Transcript 4-Contamination control - USM :: Universiti Sains Malaysia

EBB 323 Semiconductor
Fabrication Technology
Contamination
control
Dr Khairunisak Abdul Razak
Room 2.16
School of Material and Mineral Resources Engineering
Universiti Sains Malaysia
[email protected]
Topic outcomes
At the end of this topic, students should be able to:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Identify 3 major effects of contamination on
semiconductor devices and processing
Describe contamination sources in a fabrication area
Define the “class number” of a cleanroom
Describe the role of positive pressure, air showers, and
adhesive mats in maintaining cleanliness levels
List 3 techniques to minimise contamination from
fabrication personnel
Describe the differences between normal industrial
chemicals and semiconductor-grade chemicals
Name 2 problems associated with high static levels, and 2
methods of static control
Describe a typical FEOL and BEOL wafer cleaning
process
List typical wafer rinsing techniques
Cause of contamination
Forms and types of contaminants
Effects of contaminants
5 major classes of contaminants
1.
2.
3.
4.
5.
Particles
Metallic ions
Chemicals
Bacteria
Airborne molecular contaminants
(AMCs)
1. Particles
• Small feature size and thinness of deposited
layer of semiconductor devices make them
vulnerable to all kinds of contaminations
• Particle size must be 10 times smaller than the
minimum feature size e.g. 0.30m feature size
device is vulnerable to 0.03m diameter
particles
• Killer defects
– Particles present in a critical part of the device and
destroy its functioning
– Crystal defects and other process induced problems
• If contaminants present in less sensitive area
 do not harm the device
Relative sizes
Relative size of
contamination
2. Metallic ions
• Controlled resistivity is required in
semiconductor wafers; in N, P and N-P
junction
• The presence of a small amount
electrically active contaminants in the
wafer could results in
– Change device electrical characteristics
– Change performance
– Reliability parameters
• The contaminants that cause this problem
is called Mobile Ionic Contaminants (MIC)
– Metal atoms that exist in an ironic form in the
wafer
•MIC is highly mobile: metallic
ions can move inside the
device even after passing
electrical testing and shipping
 cause device fails
•MIC must be in < 1010
atoms/cm2
•Sodium is the most common
MIC especially in MOS
devices  look for lowsodium-grade chemicals
Trace Metal Parts per
Billion (ppb) Impurity
Sodium
50
Potassium
50
Iron
50
Copper
60
Nickel
60
Aluminum
60
Manganese
60
Lead
60
Zinc
60
Chlorides
1000
3. Chemicals
• Unwanted chemical contamination
could occur during process
chemicals and process water
• This may result in:
– Unwanted etching of the surface
– Create compound that cannot be
removed from the device
– Cause non-uniform process
• Chlorine is the major chemical
contaminant
Liquid chemicals in
semiconductor industries
Trace metallic impurities in some
liquid chemicals
4. Bacteria
• Can be defined as organisms that
grow in water systems or on surfaces
that are not cleaned regularly
• On semiconductor device, bacteria
acts as particulate contamination or
may contribute unwanted metallic
ions to the device surface
5. Airborne molecular contaminants
(AMCs)
• AMCs- fugitive molecules that escape from process tools,
chemical delivery systems, or are carried out into a
fabrication area on materials or personnel
• AMCs: gasses, dopants, and process chemicals used in
fabrication area e.g. oxygen, moisture, organics, acids,
bases etc..
• Problems:
– Harmful to process that requires delicate chemical
reactions such as the exposure of photoresist in the
patterning operations
– Shift etch rates
– Unwanted dopants that shift device electrical parameters
– Change the wetting characteristics of etchants leading to
incomplete etching
Relative size of airborne particles and wafer
dimensions
The effects of contamination on
semiconductor devices
1.
2.
Device processing yield
- contaminants may change the dimensions device parts
- change cleanliness of the surfaces
- pitted layers
 reduce overall yield through various quality checks
Device performance
- This may due to the presence of small pieces of
contamination that is not detectable during quality checks
- may also come from unwanted chemicals or AMCs in the
process steps  alter device dimensions or material
quality
- high amount of mobile ionic contaminants in the wafer
can change the electrical performance of the device
3. Device reliability
- Failure of device due to the presence of a small amount
of metallic contaminants that get into the wafer during
processing and not detected during device testing. These
contaminants can travel inside the device and end up in
electrically sensitive areas and cause device failure
Contamination sources
1.
2.
3.
4.
