Apparatus for controlling the pressure in a process chamber and method of operating same

An apparatus for controlling the pressure in a process chamber comprising:

• a first pump unit having an inlet for fluid connection with an outlet of said process chamber,
• a second pump unit having an inlet for fluid connection with an outlet of the first pump unit via a flow control unit, the flow control unit comprising a variable flow control device having variable conductance for controlling fluid pressure at the outlet of the first pump unit, and
• means for controlling speed of the first pump unit below a predetermined value to increase an operable range of chamber pressures over which control of the variable flow control device produces changes in said chamber pressure, wherein the range does not exceed a limit selected from the group consisting of a thermal limit and a motor stall limit of the first pump unit, wherein the means is electrically connected to the first pump unit.

An apparatus for controlling the pressure in a process chamber comprising:

• a first pump unit having an inlet for fluid connection with an outlet of said process chamber, a second pump unit having an inlet for fluid connection with an outlet of the first pump unit via a flow control unit, the flow control unit comprising a variable flow control device having variable conductance for controlling fluid pressure at the outlet of the first pump unit, and means for controlling speed of the first pump unit to increase the range of chamber pressures over which control of the variable flow control device produces changes in said chamber pressure not exceeding a limit selected from the group consisting of a thermal limit and a motor stall limit of the first pump unit, wherein the flow control unit comprises a further flow control device for decreasing the total conductance of the flow control unit thereby increasing the operative range over which control of the variable flow control device effects changes in the pressure in the process chamber.