4 - Technology of thin films
4.1. Vacuum technology
The vacuum:
Definition by standard DIN 28400: vacuum is a state of gases in which the particle density is lower than in the atmosphere of the Earth.
SI unit: pascal (Pa), which is N/m2; 105 Pa = 1 bar = 750 torr; 1 torr = 1 mmHg = ~ 133 Pa.
SI unit: pascal (Pa), which is N/m2; 105 Pa = 1 bar = 750 torr; 1 torr = 1 mmHg = ~ 133 Pa.
Role of vacoom: Mean free path of the gas particles (L): the mean length between collisions of gas particles. L = C / P, where P is pressure, C is a value, depending on the temperature and gas type.
| Pressure | 10E-10 Pa | 10E-5 Pa | 1 Pa | 10E5 Pa atmosphric |
|---|---|---|---|---|
| Mean free path | 50.000 km | 500 m | 5 mm | 50 nm |
| Particles in 1 mm3 | 24 pc. | 2,4 x 10E6 | 2,4 x 10E11 | 2,4 x 10E16 |
Evaporated particles may react with gas molecules and chemically contaminate the deposited layer therefore higher vacuum provides less impurities. Gas molecules are adsorbed on surfaces within the vacuum chamber. Glimming procedure removes them, but the surface monolayer is rebulit during a time interval determined by pressure and temperature.
| Pressure | 10E-10 Pa | 10E-5 Pa | 1 Pa | 10E5 Pa atmosphric |
|---|---|---|---|---|
| Time for one monolayer to form | 1 month | 30 s | 300 µs | 3 ns |
Main components of vacuum of the system
- Vacuum pumps (vp): two phases shall be used for generating high-vacuum
- Vacuum gauges (vg): two phases shall be used for measuring rough- and high-vacuum
- Valves (va)
- Vacuum chamber (vc)
Vacuum Pumps
Pumps operate based on three main principles (while there are hundreds of different constructions):
- principle of volume separation (mainly for rough vacuum)
- propulsive and momentum-transfer (for high-vacuum)
- principle of adsorption (mainly increase purity)
Types of vacuum pumps:
Rotary Rough Vacuum Pumps (Rotating-Sliding Blade Pump):
Operation principle: Cyclically separates a given volume of gas, compresses it, then pumps out through a valve. Operation interval: from 10E5 Pa to 1 Pa.
High vacuum pumps I. - Oil diffusion pump
Operation principle: Gas particles diffuse into the oil vapour and collide with high speed oil particles. Operation interval: from 1 Pa to 10E-7 Pa.
Pros: high pumping speed; inexpensive; durable and reliable. Cons: oil backstreamed into the vacuum chamber.
Pros: high pumping speed; inexpensive; durable and reliable. Cons: oil backstreamed into the vacuum chamber.
High vacuum pumps II. - Turbomolecular pump
Operation principle: Gas particles collide with the blades of rotor. Up to 100.000 rot. / min. Operation interval: from 10E-2 Pa to 10E-8 Pa. Pros: operates without oil and high suction speed. Cons: expensive.
Adsorbing pumps - Increasing vacuum and purity:
Oil trap: Gas/vapour particles condensate on cooled (with H2O, liquid N2) surfaces. The partial pressue is limited by the temperature of the coldest surface.
Getter pumps (selective for given/certain gases/vapours): Chemically bond or physically absorb the particles.
Getter pumps (selective for given/certain gases/vapours): Chemically bond or physically absorb the particles.
Measuring Vacuum
There are several principles and devices for vacuum measurement – their application depends on pressure range, accuracy, environment, etc. At least two gauges are necessary for a high-vacuum system (for measuring rough- and high-vacuum).
Main vacuum gauge principles in different pressure ranges:
Measuring Vacuum - Principle I: Pirani vacuum gauge
A special wire is heated, which has a temperature dependent resistance value. The wire is only cooled by the rare gas in the vacuum. The heating current necessary for keeping a constant wire temperature depends on the pressure.
Measuring Vacuum - Principle II: Ionization vacuum gauge
An electron current is generated in the vacuum, which ionizes the gas particles. Ions are then trapped by a negative electrode and they are „calculated” (~ion current). Decreasing ion current means decreasing pressure.
High-speed electrons might generate X-ray photons as they hit the anode. These photons, unfortunately, hit the ion-collector and generate photo-electrons. Photo-electron current is added to the ion current, which results in worse signal/noise ratio.
High-speed electrons might generate X-ray photons as they hit the anode. These photons, unfortunately, hit the ion-collector and generate photo-electrons. Photo-electron current is added to the ion current, which results in worse signal/noise ratio.
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