Transcript ICOPS Minicourse on Plasma Processing Technology

ICOPS Minicourse
on Plasma Processing
Technology
Part 1: Vacuum Basics
Jeff Hopwood
Northeastern University
Goals
• To review basic vacuum technology
– Pressure, pumps, gauges
• To review gas flow and conductance
• To understand the flux of vapor phase
material to a substrate
• To understand mean free path, l
Vacuum (units)
1.3x10-9
1x10-6 Torr
0.133x10-3 Pa
1.3x10-6
1 mTorr
0.133 Pa
1.3x10-3
1 Torr
133 Pa
Typical High
Pressure Plasma
1 atm.
760 Torr
101,333 Pa
1 Torr =
1 mm-Hg
1 Pascal =
1 N/m2
Typical Low Pressure
Plasma Processing
Ultrahigh Vacuum
High Vacuum
Rough Vacuum
Rough Vacuum
• “Mechanical Pumps” typically create a base
pressure of 1-10 mTorr or 0.13-1.3 Pa
Warning:
Certain process gases
are incompatible with
pump fluids and pose
severe safety risks!
Rotary Vane Pump
(Campbell)
High Vacuum Pumping
Cryopumps condense gases on cold
surfaces to produce vacuum
Typically there are three cold surfaces:
(1) Inlet array condenses water and
hydrocarbons (60-100 Kelvin)
(2) Condensing array pumps argon,
nitrogen and most other gases (10-20
K)
(3) Adsorption is needed to trap helium,
hydrogen and neon in activated carbon
at 10-12 K. These gases are pumped
very slowly!
(Campbell)
Warning: all pumped gases are trapped inside the pump, so explosive, toxic
and corrosive gases are not recommended. No mech. pump is needed until regen.
adapted from www.helixtechnology.com
High Vacuum Pumping
Turbomolecular Pump
Process chamber
High rotation speed turbine
imparts momentum to gas atoms
Inlet pressures: <10 mTorr
Foreline pressure: < 1 Torr
Requires a rough pump
Good choice for toxic and
explosive gases –
foreline
-gases are not trapped in pump
All gases are pumped at approx.
the same rate
Pumping Speeds:
20 – 2000 liters per sec
adapted from Lesker.com
High Vacuum Pumping
Process chamber
Diffusion Pump
The process gas is entrained by the
downward flow of vaporized pumping
fluid.
Watercooled
walls
Foreline
-to mech pump Benefits:
Low cost, reliable, and rugged.
High pumping speed: ~ 2000 l/s
Caution:
The process chamber will be
contaminated by pumping fluid.
A cold trap must be used between the
diffusion pump and the process chamber.
Heater/Pumping Fluid
adapted from Lesker.com
Not recommended for “clean” processes.
Flow Rate
Typically gas flows are cited in units of standard cubic
centimeters per minute (sccm) or standard liters per
minute (slm)
“Standard” refers to T=273K, P = 1 atm.
Example:
Process gas flow of 50 sccm at 5 mTorr (@300k) requires
50 cm-3min-1(760Torr/5x10-3Torr)(300/273)(1min/60sec)(1/103)
= 140 liters/sec of pumping speed at the chamber pump port
Conductance Limitation
50 sccm
Conductance depends on
geometry and pressure (use
tabulated data)
5 mTorr
140 l/s
= Q/(P1 – P2)
Fixed Throughput, Q:
Q = 0.005 Torr x 140 l/s = 0.7 Torr-l/s
> 140 l/s …since P2<P1
Corifice = ¼ (pa2)<v> l/s
Ctube = pa3 (2<v>/3L)
…if mean free path >> a, L
(see Mahan, 2000)
Pressure Measurement
Convectron Gauge:
Initial pumpdown from
1 atm, and as a
foreline monitor
Thermal Conductivity of Gas
Baratron:
Insensitive to gas
composition,
Good choice for
process pressures
True Pressure
(diaphragm displacement)
Ionization of Gas
RGA:
A simple mass
spectrometer
Ion Gauge:
Sensitive to gas
composition, but
a good choice for
base pressures
Vacuum Gauge Selection adapted from Lesker.com
Residual Gas Analysis
Low pressure systems are
dominated by water vapor as
seen in this RGA of a chamber
backfilled with 4x10-5 torr of
argon
Why? H2O is a polar molecule
that is difficult to pump from the
walls --> bake-out the chamber
Leak?
Source: Pfeiffer vacuum products
Gas Density (n)
Ideal Gas Law
PV = NkT
Gas density at 1 Pascal at room temp.
