reactive high power impulse magnetron sputtering...

Reactive High Power Impulse Magnetron Sputtering (HiPIMS)

Reactive High Power Impulse MagnetronSputtering (HiPIMS)

Jón Tómas Guðmundsson1,2, Friðrik Magnus2,Tryggvi K. Tryggvason2, Ólafur B. Sveinsson2,

S. Shayestehaminzadeh2, and Sveinn Ólafsson2

1University of Michigan – Shanghai Jiao Tong University Joint Institute,Shanghai Jiao Tong University, Shanghai, China

and2 Science Institute, University of Iceland, Iceland

[email protected]

65th Gaseous Electronics ConferenceAustin, Texas

October 26., 2012


Introduction

Magnetron sputtering has been the workhorse of plasmabased sputtering methods for over three decadesFor many applications a high degree of ionization of thesputtered vapor is desiredIn a HiPIMS discharge a high power pulse is supplied for ashort period

low frequencylow duty cyclelow average power

Ionized flux of sputtered vapor introduces an additionalcontrol parameter into the deposition process


Introduction

High power pulsed magnetronsputtering (HPPMS)HiPIMS

a pulse of very highamplitude, an impulse, isapplied to the cathode and along pause exists betweenthe pulses

Modulated pulse power (MPP)the initial stages of the pulse(few hundred µs) the powerlevel is moderate (typical fora dcMS) followed by a highpower pulse (few hundred µsup to a ms)

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

Power density limitspt = 0.05 kW/cm2 dcMS limitpt = 0.5 kW/cm2 HiPIMS limit


Reactive HiPIMS - Applications


Application – Film Resistivity

TiN as diffusion barriers forinterconnectsHiPIMS deposited films havesignificantly lower resistivity thandcMS deposited films on SiO2 atall growth temperatures due toreduced grain boundaryscatteringThus, ultrathin continuous TiNfilms with superior electricalcharacteristics and highresistance towards oxidation canbe obtained with HiPIMS atreduced temperatures

From Magnus et al. (2012) IEEE EDL 33 1045


Application – Bragg mirror

Multilayer structures containing ahigh-contrast (TiO2/SiO2) Braggmirror fabricated on fused-silicasubstrates

reactive HiPIMS TiO2 (88 nm)reactive dcMS SiO2 (163 nm)capped with semitransparent gold

Rutile TiO2 (n = 2.59) and SiO2 (n =1.45) provide a large index contrastSmooth rutile TiO2 films can beobtained by HiPIMS at relativelylow growth temperatures, withoutpost-annealingMagnus et al. (2011) Mater. Res. Soc. Symp. Proc. Vol. 1352

From Leosson et al. (2012) Opt. Lett. 37 4026


Reactive HiPIMS - Voltage - Current - Timecharacteristics


HiPIMS - Voltage - Current - time

To describe the dischargecurrent-voltage characteristicsthe current-voltage-time spaceis requiredThe early work on HiPIMSused 50 – 100 µs pulses and apulse repetition frequency inthe range 50–1000 HzThe cathode voltage and thedischarge current depend onthe discharge gas pressure

0 20 40 60 80 100 120

−1

−0.8

−0.6

−0.4

−0.2

0

0.2

t [µS]

Vd [kV]

(a)

0.4 Pa

2.7 Pa

0 20 40 60 80 100 120−50

0

50

100

150

200

250

t [µS]

Id [A]

(b) 0.4 Pa

2.7 Pa




For longer pulses the initialpressure dependent currentpeak is followed by a secondphase that is power andmaterial dependentThe initial phase is dominatedby gas ions, whereas the laterphase has a strongcontribution fromself-sputteringFor some materials, thedischarge switches into amode of sustainedself-sputtering

From Anders et al. (2007),

JAP 102 113303 and JAP 103 039901



A schematic illustration of thedischarge current assumingsquare shaped voltage pulsesThe current is generallycharacterized by an initial peakfollowed by a more or lessstable current plateau (bottomcurrent curves)In other cases it shows aninitial peak followed by asecond increase of thedischarge current (top currentcurves)




