Diffusion pump

M6 Diffusion Pump

Ulvac oil diffusion pump ULK-04 cutaway

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Calutron diffusion pumps

Diffusion pump schematic

DP pumping speed plot

Diffusion pump is a type of vacuum pump that uses a high speed jet of vapor to direct gas molecules in the pump throat down into the bottom of the pump and out through the exhaust. It is used to create extremely high vacuum environments, typically in the range of 10^-4 to 10^-8 torr. The diffusion pump is an important tool in many scientific and industrial applications where a high vacuum is required, such as in the fields of electron microscopy, semiconductor fabrication, and nuclear physics.

Principle of Operation

The operation of a diffusion pump relies on the diffusion of gas molecules into a vapor stream. The pump uses a heating element to boil a special type of oil or fluid, creating vapor. This vapor is ejected at high speed from a nozzle at the top of the pump. As the vapor travels downwards, it encounters gas molecules within the pump. These gas molecules are captured by the vapor stream and carried along with it. The momentum of the vapor stream is such that it directs the gas molecules towards the exhaust, where they are removed from the system by a secondary pump, usually a rotary vane pump or a roots blower.

Components

A typical diffusion pump consists of several key components:

• Boiler: Heats the pumping fluid to generate vapor.
• Jet nozzles: Direct the vapor stream downwards.
• Pumping fluid: Usually a type of silicone oil or polyphenyl ether, chosen for its low vapor pressure and thermal stability.
• Water cooling system: Condenses the vapor back into a liquid so it can be recycled.
• Foreline: Connects the diffusion pump to a secondary pump.

Advantages and Disadvantages

Advantages

• Capable of achieving very high vacuum levels.
• No moving parts, leading to low maintenance requirements.
• Can pump a wide range of gases.

Disadvantages

• Requires a secondary pump to initiate the vacuum process.
• Can be contaminated by backstreaming of the pumping fluid, which may affect the vacuum environment.
• Not effective at atmospheric pressure, requiring multiple stages of pumping for initial evacuation.

Applications

Diffusion pumps are widely used in various high-vacuum applications, including:

• Semiconductor fabrication, for processes such as physical vapor deposition (PVD) and chemical vapor deposition (CVD).
• Mass spectrometry, to maintain the vacuum necessary for accurate analysis.
• Electron microscopy, to ensure a high-vacuum environment for detailed imaging.
• Space simulation chambers, to replicate the vacuum of space for testing spacecraft components.

Maintenance and Safety

Regular maintenance is crucial for the optimal performance of a diffusion pump. This includes checking the integrity of the cooling system, ensuring the pumping fluid is clean and free of contaminants, and inspecting the pump for any signs of wear or damage. Safety precautions must also be observed, particularly regarding the handling of the pumping fluid, which can be hazardous.