N/V = n = P/kT
= (1 N/m2)/(1.3807x10-23J/K)(300 K)
= [1 (kg-m/s2)/m2]/[4.1x10-21 kg-m2/s2]
= 2.4x1020 atoms per m3
= 2.4x1014 cm-3 …at 1 Pa
Rule of Thumb
n(T) = 3.2x1013 cm-3 x (300/T) …at a pressure of 1 mTorr
Gas Kinetics
Maxwellian Distribution
P (v )  m 
f (v ) 


2
2
p
kT
4pv


3/ 2
  m v2 

exp
 2kT 
Average speed of an atom:
_

8kT
 v   c   v f (v)4pv dv 
pm
0
2
Flux of atoms to the x-y plane surface:
1
z  n  v z  n  vz f (v)dv  n  v 
4
vZ  0
3
Very important!
(Campbell)
Example
A vacuum chamber has a base pressure of 10-6 Torr.
Assuming that this is dominated by water vapor, what is
the flux of H2O to a substrate placed in this chamber?
n = 3.2x1013 cm-3/mTorr * 10-3 mTorr = 3.2x1010 cm-3
<v> = (8kT/pM)1/2 = 59200 cm/s
z = (¼)n<v> = 4.74x1014 molecules per cm2 per sec!
This is approximately one monolayer of H2O every second
at 10-6 Torr base pressure.
Collisions and Mean Free Path
Gas Density
n = P/ kT
Cross-section
s ~ pd2
l  1/sn
d
Rigorous Hard Sphere Collisions: l = kT / 2 pd2P
sAr 2.6 1015 cm2 
lAr(cm) ~ 8 / P (mTorr)
ICOPS Minicourse
on Plasma Processing
Technology
Part 2: Plasma Basics
Jeff Hopwood
Northeastern University
Plasma: an ionized gas consisting of
atoms, electrons, ions, molecules,
molecular fragments, and electronically
excited species (informal definition)
www.geo.mtu.edu/weather/aurora/
Plasma:
the “fourth state of matter”
plasma
energy
(electrons+ions)
gas
(steam)
solid
(ice)
energy
energy
liquid
(water)
DC Plasma
(or AC Fluorescent Lamp…why AC?)
”sputtering”
-
Argon + Mercury @ 0.05 atm.
+
-
-
+
-
++-
-
+
-
+
lamp endcap
Argon
Electron
Argon ion
-
+-
-
+-
-
+
Paschen Curve
F. Paschen, Ann. Phys. Chem., Ser. 3 37, 69 (1889).
VDC
d
Too many collisions
Electron energy<ionization energy
http://www.duniway.com/images/pdf/pg/Paschen-Curve.pdf
Too few ionizing
collisions: l>d
What do we need to know about
plasma?
light
Power
Gas flow
PLASMA
gas
(ng)
Wall
electrons
ne, Te
excited atoms
and molecules
ions
Wall
radicals,
molecular fragments
reaction
products
secondary
electrons
substrate
pumping
pumping
Power Absorbed
Power
light
Gas flow
PLASMA
gas
(ng)
Wall
electrons
ne, Te
excited atoms
and molecules
ions
Wall
radicals,
molecular fragments
reaction
products
secondary
electrons
substrate
pumping
pumping
Power Absorbed: DC
• DC power
– General electrical mobility and conductivity
– Mobility: me = q<t>/m = q/nmme
Where <t> is the average time between collisions
and nm is the collision frequency (collisions per second)
– Electron Conductivity: sDC = qneme = q2ne/nmme
– DC power absorbed:
Pabs   (s DC E  E ) dv
vol
3
Power Absorbed: RF
• RF/microwave power
– Ohmic Heating
VRF
f=13.56 MHz

1
2
3
Pabs   s DC 2
|
E
|
dv
2
2
w


m
vol
2
m
– Generic electron-neutral collision
frequency
nm ~ 5x10-8 ngasTe1/2 (s-1)
… ngas (cm-3), Te(eV).
– Example: Find the pressure at
which rf ohmic heating becomes
ineffective: nm = 0.1w, Te = 2eV
w = 13.56 MHz * 2p = 85.2Mrad/s
ngas = 0.1*85.2x106/5x10-8(2)1/2 =
1.2x1014 cm-3 = 3.7 mTorr
An electron oscillates in a rf
electric field without gaining
energy
unless
electron collisions occur
Hopwood and Mantei, JVST A21, S139 (2003)
Stochastic Heating
an electron enters and exits a region of high field for a fraction of an rf cycle
t0 << 2p/w
Reflecting Boundary (plasma sheath)
Emax
ERF
z
x
-
E~0
vx(t0) > vx(0)
The usual mechanism for heating electrons using RF electric fields at low pressures
Wave/Resonant Heating
Ex
t1
t3
t2
-
-
x
k
Electron cyclotron frequency:
BDC
wce = qB/me = 1.76x107 B(gauss)
ERF
E=0
x
v
F = q(vxB)
y
If w  wce and ERF is perpendicular to BDC,
then the electron gains energy from Ex in
the absence of collisions.
