MicroContamination

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• Unit 2
• Micro-Contamination

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Micro-Contamination
How small is small?
Cross-section of human hair 75 microns in
thickness
Smallest particle visible to the human eye 50
microns in diameter
Contamination A particle of 0.5 microns
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Typical Class Numbers for Various Environments
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Contamination Control

• The Problem
• Contamination Sources
• Clean Room Strategies
• Particulate Problems

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The Problem

• Particles
• Metallic Ions
• Chemicals
• Bacteria

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Particles

• Contaminants - particulate larger than 1/10th
  feature size
• Killer defects - particles located in critical
  part of device that destroy its functioning

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Metallic Ions

• Mobile Ionic Contaminates (mics) can electrically
  change doped silicon.
• Silicon is doped for a certain resistivity as
  either N material or P material
• A mic can change that resistivity which will
  change the electrical characteristic of the
  material

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Chemicals

• Unwanted chemicals are a major source of
  contaminate
• They can result in unwanted and undesirable
  etching of the wafer

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Bacteria

• Organisms that grow in water systems
• Occur when water is not cleaned regularly
• Result in a particulate contaminate

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Contamination Sources

• Air
• Cleanroom Personnel
• Process Water
• Process Chemicals
• Static Charge

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Clean Room Air

• Air - major problem with floating particles from
  aerosols
• Air quality designated by the number of particles
  .5 microns or greater in a cubic foot of air
• Typical city air 5 million particles per cubic
  ft.

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Minimize Contamination from Clean-room Personnel

• Adhesive floor mats
• Gowning area
• Air pressure
• Air showers
• Service bays
• Double-door pass-through
• Static control
• Shoe and glove cleaners

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Contaminants present in normal water

• Dissolved minerals
• Particulates
• Bacteria
• Organics
• Dissolved oxygen
• Silica

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Process Chemicals

• Semiconductor grade is the highest grade of
  chemicals available
• Grades are commercial, reagent electronic and
  semiconductor.
• Bulk chemical distribution systems (BCDS) provide
  such chemicals.
• Point-of-use (POU) chemical mixing systems
  provide an in-house means of maintaining chemical
  purity.

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Static Charges

• Static buildup occurs on wafers, storage boxes,
  work surfaces and equipment.
• These charges can reach as high as 50,000 volts
  and can attract aerosols out of the air.
• Attracted particles end up contaminating wafers
  and are difficult to remove.
• Static is controlled by prevention of charge
  buildup.
• Use of anti-static materials in garments,
  grounded mats and wrist straps are some of the
  ways used to control static.

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Cleanroom Strategies

• Clean work Station
• Tunnel Design
• Total Clean Room

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Clean Work Stations

• Involved the use of hoods with HEPA (High
  Efficiency Particulate Attenuation) filters -
  fragile fibers with many small holes folded into
  filter holder in an accordion design.
• Large volume of air (90-100ft/min) at low
  velocity.
• Filtering efficiency of 99.99 - filter of choice
  in all clean room designs.
• Used VLF (vertical laminar airflow) from ceiling
  to floor.
• Problem - difficult to maintain environment with
  people entering, moving and exiting the room.

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Tunnel Design

• Tunnels or bay with HEPA filters built into the
  ceilings instead of VLF hoods.
• Fewer workers in the vicinity with filtered air
  worked well.
• More expensive to construct and less versatile
  should the process be changed.

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Total Clean-Room Strategy

• Development of clean-room designs and filtering
  technology allowed the return to open fab area.
• HEPA filters in the ceiling with returns in the
  floor give continuous clean air flow.
• Work stations with perforated table tops allow
  the air to pass through them uninterrupted.

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Total Clean Room Design
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Problems caused by Particulate

• Device process yield reduction
• Device performance degraded
• Device reliability degraded

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Yield Reduction

• Can actually change the dimensions of a device
  part
• Can change the cleanliness of the surface and/or
  cause pitted layers
• There are quality checks and inspections
  throughout the process and these contaminates
  will result in lower yields through these
  stations.

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Performance

• Mics or particles can change electrical
  performance even though they are so small they
  escape inspections and defect checks.
• This problem usually appears at electrical sort

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Device Reliability

• Most insidious of the contamination failures
• Though undetected in test, mics can travel inside
  the device and end up in electrically sensitive
  areas causing failures in the field.
• This is particular concern in the fields of space
  and defense.