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Plasma Assisted Surface Modification. By Maurice Clark For Elec 7730 Dr. Tzeng. Outline. Questions Types Surface Modifications Plasma’s role in surface modification Oxidation Nitriding Carburzing Answer to questions. Questions. What are the four forms of surface modification?
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Plasma Assisted Surface Modification By Maurice Clark For Elec 7730 Dr. Tzeng
Outline • Questions • Types Surface Modifications • Plasma’s role in surface modification • Oxidation • Nitriding • Carburzing • Answer to questions
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?
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
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.
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.)
Plasma Parameters • Processing Parameters • Operating Pressure • Substrate voltage • Gas composition • Treatment Power • Treatment Time • System Parameters • Electrode Location • Reactor Design • Vacuum system
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.
Plasma Oxidation • Types of Plasma’s • Inductively coupled • Microwave • Radio Frequency • ECR • Gases • Oxygen • Nitrous Oxide • Ozone
Applications • Effective gate dielectric • Polysilicon • Thin film transistors
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.
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.
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.
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
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
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 1050°C • Plasma treatment is conducted for a short period of time. A longer heating time is need in order to diffuse carbon into steel.
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.
Oxygen Plasma • Electrons generate • Ions • UV Photons • Negative Ions • Radicals • React with Surface • Carbon dioxide • Water • Modified Surface • Tailored properties
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)
Types of Plasma’s used for Crosslinking • Helium • Argon • Krypton • Neon • Xeon • Hydrogen • Nitrogen
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.
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
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.