Ex. f=2.45 GHz --> B=875 G
W/cm3
Hopwood and Mantei, JVST A21, S139 (2003)
Electron Collisions
Power
light
Gas flow
PLASMA
gas
(ng)
Wall
electrons
ne, Te
excited atoms
and molecules
ions
Wall
radicals,
molecular fragments
reaction
products
secondary
electrons
substrate
pumping
pumping
Electron Collisions
• Elastic Collisions:
– Ar + e  Ar + e
– Gas heating: energy is coupled from e to the gas
• Excitation Collisions
– Ar + ehot  Ar* + ecold, Ar*  Ar + hn
– Responsible for the characteristic plasma “glow”
– Eelectron>Eexc (~11.55 eV for argon)
• Ionization Collisions:
– Ar + ehot  Ar+ + 2ecold
– Couples electrical energy into producing more e_
– Eelectron > Eiz (15.76 eV for argon)
• Dissociation:
– O2 + ehot  2O + ecold or O2 + ehot  O + O+ + ecold
– Creates reactive chemical species within the plasma
– Eelectron > Ediss (5.12 eV for oxygen)
Collision Cross Sections
• Unlike the hard sphere model, real collision cross
sections are a function of electron kinetic energy s(E), or
electron velocity s(v).
• We must find the expected collision frequency by
averaging over all E or v.
1
v(cm / sec)
inelastic 

 vsngas
t 
l (cm)
becomes

...where l  1 / sngas
inelastic  ngas  s v   ngas  s (v)v f (v)dv 
0
K  sv 
(cm3s-1)
Graphically
f(E) or s(E)
f(E)
sAr(E)
Note: the exponential tail of energetic
electrons is responsible for ionization
Te
Eiz
Electron energy, E
The RATE CONSTANT: Kiz(Te)  Kizoexp(-Eizo/Te)
curve fitting
Graphically
f(E) or s(E)
Hot electrons – more ionization
f(E)
sAr(E)
Note: the exponential tail of energetic
electrons is responsible for ionization
Te
Eiz
Electron energy, E
The RATE CONSTANT: Kiz(Te)  Kizoexp(-Eizo/Te)
curve fitting
Examples of Numerically Determined Rate Constants (Lieberman, 2005)
Generation Rate of Plasma
Species by Electron Collisions
y+ex+e
dnx/dt = Kxneny
For example,
e  Ar+ +
Ar +
e+e
dne/dt = Kiznengas
is the number of electrons (and ions) generated
per cm3 per second
Electron-Ion Recombination
Three-Body Problem:
e + Ar+ + M  Ar + M
the third body is needed to conserve energy and momentum in the
recombination process
wall recombination
volume recombination
-
-
M
M
M
+
+
wall recombination dominates at low pressure because three body collisions are rare
Transport to Surfaces
Power
light
Gas flow
PLASMA
gas
(ng)
Wall
electrons
ne, Te
excited atoms
and molecules
ions
Wall
radicals,
n = ¼ n<v>
molecular fragments
reaction
products
secondary
electrons
substrate
pumping
pumping
Electron and Ion Loss to the Substrate and Walls
- the plasma sheath -
-
-
-
chamber
neni
r0
-
-
-
-
-
-
electrons are much more mobile than ions
me = q<t>/me >> q<ti>/mi = mi
Electron and Ion Loss to the Substrate and Walls
- the plasma sheath s
ne<<ni
n e = ni
(sheath)
-1kV
r(x) +
+
0v
V
x
x
V(x)
+
e
x
(after Mahan, 2000)
low energy electrons are trapped within the plasma, but ions are
accelerated by the sheath potential to the chamber walls and substrate
Ion Flux
The ion flux to a solid object is determined by
the Bohm velocity (or sound speed) of the
ion:
uB = (kTe/mi)1/2 = 9.8x105 (Te/M)1/2 cm/s
=9.8x105 (3 eV/40 amu)1/2 ~ 2.5x105 cm/s
…and the ion flux is given by i = uBni (cm-2s-1)
(this is the ion speed at the edge of the sheath)
Electron Flux
• Only the most energetic electrons can
overcome the sheath potential, Vs.
• e = ¼ ne<ve> exp (qVs/kTe)
Boltzmann factor
f(E)
flux to surface
Te
qVs
Electron energy, E
Sheath Potential, Vs
In the steady state, the electron and ion fluxes to
the chamber/substrate must be equal, if there is
no external current path
e = i
¼ ne<ve> exp (qVs/kTe) = uBni = (kTe/mi)1/2 ne
giving
Vs = -Teln(mi/2pme) ~ -5Te
This is often called the floating potential:
Isolated surfaces have a negative potential relative to the plasma.
Ion Energy
Ex: Assuming argon with Te = 3 eV,
s
the ion energy at the cathode is
Ei = q(1 kV + 4.7Te) = 1014 eV
ignoring ion-neutral collision within s,
and the ion energy at the anode is
-1kV
0v
Ei = 4.7 Te = 14 eV
V
Ion mean free path:
x
li = 1/ngassi ~ 3/p (cm) for Ar+
…where p is the pressure in mTorr
Here li = 3/100 cm or 0.3 mm @ 0.1 torr
NOTE: s>>li  Ei << 1014 eV!
