2016). An ionizationregion model of the reactive Ar/O2 high power impulse magnetron sputtering discharge. Plasma SourcesScience and Technology .

Hála, M., J. Capek, O. Zabeida, J. E. Klemberg-Sapieha, and L. Martinu (2012). Hysteresis - free deposition ofniobium oxide films by HiPIMS using different pulse management strategies. Journal of Physics D: AppliedPhysics 45(5), 055204.


References

Helmersson, U., M. Lattemann, J. Bohlmark, A. P. Ehiasarian, and J. T. Gudmundsson (2006). Ionized physicalvapor deposition (IPVD): A review of technology and applications. Thin Solid Films 513(1-2), 1–24.

Magnus, F., A. S. Ingason, S. Olafsson, and J. T. Gudmundsson (2012). Nucleation and resistivity of ultrathin TiNfilms grown by high power impulse magnetron sputtering. IEEE Electron Device Letters 33(7), 1045 – 1047.

Magnus, F., O. B. Sveinsson, S. Olafsson, and J. T. Gudmundsson (2011). Current-voltage-time characteristics ofthe reactive Ar/N2 high power impulse magnetron sputtering discharge. Journal of Applied Physics 110(8),083306.

Magnus, F., T. K. Tryggvason, S. Olafsson, and J. T. Gudmundsson (2012). Current-voltage-time characteristics ofthe reactive Ar/O2 high power impulse magnetron sputtering discharge. Journal of Vacuum Science andTechnology A 30(5), 050601.

Moreira, M. A., T. Törndahl, I. Katardjiev, and T. Kubart (2015). Deposition of highly textured AlN thin films byreactive high power impulse magnetron sputtering. Journal of Vacuum Science and Technology A 33(2),021518.

Raadu, M. A., I. Axnäs, J. T. Gudmundsson, C. Huo, and N. Brenning (2011). An ionization region model for highpower impulse magnetron sputtering discharges. Plasma Sources Science and Technology 20(6), 065007.

Shimizu, T., M. Villamayor, D. Lundin, and U. Helmersson (2015). Process stabilization by peak current regulation inreactive high-power impulse magnetron sputtering of hafnium nitride. Journal of Physics D: AppliedPhysics 49(6), 065202.

Toneli, D. A., R. S. Pessoa, M. Roberto, and J. T. Gudmundsson (2015). On the formation and annihilation of thesinglet molecular metastables in an oxygen discharge. Journal of Physics D: Applied Physics 48(32), 325202.

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