A key vacuum deposition technique for making highly homogenous and high-performance solid-state thin films and materials is Chemical vapor deposition. The types of CVD systems and their key applications would also be discussed in this presentation. It is a key bottom-up processing technique, widely used in graphene fabrication, also the fabrication of various oxides, nitrides is possible, with this technique.

1. CHEMICAL VAPOR DEPOSITION
(CVD)
Aditya Bhardwaj

2. INTRODUCTION
CVD is a thin film deposition method that uses chemical
reactions to deposit high quality layers of desired material.
Vapor acronym in CVD implies that the sources used in
deposition method are in gaseous phase, in this technique solid
material is deposited from a vapor by a chemical reaction
occuring on or in the vicinity of a heated substrate surface.
It is used in microelectronics industry, to make films serving as
dielectrics, conductors, passivation layers, oxidation barriers,
used in preparation of materials such as tungsten, ceramics,
production of solar cells, fiber composites.

3. WHY CVD IS REQUIRED
• Physical deposition methods such as sputtering eject very small pieces of target
material and deposit these small pieces on the surface of the wafer. Since the
shapes of these pieces are random there are voids between these pieces and they
form pores which reduce film quality. In case of CVD, materials are deposited
through chemical reaction and this reaction happens on the deposition surface
therefore it is less likely to have pores in a CVD film hence its quality is higher
than a sputtered film.

4. DIFFERENCE BETWEEN CVD AND PVD
CVD PVD
• Material introduced in gaseous form • Material introduced in solid form
• High temperature processing, limits flexibility,
causes high stress
• Low temperature processing, increases
flexibility, reduces stress
•Higher quality and purity of films formed • Low quality of films formed
• Useful for depositing compound protective
coatings
• Useful for coating tools for applications which
demand tough cutting edge

5. MAIN STEPS IN CVD
1. Transport of reactants by forced convection to the
deposition region.
2. Transport of reactants by diffusion from the main gas
stream through the boundary layer to wafer surface.
3. Adsorption of reactants on the wafer surface.
4. Surface processes, including chemical decomposition or
reaction, surface migration to attachment sites (such as
atomic-level ledges and kinks), site incorporation, and
other surface reactions.
5. Desorption of byproducts from the surface.
6. Transport of byproducts by diffusion through the
boundary layer and back to the main gas stream.
7. Transport of byproducts by forced convection away from
the deposition region.
Ref: Plummer, J. D., M. D. Deal, and P. B. Griffin, Silicon VLSI technology 2000.

6. VARIOUS CHEMICAL REACTIONS IN CVD
Pyrolysis : SiH4(g) Si(s) + 2H2(g) at 650°C
Reduction : SiCl4(g) + 2H2(g) Si(s) + 4HCl(g) at 1200°C
Oxidation: SiH4(g) + O2(g) SiO2(g) + 2H2(g) at 450°C
Exchange reaction: SiCl4(g) + CH4(g) SiC (s) + 4HCl (g) at 1400°C
Disproportionation: 2GeI2 g) Ge(s) + GeI4(g) at 300°C
Coupled reaction: As4(g) + As2(g) + 6GaCl(g) + 3H2(g) 6GaAs(g)+
6HCl(g) at 750°C
Ref: Cao, Guozhong. Nanostructures & nanomaterials: synthesis, properties & applications. Imperial college press, 2004.

7. CLASSIFICATION OF CVD BASED ON
OPERATING PRESSURE
Atmospheric pressure CVD (APCVD) – CVD at atmospheric
pressure.
Low-pressure CVD (LPCVD) – CVD at sub-atmospheric
pressures. Reduced pressures tend to reduce unwanted gas-
phase reactions and improve film uniformity across the
wafer.
Ultrahigh vacuum CVD (UHVCVD) – CVD at very low
pressure, typically below 10−6 Pa (~10−8 torr).