(after Mahan, 2000)
Particle Conservation
and Electron Temperature
A simple model for electron temperature can
be found for a steady state plasma:
# of ions created/sec = # of ions lost/sec
KizngasneV = uBniAeff
ne=ni
Kiz/uB = Kizoe-E /kT /(kTe/mi)1/2 = Aeff/(V ngas)
=1/deffngas
iz
e
(V=plasma volume, Aeff = effective chamber area, deff = V/Aeff)
Single-step vs. Two-step Ionization
The electron temperature (Te) is a
unique function of
7
1. gas density, ngas (pressure)
2. chamber size, deff = V/Aeff
6
11
n0 = 1 x 10 cm
3. gas type: Kiz, Eiz
-3
5
Example:
Two large parallel plates separated by
2 cm are used to sustain an argon
plasma at 25 mTorr. Find Te.
Te (eV)
single-step
Ar + e  Ar+ + 2e
4
3
two-step
deff = V/Aeff ~ pR2d / (pR2 +pR2) = d/2
2
ngasdeff ~ (25*3.2x1019m-3)(0.01m)
=0.8e+19 m-2
1
Ar+eAr*+e
Ar* + e  Ar+ + 2e
0
Te = 3 eV
1e+18
1e+19
1e+20
ngdeff (m-2)
(Note: we have assume that the plasma density is uniform)
1e+21
1e+22
Power Conservation
and Electron Density, ne
Power Absorbed by the Plasma = Power Lost from the Plasma
Pion
Pelectron
Pabs = [qniuBEion+q(¼ne<ve>eV /kT )Eelec]Aeff
+(Pheat+Plight+Pdiss)
≡ qneuBAeff(Eion + Eelec + Ec)
s
qVs
e
2Te
where EC is the collisional energy lost in creating an
electron-ion pair due to ionization, light, dissociative
collisions, and heat:
EC = [nizEiz + nexEex + ndissEdiss + nm(3me/mi)Te]/niz
Collisional Energy Loss
EC
L (eV)
104
103
N2
102
Ar
101
1
2
3
4
Te (eV)
5 6 7 8 910
Electron Density Example
Continuing with the previous example
A plasma is sustained in argon at 25 mTorr between to parallel
plates separated by 2 cm. The radius of the plates is 20 cm and the
power absorbed by the plasma is 100 watts. Find ne.
100 W = qneuBAeff(Eion + Eelec + Ec)
= (1.6x10-19C)ne(2.5x105cm/s)(2px202 cm2) x
(5Te + 2Te + 55 eV)
 ne = 1.3x1010 cm-3
Find ne if the gas is N2, assuming that Te ~ 3 eV
100 W = (1.6x10-19C)ne(2.5x105cm/s)(2px202 cm2)(5Te + 2Te + 400 eV)
 ne = 2.3 x 109 cm-3
Example (cont’d)
Repeat the previous example using argon, BUT include an
electrode voltage of 1000v that is applied to one plate to
sustain the plasma.
100 W = qneuBAeff(Eion + Eelec + Ec)
= (1.6x10-19C)ne(2.5x105cm/s)(px202 cm2) x
{(5Te + 2Te + 55 eV)+[(1000 eV+5Te) + 2Te + 55 eV]}
anode
cathode
 ne = 1.7x109 cm-3
Secondary Electrons
e = gsec i , where gsec~0.1-10 and Ee ~ qVs
Power
light
Gas flow
PLASMA
gas
(ng)
Wall
excited atoms
and molecules
secondary
electrons
electrons
ne, Te
ions
Wall
radicals,
molecular fragments
reaction
products
secondary
electrons
substrate
pumping
pumping
Summary
Power
light
Gas flow
PLASMA
gas
(ng)
Wall
electrons
ne, Te
excited atoms
and molecules
ions
Wall
radicals,
molecular fragments
reaction
products
secondary
electrons
substrate
pumping
pumping
Basic Plasma Technology
Basic Plasma Technology
Sputtering Magnetron
DC
Magnetron
Pulsed
RF
S
N
Substrate
to pump
Target
N
S
S
N
Basic Plasma Technology
Capacitively Coupled Plasma
0.4 – 60 MHz
Hopwood and Mantei, JVST A21, S139 (2003)
Basic Plasma Technology
Electron Cyclotron Resonance Plasma
Hopwood and Mantei, JVST A21, S139 (2003)
Basic Plasma Technology
Inductively Coupled Plasma
0.4 – 13.56 MHz
Hopwood and Mantei, JVST A21, S139 (2003)
ICOPS Minicourse
on Plasma Processing
Technology
Part 3: Physical Vapor Deposition
Jeff Hopwood
Northeastern University
OUTLINE
• Evaporation
• Sputtering
• Ionized Physical Vapor Deposition
Deposition by Evaporation
Heating materials until the vapor pressure is
non-zero followed by condensation of the
vapor on a (relatively cold) substrate.
