Sputtering

1
Sputtering

• Sputtering is a form of PVD

PVD
Resistance-Heated (Thermal Evaporation)
Sputtering
E-beam Evaporation
2
Sputtering References
R.A. Powell and S. Rossnagel, PVD for
Microelectronics Sputter Deposition Applied to
Semiconductor Manufacturing (Academic Press,
1999) D.M. Manos and D.L. Flamm, Plasma Etching
An Introduction (Academic Press, 1989) W.N.G.
Hitchon, Plasma Processes for Semiconductor
Fabrication (Cambridge University Press,
1999) J.L. Vossen and W. Kern, Thin Film
Processes (Academic Press, 1991) M. Konuma,
Film Deposition by Plasma Techniques
(Springer-Verlag, 1992) D.M. Dobkin and M.K.
Zuraw, Principles of Chemical Vapour Deposition
(Kluwer Academic Publishers, 2003) J.E. Mahan,
Physical Vapour Deposition of Thin Films
(John-Wiley Sons, 2000) M. Ohring, The
Materials Science of Thin Films (Academic Press,
1992)
3
Sputtering Process

• A target is bombarded with inert energetic ions,
  typically Ar
• Atoms at the surface of the target are knocked
  loose by a collision cascade process analogous to
  atomic billiards

sputtered atom
incident ion
4
Sputtering Yield
Atoms are sputtered from the target with a
certain probability, Y, called the sputtering
yield Y sputtered (ejected) target atoms
incident ions Y atoms/ion
5
Sputtering Yield
Typical sputtering yields are between 0.1 and 4
From Ohring, Table 3-4, p. 113
6
Sputtering for Film Deposition

• The sputtered atoms may be deposited (condensed)
  on a substrate surface for film deposition

Target source material
Substrate for film deposition
7
Sputtering for Etching

• A sample can be placed on the target for etching
  (plasma-etching, dry-etching, reactive ion
  etching)

Target sample to be etched
8
Sputtering

• A plasma is used as the source of ions
• Other plasma-related processes PE-CVD, SIMS

modified from Mahan, Fig. VII.1, p. 200
9
Sputtering

• There exist different means of creating the
  plasma

Sputtering
RF
DC
Microwave (ECR)
Magnetron Sputtering
10
DC Sputtering Gas Conditions

• A gas is admitted into a chamber filling the
  space between two electrodes
• Typically an inert gas is used like Ar, Ne, Kr,
  and Xe
• Ar is most commonly used
• The gas pressure 0.1 1 Torr

from Mahan, Fig. VI.2, p. 155
11
DC Sputtering Anode/Cathode

• To create a plasma, a dc voltage
• ( 100s to 1000s Volts) is applied between two
  electrodes
• Cathode-anode separation few cms)
• The cathode is negatively biased and attracts
  positive ions from the plasma

from Mahan, Fig. VI.2, p. 155
12
DC Sputtering
etching
deposition
from Vossen (1991), Fig. 7, p. 24
13
Plasma Creation
electrons
ions
cathode
anode
-

photoemission
ionization

• Cosmic rays or uv light causes photoemission
  from the cathode and ionization of the neutral
  gas atoms

14
Plasma Creation
cathode
anode
-

• Electrons are accelerated toward the anode
• Ions are accelerated toward the cathode
• ? current flow

15
Plasma Creation
cathode
anode
-

• Electrons may collide with neutral gas atoms
  causing ionization

16
Plasma Creation
cathode
anode
-

• Ions striking the cathode produce secondary
  electrons

17
Plasma Creation
cathode
anode
-

• Secondary electrons accelerate toward anode and
  collide with gas atoms causing ionization
• e- Ar ? Ar 2e-

18
Plasma Creation
cathode
anode
-

• A multiplication process occurs forming a plasma
• This is known as breakdown

19
I-V Characteristic
from Mahan, Fig. VI.14, p. 185

• Ohmic Region
• Cosmic rays or uv light causes photoemission of
  the cathode or ionization of the neutral gas atoms

20
I-V Characteristic
from Mahan, Fig. VI.14, p. 185

• Saturation Region (Region A)
• All the available free electrons are collected
  at the anode as quickly as they are created
• I constant

