The Deposition Process
Top Down Manufacturing
Learning Objectives
• The Student Will Be Able to Explain
– The need for Deposition Processes in the Top
Down Manufacturing Process
– The methods used to perform physical and
chemical deposition processes
– The advantages of different deposition
processes
– The use of plasma for enhancing deposition
Thin Film Deposition
• Thin films are an integral part of IC and microsystems fabrication.
• In ICs thin films are largely used as conductors
or insulators.
• On the other hand, several types of thin films are
used in micro-systems for various purposes.
• These may be broadly classified into metals,
dielectrics (including ceramics and polymers),
and functional materials.
Purpose of Deposition
• Deposition places conductive or insulating
layers on a substrate
• Deposition processes create locally
conductive paths that can be used to
interconnect devices
• Deposition can be used to build up more
complex structures one layer at a time
Deposition Types
• Silicon Dioxide SiO2
– Insulating layers
– Protective coatings
– Gate oxides
• Silicon Nitrides
– Protective layers
– Isolation
These are typically
used as insulating
layers between
conducting layers in
MOS devices, for
diffusion and ion
implantation masks,
and for passivation to
protect devices from
impurities, moisture,
and scratches.
Deposition Types
• Polysilicon
– Heavily doped silicon
– Conductive
– Used for interconnects
and gate
• Metals
– Aluminum/AluminumCopper
– Tungsten
– Titanium Alloys
1.Polysilicon (abbreviated
as poly-Si or simply poly)
is used as a gate electrode
in metal-oxidesemiconductor (MOS)
devices, as a conductive
material for multilevel
metallization, and as a
contact material for
devices with shallow
junctions.
2.Metal films are useful in
forming low resistance
ohmic contacts and
conductivity paths for
interconnects.
Deposition Processes
• Physical or Chemical (or both?)
– Physical Processes Deposit the material
without chemical reactions
– Chemical processes utilize liquid or vapor
forms of precursors that react with the surface
to form the desired deposition
– It is possible to combine the processes and
gain the benefits of each
– Many processes are carried out in reduced
pressure (partial vacuum) environments
• Metal films can be formed by physical vapor
deposition (PVD) and chemical vapor deposition
(CVD) processes.
• In these processes, favorable conditions are
created to transfer the material from the source
(target) to the destination (substrate).
• In the PVD process, this transfer takes place by
physical means such as evaporation or impact,
while in the CVD process, this happens through
a chemical reaction.
Schematic diagram of a thermal evaporation unit for depositing metals
and their materials
• Evaporation is a relatively simple process of
deposition of metal films on a substrate.
• An evaporation system consists of an
evaporation chamber, high- vacuum pumping
system, substrate holder, crucible/filament,
and a shutter
• The source material is placed in the
crucible/filament
• The chamber is evacuated using vacuum
pumps to a pressure of 10^ (-6)–10^ (-7 )torr.
• The crucible/filament is heated by passing a heavy
current or directing an electron beam onto the
material.
• The evaporated metal condenses on the substrate.
• The thickness of the deposited film depends upon
the duration of evaporation and the distance
between the source and the crucible, and its purity
depends upon contaminations from the source,
support material (such as the substrate holder and
the crucible), and residual gases.
• Metal alloy films can be deposited by evaporating
constituent elements simultaneously from different
sources.
Summary of Evaporative Deposition
• Utilizes the principle of vapor pressure
– Metallic species are melted in a low pressure
environment
– Higher vapor pressure metals evaporate first
– Deposition of the vapor on the surface occurs
– A low temperature process on the substrate
– Alternatives include laser ablation
• Laser strikes a target, causing local melting
Evaporative Deposition (2)
• Advantages
– Uniformly covers substrate
– Simple process without chemicals or gases
• Disadvantages
– Alloys are difficult to deposit
• Different metals have different vapor pressures
– High aspect ratio features are difficult to cover
• Trajectory of evaporated particles tends to be
vertical, which may not pattern sidewalls evenly
Requirements of Deposition
• Since top-down processes may use many
layers to form a product, any deposited
layer must be compatible in many ways
with what is below it
– Film Stress
– Conformality
– Uniformity
– Step Coverage
– Thermal compatibility
Result of Non-Uniform Deposition
From MATEC Module 61
Conformal Coverage
Good Conformal
Coverage
From MATEC Module 45
Poor Conformal
Coverage
Step Coverage
From MATEC Module 45
Physical Deposition Processes
• Sputtering
– Plasma is created by RF or HV DC source
– Inert gas such as Ar is used in a low pressure
environment
– Free electrons strike Ar atoms, causing
positive ions to be formed
– Negatively charged target material attracts
ions
– Ions dislodge particles that are deposited
• Sputtering is a physical phenomenon in which
ions accelerated through a potential gradient
bombard a target which is set to be the cathode.
