Sputtering
• Sputtering is a form of PVD
PVD
Resistance-Heated
(Thermal Evaporation)
Sputtering
E-beam Evaporation
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)
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
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
Sputtering Yield
Typical sputtering yields are between
0.1 and 4
From Ohring, Table 3-4, p. 113
Sputtering for Film Deposition
• The sputtered atoms may be
deposited (condensed) on a substrate
surface for film deposition
Substrate
for film
deposition
Target =
source
material
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
Sputtering
• A plasma is used as the source of
ions
• Other plasma-related processes: PECVD, SIMS
modified from Mahan, Fig. VII.1, p. 200
Sputtering
• There exist different means of
creating the plasma
Sputtering
DC
Magnetron
Sputtering
RF
Microwave
(ECR)
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
DC Sputtering: Anode/Cathode
• To create a plasma, a dc voltage
(~ 100’s to 1000’s Volts) is applied
between two electrodes
• Cathode-anode separation ~ few
cm’s)
• The cathode is negatively biased and
attracts positive ions from the plasma
from Mahan, Fig. VI.2, p. 155
DC Sputtering
etching
deposition
from Vossen (1991), Fig. 7, p. 24
Plasma Creation
electrons
anode
ions
cathode
-
+
photoemission
ionization
• Cosmic rays or uv light causes
photoemission from the cathode and
ionization of the neutral gas atoms
Plasma Creation
anode
cathode
-
+
• Electrons are accelerated toward the
anode
• Ions are accelerated toward the
cathode
 current flow
Plasma Creation
anode
+
cathode
-
• Electrons may collide with neutral
gas atoms causing ionization
Plasma Creation
anode
+
cathode
-
• Ions striking the cathode produce
secondary electrons
Plasma Creation
anode
cathode
-
+
• Secondary electrons accelerate
toward anode and collide with gas
atoms causing ionization
e- + Ar  Ar+ + 2e-
Plasma Creation
anode
cathode
+
-
• A multiplication process occurs
forming a plasma
• This is known as breakdown
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
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
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
I-V Characteristic
Breakdown (Region C)
• Secondary electron emission
produces multiplication process
from Mahan, Fig. VI.14, p. 185
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
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
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
Collisions
• 2 ways for plasma particles
(electrons, ions) to interact and
lose energy
Collisions
Elastic
Inelastic
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)
Before
collision
M0, E0
Mr
Mr, Er
After
collision
Recoil
angle, q
M0, E0
Elastic Collisions
M0, E0
Mr, Er
Recoil
angle, q
Mr
• Conservation of energy and
momentum
 derive Er (M0, Mr, E0, q)
Elastic Collisions
M0, E0
Mr, Er
Recoil
angle, q
Mr
Er = 4 E0 M0Mrcos2q/(M0+Mr)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
Heavy Particle Collisions
Er = 4 E0 M0Mrcos2q/(M0+Mr)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
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 >> 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 ?
Light-Heavy Collisions
electron
M0, E0
Mr, Er
q
neutral gas particle
Mr
• For an electron-gas atom collision:
M0 << Mr
• Therefore,
Er ~ 4 E0 (M0/Mr) cos2q
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 < 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
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 (3001000 K)
• Electron and ion temperatures are
not equal (not in thermodynamic
equilibrium)
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
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
Inelastic Collisions
M0, E0
Mr, Er, DU
q
Mr
• For inelastic collisions :
DU = E0Mrcos2q / (M0 + Mr)
DU = change in internal energy of the
struck particle (vibrational, rotational,
electronic excitations)
Inelastic Collisions
DU / E0 = Mrcos2q / (M0 + Mr)
• For an inelastic collision between an
electron and a neutral, M0 << Mr:
DU = E0cos2q
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
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
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
Townsend Discharge
• The electron energy must exceed the
ionization energy of the gas atoms
Neutral
Ar
Ion
Ar+
Ionization
Potential (eV)
15.8
Ar+
F
H
He
Ar++
F+
H+
He+
27.6
17.4
13.6
24.6
N
O
Si
N+
O+
Si+
14.5
13.6
8.1
N2
N2+
15.6
O2
SiH4
O2+
SiH4+
12.2
12.2
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
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
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
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
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
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 << nn
ne / nn ~ 10-5
• Plasma temperature
Te ~ 10000-100000 K (1 – 10 eV)
Ti , Tn ~ 300-1000 K (0.03 – 0.1 eV)
Typical Plasma Characteristics
LD
from Manos, Fig. 3, p. 191
“Cold” Plasma
• Plasma temperature
Te ~ 10000-100000 K (1 – 10 eV)
Ti , Tn ~ 300-1000 K (0.03 – 0.1 eV)
• Te >> Ti, Tn but ne, ni << nn
• The plasma is essentially at the
neutral gas temperature which is
quite low
• “cold” plasma
“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
“Glow” Discharge
from Mahan, colorplate VI.18