Vacuum Deposition

Techniques

Edited by: Yared Daniel

1
Vacuum deposition techniques

evaporated by heating in vacuum, and can be

formed into thin films by condensing the vapor on
substrates.

It has the advantage of depositing a desired film

thickness, even in nanometer range, in the clean
environment of high vacuum, i.e., at a pressure well
below atmospheric pressure

• Thevacuum deposition techniques can be classified

based on the type of the precursor used as a vapor
source
2
 Vacuum deposition techniques

 In a chemical vapor deposition (CVD), the source of the vapor

is a chemical vapor

 In a physical vapor deposition (PVD), the source of the vapor is

either a liquid or solid precursor
The vacuum environment can be used to:
 reduce the particle density thereby increases mean free path

 reduce contaminants

 provide a means for controlling gas and vapor composition

3
 Chemical Vapor Deposition

CVD is a process in which a gaseous chemical precursor (or

precursors) has a chemical reaction on the wafer surface
and deposits a solid byproduct as a layer of thin film.

Other byproducts are gases and leave the surface.

Many materials such as metals, oxides, sulfides,

phosphides, arsenides, carbides, borides, silicides etc.
deposited using CVD and related techniques.

4
 Chemical Vapor Deposition
 CVD is a sequential process, which includes several steps:

 gas or vapor phase precursors are introduced into the reactor

 precursors diffuse across the boundary layer and reach the substrate surface
 precursors adsorb on the substrate surface
 adsorbed precursors migrate on the substrate surface
 chemical reaction occurs on the substrate surface
 solid byproducts form nuclei on the substrate surface
 nuclei grow into islands
 islands merge into the continuous thin film
 other gaseous byproducts desorb from the substrate surface
 gaseous byproducts diffuse across the boundary layer
 gaseous byproducts flow out of the reactor. 5
 Chemical Vapor Deposition
Chemical vapor deposition process

6
 Chemical Vapor Deposition
CVD mechanism and kinetics

 The ability of the precursor to migrate on the surface is called

surface mobility, which is very important for thin-film step
coverage and gap-fill properties. 7
 Chemical Vapor Deposition
Chemical vapor deposition basics
step coverage
Step coverage is a measurement of the deposited film
reproducing the slope of a step on a substrate surface.

Step coverage is determined by both the arriving angle and

precursor surface mobility.

8
 Chemical Vapor Deposition
step coverage
 Overhangs are very undesirable. If a
deposited film starts with overhangs caused
by the arriving angle effect and low surface
mobility, as the film thickness increases, the
overhang will grow faster than the film due
to the larger arriving angle.

 Very soon the overhangs seal the top of the

gap to form the void (also known as the key
hole) between the polysilicon pattern

 These voids can have processing gases

sealed inside, and the diffusion of these
gases inside the IC chip can cause problems
in later processes, causing yield issues

9
 Chemical Vapor Deposition
Effect of surface mobility on step coverage
 surface mobility can significantly affect step coverage.

 After precursors adsorb on the surface, and if the precursors have enough
energy to break the adsorption bond with the surface, they can leave the
surface and hop along it.

 If they move along the surface rapidly, migration of the precursor can
smooth out the arriving angle effect.

 Thus, precursors with high surface mobility can achieve very good step
coverage and good conformality.

10
 Chemical Vapor Deposition
Gap fill
When a deposited film hangs over a pattern, it can cause voids
if the film keeps growing.
Different methods have been developed to alleviate this issue.

argon ion sputtering etch

 chips the corner of overhangs
and tapers the gap opening to
increase the arriving angle, so
that the subsequent deposition
process can fill the gap without
voids.
 This approach is called
dep/etch/dep, short for
deposition, etchback, and
deposition 11
 Chemical Vapor Deposition
Gap fill
Conformal film deposition

If CVD precursors have very high surface mobility, the CVD
film will have good step coverage and conformality.

The film can also grow up conformally to fill the gap without
voids

12
 Chemical Vapor Deposition
Surface adsorption
When precursors reach the substrate surface after diffusion
across the boundary layer, they are adsorbed by the surface.

There are two kinds of adsorption— chemisorption and

physisorption

Relationship of bonding energy to chemical and physical adsorption.13

 Chemical Vapor Deposition
Chemisorption
Chemisorption is the shortened term for chemical adsorption.

In this case, an actual chemical bond is formed between an

atom on the surface and an atom in the adsorbed precursor
molecule.

The chemisorbed atoms or molecules are held to the surface

with energy that exceeds 2 eV.

Because of the strong chemical bonds, the chemisorbed

precursors have very low surface mobility.

14
 Chemical Vapor Deposition
Physisorption
 Physisorption is the shortened term for physical adsorption. In this
case, the adsorbed molecules are held to the surface with forces much
weaker than a chemical bonding force.

