Lab 3: Leak Rate and Outgassing

Leaks

One of the most common problems with the vacuum systems used in plasma processing is leaks. These leaks can cause problems through two means. First, if the leak is large, the base pressure – e.g. minimum pressure – of the system may be too high to operate. Second, if the leak is small the non-process gas can and often will change the process conditions. (The addition of small, <0.01%, amounts of water can radically change the plasma.)

Leaks come in two flavors, external and internal. External leaks arise from small holes in the seals of the system. These holes might occur in weld joints between metal parts or scratches in orings, oring grooves/surfaces, scratches in metal gaskets or metal gasket mating surfaces. In addition dirt on any of these mating surfaces can cause leaks. Internal leaks come in two flavors, outgassing (which is not really a leak but it might as well be) and virtual leaks. Virtual leaks arise from trapped volumes of gas that slowly leak into the system. These trapped volumes can be limited in supply or they can be fed from an outside source, such as air, making them essentially unlimited. (In general it is not good practice to create trapped volumes between the vacuum system and a high-pressure location. If the seals on both sides of the volume leak, this will result in a slow leak into the vacuum system. Because such a leak is extremely slow, this makes finding the leak very difficult.)

In this lab we are going to examine leaks in process chambers. The first type of leak that we will examine is an external leak. We will create this leak by putting a human hair across an oring and pumping the system down. We will then look at outgassing and a virtual leak with a limited gas supply.

1) Note the current pressure of the vacuum system. This is the base pressure of the system after pumping on the system for many hours – usually at least over night.

2) Bring the plasma chamber up to atmosphere and briefly open the door. This will expose the internal surfaces to air which contains a number of molecules, chiefly H2O, which tend to stick to the vacuum chamber surfaces. Leave the system open for 1 minute.

3) Pump the system down, recording the chamber pressure in 30 sec increments. Record a total of 10 minutes.

4) Vent the system again and repeat the pump down sequence after adding the following items: (Only one item is to be added at a time.)

A human hair across the oring of the door seal.
Isopropyl Alcohol soaked wipe in a cup. (The cup is make such that we do not leave any residual alcohol in the system.)
Methyl Alcohol soaked wipe in a cup.
H2O soaked wipe in a cup.
Acetone soaked wipe in a cup.
Tightly waded ball of Al food-service foil. Note that this foil is precoated with a thin layer of cooking oil. Wading the foil also serve to trap pockets of air.
Tightly waded ball of stainless steel foil. Note that there is no oil on this.
Virtual leak – a bolt into a block of Al that is trapping a pocket of air.
Virtual leak with a vented screw.

5) For your report plot the vacuum chamber pressure as a function of time. Explain the differences that you observe between the different leaks. For this explanation, you will need the vapor temperature at different pressures. These can be found in the CRC handbook of Physics and Chemistry.