objective of the milling are article size reduction morphization article size growth hape changing gglomeration hanging the properties of a material ixing or blending of two or more materials Mechanical Attrition Mechanical attrition – it is a top-down approach. The nanoparticles formed in a Mechanical devices are generally termed as mill. Energy is imparted to bulk material to result in the reduction in particle size. Nano particles – if their averarge characteristic dimension is less than 100nm Nanocrystalline – if the crystalline size after milling is between 1 to 10 nm in diameter
Transcript
The objective of the milling are
1. Particle size reduction2. Amorphization3. Particle size growth4. Shape changing5. Agglomeration6. Changing the properties of a material7. Mixing or blending of two or more materials
Mechanical Attrition
Mechanical attrition – it is a top-down approach. The nanoparticles formed in a Mechanical devices are generally termed as mill.
Energy is imparted to bulk material to result in the reduction in particle size.
Nano particles – if their averarge characteristic dimension is less than 100nm
Nanocrystalline – if the crystalline size after milling is between 1 to 10 nm in diameter
Principles of millingPrinciple - Size reduction in mechanical attrition devices lies in the energy imparted to the sample during impacts between the milling media
1. Compaction starts with rearrangement and restacking of particles.
2. Elastic and plastic deformation
3. Particle fracture, resulting in deformation and fragmentation of the particle. A continous refinement Of internal structure of the powder particles to nm scale occur during high energy mechanical attrition.
4. Formation of agglomeration
Attritor mill
Shaker millPlanetary mill
Different types of Attritor mill
Parameter influence the size of the particle1. Type of mill2. Milling atomsphere3. Milling medium4. Milling temperature5. Intensity of milling6. Ball to powder ratio – 5:107. Local temperature due to ball collision8. Overall temperature of the vessel9. The kinetic energy of the ball depends on
a. impact speed on the balls
EtchingEtching – Removal of atoms or molecules from the surface of material. The removal ofAtoms depend on the energy of atom or molecule.
This can be different ways1. Electron beam etching2. Ion beam etching3. Laser beam etching4. Corrosive chemical etching5. Plasma etching
Electron beam etching
Ion beam etching
Laser beam etching Plasma etching
• Epitaxy: Deposition and growth of monocrystalline structures/layers.
• Epitaxial growth results in monocrystalline layers differing from deposition which gives rise to polycrystalline and bulk structures.
• Epitaxy types:– Homoepitaxy: Substrate & material are of same kind.
(Si-Si)– Heteroepitaxy: Substrate & material are of different kinds. (Ga-As)
Molecular Beam Epitaxy
Atoms arriving at the substrate surface may undergo• absorption to the surface, • surface migration, • incorporation into the crystal lattice, • thermal desorption. depends strongly on the temperature of the substrate..
MBE growth mechanism. 1212
Growth modes:At very high temperature of substrate, there are many different possible surface
diffusion mechanisms:
Ehrlich-Schwoebel barrier
• very low rates of impinging atoms, • migration on the surface and • subsequent surface reactions
epitaxial growth is ensured by-
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Advantages Disadvantages Clean surfaces, free of an oxide layer Expensive (106 $ per MBE chamber)
In-situ deposition of metal seeds, semiconductor materials, and dopants
ATG instability
Low growth rate (1μm/h) Very complicated system
Precisely controllable thermal evaporation Epitaxial growth under ultra-high vacuum conditions
Seperate evaporation of each component
Substrate temperature is not high
Ultrasharp profiles
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Implantation Processes: Damage• Ion collides with lattice atoms and knock them
out of lattice grid• Implant area on substrate becomes amorphous
structure
Before Implantation After Implantation
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Stopping Mechanism
• Ions penetrate into substrate• Collide with lattice atoms• Gradually lose their energy and stop • Two stop mechanisms
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Two Stopping Mechanism
• Nuclear stopping – Collision with nuclei of the lattice atoms– Scattered significantly – Causes crystal structure damage.
• electronic stopping – Collision with electrons of the lattice atoms– Incident ion path is almost unchanged– Energy transfer is very small – Crystal structure damage is negligible
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Stopping Mechanism
• The total stopping powerStotal = Sn + Se
• Sn: nuclear stopping, Se: electronic stopping• Low E, high A ion implantation: mainly
nuclear stopping• High E, low A ion implantation, electronic
stopping mechanism is more important
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Stopping Mechanisms
Random Collisions (S=Sn+Se)
Channeling (SSe)
Back Scattering (SSn)
Ion
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Stopping Power and Ion Velocity
Nuclear Stopping
Electronic Stopping
I II III
Ion Velocity
Stop
ping
Pow
er
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Damage Process
• Implanted ions transfer energy to lattice atoms – Atoms to break free
• Freed atoms collide with other lattice atoms– Free more lattice atoms – Damage continues until all freed atoms stop
• One energetic ion can cause thousands of displacements of lattice atoms
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Lattice Damage With One Ion
Heavy Ion
Single Crystal Silicon
Damaged Region
Light Ion
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Implantation Processes: Damage• Ion collides with lattice atoms and knock them
out of lattice grid• Implant area on substrate becomes amorphous
structure
Before Implantation After Implantation
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Implantation Processes: Anneal
• Dopant atom must in single crystal structure and bond with four silicon atoms to be activated as donor (N-type) or acceptor (P-type)
• Thermal energy from high temperature helps amorphous atoms to recover single crystal structure.
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Thermal Annealing
Dopant AtomLattice Atoms
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Thermal Annealing
Dopant AtomLattice Atoms
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Thermal Annealing
Dopant AtomLattice Atoms
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Thermal Annealing
Dopant AtomLattice Atoms
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Thermal Annealing
Dopant AtomLattice Atoms
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Thermal Annealing
Dopant AtomLattice Atoms
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Thermal Annealing
Dopant AtomLattice Atoms
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Thermal Annealing
Dopant AtomLattice Atoms
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Thermal Annealing
Dopant AtomsLattice Atoms
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Implantation Processes: Annealing
Before Annealing After Annealing
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Rapid Thermal Annealing (RTA)
• At high temperature, annealing out pace diffusion
• Rapid thermal process (RTP) is widely used for post-implantation anneal
• RTA is fast (less than a minute), better WTW uniformity, better thermal budget control, and minimized the dopant diffusion
