Thin Film 2014
PVD - Evaporation
Oct 2014
Ming-Show Wong
MSE / NDHU
Vapor Phase Deposition
• Many diverse deposition techniques available
• All are more or less based on three basic methods
for providing a flux of atomic or molecular material
– evaporation (PVD): by thermal energy
– sputtering (PVD): by impact of (inert) gaseous ions
– chemical vapor deposition (CVD)
• PVD processes are generally characterized by
–
–
–
–
solid or molten sources
physical mechanism – evaporation or collisional impact
vacuum environment
absences of chemical reactions (usually)
2
Thin Film Deposition
Chemical Processes
Sol gel
Plating
Physical Processes
CVD
Evaporation
Sputter
Electroplating
MOCVD
Thermal
DC
Electroless
PECVD
Electon beam
RF
Thermal
MBE
Magnetron
Arc Evap.
Pulsed DC
Laser Ablation
Ion Beam
3
Which Process to use?
• Not always obvious
• Competition among alternative methods
• Development of hybrid processes
4
PVD - Evaporation
• The Physics and Chemistry of Evaporation
– Evaporation rate
- Vapor pressure of the elements
– Evaporation of compounds
– Evaporation of alloys
• Film Thickness Uniformity and Purity
– Deposition geometry
– Conformal Coverage
- Film thickness uniformity
- Film Purity
• Evaporation Hardware and Techniques
– Resistance-heated evaporation sources
– Electron-beam evaporation
– Other heat sources
5
EVAPORATION
• First observed by Faraday in 1857, observed thin
films from metal wires resistively-heated in an
inert gas
• (In 1852, Grove observe metal deposits sputtered
from the cathode of a glow discharge)
• Development of vacuum pumps and resistivelyheated sources (e.g. Pt and W wires) led to an
evaporated thin film technology
• Early applications: mirrors, beam splitters
6
EVAPORATION
The conversion of a substance from the liquid or solid state
(source) into the gaseous state by heating.
1: An evaporation technique utilizing a resistively
FILAMENT heated filament, usually composed of refractory
metal wire or foil, for evaporating a source material which has
been previously applied to the filament.
2: A deposition technique utilizing a resistively heated
filament composed of the source material itself to produce
sublimation of the source.
- vapor sources having high vapor pressures at temperatures
below the melting temperature and which are consequently
able to vaporize from the solid phase.
7
Evaporation Rate (Hertz Observation)
• in number of atoms (or molecules) per unit area, per
unit time
– Φe = NAαe (Pe –Ph) (2πMRT)-1/2
– Φe = 3.523x1022 Pe (MT)-1/2 molecules/cm2-s
– Φe x (M/NA) = Mass evaporation rate = Γe
– Γe = 5.84x10-2 Pe (M/T)1/2 g/cm2-s
–
–
–
–
–
–
αe is the evaporation coef., generally taken to be unity
Pe is the vapor pressure of the evaporant (in Torr)
Ph is the hydrostatic pressure surrounding the evaporant
NA is Avogadro’s number
M is the molecular weight
Γe /D = cm/s
D: film density
8
9
Vapor Pressure (of Elements)
• The rate of evaporation (or sublimation) can be
characterized by the equilibrium vapor pressure
– The equilibrium vapor pressure Pe is given by the
Clausius-Clapeyron equation, dP/dT = ΔH(T) / TΔV
dpe / dT  Pe H v / RT 2
– where Hv is the latent heat of evaporation and R is the
gas constant.
– Assuming that Hv is independent of T gives
– P = Poexp (- ΔHe/RT)
• ΔHe: : molar heat of evaporation
– [FIG. 3-1&2] shows vapor pressure data for the
common elements.
10
11
12
Evaporation of multielement materials
• Evaporation of compounds and alloys often yields films with
different composition (See Table 3-1, Ohring)
Compounds:
– Many compounds evaporate dissociatively and noncongruently (e.g. dioxides of Si, Ge, Ti, Zr)
– III-V compounds, such as GaAs, are also good examples
– Materials that evaporate non-dissociatively, e. g. CaF2,
AlN, SiO, can be evaporated to form stoichiometric films
– Some II-VI compounds, such as CdTe, evaporate
dissociatively but congruently (with equal rates), such that
compounds can be formed.
13
14
GaAs Phase Diagram at Low Pressures
1. Growth window must be As-riched
 What will happen if Ga rich?
2. At 10-6 torr, the growth temperature must be between 630
and 1000 K.
 What will happen if temperature fall out of this region?
3. Operation at a lower pressure narrows the usable deposition
range.
106 torr
109 torr
2500
2500
2000
2000
v
1500
1500
v
l v
T (K )
1000
l c
500
 v
cv
c 
0
1000
l v
500
l c
 v
0
Ga
As
cv
c 
15
Ga
As
Two-phase c(InSb) + v field is contracted compared with
that of GaAs
• Vapor pressure of Sb is less than that for As
– Solidus line at lower pressure
• Vapor pressure of In exceeds that for Ga
– Vaporous line at higher pressure
850K
c 
103
100
103
c 
l
l c
100
l c
P(Torr)
10
1000K
cv
3
cv
10
3
10 6
10 6
9
9
l v
v
10
10
v
Ga
As
Ga
As
16
Alloys:
– evaporated flux equals source composition only if solution
is ideal (i.e. Raoultian) -- seldom true
– Roaultian law: vapor pressure of component B in solution
is reduced relative to the vapor pressure of pure B (PB(0))
in proportional to its mole fraction XB. PB = XB PB(0)
– deviations from ideality are common
PB = aB PB(0) where aB = B XB
– while evaporation rates can, in principle, be calculated if
activities are known, the source composition changes
1/ 2
continually 

