EBB 323 Semiconductor Fabrication Technology

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EBB 323 Semiconductor Fabrication Technology
Contamination control
Dr Khairunisak Abdul Razak Room 2.16 School of
Material and Mineral Resources Engineering Univers
iti Sains Malaysia khairunisak_at_eng.usm.my
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Topic outcomes

• At the end of this topic, students should be able
  to
• 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

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Cause of contamination
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Forms and types of contaminants
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Effects of contaminants
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5 major classes of contaminants

• Particles
• Metallic ions
• Chemicals
• Bacteria
• Airborne molecular contaminants (AMCs)

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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

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Relative sizes
Relative size of contamination
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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

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• 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 lt 1010 atoms/cm2
• Sodium is the most common MIC especially in MOS
  devices ? look for low-sodium-grade chemicals

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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

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Liquid chemicals in semiconductor industries
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Trace metallic impurities in some liquid chemicals
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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

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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

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Relative size of airborne particles and wafer
dimensions
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The effects of contamination on semiconductor
devices

• 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

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• 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

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Contamination sources

• Air
• The production facility
• Cleanroom personnel
• Process water
• Process chemicals
• Process gasses
• Static charge

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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

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• Federal 209E
• Specify cleanliness level down to class 1 levels

Relative size of airborne particulates (in
microns)
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Typical class numbers for various environments
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Air cleanliness standard 209E
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• Clean air strategies
• Clean workstation
• Tunnel design
• Total cleanroom
• Mini-environments

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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

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• 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

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HEPA filter

• Cross-section of VLF fixed with HEPA/ULPA filter

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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

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Fabrication area with gowning area, air showers,
and service aisle
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• Cleanroom elements
• Adhesive floor mats
• At every entrance to pull off and holds dirt
  adhered at the bottom of the shoes
• 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
• 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

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• 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
• 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

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• 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
• Static control

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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

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How particles are attracted to charge particles
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Triboelectric series
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• 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

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• Eliminating static charge
• Air ioniser neutralise nonconductive materials
• Grounding of conducting surfaces
• Increasing conductivity of materials
• Humidity control
• Surface treatment with topical antistat solutions

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• 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
• 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

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Typical cleanroom garments
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Guideline for use of cleanroom garments
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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
• Flakes of dead hair
• Normal skin flaking
• Hair sprays
• Cosmetics
• Facial hair
• Exposed clothing

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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

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• 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

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• 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)
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DRAMs water spec
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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 lt 1 ppm
• To date, can obtain chemicals with 1ppb MIC
• Check assay no e.g. assay 99.9 purity
• Other steps
• Clean inside containers
• Use containers that do not dissolve
• Use particulate free labels
• Place clean bottles in bags before shipping

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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

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• 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

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Requirements for Si wafer cleaning process

• Effective removal of all types of surface
  contaminants
• Not etching or damaging Si and SiO2
• Use of contamination-free and volatilisation
  chemicals
• Relatively safe, simple, and economical for
  production applications
• Ecologically acceptable, free of toxic waste
  products
• Implementable by a variety of techniques

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Wafer surface cleaning

• 4 general types of contaminants
• Particulates
• Organic residues
• Inorganic residues
• 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
• Front end of the line (FEOL)
• Back end of the line (BEOL)

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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 lt 2.5 x 109 atoms
  /cm2
• Al and Ca contaminants need to reduce to 5 x 109
  atoms/cm2 level

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Typical FEOL cleaning process steps

• Particle removal (mechanical
• General chemical clean (such as sulphuric
  acid/H2/O2
• Oxide removal (typically dilute HF)
• Organic and metal removal
• Alkali metal and metal hydroxide removal
• Rinse steps
• Wafer drying

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BEOL

• Main concerns particles, metals, anions,
  polysilicon gate integrity, contact resistance,
  via holes cleanliness, organics, numbers of
  shorts and opens in the metal system

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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

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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

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Inorganic residues

• Organic residues- do not contain carbon e.g. HCl
  and HF from steps in wafer processing

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Chemical cleaning solutions

• For both organic and inorganic contaminants
• General chemical cleaning
• 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)

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RCA clean

• RCA clean- H2O2 is used with some base or acid
• Standard clean 1 (SC-1)
• Solution of water, hydrogen peroxide, ammonium
  hydroxide 511, 721, 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 611 to 821, 75-85?C
• Remove alkali ions and hydroxides and complex
  residual metals
• Leave a protective oxide layer

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• 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

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