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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
© Ulf Helmersson, Linköping University 2
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Sputtering is from Dutch sputteren.”To spit out small particles with a characteristic explosive sound”
Swedish: Sputtring, sputtering, katodförstoftningGerman: Katodenzerstäubung
© Ulf Helmersson, Linköping University 3
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
© Ulf Helmersson, Linköping University 4
Sputtering target, M
Implantation, I
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Can all materials be sputtered?
• Etched? Yes!
• Deposited? No!
• Polymers?
• Alloys?
C d ?
© Ulf Helmersson, Linköping University 5
• 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
© Ulf Helmersson, Linköping University 6
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:
© Ulf Helmersson, Linköping University 7
• 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
© Ulf Helmersson, Linköping University 8
well as with 1/U where U is surface binding energy.
Figure courtesy of Denis Music
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Sputtering Yield
© Ulf Helmersson, Linköping University 9
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.
© Ulf Helmersson, Linköping University 10
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
© Ulf Helmersson, Linköping University 11
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
© Ulf Helmersson, Linköping University 12
– 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.
© Ulf Helmersson, Linköping University 13
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
© Ulf Helmersson, Linköping University 14
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
© Ulf Helmersson, Linköping University 15
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
© Ulf Helmersson, Linköping University 16
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
© Ulf Helmersson, Linköping University 17
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.
© Ulf Helmersson, Linköping University 18
An alternative process (and more likely) is the production of a photon.
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Secondary electron production
© Ulf Helmersson, Linköping University 19
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
© Ulf Helmersson, Linköping University 20
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| !
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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
© Ulf Helmersson, Linköping University 23
field-line from where it originate. After ionization events the electron will move out to a field line further away.
© Ulf Helmersson, Linköping University 24
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
© Ulf Helmersson, Linköping University 25
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
© Ulf Helmersson, Linköping University 26
n 10
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Cylindrical magnetrons
a and c Cylindrical post magnetron
b and d Inverted magnetrons
© Ulf Helmersson, Linköping University 27
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
© Ulf Helmersson, Linköping University 28
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
© Ulf Helmersson, Linköping University 29
Target utilization
© Ulf Helmersson, Linköping University 30
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.
© Ulf Helmersson, Linköping University 31
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
© Ulf Helmersson, Linköping University 32
out the plasma in desired direction.
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Unbalanced magnetrons
© Ulf Helmersson, Linköping University 33
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
© Ulf Helmersson, Linköping University 34
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
© Ulf Helmersson, Linköping University 35
Figures courtesy of Jens Birch and Suzanne Rohde
Closed-field unbalanced magnetron
P.J. Kelly, R.D. Arnell, Vacuum 56, 159 (2000)
© Ulf Helmersson, Linköping University 36
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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.)
© Ulf Helmersson, Linköping University 37
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.
© Ulf Helmersson, Linköping University 38
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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
© Ulf Helmersson, Linköping University 39
Arc formation
+ + + + + + + + + ++ + + + + + + +
© Ulf Helmersson, Linköping University 40
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Oxide
Metal target
Oxide
dU
E
A
CQ
U anddCQ
E
dQ
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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.
© Ulf Helmersson, Linköping University 42
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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.
© Ulf Helmersson, Linköping University 43
Guiding of deposition material
© Ulf Helmersson, Linköping University 44
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
© Ulf Helmersson, Linköping University 45
Rf-coil enhanced sputtering
© Ulf Helmersson, Linköping University 46
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
© Ulf Helmersson, Linköping University 47
• 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).
© Ulf Helmersson, Linköping University 48
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Guiding of deposition material
D
© Ulf Helmersson, Linköping University 49
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
© Ulf Helmersson, Linköping University 51
Substrate
300nm 300nm
Sheath Plasma10 mm
Guiding of ionized metal
© Ulf Helmersson, Linköping University 52
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Ion Fluxes
© Ulf Helmersson, Linköping University 53
Ion energies – time resolved
© Ulf Helmersson, Linköping University 54
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Ion energies – time average
HIPIMS
DC
© Ulf Helmersson, Linköping University 55
After: J. Bohlmark et al. / Thin Solid Films, in press
© Ulf Helmersson, Linköping University 56
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Why is it possible to grow such a good films with HiPIMS?
© Ulf Helmersson, Linköping University 57
TiN
High adatom mobility:001-texture
Low adatom mobility:111-texture001 texture 111 texture
© Ulf Helmersson, Linköping University 58
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
© Ulf Helmersson, Linköping University 59
Increased ion energy
Ts = 300 °CP = 5.6 mTorr
Ji/JTi < 1Ei = 0 ‐ 400 eV
© Ulf Helmersson, Linköping University 60
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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
© Ulf Helmersson, Linköping University 61
Useful energy window for ion-bombardment:15 100 V15 – 100 eV
© Ulf Helmersson, Linköping University 62
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.
© Ulf Helmersson, Linköping University 63
Significant amount of ions exhibit EI ~ 10-30 I
eV and even higher
© Ulf Helmersson, Linköping University 64
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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
© Ulf Helmersson, Linköping University 65
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
© Ulf Helmersson, Linköping University 66
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
© Ulf Helmersson, Linköping University 67
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?
© Ulf Helmersson, Linköping University 68
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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
© Ulf Helmersson, Linköping University 69
+46 13 28 89 78
daniel.lundin@ionautics.com