5.
6.
7.
Air
The production facility
Cleanroom personnel
Process water
Process chemicals
Process gasses
Static charge
1. Air
• Normal air contains contaminants  must be
treated before entering a cleanroom
• Major contaminant is airborne particles;
particulates or aerosols
– They float and remain in air for long period of
time
• Air cleanliness levels of cleanroom is determined
by the
– Particulate diameters
– Density in air
• Federal standard 209E: class number of the air in
the area
– Number of particles 0.5m or larger in a cubic foot of air
• In normal city with smoke, smog and fumes can contains up
to 5 million particles per cubic foot: class number 5 million
• Federal 209E:
– Specify cleanliness level down to class
1 levels
Relative size of airborne particulates (in microns)
Typical class numbers for various environments
Environment
Class number
Projected-256 merit
0.01
Maximum particle size
(m)
<< 0.1
Mini environment
0.1
< 0.1
ULSI fab
1
0.1
VLSI fab
10
0.3
VLF station
100
0.5
Assembly area
1000-10 000
0.5
House room
100 000
Outdoors
> 500 000
Air cleanliness standard
209E
•
Clean air strategies
1.
2.
3.
4.
Clean workstation
Tunnel design
Total cleanroom
Mini-environments
2. Production facility
Clean room strategy
• Fabrication area consists of a large room with
workstations (called hoods) arranged in rows so
that the wafers could move sequentially through
the process without being exposed to dirty air
• Use high-efficiency particulate attenuation (HEPA)
filters or ultra-low-particle (ULPA) filters
– Allow passage of large volumes of air at low velocity
– Low velocity contributes to the cleanliness of the hood
by not causing air currents, and also for operators
comfort
– HEPA and ULPA filters efficiency: 99.9999+ % at
0.12micron particle size
– Typical flow 90-100 ft/min
• HEPA and ULPA filters mounted on a clean
hood
– Vertical laminar flow (VLF)  air leave the
system in a laminar pattern, and at the work
surface, it turns and exits the hood
– Horizontal laminar flow (HLF)  HEPA filter is
placed in the back of the work surface
– Both VLF and HLF stations keep the wafer
cleans:
• Filtered air inside the hood
• Cleaning action inside is the slight positive pressure
built up in the station  prevent airborne dirt from
operators and from aisle area from entering the hood
HEPA filter
Cross-section of VLF fixed
with HEPA/ULPA filter
Cleanroom construction
• Primary design is to produce a sealed room that is
supplied with clean air, build with materials that
are non contaminating, and includes the system
to prevent accidental contamination from the
outside or from operators
• All materials must be non-shedding including
wall covering, process station materials and
floors coverings
• All piping holes are sealed and all light fixtures
must have solid covers
• Design should minimise flat surfaces that can
collect dust
• Stainless steel is favourable for process stations
and work surfaces
Fabrication area with gowning area, air
showers, and service aisle
Cleanroom elements:
1. Adhesive floor mats
– At every entrance to pull off and holds dirt
adhered at the bottom of the shoes
2. Gowning area
– Buffer between cleanroom and the plant
– Always supply with filtered air from ceiling HEPA
filters
– Store cleanroom apparel and change to
cleanroom garments
3. Air pressure
– Highest pressure in cleanroom, second highest
in gowning area and the lowest in factory
hallways
– Higher pressure in cleanroom causes a low flow
of air out of the doors and blow airborne particle
back into the dirtier hall way
4. Air showers
-
Air shower is located between the governing
room and the cleanroom
High velocity air jets blow off particles from the
outside of the garments
Air shower possesses interlocking system to
prevent both doors from being opened at the
same time
5. Service bay
-
Semi-clean area for storage materials and
supplies
Service bay has Class 1000 or class 10 000
Bay area contains process chemical pipes,
electrical power lines and cleanroom materials
Critical process machines are backed up to the
wall dividing the cleanroom and the bay 
allows technician to service the equipment from
the back without entering the cleanroom
6. Double-door-pass-through
-
-
Simple double-door boxes or may have a supply
of positive-pressure filtered air with interlocking
devices to prevent both doors from being
opened at the same time
Often fitted with HEPA filters
7. Static control
7. Static charge
• Static charge  attracts smaller particles to the
wafer
• The static charge may build up on wafers, storage
boxes, work surfaces and equipment
– May generate up to 50 000V static charge  attract
aerosols out of the air and personal garment 
contaminate the wafers
• Particles held by static charge is hard to remove
using a standard brush or wet cleaning system
• Most static charge is produced by triboelectric
charging
– 2 materials initially in contact are separated
– 1 surface possesses positive charge because it losses
electron
– 1 surface becomes negative because it gains electron
How particles are attracted to charge particles
Triboelectric series
• Electrostatic Discharge (ESD):
– rapid transfer of electrostatic charge between two
objects, usually resulting when two objects at different
potentials come into direct contact with each other.
