This document discusses physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques for thin film deposition. It covers common PVD methods like thermal evaporation, sputtering, and molecular beam epitaxy. It also discusses CVD reaction mechanisms, step coverage, and overview. Key aspects include comparing evaporation and sputtering, deriving equations for mean free path and deposition rate, and factors affecting step coverage in CVD like temperature and pressure.

1. ECE614: Device Modelling
and Circuit Simulation
Unit 2 PVD & CVD
By Dr. Ghanshyam Singh

2. Content
Physical vapor deposition
(PVD)
– Thermal evaporation
– Sputtering
– Evaporation and sputtering
compared
– MBE
– Laser sputtering
– Ion Plating
– Cluster-Beam
Chemical vapor deposition
(CVD)
– Reaction mechanisms
– Step coverage
– CVD overview
Epitaxy

3. Physical vapor deposition (PVD)
The physical vapor deposition technique is based on the formation of vapor of
the material to be deposited as a thin film. The material in solid form is either
heated until evaporation (thermal evaporation) or sputtered by ions
(sputtering). In the last case, ions are generated by a plasma discharge usually
within an inert gas (argon). It is also possible to bombard the sample with an
ion beam from an external ion source. This allows to vary the energy and
intensity of ions reaching the target surface.

4. Physical vapor deposition (PVD):
thermal evaporation
Heat Sources Advantages Disadvantages
Resistance No radiation Contamination
e-beam Low contamination Radiation
RF No radiation Contamination
Laser No radiation, low
contamination
Expensive
N = No exp-
Φe
kT
6
The number of molecules
leaving a unit area of evaporant
per second
No = slowly varying function of T
ϕe = activation energy required to
evaporate one molecule of
material
k = Boltzman’s constant
T = temperature

5. Physical vapor deposition (PVD): thermal
evaporation
Si
Resist
d
ββββ
θθθθ
Evaporant container
with orifice diameter DD
Arbitrary
surface element
Kn = λλλλ/D > 1
A ~ cosββββ cos θθθθ/d2
N (molecules/unit area/unit time) =
3. 513. 1022
Pv(T)/ (MT)1/2
The cosine law
This is the relation between vapor pressure of
the evaporant and the evaporation rate. If a high
vacuum is established, most molecules/atoms will reach
the substrate without intervening collisions. Atoms and
molecules flow through the orifice in a single straight
track,or we have free molecular flow :
The fraction of particles scattered by collisions
with atoms of residual gas is proportional to:
The source-to-wafer distance must be smaller than the mean free path (e.g, 25 to 70 cm)

6. Physical vapor deposition (PVD): thermal
evaporation
ββββ2222 = 70= 70= 70= 700000
ββββ1111 = 0= 0= 0= 00000
t2
t1
Substrate
t1
t2
=
cos ββββ1
cos ββββ2
≈≈≈≈ 3
Surface feature
Source
Source
Shadow
t1/t2=cosββββ1111/cosββββ2222
λλλλ = (ππππRT/2M)1/2
ηηηη/PT
From kinetic theory the mean free path relates
to the total pressure as:
Since the thickness of the deposited film, t, is proportional
to the cos β, the ratio of the film thickness shown in the
figure on the right with θ = 0° is given as:

7. Physical vapor deposition (PVD): sputtering
W= kV i
PTd
-V working voltage
- i discharge current
- d, anode-cathode distance
- PT, gas pressure
- k proportionality constant
Momentum transfer

8. Evaporation
and
sputtering:
comparison
Evaporation Sputtering
Rate Thousandatomic layers per second
(e.g. 0.5 µm/min for Al)
One atomic layer per second
Choice of materials Limited Almost unlimited
Purity Better (no gas inclusions, very high
vacuum)
Possibility of incorporating
impurities (low-medium vacuum
range)
Substrate heating Very low Unless magnetron is usedsubstrate
heating can be substantial
Surface damage Very low, with e-beam x-ray
damage is possible
Ionic bombardment damage
In-situ cleaning Not an option Easily done with a sputter etch
Alloy compositions,
stochiometry
Little or no control Alloy composition can be tightly
controlled
X-ray damage Only with e-beam evaporation Radiation andparticle damage is
possible
Changes in source
material
Easy Expensive
Decomposition of
material
High Low
Scaling-up Difficult Good
Uniformity Difficult Easy over large areas
Capital Equipment Low cost More expensive
Number of
depositions
Only one deposition per charge Many depositions can be carried
out per target
Thickness control Not easy to control Several controls possible
Adhesion Often poor Excellent
Shadowing effect Large Small
Film properties (e.g.
grain size and step
coverage)
Difficult to control Control by bias, pressure,
substrate heat

9. Physical vapor deposition (PVD): MBE,
Laser Ablation
-
MBE
– Epitaxy: homo-epitaxy
hetero-epitaxy
– Very slow: 1µm/hr
– Very low pressure: 10-11
Torr
Laser sputter deposition
– Complex compounds (e.g.
HTSC, biocompatible
ceramics)