-
electron beam evaporation
thermal evaporation
molecular beam epitaxy
laser ablation
arc deposition
Two Types of Evaporation
Quasi-equilibrium: liquid-vapor
equilibrium within the cell
Non-equilibrium: vapor is emitted
into a low pressure volume;
no liquid-vapor equilibrium
S.M. Rossnagel, J. Vac. Sci. Technol. A 21, Sep-Oct 2003
Typical e-beam evaporator
Vapor Pressure vs. Temperature
Differing vapor
pressures make
evaporation of alloys
quite difficult
Ex: TiW
Solutions:
Rod-fed evaporation
Multiple evaporators
Evaporative Flux and Conformality
Sputtering Outline
• RF Diodes vs. Magnetrons
– electron and ion density
• Mechanism: collision cascades
– sputter yield
– angular distribution
– Thomson distribution
• Reflected neutrals and ion-assisted dep.
– film morphology and film stress
• Sputtering Alloys
• Reactive sputtering
Sputtering System
Magnetron
Magnetron Sputtering
Target
B-field traps secondary electrons near the target surface
TARGET ~300v
B
P = 0.1 – 5 mTorr
B = 200 G
VTARGET ~ 300 v
+
ne ~ 1012 cm-3
+
Ee ~ 300 eV
 300 eV / (15.76 eV/ion) = 19 ions per secondary
rce = ve/wce ~ 3 mm
ne ~ 109 cm-3
Physical Outcomes of Ion Bombardment
Surface Binding Energy, Usb
Collision Cascade
Sputtering Yield:
Y(E) = number of ejected atoms per incident ion
Empirical Yield: Y(E) = 6.4x10-3 mT g5/3 E1/4(1-Eth/E)3.5
where g = 4mgmT / (mg+mT) 2
Eth ~ 4Usb/g
J. Bohdansky, J. Roth, and H.L. Bay, “An analytical formula and important parameters for low-energy ion sputtering,” J. Appl. Phys. 51(5) 2861- (1980).
J.E. Mahan, Physical Vapor Deposition of Thin Films, p. 207ff (Wiley, New York, 2000).
Angular Distribution of Sputtered Atoms
s (E,q) = Y(E) i (cos q)/p
TARGET
i
…poor conformal coating of
trenches, vias and 3D topography
Thompson Distribution of Energies for
Sputtered Neutral Atoms
1.2
1
Sputtered Al
energy
distribution
(dn/dE)
and
integration
0.8
EsU sb
dn

dEs ( Es  U sb )3
0.6
0.4
0.2
0
0
10
20
30
40
50
60
70
Es Energy (eV)
Most probable energy is half the surface binding energy, Usb/2
There is a very broad high energy tail, such that <Es> ~ 10’s eV
Energetic Deposition
Enhances adatom surface mobility  promotes dense films, film stress
TARGET
+
d < ln
+
n
SUBSTRATE
The substrate surface may be
energetically bombarded by
1. reflected inert gas atoms
2. energetic sputtered atoms
3. inert gas ions
s < li
Sputtering of Alloys
• Alloys may be stoichiometrically sputtered,
even if the component materials have
different sputter yields, due to
conservation of mass
• The target surface must be “conditioned”
by pre-sputtering prior to actual deposition
Ya>Yb
conditioning makes the surface deficient in element “a”
Reactive Sputtering
N2, O2, (+Ar)
Metals
Metal-nitrides
and metal-oxides
Reactive Sputtering
“poisoned”
target,
e.g. TiN
metal
target,
e.g. Ti
S.M. Rossnagel, J. Vac. Sci. Technol. A 21, Sep-Oct 2003
Ionized Physical Vapor Deposition
IPVD
IPVD Outline
• Introduction
–
–
–
Definitions
Motivations
Example Application
• The physics of IPVD
• The evolution of IPVD technology
–
–
–
–
Unbalanced magnetron
geometric filtering of neutrals (collimated, long-throw)
auxiliary high density plasma
– ICP and ECR
high power density sputtering
– SIP, hollow cathode magnetron, pulsed sputtering
• Deposition and modeling
• Summary and discussion
Ionized-PVD Definition
• Physical vapor deposition in which more than
half of the deposited atoms arrive at the
substrate as ions.
• e.g., for sputtering of metals m+ > m at the
substrate.
m
m+
• Includes sputtering, evaporation, arcs, laser
ablation…
Motivation
• The energy and direction of ionized
depositing species are easily controlled with
electrostatic fields and/or sheath voltage.
• In contrast, neutral depositing species are
difficult to control.