8. CVD SYSTEMS BASED ON PHYSICAL
CHARACTERISTICS OF VAPOR
Aerosol assisted CVD (AACVD)
• The precursors are transported to the
substrate by means of a ultrasonically
generated liquid/gas aerosol.
• This technique is suitable for use with non-
volatile precursors.
Direct liquid injection CVD (DLICVD)
• The precursors are in liquid form (liquid or
solid dissolved in a convenient solvent).
• Liquid solutions are injected in a vaporization
chamber towards injectors (typically car
injectors).
• The precursor vapors are then transported to
the substrate as in classical CVD.
• This technique is suitable for use on liquid or
solid precursors. High growth rates can be
reached using this technique.

9. COMMON CVD SYSTEMS
APCVD (Atmospheric pressure CVD)
LPCVD (Low pressure CVD)
PECVD (Plasma enhanced CVD)
MOCVD (Metal organic CVD or Organometallic Vapor Phase
Epitaxy)

10. ATMOSPHERIC PRESSURE CVD (APCVD)
Quartz
reaction
chamber
RF Induction (Heating
Coils)
HC
l
H2
Vent
Silicon wafer Graphite
Susceptor
• High Temperature APCVD: used to deposit epitaxial Si and
compound films or hard metallurgical coatings like TiC and TiN.
• Low Temperature APCVD: used to deposit insulating layers on
substrate at low temperature.

11. TILTED CVD SUSCEPTOR
The Susceptor in a horizontal epitaxial reactor is tilted so that the
cross-sectional area of the chamber is decreased, increasing the gas
velocity along the Susceptor. This compensates for both the
boundary layer and depletion effects.
**** Susceptor is a material used for its ability to absorb
electromagnetic energy and convert into heat This energy is
typically radiofrequency or microwave radiation used in industrial
heating processes, and also occasionally in microwave cooking.

12. APCVD ISSUES
There are two types of non-uniformities in APCVD caused by gas flow
vortexes: (i) In wafer (ii) Wafer to Wafer
Gas flow causes vortexes at edges of wafers which causes edges to have
different thickness than centre.
Wafers in beginning of tube consume process gases, hence gas
concentration falls down towards end of the tube, which results in slower
deposition rate, hence wafer to wafer non-uniformity.
We can remove this uniformity by help of temperature gradient,
increasing temperature towards end of tube would equalize deposition
rates and reduce non-uniformity. This results in use of Induction coil or
three-zone furnaces for CVD systems.

13. LOW PRESSURE CVD (LPCVD)
• LPCVD is most successfully applied in deposition of polysilicon thin films. These
films are used for gate contact and short interconnect lines. This is done using
compounds like SiH4 in the temperature range 600- 660°C.
• When the pressure is lowered during LPCVD, the diffusion of the gas
decreases proportionally to the reciprocal of the pressure. If the pressure
is lowered from atmospheric pressure to about 100 Pa the diffusion will
decrease by almost 1000.This means that the velocity of mass transport
will decrease meaning the substrates can approach more closely and the
deposited films show better uniformity and homogeneity.

14. LPCVD PROCESS
• LPCVD has quartz tube placed in a spiral heater that starts with tube pressure at
0.1 Pa. The tube is then heated to desired temperature and gaseous species
(working gas) are inserted into tube at pressure 10-1000 Pa. The working gas
consists of dilution gas and reactive gas that reacts with substrate and create a solid
phase material on substrate. After working gas enters the tube, it spreads around
hot substrates, that are at same temperature, substrate temperature tells which
reactions will take place. A solid phase material is formed on substrate and excess
material is pumped out of tube.
3 Zone Furnace
Pressure sensor
Samples
Gas inlet Quartz Tube
Pump

15. ADVANTAGES AND DISADVANTAGES OF LPCVD
Advantages
• Excellent uniformity of thickness and purity of thin film formed.
• Simple handling
• High reliability
• High reproducibility
• Higher batch size
Disadvantages
• Lower deposition rate
• High temperature above 600°C is required
• By products formed may be harmful
• Low film density

16. THERMAL CVD
• Heat energy is supplied to activate the gas phase reactions between
reactive gases and the substrate, temperature as high as 2000 degree
celsius is required for film deposition.
• Two reactors are used for Thermal CVD process:
• Hot wall reactor
• Cold wall reactor
• A hot wall reactor is usually tubular in form, it is an isothermal
surface into which the substrates are placed. Since the whole chamber
is heated, precise temperature control can be achieved by designing
the furnace accordingly.

