The Role of Ions in Magentron Dischargesseminare.fav.zcu.cz/media/document/lundin.pdf · 2010. 11. 9. · Hollow cathode HiPIMS … RF coils: C. Nouvellon et al., J. Appl. Phys. 92,

The project is co-financed by the European Social Fund and the state budget of the Czech Republic.

The Role of Ions in MagentronDischarges

Dr. Daniel Lundin

(Linköping University, Švédsko )

2010-11-07

Linköpings universitet 1

Daniel LundinPlasma and Coatings Physics Division, LinköpingUniversity, Sweden

The role of ions in magnetron discharges

2Nov 1-3, 2010Plzen Daniel Lundin, [email protected]

Introduction

Why do we need to know about magnetron discharges and thin film growth using sputtering? It ultimately depends on our interest of course, but for people dealing with coatings on the sub-mm scale it is essential to understand the underlying process of this technology.

Two widespread methods, known to produce thin films of great quality (compared to for example electroplating) are chemical vapor deposition (CVD) and physical vapor deposition (PVD). Here we will discuss PVD of which magnetron sputtering is one important method.

N

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Objectives (the greater picture)

How do we make good coatings?What are good coatings?

What equipment do we need?

What are the important parameters to monitor?

How do we optimize those parameters (the process)?

How do we benchmark our results?

…


The deposition process

Film growth

Particle transport

Sputtering

D.J. Christie, J. Vac. Sci. Technol. A 23, 330 (2005)J. Vlcek et al., J. Vac. Sci. Technol. A 25, 42 (2007)D Lundin et al., Plasma Sources Sci. Technol. 18, 045008 (2009)

PlasmaIntroduced by I. Langmuir in 1928Collection of freely moving charged particlesOn average electrically neutralIonization

Gas dynamics

PVDCondensation of a vaporized form of a materialSputtering: physical bombardment of particles

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Results of ion bombardment

Let us start with the end results first in order to see the bigger picture. Stepwise we will break down the physics and learn how to tailor and optimize the ion bombardment.

Ts = 350 °CP = 20 mTorrJi/JTa = 1.3Ei = 20 eV

Ts = 350 °CP = 20 mTorrJi/JTa = 10.7Ei = 20 eV

Ex) TaN grown by DCMS in a UHV system.

The ratio of incoming ions (no distinction between gas and metal ions!) to incoming metal neutrals was changed while maintaining the energy of the incoming ions.

In these bright-field plan-view TEM images of 500 nm thick coatings we observe dramatic changes in microstructure.


Motivation of ion bombardment

Direct control over the sputtered flux through increased ionization of the sputtered material. Another advantage that follows is that bombarding ions can transfer momentum to the surface atoms of the coating, which in turn leads to increased adatom mobility.

Beneficial consequences for the micro-structure of the thin film regarding enhanced mechanical and chemical properties. However, in glow discharge processes such as magnetron sputtering it is relatively easy to achieve a large fraction of gas ions(like Ar+), whereas ions from the sputtered metal are rare.

Ts = 300 °CP = 5.6 mTorrJi/JTi < 1Ei = 0 - 400 eV

I. Petrov et al., J. Vac. Sci. Technol. A 21, S117 (2003)

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Metal ion bombardment

For many applications it is desired to increase the amount of metal ions, since this means greater control of the deposition flux in terms of direction and energy. One example is the use of a negative potential on the substrate for deposition of thin films into vias and trenches, where a neutral flux would tend to cover the upper part of the walls while leaving the bottom only partly covered

Incident neutral flux distribution

Tangent ruleAs a rule of thumb we can relate the angle of incident flux, α, to the column inclination, β, through: 2 tan β ≈ tan α.

β

α

Ok microstructure, with fairly dense columns

Underdense columnar microstructure due to atomic shadowing by randomly protruding columns.


Atomic shadowing

DCMS experiment using Cu target, where:

(left) Ar is used as sputtering gas, i.e. low ratio of metal ions compared to neutrals, resulting in atomic shadowing and bad wall coverage.

