Title: Sputtering
1Sputtering
- Sputtering is a form of PVD
PVD
Resistance-Heated (Thermal Evaporation)
Sputtering
E-beam Evaporation
2Sputtering 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)
3Sputtering 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
4Sputtering 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
5Sputtering Yield
Typical sputtering yields are between 0.1 and 4
From Ohring, Table 3-4, p. 113
6Sputtering for Film Deposition
- The sputtered atoms may be deposited (condensed)
on a substrate surface for film deposition
Target source material
Substrate for film deposition
7Sputtering for Etching
- A sample can be placed on the target for etching
(plasma-etching, dry-etching, reactive ion
etching)
Target sample to be etched
8Sputtering
- A plasma is used as the source of ions
- Other plasma-related processes PE-CVD, SIMS
modified from Mahan, Fig. VII.1, p. 200
9Sputtering
- There exist different means of creating the
plasma
Sputtering
RF
DC
Microwave (ECR)
Magnetron Sputtering
10DC 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
11DC 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
12DC Sputtering
etching
deposition
from Vossen (1991), Fig. 7, p. 24
13Plasma Creation
electrons
ions
cathode
anode
-
photoemission
ionization
- Cosmic rays or uv light causes photoemission
from the cathode and ionization of the neutral
gas atoms
14Plasma Creation
cathode
anode
-
- Electrons are accelerated toward the anode
- Ions are accelerated toward the cathode
- ? current flow
15Plasma Creation
cathode
anode
-
- Electrons may collide with neutral gas atoms
causing ionization
16Plasma Creation
cathode
anode
-
- Ions striking the cathode produce secondary
electrons
17Plasma Creation
cathode
anode
-
- Secondary electrons accelerate toward anode and
collide with gas atoms causing ionization - e- Ar ? Ar 2e-
18Plasma Creation
cathode
anode
-
- A multiplication process occurs forming a plasma
- This is known as breakdown
19I-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
20I-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
21I-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
22I-V Characteristic
- Breakdown (Region C)
- Secondary electron emission produces
multiplication process
from Mahan, Fig. VI.14, p. 185
23I-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
24I-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
25I-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
26Collisions
- 2 ways for plasma particles (electrons, ions) to
interact and lose energy
Collisions
Elastic
Inelastic
27Elastic 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
28Elastic Collisions
Mr, Er
M0, E0
Recoil angle, q
Mr
- Conservation of energy and momentum
- ? derive Er (M0, Mr, E0, q)
29Elastic 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
30Heavy 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
31Elastic 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 ?
32Light-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
33Elastic 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
34Elastic 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)
35Elastic 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
36Inelastic 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
37Inelastic 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)
38Inelastic Collisions
DU / E0 Mrcos2q / (M0 Mr)
- For an inelastic collision between an electron
and a neutral, M0 ltlt Mr - DU E0cos2q
39Inelastic 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
40Inelastic 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
41Townsend 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
42Townsend 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
43Townsend 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
44Paschen 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
45Paschen 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
46Paschen 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
47Paschen 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
48Typical 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)
49Typical Plasma Characteristics
LD
from Manos, Fig. 3, p. 191
50Cold 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
51Glow 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
52Glow Discharge
from Mahan, colorplate VI.18