Baking--Factor That Affects The Vacuum System

- Jul 02, 2018-

For baking, In order to achieve pressure in the ultra-high vacuum range (<10-8 hPa), the following conditions must be met:


1. The ultimate vacuum of the vacuum pump should be less than 10 times the required ultimate pressure.


2. The materials used for the vacuum chambers and components must be optimized for minimum outgassing, and it should have an appropriate level of surface treatment.


3. Metal seals (for example, CF flange connections or ISO flange standard Helicoflex seals) should be used.


4. Cleaning is essential for ultra-high vacuum. All parts must be thoroughly cleaned before installation and operators must wear grease-free gloves when installing parts.


5. Equipment and high vacuum pumps must be baked.


6. Leaks must be avoided and eliminated before starting the heater. For this purpose, a helium leak detector or a quadrupole mass spectrometer must be used.


7. Baking significantly increases the desorption rate and the diffusivity, and this leads to a drastic shortening of the suction time. As the final step of the manufacturing process, UHV chambers can be degraded at temperatures up to 900°C. The subsequent baking temperature may reach 300°C. The pump manufacturer's instructions for the highest bake temperature of the high vacuum pump flange typically limit the maximum temperature in the working to 120°C. If a heat source (for example, radiant heating) is used in a vacuum device, the permissible radiated power must be observed.


8. The equipment is put into operation after installation. After reaching a pressure of 10-5 hPa, turn on the heater. During the heating process, the vacuum gauge was operated and degassed per 10 hours. If a stainless steel container and metal seal with a suitable surface treatment grade are used, a baking temperature of 120°C and a heating time of about 48 hours are sufficient to reach a pressure range of 10-10 hPa.


9. Baking should continue until it reaches 100 times the expected ultimate pressure. Then turn off the pump and vacuum chamber heater. After cooling, the desired ultimate pressure is likely to be achieved. At pressures below 5x10-10 hPa, a large internal surface area will be advantageous to use a gas-associated pump (titanium sublimation pump) that pumps out the hydrogen escaping from the metal at a high volumetric flow rate.