(right) Cu is sputtering Cu (self-sputtering) meaning a much higher ratio of Cu ions. Here better wall coverage is achieved and one needs less material to completely cover the trench with a Cu coating.

Cu neutrals Cu ions

Z. J. Radzimski, J. Vac. Sci. Technol. B 16, 1102 (1998)

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I-PVD: definition

When the deposition flux reaching the substrate consists of moreions than neutrals the process is referred to as ionized PVD, or I-PVD. There are many different IPVD-techniques available today:

Magnetrons using post-vaporization ionization (coils)

Cathodic arc evaporation

Hollow cathode

HiPIMS

…

I-PVD: U. Helmersson et al., Thin Solid Films 513, 1 (2006)RF coils: C. Nouvellon et al., J. Appl. Phys. 92, 32 (2002)


Keys to I-PVD

Initially the sputtered material consists of neutrals. As this flux traverses out into the bulk plasma there is a probability that these neutrals will collide with the background plasma and become ionized through electron impact ionization:

As the ionization potential of the ionized process gas increases the probability of ionizing sputtered neutrals increases provided that the ionized gas has a significantly higher ionization potential than that of the sputtered neutrals.

J.A. Hopwood, Thin Films: Ionized Physical Vapor Deposition, Academic Press, San Diego (2000)

+M e M 2e − −+ → +

Ex) Sputtered Cu in an Ar plasma

Cu has an ionization potential of EIP = 7.73 eV. Arhas an ionization potential of EIP = 15.76 eV. If nAr > nCu, then the Cu will be highly ionized, because the electron temperature (Te) will be determined by the process gas ionization potential (we can see this as the gas can support a higher Te).

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Process gas as an ionizer

In order to understand how electron impact ionization works for different discharge conditions it is instructive to look at rate coefficients (kmiz) for such an event (Arrhenius form):

where k0 and E0 are constants that has to be extracted from experiments or computer simulations.

Using the above eq. we understandthat increased Te will increase the ionization of the sputtered neutrals.However, this expression does not tell us anything about the probability of having a collision between the sputtered neutral and the process gas plasma, which is required in the first place

( )0 0( ) expmiz e ek T k E T= −

Ti

Cu

C

Al

Ag

kmiz [m-3]Material6.68/14 0.54394.097 10 eT

eT e−−×6.78290.3576 /13101.3467 eT

eT e−−×12.6/13. 00 14 eTe−−×

7.13440.4840 /14103.8980 eTeT e−−×

/13 7.252.3 04 1 eTe−−×

M. Samuelsson et al., Surf. Coat. Technol. 15, 591 (2010)


Ionization mean free path

A much better expression for the overall trend of ionizing a sputtered neutral is instead the ionization mean free path for the sputtered metal neutral, which is the average distance covered by the sputtered neutral between the sputter event and ionization:

The velocity of the sputtered metal neutral is typically found to be around 500 m/s.

( )miz s miz ev k nλ = vs – velocity of sputtered neutral

kmiz – rate coeff. for electron impact ioniz.

ne – plasma densityEx) Ionization mean free path of Al and C

6.96.0×10183.6500C

0.286.4×10183.4500Al

[cm]

kmiz

[m-3]

ne

[m3 s-1]Te

[eV]vs

[m s-1]

Material mizλ

6.78290.3576 /13101.3467 eTeT e−−×

12.6/13. 00 14 eTe−−×

N. Britun et al., Appl. Phys. Lett. 92, 141503 (2008) M. Samuelsson et al., Surf. Coat. Technol. 15, 591 (2010)

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Ionized flux fraction

Degree of ionization (bulk plasma/ioniz. region):

Ionized flux fraction (to surface) :

These two fractions are not equal, since neutrals and ions are accelerated to different velocities:

Electron density, ne [ 1017 m-3 ]

Ioni

zed

flux

fract

ion Ti

Al

Cu

C


gasmetal

( )i i nn n n+

( )1 20.61i B e i ik T m nΓ =

( )1 20.25 8n B g n nk T m nπΓ =

( )i i nΓ Γ +Γ

Thermal velocity

Bohm velocity

For weakly ionized discharges the electron temperature is often significantly larger than the neutral gas temperature. This means that the ion flux fraction is larger than the fraction of ionized metal .

