IMPROVEMENT OF TURBOMOLECULAR PUMPS FOR ULTRACLEAN, LOW-PRESSURE, ANDHIGH-GAS-FLOW PROCESSING

Turbomolecular pumps (TMP) have been widely used in low-pressure proce ssing, because TMP can pump a large flow of gas as well as can exhaust the system to a suitable base pressure. A TMP, however, has the probl em that its pumping speed is severely degraded at inlet pressures high er than similar to 10 mTorr, where most of the low-pressure processing of semiconductor manufacturing is performed. In this study, it is con firmed that the occurrence of such a degradation in TMP performance is primarily determined by the inlet pressure of TMP and not by the outl et pressure. As the inlet pressure is increased to similar to 10 mTorr , the gas flow in the front blades as well as the oar blades of the TM P goes into the transition flow, and TMP pumping speed consequently be gins to decrease. Once the degradation occurs, however, the pumping sp eed is strongly dependent upon the pumping speed of the backing pumps, namely the foreline pressure. In order to improve the TMP performance under high gas flow, a characteristic dimension in the pump, particul arly at the entrance, such as the distance between each blade vane and the distance between the rotor blade and the stator blade, was shorte ned to increase Knudsen's number. This enables us to maintain the mole cular flow region at higher pressures and to improve the TMP performan ce under high gas flow. The newly developed TMP can endure a gas flow as great as similar to 1500 seem with maintaining the acceptable pumpi ng speed. We have also experimentally measured the impurity level in t he chamber due to back-diffusion through TMP, as well as the impurity compression ratio under high gas flow. It is demonstrated that the imp urity level in the chamber can be made several orders of magnitude low er than that for the best performance of TMP under ultrahigh vacuum op eration when adequate gas flow is fed into the chamber. (C) 1998 Ameri can Vacuum Society. [S0734-2101(98)11404-5].