X
P
(
0
)
M

N
3.513 10
A
A
A A
B
  
P 

2MRT
MT

 B  B X B PB (0) M 1A/ 2
• Solutions to the above problems, involving multiple
evaporation sources
A
e
e
17
22

Pe 

Al-Cu Alloy Deposition
2wt% Cu from single crucible heated to 1350 K
 A  A X A PA (0) M B1/ 2

 B  B X B PB (0) M 1A/ 2
PA (0), PB (0)

X A  A  B PB (0) M 1A/ 2


X B  B  A PA (0) M B1/ 2
X Al 98 / 27.0 2 104 (27.0)1/ 2


 15
3
1/ 2
X Cu
2 / 63.7 110 (63.7)
Not easy to maintain
uniform composition
18
Film Thickness Uniformity and Purity
Deposition geometry
Thickness control
19
Film Thickness Distribution
• The evaporated species arrive at the substrate is
determined by the geometry of the evaporation source and
deposition chamber.
• Assuming a point source (i.e. evaporated flux equal in all
directions) with total flux Jo, the fraction dJ/Jo falling on an
area dA a distance r from the source is given by [FIG. 3-4a]
– If the substrate surface area dAs is at an angle  relative to the
flux, then the projected area is dA = dAscos  , and
– dJ/Jo = dAcosθ/4  r2
• For surface source [Fig. 3-4b] (emission angle, ψ and
receiving angle, θ )
dJ / dAs  J  cos  cos  / r 2
20
Evaporation Source
dAc
Point source
Surface source
21
Point Source n = 0
dAc
dAc  dAs cos 
dM s : M e  dAc : 4r 2
dM s M e cos 

dAs
4r 2
dM s : mass falls on the substrate of dAs
M e : total evaporated mass
22
Knudsen Cell or Effusion Cell n=1
Cosine distribution flow through a hole
dM s M e cos  cos 

dAs
r 2
23
Evaporation Geometry
for highly directional source
Generally, the mass of material emitted from an evaporation source
at a fixed angle is: m () = m cosn 
(n is related to source geometry)
n
dJ / dAs  J  (n  1) cos  cos  / 2r 2
n≧0
24
Film Thickness d
Thickness
Point source
dM s M e cos 

dAs
4r 2
d
M e cos 
Me h
M eh


4r 2
4r 2 r 4 (h 2  l 2 )3 / 2
d
1

d o (1  (l / h) 2 )3 / 2
Surface source dM s  M e cos  cos 
dAs
r 2
dM s
d
dAs
do : thickness at l  0
M e cos  cos 
Me h h
M eh2
d


2
2
r
r r r  (h 2  l 2 ) 2
d
1

d o (1  (l / h) 2 ) 2
25
Film Thickness d
1.0
0.8
0.6
d / do
0.4
Point Source
Surface Source
0.2
0
0
0.5
1.0
l/h
1.5
2.0
26
Two Point Sources
27
Example 1
It is desired to coat a 150-cm-wide strip utilizing two evaporation sources oriented as
shown. If a thickness tolerance of 10% is required, what should the distance between
sources be and how far should they be located from the substrate?
D / hv 
d
 1.1  D / hv  0.6  r / hv  0.87
do
 r  150 / 2  75 cm  hv  75 / 0.87  86.2 cm
0.9 
 2 D  2  0.6  86.2  103.4 cm
It is obvious that the uniformity tolerance can always be realized by extending
the
28
source-substrate distance, but this is wasteful of evaporant.
Example 2
How high above any given source should a 25 cm diameter substrate be rotated to
maintain the desired film tolerance of 1% in thickness?
R = 20 cm, tolerance =  1%  hv/R = 1.33, r/R = 0.6,
hv = 1.3320 = 26.6 cm
29
Example 3
A clever way to achieve thickness uniformity
For Knudsen source only
dM s M e cos  cos  M e r r
Me