Ex) Degree of ionization vs. ionized flux fractionIf the degree of Ti ionization is around 44 %, the ion flux ratio is 91 % assuming that the electron temperature is 2 eV and the neutral gas is at room temperature.


Ion production mechanisms

The four most important reactions forion generation in magnetrondischarges are:

* +M e M 2e − −+ → +

Penning ionization

Charge exchange

Electron imp. ioniz. of excitec sputtered neutral

Electron imp. ioniz. of sputtered neutral

DescriptionReaction


+M e M 2e − −+ → +

* +Ar M M Ar e −+ → + +

* +M e M 2e − −+ → +

+ +Ar M M Ar+ → +

NoteWe find that electron impact ionization is the most important ionizing reaction (we will later look at charge exchange)

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Calculate the mean free path

A general formula for calculating the mean free path between collisions (general expression, not only leading to ionization):

Low Vac. High Vac. Ultra High Vac.

10-610-410-2

110210410610810101012

10-1510-1310-1110-910-710-510-30.110

mea

n fr

ee p

ath

[λ](c

m)

pressure (Torr)http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/menfre.html


Example: Ion energy effects growing TiN

Ei < 80 eV: layers consists of dense columns with open column boundaries

Ei = 120 eV: the voids along column boundaries disappear and the film becomes fully dense. But: incorporation of intragranular residual damage

Ei = 160 eV: defect density becomes so large that local epitaxial growth onindividual columns is disrupted and renucleation occurs

I. Petrov et al., J. Vac. Sci. Technol. A 21, S117 (2003)

gasmetal

Bias: -80 V

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Ion bombardment: correct bias voltage

As we have come to understand we need to know what bias voltage to use in order to optimize our ion bombardment.

Ex) TiN

< 15 eV: no atomic displacement15 – 100 eV: surface displacement> 100 eV: surface and bulk displacement

LI Wei et al., Chin. Phys. Lett. 23, 178 (2006)

ConclusionUseful energy window for ion bombardment: 15 – 100 eV


As a rule of thumb we use the following values:

Energetic bombardment: summary cont.

Slow particles (0 - 25 eV)Enhanced diffusionStimulated desorptionCluster disruption

Moderate energy particles (25 - 50 eV)Same effects as for slow particlesDisplacement cascades

High energy particles (> 50 eV)Same effects as aboveLattice damagePrimary species implantationSputtering

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Structure zone model

A. Anders, Thin Solid Films 518, 4087 (2010)

Thickness

Generalized T* = Th + Tpot

Normalized Ekin = E0 + qeVsheath


Interpretation of structure zone model

The zone model emphasizes the need to take energetic bombardment into account during film growth. The model is on one hand not that practical for everyday purpose, since it does not deal with parameters that are easily accessible during deposition, but it highlights the underlying physics really well.

Generalized temperature, T* = Th + Tpot

The potential energy is the sum of the cohesive energy and the ionization energy (which does not apply to neutrals!):

Epot = Ec + (Ei - Φ).

Ei is reduced by the work function (min energy that an electron needs to be liberated from the substrate surface and neutralize the ion).

Ec ~1 – 9 eV/atomEi ~ 4 – 10 eV/ion

Φ ~ 4 eV

Note: Tpot = Epot/(kNmoved)

Normalized Ekin = E0 + qeVsheath

The kinetic energy is the sum of a plasma component (think acceleration in the bulk plasma) and sheath acceleration

Ekin = E0 + qeVsheath.

Q is the ion charge state number and e is the elementary charge

NoteEkin causes displacement and defects followed by re-nucleation, while Epotcause atomic scale heating and annihilation of defects, i.e. we need both!

