Plasma Assisted Surface Modification

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Plasma Assisted Surface Modification

• By Maurice Clark
• For Elec 7730
• Dr. Tzeng

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Outline

• Questions
• Types Surface Modifications
• Plasmas role in surface modification
• Oxidation
• Nitriding
• Carburzing
• Answer to questions

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Questions

• What are the four forms of surface modification?
• Name one way that oxygen plasma is used to modify
  the surface of materials.
• Why is plasma used to modify the surface of
  material?

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Type of Surface modifications
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Surface Modification Categories

• Contamination Removal
• Use physical and chemical energy to remove a
  small layer of contamination.
• In this particular process positive ions hit the
  surface and dislodge the contaminates from the
  surface
• Surface Activation
• Process uses gases that dissociates when exposed
  to plasma, creating different functional groups
  on the surface.
• Functional groups bonds well with the bulk
  material
• Grafting
• Plasma excited by inert gases activate the
  surface by creating free radicals on surface,
  then the material is exposed to monomers causing
  polymer layer to grafted
• Cross Linking
• Uses inert gases such as argon and helium to
  remove atomic species for the surface and then
  generates reactive surface radials The radicals
  then react within the surface forming chemical
  bonds which result is in cross-linked surface

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Plasma Surface Modification

• Cold Plasma
• Microwave
• Radio Frequency
• Inductively Coupled
• DC Glow Discharge
• Processes such as deposition, cleaning, cross
  linking, and grafting competing within the
  plasma.
• Controlling the parameters of the plasma we can
  make one process more preferable then the others.
• Plasma is characterized by what it does to the
  substrate.

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Plasma Surface Modification

• Involves the interaction of plasma generated
  species with a solid surface.
• Result in a physical or chemical modification of
  the first few layers of the surface, while
  maintaining the properties of the bulk material.
• Materials employed in the microelectronic and
  optoelectronic industries include ceramics,
  glass, polymers ,and metal. (Au, Cu, Ni, ect.)

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Plasma Parameters

• Processing Parameters
• Operating Pressure
• Substrate voltage
• Gas composition
• Treatment Power
• Treatment Time
• System Parameters
• Electrode Location
• Reactor Design
• Vacuum system

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Plasma Oxidization
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Plasma Oxidation

• Plasma oxidation - formation of an oxide on a
  metal or semiconductor surface immersed in oxygen
  plasma at a floating potential.
• The substrate is floating therefore no current is
  flowing through it and the oxide growth occurs by
  thermal diffusion.

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Plasma Oxidation
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Plasma Oxidation

• Types of Plasmas
• Inductively coupled
• Microwave
• Radio Frequency
• ECR
• Gases
• Oxygen
• Nitrous Oxide
• Ozone

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Applications

• Effective gate dielectric
• Polysilicon
• Thin film transistors

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Advantages of Oxidation

• Reduced diffusion of heavy metals into silicon.
• Possibility of oxidation after metallization.
• Prevention of wafer warpage as a result of
  thermal stresses.
• Possibility of oxidation of hydrogenated
  amorphous silicon with reduced hydrogen loss that
  world otherwise change the properties of the
  material.

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Plasma Nitriding
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Plasma Nitriding Process

• Low pressure plasma process
• Vacuum filled with H2 and N2 at pressure between
  0.1 to 10 torr, with a large DC voltage between
  the work space as the cathode and furnace as the
  anode
• DC glow discharge ionizes the gas an ion hit the
  work surface and ion nitriding happens.

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Properties of Plasma Nitrated Surfaces

• Improvement in surface hardening
• Greater wear resistance
• Longer fatigue life
• Corrosion Resistance
• Process can be applied to wide variety of metal.
• Cast Iron
• Stainless Steel
• Titanium
• Plasma treatment is preferred over conventional
  treatment because if reduces the treatment time
  which takes about 50 hrs.

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Surface Modification using Nitrogen Plasma

• Improves the surface hardness
• Gear wheels
• Crank shafts
• Dies ECT.
• Reduced Adhesion
• Improved High-temperature Strength
• Increased part surface life

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Plasma Nitriding Systems
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Plasma Carburizing
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Schematic of Plasma Carburizing Unit

• Work piece is heated up to process temperature by
  an external heater in a vacuum furnace.
• Methane or propane is introduced into the furnace
  to a pressure 2 to 3 torr.
• A DC voltage is applied between the work piece or
  cathode and a glow discharge is generated

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Carburizing

• Plasma primarily assists in the mass transfer of
  carbon atoms to the surface of the sample.
• Carburizing of steel has been done using
  hydrocarbons gas at 1-20 torr in a DC discharge.
  This process required extra heat treatment of the
  part at 1050C
• Plasma treatment is conducted for a short period
  of time. A longer heating time is need in order
  to diffuse carbon into steel.

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Carburizing Process Units
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Surface Functionalize
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Oxygen Plasma

• Low pressure plasma
• Reactive gases are feed into a vacuum chamber and
  are ionized by electrical energy.
• Gases active in a way such that that the surface
  of the material is changed.
• The byproduct of the this reaction are carry away
  by the pump.

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Oxygen Plasma

• Electrons generate
• Ions
• UV Photons
• Negative Ions
• Radicals
• React with Surface
• Carbon dioxide
• Water
• Modified Surface
• Tailored properties

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Fictionalization

• Cross linking
• Inert gas such as Argon or Helium
• Ionized and the covalent polymer bonds are
  broken at the polymer surface .
• Three process which can occur in the inert plasma
• Dissociated molecules can revert to its previous
  state by recombining
• Reaction between adjoining free radical within
  the polymer chain forming double and triple bonds
• Molecules can form a bond with nearby free
  radical on an adjacent chain (cross linking)

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Types of Plasmas used for Crosslinking

• Helium
• Argon
• Krypton
• Neon
• Xeon
• Hydrogen
• Nitrogen

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Process Applications for Plasma Surface
Enhancement
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Fictionalization

• CASING (Crosslinks via Activated Species of Inert
  Gases.
• This process is used to enhance the performance
  of the polymers.
• The bond breaking accords on the polymer surface.
• Since there are no free radical scavengers
  (oxygen by products) in the plasma the molecules
  form bonds with nearby radicals on different
  chains.
• Depending on the chemistry of the polymer and
  the gas the surface can be made wettable or
  nonwettable.

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Answers

• What are the four forms of surface modification?
  Crosslinking, Activation, Grafting, Contamination
  Removal
• Name one way that oxygen plasma is used to modify
  the surface of materials. Oxidation
• Why is plasma used to modify the surface of
  material? Low-temperature process

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Conclusion

• Plasma has many applications in industry.
• Plasma can be used to modify a Varity of
  materials
• Plasma can functionalize the surface to create
  properties that where not present before.