– ESD can also occur when a high electrostatic field
develops between two objects in close proximity.
• Control static
– Prevent charge build up
• Use antistatic materials in garments and in-process storage
boxes
• Apply antistatic solution on certain walls to prevent charge
build up- not use in critical station due to possible
contamination
– Use discharge technique
• Use ionisers and grounded static-discharge
Eliminating static charge:
•Air ioniser – neutralise nonconductive materials
•Grounding of conducting surfaces
•Increasing conductivity of materials
•Humidity control
•Surface treatment with topical antistat solutions
6. Shoe cleaners
-
Removal of dirt from the sides of shoes and
shoes cover
Rotating brushes to remove the dirt
Typical machines feature an internal vacuum to
capture the loosened dirt, and bags to hold the
dirt for removal from the area
7. Glove cleaners
-
Discard gloves when they are contaminated or
dirty or after every shift
Some fabrication areas use glove cleaners that
clean and dry the gloves in an enclosure
Typical cleanroom garments
Guideline for use of cleanroom
garments
3. Cleanroom personnel
• Even after shower and sitting: 100 000-1
000 000 particles/minute
– Increase dramatically when moving e.g.
generate 5 million particles/min with movement
of 2 miles/hr
• Example of human contaminants:
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–
–
–
–
–
Flakes of dead hair
Normal skin flaking
Hair sprays
Cosmetics
Facial hair
Exposed clothing
4. Process water
• During fabrication process
– Repeated chemical etch and clean
– Water rinse is essential after etching/ cleaning
step  several hours in the whole system
• Unacceptable contaminants in normal city water
– Dissolves minerals
– Particles
– Bacteria
– Organics
– Dissolved O2
– Silica
• Dissolve minerals
– Comes from salt in normal water Na+ Cl– Can be removed by reverse osmosis (RO) and ion
exchange systems
• Remove electrically active ions  change water
from conductive medium to resistive medium
• It is a must to monitor resistivity of all process
water in the fabrication area
– Need to obtain between 15-18 M
• Remove contaminants
– Solid particles: sand filtration, earth filtration, membrane
– Bacteria: sterilise using UV radiation and filter out the
particles
– Organics (plant & fecal materials): carbon bed filtration
– Dissolved O2 & CO2: force draft decarbonators and
vacuum degasifiers
•Cleaning cost is a major operating cost
–Certain acceptable water quality: recycle in
a water system for clean up
–Too dirty water: treated and discharge from
plant
Resistivity of water vs concentration of
dissolved solids (ppm)
Resistivity Ohms-cm 25
Dissolved solids
(ppm)
18,000,000
0.0277
15,000,000
0.0333
10,000,000
0.0500
1,000,000
0.500
100,000
5.00
10,000
50.00
DRAMs water spec
Contaminant
64Kb
1Mb
16Mb
Resistivity (Mcm)
> 17
> 18
> 18.1
TOC (micro-g/L)
< 200
< 50
< 10
Bacteria (#/L)
< 250
< 50
<1
Particles
< 50
< 50
<1
Critical size (micron)
0.2
0.2
0.05
Dissolved oxygen (micron-g/L
< 200
< 100
< 20
Sodium (micro-g/L)
<1
<1
< 0.01
Chlorine (micro-g/L)
<5
<1
< 0.01
Manganese (micro-g/L)
N/A
<1
< 0.005
5. Process chemicals
• Highest purity of acids, bases and solvents are used for
etching and cleaning wafers and equipment
• Chemical grades:
– Commercial
– Reagent
– Electronic
– Semiconductor
• Main concerns: metallic mobile ionic contaminants (MIC) 
must be < 1 ppm
• To date, can obtain chemicals with 1ppb MIC
• Check assay no e.g. assay 99.9% purity
• Other steps:
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–
–
–
Clean inside containers
Use containers that do not dissolve
Use particulate free labels
Place clean bottles in bags before shipping
6. Process gasses
• Semiconductor fabrication uses many gases:
– Air separation gases: O2, N2, H2
– Specialty gases: arsine and carbon
tetrafluoride
• Determination of gas quality
– Percentage of purity
– Water vapour content
– Particulates
– Metallic ions
• Semicnductor fabrication requires extremely high
purity process gasses for oxidation, sputtering,
plasma etch, chemical vapour deposition (CVD),
reactive ion gas, ion implantation and diffusion
• If gas is contaminated, wafer
properties could be altered due to
chemical reaction
• Gas quality is also shown in assay
no; 99.99-99.999999. The highest
quality is called “six 9s pure”
Requirements for Si wafer
cleaning process
1. Effective removal of all types of surface
contaminants
2. Not etching or damaging Si and SiO2
3. Use of contamination-free and volatilisation
chemicals
4. Relatively safe, simple, and economical for
production applications
5. Ecologically acceptable, free of toxic waste
products
6. Implementable by a variety of techniques
Wafer surface cleaning
4 general types of contaminants:
1. Particulates
2. Organic residues
3. Inorganic residues
4. Unwanted oxide layers
•
Wafer cleaning process must
– Remove all surface contaminants
– Not etch or damage the wafer surface
– Be safe and economical in a production
setting
– Be ecological acceptable
•
2 primary wafer conditions:
1.