A. Anders, Thin Solid Films 518, 4087 (2010)

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HiPIMS

V. Kouznetsov, K. Macák, J.M. Schneider, U. Helmersson, and I. Petrov, Surf. Coat. Technol. 122, 290 (1999)

U. Helmersson, M. Lattemann, J. Bohlmark, A.P. Ehiasarian, and J.T. Gudmundsson, Thin Solid Films 513, 1 (2006)

D. Lundin, The HiPIMS Process, Linköping University, Linköping, Sweden (2010)

K. Sarakinos, J. Alami, and S. Konstantinidis, Surf. Coat. Technol. 204, 1661 (2010)


HiPIMS background

In 1999 a new IPVD technique based on magnetron sputtering, called high power impulse magnetron sputtering (HiPIMS) was developed by Kouznetsov et al., building on previous works by Mozgrin et al., Bugaev et al. and Fetisov et al. This novel technology uses high-power pulses to ionize more of the sputtered material compared to conventional techniques such as direct current magnetron sputtering (DCMS).

NotePeak power ~ kW/cm2

Average power ~ W/cm2

Frequency ~ 10-1000 HzPulse width ~ 10-500 μs

V. Kouznetsov et al., Surf. Coat. Technol. 122, 290 (1999)D.V. Mozgrin et al., Plasma Phys. Rep. 25, 255 (1999)S.P. Bugaev et al., Proceedings of the XVIIth International Symposium on Discharges and Electrical Insulation in Vacuum, p. 1074, Berkeley, CA, USA, July 21-26 (1996)I.K. Fetisov et al., Vacuum 53, 133 (1999)

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HiPIMS features

High plasma density (~1×1019 m-3) compared to conventional direct current magnetron sputtering (DCMS, ~1×1016 m-3)

Increased probability for ionizing collisions

Energetic flux of metal ions (>15 eV)

U. Helmersson et al., Thin Solid Films 513, 1 (2006)J.A. Hopwood, Thin Films: Ionized Physical Vapor Deposition, Academic Press, San Diego (2000)J. Bohlmark et al., J. Vac. Sci. Technol. A 23, 18 (2005)

Electron density, ne [ 1017 m-3 ]

Ioni

zed

flux

fract

ion Ti

Al

Cu

C

Wavelength [ nm ]

Em

issi

on in

tens

ity [

arb.

uni

ts ]

HiPIMS

dcMS


Ion energies in HiPIMS

I this case we are studying ion energies of the ions deflected sideways away from the magnetron. We can see that we have a high-energy tail for HiPIMS. (Is this important?)

z

DJ

ϕJ

B

iu

0

2

4

6

z [cm]

D. Lundin et al., Plasma Sources Sci. Technol. 17, 035021 (2008)

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Ion energies in HiPIMS cont.

NoteSignificant amount of metal ions exhibit Ei ~ 10-30 eVand even higher.Ar+ distribution is often seen as less energetic (why?)

J. Bohlmark et al., Thin Solid Films 515, 1522 (2006)


Results – HiPIMS/DCMS

Do these energetic ions have a heating effect on the growing film? Here, we compare the energy flux between HiPIMS and DCMS above the target race track in the case of sputtering Ti using Ar.

JHiPIMS / JDCMS = 30 - 50%

rateHiPIMS / rateDCMS ~ 0,2 for Ti

Tmax ~ 70 ºC

Tmax ~ 140 ºC

D. Lundin et al, J. Phys. D 42, 185202 (2009)

Conclusion~ 90% more energy/particlefor HiPIMS is deposited

Maximum equilibrium temperaturebelow the limit for many thermallysensitive substrates

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Comparison HiPIMS-DCMS

<5 %30-100 %Degree of metal ionization

20 eV

1-5 eV (hot e-:s also!)