• Because of the momentum transfer of the
accelerated ions to the atoms near the surface,
target atoms are released and are transported in
vapor form to the substrate for deposition.
• Figure below shows typical locations of targets
and substrate, and indicates the ionization and
deposition process in a sputtering system. The
vacuum chamber is evacuated to 10 ^(-6)–10^(8) torr.
• Argon gas is introduced into the chamber and
the pressure is maintained at a few milli torr.
• Argon is ionized by the application of a dc (for
conducting targets) or RF (for insulators)
voltage.
• The ions bombard the target, releasing atoms
from it. These atoms are deposited on the
substrate surface.
• The deposition rate depends on the sputtered
material, RF power applied, pressure inside the
chamber, and spacing between the electrodes.
• The sputtering process can be used for depositing one
material at a time.
• However, compound thin films can be deposited by using
co-sputtering from multiple targets.
• Since with sputtering, it is possible to obtain the same
stoichiometric composition as that of the source material,
alloys too can be used as target materials and deposited
as thin films.
• Sputtered films usually have good compositional
uniformity, adhesion to substrate surface, and grain
orientation.
• The sputtered films are amorphous, but can be made
crystalline by suitable annealing.
• Their mechanical properties and stresses depend on the
sputtering conditions.
Sputtering
Source: www.wikipedia.com
Sputtering (3)
• Advantages
– Low temperature process
– Good Conformal Coating
– Good Step Coverage
• Disadvantages
– Dielectrics require RF Source
– RF environment may affect other depositions
Practice Questions
Click once for each question.
1. What are the two main types of deposition
processes?
Physical and Chemical Deposition
2. What are commonly used metals for deposition?
Aluminum, tungsten, and copper
3. What does conformality of a deposition refer to?
The ability of the deposition to follow surface contours
evenly
Practice Questions
Click once for each question.
1. Which physical deposition process uses plasma?
Sputtering
2. What is an advantage of sputtering?
Low temperature process, good conformal coating
3. What is a disadvantage of evaporative
deposition?
Difficult to deposit alloys, difficult to get good high aspect
ratio feature deposition
Spin On Coating
• A Physical Deposition Process
– Similar to photoresist spin-on
– Si-based liquid is applied
– Coating is baked on to remove volatile liquid
• Used to planarize or flatten wafer surface
– Can be patterned and etched for contacts
• Adds steps to process
• Alternatives – Chemical Mechanical Polish
Chemical Vapor Deposition Processes(CVD)
• The CVD process involves convective heat and mass transfer and
chemical reaction at the substrate surface.
• Even though the CVD process is more complex, it gives more
effective
• control on the growth rate and the quality of deposited films.
• Most CVD processes involve low gas pressures (100–200 mtorr)
Principle of Operation
• The principle of operation involves the flow of a carrier gas with
diffused reactants over large substrate surfaces.
• The energy supplied by the heated surface triggers chemical
reaction of the reactants, thus forming thin films over the surface of
the substrates.
• The by-products of chemical reaction are vented out.
• A CVD process that requires elevated temperature (700–
800C) and near-atmospheric pressure is called
atmospheric pressure chemical vapor deposition
(APCVD).
• The low pressure chemical vapor deposition (LPCVD)
and
• plasma-enhanced chemical vapor deposition (PECVD)
processes are used to achieve higher growth rates and
better deposited film quality at lower temperatures.
• Typical reaction conditions for these methods are
provided in Table for quick comparison.