+
Vbias
Comparison of Sputtering Techniques
tt
w
d
tb
(tb/tt)
(d/w)
Ref: S.M. Rossnagel, J. Vac. Sci. Technol. B, 16(5), 2585 (1998)
Example Application
Deposition into High Aspect Ratio Trenches and Vias
K. Lai, ibid.
Metal n+1: Cu
Cu Via
TaN + Cu seed
Via
Ta
Connection
to external
package
Metal n: Cu
Via
Metal 3
Contact
Via
ILD
M2
Metal 2
Via
Via
Metal 1
M1
Contact
G
S
Well
ILD
M1
D
Si Wafer (not to scale)
J. Forster, Ionized Physical Vapor Deposition, Academic, 2000.
J. Hopwood, Ionized Physical Vapor Deposition, Academic, 2000.
Example Application
A dual damascene process
Via etch
ILD
(a)
Fill
ILD
Metal
(d)
Trench etch
Remove by CMP
Metal Interconnect
(b)
(e)
I-PVD of
Barrier and Seed layers
(c)
J. Hopwood, Ionized Physical Vapor Deposition, Academic, 2000.
ILD
ILD
Metal
Outline
• Introduction
–
–
–
Definitions
Motivations
Example Application
• The physics of IPVD
• The evolution of IPVD technology
–
–
–
–
Unbalanced magnetron
geometric filtering of neutrals (collimated, long-throw)
auxiliary high density plasma
– ICP and ECR
high power density sputtering
– SIP, hollow cathode magnetron, pulsed sputtering
• Deposition and modeling
• Summary and discussion
Ionized PVD Physics
• Ionization mechanism
• Sputter neutral energies
• Ionization mean free path
– thermalization vs. long path length
• Ionized flux vs Ionization fraction
• Spatial Distribution: Diffusion
Ionization Mechanism
Penning ionization vs. Electron collisions
• Penning ionization: Ar* + M  M+ + Ar
– Eex > 11.55 eV for argon
– Eiz = 6 - 8 eV for most metals
– Kp = sp(kTg/M)1/2
• Electron impact: M + e  M+ + 2e
–
–
–
–
for moderate sputter rates, nAr >> nM
Te is determined by argon ionization energy
Eiz(argon) >> Eiz (metal)
Kiz= ko exp(-Eiz(M)/kTe(Ar))
IPVD Physics: the metal
Ionization
of thermal
metal
atoms, M
Lifetime
of an ion:
diffusion
to walls (R,L)
Particle balance: creation rate of metal ions (Penning, electron impact, and 2-step)
is equal to the loss of metal ions due to diffusion to the chamber walls and substrate
IPVD Physics: the argon
generation and
loss of Ar+
generation and
loss of Ar*
IPVD Physics: metal ionization
electron density, from Pabs
ion lifetime
argon density (pressure)
excited argon lifetime
rate constant for excited Ar*
rate constant for Penning ionization of metal
rate constant for electron impact ionization of metal
Ionization Mechanism
Penning ionization vs. Electron collisions
Typical sputtering,
ne~1010 cm-3
Penning Dominates
Ar*+MM++Ar
Ionization is <5%
IPVD, ne~1012 cm-3
Electron Impact
Dominates
M + e  M+ + 2e
Ionization is >50%
J. Hopwood, J. Appl. Phys, 1995
Sputtered Neutral Energy
and Ionization MFP
from W.M. Holber, Ionized Physical Vapor Deposition, Academic, 2000.
MFP~5 cm
required to
ionize 50% Al
1.2
liz = vs / Kizne
5
Sputtered Al
energy
distribution
(dn/dE)
and
integration
0.8
0.6
0.4
0.2
0
0
10
20
30
40
50
60
70
Ionization Length (cm)
1
4
3
2
ne = 2x1012 cm-3
Te = 3 eV
1
Energy (eV)
50% of Al
has E>6 eV
0
0
2
4
6
8
Al-I Energy (eV)
12 over a distance of 10 cm
50% ionization requires at least ne~10Figure
3: Calculated aluminum ionization length as funct
of aluminum kinetic energy. (ne=2.e12 cm^-3, Te=3-3.5
High Pressure IPVD:
Thermalize, Ionize, Collimate
10-50 mTorr
lth ~ 24/p cm
presheath
Limitation due to the Presheath
presheath width  ion mean free path  collisions 
decreased collimation of ions
plasma
M+
V0
presheath (li)
Ar
V0 - (Te / 2)
sheath (s)
0 volts
wafer
Titanium Transverse Temperature at the Sheath Edge (s)
and Ionized Flux Fraction (%)
Target Power = 1 kW
1 kW ICP
2 kW ICP
10 mTorr
0.13 eV
50%
0.17 eV
75%
30 mTorr
0.15 eV
70%
0.18 eV
85%
- This measurement corresponds to an ion divergence of ~5o to an unbiased wafer
- Consistent with Monte-Carlo simulations
 increase V0 to narrow the angular distribution
Help from the Presheath
• Ions are accelerated by the presheath region
into the wafer
– ion flux: M+= 0.61 nM+(kTe/M)1/2
• Thermal neutrals simply diffuse to the wafer
– neutral flux: M= 1/4nM(8kTM/pM)1/2
• Ionization of the depositing metal flux is
M+/M = k(nM+/nM)(Te/TM)1/2
Typically, Te >> TM
Neutral and Ion Diffusion
Al Density (x10 10 cm -3 )
J. Hopwood, Ionized Physical Vapor Deposition, Academic, 2000.
Li, Vyvoda, and Graves, Ionized Physical Vapor Deposition, Academic, 2000.
