thin film deposition and application sputter deposition

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Thin Film Deposition and Application

Sputter deposition

© Ulf Helmersson, Linköping University 1

Content of lecture

• Mechanisms of sputtering

• Plasma for sputtering

• dc sputtering

• Pulsed dc sputtering

• rf sputtering


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Sputtering is from Dutch sputteren.”To spit out small particles with a characteristic explosive sound”

Swedish: Sputtring, sputtering, katodförstoftningGerman: Katodenzerstäubung


Ion/surface interaction

Incident particleI, I+

Elastic effects

Sputtered particles

Inelastic effects

PhotonsM, M+, M-, M*, Mn

Reflected particlesI, I+, I-, I*

X-ray

Secondary electrons


Sputtering target, M

Implantation, I

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Can all materials be sputtered?

• Etched? Yes!

• Deposited? No!

• Polymers?

• Alloys?

C d ?


• Compounds?

Energy of sputtered vs evaporated atoms

• Sputtered atoms are much more energetic as compared to evaporated atoms.

• This has important consequences in nucleation and growth of thin films


and growth of thin films.

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Terminology/definitions

• Sputtering Yield: S=Number of sputtered atoms/Incident particle

• Sputtering Threshold: Minimum energy of incident particle for

sputtering

• Secondary Electron Yield:


• Secondary Electron Yield:=Number of electrons ejected/Incident particle

Sputtering Mechanism

1. Momentum transfer is the basis for sputtering. Binary collision events does not cause sputteringevents does not cause sputtering.

2. Collision cascades can result in outward momentum of a surface atom far from the initial collision.

3. Sputtering is a stochastic event and S is a time averaged number.

4. S scales with mimt/ (mi+mt)2 as well as with 1/U where U is


well as with 1/U where U is surface binding energy.

Figure courtesy of Denis Music

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Sputtering Yield


G.K. Wehner and G.S. Anderson, ”The Nature of Physical Sputtering”, in ”Handbook of thin film technology”, ed. by L.I. Maissel and R. Glang (McGraw-Hill Book Company, New York, 1970).

Sputtering yield experimental data (Ar-ions)

Target material Sa SaET (eV)a U (eV)b

500 eV 1000 eVAg 3,12 3,8 15Al 1 05 1 13 3 39Al 1,05 1 13 3,39Au 2,4 3,6 20C 0,12Co 1,22 25Cu 2,35 2,85 17 3,49Fe 1,1 1,3 20 4,28Ge 1,1 25Mo 0,8 1,13 24 6,82Ni 1,45 2,2 21 4,44Pt 1,4 25

(a) M. Ohring, ”Materials science of thin films”, (Academic Press, London, 2002) p. 176.


ET = sputtering threshold energyU = surface binding energy

Si 0,5 0,6 4,63Ta 0,57 26Ti 0,51 20 4,85W 0,57 33 8,9

(b) H.H. Anderson and H.L. Bay, ”Sputtering yield measurements”, in ”Sputtering by particle bombardment I”, ed. by R. Behrish (Springer-Verlag, Berlin, 1981).

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Other energetic particles during sputtering

Backscattering of atoms from the sputtering gas. A reality if target atoms are more massive than sputtering gas atoms. Assuming binary collisions:

for a 90° reflection)(

)(0

gt

gt

MM

MMEE


Ar→Ti E/E0 = 9 %

Ar → Hf E/E0 = 63 %

Ar → Au E/E0 = 66 %

Other energetic particles during sputtering

• Negative atoms accelerated from the target. Often oxygen.– Electrons are easily attaching to oxygen atoms on the

sputtering target surface (negative).

– The O- ion is then accelerated away from the target with the target potential.

Accelerate perpendicular out from the target surface


– Accelerate perpendicular out from the target surface.

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Effect of incident angle

• S increases as decreases from 90° due to the fact that the collision cascade is moved closer to the surface.

• At glancing angles the sputtering ions are reflected and less energy is deposited into the target – S decreases.


Figure based on publication:

H.Oechsner, Appl.Phys. 8, 185 (1975).

Sputtering of alloycontaining elements with different yields

Altered layer

St d t t fl


Initial flux

Steady-state flux

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Ejection and gas phase transport

B

Ar+A

Different atoms - different energiesDifferent atoms different angular distribution


Different atoms - different angular distribution

• Ballistic transport• Diffusive transport• Virtual source

Plasma as a source of ions-V

Sputtering target

D kDark spaceSheathAll potential is over the sheath.

The plasma is close to neutral containing a mixture of neutrals ions


Substrate

mixture of neutrals, ions, and electrons. The charged species are highly mobile resulting that no electric field can penetrate in to the plasma.