 const
2
2
2
dAs
r
r 2ro 2ro 4ro
30
More about Thickness Uniformity
1. Physically, deposition thickness uniformity is achieved
because short source-substrate distances are offset by
unfavorably large vapor emission and deposition angles.
2. Uniformity of columnar grain microstructure, e.g., tilt, is not
preserved, however, because of variable flux incidence angle.
3. Two principal methods for optimizing film uniformity over
large areas involve varying the geometric location of the
source and interposing static as well as rotating shutters
between evaporation sources and substrates.
4. In addition to the parallel source-substrate configuration,
calculations of thickness distributions have also been made for
spherical as well as conical, parabolic, and hyperbolic
substrate surfaces.
5. Similarly, cylindrical, wire, and ring evaporation source
geometries have been treated.
31
Conformal Coverage of Steps and Trenches
32
Computer Modeling of Step Coverage
Line-of-sight motion of evaporant atoms and sticking coefficients of unity can be
assumed in estimating the extent of coverage.
1.
2.
In generating the simulated film profiles surface migration of atoms was neglected,
which is valid assumption at low substrate temperatures.
Heating the substrate increases surface diffusion of depositing atoms, thus
33
promoting coverage by filling potential voids as they form.
Film Purity
Evaporant vapor impingement rate
N Ad / M a
Gas molecule impingement rate

