With the reaction chamber at low pressure, a venting system can be employed to bring the chamber back to atmospheric pressure, as needed if the chamber is to be opened.  Venting a chamber involves two sets of actions:

1. Closing all valves which connect the chamber to pumps, other chambers, and other gas inlet connections.

For venting, pumping should be terminated, which is most easily done by closing all valves connecting the chamber to pumps. (Note: in other applications, such as maintaining a fixed pressure/flow condition in the reaction chamber, gas inlet may be used together with pumping).

Closing valves connecting to other chambers is helpful in order to isolate the pressure and usage change of the chamber to be vented. Closing valves connecting to other gas sources could be critical from a safety point of view. (E.g., if silane is flowing from another source into the chamber, it would be dangerous to venting the chamber to air, since silane is explosive and flammable in oxygen).

2. Opening a cutoff valve to admit inert gas (or air) into the chamber

Use of inert gas (nitrogen, argon, helium) is important so that reactive species present in air (e.g., water vapor, hydrocarbons) do not contaminate the chamber walls.  The gas source for venting must provide (i) a tank pressure above atmospheric, and (ii) enough gas to fill the chamber to atmospheric pressure.  The venting system should also provide means to terminate gas flow into the chamber when atmospheric pressure is reached, so that the chamber cannot fill to much higher pressures (since gas sources are often supplied at many times atmospheric pressure).

Venting system
The vacuum pump system simulator provides a nitrogen gas source and venting valve VV connecting it to the reaction chamber.  For simplicity, the source is arranged so that it will supply gas only to atmospheric pressure, or slightly higher (a slight overpressure).  Venting to slightly above atmospheric pressure is common, since the overpressure in the chamber reduces the amount of air which enters the chamber when the chamber is opened to the room.

Exercises

Exercise 1: With the reaction chamber at a low pressure, vent the chamber.   Close all valves (VB, V2, VV) connecting the reaction chamber.  Open the vent valve VV connecting the gas source to the reaction chamber.  Observation: Notice that the pressure in the reaction chamber increases. This is because the reaction chamber is completely sealed, except for the inlet of gas caused by opening the vent valve. Questions   Roughly how long does it take to vent the reaction chamber to atmospheric pressure?  For semiconductor manufacturing, do we want fast or slow venting?  Answers

 

Summary
The venting system permits bringing the reaction chamber back up to atmospheric pressure. Venting rates depend on conductance of the venting line, gas source pressure, chamber pressure, and chamber volume.

While fast venting is desired for manufacturing speed, slow initial venting helps to reduce particle generation and thus to improve yield.

Knowing how to pump down and vent the reaction chamber using the mechanical pump, bypass line, and venting system, additional experiments and exercises can be carried out as desired.