• The deposited film is usually uniform.
• Batch processing of stacked wafers is possible by this
method.
• PECVD utilizes RF plasma to transfer energy to
reactants, with the result that the substrate can
remain at lower temperature than in APCVD or
LPCVD.
• Precise temperature control of substrate surface
is necessary to ensure good deposited film
quality.
• Parameters that significantly influence the rate of
CVD are:
• 1. Temperature
• 2. Pressure of carrier gas
• 3. Velocity of gas flow
• 4. Distance along the direction of gas flow
CVD Reaction Chamber
Typical Process Conditions for various CVD
Techniques
Chemical Vapor Deposition
Processes(CVD)
• Wet or Dry?
– Wet processes use liquids and immersion
• Electroplating
• Electroless deposition
• Wet growth of SiO2 insulating layer (water vapor)
– Dry processes use chemical vapors
• Atmospheric Pressure Chemical Vapor Deposition
• Low Pressure Chemical Vapor Deposition
• Plasma Enhanced Chemical Vapor Deposition
Evaporative Deposition
• Utilizes the principle of vapor pressure
– Metallic species are melted in a low pressure
environment
– Higher vapor pressure metals evaporate first
– Deposition of the vapor on the surface occurs
– A low temperature process on the substrate
– Alternatives include laser ablation
• Laser strikes a target, causing local melting
Evaporative Deposition
• Utilizes the principle of vapor pressure
– Metallic species are melted in a low pressure
environment
– Higher vapor pressure metals evaporate first
– Deposition of the vapor on the surface occurs
– A low temperature process on the substrate
– Alternatives include laser ablation
• Laser strikes a target, causing local melting
Chemical Deposition Processes
• Atmospheric
Chemical Vapor
Deposition (CVD)
– Wafers are heated
– Chemical gases are
introduced
– A temperature
dependent deposition
rate
– Mass transport limited
at higher temperatures
Chemical Deposition Processes
• Low Pressure (CVD)
From MATEC Module 54
– Surface reaction
limited at low pressure
– Chamber may also be
heated or unheated
– Low pressure
environment increases
mean free path
– Better Step Coverage
and conformality than
APCVD
Chemical Deposition Processes
• Plasma Enhanced Low
Pressure (CVD)
– Lower Temperature
Process due to Plasma
Enhancement
– Dissociation of precursor
gas molecules
(Homogeneous reactions)
– Ions bombard surface
making it more reactive
– Higher rates of deposition
are possible than with
LPCVD
From MATEC Module 54
Chemical Deposition Processes
• Anti-reflective coatings
– Reflection from shiny layers below photoresist
causes blurred features
– Utilize thin film deposition to create coatings
that have λ/2 thickness at the exposure lamp
wavelength
– This results in destructive interference
canceling reflection in the photoresist layer
– Finer lithography is possible
Practice Questions
Click once for each question.
1. What are the advantages of atmospheric CVD?
Simple equipment requirements and batch processing is
possible
2. What is an advantage of low pressure CVD?
Improved purity of deposition and good step coverage
3. What is a principal advantage of plasma enhanced
CVD?
It is a lower temperature process than LPCVD
New Methods for
Nanomanufacturing
• Thinner layers are necessary for higher
speed transistors in IC design
– Gate oxide thickness < 50 A
– Approaches atomic layer dimensions
• Atomic Layer Deposition
– A 2 step process of deposition and re-layering
– SiOH* + SiCl4 →Si –O-SiCl3 + HCl
– SiCl* + H2O → SiOH* + HCl
New Methods for
Nanomanufacturing
• Molecular vapor deposition
– Anti-stiction layers in MEMS are needed to
avoid structures fusing to substrates
– Vapor deposition of compounds avoids
contamination found in liquid processes
– Oxygen plasma clean operation precedes
deposition process
LIGA Process
• LIGA includes X-Ray lithography, electroforming,
and plating operations that construct high aspect
ratio features on substrates
– Precision patterning of a deposited PMMA resist layer
using X-Ray lithography
– Areas remaining after development are plated with
metal
– Photo resist and excess metal removed
– Remaining features are high aspect ratio metal