Molecular pump for forming a vacuum

A molecular pump comprising:

• a casing having an interior space, an inlet port for connection to a chamber to be exhausted during operation of the molecular pump by the introduction of gas molecules from the chamber into the interior space, and an outlet port for discharging the gas from the interior space;
• a first pumping stage disposed in the interior space of the casing for compressing gas introduced into the interior space from the chamber during operation of the molecular pump, the first pumping stage having a stator and a rotor mounted to undergo rotation relative to the stator;
• a second pumping stage disposed in the interior space of the casing for further compressing the gas compressed in the first pumping stage during operation of the molecular pump, the second pumping stage including a portion of the rotor and a portion of the stator so that a surface of the portion of the rotor is disposed in confronting and spaced-apart relation to a surface of the portion of the stator, and a groove formed in at least one of the confronting and spaced-apart surfaces of the portions of the rotor and the stator;
• and
• control means for controlling a magnitude of a clearance between the confronting and spaced-apart surfaces of the portions of the rotor and the stator during operation of the molecular pump to thereby control a degree of exhaustion of the chamber.