1018-1019 m-3

0.010-0.100 T

10-3-10-2 Torr (0.1 – 1 Pa)

500-1000 V

1-10 A cm-2

1 W cm-2

103 W cm-2

HiPIMS

5 eVIon energy (average for metal ions)

1-5 eVElectron temperature

1016 m-3Electron density

0.010-0.100 TMagnetic field strength

10-3-10-2 Torr (0.1 – 1 Pa)Process gas pressure

500 VDischarge voltage

10-2-10-1 A cm-2Current density

1 W cm-2Average power density

1 W cm-2Peak power density

DCMSParameter


HiPIMS

Substrate

DCMS

Ta flux

10 mm

-V

Neutralflux

Ionflux

Benefits of HiPIMS

J. Alami et al., J. Vac. Sci. Technol. A 23, 278 (2005)

With a large fraction of ionized material we get an extra knob to turn in order to control the deposition. We can thereby deposit material in trenches, on substrates perpendicular to the incoming material flux. The HiPIMS films are dense with smooth surfaces, whereas the DCMS films are inhomogeneous due to shadowing effects, with film columns not normal to the growth surface

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TiN using HiPIMS

Ts = RTP = 4 mTorrUs = 0 V

50 nm100 nm

M. Lattemann, unpublished results


Some things to think about..

Have I really answered what is happening in the growth process using HiPIMS, which can explain the results we have been seeing?

An excellent project would be to characterize the growth (computer simulations?) and see what happens? Think about: the effect of having self-ion bombardment and what happensbetween the pulses?

Are there not any problems with HiPIMS..?

Ts =300 K

Ts =100 K

Simulation: F.H. Baumann et al., MRS Bulletin 26, 182 (2001)Rate: M. Samuelsson et al., Surf. Coat. Technol. 15, 591 (2010)

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Reduction of deposition rate

There are many ideas on why there is a reduction in the deposition rate. One is due to back-attraction of ionized sputtered material. Here is a DCMS example (Bohm) with E-fields of typically 3 - 5 V/cm

M+

Axial distance [ mm ]

Pot

entia

l [ V

]

1 cm

5 V

5 eV

J.W. Bradley et al, Plasma Sources Sci. Technol. 10, 490 (2001)


Reduction of ionization

Another problem can be seen in the following plot displaying thedischarge current when sputtering Cr in 3 mTorr Ar for different pulse lengths.

Why does the current drop?

The high-current transients cause a depletion of the working gas, and thereby a transition to a conventional DCMS-like high voltage, lower current regime, where the desired I-PVD properties are lost.

D Lundin et al., Plasma Sources Sci. Technol. 18, 045008 (2009)

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Gas depletion

Monte Carlo simulation of gas heating from sputtered Ti atoms:90 % Ar density reduction after 150 µs.

Ti/Ar-density equal to 5 close to target.

S. Kadlec, Plasma Proc. Polym. 4, S419 (2007)


Gas depletion cont.

Gas heating and expansionCollisions between gas atoms and sputtered atoms as well as reflectedgas atoms

Decrease in gas density leads to adecrease in current

Direct current lossPreviously unaccounted for

Bulk plasma processes

Refill processes (slow!)

N

RememberThe amount of available gas ions affects the whole discharge.

M. Dickson et al., J. Vac. Sci. Technol. A 15(2), 340 (1997)

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Sustained self-sputtering

On the other hand, the presence of multiply charged ions of some sputtered materials such as Ti and Al (but not C) has led to theonset of self-sputtering regimes characterized by a second increase of the current beyond the value of the initial peak current, if the pulse is sustained for a long enough time in combination with high negative discharge voltages.

HiPIMS meansGas-sputtering

Metal-sputtering

Interesting projectWhat type of sputtering is needed for good coatings? What are the advantages and disadvantages for each type?

A. Anders et al., J. Appl. Phys. 102, 113303 (2007)

Thank you

[email protected]

Documents

The Role of Ions in Magentron Dischargesseminare.fav.zcu.cz/media/document/lundin.pdf · 2010. 11. 9. · Hollow cathode HiPIMS … RF coils: C. Nouvellon et al., J. Appl. Phys. 92,