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Ion/surface interaction

Incident ionI, I+

Elastic effects

Sputtered particles

Inelastic effects

PhotonsM, M+, M-, M*, Mn

Reflected particlesI, I+, I-, I*

X-ray

Secondary electrons


Sputtering target, M

Implantation, I

Secondary electron productionThe band-structure of an ion a metal target just prior to impact:

An ion is usually neutralized just before impact with a metal surface. An electron is tunneling out to the ion and the excess energy is gained by an other electron that can leave the metal if Ee>0.


An alternative process (and more likely) is the production of a photon.

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Secondary electron production


D.B. Medved et al., Phys.Rev. 129, 2086 (1963)

2.0

Cathode regionIon production in the dark space

Ion multiplication:Consider:

60 mTorr600 V

The estimated multiplication f i 2 2


Dfactor is: 2.2

Ion production rate: 0.44

A total ion production rate of more than 1 is needed!?

Figure from B. Chapamn, ”Glow Discharge Processes”, (John Wiley & Sons, New York, 1980).

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Magnetically enhanced sputtering –Magnetron sputtering

F Qv B Lorentz force: Q

The force can effect the velocity of the particle, vbut it cannot affect its speed |v| !


Magnetic field arrangements

vB

Target


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Electron migration in the magnetic field

Secondary electrons will move along the magnetic field-line from where it


field-line from where it originate. After ionization events the electron will move out to a field line further away.


T.E. Sheridan et al., J.Vac.Sci.Technol.A 8 (1990) 30

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Magnetrons

• The B-field is typical a few hundred Gauss so it affects electrons but not ions.electrons but not ions.

• The magnetron current-voltage characteristics is usually I~Vn with n near 10. Without magnetic fields n is typically equal to 1.

• Substrate is generally only weakly connected to the plasma– Therefore, there is very little electron and ion bombardment at the

substrate compared to normal dc sputtering


substrate compared to normal dc sputtering.

• However, neutral particles originating from the target are more energetic due to generally lower operation pressure.

I-V characteristics of a sputtering plasma

I~VnI V

n 1


n 10

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Cylindrical magnetrons

a and c Cylindrical post magnetron

b and d Inverted magnetrons


J.A. Thornton and A.S. Penfold, ”Cylindrical magnetron sputtering”, in Thin Film Processes, ed. by J.L. Vossen W. Kern, (Academic Press, New York, 1978).

Cylindrical post magnetron End view


J.A. Thornton and A.S. Penfold, ”Cylindrical magnetron sputtering”, in Thin Film Processes, ed. by J.L. Vossen W. Kern, (Academic Press, New York, 1978).

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Planar magnetron


Target utilization


From: www.soleras.com/magntrn/enhance.htm

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Rotating cylindrical magnetron

Ad tAdvantages:• high target utilization (up to

90%)• better target cooling so that

higher powers can be used to increase deposition rate and plasma density.


M. Wright and T. Beardow, J.Vac.Sci.Technol.A 4, 388 (1986).

Unbalanced magnetrons

• Magnetrons concentrate the plasma close to th t tthe target.– This limits the possibility to make use of ions

for substrate preparation and ion-surface interaction.

• Solution to modify the magnetic trap to leak


out the plasma in desired direction.

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Unbalanced magnetrons


P.J. Kelly, R.D. Arnell, Vacuum 56, 159 (2000)

Unbalanced magnetron

CrN/ScN, with coilI=5 A (~60 Gauss)

#ions

Effect of coupling the magnetrons with a substrate coil

#ions#molecules = 10 - 140


Figures courtesy of Jens Birch and Suzanne Rohde

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Unbalanced magnetron

#ions0 1 1

Effect of coupling the magnetron without substrate coil

Mo/V, no coil, #atoms= 0.1 - 1


Figures courtesy of Jens Birch and Suzanne Rohde

Closed-field unbalanced magnetron

P.J. Kelly, R.D. Arnell, Vacuum 56, 159 (2000)


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Sputtering from insulating targets

You can sputter from a insulating target – but only for a very brief moment. The surface is quickly charging

i i lpositively.

Solution: Use an alternating voltage supply to the target decharging the surface during the positive part. The frequency needed depend on the thickness of the dielectric material (thick dielectric low capacitance short time to charge surface high frequency needed.)


short time to charge surface high frequency needed.)

• Alternating dc, pulsed dc (1-100 kHz)• RF (13.56 MHz)

Using dc insulating target the sputtering simply stops. For thin insulating layers on a conducting targets electric arcs develop due to breakdown of the dielectric layer.

Rf-sputtering

• Due to the higher mobility of electrons compared to ions in the plasma a net negative flow of charges occurs on a surface placed in the plasma. g p pThe result is that a self-biasing is achieved and sputtering can be performed.

• 13.56 MHz is used due to regulation regarding radio transmission.

• Rf discharges has complex impedance and a matching network is needed to avoid to much power to reflect back to the power supply.