2.
Front end of the line (FEOL)
Back end of the line (BEOL)
FEOL
• Wafer fabrication steps used to form the
active electrical components on the wafer
surface
– Wafer surface especially gate areas of MOS
transistors, are exposed and vulnerable
• Surface roughness: excessive roughness
results in degradation of device
performance and compromise the
uniformity
• Electrical conditions of bare surface
– Metal contaminants
• Na, Ni, Cu, Zn, Fe etc: cleaning process need to
reduce the concentration to < 2.5 x 109 atoms /cm2
• Al and Ca contaminants: need to reduce to 5 x 109
atoms/cm2 level
Typical FEOL cleaning process steps
1. Particle removal (mechanical
2. General chemical clean (such as
sulphuric acid/H2/O2
3. Oxide removal (typically dilute HF)
4. Organic and metal removal
5. Alkali metal and metal hydroxide
removal
6. Rinse steps
7. Wafer drying
BEOL
• Main concerns: particles, metals,
anions, polysilicon gate integrity,
contact resistance, via holes
cleanliness, organics, numbers of
shorts and opens in the metal system
Particulate removal
• Spray: blow off the water surface
using spray of filtered high pressure
nitrogen from a hand-held gun
located in the cleaning station
– In fabrication area/small particles:
nitrogen guns are fitted with ioniser that
strip static charges from the nitrogen
stream and neutralise the wafer surface
• Wafer scrubbers-combination of
brush and wafer surface.
• High pressure water cleaning
Organic residues
• Organic residues- compounds that
contain carbon such as oils in
fingerprints
• Can be removed in solvent baths
such as acetone, alcohol or TCE
• Solvent cleaning is avoided:
– difficulty of drying the solvent
completely
– Solvents always contain some
impurities that may cause contamination
Inorganic residues
• Organic residues- do not contain
carbon e.g. HCl and HF from steps in
wafer processing
Chemical cleaning solutions
•
•
For both organic and inorganic
contaminants
General chemical cleaning
1. Sulphuric acid
•
•
•
•
•
•
•
Hot sulphuric acid with added oxidant
Also a general photoresist stripper
H2SO4 is an effective cleaner in 90-125C  remove
most inorganic residues and particulates from the
surface
Oxidants are added to remove carbon residues
Chemical reaction converts C to CO2
Typical oxidants: hydrogen peroxide (H2O2),
ammonium persulfate [(NH4)2S2O8]
Nitric acid (HNO3), and ozone (O2)
RCA clean
•
•
RCA clean- H2O2 is used with some base
or acid
Standard clean 1 (SC-1)
–
–
–
•
Solution of water, hydrogen peroxide,
ammonium hydroxide = 5:1:1, 7:2:1, heated to
75-85C
SC-1 removes organic residues and set up a
condition for desorption of trace metals from
the surface
Oxide films keep forming and dissolving
SC-2
–
–
–
Solution of water, hydrogen peroxide and
hydrochloric acid = 6:1:1 to 8:2:1, 75-85C
Remove alkali ions and hydroxides and
complex residual metals
Leave a protective oxide layer
• Order of SC-1 and SC-2 can be
reversed
• If oxide-free surface is required, HF
step is used before, in between, or
after the RCA cleans