102
n Al
101
n Al+
100
0
2
4
6
8
10
12
14
Distance Below the Target, z (cm)
Experiment
T
+
2D Model
Figure 13
Hopwood
W
W
+
T
Outline
• Introduction
–
–
–
Definitions
Motivations
Example Application
• The physics of IPVD
• The evolution of IPVD technology
–
–
–
–
Unbalanced magnetron
geometric filtering of neutrals (collimated, long-throw)
auxiliary high density plasma
– ICP and ECR
high power density sputtering
– SIP, hollow cathode magnetron, pulsed sputtering
• Deposition and modeling
• Summary and discussion
The Evolution of IPVD
• Unbalanced Magnetron
• Geometric Filtering of Errant Neutrals
• Magnetron plus…
– Inductively Coupled Plasma
– ECR Plasma
• “Self-ionized” – Unbalanced Reprise
– High power density/Geometric/Pulsing
Unbalanced Magnetron
• Unbalanced magnetic field allows electrons
and ions to escape away from target
• ne ~ 1012, but not over a distance of 10 cm.
6 cm
• Metal ionization <50%,
but a much greater Ar+
flux than balanced
magnetron
• Allows large argon ionto-metal flux ratios
Windows and Savvides, J. Vac. Sci. Technol. A 4, 453 (1986)
Geometric Filtering of High-Angle Neutrals
• Collimated Sputtering
–
Rossnagel et al, J. Vac. Sci. Technol A9,
261 (1991).
• Long-throw Sputtering
–
Motegi et al, J. Vac. Sci. Technol. B 13,
1906 (1995).
ECR ionized evaporation
–Holber et al, J. Vac. Sci. Technol. A 11, 2903 (1993).
Magnetron Plus…
• Inductively Coupled Plasma (ICP)
• Electron Cyclotron Resonant Plasma
(ECR)
• Helicon Wave Plasma
– all provide ne ~ 1012 cm-3 over >10 cm
Magnetron plus…
…plus ICP from M. Yamashita, J. Vac. Sci.
Technol. A 7, 151, (1989).
…plus ECR from W.M. Holber, Ionized
Physical Vapor Deposition, Academic, 2000.
“Self-ionized” IPVD –
Unbalanced Magnetron Reprise
•
•
•
•
•
•
Increase electron density
Increase plasma length
Use geometric filtering of neutrals
Low pressure (no thermalization req’d.)
Medium-Long throw distance
Simplified operation: no auxiliary
plasma is required for ionization
Self-ionized Plasma, SIP
Fu et al, US Patent 6,251,242, June 6, 2002.
Hollow Cathode Magnetron
Helmer, Lai, and Anderson,
US Patent 5,482,611, Jan. 9, 1996
“Self-ionized” IPVD –
Unbalanced Magnetron Reprise
• Increase electron density
• Higher power (10’s kW) in smaller area of magnetic field
• Requires aggressive cooling/rotation/pulsed power
• Increase plasma length
K.Lai, Ionized Physical Vapor Deposition,
Academic Press, 2000.
• Intense plasma inside hollow cathode
• Use geometric filtering of neutrals
• Ions are extracted by magnetized electrons
• Neutrals are transported across cathode 
Target
+ -
“Self-ionized” IPVD –
Pulsed Target Power
• Pulsed Power
• 2000V x 1200 A
• 50 – 100 ms pulse
• 50 Hz rep rate
• Typically,
•
•
•
•
600 W/cm2
Ppeak ~ 100 kW
ne ~ 1-5x1012 cm-3
Pave ~ 500 W
K.Macak, V.Kouznetsov, J.Schneider, U.Helmersson,
I.Petrov, “Ionized sputter deposition using an
extremely high density plasma,”J. Vac. Sci.
Technol. A 18, 1533 (2000).
Outline
• Introduction
–
–
–
Definitions
Motivations
Example Application
• The physics of IPVD
• The evolution of IPVD technology
–
–
–
–
Unbalanced magnetron
geometric filtering of neutrals (collimated, long-throw)
auxiliary high density plasma
– ICP and ECR
high power density sputtering
– SIP, hollow cathode magnetron, pulsed sputtering
• Deposition and modeling
• Summary and discussion
Deposition Modeling
A comparison of models and
measured film characteristics
1. metal deposition by IPVD
2. metal-nitride reactive IPVD
Ionization of Metal
no substrate bias (Y=0)
• Higher ionization of
the metal results in
more deposition at
the bottom of the
trench
• Inward growth of the
overburden forms a
“keyhole” structure
S. Hamaguchi and S. M. Rossnagel, J. Vac. Sci.
Technol. B 13, 183 (1995).
