GAS FLOW in a reaction chamber

• The gas flow is given not in volume per time, but in number per time, or concentration of molecules in the chamber
  •  due to the fact that gasses are compressible (volume is not constant), unlike liquid

• On average the flow will move from high to low pressure, to reduce the pressure gradient

• The pressure gradient is created by introducing gas in a chamber and pumping out the gas (see picture below)
• A part of the gas molecules will diffuse toward the substrate and react at the surface

• The throughput (which enters the chamber) can be defined as the product between pressure and volume divided by time: Q = (pV)/time
  • because there is a relation between concentration,  volume and pressure (pV = Nk_BT), thus the gas flow (N/time) is proportional with the throughput 

• The conductance of a pipe (or a vacuum component) connecting the chamber to the pump (pipes reduces the capacity of pumping) is defined as:, therefore the pipes should be short and with a large diameter, so that the gas can be efficiently pumped
• Knudsen number gives information about the type of flow:  (defined as the ratio between mean free path and reactor's length)
  • Kn > 1 - Molecular flow (if the mean free path is larger than the reactor dimensions, the molecules do not collide at all before meeting the walls, and the gas behaves like a sum of molecules, instead of a fluid)
    • for low pressures (~ 1mTorr)
    • pumps are important to create these low pressures
  • 1 > Kn > 0.01 - Intermediate flow
  • 0.01 > Kn -  Viscuous flow (if the mean free path is much smaller than the reactor dimensions, a huge number of collisions take place and the gas behaves as a fluid)