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 needs to occur on the surfaces in a controlled environment.
• Thermochemical treatments like alcloxr carxiion, Carburixir 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 coulombian 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 allow us to understand the regime we are in:

Adimensional Number of Knudsen

Kn = λ / d_c

1. Kn > 1 → molecular
2. 0.01 < Kn < 1 → some collisions
3. Kn < 0 → viscous

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 controlled 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 / √2δP

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

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

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

• GAS → δ
• PLASMA → Larger δ due to coulombian 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:

Adimensional Number of Knudsen

K_n = λ / d_e

• 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
• Medium
• High
• Ultra High

10⁻³ ÷ 1 mbar1 ÷ 10⁻³ mbar10⁻³ ÷ 10⁻⁶ mbar< 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 / 4

Φ_net = Φ₁ - Φ₂ = (n₁ - n₂) / 4

PV = nRT

Φ ΔP

Φ_net = (P₁ - P₂) / (4RT)

Real Flow Rate = ṅ = A (P₁ - P₂) / (4RT)

Conductance of aperture

The trough AṼ is the quantity of gas that passes on plane in a unit of time.

Q = d(PV)/dt = ṅRT → more known as

Q = C (P₁ - P₂)

C = A / 4

= [8RT/(πM)]^1/2

C = [RT/2πM]^1/2 A

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

C if diameters ↑, lengths ↓, Φ₁, ΔP ↓

it is pos