33%
50%
67%
Substrate Biasing
• Energetic ions
bombard the film
and cause
– faceting
– sputtering and
redeposition
• A primary means
of coating the
sidewalls of
trenches
V.Arunachalam, S.Rauf, D.G.Coronell, and
P.L.G. Ventzek, J. Appl. Phys. 90, 64
(2001):
Lu and Kushner,
JVST A 19, 2652 (2001)
Surface Diffusion
Thermal and Ion Induced
Low density films
Comparable to experimental results
Conformal (“reflow”) coatings
-- observed at high temperatures
Lu and Kushner, J. Vac. Sci. Technol. A 19, 2652 (2001)
Reactive Sputtering
TiN deposition using IPVD
contour lines are the film resistivity
3.0
48
Area II
Area III
Area IV
52
68
RF Power [kW]
2.5
60
68
2.0
92
64
80
100
1.5
124
88
64
Area I
168
1.0
0
72
116
20
40
60
80
100
120
Bias Voltage [V]
Tanaka, Kim, Forster, and Xu, J. Vac. Sci. Technol. B 17, 416 (1999).
Reactive Sputtering
48
1.5
1.0
Area IV
32 34 36 38 40 42 44 46
Diffraction Angle 2q [°]
52
68
TiN <111>
92
TiN < 200 >
Area III
60
2.0
XRD Signal [arb. units]
Area II
32 34 36 38 40 42 44 46
Diffraction Angle 2q [°]
XRD Signal [arb. units]
RF Power [kW]
2.5
TiN < 200 >
XRD Signal [arb. units]
3.0
XRD Signal [arb. units]
TiN deposition using IPVD
32
34 36 38 40 42 44 46
Diffraction Angle 2q [°]
68
64
80
100
124
88
64
Area I
168
72
116
32 34 36 38 40 42 44 46
Diffraction Angle 2q [°]
0
20
40
60
80
100
120
Bias Voltage [V]
Tanaka, Kim, Forster, and Xu, J. Vac. Sci. Technol. B 17, 416 (1999).
Reactive Sputtering
TiN deposition using IPVD
3.0
48
Area II
Area III
Area IV
52
68
RF Power [kW]
2.5
60
68
2.0
92
64
80
100
1.5
124
88
64
Area I
168
1.0
0
72
116
20
40
60
80
100
120
Bias Voltage [V]
Tanaka, Kim, Forster, and Xu, J. Vac. Sci. Technol. B 17, 416 (1999).
Plasma Composition in Ti/N2/Ar
15-30% dissociation of N2
K. Tao, et al., J. Appl. Phys. 91, 4040 (2002)
Plasma Composition in
Ti/N2/Ar
15 mTorr, ne = 5x1011 cm-3, Tg = 400 K
1.00E+15
Density (per cc)
1.00E+14
Species
depositing
at the
bottom of
trenches
and vias:
1.00E+13
1.00E+12
1.00E+11
1.00E+10
Titanium is
greater than
1.00E+09
Nitrogen
1.00E+08
Ar
N2
N
N2*
Ar*
Ar+
Ti
Ti+
N2+
Species depositing at the top of trenches and
vias: Nitrogen is greater than Titanium
K. Tao, et al., J. Appl. Phys. 91, 4040 (2002)
N+
Film Composition in a Trench
Less nitrogen is transported to bottom of the trench (Eiz(N2)>>Eiz(Ti))
4% N2 in Ar
10% N2 in Ar
N
N
metal mode
nitride mode
D. Mao, J. Appl. Phys., Vol. 96, No. 1, 820, 1 July 2004
[0-d chemistry, 2-d fluid, Monte-Carlo sheath, geom. flux]
Experimental (RBS) and Simulated Composition Ratios
1.6
metal mode
1.4
nitride mode
Composition (TiNx)
1.2
1.0
0.8
0.6
model (no bias)
experiment (no bias)
model (-50 v)
experiment (-50 v)
0.4
0.2
1 kW/1kW/50 sccm Ar
0.0
0
1
2
3
4
5
Nitrogen Flow (sccm)
from D. Mao, PhD Thesis, Northeastern University, 2003
6
Summary
• IPVD allows for energetic deposition of dense
films into high aspect ratio microstructures
• Recent advances have provided a relatively
simple technology: Self-Ionized Plasma
• Issues with multicomponent IPVD
– disparate ionization potentials and dissociation
Some Recommended Reference Materials
• Basic Low Pressure Plasma Physics and Processing
– M.A. Lieberman and A.J. Lichtenberg, Principles of Plasma
Discharges and Materials Processing (Wiley, 2005)
– D. Manos and D. Flamm, eds., Plasma Etching (Academic
Press, 1989)
• Vacuum Science and Physical Vapor Deposition
– J. E. Mahan, Physical Vapor Deposition of Thin Films (Wiley,
2000)
– S.A. Campbell, The Science and Engineering of Microelectronic
Fabrication, (Oxford, 2001)
– J. Hopwood, ed., Ionized Physical Vapor Deposition (Academic
Press, 2000)