10. Chemical vapor deposition (CVD): reaction
mechanisms
Mass transport of the reactant in
the bulk
Gas-phase reactions
(homogeneous)
Mass transport to the surface
Adsorption on the surface
Surface reactions
(heterogeneous)
Surface migration
Incorporation of film
constituents, island formation
Desorption of by-products
Mass transport of by-produccts
in bulk
CVD: Diffusive-convective transport of
depositing species to a substrate
with many intermolecular
collisions-driven by a concentration
gradient
SiH4SiH4
Si

11. Chemical vapor deposition (CVD):
reaction mechanisms
Fl = D
∆c
∆x
δ(x) =
ηx
ρU


 


1
2
δ =
1
L
δ(x)dX =
2
30
L
∫ L
η
ρUL


 


1
2
ReL =
ρUL
η
δ = 2L
3 ReL
Energy sources for deposition:
– Thermal
– Plasma
– Laser
– Photons
Deposition rate or film growth rate
(Fick’s first law)
(gas viscosity η, gas density ρ, gas stream velocity U)
(Dimensionless Reynolds number)
Laminar flow
L
δ(x)
dx
(U)
(Boundary layer thickness)
Fl = D
∆c
2L
3 ReL (by substitution in Fick’s first law and ∆x=δ)

12. Mass flow controlled regime
(square root of gas
velocity)(e.g. AP CVD~ 100-10
kPa) : FASTER
Thermally activated regime:
rate limiting step is surface
reaction (e.g. LP CVD ~ 100
Pa----D is very large) :
SLOWER
Chemical vapor deposition (CVD)
: reaction mechanisms
Fl = D
∆c
2L
3 ReL
R = Ro e
-
Ea
kT

13. Chemical vapor deposition (CVD):
step coverage
Fl = D
∆c
2L
3 ReL
R = Ro e
-
Ea
kT
Step coverage, two factors are
important
– Mean free path and surface
migration i.e. P and T
– Mean free path: λ =
αααα
w
z
θ=180θ=180θ=180θ=1800000
θ=270θ=270θ=270θ=2700000
θ=90θ=90θ=90θ=900000
θ is angle of arrival
kT
2
1
2
PTπa2
> α
Fldθ∫
θ = arctan
w
z

14. Chemical vapor deposition (CVD) :
overview
CVD (thermal)
– APCVD (atmospheric)
– LPCVD (<10 Pa)
– VLPCVD (<1.3 Pa)
PE CVD (plasma enhanced)
Photon-assisted CVD
Laser-assisted CVD
MOCVD

15. The L-CVD method is able to fabricate
continuous thin rods by pulling the substrate
away from the stationary laser focus at the
linear growth speed of the material while
keeping the laser focus on the rod tip, as shown
in the Figure . LCVD was first demonstrated
for carbon and silicon rods. However, fibers
were grown from other substrates including
silicon, carbon, boron, oxides, nitrides,
carbides, borides, and metals such as
aluminium. The L-CVD process can operate at
low and high chamber pressures. The growth
rate is normally less than 100 µm/s at low
chamber pressure (<<1 bar). At high chamber
pressure (>1 bar), high growth rate (>1.1
mm/s) has been achieved for small-diameter (<
20 µm) amorphous boron.
Chemical vapor deposition (CVD) : L-CVD

16. Epitaxy
VPE:
– MBE (PVD) (see above)
– MOCVD (CVD) i.e.organo-metallic
CVD(e.g. trimethyl aluminum to
deposit Al) (see above)
Liquid phase epitaxy
Solid epitaxy: recrystallization of
amorphous material (e.g. poly-Si)
Liquid phase epitaxy

17. Epitaxy
Selective epitaxy
Epi-layer thickness:
– IR
– Capacitance,Voltage
– Profilometry
– Tapered groove
– Angle-lap and stain
– Weighing
Selective epitaxy

18. Homework
Homework: demonstrate equality of λ = (πRT/2M)1/2 η/PT and λ = kT/2 1/2 a 2 π PT
(where a is the molecular diameter)
What is the mean free path (MFP)? How can you increase the MFP in a vacuum
chamber? For metal deposition in an evaporation system, compare the distance
between target and evaporation source with working MFP. Which one has the
smaller dimension? 1 atmosphere pressure = ____ mm Hg =___ torr. What are the
physical dimensions of impingement rate?
Why is sputter deposition so much slower than evaporation deposition? Make a
detailed comparison of the two deposition methods.
Develop the principal equation for the material flux to a substrate in a CVD process,
and indicate how one moves from a mass transport limited to reaction-rate limited
regime. Explain why in one case wafers can be stacked close and vertically while in
the other a horizontal stacking is preferred.
Describe step coverage with CVD processes. Explain how gas pressure and surface
temperature may influence these different profiles.