The self-sputtering can operate in a self-sustained mode,when the ions of the sputtered vapor are created at highenough rate that the ions of the working gas are notneededThe condition for sustained self-sputtering is expressed as

Πss = αβtYss = 1

whereα is the probability of ionization of the sputtered atomβt in the probability that the newly formed ion of thesputtered vapor returns to the targetYss is the self-sputter yield of the ion

This is a steady state situation and the current remainsconstant



The bottom curve represents arange of low self-sputtering,Πss < 0.1 and the dischargephysics in the plateau/runawayphase is dcMS-likeThe middle range of powerdensities, with 0.1 < Πss < 1,represents partiallyself-sputtering dischargeThe top curve representsself-sputtering runaway whichrequires Πss > 1 and aself-sputter yieldYss > 1/(αβt) > 1




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 discharge ishighly dependent on the pulserepetition frequency, unlike for pureArN2 addition changes the plasmacomposition and the targetcondition can also change due tothe formation of a compound on itssurface

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



The observed changes in thedischarge current are reflected in theflux of ions impinging on thesubstrate

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



The discharge current Id is the sum of the ion current Ii andthe secondary electron current IiγSE or

Id = Ii(1 + γSE)

where γSE is the secondary electron emission coefficient ofthe target materialAlso

Ii ∝ ni ∝1ET

The total energy loss per electron-ion pair lost from thesystem ET is expected to increase with the addition ofnitrogenWe must turn to the secondary electron emission yield toexplain the self-sputtering runaway and observedfrequency dependence of the current in the reactivedischarge



HiPIMS differs significantly fromdcMS, due to the fact thatself-sputtering quickly becomesdominant and the working gas ions(mostly Ar+ and N+

2 or O+2 ) are

depleted from the area in front of thetarget, due to rarefactionThe secondary electron emissionyield is governed by the compositionof the target (Ti or TiN or TiO2) andthe type of ions that are bombardingit


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



γSE is practically zero for singlycharged metal ions impacting atarget of the same metalγSE will be higher for self sputteringfrom a TiN or TiO2 target, whereN+-ions or O+-ions are also present,than for self-sputtering from a Titarget, where multiply charged Tiions are needed to create secondaryelectrons





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 processAt 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




Summary


Summary

The current-voltage-time waveforms in a reactivedischarge exhibit similar general characteristics as thenon-reactive case

the current rises to a peak, then decays because ofrarefaction before rising to a self-sputtering dominatedphase

At low repetition frequency, the long off-time results in anitride or oxide layer being formed on the target surfaceand self-sputtering by Ti+ and N+ or O+-ions from TiN orTiO2 takes place with an increase in secondary electronemission yield and a corresponding increase in dischargecurrent


References

The slides can be downloaded athttp://langmuir.raunvis.hi.is/∼tumi/hipims.htmlAnders, A., J. Andersson, and A. Ehiasarian (2007). High power impulse magnetron sputtering:

Current-voltage-time characteristics indicate the onset of sustained self-sputtering. Journal of AppliedPhysics 102(11), 113303 and 103(3), 039901.

Gudmundsson, J. T., N. Brenning, D. Lundin, and U. Helmersson (2012). High power impulse magnetron sputteringdischarge. Journal of Vacuum Science and Technology A 30(3), 030801.

Leosson K., S. Shayestehaminzadeh, T. K. Tryggvason, A. Kossoy, B. Agnarsson, F. Magnus, S. Olafsson,J. T. Gudmundsson, E. B. Magnusson and I. A. Shelykh (2012). Optics Letters 37 (19), 4026 – 4028 .

Magnus, F., B. Agnarsson, A. S. Ingason, K. Leosson, S. Olafsson, and J. T. Gudmundsson (2011). Mater. Res.Soc. Symp. Proc. vol 1352

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., 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., 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.

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