 Physisorption involves energies less than 0.5 eV per molecule.

 The nature of forces involved in physisorption varies from long-range

Van der Waals forces to dipole–dipole forces (of which hydrogen
bonding is a special case).

 Both thermal energy at 400 ◦C and ion bombardment provide enough

energy to cause significant amounts of physisorbed precursors to break
free and leave the surface.

 The physisorbed precursors can move about the surface, so they have
much higher surface mobility than that of chemisorbed molecules. 15
 Chemical Vapor Deposition
Chemical Vapor Deposition kinetics
• The chemical reaction rate (CR) can be expressed as the Arrhenius
equation:
the activation energy

a constant Boltzmann constant substrate temperature

• Low activation energy making Ea means a low chemical reaction

barrier, which makes it easier for a reaction to happen.
16
 Chemical Vapor Deposition
Chemical Vapor Deposition kinetics

• External energy sources such as heat, rf power, or UV radiation are

needed for chemical precursors to overcome activation energy
barriers and achieve a chemical reaction

• Because the chemical reaction rate is exponentially related to

temperature, it is very sensitive to changes in temperature. 17
 Chemical Vapor Deposition
Surface-reaction-limited regime
• In a surface-reaction-limited regime, the chemical reaction rate
cannot match precursor diffusion and adsorption rates; precursors
pile up on the substrate surface and wait their turn to react

• In this case, the deposition rate (DR) is mainly determined by the

chemical reaction rate on the substrate surface:

 [B], [C], etc., are the

concentrations of the
adsorbed precursors
18
 Chemical Vapor Deposition
Mass-transport-limited regime
• When the surface chemical reaction rate is high enough, the
chemical precursors react immediately when they adsorb on the
substrate surface.

• In this case, the deposition rate is no longer determined by the

surface reaction rate but by how fast chemical precursors can
diffuse across the boundary layer, and reach and adsorb on the
surface, and the DR is given by:

dn
DR = D [B] .[C ]....
dx
concentration gradient
of the precursors in the
diffusion rate
boundary layer
19
 Chemical Vapor Deposition
Mass-transport-limited regime
• In a mass-transport-limited regime, deposition rate is not very
sensitive to temperature, and deposition is mainly controlled by
gas flow rates.
 Chemical Vapor Deposition
Metal CVD
• Protective films, reflective or conducting coating, electrodes,
microelectronics

• Commercially PVD methods are often used for metallic films

• However Al, Cu and W are often deposited by CVD methods

Aluminum deposition by CVD:

• Metallized polymer films in food packaging (gas diffusion barrier),

reflective layers (mirrors, CDs…)

• Interconnects in microelectronics

• Common precursor for CVD: tributylaluminium: AliBu3 21

 Chemical Vapor Deposition
Metal CVD
• Deposition at 200-300 ˚C (hot wall reactor): β-hydride elimination

• At temperatures > 330 °C: β-methyl elimination ---> carbon

incorporation

22
Chemical Vapor Deposition
Metal CVD

23
Chemical Vapor Deposition

24
 Atomic Layer deposition (ALD)

• The ALD process is performed in a sealed reactor

• The first processing gas flows into the chamber, and the precursor
molecules adsorb on the substrate surface

• The chamber is then purged, and the first processing gas is removed
from the chamber, leaving behind only those molecules adsorbed on
the wafer or substrate surface

• The second processing gas then flows into the chamber to react with
the first process molecules adsorbed on the surface to form a
molecular layer of compound material on the surface

25
 Atomic Layer deposition (ALD)

• is useful for deposition of thin-films atom by atom

26
 Atomic Layer deposition (ALD)
• After all of the first precursor molecules are consumed, the
chemical reaction is self-terminated, and a purge process
removes the second processing gas and byproducts of the
chemical reaction from the chamber

• The ALD cycle deposits a molecular layer of the compound

material onto the wafer surface

• Multiple circles can be performed until the required compound

film thickness is reached

27
Atomic Layer deposition (ALD)
Atomic Layer deposition (ALD) of ZnS

 deposition of ZnS
from ZnCl2 and
H2S precursors by
using ALD

28
 Atomic Layer deposition (ALD)

• ALD is capable of producing

ultra-uniform thin films even
on extremely high-aspect-ratio
surface, which are impossible
to achieve by other methods

• disadvantages: slow deposition

speed
29
Molecular beam Epitaxy (MBE)

• Evaporation of elemental sources

independently at a controlled
rate

• Molecular beams intercept at the

substrate surface

• Requires ultra high vacuum (UHV)

conditions (10-10 bar), low growth
rates

30
 Molecular beam Epitaxy (MBE)

For example, deposition of SiAs based thin film using MBE:

