Surface Technology

Vacuum technology

28 April 2019 18:09

Main uses:

• PVD Techniques to transfer atoms from a source to the object who needs a treatment.
• CVD Techniques in which chemical reactions need to occur on the surfaces in a controllable environment.
• Thermochemical treatments like alloys creation, carburizing and nitriding in which the surface has similar properties of the external layer.

Aim of Process:

Lower the MEAN FREE PATH λ. So reduce the density of the gas and avoid collisions during treatment.

PV=nRT

The mean free path is defined:

λ = ^kT/_√2σP

σ is the cross-section so the probability of interaction between atoms.

P₂ must be inside σ of P₁, to have a probability of collision.

There are different definitions of σ based on the different phases:

• GAS → σ
• PLASMA → Larger σ due to columbium interactions

P < 10^-3 mbar → λ↑ → No collisions = VACUUM

Gas Flow Regimes

• Molecular Flow: λ > D [No collisions]
• Viscous Flow: λ < D [Laminar / Turbulent]

There is a number that allows us to understand the regime we are in:

K_n = ^λ/_de

Adimensional Number of Knudsen

• K_n > 1 → molecular
• 0.01 < K_n < 1 → some collisions
• K_n < 0 → viscous

Molecular Flow

in this regime, collisions with particles are rarer than with walls

Vacuum Ranges

• Low 10^-3 ÷ 1 mbar
• Medium 1 ÷ 10^-3 mbar
• High 10^-3 ÷ 10^-6 mbar
• Ultra High <10^-6 mbar

Vacuum system produces a pressure difference to define a gas flow rate. We can define the number of moles that crosses a single area in a unit of time

Gas Flux

Φ = n_a v̅ _a

Φ_net = Φ₁ - Φ₂ = (n₁ - n₂) v̅_a/4 = Φ_net = (P₁ - P₂) √(RT/2π)

^{molecules can move either way}

so Φ is proportional to ΔP

Real flow rate:

ṁ = A (P₁ - P₂) √(RT/2π)

Conductance of Aperture

The throughput Ṫ is the quantity of gas that passes an plane in a unit of time

Q = (d(PV)/dt = PV̅ = ṁRT

→ more known as

Q = C (P₁ - P₂) C = A v̅/4

the only physical constraint is to have isothermal condition

Velocity v̅ = √(8RT/πM)

Conductance C = A √(RT/2πM)

if we consider different shapes than a cylinder, C is considered by similar expression corrected by adimensional factors

C=size size, Diameter size, Length size, Φ, pressure

Microstructure and sintering

_{19 April 2019} ^21:00

Aim of the Process:

to reduce surface area of a solid body and to reduce porosity. Interesting to see evolution in the structure and to create solid bodies out of powders (formation of net shapes) without finishing operations needed.

Main uses:

• Ceramic and refractory metals
• Porous Bodies
• Self lubricant bearings
• Matrix for composite materials
• Microstructures with no defects

Production Techniques:

1. Cold Pressing + Heat Treatment = Real Sintering
2. Heating + Pressing = Hot Pressing sintering
3. Liquid Phase - To speed up the sintering
4. Injection molding (for smaller shapes Metal + Liquid Polymer)

Cold Sintering Process

By pressing powders there occurs a plastic deformation that forms contact points which then evolve into necks that become bigger and bigger developing strong connections with near particles; any particle develops necks to form a dense material.

Atomic Diffusion

Reduction of Distance.

Densification → ∈ reduction

(Movement through less energy terms)

Faster than cold sintering

Still classified as creep because we see macroscopic deformation of atomic diffusion nature.

Second Stage

1. Plastic Yielding
2. Creep
3. Diffusional Creep

Porosity < 10%

Same steps as before but now they are applied to a continuous solid matrix with isolated pores.

Strain rate relationships are directly proportional with stresses.

Lattice   Ẽ ∝ _σDL/^ss²

Just as cold sintering we have pores inclusions in grain boundaries

Grain Boundaries   Ẽ ∝ _σDb/^ss³

We can plot sintering maps to understand which is the main mechanism for densification.

To have the best results we should have uniform bulk heating.

Liquid Sintering

Aim of Process: Mix solid components with liquid ones to aid densification. But the 2^nd element will be present in the material with a low melting point which will lead to low mechanical properties. But at the end we achieve a porous-free material.

STEP 1: Rearrangement by liquid flow through solid particles. There will form liquid binders with concave meniscuses.

Low liquid pressure → Net force push particles closer → Liquid phase over surfaces → Advance of mechanism

(Fast & Effective)

Solutions to thickness issues:

1. Extended Source or more than one Source
2. Collect atom in high thickness zones with a Shutter
3. Increase pressure in the chamber (more collisions)

Uniformity is an important parameter for optical properties.

Another issue can be the conformal coverage of the surface which means that non-planar surfaces can give geometrical shadowing to the coating. This can be solved in different ways:

• Extended or multiple sources
• Rotating systems
• Higher substrate temperature
• Modification of the substrate

Coating Purity

To have a pure coating it means that we need a very low contamination of O₂.

We can estimate the impurity atomic fraction:

X_i = (Φ_i / (Φ_v + Φ_i)) ≈ (Φ_i / Φ_v) So the aim of process is to reduce X_i maximizing vapour deposition rate but minimizing the impurity flux. We can operate by controlling two variables: P and T

To be Avoided:

• O₂, N₂, H₂O, CO₂

In vacuum evaporation we can reach 0.1 μm/s at 10^-8 mbar

The stabilization of plasma

Collisions can be:

1. Elastic: exchange of kinetic energy Fast particles collide with slow ones
2. Inelastic: potential energy transfer to exchange e^- energy to heavier species

Ionization:

Collisions between e^- and inert gas atoms, but e^- needs to have enough energy If e^- energy is under 10 eV, the recombination occurs

e^- + Ar → Ar⁺ + 2e^-

Excitation:

Collision → excited e^- state even if ionization does not occur

e^- + O₂ → O₂^* + e^-

Dissociation:

e^- in anti-bonding → unsaturated bonds = very reactive species

e^- + CF₄ → CF₃^* + F^* + e^-

e^- Attachement:

Highly electronegative species attracts e^- to form negative ions

e^- + SF₆ → SF₆^-

The charge transfer collision can occur in two different ways:

• A + A⁺ → A⁺ + A   Same species with different energies   Fast neutral species
• A + B⁺ → A⁺ + B   Chemical difference

At the cathode surface, can occur two kind of events: Sputtering or Implantation

Sputtering

Occurs when the ion impact establishes a train of collision events in the target leading to the ejection of a matrix atom.