17. • In typical cold-wall CVD reactors, substrates are directly
heated inductively by graphite susceptors, while chamber
walls are air or water-cooled.
• Even though in hot wall reactors, we have to clean the
substrate after each deposition process, as whole substrate
is heated, but due to its high throughput and multiple
wafer deposition, it is frequently used.
• The contamination of wafer significantly reduces in cold
wall reactors.

18. PRESSURE ENHANCED CVD (PECVD)
PECVD is a fabrication method used for depositing thin films of SiO2, Si3N4, (SixNy), SixOyNz
and amorphous Si films on a wafer.
Plasma is added in the deposition chamber with reactive gases to create the desired solid
surface on the substrate.
Plasma is a partially ionized gas with high free electron content (about 50%).
Plasmas are divided into two groups; cold (also called non-thermal) and thermal.
In thermal plasmas, electrons and particles in the gas are at the same temperature; however,
In cold plasmas the electrons have a much higher temperature than the neutral particles and
ions. Therefore, cold plasmas can utilize the energy of the electrons by changing just the
pressure. This allows a PECVD system to operate at low temperatures (b/w 100 and 400°C).

19. PLASMA
ELECTRODE
ELECTRODE
WAFER
HEATER
RF POWER INPUT
GAS OUTLET
GAS INLET
• PECVD systems must contain two electrodes (in a parallel plate
configuration), plasma gas, and reactive gas in a chamber. To begin the
PECVD process, a wafer is placed on the bottom electrode and reactive
gas with the deposition elements is introduced into the chamber. Plasma is
then introduced into the chamber between the two electrodes, and voltage
is applied to excite the plasma (RF voltage is applied). The excited state
plasma then bombards the reactive gas causing dissociation. This
dissociation deposits the desired element onto the wafer.

20. ADVANTAGES AND DISADVANTAGES OF PECVD
Advantages
• Low temperature
• Higher film density
• Ease of cleaning the chamber
Disadvantages
• Expensive equipment
• Films formed are not stoichiometric
• Batch size is small
• By products released are volatile due to plasma bombardment

21. METAL ORGANIC CVD (MOCVD)
• MOCVD or Organometallic Vapor Phase Epitaxy (OMVPE), is a
CVD method used to produce single or polycrystalline thin films.
• It is a highly complex process for growing crystalline layers to create
complex semiconductor multilayer structures.
• The growth of crystals is by chemical reaction under the gaseous
environment at moderate pressures (10 to 760 Torr). As such, this
technique is preferred for the formation of devices incorporating
thermodynamically metastable alloys, and it has become a major
process in the manufacture of optoelectronics.
• In this technique metal organic compounds such as trimethyl-gallium
(TmGa), trimethyl-indium (TmIn) are used as precursors, which are
volatile at moderately low temperatures, due to which there is better
control on gas flow rate and better deposition of film is possible

22. BASIC PRINCIPLES OF MOCVD
 Ultra pure gases are injected into a reactor. Surface reaction of
organic compounds or metal organics and hydrides containing the
required chemical elements creates conditions for crystalline growth.
 The semiconductors may contain combinations of Group
III and Group V, Group II and Group VI, Group IV, or Group IV, V
and VI elements. For example, indium phosphide (InP) could be
grown in a reactor on a heated substrate by introducing trimethyl
indium ((CH3)3In) and phosphine (PH3) in a first step. The heated
organic precursor molecules decompose in the absence of oxygen
(pyrolysis).
 Pyrolysis leaves the atoms on the substrate surface in the second
step. The atoms bond to the substrate surface and a new crystalline
layer is grown in the last step.