N AP
P
 3.513 1022
( MT )1/ 2
2MRT
# / cm 2 s
Impurity concentration Ci

Ci 
N A d / M a
Ci 
2
5.82 10 PM a
M gT  d
 : film density
d : deposition rate (cm / s)
M a : evaporant molecular weight
M g : evaporant molecular weight
P : residual gas vapor pressure (torr34)
Vacuum Requirements
• The chamber pressure during evaporation must be
sufficiently low to minimize:
– Scattering of evaporated species in the region between the
evaporate source and the substrate
• Minimized for pressures < 10-4 Torr, where the mean free path in
air is ~45 cm.
– background gas impurity incorporation into the film
• depends upon the incorporation probability of the impurity into the
film and the growth rate.
• typical background species present in vacuum systems.
• increasing the growth rate decreases the impurity content of
evaporated films.
• UHV systems are preferred when high purity films are required.
35
Contamination
Maximum oxygen concentration in tin films
1.
2.
3.
In order to produce very pure films, it is important to deposit at very high rates
while maintaining very low background pressures. Typical deposition rates from
electron beam sources can reach 1000Å /s at chamber pressures of ~10-8 torr.
In sputtering processes, deposition rates are typically about two orders of
magnitude lower and chamber pressures four orders of magnitude higher than for
evaporation. Therefore, the potential exists for producing films containing high gas
concentrations. (Not as “clean” a process as evaporation.)
Very high oxygen incorporation occurs at residual gas pressures of 10-3 torr.
Advantage of this fact is taken in reactive evaporation processes where
intentionally introduced oxygen serves to promote reactions with the evaporant
36
metal in the deposition of oxide films.
Types of evaporation sources
• Resistive heating of refractory metal filaments or boats
such as W, Mo, Ta, Nb, or by indirect heating of
quartz, alumina, graphite, etc., boats.
• Flash evaporation -- multicomponent materials in fine
powder form are continuously dropped into a very hot
• Arc evaporation
• Laser evaporation
• RF heating - induction heating of conducting material
in crucible.
• Electron-bombardment heating
• Knudsen cells --commonly used in MBE
37
Evaporation System Requirements
• Vacuum:
– Need 10-6 torr for medium quality films.
– Can be accomplished in UHV down to 10-9 torr.
• Cooling water:
– Hearth
– Thickness monitor
– Bell jar
• Mechanical shutter:
– Evaporation rate is set by temperature of source, but this cannot be turned on and
off rapidly. A mechanical shutter allows evaporant flux to be rapidly modulated.
• Electrical power:
– Either high current or high voltage, typically 1-10 kW.
38
Resistance Heated Evaporation
• Simple, robust, and in widespread use.
• Can achieve temperatures of about 1800°C.
• Use W, Ta, or Mo filaments to heat evaporation source.
• Typical filament currents are 200-300 Amperes.
• Exposes substrates to visible and IR radiation.
• Typical deposition rates are 1-20 Angstroms/second.
• Common evaporant materials:
– Au, Ag, Al, Sn, Cr, Sb, Ge, In, Mg, Ga
– CdS, PbS, CdSe, NaCl, KCl, AgCl, MgF2, CaF2, PbCl2
39
Crucible Sources
• Refractory metals:
– Tungsten (W); MP = 3380°C, P* = 10-2 torr at 3230°C
– Tantalum (Ta); MP = 3000°C, P* = 10-2 torr at 3060°C
– Molybdenum (Mo); MP = 2620°C, P* = 10-2 torr at 2530°C
• Refractory ceramics:
– Graphitic Carbon (C); MP = 3700°C, P* = 10-2 torr at 2600°C
– Alumina (Al2O3); MP = 2030°C, P* = 10-2 torr at 1900°C
– Boron nitride (BN); MP = 2500°C, P* = 10-2 torr at 1600°C
• Engineering considerations:
– Thermal conductivity
– Thermal expansion
– Electrical conductivity
– Wettability and reactivity
40
Resistance Heaters
41
Resistance Heaters
42
Filaments
Crucibles
Boats
Crucible Heaters
43
http://www.rdmathis.com/prodinfo.htm
Evaporation Materials
Slugs
Wire
Chunks
Pellets
Foil Rods Starter Sources
44
Low Voltage Power Supplies
45
Disadvantages of Resistivity Heating
• Contamination
by heaters, crucibles
• Low input power level
• Low melting temperature of the heaters
46
One solution is E-beam evaporation
47
E-Beam Evaporation
270 degree bent
electron beam
evaporation cones
of material
Magnetic
field
pyrolytic graphite
hearth liner
4-pocket rotary
copper hearth
(0 V)
Recirculating
cooling water
Cathode
filament
(-10,000 V)
beam
forming
aperture
48
E-Beam Evaporation Unit
49
Cosine Law Application for E-Beam Evaporation
Substrate
Molecular flow region
hv
h
Viscous region
Virtual source
Real source
50
Challenges for the E-Beam Evaporation Process
Problem associated with electron-beam source
• beam curling – cause deep drilling
• nonuniform beam density – nonuniform deposition rate
Solution
• altering the size of the focal spot
• electromagnetically scanning the beam
51
Power Density Consideration for
E-beam Evaporation
  5.67 10 8 W / m 2  K 4
k  3.1 W / cm  K
 e  1018 # / cm 2  s
Ts  1670 K
H S  3.5 eV
  0 .4
l  1 cm
Pkinetic
3
  e  k BTS
2
PSublimation  e  H S
Pk  0.034 W / cm 2
PS  0.56 W / cm 2
Pradiation   As (T 4  To4 ) Pr  17.6 W / cm 2
TS  To