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Pulsed sputtering• RF sputtering often yield low deposition rates. An alternative for reactive

sputtering is pulsed DC.• Specially useful for reactive sputtering when insulating layers form on the metallic

target surface. In this situation dc develops arcs that melt the target locally and produce liquid droplets of the target material that may be incorporated in the filmproduce liquid droplets of the target material that may be incorporated in the film produced on the substrate.

Surface decharging

Sputtering


Arc formation

+ + + + + + + + + ++ + + + + + + +


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Oxide

Metal target

Oxide

dU

E

A

CQ

U anddCQ

E

dQ


d

AC

Electric field strength is constant, independent of oxide thickness. (Assuming a homogeneous current.)

A

d

d

QE

Dual target pulsed sputtering

A problem that occurs in single target (asymmetric) pulsed reactive sputtering is that the anode (e.g. chamber walls) becomes covered with i l i i l T l hi blinsulating material. To solve this problem one can use two magnetrons and alternately sputter from one and use the other as anode: Pulsed symmetric sputtering.


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Ionized PVD

• The depositing species are ions rather than t lneutrals.

• Ions can be controlled by electric or magnetic fields in energy and direction.


Guiding of deposition material


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

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Unpenetrable plasma!

Plasma density (electron density)• Needed 1019 m-3

• Normal sputtering 1016 m-3

With sufficiently hot electrons


Rf-coil enhanced sputtering


Figure from publication by J. Hopwood

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Increased power to cathode

• More power (DC) + Higher plasma density

- Hotter target

- Gas rarefaction

• Extreme pulse energies– 200 kW (1 kV, 200 A) on a 150 mm diameter target


• Reasonable average power– Duty factor 0.5 % 1 kW

• Sputtering pressure– <1 mTorr

V. Kouznetsov, K. Macák, J.M. Schneider, U. Helmersson, and I. Petrov, Surface and Coatings Technology 122, 290 (1999).


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Guiding of deposition material

D


D ≈ 10 - 100 µm

Large structuresSmall structures

Deposition in narrow trenches


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Deposition in wider trenches

Ion flow

dc Pulsed

-V Neutral flow

Ion flow

300 300

Ti flow


Substrate

300nm 300nm

Sheath Plasma10 mm

Guiding of ionized metal


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Ion Fluxes


Ion energies – time resolved


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Ion energies – time average

HIPIMS

DC


After: J. Bohlmark et al. / Thin Solid Films, in press


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Why is it possible to grow such a good films with HiPIMS?


TiN

High adatom mobility:001-texture

Low adatom mobility:111-texture001 texture 111 texture


F. H. Baumann, D. L. Chopp, T. Díaz de la Rubia, G. H. Gilmer, J. E. Greene, H. Huang, S. Kodambaka, P. O’Sullivan, and I. Petrov, MRS Bulletin, 26 182 (2001)

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Low surface mobillity

Ts = 500 °CP = 38 mTorrJi/JTi = 0.5

Ts = 300 °CP = 5 mTorrJi/JTi = ~1

i/ Ti

Ei = 100 eVEi = 20 eV


Increased ion energy

Ts = 300 °CP = 5.6 mTorr

Ji/JTi < 1Ei = 0 ‐ 400 eV


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Increased Ji/JMe

Ts = 350 °CP = 20 mTorrJi/JTi = 1.3

Ts = 350 °CP = 20 mTorrJi/JTi = 10.7

Ei = 20 eVTaN!Ei = 20 eV


Useful energy window for ion-bombardment:15 100 V15 – 100 eV


LI Wei et al., Chin.Phys.Lett., 23, 178

(2006)

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Bombardment during HiPIMS growth

• Ji/JMe ?

F d d 10 i• For dc around 10 is needed

• For HiPIMS close to 1 is available.


Significant amount of ions exhibit EI ~ 10-30 I

eV and even higher


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HiPIMS TiN

Ts = 420 °CP = 3 mTorr

Us = ‐75 V

P 3 mTorr

Us = ‐50 V Us = ‐25 V Us = 0 V


100 nm

Preferred orientation

Ub= 0 V

Ub=-25

111 20

0

220

311

Without substrate bias002 orientation

High surface mobility?

With 100 V b t t bi


V

Ub=-50 V

Ub=-75 V

With ‐100 V substrate bias111 orientation

Reduced surface mobility?

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Room Temp. HiPIMS TiN

T = RTTs RTP = 4 mTorrUs = 0 V


50 nm100 nm

Why is HiPIMS TiN so good?

• Self ion‐bombardment?• Relaxation time between the pulses?

On 0.1 msOff 9.9 ms

• Relaxation time between the pulses?


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Thank you for the interest!

• You are welcome to contact me any time ith ti di thi l t thiwith questions regarding this lecture or thin

film growth in general.

• We are starting a company specialized in HiPIMS. Contact: Daniel Lundin: +46-13-28 89 78


+46 13 28 89 78

[email protected]

thin film deposition and application sputter deposition

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