21
I-V Characteristic

• Townsend Discharge (Region B)
• The electrons are accelerated to sufficient
  energy to cause ionization of the neutral gas
  atoms

from Mahan, Fig. VI.14, p. 185
22
I-V Characteristic

• Breakdown (Region C)
• Secondary electron emission produces
  multiplication process

from Mahan, Fig. VI.14, p. 185
23
I-V Characteristic

• Normal Glow (Region D)
• Plasma is created near edges of cathode where
  E-field is highest
• The current increases at constant voltage as the
  plasma extends over the entire cathode surface

from Mahan, Fig. VI.14, p. 185
24
I-V Characteristic

• Abnormal Glow (Region E)
• Plasma is now extended across entire cathode
  surface
• For further increases in current, the dc applied
  voltage must increase
• This is the region where most sputtering
  processes occur since it gives the highest
  sputtering rate

from Mahan, Fig. VI.14, p. 185
25
I-V Characteristic

• Arc (Region F)
• If the current is increased further, the cathode
  becomes heated which will either melt or, if the
  cathode material is refractory, will result in
  thermionic emission of electrons

from Mahan, Fig. VI.14, p. 185
26
Collisions

• 2 ways for plasma particles (electrons, ions) to
  interact and lose energy

Collisions
Elastic
Inelastic
27
Elastic Collisions

• Elastic collision
• Conserves energy and momentum
• Does not result in any internal excitations of
  the gas atoms or molecules (e.g., vibration,
  rotation, electronic)

M0, E0
Before collision
Mr
Mr, Er
After collision
Recoil angle, q
M0, E0
28
Elastic Collisions
Mr, Er
M0, E0
Recoil angle, q
Mr

• Conservation of energy and momentum
• ? derive Er (M0, Mr, E0, q)

29
Elastic Collisions
Mr, Er
M0, E0
Recoil angle, q
Mr
Er 4 E0 M0Mrcos2q/(M0Mr)2
M0 mass of incident particle Mr mass of
struck particle initially assumed to
be stationary E0 energy of incident particle Er
energy of recoiling particle (Mr) initially
assumed to be stationary q recoil angle
30
Heavy Particle Collisions
Er 4 E0 M0Mrcos2q/(M0Mr)2

• For collisions among ions and neutrals in the
  plasma, M0 Mr
• Er E0 cos2q
• Energy transfer is efficient among ions and
  neutrals
• Ions and neutrals will thermalize to the same
  temperature

31
Elastic Collisions

• The ions do not acquire kinetic energy from the
  applied field as readily as do the electrons
• v mE
• mobility, m et/m
• mi gtgt me
• Typical ion or neutral atom energies are 0.03
  0.1 eV (300-1000 K)
• Ions neutrals have insufficient energy to
  cause ionization in the gas
• So where do the ions come from ?

32
Light-Heavy Collisions
electron M0, E0
Mr, Er
q
neutral gas particle Mr

• For an electron-gas atom collision
• M0 ltlt Mr
• Therefore,
• Er 4 E0 (M0/Mr) cos2q

33
Elastic Collisions

• Er 4 E0 (M0/Mr) cos2q
• The electron will transfer a maximum energy of
• Er 4 (M0/Mr) E0
• e.g., for an electron colliding with an Ar atom,
  Er/E0 lt 1.4 x 10-4
• Very little energy is transferred in an elastic
  collision from the electron to the ions or
  neutrals in the plasma

34
Elastic Collisions

• But electrons acquire a much higher kinetic
  energy (and temperature) from the applied field
  compared to the ions or neutrals due to their
  much smaller mass
• Typical electron energy is 1-10 eV (10000-100000
  K)
• Recall that typical ion or neutral atom energies
  are 0.03 0.1 eV (300-1000 K)
• Electron and ion temperatures are not equal (not
  in thermodynamic equilibrium)

35
Elastic Collisions

• Electrons and ions/neutrals may each be
  described by a separate M-B distribution each
  with their own temperatures, Te and Ti

from Manos, Fig. 13, p. 206
36
Inelastic Collisions

• Ions have insufficient energy to cause
  ionization in the gas
• Very little energy is transferred in an elastic
  collision from electrons to the ions or neutrals
  in the plasma
• Inelastic collisions must be responsible for
  producing the plasma