• first, precursors such as dichlorosilane, SiH2Cl2, (DCS) and AsH3 are

introduced into the reactor

• precursor molecules diffuse to the surface of the

substrate,
adsorbed on the surface, dissociate, and react on the surface

• solid byproduct atoms called adatoms migrate onto the surface and
bond with other surface atoms in the same crystal structure as the
substrate crystal

• volatile byproducts desorb from the hot surface and diffuse out
31
 Molecular beam Epitaxy (MBE)

1 1

3 2

3 2

1. Diffusion of the gas precursor to the surface of the substrate

2. Adatoms migration to the reaction site to form a bond with
surface atoms
3. Adatoms attachment to the surface atoms
32
 Molecular beam Epitaxy (MBE)

 SiHCl3 and SiH2Cl2 are

less reactive than
SiH4, and thus the
former precursors can
be deposited at
higher temperatures

• two deposition regimes: one at lower temperature, with growth

rates highly sensitive to temperature, and another at higher
temperatures with growth rates less sensitive to temperature

• The first regime is called the surface-reaction-limited regime, and

the second is called the mass-transport-limited regime 33
 Quiz
1. For the silane process, if the temperature continues to increase to 1300◦C, how
will the growth rate change?

34
 Physical Vapor Deposition (PVD)

• Evaporation/sputtering of a target material onto a substrate

Steps in PVD are:

o Evaporation of a solid

o Transport of the gaseous

species to the substrate

o Condensing gaseous species

on the substrate, followed by
nucleation and growth

35
 Physical Vapor Deposition (PVD)
Evaporation/sputtering can be performed by:

• Thermal evaporation

• Electron or laser beam surface heating

• Sputtering: Atoms are removed from the target by

ion-bombardment (glow discharge or plasma)

36
 Physical Vapor Deposition (PVD)
Comparing CVD and PVD processes
The PVD process uses solid sources, while CVD processes use
gaseous or vapor precursors.

A CVD process relies on chemical reaction on the substrate

surface; the PVD process does not.

CVD film has better step coverage, and PVD film has better quality,
lower impurity concentration, and lower resistivity.

37
 Physical Vapor Deposition (PVD)
Thermal evaporation
• In the early years of IC
processing, thermal
evaporators were widely used
to deposit aluminium thin film
to form gates and
interconnections.

• Throughout the process, the

system needs to be under high
vacuum, about 10-6 torr, to
minimize residual oxygen and
moisture.

38
 Physical Vapor Deposition (PVD)
Thermal evaporation
Flowing a large amount of electric current through a tungsten
filament heats up the filament by resistive heating.

A red-hot tungsten filament heats up the aluminium charge,

melts it, and vaporizes it in the vacuum chamber.

When aluminium vapor reaches the wafer surface, it re-

condenses and forms a thin layer of aluminium film on the
surface.

 In a filament evaporation system, a shutter mechanism is placed

between the filament and wafers.

39
Physical Vapor Deposition (PVD)
Thermal evaporation
At the beginning of the deposition process, the filament is
heated to just above the metal melting point to melt all of the
metal charge while the shutter is closed.

After the temperature is stabilized and volatile impurities are

driven away from the charge surface by the heat, the current
ramps up to raise the temperature and evaporate the metal.

 Then the shutter is opened to allow the metal vapor to emit,

reach the wafer, condense on the surface, and deposit metal
thin film on the wafer.

For thermal evaporation deposition processes, the deposition

rate of aluminium is related to the heating power, which is
controlled by the electric current; usually, a higher current has a
higher deposition rate. 40
Physical Vapor Deposition (PVD)
Thermal evaporation
 One important safety issue for the thermal evaporator is
electrical shock.

 The high current (∼10 A) used by an evaporator can cause a

fatal electric shock in the case of direct contact.

 Aluminium thin film deposited with a thermal evaporator

always has a trace mount of sodium from the tungsten
filament; it is high enough to shift the threshold voltage of
MOSFETs and affect IC device reliability.

 It also has a low deposition rate and poor step coverage.

 It is also very difficult to precisely control the proper

proportions for alloyed films such as Al-Si, Al–Cu, and Al–Cu-Si.
41
Physical Vapor Deposition (PVD)
Electron beam evaporation
 To replace filament heating, which can cause contamination
and poor step coverage, electron beam (e-beam) heating
technology was developed to evaporate metals for IC
metallization.

 A beam of electrons, typically with energy of about 10 keV and

currents up to several amperes, is directed at the metal in a
water-cooled crucible in a vacuum chamber, and heats the
metal to an evaporation temperature.

42
 Physical Vapor Deposition (PVD)
Electron beam evaporation
 During the evaporation deposition process, the outer portion of
the charge does not melt and remains in a solid state,
minimizing film contamination from the trace amounts of
impurities inside the graphite or silicon carbide crucible.

43