USE OF QUADRUPOLE MASS SPECTROMETERS IN ULTRAHIGH-VACUUM SYSTEMS

In ultrahigh vacuum systems quadrupole mass spectrometers are most widely used for the determination of the residual gas composition as well as for process control, e.g. at higher pressure. Quadrupoles here have several advantages over other types of mass spectrometers such as ease of installation and operation, small size, high dynamic range, fast measuring speed and no magnetic fields. For residual gas measurements in the extreme UHV specially adapted quadrupoles must be used. They have to show adequately low outgassing rates for keeping the original gas composition undisturbed. This is obtained by the choice of special materials and by a high temperature treatment of all components. In addition, very low detection limits are needed. This requires a special detector design for the suppression of possible background signals. An overview on the detection capabilities, background effects, possible artifacts, operation software as well as multiplexing opportunities will be given. Another application for quadrupoles in uhv systems is process control. Examples here are (a) parallel determination and on-line closed loop control of the evaporation rates of several sources during, e.g. molecular beam epitaxy (MBE) processes utilizing the fast measuring speed; and (b) process gas monitoring during, e.g. sputter processes utilizing the high dynamic range.