37
Inelastic Collisions
Mr, Er, DU
M0, E0
q
Mr

• For inelastic collisions
• DU E0Mrcos2q / (M0 Mr)
• DU change in internal energy of the struck
  particle (vibrational, rotational, electronic
  excitations)

38
Inelastic Collisions
DU / E0 Mrcos2q / (M0 Mr)

• For an inelastic collision between an electron
  and a neutral, M0 ltlt Mr
• DU E0cos2q

39
Inelastic Collisions

• DU E0cos2q
• DU E0 for forward scattering (q 0)
• Practically all of the electron energy can be
  imparted to the atom or ion in an inelastic
  collision

40
Inelastic Collisions

• The energy transfer may vary from less than 0.1
  eV (for rotational excitation of molecules) to
  more than 10 eV (for ionization)

from Dunlap, Fig. 8.3, p. 194
41
Townsend Discharge

• As the voltage is increased, electrons may gain
  sufficient energy from the applied field to
  ionize a gas atom in an inelastic collision
• e- Ar ? Ar 2e-
• At this point, ions are created for the first
  time (Townsend discharge)

from Mahan, Fig. VI.14, p. 185
42
Townsend Discharge

• The electron energy must exceed the ionization
  energy of the gas atoms

Neutral Ion Ionization Potential (eV)
Ar Ar 15.8
Ar Ar 27.6
F F 17.4
H H 13.6
He He 24.6
N N 14.5
O O 13.6
Si Si 8.1
N2 N2 15.6
O2 O2 12.2
SiH4 SiH4 12.2
43
Townsend Discharge

• Typical ionization energies (10-15 eV) are
  greater than the mean electron energy (1-10 eV)
• Therefore, only electrons in the high energy
  tail of the M-B distribution will contribute to
  ionization

from Manos, Fig. 13, p. 206
44
Paschen Curve

• The breakdown voltage required to form the
  plasma is described by the Paschen curve
• The minimum in the Paschen curve is around 1
  Torr-cm

from Powell, Fig. 3.2, p. 53
45
Paschen Curve

• Low pressure or small anode-cathode spacing
  electrons can undergo only a small number of
  collisions in traversing the applied field not
  enough ionizing collisions take place to sustain
  the plasma a larger voltage is required to
  sustain the plasma

from Powell, Fig. 3.2, p. 53
46
Paschen Curve

• High pressure mean free path of electrons is
  reduced electrons cannot gain sufficient
  acceleration (i.e., sufficient energy) between
  collisions to cause ionization

from Powell, Fig. 3.2, p. 53
47
Paschen Curve

• Due to the differences in ionization energy and
  ionization cross-sections for different gases,
  the Paschen curve will have slightly different
  shapes for different gases

from Konuma, Fig. 3.1, p. 50
48
Typical dc Plasma Characteristics

• Plasma species
• Neutral atoms (or molecules depending on the
  gas), ions, electrons, radicals, and excited
  atoms
• Plasma density
• ni ne 108-1010 cm-3
• nn 3x1015 cm-3
• Degree of ionization
• ni, ne ltlt nn
• ne / nn 10-5
• Plasma temperature
• Te 10000-100000 K (1 10 eV)
• Ti , Tn 300-1000 K (0.03 0.1 eV)

49
Typical Plasma Characteristics
LD
from Manos, Fig. 3, p. 191
50
Cold Plasma

• Plasma temperature
• Te 10000-100000 K (1 10 eV)
• Ti , Tn 300-1000 K (0.03 0.1 eV)
• Te gtgt Ti, Tn but ne, ni ltlt nn
• The plasma is essentially at the neutral gas
  temperature which is quite low
• cold plasma

51
Glow Discharge

• Within the plasma, excited atoms can relax to
  lower energy states causing the emission of light
  with a wavelength that is characteristic of the
  gas used
• glow discharge

from Dunlap, Table 8.4, p. 195
52
Glow Discharge
from Mahan, colorplate VI.18