23. ILLUSTRATION OF MOCVD PROCESS AND MOCVD
APPARATUS

24. MOCVD REACTOR COMPONENTS
Reactor chamber is made of a material that does not react with the chemicals being used.
It must also withstand high temperatures.
This chamber is composed by reactor walls, liner, a susceptor, gas injection units, and
temperature control units.
The reactor walls are made from stainless steel or quartz. Ceramic or quartz, are often
used as the liner in the reactor chamber between the reactor wall and the susceptor. To
prevent overheating, cooling water must flow through the channels within the reactor
walls. A substrate sits on a susceptor which is at a controlled temperature.
The susceptor is made from a material resistant to the metal organic compounds
used; graphite is sometimes used. For growing nitrides and related materials, a special
coating on the graphite susceptor is necessary to prevent corrosion by ammonia (NH3)
gas.

25. • In a cold-wall (CW) reactor, the substrate is supported by a pedestal,
which also acts as a susceptor.
• The pedestal/susceptor is the primary origin of heat energy in the
reaction chamber. It is made of a radiation-absorbing material such
as carbon.
• The walls of the reaction chamber in a cold-wall reactor are typically
made of quartz which is largely transparent to the electromagnetic
radiation. The reaction chamber walls in a cold-wall reactor are
indirectly heated by heat radiating from the hot pedestal/susceptor,
but will remain cooler than the pedestal/susceptor.
• In contrast to CW Reactor, hot-wall CVD, the entire chamber is
heated. This may be necessary for some gases to be pre-cracked
before reaching the wafer surface to allow them to stick to the wafer.

26. • Gas is introduced via devices known as 'bubblers'. In a bubbler a carrier
gas (usually hydrogen in arsenide & phosphide growth or nitrogen for
nitride growth) is bubbled through the metal organic liquid, which picks
up some metal organic vapor and transports it to the reactor. The amount
of metal organic vapor transported depends on the rate of carrier gas flow
and the bubbler temperature, and is usually controlled automatically and
most accurately by using an ultrasonic concentration measuring feedback
gas control system. Allowance must be made for saturated vapors.
• Pressure maintenance system and gas exhaust systems are also present.
• Toxic waste products must be converted to liquid or solid wastes for
recycling (preferably) or disposal.

27. LASER ASSISTED CVD (LCVD)
Laser has also been employed to enhance or assist the chemical reactions or deposition.
Two mechanisms are involved: Pyrolytic and Photolytic process
In the Pyrolytic process, the laser heats the substrate to decompose gases above it and enhances
rates of chemical reactions, substrates which melt above temperature necessary for gas
deposition are required.
In the photolytic process, laser photons are used to directly dissociate the precursor molecules in
the gas phase. UV light sources are required because many useful parent molecules (SiH4 , Si2H6
, Si3H8 ) require absorption of photons with wavelengths less than 220 nm to initiate dissociation
reactions.
Metals such as Al, Au, Cr, Cu, Ni, Pt and W have been deposited by laser.

28. • Laser CVD is associated with the deposition of
chemical vapors using a laser beam.
• It is generated from sources like CO 2 , Nd:YAG and
Excimer.
• We can produce quality films at lower
temperatures
with better control on composition.
• For example, a silicon nitride film could be
deposited at 200°C using laser CVD, whereas it is
deposited at 850°C and 450°C by thermal CVD and
plasma-enhanced CVD, respectively.
• In the case of nanoparticles, tungsten powder of 54
nm could be synthesized from WF6/H2/M (M = Ar,
Kr, Ne, Xe) gas mixtures irradiated with an ArF
Excimer laser.

29. THANK YOU