Pconduction  k
l
PC  4.3 kW / cm 2
Most power consumed through conduction !
Increase the electron-beam power will increase the efficiency of
evaporation while increase the risk of damaging the dielectrics.
Heating efficiency can be increased by using refractory liners at
the expense of increasing the risk of contamination by liners.
52
Recommended Heating Sources and Crucible Materials
53
Columnar-Like Structure of Chromium Film
54
Columnar structure of TBCs
Grown by EB-PVD
SEM micrograph of representative cross section of partially stabilized
zirconia TBC on a N5 super alloy.
http://www.mspusch.de/Ing-Diplomarbeit/2_GeneralBackground.htm
55
Pulsed laser deposition (PLD)
a laser beam activates in pulses, striking the surface of a target
deposition disc of a specified material. A cloud of the material
then rises to create a vapor in the vacuum chamber which in
turn coats a desired surface with a thin film of the desired
material.
PLD is frequently the simplest, best, and sometimes the only
way to produce thin films of:
•complex materials, highly crystalline in structure
•multiple layers
•complex stoichiometry.
56
Types of Lasers used for Ablation
Nd3+:YAG (1064 nm)
• ~ 2 J/pulse and repetition rate ~ 30 Hz
• 1064 nm (fundamental), 355nm (2nd harmonic), 266nm (mixed)
• most nonmetals absorb light at 200-400 nm
Gas excimer lasers
• ArF (193 nm)
• KrF (248 nm)
• XeCl (308 nm)
• ~ 500 mJ/pulse and 200 Hz.
57
Pulsed Laser Deposition of Thin Films
58
Laser Types
Nd3+:YAG(1064 nm) [Yttrium aluminium garnate (YAG) containing ions of the
lanthanide metal neodymium (Nd)]
• Deliver up to ~ 2 J/pulse at a pulse repetition rate of ~ 30 Hz
• The 1064 nm radiation is frequency doubled twice and mixed so that outputs
of 355 and 266 nm are produced.
Gas excimer laser
• Popular gas excimer lasers are the ArF (193 nm), KrF (248 nm), and XeCl
(308 nm) types.
• Commercial versions of these deliver outputs of ~500 mJ/pulse at pulse rates
of several hundred Hz.
59
Nd:YAG Laser Operation
http://www.technology.niagarac.on.ca/people/mcsele/lasers/LasersYag.htm
60
http://repairfaq.ece.drexel.edu/sam/CORD/leot/course03_mod04/mod03-04.html
Energy Level Scheme of Nd:YAG
http://www.ino.it/home/pratesi/Cap%203,%203.2%20laser%20a%20neodimio.pdf
61
KrF Laser Operation
(Electron energy) + Kr + F2 => KrF* + F => Kr + F2 + light
A high-voltage pulsed-power source generates a uniform electron beam from the
cathode. The electron beam propagates through the foil support and deposits its
energy in the laser cell, filled with krypton, fluorine and argon gases. A complex set
of ionizations and chemical reactions produce the excited molecular state of KrF*.
The input laser beam them stimulates the decay of this molecule to its ground state
of separate atoms, with an enhancement of the laser intensity.
62
http://other.nrl.navy.mil/LaserFusionEnergy/lasercreation.htm
Mechanism of PLD of Thin Films
• Highly directional plume, i.e. cosn, where 8<n<12.
• Shallow melting on target
• homogeneous target required
• Stoichiometric ceramic films can be achieved
• Newton’s ring were observed
• metastable ablating surface becomes nearly transparent
• melt has higher index of refraction relative to the solid
63
Problems with PLD
Gross particulates ejection by splashing
• Rapid expansion of gas trapped beneath the target surface
• Fracture of a rough target surface by thermal shocks
• Superheating of subsurface layers before vaporization of surface
atoms
• Solution to splashing – to interpose a rapidly pinwheel-like shutter
between target and substrate
• Slower particulates can be batted back
Highly directed plume
• Non-uniform deposition of films over large substrate areas
64
Web Coating
• Need for large areas of metalized flexible
polymer film and paper sheet
dT
c p d
 qa  h(T  T1 )
dt
qa
hL
T  T2  T1  [1  exp( 
)]
h
vc p d
65
Schematic of Web Coating System
Assuming qa is constant, and qt is proportional to h(T-T1)
dT
c p d
 qa  h(T  T1 )
dt
qa
hL
T  T2  T1  [1  exp( 
)]
h
vc p d
t  L/v
Cooled roll
qt
T1
d
T2
qa L : arc length
h : heat transfer coeff .
66
Optimization of the Web Coating Process
dT
 qa  h(T  T1 )
dt
qa
hL
T  T2  T1  [1  exp( 
)]
h
vc p d
c p d
qt
T1
d
To reduce T2
1.
2.
3.
4.
qa
T2
L : arc length
Reduce L
h : heat transfer coeff .
Increase v
Increase h by intimate contact with cooled roll
Reduce qa and T1
67
Rotation
實驗方法
•置入試片
•抽真空
Auto
Match Box
⊙⊙⊙⊙⊙⊙⊙
Substrate
Holder
•通入氣體
•（外加偏壓）
•啟動電子槍
Pulse DC
Generator
T.C. Gauge A
RF
Generator
Movable
Transverse
Ion Gauge
Shutter
Magnetic
Field
V3
E-beam
C. P.
Crucible
•蒸鍍
T.C. Gauge B
E-Gun
M
F
C
M
F
C
V2
V1
Vent
M. P.
68
Ar
N
Ion Beam Assisted Evaporation
69
反應性蒸鍍系統
70
THIN FILM SCIENCE AND TECHNOLOGY
HW #3 PVD
1. List and describe three deposition methods for evaporation?
2. Calculate the evaporation rate in g/cm2-s and in nm/s of Si
metal at 1500 K under a base pressure of 10-8 torr?
3. An Al film was deposited at a rate of 1000 nm/min in
vacuum at 25 C, and it was estimated that the oxygen content
of the film was 0.1%. What was the partial pressure of
oxygen in the system?
71